[llvm-dev] [RFC] Polly Status and Integration

Fri Sep 22 06:47:00 PDT 2017

Hi Hal,

On 09/21, Hal Finkel via llvm-dev wrote:
> 
> On 09/12/2017 10:26 PM, Gerolf Hoflehner wrote:
> >
> >
> >>On Sep 11, 2017, at 10:47 PM, Hal Finkel via llvm-dev
> >><llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> >>
> >>
> >>On 09/11/2017 12:26 PM, Adam Nemet wrote:
> >>>Hi Hal, Tobias, Michael and others,
> >>>*...*
> >>>
> >>>One thing that I’d like to see more details on is what this means for
> >>>the evolution of loop transformations in LLVM.
> >>>
> >>>Our more-or-less established direction was so far to incrementally
> >>>improve and generalize the required analyses (e.g. the
> >>>LoopVectorizer’s dependence analysis + loop versioning analysis into a
> >>>stand-alone analysis pass (LoopAccessAnalysis)) and then build new
> >>>transformations (e.g. LoopDistribution, LoopLoadElimination,
> >>>LICMLoopVersioning) on top of these.
> >>>
> >>>The idea was that infrastructure would be incrementally improved from
> >>>two directions:
> >>>
> >>>- As new transformations are built analyses have to be improved (e.g.
> >>>past improvements to LAA to support the LoopVersioning utility, future
> >>>improvements for full LoopSROA beyond just store->load forwarding [1]
> >>>or the improvements to LAA for the LoopFusion proposal[2])
> >>>
> >>>- As more complex loops would have to be analyzed we either improve
> >>>LAA or make DependenceAnalysis a drop-in replacement for the memory
> >>>analysis part in LAA
> >>
> >>Or we could use Polly's dependence analysis, which I believe to be more
> >>powerful, more robust, and more correct than DependenceAnalysis. I
> >>believe that the difficult part here is actually the pairing with
> >>predicated SCEV or whatever mechanism we want to use generate runtime
> >>predicates (this applies to use of DependenceAnalysis too).
> >
> >What is a good way to measure these assertions (More powerful, more
> >robust)? Are you saying the LLVM Dependence Analysis is incorrect or do
> >you actually mean less conservative (or "more accurate" or something like
> >that)?
> 
> Sebastian's email covers the issues with the DependenceAnalysis pass pretty
> well.
> 
> Regarding what's in LoopAccessAnalysis, I believe it to be correct, but more
> limited. It is not clear to me that LAA is bad at what it does based on what
> the vectorizer can handle. LAA could do better in some cases with
> non-unit-stride loops. 

> Polly also handles piecewise-affine functions, which allows the
> modeling of loops with conditionals. Extending LAA to handle loop
> nests, moreover, seems likely to be non-trivial.
What exactly do you mean here? A loop with a conditional does not
necessarily cause a piecewise-affine (access) function. The ones that
actually do cause piecewise-affine (acces) functions are not handled by
Polly (except min/max). This is not because Polly could not handle them
but because it relies on Scalar Evolution.

> Regardless, measuring these differences certainly seems like a good idea.
It does, that's why I try to get some actual performance results soon.
If somebody does the same for Polly, e.g., by extending the GSoC'16
interface that exists, we will have actual data to talk about.

> I think that we can do this using optimization remarks. LAA already
> emits optimization remarks for loops in which it finds unsafe memory
> dependencies.  Polly also emits optimization remarks. We may need to
> iterate some in order to setup a good comparison, but we should be
> able to collect statistics (and other information) by compiling code
> using -fsave-optimization-record (in combination with some other
> flags), and then analyzing the resulting YAML files.
> 
> >>
> >>>
> >>>While this model may be slow it has all the benefits of the
> >>>incremental development model.
> >>
> >>The current model may have been slow in many areas, but I think that's
> >>mostly a question of development effort. My largest concern about the
> >>current model is that, to the extent that we're implementing classic
> >>loop transformations (e.g., fusion, distribution, interchange, skewing,
> >>tiling, and so on), we're repeating a historical design that is known to
> >>have several suboptimal properties. Chief among them is the lack of
> >>integration: many of these transformations are interconnected, and
> >>there's no good pass ordering in which to make independent decisions.
> >>Many of these transformations can be captured in a single model and we
> >>can get much better results by integrating them. There's also the matter
> >>of whether building these transformation on SCEV (or IR directly) is the
> >>best underlying infrastructure, or whether parts of Polly would be
> >>better.
> >
> >I believe that is true. What I wonder is is there a good method to reason
> >about it?
> 
> If I understand what you mean, one way to look at it is this: This is not a
> canonicalization problem. Picking an optimal way to interchange loops may
> depend on how the result can be skewed and/or tiled, picking an optimal way
> to distribute loops often depends on what can be done afterward in each
> piece. Optimal here generally involves reasoning about the memory hierarchy
> (e.g., cache properties), available prefetching streams, register-file size,
> and so on.
While I totally agree with the idea here I am not so convinced anymore.
The polyhedral scheduling technique that I have seen do not take more
than two of the above properties into account. One problem is simply the
way the scheduler works (per statement, dimension after dimension,
starting with the outermost one) and what that means for the quality of
performance prediction during the scheduling. It is also problematic
that properties might be "non-affine" which doesn't allow us to encode
them directly. At the moment the only "heuristic" you mentioned that is
actually used in Polly are cache properties for the hand-tuned
optimization (see below).

> I know that I've seen some good examples in papers over the years that
> illustrate the phase-ordering challenges. Hopefully, someone will jump in
> here with some good references. One classic one is: William Pugh. Uniform
> Techniques for Loop Optimization. 1991.
> 
> >Perhaps concrete examples or perhaps opt-viewer based comparisons on large
> >sets of benchmarks? In the big picture you could make such a modeling
> >argument for all compiler optimizations.
> 
> Certainly. However, in this case there's a well-studied unified model for
> this set of optimizations known to reduce phase-ordering effects. That's not
> true in general.
Polyhedral scheduling is not restricted by phase-ordering effects.
Though, it still has to be guided by an objective function that
realistically models "sufficient performance indicators" to actually
improve performance. I think we should talk more about how we could
get such an objective function and what might be intermediate steps as
it is (at least to me) absolutely not clear.

> >>That having been said, I think that integrating this technology into
> >>LLVM will also mean applying appropriate modularity. I think that we'll
> >>almost definitely want to make use of the dependence analysis separately
> >>as an analysis. We'll want to decide which of these transformations will
> >>be considered canonicalization (and run in the iterative pipeline) and
> >>which will be lowering (and run near the vectorizer). LoopSROA certainly
> >>sounds to me like canonicalization, but loop fusion might also fall into
> >>that category (i.e., we might want to fuse early to enable optimizations
> >>and then split late).
> >>
> >>>
> >>>Then there is the question of use cases.  It’s fairly obvious that
> >>>anybody wanting to optimize a 5-deep highly regular loop-nest
> >>>operating on arrays should use Polly.  On the other hand it’s way less
> >>>clear that we should use it for singly or doubly nested not-so-regular
> >>>loops which are the norm in non-HPC workloads.
> >>
> >>This is clearly a good question, but thinking about Polly as a set of
> >>components, not as a monolithic transformation component, I think that
> >>polyhedral analysis and transformations can underlie a lot of the
> >>transformations we need for non-HPC code (and, which I'll point out, we
> >>need for modern HPC code too). In practice, the loops that we can
> >>actually analyze have affine dependencies, and Polly does, or can do, a
> >>better job at generating runtime predicates and dealing with
> >>piecewise-linear expressions than our current infrastructure.
> >>
> >>In short, I look at Polly as two things: First, an infrastructure for
> >>dealing with loop analysis and transformation. I view this as being
> >>broadly applicable. Second, an application of that to apply
> >>cost-model-driven classic loop transformations. To some extent this is
> >>going to be more useful for HPC codes, but also applies to machine
> >>learning, signal processing, graphics, and other areas.
> >I’m wondering if it could be used for pointing out headroom for the
> >existing LLVM ecosystem (*)
> 
> Sure.
> 
> >
> >>
> >>>
> >>>And this brings me to the maintenance question.  Is it reasonable to
> >>>expect people to fix Polly when they have a seemingly unrelated change
> >>>that happens to break a Polly bot.
> >>
> >>The eventual goal here is to have this technology in appropriate parts
> >>of the main pipeline, and so the question here is not really about
> >>breaking a "Polly bot", but just about a "bot" in general. I've given
> >>this question some thought and I think it sits in a reasonable place in
> >>the risk-reward space. The answer would be, yes, we'd need to treat this
> >>like any other part of the pipeline. However, I believe that Polly has
> >>as many, or more, active contributors than essentially any other
> >>individual part of the mid-level optimizer or CodeGen. As a result,
> >>there will be people around in many time zones to help with problems
> >>with Polly-related code.
> >>
> >>> As far as I know, there were companies in the past that tried Polly
> >>>without a whole lot of prior experience.  It would be great to hear
> >>>what the experience was before adopting Polly at a much larger scale.
> >>
> >>I'm also interested, although I'll caution against over-interpreting any
> >>evidence here (positive or negative). Before a few weeks ago, Polly
> >>didn't effectively run in the pipeline after inlining, and so I doubt it
> >>would have been much use outside of embedded environments (and maybe
> >>some HPC environments) with straightforwardly-presented C code. It's
> >>only now that this has been fixed that I find the possibility of
> >>integrating this in production interesting.
> >
> >That is a good point. There are also biases independent of past
> >experiences (for disclosure mine is (*) above). But I think it is
> >objective to say a Polly integration is a big piece to swallow.Your
> >pro-Polly argument lists a number of categories that I think could be
> >reasoned about individually and partly evaluated with a data-driven
> >approach:
> >A) Architecture
> >- support for autoparallelism
> >- support for accelerators
> >- isl- rewrite? etc
> >...
> >B) Modelling
> >- polyhedral model
> >- temporal locality
> >- spatial locality
> >…
> >C) Analysis/Optimizations
> >- Dependence Analysis
> >- Transformation effective/power (loop nests, quality of transformations,
> >#vectorizable loops etc)
> >
> >A) is mostly Polly independent (except for the isl question I guess). For
> >B and C performance/ compile-time /opt-viewer data on a decent/wide range
> >of benchmarks possibly at different optimization levels (O2, O3, LTO, PGO
> >etc and combinations) should provide data-driven insight into
> >costs/benefits.
> 
> I agree. In practice, the first question is: Are will willing to take on
> Polly (+isl), in whole or in part, as a build dependency? If the answer is
> yes, the next question is: what parts should be reused or refactored for use
> in other parts of the pipeline? My argument is that we should take on Polly,
> or most of it, as a build dependency. Work on better unifying the developer
> communities as we start experimenting with other kinds of integration. This
> will, however, allow us to provide to all of our users these transformations
> through pragmas (and other kinds of optional enablement). This is an
> important first step.
> 
> I'm not sure exactly how good this is, but polly has LNT-submitting bots, so
> the website can generate a comparison (e.g.,
> http://lnt.llvm.org/db_default/v4/nts/71208?compare_to=71182). Looking at
> this comparison shows a number of potential problems but also cases where
> Polly really helps (and, FWIW, the largest two compile-time regressions are
> also associated with very large execution performance improvements). My
> first focus would certainly be on pragma-driven enablement.
To be completely honest, I think this comparisons oversell the positive
runtime effects of Polly quite a bit. (Though I do not think you try to
do that on purpose). Let me explain:

At least the first 5 of the 19 improved benchmarks are matrix-matrix
multiplications. These are not automatically scheduled by Polly but
instead pattern matched and replaced by hand-tuned, target specific
versions (ref. cache properties). Even if you argue this is a valid
"automatic optimization" it still is only one improvement shown five
times. Note also how matrix-matrix multiplications perform when there
are not matched:
  MultiSource/Benchmarks/VersaBench/bmm/bmm
  SingleSource/Benchmarks/Polybench/linear-algebra/kernels/trmm/trmm

Last time I checked the impact of Polly on the TSVC benchmarks I
realized that the effect was due to unrelated passes scheduled by Polly.
This might have changed by now but I doubt it. This leaves ten improved
Polybench benchmarks and whetstone. At least my local version of Polly
does not optimize the latter, thus the effect might be the same as for
the three TSVC benchmarks. For the Polybench benchmarks it would be
interesting to know what kind of transformation improved ten of them and
what regressed five others. I am especially interested if there is a
benefit from polyhedral scheduling or if it is only due to the tiling
that is applied afterwards.

--

I don't want to play devil's advocate here all the time but I think we
should be realistic on what we have and can expect. If not, we will
only cause unfulfilled expectations that won't help anybody.

Thanks,
  Johannes

> Thanks again,
> Hal
> 
> >
> >Cheers
> >Gerolf
> >
> >
> >
> >
> >>
> >>Thanks again,
> >>Hal
> >>
> >>>
> >>>Adam
> >>>
> >>>[1] http://lists.llvm.org/pipermail/llvm-dev/2015-November/092017.html
> >>>[2] http://lists.llvm.org/pipermail/llvm-dev/2016-March/096266.html
> >>>
> >>>
> >>>>*
> >>>>Sincerely,
> >>>>Hal (on behalf of myself, Tobias Grosser, and Michael Kruse, with
> >>>>feedback from**several other active Polly developers)
> >>>>*...**
> >>>>
> >>>>-- 
> >>>>Hal Finkel
> >>>>Lead, Compiler Technology and Programming Languages
> >>>>Leadership Computing Facility
> >>>>Argonne National Laboratory
> >>>>_______________________________________________
> >>>>LLVM Developers mailing list
> >>>>llvm-dev at lists.llvm.org <mailto: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
> >>_______________________________________________
> >>LLVM Developers mailing list
> >>llvm-dev at lists.llvm.org <mailto: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
> 

> _______________________________________________
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-- 

Johannes Doerfert
Researcher / PhD Student

Compiler Design Lab (Prof. Hack)
Saarland Informatics Campus, Germany
Building E1.3, Room 4.31

Tel. +49 (0)681 302-57521 : doerfert at cs.uni-saarland.de
Fax. +49 (0)681 302-3065  : http://www.cdl.uni-saarland.de/people/doerfert
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