# mold: A Modern Linker

mold is a faster drop-in replacement for existing Unix linkers.
It is several times faster than the LLVM lld linker, the second-fastest
open-source linker which I originally created a few years ago.
mold is designed to increase developer productivity by reducing
build time, especially in rapid debug-edit-rebuild cycles.

Here is a performance comparison of GNU gold, LLVM lld, and mold for
linking final debuginfo-enabled executables of major large programs
on a simulated 8-core 16-threads machine.

![Link speed comparison](docs/comparison.png)

| Program (linker output size)  | GNU gold | LLVM lld | mold
|-------------------------------|----------|----------|--------
| Chrome 96 (1.89 GiB)          | 53.86s   | 11.74s   | 2.21s
| Clang 13 (3.18 GiB)           | 64.12s   | 5.82s    | 2.90s
| Firefox 89 libxul (1.64 GiB)  | 32.95s   | 6.80s    | 1.42s

mold is so fast that it is only 2x _slower_ than `cp` on the same
machine. Feel free to [file a bug](https://github.com/rui314/mold/issues)
if you find mold is not faster than other linkers.

mold currently supports x86-64, i386, ARM32, ARM64 and 64-bit RISC-V.

## Why does the speed of linking matter?

If you are using a compiled language such as C, C++ or Rust, a build
consists of two phases. In the first phase, a compiler compiles
source files into object files (`.o` files). In the second phase,
a linker takes all object files to combine them into a single executable
or a shared library file.

The second phase takes a long time if your build output is large.
mold can make it faster, saving your time and keeping you from being
distracted while waiting for a long build to finish. The difference is
most noticeable when you are in rapid debug-edit-rebuild cycles.

## Install

Binary packages for the following systems are currently available.

[![Packaging status](https://repology.org/badge/vertical-allrepos/mold.svg)](https://repology.org/project/mold/versions)

## How to build

mold is written in C++20, so if you build mold yourself, you need a
recent version of a C++ compiler and a C++ standard library. GCC 10.2
or Clang 12.0.0 (or later) as well as libstdc++ 10 or libc++ 7 (or
later) are recommended.

### Install dependencies

To install build dependencies, run `./install-build-deps.sh` in this
directory. It recognizes your Linux distribution and tries to install
necessary packages. You may want to run it as root.

### Compile mold

```shell
git clone https://github.com/rui314/mold.git
cd mold
git checkout v1.4.0
make -j$(nproc) CXX=clang++
sudo make install
```

You may need to pass a C++20 compiler command name to `make`.
In the above case, `clang++` is passed. If it doesn't work for you,
try a specific version of a compiler such as `g++-10` or `clang++-12`.

By default, `mold` is installed to `/usr/local/bin`.

If you don't use a recent enough Linux distribution, or if for any reason
`make` in the above commands doesn't work for you, you can use Docker to
build it in a Docker environment. To do so, just run `./dist.sh` in this
directory instead of running `make -j$(nproc)`. The shell script pulls a
Docker image, builds mold and auxiliary files inside it, and packs
them into a single tar file `mold-$version-$arch-linux.tar.gz`.
You can extract the tar file anywhere and use `mold` executable in it.

## How to use

<details><summary>A classic way to use mold</summary>

On Unix, the linker command (which is usually `/usr/bin/ld`) is
invoked indirectly by the compiler driver (which is usually `cc`,
`gcc` or `clang`), which is typically in turn indirectly invoked by
`make` or some other build system command.

If you can specify an additional command line option to your compiler
driver by modifying build system's config files, add one of the
following flags to use `mold` instead of `/usr/bin/ld`:

- Clang: pass `-fuse-ld=mold`

- GCC 12.1.0 or later: pass `-fuse-ld=mold`

- GCC before 12.1.0: `-fuse-ld` does not accept `mold` as a valid
  argument, so you need to use `-B` option instead. `-B` is an option
  to tell GCC where to look for external commands such as `ld`.

  If you have installed mold with `make install`, there should be a
  directory named `/usr/libexec/mold` (or `/usr/local/libexec/mold`,
  depending on your `$PREFIX`), and `ld` command should be there. The
  `ld` is actually a symlink to `mold`. So, all you need is to pass
  `-B/usr/libexec/mold` (or `-B/usr/local/libexec/mold`) to GCC.

If you haven't installed `mold` to any `$PATH`, you can still pass
`-fuse-ld=/absolute/path/to/mold` to clang to use mold. GCC does not
take an absolute path as an argument for `-fuse-ld` though.

</details>

<details><summary>If you are using Rust</summary>

Create `.cargo/config.toml` in your project directory with the following:

```
[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=/path/to/mold"]
```

where `/path/to/mold` is an absolute path to `mold` exectuable.
Please make sure you have installed `clang`.

If you want to use mold for all projects, put the above snippet to
`~/.cargo/config.toml`.

</details>

<details><summary>mold -run</summary>

It is sometimes very hard to pass an appropriate command line option
to `cc` to specify an alternative linker.  To deal with the situation,
mold has a feature to intercept all invocations of `ld`, `ld.lld` or
`ld.gold` and redirect it to itself. To use the feature, run `make`
(or another build command) as a subcommand of mold as follows:

```shell
mold -run make <make-options-if-any>
```

Internally, mold invokes a given command with `LD_PRELOAD` environment
variable set to its companion shared object file. The shared object
file intercepts all function calls to `exec(3)`-family functions to
replace `argv[0]` with `mold` if it is `ld`, `ld.gold` or `ld.lld`.

</details>

<details><summary>GitHub Actions</summary>

You can use our <a href=https://github.com/rui314/setup-mold>setup-mold</a>
GitHub Action to speed up GitHub-hosted continuous build. GitHub Actions
runs on a two-core machine, but mold is still significantly faster than
the default GNU linker there especially when a program being linked is
large.

</details>

<details><summary>Verify that you are using mold</summary>

mold leaves its identification string in `.comment` section in an output
file. You can print it out to verify that you are actually using mold.

```shell
$ readelf -p .comment <executable-file>

String dump of section '.comment':
  [     0]  GCC: (Ubuntu 10.2.0-5ubuntu1~20.04) 10.2.0
  [    2b]  mold 9a1679b47d9b22012ec7dfbda97c8983956716f7
```

If `mold` is in `.comment`, the file is created by mold.

</details>

## Why is mold so fast?

One reason is because it simply uses faster algorithms and efficient
data structures than other linkers do. The other reason is that the
new linker is highly parallelized.

Here is a side-by-side comparison of per-core CPU usage of lld (left)
and mold (right). They are linking the same program, Chromium
executable.

![CPU usage comparison in htop animation](docs/htop.gif)

As you can see, mold uses all available cores throughout its execution
and finishes quickly. On the other hand, lld failed to use available
cores most of the time. In this demo, the maximum parallelism is
artificially capped to 16 so that the bars fit in the GIF.

For details, please read [design notes](docs/design.md).

## License

mold is available under AGPL. Note that that does not mean that you
have to license your program under AGPL if you use mold to link your
program. An output of the mold linker is a derived work of the object
files and libraries you pass to the linker but not a derived work of
the mold linker itself.

Besides that, you can also buy a commercial, non-AGPL license with
technical support from our company, Blue Whale Systems PTE LTD. If you
are a big company, please consider obtaining it before making hundreds
or thousands of developers of your company to depend on mold. mold is
mostly a single-person open-source project, and just like other
open-source projects, we are not legally obligated to keep maintaining
it. A legally-binding commercial license contract addresses the
concern. By purchasing a license, you are guaranteed that mold will be
maintained for you. Please [contact us](mailto:contact@bluewhale.systems)
for a commercial license inquiry.

## Acknowledgement

We accept donations via [GitHub Sponsors](https://github.com/sponsors/rui314)
and [OpenCollective](https://opencollective.com/mold-linker).
We thank you to everybody who sponsors our project. In particular,
we'd like to acknowledge the following people and organizations who
have sponsored $128/month or more:

- [300baud](https://github.com/300baud)
- [Mercury](https://github.com/MercuryTechnologies)
- [Wei Wu](https://github.com/lazyparser)
- [Signal Slot Inc.](https://github.com/signal-slot)
