[AArch64] Enable partial unrolling on cortex-a57 and 2 related improvement

[Hal Finkel]

Hi Kevin,

Regarding the first patch, you did not comment on (nor add a test for?) also adding a run of CorrelatedValuePropagation. Can you please explain the rationale?

The vectorizer/unrolling patch is now huge. What happened?

Regarding the third, this LGTM.

 -Hal

----- Original Message -----
> From: "Kevin Qin" <kevinqindev at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "llvm-commits" <llvm-commits at cs.uiuc.edu>
> Sent: Wednesday, March 4, 2015 3:09:51 AM
> Subject: Re: [AArch64] Enable partial unrolling on cortex-a57 and 2 related improvement
> 
> 
> Hi Hal,
> 
> 
> Thanks a lot for your review. I totally agree with your review
> comments, and here are the updated patches.
> 
> 
> Regards,
> Kevin
> 
> 
> 2015-03-04 11:29 GMT+08:00 Hal Finkel < hfinkel at anl.gov > :
> 
> 
> ----- Original Message -----
> > From: "Kevin Qin" < kevinqindev at gmail.com >
> > To: "llvm-commits" < llvm-commits at cs.uiuc.edu >
> > Sent: Friday, February 27, 2015 11:55:05 PM
> > Subject: [AArch64] Enable partial unrolling on cortex-a57 and 2
> > related improvement
> > 
> > 
> > 
> > Hi,
> > 
> > 
> > Previously, I made commit r219401 that try to enable partial &
> > runtime unrolling on cortex-a57, but I forgot to call base TTI
> > implementation in target specific hook, so those unrolling methods
> > are not really enabled.
> > 
> > 
> > Here are the patch to get them enabled and 2 related patches to
> > improve it.
> > 
> > 
> > 0001 - Run LICM pass after loop unrolling pass. Runtime unrollng
> > will
> > introduce a runtime check in loop prologue(you can treat it as a
> > loop preheader). If the unrolled loop is a inner loop, then the
> > proglogue will be inside the outer loop. LICM pass can help to
> > promote the runtime check out if the checked value is loop
> > invariant.
> 
> I think makes sense, at least for LICM, and is consistent with what
> James observed from the early run of the unroller. Please add a
> comment explaining why those passes are there. This file does not
> have many 'rationale' comments, and this is not a good thing. Why
> are you adding CVP? Can you please add some test cases (we normally
> don't add tests that runs the full pipeline, but for testing the
> pipeline, it is a good idea).
> 
> > 
> > 
> > 0002 - Introduce runtime unrolling disable matadata and use it to
> > mark the scalar loop from vectorization. Runtime unrolling is an
> > expensive optimization which can bring benefit only if the loop is
> > hot and iteration number is relatively large enough. For some
> > loops,
> > we know they are not worth to be runtime unrolled. The scalar loop
> > from vectorization is one of the cases.
> 
> I think this is a good idea. However, I think we might be overlooking
> something. If the purpose of the scalar loop is only to handle the
> 'left over' part of the iteration space that is not divisible by the
> vector length. However, if there are runtime safety checks, and
> those checks generally fail, then the loop could be hot. Can we
> exclude the case where we've emitted safety checks?
> 
> > 
> > 
> > 0003 - Enable partial & runtime unrolling on cortex-a57, and double
> > the unrolling threshold if the loop depth > 1. For inner one of
> > nested loops, it is more likely to be a hot loop, and the runtime
> > check can be promoted out from patch 0001, so the overhead is less,
> > we can try a larger threshold to unroll more loops.
> > 
> 
> 
> + if (L->getLoopDepth() > 1)
> + UP.PartialThreshold *= 2;
> 
> Please add a comment here.
> 
> -Hal
> 
> > 
> > 
> > 
> > Combined above changes together, we can get below performance and
> > code size changes.
> > 
> > 
> > Benchmark Execution time code bloat
> > 
> > 
> > spec.cpu2000.179_art -16.567% 8.805%
> > spec.cpu2000.177_mesa -2.771% 1.912%
> > spec.cpu2006.483_xalancbmk -2.555% 0.076%
> > spec.cpu2000.256_bzip2 -1.648% 2.414%
> > spec.cpu2006.433_milc -1.228% 1.353%
> > spec.cpu2006.456_hmmer -1.079% 2.413%
> > 
> > spec.cpu2006.462_libquantum 2.492% 1.482%
> > spec.cpu2000.253_perlbmk 1.563% 0.464%
> > spec.cpu2006.450_soplex 1.379% 1.925%
> > spec.cpu2000.186_crafty 1.242% 0.005%
> > 
> > spec.geomean -0.546% 0.952%
> > 
> > 
> > Please review. Thanks.
> > 
> > 
> > --
> > 
> > 
> > Best Regards,
> > 
> > 
> > Kevin Qin
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> > 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
> 
> 
> 
> --
> 
> 
> Best Regards,
> 
> 
> Kevin Qin

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory