[llvm] [AA] Refine ModRefInfo taking into account `errnomem` location (PR #157495)

[Nikita Popov via llvm-commits]

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@@ -997,6 +998,19 @@ 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 identified function-local object, the size
+  // of the memory access is larger than the integer size, or if TBAA proves it
+  // does not alias errno.
+  if ((ErrnoMR | Result) != Result) {
+    if (isModSet(ErrnoMR) && !isIdentifiedFunctionLocal(Object) &&
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nikic wrote:

I still don't quite get this. If the call has `memory(errno: read)` and the location can alias errno, don't we need to return `MayAlias` as the result? Why is it okay to ignore reads?

https://github.com/llvm/llvm-project/pull/157495