[PATCH] D101722: [SCEV] Don't require ControlsExit for gt/lt NoWrap

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

If I'm understanding correctly, the issue you're describing is that if %len is ULONG_MAX in the following loop, on trunk LLVM, the backedge-taken count comes out to zero.  This is because of the math in ScalarEvolution::computeBECount: we try to compute `ceil(a/b)` by converting it to `floor((a + b - 1) / b)`, which doesn't work.  There are different ways we could address this.  For example, we could change around the computation: we can compute `ceil(a/b)` by instead expanding it to `floor(a/b) + umin(a % b, 1)`.

I guess in the meantime, we should add the missing loopHasNoAbnormalExits() check.

  declare void @maybe_abort()
  define void @test1(i64 %len) {
  start:
    br label %loop
  
  loop:
    %iv = phi i64 [ 0, %start ], [ %iv.inc2, %loop ]
    %cmp1 = icmp ult i64 %iv, %len
    %iv.inc2 = add nuw i64 %iv, 2
    call void @maybe_abort()
    br i1 %cmp1, label %loop, label %exit
  
  exit:
    ret void
  }


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D101722/new/

https://reviews.llvm.org/D101722