[PATCH] Fix loop rerolling pass failure with non-consant loop lower

[David Peixotto]

Hal, thanks for the review. I made all your suggested changes and committed in r198425.

> -----Original Message-----
> From: Hal Finkel [mailto:hfinkel at anl.gov]
> Sent: Wednesday, January 01, 2014 4:39 PM
> To: David Peixotto
> Cc: llvm-commits at cs.uiuc.edu
> Subject: Re: [PATCH] Fix loop rerolling pass failure with non-consant loop
> lower
> 
> David,
> 
> Thanks for working on this! This looks good; a few minor comments:
> 
> +        // Iteration count scev minus 1
> 
> Please capitalize SCEV.
> 
> +        const SCEV *ICSCEVm1 = SE->getMinusSCEV(ICSCEV,
> + SE->getConstant(ICSCEV->getType(), 1));
> 
> This line is too long. Also, please name this variable ICMinus1SCEV for
> consistency with the other variables.
> 
> +        Value *ICm1; // Iteration count minus 1
> 
> Please name this ICMinus1.
> 
> +
> +
> +attributes #0 = { nounwind "less-precise-fpmad"="false"
> +"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
> +"no-infs-fp-math"="false" "no-nans-fp-math"="false"
> +"stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
> +"use-soft-float"="true" }
> +
> +!llvm.ident = !{!0}
> +
> +!0 = metadata !{metadata !"Clang 3.1"}
> +!1 = metadata !{metadata !2, metadata !2, i64 0}
> +!2 = metadata !{metadata !"int", metadata !3, i64 0}
> +!3 = metadata !{metadata !"omnipotent char", metadata !4, i64 0}
> +!4 = metadata !{metadata !"Simple C/C++ TBAA"}
> +!5 = metadata !{metadata !6, metadata !6, i64 0}
> +!6 = metadata !{metadata !"float", metadata !3, i64 0}
> 
> Please remove any unnecessary attributes and metadata.
> 
> With these changes, LGTM.
> 
>  -Hal
> 
> ----- Original Message -----
> > From: "David Peixotto" <dpeixott at codeaurora.org>
> > To: llvm-commits at cs.uiuc.edu
> > Cc: "Hal Finkel" <hfinkel at anl.gov>
> > Sent: Friday, December 20, 2013 7:09:41 PM
> > Subject: [PATCH] Fix loop rerolling pass failure with non-consant loop
> > lower
> >
> > Proposed fix for PR18290: http://llvm.org/bugs/show_bug.cgi?id=18290
> >
> > The loop rerolling pass was failing with an assertion failure from a
> > failed cast on loops like this:
> >
> >   void foo(int *A, int *B, int m, int n) {
> >     for (int i = m; i < n; i+=4) {
> >       A[i+0] = B[i+0] * 4;
> >       A[i+1] = B[i+1] * 4;
> >       A[i+2] = B[i+2] * 4;
> >       A[i+3] = B[i+3] * 4;
> >     }
> >   }
> >
> > The code was casting the SCEV-expanded code for the new induction
> > variable to a phi-node. When the loop had a non-constant lower bound,
> > the SCEV expander would end the code expansion with an add insted of a
> > phi node and the cast would fail.
> >
> > It looks like the cast to a phi node was only needed to get the
> > induction variable value coming from the backedge to compute the end
> > of loop condition. This patch changes the loop reroller to compare the
> > induction variable to the number of times the backedge is taken
> > instead of the iteration count of the loop. In other words, we stop
> > the loop when the current value of the induction variable ==
> > IterationCount-1. Previously, the comparison was comparing the
> > induction variable value from the next iteration == IterationCount.
> >
> > This problem only seems to occur on 32-bit targets. For some reason,
> > the loop is not rerolled on 64-bit targets.
> >
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory