Update of LLVM Getting Started page

[• BenNathanson]

Summary

Working with Philip Reames to streamline, update, and improve the focus of the Getting Started guide. Some concerns:

• Material overlaps other pages (e.g. Clang getting started), making it difficult for users to find a single authoritative build process.
• Tell users what to expect from a trunk build -- should all tests be expected to pass? what's reasonable hardware?
• Include instructions on running the tests
• Include info on pre-built releases?
• Clarify/expand list of suggested checkouts (e.g., doesn't include lldb).
• For each brief list of suggested options, give a pointer to a complete list.
• Replace list of numbered releases (which has to be maintained) with a source-control command that lists them.

The patch should be rebased to include some very recent edits by Philip.

The patch is for discussion (isn't complete) and will need revisiting if release replaces debug as the default build.

[reames]

Resigning from a stale review (2016). Feel free to re-add if thread ever revived.

GettingStarted.rst

-====================================
-Getting Started with the LLVM System
-====================================
+================
+LLVM build guide
+================
+
+This is for users who want to work with the intermediate LLVM representation. If you're interested in using LLVM-based compilers, look into `Clang <http://clang.llvm.org/>`_  instead.
 
 .. contents::
    :local:
 
-Overview
-========
-
-Welcome to LLVM! In order to get started, you first need to know some basic
-information.
-
-First, LLVM comes in three pieces. The first piece is the LLVM suite. This
-contains all of the tools, libraries, and header files needed to use LLVM.  It
-contains an assembler, disassembler, bitcode analyzer and bitcode optimizer.  It
-also contains basic regression tests that can be used to test the LLVM tools and
-the Clang front end.
 
-The second piece is the `Clang <http://clang.llvm.org/>`_ front end.  This
-component compiles C, C++, Objective C, and Objective C++ code into LLVM
-bitcode. Once compiled into LLVM bitcode, a program can be manipulated with the
-LLVM tools from the LLVM suite.
+Quickstart
+==========
 
-There is a third, optional piece called Test Suite.  It is a suite of programs
-with a testing harness that can be used to further test LLVM's functionality
-and performance.
+Check that you have the hardware_ and software_  prerequisites. **Make sure to install into the directories shown.**
 
-Getting Started Quickly (A Summary)
-===================================
+If you do additional checkouts after running cmake, first...
 
-The LLVM Getting Started documentation may be out of date.  So, the `Clang
-Getting Started <http://clang.llvm.org/get_started.html>`_ page might also be a
-good place to start.
+If your build hits a snag, see the `build problems <http://llvm.org/docs/FAQ.html#build-problems>`_ section of the `FAQ <http://llvm.org/docs/FAQ.html>`_. If the FAQ doesn't help we want to know about it; please ...
 
-Here's the short story for getting up and running quickly with LLVM:
+These instructions check out the LLVM trunk. If you want specific versions instead, see versions_.
 
-#. Read the documentation.
-#. Read the documentation.
-#. Remember that you were warned twice about reading the documentation.
-
-   * In particular, the *relative paths specified are important*.
+`Git mirrors`_ are also available for a number of LLVM subprojects.
 
-#. Checkout LLVM:
+#. Check out LLVM:
 
    * ``cd where-you-want-llvm-to-live``
    * ``svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm``
 
-#. Checkout Clang:
+#. Check out Clang:
 
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/tools``
    * ``svn co http://llvm.org/svn/llvm-project/cfe/trunk clang``
 
-#. Checkout Compiler-RT (required to build the sanitizers) **[Optional]**:
+#. *[Optional]* Check out Compiler-RT (required to build the sanitizers):
 
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/projects``
    * ``svn co http://llvm.org/svn/llvm-project/compiler-rt/trunk compiler-rt``
 
-#. Checkout Libomp (required for OpenMP support) **[Optional]**:
+#. *[Optional]* Check out Libomp (required for OpenMP support):
 
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/projects``
    * ``svn co http://llvm.org/svn/llvm-project/openmp/trunk openmp``
 
-#. Checkout libcxx and libcxxabi **[Optional]**:
+#. *[Optional]* Check out libcxx and libcxxabi:
 
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/projects``
    * ``svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx``
    * ``svn co http://llvm.org/svn/llvm-project/libcxxabi/trunk libcxxabi``
 
-#. Get the Test Suite Source Code **[Optional]**
+#. *[Optional]* Check out the test suite source:
 
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/projects``
    * ``svn co http://llvm.org/svn/llvm-project/test-suite/trunk test-suite``
 
 #. Configure and build LLVM and Clang:
 
-   *Warning:* Make sure you've checked out *all of* the source code 
-   before trying to configure with cmake.  cmake does not pickup newly
-   added source directories in incremental builds. 
-
-   The build uses `CMake <CMake.html>`_.
-   Although the build is known to work with CMake >= 2.8.8, we recommend CMake
-   >= v3.2, especially if you're generating Ninja build files.
-
    * ``cd where you want to build llvm``
    * ``mkdir build``
    * ``cd build``
    * ``cmake -G <generator> [options] <path to llvm sources>``
 
-     Some common generators are:
 
-     * ``Unix Makefiles`` --- for generating make-compatible parallel makefiles.
-     * ``Ninja`` --- for generating `Ninja <http://martine.github.io/ninja/>`
-        build files. Most llvm developers use Ninja.
-     * ``Visual Studio`` --- for generating Visual Studio projects and
+     Common generators (
+     See ??? for the full list):
+
+     * ``"Unix Makefiles"`` --- (use the quotes) for make-compatible parallel makefiles.
+     * ``Ninja`` --- for `Ninja <http://martine.github.io/ninja/>`_         build files. Most llvm developers use Ninja.
+     * ``Visual Studio`` --- for Visual Studio projects and
         solutions.
-     * ``Xcode`` --- for generating Xcode projects.
+     * ``Xcode`` --- for Xcode projects.
 
-     Some Common options:
+
+     Common options (for more, see http://llvm.org/docs/CMake.html):
 
      * ``-DCMAKE_INSTALL_PREFIX=directory`` --- Specify for *directory* the full
        pathname of where you want the LLVM tools and libraries to be installed
        (default ``/usr/local``).
 
      * ``-DCMAKE_BUILD_TYPE=type`` --- Valid options for *type* are Debug,
        Release, RelWithDebInfo, and MinSizeRel. Default is Debug.
 
-     * ``-DLLVM_ENABLE_ASSERTIONS=On`` --- Compile with assertion checks enabled
-       (default is Yes for Debug builds, No for all other build types).
-
-   * Run your build tool of choice!
-
-     * The default target (i.e. ``make``) will build all of LLVM
+     * ``-DLLVM_ENABLE_ASSERTIONS=On`` --- Compile with assertion checks enabled (default is Yes for Debug builds, No for all other build types).
 
-     * The ``check-all`` target (i.e. ``make check-all``) will run the
-       regression tests to ensure everything is in working order.
+     * ``-DLLVM_TARGETS_TO_BUILD=target,target`` --- By default LLVM will be built for all targets. Specifying fewer targets will make the build complete faster and take less space.
 
-     * CMake will generate build targets for each tool and library, and most
-       LLVM sub-projects generate their own ``check-<project>`` target.
 
-     * Running a serial build will be *slow*.  Make sure you run a 
-       parallel build; for ``make``, use ``make -j``.  
+   * Run the build tool you specified
 
-   * For more information see `CMake <CMake.html>`_
+     * The default build target (e.g. ``make``, ``ninja``) builds all of LLVM.
 
-   * If you get an "internal compiler error (ICE)" or test failures, see
-     `below`_.
+     Builds, especially for debug, can take more than an hour. Run a parallel build if possible (e.g., the -jN  in make or ninja) to save time.
 
-Consult the `Getting Started with LLVM`_ section for detailed information on
-configuring and compiling LLVM.  Go to `Program Layout`_ to learn about the 
-layout of the source code tree.
+     * The ``check-all`` target (e.g. ``make check-all``) runs the
+       regression tests to ensure everything is in working order.
 
 Requirements
 ============
 
-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 and
-software you will need.
+.. _hardware:
 
 Hardware
 --------
 
 LLVM is known to work on the following host platforms:
 
 ================== ===================== =============
 OS                 Arch                  Compilers

 Windows 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 LLVM
-     with ``-DBUILD_SHARED_LIBS=On``.
+     with ``-DBUILD_SHARED_LIBS=On`` for CMake builds or ``--enable-shared``
+     for configure builds.
   #. MCJIT not working well pre-v7, old JIT engine not supported any more.
 
-Note that Debug builds require a lot of time and disk space.  An LLVM-only build
-will need about 1-3 GB of space.  A full build of LLVM and Clang will need around
-15-20 GB of disk space.  The exact space requirements will vary by system.  (It
-is so large because of all the debugging information and the fact that the 
-libraries are statically linked into multiple tools).  
-
-If you you are space-constrained, you can build only selected tools or only 
-selected targets.  The Release build requires considerably less space.
-
-The LLVM suite *may* compile on other platforms, but it is not guaranteed to do
-so.  If compilation is successful, the LLVM utilities should be able to
-assemble, disassemble, analyze, and optimize LLVM bitcode.  Code generation
-should work as well, although the generated native code may not work on your
-platform.
+A full debug build takes 1-3 GB. If you do not need many of the tools you can save space by passing
+ ``ONLY_TOOLS="tools you need"`` to make.  The Release build requires considerably less space.
+
+.. _software:
 
 Software
 --------
 
-Compiling LLVM requires that you have several software packages installed. The
-table below lists those required packages. The Package column is the usual name
-for the software package that LLVM depends on. The Version column provides
-"known to work" versions of the package. The Notes column describes how LLVM
-uses the package and provides other details.
+Required packages:
 
 =========================================================== ============ ==========================================
 Package                                                     Version      Notes
 =========================================================== ============ ==========================================
+`cmake <http://llvm.org/docs/CMake.html>`_                  >=3.2        :sup:`1`
 `GNU Make <http://savannah.gnu.org/projects/make>`_         3.79, 3.79.1 Makefile/build processor
-`GCC <http://gcc.gnu.org/>`_                                >=4.7.0      C/C++ compiler\ :sup:`1`
-`python <http://www.python.org/>`_                          >=2.7        Automated test suite\ :sup:`2`
-`GNU M4 <http://savannah.gnu.org/projects/m4>`_             1.4          Macro processor for configuration\ :sup:`3`
-`GNU Autoconf <http://www.gnu.org/software/autoconf/>`_     2.60         Configuration script builder\ :sup:`3`
-`GNU Automake <http://www.gnu.org/software/automake/>`_     1.9.6        aclocal macro generator\ :sup:`3`
-`libtool <http://savannah.gnu.org/projects/libtool>`_       1.5.22       Shared library manager\ :sup:`3`
-`zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`4`
+`GCC <http://gcc.gnu.org/>`_                                >=4.7.0      C/C++ compiler\ :sup:`2`
+`python <http://www.python.org/>`_                          >=2.7        Automated test suite\ :sup:`3`
+`GNU M4 <http://savannah.gnu.org/projects/m4>`_             1.4          Macro processor for configuration\ :sup:`4`
+`GNU Autoconf <http://www.gnu.org/software/autoconf/>`_     2.60         Configuration script builder\ :sup:`4`
+`GNU Automake <http://www.gnu.org/software/automake/>`_     1.9.6        aclocal macro generator\ :sup:`4`
+`libtool <http://savannah.gnu.org/projects/libtool>`_       1.5.22       Shared library manager\ :sup:`4`
+`zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`5`
 =========================================================== ============ ==========================================
 
 .. note::
+   #. CMake >= v3.2 is recommended, especially if you're generating Ninja build files, though the build is known to work with CMake >= 2.8.8.
 
    #. Only the C and C++ languages are needed so there's no need to build the
       other languages for LLVM's purposes. See `below` for specific version
       info.
-   #. Only needed if you want to run the automated test suite in the
+   #. Only needed to run the automated test suite in the
       ``llvm/test`` directory.
-   #. If you want to make changes to the configure scripts, you will need GNU
+   #. To make changes to the configure scripts, you will need GNU
       autoconf (2.60), and consequently, GNU M4 (version 1.4 or higher). You
       will also need automake (1.9.6). We only use aclocal from that package.
    #. Optional, adds compression / uncompression capabilities to selected LLVM
       tools.
 
-Additionally, your compilation host is expected to have the usual plethora of
-Unix utilities. Specifically:
+Additionally, your compilation host is expected to have the usual
+Unix 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

 * **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
-------------------------------------------------------
+Known bad components
+^^^^^^^^^^^^^^^^^^^^
 
-LLVM is very demanding of the host C++ compiler, and as such tends to expose
-bugs in the compiler. We are also planning to follow improvements and
-developments in the C++ language and library reasonably closely. As such, we
-require a modern host C++ toolchain, both compiler and standard library, in
-order to build LLVM.
-
-For the most popular host toolchains we check for specific minimum versions in
-our build systems:
-
-* Clang 3.1
-* GCC 4.7
-* Visual Studio 2013
-
-Anything older than these toolchains *may* work, but will require forcing the
-build system with a special option and is not really a supported host platform.
-Also note that older versions of these compilers have often crashed or
-miscompiled LLVM.
-
-For less widely used host toolchains such as ICC or xlC, be aware that a very
-recent 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 as
-part of the host toolchain. These even include linkers at times.
+Certain software is *known* to fail in the host toolchain.
 
 **GCC 4.6.3 on ARM**: Miscompiles ``llvm-readobj`` at ``-O3``. A test failure
 in ``test/Object/readobj-shared-object.test`` is one symptom of the problem.
 
 **GNU ld 2.16.X**. Some 2.16.X versions of the ld linker will produce very long
 warning messages complaining that some "``.gnu.linkonce.t.*``" symbol was
 defined in a discarded section. You can safely ignore these messages as they are
 erroneous and the linkage is correct.  These messages disappear using ld 2.17.

 **Clang 3.0 on Mageia 2**.  There's a packaging issue: Clang can not find at
 least some (``cxxabi.h``) libstdc++ headers.
 
 **Clang in C++11 mode and libstdc++ 4.7.2**.  This version of libstdc++
 contained `a bug <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53841>`__ which
 causes Clang to refuse to compile condition_variable header file.  At the time
 of writing, this breaks LLD build.
 
-Getting a Modern Host C++ Toolchain
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-This section mostly applies to Linux and older BSDs. On Mac OS X, you should
+.. _below:
+.. _check here:
+
+Host C++ Toolchain
+^^^^^^^^^^^^^^^^^^
+
+LLVM is demanding of the host C++ compiler, which tends to expose bugs, and our project plans to incorporate new C++ features as they are introduced. Thus LLVM builds require a modern host C++ toolchain.
+
+If you are using one of these toolchains, the build system checks for specific minimum versions:
+
+* Clang 3.1
+* GCC 4.7
+* Visual Studio 2013
+
+The system can be forced to work with older verions, but these
+often crash or yield a miscompiled LLVM.
+
+On Mac OS X, you should
 have a sufficiently modern Xcode, or you will likely need to upgrade until you
-do. On Windows, just use Visual Studio 2013 as the host compiler, it is
-explicitly supported and widely available. FreeBSD 10.0 and newer have a modern
+do.
+
+On Windows, just use Visual Studio 2013 as the host compiler, it is
+explicitly supported and widely available.
+
+FreeBSD 10.0 and newer have a modern
 Clang as the system compiler.
 
-However, some Linux distributions and some other or older BSDs sometimes have
-extremely old versions of GCC. These steps attempt to help you upgrade you
-compiler even on such a system. However, if at all possible, we encourage you
-to use a recent version of a distribution with a modern system compiler that
-meets these requirements. Note that it is tempting to to install a prior
-version of Clang and libc++ to be the host compiler, however libc++ was not
-well tested or set up to build on Linux until relatively recently. As
-a consequence, this guide suggests just using libstdc++ and a modern GCC as the
+
+For less widely used host toolchains such as ICC or xlC, a very
+recent version may be required to support all of the C++ features used in LLVM.
+
+Upgrading an old GCC
+^^^^^^^^^^^^^^^^^^^^
+
+This explains how to upgrade a deeply backlevel GCC on an older
+Linux or BSD distribution. If possible, though, it's better to move to a recent
+distribution.
+
+One workaround for an older GCC might be to install an older
+version of Clang and libc++.  libc++ was not
+reliable on Linux till recently, though, so
+this guide suggests using libstdc++ and a modern GCC as the
 initial host in a bootstrap, and then using Clang (and potentially libc++).
 
-The first step is to get a recent GCC toolchain installed. The most common
-distribution on which users have struggled with the version requirements is
+The first step is to install a recent GCC toolchain. The most common
+distribution on which users have struggled with version requirements is
 Ubuntu Precise, 12.04 LTS. For this distribution, one easy option is to install
 the `toolchain testing PPA`_ and use it to install a modern GCC. There is
-a really nice discussions of this on the `ask ubuntu stack exchange`_. However,
+a really nice discussion of this on the `ask ubuntu stack exchange`_. However,
 not all users can use PPAs and there are many other distributions, so it may be
 necessary (or just useful, if you're here you *are* doing compiler development
-after all) to build and install GCC from source. It is also quite easy to do
+after all) to build and install GCC from source. It is also quite easy
 these days.
 
 .. _toolchain testing PPA:
   https://launchpad.net/~ubuntu-toolchain-r/+archive/test
 .. _ask ubuntu stack exchange:
   http://askubuntu.com/questions/271388/how-to-install-gcc-4-8-in-ubuntu-12-04-from-the-terminal
 
 Easy steps for installing GCC 4.8.2:

 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. You
 can also add an explicit prefix for Clang to look in for a GCC toolchain with
 the ``--gcc-toolchain=/opt/my/gcc/prefix`` flag, passing it to both compile and
 link 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 to
-give you some basic information about the LLVM environment.
-
-The later sections of this guide describe the `general layout`_ of the LLVM
-source tree, a `simple example`_ using the LLVM tool chain, and `links`_ to find
-more information about LLVM or to get help via e-mail.
-
-Terminology and Notation
-------------------------
+More on building
+================
 
-Throughout this manual, the following names are used to denote paths specific to
-the local system and working environment.  *These are not environment variables
-you need to set but just strings used in the rest of this document below*.  In
-any of the examples below, simply replace each of these names with the
-appropriate 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 where
-  object files and compiled programs will be placed.  It can be the same as
-  SRC_ROOT).
-
-Unpacking the LLVM Archives
----------------------------
-
-If you have the LLVM distribution, you will need to unpack it before you can
-begin to compile it.  LLVM is distributed as a set of two files: the LLVM suite
-and the LLVM GCC front end compiled for your platform.  There is an additional
-test suite that is optional.  Each file is a TAR archive that is compressed with
-the gzip program.
-
-The files are as follows, with *x.y* marking the version number:
+Bitcode execution
+-----------------
 
-``llvm-x.y.tar.gz``
+If you're on a Linux system that supports the `binfmt_misc
+<http://en.wikipedia.org/wiki/binfmt_misc>`_
+module and  have root access, you can set your system up to
+execute LLVM bitcode files directly. The
+first command may not be required if you are already using the module.
 
-  Source release for the LLVM libraries and tools.
+.. code-block:: console
 
-``llvm-test-x.y.tar.gz``
+  % 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.bc
 
-  Source release for the LLVM test-suite.
+On Debian, you can use this instead of the 'echo' command:
 
-.. _checkout:
+.. code-block:: console
 
-Checkout LLVM from Subversion
------------------------------
+  % sudo update-binfmts --install llvm /path/to/lli --magic 'BC'
 
-If you have access to our Subversion repository, you can get a fresh copy of the
-entire source code.  All you need to do is check it out from Subversion as
-follows:
+Getting specific releases
+-------------------------
 
-* ``cd where-you-want-llvm-to-live``
-* Read-Only: ``svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm``
-* Read-Write: ``svn co https://[e-mail]/svn/llvm-project/llvm/trunk llvm``
+To check out a specific release, specify the ``tags`` directory in the URL instead of ``trunk``, and append one of these subdirectories -- for example
 
-This will create an '``llvm``' directory in the current directory and fully
-populate it with the LLVM source code, Makefiles, test directories, and local
-copies of documentation files.
-
-If you want to get a specific release (as opposed to the most recent revision),
-you can checkout it from the '``tags``' directory (instead of '``trunk``'). The
-following releases are located in the following subdirectories of the '``tags``'
-directory:
+``svn co http://llvm.org/svn/llvm-project/llvm/tags/RELEASE_34/final llvm``
 
 * Release 3.4: **RELEASE_34/final**
 * Release 3.3: **RELEASE_33/final**
 * Release 3.2: **RELEASE_32/final**
 * Release 3.1: **RELEASE_31/final**
 * Release 3.0: **RELEASE_30/final**
 * Release 2.9: **RELEASE_29/final**
 * Release 2.8: **RELEASE_28**

 * Release 1.6: **RELEASE_16**
 * Release 1.5: **RELEASE_15**
 * Release 1.4: **RELEASE_14**
 * Release 1.3: **RELEASE_13**
 * Release 1.2: **RELEASE_12**
 * Release 1.1: **RELEASE_11**
 * Release 1.0: **RELEASE_1**
 
-If you would like to get the LLVM test suite (a separate package as of 1.4), you
-get it from the Subversion repository:
-
-.. code-block:: console
-
-  % cd llvm/projects
-  % svn co http://llvm.org/svn/llvm-project/test-suite/trunk test-suite
+.. _Git Mirrors:
 
-By placing it in the ``llvm/projects``, it will be automatically configured by
-the LLVM configure script as well as automatically updated when you run ``svn
-update``.
-
-Git Mirror
-----------
+Git Mirrors
+-----------
 
 Git mirrors are available for a number of LLVM subprojects. These mirrors sync
 automatically with each Subversion commit and contain all necessary git-svn
 marks (so, you can recreate git-svn metadata locally). Note that right now
 mirrors reflect only ``trunk`` for each project. You can do the read-only Git
 clone of LLVM via:
 
 .. code-block:: console

 ``dcommit``. When that happens, ``git svn dcommit`` stops working, complaining
 about files with uncommitted changes. The fix is to rebuild the metadata:
 
 .. code-block:: console
 
   % rm -rf .git/svn
   % git svn rebase -l
 
-Please, refer to the Git-SVN manual (``man git-svn``) for more information.
-
-Local LLVM Configuration
-------------------------
-
-Once checked out from the Subversion repository, the LLVM suite source code must
-be configured before being built. This process uses CMake.
-Unlinke the normal ``configure`` script, CMake
-generates the build files in whatever format you request as well as various
-``*.inc`` files, and ``llvm/include/Config/config.h``.
-
-Variables are passed to ``cmake`` on the command line using the format
-``-D<variable name>=<value>``. The following variables are some common options
-used 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.     |
-+-------------------------+----------------------------------------------------+
-| LLVM_TARGETS_TO_BUILD   | A semicolon delimited list controlling which       |
-|                         | targets will be built and linked into llc. This is |
-|                         | equivalent to the ``--enable-targets`` option in   |
-|                         | the configure script. 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, BPF, CppBackend, Hexagon,  |
-|                         | Mips, MSP430, NVPTX, PowerPC, Sparc, SystemZ       |
-|                         | X86, XCore``.                                      |
-+-------------------------+----------------------------------------------------+
-| 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_SPHINX      | Build sphinx-based documentation from the source   |
-|                         | code. This is disabled by default because it is    |
-|                         | slow and generates a lot of output.                |
-+-------------------------+----------------------------------------------------+
-| 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``.               |
-+-------------------------+----------------------------------------------------+
-| 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_INSTALL_PREFIX=prefix=/install/path
-       [other options] SRC_ROOT
-
-Compiling 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 following
-invocation:
-
-   .. code-block:: console
-
-     % cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=type SRC_ROOT
-
-Between runs, CMake preserves the values set for all options. CMake has the
-following build types defined:
-
-Debug
-
-  These builds are the default. The build system will compile the tools and
-  libraries unoptimized, with debugging information, and asserts enabled.
-
-Release
-
-  For these builds, the build system will compile the tools and libraries
-  with optimizations enabled and not generate debug info. CMakes default
-  optimization level is -O3. This can be configured by setting the
-  ``CMAKE_CXX_FLAGS_RELEASE`` variable on the CMake command line.
-
-RelWithDebInfo
-
-  These builds are useful when debugging. They generate optimized binaries with
-  debug information. CMakes default optimization level is -O2. This can be
-  configured by setting the ``CMAKE_CXX_FLAGS_RELWITHDEBINFO`` variable on the
-  CMake 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
-
-  % make
-
-If the build fails, please `check here`_ to see if you are using a version of
-GCC that is known not to compile LLVM.
-
-If you have multiple processors in your machine, you may wish to use some of the
-parallel build options provided by GNU Make.  For example, you could use the
-command:
-
-.. code-block:: console
-
-  % make -j2
-
-There are several special targets which are useful when working with the LLVM
-source 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 hierarchy
-  under ``$PREFIX``, specified with ``CMAKE_INSTALL_PREFIX``, which
-  defaults to ``/usr/local``.
-
-``make docs-llvm-html``
-
-  If configured with ``-DLLVM_ENABLE_SPHINX=On``, this will generate a directory
-  at ``OBJ_ROOT/docs/html`` which contains the HTML formatted documentation.
+See the Git-SVN manpage (``man git-svn``) for more information.
 
 Cross-Compiling LLVM
 --------------------
 
 It is possible to cross-compile LLVM itself. That is, you can create LLVM
 executables and libraries to be hosted on a platform different from the platform
-where they are built (a Canadian Cross build). To generate build files for
-cross-compiling CMake provides a variable ``CMAKE_TOOLCHAIN_FILE`` which can
-define compiler flags and variables used during the CMake test operations.
-
-The result of such a build is executables that are not runnable on on the build
-host but can be executed on the target. As an example the following CMake
-invocation can generate build files targeting iOS. This will work on Mac OS X
-with 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>
+where they are built (a Canadian Cross build). See  :doc:`HowToCrossCompileLLVM` and `Clang docs on how to cross-compile in general
+<http://clang.llvm.org/docs/CrossCompilation.html>`_.
 
-Note: There are some additional flags that need to be passed when building for
-iOS due to limitations in the iOS SDK.
-
-Check :doc:`HowToCrossCompileLLVM` and `Clang docs on how to cross-compile in general
-<http://clang.llvm.org/docs/CrossCompilation.html>`_ for more information
-about cross-compiling.
 
 The Location of LLVM Object Files
 ---------------------------------
 
 The LLVM build system is capable of sharing a single LLVM source tree among
 several LLVM builds.  Hence, it is possible to build LLVM for several different
 platforms or configurations using the same source tree.
 
 This is accomplished in the typical autoconf manner:
 
 * 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" SRC_ROOT
 
+Here the names SRC_ROOT and OBJ_ROOT *are not environment variables
+you need to set*; they stand for absolute paths on your filesystem:
+
+``SRC_ROOT``:  Top-level directory of the LLVM source tree.
+
+``OBJ_ROOT``:  Top-level directory of the LLVM object tree (i.e. the tree where
+  object files and compiled programs will be placed.  It can be the same as
+  SRC_ROOT).
+
 The LLVM build will create a structure underneath *OBJ_ROOT* that matches the
 LLVM source tree. At each level where source files are present in the source
 tree 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.o
 
-Optional 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 to
-execute LLVM bitcode files directly. To do this, use commands like this (the
-first 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.bc
+.. _versions:
 
-This allows you to execute LLVM bitcode files directly.  On Debian, you can also
-use 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:
 
-Program Layout
-==============
+Source tree
+===========
 
 One useful source of information about the LLVM source base is the LLVM `doxygen
 <http://www.doxygen.org/>`_ documentation available at
 `<http://llvm.org/doxygen/>`_.  The following is a brief introduction to code
 layout:
 
 ``llvm/examples``
 -----------------
 
-This directory contains some simple examples of how to use the LLVM IR and JIT.
+Simple examples using the LLVM IR and JIT.
 
 ``llvm/include``
 ----------------
 
-This directory contains public header files exported from the LLVM library. The
-three main subdirectories of this directory are:
+Public header files exported from the LLVM library. The
+three main subdirectories:
 
 ``llvm/include/llvm``
 
-  This directory contains all of the LLVM specific header files.  This directory
-  also has subdirectories for different portions of LLVM: ``Analysis``,
+  All LLVM-specific header files, and  subdirectories for different portions of LLVM: ``Analysis``,
   ``CodeGen``, ``Target``, ``Transforms``, etc...
 
 ``llvm/include/llvm/Support``
 
-  This directory contains generic support libraries that are provided with LLVM
+  Generic support libraries provided with LLVM
   but not necessarily specific to LLVM. For example, some C++ STL utilities and
-  a Command Line option processing library store their header files here.
+  a Command Line option processing library store header files here.
 
 ``llvm/include/llvm/Config``
 
-  This directory contains header files configured by the ``configure`` script.
+  Header files configured by the ``configure`` script.
   They wrap "standard" UNIX and C header files.  Source code can include these
   header files which automatically take care of the conditional #includes that
   the ``configure`` script generates.
 
 ``llvm/lib``
 ------------
 
-This directory contains most of the source files of the LLVM system. In LLVM,
-almost all code exists in libraries, making it very easy to share code among the
-different `tools`_.
+Most source files are here. By putting   code  in libraries, LLVM makes it  easy to share code among the
+ `tools`_.
 
 ``llvm/lib/IR/``
 
-  This directory holds the core LLVM source files that implement core classes
+  Core LLVM source files that implement core classes
   like Instruction and BasicBlock.
 
 ``llvm/lib/AsmParser/``
 
-  This directory holds the source code for the LLVM assembly language parser
+  Source code for the LLVM assembly language parser
   library.
 
 ``llvm/lib/Bitcode/``
 
-  This directory holds code for reading and write LLVM bitcode.
+  Code for reading and writing bitcode.
 
 ``llvm/lib/Analysis/``
 
-  This directory contains a variety of different program analyses, such as
+  A variety of program analyses, such as
   Dominator Information, Call Graphs, Induction Variables, Interval
   Identification, Natural Loop Identification, etc.
 
 ``llvm/lib/Transforms/``
 
-  This directory contains the source code for the LLVM to LLVM program
+  LLVM-to-LLVM 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/``
 
-  This directory contains files that describe various target architectures for
-  code generation.  For example, the ``llvm/lib/Target/X86`` directory holds the
-  X86 machine description while ``llvm/lib/Target/ARM`` implements the ARM
-  backend.
+  Files describing  target architectures for
+  code generation.  For example,  ``llvm/lib/Target/X86`` holds the
+  X86 machine description.
 
 ``llvm/lib/CodeGen/``
 
-  This directory contains the major parts of the code generator: Instruction
+  The major parts of the code generator: Instruction
   Selector, Instruction Scheduling, and Register Allocation.
 
 ``llvm/lib/MC/``
 
-  (FIXME: T.B.D.)
+  (FIXME: T.B.D.)  ....?
 
 ``llvm/lib/Debugger/``
 
-  This directory contains the source level debugger library that makes it
-  possible to instrument LLVM programs so that a debugger could identify source
-  code locations at which the program is executing.
+  Source-level debugger.
 
 ``llvm/lib/ExecutionEngine/``
 
-  This directory contains libraries for executing LLVM bitcode directly at
-  runtime in both interpreted and JIT compiled fashions.
+  Libraries for directly executing  bitcode  at
+  runtime in interpreted and JIT-compiled scenarios.
 
 ``llvm/lib/Support/``
 
-  This directory contains the source code that corresponds to the header files
-  located in ``llvm/include/ADT/`` and ``llvm/include/Support/``.
+  Source code that corresponding to the header files
+  in ``llvm/include/ADT/`` and ``llvm/include/Support/``.
 
 ``llvm/projects``
 -----------------
 
-This directory contains projects that are not strictly part of LLVM but are
-shipped with LLVM. This is also the directory where you should create your own
+Projects not strictly part of LLVM but
+shipped with LLVM. This is also the directory for creating your own
 LLVM-based projects.
 
 ``llvm/runtime``
 ----------------
 
-This directory contains libraries which are compiled into LLVM bitcode and used
-when linking programs with the Clang front end.  Most of these libraries are
+Libraries compiled into  bitcode and used
+when linking programs with the Clang front end.  Most of these  are
 skeleton versions of real libraries; for example, libc is a stripped down
 version of glibc.
 
 Unlike the rest of the LLVM suite, this directory needs the LLVM GCC front end
 to compile.
 
 ``llvm/test``
 -------------
 
-This directory contains feature and regression tests and other basic sanity
-checks on the LLVM infrastructure. These are intended to run quickly and cover a
+Feature and regression tests and other  sanity
+checks on LLVM infrastructure. These are intended to run quickly and cover a
 lot of territory without being exhaustive.
 
 ``test-suite``
 --------------
 
-This is not a directory in the normal llvm module; it is a separate Subversion
-module that must be checked out (usually to ``projects/test-suite``).  This
-module contains a comprehensive correctness, performance, and benchmarking test
-suite for LLVM. It is a separate Subversion module because not every LLVM user
-is interested in downloading or building such a comprehensive test suite. For
-further details on this test suite, please see the :doc:`Testing Guide
+A comprehensive correctness, performance, and benchmarking test
+suite for LLVM. Comes in a separate Subversion module because not every LLVM user
+is interested in such a comprehensive suite. For
+details see the :doc:`Testing Guide
 <TestingGuide>` document.
 
 .. _tools:
 
 ``llvm/tools``
 --------------
 
-The **tools** directory contains the executables built out of the libraries
+Executables built out of the libraries
 above, which form the main part of the user interface.  You can always get help
 for a tool by typing ``tool_name -help``.  The following is a brief introduction
 to the most important tools.  More detailed information is in
 the `Command Guide <CommandGuide/index.html>`_.
 
 ``bugpoint``
 
   ``bugpoint`` is used to debug optimization passes or code generation backends

 ``llc``
 
   ``llc`` is the LLVM backend compiler, which translates LLVM bitcode to a
   native code assembly file or to C code (with the ``-march=c`` option).
 
 ``opt``
 
   ``opt`` reads LLVM bitcode, applies a series of LLVM to LLVM transformations
-  (which are specified on the command line), and then outputs the resultant
-  bitcode.  The '``opt -help``' command is a good way to get a list of the
+  (which are specified on the command line), and outputs the resultant
+  bitcode.   '``opt -help``'  is a good way to get a list of the
   program transformations available in LLVM.
 
-  ``opt`` can also be used to run a specific analysis on an input LLVM bitcode
-  file and print out the results.  It is primarily useful for debugging
+  ``opt`` can also  run a specific analysis on an input LLVM bitcode
+  file and print  the results.  Primarily useful for debugging
   analyses, or familiarizing yourself with what an analysis does.
 
 ``llvm/utils``
 --------------
 
-This directory contains utilities for working with LLVM source code, and some of
-the utilities are actually required as part of the build process because they
-are code generators for parts of LLVM infrastructure.
+Utilities for working with LLVM source code; some are part of the build process because they
+are code generators for parts of the infrastructure.
 
 
 ``codegen-diff``
 
-  ``codegen-diff`` is a script that finds differences between code that LLC
-  generates and code that LLI generates. This is a useful tool if you are
+  ``codegen-diff`` finds differences between code that LLC
+  generates and code that LLI generates. This is useful if you are
   debugging one of them, assuming that the other generates correct output. For
   the full user manual, run ```perldoc codegen-diff'``.
 
 ``emacs/``
 
-  The ``emacs`` directory contains syntax-highlighting files which will work
-  with Emacs and XEmacs editors, providing syntax highlighting support for LLVM
-  assembly files and TableGen description files. For information on how to use
-  the syntax files, consult the ``README`` file in that directory.
+   Emacs and XEmacs syntax highlighting  for LLVM   assembly files and TableGen description files.  See the ``README`` for information on using them.
 
 ``getsrcs.sh``
 
-  The ``getsrcs.sh`` script finds and outputs all non-generated source files,
-  which is useful if one wishes to do a lot of development across directories
-  and does not want to individually find each file. One way to use it is to run,
-  for example: ``xemacs `utils/getsources.sh``` from the top of your LLVM source
+  Finds and outputs all non-generated source files,
+  useful if one wishes to do a lot of development across directories
+  and 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 source
   tree.
 
 ``llvmgrep``
 
-  This little tool performs an ``egrep -H -n`` on each source file in LLVM and
+  Performs an ``egrep -H -n`` on each source file in LLVM and
   passes to it a regular expression provided on ``llvmgrep``'s command
-  line. This is a very efficient way of searching the source base for a
+  line. This is an efficient way of searching the source base for a
   particular regular expression.
 
 ``makellvm``
 
-  The ``makellvm`` script compiles all files in the current directory and then
+  Compiles all files in the current directory, then
   compiles and links the tool that is the first argument. For example, assuming
-  you are in the directory ``llvm/lib/Target/Sparc``, if ``makellvm`` is in your
-  path, simply running ``makellvm llc`` will make a build of the current
+  you are in  ``llvm/lib/Target/Sparc``, if ``makellvm`` is in your
+  path,  running ``makellvm llc`` will make a build of the current
   directory, switch to directory ``llvm/tools/llc`` and build it, causing a
   re-linking of LLC.
 
 ``TableGen/``
 
-  The ``TableGen`` directory contains the tool used to generate register
+  Contains the tool used to generate register
   descriptions, instruction set descriptions, and even assemblers from common
   TableGen description files.
 
 ``vim/``
 
-  The ``vim`` directory contains syntax-highlighting files which will work with
-  the VIM editor, providing syntax highlighting support for LLVM assembly files
-  and TableGen description files. For information on how to use the syntax
-  files, consult the ``README`` file in that directory.
+  vim syntax-highlighting for LLVM assembly files
+  and 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 arguments
-     work 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.bc
-
-   The -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 use
-   the `standard LLVM tools <CommandGuide/index.html>`_ on the bitcode file.
-
-#. Run the program in both forms. To run the program, use:
-
-   .. code-block:: console
-
-      % ./hello
-
-   and
-
-   .. code-block:: console
-
-     % lli hello.bc
-
-   The 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.native
-
-   Note 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 other
-general questions about LLVM, please consult the `Frequently Asked
-Questions <FAQ.html>`_ page.
-
-.. _links:
-
-Links
-=====
-
-This document is just an **introduction** on how to use LLVM to do some simple
-things... there are many more interesting and complicated things that you can do
-that aren't documented here (but we'll gladly accept a patch if you want to
-write something up!).  For more information about LLVM, check out:
-
-* `LLVM Homepage <http://llvm.org/>`_
-* `LLVM Doxygen Tree <http://llvm.org/doxygen/>`_
-* `Starting a Project that Uses LLVM <http://llvm.org/docs/Projects.html>`_