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

[Jameson Nash via llvm-commits]

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@@ -3382,6 +3418,17 @@ 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) {
+            IRBuilderBase::InsertPointGuard Guard(Builder);
+            Builder.SetInsertPoint(MTI);
+            auto *M = Builder.CreateMemSet(MTI->getRawDest(),
+                    ConstantInt::get(Type::getInt8Ty(MI.getContext()), 0),
+                    MTI->getLength(),
+                    MTI->getDestAlign());
+            M->copyMetadata(*MTI, LLVMContext::MD_DIAssignID);
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vtjnash wrote:

I was wondering about that too. The current MemCpyOptimizer pass usually drops all metadata information (which is where I copied this from to be consistent) but I couldn't really justify that to myself either.

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