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

[Vasileios Porpodas via Phabricator via llvm-commits]

vporpo added a comment.

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

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.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D126895/new/

https://reviews.llvm.org/D126895