[clang] [llvm] [clang][CodeGen][AA] Introduce `!llvm.errno.tbaa` for errno alias disambiguation (PR #125258)

[Nikita Popov via cfe-commits]

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@@ -970,6 +971,19 @@ ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call,
   }
 
   ModRefInfo Result = ArgMR | OtherMR;
+
+  // Refine writes to errno memory. We can safely exclude the errno location if
+  // the given memory location is an alloca, the size of the memory access is
+  // larger than `sizeof(int)` or if TBAA proves it does not alias errno.
+  if ((ErrnoMR | OtherMR) != OtherMR) {
+    bool IsLocSizeUnknown = Loc.Size == MemoryLocation::UnknownSize;
+    if (ErrnoMR == ModRefInfo::Mod && !isa<AllocaInst>(Object) &&
+        (IsLocSizeUnknown ||
+         (!IsLocSizeUnknown && Loc.Size.getValue() <= sizeof(int))) &&
+        AAQI.AAR.aliasErrno(Loc, Call->getModule()) != AliasResult::NoAlias)
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nikic wrote:

Also, possibly these extra checks should be in BasicAA::aliasErrno instead?

https://github.com/llvm/llvm-project/pull/125258