forked from OSchip/llvm-project
180 lines
7.6 KiB
ReStructuredText
180 lines
7.6 KiB
ReStructuredText
LLD - The LLVM Linker
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=====================
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LLD is a linker from the LLVM project that is a drop-in replacement
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for system linkers and runs much faster than them. It also provides
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features that are useful for toolchain developers.
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The linker supports ELF (Unix), PE/COFF (Windows), Mach-O (macOS) and
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WebAssembly in descending order of completeness. Internally, LLD consists of
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several different linkers. The ELF port is the one that will be described in
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this document. The PE/COFF port is complete, including
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Windows debug info (PDB) support. The WebAssembly port is still a work in
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progress (See :doc:`WebAssembly`). The Mach-O port is built based on a
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different architecture than the others. For the details about Mach-O, please
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read :doc:`AtomLLD`.
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Features
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--------
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- LLD is a drop-in replacement for the GNU linkers that accepts the
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same command line arguments and linker scripts as GNU.
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We are currently working closely with the FreeBSD project to make
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LLD default system linker in future versions of the operating
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system, so we are serious about addressing compatibility issues. As
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of February 2017, LLD is able to link the entire FreeBSD/amd64 base
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system including the kernel. With a few work-in-progress patches it
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can link approximately 95% of the ports collection on AMD64. For the
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details, see `FreeBSD quarterly status report
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<https://www.freebsd.org/news/status/report-2016-10-2016-12.html#Using-LLVM%27s-LLD-Linker-as-FreeBSD%27s-System-Linker>`_.
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- LLD is very fast. When you link a large program on a multicore
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machine, you can expect that LLD runs more than twice as fast as the GNU
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gold linker. Your mileage may vary, though.
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- It supports various CPUs/ABIs including AArch64, AMDGPU, ARM, Hexagon, MIPS
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32/64 big/little-endian, PowerPC, PowerPC64, RISC-V, SPARC V9, x86-32 and
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x86-64. Among these, AArch64, ARM (>= v6), PowerPC, PowerPC64, x86-32 and
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x86-64 have production quality. MIPS seems decent too.
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- It is always a cross-linker, meaning that it always supports all the
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above targets however it was built. In fact, we don't provide a
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build-time option to enable/disable each target. This should make it
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easy to use our linker as part of a cross-compile toolchain.
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- You can embed LLD in your program to eliminate dependencies on
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external linkers. All you have to do is to construct object files
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and command line arguments just like you would do to invoke an
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external linker and then call the linker's main function,
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``lld::elf::link``, from your code.
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- It is small. We are using LLVM libObject library to read from object
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files, so it is not a completely fair comparison, but as of February
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2017, LLD/ELF consists only of 21k lines of C++ code while GNU gold
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consists of 198k lines of C++ code.
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- Link-time optimization (LTO) is supported by default. Essentially,
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all you have to do to do LTO is to pass the ``-flto`` option to clang.
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Then clang creates object files not in the native object file format
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but in LLVM bitcode format. LLD reads bitcode object files, compile
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them using LLVM and emit an output file. Because in this way LLD can
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see the entire program, it can do the whole program optimization.
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- Some very old features for ancient Unix systems (pre-90s or even
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before that) have been removed. Some default settings have been
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tuned for the 21st century. For example, the stack is marked as
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non-executable by default to tighten security.
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Performance
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-----------
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This is a link time comparison on a 2-socket 20-core 40-thread Xeon
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E5-2680 2.80 GHz machine with an SSD drive. We ran gold and lld with
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or without multi-threading support. To disable multi-threading, we
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added ``-no-threads`` to the command lines.
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============ =========== ============ ==================== ================== =============== =============
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Program Output size GNU ld GNU gold w/o threads GNU gold w/threads lld w/o threads lld w/threads
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ffmpeg dbg 92 MiB 1.72s 1.16s 1.01s 0.60s 0.35s
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mysqld dbg 154 MiB 8.50s 2.96s 2.68s 1.06s 0.68s
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clang dbg 1.67 GiB 104.03s 34.18s 23.49s 14.82s 5.28s
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chromium dbg 1.14 GiB 209.05s [1]_ 64.70s 60.82s 27.60s 16.70s
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============ =========== ============ ==================== ================== =============== =============
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As you can see, lld is significantly faster than GNU linkers.
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Note that this is just a benchmark result of our environment.
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Depending on number of available cores, available amount of memory or
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disk latency/throughput, your results may vary.
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.. [1] Since GNU ld doesn't support the ``-icf=all`` and
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``-gdb-index`` options, we removed them from the command line
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for GNU ld. GNU ld would have been slower than this if it had
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these options.
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Build
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-----
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If you have already checked out LLVM using SVN, you can check out LLD
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under ``tools`` directory just like you probably did for clang. For the
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details, see `Getting Started with the LLVM System
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<https://llvm.org/docs/GettingStarted.html>`_.
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If you haven't checked out LLVM, the easiest way to build LLD is to
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check out the entire LLVM projects/sub-projects from a git mirror and
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build that tree. You need `cmake` and of course a C++ compiler.
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.. code-block:: console
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$ git clone https://github.com/llvm/llvm-project llvm-project
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$ mkdir build
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$ cd build
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$ cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=lld -DCMAKE_INSTALL_PREFIX=/usr/local ../llvm-project/llvm
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$ make install
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Using LLD
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---------
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LLD is installed as ``ld.lld``. On Unix, linkers are invoked by
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compiler drivers, so you are not expected to use that command
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directly. There are a few ways to tell compiler drivers to use ld.lld
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instead of the default linker.
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The easiest way to do that is to overwrite the default linker. After
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installing LLD to somewhere on your disk, you can create a symbolic
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link by doing ``ln -s /path/to/ld.lld /usr/bin/ld`` so that
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``/usr/bin/ld`` is resolved to LLD.
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If you don't want to change the system setting, you can use clang's
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``-fuse-ld`` option. In this way, you want to set ``-fuse-ld=lld`` to
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LDFLAGS when building your programs.
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LLD leaves its name and version number to a ``.comment`` section in an
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output. If you are in doubt whether you are successfully using LLD or
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not, run ``readelf --string-dump .comment <output-file>`` and examine the
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output. If the string "Linker: LLD" is included in the output, you are
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using LLD.
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History
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-------
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Here is a brief project history of the ELF and COFF ports.
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- May 2015: We decided to rewrite the COFF linker and did that.
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Noticed that the new linker is much faster than the MSVC linker.
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- July 2015: The new ELF port was developed based on the COFF linker
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architecture.
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- September 2015: The first patches to support MIPS and AArch64 landed.
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- October 2015: Succeeded to self-host the ELF port. We have noticed
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that the linker was faster than the GNU linkers, but we weren't sure
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at the time if we would be able to keep the gap as we would add more
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features to the linker.
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- July 2016: Started working on improving the linker script support.
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- December 2016: Succeeded to build the entire FreeBSD base system
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including the kernel. We had widen the performance gap against the
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GNU linkers.
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Internals
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---------
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For the internals of the linker, please read :doc:`NewLLD`. It is a bit
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outdated but the fundamental concepts remain valid. We'll update the
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document soon.
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.. toctree::
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:maxdepth: 1
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NewLLD
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AtomLLD
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WebAssembly
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windows_support
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missingkeyfunction
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Partitions
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ReleaseNotes
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ELF/linker_script
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