[PATCH] GVN: If a branch has two identical successors, we cannot declare either dead.

[Sanjay Patel]

I'm by no means a GVN expert, but the code change looks trivially safer, so LGTM.

I'd like to improve the test case though:

1. Please minimize/edit to make it more obvious how this can cause a miscompile.
2. Add a comment to explain why the test exists (something similar to the code comment should do it).

This may not be minimal, and with the 'undefs' I don't think it can actually cause a miscompile, but I whittled it down this far to see what happens before and after the patch:

  define void @widget() {
  entry:
  br label %bb2
  
  bb2:
  %t1 = phi i64 [ 0, %entry ], [ %t5, %bb7 ]
  %t2 = add i64 %t1, 1
  %t3 = icmp ult i64 0, %t2
  br i1 %t3, label %bb3, label %bb4
  
  bb3:
  %t4 = call i64 @f()
  br label %bb4
  
  bb4:
  ; CHECK-NOT: phi {{.*}} undef
  %foo = phi i64 [ %t4, %bb3 ], [ 0, %bb2 ]
  br i1 undef, label %bb5, label %bb6
  
  bb5:
  br i1 true, label %bb7, label %bb7
  
  bb6:
  br i1 true, label %bb7, label %bb7
  
  bb7:
  %t5 = add i64 %t1, 1
  br i1 undef, label %bb2, label %bb8
  
  bb8:
  ret void

}


http://reviews.llvm.org/D10726

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