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

[Mehdi AMINI via llvm-dev]

Hi Hal,


2017-09-21 20:59 GMT-07:00 Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org>
:

>
> 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> 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.
>
> Regardless, measuring these differences certainly seems like a good idea.
> 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.
>
> 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.
>
>
> 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.
>

When you write that we should take Polly as a build dependency: are you
envisioning this to be tied into LLVM to the point where we can't build
LLVM without Polly?
I thought the approach would rather be that a default build of LLVM would
have Polly available but that there will "forever" be a
`-DENABLE_POLLY=OFF` option?
This seems like an important distinction to me, as making it more
integrated but still optional for the correct behavior of LLVM means that
people can continue to work and maintain "forever" an optimization pipeline
which operates without Polly, while starting to get more integration in a
pluggable form. I believe this is the direction that was taken with the
GSoC last year, trying to get Polly as a "pluggable" dependency analysis.


>
> 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.
>

I'll add: PGO-driven enablement: spending compile-time where we know it can
have an impact. What do you think?

Best,

-- 
Mehdi
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