forked from OSchip/llvm-project
179 lines
7.6 KiB
ReStructuredText
179 lines
7.6 KiB
ReStructuredText
LLD - The LLVM Linker
|
|
=====================
|
|
|
|
LLD is a linker from the LLVM project that is a drop-in replacement
|
|
for system linkers and runs much faster than them. It also provides
|
|
features that are useful for toolchain developers.
|
|
|
|
The linker supports ELF (Unix), PE/COFF (Windows), Mach-O (macOS) and
|
|
WebAssembly in descending order of completeness. Internally, LLD consists of
|
|
several different linkers. The ELF port is the one that will be described in
|
|
this document. The PE/COFF port is complete, including
|
|
Windows debug info (PDB) support. The WebAssembly port is still a work in
|
|
progress (See :doc:`WebAssembly`). The Mach-O port is built based on a
|
|
different architecture than the others. For the details about Mach-O, please
|
|
read :doc:`AtomLLD`.
|
|
|
|
Features
|
|
--------
|
|
|
|
- LLD is a drop-in replacement for the GNU linkers that accepts the
|
|
same command line arguments and linker scripts as GNU.
|
|
|
|
We are currently working closely with the FreeBSD project to make
|
|
LLD default system linker in future versions of the operating
|
|
system, so we are serious about addressing compatibility issues. As
|
|
of February 2017, LLD is able to link the entire FreeBSD/amd64 base
|
|
system including the kernel. With a few work-in-progress patches it
|
|
can link approximately 95% of the ports collection on AMD64. For the
|
|
details, see `FreeBSD quarterly status report
|
|
<https://www.freebsd.org/news/status/report-2016-10-2016-12.html#Using-LLVM%27s-LLD-Linker-as-FreeBSD%27s-System-Linker>`_.
|
|
|
|
- LLD is very fast. When you link a large program on a multicore
|
|
machine, you can expect that LLD runs more than twice as fast as the GNU
|
|
gold linker. Your milage may vary, though.
|
|
|
|
- It supports various CPUs/ABIs including x86-64, x86, x32, AArch64,
|
|
ARM, MIPS 32/64 big/little-endian, PowerPC, PowerPC 64 and AMDGPU.
|
|
Among these, x86-64, AArch64, and ARM (>= v6) are production quality.
|
|
MIPS seems decent too. x86 should be OK but is not well tested yet.
|
|
|
|
- It is always a cross-linker, meaning that it always supports all the
|
|
above targets however it was built. In fact, we don't provide a
|
|
build-time option to enable/disable each target. This should make it
|
|
easy to use our linker as part of a cross-compile toolchain.
|
|
|
|
- You can embed LLD in your program to eliminate dependencies on
|
|
external linkers. All you have to do is to construct object files
|
|
and command line arguments just like you would do to invoke an
|
|
external linker and then call the linker's main function,
|
|
``lld::elf::link``, from your code.
|
|
|
|
- It is small. We are using LLVM libObject library to read from object
|
|
files, so it is not a completely fair comparison, but as of February
|
|
2017, LLD/ELF consists only of 21k lines of C++ code while GNU gold
|
|
consists of 198k lines of C++ code.
|
|
|
|
- Link-time optimization (LTO) is supported by default. Essentially,
|
|
all you have to do to do LTO is to pass the ``-flto`` option to clang.
|
|
Then clang creates object files not in the native object file format
|
|
but in LLVM bitcode format. LLD reads bitcode object files, compile
|
|
them using LLVM and emit an output file. Because in this way LLD can
|
|
see the entire program, it can do the whole program optimization.
|
|
|
|
- Some very old features for ancient Unix systems (pre-90s or even
|
|
before that) have been removed. Some default settings have been
|
|
tuned for the 21st century. For example, the stack is marked as
|
|
non-executable by default to tighten security.
|
|
|
|
Performance
|
|
-----------
|
|
|
|
This is a link time comparison on a 2-socket 20-core 40-thread Xeon
|
|
E5-2680 2.80 GHz machine with an SSD drive. We ran gold and lld with
|
|
or without multi-threading support. To disable multi-threading, we
|
|
added ``-no-threads`` to the command lines.
|
|
|
|
============ =========== ============ ==================== ================== =============== =============
|
|
Program Output size GNU ld GNU gold w/o threads GNU gold w/threads lld w/o threads lld w/threads
|
|
ffmpeg dbg 92 MiB 1.72s 1.16s 1.01s 0.60s 0.35s
|
|
mysqld dbg 154 MiB 8.50s 2.96s 2.68s 1.06s 0.68s
|
|
clang dbg 1.67 GiB 104.03s 34.18s 23.49s 14.82s 5.28s
|
|
chromium dbg 1.14 GiB 209.05s [1]_ 64.70s 60.82s 27.60s 16.70s
|
|
============ =========== ============ ==================== ================== =============== =============
|
|
|
|
As you can see, lld is significantly faster than GNU linkers.
|
|
Note that this is just a benchmark result of our environment.
|
|
Depending on number of available cores, available amount of memory or
|
|
disk latency/throughput, your results may vary.
|
|
|
|
.. [1] Since GNU ld doesn't support the ``-icf=all`` and
|
|
``-gdb-index`` options, we removed them from the command line
|
|
for GNU ld. GNU ld would have been slower than this if it had
|
|
these options.
|
|
|
|
Build
|
|
-----
|
|
|
|
If you have already checked out LLVM using SVN, you can check out LLD
|
|
under ``tools`` directory just like you probably did for clang. For the
|
|
details, see `Getting Started with the LLVM System
|
|
<http://llvm.org/docs/GettingStarted.html>`_.
|
|
|
|
If you haven't checked out LLVM, the easiest way to build LLD is to
|
|
check out the entire LLVM projects/sub-projects from a git mirror and
|
|
build that tree. You need `cmake` and of course a C++ compiler.
|
|
|
|
.. code-block:: console
|
|
|
|
$ git clone https://github.com/llvm/llvm-project llvm-project
|
|
$ mkdir build
|
|
$ cd build
|
|
$ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=lld -DCMAKE_INSTALL_PREFIX=/usr/local ../llvm-project/llvm
|
|
$ make install
|
|
|
|
Using LLD
|
|
---------
|
|
|
|
LLD is installed as ``ld.lld``. On Unix, linkers are invoked by
|
|
compiler drivers, so you are not expected to use that command
|
|
directly. There are a few ways to tell compiler drivers to use ld.lld
|
|
instead of the default linker.
|
|
|
|
The easiest way to do that is to overwrite the default linker. After
|
|
installing LLD to somewhere on your disk, you can create a symbolic
|
|
link by doing ``ln -s /path/to/ld.lld /usr/bin/ld`` so that
|
|
``/usr/bin/ld`` is resolved to LLD.
|
|
|
|
If you don't want to change the system setting, you can use clang's
|
|
``-fuse-ld`` option. In this way, you want to set ``-fuse-ld=lld`` to
|
|
LDFLAGS when building your programs.
|
|
|
|
LLD leaves its name and version number to a ``.comment`` section in an
|
|
output. If you are in doubt whether you are successfully using LLD or
|
|
not, run ``readelf --string-dump .comment <output-file>`` and examine the
|
|
output. If the string "Linker: LLD" is included in the output, you are
|
|
using LLD.
|
|
|
|
History
|
|
-------
|
|
|
|
Here is a brief project history of the ELF and COFF ports.
|
|
|
|
- May 2015: We decided to rewrite the COFF linker and did that.
|
|
Noticed that the new linker is much faster than the MSVC linker.
|
|
|
|
- July 2015: The new ELF port was developed based on the COFF linker
|
|
architecture.
|
|
|
|
- September 2015: The first patches to support MIPS and AArch64 landed.
|
|
|
|
- October 2015: Succeeded to self-host the ELF port. We have noticed
|
|
that the linker was faster than the GNU linkers, but we weren't sure
|
|
at the time if we would be able to keep the gap as we would add more
|
|
features to the linker.
|
|
|
|
- July 2016: Started working on improving the linker script support.
|
|
|
|
- December 2016: Succeeded to build the entire FreeBSD base system
|
|
including the kernel. We had widen the performance gap against the
|
|
GNU linkers.
|
|
|
|
Internals
|
|
---------
|
|
|
|
For the internals of the linker, please read :doc:`NewLLD`. It is a bit
|
|
outdated but the fundamental concepts remain valid. We'll update the
|
|
document soon.
|
|
|
|
.. toctree::
|
|
:maxdepth: 1
|
|
|
|
NewLLD
|
|
AtomLLD
|
|
WebAssembly
|
|
windows_support
|
|
missingkeyfunction
|
|
Partitions
|
|
ReleaseNotes
|