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gh-90536: Add support for the BOLT post-link binary optimizer (gh-95908)

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* Add support for the BOLT post-link binary optimizer

Using [bolt](https://github.com/llvm/llvm-project/tree/main/bolt)
provides a fairly large speedup without any code or functionality
changes. It provides roughly a 1% speedup on pyperformance, and a
4% improvement on the Pyston web macrobenchmarks.

It is gated behind an `--enable-bolt` configure arg because not all
toolchains and environments are supported. It has been tested on a
Linux x86_64 toolchain, using llvm-bolt built from the LLVM 14.0.6
sources (their binary distribution of this version did not include bolt).

Compared to [a previous attempt](https://github.com/faster-cpython/ideas/issues/224),
this commit uses bolt's preferred "instrumentation" approach, as well as adds some non-PIE
flags which enable much better optimizations from bolt.

The effects of this change are a bit more dependent on CPU microarchitecture
than other changes, since it optimizes i-cache behavior which seems
to be a bit more variable between architectures. The 1%/4% numbers
were collected on an Intel Skylake CPU, and on an AMD Zen 3 CPU I
got a slightly larger speedup (2%/4%), and on a c6i.xlarge EC2 instance
I got a slightly lower speedup (1%/3%).

The low speedup on pyperformance is not entirely unexpected, because
BOLT improves i-cache behavior, and the benchmarks in the pyperformance
suite are small and tend to fit in i-cache.

This change uses the existing pgo profiling task (`python -m test --pgo`),
though I was able to measure about a 1% macrobenchmark improvement by
using the macrobenchmarks as the training task. I personally think that
both the PGO and BOLT tasks should be updated to use macrobenchmarks,
but for the sake of splitting up the work this PR uses the existing pgo task.

* Simplify the build flags

* Add a NEWS entry

* Update Makefile.pre.in

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>

* Update configure.ac

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>

* Add myself to ACKS

* Add docs

* Other review comments

* fix tab/space issue

* Make it more clear that --enable-bolt is experimental

* Add link to bolt's github page

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>
This commit is contained in:
Kevin Modzelewski 2022-08-18 17:33:54 -04:00 • committed by GitHub
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21
Doc/using/configure.rst
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@ -191,7 +191,8 @@ Performance options
-------------------
Configuring Python using ``--enable-optimizations --with-lto`` (PGO + LTO) is
recommended for best performance.
recommended for best performance. The experimental ``--enable-bolt`` flag can
also be used to improve performance.
.. cmdoption:: --enable-optimizations
@ -231,6 +232,24 @@ recommended for best performance.
.. versionadded:: 3.11
To use ThinLTO feature, use ``--with-lto=thin`` on Clang.
.. cmdoption:: --enable-bolt
Enable usage of the `BOLT post-link binary optimizer
<https://github.com/llvm/llvm-project/tree/main/bolt>` (disabled by
default).
BOLT is part of the LLVM project but is not always included in their binary
distributions. This flag requires that ``llvm-bolt`` and ``merge-fdata``
are available.
BOLT is still a fairly new project so this flag should be considered
experimental for now. Because this tool operates on machine code its success
is dependent on a combination of the build environment + the other
optimization configure args + the CPU architecture, and not all combinations
are supported.
.. versionadded:: 3.12
.. cmdoption:: --with-computed-gotos
Enable computed gotos in evaluation loop (enabled by default on supported