[llvm] [memcpyopt] handle memcpy from memset in more cases (PR #140954)

[Jameson Nash via llvm-commits]

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@@ -1418,51 +1437,52 @@ static bool hasUndefContents(MemorySSA *MSSA, BatchAAResults &AA, Value *V,
 bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
                                                MemSetInst *MemSet,
                                                BatchAAResults &BAA) {
-  // Make sure that memcpy(..., memset(...), ...), that is we are memsetting and
-  // memcpying from the same address. Otherwise it is hard to reason about.
-  if (!BAA.isMustAlias(MemSet->getRawDest(), MemCpy->getRawSource()))
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vtjnash wrote:

No, that is the improvement being implemented by the PR (mustalias means that they are exactly the same pointer, without offset)

https://github.com/llvm/llvm-project/pull/140954