docs/windows_build_instructions.md

# Checking out and Building Chromium for Windows

There are instructions for other platforms linked from the
[get the code](get_the_code.md) page.

## Instructions for Google Employees

Are you a Google employee? See
[go/building-chrome-win](https://goto.google.com/building-chrome-win) instead.

[TOC]

## System requirements

* A 64-bit Intel machine with at least 8GB of RAM. More than 16GB is highly
  recommended.
* At least 100GB of free disk space on an NTFS-formatted hard drive. FAT32
  will not work, as some of the Git packfiles are larger than 4GB.
* An appropriate version of Visual Studio, as described below.
* Windows 10 or newer.

## Setting up Windows

### Visual Studio

Chromium requires [Visual Studio 2022](https://learn.microsoft.com/en-us/visualstudio/releases/2022/release-notes) (>=17.0.0)
to build. Visual Studio can also be used to debug Chromium.
The clang-cl compiler is used but Visual Studio's header files, libraries, and
some tools are required. Visual Studio Community Edition should work if its
license is appropriate for you. You must install the "Desktop development with
C++" component and the "MFC/ATL support" sub-components. This can be done from
the command line by passing these arguments to the Visual Studio installer (see
below for ARM64 instructions):
```shell
$ PATH_TO_INSTALLER.EXE ^
--add Microsoft.VisualStudio.Workload.NativeDesktop ^
--add Microsoft.VisualStudio.Component.VC.ATLMFC ^
--includeRecommended
```

If you want to build for ARM64 Win32 then some extra arguments are needed. The
full set for that case is:
```shell
$ PATH_TO_INSTALLER.EXE ^
--add Microsoft.VisualStudio.Workload.NativeDesktop ^
--add Microsoft.VisualStudio.Component.VC.ATLMFC ^
--add Microsoft.VisualStudio.Component.VC.Tools.ARM64 ^
--add Microsoft.VisualStudio.Component.VC.MFC.ARM64 ^
--includeRecommended
```

Required

* [Windows 11 SDK](https://developer.microsoft.com/en-us/windows/downloads/windows-sdk/)
version 10.0.22621.2428. This can be installed separately or by checking the
appropriate box in the Visual Studio Installer.
* (Windows 11) SDK Debugging Tools 10.0.22621.755 or higher. This version of the
Debugging tools is needed in order to support reading the large-page PDBs that
Chrome uses to allow greater-than 4 GiB PDBs. This can be installed after the
matching Windows SDK version is installed, from: Control Panel -> Programs and
Features
-> Windows Software Development Kit [version] -> Change -> Debugging Tools for
Windows. If building on ARM64 Windows then you will need to manually copy the
Debuggers\x64 directory from another machine because it does not get installed
on ARM64 and is needed, whether you are building Chromium for x64 or ARM64 on
ARM64.

## Install `depot_tools`

Download the
[depot_tools bundle](https://storage.googleapis.com/chrome-infra/depot_tools.zip)
and extract it somewhere (eg: C:\src\depot_tools).

***
**Warning:** **DO NOT** use drag-n-drop or copy-n-paste extract from Explorer,
this will not extract the hidden “.git” folder which is necessary for
depot_tools to autoupdate itself. You can use “Extract all…” from the
context menu though.
***

Add depot_tools to the start of your PATH (must be ahead of any installs of
Python. Note that environment variable names are case insensitive).
* Assuming you unzipped the bundle to C:\src\depot_tools, open: Control Panel → System and Security → System → Advanced system settings
* If you have Administrator access, Modify the PATH system variable and put
`C:\src\depot_tools` at the front (or at least in front of any directory that
might already have a copy of Python or Git).
* If you don't have Administrator access, you can add a user-level PATH
environment variable by opening: Control Panel → System and Security → System →
Search for "Edit environment variables for your account"
* Add `C:\src\depot_tools` at the front. Note: If your system PATH has a Python
in it, you will be out of luck.

Also, add a DEPOT_TOOLS_WIN_TOOLCHAIN environment variable in the same way, and set
it to 0. This tells depot_tools to use your locally installed version of Visual
Studio (by default, depot_tools will try to use a google-internal version).

You may also have to set variable `vs2022_install` to your installation path of
Visual Studio 2022, like
`set vs2022_install=C:\Program Files\Microsoft Visual Studio\2022\Professional`.

From a cmd.exe shell, run:
```shell
$ gclient
```

On first run, gclient will install all the Windows-specific bits needed to work
with the code, including msysgit and python.

* If you run gclient from a non-cmd shell (e.g., cygwin, PowerShell),
  it may appear to run properly, but msysgit, python, and other tools
  may not get installed correctly.
* If you see strange errors with the file system on the first run of gclient,
  you may want to [disable Windows Indexing](https://tortoisesvn.net/faq.html#cantmove2).

## Check python install

After running gclient open a command prompt and type `where python3` and
confirm that the depot_tools `python3.bat` comes ahead of any copies of
python3.exe. Failing to ensure this can lead to overbuilding when
using gn - see [crbug.com/611087](https://crbug.com/611087).

[App Execution Aliases](https://docs.microsoft.com/en-us/windows/apps/desktop/modernize/desktop-to-uwp-extensions#alias)
can conflict with other installations of python on the system so disable
these for 'python.exe' and 'python3.exe' by opening 'App execution aliases'
section of Control Panel and unticking the boxes next to both of these
that point to 'App Installer'.

## Get the code

First, configure Git:

```shell
$ git config --global user.name "My Name"
$ git config --global user.email "my-name@chromium.org"
$ git config --global core.autocrlf false
$ git config --global core.filemode false
$ git config --global branch.autosetuprebase always
```

While not necessarily required it can be helpful to configure git to allow long
path support (beyond the Windows MAX_PATH limit):

```shell
git config --global core.longpaths true
```

Create a `chromium` directory for the checkout and change to it. You can call
this whatever you like and put it wherever you like, as long as the full path
has no spaces. However there are some performance benefits for Googlers in
placing the directory under `C:\src\`
(See [Why is my build slow?](https://chromium.googlesource.com/chromium/src/+/main/docs/windows_build_instructions.md#why-is-my-build-slow)).

```shell
$ mkdir chromium && cd chromium
```

Run the `fetch` tool from `depot_tools` to check out the code and its
dependencies.

```shell
$ fetch chromium
```

If you don't want the full repo history, you can save a lot of time by
adding the `--no-history` flag to `fetch`.

Expect the command to take over an hour on even a fast connection, and many
hours on slower ones. You should configure your PC so that it doesn't sleep
or hibernate during the fetch or else errors may occur. If errors occur while
fetching sub-repos then you can start over, or you may be able to correct them
by going to the chromium/src directory and running this command:

```shell
$ gclient sync
```

When `fetch` completes, it will have created a hidden `.gclient` file and a
directory called `src` in the working directory. The remaining instructions
assume you have switched to the `src` directory:

```shell
$ cd src
```

*Optional*: You can also [install API
keys](https://www.chromium.org/developers/how-tos/api-keys) if you want your
build to talk to some Google services, but this is not necessary for most
development and testing purposes.

## Setting up the build

Chromium uses [Ninja](https://ninja-build.org) as its main build tool along with
a tool called [GN](https://gn.googlesource.com/gn/+/main/docs/quick_start.md)
to generate `.ninja` files. You can create any number of *build directories*
with different configurations. To create a build directory:

```shell
$ gn gen out\Default
```

* You only have to run this once for each new build directory, Ninja will
  update the build files as needed.
* You can replace `Default` with another name, but
  it should be a subdirectory of `out`.
* For other build arguments, including release settings or using an alternate
  version of Visual Studio, see [GN build
  configuration](https://www.chromium.org/developers/gn-build-configuration).
  The default will be a debug component build matching the current host
  operating system and CPU.
* For more info on GN, run `gn help` on the command line or read the [quick
  start guide](https://gn.googlesource.com/gn/+/main/docs/quick_start.md).

### Faster builds

* Reduce file system overhead by excluding build directories from
  antivirus and indexing software.
* Store the build tree on a fast disk (preferably SSD).
* The more cores the better (20+ is not excessive) and lots of RAM is needed
(64 GB is not excessive).

There are some gn flags that can improve build speeds. You can specify these
in the editor that appears when you create your output directory
(`gn args out\Default`) or on the gn gen command line
(`gn gen out\Default --args="is_component_build = true is_debug = true"`).
Some helpful settings to consider using include:
* `is_component_build = true` - this uses more, smaller DLLs, and may avoid
having to relink chrome.dll after every change.
* `enable_nacl = false` - this disables Native Client which is usually not
needed for local builds.
* `target_cpu = "x86"` - x86 builds may be slightly faster than x64 builds. Note
that if you set this but don't set `enable_nacl = false` then build times may
get worse.
* `blink_symbol_level = 0` - turn off source-level debugging for
blink to reduce build times, appropriate if you don't plan to debug blink.
* `v8_symbol_level = 0` - turn off source-level debugging for v8 to reduce
build times, appropriate if you don't plan to debug v8.

In order to speed up linking you can set `symbol_level = 1` or
`symbol_level = 0` - these options reduce the work the compiler and linker have
to do. With `symbol_level = 1` the compiler emits file name and line number
information so you can still do source-level debugging but there will be no
local variable or type information. With `symbol_level = 0` there is no
source-level debugging but call stacks still have function names. Changing
`symbol_level` requires recompiling everything.

When invoking ninja, specify 'chrome' as the target to avoid building all test
binaries as well.

#### Use Reclient

In addition, Google employees should use Reclient, a distributed compilation system.
Detailed information is available internally but the relevant gn arg is:
* `use_remoteexec = true`

Google employees can visit
[go/building-chrome-win#setup-remote-execution](https://goto.google.com/building-chrome-win#setup-remote-execution)
for more information. For external contributors, Reclient does not support
Windows builds.

#### Use Goma (deprecated)

In addition, Google employees should use goma, a distributed compilation system.
Detailed information is available internally but the relevant gn arg is:
* `use_goma = true`

To get any benefit from goma it is important to pass a large -j value to ninja.
A good default is 10\*numCores to 20\*numCores. If you run autoninja then it
will automatically pass an appropriate -j value to ninja for goma or not.

```shell
$ autoninja -C out\Default chrome
```

When invoking ninja, specify 'chrome' as the target to avoid building all test
binaries as well.

Still, builds will take many hours on many machines.

#### Use SCCACHE

You might be able to use [sccache](https://github.com/mozilla/sccache) for the
build process by enabling the following arguments:

* `cc_wrapper = "sccache"` - assuming the `sccache` binary is in your `%PATH%`
* `chrome_pgo_phase = 0`

### Why is my build slow?

Many things can make builds slow, with Windows Defender slowing process startups
being a frequent culprit. Have you ensured that the entire Chromium src
directory is excluded from antivirus scanning (on Google machines this means
putting it in a ``src`` directory in the root of a drive)? Have you tried the
different settings listed above, including different link settings and -j
values? Have you asked on the chromium-dev mailing list to see if your build is
slower than expected for your machine's specifications?

If you suspect that Defender is slowing your build then you can try Microsoft's
[Performance analyzer for Microsoft Defender Antivirus](https://learn.microsoft.com/en-us/microsoft-365/security/defender-endpoint/tune-performance-defender-antivirus?view=o365-worldwide)
to investigate in detail.

The next step is to gather some data. If you set the ``NINJA_SUMMARIZE_BUILD``
environment variable to 1 then ``autoninja`` will do three things. First, it
will set the [NINJA_STATUS](https://ninja-build.org/manual.html#_environment_variables)
environment variable so that ninja will print additional information while
building Chrome. It will show how many build processes are running at any given
time, how many build steps have completed, how many build steps have completed
per second, and how long the build has been running, as shown here:

```shell
$ set NINJA_SUMMARIZE_BUILD=1
$ autoninja -C out\Default base
ninja: Entering directory `out\Default'
[1 processes, 86/86 @ 2.7/s : 31.785s ] LINK(DLL) base.dll base.dll.lib base.dll.pdb
```

This makes slow process creation immediately obvious and lets you tell quickly
if a build is running more slowly than normal.

In addition, setting ``NINJA_SUMMARIZE_BUILD=1`` tells ``autoninja`` to print a
build performance summary when the build completes, showing the slowest build
steps and slowest build-step types, as shown here:

```shell
$ set NINJA_SUMMARIZE_BUILD=1
$ autoninja -C out\Default base
Longest build steps:
       0.1 weighted s to build obj/base/base/trace_log.obj (6.7 s elapsed time)
       0.2 weighted s to build nasm.exe, nasm.exe.pdb (0.2 s elapsed time)
       0.3 weighted s to build obj/base/base/win_util.obj (12.4 s elapsed time)
       1.2 weighted s to build base.dll, base.dll.lib (1.2 s elapsed time)
Time by build-step type:
       0.0 s weighted time to generate 6 .lib files (0.3 s elapsed time sum)
       0.1 s weighted time to generate 25 .stamp files (1.2 s elapsed time sum)
       0.2 s weighted time to generate 20 .o files (2.8 s elapsed time sum)
       1.7 s weighted time to generate 4 PEFile (linking) files (2.0 s elapsed
time sum)
      23.9 s weighted time to generate 770 .obj files (974.8 s elapsed time sum)
26.1 s weighted time (982.9 s elapsed time sum, 37.7x parallelism)
839 build steps completed, average of 32.17/s
```

The "weighted" time is the elapsed time of each build step divided by the number
of tasks that were running in parallel. This makes it an excellent approximation
of how "important" a slow step was. A link that is entirely or mostly serialized
will have a weighted time that is the same or similar to its elapsed time. A
compile that runs in parallel with 999 other compiles will have a weighted time
that is tiny.

You can also generate these reports by manually running the script after a build:

```shell
$ python depot_tools\post_build_ninja_summary.py -C out\Default
```

Finally, setting ``NINJA_SUMMARIZE_BUILD=1`` tells autoninja to tell Ninja to
report on its own overhead by passing "-d stats". This can be helpful if, for
instance, process creation (which shows up in the StartEdge metric) is making
builds slow, perhaps due to antivirus interference due to clang-cl not being in
an excluded directory:

```shell
$ set NINJA_SUMMARIZE_BUILD=1
$ autoninja -C out\Default base
metric                  count   avg (us)        total (ms)
.ninja parse            3555    1539.4          5472.6
canonicalize str        1383032 0.0             12.7
canonicalize path       1402349 0.0             11.2
lookup node             1398245 0.0             8.1
.ninja_log load         2       118.0           0.2
.ninja_deps load        2       67.5            0.1
node stat               2516    29.6            74.4
depfile load            2       1132.0          2.3
StartEdge               88      3508.1          308.7
FinishCommand           87      1670.9          145.4
CLParser::Parse         45      1889.1          85.0
```

You can also get a visual report of the build performance with
[ninjatracing](https://github.com/nico/ninjatracing). This converts the
.ninja_log file into a .json file which can be loaded into [chrome://tracing](chrome://tracing):

```shell
$ python ninjatracing out\Default\.ninja_log >build.json
```

## Build Chromium

Build Chromium (the "chrome" target) with Ninja using the command:

```shell
$ autoninja -C out\Default chrome
```

`autoninja` is a wrapper that automatically provides optimal values for the
arguments passed to `ninja`.

You can get a list of all of the other build targets from GN by running
`gn ls out\Default` from the command line. To compile one, pass to Ninja
the GN label with no preceding "//" (so for `//chrome/test:unit_tests`
use `autoninja -C out\Default chrome/test:unit_tests`).

## Compile a single file

Ninja supports a special [syntax `^`][ninja hat syntax] to compile a single object file specyfing
the source file. For example, `ninja -C out/Default ../../base/logging.cc^`
compiles `obj/base/base/logging.o`.

[ninja hat syntax]: https://ninja-build.org/manual.html#:~:text=There%20is%20also%20a%20special%20syntax%20target%5E%20for%20specifying%20a%20target%20as%20the%20first%20output%20of%20some%20rule%20containing%20the%20source%20you%20put%20in%20the%20command%20line%2C%20if%20one%20exists.%20For%20example%2C%20if%20you%20specify%20target%20as%20foo.c%5E%20then%20foo.o%20will%20get%20built%20(assuming%20you%20have%20those%20targets%20in%20your%20build%20files)

With autoninja, you need to add  `^^` to preserve the trailing `^`.

```shell
$ autoninja -C out\Default ..\..\base\logging.cc^^
```

In addition to `foo.cc^^`, Siso also supports `foo.h^^` syntax to compile
the corresponding `foo.o` if it exists.

## Run Chromium

Once it is built, you can simply run the browser:

```shell
$ out\Default\chrome.exe
```

(The ".exe" suffix in the command is actually optional).

## Running test targets

Tests are split into multiple test targets based on their type and where they
exist in the directory structure. To see what target a given unit test or
browser test file corresponds to, the following command can be used:

```shell
$ gn refs out\Default --testonly=true --type=executable --all chrome\browser\ui\browser_list_unittest.cc
//chrome/test:unit_tests
```

In the example above, the target is unit_tests. The unit_tests binary can be
built by running the following command:

```shell
$ autoninja -C out\Default unit_tests
```

You can run the tests by running the unit_tests binary. You can also limit which
tests are run using the `--gtest_filter` arg, e.g.:

```shell
$ out\Default\unit_tests.exe --gtest_filter="BrowserListUnitTest.*"
```

You can find out more about GoogleTest at its
[GitHub page](https://github.com/google/googletest).

## Update your checkout

To update an existing checkout, you can run

```shell
$ git rebase-update
$ gclient sync -D
```

The first command updates the primary Chromium source repository and rebases
any of your local branches on top of tip-of-tree (aka the Git branch
`origin/main`). If you don't want to use this script, you can also just use
`git pull` or other common Git commands to update the repo.

The second command syncs the subrepositories to the appropriate versions,
deleting those that are no longer needed, and re-runs the hooks as needed.

### Editing and Debugging With the Visual Studio IDE

You can use the Visual Studio IDE to edit and debug Chrome, with or without
Intellisense support.

#### Using Visual Studio Intellisense

If you want to use Visual Studio Intellisense when developing Chromium, use the
`--ide` command line argument to `gn gen` when you generate your output
directory (as described on the [get the code](https://dev.chromium.org/developers/how-tos/get-the-code)
page). This is an example when your checkout is `C:\src\chromium` and your
output directory is `out\Default`:

```shell
$ gn gen --ide=vs --ninja-executable=C:\src\chromium\src\third_party\ninja\ninja.exe out\Default
$ devenv out\Default\all.sln
```

GN will produce a file `all.sln` in your build directory. It will internally
use Ninja to compile while still allowing most IDE functions to work (there is
no native Visual Studio compilation mode). If you manually run "gen" again you
will need to resupply this argument, but normally GN will keep the build and
IDE files up to date automatically when you build.

The generated solution will contain several thousand projects and will be very
slow to load. Use the `--filters` argument to restrict generating project files
for only the code you're interested in. Although this will also limit what
files appear in the project explorer, debugging will still work and you can
set breakpoints in files that you open manually. A minimal solution that will
let you compile and run Chrome in the IDE but will not show any source files
is:

```
$ gn gen --ide=vs --ninja-executable=C:\src\chromium\src\third_party\ninja\ninja.exe --filters=//chrome --no-deps out\Default
```

You can selectively add other directories you care about to the filter like so:
`--filters=//chrome;//third_party/WebKit/*;//gpu/*`.

There are other options for controlling how the solution is generated, run `gn
help gen` for the current documentation.

#### Using Visual Studio without Intellisense

It is also possible to debug and develop Chrome in Visual Studio without the
overhead of a multi-project solution file. Simply "open" your chrome.exe binary
with `File->Open->Project/Solution`, or from a Visual Studio command prompt like
so: `devenv /debugexe out\Debug\chrome.exe <your arguments>`. Many of Visual
Studio's code exploration features will not work in this configuration, but by
installing the [VsChromium Visual Studio Extension](https://chromium.github.io/vs-chromium/)
you can get the source code to appear in the solution explorer window along
with other useful features such as code search. You can add multiple executables
of interest (base_unittests.exe, browser_tests.exe) to your solution with
`File->Add->Existing Project...` and change which one will be debugged by
right-clicking on them in `Solution Explorer` and selecting `Set as Startup
Project`. You can also change their properties, including command line
arguments, by right-clicking on them in `Solution Explorer` and selecting
`Properties`.

By default when you start debugging in Visual Studio the debugger will only
attach to the main browser process. To debug all of Chrome, install
[Microsoft's Child Process Debugging Power Tool](https://blogs.msdn.microsoft.com/devops/2014/11/24/introducing-the-child-process-debugging-power-tool/).
You will also need to run Visual Studio as administrator, or it will silently
fail to attach to some of Chrome's child processes.

### Improving performance of git commands

#### Configure git to use an untracked cache

Try running

```shell
$ git update-index --test-untracked-cache
```

If the output ends with `OK`, then the following may also improve performance of
`git status`:

```shell
$ git config core.untrackedCache true
```

#### Configure git to use fsmonitor

You can significantly speed up git by using [fsmonitor.](https://github.blog/2022-06-29-improve-git-monorepo-performance-with-a-file-system-monitor/)
You should enable fsmonitor in large repos, such as Chromium and v8. Enabling
it globally will launch many processes and consume excess commit/memory and
probably isn't worthwhile. The command to enable fsmonitor in the current repo
is:

```shell
$ git config core.fsmonitor true
```