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

[Johannes Doerfert via llvm-commits]

On 08/28, Tobias Grosser wrote:
> On 08/28/2015 08:53 AM, Johannes Doerfert wrote:
> >On 08/28, Tobias Grosser via llvm-commits wrote:
> >>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.
> >While this sounds good, I think we do miss too many SCoPs with this
> >patch. Especially ones that have two or more affine loops but the region
> >entry is just not part of them.
> 
> Yes, I also realized we block too many loops here. Polybench's 3mm kernel
> is e.g. split into three kernels now. :(
> 
> >To this end I think we need to change
> >the way we count loops. My proposal:
> >
> >   Instead of a boolean Context.hasAffineLoop we keep a integer that we
> >   increase for each affine loop. In the end we do not check if
> >   hasAffineLoop is true or false but the integer is bigger than 1.
> >   This will catch all affine loops in the region.
> 
> >@Tobias, @Pratik:
> >   Do you agree that we might miss SCoPs with multiple loops?
> >   Do you think my proposal is OK or do you see any potential for
> >   improvement?
> 
> 
> Right. That would definitely work. However, we only have this number after
> verifying that the full scop is indeed a scop. My hope would be that we
> could bail out even before running scop-detection over the offending region.
> 
> Maybe by iterating over the outermost loops and checking if more of them
> are contained in the region?
It's not as easy in general I think. Maybe building a mapping from
regions to loops when we iterate over __all__ loops. That should work
but outermost loops will probably not be precise.

> >@Pratik:
> >   If we need to change this again, will you implment the new patch?
> 
> I leave this to Pratik?
> 
> Tobias
> 
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-- 

Johannes Doerfert
Researcher / PhD Student

Compiler Design Lab (Prof. Hack)
Saarland University, Computer Science
Building E1.3, Room 4.31

Tel. +49 (0)681 302-57521 : doerfert at cs.uni-saarland.de
Fax. +49 (0)681 302-3065  : http://www.cdl.uni-saarland.de/people/doerfert
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