[llvm] [AA] Improve precision for monotonic atomic load/store operations (PR #158169)

[Jin Huang via llvm-commits]

================
@@ -421,9 +421,18 @@ ModRefInfo AAResults::getModRefInfo(const LoadInst *L,
                                     const MemoryLocation &Loc,
                                     AAQueryInfo &AAQI) {
   // Be conservative in the face of atomic.
-  if (isStrongerThan(L->getOrdering(), AtomicOrdering::Unordered))
+  if (isStrongerThan(L->getOrdering(), AtomicOrdering::Monotonic))
     return ModRefInfo::ModRef;
 
+  // For Monotonic and unordered atomic loads, if the locations are not NoAlias,
+  // we must be conservative and return ModRef to prevent unsafe reordering of
+  // accesses to the same memory.
+  if (L->isAtomic()){
+    if (Loc.Ptr &&
+        alias(MemoryLocation::get(L), Loc, AAQI, L) != AliasResult::NoAlias)
+      return ModRefInfo::ModRef;
+  }
----------------
jinhuang1102 wrote:

I see. You means that treat `unordered` like the non-atomic load and add the following code:
```c++
if (isStrongerThanUnordered(L->getOrdering())) 
  return ModRefInfo::ModRef;
```
After the `AliasResult AR` check:
```
  if (Loc.Ptr) {
    AliasResult AR = alias(MemoryLocation::get(L), Loc, AAQI, L);
    if (AR == AliasResult::NoAlias)
      return ModRefInfo::NoModRef;
  }
```

Am I right?

https://github.com/llvm/llvm-project/pull/158169