[llvm-dev] [Proposal][RFC] Epilog loop vectorization
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Tue Mar 14 11:40:53 PDT 2017
On 03/14/2017 11:58 AM, Michael Kuperstein wrote:
> 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.
I agree that sounds ideal. Identifying the effective vectorization
factor of the hand-vectorized loop seems fragile. However, if someone is
hand vectorizing then it seems like a small price to add a pragma to the
scalar loop restricting the vectorization factor (and/or specifying that
it is safe to execute). As a result, I'm not sure how much effort we
should make here.
>
> 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.
I think that the reason we have that limit, however, is that we don't
model the costs of the checks. Not that the cost model is otherwise too
inaccurate for low-trip-count loops. If we modeled the costs of the
checks, then I don't think this would be a problem.
>
> 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 agree, although we do want to reuse the cost and legality analysis
(which I think is a worthwhile engineering decision because that
analysis involves SCEV, AA, and TTI, all of which can get expensive). If
we can do that and also just add the new loop to the work queue, that
certainly might be cleaner.
-Hal
>
> 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
> <mailto: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
> <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
> <mailto:dberlin at dberlin.org>>
> *Cc:* anemet at apple.com <mailto:anemet at apple.com>; Hal Finkel
> <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>; Zaks, Ayal
> <ayal.zaks at intel.com <mailto:ayal.zaks at intel.com>>; Renato Golin
> <renato.golin at linaro.org <mailto:renato.golin at linaro.org>>;
> mkuper at google.com <mailto:mkuper at google.com>; Mehdi Amini
> <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>>; llvm-dev
> <llvm-dev at lists.llvm.org <mailto: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]
> *Sent:* Tuesday, February 28, 2017 6:58 PM
> *To:* Nema, Ashutosh <Ashutosh.Nema at amd.com
> <mailto:Ashutosh.Nema at amd.com>>
> *Cc:* anemet at apple.com <mailto:anemet at apple.com>; Hal Finkel
> <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>; Zaks, Ayal
> <ayal.zaks at intel.com <mailto:ayal.zaks at intel.com>>; Renato Golin
> <renato.golin at linaro.org <mailto:renato.golin at linaro.org>>;
> mkuper at google.com <mailto:mkuper at google.com>; Mehdi Amini
> <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>>; llvm-dev
> <llvm-dev at lists.llvm.org <mailto: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 <mailto: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>
> [mailto:anemet at apple.com <mailto:anemet at apple.com>]
> *Sent:* Tuesday, February 28, 2017 1:33 AM
> *To:* Hal Finkel <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>
> *Cc:* Daniel Berlin <dberlin at dberlin.org
> <mailto:dberlin at dberlin.org>>; Nema, Ashutosh
> <Ashutosh.Nema at amd.com <mailto:Ashutosh.Nema at amd.com>>; Zaks,
> Ayal <ayal.zaks at intel.com <mailto:ayal.zaks at intel.com>>;
> Renato Golin <renato.golin at linaro.org
> <mailto:renato.golin at linaro.org>>; mkuper at google.com
> <mailto:mkuper at google.com>; Mehdi Amini <mehdi.amini at apple.com
> <mailto:mehdi.amini at apple.com>>; llvm-dev
> <llvm-dev at lists.llvm.org <mailto: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
> <mailto: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 <mailto:anemet at apple.com>> wrote:
>
> On Feb 27, 2017, at 10:11 AM, Hal Finkel
> <hfinkel at anl.gov <mailto: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
> <mailto:dberlin at dberlin.org>> wrote:
>
> On Mon, Feb 27, 2017 at 9:29 AM, Adam
> Nemet <anemet at apple.com
> <mailto:anemet at apple.com>> wrote:
>
> On Feb 27, 2017, at 7:27 AM,
> Hal Finkel <hfinkel at anl.gov
> <mailto: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
>
--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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