From 65c1978965bc2f00d07765978f977617ba40b28f Mon Sep 17 00:00:00 2001
From: Ayke van Laethem <aykevanlaethem@gmail.com>
Date: Sun, 25 Jul 2021 14:30:16 +0200
Subject: [PATCH] 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 | 6 ++++--
 src/runtime/gc_conservative.go | 8 ++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/runtime/arch_tinygowasm.go b/src/runtime/arch_tinygowasm.go
index 753591e9..a19a1448 100644
--- a/src/runtime/arch_tinygowasm.go
+++ b/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
diff --git a/src/runtime/gc_conservative.go b/src/runtime/gc_conservative.go
index e9682592..67fbfdb2 100644
--- a/src/runtime/gc_conservative.go
+++ b/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.