[polly] r246161 - Do not detect Scops with only one loop.

[Tobias Grosser via llvm-commits]

On 08/27/2015 06:55 PM, Tobias Grosser via llvm-commits wrote:
> Author: grosser
> Date: Thu Aug 27 11:55:18 2015
> New Revision: 246161
>
> URL: http://llvm.org/viewvc/llvm-project?rev=246161&view=rev
> Log:
> Do not detect Scops with only one loop.
>
> If a region does not have more than one loop, we do not identify it as
> a Scop in ScopDetection. The main optimizations Polly is currently performing
> (tiling, preparation for outer-loop vectorization and loop fusion) are unlikely
> to have a positive impact on individual loops. In some cases, Polly's run-time
> alias checks or conditional hoisting may still have a positive impact, but those
> are mostly enabling transformations which LLVM already performs for individual
> loops. As we do not focus on individual loops, we leave them untouched to not
> introduce compile time regressions and execution time noise. This results in
> good compile time reduction (oourafft: -73.99%, smg2000: -56.25%).
>
> Contributed-by: Pratik Bhatu <cs12b1010 at iith.ac.in>
>
> Reviewers: grosser
>
> Differential Revision: http://reviews.llvm.org/D12268

This change significantly improves compile-time performance on LNT without
causing any execution-time regressions:

clang -O3 -polly - Before vs. After:
	
http://llvm.org/perf/db_default/v4/nts/38436

clang -O3 vs. clang -O3 -polly (before):

http://llvm.org/perf/db_default/v4/nts/38423?compare_to=38469

clang -O3 vs. clang -O3 -polly (after):

http://llvm.org/perf/db_default/v4/nts/38436?compare_to=38469

We see 78 compile time improvements with 14 of them showing more than 25%
percent compile time reduction. The overall benchmarks which show an
increase in compile time due to Polly are reduced from 140 to 111.

Thanks Pratik!

Best,
Tobias