[PATCH] D84220: [IPSCCP] Fix a bug that the "returned" attribute is not cleared when function is optimized to return undef

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added a comment.

In D84220#2179859 <https://reviews.llvm.org/D84220#2179859>, @efriedma wrote:

>> I'm not sure I understand the motivation. My problem is that if we replace the return values with undef, it should mean there is no use of the returned value of the function live anymore. What am I missing? Maybe provide the motivating example as a test case?
>
> We use "returned" in codegen even if the value has no uses; see https://reviews.llvm.org/D64986 .

Argh, ... I'm certain this is a bad idea and we are just hiding the problem one level deeper here. I still believe violation of the attributes means the value is poison, not immediate UB. With that semantic the codegen trick is not valid in general but the IPSCCP transformation is. As I argued before, IPSCCP is only one reason things can become undef. FWIW, I am actually expecting a patch + RFC on the attribute semantic is poison not UB stuff "soonish". If you feel this is the right way for now, I will not stop you. If you want me to come up with other examples where the codegen logic will break, let me know.


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https://reviews.llvm.org/D84220