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compile-size Builder

[TOC]

About

The compile-size trybot measures and prevents unexpected growth in the size of the C++ compiler input size, which closely correlates with time spent in the build.

Overview

This builder does the following:

  1. With the CL and without the CL applied:
    1. Builds chrome on Linux
    2. Queries siso for build deps and commands
    3. Generates a compile size report broken down by TU (translation unit)
  2. Compares the two reports and writes a diff report with a summary and TU deltas
  3. If a size increase threshold is exceeded and there is no override then fail the build

What to do if the check fails?

  1. Inspect the report
  2. Iterate on a fix
    1. Run locally to collect a baseline (or skip this to iterate remotely)
    2. If applicable, remove transitive includes by using forward declarations
    3. If stuck, ask for help
      1. If still stuck, override the builder
  3. Confirm results reproduce remotely

Inspect the report

  1. On the compile-size builder's overview or infra page look for the summary and the link to the compile_size_deltas.txt report. alt text
  2. The report shows translation units with the largest increases. These can give clues about the source of the increase. For example, when many TUs show a similar increase in size, it could mean that a header was added to be transitively included in all of those TUs. alt text

Run a local compile-size check

If you want to check TU sizes locally to iterate faster, do the following:

# Set up a new build directory with the builder's GN args
tools/mb/mb gen -m tryserver.chromium.linux -b compile-size out/compile_size

# Build the `chrome` target
autoninja -C out/compile_size chrome

# Collect the size info
siso query commands -C out/compile_size chrome > /tmp/commands.txt
siso query deps -C out/compile_size > /tmp/deps.txt
tools/clang/scripts/compiler_inputs_size.py out/compile_size \
  /tmp/commands.txt /tmp/deps.txt > /tmp/report.txt

The report will contain lines with compile sizes in bytes per TU, like

apps/app_lifetime_monitor.cc 9,034,754
apps/app_lifetime_monitor_factory.cc 5,863,660
apps/app_restore_service.cc 9,198,130

As you make changes, you can re-build, collect size info, and generate reports to see how the per-TU numbers change. Two reports can be diffed with compiler_inputs_size_diff.py.

tools/clang/scripts/compiler_inputs_size_diff.py /tmp/report-old.txt \
  /tmp/report-new.txt > /tmp/diff-report.txt

Run remotely

Once done iterating on a fix locally (or if you skipped local iterations), you can upload a new patchset or CL and trigger the compile-size builder to get a new result and report.

How to reduce your compiler input size

Forward Declarations

A common source of compiler input size growth is adding a header to another widely included header. This can result in the added header getting compiled an additional thousands or tens of thousands of times.

Chromium style prefers forward declarations over includes when a full definition isn't required. For advice on when to use forward declarations and how to minimize code in headers see these tips.

Example CLs replacing header inclusion with forward declarations resulting in large compiler input size decreases:

Get help

For help, post to [email protected]. They're expert build system developers. You may also ask #cxx or #halp on Chromium slack.

Skip the check

Not all checks are perfect and sometimes you want to bypass the builder (for example, if you did your best and are unable to reduce compiler input size any further).

Adding a Compile-Size: $ANY_TEXT_HERE footer to your CL description (above or below Bug: ) will bypass the builder assertions.

Examples:

  • Compile-Size: Size increase is unavoidable.
  • Compile-Size: Increase is temporary.
  • Compile-Size: See commit description. <-- use this if longer than one line.

***note Note: Make sure there are no blank lines between Compile-Size: and other footers.

Builder Implementation

The analysis is done with the following scripts:

The GN args are in the builder's gn-args.json file.

References