[PATCH] D146898: [Docs][typo] Done the required fix for the #61690
Johannes Doerfert via Phabricator via llvm-commits
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Fri Mar 31 17:53:12 PDT 2023
jdoerfert added inline comments.
================
Comment at: llvm/docs/GettingStarted.rst:2
+====================================Getting Started with the LLVM System====================================.. contents:::local:Overview========Welcome to the LLVM project!The LLVM project has multiple components. The core of the project isitself called "LLVM". This contains all of the tools, libraries, and headerfiles needed to process intermediate representations and converts it intoobject files. Tools include an assembler, disassembler, bitcode analyzer, andbitcode optimizer. It also contains basic regression tests.C-like languages use the `Clang <https://clang.llvm.org/>`_ front end. Thiscomponent compiles C, C++, Objective C, and Objective C++ code into LLVM bitcode-- and from there into object files, using LLVM.Other components include:the `libc++ C++ standard library <https://libcxx.llvm.org>`_,the `LLD linker <https://lld.llvm.org>`_, and more.Getting the Source Code and Building LLVM=========================================#. Check out LLVM (including subprojects like Clang):* ``git clone https://github.com/llvm/llvm-project.git``* Or, on windows:``git clone --config core.autocrlf=falsehttps://github.com/llvm/llvm-project.git``* To save storage and speed-up the checkout time, you may want to do a`shallow clone <https://git-scm.com/docs/git-clone#Documentation/git-clone.txt---depthltdepthgt>`_.For example, to get the latest revision of the LLVM project, use``git clone --depth 1 https://github.com/llvm/llvm-project.git``#. Configure and build LLVM and Clang:* ``cd llvm-project``* ``cmake -S llvm -B build -G <generator> [options]``Some common build system generators are:* ``Ninja`` --- for generating `Ninja <https://ninja-build.org>`_build files. Most llvm developers use Ninja.* ``Unix Makefiles`` --- for generating make-compatible parallel makefiles.* ``Visual Studio`` --- for generating Visual Studio projects andsolutions.* ``Xcode`` --- for generating Xcode projects.* See the `CMake docs<https://cmake.org/cmake/help/latest/manual/cmake-generators.7.html>`_for a more comprehensive list.Some common options:* ``-DLLVM_ENABLE_PROJECTS='...'`` --- semicolon-separated list of the LLVMsubprojects you'd like to additionally build. Can include any of: clang,clang-tools-extra, lldb, lld, polly, or cross-project-tests.For example, to build LLVM, Clang, and LLD, use``-DLLVM_ENABLE_PROJECTS="clang;lld"``.* ``-DCMAKE_INSTALL_PREFIX=directory`` --- Specify for *directory* the fullpathname of where you want the LLVM tools and libraries to be installed(default ``/usr/local``).* ``-DCMAKE_BUILD_TYPE=type`` --- Controls optimization level and debuginformation of the build. Valid options for *type* are ``Debug``,``Release``, ``RelWithDebInfo``, and ``MinSizeRel``. For more detailedinformation see :ref:`CMAKE_BUILD_TYPE <cmake_build_type>`.* ``-DLLVM_ENABLE_ASSERTIONS=ON`` --- Compile with assertion checks enabled(default is ON for Debug builds, OFF for all other build types).* ``-DLLVM_USE_LINKER=lld`` --- Link with the `lld linker`_, assuming itis installed on your system. This can dramatically speed up link timesif the default linker is slow.* ``-DLLVM_PARALLEL_{COMPILE,LINK}_JOBS=N`` --- Limit the number ofcompile/link jobs running in parallel at the same time. This isespecially important for linking since linking can use lots of memory. Ifyou run into memory issues building LLVM, try setting this to limit themaximum number of compile/link jobs running at the same time.* ``cmake --build . [--target <target>]`` or the build system specifiedabove directly.* The default target (i.e. ``cmake --build .`` or ``make``) will build all ofLLVM.* The ``check-all`` target (i.e. ``ninja check-all``) will run theregression tests to ensure everything is in working order.* CMake will generate build targets for each tool and library, and mostLLVM sub-projects generate their own ``check-<project>`` target.* Running a serial build will be **slow**. To improve speed, try running aparallel build. That's done by default in Ninja; for ``make``, use theoption ``-j NN``, where ``NN`` is the number of parallel jobs, e.g. thenumber of available CPUs.* A basic CMake and build/test invocation which only builds LLVM and no othersubprojects:``cmake -S llvm -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug````ninja -C build check-llvm``This will setup an LLVM build with debugging info, then compile LLVM andrun LLVM tests.* For more detailed information on CMake options, see `CMake <CMake.html>`__* If you get build or test failures, see `below`_.Consult the `Getting Started with LLVM`_ section for detailed information onconfiguring and compiling LLVM. Go to `Directory Layout`_ to learn about thelayout of the source code tree.Stand-alone Builds------------------Stand-alone builds allow you to build a sub-project against a pre-builtversion of the clang or llvm libraries that is already present on yoursystem.You can use the source code from a standard checkout of the llvm-project(as described above) to do stand-alone builds, but you may also buildfrom a :ref:`sparse checkout<workflow-multicheckout-nocommit>` or from thetarballs available on the `releases <https://github.com/llvm/llvm-project/releases/>`_page.For stand-alone builds, you must have an llvm install that is configuredproperly to be consumable by stand-alone builds of the other projects.This could be a distro provided LLVM install, or you can build it yourself,like this:.. code-block:: consolecmake -G Ninja -S path/to/llvm-project/llvm -B $builddir \-DLLVM_INSTALL_UTILS=ON \-DCMAKE_INSTALL_PREFIX=/path/to/llvm/install/prefix \< other options >ninja -C $builddir installOnce llvm is installed, to configure a project for a stand-alone build, invoke CMake like this:.. code-block:: consolecmake -G Ninja -S path/to/llvm-project/$subproj \-B $buildir_subproj \-DLLVM_EXTERNAL_LIT=/path/to/lit \-DLLVM_ROOT=/path/to/llvm/install/prefixNotice that:* The stand-alone build needs to happen in a folder that is not theoriginal folder where LLVMN was built(`$builddir!=$builddir_subproj`).* ``LLVM_ROOT`` should point to the prefix of your llvm installation,so for example, if llvm is installed into ``/usr/bin`` and``/usr/lib64``, then you should pass ``-DLLVM_ROOT=/usr/``.* Both the ``LLVM_ROOT`` and ``LLVM_EXTERNAL_LIT`` options arerequired to do stand-alone builds for all sub-projects. Additionalrequired options for each sub-project can be found in the tablebelow.The ``check-$subproj`` and ``install`` build targets are supported for thesub-projects listed in the table below.============ ======================== ======================Sub-Project Required Sub-Directories Required CMake Options============ ======================== ======================llvm llvm, cmake, third-party LLVM_INSTALL_UTILS=ONclang clang, cmake CLANG_INCLUDE_TESTS=ON (Required for check-clang only)lld lld, cmake============ ======================== ======================Example for building stand-alone `clang`:.. code-block:: console#!/bin/shbuild_llvm=`pwd`/build-llvmbuild_clang=`pwd`/build-clanginstallprefix=`pwd`/installllvm=`pwd`/llvm-projectmkdir -p $build_llvmmkdir -p $installprefixcmake -G Ninja -S $llvm/llvm -B $build_llvm \-DLLVM_INSTALL_UTILS=ON \-DCMAKE_INSTALL_PREFIX=$installprefix \-DCMAKE_BUILD_TYPE=Releaseninja -C $build_llvm installcmake -G Ninja -S $llvm/clang -B $build_clang \-DLLVM_EXTERNAL_LIT=$build_llvm/utils/lit \-DLLVM_ROOT=$installprefixninja -C $build_clangRequirements============Before you begin to use the LLVM system, review the requirements given below.This may save you some trouble by knowing ahead of time what hardware andsoftware you will need.Hardware--------LLVM is known to work on the following host platforms:================== ===================== =============OS Arch Compilers================== ===================== =============Linux x86\ :sup:`1` GCC, ClangLinux amd64 GCC, ClangLinux ARM GCC, ClangLinux Mips GCC, ClangLinux PowerPC GCC, ClangLinux SystemZ GCC, ClangSolaris V9 (Ultrasparc) GCCDragonFlyBSD amd64 GCC, ClangFreeBSD x86\ :sup:`1` GCC, ClangFreeBSD amd64 GCC, ClangNetBSD x86\ :sup:`1` GCC, ClangNetBSD amd64 GCC, ClangOpenBSD x86\ :sup:`1` GCC, ClangOpenBSD amd64 GCC, ClangmacOS\ :sup:`2` PowerPC GCCmacOS x86 GCC, ClangCygwin/Win32 x86\ :sup:`1, 3` GCCWindows x86\ :sup:`1` Visual StudioWindows x64 x86-64 Visual Studio================== ===================== =============.. note::#. Code generation supported for Pentium processors and up#. Code generation supported for 32-bit ABI only#. To use LLVM modules on Win32-based system, you may configure LLVMwith ``-DBUILD_SHARED_LIBS=On``.Note that Debug builds require a lot of time and disk space. An LLVM-only buildwill need about 1-3 GB of space. A full build of LLVM and Clang will need around15-20 GB of disk space. The exact space requirements will vary by system. (Itis so large because of all the debugging information and the fact that thelibraries are statically linked into multiple tools).If you are space-constrained, you can build only selected tools or onlyselected targets. The Release build requires considerably less space.The LLVM suite *may* compile on other platforms, but it is not guaranteed to doso. If compilation is successful, the LLVM utilities should be able toassemble, disassemble, analyze, and optimize LLVM bitcode. Code generationshould work as well, although the generated native code may not work on yourplatform.Software--------Compiling LLVM requires that you have several software packages installed. Thetable below lists those required packages. The Package column is the usual namefor the software package that LLVM depends on. The Version column provides"known to work" versions of the package. The Notes column describes how LLVMuses the package and provides other details.=========================================================== ============ ==========================================Package Version Notes=========================================================== ============ ==========================================`CMake <http://cmake.org/>`__ >=3.13.4 Makefile/workspace generator`GCC <http://gcc.gnu.org/>`_ >=7.1.0 C/C++ compiler\ :sup:`1``python <http://www.python.org/>`_ >=3.6 Automated test suite\ :sup:`2``zlib <http://zlib.net>`_ >=1.2.3.4 Compression library\ :sup:`3``GNU Make <http://savannah.gnu.org/projects/make>`_ 3.79, 3.79.1 Makefile/build processor\ :sup:`4`=========================================================== ============ ==========================================.. note::#. Only the C and C++ languages are needed so there's no need to build theother languages for LLVM's purposes. See `below` for specific versioninfo.#. Only needed if you want to run the automated test suite in the``llvm/test`` directory.#. Optional, adds compression / uncompression capabilities to selected LLVMtools.#. Optional, you can use any other build tool supported by CMake.Additionally, your compilation host is expected to have the usual plethora ofUnix utilities. Specifically:* **ar** --- archive library builder* **bzip2** --- bzip2 command for distribution generation* **bunzip2** --- bunzip2 command for distribution checking* **chmod** --- change permissions on a file* **cat** --- output concatenation utility* **cp** --- copy files* **date** --- print the current date/time* **echo** --- print to standard output* **egrep** --- extended regular expression search utility* **find** --- find files/dirs in a file system* **grep** --- regular expression search utility* **gzip** --- gzip command for distribution generation* **gunzip** --- gunzip command for distribution checking* **install** --- install directories/files* **mkdir** --- create a directory* **mv** --- move (rename) files* **ranlib** --- symbol table builder for archive libraries* **rm** --- remove (delete) files and directories* **sed** --- stream editor for transforming output* **sh** --- Bourne shell for make build scripts* **tar** --- tape archive for distribution generation* **test** --- test things in file system* **unzip** --- unzip command for distribution checking* **zip** --- zip command for distribution generation.. _below:.. _check here:Host C++ Toolchain, both Compiler and Standard Library------------------------------------------------------LLVM is very demanding of the host C++ compiler, and as such tends to exposebugs in the compiler. We also attempt to follow improvements and developments inthe C++ language and library reasonably closely. As such, we require a modernhost C++ toolchain, both compiler and standard library, in order to build LLVM.LLVM is written using the subset of C++ documented in :doc:`codingstandards<CodingStandards>`. To enforce this language version, we check the mostpopular host toolchains for specific minimum versions in our build systems:* Clang 5.0* Apple Clang 10.0* GCC 7.1* Visual Studio 2019 16.7Anything older than these toolchains *may* work, but will require forcing thebuild system with a special option and is not really a supported host platform.Also note that older versions of these compilers have often crashed ormiscompiled LLVM.For less widely used host toolchains such as ICC or xlC, be aware that a veryrecent version may be required to support all of the C++ features used in LLVM.We track certain versions of software that are *known* to fail when used aspart of the host toolchain. These even include linkers at times.**GNU ld 2.16.X**. Some 2.16.X versions of the ld linker will produce very longwarning messages complaining that some "``.gnu.linkonce.t.*``" symbol wasdefined in a discarded section. You can safely ignore these messages as they areerroneous and the linkage is correct. These messages disappear using ld 2.17.**GNU binutils 2.17**: Binutils 2.17 contains `a bug<http://sourceware.org/bugzilla/show_bug.cgi?id=3111>`__ which causes huge linktimes (minutes instead of seconds) when building LLVM. We recommend upgradingto a newer version (2.17.50.0.4 or later).**GNU Binutils 2.19.1 Gold**: This version of Gold contained `a bug<http://sourceware.org/bugzilla/show_bug.cgi?id=9836>`__ which causesintermittent failures when building LLVM with position independent code. Thesymptom is an error about cyclic dependencies. We recommend upgrading to anewer version of Gold.Getting a Modern Host C++ Toolchain^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^This section mostly applies to Linux and older BSDs. On macOS, you shouldhave a sufficiently modern Xcode, or you will likely need to upgrade until youdo. Windows does not have a "system compiler", so you must install either VisualStudio 2019 (or later), or a recent version of mingw64. FreeBSD 10.0 and newerhave a modern Clang as the system compiler.However, some Linux distributions and some other or older BSDs sometimes haveextremely old versions of GCC. These steps attempt to help you upgrade youcompiler even on such a system. However, if at all possible, we encourage youto use a recent version of a distribution with a modern system compiler thatmeets these requirements. Note that it is tempting to install a priorversion of Clang and libc++ to be the host compiler, however libc++ was notwell tested or set up to build on Linux until relatively recently. Asa consequence, this guide suggests just using libstdc++ and a modern GCC as theinitial host in a bootstrap, and then using Clang (and potentially libc++).The first step is to get a recent GCC toolchain installed. The most commondistribution on which users have struggled with the version requirements isUbuntu Precise, 12.04 LTS. For this distribution, one easy option is to installthe `toolchain testing PPA`_ and use it to install a modern GCC. There isa really nice discussions of this on the `ask ubuntu stack exchange`_ and a`github gist`_ with updated commands. However, not all users can use PPAs andthere are many other distributions, so it may be necessary (or just useful, ifyou're here you *are* doing compiler development after all) to build and installGCC from source. It is also quite easy to do these days... _toolchain testing PPA:https://launchpad.net/~ubuntu-toolchain-r/+archive/test.. _ask ubuntu stack exchange:https://askubuntu.com/questions/466651/how-do-i-use-the-latest-gcc-on-ubuntu/581497#58149.. _github gist:https://gist.github.com/application2000/73fd6f4bf1be6600a2cf9f56315a2d91Easy steps for installing GCC 7.1.0:.. code-block:: console% gcc_version=7.1.0% wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2% wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2.sig% wget https://ftp.gnu.org/gnu/gnu-keyring.gpg% signature_invalid=`gpg --verify --no-default-keyring --keyring ./gnu-keyring.gpg gcc-${gcc_version}.tar.bz2.sig`% if [ $signature_invalid ]; then echo "Invalid signature" ; exit 1 ; fi% tar -xvjf gcc-${gcc_version}.tar.bz2% cd gcc-${gcc_version}% ./contrib/download_prerequisites% cd ..% mkdir gcc-${gcc_version}-build% cd gcc-${gcc_version}-build% $PWD/../gcc-${gcc_version}/configure --prefix=$HOME/toolchains --enable-languages=c,c++% make -j$(nproc)% make installFor more details, check out the excellent `GCC wiki entry`_, where I got mostof this information from... _GCC wiki entry:https://gcc.gnu.org/wiki/InstallingGCCOnce you have a GCC toolchain, configure your build of LLVM to use the newtoolchain for your host compiler and C++ standard library. Because the newversion of libstdc++ is not on the system library search path, you need to passextra linker flags so that it can be found at link time (``-L``) and at runtime(``-rpath``). If you are using CMake, this invocation should produce workingbinaries:.. code-block:: console% mkdir build% cd build% CC=$HOME/toolchains/bin/gcc CXX=$HOME/toolchains/bin/g++ \cmake .. -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,$HOME/toolchains/lib64 -L$HOME/toolchains/lib64"If you fail to set rpath, most LLVM binaries will fail on startup with a messagefrom the loader similar to ``libstdc++.so.6: version `GLIBCXX_3.4.20' notfound``. This means you need to tweak the -rpath linker flag.This method will add an absolute path to the rpath of all executables. That'sfine for local development. If you want to distribute the binaries you buildso that they can run on older systems, copy ``libstdc++.so.6`` into the``lib/`` directory. All of LLVM's shipping binaries have an rpath pointing at``$ORIGIN/../lib``, so they will find ``libstdc++.so.6`` there. Non-distributedbinaries don't have an rpath set and won't find ``libstdc++.so.6``. Pass``-DLLVM_LOCAL_RPATH="$HOME/toolchains/lib64"`` to cmake to add an absolutepath to ``libstdc++.so.6`` as above. Since these binaries are not distributed,having an absolute local path is fine for them.When you build Clang, you will need to give *it* access to modern C++standard library in order to use it as your new host in part of a bootstrap.There are two easy ways to do this, either build (and install) libc++ alongwith Clang and then use it with the ``-stdlib=libc++`` compile and link flag,or install Clang into the same prefix (``$HOME/toolchains`` above) as GCC.Clang will look within its own prefix for libstdc++ and use it if found. Youcan also add an explicit prefix for Clang to look in for a GCC toolchain withthe ``--gcc-toolchain=/opt/my/gcc/prefix`` flag, passing it to both compile andlink commands when using your just-built-Clang to bootstrap... _Getting Started with LLVM:Getting Started with LLVM=========================The remainder of this guide is meant to get you up and running with LLVM and togive you some basic information about the LLVM environment.The later sections of this guide describe the `general layout`_ of the LLVMsource tree, a `simple example`_ using the LLVM tool chain, and `links`_ to findmore information about LLVM or to get help via e-mail.Terminology and Notation------------------------Throughout this manual, the following names are used to denote paths specific tothe local system and working environment. *These are not environment variablesyou need to set but just strings used in the rest of this document below*. Inany of the examples below, simply replace each of these names with theappropriate pathname on your local system. All these paths are absolute:``SRC_ROOT``This is the top level directory of the LLVM source tree.``OBJ_ROOT``This is the top level directory of the LLVM object tree (i.e. the tree whereobject files and compiled programs will be placed. It can be the same asSRC_ROOT).Unpacking the LLVM Archives---------------------------If you have the LLVM distribution, you will need to unpack it before you canbegin to compile it. LLVM is distributed as a number of differentsubprojects. Each one has its own download which is a TAR archive that iscompressed with the gzip program.The files are as follows, with *x.y* marking the version number:``llvm-x.y.tar.gz``Source release for the LLVM libraries and tools.``cfe-x.y.tar.gz``Source release for the Clang frontend... _checkout:Checkout LLVM from Git----------------------You can also checkout the source code for LLVM from Git... note::Passing ``--config core.autocrlf=false`` should not be required inthe future after we adjust the .gitattribute settings correctly, butis required for Windows users at the time of this writing.Simply run:.. code-block:: console% git clone https://github.com/llvm/llvm-project.gitor on Windows,.. code-block:: console% git clone --config core.autocrlf=false https://github.com/llvm/llvm-project.gitThis will create an '``llvm-project``' directory in the current directory andfully populate it with all of the source code, test directories, and localcopies of documentation files for LLVM and all the related subprojects. Notethat unlike the tarballs, which contain each subproject in a separate file, thegit repository contains all of the projects together.If you want to get a specific release (as opposed to the most recent revision),you can check out a tag after cloning the repository. E.g., `git checkoutllvmorg-6.0.1` inside the ``llvm-project`` directory created by the abovecommand. Use `git tag -l` to list all of them.Sending patches^^^^^^^^^^^^^^^See :ref:`Contributing <submit_patch>`.Bisecting commits^^^^^^^^^^^^^^^^^See `Bisecting LLVM code <GitBisecting.html>`_ for how to use ``git bisect``on LLVM.Reverting a change^^^^^^^^^^^^^^^^^^When reverting changes using git, the default message will say "This revertscommit XYZ". Leave this at the end of the commit message, but add some detailsbefore it as to why the commit is being reverted. A brief explanation and/orlinks to bots that demonstrate the problem are sufficient.Local LLVM Configuration------------------------Once checked out repository, the LLVM suite source code must be configuredbefore being built. This process uses CMake. Unlinke the normal ``configure``script, CMake generates the build files in whatever format you request as wellas various ``*.inc`` files, and ``llvm/include/llvm/Config/config.h.cmake``.Variables are passed to ``cmake`` on the command line using the format``-D<variable name>=<value>``. The following variables are some common optionsused by people developing LLVM.+-------------------------+----------------------------------------------------+| Variable | Purpose |+=========================+====================================================+| CMAKE_C_COMPILER | Tells ``cmake`` which C compiler to use. By || | default, this will be /usr/bin/cc. |+-------------------------+----------------------------------------------------+| CMAKE_CXX_COMPILER | Tells ``cmake`` which C++ compiler to use. By || | default, this will be /usr/bin/c++. |+-------------------------+----------------------------------------------------+| CMAKE_BUILD_TYPE | Tells ``cmake`` what type of build you are trying || | to generate files for. Valid options are Debug, || | Release, RelWithDebInfo, and MinSizeRel. Default || | is Debug. |+-------------------------+----------------------------------------------------+| CMAKE_INSTALL_PREFIX | Specifies the install directory to target when || | running the install action of the build files. |+-------------------------+----------------------------------------------------+| Python3_EXECUTABLE | Forces CMake to use a specific Python version by || | passing a path to a Python interpreter. By default || | the Python version of the interpreter in your PATH || | is used. |+-------------------------+----------------------------------------------------+| LLVM_TARGETS_TO_BUILD | A semicolon delimited list controlling which || | targets will be built and linked into llvm. || | The default list is defined as || | ``LLVM_ALL_TARGETS``, and can be set to include || | out-of-tree targets. The default value includes: || | ``AArch64, AMDGPU, ARM, AVR, BPF, Hexagon, Lanai, || | Mips, MSP430, NVPTX, PowerPC, RISCV, Sparc, || | SystemZ, WebAssembly, X86, XCore``. Setting this || | to ``"host"`` will only compile the host || | architecture (e.g. equivalent to specifying ``X86``|| | on an x86 host machine) can || | significantly speed up compile and test times. |+-------------------------+----------------------------------------------------+| LLVM_ENABLE_DOXYGEN | Build doxygen-based documentation from the source || | code This is disabled by default because it is || | slow and generates a lot of output. |+-------------------------+----------------------------------------------------+| LLVM_ENABLE_PROJECTS | A semicolon-delimited list selecting which of the || | other LLVM subprojects to additionally build. (Only|| | effective when using a side-by-side project layout || | e.g. via git). The default list is empty. Can || | include: clang, clang-tools-extra, || | cross-project-tests, flang, libc, libclc, lld, || | lldb, mlir, openmp, polly, or pstl. |+-------------------------+----------------------------------------------------+| LLVM_ENABLE_RUNTIMES | A semicolon-delimited list selecting which of the || | runtimes to build. (Only effective when using the || | full monorepo layout). The default list is empty. || | Can include: compiler-rt, libc, libcxx, libcxxabi, || | libunwind, or openmp. |+-------------------------+----------------------------------------------------+| LLVM_ENABLE_SPHINX | Build sphinx-based documentation from the source || | code. This is disabled by default because it is || | slow and generates a lot of output. Sphinx version || | 1.5 or later recommended. |+-------------------------+----------------------------------------------------+| LLVM_BUILD_LLVM_DYLIB | Generate libLLVM.so. This library contains a || | default set of LLVM components that can be || | overridden with ``LLVM_DYLIB_COMPONENTS``. The || | default contains most of LLVM and is defined in || | ``tools/llvm-shlib/CMakelists.txt``. This option is|| | not available on Windows. |+-------------------------+----------------------------------------------------+| LLVM_OPTIMIZED_TABLEGEN | Builds a release tablegen that gets used during || | the LLVM build. This can dramatically speed up || | debug builds. |+-------------------------+----------------------------------------------------+To configure LLVM, follow these steps:#. Change directory into the object root directory:.. code-block:: console% cd OBJ_ROOT#. Run the ``cmake``:.. code-block:: console% cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=<type> -DCMAKE_INSTALL_PREFIX=/install/path[other options] SRC_ROOTCompiling the LLVM Suite Source Code------------------------------------Unlike with autotools, with CMake your build type is defined at configuration.If you want to change your build type, you can re-run cmake with the followinginvocation:.. code-block:: console% cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=<type> -DCMAKE_BUILD_TYPE=type SRC_ROOTBetween runs, CMake preserves the values set for all options. CMake has thefollowing build types defined:DebugThese builds are the default. The build system will compile the tools andlibraries unoptimized, with debugging information, and asserts enabled.ReleaseFor these builds, the build system will compile the tools and librarieswith optimizations enabled and not generate debug info. CMakes defaultoptimization level is -O3. This can be configured by setting the``CMAKE_CXX_FLAGS_RELEASE`` variable on the CMake command line.RelWithDebInfoThese builds are useful when debugging. They generate optimized binaries withdebug information. CMakes default optimization level is -O2. This can beconfigured by setting the ``CMAKE_CXX_FLAGS_RELWITHDEBINFO`` variable on theCMake command line.Once you have LLVM configured, you can build it by entering the *OBJ_ROOT*directory and issuing the following command:.. code-block:: console% makeIf the build fails, please `check here`_ to see if you are using a version ofGCC that is known not to compile LLVM.If you have multiple processors in your machine, you may wish to use some of theparallel build options provided by GNU Make. For example, you could use thecommand:.. code-block:: console% make -j2There are several special targets which are useful when working with the LLVMsource code:``make clean``Removes all files generated by the build. This includes object files,generated C/C++ files, libraries, and executables.``make install``Installs LLVM header files, libraries, tools, and documentation in a hierarchyunder ``$PREFIX``, specified with ``CMAKE_INSTALL_PREFIX``, whichdefaults to ``/usr/local``.``make docs-llvm-html``If configured with ``-DLLVM_ENABLE_SPHINX=On``, this will generate a directoryat ``OBJ_ROOT/docs/html`` which contains the HTML formatted documentation.Cross-Compiling LLVM--------------------It is possible to cross-compile LLVM itself. That is, you can create LLVMexecutables and libraries to be hosted on a platform different from the platformwhere they are built (a Canadian Cross build). To generate build files forcross-compiling CMake provides a variable ``CMAKE_TOOLCHAIN_FILE`` which candefine compiler flags and variables used during the CMake test operations.The result of such a build is executables that are not runnable on the buildhost but can be executed on the target. As an example the following CMakeinvocation can generate build files targeting iOS. This will work on macOSwith the latest Xcode:.. code-block:: console% cmake -G "Ninja" -DCMAKE_OSX_ARCHITECTURES="armv7;armv7s;arm64"-DCMAKE_TOOLCHAIN_FILE=<PATH_TO_LLVM>/cmake/platforms/iOS.cmake-DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_RUNTIME=Off -DLLVM_INCLUDE_TESTS=Off-DLLVM_INCLUDE_EXAMPLES=Off -DLLVM_ENABLE_BACKTRACES=Off [options]<PATH_TO_LLVM>Note: There are some additional flags that need to be passed when building foriOS due to limitations in the iOS SDK.Check :doc:`HowToCrossCompileLLVM` and `Clang docs on how to cross-compile in general<https://clang.llvm.org/docs/CrossCompilation.html>`_ for more informationabout cross-compiling.The Location of LLVM Object Files---------------------------------The LLVM build system is capable of sharing a single LLVM source tree amongseveral LLVM builds. Hence, it is possible to build LLVM for several differentplatforms or configurations using the same source tree.* Change directory to where the LLVM object files should live:.. code-block:: console% cd OBJ_ROOT* Run ``cmake``:.. code-block:: console% cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release SRC_ROOTThe LLVM build will create a structure underneath *OBJ_ROOT* that matches theLLVM source tree. At each level where source files are present in the sourcetree there will be a corresponding ``CMakeFiles`` directory in the *OBJ_ROOT*.Underneath that directory there is another directory with a name ending in``.dir`` under which you'll find object files for each source.For example:.. code-block:: console% cd llvm_build_dir% find lib/Support/ -name APFloat*lib/Support/CMakeFiles/LLVMSupport.dir/APFloat.cpp.oOptional Configuration Items----------------------------If you're running on a Linux system that supports the `binfmt_misc<http://en.wikipedia.org/wiki/binfmt_misc>`_module, and you have root access on the system, you can set your system up toexecute LLVM bitcode files directly. To do this, use commands like this (thefirst command may not be required if you are already using the module):.. code-block:: console% mount -t binfmt_misc none /proc/sys/fs/binfmt_misc% echo ':llvm:M::BC::/path/to/lli:' > /proc/sys/fs/binfmt_misc/register% chmod u+x hello.bc (if needed)% ./hello.bcThis allows you to execute LLVM bitcode files directly. On Debian, you can alsouse this command instead of the 'echo' command above:.. code-block:: console% sudo update-binfmts --install llvm /path/to/lli --magic 'BC'.. _Program Layout:.. _general layout:Directory Layout================One useful source of information about the LLVM source base is the LLVM `doxygen<http://www.doxygen.org/>`_ documentation available at`<https://llvm.org/doxygen/>`_. The following is a brief introduction to codelayout:``llvm/cmake``--------------Generates system build files.``llvm/cmake/modules``Build configuration for llvm user defined options. Checks compiler version andlinker flags.``llvm/cmake/platforms``Toolchain configuration for Android NDK, iOS systems and non-Windows hosts totarget MSVC.``llvm/examples``------------------ Some simple examples showing how to use LLVM as a compiler for a customlanguage - including lowering, optimization, and code generation.- Kaleidoscope Tutorial: Kaleidoscope language tutorial run through theimplementation of a nice little compiler for a non-trivial languageincluding a hand-written lexer, parser, AST, as well as code generationsupport using LLVM- both static (ahead of time) and various approaches toJust In Time (JIT) compilation.`Kaleidoscope Tutorial for complete beginner<https://llvm.org/docs/tutorial/MyFirstLanguageFrontend/index.html>`_.- BuildingAJIT: Examples of the `BuildingAJIT tutorial<https://llvm.org/docs/tutorial/BuildingAJIT1.html>`_ that shows how LLVM’sORC JIT APIs interact with other parts of LLVM. It also, teaches how torecombine them to build a custom JIT that is suited to your use-case.``llvm/include``----------------Public header files exported from the LLVM library. The three main subdirectories:``llvm/include/llvm``All LLVM-specific header files, and subdirectories for different portions ofLLVM: ``Analysis``, ``CodeGen``, ``Target``, ``Transforms``, etc...``llvm/include/llvm/Support``Generic support libraries provided with LLVM but not necessarily specific toLLVM. For example, some C++ STL utilities and a Command Line option processinglibrary store header files here.``llvm/include/llvm/Config``Header files configured by ``cmake``. They wrap "standard" UNIX andC header files. Source code can include these header files whichautomatically take care of the conditional #includes that ``cmake``generates.``llvm/lib``------------Most source files are here. By putting code in libraries, LLVM makes it easy toshare code among the `tools`_.``llvm/lib/IR/``Core LLVM source files that implement core classes like Instruction andBasicBlock.``llvm/lib/AsmParser/``Source code for the LLVM assembly language parser library.``llvm/lib/Bitcode/``Code for reading and writing bitcode.``llvm/lib/Analysis/``A variety of program analyses, such as Call Graphs, Induction Variables,Natural Loop Identification, etc.``llvm/lib/Transforms/``IR-to-IR program transformations, such as Aggressive Dead Code Elimination,Sparse Conditional Constant Propagation, Inlining, Loop Invariant Code Motion,Dead Global Elimination, and many others.``llvm/lib/Target/``Files describing target architectures for code generation. For example,``llvm/lib/Target/X86`` holds the X86 machine description.``llvm/lib/CodeGen/``The major parts of the code generator: Instruction Selector, InstructionScheduling, and Register Allocation.``llvm/lib/MC/``The libraries represent and process code at machine code level. Handlesassembly and object-file emission.``llvm/lib/ExecutionEngine/``Libraries for directly executing bitcode at runtime in interpreted andJIT-compiled scenarios.``llvm/lib/Support/``Source code that corresponding to the header files in ``llvm/include/ADT/``and ``llvm/include/Support/``.``llvm/bindings``----------------------Contains bindings for the LLVM compiler infrastructure to allowprograms written in languages other than C or C++ to take advantage of the LLVMinfrastructure.LLVM project provides language bindings for OCaml and Python.``llvm/projects``-----------------Projects not strictly part of LLVM but shipped with LLVM. This is also thedirectory for creating your own LLVM-based projects which leverage the LLVMbuild system.``llvm/test``-------------Feature and regression tests and other sanity checks on LLVM infrastructure. Theseare intended to run quickly and cover a lot of territory without being exhaustive.``test-suite``--------------A comprehensive correctness, performance, and benchmarking test suitefor LLVM. This comes in a ``separate git repository<https://github.com/llvm/llvm-test-suite>``, because it contains alarge amount of third-party code under a variety of licenses. Fordetails see the :doc:`Testing Guide <TestingGuide>` document... _tools:``llvm/tools``--------------Executables built out of the librariesabove, which form the main part of the user interface. You can always get helpfor a tool by typing ``tool_name -help``. The following is a brief introductionto the most important tools. More detailed information is inthe `Command Guide <CommandGuide/index.html>`_.``bugpoint````bugpoint`` is used to debug optimization passes or code generation backendsby narrowing down the given test case to the minimum number of passes and/orinstructions that still cause a problem, whether it is a crash ormiscompilation. See `<HowToSubmitABug.html>`_ for more information on using``bugpoint``.``llvm-ar``The archiver produces an archive containing the given LLVM bitcode files,optionally with an index for faster lookup.``llvm-as``The assembler transforms the human readable LLVM assembly to LLVM bitcode.``llvm-dis``The disassembler transforms the LLVM bitcode to human readable LLVM assembly.``llvm-link````llvm-link``, not surprisingly, links multiple LLVM modules into a singleprogram.``lli````lli`` is the LLVM interpreter, which can directly execute LLVM bitcode(although very slowly...). For architectures that support it (currently x86,Sparc, and PowerPC), by default, ``lli`` will function as a Just-In-Timecompiler (if the functionality was compiled in), and will execute the code*much* faster than the interpreter.``llc````llc`` is the LLVM backend compiler, which translates LLVM bitcode to anative code assembly file.``opt````opt`` reads LLVM bitcode, applies a series of LLVM to LLVM transformations(which are specified on the command line), and outputs the resultantbitcode. '``opt -help``' is a good way to get a list of theprogram transformations available in LLVM.``opt`` can also run a specific analysis on an input LLVM bitcodefile and print the results. Primarily useful for debugginganalyses, or familiarizing yourself with what an analysis does.``llvm/utils``--------------Utilities for working with LLVM source code; some are part of the build processbecause they are code generators for parts of the infrastructure.``codegen-diff````codegen-diff`` finds differences between code that LLCgenerates and code that LLI generates. This is useful if you aredebugging one of them, assuming that the other generates correct output. Forthe full user manual, run ```perldoc codegen-diff'``.``emacs/``Emacs and XEmacs syntax highlighting for LLVM assembly files and TableGendescription files. See the ``README`` for information on using them.``getsrcs.sh``Finds and outputs all non-generated source files,useful if one wishes to do a lot of development across directoriesand does not want to find each file. One way to use it is to run,for example: ``xemacs `utils/getsources.sh``` from the top of the LLVM sourcetree.``llvmgrep``Performs an ``egrep -H -n`` on each source file in LLVM andpasses to it a regular expression provided on ``llvmgrep``'s commandline. This is an efficient way of searching the source base for aparticular regular expression.``TableGen/``Contains the tool used to generate registerdescriptions, instruction set descriptions, and even assemblers from commonTableGen description files.``vim/``vim syntax-highlighting for LLVM assembly filesand TableGen description files. See the ``README`` for how to use them... _simple example:An Example Using the LLVM Tool Chain====================================This section gives an example of using LLVM with the Clang front end.Example with clang------------------#. First, create a simple C file, name it 'hello.c':.. code-block:: c#include <stdio.h>int main() {printf("hello world\n");return 0;}#. Next, compile the C file into a native executable:.. code-block:: console% clang hello.c -o hello.. note::Clang works just like GCC by default. The standard -S and -c argumentswork as usual (producing a native .s or .o file, respectively).#. Next, compile the C file into an LLVM bitcode file:.. code-block:: console% clang -O3 -emit-llvm hello.c -c -o hello.bcThe -emit-llvm option can be used with the -S or -c options to emit an LLVM``.ll`` or ``.bc`` file (respectively) for the code. This allows you to usethe `standard LLVM tools <CommandGuide/index.html>`_ on the bitcode file.#. Run the program in both forms. To run the program, use:.. code-block:: console% ./helloand.. code-block:: console% lli hello.bcThe second examples shows how to invoke the LLVM JIT, :doc:`lli<CommandGuide/lli>`.#. Use the ``llvm-dis`` utility to take a look at the LLVM assembly code:.. code-block:: console% llvm-dis < hello.bc | less#. Compile the program to native assembly using the LLC code generator:.. code-block:: console% llc hello.bc -o hello.s#. Assemble the native assembly language file into a program:.. code-block:: console% /opt/SUNWspro/bin/cc -xarch=v9 hello.s -o hello.native # On Solaris% gcc hello.s -o hello.native # On others#. Execute the native code program:.. code-block:: console% ./hello.nativeNote that using clang to compile directly to native code (i.e. when the``-emit-llvm`` option is not present) does steps 6/7/8 for you.Common Problems===============If you are having problems building or using LLVM, or if you have any othergeneral questions about LLVM, please consult the `Frequently AskedQuestions <FAQ.html>`_ page.If you are having problems with limited memory and build time, please trybuilding with ninja instead of make. Please consider configuring thefollowing options with cmake:* -G NinjaSetting this option will allow you to build with ninja instead of make.Building with ninja significantly improves your build time, especially withincremental builds, and improves your memory usage.* -DLLVM_USE_LINKERSetting this option to lld will significantly reduce linking time for LLVMexecutables on ELF-based platforms, such as Linux. If you are building LLVMfor the first time and lld is not available to you as a binary package, thenyou may want to use the gold linker as a faster alternative to GNU ld.* -DCMAKE_BUILD_TYPEControls optimization level and debug information of the build. This settingcan affect RAM and disk usage, see :ref:`CMAKE_BUILD_TYPE <cmake_build_type>`for more information.* -DLLVM_ENABLE_ASSERTIONSThis option defaults to ON for Debug builds and defaults to OFF for Releasebuilds. As mentioned in the previous option, using the Release build type andenabling assertions may be a good alternative to using the Debug build type.* -DLLVM_PARALLEL_LINK_JOBSSet this equal to number of jobs you wish to run simultaneously. This issimilar to the -j option used with make, but only for link jobs. This optioncan only be used with ninja. You may wish to use a very low number of jobs,as this will greatly reduce the amount of memory used during the buildprocess. If you have limited memory, you may wish to set this to 1.* -DLLVM_TARGETS_TO_BUILDSet this equal to the target you wish to build. You may wish to set this toX86; however, you will find a full list of targets within thellvm-project/llvm/lib/Target directory.* -DLLVM_OPTIMIZED_TABLEGENSet this to ON to generate a fully optimized tablegen during your build. Thiswill significantly improve your build time. This is only useful if you areusing the Debug build type.* -DLLVM_ENABLE_PROJECTSSet this equal to the projects you wish to compile (e.g. clang, lld, etc.) Ifcompiling more than one project, separate the items with a semicolon. Shouldyou run into issues with the semicolon, try surrounding it with single quotes.* -DLLVM_ENABLE_RUNTIMESSet this equal to the runtimes you wish to compile (e.g. libcxx, libcxxabi, etc.)If compiling more than one runtime, separate the items with a semicolon. Shouldyou run into issues with the semicolon, try surrounding it with single quotes.* -DCLANG_ENABLE_STATIC_ANALYZERSet this option to OFF if you do not require the clang static analyzer. Thisshould improve your build time slightly.* -DLLVM_USE_SPLIT_DWARFConsider setting this to ON if you require a debug build, as this will easememory pressure on the linker. This will make linking much faster, as thebinaries will not contain any of the debug information; however, this willgenerate the debug information in the form of a DWARF object file (with theextension .dwo). This only applies to host platforms using ELF, such as Linux... _links:Links=====This document is just an **introduction** on how to use LLVM to do some simplethings... there are many more interesting and complicated things that you can dothat aren't documented here (but we'll gladly accept a patch if you want towrite something up!). For more information about LLVM, check out:* `LLVM Homepage <https://llvm.org/>`_* `LLVM Doxygen Tree <https://llvm.org/doxygen/>`_* `Starting a Project that Uses LLVM <https://llvm.org/docs/Projects.html>`_
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