[LLVMdev] Enabling the SLP vectorizer by default for -O3

Tue Jul 23 16:33:41 PDT 2013

----- Original Message -----
> 
> 
> Hi,
> 
> 
> Sorry for the delay in response. I measured the code size change and
> noticed small changes in both directions for individual programs. I
> found a 30k binary size growth for the entire testsuite + SPEC. I
> attached an updated performance report that includes both compile
> time and performance measurements.
> 

I think that these number look good. Regarding the performance regressions:

This looks like noise:
MultiSource/Benchmarks/McCat/08-main/main 44.40% 0.0277 0.0400 0.0000

For these two:
MultiSource/Benchmarks/Olden/bh/bh 19.73% 1.1547 1.3825 0.0017
MultiSource/Benchmarks/Bullet/bullet 7.30% 3.6130 3.8767 0.0069
can you run them on a different CPU and see how generic these slowdowns are?

Thanks again,
Hal

> 
> Thanks,
> Nadav
> 
> 
> 
> On Jul 14, 2013, at 10:55 PM, Nadav Rotem < nrotem at apple.com > wrote:
> 
> 
> 
> 
> 
> On Jul 14, 2013, at 9:52 PM, Chris Lattner < clattner at apple.com >
> wrote:
> 
> 
> 
> 
> On Jul 13, 2013, at 11:30 PM, Nadav Rotem < nrotem at apple.com > wrote:
> 
> 
> 
> Hi,
> 
> LLVM’s SLP-vectorizer is a new pass that combines similar independent
> instructions in a straight-line code. It is currently not enabled by
> default, and people who want to experiment with it can use the clang
> command line flag “-fslp-vectorize”. I ran LLVM’s test suite with
> and without the SLP vectorizer on a Sandybridge mac (using SSE4, w/o
> AVX). Based on my performance measurements (below) I would like to
> enable the SLP-vectorizer by default on -O3. I would like to hear
> what others in the community think about this and give other people
> the opportunity to perform their own performance measurements.
> 
> This looks great Nadav. The performance wins are really big. How you
> investigated the bh and bullet regression though?
> 
> 
> Thanks. Yes, I looked at both. The hot function in BH is “gravsub”.
> The vectorized IR looks fine and the assembly looks fine, but for
> some reason Instruments reports that the first vector-subtract
> instruction takes 18% of the time. The regression happens both with
> the VEX prefix and without. I suspected that the problem is the
> movupd's that load xmm0 and xmm1. I started looking at some
> performance counters on Friday, but I did not find anything
> suspicious yet.
> 
> +0x00 movupd 16(%rsi), %xmm0
> +0x05 movupd 16(%rsp), %xmm1
> +0x0b subpd %xmm1, %xmm0 <———— 18% of the runtime of bh ?
> +0x0f movapd %xmm0, %xmm2
> +0x13 mulsd %xmm2, %xmm2
> +0x17 xorpd %xmm1, %xmm1
> 
> +0x1b addsd %xmm2, %xmm1
> 
> 
> I spent less time on Bullet. Bullet also has one hot function
> (“resolveSingleConstraintRowLowerLimit”). On this code the
> vectorizer generates several trees that use the <3 x float> type.
> This is risky because the loads/stores are inefficient, but
> unfortunately triples of RGB and XYZ are very popular in some
> domains and we do want to vectorize them. I skimmed through the IR
> and the assembly and I did not see anything too bad. The next step
> would be to do a binary search on the places where the vectorizer
> fires to locate the bad pattern.
> 
> 
> On AVX we have another regression that I did not mention: Flops-7.
> When we vectorize we cause more spills because we do a poor job
> scheduling non-destructive source instructions (related to PR10928).
> Hopefully Andy’s scheduler will fix this regression once it is
> enabled.
> 
> 
> I did not measure code size, but I did measure compile time. There
> are 4-5 workloads (not counting workloads that run below 0.5
> seconds) where the compile time increase is more than 5%. I am aware
> of a problem in the (quadratic) code that looks for consecutive
> stores. This code calls SCEV too many times. I plan to fix this.
> 
> 
> Thanks,
> Nadav
> 
> 
> 
> 
> 
> 
> We should at least understand what is going wrong there. bh is pretty
> tiny, so it should be straight-forward. It would also be really
> useful to see what the code size and compile time impact is.
> 
> -Chris
> 
> 
> 
> 
> — Performance Gains —
> SingleSource/Benchmarks/Misc/matmul_f64_4x4 -53.68%
> MultiSource/Benchmarks/Olden/power/power -18.55%
> MultiSource/Benchmarks/TSVC/LoopRerolling-flt/LoopRerolling-flt
> -14.71%
> SingleSource/Benchmarks/Misc/flops-6 -11.02%
> SingleSource/Benchmarks/Misc/flops-5 -10.03%
> MultiSource/Benchmarks/TSVC/LinearDependence-flt/LinearDependence-flt
> -8.37%
> External/Nurbs/nurbs -7.98%
> SingleSource/Benchmarks/Misc/pi -7.29%
> External/SPEC/CINT2000/252_eon/252_eon -5.78%
> External/SPEC/CFP2006/444_namd/444_namd -4.52%
> External/SPEC/CFP2000/188_ammp/188_ammp -4.45%
> MultiSource/Applications/SIBsim4/SIBsim4 -3.58%
> MultiSource/Benchmarks/TSVC/LoopRerolling-dbl/LoopRerolling-dbl
> -3.52%
> SingleSource/Benchmarks/Misc-C++/Large/sphereflake -2.96%
> MultiSource/Benchmarks/TSVC/LinearDependence-dbl/LinearDependence-dbl
> -2.75%
> MultiSource/Benchmarks/VersaBench/beamformer/beamformer -2.70%
> MultiSource/Benchmarks/TSVC/NodeSplitting-dbl/NodeSplitting-dbl
> -1.95%
> SingleSource/Benchmarks/Misc/flops -1.89%
> SingleSource/Benchmarks/Misc/oourafft -1.71%
> MultiSource/Benchmarks/mafft/pairlocalalign -1.16%
> External/SPEC/CFP2006/447_dealII/447_dealII -1.06%
> 
> — Regressions —
> MultiSource/Benchmarks/Olden/bh/bh 22.47%
> MultiSource/Benchmarks/Bullet/bullet 7.31%
> SingleSource/Benchmarks/Misc-C++-EH/spirit 5.68%
> SingleSource/Benchmarks/SmallPT/smallpt 3.91%
> 
> Thanks,
> Nadav
> 
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory