mold is a faster replacement for existing Unix linkers.

It is several times faster than the LLVM lld and GNU ld linker.

mold aims to increase developer productivity by minimizing build time, especially in fast debug, edit and rebuild cycles.

Here is a performance comparison of GNU ld, GNU gold, LLVM lld, and mold when linking final debuginfo-enabled executables for large programs on a simulated 16-core, 32-thread machine.

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, 64/32-bit LoongArch, SPARC64, m68k, SH-4 and DEC Alpha.

Here is a side-by-side comparison of CPU usage per core for lld (left) and mold (right), linking the same program, a Chromium executable.

Why is link speed important?

If you are using a compiled language like C, C++ or Rust, a compilation consists of two phases.

In the first phase, a compiler compiles the 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 the build output is large.

Mold can speed up this process, saving time and avoiding distractions while waiting for a long build to be completed.

The difference is most noticeable during rapid debug, edit, and rebuild cycles.

Installation

mold is available in almost all package manager repositories, example:

sudo brew install mold # Brew
sudo apt install mold # APT
sudo emerge mold # Portage
sudo dnf install mold # DNF

And among many others! Use search in your package manager to find it!

Usage

If you are going to compile C/C++ files, for example use the flag: fuse-ld=mold:

# Clang
clang++ -fuse-ld=mold main.cpp

# GCC
g++ -fuse-ld=mold main.cpp

If you are going to compile Assembly files, example hello.s (GNU Assembler - GAS):

.section.data
hello:
     .ascii "Hello, World!\n"

.section.text
.globl_start

_start:
     mov $1, %rax # syscall: sys_write
     mov $1, %rdi # file descriptor: stdout
     mov $hello, %rsi # string address
     mov $14, %rdx # string length
     syscall # calls the kernel

     mov $60, %rax # syscall: sys_exit
     xor %rdi, %rdi # exit status: 0
     syscall # calls the kernel

Compile normally:

as --64 -o hello.o hello.s

And use the template to link:

mold -o hello hello.o

Checking if elf was linked with mold:

readelf -p .comment a.out

# Example output

Verbatim dump from '.comment' section:
   [ 0] GCC: (Ubuntu 12.3.0-1ubuntu1~23.04) 12.3.0
   [2b] mold 1.10.1 (compatible with GNU ld)

For more information access the repository on GiiHub.

cpp clanguage rust

Marcos Oliveira

Software developer

https://github.com/terroo