mold

Mold: A Modern Linker 🦠

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Quick Overview

mold is a high-performance linker for ELF, Mach-O, and PE executables. It is designed to be a drop-in replacement for the GNU linker (ld) and the LLVM linker (lld), providing faster build times and smaller binary sizes.

Pros

Fast Linking: mold is designed to be significantly faster than traditional linkers, with benchmarks showing up to 10x faster linking times.
Smaller Binary Sizes: mold can produce smaller executable files compared to other linkers, reducing the overall size of the final build.
Cross-Platform: mold supports multiple executable formats (ELF, Mach-O, PE) and can be used on various operating systems, including Linux, macOS, and Windows.
Compatibility: mold aims to be a drop-in replacement for existing linkers, allowing for easy integration into existing build systems.

Cons

Limited Documentation: The project's documentation is relatively sparse, which may make it challenging for new users to get started.
Experimental Status: As a relatively new project, mold is still considered experimental and may not have the same level of maturity and stability as more established linkers.
Limited Ecosystem: Since mold is a relatively new project, the ecosystem of tools and libraries that integrate with it may be more limited compared to other linkers.
Potential Compatibility Issues: While mold aims to be a drop-in replacement, there may be some compatibility issues or edge cases that users need to be aware of when transitioning from other linkers.

Code Examples

N/A (mold is a linker, not a code library)

Getting Started

N/A (mold is a linker, not a code library)

Competitor Comparisons

8cc

6,127

A Small C Compiler

Pros of 8cc

8cc is a small and simple C compiler, making it a good learning resource for understanding compiler internals.
The codebase is well-documented and easy to navigate, making it a good starting point for those interested in compiler development.
8cc supports a wide range of target architectures, including x86, ARM, and RISC-V.

Cons of 8cc

8cc is primarily focused on being a learning tool and may not be suitable for large-scale production use.
The compiler does not support advanced language features like C++, which may limit its usefulness for some projects.
The development of 8cc appears to be less active compared to mold, with fewer recent commits and updates.

Code Comparison

Here's a brief comparison of the code structure between 8cc and mold:

8cc (main.c):

int main(int argc, char **argv) {
    if (argc < 2) {
        fprintf(stderr, "usage: %s <file>\n", argv[0]);
        return 1;
    }
    return compile(argv[1]);
}

mold (main.cpp):

int main(int argc, char **argv) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <object-file>...\n", argv[0]);
        return 1;
    }
    return link(argc - 1, argv + 1);
}

As you can see, the main function in 8cc is more focused on the compilation process, while the main function in mold is focused on the linking process.

chibicc

9,556

A small C compiler

Pros of chibicc

chibicc is a self-contained, single-file C compiler, making it easy to understand and modify.
chibicc supports a wide range of C language features, including structs, functions, and pointers.
chibicc has a simple and straightforward codebase, making it a good learning resource for those interested in compiler design.

Cons of chibicc

chibicc is primarily focused on being a learning resource, and may not be as optimized or feature-rich as other C compilers.
chibicc has a limited set of target architectures, primarily focusing on x86-64.
chibicc may not be suitable for large-scale, production-ready projects due to its limited feature set and optimization capabilities.

Code Comparison

Here's a brief comparison of the code structure between chibicc and mold:

chibicc (main.c):

int main(int argc, char **argv) {
    if (argc != 2) {
        fprintf(stderr, "usage: chibicc <source>\n");
        return 1;
    }

    // Parse the input file
    user_input = argv[1];
    token = tokenize();
    program();

    // Traverse the AST and generate assembly
    codegen();
    return 0;
}

mold (main.cpp):

int main(int argc, char **argv) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <object files or archives>\n", argv[0]);
        return 1;
    }

    // Initialize the linker
    LdDriver ld;
    ld.parseCommandLine(argc, argv);
    ld.run();
    return 0;
}

As you can see, the main functions of the two projects have a similar structure, but the implementation details and the overall scope of the projects are quite different.

9cc

1,806

A Small C Compiler

Pros of 9cc

9cc is a simple and lightweight C compiler, making it a good choice for learning and understanding the fundamentals of compiler design.
The codebase is well-documented and easy to navigate, making it a valuable resource for those interested in compiler development.
9cc supports a subset of the C language, which can be useful for specific projects or educational purposes.

Cons of 9cc

9cc has a more limited feature set compared to mold, as it only supports a subset of the C language.
The performance of 9cc may not be as optimized as mold, as it is a simpler and more lightweight compiler.
9cc may not be suitable for large-scale or complex projects that require a more robust and feature-rich compiler.

Code Comparison

9cc (5 lines):

int main() {
    printf("Hello, world!\n");
    return 0;
}

mold (5 lines):

int main() {
    printf("Hello, world!\n");
    return 0;
}

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README

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, SH-4, and DEC Alpha.

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:

[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=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.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.

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.

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Quick Overview

Pros

Cons

Code Examples

Getting Started

Competitor Comparisons

Pros of 8cc

Cons of 8cc

Code Comparison

Pros of chibicc

Cons of chibicc

Code Comparison

Pros of 9cc

Cons of 9cc

Code Comparison

Convert designs to code with AI

README

mold: A Modern Linker

Why does linking speed matter?

Installation

How to Build

Install Dependencies

Compile mold

How to use

Why is mold so fast?

Sponsors

Corporate sponsors

Individual sponsors

Top Related Projects

Convert designs to code with AI