runtime: support panic recovery on Xtensa

Use picolibc setjmp and longjmp to save and restore the complete
Xtensa register-window state at TinyGo panic checkpoints.

Provide the target configuration and window spill helper required by
picolibc. Increment its cache version because Xtensa archives now
include another source file.

builder/picolibc.go

@@ -4,6 +4,7 @@ import (
 	"os"
 	"path/filepath"
 
+	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/goenv"
 )
 
@@ -19,8 +20,9 @@ var libPicolibc = Library{
 		return f.Close()
 	},
 	cflags: func(target, headerPath string) []string {
-		picolibcDir := filepath.Join(goenv.Get("TINYGOROOT"), "lib/picolibc")
-		return []string{
+		root := goenv.Get("TINYGOROOT")
+		picolibcDir := filepath.Join(root, "lib/picolibc")
+		flags := []string{
 			"-Werror",
 			"-Wall",
 			"-std=gnu11",
@@ -39,10 +41,22 @@ var libPicolibc = Library{
 			"-I" + picolibcDir + "/libm/common",
 			"-I" + headerPath,
 		}
+		if compileopts.CanonicalArchName(target) == "xtensa" {
+			flags = append(flags,
+				"-I"+filepath.Join(root, "lib/xtensa/include"),
+				"-I"+picolibcDir+"/libc/machine/xtensa",
+				"-D_XTENSA_HAVE_CONFIG_CORE_ISA_H",
+			)
+		}
+		return flags
 	},
 	sourceDir: func() string { return filepath.Join(goenv.Get("TINYGOROOT"), "lib/picolibc") },
 	librarySources: func(target string, _ bool) ([]string, error) {
-		return append([]string(nil), picolibcSources...), nil
+		sources := append([]string(nil), picolibcSources...)
+		if compileopts.CanonicalArchName(target) == "xtensa" {
+			sources = append(sources, "libc/machine/xtensa/setjmp.S")
+		}
+		return sources, nil
 	},
 }
 

compileopts/config.go

@@ -25,7 +25,7 @@ import (
 var libVersions = map[string]int{
 	"musl":         3,
 	"bdwgc":        2,
-	"picolibc":     1,
+	"picolibc":     2,
 	"wasmbuiltins": 1,
 }
 

compiler/defer.go

@@ -32,9 +32,6 @@ func (b *builder) supportsRecover() bool {
 		// proposal of WebAssembly:
 		// https://github.com/WebAssembly/exception-handling
 		return false
-	case "xtensa":
-		// TODO: add support for these architectures
-		return false
 	default:
 		return true
 	}
@@ -109,11 +106,21 @@ func (b *builder) createLandingPad() {
 	b.CreateBr(b.blockInfo[b.fn.Recover.Index].entry)
 }
 
-// Create a checkpoint (similar to setjmp). This emits inline assembly that
-// stores the current program counter inside the ptr address (actually
-// ptr+sizeof(ptr)) and then returns a boolean indicating whether this is the
-// normal flow (false) or we jumped here from somewhere else (true).
+// Create a checkpoint (similar to setjmp). It returns whether execution is
+// continuing normally instead of resuming after a longjmp.
 func (b *builder) createCheckpoint(ptr llvm.Value) llvm.Value {
+	if b.archFamily() == "xtensa" {
+		fnType := llvm.FunctionType(b.ctx.Int32Type(), []llvm.Type{b.dataPtrType}, false)
+		fn := b.mod.NamedFunction("setjmp")
+		if fn.IsNil() {
+			fn = llvm.AddFunction(b.mod, "setjmp", fnType)
+			fn.AddFunctionAttr(b.ctx.CreateEnumAttribute(llvm.AttributeKindID("returns_twice"), 0))
+		}
+		result := b.CreateCall(fnType, fn, []llvm.Value{ptr}, "setjmp")
+		result.AddCallSiteAttribute(-1, b.ctx.CreateEnumAttribute(llvm.AttributeKindID("returns_twice"), 0))
+		return b.CreateICmp(llvm.IntEQ, result, llvm.ConstInt(b.ctx.Int32Type(), 0, false), "setjmp.result")
+	}
+
 	// Construct inline assembly equivalents of setjmp.
 	// The assembly works as follows:
 	//   * Registers are either clobbered or, on 386, saved for longjmp to

lib/xtensa/include/xtensa/config/core-isa.h

@@ -0,0 +1,18 @@
+#ifndef TINYGO_XTENSA_CONFIG_CORE_ISA_H
+#define TINYGO_XTENSA_CONFIG_CORE_ISA_H
+
+#ifdef __XTENSA_WINDOWED_ABI__
+#define XCHAL_HAVE_WINDOWED 1
+#else
+#define XCHAL_HAVE_WINDOWED 0
+#endif
+
+#ifdef __XTENSA_EB__
+#define XCHAL_HAVE_BE 1
+#else
+#define XCHAL_HAVE_BE 0
+#endif
+
+#define XCHAL_MAYHAVE_ERRATUM_XEA1KWIN 1
+
+#endif

make/release.mk

@@ -15,12 +15,14 @@ build/release: tinygo gen-device $(if $(filter 1,$(USE_SYSTEM_BINARYEN)),,binary
 	@mkdir -p build/release/tinygo/lib/musl/src
 	@mkdir -p build/release/tinygo/lib/nrfx
 	@mkdir -p build/release/tinygo/lib/picolibc/libc
+	@mkdir -p build/release/tinygo/lib/picolibc/libc/machine
 	@mkdir -p build/release/tinygo/lib/picolibc/libm
 	@mkdir -p build/release/tinygo/lib/wasi-libc/dlmalloc
 	@mkdir -p build/release/tinygo/lib/wasi-libc/libc-bottom-half
 	@mkdir -p build/release/tinygo/lib/wasi-libc/libc-top-half/musl/arch
 	@mkdir -p build/release/tinygo/lib/wasi-libc/libc-top-half/musl/src
 	@mkdir -p build/release/tinygo/lib/wasi-cli/
+	@mkdir -p build/release/tinygo/lib/xtensa
 	@echo copying source files
 	@cp -p  build/tinygo$(EXE)           build/release/tinygo/bin
 ifneq ($(USE_SYSTEM_BINARYEN),1)
@@ -83,6 +85,7 @@ endif
 	@cp -rp lib/picolibc/libc/ctype              build/release/tinygo/lib/picolibc/libc
 	@cp -rp lib/picolibc/libc/include            build/release/tinygo/lib/picolibc/libc
 	@cp -rp lib/picolibc/libc/locale             build/release/tinygo/lib/picolibc/libc
+	@cp -rp lib/picolibc/libc/machine/xtensa     build/release/tinygo/lib/picolibc/libc/machine
 	@cp -rp lib/picolibc/libc/stdlib             build/release/tinygo/lib/picolibc/libc
 	@cp -rp lib/picolibc/libc/string             build/release/tinygo/lib/picolibc/libc
 	@cp -rp lib/picolibc/libc/stdio              build/release/tinygo/lib/picolibc/libc
@@ -121,6 +124,7 @@ endif
 	@cp -rp lib/wasi-libc/libc-top-half/musl/src/unistd             build/release/tinygo/lib/wasi-libc/libc-top-half/musl/src
 	@cp -rp lib/wasi-libc/libc-top-half/sources                     build/release/tinygo/lib/wasi-libc/libc-top-half
 	@cp -rp lib/wasi-cli/wit                                        build/release/tinygo/lib/wasi-cli/wit
+	@cp -rp lib/xtensa/include                                      build/release/tinygo/lib/xtensa
 	@cp -rp ${LLVM_PROJECTDIR}/compiler-rt/lib/builtins build/release/tinygo/lib/compiler-rt-builtins
 	@cp -rp ${LLVM_PROJECTDIR}/compiler-rt/LICENSE.TXT  build/release/tinygo/lib/compiler-rt-builtins
 	@cp -rp src                          build/release/tinygo/src

src/runtime/arch_xtensa.go

@@ -11,7 +11,9 @@ const zeroSizeAllocPtr uintptr = 16 // part of early flash: partition table, etc
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 32
 
-const deferExtraRegs = 0
+// Xtensa's windowed ABI uses a 17-word jmp_buf. JumpSP and JumpPC provide the
+// first two words.
+const deferExtraRegs = 15
 
 const callInstSize = 3 // "callx0 someFunction" (and similar) is 3 bytes
 

src/runtime/asm_xtensa.S

@@ -0,0 +1,34 @@
+#ifdef __XTENSA_WINDOWED_ABI__
+.section .text.__xtensa_libgcc_window_spill,"ax",@progbits
+.global __xtensa_libgcc_window_spill
+.type __xtensa_libgcc_window_spill, %function
+__xtensa_libgcc_window_spill:
+    entry sp, 16
+    movi a6, 15
+    call4 .Lspill_windows
+    retw
+.size __xtensa_libgcc_window_spill, .-__xtensa_libgcc_window_spill
+
+.balign 4
+.Lspill_windows:
+    entry sp, 16
+    beqz a2, .Lspill_done
+    addi a2, a2, -1
+    mov a6, a2
+    call4 .Lspill_windows
+.Lspill_done:
+    retw
+#endif
+
+.section .text.tinygo_longjmp,"ax",@progbits
+.global tinygo_longjmp
+.type tinygo_longjmp, %function
+tinygo_longjmp:
+#ifdef __XTENSA_WINDOWED_ABI__
+    // longjmp's entry rotates a10/a11 into its a2/a3 arguments.
+    movi a11, 1
+#else
+    movi a3, 1
+#endif
+    j longjmp
+.size tinygo_longjmp, .-tinygo_longjmp

targets/xtensa.json

@@ -14,5 +14,6 @@
 	],
 	"ldflags": [
 		"--gc-sections"
-	]
+	],
+	"extra-files": ["src/runtime/asm_xtensa.S"]
 }