[llvm-dev] [Proposal][RFC] Epilog loop vectorization

Tue Mar 14 09:58:33 PDT 2017

I'm still not sure about this, for a few reasons:

1) I'd like to try to treat epilogue loops the same way regardless of
whether the main loop was vectorized by hand or automatically. So if
someone hand-wrote an avx-512 16-wide loop, with alias checks, and we
decide it's profitable to vectorize the epilogue loop by 4 and re-use the
checks, it ought to be done the same way. I realize this may be a
pipe-dream, though.

2) I'm still somewhat worried about "tiny loops". As I wrote before, we
explicitly refuse to vectorize loops we know have a trip-count less than
16, because our profitability heuristic for such loops is probably bad.
IIUC the only reason we don't bail due to the threshold is because we use
the same loop for "failed min iters check" and "failed alias check". So,
because it's reachable through the alias-check path, the max trip count
isn't actually known, even though the typical trip count is probably small.
It's true that you currently don't try to vectorize the epilogue if the
original VF is below 16, but this is a somewhat different condition.

3) Technically speaking, constructing a new InnerLoopVectorizer to
vectorize this one loop sounds weird. We already have a worklist in the
vectorizer that's currently running.

I don't think (1) is a blocker, and (3) should be easy to fix, but I'm not
sure whether the way this is going to handle (2) is sufficient.  If I'm the
only one that this bothers, I won't stand in the way, but I'd like to at
least make sure we've fully considered this.

On Mar 14, 2017 06:00, "Nema, Ashutosh" <Ashutosh.Nema at amd.com> wrote:

> Summarizing the discussion on the implementation approaches.
>
>
>
> Discussed about two approaches, first running ‘InnerLoopVectorizer’ again
> on the epilog loop immediately after vectorizing the original loop within
> the same vectorization pass, the second approach where re-running
> vectorization pass and limiting vectorization factor of epilog loop by
> metadata.
>
>
>
> <Approach-2>
>
> Challenges with re-running the vectorizer pass:
>
> 1)      Reusing alias check result:
>
> When vectorizer pass runs again it finds the epilog loop as a new loop and
> it may generates alias check, this new alias check may overkill the gains
> of epilog vectorization.
>
> We should use the already computed alias check result instead of re
> computing again.
>
> 2)      Rerun the vectorizer and hoist the new alias check:
>
> It’s not possible to hoist alias checks as its not fully redundant (not
> dominated by other checks), it’s not getting execute in all paths.
>
>
>
>
>
> NOTE: We cannot prepone alias check as its expensive compared to other
> checks.
>
>
>
> <Approach-1>
>
> 1)      Current patch depends on the existing functionality of
> LoopVectorizer, it uses ‘InnerLoopVectorizer’ again to vectorize the epilog
> loop, as it happens in the same vectorization pass we have flexibility to
> reuse already computed alias result check & limit vectorization factor for
> the epilog loop.
>
> 2)      It does not generate the blocks for new block layout explicitly,
> rather it depends on ‘InnerLoopVectorizer::createEmptyLoop’ to generate
> new block layout. The new block layout get automatically generated by
> calling the ‘InnerLoopVectorizer:: vectorize’ again.
>
> 3)      Block layout description with epilog loop vectorization is
> available at
>
> https://reviews.llvm.org/file/data/fxg5vx3capyj257rrn5j/PHID
> -FILE-x6thnbf6ub55ep5yhalu/LayoutDescription.png
>
>
>
> Approach-1 looks feasible, please comment if any objections.
>
>
>
> Regards,
>
> Ashutosh
>
>
>
>
>
> *From:* Nema, Ashutosh
> *Sent:* Wednesday, March 1, 2017 10:42 AM
> *To:* 'Daniel Berlin' <dberlin at dberlin.org>
> *Cc:* anemet at apple.com; Hal Finkel <hfinkel at anl.gov>; Zaks, Ayal <
> ayal.zaks at intel.com>; Renato Golin <renato.golin at linaro.org>;
> mkuper at google.com; Mehdi Amini <mehdi.amini at apple.com>; llvm-dev <
> llvm-dev at lists.llvm.org>
> *Subject:* RE: [llvm-dev] [Proposal][RFC] Epilog loop vectorization
>
>
>
> Sorry I misunderstood, gvn/newgvn/gvnhoist cannot help here as these
> checks are not dominated by all paths.
>
>
>
> Regards,
>
> Ashutosh
>
>
>
> *From:* Daniel Berlin [mailto:dberlin at dberlin.org <dberlin at dberlin.org>]
> *Sent:* Tuesday, February 28, 2017 6:58 PM
> *To:* Nema, Ashutosh <Ashutosh.Nema at amd.com>
> *Cc:* anemet at apple.com; Hal Finkel <hfinkel at anl.gov>; Zaks, Ayal <
> ayal.zaks at intel.com>; Renato Golin <renato.golin at linaro.org>;
> mkuper at google.com; Mehdi Amini <mehdi.amini at apple.com>; llvm-dev <
> llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] [Proposal][RFC] Epilog loop vectorization
>
>
>
> Hoisting or removing?
> Neither pass does hoisting, you'd need gvnhoist for that.
>
>
>
> Even then:
> Staring at your example, none of the checks are fully redundant  (IE they
> are not dominated by other checks) or execute on all paths.
>
>
>
> Thus, hoisting them  would be purely speculative code motion, which none
> of our passes do.
>
>
>
> If you would like these sets of checks to be removed, you would need to
> place them in a place that they execute unconditionally.
>
>
>
> Otherwise, this is not a standard code hoisting/removal transform.
>
>
>
> The only redundancy i can see here at all is the repeated getelementptr
> computation.
>
> If you move it to the preheader, it will be eliminated.
>
> Otherwise, none of the checks are redundant.
>
>
> What would you hope to happen in this case?
>
>
>
> On Tue, Feb 28, 2017 at 5:09 AM, Nema, Ashutosh <Ashutosh.Nema at amd.com>
> wrote:
>
> I have tried running both gvn and newgvn but it did not helped in hoisting
> the alias checks:
>
>
>
> Please check, maybe I have missed something.
>
>
>
> <TestCase>
>
> void foo (char *A, char *B, char *C, int len) {
>
>   int i = 0;
>
>   for (i=0 ; i< len; i++)
>
>     A[i] = B[i] + C[i];
>
> }
>
>
>
> <Command>
>
>   $ opt –O3 –gvn test.ll –o test.opt.ll
>
>   $ opt –O3 –newgvn test.ll –o test.opt.ll
>
>
>
> “test.ll” is attached, it got already vectorized by the approach running
> vectorizer twice by annotate the remainder loop with metadata to limit the
> vectorization factor for epilog vector loop.
>
>
>
> Regards,
>
> Ashutosh
>
>
>
> *From:* anemet at apple.com [mailto:anemet at apple.com]
> *Sent:* Tuesday, February 28, 2017 1:33 AM
> *To:* Hal Finkel <hfinkel at anl.gov>
> *Cc:* Daniel Berlin <dberlin at dberlin.org>; Nema, Ashutosh <
> Ashutosh.Nema at amd.com>; Zaks, Ayal <ayal.zaks at intel.com>; Renato Golin <
> renato.golin at linaro.org>; mkuper at google.com; Mehdi Amini <
> mehdi.amini at apple.com>; llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] [Proposal][RFC] Epilog loop vectorization
>
>
>
>
>
> On Feb 27, 2017, at 12:01 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>
>
>
>
> On 02/27/2017 01:47 PM, Daniel Berlin wrote:
>
>
>
>
>
> On Mon, Feb 27, 2017 at 11:29 AM, Adam Nemet <anemet at apple.com> wrote:
>
>
>
> On Feb 27, 2017, at 10:11 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>
>
>
>
> On 02/27/2017 11:47 AM, Adam Nemet wrote:
>
>
>
> On Feb 27, 2017, at 9:39 AM, Daniel Berlin <dberlin at dberlin.org> wrote:
>
>
>
>
>
>
>
> On Mon, Feb 27, 2017 at 9:29 AM, Adam Nemet <anemet at apple.com> wrote:
>
>
>
> On Feb 27, 2017, at 7:27 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>
>
>
> On 02/27/2017 06:29 AM, Nema, Ashutosh wrote:
>
> Thanks for looking into this.
>
>
>
> 1) Issues with re running vectorizer:
>
> Vectorizer might generate redundant alias checks while vectorizing epilog
> loop.
>
> Redundant alias checks are expensive, we like to reuse the results of
> already computed alias checks.
>
> With metadata we can limit the width of epilog loop, but not sure about
> reusing alias check result.
>
> Any thoughts on rerunning vectorizer with reusing the alias check result ?
>
>
> One way of looking at this is: Reusing the alias-check result is really
> just a conditional propagation problem; if we don't already have an
> optimization that can combine these after the fact, then we should.
>
>
>
> +Danny
>
>
>
> Isn’t Extended SSA supposed to help with this?
>
>
>
> Yes, it will solve this with no issue already.  GVN probably does already
> too.
>
>
>
> even if if you have
>
>
>
> if (a == b)
>
> if (a == c)
>
>  if (a == d)
>
>  if (a == e)
>
>  if (a == g)
>
>
>
>
>
> and  we can prove a ... g equivalent, newgvn will eliminate them all and
> set all the branches true.
>
>
>
> If you need a simpler clean up pass, we could run it on sub-graphs.
>
>
>
> Yes we probably don’t want to run a full GVN after the “loop-scheduling”
> passes.
>
>
> FWIW, we could, just without the memory-dependence analysis enabled (i.e.
> set the NoLoads constructor parameter to true). GVN is pretty fast in that
> mode.
>
>
>
> OK.  Another data point is that I’ve seen cases in the past where the
> alias checks required for the loop passes could enable GVN to remove
> redundant loads/stores.  Currently we can only pick these up with LTO when
> GVN is rerun.
>
>
>
> This is just GVN brokenness, newgvn should not have this problem.
>
> If it does, i'd love to see it.
>
>
> I thought that the problem is that we just don't run GVN after that point
> in the pipeline.
>
>
>
> Yeah, that is the problem but I think Danny misunderstood what I was
> trying to say.
>
>
>
> This was a datapoint to possibly rerun GVN with memory-awareness.
>
>
>
>
>  -Hal
>
>
>
> (I'm working on the last few parts of turning it on by default, but it
> requires a new getModRefInfo interface to be able to get the last few
> testcases)
>
>
>
>
>
> --
>
> Hal Finkel
>
> Lead, Compiler Technology and Programming Languages
>
> Leadership Computing Facility
>
> Argonne National Laboratory
>
>
>
>
>
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