[clang] [llvm] [mlir] [IR][ModRef] Introduce `errno` memory location (PR #120783)

[Antonio Frighetto via llvm-commits]

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@@ -82,7 +82,7 @@ define void @test_store(ptr %p) {
 
 @G = external global ptr
 define i8 @test_store_capture(ptr %p) {
-; FNATTRS: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(readwrite, argmem: read, inaccessiblemem: none)
+; FNATTRS: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(readwrite, argmem: read, inaccessiblemem: none, errnomem: none)
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antoniofrighetto wrote:

While walking the call-graph for deducing attrs, in `checkFunctionMemoryAccess` we initialize a ME object that doesn't access/modify memory and gradually refine it (currently says ErrnoMem: NoModRef). This ME is intersected with the original ME based on AA results (which says ErrnoMem: ModRef), so we get NoModRef, as per the meet of the lattice. We miss logic to infer if the function clobbers errno, but I think it should be fine as the PR only introduces the location (it might be better to conservatively always say ModRef, but the location is currently unused, so it should be okay to address this in an upcoming PR?). Rebased to main too.

https://github.com/llvm/llvm-project/pull/120783