[PATCH] D42417: [SCEV] Fix isLoopEntryGuardedByCond

[Serguei Katkov via llvm-commits]

Hi Sanjoy, thank you for looking into this.

Hmmm, why do look into howManyLessThans?

I guess the (2) is about ScalarEvolution::getSignExtendExpr...


do I miss anything?

Thank you,
Serguei.

> -----Original Message-----
> From: Sanjoy Das [mailto:sanjoy at playingwithpointers.com]
> Sent: Friday, January 26, 2018 12:15 PM
> To: reviews+D42417+public+54a7b9b7b0be118c at reviews.llvm.org
> Cc: Serguei Katkov <serguei.katkov at azul.com>; Maxim Kazantsev
> <max.kazantsev at azul.com>; Anna Thomas <anna at azul.com>;
> dorit.nuzman at intel.com; Philip Reames <listmail at philipreames.com>; llvm-
> commits <llvm-commits at lists.llvm.org>; Junbum Lim
> <junbuml at codeaurora.org>; Wei Mi <wmi at google.com>;
> florian.hahn at arm.com
> Subject: Re: [PATCH] D42417: [SCEV] Fix isLoopEntryGuardedByCond
> 
> 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/st
> > eps/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/b
> > uilds/3854/steps/test-suite/logs/test.log
> >
> > I hope it helps.
> >
> >
> > https://reviews.llvm.org/D42417
> >
> >
> >