[clang] [llvm] [InstCombine] remove dead loads, such as memcpy from undef (PR #143958)

[Nikita Popov via llvm-commits]

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@@ -3382,6 +3422,17 @@ Instruction *InstCombinerImpl::visitAllocSite(Instruction &MI) {
           eraseInstFromFunction(*I);
           Users[i] = nullptr; // Skip examining in the next loop.
         }
+        if (auto *MTI = dyn_cast<MemTransferInst>(I)) {
+          if (KnowInitZero && getUnderlyingObject(MTI->getRawDest()) != &MI) {
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nikic wrote:

I think this check isn't entirely right, in case we can't see through to the underlying object (e.g. a very long gep chain). In that case even though we're writing to the alloca, we'd leave a memset here.

I think it would be more robust if isAllocSiteRemovable returned the ModRefInfo and then you made the decision here on whether this is a Mod or Ref alloca.

https://github.com/llvm/llvm-project/pull/143958