mold: A Modern Linker

This repository contains a free version of the mold linker. If you are looking for a commercial version that supports macOS please visit the repository of the sold 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 gold, LLVM lld, and mold when linking final debuginfo-enabled executables for major large programs on a simulated 8-core, 16-thread machine.

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 the cp command on the same machine. If you find that mold is not faster than other linkers, please 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, SH-4, and DEC Alpha.

mold/macOS is commercial software. For mold/macOS, please visit https://github.com/bluewhalesystems/sold.

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 https://github.com/rui314/mold.git
mkdir mold/build
cd mold/build
git checkout v2.4.0
../install-build-deps.sh
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=c++ ..
cmake --build . -j $(nproc)
sudo cmake --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 another build system command.

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:

[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.

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

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.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:

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

Internally, mold invokes the 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.gold, or ld.lld.

GitHub Actions

You can use our setup-mold GitHub Action to speed up GitHub-hosted continuous builds. Although GitHub Actions run on a two-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.

$ 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 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 is available. 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.

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.

habbyge/mold