wasm: implement a growable heap

On WebAssembly it is possible to grow the heap with the memory.grow
instruction. This commit implements this feature and with that also
removes the -heap-size flag that was reportedly broken (I haven't
verified that). This should make it easier to use TinyGo for
WebAssembly, where there was no good reason to use a fixed heap size.

This commit has no effect on baremetal targets with optimizations
enabled.

compileopts/config.go

@@ -7,7 +7,6 @@ import (
 	"fmt"
 	"path/filepath"
 	"regexp"
-	"strconv"
 	"strings"
 
 	"github.com/tinygo-org/tinygo/goenv"
@@ -203,12 +202,6 @@ func (c *Config) LDFlags() []string {
 		ldflags = append(ldflags, strings.Replace(flag, "{root}", root, -1))
 	}
 	ldflags = append(ldflags, "-L", root)
-	if c.Target.GOARCH == "wasm" {
-		// Round heap size to next multiple of 65536 (the WebAssembly page
-		// size).
-		heapSize := (c.Options.HeapSize + (65536 - 1)) &^ (65536 - 1)
-		ldflags = append(ldflags, "--initial-memory="+strconv.FormatInt(heapSize, 10))
-	}
 	if c.Target.LinkerScript != "" {
 		ldflags = append(ldflags, "-T", c.Target.LinkerScript)
 	}

compileopts/options.go

@@ -30,7 +30,6 @@ type Options struct {
 	LDFlags       []string
 	Tags          string
 	WasmAbi       string
-	HeapSize      int64
 	TestConfig    TestConfig
 	Programmer    string
 }

main.go

@@ -14,7 +14,6 @@ import (
 	"os/signal"
 	"path/filepath"
 	"runtime"
-	"strconv"
 	"strings"
 	"syscall"
 	"time"
@@ -650,30 +649,6 @@ func windowsFindUSBDrive(volume string) (string, error) {
 	return "", errors.New("unable to locate a USB device to be flashed")
 }
 
-// parseSize converts a human-readable size (with k/m/g suffix) into a plain
-// number.
-func parseSize(s string) (int64, error) {
-	s = strings.ToLower(strings.TrimSpace(s))
-	if len(s) == 0 {
-		return 0, errors.New("no size provided")
-	}
-	multiply := int64(1)
-	switch s[len(s)-1] {
-	case 'k':
-		multiply = 1 << 10
-	case 'm':
-		multiply = 1 << 20
-	case 'g':
-		multiply = 1 << 30
-	}
-	if multiply != 1 {
-		s = s[:len(s)-1]
-	}
-	n, err := strconv.ParseInt(s, 0, 64)
-	n *= multiply
-	return n, err
-}
-
 // getDefaultPort returns the default serial port depending on the operating system.
 func getDefaultPort() (port string, err error) {
 	var portPath string
@@ -839,7 +814,6 @@ func main() {
 	cFlags := flag.String("cflags", "", "additional cflags for compiler")
 	ldFlags := flag.String("ldflags", "", "additional ldflags for linker")
 	wasmAbi := flag.String("wasm-abi", "", "WebAssembly ABI conventions: js (no i64 params) or generic")
-	heapSize := flag.String("heap-size", "1M", "default heap size in bytes (only supported by WebAssembly)")
 
 	var flagJSON, flagDeps *bool
 	if command == "help" || command == "list" {
@@ -893,16 +867,9 @@ func main() {
 		options.LDFlags = strings.Split(*ldFlags, " ")
 	}
 
-	var err error
-	if options.HeapSize, err = parseSize(*heapSize); err != nil {
-		fmt.Fprintln(os.Stderr, "Could not read heap size:", *heapSize)
-		usage()
-		os.Exit(1)
-	}
-
 	os.Setenv("CC", "clang -target="+*target)
 
-	err = options.Verify()
+	err := options.Verify()
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err.Error())
 		usage()

src/runtime/arch_wasm.go

@@ -17,6 +17,9 @@ var heapStartSymbol [0]byte
 //export llvm.wasm.memory.size.i32
 func wasm_memory_size(index int32) int32
 
+//export llvm.wasm.memory.grow.i32
+func wasm_memory_grow(index int32, delta int32) int32
+
 var (
 	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
 	heapEnd   = uintptr(wasm_memory_size(0) * wasmPageSize)
@@ -30,3 +33,20 @@ func align(ptr uintptr) uintptr {
 }
 
 func getCurrentStackPointer() uintptr
+
+// growHeap tries to grow the heap size. It returns true if it succeeds, false
+// otherwise.
+func growHeap() bool {
+	// Grow memory by the available size, which means the heap size is doubled.
+	memorySize := wasm_memory_size(0)
+	result := wasm_memory_grow(0, memorySize)
+	if result == -1 {
+		// Grow failed.
+		return false
+	}
+
+	setHeapEnd(uintptr(wasm_memory_size(0) * wasmPageSize))
+
+	// Heap has grown successfully.
+	return true
+}

src/runtime/baremetal.go

@@ -29,6 +29,13 @@ var (
 	stackTop     = uintptr(unsafe.Pointer(&stackTopSymbol))
 )
 
+// growHeap tries to grow the heap size. It returns true if it succeeds, false
+// otherwise.
+func growHeap() bool {
+	// On baremetal, there is no way the heap can be grown.
+	return false
+}
+
 //export malloc
 func libc_malloc(size uintptr) unsafe.Pointer {
 	return alloc(size)

src/runtime/gc_conservative.go

@@ -184,6 +184,50 @@ func (b gcBlock) unmark() {
 // any packages the runtime depends upon may not allocate memory during package
 // initialization.
 func initHeap() {
+	calculateHeapAddresses()
+
+	// Set all block states to 'free'.
+	metadataSize := heapEnd - uintptr(metadataStart)
+	memzero(unsafe.Pointer(metadataStart), metadataSize)
+}
+
+// setHeapEnd is called to expand the heap. The heap can only grow, not shrink.
+// Also, the heap should grow substantially each time otherwise growing the heap
+// will be expensive.
+func setHeapEnd(newHeapEnd uintptr) {
+	if gcAsserts && newHeapEnd <= heapEnd {
+		panic("gc: setHeapEnd didn't grow the heap")
+	}
+
+	// Save some old variables we need later.
+	oldMetadataStart := metadataStart
+	oldMetadataSize := heapEnd - uintptr(metadataStart)
+
+	// Increase the heap. After setting the new heapEnd, calculateHeapAddresses
+	// will update metadataStart and the memcpy will copy the metadata to the
+	// new location.
+	// The new metadata will be bigger than the old metadata, but a simple
+	// memcpy is fine as it only copies the old metadata and the new memory will
+	// have been zero initialized.
+	heapEnd = newHeapEnd
+	calculateHeapAddresses()
+	memcpy(metadataStart, oldMetadataStart, oldMetadataSize)
+
+	// Note: the memcpy above assumes the heap grows enough so that the new
+	// metadata does not overlap the old metadata. If that isn't true, memmove
+	// should be used to avoid corruption.
+	// This assert checks whether that's true.
+	if gcAsserts && uintptr(metadataStart) < uintptr(oldMetadataStart)+oldMetadataSize {
+		panic("gc: heap did not grow enough at once")
+	}
+}
+
+// calculateHeapAddresses initializes variables such as metadataStart and
+// numBlock based on heapStart and heapEnd.
+//
+// This function can be called again when the heap size increases. The caller is
+// responsible for copying the metadata to the new location.
+func calculateHeapAddresses() {
 	totalSize := heapEnd - heapStart
 
 	// Allocate some memory to keep 2 bits of information about every block.
@@ -206,9 +250,6 @@ func initHeap() {
 		// sanity check
 		runtimePanic("gc: metadata array is too small")
 	}
-
-	// Set all block states to 'free'.
-	memzero(metadataStart, metadataSize)
 }
 
 // alloc tries to find some free space on the heap, possibly doing a garbage
@@ -238,7 +279,16 @@ func alloc(size uintptr) unsafe.Pointer {
 				GC()
 			} else {
 				// Even after garbage collection, no free memory could be found.
-				runtimePanic("out of memory")
+				// 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).
+					runtimePanic("out of memory")
+				}
 			}
 		}
 

src/runtime/gc_leaking.go

@@ -20,7 +20,12 @@ func alloc(size uintptr) unsafe.Pointer {
 	size = align(size)
 	addr := heapptr
 	heapptr += size
-	if heapptr >= heapEnd {
+	for heapptr >= heapEnd {
+		// Try to increase the heap and check again.
+		if growHeap() {
+			continue
+		}
+		// Failed to make the heap bigger, so we must really be out of memory.
 		runtimePanic("out of memory")
 	}
 	for i := uintptr(0); i < uintptr(size); i += 4 {
@@ -49,3 +54,10 @@ func SetFinalizer(obj interface{}, finalizer interface{}) {
 func initHeap() {
 	// Nothing to initialize.
 }
+
+// setHeapEnd sets a new (larger) heapEnd pointer.
+func setHeapEnd(newHeapEnd uintptr) {
+	// This "heap" is so simple that simply assigning a new value is good
+	// enough.
+	heapEnd = newHeapEnd
+}

src/runtime/gc_none.go

@@ -31,3 +31,7 @@ func SetFinalizer(obj interface{}, finalizer interface{}) {
 func initHeap() {
 	// Nothing to initialize.
 }
+
+func setHeapEnd(newHeapEnd uintptr) {
+	// Nothing to do here, this function is never actually called.
+}

src/runtime/runtime_nintendoswitch.go

@@ -217,6 +217,13 @@ func setupHeap() {
 	}
 }
 
+// growHeap tries to grow the heap size. It returns true if it succeeds, false
+// otherwise.
+func growHeap() bool {
+	// Growing the heap is unimplemented.
+	return false
+}
+
 // getHeapBase returns the start address of the heap
 // this is externally linked by gonx
 func getHeapBase() uintptr {

src/runtime/runtime_unix_heap.go

@@ -13,3 +13,12 @@ func preinit() {
 	heapStart = uintptr(malloc(heapSize))
 	heapEnd = heapStart + heapSize
 }
+
+// growHeap tries to grow the heap size. It returns true if it succeeds, false
+// otherwise.
+func growHeap() bool {
+	// At the moment, this is not possible. However it shouldn't be too
+	// difficult (at least on Linux) to allocate a large amount of virtual
+	// memory at startup that is then slowly used.
+	return false
+}