[PATCH] D103077: [DAGCombine] Poison-prove scalarizeExtractedVectorLoad.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D103077#2780921 <https://reviews.llvm.org/D103077#2780921>, @jonpa wrote:

> In D103077#2779949 <https://reviews.llvm.org/D103077#2779949>, @fhahn wrote:
>
>> Adjust failing SystemZ test. It would be great if someone familiar with SystemZ could take a look to double-check this makes sense. Perhaps @jonpa?
>
> Yes, this looks like a correct test update to me: the changed immediate operand reflects that now only two shifted bits are inserted into %r1 (instead of all 32).
>
> I wonder what the rationale behind this patch is: if the instruction is poison to begin with, then the program is broken and I don't understand how changing the argument value can change that..?

Whether the whole program is broken (has UB) depends on how the returned value is used in the callers. Returning poison is not UB, unless the function has the `noundef` attribute.


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