[PATCH] D79452: [VectorCombine] scalarize binop of inserted elements into undef

Wed May 6 17:04:24 PDT 2020

aqjune added a comment.

In D79452#2022815 <https://reviews.llvm.org/D79452#2022815>, @spatel wrote:

> In D79452#2022567 <https://reviews.llvm.org/D79452#2022567>, @spatel wrote:
>
> > undef ^ undef is actually undef:
> >  http://volta.cs.utah.edu:8080/z/GAs5QJ
> >
> > But I think we choose to fold to "0" on that to avoid breaking too much existing source code and/or expose possible semantic incompatibility with source code?
>
>
> https://groups.google.com/forum/#!topic/llvm-dev/C5ydxnn-r0o

This seems interesting.

I modified llvm/lib/IR/ConstantFold.cpp to return undef on xor undef undef, and it did not miscompile testsuite & SPEC on my machine, at least.
I can make a patch for this.

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https://reviews.llvm.org/D79452