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 the 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, AArch64, and ARM (>= v6) are production quality.
39 MIPS seems decent too. x86 should be OK but is not well tested yet.
41 - It is always a cross-linker, meaning that it always supports all the
42 above targets however it was built. In fact, we don't provide a
43 build-time option to enable/disable each target. This should make it
44 easy to use our linker as part of a cross-compile toolchain.
46 - You can embed LLD in your program to eliminate dependencies on
47 external linkers. All you have to do is to construct object files
48 and command line arguments just like you would do to invoke an
49 external linker and then call the linker's main function,
50 ``lld::elf::link``, from your code.
52 - It is small. We are using LLVM libObject library to read from object
53 files, so it is not a completely fair comparison, but as of February
54 2017, LLD/ELF consists only of 21k lines of C++ code while GNU gold
55 consists of 198k lines of C++ code.
57 - Link-time optimization (LTO) is supported by default. Essentially,
58 all you have to do to do LTO is to pass the ``-flto`` option to clang.
59 Then clang creates object files not in the native object file format
60 but in LLVM bitcode format. LLD reads bitcode object files, compile
61 them using LLVM and emit an output file. Because in this way LLD can
62 see the entire program, it can do the whole program optimization.
64 - Some very old features for ancient Unix systems (pre-90s or even
65 before that) have been removed. Some default settings have been
66 tuned for the 21st century. For example, the stack is marked as
67 non-executable by default to tighten security.
72 This is a link time comparison on a 2-socket 20-core 40-thread Xeon
73 E5-2680 2.80 GHz machine with an SSD drive. We ran gold and lld with
74 or without multi-threading support. To disable multi-threading, we
75 added ``-no-threads`` to the command lines.
77 ============ =========== ============ ==================== ================== =============== =============
78 Program Output size GNU ld GNU gold w/o threads GNU gold w/threads lld w/o threads lld w/threads
79 ffmpeg dbg 92 MiB 1.72s 1.16s 1.01s 0.60s 0.35s
80 mysqld dbg 154 MiB 8.50s 2.96s 2.68s 1.06s 0.68s
81 clang dbg 1.67 GiB 104.03s 34.18s 23.49s 14.82s 5.28s
82 chromium dbg 1.14 GiB 209.05s [1]_ 64.70s 60.82s 27.60s 16.70s
83 ============ =========== ============ ==================== ================== =============== =============
85 As you can see, lld is significantly faster than GNU linkers.
86 Note that this is just a benchmark result of our environment.
87 Depending on number of available cores, available amount of memory or
88 disk latency/throughput, your results may vary.
90 .. [1] Since GNU ld doesn't support the ``-icf=all`` and
91 ``-gdb-index`` options, we removed them from the command line
92 for GNU ld. GNU ld would have been slower than this if it had
98 If you have already checked out LLVM using SVN, you can check out LLD
99 under ``tools`` directory just like you probably did for clang. For the
100 details, see `Getting Started with the LLVM System
101 <http://llvm.org/docs/GettingStarted.html>`_.
103 If you haven't checked out LLVM, the easiest way to build LLD is to
104 check out the entire LLVM projects/sub-projects from a git mirror and
105 build that tree. You need `cmake` and of course a C++ compiler.
107 .. code-block:: console
109 $ git clone https://github.com/llvm-project/llvm-project-20170507 llvm-project
112 $ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=lld -DCMAKE_INSTALL_PREFIX=/usr/local ../llvm-project/llvm
118 LLD is installed as ``ld.lld``. On Unix, linkers are invoked by
119 compiler drivers, so you are not expected to use that command
120 directly. There are a few ways to tell compiler drivers to use ld.lld
121 instead of the default linker.
123 The easiest way to do that is to overwrite the default linker. After
124 installing LLD to somewhere on your disk, you can create a symbolic
125 link by doing ``ln -s /path/to/ld.lld /usr/bin/ld`` so that
126 ``/usr/bin/ld`` is resolved to LLD.
128 If you don't want to change the system setting, you can use clang's
129 ``-fuse-ld`` option. In this way, you want to set ``-fuse-ld=lld`` to
130 LDFLAGS when building your programs.
132 LLD leaves its name and version number to a ``.comment`` section in an
133 output. If you are in doubt whether you are successfully using LLD or
134 not, run ``readelf --string-dump .comment <output-file>`` and examine the
135 output. If the string "Linker: LLD" is included in the output, you are
141 Here is a brief project history of the ELF and COFF ports.
143 - May 2015: We decided to rewrite the COFF linker and did that.
144 Noticed that the new linker is much faster than the MSVC linker.
146 - July 2015: The new ELF port was developed based on the COFF linker
149 - September 2015: The first patches to support MIPS and AArch64 landed.
151 - October 2015: Succeeded to self-host the ELF port. We have noticed
152 that the linker was faster than the GNU linkers, but we weren't sure
153 at the time if we would be able to keep the gap as we would add more
154 features to the linker.
156 - July 2016: Started working on improving the linker script support.
158 - December 2016: Succeeded to build the entire FreeBSD base system
159 including the kernel. We had widen the performance gap against the
165 For the internals of the linker, please read :doc:`NewLLD`. It is a bit
166 outdated but the fundamental concepts remain valid. We'll update the