[llvm-dev] question about llvm partial unrolling/runtime unrolling

[Frances Tzeng via llvm-dev]

Hi Hal,

  Thanks for the response. I think I found the reason. ( not the debug
message above)
  My loop count is from -n to n, and it fails the
"isa<SCEVCouldNotCompute>(TripCountSC)" check and exit.


Thanks,
Frances


On Fri, Oct 16, 2015 at 9:29 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Frances Tzeng" <francestzeng at gmail.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: llvm-dev at lists.llvm.org
> > Sent: Friday, October 16, 2015 11:19:25 AM
> > Subject: Re: [llvm-dev] question about llvm partial unrolling/runtime
> unrolling
> >
> >
> > Hi Hal,
> >
> >
> >
> > I did
> >
> >
> > opt.exe -S -debug -loop-unroll -unroll-runtime=true -unroll-count=4
> > csShader.ll
> >
> >
> >
> > and it prints out:
> >
> >
> > Args: opt.exe -S -debug -loop-unroll -unroll-runtime=true
> > -unroll-count=4 csShader.ll
> >
> >
> > Loop Unroll: F[build_cs_5_0] Loop %loop_entry
> > Loop Size = 82
> > partially unrolling with count: 1
> >
> >
>
> Why does it compute a count of 1? If you use a debugger (and/or insert
> some additional print statements) around here in
> lib/Transforms/Scalar/LoopUnrollPass.cpp you should be able to figure it
> out:
>
>   } else if (Unrolling == Runtime) {
>     ...
>     // 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;
>     }
>     if (Count > UP.MaxCount)
>       Count = UP.MaxCount;
>     DEBUG(dbgs() << "  partially unrolling with count: " << Count << "\n");
>   }
>
>  -Hal
>
> >
> >
> > Thanks,
> > Frances
> >
> >
> > On Thu, Oct 15, 2015 at 9:35 PM, Hal Finkel < hfinkel at anl.gov >
> > wrote:
> >
> >
> >
> >
> >
> >
> >
> >
> > From: "Frances Tzeng via llvm-dev" < llvm-dev at lists.llvm.org >
> > To: llvm-dev at lists.llvm.org
> > Sent: Monday, October 12, 2015 6:13:25 PM
> > Subject: [llvm-dev] question about llvm partial unrolling/runtime
> > unrolling
> >
> >
> > Hi,
> >
> >
> >
> > I am trying to do loop unrolling with loops that don't have constant
> > loop counter. It is highly appreciated if anyone can help me on
> > this.
> >
> >
> >
> >
> > What I want to do is to turn
> >
> >
> > loop (n)
> > {
> > <loop body>
> > }
> >
> >
> > into
> >
> >
> >
> > loop (n/4)
> > {
> > <loop body>
> > <loop body>
> > <loop body>
> > <loop body>
> > }
> >
> >
> > loop (n%4)
> > {
> > <loop body>
> > }
> >
> >
> > I set allowPartial and Runtime to both 1 (
> > llvm::createLoopUnrollPass(Threshold, count, 1, 1) )
> > Also overload the UnrollingPreferences structure to gives values to
> > all the Partial* members, but the loop still doesn't unroll.
> >
> >
> > The unrolling process hits this code in LoopUnrollRuntime.cpp
> >
> >
> >
> > // Only unroll loops with a computable trip count and the trip count
> > needs
> > // to be an int value (allowing a pointer type is a TODO item)
> > const SCEV *BECountSC = SE->getBackedgeTakenCount(L);
> > if (isa<SCEVCouldNotCompute>(BECountSC) ||
> > !BECountSC->getType()->isIntegerTy())
> > return false;
> >
> >
> > BECountSC=0xcccccccc and returns false here.
> >
> >
> > Based on the comments it looks like I still need a constant loop
> > counter. Is there a way to unroll with non-constant loop counter as
> > in the example above? Computable is not the same as constant. With
> > runtime loop unrolling enabled, you can certainly unroll a loop with
> > a runtime trip count. If you run with -debug=loop-unroll, what does
> > it say regarding your loop?
> >
> > -Hal
> >
> >
> >
> >
> >
> >
> >
> >
> > Thanks,
> > Frances
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
> >
> > --
> >
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> >
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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