[llvm] r207940 - LoopUnroll: If we're doing partial unrolling, use the PartialThreshold to limit unrolling.
Hal Finkel
hfinkel at anl.gov
Tue May 6 00:26:07 PDT 2014
----- Original Message -----
> From: "Louis Gerbarg" <lgg at apple.com>
> To: "Benjamin Kramer" <benny.kra at gmail.com>
> Cc: "llvm-commits" <llvm-commits at cs.uiuc.edu>
> Sent: Monday, May 5, 2014 9:21:32 PM
> Subject: Re: [llvm] r207940 - LoopUnroll: If we're doing partial unrolling, use the PartialThreshold to limit
> unrolling.
>
> I am seeing what appears to be significant regressions on some of the
> nightly tests from this patch. For example the following benches all
> show 50-100% slowdowns when I apply the patch:
>
> SingleSource/Benchmarks/Stanford/Bubblesort
> SingleSource/Benchmarks/Polybench/linear-algebra/solvers/dynprog/dynprog
> MultiSource/Benchmarks/FreeBench/neural/neural
To be clear, on what system is that?
Generally speaking, if you use -mllvm -x86-partial-unrolling-threshold=N as you increase the value of N does that improve things for you? The current values for the unrolling were taken from the optimization manual (and I'm guessing is some value like 28 for your system; see X86TTI::getUnrollingPreferences in lib/Target/X86/X86TargetTransformInfo.cpp), but was not actually tuned experimentally. Perhaps this could use some improvement.
-Hal
>
> Louis
>
> On May 5, 2014, at 3:09 AM, Benjamin Kramer <benny.kra at gmail.com>
> wrote:
>
> >
> > On 05.05.2014, at 01:18, Nadav Rotem <nrotem at apple.com> wrote:
> >
> >> 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?
> >
> > I didn't find any significant runtime change in the test suite or
> > when trying some of the synthetic benchmarks that were showing
> > extreme unrolling. Code size is a bit better though.
> >
> > I initially observed this behavior when looking into the vectorizer
> > ( http://llvm.org/bugs/show_bug.cgi?id=14985 ) For the trivial
> > loop in the test case we used to unroll 2x in the loop vectorizer
> > (that's a good thing) and then up to another 8x in the loop
> > unroller, when we're targeting core2 or higher. I asked Hal and he
> > agreed that we were unrolling too much.
> >
> > I guess it makes sense to actually use the threshold derived from
> > the processor manuals to drive unrolling instead of assuming that
> > more unrolling is better :)
> >
> > - Ben
> >
> >>
> >> 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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> >>
> >
> >
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--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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