More efficient Value implementation.

Using a union to store all possible inner data-types means less
work whenever creating / copying / moving / destroying values,
and small size for each instance.

This results in a slightly faster "gn gen" operation, using
slightly less memory, i.e. when used with a Chromium checkout:

  $ /usr/bin/time -f "%M %U" /tmp/aa/gn-origin gen out/Release
  Done. Made 21651 targets from 1614 files in 6339ms
  764468 25.30

  $ /usr/bin/time -f "%M %U" /tmp/aa/gn-better-value gen out/Release
  Done. Made 21651 targets from 1614 files in 6014ms
  738132 23.27,

Change-Id: I6804857b1e69653713f038903f9abbbcae6ec62c
Reviewed-by: Brett Wilson <>
Commit-Queue: Brett Wilson <>
2 files changed
tree: b03ac2bc9433237324b910cfdec1bfc293767663
  1. .clang-format
  2. .editorconfig
  3. .gitignore
  4. .style.yapf
  9. base/
  10. build/
  11. docs/
  12. infra/
  13. tools/
  14. util/


GN is a meta-build system that generates build files for Ninja. There is documentation in docs/ and a presentation on it.

Getting started

You can download the latest version of GN binary for Linux, macOS and Windows.

Alternatively, you can build GN from source:

git clone
cd gn
python build/
ninja -C out
# To run tests:

On Windows, it is expected that cl.exe, link.exe, and lib.exe can be found in PATH, so you'll want to run from a Visual Studio command prompt, or similar.

On Linux and Mac, the default compiler is clang++, a recent version is expected to be found in PATH. This can be overridden by setting CC, CXX, and AR.

Reporting bugs

If you find a bug, you can see if it is known or report it in the bug database.

Sending patches

GN uses Gerrit for code review. The short version of how to patch is:

Register at

... edit code ...
ninja -C out && out/gn_unittests

Then, to upload a change for review:

git commit
git cl upload --gerrit

When revising a change, use:

git commit --amend
git cl upload --gerrit

which will add the new changes to the existing code review, rather than creating a new one.

We ask that all contributors sign Google's Contributor License Agreement (either individual or corporate as appropriate, select ‘any other Google project’).


You may ask questions and follow along w/ GN‘s development on Chromium’s gn-dev@ Google Group.