mold is a faster drop-in replacement for existing Unix linkers. It is several times faster than LLVM lld linker, the second-fastest open-source linker which I originally created a few years ago. mold is created for increasing 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.
| 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 if you find mold is not faster than other linkers.
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 a
source file into an object file (.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.
Binary packages for the following distros are currently available.
mold is written in C++20, so if you build mold yourself, you need a very recent version of GCC or Clang. I'm using Ubuntu 20.04 as a development platform. In that environment, you can build mold by the following commands.
sudo apt-get update
sudo apt-get install -y build-essential git clang cmake libstdc++-10-dev libssl-dev libxxhash-dev zlib1g-devsudo dnf install -y git clang cmake openssl-devel xxhash-devel zlib-devel libstdc++-develgit clone https://github.com/rui314/mold.git
cd mold
git checkout v1.0.1
make -j$(nproc)
sudo make installBy default, mold is installed to /usr/local/bin.
If you don't use a recent enough Linux distribution, or if for any reason make
in the above commands doesn't work for you, you can use Docker to build it in
a Docker environment. To do so, just run ./build-static.sh in this
directory instead of running make -j$(nproc). The shell script creates a
Ubuntu 20.04 Docker image, installs necessary tools and libraries to it,
and builds mold as a statically-linked executable.
make test depends on a few more packages. To install, run the following commands:
sudo dpkg --add-architecture i386
sudo apt update
sudo apt-get install bsdmainutils dwarfdump libc6-dev:i386 lib32gcc-10-dev libstdc++-10-dev-arm64-cross gcc-10-aarch64-linux-gnu g++-10-aarch64-linux-gnuA classic way to use mold
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-lddoes not acceptmoldas a valid argument, so you need to use-Boption instead.-Bis an option to tell GCC where to look for external commands such asld.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), andldcommand should be there. Theldis actually a symlink tomold. 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.
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 mold exectuable.
If you want to use mold for all projects, put the above snippet to
~/.cargo/config.toml.
mold -run
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:
mold -run make <make-options-if-any>Here's an example showing how to link Rust code when using the cargo package manager:
mold -run cargo buildInternally, 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.
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.
readelf -p .comment <executable-file>
String dump of section '.comment':
[ 0] GCC: (Ubuntu 10.2.0-5ubuntu1~20.04) 10.2.0
[ 2b] mold 9a1679b47d9b22012ec7dfbda97c8983956716f7If mold is in .comment, the file is created by mold.
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.
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.
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