# LLD - The LLVM Linker

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.

The linker supports ELF (Unix), PE/COFF (Windows), Mach-O (macOS) and WebAssembly in descending order of completeness. Internally, LLD consists of several different linkers. The ELF port is the one that will be described in this document. The PE/COFF port is complete, including Windows debug info (PDB) support. The WebAssembly port is still a work in progress (See WebAssembly lld port).

## Features

- LLD is a drop-in replacement for the GNU linkers that accepts the same command line arguments and linker scripts as GNU.

- 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 GNU gold linker. Your mileage may vary, though.

- It supports various CPUs/ABIs including AArch64, AMDGPU, ARM, Hexagon, LoongArch, MIPS 32/64 big/little-endian, PowerPC, PowerPC64, RISC-V, SPARC V9, x86-32 and x86-64. Among these, AArch64, ARM (>= v4), LoongArch, PowerPC, PowerPC64, RISC-V, x86-32 and x86-64 have production quality. MIPS seems decent too.

- It is always a cross-linker, meaning that it always supports all the above targets however it was built. In fact, we don’t provide a build-time option to enable/disable each target. This should make it easy to use our linker as part of a cross-compile toolchain.

- You can embed LLD in your program to eliminate dependencies on external linkers. All you have to do is to construct object files and command line arguments just like you would do to invoke an external linker and then call the linker’s main function, `lld::lldMain`, from your code.

- It is small. We are using LLVM libObject library to read from object files, so it is not a completely fair comparison, but as of February 2017, LLD/ELF consists only of 21k lines of C++ code while GNU gold consists of 198k lines of C++ code.

- Link-time optimization (LTO) is supported by default. Essentially, all you have to do to do LTO is to pass the `-flto` option to clang. Then clang creates object files not in the native object file format but in LLVM bitcode format. LLD reads bitcode object files, compile them using LLVM and emit an output file. Because in this way LLD can see the entire program, it can do the whole program optimization.

- Some very old features for ancient Unix systems (pre-90s or even before that) have been removed. Some default settings have been tuned for the 21st century. For example, the stack is marked as non-executable by default to tighten security.

## Performance

This is a link time comparison on a 2-socket 20-core 40-thread Xeon E5-2680 2.80 GHz machine with an SSD drive. We ran gold and lld with or without multi-threading support. To disable multi-threading, we added `-no-threads` to the command lines.

Program | Output size | GNU ld | GNU gold w/o threads | GNU gold w/threads | lld w/o threads | lld w/threads
ffmpeg dbg | 92 MiB | 1.72s | 1.16s | 1.01s | 0.60s | 0.35s
mysqld dbg | 154 MiB | 8.50s | 2.96s | 2.68s | 1.06s | 0.68s
clang dbg | 1.67 GiB | 104.03s | 34.18s | 23.49s | 14.82s | 5.28s
chromium dbg | 1.14 GiB | 209.05s [1] | 64.70s | 60.82s | 27.60s | 16.70s

As you can see, lld is significantly faster than GNU linkers. Note that this is just a benchmark result of our environment. Depending on number of available cores, available amount of memory or disk latency/throughput, your results may vary.

[1]

Since GNU ld doesn’t support the `-icf=all` and `-gdb-index` options, we removed them from the command line for GNU ld. GNU ld would have been slower than this if it had these options.

## Build

If you have already checked out LLVM using SVN, you can check out LLD under `tools` directory just like you probably did for clang. For the details, see Getting Started with the LLVM System.

If you haven’t checked out LLVM, 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 -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=lld -DCMAKE_INSTALL_PREFIX=/usr/local ../llvm-project/llvm
    $ make install

## Using LLD

LLD is installed as `ld.lld`. On Unix, linkers are invoked by compiler drivers, so you are not expected to use that command directly. There are a few ways to tell compiler drivers to use ld.lld instead of the default linker.

The easiest way to do that is to overwrite the default linker. After installing LLD to somewhere on your disk, you can create a symbolic link by doing `ln -s /path/to/ld.lld /usr/bin/ld` so that `/usr/bin/ld` is resolved to LLD.

If you don’t want to change the system setting, you can use clang’s `-fuse-ld` option. In this way, you want to set `-fuse-ld=lld` to LDFLAGS when building your programs.

LLD leaves its name and version number to a `.comment` section in an output. If you are in doubt whether you are successfully using LLD or not, run `readelf --string-dump .comment <output-file>` and examine the output. If the string “Linker: LLD” is included in the output, you are using LLD.

## Internals

For the internals of the linker, please read The ELF, COFF and Wasm Linkers. It is a bit outdated but the fundamental concepts remain valid. We’ll update the document soon.

- The ELF, COFF and Wasm Linkers
- Design
- WebAssembly lld port
- Windows support
- Missing Key Function
- Error Handling Script
- Partitions
- lld 21.1.5 Release Notes
- Large data sections
- Linker Script implementation notes and policy
- -z start-stop-gc
- –warn-backrefs
- Mach-O LLD Port
- Integrated Distributed ThinLTO (DTLTO)