runtime, cmd/link: optimize memory allocation on wasm

WebAssembly's memory is contiguous. Allocating memory at a high address
also allocates all memory up to that address. This change reduces
the initial memory allocated on wasm from 1GB to 16MB by using multiple
heap arenas and reducing the size of a heap arena.

Fixes #27462.

Change-Id: Ic941e6edcadd411e65a14cb2f9fd6c8eae02fc7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/170950
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/internal/wasm/asm.go

@@ -297,18 +297,18 @@ func writeTableSec(ctxt *ld.Link, fns []*wasmFunc) {
 }
 
 // writeMemorySec writes the section that declares linear memories. Currently one linear memory is being used.
+// Linear memory always starts at address zero. More memory can be requested with the GrowMemory instruction.
 func writeMemorySec(ctxt *ld.Link) {
 	sizeOffset := writeSecHeader(ctxt, sectionMemory)
 
-	// Linear memory always starts at address zero.
-	// The unit of the sizes is "WebAssembly page size", which is 64Ki.
-	// The minimum is currently set to 1GB, which is a lot.
-	// More memory can be requested with the grow_memory instruction,
-	// but this operation currently is rather slow, so we avoid it for now.
-	// TODO(neelance): Use lower initial memory size.
+	const (
+		initialSize  = 16 << 20 // 16MB, enough for runtime init without growing
+		wasmPageSize = 64 << 10 // 64KB
+	)
+
 	writeUleb128(ctxt.Out, 1)                        // number of memories
 	ctxt.Out.WriteByte(0x00)                         // no maximum memory size
-	writeUleb128(ctxt.Out, 1024*16) // minimum (initial) memory size
+	writeUleb128(ctxt.Out, initialSize/wasmPageSize) // minimum (initial) memory size
 
 	writeSecSize(ctxt, sizeOffset)
 }

src/runtime/malloc.go

@@ -248,7 +248,7 @@ const (
 	// logHeapArenaBytes is log_2 of heapArenaBytes. For clarity,
 	// prefer using heapArenaBytes where possible (we need the
 	// constant to compute some other constants).
-	logHeapArenaBytes = (6+20)*(_64bit*(1-sys.GoosWindows)*(1-sys.GoosAix)) + (2+20)*(_64bit*sys.GoosWindows) + (2+20)*(1-_64bit) + (8+20)*sys.GoosAix
+	logHeapArenaBytes = (6+20)*(_64bit*(1-sys.GoosWindows)*(1-sys.GoosAix)*(1-sys.GoarchWasm)) + (2+20)*(_64bit*sys.GoosWindows) + (2+20)*(1-_64bit) + (8+20)*sys.GoosAix + (2+20)*sys.GoarchWasm
 
 	// heapArenaBitmapBytes is the size of each heap arena's bitmap.
 	heapArenaBitmapBytes = heapArenaBytes / (sys.PtrSize * 8 / 2)
@@ -394,7 +394,7 @@ func mallocinit() {
 	_g_.m.mcache = allocmcache()
 
 	// Create initial arena growth hints.
-	if sys.PtrSize == 8 && GOARCH != "wasm" {
+	if sys.PtrSize == 8 {
 		// On a 64-bit machine, we pick the following hints
 		// because:
 		//