4 LLD is a linker from the LLVM project. That is a drop-in replacement
5 for system linkers and runs much faster than them. It also provides
6 features that are useful for toolchain developers.
8 The linker supports ELF (Unix), PE/COFF (Windows), Mach-O (macOS) and
9 WebAssembly in descending order of completeness. Internally, LLD consists of
10 several different linkers. The ELF port is the one that will be described in
11 this document. The PE/COFF port is complete, including
12 Windows debug info (PDB) support. The WebAssembly port is still a work in
13 progress (See :doc:`WebAssembly`). The Mach-O port is built based on a
14 different architecture than the others. For the details about Mach-O, please
20 - LLD is a drop-in replacement for the GNU linkers. That accepts the
21 same command line arguments and linker scripts as GNU.
23 We are currently working closely with the FreeBSD project to make
24 LLD default system linker in future versions of the operating
25 system, so we are serious about addressing compatibility issues. As
26 of February 2017, LLD is able to link the entire FreeBSD/amd64 base
27 system including the kernel. With a few work-in-progress patches it
28 can link approximately 95% of the ports collection on AMD64. For the
29 details, see `FreeBSD quarterly status report
30 <https://www.freebsd.org/news/status/report-2016-10-2016-12.html#Using-LLVM%27s-LLD-Linker-as-FreeBSD%27s-System-Linker>`_.
32 - LLD is very fast. When you link a large program on a multicore
33 machine, you can expect that LLD runs more than twice as fast as GNU
34 gold linker. Your milage may vary, though.
36 - It supports various CPUs/ABIs including x86-64, x86, x32, AArch64,
37 ARM, MIPS 32/64 big/little-endian, PowerPC, PowerPC 64 and AMDGPU.
38 Among these, x86-64 is the most well-supported target and have
39 reached production quality. AArch64 and MIPS seem decent too. x86
40 should be OK but not well tested yet. ARM support is being developed
43 - It is always a cross-linker, meaning that it always supports all the
44 above targets however it was built. In fact, we don't provide a
45 build-time option to enable/disable each target. This should make it
46 easy to use our linker as part of a cross-compile toolchain.
48 - You can embed LLD to your program to eliminate dependency to
49 external linkers. All you have to do is to construct object files
50 and command line arguments just like you would do to invoke an
51 external linker and then call the linker's main function,
52 ``lld::elf::link``, from your code.
54 - It is small. We are using LLVM libObject library to read from object
55 files, so it is not completely a fair comparison, but as of February
56 2017, LLD/ELF consists only of 21k lines of C++ code while GNU gold
57 consists of 198k lines of C++ code.
59 - Link-time optimization (LTO) is supported by default. Essentially,
60 all you have to do to do LTO is to pass the ``-flto`` option to clang.
61 Then clang creates object files not in the native object file format
62 but in LLVM bitcode format. LLD reads bitcode object files, compile
63 them using LLVM and emit an output file. Because in this way LLD can
64 see the entire program, it can do the whole program optimization.
66 - Some very old features for ancient Unix systems (pre-90s or even
67 before that) have been removed. Some default settings have been
68 tuned for the 21st century. For example, the stack is marked as
69 non-executable by default to tighten security.
74 This is a link time comparison on a 2-socket 20-core 40-thread Xeon
75 E5-2680 2.80 GHz machine with an SSD drive. We ran gold and lld with
76 or without multi-threading support. To disable multi-threading, we
77 added ``-no-threads`` to the command lines.
79 ============ =========== ============ ==================== ================== =============== =============
80 Program Output size GNU ld GNU gold w/o threads GNU gold w/threads lld w/o threads lld w/threads
81 ffmpeg dbg 92 MiB 1.72s 1.16s 1.01s 0.60s 0.35s
82 mysqld dbg 154 MiB 8.50s 2.96s 2.68s 1.06s 0.68s
83 clang dbg 1.67 GiB 104.03s 34.18s 23.49s 14.82s 5.28s
84 chromium dbg 1.14 GiB 209.05s [1]_ 64.70s 60.82s 27.60s 16.70s
85 ============ =========== ============ ==================== ================== =============== =============
87 As you can see, lld is significantly faster than GNU linkers.
88 Note that this is just a benchmark result of our environment.
89 Depending on number of available cores, available amount of memory or
90 disk latency/throughput, your results may vary.
92 .. [1] Since GNU ld doesn't support the ``-icf=all`` and
93 ``-gdb-index`` options, we removed them from the command line
94 for GNU ld. GNU ld would have been slower than this if it had
100 If you have already checked out LLVM using SVN, you can check out LLD
101 under ``tools`` directory just like you probably did for clang. For the
102 details, see `Getting Started with the LLVM System
103 <http://llvm.org/docs/GettingStarted.html>`_.
105 If you haven't checkout out LLVM, the easiest way to build LLD is to
106 checkout the entire LLVM projects/sub-projects from a git mirror and
107 build that tree. You need `cmake` and of course a C++ compiler.
109 .. code-block:: console
111 $ git clone https://github.com/llvm-project/llvm-project-20170507 llvm-project
114 $ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=lld -DCMAKE_INSTALL_PREFIX=/usr/local ../llvm-project/llvm
120 LLD is installed as ``ld.lld``. On Unix, linkers are invoked by
121 compiler drivers, so you are not expected to use that command
122 directly. There are a few ways to tell compiler drivers to use ld.lld
123 instead of the default linker.
125 The easiest way to do that is to overwrite the default linker. After
126 installing LLD to somewhere on your disk, you can create a symbolic
127 link by doing ``ln -s /path/to/ld.lld /usr/bin/ld`` so that
128 ``/usr/bin/ld`` is resolved to LLD.
130 If you don't want to change the system setting, you can use clang's
131 ``-fuse-ld`` option. In this way, you want to set ``-fuse-ld=lld`` to
132 LDFLAGS when building your programs.
134 LLD leaves its name and version number to a ``.comment`` section in an
135 output. If you are in doubt whether you are successfully using LLD or
136 not, run ``readelf --string-dump .comment <output-file>`` and examine the
137 output. If the string "Linker: LLD" is included in the output, you are
143 Here is a brief project history of the ELF and COFF ports.
145 - May 2015: We decided to rewrite the COFF linker and did that.
146 Noticed that the new linker is much faster than the MSVC linker.
148 - July 2015: The new ELF port was developed based on the COFF linker
151 - September 2015: The first patches to support MIPS and AArch64 landed.
153 - October 2015: Succeeded to self-host the ELF port. We have noticed
154 that the linker was faster than the GNU linkers, but we weren't sure
155 at the time if we would be able to keep the gap as we would add more
156 features to the linker.
158 - July 2016: Started working on improving the linker script support.
160 - December 2016: Succeeded to build the entire FreeBSD base system
161 including the kernel. We had widen the performance gap against the
167 For the internals of the linker, please read :doc:`NewLLD`. It is a bit
168 outdated but the fundamental concepts remain valid. We'll update the