[llvm-dev] [RFC][LV][VPlan] Proposal for Outer Loop Vectorization Implementation Plan

[Hal Finkel via llvm-dev]

On 01/15/2018 03:52 PM, Philip Reames wrote:
> To revive the discussion around vectorizer testing, here's a quick
> sample of a few of the issues hit recently in the loop vectorizer.  I
> want to be careful to say that I am not stating these are the result
> of any recent work, just that they're issues that have been triaged
> down to the loop vectorizer doing something incorrect or questionable
> from a performance perspective.
>
> https://bugs.llvm.org/show_bug.cgi?id=35282
> https://bugs.llvm.org/show_bug.cgi?id=35687
> https://bugs.llvm.org/show_bug.cgi?id=35734
> https://bugs.llvm.org/show_bug.cgi?id=35773
> https://reviews.llvm.org/D41939
>
> I also see another 10 or so correctness related ones in a a simple
> search:
> https://bugs.llvm.org/buglist.cgi?quicksearch=LoopVectorize&list_id=131629
>
> The loop vectorizer is currently a serious pain point in our
> regression testing.  As the rest of the optimizer changes, we are
> finding that existing bugs in the vectorizer are being exposed with
> disturbing frequency and that some new regressions are landing as well.

I certainly understand what you're saying, but, as you point out, many
of these are existing bugs that are being exposed by other changes (and
they're seemingly all over the map). My general feeling is that the more
limited the applicability of a particular transform the buggier it will
tend to be. The work here to allow vectorization of an every-wider set
of inputs will really help to expose, and thus help us eliminate, bugs.
As such, one of the largest benefits of adding the
function-vectorization work (https://reviews.llvm.org/D22792), and
outer-loop vectorization capabilities, will be making it easier to throw
essentially-arbitrary inputs at the vectorizer (and have it do
something), and thus, hit it more effectively with automated testing.

Maybe we can do a better job, even with the current capabilities, of
automatically generating data-parallel loops with reductions of various
kinds? I'm thinking about automated testing because, AFAIK, the
vectorizer is already run through almost all of the relevant benchmarks
and test suites, and even if we add a few more, we probably need to
increase the test coverage by a lot more than that.

Do you have ideas about how we can have better testing in this area
otherwise?

 -Hal

>
> Philip
>
> On 12/14/2017 01:49 PM, Saito, Hideki wrote:
>>> Another might be to introduce changes under feature flags to ease
>>> the revert/reintroduce/revert cycle.
>> This is essentially the first guard. We plan to have flags/settings
>> to control which types of outer loops are handled.
>> The new code path is initially exclusive to outer loop vectorization.
>> If we disable all types of outer loops
>> (and that's the initial default), LV continues to be good old
>> innermost loop vectorizer.
>>
>>> I'd like to hear what plan is in place to ensure we don't
>>> destabilize the vectorizer while working on this.
>> W.r.t. this project, this matters when we touch the code that is
>> shared with innermost loop vectorization
>> and outer loop vectorization. It's certainly good to ensure that such
>> places to have good test coverage
>> at the time of commit.
>>
>> This also matters (and matters greatly) when we start guiding
>> innermost loops towards the new code path (when
>> we are ready). For this, another flag control would be there for
>> everyone to try before the flipping of the default could
>> land in the trunk (somewhat analogous to Chandler asking people to
>> test new Pass Manager prior to the switch).
>> We might be able to control which kind of innermost loops would take
>> new code path, but that's TBD.
>>
>> Fuzz testing or not, I fully agree that good testing coverage of
>> vectorizer is desired.
>>
>> Thanks,
>> Hideki
>>
>> -----Original Message-----
>> From: Philip Reames [mailto:listmail at philipreames.com]
>> Sent: Thursday, December 14, 2017 1:10 PM
>> To: Saito, Hideki <hideki.saito at intel.com>; llvm-dev at lists.llvm.org;
>> renato.golin at linaro.org; aemerson at apple.com; mkuper at google.com;
>> mssimpso at codeaurora.org; Simon Moll <moll at cs.uni-saarland.de>;
>> hfinkel at anl.gov
>> Subject: Re: [llvm-dev] [RFC][LV][VPlan] Proposal for Outer Loop
>> Vectorization Implementation Plan
>>
>> One area that needs a bit of attention before work on this proceeds
>> much further is testing.  The introduction of VPlan appears to have
>> introduced a couple of bugs and exposed a couple of others.  Most of
>> those are now fixed, but the process did point out a lack of test
>> coverage around the changes which is concerning.
>>
>> I'd like to hear what plan is in place to ensure we don't destabilize
>> the vectorizer while working on this.
>>
>> One thing we could consider is leveraging the new IR fuzzer to help
>> find assertion failures either before submission or shortly there
>> after. Another might be to introduce changes under feature flags to
>> ease the revert/reintroduce/revert cycle.
>>
>> Philip
>>
>>
>> On 12/05/2017 05:09 PM, Saito, Hideki via llvm-dev wrote:
>>> Proposal for Outer Loop Vectorization Implementation Plan
>>> =============================================
>>>
>>> =====
>>> Goal:
>>> =====
>>> Extending Loop Vectorizer (LV) such that it can handle outer loops,
>>> via VPlan infrastructure enhancements.
>>> Understand the trade-offs in trying to make concurrent progress with
>>> moving remaining inner loop vectorization functionality to VPlan
>>> infrastructure
>>>    ===========
>>> Background:
>>> ===========
>>> This is related to VPlan infrastructure project we started a while
>>> back, a project to extend the (inner loop vectorization focused)
>>> Loop Vectorizer to support outer loop vectorization.
>>> VPlan is the vectorization planner that records the decisions and
>>> candidate directions to pursue in order to drive cost modeling and
>>> vector code generation. When it is fully integrated into LV (i.e.,
>>> at the end of this big project), VPlan will use a Hierarchical-CFG
>>> (HCFG) and transform it starting from the abstraction of the input
>>> IR to reflect current vectorization decisions being made. The HCFG
>>> eventually becomes the abstraction of the output IR, and the vector
>>> code generation is driven by this abstract representation.
>>>    This is a follow up of the previous RFCs and LLVM Developer
>>> Conference presentations:
>>>            
>>> http://lists.llvm.org/pipermail/llvm-dev/2016-September/105057.html
>>> (RFC: Extending LV to vectorize outerloops)
>>>            
>>> http://lists.llvm.org/pipermail/llvm-dev/2017-February/110159.html
>>> (RFC: Introducing VPlan to model the vectorized code and drive its
>>> transformation)
>>>             https://www.youtube.com/watch?v=XXAvdUwO7kQ (Extending
>>> LoopVectorizer: OpenMP4.5 SIMD and Outer Loop Auto-Vectorization)
>>>             https://www.youtube.com/watch?v=IqzJRs6tb7Y (Introducing
>>> VPlan to the LoopVectorizer)
>>>             https://www.youtube.com/watch?v=BjBSJFzYDVk (Vectorizing
>>> Loops with VPlan - Current State and Next Steps) Please also refer
>>> to the separately sent out status update
>>> http://lists.llvm.org/pipermail/llvm-dev/2017-December/119522.html.
>>> .
>>>    So far, two big patches related to VPlan implementation have been
>>> committed.
>>> https://reviews.llvm.org/D28975 by Gil Rapaport. (Introducing VPlan to
>>> model the vectorized code and drive its transformation)
>>>        Has been broken down to a series of smaller patches and went in.
>>> The last (re)commit of the series is
>>>        https://reviews.llvm.org/rL311849
>>> https://reviews.llvm.org/D38676 by Gil Rapaport. (Modeling masking in
>>> VPlan, introducing VPInstructions)
>>>        This is also broken down to a series of smaller patches to
>>> facilitate the review.
>>>        Committed as https://reviews.llvm.org/rL318645
>>>    With the first patch, we introduced the concept of VPlan to LV
>>> and started explicitly recording decisions like interleave memory
>>> access optimization and serialization.
>>> With the second patch, we also introduced the concept of
>>> VPInstruction (to model newly introduced use-def) and started
>>> explicitly modeling masking and mask manipulations.
>>>    So far, work in this area has been mostly restricted to
>>> refactoring of Loop Vectorizer's existing functionality with several
>>> remaining areas of LV that still need to be refactored into the new
>>> VPlan based representation. The following is a non-exhaustive list
>>> of areas that need to be worked on and gives an idea of the amount
>>> of remaining work.
>>> * Predication
>>> * Cost model
>>> * Remainder Loop
>>> * Runtime Guards
>>> * External Users
>>> * Reduction Epilog
>>> * Interleave Grouping
>>> * Sink Scalar Operands
>>>    While it is important to keep refactoring the above mentioned
>>> items such that entire LV becomes fully VPlan based (which completes
>>> the transition into VPlan infrastructure), we are aware that there
>>> is also a need to make progress in outer loop vectorization.
>>> Function vectorization via VecClone pass
>>> (https://reviews.llvm.org/D22792) also depends on the progress of
>>> outer loop vectorization.
>>>    ==============
>>> Proposed Plan:
>>> ==============
>>> Instead of waiting for most of the above mentioned refactoring to
>>> finish before working on outer loop vectorization, we believe it
>>> serves the best interest of the community if we start making
>>> concurrent progress on continued refactoring effort and outer loop
>>> vectorization effort.
>>> To implement outer loop vectorization in incremental steps, we
>>> propose adding new code paths that would be exercised initially for
>>> only outer loop vectorization cases. This would ensure that the
>>> changes are NFC for innermost loop vectorization cases until we
>>> (slowly) transition them to also go through these new code paths.
>>>    Given that LV never had the ability to vectorize outer level
>>> loops, letting outer loop vectorization take slightly different code
>>> paths, compared to innermost loop vectorization, won't cause any
>>> issues in innermost loop vectorization. The new paths are what inner
>>> loop vectorization will eventually go through (i.e.,
>>> post-refactoring), after the kinks are worked out so that doing so
>>> will not regress any of the existing functionality and performance.
>>> Having the new code paths exposed early would encourage more
>>> community participating in maturing the new code paths, makes it
>>> clearer what still needs to be moved over to VPlan infrastructure,
>>> and hopefully also encourage more community participation in the
>>> remaining refactoring effort.
>>>    ======================
>>> Proposed Patch Series:
>>> ======================
>>> There will be a lot of new development before declaring "feature
>>> complete". As such, we propose to break it into five or more series
>>> of patches ---- starting from vectorizing trivial outer loops. Along
>>> the way, we expect to include NFC patches to make LV more modular,
>>> in order to keep LoopVectorize.cpp file size from growing too much.
>>>        Patch Series #1
>>>     ---------------
>>>       Trivial outer loop vectorization
>>>       * Inner loops are trivially lock-step among all vector elements
>>>       * No other branches inside the outer loop (to avoid touching
>>> divergence analysis in this series)
>>>       This should require
>>>       1) inner loop uniformity check (SCEV based invariance will do)
>>> --- the first small patch https://reviews.llvm.org/D40874 is a
>>> subset of this.
>>>       2) VPlan construction
>>>       3) Vector code generation (w/ unmodified uniform inner control
>>> flow) from VPlan
>>>          for (i=0;i<N;i++){ // vectorize here
>>>         // no branches here
>>>         for (j=0;j<loop_invariant_value; j++){
>>>           // no branches here
>>>         }
>>>         // no branches here
>>>       }
>>>        Patch Series #2
>>>     ---------------
>>>       Simple outer loop vectorization
>>>       * Inner loops are still trivially lock-step among all vector
>>> elements
>>>       * Non-loop branches are blindly assumed as divergent
>>>       This should require
>>>       1) Patch Series #1
>>>       2) Predication analysis and masking on VPlan
>>>       3) Vector code generation support for masking
>>>          for (i=0;i<N;i++){ // vectorize here
>>>         // branches may be here
>>>         for (j=0;j<loop_invariant_value; j++){ // unconditional w.r.t.
>>> i-loop
>>>           // branches may be here
>>>         }
>>>         // branches may be here
>>>       }
>>>        Patch Series #3
>>>     ---------------
>>>       Better simple outer loop vectorization
>>>       * Inner loops are still trivially lock-step among all vector
>>> elements
>>>       * Non-loop branches may be uniform/non-uniform
>>>       This should require
>>>       1) Patch Series #1, #2
>>>       2) We first use SCEV invariance for uniformity analysis.
>>>       3) Vector code generation support for uniform forward branches
>>>       4) We are working with Simon Moll (U-Saarland) to improve
>>> uniformity analysis
>>> (https://www.youtube.com/watch?v=svMEphbFukw&feature=youtu.be).
>>>          Not a hard dependency, but improved analysis is good for
>>> performance.
>>>          Same kind of loop as Patch Series #2, but uniform branches
>>> are optimized (i.e., less masking).
>>>        Patch Series #4
>>>     ---------------
>>>       Less simple outer loop vectorization
>>>       * Inner loops may not be lock-step among all vector elements, but
>>> single-entry/single-exit
>>>       This should require
>>>       1) Patch Series #1, #2, #3
>>>       2) VPlan to VPlan transformation to make inner loop lock-step.
>>> After the transform, the VPlan should look like the ones #3 can handle.
>>>          We'll look into porting code from U-Saarland RV project,
>>> which performs this transformation in IR to IR manner.
>>>          for (i=0;i<N;i++){ // vectorize here
>>>         // branches may be here
>>>         if (cond(i)) {
>>>           // branches may be here
>>>           while (cond1(i)){
>>>             // branches may be here
>>>           }
>>>           // branches may be here
>>>         }
>>>         // branch may be here
>>>       }
>>>        Patch Series #5
>>>     ---------------
>>>       Outer loop vectorization "Feature Complete"
>>>       * Inner loops must be redusible, allows multiple exits.
>>>       This should require
>>>       1) Patch Series #1, #2, #3, #4
>>>       2) VPlan to VPlan transformation to make inner loop lock-step.
>>>       3) Vector code generation support for inner loop with multiple
>>> exits
>>>          for (i=0;i<N;i++){ // vectorize here
>>>         // branches may be here
>>>         if (cond(1)) {
>>>           // branches may be here
>>>           while (cond1(i)){
>>>             .
>>>             if (cond2) break;
>>>             .
>>>             if (cond3) break;
>>>             .
>>>           }
>>>           // branches may be here
>>>         }
>>>         // branch may be here
>>>       }
>>>    ============================
>>> Future Work beyond this RFC:
>>> ============================
>>>     * Outer loop auto-vectorization legality analysis
>>>     * Outer loop auto-vectorization cost model
>>>     * Cost model to compute VF/UF for explicit outer loop vectorization
>>> (when not specified)
>>>     * (lower in priority) Irrducible inner loop handling (VPlan to
>>> VPlan transform to make inner loops reducible. After the transform,
>>> the VPlan should look like the ones #5 can handle.)
>>>    ===========
>>> Trade-Offs:
>>> ===========
>>> The new code path also has the ability to perform innermost loop
>>> vectorization (as a trivial subset of outer loop vectorization if we
>>> want to exercise that path for such purposes). It's good to be able
>>> to compare, side-by-side, what existing code path does and what new
>>> path does so that we can incrementally build up
>>> confidence/experience levels on the new code path. For the time
>>> being, however, that can be seen as extra maintenance needs. Instead
>>> of trying to do this on the trunk, we can do the same thing on the
>>> side, using github, until "outer loop vectorization feature" becomes
>>> mature enough. Amount of code review required, to get into the
>>> trunk, most likely would be comparable.
>>>    ========
>>> Summary:
>>> ========
>>> Outer loop vectorization feature implementation plan is presented. We
>>> think concurrent progress has a good enough chance of expediting
>>> overall transition of LV into the new infrastructure while
>>> benefiting from delivering new functionality to those in need
>>> sooner, with wider community participation in the overall development.
>>>    Thanks,
>>> Hideki Saito
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory