[llvm-dev] RFC: Import of Integer Set Library into LLVM source tree

Sun Jan 21 20:37:17 PST 2018

Hi, Nadav, Chris, et al.,

If you've not already seen it, we had a long discussion about
incorporating Polly into LLVM on llvm-dev,
http://lists.llvm.org/pipermail/llvm-dev/2017-September/117063.html
(with a continuation in October:
http://lists.llvm.org/pipermail/llvm-dev/2017-October/118125.html) with
a lot of detailed information.

I think it is important, first, that we agree on the goals of this
effort. While Polly is currently used as an isolated
analysis-and-transformation engine, it won't stay that way. The goal is
to split, refactor, and rewrite (as needed) the current components so
that we have four things:

 1. A dependence analysis driven by state-of-the-art polyhedral analysis.

 2. A framework for easily constructing loop transformations on LLVM IR.

 3. Loop transformations built on these capabilities. These might be
canonicalization transformations in addition to cost-model-driven
optimization transformations.

 4. A generic (e.g., ILP-driven) scheduling capability for a wide class
of transformations.

Especially as we leave the realm of inner loops, we need a better
dependence analysis than what we have currently in LoopAccessAnalysis
and DependenceAnalysis. Moreover, this dependence analysis needs the
ability to interact with a capability for generating dynamic legality
predicates (for multiversioning; something that Polly can do).
Dependence analysis using polyhedral modeling is appealing for several
reasons. It provides a robust solution that seems likely easier to
maintain than an every-expanding set of special tests. Moreover, it
isolates the mathematical reasoning behind a well-defined interface
allowing for proper separation of concerns.

Working with loops in LLVM currently is unnecessarily difficult. Just
last week I was writing a transformation which split a loop into several
consecutive loops (where each of the p loops had ~n/p iterations). This
seems like a simple task, but even using (what I believe to be) our best
utilities from SCEV, LoopUtils, etc., by the time I dealt with multiple
induction variables, reductions, and more, plus keeping needed analyses
up to date, this transformation took hundreds of lines of code. We need
a better way of doing this. One of the capabilities within Polly is,
essentially, an infrastructure for rewriting loop nests. Our plan
includes refactoring this capability into a generic loop-rewriting
utility that can be used to implement a variety of different loop
transformations. This should make of our loop transformations, current
and future, easier to write, more robust, and easier to maintain.

One thing we can do immediately with this infrastructure, even in its
current form, is provide a user-driven (i.e., metadata-directed) set of
transformations encompassing nearly all common loop-nest transforms
(interchange, fission/fusion, skewing, tiling). Importantly, we can do
this, not only where the user assert safety, but using polyhedral
analysis, also where we generate legality predicates. This would be a
significant step toward making LLVM-based compilers in the top tier of
HPC compilers, but having such capabilities only conditionally supported
in the backend would be an unfortunate complication for our users.

The automated loop-nest rewriting (using an ILP-solver to pick some
optimal schedule), which is what many people might think of as the
primary usage modality of polyhedral loop optimizations, and thus Polly,
is only a part of this overall program for improving LLVM's
infrastructure for handling loops. This is an important capability, but
among many other factors, depends on good cost modeling (an area in
which Polly currently needs work). If at some point the cost modeling,
etc. improves to the point where we could have something like this in
the default optimization pipeline, that would be great (from my
perspective), but that's only a small potential benefit from this
integration.

In addition to these technical aspects, there are community
considerations, but with respect to Polly, and with respect to the isl
library on which it depends. Obviously there are different ways we can
develop these capabilities in LLVM, but I prefer a way that takes as
much of the existing Polly community as possible and merges it into the
LLVM community. This requires preserving Polly's current capabilities
during this development process, and ideally, not partitioning between
the "old" separate-repository code base and the "new" in-tree code base.
The tricky part here is what to do with isl. To be clear, isl has
already been relicensed once with the specific goal of better LLVM
integration, the dependency on GMP has been removed, and further changes
along these lines are not unthinkable. isl has its own developer
community, but many of the core Polly developers are also significant
isl contributors, and so there's significant overlap. The remainder of
the isl community is a significant resource, and whether or not we wish
to separate it from our development in this area is something we should
specifically consider. I personally suspect that we'll end up rewriting
a lot of what's in isl eventually, but we'd need to figure out what to
do in the mean time. isl is a moving target, like LLVM itself, and given
the developer overlap between Polly and isl, I don't think it makes
sense to think of isl as some kind of truly external dependency. We'd
likely need to version lock anyway, so the benefit of having isl in a
separate repository on separate servers seems like a system providing
all of the disadvantages of an external dependency with few/none of the
benefits. This is why Polly currently has the needed version of isl in
its repository, and in order to move Polly into LLVM itself, moving isl
seems like the first step.

Finally, LLVM is a major dependency of many external projects, and
binary side is certainly a concern. In general, we need a better way to
enable not compiling transformations not needed by a particular project.
Polyhedral, or any other, loop transformations are no different from CFL
alias analysis, sanitizer instrumentation passes, or any number of other
things in this regard. We should have a better infrastructure for
partial builds but we shouldn't hold up any particular contribution
because of deficiencies in this area. Also, I believe that the
infrastructure added will be useful for transformations important to
many different frontends (e.g., bounds-check removal and canonicalizing
transformations).

Thanks again,
Hal

On 01/20/2018 12:47 PM, Nadav Rotem via llvm-dev wrote:
>
> Hi Tobi, 
>
> I have some concerns about adding Polly into LLVM proper. I think that
> it's great that Polly is a part of the LLVM umbrella of projects, like
> Clang and LLDB. However, I am not convinced that Polly belongs in the
> LLVM compiler library. LLVM is a major dependency for so many external
> projects. Rust, Swift, GPU drivers by different vendors, and JIT
> compilers all rely on LLVM. Projects that depend on LLVM are going to
> pay the cost of adding Polly into the LLVM library. These projects
> operate under a different set of constraints and optimize different
> metrics. One of my main concerns is binary size. The size of the LLVM
> compiler library matters, especially on mobile, especially for JIT
> compilers. Growing the size of the LLVM binary increases the app load
> time (because the shared object needs to be read from disk). Moreover,
> the current size of the LLVM library prevents people from bundling a
> copy of LLVM with mobile apps because app stores limit the size of
> apps. Yes, I know that it's possible to disable Polly in production
> scenarios, but this looks like an unnecessary hurdle. 
>   
> Would it be possible to use the LLVM plugin mechanism to load Polly
> dynamically into the clang pass manager instead of integrating it into
> the LLVM tree? 
>
> Thanks,
> Nadav 
>
>
> On Jan 15, 2018, at 01:33 PM, Tobias Grosser via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>
>> Dear LLVM community,
>>
>> hope all of you had a good start into 2018 and a quiet branching of
>> LLVM 6.0.
>>
>> With the latest LLVM release out of the way and a longer development
>> phase starting, we would like to restart the process of including
>> Polly and isl into core LLVM to bring changes in early on before the
>> next LLVM release.
>>
>> Short summary:
>>
>> * Today Polly is already part of each LLVM release (and will be
>> shipping with LLVM 6.0) for everybody to try, with conservative defaults.
>> * We proposed to include Polly and isl into LLVM to provide modern
>> high-level loop optimizations into LLVM
>> * We suggested to develop Polly and isl as part of core LLVM to make
>> interactions with the core LLVM community easier and to allow us to
>> better integrate Polly with the new pass manager.
>>
>> Let me briefly summarize the current status:
>>
>> * Michael sent out an official email to discuss how to best include
>> isl into LLVM
>> (http://lists.llvm.org/pipermail/llvm-dev/2018-January/120408.html)
>> * We sent out the LLVM developers meeting notes
>> (_http://lists.llvm.org/pipermail/llvm-dev/2018-January/120419.html_)
>> * Philip Pfaffe prepared a preliminary patch set for integrating
>> Polly closer into LLVM:
>> _https://github.com/pfaffe/llvm-project-20170507/commits/merge-polly-into-upstream_
>> (further cleanup needed)
>> * We are working further with ARM (Florian Hahn and Francesco) to
>> upstream the inliner changes needed for the end-to-end optimization
>> of SPEC 2006 libquantum.   _https://reviews.llvm.org/D38585_
>> * Oleksandr, Sven and Manasij Mukherjee started to look into spatial
>> locality
>> * We worked on expanding the isl C++ bindings
>> (_http://repo.or.cz/isl.git/shortlog_). While a first set of patches
>> is already open, further patches will follow over the next couple of
>> weeks.
>>
>> Let me briefly summarize the LLVM developer meeting comments on our
>> proposal (subjective summary)
>>
>> * Most people were interested in having polyhedral loop optimizations
>> being part of LLVM.
>> * Ideas of uses of isl beyond polyhedral loop scheduling were raised
>> (e.g., for polyhedral value analysis, dependence analysis, or broader
>> assumption tracking). Others were interested in the use of polyhedral
>> loop optimization with “learned” scheduling strategies.
>> * Specific concerns were raised that an integration of Polly into
>> LLVM may be an implicit choice of LLVMs loop optimization future.
>> This is not the case. While Polly is today the only end-to-end
>> high-level loop optimization, other approaches can and should
>> explored (e.g., can there be synergies with the region vectorizer?)
>> * How stable/fast/… is Polly today
>> * We build all of AOSP with rather restrictive compile-time limits
>> * Bootstrapping time of clang is regressed by 6% (at most)
>> * Removal of scalar dependences is today very generic and must be
>> sped up in the future
>> * Polly still shows up at the top of the middle-end, but larger
>> compile time regressions are often due to increased code size (and
>> the LLVM backend)
>> * We see non-trivial speedups for hmmer, libquantum, and various
>> linear-algebra kernels (we use gemm-specific optimizations). The
>> first two require additional flags to be enabled.
>>
>> The precise inclusion agenda has been presented here:
>>
>> http://lists.llvm.org/pipermail/llvm-dev/2017-September/117698.html
>>
>> After having merged communities, I suggest to form a loop
>> optimization working group which jointly discusses how LLVM’s loop
>> optimizations should evolve.
>>
>> I would like to invite comments regarding this proposal.
>> Are there any specific concerns we should address before requesting
>> the initial svn move?
>>
>> Best,
>> Tobias
>>
>>
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>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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