[LLVMdev] [Polly] Update of Polly compile-time performance on LLVM test-suite

Star Tan tanmx_star at yeah.net
Wed Jul 31 21:23:21 PDT 2013

At 2013-07-31 22:50:57,"Tobias Grosser" <tobias at grosser.es> wrote:

>On 07/30/2013 10:03 AM, Star Tan wrote:
>> Hi Tobias and all Polly developers,
>> I have re-evaluated the Polly compile-time performance using newest
>> LLVM/Polly source code.  You can view the results on
>> <>.
>> Especially, I also evaluated ourr187102 patch file that avoids expensive
>> failure string operations in normal execution. Specifically, I evaluated
>> two cases for it:
>> Polly-NoCodeGen: clang -O3 -load LLVMPolly.so -mllvm
>> -polly-optimizer=none -mllvm -polly-code-generator=none
>> Polly-Opt: clang -O3 -load LLVMPolly.so -mllvm -polly
>> The "Polly-NoCodeGen" case is mainly used to compare the compile-time
>> performance for the polly-detect pass. As shown in the results, our
>> patch file could significantly reduce the compile-time overhead for some
>> benchmarks such as tramp3dv4
>> <> (24.2%), simple_types_constant_folding
>> <>(12.6%),
>> oggenc
>> <>(9.1%),
>> loop_unroll
>> <>(7.8%)
>Very nice!
>Though I am surprised to also see performance regressions. They are all 
>in very shortly executing kernels, so they may very well be measuring 
>noice. Is this really the case?
Yes, it seems that shortly executing benchmarks always show huge unexpected noise even we run 10 samples for a test. 
I have changed the ignore_small abs value to 0.05 from the original 0.01, which means benchmarks with the performance delta less then 0.05s would be skipped. In that case, the results seem to be much more stable. 
However, I have noticed that there are many other Polly patches between the two version r185399 and r187116. They may also affect the compile-time performance. I would re-evaluate LLVM-testsuite to see the performance improvements caused only by our  
>Also, it may be interesting to compare against the non-polly case to see
>how much overhead there is still due to our scop detetion.
>> The "Polly-opt" case is used to compare the whole compile-time
>> performance of Polly. Since our patch file mainly affects the
>> Polly-Detect pass, it shows similar performance to "Polly-NoCodeGen". As
>> shown in results, it reduces the compile-time overhead of some
>> benchmarks such as tramp3dv4
>> <> (23.7%), simple_types_constant_folding
>> <>(12.9%),
>> oggenc
>> <>(8.3%),
>> loop_unroll
>> <>(7.5%)
>> At last, I also evaluated the performance of the ScopBottomUp patch that
>> changes the up-down scop detection into bottom-up scop detection.
>> Results can be viewed by:
>> pNoCodeGen-ScopBottomUp: clang -O3 -load LLVMPolly.so (v.s.
>> LLVMPolly-ScopBottomUp.so)  -mllvm -polly-optimizer=none -mllvm
>> -polly-code-generator=none
>> pOpt-ScopBottomUp: clang -O3 -load LLVMPolly.so (v.s.
>> LLVMPolly-ScopBottomUp.so)  -mllvm -polly
>> (*Both of these results are based on LLVM r187116, which has included
>> the r187102 patch file that we discussed above)
>> Please notice that this patch file will lead to some errors in
>> Polly-tests, so the data shown here can not be regards as confident
>> results. For example, this patch can significantly reduce the
>> compile-time overhead of SingleSource/Benchmarks/Shootout/nestedloop
>> <> only
>> because it regards the nested loop as an invalid scop and skips all
>> following transformations and optimizations. However, I evaluated it
>> here to see its potential performance impact.  Based on the results
>> shown on
>> we can see detecting scops bottom-up may further reduce Polly
>> compile-time by more than 10%.
>Interesting. For some reason it also regresses huffbench quite a bit. 
This is because the ScopBottomUp patch file invalids the scop detection for huffbench. The run-time of huffbench with different options are shown as follows:
clang: 19.1680s  (see runid=14)
polly without ScopBottomUp patch file: 14.8340s (see runid=16)
polly with ScopBottomUp patch file: 19.2920s (see runid=21)
As you can see, the ScopBottomUp patch file shows almost the same execution performance with clang. That is because no invalid scops is detected with this patch file at all.

>:-( I think here an up-to-date non-polly to polly comparision would come 
>handy to see which benchmarks we still see larger performance 
>regressions. And if the bottom-up scop detection actually helps here.
>As this is a larger patch, we should really have a need for it before 
>switching to it.
I have evaluated Polly compile-time performance for the following options:
  clang: clang -O3  (runid: 14) 
  pBasic: clang -O3 -load LLVMPolly.so (runid:15) 
  pNoGen: pollycc -O3 -mllvm -polly-optimizer=none -mllvm -polly-code-generator=none (runid:16) 
  pNoOpt: pollycc -O3 -mllvm -polly-optimizer=none (runid:17) 
  pOpt: pollycc -O3 (runid:18)
For example, you can view the comparison between "clang" and "pNoGen" with:
It shows that without optimizer and code generator, Polly would lead to less then 30% extra compile-time overhead. 
For the execution performance, it is interesting that pNoGen not only significantly improves the execution performance for some benchmarks (nestedloop/huffbench) but also significantly reduces the execution performance for another set of benchmarks (gcc-loops/lpbench).

Star Tan
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