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