[PATCH] D71799: [Attributor] AAUndefinedBehavior: Check for branches on undef value.

[Stefanos Baziotis via Phabricator via llvm-commits]

baziotis added a comment.

In D71799#1794602 <https://reviews.llvm.org/D71799#1794602>, @uenoku wrote:

> In D71799#1794464 <https://reviews.llvm.org/D71799#1794464>, @baziotis wrote:
>
> > Alright, that makes sense!
> >  So, one quick question that I hope will help solve some issues: If `!SimplifiedV.hasValue()` but `ValueSimplifyAA.isKnown()`, then it is //known// that the value is `undef`?
>
>
> It can't happen but it is semantically correct.


It happens though :) Unless I messed up something. It happens with e.g. this code:

  define i1 @ret_undef() {
    ret i1 undef
  }
  
  define void @cond_br() {
    %cond = call i1 @ret_undef()
    br i1 %cond, label %t, label %e
  t:
    ret void
  e:
    ret void
  }

And in the Attributor:

  if (!SimplifiedV.hasValue()) {
    if (ValueSimplifyAA.isKnown())
      dbgs() << "IS IT UNDEF?\n";
    ...
  }

I see the message. Sorry btw that I don't know exactly how `AAValueSimplify` works. When I started this patch, I assumed it was in all's best interest
to not spend time in it right now, so I'm guessing from looking small pieces of its code.


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