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211 lines
7.5 KiB
Markdown
211 lines
7.5 KiB
Markdown
# mold: A Modern Linker
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mold is a faster drop-in replacement for existing Unix linkers.
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It is several times faster than the LLVM lld linker, the second-fastest
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open-source linker which I originally created a few years ago.
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mold is designed to increase developer productivity by reducing
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build time, especially in rapid debug-edit-rebuild cycles.
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Here is a performance comparison of GNU gold, LLVM lld, and mold for
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linking final debuginfo-enabled executables of major large programs
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on a simulated 8-core 16-threads machine.
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| Program (linker output size) | GNU gold | LLVM lld | mold
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|-------------------------------|----------|----------|--------
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| Chrome 96 (1.89 GiB) | 53.86s | 11.74s | 2.21s
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| Clang 13 (3.18 GiB) | 64.12s | 5.82s | 2.90s
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| Firefox 89 libxul (1.64 GiB) | 32.95s | 6.80s | 1.42s
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mold is so fast that it is only 2x _slower_ than `cp` on the same
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machine. Feel free to [file a bug](https://github.com/rui314/mold/issues)
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if you find mold is not faster than other linkers.
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mold currently supports x86-64, i386, ARM64 and 64-bit RISC-V.
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## Why does the speed of linking matter?
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If you are using a compiled language such as C, C++ or Rust, a build
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consists of two phases. In the first phase, a compiler compiles a
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source file into an object file (`.o` files). In the second phase,
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a linker takes all object files to combine them into a single executable
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or a shared library file.
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The second phase takes a long time if your build output is large.
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mold can make it faster, saving your time and keeping you from being
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distracted while waiting for a long build to finish. The difference is
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most noticeable when you are in rapid debug-edit-rebuild cycles.
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## Install
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Binary packages for the following distros are currently available.
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[](https://repology.org/project/mold/versions)
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## How to build
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mold is written in C++20, so if you build mold yourself, you need a
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recent version of a C++ compiler and a C++ standard library. GCC 10.2
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or Clang 12.0.0 as well as libstdc++ 10 or libc++ 7 are recommended.
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I'm using Ubuntu 20.04 as a development platform. In that environment,
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you can build mold by the following commands.
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### Install dependencies
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#### Ubuntu 20.04 and later / Debian 11 and later
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```shell
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sudo apt-get update
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sudo apt-get install -y build-essential git clang cmake libstdc++-10-dev libssl-dev libxxhash-dev zlib1g-dev pkg-config
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```
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#### Fedora 34 and later
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```shell
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sudo dnf install -y git clang cmake openssl-devel xxhash-devel zlib-devel libstdc++-devel
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```
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### Compile mold
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```shell
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git clone https://github.com/rui314/mold.git
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cd mold
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git checkout v1.0.3
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make -j$(nproc) CXX=clang++
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sudo make install
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```
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You may need to pass a C++20 compiler command name to `make`.
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In the above case, `clang++` is passed. If it doesn't work for you,
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try a specific version of a compiler such as `g++-10` or `clang++-12`.
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By default, `mold` is installed to `/usr/local/bin`.
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If you don't use a recent enough Linux distribution, or if for any reason `make`
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in the above commands doesn't work for you, you can use Docker to build it in
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a Docker environment. To do so, just run `./build-static.sh` in this
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directory instead of running `make -j$(nproc)`. The shell script creates a
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Ubuntu 20.04 Docker image, installs necessary tools and libraries to it,
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and builds mold as a statically-linked executable.
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(If `build-static.sh` script didn't work, please apply [3232c1f8](https://github.com/rui314/mold/commit/3232c1f8840885e5656c8cee7fab5f8e52926bd8).)
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`make test` depends on a few more packages. To install, run the following commands:
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```shell
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sudo dpkg --add-architecture i386
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sudo apt update
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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-gnu
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```
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## How to use
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<details><summary>A classic way to use mold</summary>
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On Unix, the linker command (which is usually `/usr/bin/ld`) is
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invoked indirectly by the compiler driver (which is usually `cc`,
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`gcc` or `clang`), which is typically in turn indirectly invoked by
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`make` or some other build system command.
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If you can specify an additional command line option to your compiler
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driver by modifying build system's config files, add one of the
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following flags to use `mold` instead of `/usr/bin/ld`:
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- Clang: pass `-fuse-ld=mold`
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- GCC 12.1.0 (upcoming version) or later: pass `-fuse-ld=mold`
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- GCC before 12.1.0: `-fuse-ld` does not accept `mold` as a valid
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argument, so you need to use `-B` option instead. `-B` is an option
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to tell GCC where to look for external commands such as `ld`.
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If you have installed mold with `make install`, there should be a
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directory named `/usr/libexec/mold` (or `/usr/local/libexec/mold`,
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depending on your $PREFIX), and `ld` command should be there. The
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`ld` is actually a symlink to `mold`. So, all you need is to pass
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`-B/usr/libexec/mold` (or `-B/usr/local/libexec/mold`) to GCC.
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If you haven't installed `mold` to any `$PATH`, you can still pass
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`-fuse-ld=/absolute/path/to/mold` to clang to use mold. GCC does not
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take an absolute path as an argument for `-fuse-ld` though.
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</details>
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<details><summary>If you are using Rust</summary>
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Create `.cargo/config.toml` in your project directory with the following:
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```
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[target.x86_64-unknown-linux-gnu]
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linker = "clang"
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rustflags = ["-C", "link-arg=-fuse-ld=/path/to/mold"]
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```
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where `/path/to/mold` is an absolute path to `mold` exectuable.
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Please make sure you have installed `clang`.
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If you want to use mold for all projects, put the above snippet to
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`~/.cargo/config.toml`.
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</details>
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<details><summary>mold -run</summary>
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It is sometimes very hard to pass an appropriate command line option
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to `cc` to specify an alternative linker. To deal with the situation,
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mold has a feature to intercept all invocations of `ld`, `ld.lld` or
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`ld.gold` and redirect it to itself. To use the feature, run `make`
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(or another build command) as a subcommand of mold as follows:
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```shell
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mold -run make <make-options-if-any>
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```
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Internally, mold invokes a given command with `LD_PRELOAD` environment
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variable set to its companion shared object file. The shared object
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file intercepts all function calls to `exec(3)`-family functions to
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replace `argv[0]` with `mold` if it is `ld`, `ld.gold` or `ld.lld`.
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</details>
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<details><summary>Verify that you are using mold</summary>
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mold leaves its identification string in `.comment` section in an output
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file. You can print it out to verify that you are actually using mold.
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```shell
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readelf -p .comment <executable-file>
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String dump of section '.comment':
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[ 0] GCC: (Ubuntu 10.2.0-5ubuntu1~20.04) 10.2.0
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[ 2b] mold 9a1679b47d9b22012ec7dfbda97c8983956716f7
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```
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If `mold` is in `.comment`, the file is created by mold.
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</details>
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## Why is mold so fast?
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One reason is because it simply uses faster algorithms and efficient
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data structures than other linkers do. The other reason is that the
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new linker is highly parallelized.
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Here is a side-by-side comparison of per-core CPU usage of lld (left)
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and mold (right). They are linking the same program, Chromium
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executable.
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As you can see, mold uses all available cores throughout its execution
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and finishes quickly. On the other hand, lld failed to use available
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cores most of the time. In this demo, the maximum parallelism is
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artificially capped to 16 so that the bars fit in the GIF.
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For details, please read [design notes](docs/design.md).
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# Logo
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-
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