wasm: align heap to 16 bytes

This commit fixes two things:

  * It changes the alignment to 16 bytes (from 4), to match max_align_t
    in C.
  * It manually aligns heapStart on WebAssembly, to work around a bug in
    wasm-ld with --stack-first (see https://reviews.llvm.org/D106499).

src/runtime/arch_tinygowasm.go

@@ -32,9 +32,11 @@ var (
 
 const wasmPageSize = 64 * 1024
 
-// Align on word boundary.
 func align(ptr uintptr) uintptr {
-	return (ptr + 3) &^ 3
+	// Align to 16, which is the alignment of max_align_t:
+	// https://godbolt.org/z/dYqTsWrGq
+	const heapAlign = 16
+	return (ptr + heapAlign - 1) &^ (heapAlign - 1)
 }
 
 func getCurrentStackPointer() uintptr

src/runtime/gc_conservative.go

@@ -228,6 +228,14 @@ func setHeapEnd(newHeapEnd uintptr) {
 // 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() {
+	if GOARCH == "wasm" {
+		// This is a workaround for a bug in wasm-ld: wasm-ld doesn't always
+		// align __heap_base and when this memory is shared through an API, it
+		// might result in unaligned memory. For details, see:
+		// https://reviews.llvm.org/D106499
+		// It should be removed once we switch to LLVM 13, where this is fixed.
+		heapStart = align(heapStart)
+	}
 	totalSize := heapEnd - heapStart
 
 	// Allocate some memory to keep 2 bits of information about every block.