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[lld][WebAssembly] Align __heap_base

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__heap_base was not aligned. In practice, it will often be aligned
simply because it follows the stack, but when the stack is placed at the
beginning (with the --stack-first option), the __heap_base might be
unaligned. It could even be byte-aligned.

At least wasi-libc appears to expect that __heap_base is aligned:
659ff41456/dlmalloc/src/malloc.c (L5224)

While WebAssembly itself does not appear to require any alignment for
memory accesses, it is sometimes required when sharing a pointer
externally. For example, WASI might expect alignment up to 8:
https://github.com/WebAssembly/WASI/blob/main/phases/snapshot/docs.md#-timestamp-u64

This issue got introduced with the addition of the --stack-first flag:
https://reviews.llvm.org/D46141
I suspect the lack of alignment wasn't intentional here.

Differential Revision: https://reviews.llvm.org/D106499
This commit is contained in:
Ayke van Laethem
2021-07-21 23:35:10 +02:00
parent b7a4649899
commit 13ca0c87ed
3 changed files with 33 additions and 8 deletions
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10
lld/wasm/Writer.cpp
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@@ -43,6 +43,7 @@ using namespace llvm::wasm;
namespace lld {
namespace wasm {
static constexpr int stackAlignment = 16;
static constexpr int heapAlignment = 16;
namespace {
@@ -310,9 +311,12 @@ void Writer::layoutMemory() {
placeStack();
if (WasmSym::heapBase) {
// Set `__heap_base` to directly follow the end of the stack or global data.
// The fact that this comes last means that a malloc/brk implementation
// can grow the heap at runtime.
// Set `__heap_base` to follow the end of the stack or global data. The
// fact that this comes last means that a malloc/brk implementation can
// grow the heap at runtime.
// We'll align the heap base here because memory allocators might expect
// __heap_base to be aligned already.
memoryPtr = alignTo(memoryPtr, heapAlignment);
log("mem: heap base = " + Twine(memoryPtr));
WasmSym::heapBase->setVA(memoryPtr);
}