[polly] r240689 - Enable ISL's small integer optimization

[Ramakrishna Upadrasta]

On Fri, Jun 26, 2015 at 2:20 PM, Tobias Grosser <tobias at grosser.es> wrote:

> On 06/26/2015 08:55 AM, Tobias Grosser wrote:
>
>> On 06/25/2015 10:47 PM, Michael Kruse wrote:
>>
>>> Author: meinersbur
>>> Date: Thu Jun 25 15:47:35 2015
>>> New Revision: 240689
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=240689&view=rev
>>> Log:
>>> Enable ISL's small integer optimization
>>>
>>> Summary:
>>> With small integer optimization (short: sio) enabled, ISL uses 32 bit
>>> integers for its arithmetic and only falls back to a big integer library
>>> (in the case of Polly: IMath) if an operation's result is too large.
>>> This gives a massive performance boost for most application using ISL.
>>> For instance, experiments with ppcg (polyhedral source-to-source
>>> compiler) show speed-ups of 5.8 (compared to plain IMath), respectively
>>> 2.7 (compared to GMP).
>>>
>>> In Polly, a smaller fraction of the total compile time is taken by ISL,
>>> but the speed-ups are still very significant. The buildbots measure
>>> compilation speed-up up to 1.8 (oourafft, floyd-warshall, symm). All
>>> Polybench benchmarks compile in at least 9% less time, and about 20%
>>> less on average.
>>>
>>> Detailed Polybench compile time results (median of 10):
>>> correlation     -25.51%
>>> covariance      -24.82%
>>> 2mm             -26.64%
>>> 3mm             -28.69%
>>> atax            -13.70%
>>> bicg            -10.78%
>>> cholesky        -40.67%
>>> doitgen         -11.60%
>>> gemm            -11.54%
>>> gemver          -10.63%
>>> gesummv         -11.54%
>>> mvt              -9.43%
>>> symm            -41.25%
>>> syr2k           -14.71%
>>> syrk            -14.52%
>>> trisolv         -17.65%
>>> trmm             -9.78%
>>> durbin          -19.32%
>>> dynprog          -9.09%
>>> gramschmidt     -15.38%
>>> lu              -21.77%
>>> floyd-warshall  -42.71%
>>> reg_detect      -41.17%
>>> adi             -36.69%
>>> fdtd-2d         -32.61%
>>> fdtd-apml       -21.90%
>>> jacobi-1d-imper  -9.41%
>>> jacobi-2d-imper -27.65%
>>> seidel-2d       -31.00%
>>>
>>
>>
>> Very nice. Here the corresponding performance bot results:
>>
>> "clang -O3 -mllvm -polly" BEFORE
>> vs.
>> "clang -O3 -mllvm -polly" AFTER
>>
>> http://llvm.org/perf/db_default/v4/nts/27877?compare_to=27869
>>
>> "clang -O3"
>> vs.
>> "clang -O3 -mllvm -polly" BEFORE
>>
>> http://llvm.org/perf/db_default/v4/nts/27869?compare_to=27876
>>
>> "clang -O3"
>> vs.
>> "clang -O3 -mllvm -polly" AFTER
>>
>> http://llvm.org/perf/db_default/v4/nts/27877?compare_to=27876
>>
>>
>>



> Instead of a 450% slowdown compared to LLVM in terms of compile time,
>> the largest slowdown is now 200% (0.13 s to 0.44s) and most kernels show
>> even less slowdown. A larger part of the remaining slowdown (about half
>> of it) is increased LLVM codegen time due to code versioning.
>> 200% slowdown may sound a lot, but these are indeed the loop kernels we
>> optimize, meaning their compile-time overall is commonly less than a
>> second. Compile time impact on entire applications is commonly a lot less.
>>
>
Great indeed. I guess we can target for a max of 100% slowdown next?


There is still some headroom for further optimizations, which we are
>> aiming for. However, for now, thanks again to Michael Kruse for this
>> great patch and to Pratik Bhatu for the preparations and preliminary
>> studies that allowed us to understand the performance benefits such an
>> optimization give.
>>
>
> For the record, besides Pratik's nice prepartions, Michael had already
> worked on a similar optimization for the gmp backend in the context of his
> Molly project and as a result also obtained valuable insights (and
> experience) that have proven very useful when doing this work.


I totally agree. Having Michael's prior work and experience was indeed the
major factor!
Merci beaucoup/Danke Michael!

Best Regards
Ramakrishna
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