WIP shadow-stack based mark/sweep collector

compiler/gc-pass-shadowstack.go

@@ -0,0 +1,199 @@
+package compiler
+
+// This file implements a compiler pass to move GC pointers to a "shadow stack"
+// that can easily be scanned by a garbage collector, even without platform
+// support.
+// For more information, see:
+//    https://llvm.org/docs/GarbageCollection.html#the-shadow-stack-gc
+
+import (
+	"github.com/aykevl/go-llvm"
+)
+
+// AddGCRoots moves pointer values to shadow stack frames when this function (or
+// any function it calls) may allocate something. This allows the GC to scan the
+// stack in a highly portable way.
+func (c *Compiler) AddGCRoots() {
+	alloc := c.mod.NamedFunction("runtime.alloc")
+	if alloc.IsNil() {
+		return
+	}
+
+	// Find all functions that do memory allocation.
+	worklist := []llvm.Value{alloc}
+	allocSet := make(map[llvm.Value]struct{})
+	allocList := make([]llvm.Value, 0, 4)
+	for len(worklist) != 0 {
+		// Pick the topmost.
+		f := worklist[len(worklist)-1]
+		worklist = worklist[:len(worklist)-1]
+		if _, ok := allocSet[f]; ok {
+			continue // already added to list
+		}
+		// Add to set of allocating functions.
+		allocSet[f] = struct{}{}
+		allocList = append(allocList, f)
+
+		// Add all callees to the worklist.
+		for _, use := range getUses(f) {
+			if use.IsACallInst().IsNil() {
+				// TODO: function pointers
+				panic("allocating function " + f.Name() + " used as function pointer")
+			}
+			parent := use.InstructionParent().Parent()
+			for i := 0; i < use.OperandsCount()-1; i++ {
+				if use.Operand(i) == f {
+					// TODO: function pointers
+					panic("allocating function " + f.Name() + " used as function pointer in " + parent.Name())
+				}
+			}
+			worklist = append(worklist, parent)
+		}
+	}
+
+	i8ptrPtrType := llvm.PointerType(c.i8ptrType, 0)
+	gcrootType := llvm.FunctionType(c.ctx.VoidType(), []llvm.Type{i8ptrPtrType, c.i8ptrType}, false)
+	gcroot := llvm.AddFunction(c.mod, "llvm.gcroot", gcrootType)
+
+	// Process every function that needs to save pointers to the shadow stack.
+	for _, fn := range allocList {
+		if fn == alloc {
+			// runtime.alloc itself should not be treated this way, it is a
+			// special case.
+			continue
+		}
+
+		// Check all instructions in this function and see whether the value
+		// needs to be kept on the shadow stack.
+		var values []llvm.Value // values to be kept in the shadow stack
+		for bb := fn.EntryBasicBlock(); !bb.IsNil(); bb = llvm.NextBasicBlock(bb) {
+			for inst := bb.FirstInstruction(); !inst.IsNil(); inst = llvm.NextInstruction(inst) {
+				if !typeHasPointer(inst.Type()) {
+					// This instruction does not result in a pointer value.
+					continue
+				}
+				// Check whether any of the uses may occur after a call to
+				// runtime.alloca. For example, if there are no call
+				// instructions between the definition and the use, then the
+				// pointer does not have to be stored in the shadow stack.
+				for _, use := range getUses(inst) {
+					if crossesAllocatingInst(inst, use, allocSet) {
+						values = append(values, inst)
+						break
+					}
+				}
+			}
+		}
+		if len(values) == 0 {
+			// The children of this function do allocations, but there is
+			// nothing to keep in a stack frame for this function.
+			continue
+		}
+		fn.SetGC("shadow-stack")
+
+		// Convert all values to be kept in the shadow stack to actually be in
+		// the shadow stack.
+		firstInst := fn.EntryBasicBlock().FirstInstruction()
+		for _, value := range values {
+			valueUses := getUses(value)
+			c.builder.SetInsertPointBefore(firstInst)
+			alloca := c.builder.CreateAlloca(value.Type(), "gcroot.value")
+			c.builder.SetInsertPointBefore(llvm.NextInstruction(value))
+			c.builder.CreateStore(value, alloca)
+			metadata := c.gcTypeMetadata(alloca.Type().ElementType())
+			allocaCast := alloca
+			if alloca.Type() != i8ptrPtrType {
+				allocaCast = c.builder.CreateBitCast(alloca, i8ptrPtrType, "")
+			}
+			c.builder.CreateCall(gcroot, []llvm.Value{allocaCast, metadata}, "")
+			for _, use := range valueUses {
+				c.builder.SetInsertPointBefore(use)
+				load := c.builder.CreateLoad(alloca, "")
+				for i := 0; i < use.OperandsCount(); i++ {
+					if use.Operand(i) == value {
+						use.SetOperand(i, load)
+					}
+				}
+			}
+		}
+	}
+	println(c.IR())
+}
+
+// typeHasPointer returns true if (and only if) the given type contains a
+// pointer value.
+func typeHasPointer(typ llvm.Type) bool {
+	switch typ.TypeKind() {
+	case llvm.PointerTypeKind:
+		return true
+	case llvm.ArrayTypeKind, llvm.VectorTypeKind:
+		return typeHasPointer(typ.ElementType())
+	case llvm.StructTypeKind:
+		return false
+		for _, subtyp := range typ.StructElementTypes() {
+			if typeHasPointer(subtyp) {
+				return true
+			}
+		}
+		return false
+	default:
+		return false
+	}
+}
+
+// gcTypeMetadata returns a pointer value to be used in the llvm.gcroot
+// intrinsic. It is either a null pointer or a number which is the number of
+// words in the stack slot for this value.
+func (c *Compiler) gcTypeMetadata(typ llvm.Type) llvm.Value {
+	if typ.TypeKind() == llvm.PointerTypeKind {
+		// Simple pointer. This is a common case, so signal this fact by setting
+		// the pointer to null.
+		return llvm.ConstPointerNull(c.i8ptrType)
+	}
+	if typ.TypeKind() == llvm.StructTypeKind {
+		// Check for structs that only contain a pointer at the start.
+		// We can pretend that such structs are a simple pointer, as the GC only
+		// needs to read the first word.
+		subTypes := typ.StructElementTypes()
+		onlyFirstPointer := subTypes[0].TypeKind() == llvm.PointerTypeKind
+		for _, subType := range subTypes[1:] {
+			if typeHasPointer(subType) {
+				onlyFirstPointer = false
+			}
+		}
+		if onlyFirstPointer {
+			// Types like string and slice.
+			return llvm.ConstPointerNull(c.i8ptrType)
+		}
+	}
+	allocaSize := c.targetData.TypeAllocSize(typ)
+	pointerAlignment := uint64(c.targetData.PrefTypeAlignment(c.i8ptrType))
+	numWords := allocaSize / pointerAlignment
+	// TODO: only return the number until all pointers are included in this
+	// struct, not more.
+	metadata := llvm.ConstIntToPtr(llvm.ConstInt(c.uintptrType, numWords, false), c.i8ptrType)
+	return metadata
+}
+
+// crossesAllocatingInst returns true if the given value may be used across a
+// call to runtime.alloc. This check is very conservative.
+func crossesAllocatingInst(from, to llvm.Value, allocSet map[llvm.Value]struct{}) bool {
+	if from.InstructionParent() != to.InstructionParent() {
+		// Don't try to check the CFG, conservatively assume there is an alloca
+		// in between these instructions.
+		return true
+	}
+	for inst := llvm.NextInstruction(from); inst != to; inst = llvm.NextInstruction(inst) {
+		if inst.IsACallInst().IsNil() {
+			// Not a call instruction thus not an alloca instruction.
+			continue
+		}
+		if _, ok := allocSet[inst.CalledValue()]; ok {
+			// This call is to a function that may do an allocation, or is even
+			// runtime.alloc itself.
+			// TODO: function pointers
+			return true
+		}
+	}
+	return false
+}

compiler/optimizer.go

@@ -52,6 +52,10 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) erro
 		// Run TinyGo-specific interprocedural optimizations.
 		c.OptimizeAllocs()
 		c.OptimizeStringToBytes()
+
+		if c.selectGC() == "shadowstack" {
+			c.AddGCRoots()
+		}
 	} else {
 		// Must be run at any optimization level.
 		c.LowerInterfaces()

src/runtime/gc.go

@@ -0,0 +1,366 @@
+package runtime
+
+// This memory manager is a textbook mark/sweep implementation, heavily inspired
+// by the MicroPython garbage collector.
+//
+// The memory manager internally uses blocks of 4 pointers big (see
+// bytesPerBlock). Every allocation first rounds up to this size to align every
+// block. It will first try to find a chain of blocks that is big enough to
+// satisfy the allocation. If it finds one, it marks the first one as the "head"
+// and the following ones (if any) as the "tail" (see below). If it cannot find
+// any free space, it will perform a garbage collection cycle and try again. If
+// it still cannot find any free space, it gives up.
+//
+// Every block has some metadata, which is stored at the beginning of the heap.
+// The four states are "free", "head", "tail", and "mark". During normal
+// operation, there are no marked blocks. Every allocated object starts with a
+// "head" and is followed by "tail" blocks. The reason for this distinction is
+// that this way, the start and end of every object can be found easily.
+//
+// Metadata is stored in a special area at the beginning of the heap, in the
+// area heapStart..poolStart. The actual blocks are stored in
+// poolStart..heapEnd.
+//
+// More information:
+// https://github.com/micropython/micropython/wiki/Memory-Manager
+// "The Garbage Collection Handbook" by Richard Jones, Antony Hosking, Eliot
+// Moss.
+
+import (
+	"unsafe"
+)
+
+// Set gcDebug to true to print debug information.
+const (
+	gcDebug   = false   // print debug info
+	gcAsserts = gcDebug // perform sanity checks
+)
+
+// Some globals + constants for the entire GC.
+
+const (
+	wordsPerBlock      = 4 // number of pointers in an allocated block
+	bytesPerBlock      = wordsPerBlock * unsafe.Sizeof(heapStart)
+	stateBits          = 2 // how many bits a block state takes (see blockState type)
+	blocksPerStateByte = 8 / stateBits
+)
+
+var (
+	poolStart uintptr // the first heap pointer
+	nextAlloc gcBlock // the next block that should be tried by the allocator
+	endBlock  gcBlock // the block just past the end of the available space
+)
+
+// zeroSizedAlloc is just a sentinel that gets returned when allocating 0 bytes.
+var zeroSizedAlloc uint8
+
+// Provide some abstraction over heap blocks.
+
+// blockState stores the four states in which a block can be. It is two bits in
+// size.
+type blockState uint8
+
+const (
+	blockStateFree blockState = 0 // 00
+	blockStateHead blockState = 1 // 01
+	blockStateTail blockState = 2 // 10
+	blockStateMark blockState = 3 // 11
+	blockStateMask blockState = 3 // 11
+)
+
+// String returns a human-readable version of the block state, for debugging.
+func (s blockState) String() string {
+	switch s {
+	case blockStateFree:
+		return "free"
+	case blockStateHead:
+		return "head"
+	case blockStateTail:
+		return "tail"
+	case blockStateMark:
+		return "mark"
+	default:
+		// must never happen
+		return "!err"
+	}
+}
+
+// The block number in the pool.
+type gcBlock uintptr
+
+// blockFromAddr returns a block given an address somewhere in the heap (which
+// might not be heap-aligned).
+func blockFromAddr(addr uintptr) gcBlock {
+	return gcBlock((addr - poolStart) / bytesPerBlock)
+}
+
+// Return a pointer to the start of the allocated object.
+func (b gcBlock) pointer() unsafe.Pointer {
+	return unsafe.Pointer(b.address())
+}
+
+// Return the address of the start of the allocated object.
+func (b gcBlock) address() uintptr {
+	return poolStart + uintptr(b)*bytesPerBlock
+}
+
+// findHead returns the head (first block) of an object, assuming the block
+// points to an allocated object. It returns the same block if this block
+// already points to the head.
+func (b gcBlock) findHead() gcBlock {
+	for b.state() == blockStateTail {
+		b--
+	}
+	return b
+}
+
+// findNext returns the first block just past the end of the tail. This may or
+// may not be the head of an object.
+func (b gcBlock) findNext() gcBlock {
+	if b.state() == blockStateHead {
+		b++
+	}
+	for b.state() == blockStateTail {
+		b++
+	}
+	return b
+}
+
+// State returns the current block state.
+func (b gcBlock) state() blockState {
+	stateBytePtr := (*uint8)(unsafe.Pointer(heapStart + uintptr(b/blocksPerStateByte)))
+	return blockState(*stateBytePtr>>((b%blocksPerStateByte)*2)) % 4
+}
+
+// setState sets the current block to the given state, which must contain more
+// bits than the current state. Allowed transitions: from free to any state and
+// from head to mark.
+func (b gcBlock) setState(newState blockState) {
+	stateBytePtr := (*uint8)(unsafe.Pointer(heapStart + uintptr(b/blocksPerStateByte)))
+	*stateBytePtr |= uint8(newState << ((b % blocksPerStateByte) * 2))
+	if gcAsserts && b.state() != newState {
+		runtimePanic("gc: setState() was not successful")
+	}
+}
+
+// markFree sets the block state to free, no matter what state it was in before.
+func (b gcBlock) markFree() {
+	stateBytePtr := (*uint8)(unsafe.Pointer(heapStart + uintptr(b/blocksPerStateByte)))
+	*stateBytePtr &^= uint8(blockStateMask << ((b % blocksPerStateByte) * 2))
+	if gcAsserts && b.state() != blockStateFree {
+		runtimePanic("gc: markFree() was not successful")
+	}
+}
+
+// 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")
+	}
+	clearMask := blockStateMask ^ blockStateHead // the bits to clear from the state
+	stateBytePtr := (*uint8)(unsafe.Pointer(heapStart + uintptr(b/blocksPerStateByte)))
+	*stateBytePtr &^= uint8(clearMask << ((b % blocksPerStateByte) * 2))
+	if gcAsserts && b.state() != blockStateHead {
+		runtimePanic("gc: unmark() was not successful")
+	}
+}
+
+// Initialize the memory allocator.
+// No memory may be allocated before this is called. That means the runtime and
+// any packages the runtime depends upon may not allocate memory during package
+// initialization.
+func initHeap() {
+	totalSize := heapEnd - heapStart
+
+	// Allocate some memory to keep 2 bits of information about every block.
+	metadataSize := totalSize / (blocksPerStateByte * bytesPerBlock)
+
+	// Align the pool.
+	poolStart = (heapStart + metadataSize + (bytesPerBlock - 1)) &^ (bytesPerBlock - 1)
+	poolEnd := heapEnd &^ (bytesPerBlock - 1)
+	numBlocks := (poolEnd - poolStart) / bytesPerBlock
+	endBlock = gcBlock(numBlocks)
+	if gcDebug {
+		println("heapStart:        ", heapStart)
+		println("heapEnd:          ", heapEnd)
+		println("total size:       ", totalSize)
+		println("metadata size:    ", metadataSize)
+		println("poolStart:        ", poolStart)
+		println("# of blocks:      ", numBlocks)
+		println("# of block states:", metadataSize*blocksPerStateByte)
+	}
+	if gcAsserts && metadataSize*blocksPerStateByte < numBlocks {
+		// sanity check
+		runtimePanic("gc: metadata array is too small")
+	}
+
+	// Set all block states to 'free'.
+	memzero(unsafe.Pointer(heapStart), metadataSize)
+}
+
+// heapAlloc tries to find some free space on the heap, possibly doing a garbage
+// collection cycle if needed. If no space is free, it panics.
+func heapAlloc(size uintptr) unsafe.Pointer {
+	if size == 0 {
+		return unsafe.Pointer(&zeroSizedAlloc)
+	}
+
+	neededBlocks := (size + (bytesPerBlock - 1)) / bytesPerBlock
+
+	// 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 {
+		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
+				GC()
+			} else {
+				// Even after garbage collection, no free memory could be found.
+				runtimePanic("out of memory")
+			}
+		}
+
+		// Wrap around the end of the heap.
+		if index == endBlock {
+			index = 0
+			// Reset numFreeBlocks as allocations cannot wrap.
+			numFreeBlocks = 0
+		}
+
+		// 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++
+
+		// 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)
+			}
+
+			// Return a pointer to this allocation.
+			pointer := thisAlloc.pointer()
+			memzero(pointer, size)
+			return pointer
+		}
+	}
+}
+
+func heapFree(ptr unsafe.Pointer) {
+	// TODO: free blocks on request, when the compiler knows they're unused.
+}
+
+// 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 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))
+	}
+
+	for addr := start; addr != end; addr += unsafe.Sizeof(addr) {
+		root := *(*uintptr)(unsafe.Pointer(addr))
+		if looksLikePointer(root) {
+			markRoot(root, addr)
+		}
+	}
+}
+
+func markRoot(root, parent uintptr) {
+	block := blockFromAddr(root)
+	head := block.findHead()
+	if head.state() != blockStateMark {
+		if gcDebug {
+			println("found unmarked pointer", root, "at address", parent)
+		}
+		head.setState(blockStateMark)
+		next := block.findNext()
+		// TODO: avoid recursion as much as possible
+		markRoots(head.address(), next.address())
+	}
+}
+
+// Sweep goes through all memory and frees unmarked memory.
+func sweep() {
+	freeCurrentObject := false
+	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
+		case blockStateTail:
+			if freeCurrentObject {
+				// This is a tail object following an unmarked head.
+				// Free it now.
+				block.markFree()
+			}
+		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
+		}
+	}
+}
+
+// looksLikePointer returns whether this could be a pointer. Currently, it
+// simply returns whether it lies anywhere in the heap. Go allows interior
+// pointers so we can't check alignment or anything like that.
+func looksLikePointer(ptr uintptr) bool {
+	return ptr >= poolStart && ptr < heapEnd
+}
+
+// dumpHeap can be used for debugging purposes. It dumps the state of each heap
+// block to standard output.
+func dumpHeap() {
+	println("heap:")
+	for block := gcBlock(0); block < endBlock; block++ {
+		switch block.state() {
+		case blockStateHead:
+			print("*")
+		case blockStateTail:
+			print("-")
+		case blockStateMark:
+			print("#")
+		default: // free
+			print("·")
+		}
+		if block%64 == 63 || block+1 == endBlock {
+			println()
+		}
+	}
+}
+
+func KeepAlive(x interface{}) {
+	// Unimplemented. Only required with SetFinalizer().
+}
+
+func SetFinalizer(obj interface{}, finalizer interface{}) {
+	// Unimplemented.
+}

src/runtime/gc_dumb.go

@@ -37,11 +37,3 @@ func free(ptr unsafe.Pointer) {
 func GC() {
 	// No-op.
 }
-
-func KeepAlive(x interface{}) {
-	// Unimplemented. Only required with SetFinalizer().
-}
-
-func SetFinalizer(obj interface{}, finalizer interface{}) {
-	// Unimplemented.
-}

src/runtime/gc_marksweep.go

@@ -2,277 +2,20 @@
 
 package runtime
 
-// This memory manager is a textbook mark/sweep implementation, heavily inspired
-// by the MicroPython garbage collector.
-//
-// The memory manager internally uses blocks of 4 pointers big (see
-// bytesPerBlock). Every allocation first rounds up to this size to align every
-// block. It will first try to find a chain of blocks that is big enough to
-// satisfy the allocation. If it finds one, it marks the first one as the "head"
-// and the following ones (if any) as the "tail" (see below). If it cannot find
-// any free space, it will perform a garbage collection cycle and try again. If
-// it still cannot find any free space, it gives up.
-//
-// Every block has some metadata, which is stored at the beginning of the heap.
-// The four states are "free", "head", "tail", and "mark". During normal
-// operation, there are no marked blocks. Every allocated object starts with a
-// "head" and is followed by "tail" blocks. The reason for this distinction is
-// that this way, the start and end of every object can be found easily.
-//
-// Metadata is stored in a special area at the beginning of the heap, in the
-// area heapStart..poolStart. The actual blocks are stored in
-// poolStart..heapEnd.
-//
-// More information:
-// https://github.com/micropython/micropython/wiki/Memory-Manager
-// "The Garbage Collection Handbook" by Richard Jones, Antony Hosking, Eliot
-// Moss.
-
 import (
 	"unsafe"
 )
 
-// Set gcDebug to true to print debug information.
-const (
-	gcDebug   = false   // print debug info
-	gcAsserts = gcDebug // perform sanity checks
-)
-
-// Some globals + constants for the entire GC.
-
-const (
-	wordsPerBlock      = 4 // number of pointers in an allocated block
-	bytesPerBlock      = wordsPerBlock * unsafe.Sizeof(heapStart)
-	stateBits          = 2 // how many bits a block state takes (see blockState type)
-	blocksPerStateByte = 8 / stateBits
-)
-
-var (
-	poolStart uintptr // the first heap pointer
-	nextAlloc gcBlock // the next block that should be tried by the allocator
-	endBlock  gcBlock // the block just past the end of the available space
-)
-
-// zeroSizedAlloc is just a sentinel that gets returned when allocating 0 bytes.
-var zeroSizedAlloc uint8
-
-// Provide some abstraction over heap blocks.
-
-// blockState stores the four states in which a block can be. It is two bits in
-// size.
-type blockState uint8
-
-const (
-	blockStateFree blockState = 0 // 00
-	blockStateHead blockState = 1 // 01
-	blockStateTail blockState = 2 // 10
-	blockStateMark blockState = 3 // 11
-	blockStateMask blockState = 3 // 11
-)
-
-// String returns a human-readable version of the block state, for debugging.
-func (s blockState) String() string {
-	switch s {
-	case blockStateFree:
-		return "free"
-	case blockStateHead:
-		return "head"
-	case blockStateTail:
-		return "tail"
-	case blockStateMark:
-		return "mark"
-	default:
-		// must never happen
-		return "!err"
-	}
-}
-
-// The block number in the pool.
-type gcBlock uintptr
-
-// blockFromAddr returns a block given an address somewhere in the heap (which
-// might not be heap-aligned).
-func blockFromAddr(addr uintptr) gcBlock {
-	return gcBlock((addr - poolStart) / bytesPerBlock)
-}
-
-// Return a pointer to the start of the allocated object.
-func (b gcBlock) pointer() unsafe.Pointer {
-	return unsafe.Pointer(b.address())
-}
-
-// Return the address of the start of the allocated object.
-func (b gcBlock) address() uintptr {
-	return poolStart + uintptr(b)*bytesPerBlock
-}
-
-// findHead returns the head (first block) of an object, assuming the block
-// points to an allocated object. It returns the same block if this block
-// already points to the head.
-func (b gcBlock) findHead() gcBlock {
-	for b.state() == blockStateTail {
-		b--
-	}
-	return b
-}
-
-// findNext returns the first block just past the end of the tail. This may or
-// may not be the head of an object.
-func (b gcBlock) findNext() gcBlock {
-	if b.state() == blockStateHead {
-		b++
-	}
-	for b.state() == blockStateTail {
-		b++
-	}
-	return b
-}
-
-// State returns the current block state.
-func (b gcBlock) state() blockState {
-	stateBytePtr := (*uint8)(unsafe.Pointer(heapStart + uintptr(b/blocksPerStateByte)))
-	return blockState(*stateBytePtr>>((b%blocksPerStateByte)*2)) % 4
-}
-
-// setState sets the current block to the given state, which must contain more
-// bits than the current state. Allowed transitions: from free to any state and
-// from head to mark.
-func (b gcBlock) setState(newState blockState) {
-	stateBytePtr := (*uint8)(unsafe.Pointer(heapStart + uintptr(b/blocksPerStateByte)))
-	*stateBytePtr |= uint8(newState << ((b % blocksPerStateByte) * 2))
-	if gcAsserts && b.state() != newState {
-		runtimePanic("gc: setState() was not successful")
-	}
-}
-
-// markFree sets the block state to free, no matter what state it was in before.
-func (b gcBlock) markFree() {
-	stateBytePtr := (*uint8)(unsafe.Pointer(heapStart + uintptr(b/blocksPerStateByte)))
-	*stateBytePtr &^= uint8(blockStateMask << ((b % blocksPerStateByte) * 2))
-	if gcAsserts && b.state() != blockStateFree {
-		runtimePanic("gc: markFree() was not successful")
-	}
-}
-
-// 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")
-	}
-	clearMask := blockStateMask ^ blockStateHead // the bits to clear from the state
-	stateBytePtr := (*uint8)(unsafe.Pointer(heapStart + uintptr(b/blocksPerStateByte)))
-	*stateBytePtr &^= uint8(clearMask << ((b % blocksPerStateByte) * 2))
-	if gcAsserts && b.state() != blockStateHead {
-		runtimePanic("gc: unmark() was not successful")
-	}
-}
-
-// Initialize the memory allocator.
-// No memory may be allocated before this is called. That means the runtime and
-// any packages the runtime depends upon may not allocate memory during package
-// initialization.
 func init() {
-	totalSize := heapEnd - heapStart
-
-	// Allocate some memory to keep 2 bits of information about every block.
-	metadataSize := totalSize / (blocksPerStateByte * bytesPerBlock)
-
-	// Align the pool.
-	poolStart = (heapStart + metadataSize + (bytesPerBlock - 1)) &^ (bytesPerBlock - 1)
-	poolEnd := heapEnd &^ (bytesPerBlock - 1)
-	numBlocks := (poolEnd - poolStart) / bytesPerBlock
-	endBlock = gcBlock(numBlocks)
-	if gcDebug {
-		println("heapStart:        ", heapStart)
-		println("heapEnd:          ", heapEnd)
-		println("total size:       ", totalSize)
-		println("metadata size:    ", metadataSize)
-		println("poolStart:        ", poolStart)
-		println("# of blocks:      ", numBlocks)
-		println("# of block states:", metadataSize*blocksPerStateByte)
-	}
-	if gcAsserts && metadataSize*blocksPerStateByte < numBlocks {
-		// sanity check
-		runtimePanic("gc: metadata array is too small")
-	}
-
-	// Set all block states to 'free'.
-	memzero(unsafe.Pointer(heapStart), metadataSize)
+	initHeap()
 }
 
-// alloc tries to find some free space on the heap, possibly doing a garbage
-// collection cycle if needed. If no space is free, it panics.
 func alloc(size uintptr) unsafe.Pointer {
-	if size == 0 {
-		return unsafe.Pointer(&zeroSizedAlloc)
-	}
-
-	neededBlocks := (size + (bytesPerBlock - 1)) / bytesPerBlock
-
-	// 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 {
-		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
-				GC()
-			} else {
-				// Even after garbage collection, no free memory could be found.
-				runtimePanic("out of memory")
-			}
-		}
-
-		// Wrap around the end of the heap.
-		if index == endBlock {
-			index = 0
-			// Reset numFreeBlocks as allocations cannot wrap.
-			numFreeBlocks = 0
-		}
-
-		// 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++
-
-		// 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)
-			}
-
-			// Return a pointer to this allocation.
-			pointer := thisAlloc.pointer()
-			memzero(pointer, size)
-			return pointer
-		}
-	}
+	return heapAlloc(size)
 }
 
 func free(ptr unsafe.Pointer) {
-	// TODO: free blocks on request, when the compiler knows they're unused.
+	heapFree(ptr)
 }
 
 // GC performs a garbage collection cycle.
@@ -294,91 +37,3 @@ func GC() {
 		dumpHeap()
 	}
 }
-
-// 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 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))
-	}
-
-	for addr := start; addr != end; addr += unsafe.Sizeof(addr) {
-		root := *(*uintptr)(unsafe.Pointer(addr))
-		if looksLikePointer(root) {
-			block := blockFromAddr(root)
-			head := block.findHead()
-			if head.state() != blockStateMark {
-				if gcDebug {
-					println("found unmarked pointer", root, "at address", addr)
-				}
-				head.setState(blockStateMark)
-				next := block.findNext()
-				// TODO: avoid recursion as much as possible
-				markRoots(head.address(), next.address())
-			}
-		}
-	}
-}
-
-// Sweep goes through all memory and frees unmarked memory.
-func sweep() {
-	freeCurrentObject := false
-	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
-		case blockStateTail:
-			if freeCurrentObject {
-				// This is a tail object following an unmarked head.
-				// Free it now.
-				block.markFree()
-			}
-		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
-		}
-	}
-}
-
-// looksLikePointer returns whether this could be a pointer. Currently, it
-// simply returns whether it lies anywhere in the heap. Go allows interior
-// pointers so we can't check alignment or anything like that.
-func looksLikePointer(ptr uintptr) bool {
-	return ptr >= poolStart && ptr < heapEnd
-}
-
-// dumpHeap can be used for debugging purposes. It dumps the state of each heap
-// block to standard output.
-func dumpHeap() {
-	println("heap:")
-	for block := gcBlock(0); block < endBlock; block++ {
-		switch block.state() {
-		case blockStateHead:
-			print("*")
-		case blockStateTail:
-			print("-")
-		case blockStateMark:
-			print("#")
-		default: // free
-			print("·")
-		}
-		if block%64 == 63 || block+1 == endBlock {
-			println()
-		}
-	}
-}
-
-func KeepAlive(x interface{}) {
-	// Unimplemented. Only required with SetFinalizer().
-}
-
-func SetFinalizer(obj interface{}, finalizer interface{}) {
-	// Unimplemented.
-}

src/runtime/gc_none.go

@@ -19,11 +19,3 @@ func free(ptr unsafe.Pointer) {
 func GC() {
 	// Unimplemented.
 }
-
-func KeepAlive(x interface{}) {
-	// Unimplemented. Only required with SetFinalizer().
-}
-
-func SetFinalizer(obj interface{}, finalizer interface{}) {
-	// Unimplemented.
-}

src/runtime/gc_shadowstack.go

@@ -0,0 +1,106 @@
+// +build gc.shadowstack
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+//go:extern __data_start
+var dataStartSymbol unsafe.Pointer
+
+//go:extern _edata
+var dataEndSymbol unsafe.Pointer
+
+//go:extern __bss_start
+var bssStartSymbol unsafe.Pointer
+
+//go:extern _ebss
+var bssEndSymbol unsafe.Pointer
+
+var (
+	dataStart = uintptr(unsafe.Pointer(&dataStartSymbol))
+	dataEnd   = uintptr(unsafe.Pointer(&dataEndSymbol))
+	bssStart  = uintptr(unsafe.Pointer(&bssStartSymbol))
+	bssEnd    = uintptr(unsafe.Pointer(&bssEndSymbol))
+)
+
+// Constant value with metadata about a given function.
+type frameMap struct {
+	numRoots int32
+	numMeta  int32
+	meta     [0]uintptr // unknown number of 'meta' pointers
+}
+
+// Stack-allocated struct with live pointers.
+type stackEntry struct {
+	next  *stackEntry
+	frame *frameMap
+	roots [0]uintptr // unknown number of root pointers
+}
+
+// Note: this symbol must be redefined to llvm_gc_root_chain by the linker.
+// For example, you can use this linker option:
+//     --defsym=tinygo_gc_root_chain=llvm_gc_root_chain
+// The reason is that otherwise the shadow stack GC pass in LLVM tries to set an
+// initial value to llvm_gc_root_chain of a different type.
+//go:extern tinygo_gc_root_chain
+var gcRootChain *stackEntry
+
+func init() {
+	initHeap()
+}
+
+func alloc(size uintptr) unsafe.Pointer {
+	GC()
+	return heapAlloc(size)
+}
+
+func free(ptr unsafe.Pointer) {
+	heapFree(ptr)
+}
+
+// GC performs a garbage collection cycle.
+func GC() {
+	if gcDebug {
+		println("running collection cycle...")
+	}
+
+	// Mark phase: mark all reachable objects, recursively.
+	markRoots(globalsStart, globalsEnd)
+	markStack()
+
+	// Sweep phase: free all non-marked objects and unmark marked objects for
+	// the next collection cycle.
+	sweep()
+
+	// Show how much has been sweeped, for debugging.
+	if gcDebug {
+		dumpHeap()
+	}
+}
+
+// Mark all pointers from the stack using the LLVM shadow stack.
+//go:nobounds
+func markStack() {
+	chain := gcRootChain
+	// Traverse the linked-list of stack roots.
+	for chain != nil {
+		// Check each root.
+		for i := int32(0); i < chain.frame.numRoots; i++ {
+			root := chain.roots[i]
+			size := uintptr(1)
+			if i < chain.frame.numMeta {
+				// This root has no metadata associated with it.
+				// Therefore, it must be a single pointer.
+				size = chain.frame.meta[i]
+			}
+			for i := uintptr(0); i < size; i++ {
+				if looksLikePointer(root) {
+					markRoot(root, 0)
+				}
+			}
+		}
+		chain = chain.next
+	}
+}

target.go

@@ -210,7 +210,7 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 		BuildTags: []string{goos, goarch},
 		Compiler:  commands["clang"],
 		Linker:    "cc",
-		LDFlags:   []string{"-no-pie"}, // WARNING: clang < 5.0 requires -nopie
+		LDFlags:   []string{"-Wl,--defsym=tinygo_gc_root_chain=llvm_gc_root_chain", "-no-pie"}, // WARNING: clang < 5.0 requires -nopie
 		Objcopy:   "objcopy",
 		GDB:       "gdb",
 		GDBCmds:   []string{"run"},