[PATCH] D105344: [DAGCombiner] Fold SETCC(FREEZE(x), y) to SETCC(x, y) if used by BRCOND

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

>     %z = op1(FREEZE(x), y)
>     op2(%z)
>   => // step 1
>     %z = op1(FREEZE(x), FREEZE(y))
>     op2(%z)
>   => // step 2
>     %z = op1(x, y)
>     %z.fr = FREEZE(%z)
>     op2(%z.fr)
>   => // step 3
>     %z = op1(x, y)
>     op2(%z)
>
> Step 2: Relying on `canCreateUndefOrPoison` makes checking this step simpler, maybe?

I found that `canCreateUndefOrPoison` isn't enough. A non-poison-generating operation can be problematic as well.
Look at this example: https://alive2.llvm.org/ce/z/xGDGHF

      %n.fr = freeze i8 %n           // Assume that %n = poison
      %m.fr = freeze i8 %m           // Assume that %m = -128
      %v = icmp slt i8 %n.fr, %m.fr  // This is always false, but...
  =>
      %v = icmp slt i8 %n, %m        // This is poison, so
      %v.fr = freeze i1 %v           // This (freeze poison) can be true.
      ret i1 %v.fr

So, even in SETCC (which is analogous to icmp) pushing freeze should have been carefully done.

Fortunately, we only need to deal with one operand being constant, so this can be checked in advance.


Repository:
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  https://reviews.llvm.org/D105344/new/

https://reviews.llvm.org/D105344