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diff --git a/contrib/llvm/tools/lld/docs/index.rst b/contrib/llvm/tools/lld/docs/index.rst new file mode 100644 index 000000000000..2821ce4d214e --- /dev/null +++ b/contrib/llvm/tools/lld/docs/index.rst @@ -0,0 +1,178 @@ +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 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 is the most well-supported target and have + reached production quality. AArch64 and MIPS seem decent too. x86 + should be OK but not well tested yet. ARM support is being developed + actively. + +- 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 to your program to eliminate dependency to + 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 completely a 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 checkout out LLVM, the easiest way to build LLD is to +checkout 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-project/llvm-project-20170507 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 + ReleaseNotes |