testing: switch to upstream version

2026-07-29 16:18:41 +00:00 · 2025-10-05 15:33:49 +02:00
120 changed files with 1075 additions and 5321 deletions
@@ -16,15 +16,14 @@ jobs:
    name: build-macos
    strategy:
      matrix:
-        # macos-14: arm64 (oldest supported version as of 18-11-2025)
-        # macos-15-intel: amd64 (last intel version to be supported by github runners)
-        # See https://github.com/actions/runner-images/issues/13046
-        os: [macos-14, macos-15-intel]
+        # macos-13: amd64 (oldest supported version as of 18-10-2024)
+        # macos-14: arm64 (oldest arm64 version)
+        os: [macos-13, macos-14]
        include:
+          - os: macos-13
+            goarch: amd64
          - os: macos-14
            goarch: arm64
-          - os: macos-15-intel
-            goarch: amd64
    runs-on: ${{ matrix.os }}
    steps:
      - name: Install Dependencies
@@ -40,7 +39,7 @@ jobs:
      - name: Install Go
        uses: actions/setup-go@v6
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.1'
          cache: true
      - name: Restore LLVM source cache
        uses: actions/cache/restore@v4
@@ -135,7 +134,7 @@ jobs:
      - name: Install Go
        uses: actions/setup-go@v6
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.1'
          cache: true
      - name: Build TinyGo (LLVM ${{ matrix.version }})
        run: go install -tags=llvm${{ matrix.version }}
@@ -31,7 +31,7 @@ jobs:
          sudo rm -rf /usr/local/share/boost
          df -h
      - name: Check out the repo
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
        with:
          submodules: recursive
      - name: Set up Docker Buildx
@@ -58,7 +58,7 @@ jobs:
          username: ${{ github.actor }}
          password: ${{ secrets.GITHUB_TOKEN }}
      - name: Build and push
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v5
        with:
          context: .
          push: true
@@ -66,3 +66,45 @@ jobs:
          labels: ${{ steps.meta.outputs.labels }}
          cache-from: type=gha
          cache-to: type=gha,mode=max
+      - name: Trigger Drivers repo build on Github Actions
+        run: |
+          curl -X POST \
+          -H "Authorization: Bearer ${{secrets.GHA_ACCESS_TOKEN}}" \
+          -H "Accept: application/vnd.github.v3+json" \
+          https://api.github.com/repos/tinygo-org/drivers/actions/workflows/build.yml/dispatches \
+          -d '{"ref": "dev"}'
+      - name: Trigger Bluetooth repo build on Github Actions
+        run: |
+          curl -X POST \
+          -H "Authorization: Bearer ${{secrets.GHA_ACCESS_TOKEN}}" \
+          -H "Accept: application/vnd.github.v3+json" \
+          https://api.github.com/repos/tinygo-org/bluetooth/actions/workflows/linux.yml/dispatches \
+          -d '{"ref": "dev"}'
+      - name: Trigger TinyFS repo build on Github Actions
+        run: |
+          curl -X POST \
+          -H "Authorization: Bearer ${{secrets.GHA_ACCESS_TOKEN}}" \
+          -H "Accept: application/vnd.github.v3+json" \
+          https://api.github.com/repos/tinygo-org/tinyfs/actions/workflows/build.yml/dispatches \
+          -d '{"ref": "dev"}'
+      - name: Trigger TinyFont repo build on Github Actions
+        run: |
+          curl -X POST \
+          -H "Authorization: Bearer ${{secrets.GHA_ACCESS_TOKEN}}" \
+          -H "Accept: application/vnd.github.v3+json" \
+          https://api.github.com/repos/tinygo-org/tinyfont/actions/workflows/build.yml/dispatches \
+          -d '{"ref": "dev"}'
+      - name: Trigger TinyDraw repo build on Github Actions
+        run: |
+          curl -X POST \
+          -H "Authorization: Bearer ${{secrets.GHA_ACCESS_TOKEN}}" \
+          -H "Accept: application/vnd.github.v3+json" \
+          https://api.github.com/repos/tinygo-org/tinydraw/actions/workflows/build.yml/dispatches \
+          -d '{"ref": "dev"}'
+      - name: Trigger TinyTerm repo build on Github Actions
+        run: |
+          curl -X POST \
+          -H "Authorization: Bearer ${{secrets.GHA_ACCESS_TOKEN}}" \
+          -H "Accept: application/vnd.github.v3+json" \
+          https://api.github.com/repos/tinygo-org/tinyterm/actions/workflows/build.yml/dispatches \
+          -d '{"ref": "dev"}'
@@ -31,7 +31,7 @@ jobs:
        # We're not on a multi-user machine, so this is safe.
        run: git config --global --add safe.directory "$GITHUB_WORKSPACE"
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
        with:
          submodules: true
      - name: Extract TinyGo version
@@ -131,13 +131,13 @@ jobs:
    needs: build-linux
    steps:
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
        with:
          submodules: true
      - name: Install Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@v5
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.0'
          cache: true
      - name: Install wasmtime
        uses: bytecodealliance/actions/wasmtime/setup@v1
@@ -164,7 +164,7 @@ jobs:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
        with:
          submodules: true
      - name: Install apt dependencies
@@ -179,9 +179,9 @@ jobs:
              simavr \
              ninja-build
      - name: Install Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@v5
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.0'
          cache: true
      - name: Install Node.js
        uses: actions/setup-node@v4
@@ -284,7 +284,7 @@ jobs:
    needs: build-linux
    steps:
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
      - name: Get TinyGo version
        id: version
        run: ./.github/workflows/tinygo-extract-version.sh | tee -a "$GITHUB_OUTPUT"
@@ -296,9 +296,9 @@ jobs:
              g++-${{ matrix.toolchain }} \
              libc6-dev-${{ matrix.libc }}-cross
      - name: Install Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@v5
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.0'
          cache: true
      - name: Restore LLVM source cache
        uses: actions/cache/restore@v4
@@ -25,7 +25,7 @@ jobs:
      contents: read
    steps:
      - name: Check out the repo
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
        with:
          submodules: recursive
      - name: Set up Docker Buildx
@@ -52,7 +52,7 @@ jobs:
          username: ${{ github.actor }}
          password: ${{ secrets.GITHUB_TOKEN }}
      - name: Build and push
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v5
        with:
          target: tinygo-llvm-build
          context: .
@@ -21,7 +21,7 @@ jobs:
        # See: https://github.com/tinygo-org/tinygo/pull/4516#issuecomment-2416363668
        run: sudo apt-get remove llvm-18
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
      - name: Pull musl, bdwgc
        run: |
            git submodule update --init lib/musl lib/bdwgc
@@ -20,7 +20,7 @@ jobs:
        run: |
          echo "$HOME/go/bin" >> $GITHUB_PATH
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
        with:
          fetch-depth: 0 # fetch all history (no sparse checkout)
          submodules: true
@@ -31,7 +31,7 @@ jobs:
        run: |
          scoop install ninja binaryen
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
        with:
          submodules: true
      - name: Extract TinyGo version
@@ -39,9 +39,9 @@ jobs:
        shell: bash
        run: ./.github/workflows/tinygo-extract-version.sh | tee -a "$GITHUB_OUTPUT"
      - name: Install Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@v5
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.0'
          cache: true
      - name: Restore cached LLVM source
        uses: actions/cache/restore@v4
@@ -143,11 +143,11 @@ jobs:
        run: |
          scoop install binaryen
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
      - name: Install Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@v5
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.0'
          cache: true
      - name: Download TinyGo build
        uses: actions/download-artifact@v4
@@ -173,11 +173,11 @@ jobs:
          maximum-size: 24GB
          disk-root: "C:"
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
      - name: Install Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@v5
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.0'
          cache: true
      - name: Download TinyGo build
        uses: actions/download-artifact@v4
@@ -209,11 +209,11 @@ jobs:
        run: |
          scoop install binaryen && scoop install wasmtime@29.0.1
      - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v4
      - name: Install Go
-        uses: actions/setup-go@v6
+        uses: actions/setup-go@v5
        with:
-          go-version: '1.25.5'
+          go-version: '1.25.0'
          cache: true
      - name: Download TinyGo build
        uses: actions/download-artifact@v4
@@ -16,7 +16,7 @@
 	url = https://github.com/WebAssembly/wasi-libc
 [submodule "lib/picolibc"]
 	path = lib/picolibc
-	url = https://github.com/picolibc/picolibc.git
+	url = https://github.com/keith-packard/picolibc.git
 [submodule "lib/stm32-svd"]
 	path = lib/stm32-svd
 	url = https://github.com/tinygo-org/stm32-svd
@@ -1,104 +1,3 @@
-0.40.1
---
-* **machine**
-  - nrf: fix flash writes when SoftDevice is enabled
-* **runtime**
-  - runtime: avoid fixed math/rand sequence on RP2040/RP2350 (#5124)
-  - runtime: add calls to initRand() during run() for all schedulers
-  - runtime: call initRand() before initHeap() during initialization
-  - runtime: use rand_hwrng hardwareRand for RP2040/RP2350 (#5135)
-* **libs**
-  - picolibc: use updated location for git repo
-
-0.40.0
---
-* **general**
-  - all: add full LLVM 20 support
-  - core: feat: enable //go:linkname pragma for globals
-  - core: feature: Add flag to ignore go compatibility matrix (#5078)
-  - chore: update version for 0.40 development cycle
-* **compiler**
-  - emit an error when the actual arch doesn't match GOARCH
-  - mark string parameters as readonly
-  - use Tarjan's SCC algorithm to detect loops for defer
-  - lower "large stack" limit to 16kb
-* **core**
-  - shrink bdwgc library
-  - Fix linker errors for runtime.vgetrandom and crypto/internal/sysrand.fatal
-  - fix: add TryLock to sync.RWMutex
-  - fix: correct linter issues exposed by the fix in #4679
-  - fix: don't hardcode success return state
-  - fix: expand RTT debugger compatibility
-  - internal/task (threads): save stack bounds instead of scanning under a lock
-  - internal/task: create detached threads and fix error handling
-  - interp: better errors when debugging interp
-  - transform (gc): create stack slots in callers of external functions
-  - internal/task: prevent semaphore resource leak for threads scheduler
-* **machine**
-  - cortexm: optimize code size for the HardFault_Handler
-  - fe310: add I2C pins for the HiFive1b
-  - clarify WriteAt semantics of BlockDevice
-  - fix deprecated AsmFull comment (#5005)
-  - make sure DMA buffers do not escape unnecessarily
-  - only enable USB-CDC when needed
-  - use larger SPI MAXCNT on nrf52833 and nrf52840
-  - fix: update m.queuedBytes when clamping output to avoid corrupting sentBytes
-  - fix: use int64 in ReadTemperature to avoid overflow
-  - fix(rp2): disable DBGPAUSE on startup
-  - fix(rp2): possible integer overflow while computing factors for SPI baudrate
-  - fix(rp2): reset spinlocks at startup
-  - fix(rp2): switch spinlock busy loop to wfe
-  - fix(rp2): use side-effect-free spinlocks
-  - nrf: add ADC_VDDH which is an ADC pin for VDDH
-  - nrf: don't block SPI transfer
-  - nrf: don't set PSELN, it's ignored in single ended mode anyway
-  - nrf: fix typo in ADC configuration
-  - nrf: refactor SoftDevice enabled check
-  - nrf: rename pwmPin to adcPin
-  - nrf: support flash operations while the SoftDevice is enabled
-  - rp2040: allow writing to the UART inside interrupts
-  - machine,nrf528: stop the bus only once on I2C bus error and ensures the first error is returned
-* **net**
-  - update submodule to latest commits
-* **runtime**
-  - (avr): fix infinite longjmp loop if stack is aligned to 256 bytes
-  - (gc_blocks.go): clear full size of allocation
-  - (gc_blocks.go): make sweep branchless
-  - (gc_blocks.go): simplify scanning logic
-  - (gc_blocks.go): use a linked stack to scan marked objects
-  - (gc_blocks.go): use best-fit allocation
-  - (gc_boehm.go): fix world already stopped check
-  - (wasm): scan the system stack
-  - fix sleep duration for long sleeps
-  - remove copied code for nrf52840
-  - src/syscall: update src buffer after write
-  - wasm: fix C realloc and optimize it a bit
-* **targets**
-  - add xiao-esp32s3 board target
-  - Add ESP32-S3 support (#5091)
-  - Added Gopher ARCADE board
-  - Create "pico2-ice" target board (#5062)
-* **build/test**
-  - Add testing.T.Context() and testing.B.Context()
-  - create separate go.mod file for testing dependencies for wasm tests that use Chromium headless browser to avoid use of older incompatible version.
-  - go back to normal scheduler instead of tasks scheduler for macos CI
-  - update CI to use Go 1.25.5
-  - update macOS GH actions builds to handle sunset of macOS 13
-  - use task scheduler on macOS builds to avoid test race condition lockups
-  - update all CI builds to use latest stable Go release. Also update some of the actions to their latest releases.
-  - update GH actions builds to use Go 1.25.4
-  - uninstall cmake before install
-  - fix: point the submodule for musl-lib to a mirror in the TinyGo GitHub org
-  - fix: remove macOS 15 from CI build matrix (conflicts with macOS 14 build)
-  - fix: separate host expected bytes from device intended bytes
-  - fix/typo: makeESPFirmwareImage
-  - make: GNUmakefile: shrink TinyGo binaries on Linux
-  - move the directory list into a variable
-  - several improvements to the macOS GH actions build
-  - Fix for #4678: top-level 'make lint' wasn't working
-  - fix: increase the timeout for chromedp to connect to the headless browser used for running the wasm tests.
-  - testdata: some more packages for the test corpus
-
 0.39.0
 ---
 * **general**
@@ -363,7 +363,6 @@ TEST_PACKAGES_FAST = \
 	path \
 	reflect \
 	sync \
-	testing \
 	testing/iotest \
 	text/scanner \
 	unicode \
@@ -480,8 +479,7 @@ TEST_PACKAGES_HOST := $(TEST_PACKAGES_FAST) $(TEST_PACKAGES_WINDOWS)
 TEST_IOFS := false
 endif

-TEST_SKIP_FLAG := -skip='TestExtraMethods|TestParseAndBytesRoundTrip/P256/Generic'
-TEST_ADDITIONAL_FLAGS ?=
+TEST_SKIP_FLAG := -skip='TestExtraMethods|TestParseAndBytesRoundTrip/P256/Generic|^Fuzz'

 # Test known-working standard library packages.
 # TODO: parallelize, and only show failing tests (no implied -v flag).
@@ -489,7 +487,7 @@ TEST_ADDITIONAL_FLAGS ?=
 tinygo-test:
 	@# TestExtraMethods: used by many crypto packages and uses reflect.Type.Method which is not implemented.
 	@# TestParseAndBytesRoundTrip/P256/Generic: relies on t.Skip() which is not implemented
-	$(TINYGO) test $(TEST_ADDITIONAL_FLAGS) $(TEST_SKIP_FLAG) $(TEST_PACKAGES_HOST) $(TEST_PACKAGES_SLOW)
+	$(TINYGO) test $(TEST_SKIP_FLAG) $(TEST_PACKAGES_HOST) $(TEST_PACKAGES_SLOW)
 	@# io/fs requires os.ReadDir, not yet supported on windows or wasi. It also
 	@# requires a large stack-size. Hence, io/fs is only run conditionally.
 	@# For more details, see the comments on issue #3143.
@@ -622,8 +620,6 @@ smoketest: testchdir
 	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=feather-rp2040      examples/device-id
 	@$(MD5SUM) test.hex
-	$(TINYGO) build -size short -o test.hex -target=pico2-ice           examples/blinky1
-	@$(MD5SUM) test.hex
 	# test simulated boards on play.tinygo.org
 ifneq ($(WASM), 0)
 	GOOS=js GOARCH=wasm $(TINYGO) build -size short -o test.wasm -tags=arduino              examples/blinky1
@@ -915,8 +911,6 @@ ifneq ($(XTENSA), 0)
 	@$(MD5SUM) test.bin
 	$(TINYGO) build -size short -o test.bin -target mch2022             examples/machinetest
 	@$(MD5SUM) test.bin
-	$(TINYGO) build -size short -o test.bin -target=xiao-esp32s3   		examples/blinky1
-	@$(MD5SUM) test.bin
 endif
 	$(TINYGO) build -size short -o test.bin -target=esp-c3-32s-kit      examples/blinky1
 	@$(MD5SUM) test.bin
@@ -1042,7 +1042,7 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		if err != nil {
 			return result, err
 		}
-	case "esp32", "esp32-img", "esp32c3", "esp32s3", "esp8266":
+	case "esp32", "esp32-img", "esp32c3", "esp8266":
 		// Special format for the ESP family of chips (parsed by the ROM
 		// bootloader).
 		result.Binary = filepath.Join(tmpdir, "main"+outext)
@@ -28,7 +28,6 @@ func TestClangAttributes(t *testing.T) {
 		"cortex-m4",
 		"cortex-m7",
 		"esp32c3",
-		"esp32s3",
 		"fe310",
 		"gameboy-advance",
 		"k210",
@@ -33,13 +33,10 @@ func NewConfig(options *compileopts.Options) (*compileopts.Config, error) {
 	if err != nil {
 		return nil, err
 	}
-
-	if options.GoCompatibility {
-		if gorootMajor != 1 || gorootMinor < minorMin || gorootMinor > minorMax {
-			// Note: when this gets updated, also update the Go compatibility matrix:
-			// https://github.com/tinygo-org/tinygo-site/blob/dev/content/docs/reference/go-compat-matrix.md
-			return nil, fmt.Errorf("requires go version 1.%d through 1.%d, got go%d.%d", minorMin, minorMax, gorootMajor, gorootMinor)
-		}
+	if gorootMajor != 1 || gorootMinor < minorMin || gorootMinor > minorMax {
+		// Note: when this gets updated, also update the Go compatibility matrix:
+		// https://github.com/tinygo-org/tinygo-site/blob/dev/content/docs/reference/go-compat-matrix.md
+		return nil, fmt.Errorf("requires go version 1.%d through 1.%d, got go%d.%d", minorMin, minorMax, gorootMajor, gorootMinor)
 	}

 	// Check that the Go toolchain version isn't too new, if we haven't been
@@ -100,12 +100,11 @@ func makeESPFirmwareImage(infile, outfile, format string) error {
 	chip_id := map[string]uint16{
 		"esp32":   0x0000,
 		"esp32c3": 0x0005,
-		"esp32s3": 0x0009,
 	}[chip]

 	// Image header.
 	switch chip {
-	case "esp32", "esp32c3", "esp32s3":
+	case "esp32", "esp32c3":
 		// Header format:
 		// https://github.com/espressif/esp-idf/blob/v4.3/components/bootloader_support/include/esp_app_format.h#L71
 		// Note: not adding a SHA256 hash as the binary is modified by
@@ -42,9 +42,9 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 3668, 280, 0, 2244},
-		{"microbit", "examples/serial", 2694, 342, 8, 2248},
-		{"wioterminal", "examples/pininterrupt", 6833, 1491, 120, 6888},
+		{"hifive1b", "examples/echo", 3884, 280, 0, 2268},
+		{"microbit", "examples/serial", 2852, 360, 8, 2272},
+		{"wioterminal", "examples/pininterrupt", 7337, 1491, 116, 6912},

 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the
@@ -226,7 +226,7 @@ func (c *Config) StackSize() uint64 {

 // MaxStackAlloc returns the size of the maximum allocation to put on the stack vs heap.
 func (c *Config) MaxStackAlloc() uint64 {
-	if c.StackSize() >= 16*1024 {
+	if c.StackSize() > 32*1024 {
 		return 1024
 	}

@@ -59,7 +59,6 @@ type Options struct {
 	WITPackage      string // pass through to wasm-tools component embed invocation
 	WITWorld        string // pass through to wasm-tools component embed -w option
 	ExtLDFlags      []string
-	GoCompatibility bool // enable to check for Go version compatibility
 }

 // Verify performs a validation on the given options, raising an error if options are not valid.
@@ -245,7 +245,7 @@ func (b *builder) createRuntimeAssert(assert llvm.Value, blockPrefix, assertFunc
 	// current insert position.
 	faultBlock := b.ctx.AddBasicBlock(b.llvmFn, blockPrefix+".throw")
 	nextBlock := b.insertBasicBlock(blockPrefix + ".next")
-	b.currentBlockInfo.exit = nextBlock // adjust outgoing block for phi nodes
+	b.blockExits[b.currentBlock] = nextBlock // adjust outgoing block for phi nodes

 	// Now branch to the out-of-bounds or the regular block.
 	b.CreateCondBr(assert, faultBlock, nextBlock)
@@ -32,9 +32,6 @@ const (
 	// Whether this is a full or partial Go parameter (int, slice, etc).
 	// The extra context parameter is not a Go parameter.
 	paramIsGoParam = 1 << iota
-
-	// Whether this is a readonly parameter (for example, a string pointer).
-	paramIsReadonly
 )

 // createRuntimeCallCommon creates a runtime call. Use createRuntimeCall or
@@ -170,7 +167,6 @@ func (c *compilerContext) flattenAggregateType(t llvm.Type, name string, goType
 				continue
 			}
 			suffix := strconv.Itoa(i)
-			isString := false
 			if goType != nil {
 				// Try to come up with a good suffix for this struct field,
 				// depending on which Go type it's based on.
@@ -187,16 +183,12 @@ func (c *compilerContext) flattenAggregateType(t llvm.Type, name string, goType
 						suffix = []string{"r", "i"}[i]
 					case types.String:
 						suffix = []string{"data", "len"}[i]
-						isString = true
 					}
 				case *types.Signature:
 					suffix = []string{"context", "funcptr"}[i]
 				}
 			}
 			subInfos := c.flattenAggregateType(subfield, name+"."+suffix, extractSubfield(goType, i))
-			if isString {
-				subInfos[0].flags |= paramIsReadonly
-			}
 			paramInfos = append(paramInfos, subInfos...)
 		}
 		return paramInfos
@@ -152,12 +152,10 @@ type builder struct {
 	llvmFnType        llvm.Type
 	llvmFn            llvm.Value
 	info              functionInfo
-	locals            map[ssa.Value]llvm.Value // local variables
-	blockInfo         []blockInfo
+	locals            map[ssa.Value]llvm.Value            // local variables
+	blockEntries      map[*ssa.BasicBlock]llvm.BasicBlock // a *ssa.BasicBlock may be split up
+	blockExits        map[*ssa.BasicBlock]llvm.BasicBlock // these are the exit blocks
 	currentBlock      *ssa.BasicBlock
-	currentBlockInfo  *blockInfo
-	tarjanStack       []uint
-	tarjanIndex       uint
 	phis              []phiNode
 	deferPtr          llvm.Value
 	deferFrame        llvm.Value
@@ -189,22 +187,11 @@ func newBuilder(c *compilerContext, irbuilder llvm.Builder, f *ssa.Function) *bu
 		info:            c.getFunctionInfo(f),
 		locals:          make(map[ssa.Value]llvm.Value),
 		dilocals:        make(map[*types.Var]llvm.Metadata),
+		blockEntries:    make(map[*ssa.BasicBlock]llvm.BasicBlock),
+		blockExits:      make(map[*ssa.BasicBlock]llvm.BasicBlock),
 	}
 }

-type blockInfo struct {
-	// entry is the LLVM basic block corresponding to the start of this *ssa.Block.
-	entry llvm.BasicBlock
-
-	// exit is the LLVM basic block corresponding to the end of this *ssa.Block.
-	// It will be different than entry if any of the block's instructions contain internal branches.
-	exit llvm.BasicBlock
-
-	// tarjan holds state for applying Tarjan's strongly connected components algorithm to the CFG.
-	// This is used by defer.go to determine whether to stack- or heap-allocate defer data.
-	tarjan tarjanNode
-}
-
 type deferBuiltin struct {
 	callName string
 	pos      token.Pos
@@ -1233,29 +1220,14 @@ func (b *builder) createFunctionStart(intrinsic bool) {
 		// intrinsic (like an atomic operation). Create the entry block
 		// manually.
 		entryBlock = b.ctx.AddBasicBlock(b.llvmFn, "entry")
-		// Intrinsics may create internal branches (e.g. nil checks).
-		// They will attempt to access b.currentBlockInfo to update the exit block.
-		// Create some fake block info for them to access.
-		blockInfo := []blockInfo{
-			{
-				entry: entryBlock,
-				exit:  entryBlock,
-			},
-		}
-		b.blockInfo = blockInfo
-		b.currentBlockInfo = &blockInfo[0]
 	} else {
-		blocks := b.fn.Blocks
-		blockInfo := make([]blockInfo, len(blocks))
 		for _, block := range b.fn.DomPreorder() {
-			info := &blockInfo[block.Index]
 			llvmBlock := b.ctx.AddBasicBlock(b.llvmFn, block.Comment)
-			info.entry = llvmBlock
-			info.exit = llvmBlock
+			b.blockEntries[block] = llvmBlock
+			b.blockExits[block] = llvmBlock
 		}
-		b.blockInfo = blockInfo
 		// Normal functions have an entry block.
-		entryBlock = blockInfo[0].entry
+		entryBlock = b.blockEntries[b.fn.Blocks[0]]
 	}
 	b.SetInsertPointAtEnd(entryBlock)

@@ -1351,9 +1323,8 @@ func (b *builder) createFunction() {
 		if b.DumpSSA {
 			fmt.Printf("%d: %s:\n", block.Index, block.Comment)
 		}
+		b.SetInsertPointAtEnd(b.blockEntries[block])
 		b.currentBlock = block
-		b.currentBlockInfo = &b.blockInfo[block.Index]
-		b.SetInsertPointAtEnd(b.currentBlockInfo.entry)
 		for _, instr := range block.Instrs {
 			if instr, ok := instr.(*ssa.DebugRef); ok {
 				if !b.Debug {
@@ -1413,7 +1384,7 @@ func (b *builder) createFunction() {
 		block := phi.ssa.Block()
 		for i, edge := range phi.ssa.Edges {
 			llvmVal := b.getValue(edge, getPos(phi.ssa))
-			llvmBlock := b.blockInfo[block.Preds[i].Index].exit
+			llvmBlock := b.blockExits[block.Preds[i]]
 			phi.llvm.AddIncoming([]llvm.Value{llvmVal}, []llvm.BasicBlock{llvmBlock})
 		}
 	}
@@ -1527,11 +1498,11 @@ func (b *builder) createInstruction(instr ssa.Instruction) {
 	case *ssa.If:
 		cond := b.getValue(instr.Cond, getPos(instr))
 		block := instr.Block()
-		blockThen := b.blockInfo[block.Succs[0].Index].entry
-		blockElse := b.blockInfo[block.Succs[1].Index].entry
+		blockThen := b.blockEntries[block.Succs[0]]
+		blockElse := b.blockEntries[block.Succs[1]]
 		b.CreateCondBr(cond, blockThen, blockElse)
 	case *ssa.Jump:
-		blockJump := b.blockInfo[instr.Block().Succs[0].Index].entry
+		blockJump := b.blockEntries[instr.Block().Succs[0]]
 		b.CreateBr(blockJump)
 	case *ssa.MapUpdate:
 		m := b.getValue(instr.Map, getPos(instr))
@@ -1599,8 +1570,7 @@ func (b *builder) createBuiltin(argTypes []types.Type, argValues []llvm.Value, c
 		elemsLen := b.CreateExtractValue(elems, 1, "append.elemsLen")
 		elemType := b.getLLVMType(argTypes[0].Underlying().(*types.Slice).Elem())
 		elemSize := llvm.ConstInt(b.uintptrType, b.targetData.TypeAllocSize(elemType), false)
-		elemLayout := b.createObjectLayout(elemType, pos)
-		result := b.createRuntimeCall("sliceAppend", []llvm.Value{srcBuf, elemsBuf, srcLen, srcCap, elemsLen, elemSize, elemLayout}, "append.new")
+		result := b.createRuntimeCall("sliceAppend", []llvm.Value{srcBuf, elemsBuf, srcLen, srcCap, elemsLen, elemSize}, "append.new")
 		newPtr := b.CreateExtractValue(result, 0, "append.newPtr")
 		newLen := b.CreateExtractValue(result, 1, "append.newLen")
 		newCap := b.CreateExtractValue(result, 2, "append.newCap")
@@ -100,7 +100,7 @@ func (b *builder) createLandingPad() {

 	// Continue at the 'recover' block, which returns to the parent in an
 	// appropriate way.
-	b.CreateBr(b.blockInfo[b.fn.Recover.Index].entry)
+	b.CreateBr(b.blockEntries[b.fn.Recover])
 }

 // Create a checkpoint (similar to setjmp). This emits inline assembly that
@@ -234,108 +234,41 @@ func (b *builder) createInvokeCheckpoint() {
 	continueBB := b.insertBasicBlock("")
 	b.CreateCondBr(isZero, continueBB, b.landingpad)
 	b.SetInsertPointAtEnd(continueBB)
-	b.currentBlockInfo.exit = continueBB
+	b.blockExits[b.currentBlock] = continueBB
 }

-// isInLoop checks if there is a path from the current block to itself.
-// Use Tarjan's strongly connected components algorithm to search for cycles.
-// A one-node SCC is a cycle iff there is an edge from the node to itself.
-// A multi-node SCC is always a cycle.
-// https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm
-func (b *builder) isInLoop() bool {
-	if b.currentBlockInfo.tarjan.lowLink == 0 {
-		b.strongConnect(b.currentBlock)
-	}
-	return b.currentBlockInfo.tarjan.cyclic
-}
+// isInLoop checks if there is a path from a basic block to itself.
+func isInLoop(start *ssa.BasicBlock) bool {
+	// Use a breadth-first search to scan backwards through the block graph.
+	queue := []*ssa.BasicBlock{start}
+	checked := map[*ssa.BasicBlock]struct{}{}

-func (b *builder) strongConnect(block *ssa.BasicBlock) {
-	// Assign a new index.
-	// Indices start from 1 so that 0 can be used as a sentinel.
-	assignedIndex := b.tarjanIndex + 1
-	b.tarjanIndex = assignedIndex
+	for len(queue) > 0 {
+		// pop a block off of the queue
+		block := queue[len(queue)-1]
+		queue = queue[:len(queue)-1]

-	// Apply the new index.
-	blockIndex := block.Index
-	node := &b.blockInfo[blockIndex].tarjan
-	node.lowLink = assignedIndex
-
-	// Push the node onto the stack.
-	node.onStack = true
-	b.tarjanStack = append(b.tarjanStack, uint(blockIndex))
-
-	// Process the successors.
-	for _, successor := range block.Succs {
-		// Look up the successor's state.
-		successorIndex := successor.Index
-		if successorIndex == blockIndex {
-			// Handle a self-cycle specially.
-			node.cyclic = true
-			continue
-		}
-		successorNode := &b.blockInfo[successorIndex].tarjan
-
-		switch {
-		case successorNode.lowLink == 0:
-			// This node has not yet been visisted.
-			b.strongConnect(successor)
-
-		case !successorNode.onStack:
-			// This node has been visited, but is in a different SCC.
-			// Ignore it, and do not update lowLink.
-			continue
-		}
-
-		// Update the lowLink index.
-		// This always uses the min-of-lowlink instead of using index in the on-stack case.
-		// This is done for two reasons:
-		// 1. The lowLink update can be shared between the new-node and on-stack cases.
-		// 2. The assigned index does not need to be saved - it is only needed for root node detection.
-		if successorNode.lowLink < node.lowLink {
-			node.lowLink = successorNode.lowLink
-		}
-	}
-
-	if node.lowLink == assignedIndex {
-		// This is a root node.
-		// Pop the SCC off the stack.
-		stack := b.tarjanStack
-		top := stack[len(stack)-1]
-		stack = stack[:len(stack)-1]
-		blocks := b.blockInfo
-		topNode := &blocks[top].tarjan
-		topNode.onStack = false
-
-		if top != uint(blockIndex) {
-			// The root node is not the only node in the SCC.
-			// Mark all nodes in this SCC as cyclic.
-			topNode.cyclic = true
-			for top != uint(blockIndex) {
-				top = stack[len(stack)-1]
-				stack = stack[:len(stack)-1]
-				topNode = &blocks[top].tarjan
-				topNode.onStack = false
-				topNode.cyclic = true
+		// Search through predecessors.
+		// Searching backwards means that this is pretty fast when the block is close to the start of the function.
+		// Defers are often placed near the start of the function.
+		for _, pred := range block.Preds {
+			if pred == start {
+				// cycle found
+				return true
 			}
+
+			if _, ok := checked[pred]; ok {
+				// block already checked
+				continue
+			}
+
+			// add to queue and checked map
+			queue = append(queue, pred)
+			checked[pred] = struct{}{}
 		}
-
-		b.tarjanStack = stack
 	}
-}

-// tarjanNode holds per-block state for isInLoop and strongConnect.
-type tarjanNode struct {
-	// lowLink is the index of the first visited node that is reachable from this block.
-	// The lowLink indices are assigned by the SCC search, and do not correspond to b.Index.
-	// A lowLink of 0 is used as a sentinel to mark a node which has not yet been visited.
-	lowLink uint
-
-	// onStack tracks whether this node is currently on the SCC search stack.
-	onStack bool
-
-	// cyclic indicates whether this block is in a loop.
-	// If lowLink is 0, strongConnect must be called before reading this field.
-	cyclic bool
+	return false
 }

 // createDefer emits a single defer instruction, to be run when this function
@@ -477,10 +410,7 @@ func (b *builder) createDefer(instr *ssa.Defer) {

 	// Put this struct in an allocation.
 	var alloca llvm.Value
-	if instr.Block() != b.currentBlock {
-		panic("block mismatch")
-	}
-	if !b.isInLoop() {
+	if !isInLoop(instr.Block()) {
 		// This can safely use a stack allocation.
 		alloca = llvmutil.CreateEntryBlockAlloca(b.Builder, deferredCallType, "defer.alloca")
 	} else {
@@ -737,7 +737,7 @@ func (b *builder) createTypeAssert(expr *ssa.TypeAssert) llvm.Value {
 	prevBlock := b.GetInsertBlock()
 	okBlock := b.insertBasicBlock("typeassert.ok")
 	nextBlock := b.insertBasicBlock("typeassert.next")
-	b.currentBlockInfo.exit = nextBlock // adjust outgoing block for phi nodes
+	b.blockExits[b.currentBlock] = nextBlock // adjust outgoing block for phi nodes
 	b.CreateCondBr(commaOk, okBlock, nextBlock)

 	// Retrieve the value from the interface if the type assert was
@@ -27,8 +27,6 @@ func (b *builder) defineIntrinsicFunction() {
 		b.createStackSaveImpl()
 	case name == "runtime.KeepAlive":
 		b.createKeepAliveImpl()
-	case name == "machine.keepAliveNoEscape":
-		b.createMachineKeepAliveImpl()
 	case strings.HasPrefix(name, "runtime/volatile.Load"):
 		b.createVolatileLoad()
 	case strings.HasPrefix(name, "runtime/volatile.Store"):
@@ -146,20 +144,6 @@ func (b *builder) createAbiEscapeImpl() {
 	b.CreateRet(result)
 }

-// Implement machine.keepAliveNoEscape, which makes sure the compiler keeps the
-// pointer parameter alive until this point (for GC).
-func (b *builder) createMachineKeepAliveImpl() {
-	b.createFunctionStart(true)
-	pointerValue := b.getValue(b.fn.Params[0], getPos(b.fn))
-
-	// See createKeepAliveImpl for details.
-	asmType := llvm.FunctionType(b.ctx.VoidType(), []llvm.Type{b.dataPtrType}, false)
-	asmFn := llvm.InlineAsm(asmType, "", "r", true, false, 0, false)
-	b.createCall(asmType, asmFn, []llvm.Value{pointerValue}, "")
-
-	b.CreateRetVoid()
-}
-
 var mathToLLVMMapping = map[string]string{
 	"math.Ceil":  "llvm.ceil.f64",
 	"math.Exp":   "llvm.exp.f64",
@@ -250,9 +250,6 @@ func (b *builder) createMapIteratorNext(rangeVal ssa.Value, llvmRangeVal, it llv
 func hashmapIsBinaryKey(keyType types.Type) bool {
 	switch keyType := keyType.Underlying().(type) {
 	case *types.Basic:
-		// TODO: unsafe.Pointer is also a binary key, but to support that we
-		// need to fix an issue with interp first (see
-		// https://github.com/tinygo-org/tinygo/pull/4898).
 		return keyType.Info()&(types.IsBoolean|types.IsInteger) != 0
 	case *types.Pointer:
 		return true
@@ -142,11 +142,6 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 			nocapture := c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0)
 			llvmFn.AddAttributeAtIndex(i+1, nocapture)
 		}
-		if paramInfo.flags&paramIsReadonly != 0 && paramInfo.llvmType.TypeKind() == llvm.PointerTypeKind {
-			// Readonly pointer parameters (like strings) benefit from being marked as readonly.
-			readonly := c.ctx.CreateEnumAttribute(llvm.AttributeKindID("readonly"), 0)
-			llvmFn.AddAttributeAtIndex(i+1, readonly)
-		}
 	}

 	// Set a number of function or parameter attributes, depending on the
@@ -159,8 +154,6 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 		llvmFn.AddFunctionAttr(c.ctx.CreateEnumAttribute(llvm.AttributeKindID("noreturn"), 0))
 	case "internal/abi.NoEscape":
 		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
-	case "machine.keepAliveNoEscape", "machine.unsafeNoEscape":
-		llvmFn.AddAttributeAtIndex(1, c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0))
 	case "runtime.alloc":
 		// Tell the optimizer that runtime.alloc is an allocator, meaning that it
 		// returns values that are never null and never alias to an existing value.
@@ -74,14 +74,6 @@ func (b *builder) createRawSyscall(call *ssa.CallCommon) (llvm.Value, error) {
 		return b.CreateCall(fnType, target, args, ""), nil

 	case b.GOARCH == "arm" && b.GOOS == "linux":
-		if arch := b.archFamily(); arch != "arm" {
-			// Some targets pretend to be linux/arm for compatibility but aren't
-			// actually such a system. Make sure we emit an error instead of
-			// creating inline assembly that will fail to compile.
-			// See: https://github.com/tinygo-org/tinygo/issues/4959
-			return llvm.Value{}, b.makeError(call.Pos(), "system calls are not supported: target emulates a linux/arm system on "+arch)
-		}
-
 		// Implement the EABI system call convention for Linux.
 		// Source: syscall(2) man page.
 		args := []llvm.Value{}
@@ -270,241 +270,6 @@ entry:
  ret void
 }

-; Function Attrs: nounwind
-define hidden void @main.deferInfiniteLoop(ptr %context) unnamed_addr #1 {
-entry:
-  %deferPtr = alloca ptr, align 4
-  store ptr null, ptr %deferPtr, align 4
-  %deferframe.buf = alloca %runtime.deferFrame, align 4
-  %0 = call ptr @llvm.stacksave.p0()
-  call void @runtime.setupDeferFrame(ptr nonnull %deferframe.buf, ptr %0, ptr undef) #4
-  br label %for.body
-
-for.body:                                         ; preds = %for.body, %entry
-  %defer.next = load ptr, ptr %deferPtr, align 4
-  %defer.alloc.call = call dereferenceable(12) ptr @runtime.alloc(i32 12, ptr null, ptr undef) #4
-  store i32 0, ptr %defer.alloc.call, align 4
-  %defer.alloc.call.repack1 = getelementptr inbounds nuw i8, ptr %defer.alloc.call, i32 4
-  store ptr %defer.next, ptr %defer.alloc.call.repack1, align 4
-  %defer.alloc.call.repack3 = getelementptr inbounds nuw i8, ptr %defer.alloc.call, i32 8
-  store i32 8, ptr %defer.alloc.call.repack3, align 4
-  store ptr %defer.alloc.call, ptr %deferPtr, align 4
-  br label %for.body
-
-recover:                                          ; preds = %rundefers.end
-  ret void
-
-lpad:                                             ; No predecessors!
-  br label %rundefers.loophead
-
-rundefers.loophead:                               ; preds = %rundefers.callback0, %lpad
-  br i1 poison, label %rundefers.end, label %rundefers.loop
-
-rundefers.loop:                                   ; preds = %rundefers.loophead
-  switch i32 poison, label %rundefers.default [
-    i32 0, label %rundefers.callback0
-  ]
-
-rundefers.callback0:                              ; preds = %rundefers.loop
-  br label %rundefers.loophead
-
-rundefers.default:                                ; preds = %rundefers.loop
-  unreachable
-
-rundefers.end:                                    ; preds = %rundefers.loophead
-  br label %recover
-}
-
-; Function Attrs: nounwind
-define hidden void @main.deferLoop(ptr %context) unnamed_addr #1 {
-entry:
-  %deferPtr = alloca ptr, align 4
-  store ptr null, ptr %deferPtr, align 4
-  %deferframe.buf = alloca %runtime.deferFrame, align 4
-  %0 = call ptr @llvm.stacksave.p0()
-  call void @runtime.setupDeferFrame(ptr nonnull %deferframe.buf, ptr %0, ptr undef) #4
-  br label %for.loop
-
-for.loop:                                         ; preds = %for.body, %entry
-  %1 = phi i32 [ 0, %entry ], [ %3, %for.body ]
-  %2 = icmp slt i32 %1, 10
-  br i1 %2, label %for.body, label %for.done
-
-for.body:                                         ; preds = %for.loop
-  %defer.next = load ptr, ptr %deferPtr, align 4
-  %defer.alloc.call = call dereferenceable(12) ptr @runtime.alloc(i32 12, ptr null, ptr undef) #4
-  store i32 0, ptr %defer.alloc.call, align 4
-  %defer.alloc.call.repack13 = getelementptr inbounds nuw i8, ptr %defer.alloc.call, i32 4
-  store ptr %defer.next, ptr %defer.alloc.call.repack13, align 4
-  %defer.alloc.call.repack15 = getelementptr inbounds nuw i8, ptr %defer.alloc.call, i32 8
-  store i32 %1, ptr %defer.alloc.call.repack15, align 4
-  store ptr %defer.alloc.call, ptr %deferPtr, align 4
-  %3 = add i32 %1, 1
-  br label %for.loop
-
-for.done:                                         ; preds = %for.loop
-  br label %rundefers.block
-
-rundefers.after:                                  ; preds = %rundefers.end
-  call void @runtime.destroyDeferFrame(ptr nonnull %deferframe.buf, ptr undef) #4
-  ret void
-
-rundefers.block:                                  ; preds = %for.done
-  br label %rundefers.loophead
-
-rundefers.loophead:                               ; preds = %rundefers.callback0, %rundefers.block
-  %4 = load ptr, ptr %deferPtr, align 4
-  %stackIsNil = icmp eq ptr %4, null
-  br i1 %stackIsNil, label %rundefers.end, label %rundefers.loop
-
-rundefers.loop:                                   ; preds = %rundefers.loophead
-  %stack.next.gep = getelementptr inbounds nuw i8, ptr %4, i32 4
-  %stack.next = load ptr, ptr %stack.next.gep, align 4
-  store ptr %stack.next, ptr %deferPtr, align 4
-  %callback = load i32, ptr %4, align 4
-  switch i32 %callback, label %rundefers.default [
-    i32 0, label %rundefers.callback0
-  ]
-
-rundefers.callback0:                              ; preds = %rundefers.loop
-  %gep = getelementptr inbounds nuw i8, ptr %4, i32 8
-  %param = load i32, ptr %gep, align 4
-  call void @runtime.printlock(ptr undef) #4
-  call void @runtime.printint32(i32 %param, ptr undef) #4
-  call void @runtime.printunlock(ptr undef) #4
-  br label %rundefers.loophead
-
-rundefers.default:                                ; preds = %rundefers.loop
-  unreachable
-
-rundefers.end:                                    ; preds = %rundefers.loophead
-  br label %rundefers.after
-
-recover:                                          ; preds = %rundefers.end1
-  ret void
-
-lpad:                                             ; No predecessors!
-  br label %rundefers.loophead4
-
-rundefers.loophead4:                              ; preds = %rundefers.callback010, %lpad
-  br i1 poison, label %rundefers.end1, label %rundefers.loop3
-
-rundefers.loop3:                                  ; preds = %rundefers.loophead4
-  switch i32 poison, label %rundefers.default2 [
-    i32 0, label %rundefers.callback010
-  ]
-
-rundefers.callback010:                            ; preds = %rundefers.loop3
-  br label %rundefers.loophead4
-
-rundefers.default2:                               ; preds = %rundefers.loop3
-  unreachable
-
-rundefers.end1:                                   ; preds = %rundefers.loophead4
-  br label %recover
-}
-
-; Function Attrs: nounwind
-define hidden void @main.deferBetweenLoops(ptr %context) unnamed_addr #1 {
-entry:
-  %defer.alloca = alloca { i32, ptr, i32 }, align 4
-  %deferPtr = alloca ptr, align 4
-  store ptr null, ptr %deferPtr, align 4
-  %deferframe.buf = alloca %runtime.deferFrame, align 4
-  %0 = call ptr @llvm.stacksave.p0()
-  call void @runtime.setupDeferFrame(ptr nonnull %deferframe.buf, ptr %0, ptr undef) #4
-  br label %for.loop
-
-for.loop:                                         ; preds = %for.body, %entry
-  %1 = phi i32 [ 0, %entry ], [ %3, %for.body ]
-  %2 = icmp slt i32 %1, 10
-  br i1 %2, label %for.body, label %for.done
-
-for.body:                                         ; preds = %for.loop
-  %3 = add i32 %1, 1
-  br label %for.loop
-
-for.done:                                         ; preds = %for.loop
-  %defer.next = load ptr, ptr %deferPtr, align 4
-  store i32 0, ptr %defer.alloca, align 4
-  %defer.alloca.repack16 = getelementptr inbounds nuw i8, ptr %defer.alloca, i32 4
-  store ptr %defer.next, ptr %defer.alloca.repack16, align 4
-  %defer.alloca.repack18 = getelementptr inbounds nuw i8, ptr %defer.alloca, i32 8
-  store i32 1, ptr %defer.alloca.repack18, align 4
-  store ptr %defer.alloca, ptr %deferPtr, align 4
-  br label %for.loop1
-
-for.loop1:                                        ; preds = %for.body2, %for.done
-  %4 = phi i32 [ 0, %for.done ], [ %6, %for.body2 ]
-  %5 = icmp slt i32 %4, 10
-  br i1 %5, label %for.body2, label %for.done3
-
-for.body2:                                        ; preds = %for.loop1
-  %6 = add i32 %4, 1
-  br label %for.loop1
-
-for.done3:                                        ; preds = %for.loop1
-  br label %rundefers.block
-
-rundefers.after:                                  ; preds = %rundefers.end
-  call void @runtime.destroyDeferFrame(ptr nonnull %deferframe.buf, ptr undef) #4
-  ret void
-
-rundefers.block:                                  ; preds = %for.done3
-  br label %rundefers.loophead
-
-rundefers.loophead:                               ; preds = %rundefers.callback0, %rundefers.block
-  %7 = load ptr, ptr %deferPtr, align 4
-  %stackIsNil = icmp eq ptr %7, null
-  br i1 %stackIsNil, label %rundefers.end, label %rundefers.loop
-
-rundefers.loop:                                   ; preds = %rundefers.loophead
-  %stack.next.gep = getelementptr inbounds nuw i8, ptr %7, i32 4
-  %stack.next = load ptr, ptr %stack.next.gep, align 4
-  store ptr %stack.next, ptr %deferPtr, align 4
-  %callback = load i32, ptr %7, align 4
-  switch i32 %callback, label %rundefers.default [
-    i32 0, label %rundefers.callback0
-  ]
-
-rundefers.callback0:                              ; preds = %rundefers.loop
-  %gep = getelementptr inbounds nuw i8, ptr %7, i32 8
-  %param = load i32, ptr %gep, align 4
-  call void @runtime.printlock(ptr undef) #4
-  call void @runtime.printint32(i32 %param, ptr undef) #4
-  call void @runtime.printunlock(ptr undef) #4
-  br label %rundefers.loophead
-
-rundefers.default:                                ; preds = %rundefers.loop
-  unreachable
-
-rundefers.end:                                    ; preds = %rundefers.loophead
-  br label %rundefers.after
-
-recover:                                          ; preds = %rundefers.end4
-  ret void
-
-lpad:                                             ; No predecessors!
-  br label %rundefers.loophead7
-
-rundefers.loophead7:                              ; preds = %rundefers.callback013, %lpad
-  br i1 poison, label %rundefers.end4, label %rundefers.loop6
-
-rundefers.loop6:                                  ; preds = %rundefers.loophead7
-  switch i32 poison, label %rundefers.default5 [
-    i32 0, label %rundefers.callback013
-  ]
-
-rundefers.callback013:                            ; preds = %rundefers.loop6
-  br label %rundefers.loophead7
-
-rundefers.default5:                               ; preds = %rundefers.loop6
-  unreachable
-
-rundefers.end4:                                   ; preds = %rundefers.loophead7
-  br label %recover
-}
-
 attributes #0 = { allockind("alloc,zeroed") allocsize(0) "alloc-family"="runtime.alloc" "target-features"="+armv7-m,+hwdiv,+soft-float,+thumb-mode,-aes,-bf16,-cdecp0,-cdecp1,-cdecp2,-cdecp3,-cdecp4,-cdecp5,-cdecp6,-cdecp7,-crc,-crypto,-d32,-dotprod,-dsp,-fp-armv8,-fp-armv8d16,-fp-armv8d16sp,-fp-armv8sp,-fp16,-fp16fml,-fp64,-fpregs,-fullfp16,-hwdiv-arm,-i8mm,-lob,-mve,-mve.fp,-neon,-pacbti,-ras,-sb,-sha2,-vfp2,-vfp2sp,-vfp3,-vfp3d16,-vfp3d16sp,-vfp3sp,-vfp4,-vfp4d16,-vfp4d16sp,-vfp4sp" }
 attributes #1 = { nounwind "target-features"="+armv7-m,+hwdiv,+soft-float,+thumb-mode,-aes,-bf16,-cdecp0,-cdecp1,-cdecp2,-cdecp3,-cdecp4,-cdecp5,-cdecp6,-cdecp7,-crc,-crypto,-d32,-dotprod,-dsp,-fp-armv8,-fp-armv8d16,-fp-armv8d16sp,-fp-armv8sp,-fp16,-fp16fml,-fp64,-fpregs,-fullfp16,-hwdiv-arm,-i8mm,-lob,-mve,-mve.fp,-neon,-pacbti,-ras,-sb,-sha2,-vfp2,-vfp2sp,-vfp3,-vfp3d16,-vfp3d16sp,-vfp3sp,-vfp4,-vfp4d16,-vfp4d16sp,-vfp4sp" }
 attributes #2 = { "target-features"="+armv7-m,+hwdiv,+soft-float,+thumb-mode,-aes,-bf16,-cdecp0,-cdecp1,-cdecp2,-cdecp3,-cdecp4,-cdecp5,-cdecp6,-cdecp7,-crc,-crypto,-d32,-dotprod,-dsp,-fp-armv8,-fp-armv8d16,-fp-armv8d16sp,-fp-armv8sp,-fp16,-fp16fml,-fp64,-fpregs,-fullfp16,-hwdiv-arm,-i8mm,-lob,-mve,-mve.fp,-neon,-pacbti,-ras,-sb,-sha2,-vfp2,-vfp2sp,-vfp3,-vfp3d16,-vfp3d16sp,-vfp3sp,-vfp4,-vfp4d16,-vfp4d16sp,-vfp4sp" }
@@ -18,23 +18,3 @@ func deferMultiple() {
 	}()
 	external()
 }
-
-func deferInfiniteLoop() {
-	for {
-		defer print(8)
-	}
-}
-
-func deferLoop() {
-	for i := 0; i < 10; i++ {
-		defer print(i)
-	}
-}
-
-func deferBetweenLoops() {
-	for i := 0; i < 10; i++ {
-	}
-	defer print(1)
-	for i := 0; i < 10; i++ {
-	}
-}
@@ -50,7 +50,7 @@ unsafe.String.throw:                              ; preds = %entry
 declare void @runtime.unsafeSlicePanic(ptr) #1

 ; Function Attrs: nounwind
-define hidden ptr @main.unsafeStringData(ptr readonly %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
+define hidden ptr @main.unsafeStringData(ptr %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
 entry:
  %stackalloc = alloca i8, align 1
  call void @runtime.trackPointer(ptr %s.data, ptr nonnull %stackalloc, ptr undef) #3
@@ -77,7 +77,7 @@ entry:
 }

 ; Function Attrs: nounwind
-define hidden %runtime._string @main.minString(ptr readonly %a.data, i32 %a.len, ptr readonly %b.data, i32 %b.len, ptr %context) unnamed_addr #2 {
+define hidden %runtime._string @main.minString(ptr %a.data, i32 %a.len, ptr %b.data, i32 %b.len, ptr %context) unnamed_addr #2 {
 entry:
  %0 = insertvalue %runtime._string zeroinitializer, ptr %a.data, 0
  %1 = insertvalue %runtime._string %0, i32 %a.len, 1
@@ -91,7 +91,7 @@ entry:
  ret %runtime._string %5
 }

-declare i1 @runtime.stringLess(ptr readonly, i32, ptr readonly, i32, ptr) #1
+declare i1 @runtime.stringLess(ptr, i32, ptr, i32, ptr) #1

 ; Function Attrs: nounwind
 define hidden i32 @main.maxInt(i32 %a, i32 %b, ptr %context) unnamed_addr #2 {
@@ -116,7 +116,7 @@ entry:
 }

 ; Function Attrs: nounwind
-define hidden %runtime._string @main.maxString(ptr readonly %a.data, i32 %a.len, ptr readonly %b.data, i32 %b.len, ptr %context) unnamed_addr #2 {
+define hidden %runtime._string @main.maxString(ptr %a.data, i32 %a.len, ptr %b.data, i32 %b.len, ptr %context) unnamed_addr #2 {
 entry:
  %0 = insertvalue %runtime._string zeroinitializer, ptr %a.data, 0
  %1 = insertvalue %runtime._string %0, i32 %a.len, 1
@@ -55,7 +55,7 @@ entry:
  store i32 2, ptr %0, align 4
  %1 = getelementptr inbounds nuw i8, ptr %varargs, i32 8
  store i32 3, ptr %1, align 4
-  %append.new = call { ptr, i32, i32 } @runtime.sliceAppend(ptr %ints.data, ptr nonnull %varargs, i32 %ints.len, i32 %ints.cap, i32 3, i32 4, ptr nonnull inttoptr (i32 3 to ptr), ptr undef) #3
+  %append.new = call { ptr, i32, i32 } @runtime.sliceAppend(ptr %ints.data, ptr nonnull %varargs, i32 %ints.len, i32 %ints.cap, i32 3, i32 4, ptr undef) #3
  %append.newPtr = extractvalue { ptr, i32, i32 } %append.new, 0
  %append.newLen = extractvalue { ptr, i32, i32 } %append.new, 1
  %append.newCap = extractvalue { ptr, i32, i32 } %append.new, 2
@@ -66,13 +66,13 @@ entry:
  ret { ptr, i32, i32 } %4
 }

-declare { ptr, i32, i32 } @runtime.sliceAppend(ptr, ptr nocapture readonly, i32, i32, i32, i32, ptr, ptr) #1
+declare { ptr, i32, i32 } @runtime.sliceAppend(ptr, ptr nocapture readonly, i32, i32, i32, i32, ptr) #1

 ; Function Attrs: nounwind
 define hidden { ptr, i32, i32 } @main.sliceAppendSlice(ptr %ints.data, i32 %ints.len, i32 %ints.cap, ptr %added.data, i32 %added.len, i32 %added.cap, ptr %context) unnamed_addr #2 {
 entry:
  %stackalloc = alloca i8, align 1
-  %append.new = call { ptr, i32, i32 } @runtime.sliceAppend(ptr %ints.data, ptr %added.data, i32 %ints.len, i32 %ints.cap, i32 %added.len, i32 4, ptr nonnull inttoptr (i32 3 to ptr), ptr undef) #3
+  %append.new = call { ptr, i32, i32 } @runtime.sliceAppend(ptr %ints.data, ptr %added.data, i32 %ints.len, i32 %ints.cap, i32 %added.len, i32 4, ptr undef) #3
  %append.newPtr = extractvalue { ptr, i32, i32 } %append.new, 0
  %append.newLen = extractvalue { ptr, i32, i32 } %append.new, 1
  %append.newCap = extractvalue { ptr, i32, i32 } %append.new, 2
@@ -31,13 +31,13 @@ entry:
 }

 ; Function Attrs: nounwind
-define hidden i32 @main.stringLen(ptr readonly %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
+define hidden i32 @main.stringLen(ptr %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
 entry:
  ret i32 %s.len
 }

 ; Function Attrs: nounwind
-define hidden i8 @main.stringIndex(ptr readonly %s.data, i32 %s.len, i32 %index, ptr %context) unnamed_addr #2 {
+define hidden i8 @main.stringIndex(ptr %s.data, i32 %s.len, i32 %index, ptr %context) unnamed_addr #2 {
 entry:
  %.not = icmp ult i32 %index, %s.len
  br i1 %.not, label %lookup.next, label %lookup.throw
@@ -55,16 +55,16 @@ lookup.throw:                                     ; preds = %entry
 declare void @runtime.lookupPanic(ptr) #1

 ; Function Attrs: nounwind
-define hidden i1 @main.stringCompareEqual(ptr readonly %s1.data, i32 %s1.len, ptr readonly %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
+define hidden i1 @main.stringCompareEqual(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
 entry:
  %0 = call i1 @runtime.stringEqual(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr undef) #3
  ret i1 %0
 }

-declare i1 @runtime.stringEqual(ptr readonly, i32, ptr readonly, i32, ptr) #1
+declare i1 @runtime.stringEqual(ptr, i32, ptr, i32, ptr) #1

 ; Function Attrs: nounwind
-define hidden i1 @main.stringCompareUnequal(ptr readonly %s1.data, i32 %s1.len, ptr readonly %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
+define hidden i1 @main.stringCompareUnequal(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
 entry:
  %0 = call i1 @runtime.stringEqual(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr undef) #3
  %1 = xor i1 %0, true
@@ -72,16 +72,16 @@ entry:
 }

 ; Function Attrs: nounwind
-define hidden i1 @main.stringCompareLarger(ptr readonly %s1.data, i32 %s1.len, ptr readonly %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
+define hidden i1 @main.stringCompareLarger(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
 entry:
  %0 = call i1 @runtime.stringLess(ptr %s2.data, i32 %s2.len, ptr %s1.data, i32 %s1.len, ptr undef) #3
  ret i1 %0
 }

-declare i1 @runtime.stringLess(ptr readonly, i32, ptr readonly, i32, ptr) #1
+declare i1 @runtime.stringLess(ptr, i32, ptr, i32, ptr) #1

 ; Function Attrs: nounwind
-define hidden i8 @main.stringLookup(ptr readonly %s.data, i32 %s.len, i8 %x, ptr %context) unnamed_addr #2 {
+define hidden i8 @main.stringLookup(ptr %s.data, i32 %s.len, i8 %x, ptr %context) unnamed_addr #2 {
 entry:
  %0 = zext i8 %x to i32
  %.not = icmp ugt i32 %s.len, %0
@@ -10,7 +10,7 @@ import (

 // Version of TinyGo.
 // Update this value before release of new version of software.
-const version = "0.40.1"
+const version = "0.40.0-dev"

 // Return TinyGo version, either in the form 0.30.0 or as a development version
 // (like 0.30.0-dev-abcd012).
@@ -256,7 +256,6 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 		"reflect/":                    false,
 		"runtime/":                    false,
 		"sync/":                       true,
-		"testing/":                    true,
 		"tinygo/":                     false,
 		"unique/":                     false,
 	}
@@ -1635,7 +1635,6 @@ func main() {
 	cpuprofile := flag.String("cpuprofile", "", "cpuprofile output")
 	monitor := flag.Bool("monitor", false, "enable serial monitor")
 	baudrate := flag.Int("baudrate", 115200, "baudrate of serial monitor")
-	gocompatibility := flag.Bool("go-compatibility", true, "enable to check for Go versions compatibility, you can also configure this by setting the TINYGO_GOCOMPATIBILITY environment variable")

 	// Internal flags, that are only intended for TinyGo development.
 	printIR := flag.Bool("internal-printir", false, "print LLVM IR")
@@ -1713,16 +1712,6 @@ func main() {
 		ocdCommands = strings.Split(*ocdCommandsString, ",")
 	}

-	val, ok := os.LookupEnv("TINYGO_GOCOMPATIBILITY")
-	if ok {
-		b, err := strconv.ParseBool(val)
-		if err != nil {
-			fmt.Fprintf(os.Stderr, "could not parse TINYGO_GOCOMPATIBILITY value %q: %v\n", val, err)
-			os.Exit(1)
-		}
-		*gocompatibility = b
-	}
-
 	options := &compileopts.Options{
 		GOOS:            goenv.Get("GOOS"),
 		GOARCH:          goenv.Get("GOARCH"),
@@ -1759,7 +1748,6 @@ func main() {
 		Timeout:         *timeout,
 		WITPackage:      witPackage,
 		WITWorld:        witWorld,
-		GoCompatibility: *gocompatibility,
 	}
 	if *printCommands {
 		options.PrintCommands = printCommand
@@ -1745,7 +1745,7 @@ func memzero(ptr unsafe.Pointer, size uintptr)
 func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer

 //go:linkname sliceAppend runtime.sliceAppend
-func sliceAppend(srcBuf, elemsBuf unsafe.Pointer, srcLen, srcCap, elemsLen uintptr, elemSize uintptr, layout unsafe.Pointer) (unsafe.Pointer, uintptr, uintptr)
+func sliceAppend(srcBuf, elemsBuf unsafe.Pointer, srcLen, srcCap, elemsLen uintptr, elemSize uintptr) (unsafe.Pointer, uintptr, uintptr)

 //go:linkname sliceCopy runtime.sliceCopy
 func sliceCopy(dst, src unsafe.Pointer, dstLen, srcLen uintptr, elemSize uintptr) int
@@ -1810,7 +1810,7 @@ func buflen(v Value) (unsafe.Pointer, uintptr) {
 }

 //go:linkname sliceGrow runtime.sliceGrow
-func sliceGrow(buf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr, layout unsafe.Pointer) (unsafe.Pointer, uintptr, uintptr)
+func sliceGrow(buf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr) (unsafe.Pointer, uintptr, uintptr)

 // extend slice to hold n new elements
 func extendSlice(v Value, n int) sliceHeader {
@@ -1823,10 +1823,7 @@ func extendSlice(v Value, n int) sliceHeader {
 		old = *(*sliceHeader)(v.value)
 	}

-	elem := v.typecode.elem()
-	elemSize := elem.Size()
-	elemLayout := elem.gcLayout()
-	nbuf, nlen, ncap := sliceGrow(old.data, old.len, old.cap, old.len+uintptr(n), elemSize, elemLayout)
+	nbuf, nlen, ncap := sliceGrow(old.data, old.len, old.cap, old.len+uintptr(n), v.typecode.elem().Size())

 	return sliceHeader{
 		data: nbuf,
@@ -1865,10 +1862,8 @@ func AppendSlice(s, t Value) Value {
 	}
 	sSlice := (*sliceHeader)(s.value)
 	tSlice := (*sliceHeader)(t.value)
-	elem := s.typecode.elem()
-	elemSize := elem.Size()
-	elemLayout := elem.gcLayout()
-	ptr, len, cap := sliceAppend(sSlice.data, tSlice.data, sSlice.len, sSlice.cap, tSlice.len, elemSize, elemLayout)
+	elemSize := s.typecode.elem().Size()
+	ptr, len, cap := sliceAppend(sSlice.data, tSlice.data, sSlice.len, sSlice.cap, tSlice.len, elemSize)
 	result := &sliceHeader{
 		data: ptr,
 		len:  len,
@@ -108,10 +108,6 @@ func (*stackState) unwind()
 func (t *Task) Resume() {
 	// The current task must be saved and restored because this can nest on WASM with JS.
 	prevTask := currentTask
-	if prevTask == nil {
-		// Save the system stack pointer.
-		saveStackPointer()
-	}
 	t.gcData.swap()
 	currentTask = t
 	if !t.state.launched {
@@ -127,9 +123,6 @@ func (t *Task) Resume() {
 	}
 }

-//go:linkname saveStackPointer runtime.saveStackPointer
-func saveStackPointer()
-
 //export tinygo_rewind
 func (*state) rewind()

@@ -1,4 +1,4 @@
-//go:build scheduler.tasks && (esp32 || esp32s3)
+//go:build scheduler.tasks && esp32

 package task

@@ -120,21 +120,15 @@ int tinygo_task_start(uintptr_t fn, void *args, void *task, pthread_t *thread, u
    #endif
    pthread_attr_t attrs;
    pthread_attr_init(&attrs);
-	pthread_attr_setdetachstate(&attrs, PTHREAD_CREATE_DETACHED);
    pthread_attr_setstacksize(&attrs, stackSize);
    int result = pthread_create(thread, &attrs, &start_wrapper, &state);
    pthread_attr_destroy(&attrs);
-	if (result != 0) {
-		return result;
-	}

    // Wait until the thread has been created and read all state_pass variables.
    #if __APPLE__
    dispatch_semaphore_wait(state.startlock, DISPATCH_TIME_FOREVER);
-    dispatch_release(state.startlock);
    #else
    sem_wait(&state.startlock);
-    sem_destroy(&state.startlock);
    #endif

    return result;
@@ -23,15 +23,16 @@ type state struct {
 	// is needed to be able to scan the stack.
 	stackTop uintptr

-	// Lowest address of the stack.
-	// This is populated when the thread is stopped by the GC.
-	stackBottom uintptr
-
 	// Next task in the activeTasks queue.
 	QueueNext *Task

 	// Semaphore to pause/resume the thread atomically.
 	pauseSem Semaphore
+
+	// Semaphore used for stack scanning.
+	// We can't reuse pauseSem here since the thread might have been paused for
+	// other reasons (for example, because it was waiting on a channel).
+	gcSem Semaphore
 }

 // Goroutine counter, starting at 0 for the main goroutine.
@@ -95,9 +96,6 @@ func (t *Task) Resume() {
 	t.state.pauseSem.Post()
 }

-// otherGoroutines is the total number of live goroutines minus one.
-var otherGoroutines uint32
-
 // Start a new OS thread.
 func start(fn uintptr, args unsafe.Pointer, stackSize uintptr) {
 	t := &Task{}
@@ -117,7 +115,6 @@ func start(fn uintptr, args unsafe.Pointer, stackSize uintptr) {
 	}
 	t.state.QueueNext = activeTasks
 	activeTasks = t
-	otherGoroutines++
 	activeTaskLock.Unlock()
 }

@@ -138,7 +135,6 @@ func taskExited(t *Task) {
 			break
 		}
 	}
-	otherGoroutines--
 	activeTaskLock.Unlock()

 	// Sanity check.
@@ -147,42 +143,9 @@ func taskExited(t *Task) {
 	}
 }

-// scanWaitGroup is used to wait on until all threads have finished the current state transition.
-var scanWaitGroup waitGroup
-
-type waitGroup struct {
-	f Futex
-}
-
-func initWaitGroup(n uint32) waitGroup {
-	var wg waitGroup
-	wg.f.Store(n)
-	return wg
-}
-
-func (wg *waitGroup) done() {
-	if wg.f.Add(^uint32(0)) == 0 {
-		wg.f.WakeAll()
-	}
-}
-
-func (wg *waitGroup) wait() {
-	for {
-		val := wg.f.Load()
-		if val == 0 {
-			return
-		}
-		wg.f.Wait(val)
-	}
-}
-
-// gcState is used to track and notify threads when the GC is stopping/resuming.
-var gcState Futex
-
-const (
-	gcStateResumed = iota
-	gcStateStopped
-)
+// Futex to wait on until all tasks have finished scanning the stack.
+// This is basically a sync.WaitGroup.
+var scanDoneFutex Futex

 // GC scan phase. Because we need to stop the world while scanning, this kinda
 // needs to be done in the tasks package.
@@ -192,71 +155,65 @@ const (
 func GCStopWorldAndScan() {
 	current := Current()

-	// NOTE: This does not need to be atomic.
-	if gcState.Load() == gcStateResumed {
-		// Don't allow new goroutines to be started while pausing/resuming threads
-		// in the stop-the-world phase.
-		activeTaskLock.Lock()
+	// Don't allow new goroutines to be started while pausing/resuming threads
+	// in the stop-the-world phase.
+	activeTaskLock.Lock()

-		// Wait for threads to finish resuming.
-		scanWaitGroup.wait()
-
-		// Change the gc state to stopped.
-		// NOTE: This does not need to be atomic.
-		gcState.Store(gcStateStopped)
-
-		// Set the number of threads to wait for.
-		scanWaitGroup = initWaitGroup(otherGoroutines)
-
-		// Pause all other threads.
-		for t := activeTasks; t != nil; t = t.state.QueueNext {
-			if t != current {
-				tinygo_task_send_gc_signal(t.state.thread)
-			}
-		}
-
-		// Wait for the threads to finish stopping.
-		scanWaitGroup.wait()
-	}
-
-	// Scan other thread stacks.
+	// Pause all other threads.
+	numOtherThreads := uint32(0)
 	for t := activeTasks; t != nil; t = t.state.QueueNext {
 		if t != current {
-			markRoots(t.state.stackBottom, t.state.stackTop)
+			numOtherThreads++
+			tinygo_task_send_gc_signal(t.state.thread)
 		}
 	}

+	// Store the number of threads to wait for in the futex.
+	// This is the equivalent of doing an initial wg.Add(numOtherThreads).
+	scanDoneFutex.Store(numOtherThreads)
+
 	// Scan the current stack, and all current registers.
 	scanCurrentStack()

+	// Wake each paused thread for the first time so it will scan the stack.
+	for t := activeTasks; t != nil; t = t.state.QueueNext {
+		if t != current {
+			t.state.gcSem.Post()
+		}
+	}
+
+	// Wait until all threads have finished scanning their stack.
+	// This is the equivalent of wg.Wait()
+	for {
+		val := scanDoneFutex.Load()
+		if val == 0 {
+			break
+		}
+		scanDoneFutex.Wait(val)
+	}
+
 	// Scan all globals (implemented in the runtime).
 	gcScanGlobals()
 }

 // After the GC is done scanning, resume all other threads.
+//
+// This must only be called after a GCStopWorldAndScan call.
 func GCResumeWorld() {
-	// NOTE: This does not need to be atomic.
-	if gcState.Load() == gcStateResumed {
-		// This is already resumed.
-		return
+	current := Current()
+
+	// Wake each paused thread for the second time, so they will resume normal
+	// operation.
+	for t := activeTasks; t != nil; t = t.state.QueueNext {
+		if t != current {
+			t.state.gcSem.Post()
+		}
 	}

-	// Set the wait group to track resume progress.
-	scanWaitGroup = initWaitGroup(otherGoroutines)
-
-	// Set the state to resumed.
-	gcState.Store(gcStateResumed)
-
-	// Wake all of the stopped threads.
-	gcState.WakeAll()
-
 	// Allow goroutines to start and exit again.
 	activeTaskLock.Unlock()
 }

-//go:linkname markRoots runtime.markRoots
-func markRoots(start, end uintptr)
-
 // Scan globals, implemented in the runtime package.
 func gcScanGlobals()

@@ -264,27 +221,34 @@ var stackScanLock PMutex

 //export tinygo_task_gc_pause
 func tingyo_task_gc_pause(sig int32) {
-	// Write the entrty stack pointer to the state.
-	Current().state.stackBottom = uintptr(stacksave())
+	// Wait until we get the signal to start scanning the stack.
+	Current().state.gcSem.Wait()

-	// Notify the GC that we are stopped.
-	scanWaitGroup.done()
+	// Scan the thread stack.
+	// Only scan a single thread stack at a time, because the GC marking phase
+	// doesn't support parallelism.
+	// TODO: it may be possible to call markRoots directly (without saving
+	// registers) since we are in a signal handler that already saved a bunch of
+	// registers. This is an optimization left for a future time.
+	stackScanLock.Lock()
+	scanCurrentStack()
+	stackScanLock.Unlock()

-	// Wait for the GC to resume.
-	for gcState.Load() == gcStateStopped {
-		gcState.Wait(gcStateStopped)
+	// Equivalent of wg.Done(): subtract one from the futex and if the result is
+	// 0 (meaning we were the last in the waitgroup), wake the waiting thread.
+	n := uint32(1)
+	if scanDoneFutex.Add(-n) == 0 {
+		scanDoneFutex.Wake()
 	}

-	// Notify the GC that we have resumed.
-	scanWaitGroup.done()
+	// Wait until we get the signal we can resume normally (after the mark phase
+	// has finished).
+	Current().state.gcSem.Wait()
 }

 //go:export tinygo_scanCurrentStack
 func scanCurrentStack()

-//go:linkname stacksave runtime.stacksave
-func stacksave() unsafe.Pointer
-
 // Return the highest address of the current stack.
 func StackTop() uintptr {
 	return Current().state.stackTop
@@ -1,173 +0,0 @@
-//go:build pico2_ice
-
-// Most of the info is from
-// https://pico2-ice.tinyvision.ai/md_pinout.html although
-// (2025-09-07) RP4 appears twice in that pinout - the schematic is
-// more clear. Consistent with other RPi boards, we use GPn instead of
-// RPn to reference the RPi connected pins.
-
-package machine
-
-// GPIO pins
-const (
-	GP0  = GPIO0
-	GP1  = GPIO1
-	GP2  = GPIO2
-	GP3  = GPIO3
-	GP4  = GPIO4
-	GP5  = GPIO5
-	GP6  = GPIO6
-	GP7  = GPIO7
-	GP8  = GPIO8
-	GP9  = GPIO9
-	GP10 = GPIO10
-	GP11 = GPIO11
-	GP12 = GPIO12
-	GP13 = GPIO13
-	GP14 = GPIO14
-	GP15 = GPIO15
-	GP16 = GPIO16
-	GP17 = GPIO17
-	GP18 = GPIO18
-	GP19 = GPIO19
-	GP20 = GPIO20
-	GP21 = GPIO21
-	GP22 = GPIO22
-	GP23 = GPIO23
-	GP24 = GPIO24
-	GP25 = GPIO25
-	GP26 = GPIO26
-	GP27 = GPIO27
-	GP28 = GPIO28
-	GP29 = GPIO29
-	GP30 = GPIO30
-	GP31 = GPIO31
-	GP32 = GPIO32
-	GP33 = GPIO33
-	GP34 = GPIO34
-	GP35 = GPIO35
-	GP36 = GPIO36
-	GP37 = GPIO37
-	GP38 = GPIO38
-	GP39 = GPIO39
-	GP40 = GPIO40
-	GP41 = GPIO41
-	GP42 = GPIO42
-	GP43 = GPIO43
-	GP44 = GPIO44
-	GP45 = GPIO45
-	GP46 = GPIO46
-	GP47 = GPIO47
-
-	// RPi pins shared with ICE. The ICE number is what appears on
-	// the board silkscreen.
-	ICE9  = GP28
-	ICE11 = GP29
-	ICE14 = GP7
-	ICE15 = GP6
-	ICE16 = GP5
-	ICE17 = GP4
-	ICE18 = GP27
-	ICE19 = GP23
-	ICE20 = GP22
-	ICE21 = GP26
-	ICE23 = GP25
-	ICE25 = GP30
-	ICE26 = GP24
-	ICE27 = GP20
-
-	// FPGA Clock pin.
-	ICE35_G0 = GP21
-
-	// Silkscreen & Pinout names
-	ICE_SSN = ICE16
-	ICE_SO  = ICE14
-	ICE_SI  = ICE17
-	ICE_CK  = ICE15
-	SD      = GP2
-	SC      = GP3
-
-	FPGA_RSTN = GP31
-	A3        = GP32
-	A1        = GP33
-	A4        = GP34
-	A2        = GP35
-	B3        = GP36
-	B1        = GP37
-	B4        = GP38
-	B2        = GP39
-	N0        = GP40 // On the board these are labeled "~0"
-	N1        = GP41
-	N2        = GP42
-	N3        = GP43
-	N4        = GP44
-	N5        = GP45
-	N6        = GP46
-
-	// Functions from Schematic.
-	ICE_DONE = GP40
-	USB_BOOT = GP42
-
-	// Button
-	SW1     = GP42
-	BOOTSEL = GP42
-
-	// Tricolor LEDs
-	LED_RED   = GP1
-	LED_GREEN = GP0
-	LED_BLUE  = GP9
-
-	// Onboard LED
-	LED = LED_GREEN
-
-	// Onboard crystal oscillator frequency, in MHz.
-	xoscFreq = 12 // MHz
-)
-
-// This board does not define default i2c pins.
-const (
-	I2C0_SDA_PIN = NoPin
-	I2C0_SCL_PIN = NoPin
-	I2C1_SDA_PIN = NoPin
-	I2C1_SCL_PIN = NoPin
-)
-
-// SPI default pins
-const (
-	// Default Serial Clock Bus 0 for SPI communications
-	SPI0_SCK_PIN = GPIO18
-	// Default Serial Out Bus 0 for SPI communications
-	SPI0_SDO_PIN = GPIO19 // Tx
-	// Default Serial In Bus 0 for SPI communications
-	SPI0_SDI_PIN = GPIO16 // Rx
-
-	// Default Serial Clock Bus 1 for SPI communications
-	SPI1_SCK_PIN = GPIO10
-	// Default Serial Out Bus 1 for SPI communications
-	SPI1_SDO_PIN = GPIO11 // Tx
-	// Default Serial In Bus 1 for SPI communications
-	SPI1_SDI_PIN = GPIO12 // Rx
-)
-
-// UART pins
-const (
-	UART0_TX_PIN = GPIO0
-	UART0_RX_PIN = GPIO1
-	UART1_TX_PIN = GPIO8
-	UART1_RX_PIN = GPIO9
-	UART_TX_PIN  = UART0_TX_PIN
-	UART_RX_PIN  = UART0_RX_PIN
-)
-
-var DefaultUART = UART0
-
-// USB identifiers
-const (
-	usb_STRING_PRODUCT      = "Pico2"
-	usb_STRING_MANUFACTURER = "Raspberry Pi"
-)
-
-var (
-	usb_VID uint16 = 0x2E8A
-	usb_PID uint16 = 0x000A
-)
@@ -1,58 +0,0 @@
-//go:build xiao_esp32s3
-
-// This file contains the pin mappings for the Seeed XIAO ESP32S3 boards.
-//
-// Seeed Studio XIAO ESP32S3 is an IoT mini development board based on
-// the Espressif ESP32-S3 WiFi/Bluetooth dual-mode chip.
-//
-// - https://www.seeedstudio.com/XIAO-ESP32S3-p-5627.html
-// - https://wiki.seeedstudio.com/xiao_esp32s3_getting_started/
-
-package machine
-
-// Digital Pins
-const (
-	D0  = GPIO1
-	D1  = GPIO2
-	D2  = GPIO3
-	D3  = GPIO4
-	D4  = GPIO5
-	D5  = GPIO6
-	D6  = GPIO43
-	D7  = GPIO44
-	D8  = GPIO7
-	D9  = GPIO8
-	D10 = GPIO9
-)
-
-// Analog pins
-const (
-	A0 = GPIO1
-	A1 = GPIO2
-	A2 = GPIO3
-	A3 = GPIO4
-)
-
-// UART pins
-const (
-	UART_RX_PIN = GPIO44
-	UART_TX_PIN = GPIO43
-)
-
-// I2C pins
-const (
-	SDA_PIN = GPIO5
-	SCL_PIN = GPIO6
-)
-
-// SPI pins
-const (
-	SPI_SCK_PIN = GPIO7
-	SPI_SDI_PIN = GPIO9
-	SPI_SDO_PIN = GPIO8
-)
-
-// Onboard LEDs
-const (
-	LED = GPIO21
-)
@@ -43,11 +43,6 @@ type BlockDevice interface {
 	io.ReaderAt

 	// WriteAt writes the given number of bytes to the block device.
-	//
-	// This interface directly writes data to the underlying block device.
-	// Different kinds of devices have different requirements: most can only
-	// write data after the page has been erased, and many can only write data
-	// with specific alignment (such as 4-byte alignment).
 	io.WriterAt

 	// Size returns the number of bytes in this block device.
@@ -1,9 +1,6 @@
 package machine

-import (
-	"errors"
-	"unsafe"
-)
+import "errors"

 var (
 	ErrTimeoutRNG         = errors.New("machine: RNG Timeout")
@@ -65,30 +62,3 @@ func (p Pin) Low() {
 type ADC struct {
 	Pin Pin
 }
-
-// Convert the pointer to a uintptr, to be used for memory I/O (DMA for
-// example). It also means the pointer is "gone" as far as the compiler is
-// concerned, and a GC cycle might deallocate the object. To prevent this from
-// happening, also call keepAliveNoEscape at a point after the address isn't
-// accessed anymore by the hardware.
-// The only exception is if the pointer is accessed later in a volatile way
-// (volatile read/write), which also forces the value to stay alive until that
-// point.
-//
-// This function is treated specially by the compiler to mark the 'ptr'
-// parameter as not escaping.
-//
-// TODO: this function should eventually be replaced with the proposed ptrtoaddr
-// instruction in LLVM. See:
-// https://discourse.llvm.org/t/clarifiying-the-semantics-of-ptrtoint/83987/10
-// https://github.com/llvm/llvm-project/pull/139357
-func unsafeNoEscape(ptr unsafe.Pointer) uintptr {
-	return uintptr(ptr)
-}
-
-// Make sure the given pointer stays alive until this point. This is similar to
-// runtime.KeepAlive, with the difference that it won't let the pointer escape.
-// This is typically used together with unsafeNoEscape.
-//
-// This is a compiler intrinsic.
-func keepAliveNoEscape(ptr unsafe.Pointer)
@@ -337,7 +337,7 @@ func handleUSBSetAddress(setup usb.Setup) bool {

 // SendUSBInPacket sends a packet for USB (interrupt in / bulk in).
 func SendUSBInPacket(ep uint32, data []byte) bool {
-	sendUSBPacket(ep, data)
+	sendUSBPacket(ep, data, 0)

 	// clear transfer complete flag
 	setEPINTFLAG(ep, sam.USB_DEVICE_EPINTFLAG_TRCPT1)
@@ -351,28 +351,27 @@ func SendUSBInPacket(ep uint32, data []byte) bool {
 // Prevent file size increases: https://github.com/tinygo-org/tinygo/pull/998
 //
 //go:noinline
-func sendUSBPacket(ep uint32, data []byte) {
-	// Select the corresponding buffer.
-	buffer := udd_ep_control_cache_buffer[:]
-	if ep != 0 {
-		buffer = udd_ep_in_cache_buffer[ep][:]
+func sendUSBPacket(ep uint32, data []byte, maxsize uint16) {
+	l := uint16(len(data))
+	if 0 < maxsize && maxsize < l {
+		l = maxsize
 	}

-	// Copy the packet to the buffer.
-	copy(buffer[:len(data)], data)
-
-	// Select the corresponding endpoint descriptor.
-	endpoint := &usbEndpointDescriptors[ep].DeviceDescBank[1]
-
-	// Set the endpoint address.
-	endpoint.ADDR.Set(uint32(uintptr(unsafe.Pointer(unsafe.SliceData(buffer)))))
+	// Set endpoint address for sending data
+	if ep == 0 {
+		copy(udd_ep_control_cache_buffer[:], data[:l])
+		usbEndpointDescriptors[ep].DeviceDescBank[1].ADDR.Set(uint32(uintptr(unsafe.Pointer(&udd_ep_control_cache_buffer))))
+	} else {
+		copy(udd_ep_in_cache_buffer[ep][:], data[:l])
+		usbEndpointDescriptors[ep].DeviceDescBank[1].ADDR.Set(uint32(uintptr(unsafe.Pointer(&udd_ep_in_cache_buffer[ep]))))
+	}

 	// clear multi-packet size which is total bytes already sent
-	endpoint.PCKSIZE.ClearBits(usb_DEVICE_PCKSIZE_MULTI_PACKET_SIZE_Mask << usb_DEVICE_PCKSIZE_MULTI_PACKET_SIZE_Pos)
+	usbEndpointDescriptors[ep].DeviceDescBank[1].PCKSIZE.ClearBits(usb_DEVICE_PCKSIZE_MULTI_PACKET_SIZE_Mask << usb_DEVICE_PCKSIZE_MULTI_PACKET_SIZE_Pos)

 	// set byte count, which is total number of bytes to be sent
-	endpoint.PCKSIZE.ClearBits(usb_DEVICE_PCKSIZE_BYTE_COUNT_Mask << usb_DEVICE_PCKSIZE_BYTE_COUNT_Pos)
-	endpoint.PCKSIZE.SetBits((uint32(len(data)) & usb_DEVICE_PCKSIZE_BYTE_COUNT_Mask) << usb_DEVICE_PCKSIZE_BYTE_COUNT_Pos)
+	usbEndpointDescriptors[ep].DeviceDescBank[1].PCKSIZE.ClearBits(usb_DEVICE_PCKSIZE_BYTE_COUNT_Mask << usb_DEVICE_PCKSIZE_BYTE_COUNT_Pos)
+	usbEndpointDescriptors[ep].DeviceDescBank[1].PCKSIZE.SetBits((uint32(l) & usb_DEVICE_PCKSIZE_BYTE_COUNT_Mask) << usb_DEVICE_PCKSIZE_BYTE_COUNT_Pos)
 }

 func ReceiveUSBControlPacket() ([cdcLineInfoSize]byte, error) {
@@ -340,7 +340,7 @@ func handleUSBSetAddress(setup usb.Setup) bool {

 // SendUSBInPacket sends a packet for USB (interrupt in / bulk in).
 func SendUSBInPacket(ep uint32, data []byte) bool {
-	sendUSBPacket(ep, data)
+	sendUSBPacket(ep, data, 0)

 	// clear transfer complete flag
 	setEPINTFLAG(ep, sam.USB_DEVICE_ENDPOINT_EPINTFLAG_TRCPT1)
@@ -354,28 +354,27 @@ func SendUSBInPacket(ep uint32, data []byte) bool {
 // Prevent file size increases: https://github.com/tinygo-org/tinygo/pull/998
 //
 //go:noinline
-func sendUSBPacket(ep uint32, data []byte) {
-	// Select the corresponding buffer.
-	buffer := udd_ep_control_cache_buffer[:]
-	if ep != 0 {
-		buffer = udd_ep_in_cache_buffer[ep][:]
+func sendUSBPacket(ep uint32, data []byte, maxsize uint16) {
+	l := uint16(len(data))
+	if 0 < maxsize && maxsize < l {
+		l = maxsize
 	}

-	// Copy the packet to the buffer.
-	copy(buffer[:len(data)], data)
-
-	// Select the corresponding endpoint descriptor.
-	endpoint := &usbEndpointDescriptors[ep].DeviceDescBank[1]
-
-	// Set the endpoint address.
-	endpoint.ADDR.Set(uint32(uintptr(unsafe.Pointer(unsafe.SliceData(buffer)))))
+	// Set endpoint address for sending data
+	if ep == 0 {
+		copy(udd_ep_control_cache_buffer[:], data[:l])
+		usbEndpointDescriptors[ep].DeviceDescBank[1].ADDR.Set(uint32(uintptr(unsafe.Pointer(&udd_ep_control_cache_buffer))))
+	} else {
+		copy(udd_ep_in_cache_buffer[ep][:], data[:l])
+		usbEndpointDescriptors[ep].DeviceDescBank[1].ADDR.Set(uint32(uintptr(unsafe.Pointer(&udd_ep_in_cache_buffer[ep]))))
+	}

 	// clear multi-packet size which is total bytes already sent
-	endpoint.PCKSIZE.ClearBits(usb_DEVICE_PCKSIZE_MULTI_PACKET_SIZE_Mask << usb_DEVICE_PCKSIZE_MULTI_PACKET_SIZE_Pos)
+	usbEndpointDescriptors[ep].DeviceDescBank[1].PCKSIZE.ClearBits(usb_DEVICE_PCKSIZE_MULTI_PACKET_SIZE_Mask << usb_DEVICE_PCKSIZE_MULTI_PACKET_SIZE_Pos)

 	// set byte count, which is total number of bytes to be sent
-	endpoint.PCKSIZE.ClearBits(usb_DEVICE_PCKSIZE_BYTE_COUNT_Mask << usb_DEVICE_PCKSIZE_BYTE_COUNT_Pos)
-	endpoint.PCKSIZE.SetBits((uint32(len(data)) & usb_DEVICE_PCKSIZE_BYTE_COUNT_Mask) << usb_DEVICE_PCKSIZE_BYTE_COUNT_Pos)
+	usbEndpointDescriptors[ep].DeviceDescBank[1].PCKSIZE.ClearBits(usb_DEVICE_PCKSIZE_BYTE_COUNT_Mask << usb_DEVICE_PCKSIZE_BYTE_COUNT_Pos)
+	usbEndpointDescriptors[ep].DeviceDescBank[1].PCKSIZE.SetBits((uint32(l) & usb_DEVICE_PCKSIZE_BYTE_COUNT_Mask) << usb_DEVICE_PCKSIZE_BYTE_COUNT_Pos)
 }

 func ReceiveUSBControlPacket() ([cdcLineInfoSize]byte, error) {
@@ -519,10 +519,6 @@ type Serialer interface {
 	RTS() bool
 }

-func initUSB() {
-	// nothing to do here
-}
-
 // USB Serial/JTAG Controller
 // See esp32-c3_technical_reference_manual_en.pdf
 // pg. 736
@@ -1,312 +0,0 @@
-//go:build esp32s3
-
-package machine
-
-import (
-	"device/esp"
-	"errors"
-	"runtime/volatile"
-	"unsafe"
-)
-
-const deviceName = esp.Device
-
-const xtalClock = 40_000000       // 40MHz
-const apbClock = 80_000000        // 80MHz
-const cryptoPWMClock = 160_000000 // 160MHz
-
-// GetCPUFrequency returns the current CPU frequency of the chip.
-func GetCPUFrequency() (uint32, error) {
-	switch esp.SYSTEM.GetSYSCLK_CONF_SOC_CLK_SEL() {
-	case 0:
-		return xtalClock / (esp.SYSTEM.GetSYSCLK_CONF_PRE_DIV_CNT() + 1), nil
-	case 1:
-		switch esp.SYSTEM.GetCPU_PER_CONF_CPUPERIOD_SEL() {
-		case 0:
-			return 80e6, nil
-		case 1:
-			return 160e6, nil
-		case 2:
-			// If esp.SYSTEM.GetCPU_PER_CONF_PLL_FREQ_SEL() == 1, this is undefined
-			return 240e6, nil
-		}
-	case 2:
-		//RC Fast Clock
-		return (175e5) / (esp.SYSTEM.GetSYSCLK_CONF_PRE_DIV_CNT() + 1), nil
-	}
-	return 0, errors.New("machine: Unable to determine current cpu frequency")
-}
-
-// SetCPUFrequency sets the frequency of the CPU to one of several targets
-func SetCPUFrequency(frequency uint32) error {
-	// Always assume we are on PLL. Lower frequencies can be set with a different
-	// clock source, but this will change the behavior of APB clock and Crypto PWM
-	// clock
-	//esp.SYSTEM.SetSYSCLK_CONF_SOC_CLK_SEL(1)
-
-	switch frequency {
-	case 80_000000:
-		esp.SYSTEM.SetCPU_PER_CONF_CPUPERIOD_SEL(0)
-		esp.SYSTEM.SetCPU_PER_CONF_PLL_FREQ_SEL(0) // Reduce PLL freq when possible
-		return nil
-	case 160_000000:
-		esp.SYSTEM.SetCPU_PER_CONF_CPUPERIOD_SEL(1)
-		esp.SYSTEM.SetCPU_PER_CONF_PLL_FREQ_SEL(0)
-		return nil
-	case 240_000000:
-		esp.SYSTEM.SetCPU_PER_CONF_PLL_FREQ_SEL(1) // Increase PLL freq when needed
-		esp.SYSTEM.SetCPU_PER_CONF_CPUPERIOD_SEL(2)
-		return nil
-	}
-	return errors.New("machine: Unsupported CPU frequency selected. Supported: 80, 160, 240 MHz")
-}
-
-var (
-	ErrInvalidSPIBus = errors.New("machine: invalid SPI bus")
-)
-
-const (
-	PinOutput PinMode = iota
-	PinInput
-	PinInputPullup
-	PinInputPulldown
-)
-
-// Hardware pin numbers
-const (
-	GPIO0  Pin = 0
-	GPIO1  Pin = 1
-	GPIO2  Pin = 2
-	GPIO3  Pin = 3
-	GPIO4  Pin = 4
-	GPIO5  Pin = 5
-	GPIO6  Pin = 6
-	GPIO7  Pin = 7
-	GPIO8  Pin = 8
-	GPIO9  Pin = 9
-	GPIO10 Pin = 10
-	GPIO11 Pin = 11
-	GPIO12 Pin = 12
-	GPIO13 Pin = 13
-	GPIO14 Pin = 14
-	GPIO15 Pin = 15
-	GPIO16 Pin = 16
-	GPIO17 Pin = 17
-	GPIO18 Pin = 18
-	GPIO19 Pin = 19
-	GPIO20 Pin = 20
-	GPIO21 Pin = 21
-	GPIO26 Pin = 26
-	GPIO27 Pin = 27
-	GPIO28 Pin = 28
-	GPIO29 Pin = 29
-	GPIO30 Pin = 30
-	GPIO31 Pin = 31
-	GPIO32 Pin = 32
-	GPIO33 Pin = 33
-	GPIO34 Pin = 34
-	GPIO35 Pin = 35
-	GPIO36 Pin = 36
-	GPIO37 Pin = 37
-	GPIO38 Pin = 38
-	GPIO39 Pin = 39
-	GPIO40 Pin = 40
-	GPIO41 Pin = 41
-	GPIO42 Pin = 42
-	GPIO43 Pin = 43
-	GPIO44 Pin = 44
-	GPIO45 Pin = 45
-	GPIO46 Pin = 46
-	GPIO47 Pin = 47
-	GPIO48 Pin = 48
-)
-
-// Configure this pin with the given configuration.
-func (p Pin) Configure(config PinConfig) {
-	// Output function 256 is a special value reserved for use as a regular GPIO
-	// pin. Peripherals (SPI etc) can set a custom output function by calling
-	// lowercase configure() instead with a signal name.
-	p.configure(config, 256)
-}
-
-// configure is the same as Configure, but allows for setting a specific input
-// or output signal.
-// Signals are always routed through the GPIO matrix for simplicity. Output
-// signals are configured in FUNCx_OUT_SEL_CFG which selects a particular signal
-// to output on a given pin. Input signals are configured in FUNCy_IN_SEL_CFG,
-// which sets the pin to use for a particular input signal.
-func (p Pin) configure(config PinConfig, signal uint32) {
-	if p == NoPin {
-		// This simplifies pin configuration in peripherals such as SPI.
-		return
-	}
-
-	ioConfig := uint32(0)
-
-	// MCU_SEL: Function 1 is always GPIO
-	ioConfig |= (1 << esp.IO_MUX_GPIO_MCU_SEL_Pos)
-
-	// FUN_IE: Make this pin an input pin (always set for GPIO operation)
-	ioConfig |= esp.IO_MUX_GPIO_FUN_IE
-
-	// DRV: Set drive strength to 20 mA as a default. Pins 17 and 18 are special
-	var drive uint32
-	if p == GPIO17 || p == GPIO18 {
-		drive = 1 // 20 mA
-	} else {
-		drive = 2 // 20 mA
-	}
-	ioConfig |= (drive << esp.IO_MUX_GPIO_FUN_DRV_Pos)
-
-	// WPU/WPD: Select pull mode.
-	if config.Mode == PinInputPullup {
-		ioConfig |= esp.IO_MUX_GPIO_FUN_WPU
-	} else if config.Mode == PinInputPulldown {
-		ioConfig |= esp.IO_MUX_GPIO_FUN_WPD
-	}
-
-	// Set configuration
-	ioRegister := p.ioMuxReg()
-	ioRegister.Set(ioConfig)
-
-	switch config.Mode {
-	case PinOutput:
-		// Set the 'output enable' bit.
-		if p < 32 {
-			esp.GPIO.ENABLE_W1TS.Set(1 << p)
-		} else {
-			esp.GPIO.ENABLE1_W1TS.Set(1 << (p - 32))
-		}
-		// Set the signal to read the output value from. It can be a peripheral
-		// output signal, or the special value 256 which indicates regular GPIO
-		// usage.
-		p.outFunc().Set(signal)
-	case PinInput, PinInputPullup, PinInputPulldown:
-		// Clear the 'output enable' bit.
-		if p < 32 {
-			esp.GPIO.ENABLE_W1TC.Set(1 << p)
-		} else {
-			esp.GPIO.ENABLE1_W1TC.Set(1 << (p - 32))
-		}
-		if signal != 256 {
-			// Signal is a peripheral function (not a simple GPIO). Connect this
-			// signal to the pin.
-			// Note that outFunc and inFunc work in the opposite direction.
-			// outFunc configures a pin to use a given output signal, while
-			// inFunc specifies a pin to use to read the signal from.
-			inFunc(signal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(p)<<esp.GPIO_FUNC_IN_SEL_CFG_IN_SEL_Pos)
-		}
-	}
-}
-
-// ioMuxReg returns the IO_MUX_n_REG register used for configuring the io mux for
-// this pin
-func (p Pin) ioMuxReg() *volatile.Register32 {
-	return (*volatile.Register32)(unsafe.Add(unsafe.Pointer(&esp.IO_MUX.GPIO0), uintptr(p)*4))
-}
-
-// outFunc returns the FUNCx_OUT_SEL_CFG register used for configuring the
-// output function selection.
-func (p Pin) outFunc() *volatile.Register32 {
-	return (*volatile.Register32)(unsafe.Add(unsafe.Pointer(&esp.GPIO.FUNC0_OUT_SEL_CFG), uintptr(p)*4))
-}
-
-// inFunc returns the FUNCy_IN_SEL_CFG register used for configuring the input
-// function selection.
-func inFunc(signal uint32) *volatile.Register32 {
-	return (*volatile.Register32)(unsafe.Add(unsafe.Pointer(&esp.GPIO.FUNC0_IN_SEL_CFG), uintptr(signal)*4))
-}
-
-// Set the pin to high or low.
-// Warning: only use this on an output pin!
-func (p Pin) Set(value bool) {
-	if value {
-		reg, mask := p.portMaskSet()
-		reg.Set(mask)
-	} else {
-		reg, mask := p.portMaskClear()
-		reg.Set(mask)
-	}
-}
-
-// Return the register and mask to enable a given GPIO pin. This can be used to
-// implement bit-banged drivers.
-//
-// Warning: only use this on an output pin!
-func (p Pin) PortMaskSet() (*uint32, uint32) {
-	reg, mask := p.portMaskSet()
-	return &reg.Reg, mask
-}
-
-// Return the register and mask to disable a given GPIO pin. This can be used to
-// implement bit-banged drivers.
-//
-// Warning: only use this on an output pin!
-func (p Pin) PortMaskClear() (*uint32, uint32) {
-	reg, mask := p.portMaskClear()
-	return &reg.Reg, mask
-}
-
-func (p Pin) portMaskSet() (*volatile.Register32, uint32) {
-	if p < 32 {
-		return &esp.GPIO.OUT_W1TS, 1 << p
-	} else {
-		return &esp.GPIO.OUT1_W1TS, 1 << (p - 32)
-	}
-}
-
-func (p Pin) portMaskClear() (*volatile.Register32, uint32) {
-	if p < 32 {
-		return &esp.GPIO.OUT_W1TC, 1 << p
-	} else {
-		return &esp.GPIO.OUT1_W1TC, 1 << (p - 32)
-	}
-}
-
-// Get returns the current value of a GPIO pin when the pin is configured as an
-// input or as an output.
-func (p Pin) Get() bool {
-	if p < 32 {
-		return esp.GPIO.IN.Get()&(1<<p) != 0
-	} else {
-		return esp.GPIO.IN1.Get()&(1<<(p-32)) != 0
-	}
-}
-
-var DefaultUART = UART0
-
-var (
-	UART0  = &_UART0
-	_UART0 = UART{Bus: esp.UART0, Buffer: NewRingBuffer()}
-	UART1  = &_UART1
-	_UART1 = UART{Bus: esp.UART1, Buffer: NewRingBuffer()}
-	UART2  = &_UART2
-	_UART2 = UART{Bus: esp.UART2, Buffer: NewRingBuffer()}
-)
-
-type UART struct {
-	Bus    *esp.UART_Type
-	Buffer *RingBuffer
-}
-
-func (uart *UART) Configure(config UARTConfig) {
-	if config.BaudRate == 0 {
-		config.BaudRate = 115200
-	}
-	// Crystal clock source is selected by default
-	uart.Bus.CLKDIV.Set(xtalClock / config.BaudRate)
-}
-
-func (uart *UART) writeByte(b byte) error {
-	for (uart.Bus.STATUS.Get()>>16)&0xff >= 128 {
-		// Read UART_TXFIFO_CNT from the status register, which indicates how
-		// many bytes there are in the transmit buffer. Wait until there are
-		// less than 128 bytes in this buffer (the default buffer size).
-	}
-	uart.Bus.FIFO.Set(uint32(b))
-	return nil
-}
-
-func (uart *UART) flush() {}
-
-// TODO: SPI
@@ -3,9 +3,7 @@
 package machine

 import (
-	"device/arm"
 	"device/nrf"
-	"errors"
 	"internal/binary"
 	"runtime/interrupt"
 	"unsafe"
@@ -382,10 +380,6 @@ func (f flashBlockDevice) ReadAt(p []byte, off int64) (n int, err error) {
 // If the length of p is not long enough it will be padded with 0xFF bytes.
 // This method assumes that the destination is already erased.
 func (f flashBlockDevice) WriteAt(p []byte, off int64) (n int, err error) {
-	if len(p) == 0 {
-		return 0, nil // nothing to do (and it would fail in sd_flash_write)
-	}
-
 	if FlashDataStart()+uintptr(off)+uintptr(len(p)) > FlashDataEnd() {
 		return 0, errFlashCannotWritePastEOF
 	}
@@ -393,35 +387,6 @@ func (f flashBlockDevice) WriteAt(p []byte, off int64) (n int, err error) {
 	address := FlashDataStart() + uintptr(off)
 	padded := flashPad(p, int(f.WriteBlockSize()))

-	// When the SoftDevice is enabled, access to the flash is restricted and
-	// must go through the SoftDevice API.
-	if isSoftDeviceEnabled() {
-		// Call sd_flash_write, which is SVC_SOC_BASE + 9 in all the
-		// SoftDevices I've checked.
-		// Documentation:
-		// https://docs.nordicsemi.com/bundle/s140_v6.0.0_api/page/group_n_r_f_s_o_c_f_u_n_c_t_i_o_n_s.html
-		numberOfWords := len(padded) / 4 // flash access goes in 32-bit words
-		result := arm.SVCall3(0x20+9, address, &padded[0], uint32(numberOfWords))
-		if result != 0 {
-			// Could not queue flash operation? Not sure when this can
-			// happen.
-			return 0, flashError
-		}
-
-		// Wait until the SoftDevice is finished.
-		flashStatus = flashStatusBusy
-		for flashStatus == flashStatusBusy {
-			handleSoftDeviceEvents()
-		}
-
-		// Check whether the operation was successful.
-		if flashStatus != flashStatusOk {
-			flashStatus = flashStatusOk
-			return 0, flashError
-		}
-		return len(p), nil
-	}
-
 	waitWhileFlashBusy()

 	nrf.NVMC.SetCONFIG_WEN(nrf.NVMC_CONFIG_WEN_Wen)
@@ -463,40 +428,6 @@ func (f flashBlockDevice) EraseBlockSize() int64 {
 // supports this. The start and len parameters are in block numbers, use
 // EraseBlockSize to map addresses to blocks.
 func (f flashBlockDevice) EraseBlocks(start, len int64) error {
-	// When the SoftDevice is enabled, access to the flash is restricted and
-	// must go through the SoftDevice API.
-	if isSoftDeviceEnabled() {
-		for i := range uint32(len) {
-			flashPage := uint32(FlashDataStart())/eraseBlockSizeValue + uint32(start) + i
-
-			// Call sd_flash_page_erase, which is SVC_SOC_BASE + 8 in all the
-			// SoftDevices I've checked.
-			// Documentation:
-			// https://docs.nordicsemi.com/bundle/s140_v6.0.0_api/page/group_n_r_f_s_o_c_f_u_n_c_t_i_o_n_s.html#ga9c93dd94a138ad8b5ed3693ea38ffb3e
-			result := arm.SVCall1(0x20+8, flashPage)
-			if result != 0 {
-				// Could not queue flash operation? Not sure when this can
-				// happen.
-				return flashError
-			}
-
-			// Wait until the SoftDevice is finished.
-			flashStatus = flashStatusBusy
-			for flashStatus == flashStatusBusy {
-				handleSoftDeviceEvents()
-			}
-
-			// Check whether the operation was successful.
-			if flashStatus != flashStatusOk {
-				flashStatus = flashStatusOk
-				return flashError
-			}
-		}
-		return nil
-	}
-
-	// SoftDevice is not used or enabled. Use NVIC directly.
-
 	address := FlashDataStart() + uintptr(start*f.EraseBlockSize())
 	waitWhileFlashBusy()

@@ -516,52 +447,3 @@ func waitWhileFlashBusy() {
 	for nrf.NVMC.GetREADY() != nrf.NVMC_READY_READY_Ready {
 	}
 }
-
-var flashError = errors.New("machine: flash operation failed")
-
-const (
-	flashStatusOk = iota
-	flashStatusError
-	flashStatusBusy
-)
-
-var flashStatus uint8 = flashStatusOk
-
-var sdEvent uint32
-
-// Process all queued SoftDevice events. May only be called when the SoftDevice is enabled.
-//
-// Normally these are handled in the same interrupt where Bluetooth events are
-// handled. But in TinyGo, that's complicated. One option would be to put it in
-// the tinygo.org/x/bluetooth package, but that would cause a circular
-// dependency between the machine and the bluetooth package. Another would be to
-// put it here, but let the bluetooth package call handleSoftDeviceEvents, but
-// that relies on updating the bluetooth package at the same time. As a
-// compromise, these events are handled directly where they are expected (here
-// in the machine package). This works in practice since there are only very few
-// of such events (at the moment, only flash-related ones which is in the
-// machine package anyway).
-func handleSoftDeviceEvents() {
-	for {
-		var result uintptr
-		if nrf.Device == "nrf52" || nrf.Device == "nrf52840" || nrf.Device == "nrf52833" {
-			// sd_evt_get: SOC_SVC_BASE_NOT_AVAILABLE + 31
-			result = arm.SVCall1(0x2C+31, &sdEvent)
-		} else {
-			return // TODO: nrf51 etc
-		}
-		if result != 0 {
-			// Some error occured. The only possible error is
-			// NRF_ERROR_NOT_FOUND, which means there are no more events.
-			return
-		}
-
-		// The following events are the same numbers in all SoftDevices I've checked.
-		switch sdEvent {
-		case 2: // NRF_EVT_FLASH_OPERATION_SUCCESS
-			flashStatus = flashStatusOk
-		case 3: // NRF_EVT_FLASH_OPERATION_ERROR
-			flashStatus = flashStatusError
-		}
-	}
-}
@@ -69,5 +69,3 @@ const eraseBlockSizeValue = 4096
 func eraseBlockSize() int64 {
 	return eraseBlockSizeValue
 }
-
-const spiMaxBufferSize = 255 // from the datasheet: TXD.MAXCNT and RXD.MAXCNT
@@ -90,5 +90,3 @@ const eraseBlockSizeValue = 4096
 func eraseBlockSize() int64 {
 	return eraseBlockSizeValue
 }
-
-const spiMaxBufferSize = 0xffff // from the datasheet: TXD.MAXCNT and RXD.MAXCNT
@@ -108,9 +108,3 @@ const eraseBlockSizeValue = 4096
 func eraseBlockSize() int64 {
 	return eraseBlockSizeValue
 }
-
-const spiMaxBufferSize = 0xffff // from the datasheet: TXD.MAXCNT and RXD.MAXCNT
-
-// ADC instance for the VDDH input pin. This pin is typically connected to USB
-// input voltage (~5V) or directly to a battery.
-var ADC_VDDH = ADC{adcVDDHPin}
@@ -256,7 +256,7 @@ func initEndpoint(ep, config uint32) {

 // SendUSBInPacket sends a packet for USBHID (interrupt in / bulk in).
 func SendUSBInPacket(ep uint32, data []byte) bool {
-	sendUSBPacket(ep, data)
+	sendUSBPacket(ep, data, 0)

 	// clear transfer complete flag
 	nrf.USBD.INTENCLR.Set(nrf.USBD_INTENCLR_ENDEPOUT0 << 4)
@@ -267,32 +267,33 @@ func SendUSBInPacket(ep uint32, data []byte) bool {
 // Prevent file size increases: https://github.com/tinygo-org/tinygo/pull/998
 //
 //go:noinline
-func sendUSBPacket(ep uint32, data []byte) {
-	// Select the corresponding buffer.
+func sendUSBPacket(ep uint32, data []byte, maxsize uint16) {
 	count := len(data)
-	var buffer []byte
+	if 0 < int(maxsize) && int(maxsize) < count {
+		count = int(maxsize)
+	}
+
 	if ep == 0 {
-		buffer = udd_ep_control_cache_buffer[:]
+		copy(udd_ep_control_cache_buffer[:], data[:count])
 		if count > usb.EndpointPacketSize {
-			// The packet must be sent in chunks.
 			sendOnEP0DATADONE.offset = usb.EndpointPacketSize
-			sendOnEP0DATADONE.ptr = &udd_ep_control_cache_buffer[usb.EndpointPacketSize]
+			sendOnEP0DATADONE.ptr = &udd_ep_control_cache_buffer[sendOnEP0DATADONE.offset]
 			sendOnEP0DATADONE.count = count - usb.EndpointPacketSize
 			count = usb.EndpointPacketSize
 		}
+		sendViaEPIn(
+			ep,
+			&udd_ep_control_cache_buffer[0],
+			count,
+		)
 	} else {
-		buffer = udd_ep_in_cache_buffer[ep][:]
+		copy(udd_ep_in_cache_buffer[ep][:], data[:count])
+		sendViaEPIn(
+			ep,
+			&udd_ep_in_cache_buffer[ep][0],
+			count,
+		)
 	}
-
-	// Copy the packet to the buffer.
-	copy(buffer[:len(data)], data)
-
-	// Send the first chunk of the packet.
-	sendViaEPIn(
-		ep,
-		&buffer[0],
-		count,
-	)
 }

 func handleEndpointRx(ep uint32) []byte {
@@ -49,7 +49,7 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) (err error) {

 	// Configure for a single shot to perform both write and read (as applicable)
 	if len(w) != 0 {
-		i2c.Bus.TXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(w)))))
+		i2c.Bus.TXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&w[0]))))
 		i2c.Bus.TXD.MAXCNT.Set(uint32(len(w)))

 		// If no read, immediately signal stop after TX
@@ -58,7 +58,7 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) (err error) {
 		}
 	}
 	if len(r) != 0 {
-		i2c.Bus.RXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(r)))))
+		i2c.Bus.RXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&r[0]))))
 		i2c.Bus.RXD.MAXCNT.Set(uint32(len(r)))

 		// Auto-start Rx after Tx and Stop after Rx
@@ -89,11 +89,6 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) (err error) {
 		}
 	}

-	// Make sure the w and r buffers stay alive until this point, so they won't
-	// be garbage collected while the buffers are used by the hardware.
-	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(w)))
-	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(r)))
-
 	return
 }

@@ -122,7 +117,7 @@ func (i2c *I2C) Listen(addr uint8) error {
 //
 // For request events, the caller MUST call `Reply` to avoid hanging the i2c bus indefinitely.
 func (i2c *I2C) WaitForEvent(buf []byte) (evt I2CTargetEvent, count int, err error) {
-	i2c.BusT.RXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(buf)))))
+	i2c.BusT.RXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&buf[0]))))
 	i2c.BusT.RXD.MAXCNT.Set(uint32(len(buf)))

 	i2c.BusT.TASKS_PREPARERX.Set(nrf.TWIS_TASKS_PREPARERX_TASKS_PREPARERX_Trigger)
@@ -139,10 +134,6 @@ func (i2c *I2C) WaitForEvent(buf []byte) (evt I2CTargetEvent, count int, err err
 		}
 	}

-	// Make sure buf stays alive until this point, so it won't be garbage
-	// collected while it is used by the hardware.
-	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(buf)))
-
 	count = 0
 	evt = I2CFinish
 	err = nil
@@ -172,7 +163,7 @@ func (i2c *I2C) WaitForEvent(buf []byte) (evt I2CTargetEvent, count int, err err

 // Reply supplies the response data the controller.
 func (i2c *I2C) Reply(buf []byte) error {
-	i2c.BusT.TXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(buf)))))
+	i2c.BusT.TXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&buf[0]))))
 	i2c.BusT.TXD.MAXCNT.Set(uint32(len(buf)))

 	i2c.BusT.EVENTS_STOPPED.Set(0)
@@ -189,10 +180,6 @@ func (i2c *I2C) Reply(buf []byte) error {
 		}
 	}

-	// Make sure the buffer stays alive until this point, so it won't be garbage
-	// collected while it is used by the hardware.
-	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(buf)))
-
 	i2c.BusT.EVENTS_STOPPED.Set(0)

 	return nil
@@ -12,8 +12,6 @@ func CPUFrequency() uint32 {
 	return 64000000
 }

-var adcVDDHPin = Pin(254) // special pin number for VDDH on the nrf52840
-
 // InitADC initializes the registers needed for ADC.
 func InitADC() {
 	// Enable ADC.
@@ -28,7 +26,7 @@ func InitADC() {
 // Samples can be 1(default), 2, 4, 8, 16, 32, 64, 128, 256 samples
 func (a *ADC) Configure(config ADCConfig) {
 	var configVal uint32 = nrf.SAADC_CH_CONFIG_RESP_Bypass<<nrf.SAADC_CH_CONFIG_RESP_Pos |
-		nrf.SAADC_CH_CONFIG_RESN_Bypass<<nrf.SAADC_CH_CONFIG_RESN_Pos |
+		nrf.SAADC_CH_CONFIG_RESP_Bypass<<nrf.SAADC_CH_CONFIG_RESN_Pos |
 		nrf.SAADC_CH_CONFIG_REFSEL_Internal<<nrf.SAADC_CH_CONFIG_REFSEL_Pos |
 		nrf.SAADC_CH_CONFIG_MODE_SE<<nrf.SAADC_CH_CONFIG_MODE_Pos

@@ -118,43 +116,36 @@ func (a *ADC) Configure(config ADCConfig) {

 // Get returns the current value of an ADC pin in the range 0..0xffff.
 func (a *ADC) Get() uint16 {
-	var adcPin uint32
+	var pwmPin uint32
 	var rawValue volatile.Register16

 	switch a.Pin {
 	case 2:
-		adcPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput0
+		pwmPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput0
 	case 3:
-		adcPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput1
+		pwmPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput1
 	case 4:
-		adcPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput2
+		pwmPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput2
 	case 5:
-		adcPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput3
+		pwmPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput3
 	case 28:
-		adcPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput4
+		pwmPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput4
 	case 29:
-		adcPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput5
+		pwmPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput5
 	case 30:
-		adcPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput6
+		pwmPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput6
 	case 31:
-		adcPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput7
-	case adcVDDHPin:
-		if Device == "nrf52840" {
-			adcPin = 0x0D // VDDHDIV5 on the nrf52840
-		} else {
-			return 0
-		}
+		pwmPin = nrf.SAADC_CH_PSELP_PSELP_AnalogInput7
 	default:
 		return 0
 	}

 	// Set pin to read.
-	nrf.SAADC.CH[0].PSELP.Set(adcPin)
+	nrf.SAADC.CH[0].PSELN.Set(pwmPin)
+	nrf.SAADC.CH[0].PSELP.Set(pwmPin)

 	// Destination for sample result.
-	// Note: rawValue doesn't need to be kept alive for the GC, since the
-	// volatile read later will force it to stay alive.
-	nrf.SAADC.RESULT.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(&rawValue))))
+	nrf.SAADC.RESULT.PTR.Set(uint32(uintptr(unsafe.Pointer(&rawValue))))
 	nrf.SAADC.RESULT.MAXCNT.Set(1) // One sample

 	// Start tasks.
@@ -310,18 +301,20 @@ func (spi *SPI) Transfer(w byte) (byte, error) {
 // padded until they fit: if len(w) > len(r) the extra bytes received will be
 // dropped and if len(w) < len(r) extra 0 bytes will be sent.
 func (spi *SPI) Tx(w, r []byte) error {
-	// Unfortunately the hardware (on the nrf52832) only supports a limited
-	// amount of bytes in the buffers (depending on the chip), so if either w or
-	// r is longer than that the transfer needs to be broken up in pieces.
+	// Unfortunately the hardware (on the nrf52832) only supports up to 255
+	// bytes in the buffers, so if either w or r is longer than that the
+	// transfer needs to be broken up in pieces.
+	// The nrf52840 supports far larger buffers however, which isn't yet
+	// supported.
 	for len(r) != 0 || len(w) != 0 {
 		// Prepare the SPI transfer: set the DMA pointers and lengths.
 		// read buffer
 		nr := uint32(len(r))
 		if nr > 0 {
-			if nr > spiMaxBufferSize {
-				nr = spiMaxBufferSize
+			if nr > 255 {
+				nr = 255
 			}
-			spi.Bus.RXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(r)))))
+			spi.Bus.RXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&r[0]))))
 			r = r[nr:]
 		}
 		spi.Bus.RXD.MAXCNT.Set(nr)
@@ -329,10 +322,10 @@ func (spi *SPI) Tx(w, r []byte) error {
 		// write buffer
 		nw := uint32(len(w))
 		if nw > 0 {
-			if nw > spiMaxBufferSize {
-				nw = spiMaxBufferSize
+			if nw > 255 {
+				nw = 255
 			}
-			spi.Bus.TXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(w)))))
+			spi.Bus.TXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&w[0]))))
 			w = w[nw:]
 		}
 		spi.Bus.TXD.MAXCNT.Set(nw)
@@ -342,16 +335,10 @@ func (spi *SPI) Tx(w, r []byte) error {
 		// finished if the transfer is send-only (a common case).
 		spi.Bus.TASKS_START.Set(1)
 		for spi.Bus.EVENTS_END.Get() == 0 {
-			gosched()
 		}
 		spi.Bus.EVENTS_END.Set(0)
 	}

-	// Make sure the w and r buffers stay alive for the GC until this point,
-	// since they are used by the hardware but not otherwise visible.
-	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(r)))
-	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(w)))
-
 	return nil
 }

@@ -7,7 +7,3 @@ package machine
 func GetRNG() (ret uint32, err error) {
 	return getRNG()
 }
-
-func isSoftDeviceEnabled() bool {
-	return false
-}
@@ -11,8 +11,9 @@ import (

 // avoid a heap allocation in GetRNG.
 var (
-	bytesAvailable uint8
-	buf            [4]uint8
+	softdeviceEnabled uint8
+	bytesAvailable    uint8
+	buf               [4]uint8

 	errNoSoftDeviceSupport = errors.New("rng: softdevice not supported on this device")
 	errNotEnoughRandomData = errors.New("rng: not enough random data available")
@@ -23,7 +24,9 @@ var (
 func GetRNG() (ret uint32, err error) {
 	// First check whether the SoftDevice is enabled.
 	// sd_rand_application_bytes_available_get cannot be called when the SoftDevice is not enabled.
-	if !isSoftDeviceEnabled() {
+	arm.SVCall1(0x12, &softdeviceEnabled) // sd_softdevice_is_enabled
+
+	if softdeviceEnabled == 0 {
 		return getRNG()
 	}

@@ -54,8 +57,3 @@ func GetRNG() (ret uint32, err error) {

 	return uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24, nil
 }
-
-// This function is defined in the runtime, but we need it too.
-//
-//go:linkname isSoftDeviceEnabled runtime.isSoftDeviceEnabled
-func isSoftDeviceEnabled() bool
@@ -128,7 +128,7 @@ func handleUSBSetAddress(setup usb.Setup) bool {
 	const ackTimeout = 570

 	rp.USBCTRL_REGS.SIE_STATUS.Set(rp.USBCTRL_REGS_SIE_STATUS_ACK_REC)
-	sendUSBPacket(0, []byte{})
+	sendUSBPacket(0, []byte{}, 0)

 	// Wait for transfer to complete with a timeout.
 	t := timer.timeElapsed()
@@ -131,7 +131,7 @@ func handleUSBSetAddress(setup usb.Setup) bool {
 	const ackTimeout = 570

 	rp.USB.SIE_STATUS.Set(rp.USB_SIE_STATUS_ACK_REC)
-	sendUSBPacket(0, []byte{})
+	sendUSBPacket(0, []byte{}, 0)

 	// Wait for transfer to complete with a timeout.
 	t := timer.timeElapsed()
@@ -124,31 +124,6 @@ const (
 	fnXIP pinFunc = 0
 )

-// validPins confirms that the SPI pin selection is a legitimate one
-// for the the 2040 chip.
-func (spi *SPI) validPins(config SPIConfig) error {
-	var okSDI, okSDO, okSCK bool
-	switch spi.Bus {
-	case rp.SPI0:
-		okSDI = config.SDI == 0 || config.SDI == 4 || config.SDI == 16 || config.SDI == 20
-		okSDO = config.SDO == 3 || config.SDO == 7 || config.SDO == 19 || config.SDO == 23
-		okSCK = config.SCK == 2 || config.SCK == 6 || config.SCK == 18 || config.SCK == 22
-	case rp.SPI1:
-		okSDI = config.SDI == 8 || config.SDI == 12 || config.SDI == 24 || config.SDI == 28
-		okSDO = config.SDO == 11 || config.SDO == 15 || config.SDO == 27
-		okSCK = config.SCK == 10 || config.SCK == 14 || config.SCK == 26
-	}
-	switch {
-	case !okSDI:
-		return errSPIInvalidSDI
-	case !okSDO:
-		return errSPIInvalidSDO
-	case !okSCK:
-		return errSPIInvalidSCK
-	}
-	return nil
-}
-
 // Configure configures the gpio pin as per mode.
 func (p Pin) Configure(config PinConfig) {
 	if p == NoPin {
@@ -2,8 +2,6 @@

 package machine

-import "device/rp"
-
 // Analog pins on RP2350a.
 const (
 	ADC0 Pin = GPIO26
@@ -14,28 +12,3 @@ const (
 	// fifth ADC channel.
 	thermADC = 30
 )
-
-// validPins confirms that the SPI pin selection is a legitimate one
-// for the the 2350a chip.
-func (spi *SPI) validPins(config SPIConfig) error {
-	var okSDI, okSDO, okSCK bool
-	switch spi.Bus {
-	case rp.SPI0:
-		okSDI = config.SDI == 0 || config.SDI == 4 || config.SDI == 16 || config.SDI == 20
-		okSDO = config.SDO == 3 || config.SDO == 7 || config.SDO == 19 || config.SDO == 23
-		okSCK = config.SCK == 2 || config.SCK == 6 || config.SCK == 18 || config.SCK == 22
-	case rp.SPI1:
-		okSDI = config.SDI == 8 || config.SDI == 12 || config.SDI == 24 || config.SDI == 28
-		okSDO = config.SDO == 11 || config.SDO == 15 || config.SDO == 27
-		okSCK = config.SCK == 10 || config.SCK == 14 || config.SCK == 26
-	}
-	switch {
-	case !okSDI:
-		return errSPIInvalidSDI
-	case !okSDO:
-		return errSPIInvalidSDO
-	case !okSCK:
-		return errSPIInvalidSCK
-	}
-	return nil
-}
@@ -2,8 +2,6 @@

 package machine

-import "device/rp"
-
 // RP2350B has additional pins.

 const (
@@ -48,28 +46,3 @@ var (
 	PWM10 = getPWMGroup(10)
 	PWM11 = getPWMGroup(11)
 )
-
-// validPins confirms that the SPI pin selection is a legitimate one
-// for the the 2350b chip.
-func (spi *SPI) validPins(config SPIConfig) error {
-	var okSDI, okSDO, okSCK bool
-	switch spi.Bus {
-	case rp.SPI0:
-		okSDI = config.SDI == 0 || config.SDI == 4 || config.SDI == 16 || config.SDI == 20 || config.SDI == 32 || config.SDI == 36
-		okSDO = config.SDO == 3 || config.SDO == 7 || config.SDO == 19 || config.SDO == 23 || config.SDO == 35 || config.SDO == 39
-		okSCK = config.SCK == 2 || config.SCK == 6 || config.SCK == 18 || config.SCK == 22 || config.SCK == 34 || config.SCK == 38
-	case rp.SPI1:
-		okSDI = config.SDI == 8 || config.SDI == 12 || config.SDI == 24 || config.SDI == 28 || config.SDI == 40 || config.SDI == 44
-		okSDO = config.SDO == 11 || config.SDO == 15 || config.SDO == 27 || config.SDO == 31 || config.SDO == 43 || config.SDO == 47
-		okSCK = config.SCK == 10 || config.SCK == 14 || config.SCK == 26 || config.SCK == 30 || config.SCK == 42 || config.SCK == 46
-	}
-	switch {
-	case !okSDI:
-		return errSPIInvalidSDI
-	case !okSDO:
-		return errSPIInvalidSDO
-	case !okSCK:
-		return errSPIInvalidSCK
-	}
-	return nil
-}
@@ -108,7 +108,7 @@ func (spi *SPI) SetBaudRate(br uint32) error {
 	var prescale, postdiv uint32
 	freq := CPUFrequency()
 	for prescale = 2; prescale < 255; prescale += 2 {
-		if uint64(freq) < uint64((prescale+2)*256)*uint64(br) {
+		if freq < (prescale+2)*256*br {
 			break
 		}
 	}
@@ -165,9 +165,27 @@ func (spi *SPI) Configure(config SPIConfig) error {
 			config.SDI = SPI1_SDI_PIN
 		}
 	}
-	if err := spi.validPins(config); err != nil {
-		return err
+	var okSDI, okSDO, okSCK bool
+	switch spi.Bus {
+	case rp.SPI0:
+		okSDI = config.SDI == 0 || config.SDI == 4 || config.SDI == 16 || config.SDI == 20
+		okSDO = config.SDO == 3 || config.SDO == 7 || config.SDO == 19 || config.SDO == 23
+		okSCK = config.SCK == 2 || config.SCK == 6 || config.SCK == 18 || config.SCK == 22
+	case rp.SPI1:
+		okSDI = config.SDI == 8 || config.SDI == 12 || config.SDI == 24 || config.SDI == 28
+		okSDO = config.SDO == 11 || config.SDO == 15 || config.SDO == 27
+		okSCK = config.SCK == 10 || config.SCK == 14 || config.SCK == 26
 	}
+
+	switch {
+	case !okSDI:
+		return errSPIInvalidSDI
+	case !okSDO:
+		return errSPIInvalidSDO
+	case !okSCK:
+		return errSPIInvalidSCK
+	}
+
 	if config.Frequency == 0 {
 		config.Frequency = defaultBaud
 	}
@@ -291,7 +309,7 @@ func (spi *SPI) tx(tx []byte) error {
 	//   - set data size to single bytes
 	//   - set the DREQ so that the DMA will fill the SPI FIFO as needed
 	//   - start the transfer
-	ch.READ_ADDR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(tx)))))
+	ch.READ_ADDR.Set(uint32(uintptr(unsafe.Pointer(&tx[0]))))
 	ch.WRITE_ADDR.Set(uint32(uintptr(unsafe.Pointer(&spi.Bus.SSPDR))))
 	ch.TRANS_COUNT.Set(uint32(len(tx)))
 	ch.CTRL_TRIG.Set(rp.DMA_CH0_CTRL_TRIG_INCR_READ |
@@ -310,11 +328,6 @@ func (spi *SPI) tx(tx []byte) error {
 	for ch.CTRL_TRIG.Get()&rp.DMA_CH0_CTRL_TRIG_BUSY != 0 {
 	}

-	// Make sure the read buffer stays alive until this point (in the unlikely
-	// case the tx slice wasn't read after this function returns and a GC cycle
-	// happened inbetween).
-	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(tx)))
-
 	// We didn't read any result values, which means the RX FIFO has likely
 	// overflown. We have to clean up this mess now.

@@ -72,15 +72,19 @@ func initEndpoint(ep, config uint32) {

 // SendUSBInPacket sends a packet for USB (interrupt in / bulk in).
 func SendUSBInPacket(ep uint32, data []byte) bool {
-	sendUSBPacket(ep, data)
+	sendUSBPacket(ep, data, 0)
 	return true
 }

 // Prevent file size increases: https://github.com/tinygo-org/tinygo/pull/998
 //
 //go:noinline
-func sendUSBPacket(ep uint32, data []byte) {
+func sendUSBPacket(ep uint32, data []byte, maxsize uint16) {
 	count := len(data)
+	if 0 < int(maxsize) && int(maxsize) < count {
+		count = int(maxsize)
+	}
+
 	if ep == 0 {
 		if count > usb.EndpointPacketSize {
 			count = usb.EndpointPacketSize
@@ -141,7 +145,7 @@ func setEPDataPID(ep uint32, dataOne bool) {
 }

 func SendZlp() {
-	sendUSBPacket(0, []byte{})
+	sendUSBPacket(0, []byte{}, 0)
 }

 func sendViaEPIn(ep uint32, data []byte, count int) {
@@ -6,6 +6,5 @@ package machine
 var Serial Serialer

 func InitSerial() {
-	initUSB()
 	Serial = USBCDC
 }
@@ -19,17 +19,6 @@ var (
 	USBCDC Serialer
 )

-func initUSB() {
-	enableUSBCDC()
-	USBDev.Configure(UARTConfig{})
-}
-
-// Using go:linkname here because there's a circular dependency between the
-// machine package and the machine/usb/cdc package.
-//
-//go:linkname enableUSBCDC machine/usb/cdc.EnableUSBCDC
-func enableUSBCDC()
-
 type Serialer interface {
 	WriteByte(c byte) error
 	Write(data []byte) (n int, err error)
@@ -81,6 +70,21 @@ func usbSerial() string {
 	return ""
 }

+// strToUTF16LEDescriptor converts a utf8 string into a string descriptor
+// note: the following code only converts ascii characters to UTF16LE. In order
+// to do a "proper" conversion, we would need to pull in the 'unicode/utf16'
+// package, which at the time this was written added 512 bytes to the compiled
+// binary.
+func strToUTF16LEDescriptor(in string, out []byte) {
+	out[0] = byte(len(out))
+	out[1] = descriptor.TypeString
+	for i, rune := range in {
+		out[(i<<1)+2] = byte(rune)
+		out[(i<<1)+3] = 0
+	}
+	return
+}
+
 const cdcLineInfoSize = 7

 var (
@@ -122,53 +126,51 @@ var (
 	usbStallHandler [NumberOfUSBEndpoints]func(usb.Setup) bool
 )

-var usbLangInfo = [4]byte{
-	// length = 4 bytes
-	0x04,
-	// descriptor type = string
-	0x03,
-	// language codes
-	// 0x0409 = English (United States)
-	0x09, 0x04,
-}
-
 // sendDescriptor creates and sends the various USB descriptor types that
 // can be requested by the host.
 func sendDescriptor(setup usb.Setup) {
 	switch setup.WValueH {
 	case descriptor.TypeConfiguration:
-		sendDescriptorData(usbDescriptor.Configuration, setup.WLength)
+		sendUSBPacket(0, usbDescriptor.Configuration, setup.WLength)
 		return
-
 	case descriptor.TypeDevice:
 		usbDescriptor.Configure(usbVendorID(), usbProductID())
-		sendDescriptorData(usbDescriptor.Device, setup.WLength)
+		sendUSBPacket(0, usbDescriptor.Device, setup.WLength)
 		return

 	case descriptor.TypeString:
 		switch setup.WValueL {
 		case 0:
-			sendDescriptorData(usbLangInfo[:], setup.WLength)
+			usb_trans_buffer[0] = 0x04
+			usb_trans_buffer[1] = 0x03
+			usb_trans_buffer[2] = 0x09
+			usb_trans_buffer[3] = 0x04
+			sendUSBPacket(0, usb_trans_buffer[:4], setup.WLength)

 		case usb.IPRODUCT:
-			sendDescriptorString(usbProduct(), setup.WLength)
+			b := usb_trans_buffer[:(len(usbProduct())<<1)+2]
+			strToUTF16LEDescriptor(usbProduct(), b)
+			sendUSBPacket(0, b, setup.WLength)

 		case usb.IMANUFACTURER:
-			sendDescriptorString(usbManufacturer(), setup.WLength)
+			b := usb_trans_buffer[:(len(usbManufacturer())<<1)+2]
+			strToUTF16LEDescriptor(usbManufacturer(), b)
+			sendUSBPacket(0, b, setup.WLength)

 		case usb.ISERIAL:
-			serial := usbSerial()
-			if len(serial) == 0 {
+			sz := len(usbSerial())
+			if sz == 0 {
 				SendZlp()
 			} else {
-				sendDescriptorString(serial, setup.WLength)
+				b := usb_trans_buffer[:(sz<<1)+2]
+				strToUTF16LEDescriptor(usbSerial(), b)
+				sendUSBPacket(0, b, setup.WLength)
 			}
 		}
-		// TODO: why do we do this when WValueL is unknown?
 		return
 	case descriptor.TypeHIDReport:
 		if h, ok := usbDescriptor.HID[setup.WIndex]; ok {
-			sendDescriptorData(h, setup.WLength)
+			sendUSBPacket(0, h, setup.WLength)
 			return
 		}
 	case descriptor.TypeDeviceQualifier:
@@ -181,39 +183,6 @@ func sendDescriptor(setup usb.Setup) {
 	return
 }

-// sendDescriptorString sends a string descriptor, truncating it to fit maxLen or the buffer size.
-// note: the following code only converts ascii characters to UTF16LE. In order
-// to do a "proper" conversion, we would need to pull in the 'unicode/utf16'
-// package, which at the time this was written added 512 bytes to the compiled
-// binary.
-// TODO: old comment, re-evaluate
-func sendDescriptorString(data string, maxLen uint16) {
-	if maxLen < 2 {
-		// Something has gone horribly wrong.
-		SendZlp()
-		return
-	}
-
-	// Clamp the length.
-	maxEncBytes := min(len(usb_trans_buffer), len(udd_ep_control_cache_buffer), int(maxLen))
-	data = data[:min(len(data), (maxEncBytes-2)/2)]
-
-	// Write the header.
-	buf := usb_trans_buffer[:2*len(data)+2]
-	hdr, body := buf[:2], buf[2:]
-	hdr[0] = byte(len(buf))
-	hdr[1] = descriptor.TypeString
-
-	// Convert the string to UTF16.
-	// NOTE: Using range here would cause the length to disagree when multibyte codepoints are present.
-	for i := 0; i < len(data); i++ {
-		body[2*i] = byte(data[i])
-		body[2*i+1] = 0
-	}
-
-	sendUSBPacket(0, buf)
-}
-
 func handleStandardSetup(setup usb.Setup) bool {
 	switch setup.BRequest {
 	case usb.GET_STATUS:
@@ -226,7 +195,7 @@ func handleStandardSetup(setup usb.Setup) bool {
 			}
 		}

-		sendDescriptorData(usb_trans_buffer[:2], setup.WLength)
+		sendUSBPacket(0, usb_trans_buffer[:2], setup.WLength)
 		return true

 	case usb.CLEAR_FEATURE:
@@ -271,7 +240,7 @@ func handleStandardSetup(setup usb.Setup) bool {

 	case usb.GET_CONFIGURATION:
 		usb_trans_buffer[0] = usbConfiguration
-		sendDescriptorData(usb_trans_buffer[:1], setup.WLength)
+		sendUSBPacket(0, usb_trans_buffer[:1], setup.WLength)
 		return true

 	case usb.SET_CONFIGURATION:
@@ -291,7 +260,7 @@ func handleStandardSetup(setup usb.Setup) bool {

 	case usb.GET_INTERFACE:
 		usb_trans_buffer[0] = usbSetInterface
-		sendDescriptorData(usb_trans_buffer[:1], setup.WLength)
+		sendUSBPacket(0, usb_trans_buffer[:1], setup.WLength)
 		return true

 	case usb.SET_INTERFACE:
@@ -305,21 +274,6 @@ func handleStandardSetup(setup usb.Setup) bool {
 	}
 }

-// sendDescriptorData sends a descriptor, truncating it to fit maxLen or the buffer size.
-func sendDescriptorData(data []byte, maxLen uint16) {
-	data = lenToCap(data)
-	data = data[:min(len(data), len(udd_ep_control_cache_buffer), int(maxLen))]
-	sendUSBPacket(0, data)
-}
-
-// Set the cap of the slice to the length.
-// This is safe, but cannot be proven by the compiler.
-//
-//go:nobounds
-func lenToCap(b []byte) []byte {
-	return b[:len(b):len(b)]
-}
-
 func EnableCDC(txHandler func(), rxHandler func([]byte), setupHandler func(usb.Setup) bool) {
 	if len(usbDescriptor.Device) == 0 {
 		usbDescriptor = descriptor.CDC
@@ -35,12 +35,12 @@ type msc struct {
 	respStatus    csw.Status // Response status for the last command
 	sendZLP       bool       // Flag to indicate if a zero-length packet should be sent before sending CSW

-	cbw           *CBW   // Last received Command Block Wrapper
-	queuedBytes   uint32 // Number of bytes queued for sending
-	sentBytes     uint32 // Number of bytes sent
-	transferBytes uint32 // Total bytes to send
-	cswBuf        []byte // CSW response buffer
-	state         mscState
+	cbw         *CBW   // Last received Command Block Wrapper
+	queuedBytes uint32 // Number of bytes queued for sending
+	sentBytes   uint32 // Number of bytes sent
+	totalBytes  uint32 // Total bytes to send
+	cswBuf      []byte // CSW response buffer
+	state       mscState

 	maxLUN       uint8 // Maximum Logical Unit Number (n-1 for n LUNs)
 	dev          machine.BlockDevice
@@ -160,9 +160,8 @@ func (m *msc) sendUSBPacket(b []byte) {
 func (m *msc) sendCSW(status csw.Status) {
 	// Generate CSW packet into m.cswBuf and send it
 	residue := uint32(0)
-	expected := m.cbw.transferLength()
-	if expected >= m.sentBytes {
-		residue = expected - m.sentBytes
+	if m.totalBytes >= m.sentBytes {
+		residue = m.totalBytes - m.sentBytes
 	}
 	m.cbw.CSW(status, residue, m.cswBuf)
 	m.state = mscStateStatusSent
@@ -246,7 +245,7 @@ func (m *msc) run(b []byte, isEpOut bool) bool {

 		// Move on to the data transfer phase next go around (after sending the first message)
 		m.state = mscStateData
-		m.transferBytes = cbw.transferLength()
+		m.totalBytes = cbw.transferLength()
 		m.queuedBytes = 0
 		m.sentBytes = 0
 		m.respStatus = csw.StatusPassed
@@ -282,7 +281,7 @@ func (m *msc) run(b []byte, isEpOut bool) bool {
 	// to cycle back through this block, e.g. with TEST UNIT READY which sends only a CSW after
 	// setting the sense key/add'l code/qualifier internally
 	if m.state == mscStateStatus && !m.txStalled {
-		if m.cbw.transferLength() > m.sentBytes && m.cbw.isIn() {
+		if m.totalBytes > m.sentBytes && m.cbw.isIn() {
 			// 6.7.2 The Thirteen Cases - Case 5 (Hi > Di): STALL before status
 			m.stallEndpoint(usb.MSC_ENDPOINT_IN)
 		} else if m.sendZLP {
@@ -21,9 +21,9 @@ func (m *msc) scsiCmdBegin() {
 		return
 	}

-	if m.transferBytes > 0 && m.cbw.isOut() {
+	if m.totalBytes > 0 && m.cbw.isOut() {
 		// Reject any other multi-packet commands
-		if m.transferBytes > m.maxPacketSize {
+		if m.totalBytes > m.maxPacketSize {
 			m.sendScsiError(csw.StatusFailed, scsi.SenseIllegalRequest, scsi.SenseCodeInvalidCmdOpCode)
 			return
 		} else {
@@ -53,7 +53,7 @@ func (m *msc) scsiCmdBegin() {
 	}

 	if len(m.buf) == 0 {
-		if m.transferBytes > 0 {
+		if m.totalBytes > 0 {
 			// 6.7.2 The Thirteen Cases - Case 4 (Hi > Dn)
 			// https://usb.org/sites/default/files/usbmassbulk_10.pdf
 			m.sendScsiError(csw.StatusFailed, scsi.SenseIllegalRequest, 0)
@@ -63,7 +63,7 @@ func (m *msc) scsiCmdBegin() {
 			m.state = mscStateStatus
 		}
 	} else {
-		if m.transferBytes == 0 {
+		if m.totalBytes == 0 {
 			// 6.7.1 The Thirteen Cases - Case 2 (Hn < Di)
 			// https://usb.org/sites/default/files/usbmassbulk_10.pdf
 			m.sendScsiError(csw.StatusFailed, scsi.SenseIllegalRequest, 0)
@@ -72,7 +72,6 @@ func (m *msc) scsiCmdBegin() {
 			if m.cbw.transferLength() < uint32(len(m.buf)) {
 				m.buf = m.buf[:m.cbw.transferLength()]
 			}
-			m.queuedBytes = uint32(len(m.buf))
 			m.sendUSBPacket(m.buf)
 		}
 	}
@@ -94,7 +93,7 @@ func (m *msc) scsiDataTransfer(b []byte) bool {
 	// Update our sent bytes count to include the just-confirmed bytes
 	m.sentBytes += m.queuedBytes

-	if m.sentBytes >= m.transferBytes {
+	if m.sentBytes >= m.totalBytes {
 		// Transfer complete, send CSW after transfer confirmed
 		m.state = mscStateStatus
 	} else if cmdType == scsi.CmdRead {
@@ -159,8 +158,8 @@ func (m *msc) scsiCmdModeSense(cmd scsi.Cmd) {

 	// The host allows a good amount of leeway in response size
 	// Reset total bytes to what we'll actually send
-	if m.transferBytes > respLen {
-		m.transferBytes = respLen
+	if m.totalBytes > respLen {
+		m.totalBytes = respLen
 		m.sendZLP = true
 	}

@@ -211,7 +210,7 @@ func (m *msc) scsiCmdRequestSense() {
 	// Set the buffer size to the SCSI sense message size and clear
 	m.resetBuffer(scsi.RequestSenseRespLen)
 	m.queuedBytes = scsi.RequestSenseRespLen
-	m.transferBytes = scsi.RequestSenseRespLen
+	m.totalBytes = scsi.RequestSenseRespLen

 	// 0x70 - current error, 0x71 - deferred error (not used)
 	m.buf[0] = 0xF0 // 0x70 for current error plus 0x80 for valid flag bit
@@ -267,7 +266,7 @@ func (m *msc) scsiQueueTask(cmdType scsi.CmdType, b []byte) bool {
 	switch cmdType {
 	case scsi.CmdWrite:
 		// If we're writing data wait until we have a full write block of data that can be processed.
-		if m.queuedBytes == uint32(cap(m.blockCache)) || (m.sentBytes+m.queuedBytes >= m.transferBytes) {
+		if m.queuedBytes == uint32(cap(m.blockCache)) {
 			m.taskQueued = true
 		}
 	case scsi.CmdUnmap:
@@ -280,11 +279,7 @@ func (m *msc) scsiQueueTask(cmdType scsi.CmdType, b []byte) bool {

 func (m *msc) sendScsiError(status csw.Status, key scsi.Sense, code scsi.SenseCode) {
 	// Generate CSW into m.cswBuf
-	expected := m.cbw.transferLength()
-	residue := uint32(0)
-	if expected > m.sentBytes {
-		residue = expected - m.sentBytes
-	}
+	residue := m.totalBytes - m.sentBytes

 	// Prepare to send CSW
 	m.sendZLP = true // Ensure the transaction is signaled as ended before a CSW is sent
@@ -296,7 +291,7 @@ func (m *msc) sendScsiError(status csw.Status, key scsi.Sense, code scsi.SenseCo
 	m.addlSenseCode = code
 	m.addlSenseQualifier = 0x00 // Not used

-	if expected > 0 && residue > 0 {
+	if m.totalBytes > 0 && residue > 0 {
 		if m.cbw.isIn() {
 			m.stallEndpoint(usb.MSC_ENDPOINT_IN)
 		} else {
@@ -136,13 +136,13 @@ func (m *msc) scsiEvpdInquiry(cmd scsi.Cmd, pageCode uint8) {

 	// Set total bytes to the length of our response
 	m.queuedBytes = uint32(len(m.buf))
-	m.transferBytes = uint32(len(m.buf))
+	m.totalBytes = uint32(len(m.buf))
 }

 func (m *msc) scsiStdInquiry(cmd scsi.Cmd) {
 	m.resetBuffer(scsi.InquiryRespLen)
 	m.queuedBytes = scsi.InquiryRespLen
-	m.transferBytes = scsi.InquiryRespLen
+	m.totalBytes = scsi.InquiryRespLen

 	// byte 0 - Device Type (0x00 for direct access block device)
 	// byte 1 - Removable media bit
@@ -12,7 +12,7 @@ func (m *msc) scsiCmdReadWrite(cmd scsi.Cmd) {
 	status := m.validateScsiReadWrite(cmd)
 	if status != csw.StatusPassed {
 		m.sendScsiError(status, scsi.SenseIllegalRequest, scsi.SenseCodeInvalidCmdOpCode)
-	} else if m.transferBytes > 0 {
+	} else if m.totalBytes > 0 {
 		if cmd.CmdType() == scsi.CmdRead {
 			m.scsiRead(cmd)
 		} else {
@@ -28,7 +28,7 @@ func (m *msc) scsiCmdReadWrite(cmd scsi.Cmd) {
 func (m *msc) validateScsiReadWrite(cmd scsi.Cmd) csw.Status {
 	blockCount := cmd.BlockCount()
 	// CBW wrapper transfer length
-	if m.transferBytes == 0 {
+	if m.totalBytes == 0 {
 		// If the SCSI command's block count doesn't loosely match the wrapper's transfer length something's wrong
 		if blockCount > 0 {
 			return csw.StatusPhaseError
@@ -50,7 +50,7 @@ func (m *msc) validateScsiReadWrite(cmd scsi.Cmd) csw.Status {
 		// https://usb.org/sites/default/files/usbmassbulk_10.pdf
 		return csw.StatusFailed
 	}
-	if m.transferBytes/blockCount == 0 {
+	if m.totalBytes/blockCount == 0 {
 		// Block size shouldn't be small enough to round to zero
 		// 6.7.2 The Thirteen Cases - Case 7 (Hi < Di) READ(10) or
 		// 6.7.3 The Thirteen Cases - Case 13 (Ho < Do) WRITE(10)
@@ -103,7 +103,7 @@ func (m *msc) writeBlock(b []byte, lba, offset uint32) (n int, err error) {

 func (m *msc) scsiRead(cmd scsi.Cmd) {
 	// Make sure we don't exceed the buffer size
-	readEnd := m.transferBytes - m.sentBytes
+	readEnd := m.totalBytes - m.sentBytes
 	if readEnd > m.maxPacketSize {
 		readEnd = m.maxPacketSize
 	}
@@ -136,7 +136,7 @@ func (m *msc) scsiWrite(cmd scsi.Cmd, b []byte) {
 		m.sentBytes += uint32(len(b))
 	}

-	if m.sentBytes >= m.transferBytes {
+	if m.sentBytes >= m.totalBytes {
 		// Data transfer is complete, send CSW
 		m.state = mscStateStatus
 		m.run([]byte{}, true)
@@ -60,7 +60,7 @@ func (m *msc) scsiUnmap(b []byte) {
 	// FIXME: We need to handle erase block alignment

 	m.sentBytes += uint32(len(b))
-	if m.sentBytes >= m.transferBytes {
+	if m.sentBytes >= m.totalBytes {
 		// Order 66 complete, send CSW to establish galactic empire
 		m.state = mscStateStatus
 		m.run([]byte{}, true)
@@ -3,6 +3,7 @@ package msc
 import (
 	"machine"
 	"machine/usb"
+	"machine/usb/msc/csw"
 )

 func setupPacketHandler(setup usb.Setup) bool {
@@ -17,17 +18,11 @@ func (m *msc) setupPacketHandler(setup usb.Setup) bool {
 	wValue := (uint16(setup.WValueH) << 8) | uint16(setup.WValueL)
 	switch setup.BRequest {
 	case usb.CLEAR_FEATURE:
-		if setup.BmRequestType == 0x02 { // Host-to-Device | Standard | Endpoint
-			ok = m.handleClearFeature(setup, wValue)
-		}
+		ok = m.handleClearFeature(setup, wValue)
 	case usb.GET_MAX_LUN:
-		if setup.BmRequestType == 0xA1 { // Device-to-Host | Class | Interface
-			ok = m.handleGetMaxLun(setup, wValue)
-		}
+		ok = m.handleGetMaxLun(setup, wValue)
 	case usb.MSC_RESET:
-		if setup.BmRequestType == 0x21 { // Host-to-Device | Class | Interface
-			ok = m.handleReset(setup, wValue)
-		}
+		ok = m.handleReset(setup, wValue)
 	}
 	return ok
 }
@@ -58,25 +53,24 @@ func (m *msc) handleClearFeature(setup usb.Setup, wValue uint16) bool {
 		} else if wIndex == usb.MSC_ENDPOINT_OUT {
 			m.stallEndpoint(usb.MSC_ENDPOINT_OUT)
 		}
-		machine.SendZlp()
-		return true
+		return ok
 	}

 	// Clear the direction bit from the endpoint address for comparison
 	wIndex := setup.WIndex & 0x7F

-	// Clear the IN/OUT stalls if addressed to the endpoint
-	if wIndex == usb.MSC_ENDPOINT_IN {
+	// Clear the IN/OUT stalls if addressed to the endpoint, or both if addressed to the interface
+	if wIndex == usb.MSC_ENDPOINT_IN || wIndex == mscInterface {
 		m.clearStallEndpoint(usb.MSC_ENDPOINT_IN)
 		ok = true
 	}
-	if wIndex == usb.MSC_ENDPOINT_OUT {
+	if wIndex == usb.MSC_ENDPOINT_OUT || wIndex == mscInterface {
 		m.clearStallEndpoint(usb.MSC_ENDPOINT_OUT)
 		ok = true
 	}
 	// Send a CSW if needed to resume after the IN endpoint stall is cleared
 	if m.state == mscStateStatus && wIndex == usb.MSC_ENDPOINT_IN {
-		m.sendCSW(m.respStatus)
+		m.sendCSW(csw.StatusPassed)
 		ok = true
 	}

@@ -73,14 +73,6 @@ func align(ptr uintptr) uintptr {
 //export tinygo_getCurrentStackPointer
 func getCurrentStackPointer() uintptr

-// savedStackPointer is used to save the system stack pointer while in a goroutine.
-var savedStackPointer uintptr
-
-// saveStackPointer is called by internal/task.state.Resume() to save the stack pointer before entering a goroutine from the system stack.
-func saveStackPointer() {
-	savedStackPointer = getCurrentStackPointer()
-}
-
 // growHeap tries to grow the heap size. It returns true if it succeeds, false
 // otherwise.
 func growHeap() bool {
@@ -8,21 +8,16 @@ import "unsafe"
 // code linked from other languages can allocate memory without colliding with
 // our GC allocations.

-// Map of allocations, where the key is the allocated pointer and the value is
-// the size of the allocation.
-// TODO: make this a map[unsafe.Pointer]uintptr, since that results in slightly
-// smaller binaries. But for that to work, unsafe.Pointer needs to be seen as a
-// binary key (which it is not at the moment).
-// See https://github.com/tinygo-org/tinygo/pull/4898 for details.
-var allocs = make(map[*byte]uintptr)
+var allocs = make(map[uintptr][]byte)

 //export malloc
 func libc_malloc(size uintptr) unsafe.Pointer {
 	if size == 0 {
 		return nil
 	}
-	ptr := alloc(size, nil)
-	allocs[(*byte)(ptr)] = size
+	buf := make([]byte, size)
+	ptr := unsafe.Pointer(&buf[0])
+	allocs[uintptr(ptr)] = buf
 	return ptr
 }

@@ -31,8 +26,8 @@ func libc_free(ptr unsafe.Pointer) {
 	if ptr == nil {
 		return
 	}
-	if _, ok := allocs[(*byte)(ptr)]; ok {
-		delete(allocs, (*byte)(ptr))
+	if _, ok := allocs[uintptr(ptr)]; ok {
+		delete(allocs, uintptr(ptr))
 	} else {
 		panic("free: invalid pointer")
 	}
@@ -53,20 +48,18 @@ func libc_realloc(oldPtr unsafe.Pointer, size uintptr) unsafe.Pointer {

 	// It's hard to optimize this to expand the current buffer with our GC, but
 	// it is theoretically possible. For now, just always allocate fresh.
-	// TODO: we could skip this if the new allocation is smaller than the old.
-	ptr := alloc(size, nil)
+	buf := make([]byte, size)

 	if oldPtr != nil {
-		if oldSize, ok := allocs[(*byte)(oldPtr)]; ok {
-			oldBuf := unsafe.Slice((*byte)(oldPtr), oldSize)
-			newBuf := unsafe.Slice((*byte)(ptr), size)
-			copy(newBuf, oldBuf)
-			delete(allocs, (*byte)(oldPtr))
+		if oldBuf, ok := allocs[uintptr(oldPtr)]; ok {
+			copy(buf, oldBuf)
+			delete(allocs, uintptr(oldPtr))
 		} else {
 			panic("realloc: invalid pointer")
 		}
 	}

-	allocs[(*byte)(ptr)] = size
+	ptr := unsafe.Pointer(&buf[0])
+	allocs[uintptr(ptr)] = buf
 	return ptr
 }
@@ -78,8 +78,5 @@ tinygo_longjmp:
    ld r28, X+
    ld r29, X+

-	; Mark that we returned from a longjmp.
-	ldi r24, 1
-
    ; Jump to the PC (stored in the Z register)
    icall
@@ -30,7 +30,9 @@ func SetPanicOnFault(enabled bool) bool {

 func WriteHeapDump(fd uintptr)

-func SetTraceback(level string)
+// Unimplemented.
+func SetTraceback(level string) {
+}

 func SetMemoryLimit(limit int64) int64 {
 	return limit
@@ -1,6 +1,11 @@
 package runtime

 func Callers(skip int, pc []uintptr) int {
+	if len(pc) > 0 {
+		// The testing package expects at least one caller in all cases.
+		pc[0] = 0
+		return 1
+	}
 	return 0
 }

@@ -46,15 +46,18 @@ const (
 	bytesPerBlock      = wordsPerBlock * unsafe.Sizeof(heapStart)
 	stateBits          = 2 // how many bits a block state takes (see blockState type)
 	blocksPerStateByte = 8 / stateBits
+	markStackSize      = 8 * unsafe.Sizeof((*int)(nil)) // number of to-be-marked blocks to queue before forcing a rescan
 )

 var (
 	metadataStart unsafe.Pointer // pointer to the start of the heap metadata
-	scanList      *objHeader     // scanList is a singly linked list of heap objects that have been marked but not scanned
-	freeRanges    *freeRange     // freeRanges is a linked list of free block ranges
+	nextAlloc     gcBlock        // the next block that should be tried by the allocator
 	endBlock      gcBlock        // the block just past the end of the available space
 	gcTotalAlloc  uint64         // total number of bytes allocated
+	gcTotalBlocks uint64         // total number of allocated blocks
 	gcMallocs     uint64         // total number of allocations
+	gcFrees       uint64         // total number of objects freed
+	gcFreedBlocks uint64         // total number of freed blocks
 	gcLock        task.PMutex    // lock to avoid race conditions on multicore systems
 )

@@ -63,28 +66,24 @@ var zeroSizedAlloc uint8

 // Provide some abstraction over heap blocks.

-// blockState stores the four states in which a block can be.
-// It holds 1 bit in each nibble.
-// When stored into a state byte, each bit in a nibble corresponds to a different block.
-// For blocks A-D, a state byte would be laid out as 0bDCBA_DCBA.
+// blockState stores the four states in which a block can be. It is two bits in
+// size.
 type blockState uint8

 const (
-	blockStateLow  blockState = 1
-	blockStateHigh blockState = 1 << blocksPerStateByte
-
-	blockStateFree blockState = 0
-	blockStateHead blockState = blockStateLow
-	blockStateTail blockState = blockStateHigh
-	blockStateMark blockState = blockStateLow | blockStateHigh
-	blockStateMask blockState = blockStateLow | blockStateHigh
+	blockStateFree blockState = 0 // 00
+	blockStateHead blockState = 1 // 01
+	blockStateTail blockState = 2 // 10
+	blockStateMark blockState = 3 // 11
+	blockStateMask blockState = 3 // 11
 )

-// blockStateEach is a mask that can be used to extract a nibble from the block state.
-const blockStateEach = 1<<blocksPerStateByte - 1
-
 // The byte value of a block where every block is a 'tail' block.
-const blockStateByteAllTails = byte(blockStateTail) * blockStateEach
+const blockStateByteAllTails = 0 |
+	uint8(blockStateTail<<(stateBits*3)) |
+	uint8(blockStateTail<<(stateBits*2)) |
+	uint8(blockStateTail<<(stateBits*1)) |
+	uint8(blockStateTail<<(stateBits*0))

 // String returns a human-readable version of the block state, for debugging.
 func (s blockState) String() string {
@@ -181,7 +180,7 @@ func (b gcBlock) stateByte() byte {
 // Return the block state given a state byte. The state byte must have been
 // obtained using b.stateByte(), otherwise the result is incorrect.
 func (b gcBlock) stateFromByte(stateByte byte) blockState {
-	return blockState(stateByte>>(b%blocksPerStateByte)) & blockStateMask
+	return blockState(stateByte>>((b%blocksPerStateByte)*stateBits)) & blockStateMask
 }

 // State returns the current block state.
@@ -194,112 +193,36 @@ func (b gcBlock) state() blockState {
 // from head to mark.
 func (b gcBlock) setState(newState blockState) {
 	stateBytePtr := (*uint8)(unsafe.Add(metadataStart, b/blocksPerStateByte))
-	*stateBytePtr |= uint8(newState << (b % blocksPerStateByte))
+	*stateBytePtr |= uint8(newState << ((b % blocksPerStateByte) * stateBits))
 	if gcAsserts && b.state() != newState {
 		runtimePanic("gc: setState() was not successful")
 	}
 }

-// objHeader is a structure prepended to every heap object to hold metadata.
-type objHeader struct {
-	// next is the next object to scan after this.
-	next *objHeader
-
-	// layout holds the layout bitmap used to find pointers in the object.
-	layout gcLayout
-}
-
-// freeRange is a node on the outer list of range lengths.
-// The free ranges are structured as two nested singly-linked lists:
-// - The outer level (freeRange) has one entry for each unique range length.
-// - The inner level (freeRangeMore) has one entry for each additional range of the same length.
-// This two-level structure ensures that insertion/removal times are proportional to the requested length.
-type freeRange struct {
-	// len is the length of this free range.
-	len uintptr
-
-	// nextLen is the next longer free range.
-	nextLen *freeRange
-
-	// nextWithLen is the next free range with this length.
-	nextWithLen *freeRangeMore
-}
-
-// freeRangeMore is a node on the inner list of equal-length ranges.
-type freeRangeMore struct {
-	next *freeRangeMore
-}
-
-// insertFreeRange inserts a range of len blocks starting at ptr into the free list.
-func insertFreeRange(ptr unsafe.Pointer, len uintptr) {
-	if gcAsserts && len == 0 {
-		runtimePanic("gc: insert 0-length free range")
+// markFree sets the block state to free, no matter what state it was in before.
+func (b gcBlock) markFree() {
+	stateBytePtr := (*uint8)(unsafe.Add(metadataStart, b/blocksPerStateByte))
+	*stateBytePtr &^= uint8(blockStateMask << ((b % blocksPerStateByte) * stateBits))
+	if gcAsserts && b.state() != blockStateFree {
+		runtimePanic("gc: markFree() was not successful")
 	}
-
-	// Find the insertion point by length.
-	// Skip until the next range is at least the target length.
-	insDst := &freeRanges
-	for *insDst != nil && (*insDst).len < len {
-		insDst = &(*insDst).nextLen
-	}
-
-	// Create the new free range.
-	next := *insDst
-	if next != nil && next.len == len {
-		// Insert into the list with this length.
-		newRange := (*freeRangeMore)(ptr)
-		newRange.next = next.nextWithLen
-		next.nextWithLen = newRange
-	} else {
-		// Insert into the list of lengths.
-		newRange := (*freeRange)(ptr)
-		*newRange = freeRange{
-			len:         len,
-			nextLen:     next,
-			nextWithLen: nil,
-		}
-		*insDst = newRange
+	if gcAsserts {
+		*(*[wordsPerBlock]uintptr)(unsafe.Pointer(b.address())) = [wordsPerBlock]uintptr{}
 	}
 }

-// popFreeRange removes a range of len blocks from the freeRanges list.
-// It returns nil if there are no sufficiently long ranges.
-func popFreeRange(len uintptr) unsafe.Pointer {
-	if gcAsserts && len == 0 {
-		runtimePanic("gc: pop 0-length free range")
+// unmark changes the state of the block from mark to head. It must be marked
+// before calling this function.
+func (b gcBlock) unmark() {
+	if gcAsserts && b.state() != blockStateMark {
+		runtimePanic("gc: unmark() on a block that is not marked")
 	}
-
-	// Find the removal point by length.
-	// Skip until the next range is at least the target length.
-	remDst := &freeRanges
-	for *remDst != nil && (*remDst).len < len {
-		remDst = &(*remDst).nextLen
+	clearMask := blockStateMask ^ blockStateHead // the bits to clear from the state
+	stateBytePtr := (*uint8)(unsafe.Add(metadataStart, b/blocksPerStateByte))
+	*stateBytePtr &^= uint8(clearMask << ((b % blocksPerStateByte) * stateBits))
+	if gcAsserts && b.state() != blockStateHead {
+		runtimePanic("gc: unmark() was not successful")
 	}
-
-	rangeWithLength := *remDst
-	if rangeWithLength == nil {
-		// No ranges are long enough.
-		return nil
-	}
-	removedLen := rangeWithLength.len
-
-	// Remove the range.
-	var ptr unsafe.Pointer
-	if nextWithLen := rangeWithLength.nextWithLen; nextWithLen != nil {
-		// Remove from the list with this length.
-		rangeWithLength.nextWithLen = nextWithLen.next
-		ptr = unsafe.Pointer(nextWithLen)
-	} else {
-		// Remove from the list of lengths.
-		*remDst = rangeWithLength.nextLen
-		ptr = unsafe.Pointer(rangeWithLength)
-	}
-
-	if removedLen > len {
-		// Insert the leftover range.
-		insertFreeRange(unsafe.Add(ptr, len*bytesPerBlock), removedLen-len)
-	}
-	return ptr
 }

 func isOnHeap(ptr uintptr) bool {
@@ -316,9 +239,6 @@ func initHeap() {
 	// Set all block states to 'free'.
 	metadataSize := heapEnd - uintptr(metadataStart)
 	memzero(unsafe.Pointer(metadataStart), metadataSize)
-
-	// Rebuild the free ranges list.
-	buildFreeRanges()
 }

 // setHeapEnd is called to expand the heap. The heap can only grow, not shrink.
@@ -350,9 +270,6 @@ func setHeapEnd(newHeapEnd uintptr) {
 	if gcAsserts && uintptr(metadataStart) < uintptr(oldMetadataStart)+oldMetadataSize {
 		runtimePanic("gc: heap did not grow enough at once")
 	}
-
-	// Rebuild the free ranges list.
-	buildFreeRanges()
 }

 // calculateHeapAddresses initializes variables such as metadataStart and
@@ -394,88 +311,118 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 		return unsafe.Pointer(&zeroSizedAlloc)
 	}

+	if preciseHeap {
+		size += align(unsafe.Sizeof(layout))
+	}
+
 	if interrupt.In() {
 		runtimePanicAt(returnAddress(0), "heap alloc in interrupt")
 	}

-	// Round the size up to a multiple of blocks, adding space for the header.
-	rawSize := size
-	size += align(unsafe.Sizeof(objHeader{}))
-	size += bytesPerBlock - 1
-	if size < rawSize {
-		// The size overflowed.
-		runtimePanicAt(returnAddress(0), "out of memory")
-	}
-	neededBlocks := size / bytesPerBlock
-	size = neededBlocks * bytesPerBlock
-
 	// Make sure there are no concurrent allocations. The heap is not currently
 	// designed for concurrent alloc/GC.
 	gcLock.Lock()

-	// Update the total allocation counters.
-	gcTotalAlloc += uint64(rawSize)
+	gcTotalAlloc += uint64(size)
 	gcMallocs++

-	// Acquire a range of free blocks.
-	var ranGC bool
-	var grewHeap bool
-	var pointer unsafe.Pointer
+	neededBlocks := (size + (bytesPerBlock - 1)) / bytesPerBlock
+	gcTotalBlocks += uint64(neededBlocks)
+
+	// Continue looping until a run of free blocks has been found that fits the
+	// requested size.
+	index := nextAlloc
+	numFreeBlocks := uintptr(0)
+	heapScanCount := uint8(0)
 	for {
-		pointer = popFreeRange(neededBlocks)
-		if pointer != nil {
-			break
-		}
-
-		if !ranGC {
-			// Run the collector and try again.
-			freeBytes := runGC()
-			ranGC = true
-			heapSize := uintptr(metadataStart) - heapStart
-			if freeBytes < heapSize/3 {
-				// Ensure there is at least 33% headroom.
-				// This percentage was arbitrarily chosen, and may need to
-				// be tuned in the future.
-				growHeap()
+		if index == nextAlloc {
+			if heapScanCount == 0 {
+				heapScanCount = 1
+			} else if heapScanCount == 1 {
+				// The entire heap has been searched for free memory, but none
+				// could be found. Run a garbage collection cycle to reclaim
+				// free memory and try again.
+				heapScanCount = 2
+				freeBytes := runGC()
+				heapSize := uintptr(metadataStart) - heapStart
+				if freeBytes < heapSize/3 {
+					// Ensure there is at least 33% headroom.
+					// This percentage was arbitrarily chosen, and may need to
+					// be tuned in the future.
+					growHeap()
+				}
+			} else {
+				// Even after garbage collection, no free memory could be found.
+				// Try to increase heap size.
+				if growHeap() {
+					// Success, the heap was increased in size. Try again with a
+					// larger heap.
+				} else {
+					// Unfortunately the heap could not be increased. This
+					// happens on baremetal systems for example (where all
+					// available RAM has already been dedicated to the heap).
+					runtimePanicAt(returnAddress(0), "out of memory")
+				}
 			}
+		}
+
+		// Wrap around the end of the heap.
+		if index == endBlock {
+			index = 0
+			// Reset numFreeBlocks as allocations cannot wrap.
+			numFreeBlocks = 0
+			// In rare cases, the initial heap might be so small that there are
+			// no blocks at all. In this case, it's better to jump back to the
+			// start of the loop and try again, until the GC realizes there is
+			// no memory and grows the heap.
+			// This can sometimes happen on WebAssembly, where the initial heap
+			// is created by whatever is left on the last memory page.
 			continue
 		}

-		if gcDebug && !grewHeap {
-			println("grow heap for request:", uint(neededBlocks))
-			dumpFreeRangeCounts()
-		}
-		if growHeap() {
-			grewHeap = true
+		// Is the block we're looking at free?
+		if index.state() != blockStateFree {
+			// This block is in use. Try again from this point.
+			numFreeBlocks = 0
+			index++
 			continue
 		}
+		numFreeBlocks++
+		index++

-		// Unfortunately the heap could not be increased. This
-		// happens on baremetal systems for example (where all
-		// available RAM has already been dedicated to the heap).
-		runtimePanicAt(returnAddress(0), "out of memory")
+		// Are we finished?
+		if numFreeBlocks == neededBlocks {
+			// Found a big enough range of free blocks!
+			nextAlloc = index
+			thisAlloc := index - gcBlock(neededBlocks)
+			if gcDebug {
+				println("found memory:", thisAlloc.pointer(), int(size))
+			}
+
+			// Set the following blocks as being allocated.
+			thisAlloc.setState(blockStateHead)
+			for i := thisAlloc + 1; i != nextAlloc; i++ {
+				i.setState(blockStateTail)
+			}
+
+			// We've claimed this allocation, now we can unlock the heap.
+			gcLock.Unlock()
+
+			// Return a pointer to this allocation.
+			pointer := thisAlloc.pointer()
+			if preciseHeap {
+				// Store the object layout at the start of the object.
+				// TODO: this wastes a little bit of space on systems with
+				// larger-than-pointer alignment requirements.
+				*(*unsafe.Pointer)(pointer) = layout
+				add := align(unsafe.Sizeof(layout))
+				pointer = unsafe.Add(pointer, add)
+				size -= add
+			}
+			memzero(pointer, size)
+			return pointer
+		}
 	}
-
-	// Set the backing blocks as being allocated.
-	block := blockFromAddr(uintptr(pointer))
-	block.setState(blockStateHead)
-	for i := block + 1; i != block+gcBlock(neededBlocks); i++ {
-		i.setState(blockStateTail)
-	}
-
-	// Create the object header.
-	header := (*objHeader)(pointer)
-	header.layout = parseGCLayout(layout)
-
-	// We've claimed this allocation, now we can unlock the heap.
-	gcLock.Unlock()
-
-	// Return a pointer to this allocation.
-	add := align(unsafe.Sizeof(objHeader{}))
-	pointer = unsafe.Add(pointer, add)
-	size -= add
-	memzero(pointer, size)
-	return pointer
 }

 func realloc(ptr unsafe.Pointer, size uintptr) unsafe.Pointer {
@@ -560,10 +507,7 @@ func runGC() (freeBytes uintptr) {

 	// Sweep phase: free all non-marked objects and unmark marked objects for
 	// the next collection cycle.
-	sweep()
-
-	// Rebuild the free ranges list.
-	freeBytes = buildFreeRanges()
+	freeBytes = sweep()

 	// Show how much has been sweeped, for debugging.
 	if gcDebug {
@@ -575,7 +519,8 @@ func runGC() (freeBytes uintptr) {

 // markRoots reads all pointers from start to end (exclusive) and if they look
 // like a heap pointer and are unmarked, marks them and scans that object as
-// well (recursively). The starting address must be valid and aligned.
+// well (recursively). The start and end parameters must be valid pointers and
+// must be aligned.
 func markRoots(start, end uintptr) {
 	if gcDebug {
 		println("mark from", start, "to", end, int(end-start))
@@ -587,21 +532,18 @@ func markRoots(start, end uintptr) {
 		if start%unsafe.Alignof(start) != 0 {
 			runtimePanic("gc: unaligned start pointer")
 		}
+		if end%unsafe.Alignof(end) != 0 {
+			runtimePanic("gc: unaligned end pointer")
+		}
 	}

-	// Scan the range conservatively.
-	scanConservative(start, end-start)
-}
+	// Reduce the end bound to avoid reading too far on platforms where pointer alignment is smaller than pointer size.
+	// If the size of the range is 0, then end will be slightly below start after this.
+	end -= unsafe.Sizeof(end) - unsafe.Alignof(end)

-// scanConservative scans all possible pointer locations in a range and marks referenced heap allocations.
-// The starting address must be valid and pointer-aligned.
-func scanConservative(addr, len uintptr) {
-	for len >= unsafe.Sizeof(addr) {
+	for addr := start; addr < end; addr += unsafe.Alignof(addr) {
 		root := *(*uintptr)(unsafe.Pointer(addr))
 		markRoot(addr, root)
-
-		addr += unsafe.Alignof(addr)
-		len -= unsafe.Alignof(addr)
 	}
 }

@@ -611,164 +553,159 @@ func markCurrentGoroutineStack(sp uintptr) {
 	markRoot(0, sp)
 }

-// finishMark finishes the marking process by scanning all heap objects on scanList.
-func finishMark() {
-	for {
-		// Remove an object from the scan list.
-		obj := scanList
-		if obj == nil {
-			return
-		}
-		scanList = obj.next
+// stackOverflow is a flag which is set when the GC scans too deep while marking.
+// After it is set, all marked allocations must be re-scanned.
+var stackOverflow bool

-		// Check if the object may contain pointers.
-		if obj.layout.pointerFree() {
+// startMark starts the marking process on a root and all of its children.
+func startMark(root gcBlock) {
+	var stack [markStackSize]gcBlock
+	stack[0] = root
+	root.setState(blockStateMark)
+	stackLen := 1
+	for stackLen > 0 {
+		// Pop a block off of the stack.
+		stackLen--
+		block := stack[stackLen]
+		if gcDebug {
+			println("stack popped, remaining stack:", stackLen)
+		}
+
+		// Scan all pointers inside the block.
+		scanner := newGCObjectScanner(block)
+		if scanner.pointerFree() {
 			// This object doesn't contain any pointers.
 			// This is a fast path for objects like make([]int, 4096).
-			// It skips the length calculation.
 			continue
 		}
+		start, end := block.address(), block.findNext().address()
+		if preciseHeap {
+			// The first word of the object is just the pointer layout value.
+			// Skip it.
+			start += align(unsafe.Sizeof(uintptr(0)))
+		}
+		for addr := start; addr != end; addr += unsafe.Alignof(addr) {
+			// Load the word.
+			word := *(*uintptr)(unsafe.Pointer(addr))

-		// Compute the scan bounds.
-		objAddr := uintptr(unsafe.Pointer(obj))
-		start := objAddr + align(unsafe.Sizeof(objHeader{}))
-		end := blockFromAddr(objAddr).findNext().address()
+			if !scanner.nextIsPointer(word, root.address(), addr) {
+				// Not a heap pointer.
+				continue
+			}

-		// Scan the object.
-		obj.layout.scan(start, end-start)
+			// Find the corresponding memory block.
+			referencedBlock := blockFromAddr(word)
+
+			if referencedBlock.state() == blockStateFree {
+				// The to-be-marked object doesn't actually exist.
+				// This is probably a false positive.
+				if gcDebug {
+					println("found reference to free memory:", word, "at:", addr)
+				}
+				continue
+			}
+
+			// Move to the block's head.
+			referencedBlock = referencedBlock.findHead()
+
+			if referencedBlock.state() == blockStateMark {
+				// The block has already been marked by something else.
+				continue
+			}
+
+			// Mark block.
+			if gcDebug {
+				println("marking block:", referencedBlock)
+			}
+			referencedBlock.setState(blockStateMark)
+
+			if stackLen == len(stack) {
+				// The stack is full.
+				// It is necessary to rescan all marked blocks once we are done.
+				stackOverflow = true
+				if gcDebug {
+					println("gc stack overflowed")
+				}
+				continue
+			}
+
+			// Push the pointer onto the stack to be scanned later.
+			stack[stackLen] = referencedBlock
+			stackLen++
+		}
+	}
+}
+
+// finishMark finishes the marking process by processing all stack overflows.
+func finishMark() {
+	for stackOverflow {
+		// Re-mark all blocks.
+		stackOverflow = false
+		for block := gcBlock(0); block < endBlock; block++ {
+			if block.state() != blockStateMark {
+				// Block is not marked, so we do not need to rescan it.
+				continue
+			}
+
+			// Re-mark the block.
+			startMark(block)
+		}
 	}
 }

 // mark a GC root at the address addr.
 func markRoot(addr, root uintptr) {
-	// Find the heap block corresponding to the root.
-	if !isOnHeap(root) {
-		// This is not a heap pointer.
-		return
+	if isOnHeap(root) {
+		block := blockFromAddr(root)
+		if block.state() == blockStateFree {
+			// The to-be-marked object doesn't actually exist.
+			// This could either be a dangling pointer (oops!) but most likely
+			// just a false positive.
+			return
+		}
+		head := block.findHead()
+		if head.state() != blockStateMark {
+			if gcDebug {
+				println("found unmarked pointer", root, "at address", addr)
+			}
+			startMark(head)
+		}
 	}
-	block := blockFromAddr(root)
-
-	// Find the head of the corresponding object.
-	if block.state() == blockStateFree {
-		// The to-be-marked object doesn't actually exist.
-		// This could either be a dangling pointer (oops!) but most likely
-		// just a false positive.
-		return
-	}
-	head := block.findHead()
-
-	// Mark the object.
-	if head.state() == blockStateMark {
-		// This object is already marked.
-		return
-	}
-	if gcDebug {
-		println("found unmarked pointer", root, "at address", addr)
-	}
-	head.setState(blockStateMark)
-
-	// Add the object to the scan list.
-	header := (*objHeader)(head.pointer())
-	header.next = scanList
-	scanList = header
 }

 // Sweep goes through all memory and frees unmarked memory.
-func sweep() {
-	metadataEnd := unsafe.Add(metadataStart, (endBlock+(blocksPerStateByte-1))/blocksPerStateByte)
-	var carry byte
-	for meta := metadataStart; meta != metadataEnd; meta = unsafe.Add(meta, 1) {
-		// Fetch the state byte.
-		stateBytePtr := (*byte)(unsafe.Pointer(meta))
-		stateByte := *stateBytePtr
-
-		// Separate blocks by type.
-		// Split the nibbles.
-		// Each nibble is a mask of blocks.
-		high := stateByte >> blocksPerStateByte
-		low := stateByte & blockStateEach
-		// Marked heads are in both nibbles.
-		markedHeads := low & high
-		// Unmarked heads are in the low nibble but not the high nibble.
-		unmarkedHeads := low &^ high
-		// Tails are in the high nibble but not the low nibble.
-		tails := high &^ low
-
-		// Clear all tail runs after unmarked (freed) heads.
-		//
-		// Adding 1 to the start of a bit run will clear the run and set the next bit:
-		//   (2^k - 1) + 1 = 2^k
-		//   e.g. 0b0011 + 1 = 0b0100
-		// Bitwise-and with the original mask to clear the newly set bit.
-		//   e.g. (0b0011 + 1) & 0b0011 = 0b0100 & 0b0011 = 0b0000
-		// This will not clear bits after the run because the gap stops the carry:
-		//   e.g. (0b1011 + 1) & 0b1011 = 0b1100 & 0b1011 = 0b1000
-		// This can clear multiple runs in a single addition:
-		//   e.g. (0b1101 + 0b0101) & 0b1101 = 0b10010 & 0b1101 = 0b0000
-		//
-		// In order to find tail run starts after unmarked heads we could use tails & (unmarkedHeads << 1).
-		// It is possible omit the bitwise-and because the clear still works if the next block is not a tail.
-		// A head is not a tail, so corresponding missing tail bit will stop the carry from a previous tail run.
-		// As such it will set the next bit which will be cleared back away later.
-		// e.g. HHTH: (0b0010 + (0b1101 << 1)) & 0b0010 = 0b11100 & 0b0010 = 0b0000
-		//
-		// Treat the whole heap as a single pair of integer masks.
-		// This is accomplished for addition by carrying the overflow to the next state byte.
-		// The unmarkedHeads << 1 is equivalent to unmarkedHeads + unmarkedHeads, so it can be merged with the sum.
-		// This does not require any special work for the bitwise-and because it operates bitwise.
-		tailClear := tails + (unmarkedHeads << 1) + carry
-		carry = tailClear >> blocksPerStateByte
-		tails &= tailClear
-
-		// Construct the new state byte.
-		*stateBytePtr = markedHeads | (tails << blocksPerStateByte)
-	}
-}
-
-// buildFreeRanges rebuilds the freeRanges list.
-// This must be called after a GC sweep or heap grow.
-// It returns how many bytes are free in the heap.
-func buildFreeRanges() uintptr {
-	freeRanges = nil
-	block := endBlock
-	var totalBlocks uintptr
-	for {
-		// Skip backwards over occupied blocks.
-		for block > 0 && (block-1).state() != blockStateFree {
-			block--
+// It returns how many bytes are free in the heap after the sweep.
+func sweep() (freeBytes uintptr) {
+	freeCurrentObject := false
+	var freed uint64
+	for block := gcBlock(0); block < endBlock; block++ {
+		switch block.state() {
+		case blockStateHead:
+			// Unmarked head. Free it, including all tail blocks following it.
+			block.markFree()
+			freeCurrentObject = true
+			gcFrees++
+			freed++
+		case blockStateTail:
+			if freeCurrentObject {
+				// This is a tail object following an unmarked head.
+				// Free it now.
+				block.markFree()
+				freed++
+			}
+		case blockStateMark:
+			// This is a marked object. The next tail blocks must not be freed,
+			// but the mark bit must be removed so the next GC cycle will
+			// collect this object if it is unreferenced then.
+			block.unmark()
+			freeCurrentObject = false
+		case blockStateFree:
+			freeBytes += bytesPerBlock
 		}
-		if block == 0 {
-			break
-		}
-
-		// Find the start of the free range.
-		end := block
-		for block > 0 && (block-1).state() == blockStateFree {
-			block--
-		}
-
-		// Insert the free range.
-		len := uintptr(end - block)
-		totalBlocks += len
-		insertFreeRange(block.pointer(), len)
-	}
-
-	if gcDebug {
-		println("free ranges after rebuild:")
-		dumpFreeRangeCounts()
-	}
-
-	return totalBlocks * bytesPerBlock
-}
-
-func dumpFreeRangeCounts() {
-	for rangeWithLength := freeRanges; rangeWithLength != nil; rangeWithLength = rangeWithLength.nextLen {
-		totalRanges := uintptr(1)
-		for nextWithLen := rangeWithLength.nextWithLen; nextWithLen != nil; nextWithLen = nextWithLen.next {
-			totalRanges++
-		}
-		println("-", uint(rangeWithLength.len), "x", uint(totalRanges))
 	}
+	gcFreedBlocks += freed
+	freeBytes += uintptr(freed) * bytesPerBlock
+	return
 }

 // dumpHeap can be used for debugging purposes. It dumps the state of each heap
@@ -798,75 +735,28 @@ func dumpHeap() {
 // call to ReadMemStats. This would not do GC implicitly for you.
 func ReadMemStats(m *MemStats) {
 	gcLock.Lock()
-
-	// Calculate the raw size of the heap.
-	heapEnd := heapEnd
-	heapStart := heapStart
-	m.Sys = uint64(heapEnd - heapStart)
-	m.HeapSys = uint64(uintptr(metadataStart) - heapStart)
-	metadataStart := metadataStart
-	// TODO: should GCSys include objHeaders?
-	m.GCSys = uint64(heapEnd - uintptr(metadataStart))
-	m.HeapReleased = 0 // always 0, we don't currently release memory back to the OS.
-
-	// Count live heads and tails.
-	var liveHeads, liveTails uintptr
-	endBlock := endBlock
-	metadataEnd := unsafe.Add(metadataStart, (endBlock+(blocksPerStateByte-1))/blocksPerStateByte)
-	for meta := metadataStart; meta != metadataEnd; meta = unsafe.Add(meta, 1) {
-		// Since we are outside of a GC, nothing is marked.
-		// A bit in the low nibble implies a head.
-		// A bit in the high nibble implies a tail.
-		stateByte := *(*byte)(unsafe.Pointer(meta))
-		liveHeads += uintptr(count4LUT[stateByte&blockStateEach])
-		liveTails += uintptr(count4LUT[stateByte>>blocksPerStateByte])
+	m.HeapIdle = 0
+	m.HeapInuse = 0
+	for block := gcBlock(0); block < endBlock; block++ {
+		bstate := block.state()
+		if bstate == blockStateFree {
+			m.HeapIdle += uint64(bytesPerBlock)
+		} else {
+			m.HeapInuse += uint64(bytesPerBlock)
+		}
 	}
-
-	// Add heads and tails to count live blocks.
-	liveBlocks := liveHeads + liveTails
-	liveBytes := uint64(liveBlocks * bytesPerBlock)
-	m.HeapInuse = liveBytes
-	m.HeapAlloc = liveBytes
-	m.Alloc = liveBytes
-
-	// Subtract live blocks from total blocks to count free blocks.
-	freeBlocks := uintptr(endBlock) - liveBlocks
-	m.HeapIdle = uint64(freeBlocks * bytesPerBlock)
-
-	// Record the number of allocated objects.
-	gcMallocs := gcMallocs
-	m.Mallocs = gcMallocs
-
-	// Subtract live objects from allocated objects to count freed objects.
-	m.Frees = gcMallocs - uint64(liveHeads)
-
-	// Record the total allocated bytes.
+	m.HeapReleased = 0 // always 0, we don't currently release memory back to the OS.
+	m.HeapSys = m.HeapInuse + m.HeapIdle
+	m.GCSys = uint64(heapEnd - uintptr(metadataStart))
 	m.TotalAlloc = gcTotalAlloc
-
+	m.Mallocs = gcMallocs
+	m.Frees = gcFrees
+	m.Sys = uint64(heapEnd - heapStart)
+	m.HeapAlloc = (gcTotalBlocks - gcFreedBlocks) * uint64(bytesPerBlock)
+	m.Alloc = m.HeapAlloc
 	gcLock.Unlock()
 }

-// count4LUT is a lookup table used to count set bits in a 4-bit mask.
-// TODO: replace with popcnt when available
-var count4LUT = [16]uint8{
-	0b0000: 0,
-	0b0001: 1,
-	0b0010: 1,
-	0b0011: 2,
-	0b0100: 1,
-	0b0101: 2,
-	0b0110: 2,
-	0b0111: 3,
-	0b1000: 1,
-	0b1001: 2,
-	0b1010: 2,
-	0b1011: 3,
-	0b1100: 2,
-	0b1101: 3,
-	0b1110: 3,
-	0b1111: 4,
-}
-
 func SetFinalizer(obj interface{}, finalizer interface{}) {
 	// Unimplemented.
 }
@@ -31,6 +31,10 @@ var zeroSizedAlloc uint8

 var gcLock task.PMutex

+// Normally false, set to true during a GC scan when all other threads get
+// paused.
+var needsResumeWorld bool
+
 func initHeap() {
 	libgc_init()

@@ -46,6 +50,18 @@ func gcInit()
 func gcCallback() {
 	// Mark globals and all stacks, and stop the world if we're using threading.
 	gcMarkReachable()
+
+	// If we use a scheduler with parallelism (the threads scheduler for
+	// example), we need to call gcResumeWorld() after scanning has finished.
+	if hasParallelism {
+		if needsResumeWorld {
+			// Should never happen, check for it anyway.
+			runtimePanic("gc: world already stopped")
+		}
+
+		// Note that we need to resume the world after finishing the GC call.
+		needsResumeWorld = true
+	}
 }

 func markRoots(start, end uintptr) {
@@ -71,6 +87,7 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 	}

 	gcLock.Lock()
+	needsResumeWorld = false
 	var ptr unsafe.Pointer
 	if layout == gclayout.NoPtrs.AsPtr() {
 		// This object is entirely pointer free, for example make([]int, ...).
@@ -87,7 +104,9 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 		// Memory returned from libgc_malloc has already been zeroed, so nothing
 		// to do here.
 	}
-	gcResumeWorld()
+	if needsResumeWorld {
+		gcResumeWorld()
+	}
 	gcLock.Unlock()
 	if ptr == nil {
 		runtimePanic("gc: out of memory")
@@ -102,8 +121,11 @@ func free(ptr unsafe.Pointer) {

 func GC() {
 	gcLock.Lock()
+	needsResumeWorld = false
 	libgc_gcollect()
-	gcResumeWorld()
+	if needsResumeWorld {
+		gcResumeWorld()
+	}
 	gcLock.Unlock()
 }

@@ -6,25 +6,24 @@

 package runtime

-import "unsafe"
+const preciseHeap = false

-// parseGCLayout stores the layout information passed to alloc into a gcLayout value.
-// The conservative GC discards this information.
-func parseGCLayout(layout unsafe.Pointer) gcLayout {
-	return gcLayout{}
+type gcObjectScanner struct {
 }

-// gcLayout tracks pointer locations in a heap object.
-// The conservative GC treats all locations as potential pointers, so this doesn't need to store anything.
-type gcLayout struct {
+func newGCObjectScanner(block gcBlock) gcObjectScanner {
+	return gcObjectScanner{}
 }

-func (l gcLayout) pointerFree() bool {
+func (scanner *gcObjectScanner) pointerFree() bool {
 	// We don't know whether this object contains pointers, so conservatively
 	// return false.
 	return false
 }

-func (l gcLayout) scan(start, len uintptr) {
-	scanConservative(start, len)
+// nextIsPointer returns whether this could be a pointer. Because the GC is
+// conservative, we can't do much more than check whether the object lies
+// somewhere in the heap.
+func (scanner gcObjectScanner) nextIsPointer(ptr, parent, addrOfWord uintptr) bool {
+	return isOnHeap(ptr)
 }
@@ -57,96 +57,91 @@ package runtime

 import "unsafe"

-const sizeFieldBits = 4 + (unsafe.Sizeof(uintptr(0)) / 4)
+const preciseHeap = true

-// parseGCLayout stores the layout information passed to alloc into a gcLayout value.
-func parseGCLayout(layout unsafe.Pointer) gcLayout {
-	return gcLayout(layout)
+type gcObjectScanner struct {
+	index      uintptr
+	size       uintptr
+	bitmap     uintptr
+	bitmapAddr unsafe.Pointer
 }

-// gcLayout tracks pointer locations in a heap object.
-type gcLayout uintptr
+func newGCObjectScanner(block gcBlock) gcObjectScanner {
+	if gcAsserts && block != block.findHead() {
+		runtimePanic("gc: object scanner must start at head")
+	}
+	scanner := gcObjectScanner{}
+	layout := *(*uintptr)(unsafe.Pointer(block.address()))
+	if layout == 0 {
+		// Unknown layout. Assume all words in the object could be pointers.
+		// This layout value below corresponds to a slice of pointers like:
+		//     make(*byte, N)
+		scanner.size = 1
+		scanner.bitmap = 1
+	} else if layout&1 != 0 {
+		// Layout is stored directly in the integer value.
+		// Determine format of bitfields in the integer.
+		const layoutBits = uint64(unsafe.Sizeof(layout) * 8)
+		var sizeFieldBits uint64
+		switch layoutBits { // note: this switch should be resolved at compile time
+		case 16:
+			sizeFieldBits = 4
+		case 32:
+			sizeFieldBits = 5
+		case 64:
+			sizeFieldBits = 6
+		default:
+			runtimePanic("unknown pointer size")
+		}

-func (layout gcLayout) pointerFree() bool {
-	return layout&1 != 0 && layout>>(sizeFieldBits+1) == 0
-}
-
-// scan an object with this element layout.
-// The starting address must be valid and pointer-aligned.
-// The length is rounded down to a multiple of the element size.
-func (layout gcLayout) scan(start, len uintptr) {
-	switch {
-	case layout == 0:
-		// This is an unknown layout.
-		// Scan conservatively.
-		// NOTE: This is *NOT* equivalent to a slice of pointers on AVR.
-		scanConservative(start, len)
-
-	case layout&1 != 0:
-		// The layout is stored directly in the integer value.
-		// Extract the bitfields.
-		size := uintptr(layout>>1) & (1<<sizeFieldBits - 1)
-		mask := uintptr(layout) >> (1 + sizeFieldBits)
-
-		// Scan with the extracted mask.
-		scanSimple(start, len, size*unsafe.Alignof(start), mask)
-
-	default:
-		// The layout is stored seperately in a global object.
-		// Extract the size and bitmap.
+		// Extract values from the bitfields.
+		// See comment at the top of this file for more information.
+		scanner.size = (layout >> 1) & (1<<sizeFieldBits - 1)
+		scanner.bitmap = layout >> (1 + sizeFieldBits)
+	} else {
+		// Layout is stored separately in a global object.
 		layoutAddr := unsafe.Pointer(layout)
-		size := *(*uintptr)(layoutAddr)
-		bitmapPtr := unsafe.Add(layoutAddr, unsafe.Sizeof(uintptr(0)))
-		bitmapLen := (size + 7) / 8
-		bitmap := unsafe.Slice((*byte)(bitmapPtr), bitmapLen)
-
-		// Scan with the bitmap.
-		scanComplex(start, len, size*unsafe.Alignof(start), bitmap)
+		scanner.size = *(*uintptr)(layoutAddr)
+		scanner.bitmapAddr = unsafe.Add(layoutAddr, unsafe.Sizeof(uintptr(0)))
 	}
+	return scanner
 }

-// scanSimple scans an object with an integer bitmask of pointer locations.
-// The starting address must be valid and pointer-aligned.
-func scanSimple(start, len, size, mask uintptr) {
-	for len >= size {
-		// Scan this element.
-		scanWithMask(start, mask)
-
-		// Move to the next element.
-		start += size
-		len -= size
+func (scanner *gcObjectScanner) pointerFree() bool {
+	if scanner.bitmapAddr != nil {
+		// While the format allows for large objects without pointers, this is
+		// optimized by the compiler so if bitmapAddr is set, we know that there
+		// are at least some pointers in the object.
+		return false
 	}
+	// If the bitmap is zero, there are definitely no pointers in the object.
+	return scanner.bitmap == 0
 }

-// scanComplex scans an object with a bitmap of pointer locations.
-// The starting address must be valid and pointer-aligned.
-func scanComplex(start, len, size uintptr, bitmap []byte) {
-	for len >= size {
-		// Scan this element.
-		for i, mask := range bitmap {
-			addr := start + 8*unsafe.Alignof(start)*uintptr(i)
-			scanWithMask(addr, uintptr(mask))
+func (scanner *gcObjectScanner) nextIsPointer(word, parent, addrOfWord uintptr) bool {
+	index := scanner.index
+	scanner.index++
+	if scanner.index == scanner.size {
+		scanner.index = 0
+	}
+
+	if !isOnHeap(word) {
+		// Definitely isn't a pointer.
+		return false
+	}
+
+	// Might be a pointer. Now look at the object layout to know for sure.
+	if scanner.bitmapAddr != nil {
+		if (*(*uint8)(unsafe.Add(scanner.bitmapAddr, index/8))>>(index%8))&1 == 0 {
+			return false
 		}
-
-		// Move to the next element.
-		start += size
-		len -= size
+		return true
 	}
-}
-
-// scanWithMask scans a portion of an object with a mask of pointer locations.
-// The address must be valid and pointer-aligned.
-func scanWithMask(addr, mask uintptr) {
-	// TODO: use ctz when available
-	for mask != 0 {
-		if mask&1 != 0 {
-			// Load and mark this pointer.
-			root := *(*uintptr)(unsafe.Pointer(addr))
-			markRoot(addr, root)
-		}
-
-		// Move to the next offset.
-		mask >>= 1
-		addr += unsafe.Alignof(addr)
+	if (scanner.bitmap>>index)&1 == 0 {
+		// not a pointer!
+		return false
 	}
+
+	// Probably a pointer.
+	return true
 }
@@ -29,6 +29,16 @@ type stackChainObject struct {
 //   - The system stack (aka startup stack) is not heap allocated, so even
 //     though it may be referenced it will not be scanned by default.
 //
+// Therefore, we only need to scan the system stack.
+// It is relatively easy to scan the system stack while we're on it: we can
+// simply read __stack_pointer and __global_base and scan the area in between.
+// Unfortunately, it's hard to get the system stack pointer while we're on a
+// goroutine stack. But when we're on a goroutine stack, the system stack is in
+// the scheduler which means there shouldn't be anything on the system stack
+// anyway.
+// ...I hope this assumption holds, otherwise we will need to store the system
+// stack in a global or something.
+//
 // The compiler also inserts code to store all globals in a chain via
 // stackChainStart. Luckily we don't need to scan these, as these globals are
 // stored on the goroutine stack and are therefore already getting scanned.
@@ -40,18 +50,9 @@ func markStack() {
 	// live.
 	volatile.LoadUint32((*uint32)(unsafe.Pointer(&stackChainStart)))

-	// Scan the system stack.
-	var sysSP uintptr
 	if task.OnSystemStack() {
-		// We are on the system stack.
-		// Use the current stack pointer.
-		sysSP = getCurrentStackPointer()
-	} else {
-		// We are in a goroutine.
-		// Use the saved stack pointer.
-		sysSP = savedStackPointer
+		markRoots(getCurrentStackPointer(), stackTop)
 	}
-	markRoots(sysSP, stackTop)
 }

 // trackPointer is a stub function call inserted by the compiler during IR
@@ -1,12 +1,7 @@
-//go:build baremetal && (nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey || (tinygo.riscv32 && virt) || rp2040 || rp2350)
+//go:build baremetal && (nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey || (tinygo.riscv32 && virt))

 // If you update the above build constraint, you'll probably also need to update
 // src/crypto/rand/rand_baremetal.go.
-//
-// The rp2040 and rp2350 implementations are not included in src/crypto/rand/rand_baremetal.go
-// due to not being sufficiently random for the Go crypto libs.
-// However since the randomness here does not provide those same guarantees,
-// they are included in the list for hardwareRand() implementations.

 package runtime

@@ -1,4 +1,4 @@
-//go:build baremetal && !(nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey || (tinygo.riscv32 && virt) || rp2040 || rp2350)
+//go:build baremetal && !(nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey || (tinygo.riscv32 && virt))

 package runtime

@@ -181,3 +181,19 @@ func getAuxv() []uintptr {
 func cgo_errno() uintptr {
 	return uintptr(*libc_errno_location())
 }
+
+// Unimplemented.
+var MemProfileRate int = 0
+
+// Unimplemented.
+func SetBlockProfileRate(rate int) {
+}
+
+var mutexProfileFraction int
+
+// Unimplemented.
+func SetMutexProfileFraction(rate int) int {
+	previous := mutexProfileFraction
+	mutexProfileFraction = rate
+	return previous
+}
@@ -6,7 +6,7 @@ import (
 	"device/arm"
 	"device/sam"
 	"machine"
-	_ "machine/usb/cdc"
+	"machine/usb/cdc"
 	"runtime/interrupt"
 	"runtime/volatile"
 	"unsafe"
@@ -26,6 +26,8 @@ func init() {
 	initUSBClock()
 	initADCClock()

+	cdc.EnableUSBCDC()
+	machine.USBDev.Configure(machine.UARTConfig{})
 	machine.InitSerial()
 }

@@ -271,9 +273,6 @@ func nanosecondsToTicks(ns int64) timeUnit {
 func sleepTicks(d timeUnit) {
 	for d != 0 {
 		ticks := uint32(d)
-		if d > 0xffff_ffff {
-			ticks = 0xffff_ffff
-		}
 		if !timerSleep(ticks) {
 			// Bail out early to handle a non-time interrupt.
 			return
@@ -6,7 +6,7 @@ import (
 	"device/arm"
 	"device/sam"
 	"machine"
-	_ "machine/usb/cdc"
+	"machine/usb/cdc"
 	"runtime/interrupt"
 	"runtime/volatile"
 )
@@ -27,6 +27,8 @@ func init() {
 	initADCClock()
 	enableCache()

+	cdc.EnableUSBCDC()
+	machine.USBDev.Configure(machine.UARTConfig{})
 	machine.InitSerial()
 }

@@ -264,9 +266,6 @@ func nanosecondsToTicks(ns int64) timeUnit {
 func sleepTicks(d timeUnit) {
 	for d != 0 {
 		ticks := uint32(d)
-		if d > 0xffff_ffff {
-			ticks = 0xffff_ffff
-		}
 		if !timerSleep(ticks) {
 			return
 		}
@@ -1,82 +0,0 @@
-//go:build esp32s3
-
-package runtime
-
-import (
-	"device/esp"
-)
-
-// This is the function called on startup after the flash (IROM/DROM) is
-// initialized and the stack pointer has been set.
-//
-//export main
-func main() {
-	// This initialization configures the following things:
-	// * It disables all watchdog timers. They might be useful at some point in
-	//   the future, but will need integration into the scheduler. For now,
-	//   they're all disabled.
-	// * It sets the CPU frequency to 240MHz, which is the maximum speed allowed
-	//   for this CPU. Lower frequencies might be possible in the future, but
-	//   running fast and sleeping quickly is often also a good strategy to save
-	//   power.
-	// TODO: protect certain memory regions, especially the area below the stack
-	// to protect against stack overflows. See
-	// esp_cpu_configure_region_protection in ESP-IDF.
-
-	// Disable RTC watchdog.
-	esp.RTC_CNTL.WDTWPROTECT.Set(0x50D83AA1)
-	esp.RTC_CNTL.WDTCONFIG0.Set(0)
-	esp.RTC_CNTL.WDTWPROTECT.Set(0x0) // Re-enable write protect
-
-	// Disable Timer 0 watchdog.
-	esp.TIMG1.WDTWPROTECT.Set(0x50D83AA1) // write protect
-	esp.TIMG1.WDTCONFIG0.Set(0)           // disable TG0 WDT
-	esp.TIMG1.WDTWPROTECT.Set(0x0)        // Re-enable write protect
-
-	esp.TIMG0.WDTWPROTECT.Set(0x50D83AA1) // write protect
-	esp.TIMG0.WDTCONFIG0.Set(0)           // disable TG0 WDT
-	esp.TIMG0.WDTWPROTECT.Set(0x0)        // Re-enable write protect
-
-	// Disable super watchdog.
-	esp.RTC_CNTL.SWD_WPROTECT.Set(0x8F1D312A)
-	esp.RTC_CNTL.SWD_CONF.Set(esp.RTC_CNTL_SWD_CONF_SWD_DISABLE)
-	esp.RTC_CNTL.SWD_WPROTECT.Set(0x0) // Re-enable write protect
-
-	// Change CPU frequency from 20MHz to 80MHz, by switching from the XTAL to the
-	// PLL clock source (see table "CPU Clock Frequency" in the reference manual).
-	esp.SYSTEM.SetSYSCLK_CONF_SOC_CLK_SEL(1)
-
-	// Change CPU frequency from 80MHz to 240MHz by setting SYSTEM_PLL_FREQ_SEL to
-	// 1 and SYSTEM_CPUPERIOD_SEL to 2 (see table "CPU Clock Frequency" in the
-	// reference manual).
-	esp.SYSTEM.SetCPU_PER_CONF_PLL_FREQ_SEL(1)
-	esp.SYSTEM.SetCPU_PER_CONF_CPUPERIOD_SEL(2)
-
-	// Clear bss. Repeat many times while we wait for cpu/clock to stabilize
-	for x := 0; x < 30; x++ {
-		clearbss()
-	}
-
-	// Initialize main system timer used for time.Now.
-	initTimer()
-
-	// Initialize the heap, call main.main, etc.
-	run()
-
-	// Fallback: if main ever returns, hang the CPU.
-	exit(0)
-}
-
-func abort() {
-	// lock up forever
-	print("abort called\n")
-}
-
-//go:extern _vector_table
-var _vector_table [0]uintptr
-
-//go:extern _sbss
-var _sbss [0]byte
-
-//go:extern _ebss
-var _ebss [0]byte
@@ -1,86 +0,0 @@
-//go:build esp32s3
-
-package runtime
-
-import (
-	"device/esp"
-	"machine"
-	"unsafe"
-)
-
-//type timeUnit int64
-
-func putchar(c byte) {
-	machine.Serial.WriteByte(c)
-}
-
-func getchar() byte {
-	for machine.Serial.Buffered() == 0 {
-		Gosched()
-	}
-	v, _ := machine.Serial.ReadByte()
-	return v
-}
-
-func buffered() int {
-	return machine.Serial.Buffered()
-}
-
-// Initialize .bss: zero-initialized global variables.
-// The .data section has already been loaded by the ROM bootloader.
-func clearbss() {
-	ptr := unsafe.Pointer(&_sbss)
-	for ptr != unsafe.Pointer(&_ebss) {
-		*(*uint32)(ptr) = 0
-		ptr = unsafe.Add(ptr, 4)
-	}
-}
-
-func initTimer() {
-	// Configure timer 0 in timer group 0, for timekeeping.
-	//   EN:       Enable the timer.
-	//   INCREASE: Count up every tick (as opposed to counting down).
-	//   DIVIDER:  16-bit prescaler, set to 2 for dividing the APB clock by two
-	//             (40MHz).
-	// esp.TIMG0.T0CONFIG.Set(0 << esp.TIMG_T0CONFIG_T0_EN_Pos)
-	esp.TIMG0.T0CONFIG.Set(esp.TIMG_TCONFIG_EN | esp.TIMG_TCONFIG_INCREASE | 2<<esp.TIMG_TCONFIG_DIVIDER_Pos)
-	// esp.TIMG0.T0CONFIG.Set(1 << esp.TIMG_T0CONFIG_T0_DIVCNT_RST_Pos)
-	// esp.TIMG0.T0CONFIG.Set(esp.TIMG_T0CONFIG_T0_EN)
-
-	// Set the timer counter value to 0.
-	esp.TIMG0.T0LOADLO.Set(0)
-	esp.TIMG0.T0LOADHI.Set(0)
-	esp.TIMG0.T0LOAD.Set(0) // value doesn't matter.
-}
-
-func ticks() timeUnit {
-	// First, update the LO and HI register pair by writing any value to the
-	// register. This allows reading the pair atomically.
-	esp.TIMG0.T0UPDATE.Set(0)
-	// Then read the two 32-bit parts of the timer.
-	return timeUnit(uint64(esp.TIMG0.T0LO.Get()) | uint64(esp.TIMG0.T0HI.Get())<<32)
-}
-
-func nanosecondsToTicks(ns int64) timeUnit {
-	// Calculate the number of ticks from the number of nanoseconds. At a 80MHz
-	// APB clock, that's 25 nanoseconds per tick with a timer prescaler of 2:
-	// 25 = 1e9 / (80MHz / 2)
-	return timeUnit(ns / 25)
-}
-
-func ticksToNanoseconds(ticks timeUnit) int64 {
-	// See nanosecondsToTicks.
-	return int64(ticks) * 25
-}
-
-// sleepTicks busy-waits until the given number of ticks have passed.
-func sleepTicks(d timeUnit) {
-	sleepUntil := ticks() + d
-	for ticks() < sleepUntil {
-		// TODO: suspend the CPU to not burn power here unnecessarily.
-	}
-}
-
-func exit(code int) {
-	abort()
-}
@@ -1,4 +1,4 @@
-//go:build nrf
+//go:build nrf && !nrf52840

 package runtime

@@ -78,9 +78,6 @@ func buffered() int {
 func sleepTicks(d timeUnit) {
 	for d != 0 {
 		ticks := uint32(d) & 0x7fffff // 23 bits (to be on the safe side)
-		if d > 0x7fffff {
-			ticks = 0x7fffff
-		}
 		rtc_sleep(ticks)
 		d -= timeUnit(ticks)
 	}
@@ -2,6 +2,147 @@

 package runtime

-// This package needs to be present so that the machine package can go:linkname
-// EnableUSBCDC from it.
-import _ "machine/usb/cdc"
+import (
+	"device/arm"
+	"device/nrf"
+	"machine"
+	"machine/usb/cdc"
+	"runtime/interrupt"
+	"runtime/volatile"
+)
+
+//go:linkname systemInit SystemInit
+func systemInit()
+
+//export Reset_Handler
+func main() {
+	if nrf.FPUPresent {
+		arm.SCB.CPACR.Set(0) // disable FPU if it is enabled
+	}
+	systemInit()
+	preinit()
+	run()
+	exit(0)
+}
+
+func init() {
+	cdc.EnableUSBCDC()
+	machine.USBDev.Configure(machine.UARTConfig{})
+	machine.InitSerial()
+	initLFCLK()
+	initRTC()
+}
+
+func initLFCLK() {
+	if machine.HasLowFrequencyCrystal {
+		nrf.CLOCK.LFCLKSRC.Set(nrf.CLOCK_LFCLKSTAT_SRC_Xtal)
+	}
+	nrf.CLOCK.TASKS_LFCLKSTART.Set(1)
+	for nrf.CLOCK.EVENTS_LFCLKSTARTED.Get() == 0 {
+	}
+	nrf.CLOCK.EVENTS_LFCLKSTARTED.Set(0)
+}
+
+func initRTC() {
+	nrf.RTC1.TASKS_START.Set(1)
+	intr := interrupt.New(nrf.IRQ_RTC1, func(intr interrupt.Interrupt) {
+		if nrf.RTC1.EVENTS_COMPARE[0].Get() != 0 {
+			nrf.RTC1.EVENTS_COMPARE[0].Set(0)
+			nrf.RTC1.INTENCLR.Set(nrf.RTC_INTENSET_COMPARE0)
+			nrf.RTC1.EVENTS_COMPARE[0].Set(0)
+			rtc_wakeup.Set(1)
+		}
+		if nrf.RTC1.EVENTS_OVRFLW.Get() != 0 {
+			nrf.RTC1.EVENTS_OVRFLW.Set(0)
+			rtcOverflows.Set(rtcOverflows.Get() + 1)
+		}
+	})
+	nrf.RTC1.INTENSET.Set(nrf.RTC_INTENSET_OVRFLW)
+	intr.SetPriority(0xc0) // low priority
+	intr.Enable()
+}
+
+func putchar(c byte) {
+	machine.Serial.WriteByte(c)
+}
+
+func getchar() byte {
+	for machine.Serial.Buffered() == 0 {
+		Gosched()
+	}
+	v, _ := machine.Serial.ReadByte()
+	return v
+}
+
+func buffered() int {
+	return machine.Serial.Buffered()
+}
+
+func sleepTicks(d timeUnit) {
+	for d != 0 {
+		ticks := uint32(d) & 0x7fffff // 23 bits (to be on the safe side)
+		rtc_sleep(ticks)
+		d -= timeUnit(ticks)
+	}
+}
+
+var rtcOverflows volatile.Register32 // number of times the RTC wrapped around
+
+// ticksToNanoseconds converts RTC ticks (at 32768Hz) to nanoseconds.
+func ticksToNanoseconds(ticks timeUnit) int64 {
+	// The following calculation is actually the following, but with both sides
+	// reduced to reduce the risk of overflow:
+	//     ticks * 1e9 / 32768
+	return int64(ticks) * 1953125 / 64
+}
+
+// nanosecondsToTicks converts nanoseconds to RTC ticks (running at 32768Hz).
+func nanosecondsToTicks(ns int64) timeUnit {
+	// The following calculation is actually the following, but with both sides
+	// reduced to reduce the risk of overflow:
+	//     ns * 32768 / 1e9
+	return timeUnit(ns * 64 / 1953125)
+}
+
+// Monotonically increasing number of ticks since start.
+func ticks() timeUnit {
+	// For some ways of capturing the time atomically, see this thread:
+	// https://www.eevblog.com/forum/microcontrollers/correct-timing-by-timer-overflow-count/msg749617/#msg749617
+	// Here, instead of re-reading the counter register if an overflow has been
+	// detected, we simply try again because that results in (slightly) smaller
+	// code and is perhaps easier to prove correct.
+	for {
+		mask := interrupt.Disable()
+		counter := uint32(nrf.RTC1.COUNTER.Get())
+		overflows := rtcOverflows.Get()
+		hasOverflow := nrf.RTC1.EVENTS_OVRFLW.Get() != 0
+		interrupt.Restore(mask)
+
+		if hasOverflow {
+			// There was an overflow. Try again.
+			continue
+		}
+
+		// The counter is 24 bits in size, so the number of overflows form the
+		// upper 32 bits (together 56 bits, which covers 71493 years at
+		// 32768kHz: I'd argue good enough for most purposes).
+		return timeUnit(overflows)<<24 + timeUnit(counter)
+	}
+}
+
+var rtc_wakeup volatile.Register8
+
+func rtc_sleep(ticks uint32) {
+	nrf.RTC1.INTENSET.Set(nrf.RTC_INTENSET_COMPARE0)
+	rtc_wakeup.Set(0)
+	if ticks == 1 {
+		// Race condition (even in hardware) at ticks == 1.
+		// TODO: fix this in a better way by detecting it, like the manual
+		// describes.
+		ticks = 2
+	}
+	nrf.RTC1.CC[0].Set((nrf.RTC1.COUNTER.Get() + ticks) & 0x00ffffff)
+	for rtc_wakeup.Get() == 0 {
+		waitForEvents()
+	}
+}
@@ -13,24 +13,17 @@ func sd_app_evt_wait()
 // This is a global variable to avoid a heap allocation in waitForEvents.
 var softdeviceEnabled uint8

-// Check whether the SoftDevice is currently enabled.
-// This function is also called from the machine package, so the signature has
-// to stay consistent.
-func isSoftDeviceEnabled() bool {
-	// First check whether the SoftDevice is enabled. Unfortunately,
-	// sd_app_evt_wait cannot be called when the SoftDevice is not enabled.
-	arm.SVCall1(0x12, &softdeviceEnabled) // sd_softdevice_is_enabled
-
-	return softdeviceEnabled != 0
-}
-
 func waitForEvents() {
 	// Call into the SoftDevice to sleep. This is necessary here because a
 	// normal wfe will not put the chip in low power mode (it still consumes
 	// 500µA-1mA). It is really needed to call sd_app_evt_wait for low power
 	// consumption.

-	if isSoftDeviceEnabled() {
+	// First check whether the SoftDevice is enabled. Unfortunately,
+	// sd_app_evt_wait cannot be called when the SoftDevice is not enabled.
+	arm.SVCall1(0x12, &softdeviceEnabled) // sd_softdevice_is_enabled
+
+	if softdeviceEnabled != 0 {
 		// Now pick the appropriate SVCall number. Hopefully they won't change
 		// in the future with a different SoftDevice version.
 		if nrf.Device == "nrf51" {
@@ -7,7 +7,7 @@ import (
 	"device/rp"
 	"internal/task"
 	"machine"
-	_ "machine/usb/cdc"
+	"machine/usb/cdc"
 	"runtime/interrupt"
 	"runtime/volatile"
 	"unsafe"
@@ -360,6 +360,8 @@ func machineInit()
 func init() {
 	machineInit()

+	cdc.EnableUSBCDC()
+	machine.USBDev.Configure(machine.UARTConfig{})
 	machine.InitSerial()
 }

@@ -34,8 +34,8 @@ func wasmEntryReactor() {
 	// Initialize the heap.
 	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
 	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
-	initRand()
 	initHeap()
+	initRand()

 	if hasScheduler {
 		// A package initializer might do funky stuff like start a goroutine and
@@ -243,8 +243,8 @@ func sleep(duration int64) {
 // run is called by the program entry point to execute the go program.
 // With a scheduler, init and the main function are invoked in a goroutine before starting the scheduler.
 func run() {
-	initRand()
 	initHeap()
+	initRand()
 	go func() {
 		initAll()
 		callMain()
@@ -135,7 +135,6 @@ func sleep(duration int64) {
 // This function is called on the first core in the system. It will wake up the
 // other cores when ready.
 func run() {
-	initRand()
 	initHeap()

 	go func() {
@@ -21,8 +21,8 @@ var schedulerExit bool
 // run is called by the program entry point to execute the go program.
 // With the "none" scheduler, init and the main function are invoked directly.
 func run() {
-	initRand()
 	initHeap()
+	initRand()
 	initAll()
 	callMain()
 	mainExited = true
@@ -21,7 +21,6 @@ var (
 // Because we just use OS threads, we don't need to do anything special here. We
 // can just initialize everything and run main.main on the main thread.
 func run() {
-	initRand()
 	initHeap()
 	task.Init(stackTop)
 	initAll()
@@ -3,18 +3,19 @@ package runtime
 // This file implements compiler builtins for slices: append() and copy().

 import (
+	"internal/gclayout"
 	"math/bits"
 	"unsafe"
 )

 // Builtin append(src, elements...) function: append elements to src and return
 // the modified (possibly expanded) slice.
-func sliceAppend(srcBuf, elemsBuf unsafe.Pointer, srcLen, srcCap, elemsLen, elemSize uintptr, layout unsafe.Pointer) (unsafe.Pointer, uintptr, uintptr) {
+func sliceAppend(srcBuf, elemsBuf unsafe.Pointer, srcLen, srcCap, elemsLen, elemSize uintptr) (unsafe.Pointer, uintptr, uintptr) {
 	newLen := srcLen + elemsLen
 	if elemsLen > 0 {
 		// Allocate a new slice with capacity for elemsLen more elements, if necessary;
 		// otherwise, reuse the passed slice.
-		srcBuf, _, srcCap = sliceGrow(srcBuf, srcLen, srcCap, newLen, elemSize, layout)
+		srcBuf, _, srcCap = sliceGrow(srcBuf, srcLen, srcCap, newLen, elemSize)

 		// Append the new elements in-place.
 		memmove(unsafe.Add(srcBuf, srcLen*elemSize), elemsBuf, elemsLen*elemSize)
@@ -35,7 +36,7 @@ func sliceCopy(dst, src unsafe.Pointer, dstLen, srcLen uintptr, elemSize uintptr
 }

 // sliceGrow returns a new slice with space for at least newCap elements
-func sliceGrow(oldBuf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr, layout unsafe.Pointer) (unsafe.Pointer, uintptr, uintptr) {
+func sliceGrow(oldBuf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr) (unsafe.Pointer, uintptr, uintptr) {
 	if oldCap >= newCap {
 		// No need to grow, return the input slice.
 		return oldBuf, oldLen, oldCap
@@ -47,6 +48,12 @@ func sliceGrow(oldBuf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr,
 	// memory allocators, this causes some difficult to debug issues.
 	newCap = 1 << bits.Len(uint(newCap))

+	var layout unsafe.Pointer
+	// less type info here; can only go off element size
+	if elemSize < unsafe.Sizeof(uintptr(0)) {
+		layout = gclayout.NoPtrs.AsPtr()
+	}
+
 	buf := alloc(newCap*elemSize, layout)
 	if oldLen > 0 {
 		// copy any data to new slice
@@ -212,7 +212,6 @@ func writeStream(stream *wasiStream, buf *byte, count uint, offset int64) int {
 			return -1
 		}
 		remaining -= len
-		src = src[len:]
 	}

 	return int(count)
--- a/Show More
+++ b/Show More