[PATCH] D129753: [SCEV] Prove condition invariance via context

[Mikael Holmén via Phabricator via llvm-commits]

uabelho added a comment.

Hello,

I think I'm seeing a miscompile with this patch.
Reproduce with:

  opt --passes="indvars" bbi-72689.ll -S -o -

indvars does

  <   %cmp22.not.i = icmp ult i32 %storemerge13.i, 2
  ---
  >   %cmp22.not.i = icmp ult i32 0, 2

which I think is incorrect.
We have

  [...]
  %storemerge13.i = phi i32 [ 0, %entry ], [ %dec.i, %for.inc32.i ]
  %cmp22.not.i = icmp ult i32 %storemerge13.i, 2
  [...]
  %dec.i = add i32 %storemerge13.i, -1
  [...]

so we start at 0, then add -1 so we get -1, and then do an ult compare between -1 (0xffffffff) and 2, which I suppose should be false?
But indvars changes the compare to icmp ult i32 0, 2 which is always true?

The end result is that the function loops forever instead of returning -1.

F24200119: bbi-72689.ll <https://reviews.llvm.org/F24200119>


Repository:
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https://reviews.llvm.org/D129753