1LLD - The LLVM Linker 2===================== 3 4LLD is a linker from the LLVM project that is a drop-in replacement 5for system linkers and runs much faster than them. It also provides 6features that are useful for toolchain developers. 7 8The linker supports ELF (Unix), PE/COFF (Windows), Mach-O (macOS) and 9WebAssembly in descending order of completeness. Internally, LLD consists of 10several different linkers. The ELF port is the one that will be described in 11this document. The PE/COFF port is complete, including 12Windows debug info (PDB) support. The WebAssembly port is still a work in 13progress (See :doc:`WebAssembly`). 14 15Features 16-------- 17 18- LLD is a drop-in replacement for the GNU linkers that accepts the 19 same command line arguments and linker scripts as GNU. 20 21- LLD is very fast. When you link a large program on a multicore 22 machine, you can expect that LLD runs more than twice as fast as the GNU 23 gold linker. Your mileage may vary, though. 24 25- It supports various CPUs/ABIs including AArch64, AMDGPU, ARM, Hexagon, 26 LoongArch, MIPS 32/64 big/little-endian, PowerPC, PowerPC64, RISC-V, 27 SPARC V9, x86-32 and x86-64. Among these, AArch64, ARM (>= v4), LoongArch, 28 PowerPC, PowerPC64, RISC-V, x86-32 and x86-64 have production quality. 29 MIPS seems decent too. 30 31- It is always a cross-linker, meaning that it always supports all the 32 above targets however it was built. In fact, we don't provide a 33 build-time option to enable/disable each target. This should make it 34 easy to use our linker as part of a cross-compile toolchain. 35 36- You can embed LLD in your program to eliminate dependencies on 37 external linkers. All you have to do is to construct object files 38 and command line arguments just like you would do to invoke an 39 external linker and then call the linker's main function, 40 ``lld::lldMain``, from your code. 41 42- It is small. We are using LLVM libObject library to read from object 43 files, so it is not a completely fair comparison, but as of February 44 2017, LLD/ELF consists only of 21k lines of C++ code while GNU gold 45 consists of 198k lines of C++ code. 46 47- Link-time optimization (LTO) is supported by default. Essentially, 48 all you have to do to do LTO is to pass the ``-flto`` option to clang. 49 Then clang creates object files not in the native object file format 50 but in LLVM bitcode format. LLD reads bitcode object files, compile 51 them using LLVM and emit an output file. Because in this way LLD can 52 see the entire program, it can do the whole program optimization. 53 54- Some very old features for ancient Unix systems (pre-90s or even 55 before that) have been removed. Some default settings have been 56 tuned for the 21st century. For example, the stack is marked as 57 non-executable by default to tighten security. 58 59Performance 60----------- 61 62This is a link time comparison on a 2-socket 20-core 40-thread Xeon 63E5-2680 2.80 GHz machine with an SSD drive. We ran gold and lld with 64or without multi-threading support. To disable multi-threading, we 65added ``-no-threads`` to the command lines. 66 67============ =========== ============ ==================== ================== =============== ============= 68Program Output size GNU ld GNU gold w/o threads GNU gold w/threads lld w/o threads lld w/threads 69ffmpeg dbg 92 MiB 1.72s 1.16s 1.01s 0.60s 0.35s 70mysqld dbg 154 MiB 8.50s 2.96s 2.68s 1.06s 0.68s 71clang dbg 1.67 GiB 104.03s 34.18s 23.49s 14.82s 5.28s 72chromium dbg 1.14 GiB 209.05s [1]_ 64.70s 60.82s 27.60s 16.70s 73============ =========== ============ ==================== ================== =============== ============= 74 75As you can see, lld is significantly faster than GNU linkers. 76Note that this is just a benchmark result of our environment. 77Depending on number of available cores, available amount of memory or 78disk latency/throughput, your results may vary. 79 80.. [1] Since GNU ld doesn't support the ``-icf=all`` and 81 ``-gdb-index`` options, we removed them from the command line 82 for GNU ld. GNU ld would have been slower than this if it had 83 these options. 84 85Build 86----- 87 88If you have already checked out LLVM using SVN, you can check out LLD 89under ``tools`` directory just like you probably did for clang. For the 90details, see `Getting Started with the LLVM System 91<https://llvm.org/docs/GettingStarted.html>`_. 92 93If you haven't checked out LLVM, the easiest way to build LLD is to 94check out the entire LLVM projects/sub-projects from a git mirror and 95build that tree. You need `cmake` and of course a C++ compiler. 96 97.. code-block:: console 98 99 $ git clone https://github.com/llvm/llvm-project llvm-project 100 $ mkdir build 101 $ cd build 102 $ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=lld -DCMAKE_INSTALL_PREFIX=/usr/local ../llvm-project/llvm 103 $ make install 104 105Using LLD 106--------- 107 108LLD is installed as ``ld.lld``. On Unix, linkers are invoked by 109compiler drivers, so you are not expected to use that command 110directly. There are a few ways to tell compiler drivers to use ld.lld 111instead of the default linker. 112 113The easiest way to do that is to overwrite the default linker. After 114installing LLD to somewhere on your disk, you can create a symbolic 115link by doing ``ln -s /path/to/ld.lld /usr/bin/ld`` so that 116``/usr/bin/ld`` is resolved to LLD. 117 118If you don't want to change the system setting, you can use clang's 119``-fuse-ld`` option. In this way, you want to set ``-fuse-ld=lld`` to 120LDFLAGS when building your programs. 121 122LLD leaves its name and version number to a ``.comment`` section in an 123output. If you are in doubt whether you are successfully using LLD or 124not, run ``readelf --string-dump .comment <output-file>`` and examine the 125output. If the string "Linker: LLD" is included in the output, you are 126using LLD. 127 128History 129------- 130 131Here is a brief project history of the ELF and COFF ports. 132 133- May 2015: We decided to rewrite the COFF linker and did that. 134 Noticed that the new linker is much faster than the MSVC linker. 135 136- July 2015: The new ELF port was developed based on the COFF linker 137 architecture. 138 139- September 2015: The first patches to support MIPS and AArch64 landed. 140 141- October 2015: Succeeded to self-host the ELF port. We have noticed 142 that the linker was faster than the GNU linkers, but we weren't sure 143 at the time if we would be able to keep the gap as we would add more 144 features to the linker. 145 146- July 2016: Started working on improving the linker script support. 147 148- December 2016: Succeeded to build the entire FreeBSD base system 149 including the kernel. We had widen the performance gap against the 150 GNU linkers. 151 152Internals 153--------- 154 155For the internals of the linker, please read :doc:`NewLLD`. It is a bit 156outdated but the fundamental concepts remain valid. We'll update the 157document soon. 158 159.. toctree:: 160 :maxdepth: 1 161 162 NewLLD 163 WebAssembly 164 windows_support 165 missingkeyfunction 166 error_handling_script 167 Partitions 168 ReleaseNotes 169 ELF/linker_script 170 ELF/start-stop-gc 171 ELF/warn_backrefs 172 MachO/index 173