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.
| 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 16.0.0 (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.
Compile mold
git clone --branch stable https://github.com/rui314/mold.git
cd mold
sudo ./install-build-deps.sh
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=c++ -B build
cmake --build build -j$(nproc)
sudo cmake --build build --target install
You might need to pass a C++20 compiler command name to cmake. In the
example above, c++ is passed. If that doesn't work for you, try a specific
version of a compiler, such as g++-10 or clang++-12.
By default, mold is installed to /usr/local/bin. You can change the
installation location by passing -DCMAKE_INSTALL_PREFIX=<directory>.
For other cmake options, see the comments in CMakeLists.txt.
If you are not using a recent enough Linux distribution, or if cmake does
not work for you for any reason, you can use Podman to build mold in a
container. To do so, run ./dist.sh in this directory instead of using
cmake. The shell script will pull a container image, build mold and auxiliary
files inside it, and package them into a single tar file named
dist/mold-$version-$arch-linux.tar.gz. You can extract the tar file anywhere
and use the mold executable in 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:
[target.'cfg(target_os = "linux")']
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.'cfg(target_os = "linux")']
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:
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.
If you are using Conan package manager
You can configure Conan to download the latest
version of mold and use it as the linker when building your dependencies and
projects from source. Please see the instructions here.
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:
mold -run make <make-options-if-any>
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 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.
$ 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 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.
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.
Sponsors
It is taken for granted nowadays that compiler toolchains can be easily
installed and used for free, and people may not think too much about the
individuals behind these "free tools". mold supports many projects, but it
is essentially a one-person project. This situation is similar to the one
depicted in the following xkcd illustration.
If you think that the "Nebraska guy" should be rewarded, please consider
becoming our GitHub sponsor!
We thank everyone who sponsors our project. In particular, we'd like to acknowledge
the following people and organizations who have sponsored $128/month or more:
Corporate sponsors
- G-Research
- Signal Slot Inc.
- GlareDB
Individual sponsors
- Wei Wu
- kyle-elliott
- Bryant Biggs
- kraptor23
- Jinkyu Yi
- Pedro Navarro
