[all-commits] [llvm/llvm-project] 550f61: [SimplifyCFG] Preserve atomicity when merging atom...

[Justin Lebar via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 550f61f191e57660ce9a55da22304b736ff85606
      https://github.com/llvm/llvm-project/commit/550f61f191e57660ce9a55da22304b736ff85606
  Author: Justin Lebar <justin.lebar at gmail.com>
  Date:   2026-05-29 (Fri, 29 May 2026)

  Changed paths:
    M llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    M llvm/test/Transforms/SimplifyCFG/merge-cond-stores.ll

  Log Message:
  -----------
  [SimplifyCFG] Preserve atomicity when merging atomic conditional stores (#200327)

mergeConditionalStoreToAddress() merges two stores into one.  It does
this for non-atomic and atomic-unordered stores, but when merging
unordered stores, it would downgrade them to non-atomic!

This bug isn't accessible from C because C doesn't expose unordered
atomics. But you can access it from e.g. Objective-C with something like

```
// repro.m — clang -fno-objc-arc -O2
__attribute__((objc_root_class))
@interface C { int _value; }
@property(atomic, direct) int value;
@end
@implementation C
@end

void f(C *obj, _Bool c1, _Bool c2, int v1, int v2) {
    if (!obj) __builtin_unreachable();
    if (c1) obj.value = v1;
    if (c2) obj.value = v2;
}
```

LLVM merges these into a single store.  The store is non-atomic without
this change.

This bug was found by a large run of Opus 4.7 looking for bugs in LLVM.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply at anthropic.com>



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