win: Only use ICF and LTO if explicitly opted in.

The bots pass --use-icf and --use-lto, so it doesn't affect them.
Makes the Windows build more like the non-Windows build, and
makes incremental build times (== local dev) in the default build
config 20x faster.

(It also makes it possible to set CC/CXX to clang-cl without
getting warnings about /GL, but that's not my motivation here.
But I might want to use goma to build gn in the future, at least
on my laptop, and then that'll come in handy.)

Also add /Zc:inline to cflags in release builds on Windows.

Also change non-Win to use LTO only in release builds -- LTO in
debug builds doesn't make much sense.

Release build times (always `ninja -C out`, i.e. linking both
gn.exe and gn_unittests.exe):

With this patch:
Full: 4m25s
Touch just src\gn\ 3.8s
Size of out\: 844 MB
Peak memory use during build: 250 MB

With this patch but no /Zc:inline:
Full: 4m25s
Touch just src\gn\ 4.8s
Size of out\: 1.05 GB
Peak memory use during build: 345 MB

Without this patch (== this patch with --use-icf --use-lto;
/Zc:inline seems to have no effect when used with /GL):
Full: 4m33s
Touch just src\gn\ 1m55s (!)
Size of out\: 1.52 GB
Peak memory use during build: 678 MB

Change-Id: Idd87389c7537828fe6523a64de6031f9cbbc0fc6
Reviewed-by: Scott Graham <>
Commit-Queue: Nico Weber <>
1 file changed
tree: 91143a5c0fce4ac58135d009f3503ad9289be3e7
  1. build/
  2. docs/
  3. examples/
  4. infra/
  5. misc/
  6. src/
  7. .clang-format
  8. .editorconfig
  9. .gitignore
  10. .style.yapf
  13. OWNERS


GN is a meta-build system that generates build files for Ninja.

Related resources:

Getting a binary

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.


There is a simple example in examples/simple_build directory that is a good place to get started with the minimal configuration.

For a maximal configuration see the Chromium setup:

and the Fuchsia setup:

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 with GN‘s development on Chromium’s gn-dev@ Google Group.