[llvm-dev] (RFC) Adjusting default loop fully unroll threshold

[Mehdi Amini via llvm-dev]

> On Jan 30, 2017, at 10:49 AM, Dehao Chen via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Currently, loop fully unroller shares the same default threshold as loop dynamic unroller and partial unroller. This seems conservative because unlike dynamic/partial unrolling, fully unrolling will not affect LSD/ICache performance. In https://reviews.llvm.org/D28368 <https://reviews.llvm.org/D28368>, I proposed to double the threshold for loop fully unroller. This will change the codegen of several SPECCPU benchmarks:
> 
> Code size:
> 447.dealII 0.50%
> 453.povray 0.42%
> 433.milc 0.20%
> 445.gobmk 0.32%
> 403.gcc 0.05%
> 464.h264ref 3.62%
> 
> Compile Time:
> 447.dealII 0.22%
> 453.povray -0.16%
> 433.milc 0.09%
> 445.gobmk -2.43%
> 403.gcc 0.06%
> 464.h264ref 3.21%
> 
> Performance (on intel sandybridge):
> 447.dealII +0.07%
> 453.povray +1.79%
> 433.milc +1.02%
> 445.gobmk +0.56%
> 403.gcc -0.16%
> 464.h264ref -0.41%
> 

Can you clarify how to read these numbers? (I’m using +xx% to indicates a slowdown usually, it seems you’re doing the opposite?).

So considering 464.h264ref, does it mean it is 3.6% slower to compile, gets 3.2% larger, and 0.4% slower?

Another question is about PGO integration: is it already hooked there? Should we have a more aggressive threshold in a hot function? (Assuming we’re willing to spend some binary size there but not on the cold path).

Thanks,

— 
Mehdi


> Looks like the change has overall positive performance impact with very small code size/compile time overhead. Now the question is shall we make this change default in O2, or shall we leave it in O3. We would like to have more input from the community to make the decision.
> 
> Thanks,
> 
> Dehao
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170130/e3ec0873/attachment.html>