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corona10 requested a review from erlend-aasland as a code owner April 16, 2023 04:41

erlend-aasland reviewed Apr 16, 2023

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erlend-aasland approved these changes Apr 17, 2023

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This commit overhauls the make-based build system's rules for
building optimized binaries. Along the way it fixes a myriad of
bugs and shortcomings with the prior approach.

The old way of producing optimized binaries had various limitations:

* `make [all]` would do work when PGO was enabled because the phony
  `profile-opt` rule was non-empty. This prevented no-op PGO builds
   from working at all. This meant workflows like `make; make install`
   either incurred extra work or failed due to race conditions.
* Same thing for BOLT, as its `bolt-opt` rule was also non-empty
  and always ran during `make [all]`.
* BOLT could not be run multiple times without a full rebuild because
  `llvm-bolt` can't instrument binaries that have already received
  BOLT optimizations.
* It was difficult to run BOLT on its own because of how various make
  targets and their dependencies were structured.
* I found the old way that configure and make communicated the default
  targets to be confusing and hard to understand.

There are essentially 2 major changes going on in this commit:

1. A rework of the high-level make targets for performing a build and
   how they are defined.
2. A rework of all the make logic related to profile-based optimization
   (read: PGO and BOLT).

Build Target Rework
===================

Before, we essentially had `build_all`, `profile-opt`, `bolt-opt` and
`build_wasm` as our 3 targets for performing a build. `all` would alias
to one of these, as appropriate.

And there was another definition for which _simple_ make target to
evaluate for non-optimized builds. This was likely `build_all` or
`all`.

In the rework, we introduce 2 new high-level targets:

* `build-plain` - Perform a build without optimizations.
* `build-optimized` - Perform a build with optimizations.

`build-plain` is aliased to `build_all` in all configurations except
WASM, where it is `build_wasm`.

`build-optimized` by default is aliased to a target that prints an error
message when optimizations aren't enabled. If PGO or BOLT are enabled,
it is aliased to their respective target.

`build-optimized` is the logical successor to `profile-opt`.

I felt it best to delete `profile-opt` completely, as the new `build-*`
high-level targets feel more friendly to use. But if people lament its
loss, we can add a `profile-opt: build-optimized` to achieve almost the
same result.

Profiled-Based Optimization Rework
==================================

Most of the make logic related to profile-based optimization (read: PGO
and BOLT) has been touched in this change.

A major issue with the old way of doing things was we used phony,
always-executed make rules. This is a bad practice in make because it
undermines no-op builds.

Another issue is that the separation between the rules and what order
they ran in wasn't always clear. Both PGO and BOLT consist of the same
4 phase solution: instrument, run, analyze, and apply. However, these
steps weren't clearly expressed in the make logic. This is especially
true for BOLT, which only had 1 make rule.

Another issue with BOLT is that it was really easy to get things into
a bad state. e.g. if you applied BOLT to `pythonX.Y` you could not
run BOLT again unless you rebuilt `pythonX.Y` from source.

In the new world, we have separate `profile-<tool>-<stage>-stamp`
rules defining the 4 distinct `instrument`, `run`, `analyze`, and
`apply` stages for both PGO and BOLT. Each of these stages is tracked
by a _stamp_ semaphore file so progress can be captured. This should
all be pretty straightforward.

There is some minimal complexity here to handle BOLT's optional
dependency on PGO, as BOLT either depends on `build_all` or
`profile-pgo-apply-stamp`.

As part of the refactor to BOLT we also preserve the original input
binary before BOLT is applied. This original file is restored if
BOLT runs again. This greatly simplifies repeated BOLT invocations,
as make doesn't perform needless work. However, this is all best
effort, as it is possible for some make target evaluations to still
get things in a bad state.

Other Remarks
=============

This change effectively reverts pythongh-103574. The readelf based mechanism
inserted by that change was effectively working around shortcomings
in the make DAG. This change addresses those shortcomings so the
readelf integration is no longer required.

If this change perturbs any bugs, they are likely around cleaning
behavior. The cleaning rules are a bit complicated and not clearly
documented. And I'm unsure which targets CPython developers often
iterate on. It is highly possible that state cleanup of PGO and/or
BOLT files isn't as robust as it needs to be.

I explicitly deleted some calls to PGO cleanup because those calls
prevented no-op `make [all]` from working. It is certainly possible
something somewhere (release automation?) relied on these files being
deleted when they no longer are. We still have targets to purge profile
files and it should be trivial to add these to appropriate make rules.

What I'm trying to say is there are likely subtle workflow regressions
with this refactor. But in my mind it is 3 steps forward 1 step back.
It should be pretty straightforward to fix any regressions once people
complain.