[llvm-dev] Enable vectorizer-maximize-bandwidth by default?

[Dehao Chen via llvm-dev]

On Fri, May 19, 2017 at 4:01 PM, Adam Nemet <anemet at apple.com> wrote:

>
> On May 18, 2017, at 3:30 PM, Dehao Chen via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> I'm proposing to make vectorizer-maximize-bandwidth on by default for
> loop vectorizer because it should generally help performance.
>
> I've tested the performance impact on Intel sandybridge machine with
> speccpu benchmarks:
>
>            Benchmark             Base:Reference   (1)
> -------------------------------------------------------
> spec/2006/fp/C++/444.namd                 26.84  -0.31%
> spec/2006/fp/C++/447.dealII               46.19  +0.89%
> spec/2006/fp/C++/450.soplex               42.92  -0.44%
> spec/2006/fp/C++/453.povray               38.57  -2.25%
> spec/2006/fp/C/433.milc                   24.54  -0.76%
> spec/2006/fp/C/470.lbm                    41.08  +0.26%
> spec/2006/fp/C/482.sphinx3                47.58  -0.99%
> spec/2006/int/C++/471.omnetpp             22.06  +1.87%
> spec/2006/int/C++/473.astar               22.65  -0.12%
> spec/2006/int/C++/483.xalancbmk           33.69  +4.97%
> spec/2006/int/C/400.perlbench             33.43  +1.70%
> spec/2006/int/C/401.bzip2                 23.02  -0.19%
> spec/2006/int/C/403.gcc                   32.57  -0.43%
> spec/2006/int/C/429.mcf                   40.35  +0.27%
> spec/2006/int/C/445.gobmk                 26.96  +0.06%
> spec/2006/int/C/456.hmmer                  24.4  +0.19%
> spec/2006/int/C/458.sjeng                 27.91  -0.08%
> spec/2006/int/C/462.libquantum            57.47  -0.20%
> spec/2006/int/C/464.h264ref               46.52  +1.35%
>
> geometric mean                                   +0.29%
>
>   Scores are benchmark specific.
>
> We do have regression on 453.povray, but it's due to secondary effects as
> all hot functions are the same. I've also tested the code size impact, it
> does not change for tested speccpu benchmarks.
>
>
> Can you please describe the config for the runs (optimization level,
> PGO/no-PGO, etc).
>

This is O2 build without PGO.


>
> It would be good to provide analysis for the changes >1%. I.e. we need to
> make sure that the improvements are not noise either ;).
>

Good point. I just examined all benchmarks with >1% "improvement". Turns
out they are all noises: the hot functions (with >1% total cycles) are all
identical. So the conclusion is: this change does not affect speccpu2006
performance.

Thanks,
Dehao


>
>
> I've prepared https://reviews.llvm.org/D33341 to do this.
>
> I really appreciate if the community can help test the performance impact
> of this change on other architectures so that we can decide if this should
> go target-dependent.
>
>
> I will run it on Cyclone/AArch64 next week.
>
> Adam
>
>
> Any comments/concerns?
>
> Thanks,
> Dehao
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