[PATCH] D126895: [SLP] Phi inputs that come from an unreachable block should be undef.

[Harald van Dijk via Phabricator via llvm-commits]

hvdijk added a comment.

In D126895#3554064 <https://reviews.llvm.org/D126895#3554064>, @vporpo wrote:

> I think that the issue boils down to whether: `%zext = zext i8 poison to i32` is an i32 poison.

It is, yes.

> Instcombine is folding zext into phis like so:
>
> For a phi with an undef input:
>
>   bb1:
>     br label %bb3
>   bb2:
>     br label %bb3
>   bb3:
>     %phi = phi <2 x i8> [ %arg, %bb1 ], [ <i8 1, i8 undef>, %bb2 ]
>     %zext = zext <2 x i8> %phi to <2 x i32>
>
> Instcombine will fold the zext into phi like this:
>
>   bb1:
>     %phi.cast = zext <2 x i8> %arg to <2 x i32>
>     br label %bb3
>   bb2:                                              ; No predecessors!
>     br label %bb3
>   bb3:                                              ; preds = %bb2, %bb1
>     %phi = phi <2 x i32> [ %phi.cast, %bb1 ], [ <i32 1, i32 0>, %bb2 ]
>
> But for a phi with a poison input it will generate this:
>
>   bb1:
>     %phi.cast = zext <2 x i8> %arg to <2 x i32>
>     br label %bb3
>   bb2:                                              ; No predecessors!
>     br label %bb3
>   bb3:                                              ; preds = %bb2, %bb1   
>     %phi = phi <2 x i32> [ %phi.cast, %bb1 ], [ <i32 1, i32 poison>, %bb2 ]
>
> The input from %bb2 is now an i32 poison, which does not seem to hold the same properties as a zero-extended i8 poison.

It holds the same properties in all cases where we reach the block through bb.2, all zero of them. That's why poison should be fine: we know it's only for a case that cannot ever happen.


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