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[lld][WebAssembly] Align __heap_base 

__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
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GPG Key ID: E97FF5335DFDFDED
3 changed files with 33 additions and 8 deletions
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11
lld/test/wasm/Inputs/stack-first.s Normal file
View File

@ -0,0 +1,11 @@
.globl _start
_start:
.functype _start () -> ()
end_function
.globl someByte
.type someByte,@object
.section .data.someByte,"",@
someByte:
.int8 42
.size someByte, 1

20
lld/test/wasm/stack-first.test
View File

@ -2,9 +2,10 @@
; memory. In this case the --stack-first option is being passed along with a
; stack size of 512. This means (since the stack grows down) the stack pointer
; global should be initialized to 512.
; Also test that __heap_base is still aligned with the --stack-first option.
RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/start.s -o %t.o
RUN: wasm-ld -z stack-size=512 --stack-first --export=__data_end --export=__heap_base -o %t.wasm %t.o
RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/stack-first.s -o %t.o
RUN: wasm-ld -z stack-size=512 --stack-first --export=__data_end --export=__heap_base --export=someByte -o %t.wasm %t.o
RUN: obj2yaml %t.wasm | FileCheck %s
CHECK: - Type: GLOBAL
@ -26,7 +27,13 @@ CHECK-NEXT: Type: I32
CHECK-NEXT: Mutable: false
CHECK-NEXT: InitExpr:
CHECK-NEXT: Opcode: I32_CONST
CHECK-NEXT: Value: 512
CHECK-NEXT: Value: 513
CHECK-NEXT: - Index: 3
CHECK-NEXT: Type: I32
CHECK-NEXT: Mutable: false
CHECK-NEXT: InitExpr:
CHECK-NEXT: Opcode: I32_CONST
CHECK-NEXT: Value: 528
CHECK-NEXT: - Type: EXPORT
CHECK-NEXT: Exports:
CHECK-NEXT: - Name: memory
@ -35,9 +42,12 @@ CHECK-NEXT: Index: 0
CHECK-NEXT: - Name: _start
CHECK-NEXT: Kind: FUNCTION
CHECK-NEXT: Index: 0
CHECK-NEXT: - Name: __data_end
CHECK-NEXT: - Name: someByte
CHECK-NEXT: Kind: GLOBAL
CHECK-NEXT: Index: 1
CHECK-NEXT: - Name: __heap_base
CHECK-NEXT: - Name: __data_end
CHECK-NEXT: Kind: GLOBAL
CHECK-NEXT: Index: 2
CHECK-NEXT: - Name: __heap_base
CHECK-NEXT: Kind: GLOBAL
CHECK-NEXT: Index: 3

10
lld/wasm/Writer.cpp
View File

@ -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);
}