SLP/Loop vectorizer pass ordering

Chandler Carruth chandlerc at google.com
Thu Oct 9 13:51:34 PDT 2014 

On Thu, Oct 9, 2014 at 1:46 PM, Gerolf Hoflehner <ghoflehner at apple.com>
wrote:

> Are you going to test ARM and x86? Otherwise could you send out your patch
> even though it is preliminary?
>

Only x86 sadly. I'll send it out later today hopefully.

>
> Thanks
> Gerolf
>
> On Oct 9, 2014, at 12:44 PM, Chandler Carruth <chandlerc at google.com>
> wrote:
>
> I have a patch I've been testing to clean up a lot of the passes around
> the vectorizers. I'll add this in and finish testing it, then send it out
> with numbers.
> On Oct 9, 2014 12:40 PM, "Andrew Trick" <atrick at apple.com> wrote:
>
>>
>> On Oct 9, 2014, at 8:48 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>>
>> ----- 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.
>>
>>
>> Well, that is a classic candidate for rotate. So assuming whatever GVN is
>> doing is sane, then I’d say it makes sense to rerun rotation.
>>
>> -Andy
>>
>>
>>
>>
>> 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.
>>
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>>
>> --
>> 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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