[PATCH] D63489: [InstSimplify] LCSSA PHIs should not be simplified away

[Alexander via Phabricator via llvm-commits]

alex-t added a comment.

In D63489#1548288 <https://reviews.llvm.org/D63489#1548288>, @lebedev.ri wrote:

> > This causes generation of incorrect code in AMDGPU backend.
>
> This sounds like some other check is missing elsewhere?
>  What happens if you feed it such an ir as-if after this transform, but manually written?
>  ("that will result in broken asm/crashes" is hopefully not the answer)
>
> That being said, why is `LCSSAPass` not sufficient?
>  It's already supposed to undo transforms like this.


Manually written IR w/o lcssa phis will result in syntactically correct asm and no crashes. In runtime we'll get incorrect result though :)  
Adding LCSSA pass again later on is difficult in the sense of the pass dependencies.
So, it's better to fix the explicit bug in SimplifyPHI....


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