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

[Hal Finkel via llvm-dev]

----- 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
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> 
> 
> 
> --
> 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory