[PATCH] D120551: [SCEV] Enable verification in LoopPM

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

The pr35406.ll failure can be reduced to this:

  define i32 @test() {
  entry:
    br label %loop1
  
  loop1:
    br i1 false, label %loop2, label %loop1.latch
  
  loop2:
    %iv = phi i32 [ 0, %loop1 ], [ %iv.next, %loop2 ]
    %iv.next = add nuw nsw i32 %iv, 1
    %cmp = icmp ne i32 %iv.next, 1
    br i1 %cmp, label %loop1.latch, label %loop2
  
  loop1.latch:
    br label %loop1
  }
  
  ; opt -indvars
  
  define i32 @test() {
  entry:
    br label %loop1
  
  loop1:                                            ; preds = %loop1.latch, %entry
    br i1 true, label %loop2.preheader, label %loop1.latch
  
  loop2.preheader:                                  ; preds = %loop1
    br label %loop2
  
  loop2:                                            ; preds = %loop2.preheader, %loop2
    br i1 true, label %loop1.latch.loopexit, label %loop2
  
  loop1.latch.loopexit:                             ; preds = %loop2
    br label %loop1.latch
  
  loop1.latch:                                      ; preds = %loop1.latch.loopexit, %loop1
    br label %loop1
  }

In this case the BECount of the inner loop legitimately changes from 1 to 0, and this isn't wrong, because the loop is unreachable.

We make use of the fact that the loop is unreachable due to https://github.com/llvm/llvm-project/blob/20a093e2bc310283ec216ab47fddb68f0bdaa6f0/llvm/lib/Analysis/ScalarEvolution.cpp#L11024-L11027, introduced in D98706 <https://reviews.llvm.org/D98706>. And apparently this is already a known issue, with a variant reported in: https://github.com/llvm/llvm-project/issues/50523

I'm not really sure what to do about that. I think the options are a) give up on BECount verification entirely, b) drop that isImpliedCond() change, as I think it is of very limited practical value (the next SimplifyCFG will eliminate that code entirely, so it is irrelevant how we optimize it) or c) try to limit BECount verification to loops that are reachable, while taking into account constant branches.


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  https://reviews.llvm.org/D120551/new/

https://reviews.llvm.org/D120551