mold: A Modern Linker

mold is a faster drop-in replacement for existing Unix linkers. It is several times quicker than the LLVM lld linker, the second-fastest open-source linker, which I initially developed a few years ago. mold aims to enhance developer productivity by minimizing build time, particularly in rapid debug-edit-rebuild cycles.

Here is a performance comparison of GNU ld, GNU gold, LLVM lld, and mold when linking final debuginfo-enabled executables for major large programs on a simulated 16-core, 32-thread machine.

Link speed comparison

Program (linker output size)	GNU ld	GNU gold	LLVM lld	mold
MySQL 8.3 (0.47 GiB)	10.84s	7.47s	1.64s	0.46s
Clang 19 (1.56 GiB)	42.07s	33.13s	5.20s	1.35s
Chromium 124 (1.35 GiB)	N/A	27.40s	6.10s	1.52s

mold is so fast that it is only 2x slower than the cp command on the same machine. If you find that mold is not faster than other linkers, feel free to file a bug report.

mold supports x86-64, i386, ARM64, ARM32, 64-bit/32-bit little/big-endian RISC-V, 32-bit PowerPC, 64-bit big-endian PowerPC ELFv1, 64-bit little-endian PowerPC ELFv2, s390x, 64-bit/32-bit LoongArch, SPARC64, m68k, and SH-4.

Why does linking speed 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 and combines them into a single executable or shared library file.

The second phase can be time-consuming if your build output is large. mold can speed up this process, saving you time and preventing distractions while waiting for a lengthy build to finish. The difference is most noticeable during rapid debug-edit-rebuild cycles.

Installation

Binary packages for the following systems are currently available:

How to Build

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

Install Dependencies

To install build dependencies, run ./install-build-deps.sh in this directory. It will detect your Linux distribution and attempt to install the necessary packages. You may need to run it as root.

Compile mold

git clone --branch stable https://github.com/rui314/mold.git
cd mold
./install-build-deps.sh
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=c++ -B build
cmake --build build -j$(nproc)
sudo cmake --build build --target install

You might need to pass a C++20 compiler command name to cmake. In the example above, c++ is passed. If that 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. You can change the installation location by passing -DCMAKE_INSTALL_PREFIX=<directory>. For other cmake options, see the comments in CMakeLists.txt.

If you are not using a recent enough Linux distribution, or if cmake does not work for you for any reason, you can use Docker to build mold in a Docker environment. To do so, run ./dist.sh in this directory instead of using cmake. The shell script will pull a Docker image, build mold and auxiliary files inside it, and package them into a single tar file named mold-$version-$arch-linux.tar.gz. You can extract the tar file anywhere and use the mold executable within it.

How to use

A classic way to use mold

On Unix, the linker command (usually `/usr/bin/ld`) is indirectly invoked by the compiler driver (typically `cc`, `gcc`, or `clang`), which is in turn indirectly invoked by `make` or other build system commands. If you can specify an additional command line option for your compiler driver by modifying the build system's config files, add one of the following flags to use mold instead of `/usr/bin/ld`: - For Clang: pass `-fuse-ld=mold` - For GCC 12.1.0 or later: pass `-fuse-ld=mold` - For GCC before 12.1.0: the `-fuse-ld` option does not accept `mold` as a valid argument, so you need to use the `-B` option instead. The `-B` option tells GCC where to look for external commands like `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 `$PREFIX`), and the `ld` command should be there. The `ld` is actually a symlink to `mold`. So, all you need is to pass `-B/usr/libexec/mold` (or `-B/usr/local/libexec/mold`) to GCC. If you haven't installed `ld.mold` to any `$PATH`, you can still pass `-fuse-ld=/absolute/path/to/mold` to clang to use mold. However, GCC does not accept an absolute path as an argument for `-fuse-ld`.

If you are using Rust

Create `.cargo/config.toml` in your project directory with the following: ```toml [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 the mold executable. In the example above, we use `clang` as a linker driver since it always accepts the `-fuse-ld` option. If your GCC is recent enough to recognize the option, you may be able to remove the `linker = "clang"` line. ```toml [target.x86_64-unknown-linux-gnu] rustflags = ["-C", "link-arg=-fuse-ld=mold"] ``` If you want to use mold for all projects, add the above snippet to `~/.cargo/config.toml`.

If you are using Nim

Create `config.nims` in your project directory with the following: ```nim when findExe("mold").len > 0 and defined(linux): switch("passL", "-fuse-ld=mold") ``` where `mold` must be included in the `PATH` environment variable. In this example, `gcc` is used as the linker driver. Use the `-fuse-ld` option if your GCC is recent enough to recognize this option. If you want to use mold for all projects, add the above snippet to `~/.config/config.nims`.

mold -run

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

GitHub Actions

You can use our [setup-mold](https://github.com/rui314/setup-mold) GitHub Action to speed up GitHub-hosted continuous builds. Although GitHub Actions run on a 4 core machine, mold is still significantly faster than the default GNU linker, especially when linking large programs.

Verify that you are using mold

mold leaves its identification string in the `.comment` section of an output file. You can print it out to verify that you are actually using mold. ```shell $ readelf -p .comment String dump of section '.comment': [ 0] GCC: (Ubuntu 10.2.0-5ubuntu1~20.04) 10.2.0 [ 2b] mold 9a1679b47d9b22012ec7dfbda97c8983956716f7 ``` If `mold` is present in the `.comment` section, the file was created by mold.

Online manual

Since mold is a drop-in replacement, you should be able to use it without reading its manual. However, if you need it, [mold's man page](docs/mold.md) is available online. You can read the same manual by running `man mold`.

Why is mold so fast?

One reason is that it utilizes faster algorithms and more efficient data structures compared to other linkers. Another reason is that mold is highly parallelized.

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

CPU usage comparison in htop animation

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

For details, please see the design notes.

rui314 / mold

readme