llvm dev - Oct 2015 - 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?

Thanks,
Frances
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----- Original Message -----
> 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 
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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


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
>
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