SLP/Loop vectorizer pass ordering
Hal Finkel
hfinkel at anl.gov
Thu Oct 9 08:48:25 PDT 2014
----- Original Message -----
> From: "Arnold Schwaighofer" <aschwaighofer at apple.com>
> To: "Zinovy Nis" <zinovy.nis at gmail.com>
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "LLVM Commits" <llvm-commits at cs.uiuc.edu>, "Tobias Grosser" <tobias at grosser.es>,
> "Chandler Carruth" <chandlerc at google.com>, "Nadav Rotem" <nrotem at apple.com>
> Sent: Thursday, October 9, 2014 10:07:42 AM
> Subject: Re: SLP/Loop vectorizer pass ordering
>
>
> The loop vectorizer now sees this loop:
>
> define void
> @_Z21ambient_occlusion_vecP6_IsectR5vrandILm8EE(%struct._Isect*
> nocapture %isect, %class.vrand* nocapture readonly
> dereferenceable(32) %rng) #0 {
> entry:
> br label %for.body
>
> for.body: ; preds =
> %for.inc.for.body_crit_edge, %entry
> %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next,
> %for.inc.for.body_crit_edge ]
> %occlusion.017 = phi float [ 1.000000e+00, %entry ], [ %phitmp,
> %for.inc.for.body_crit_edge ]
> %exitcond = icmp eq i64 %indvars.iv, 63
> br i1 %exitcond, label %for.end, label %for.inc.for.body_crit_edge
>
> for.inc.for.body_crit_edge: ; preds = %for.body
> %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
> %phitmp = fadd fast float %occlusion.017, 1.000000e+00
> br label %for.body
>
> for.end: ; preds = %for.body
> %occlusion.017.lcssa = phi float [ %occlusion.017, %for.body ]
> %t5 = getelementptr inbounds %struct._Isect* %isect, i64 0, i32 0
> store float %occlusion.017.lcssa, float* %t5, align 4, !tbaa !1
> ret void
> }
>
> Notice that the loop exit block is the loop header and the latch is
> not guaranteed to be executed. The loop vectorizer assumes such
> loops have been rotated.
>
>
> If we send this IR through loop-rotate it will vectorize.
>
> The farther away we move the loop vectorizer from loop rotate the
> likelier some optimization will destroy the rotated from. We might
> just want to run loop rotate before the loop vectorizer ...
>
I think that makes sense -- and I don't recall loop rotation being expensive, plus is preserves just about everything (and I think does a reasonable job cleaning up after itself) ;)
I'd say we run some benchmarks, and barring any issues, we just do it.
-Hal
>
>
> > On Oct 9, 2014, at 1:15 AM, Zinovy Nis <zinovy.nis at gmail.com>
> > wrote:
> >
> > Hi.
> >
> > Did you have a chance to look at my reproducer?
> >
> > 2014-10-07 21:34 GMT+04:00 Zinovy Nis <zinovy.nis at gmail.com>:
> >> Hi.
> >>
> >> I attached a reduced sample, based on
> >> https://code.google.com/p/aobench/.
> >>
> >> Run it first with an old SLP order:
> >>
> >> 1) clang -c -Ofast -static -march=core-avx2 aobench.cpp -Rpass=.
> >> -mllvm -debug-only=loop-vectorize -mllvm
> >> -run-slp-after-loop-vectorization=0
> >>
> >> and then with a new order:
> >>
> >> 2) clang -c -Ofast -static -march=core-avx2 aobench.cpp -Rpass=.
> >> -debug-only=loop-vectorize -mllvm
> >> -run-slp-after-loop-vectorization=1
> >>
> >> and see the logs:
> >>
> >> 1) aobench.cpp:59:9: remark: vectorized loop (vectorization
> >> factor: 8,
> >> unrolling interleave factor: 1) [-Rpass=loop-vectorize]
> >> 2) aobench.cpp:59:9: remark: loop ***not*** vectorized: use
> >> -Rpass-analysis=loop-vectorize for more info
> >> [-Rpass-missed=loop-vectorize]
> >>
> >> LV: Found an unidentified PHI. %occlusion.017 = phi float [
> >> 1.000000e+00, %entry ], [ %phitmp, %for.inc.for.body_crit_edge ]
> >> LV: Can't vectorize the instructions or CFG
> >> LV: Not vectorizing: Cannot prove legality.
> >>
> >> 2014-10-06 17:46 GMT+04:00 Hal Finkel <hfinkel at anl.gov>:
> >>> ----- Original Message -----
> >>>> From: "Zinovy Nis" <zinovy.nis at gmail.com>
> >>>> To: "Hal Finkel" <hfinkel at anl.gov>
> >>>> Cc: "LLVM Commits" <llvm-commits at cs.uiuc.edu>, "Tobias Grosser"
> >>>> <tobias at grosser.es>, "Chandler Carruth"
> >>>> <chandlerc at google.com>, "Nadav Rotem" <nrotem at apple.com>,
> >>>> "Arnold Schwaighofer" <aschwaighofer at apple.com>
> >>>> Sent: Monday, October 6, 2014 8:44:28 AM
> >>>> Subject: Re: SLP/Loop vectorizer pass ordering
> >>>>
> >>>> A bit later. At least GVN creates critical edges which are not
> >>>> handled
> >>>> by loop vectorizer then.
> >>>
> >>> Okay, please do (this is fairly important) -- if you can extract
> >>> some relevant IR, filing a bug report would be great. Are you
> >>> saying that running SLP early inhibits GVN from creating
> >>> critical edges that the loop vectorizer does not understand?
> >>>
> >>> Thanks again,
> >>> Hal
> >>>
> >>>>
> >>>> 2014-10-06 17:33 GMT+04:00 Hal Finkel <hfinkel at anl.gov>:
> >>>>> ----- Original Message -----
> >>>>>> From: "Zinovy Nis" <zinovy.nis at gmail.com>
> >>>>>> To: "Chandler Carruth" <chandlerc at google.com>
> >>>>>> Cc: "LLVM Commits" <llvm-commits at cs.uiuc.edu>, "Tobias
> >>>>>> Grosser"
> >>>>>> <tobias at grosser.es>
> >>>>>> Sent: Monday, October 6, 2014 8:19:24 AM
> >>>>>> Subject: Re: SLP/Loop vectorizer pass ordering
> >>>>>>
> >>>>>> Please wait a while, I'm using it to revert the new order as
> >>>>>> it
> >>>>>> introduces regression in our internal benchmark: SLP was
> >>>>>> creating
> >>>>>> loop
> >>>>>> vectorization opportunities when was called before LV. Now no
> >>>>>> such
> >>>>>> opportunities are available, so we've got a regression.
> >>>>>
> >>>>> Interesting. Can you provide any further details?
> >>>>>
> >>>>> -Hal
> >>>>>
> >>>>>>
> >>>>>> 2014-10-06 3:28 GMT+04:00 Chandler Carruth
> >>>>>> <chandlerc at google.com>:
> >>>>>>>
> >>>>>>> On Thu, Sep 4, 2014 at 6:32 AM, James Molloy
> >>>>>>> <james at jamesmolloy.co.uk>
> >>>>>>> wrote:
> >>>>>>>>
> >>>>>>>> Hi Hal, Chandler,
> >>>>>>>>
> >>>>>>>> r217144.
> >>>>>>>
> >>>>>>>
> >>>>>>> Is anyone still using the option to disable this? If I don't
> >>>>>>> hear
> >>>>>>> anything,
> >>>>>>> I'll remove this option entirely in the next week.
> >>>>>>>
> >>>>>>> _______________________________________________
> >>>>>>> llvm-commits mailing list
> >>>>>>> llvm-commits at cs.uiuc.edu
> >>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >>>>>>>
> >>>>>> _______________________________________________
> >>>>>> 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
> >>>>
> >>>
> >>> --
> >>> Hal Finkel
> >>> Assistant Computational Scientist
> >>> Leadership Computing Facility
> >>> Argonne National Laboratory
>
>
--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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