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<div class="moz-cite-prefix">On 11/10/2017 11:12 AM, Amara Emerson
via llvm-dev wrote:<br>
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Hi everyone,
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<div class="">This RFC concerns the design and architecture of a
generic machine instruction combiner/optimizer framework to be
developed as part of the GISel pipeline. As we transition from
correctness and reducing the fallback rate to SelectionDAG at
-O0, we’re now starting to think about using GlobalISel with
optimizations enabled. There are obviously many parts to this
story as optimizations happen at various stages of the codegen
pipeline. The focus of this RFC is the replacement of the
equivalent of the DAGCombiner in SDAG land. Despite the focus on
the DAGCombiner, since there aren’t perfect 1-1 mappings between
SDAG and GlobalISel components, this may also include features
that are currently implemented as part of the target lowerings,
and tablegen isel patterns. As we’re starting from a blank
slate, we have an opportunity here to think about what we might
need from such a framework without the legacy cruft (although we
still have the high performance bar to meet).</div>
<div class=""><br class="">
</div>
<div class="">I want to poll the community about what future
requirements we have for the GISel G_MI optimizer/combiner. The
following are the general requirements we have so far:</div>
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</div>
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<ol class="MailOutline">
<li class="">It should have at least equivalent, but hopefully
better runtime/compile time trade off than the DAGCombiner.</li>
<li class="">There needs to be flexibility in the design to
allow targets to run subsets of the overall optimizer. For
example, some targets may want to avoid trying to run
certain types of optimizations like vector or FP combines if
they’re either not applicable, or not worth the compile
time.</li>
<li class="">Have a reasonably concise way to write most
optimizations. Hand written C++ will always be an option,
but there’s value in having easy to read and reason about
descriptions of transforms.</li>
</ol>
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<div class="">These requirements aren’t set in stone nor complete,
but using them as a starting point: a single monolithic “Generic
MI combiner” component doesn’t look like the right approach. Our
current thinking is that, like we’ve done with the Legalizer,
the specific mechanics of the actual optimization should be
separated into it’s own unit. This would allow the combines to
be re-used at different stages of the pipeline according to
target needs. Using the current situation with instcombine as an
example, there is no way to explicitly pick and choose a
specific subset of IC, it’s only available as a whole pass with
all the costs that entails.</div>
<div class=""><br class="">
</div>
<div class="">The reasoning behind req 3 is that there may be
compile time savings available if we can describe in a
declarative style the combines we want to do, like it’s
currently possible with tablegen patterns. This hasn’t been
proven out yet, but consider an alternative where we use the
machine instruction equivalent of the IR/PatternMatch tooling
which allows easy and expressive matching of IR sub-trees. A
concern I have with using that as the main approach to writing
combines is that it’s easy to add new matchers in an routine
which re-computes information that’s previously been computed in
previous match() attempts.</div>
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<br>
I share this concern.<br>
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<div class=""> This form of back-tracking might be avoided if we
can reason about a group of combines together automatically (or
perhaps we could add caching capabilities to PatternMatch).</div>
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<div class=""><br class="">
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<div class="">What would everyone else like to see from this?</div>
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<br>
The current DAGCombine, being constructed on top of SDAG, has a kind
of built-in CSE and automatic DCE. How will things change, if
they'll change, in this new model?<br>
<br>
Thanks again,<br>
Hal<br>
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<div class="">Thanks,</div>
<div class="">Amara</div>
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</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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