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 We are currently working closely with the FreeBSD project to make 22 LLD default system linker in future versions of the operating 23 system, so we are serious about addressing compatibility issues. As 24 of February 2017, LLD is able to link the entire FreeBSD/amd64 base 25 system including the kernel. With a few work-in-progress patches it 26 can link approximately 95% of the ports collection on AMD64. For the 27 details, see `FreeBSD quarterly status report 28 <https://www.freebsd.org/news/status/report-2016-10-2016-12.html#Using-LLVM%27s-LLD-Linker-as-FreeBSD%27s-System-Linker>`_. 29 30- LLD is very fast. When you link a large program on a multicore 31 machine, you can expect that LLD runs more than twice as fast as the GNU 32 gold linker. Your mileage may vary, though. 33 34- It supports various CPUs/ABIs including AArch64, AMDGPU, ARM, Hexagon, MIPS 35 32/64 big/little-endian, PowerPC, PowerPC64, RISC-V, SPARC V9, x86-32 and 36 x86-64. Among these, AArch64, ARM (>= v6), PowerPC, PowerPC64, x86-32 and 37 x86-64 have production quality. MIPS seems decent too. 38 39- It is always a cross-linker, meaning that it always supports all the 40 above targets however it was built. In fact, we don't provide a 41 build-time option to enable/disable each target. This should make it 42 easy to use our linker as part of a cross-compile toolchain. 43 44- You can embed LLD in your program to eliminate dependencies on 45 external linkers. All you have to do is to construct object files 46 and command line arguments just like you would do to invoke an 47 external linker and then call the linker's main function, 48 ``lld::elf::link``, from your code. 49 50- It is small. We are using LLVM libObject library to read from object 51 files, so it is not a completely fair comparison, but as of February 52 2017, LLD/ELF consists only of 21k lines of C++ code while GNU gold 53 consists of 198k lines of C++ code. 54 55- Link-time optimization (LTO) is supported by default. Essentially, 56 all you have to do to do LTO is to pass the ``-flto`` option to clang. 57 Then clang creates object files not in the native object file format 58 but in LLVM bitcode format. LLD reads bitcode object files, compile 59 them using LLVM and emit an output file. Because in this way LLD can 60 see the entire program, it can do the whole program optimization. 61 62- Some very old features for ancient Unix systems (pre-90s or even 63 before that) have been removed. Some default settings have been 64 tuned for the 21st century. For example, the stack is marked as 65 non-executable by default to tighten security. 66 67Performance 68----------- 69 70This is a link time comparison on a 2-socket 20-core 40-thread Xeon 71E5-2680 2.80 GHz machine with an SSD drive. We ran gold and lld with 72or without multi-threading support. To disable multi-threading, we 73added ``-no-threads`` to the command lines. 74 75============ =========== ============ ==================== ================== =============== ============= 76Program Output size GNU ld GNU gold w/o threads GNU gold w/threads lld w/o threads lld w/threads 77ffmpeg dbg 92 MiB 1.72s 1.16s 1.01s 0.60s 0.35s 78mysqld dbg 154 MiB 8.50s 2.96s 2.68s 1.06s 0.68s 79clang dbg 1.67 GiB 104.03s 34.18s 23.49s 14.82s 5.28s 80chromium dbg 1.14 GiB 209.05s [1]_ 64.70s 60.82s 27.60s 16.70s 81============ =========== ============ ==================== ================== =============== ============= 82 83As you can see, lld is significantly faster than GNU linkers. 84Note that this is just a benchmark result of our environment. 85Depending on number of available cores, available amount of memory or 86disk latency/throughput, your results may vary. 87 88.. [1] Since GNU ld doesn't support the ``-icf=all`` and 89 ``-gdb-index`` options, we removed them from the command line 90 for GNU ld. GNU ld would have been slower than this if it had 91 these options. 92 93Build 94----- 95 96If you have already checked out LLVM using SVN, you can check out LLD 97under ``tools`` directory just like you probably did for clang. For the 98details, see `Getting Started with the LLVM System 99<https://llvm.org/docs/GettingStarted.html>`_. 100 101If you haven't checked out LLVM, the easiest way to build LLD is to 102check out the entire LLVM projects/sub-projects from a git mirror and 103build that tree. You need `cmake` and of course a C++ compiler. 104 105.. code-block:: console 106 107 $ git clone https://github.com/llvm/llvm-project llvm-project 108 $ mkdir build 109 $ cd build 110 $ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=lld -DCMAKE_INSTALL_PREFIX=/usr/local ../llvm-project/llvm 111 $ make install 112 113Using LLD 114--------- 115 116LLD is installed as ``ld.lld``. On Unix, linkers are invoked by 117compiler drivers, so you are not expected to use that command 118directly. There are a few ways to tell compiler drivers to use ld.lld 119instead of the default linker. 120 121The easiest way to do that is to overwrite the default linker. After 122installing LLD to somewhere on your disk, you can create a symbolic 123link by doing ``ln -s /path/to/ld.lld /usr/bin/ld`` so that 124``/usr/bin/ld`` is resolved to LLD. 125 126If you don't want to change the system setting, you can use clang's 127``-fuse-ld`` option. In this way, you want to set ``-fuse-ld=lld`` to 128LDFLAGS when building your programs. 129 130LLD leaves its name and version number to a ``.comment`` section in an 131output. If you are in doubt whether you are successfully using LLD or 132not, run ``readelf --string-dump .comment <output-file>`` and examine the 133output. If the string "Linker: LLD" is included in the output, you are 134using LLD. 135 136History 137------- 138 139Here is a brief project history of the ELF and COFF ports. 140 141- May 2015: We decided to rewrite the COFF linker and did that. 142 Noticed that the new linker is much faster than the MSVC linker. 143 144- July 2015: The new ELF port was developed based on the COFF linker 145 architecture. 146 147- September 2015: The first patches to support MIPS and AArch64 landed. 148 149- October 2015: Succeeded to self-host the ELF port. We have noticed 150 that the linker was faster than the GNU linkers, but we weren't sure 151 at the time if we would be able to keep the gap as we would add more 152 features to the linker. 153 154- July 2016: Started working on improving the linker script support. 155 156- December 2016: Succeeded to build the entire FreeBSD base system 157 including the kernel. We had widen the performance gap against the 158 GNU linkers. 159 160Internals 161--------- 162 163For the internals of the linker, please read :doc:`NewLLD`. It is a bit 164outdated but the fundamental concepts remain valid. We'll update the 165document soon. 166 167.. toctree:: 168 :maxdepth: 1 169 170 NewLLD 171 WebAssembly 172 windows_support 173 missingkeyfunction 174 error_handling_script 175 Partitions 176 ReleaseNotes 177 ELF/linker_script 178 ELF/start-stop-gc 179 ELF/warn_backrefs 180