2020-10-21 08:55:50 +03:00
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# mold: A Modern Linker
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2020-10-21 06:52:11 +03:00
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2021-12-12 15:19:12 +03:00
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mold is a faster drop-in replacement for existing Unix linkers.
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2022-02-13 00:53:17 +03:00
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It is several times faster than the LLVM lld linker, the second-fastest
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2021-12-12 15:19:12 +03:00
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open-source linker which I originally created a few years ago.
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2022-02-13 00:53:17 +03:00
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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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2020-10-21 06:52:11 +03:00
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2021-08-06 02:55:25 +03:00
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Here is a performance comparison of GNU gold, LLVM lld, and mold for
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2021-12-12 15:19:12 +03:00
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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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2021-03-21 07:51:34 +03:00
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2021-12-12 15:19:12 +03:00
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![Link speed comparison](docs/comparison.png)
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2021-12-13 09:28:26 +03:00
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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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2021-12-12 15:19:12 +03:00
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mold is so fast that it is only 2x _slower_ than `cp` on the same
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2022-01-10 18:12:12 +03:00
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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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2021-03-21 07:51:34 +03:00
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2022-04-08 13:21:50 +03:00
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mold currently supports x86-64, i386, ARM32, ARM64 and 64-bit RISC-V.
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2021-12-15 04:44:10 +03:00
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2021-12-12 15:19:12 +03:00
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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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2022-04-20 05:14:42 +03:00
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consists of two phases. In the first phase, a compiler compiles
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source files into object files (`.o` files). In the second phase,
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2021-12-12 15:19:12 +03:00
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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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2021-04-18 15:41:36 +03:00
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2021-12-12 15:19:12 +03:00
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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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2021-04-19 16:25:13 +03:00
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2021-12-22 13:59:39 +03:00
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## Install
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2022-04-28 15:23:55 +03:00
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Binary packages for the following systems are currently available.
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2021-12-22 13:59:39 +03:00
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[![Packaging status](https://repology.org/badge/vertical-allrepos/mold.svg)](https://repology.org/project/mold/versions)
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2021-03-16 19:35:32 +03:00
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## How to build
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2021-12-22 13:59:39 +03:00
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mold is written in C++20, so if you build mold yourself, you need a
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2022-01-12 07:46:44 +03:00
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recent version of a C++ compiler and a C++ standard library. GCC 10.2
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2022-04-20 05:14:42 +03:00
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or Clang 12.0.0 (or later) as well as libstdc++ 10 or libc++ 7 (or
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later) are recommended.
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2021-03-16 19:35:32 +03:00
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2021-10-27 20:22:38 +03:00
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### Install dependencies
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2022-04-23 13:22:53 +03:00
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To install build dependencies, run `./install-build-deps.sh` in this
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directory. It recognizes your Linux distribution and tries to install
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necessary packages. You may want to run it as root.
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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 v1.4.1
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2022-01-07 12:42:40 +03:00
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make -j$(nproc) CXX=clang++
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sudo make install
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```
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2022-01-07 12:42:40 +03:00
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You may need to pass a C++20 compiler command name to `make`.
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In the above case, `clang++` is passed. If it doesn't work for you,
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try a specific version of a compiler such as `g++-10` or `clang++-12`.
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2021-12-13 09:28:26 +03:00
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By default, `mold` is installed to `/usr/local/bin`.
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2021-03-24 12:38:08 +03:00
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2022-02-19 05:56:20 +03:00
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If you don't use a recent enough Linux distribution, or if for any reason
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`make` in the above commands doesn't work for you, you can use Docker to
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build it in a Docker environment. To do so, just run `./dist.sh` in this
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directory instead of running `make -j$(nproc)`. The shell script pulls a
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Docker image, builds mold and auxiliary files inside it, and packs
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them into a single tar file `mold-$version-$arch-linux.tar.gz`.
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You can extract the tar file anywhere and use `mold` executable in it.
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2021-04-19 17:08:29 +03:00
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2021-03-26 11:06:35 +03:00
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## How to use
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<details><summary>A classic way to use mold</summary>
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2021-03-26 11:06:35 +03:00
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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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2021-06-30 20:45:04 +03:00
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- Clang: pass `-fuse-ld=mold`
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- GCC 12.1.0 or later: pass `-fuse-ld=mold`
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2021-12-29 15:21:06 +03:00
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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 `$PREFIX`), and `ld` command should be there. The
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`ld` is actually a symlink to `mold`. So, all you need is to pass
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`-B/usr/libexec/mold` (or `-B/usr/local/libexec/mold`) to GCC.
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2021-12-29 15:36:06 +03:00
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If you haven't installed `mold` to any `$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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2021-12-27 06:14:30 +03:00
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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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2021-09-23 08:31:08 +03:00
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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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2021-10-27 20:22:38 +03:00
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```shell
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mold -run make <make-options-if-any>
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```
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2021-03-26 11:06:35 +03:00
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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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2021-12-27 06:14:30 +03:00
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</details>
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2022-08-17 12:40:16 +03:00
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<details><summary>On macOS</summary>
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mold/macOS is available as an alpha version. It can be used to build not
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only macOS apps but also iOS apps because their binary formats are the same.
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The command name of mold/macOS is `ld64.mold`. If you build mold on macOS,
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it still produces `mold` and `ld.mold`, but these executables are useful
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only for cross compilation (i.e. building Linux apps on macOS.)
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If you find any issue with mold/macOS, please file it to
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<a href=https://github.com/rui314/mold/issues>our GitHub Issues</a>.
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</details>
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2022-06-03 14:38:05 +03:00
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<details><summary>GitHub Actions</summary>
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You can use our <a href=https://github.com/rui314/setup-mold>setup-mold</a>
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GitHub Action to speed up GitHub-hosted continuous build. GitHub Actions
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runs on a two-core machine, but mold is still significantly faster than
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the default GNU linker there especially when a program being linked is
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large.
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2022-04-10 04:03:06 +03:00
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</details>
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2022-02-13 09:16:05 +03:00
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<details><summary>Verify that you are using mold</summary>
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2021-03-24 12:38:08 +03:00
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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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2021-10-27 20:22:38 +03:00
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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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2022-02-13 09:16:05 +03:00
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</details>
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2022-08-14 10:06:15 +03:00
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<details><summary>Online manual</summary>
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Since mold is a drop-in replacement, you should be able to use it
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without reading its manual. But just in case you need it, it's available
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online at <a href=https://rui314.github.io/mold.html>here</a>.
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You can also read the same manual by `man mold`.
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</details>
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2021-12-12 15:19:12 +03:00
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## Why is mold so fast?
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2021-12-13 09:28:26 +03:00
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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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2021-12-12 15:19:12 +03:00
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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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![CPU usage comparison in htop animation](docs/htop.gif)
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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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2021-12-13 09:28:26 +03:00
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For details, please read [design notes](docs/design.md).
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2022-05-05 16:37:42 +03:00
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## License
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mold is available under AGPL. Note that that does not mean that you
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have to license your program under AGPL if you use mold to link your
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2022-05-05 16:37:42 +03:00
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program. An output of the mold linker is a derived work of the object
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files and libraries you pass to the linker but not a derived work of
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the mold linker itself.
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2022-05-14 09:47:38 +03:00
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Besides that, you can also buy a commercial, non-AGPL license with
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technical support from our company, Blue Whale Systems PTE LTD. If you
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are a big company, please consider obtaining it before making hundreds
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or thousands of developers of your company to depend on mold. mold is
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mostly a single-person open-source project, and just like other
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open-source projects, we are not legally obligated to keep maintaining
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it. A legally-binding commercial license contract addresses the
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concern. By purchasing a license, you are guaranteed that mold will be
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maintained for you. Please [contact us](mailto:contact@bluewhale.systems)
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for a commercial license inquiry.
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2022-08-04 10:22:01 +03:00
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## Acknowledgement
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We accept donations via [GitHub Sponsors](https://github.com/sponsors/rui314)
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and [OpenCollective](https://opencollective.com/mold-linker).
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We thank you to everybody who sponsors our project. In particular,
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we'd like to acknowledge the following people and organizations who
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have sponsored $128/month or more:
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- [300baud](https://github.com/300baud)
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- [Mercury](https://github.com/MercuryTechnologies)
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- [Wei Wu](https://github.com/lazyparser)
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- [Signal Slot Inc.](https://github.com/signal-slot)
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