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). The Mach-O port is built based on a different architecture than the others. 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 the GNU gold linker. Your mileage 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, AArch64, and ARM (>= v6) are production quality. MIPS seems decent too. x86 should be OK but is not well tested yet.

  • 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::elf::link, 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.

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, COFF and Wasm Linkers. It is a bit outdated but the fundamental concepts remain valid. We’ll update the document soon.