Cross-compiling Chrome/win

It's possible to build most parts of the codebase on a Linux or Mac host while targeting Windows. This document describes how to set that up, and current restrictions.

What does not work:

goma. Sorry. (internal bug) You can use the jumbo build for faster build times.
64-bit renderer processes don't use V8 snapshots, slowing down their startup (bug)
on Mac hosts, building a 32-bit chrome (bug)

All other targets build fine (including chrome, browser_tests, ...).

Uses of .asm files have been stubbed out. As a result, some of Skia's software rendering fast paths are not present in cross builds, Crashpad cannot report crashes, and NaCl defaults to disabled and cannot be enabled in cross builds (.asm bug).

.gclient setup

Tell gclient that you need Windows build dependencies by adding target_os = ['win'] to the end of your .gclient. (If you already have a target_os line in there, just add 'win' to the list.) e.g.
```
solutions = [
  {
    ...
  }
]
target_os = ['android', 'win']
```
gclient sync, follow instructions on screen.

If you're at Google, this will automatically download the Windows SDK for you. If this fails with an error:

Please follow the instructions at
https://chromium.googlesource.com/chromium/src/+/master/docs/windows_build_instructions.md

then you may need to re-authenticate via:

cd path/to/chrome/src
# Follow instructions, enter 0 as project id.
download_from_google_storage --config

If you are not at Google, you can package your Windows SDK installation into a zip file by running the following on a Windows machine:

cd path/to/depot_tools/win_toolchain
# customize the Windows SDK version numbers
python package_from_installed.py 2017 -w 10.0.17134.0

These commands create a zip file named <hash value>.zip. Then, to use the generated file in a Linux or Mac host, the following environment variables need to be set:

export DEPOT_TOOLS_WIN_TOOLCHAIN_BASE_URL=<path/to/sdk/zip/file>
export GYP_MSVS_HASH_<toolchain hash>=<hash value>

<toolchain hash> is hardcoded in src/build/vs_toolchain.py and can be found by setting DEPOT_TOOLS_WIN_TOOLCHAIN_BASE_URL and running gclient sync:

gclient sync
...
Running hooks:  17% (11/64) win_toolchain
________ running '/usr/bin/python src/build/vs_toolchain.py update --force' in <chromium dir>
Windows toolchain out of date or doesn't exist, updating (Pro)...
current_hashes:
desired_hash: <toolchain hash>

GN setup

Add target_os = "win" to your args.gn. Then just build, e.g.

ninja -C out/gnwin base_unittests.exe

Copying and running chrome

A convenient way to copy chrome over to a Windows box is to build the mini_installer target. Then, copy just mini_installer.exe over to the Windows box and run it to install the chrome you just built.

Running tests on swarming

You can run the Windows binaries you built on swarming, like so:

tools/run-swarmed.py -C out/gnwin -t base_unittests [ --gtest_filter=... ]

See the contents of run-swarmed.py for how to do this manually.

The linux-win_cross-rel buildbot does 64-bit release cross builds, and also runs tests. You can look at it to get an idea of which tests pass in the cross build.