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