[PATCH] D124078: [MemCpyOpt] Avoid infinite loop in processMemSetMemCpyDependence (PR54983)

[Florian Hahn via Phabricator via llvm-commits]

fhahn added inline comments.


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Comment at: llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp:1298
           Builder.CreatePointerCast(Dest, Builder.getInt8PtrTy(DestAS)),
           SrcSize),
       MemSet->getOperand(1), MemsetLen, Alignment);
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I think the underlying issue here is that the builder folds this GEP to `Dest` is `SrcSize` is poison. This in turn leads to the new MemSet to `must-alias` the original memcpy, causing the cycle IIUC. Could we break the cycle here by bailing out if the pointer folds to `Dest` as more general solution and leave the cleanup of the cases where the size simplifies to 0 to instcombine?


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Comment at: llvm/test/Transforms/MemCpyOpt/memcpy-zero-size.ll:34
+  call void @llvm.memset.p0.i64(ptr inttoptr (i64 -1 to ptr), i8 0, i64 %len, i1 false)
+  call void @llvm.memcpy.p0.p0.i64(ptr inttoptr (i64 -1 to ptr), ptr null, i64 poison, i1 false)
+  ret void
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might be good to use pointer arguments passed to the function here, instead of `null`/`inttoptr` so the `poison` is the only issue here 


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  https://reviews.llvm.org/D124078/new/

https://reviews.llvm.org/D124078