[PATCH] D42417: [SCEV] Fix isLoopEntryGuardedByCond

[Sanjoy Das via llvm-commits]

I clicked through (2) and that does look suspicious -- the only call
to isLoopEntryGuardedByCond in howManyLessThans is

  // as if the backedge is taken at least once max(End,Start) is End and so the
  // result is as above, and if not max(End,Start) is Start so we get a backedge
  // count of zero.
  const SCEV *BECount;
  if (isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(Start, Stride), RHS))
    BECount = BECountIfBackedgeTaken;
  else {
    End = IsSigned ? getSMaxExpr(RHS, Start) : getUMaxExpr(RHS, Start);
    BECount = computeBECount(getMinusSCEV(End, Start), Stride, false);

but we have

  // Avoid weird loops
  if (!IV || IV->getLoop() != L || !IV->isAffine())
    return getCouldNotCompute();

  ...
  const SCEV *Start = IV->getStart();

  ...
  const SCEV *Stride = IV->getStepRecurrence(*this);


I may be missing something, but altogether this should imply that
getMinusSCEV(Start, Stride) is, in fact, available at the loop entry.

-- Sanjoy

On Wed, Jan 24, 2018 at 9:19 PM, Serguei Katkov via Phabricator
<reviews at reviews.llvm.org> wrote:
> skatkov added a comment.
>
> Hi Sanjoy,
>
> The number of buildbot failures were so big but I remember the following case (if I do not miss anything):
>
> 1. All Usages in IRCE caused an assert in unit testing
> 2. Crash was in getSignExtendExpr: the prblem was with LHS. Might be it is the same issue as 3rd one. Example: http://lab.llvm.org:8011/builders/clang-s390x-linux/builds/14076/steps/ninja%20check%201/logs/stdio or http://lab.llvm.org:8011/builders/clang-s390x-linux-lnt/builds/4265/steps/ninja%20check%201/logs/stdio
> 3. Most popular ScalarEvolution::isImpliedCondOperandsViaNoOverflow (LHS). Example : http://lab.llvm.org:8011/builders/clang-cmake-x86_64-avx2-linux-perf/builds/3854/steps/test-suite/logs/test.log
>
> I hope it helps.
>
>
> https://reviews.llvm.org/D42417
>
>
>