[LLVMdev] Is this a bug with loop unrolling and TargetTransformInfo ?

[Tom Stellard]

On Wed, Feb 04, 2015 at 02:05:11PM -0600, Hal Finkel wrote:
> ----- Original Message -----
> > From: "Tom Stellard" <tom at stellard.net>
> > To: llvmdev at cs.uiuc.edu
> > Sent: Wednesday, February 4, 2015 1:45:26 PM
> > Subject: [LLVMdev] Is this a bug with loop unrolling and TargetTransformInfo	?
> > 
> > Hi,
> > 
> > I ran into this issue recently and wanted to know if it was a bug or
> > expected behavior.
> > 
> > In the R600 backend's TargetTransformInfo implementation, we were
> > setting
> > UnrollingPreferences::Count = UINT_MAX.  This was a mistake as we
> > should have been
> > setting UnrollingPreferences::MaxCount instead.  However, as a result
> > of setting
> > Count to UINT_MAX, this loop would be unrolled 15 times:
> 
> A bug regardless? Where did 15 come from?
> 

It's the value computed by this loop on line 453 of the LoopUnrolPass.
Inputs are:

Count = 4294967295
PartialThreshold = 150
UnrolledSize = 34359738362

// Reduce unroll count to be the largest power-of-two factor of
// the original count which satisfies the threshold limit.
while (Count != 0 && UnrolledSize > PartialThreshold) {
  Count >>= 1;
  UnrolledSize = (LoopSize-2) * Count + 2;
}

I think the bug here may actually be in
LoopUnroll::selectUnrollCount().  It looks like it ends up returning UP.Count
in some cases when it is larger than TripCount.

-Tom
> 
> > 
> > if (b + 4 > a) {
> >   for (int i = 0; i < 4; i++, b++) {
> >     if (b + 1 <= a)
> >       *(dst + c + b) = 0;
> >      else
> >        break;
> >   }
> > }
> > 
> > Is this the expected behavior?  Is the loop unroll pass supposed to
> > use the
> > value of UnrollingPreferecnes::Count even if it is clearly wrong?
> > 
> > I have an LLVM IR test case for this issue attached to this patch:
> > http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150202/257276.html
> > 
> > -Tom
> > _______________________________________________
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > 
> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory