[llvm] [AA] A conservative fix for atomic store instruction. (PR #155032)

[Jin Huang via llvm-commits]

jinhuang1102 wrote:

Hi, I am back from vacation and have just pushed some updates to address the feedback from the previous review. The main functional change is in `llvm/lib/Analysis/AliasAnalysis.cpp` to refine the logic for atomic load instructions see below.
```
  // Be conservative in the face of atomic.
  if (isStrongerThan(L->getOrdering(), AtomicOrdering::Monotonic))
    return ModRefInfo::ModRef;

  // For Monotonic and unordered loads, we only need to be more conservative
  // when the locations are MustAlias to prevent unsafe reordering of accesses
  // to the same memory. For MayAlias, we can treat it as a normal read.
  if (L->isAtomic()) {
    if (Loc.Ptr &&
        alias(MemoryLocation::get(L), Loc, AAQI, L) == AliasResult::MustAlias)
      return ModRefInfo::ModRef;
  }
```
Two new tests (`MonotonicAtomicLoadIsModRefOnlyForMustAlias` and `MonotonicAtomicStoreAliasBehavior`) are also added to `AliasAnalysisTest.cpp`.

The only functional changes are in these two files; other modifications are NFC (No Functional Change) from `clang-format`.

https://github.com/llvm/llvm-project/pull/155032