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

[Matthias Braun 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%
> 
> 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.
> 
Intuitively (correct me if I am wrong) I would think loop unrolling is a more risky operation that is good in many/most cases but can be detrimental to performance (by blowing up code sizes and I-Caches). So I would rather put that into -O3.

- Matthias

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