llvm dev - Nov 2021 - [RFC] MLGO Regalloc: learned eviction policy for regalloc

[RFC] MLGO for Regalloc: learned eviction policy for regalloc

alphabetical list of authors: Eugene Brevdo, David Li, Yundi Qian, Mircea
Trofin

Summary: continuing from our experience with the inliner for size
<https://lists.llvm.org/pipermail/llvm-dev/2020-April/140763.html>, we
were
able to learn live range eviction for the ‘greedy’ register allocator, with
measurable, positive results on some datacenter-type applications (from
0.3% up to 1.5% improvement in QPS (query per second) on selected internal
ThinLTO + FDO apps).

Just like in the inliner case, and salient to our approach, opting in to
the learned policy maintains critical compiler invariants, like determinism
and timeliness, and does not add new runtime dependencies.

Details:

The proposed changes are:

   -

   the main change (https://reviews.llvm.org/D113525) is a NFC - factoring
   eviction as an ImmutablePass, RegAllocEvictionAdvisorAnalysis. It can be
   queried for an implementation of RegAllocEvictionAdvisor. By default,
   that implementation is today’s behavior.

This design is similar to what we did for the inliner with InlineAdvisor.
We expect the factoring to create opportunities for other initiatives, just
like it did in the inliner case (see @modimo’s inline replay work, for
example).

   -

   all the ML stuff is an alternative implementation of
   RegAllocEvictionAdvisor, and needs to be opted in. Size of change -
   wise, the ML support will be relatively small, as we are leveraging the
   existing infrastructure we have already introduced with the inliner work
   (including leveraging the same build bots)


MLGO-specific details:

We plan to first introduce ‘development mode’, i.e. the support for
training a policy, then ‘release mode’, i.e. the way in which a pre-trained
policy is meant to be leveraged in production. This greatly simplifies
integration on our side, and should also allow anyone wishing to train a
policy with their training algorithm to do so. Separately, we have been
upstreaming the training infrastructure pertinent to the regalloc work to
github.com/google/ml-compiler-opt.
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