[llvm] r207940 - LoopUnroll: If we're doing partial unrolling, use the PartialThreshold to limit unrolling.
Louis Gerbarg
lgg at apple.com
Mon May 5 19:21:32 PDT 2014
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
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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