# Mach-O LLD Port

LLD is a linker from the LLVM project that is a drop-in replacement for system linkers and runs much faster than them. It also provides features that are useful for toolchain developers. This document will describe the Mach-O port.

## Features

- LLD is a drop-in replacement for Apple’s Mach-O linker, ld64, that accepts the same command line arguments.

- LLD is very fast. When you link a large program on a multicore machine, you can expect that LLD runs more than twice as fast as the ld64 linker.

## Download

LLD is available as a pre-built binary by going to the latest release, downloading the appropriate bundle (`clang+llvm-<version>-<your architecture>-<your platform>.tar.xz`), decompressing it, and locating the binary at `bin/ld64.lld`. Note that if `ld64.lld` is moved out of `bin`, it must still be accompanied by its sibling file `lld`, as `ld64.lld` is technically a symlink to `lld`.

## Build

The easiest way to build LLD is to check out the entire LLVM projects/sub-projects from a git mirror and build that tree. You need `cmake` and of course a C++ compiler.

    $ git clone https://github.com/llvm/llvm-project llvm-project
    $ mkdir build
    $ cd build
    $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS='lld' ../llvm-project/llvm
    $ ninja check-lld-macho

Then you can find output binary at `build/bin/ld64.lld`. Note that if `ld64.lld` is moved out of `bin`, it must still be accompanied by its sibling file `lld`, as `ld64.lld` is technically a symlink to `lld`.

## Using LLD

LLD can be used by adding `-fuse-ld=/path/to/ld64.lld` to the linker flags. For Xcode, this can be done by adding it to “Other linker flags” in the build settings. For Bazel, this can be done with `--linkopt` or with rules_apple_linker.

See also

ld64 vs LLD-MachO has more info on the differences between the two linkers.