LLD - The LLVM Linker — lld 5 documentation

# 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) and Mach-O (macOS) in descending order of completeness. Internally, LLD consists of three different linkers. The ELF port is the one that will be described in this document. The PE/COFF port is almost complete except the lack of the Windows debug info (PDB) support. The Mach-O port is built based on a different architecture than the ELF or COFF ports. For the details about Mach-O, please read ATOM-based lld.

## Features

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

  We are currently working closely with the FreeBSD project to make LLD default system linker in future versions of the operating system, so we are serious about addressing compatibility issues. As of February 2017, LLD is able to link the entire FreeBSD/amd64 base system including the kernel. With a few work-in-progress patches it can link approximately 95% of the ports collection on AMD64. For the details, see FreeBSD quarterly status report.

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

- It supports various CPUs/ABIs including x86-64, x86, x32, AArch64, ARM, MIPS 32/64 big/little-endian, PowerPC, PowerPC 64 and AMDGPU. Among these, x86-64 is the most well-supported target and have reached production quality. AArch64 and MIPS seem decent too. x86 should be OK but not well tested yet. ARM support is being developed actively.

- 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 to your program to eliminate dependency to 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::elf::link`, from your code.

- It is small. We are using LLVM libObject library to read from object files, so it is not completely a 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.

LLD is much faster than the GNU linkers for large programs. That’s fast for small programs too, but because the link time is short anyway, the difference is not very noticeable in that case.

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.

Program | Output size | GNU ld | GNU gold [1] | LLD
ffmpeg dbg | 91 MiB | 1.59s | 1.15s | 0.78s
mysqld dbg | 157 MiB | 7.09s | 2.49s | 1.31s
clang dbg | 1.45 GiB | 86.76s | 21.93s | 8.38s
chromium dbg | 1.52 GiB | 142.30s [2] | 40.86s | 12.69s

[1] | With the `--threads` option to enable multi-threading support.

[2] | Since GNU ld doesn’t support the `-icf=all` option, we removed that from the command line for GNU ld. GNU ld would be slower than this if it had that option support. For gold and LLD, we use `-icf=all`.

## 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 checkout out LLVM, the easiest way to build LLD is to checkout 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-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.

## History

Here is a brief project history of the ELF and COFF ports.

- May 2015: We decided to rewrite the COFF linker and did that. Noticed that the new linker is much faster than the MSVC linker.
- July 2015: The new ELF port was developed based on the COFF linker architecture.
- September 2015: The first patches to support MIPS and AArch64 landed.
- October 2015: Succeeded to self-host the ELF port. We have noticed that the linker was faster than the GNU linkers, but we weren’t sure at the time if we would be able to keep the gap as we would add more features to the linker.
- July 2016: Started working on improving the linker script support.
- December 2016: Succeeded to build the entire FreeBSD base system including the kernel. We had widen the performance gap against the GNU linkers.

## Internals

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

- The ELF and COFF Linkers
- Design
- ATOM-based lld
- Windows support
- lld 5.0.0 Release Notes