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 7 or newer.

## Setting up Windows

### Visual Studio

Chromium requires Visual Studio 2017 (>=15.7.2) to build, but VS2019 (>=16.0.0)
is preferred. Visual Studio can also be used to debug Chromium, and VS2019 is
preferred for this as it handles Chromium's large debug information much better.
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
```

You must have the version 10.0.18362 or higher Windows 10 SDK installed. This
can be installed separately or by checking the appropriate box in the Visual
Studio Installer.

The SDK Debugging Tools must also be installed. If the Windows 10 SDK was
installed via the Visual Studio installer, then they can be installed by going
to: Control Panel → Programs → Programs and Features → Select the "Windows
Software Development Kit" → Change → Change → Check "Debugging Tools For
Windows" → Change. Or, you can download the standalone SDK installer and use it
to install the Debugging Tools.

## Install `depot_tools`

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

*** note
**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). 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 and put `C:\src\depot_tools` at the front, but
if your system PATH has a Python in it, you will be out of luck.

Also, add a DEPOT_TOOLS_WIN_TOOLCHAIN system 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).

From a cmd.exe shell, run the command gclient (without arguments). 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](http://tortoisesvn.tigris.org/faq.html#cantmove2).

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

## 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
```

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):

```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 30 minutes on even a fast connection, and many
hours on slower ones.

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/+/master/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/+/master/docs/quick_start.md).

### Using the Visual Studio IDE

If you want to use the Visual Studio IDE, 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):

```shell
$ gn gen --ide=vs 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 --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.

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.

It is also possible to debug and develop Chrome in Visual Studio without a
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 editing 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.

### 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 incremental
linking.
* `enable_nacl = false` - this disables Native Client which is usually not
needed for local builds.
* `target_cpu = "x86"` - x86 builds are slightly faster than x64 builds and
support incremental linking for more targets. 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.

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.

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.

### 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?

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
"c:\src\depot_tools\ninja.exe" -C out\Default base -j 10 -d stats
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:

```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 ninja -C out/Default chrome/test:unit_tests`).

## 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

You can run the tests in the same way. You can also limit which tests are
run using the `--gtest_filter` arg, e.g.:

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

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
```

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/master`).
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 and
re-runs the hooks as needed.