Speed up label comparison

The Label::operator< function appears high in the profile of "gn gen"
for the Chromium and Fuchsia build, so this CL optimizes its
implementation by using the fact that StringAtom and SourceDir are
both unique/interned string pointer values, with very fast equality
comparison.

This saves about 1s from the Fuchsia 'gn gen' step.

Measurements:

/work/chromium0/src$ repeat_cmd 5 /tmp/gn-master gen --check out/Release
Done. Made 13413 targets from 2220 files in 6620ms
Done. Made 13413 targets from 2220 files in 6552ms
Done. Made 13413 targets from 2220 files in 6450ms
Done. Made 13413 targets from 2220 files in 6532ms
Done. Made 13413 targets from 2220 files in 6346ms (best)

/work/chromium0/src$ repeat_cmd 5 /tmp/gn-fast-label gen --check out/Release
Done. Made 13413 targets from 2220 files in 6785ms
Done. Made 13413 targets from 2220 files in 6369ms
Done. Made 13413 targets from 2220 files in 6285ms (best)
Done. Made 13413 targets from 2220 files in 6285ms
Done. Made 13413 targets from 2220 files in 6391ms

/work/fx-gn$ GN=prebuilt/third_party/gn/linux-x64/gn
/work/fx-gn$ cp -f /tmp/gn-master $GN
/work/fx-gn$ repeat_cmd 5 $GN gen --check out/default
Done. Made 42114 targets from 3695 files in 12574ms (best)
Done. Made 42114 targets from 3695 files in 12932ms
Done. Made 42114 targets from 3695 files in 13155ms
Done. Made 42114 targets from 3695 files in 13238ms
Done. Made 42114 targets from 3695 files in 13108ms

/work/fx-gn$ cp -f /tmp/gn-fast-label $GN
/work/fx-gn$ repeat_cmd 5 $GN gen --check out/default
Done. Made 42114 targets from 3695 files in 12252ms
Done. Made 42114 targets from 3695 files in 12336ms
Done. Made 42114 targets from 3695 files in 12438ms
Done. Made 42114 targets from 3695 files in 11938ms
Done. Made 42114 targets from 3695 files in 11601ms (best)

Change-Id: I80c0bd47e89c101c7c42fe57cc71592b2b27713e
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/8000
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>
2 files changed
tree: 2c8ce7a06ad36e2794ed513bd3d47cd3ab852bad
  1. .clang-format
  2. .editorconfig
  3. .gitignore
  4. .style.yapf
  5. AUTHORS
  6. LICENSE
  7. OWNERS
  8. README.md
  9. build/
  10. docs/
  11. examples/
  12. infra/
  13. misc/
  14. src/
README.md

GN

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 https://gn.googlesource.com/gn
cd gn
python build/gen.py
ninja -C out
# To run tests:
out/gn_unittests

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.

Examples

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

To build and run the simple example with the default gcc compiler:

cd examples/simple_build
../../out/gn gen -C out
ninja -C out
./out/hello

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 https://gn-review.googlesource.com.

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

Then, to upload a change for review:

git commit
git push origin HEAD:refs/for/master

The first time you do this you'll get an error from the server about a missing change-ID. Follow the directions in the error message to install the change-ID hook and run git commit --amend to apply the hook to the current commit.

When revising a change, use:

git commit --amend
git push origin HEAD:refs/for/master

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’).

Community

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