[llvm] r207940 - LoopUnroll: If we're doing partial unrolling, use the PartialThreshold to limit unrolling.

[Nadav Rotem]

Hi Ben, 

Thanks for working on this. Overall it sounds like a good change and unrolling 8 times sounds way too high, even for small loops. Did you get a chance to measure the performance difference of this patch?  

Thanks,
Nadav


On May 4, 2014, at 12:12 PM, Benjamin Kramer <benny.kra at googlemail.com> wrote:

> Author: d0k
> Date: Sun May  4 14:12:38 2014
> New Revision: 207940
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=207940&view=rev
> Log:
> LoopUnroll: If we're doing partial unrolling, use the PartialThreshold to limit unrolling.
> 
> Otherwise we use the same threshold as for complete unrolling, which is
> way too high. This made us unroll any loop smaller than 150 instructions
> by 8 times, but only if someone specified -march=core2 or better,
> which happens to be the default on darwin.
> 
> Modified:
>    llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
>    llvm/trunk/test/Transforms/LoopUnroll/X86/partial.ll
> 
> Modified: llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp?rev=207940&r1=207939&r2=207940&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp Sun May  4 14:12:38 2014
> @@ -238,9 +238,12 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPa
>       return false;
>     }
>     uint64_t Size = (uint64_t)LoopSize*Count;
> -    if (TripCount != 1 && Size > Threshold) {
> -      DEBUG(dbgs() << "  Too large to fully unroll with count: " << Count
> -            << " because size: " << Size << ">" << Threshold << "\n");
> +    if (TripCount != 1 &&
> +        (Size > Threshold || (Count != TripCount && Size > PartialThreshold))) {
> +      if (Size > Threshold)
> +        DEBUG(dbgs() << "  Too large to fully unroll with count: " << Count
> +                     << " because size: " << Size << ">" << Threshold << "\n");
> +
>       bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial;
>       if (!AllowPartial && !(Runtime && TripCount == 0)) {
>         DEBUG(dbgs() << "  will not try to unroll partially because "
> 
> Modified: llvm/trunk/test/Transforms/LoopUnroll/X86/partial.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopUnroll/X86/partial.ll?rev=207940&r1=207939&r2=207940&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/LoopUnroll/X86/partial.ll (original)
> +++ llvm/trunk/test/Transforms/LoopUnroll/X86/partial.ll Sun May  4 14:12:38 2014
> @@ -76,5 +76,52 @@ for.end:
>   ret void
> }
> 
> +define zeroext i16 @test1(i16* nocapture readonly %arr, i32 %n) #0 {
> +entry:
> +  %cmp25 = icmp eq i32 %n, 0
> +  br i1 %cmp25, label %for.end, label %for.body
> +
> +for.body:                                         ; preds = %entry, %for.body
> +  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
> +  %reduction.026 = phi i16 [ %add14, %for.body ], [ 0, %entry ]
> +  %arrayidx = getelementptr inbounds i16* %arr, i64 %indvars.iv
> +  %0 = load i16* %arrayidx, align 2
> +  %add = add i16 %0, %reduction.026
> +  %sext = mul i64 %indvars.iv, 12884901888
> +  %idxprom3 = ashr exact i64 %sext, 32
> +  %arrayidx4 = getelementptr inbounds i16* %arr, i64 %idxprom3
> +  %1 = load i16* %arrayidx4, align 2
> +  %add7 = add i16 %add, %1
> +  %sext28 = mul i64 %indvars.iv, 21474836480
> +  %idxprom10 = ashr exact i64 %sext28, 32
> +  %arrayidx11 = getelementptr inbounds i16* %arr, i64 %idxprom10
> +  %2 = load i16* %arrayidx11, align 2
> +  %add14 = add i16 %add7, %2
> +  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
> +  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
> +  %exitcond = icmp eq i32 %lftr.wideiv, %n
> +  br i1 %exitcond, label %for.end, label %for.body
> +
> +for.end:                                          ; preds = %for.body, %entry
> +  %reduction.0.lcssa = phi i16 [ 0, %entry ], [ %add14, %for.body ]
> +  ret i16 %reduction.0.lcssa
> +
> +; This loop is too large to be partially unrolled (size=16)
> +
> +; CHECK-LABEL: @test1
> +; CHECK: br
> +; CHECK: br
> +; CHECK: br
> +; CHECK: br
> +; CHECK-NOT: br
> +
> +; CHECK-NOUNRL-LABEL: @test1
> +; CHECK-NOUNRL: br
> +; CHECK-NOUNRL: br
> +; CHECK-NOUNRL: br
> +; CHECK-NOUNRL: br
> +; CHECK-NOUNRL-NOT: br
> +}
> +
> attributes #0 = { nounwind uwtable }
> 
> 
> 
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