[docs] Update README and GettingStarted

[aeubanks]

Summary

Funnel fetching and building LLVM instructions into GettingStarted.

Modernize the build steps a little.

Remove comments saying CMAKE_BUILD_TYPE defaults to Debug as that's not true anymore (must explicitly pass it).

[MaskRay]

LGTM. Deduplicating README.md makes sense and removes one place to change when we update instructions.
Changes like cmake -S LG, too.

llvm/docs/GettingStarted.rst

30

Check out

[MaskRay]

llvm/docs/GettingStarted.rst

70

Keep double backquotes for Debug, Release, ...

95–105

... building just llvm/, not other subprojects

[MaskRay]

llvm/docs/GettingStarted.rst

607–611

LLVM_TARGETS_TO_BUILD=host is supported and seems preferred to include just the host architecture.

[aeubanks]

address comments
also remove comment saying CMAKE_BUILD_TYPE defaults to Debug as that's not true anymore

[aeubanks]

llvm/docs/GettingStarted.rst

607–611

TIL, thanks!

[MaskRay]

Best to wait for a second opinion.

llvm/docs/GettingStarted.rst

98

Are the two code blocks look good when you read them in a browser? ninja docs-llvm-html

[aeubanks]

llvm/docs/GettingStarted.rst

98

yup, I had checked that way

[MaskRay]

Seems fine to land if nobody else has an opinion in one or two days.

[hans]

Looks great, thanks for updating!

[aaron.ballman]

LGTM as-is, but I did have some comments you can feel free to take or leave.

llvm/docs/GettingStarted.rst

48

Should we link to CMake's generator documentation here?

75–76

Should we advertise LLVM_PARALLEL_{COMPILE,LINK}_JOBS and LLVM_USE_LINKER given how important those are to getting a working build on some platforms? (This can be done later if you don't want to do it as part of this patch.)

[aeubanks]

link to cmake generator docs

llvm/docs/GettingStarted.rst

75–76

will do that in a followup change

README.md

 # The LLVM Compiler Infrastructure
 
-This directory and its sub-directories contain the source code for LLVM,
-a toolkit for the construction of highly optimized compilers,
-optimizers, and run-time environments.
-
-The README briefly describes how to get started with building LLVM.
-For more information on how to contribute to the LLVM project, please
-take a look at the
-[Contributing to LLVM](https://llvm.org/docs/Contributing.html) guide.
-
-## Getting Started with the LLVM System
-
-Taken from [here](https://llvm.org/docs/GettingStarted.html).
-
-### Overview
-
 Welcome to the LLVM project!
 
+This repository contains the source code for LLVM, a toolkit for the
+construction of highly optimized compilers, optimizers, and run-time
+environments.
+
 The LLVM project has multiple components. The core of the project is
 itself called "LLVM". This contains all of the tools, libraries, and header
 files needed to process intermediate representations and convert them into
 object files. Tools include an assembler, disassembler, bitcode analyzer, and
-bitcode optimizer. It also contains basic regression tests.
+bitcode optimizer.
 
 C-like languages use the [Clang](http://clang.llvm.org/) frontend. This
 component 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
-
-The LLVM Getting Started documentation may be out of date. The [Clang
-Getting Started](http://clang.llvm.org/get_started.html) page might have more
-accurate information.
-
-This is an example work-flow and configuration to get and build the LLVM source:
-
-1. Checkout LLVM (including related sub-projects like Clang):
-
-     * ``git clone https://github.com/llvm/llvm-project.git``
-
-     * Or, on windows, ``git clone --config core.autocrlf=false
-    https://github.com/llvm/llvm-project.git``
-
-2. 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 and
-          solutions.
-        * ``Xcode`` --- for generating Xcode projects.
-
-        Some common options:
-
-        * ``-DLLVM_ENABLE_PROJECTS='...'`` and ``-DLLVM_ENABLE_RUNTIMES='...'`` ---
-          semicolon-separated list of the LLVM sub-projects and runtimes you'd like to
-          additionally build. ``LLVM_ENABLE_PROJECTS`` can include any of: clang,
-          clang-tools-extra, cross-project-tests, flang, libc, libclc, lld, lldb,
-          mlir, openmp, polly, or pstl. ``LLVM_ENABLE_RUNTIMES`` can include any of
-          libcxx, libcxxabi, libunwind, compiler-rt, libc or openmp. Some runtime
-          projects can be specified either in ``LLVM_ENABLE_PROJECTS`` or in
-          ``LLVM_ENABLE_RUNTIMES``.
-
-          For example, to build LLVM, Clang, libcxx, and libcxxabi, use
-          ``-DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"``.
-
-        * ``-DCMAKE_INSTALL_PREFIX=directory`` --- Specify for *directory* the full
-          path name of where you want the LLVM tools and libraries to be installed
-          (default ``/usr/local``). Be careful if you install runtime libraries: if
-          your system uses those provided by LLVM (like libc++ or libc++abi), you
-          must not overwrite your system's copy of those libraries, since that
-          could render your system unusable. In general, using something like
-          ``/usr`` is not advised, but ``/usr/local`` is fine.
-
-        * ``-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).
-
-      * ``cmake --build build [-- [options] <target>]`` or your build system specified above
-        directly.
-
-        * The default target (i.e. ``ninja`` or ``make``) will build all of LLVM.
-
-        * The ``check-all`` target (i.e. ``ninja check-all``) will run the
-          regression tests to ensure everything is in working order.
-
-        * CMake will generate targets for each tool and library, and most
-          LLVM sub-projects generate their own ``check-<project>`` target.
-
-        * Running a serial build will be **slow**. To improve speed, try running a
-          parallel build. That's done by default in Ninja; for ``make``, use the option
-          ``-j NNN``, where ``NNN`` is the number of parallel jobs to run.
-          In most cases, you get the best performance if you specify the number of CPU threads you have.
-          On some Unix systems, you can specify this with ``-j$(nproc)``.
-
-      * For more information see [CMake](https://llvm.org/docs/CMake.html).
+## Getting the Source Code and Building LLVM
 
 Consult the
-[Getting Started with LLVM](https://llvm.org/docs/GettingStarted.html#getting-started-with-llvm)
-page for detailed information on configuring and compiling LLVM. You can visit
-[Directory Layout](https://llvm.org/docs/GettingStarted.html#directory-layout)
-to learn about the layout of the source code tree.
+[Getting Started with LLVM](https://llvm.org/docs/GettingStarted.html#getting-the-source-code-and-building-llvm)
+page for information on building and running LLVM.
+
+For information on how to contribute to the LLVM project, please take a look at
+the [Contributing to LLVM](https://llvm.org/docs/Contributing.html) guide.
 
 ## Getting in touch
 
-Join [LLVM Discourse forums](https://discourse.llvm.org/), [discord chat](https://discord.gg/xS7Z362) or #llvm IRC channel on [OFTC](https://oftc.net/).
+Join the [LLVM Discourse forums](https://discourse.llvm.org/), [Discord
+chat](https://discord.gg/xS7Z362), or #llvm IRC channel on
+[OFTC](https://oftc.net/).
 
 The LLVM project has adopted a [code of conduct](https://llvm.org/docs/CodeOfConduct.html) for
 participants to all modes of communication within the project.

llvm/docs/CMake.rst

 or execute ``cmake --help-variable VARIABLE_NAME``.  See `Frequently
 Used LLVM-related Variables`_ below for information about commonly
 used variables that control features of LLVM and enabled subprojects.
 
 .. _cmake_build_type:
 
 **CMAKE_BUILD_TYPE**:STRING
   This configures the optimization level for ``make`` or ``ninja`` builds.
-  The default ``CMAKE_BUILD_TYPE`` is set to ``Debug`` but you should
-  carefully read the list below to figure out what configuration matches
-  your use case the best.
 
   Possible values:
 
   =========================== ============= ========== ========== ==========================
   Build Type                  Optimizations Debug Info Assertions Best suited for
   =========================== ============= ========== ========== ==========================
   **Release**                 For Speed     No         No         Users of LLVM and Clang
   **Debug**                   None          Yes        Yes        Developers of LLVM

llvm/docs/GettingStarted.rst

 
 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
 =========================================
 
-The LLVM Getting Started documentation may be out of date.  The `Clang
-Getting Started <https://clang.llvm.org/get_started.html>`_ page might have more
-accurate information.
-
-This is an example workflow and configuration to get and build the LLVM source:
-
-#. Checkout LLVM (including related subprojects like Clang):
+#. Check out LLVM (including subprojects like Clang):
 
    * ``git clone https://github.com/llvm/llvm-project.git``
-   * Or, on windows, ``git clone --config core.autocrlf=false
+   * Or, on windows:
+
+     ``git clone --config core.autocrlf=false
      https://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``
-   * ``mkdir build``
-   * ``cd build``
-   * ``cmake -G <generator> -DCMAKE_BUILD_TYPE=<type> [options] ../llvm``
+   * ``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 and
        solutions.
      * ``Xcode`` --- for generating Xcode projects.
 
      Some Common options:
 
      * ``-DLLVM_ENABLE_PROJECTS='...'`` --- semicolon-separated list of the LLVM
        subprojects you'd like to additionally build. Can include any of: clang,
-       clang-tools-extra, lldb, compiler-rt, lld, polly, or cross-project-tests.
+       clang-tools-extra, lldb, lld, polly, or cross-project-tests.
 
-       For example, to build LLVM, Clang, libcxx, and libcxxabi, use
-       ``-DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"``.
+       For example, to build LLVM, Clang, and LLD, use
+       ``-DLLVM_ENABLE_PROJECTS="clang;lld"``.
 
      * ``-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`` --- Controls optimization level and debug information
-       of the build. The default value is ``Debug`` which fits people who want
-       to work on LLVM or its libraries. ``Release`` is a better fit for most
-       users of LLVM and Clang. For more detailed information see
-       :ref:`CMAKE_BUILD_TYPE <cmake_build_type>`.
+     * ``-DCMAKE_BUILD_TYPE=type`` --- Controls optimization level and debug
+       information of the build. Valid options for *type* are ``Debug``,
+       ``Release``, ``RelWithDebInfo``, and ``MinSizeRel``. For more detailed
+       information see :ref:`CMAKE_BUILD_TYPE <cmake_build_type>`.
 
-     * ``-DLLVM_ENABLE_ASSERTIONS=On`` --- Compile with assertion checks enabled
-       (default is Yes for Debug builds, No for all other build types).
+     * ``-DLLVM_ENABLE_ASSERTIONS=ON`` --- Compile with assertion checks enabled
+       (default is ON for Debug builds, OFF for all other build types).
 
    * ``cmake --build . [--target <target>]`` or the build system specified
      above directly.
 
      * The default target (i.e. ``cmake --build .`` or ``make``) will build all of
        LLVM.
 
      * The ``check-all`` target (i.e. ``ninja check-all``) will run the
        regression tests to ensure everything is in working order.
 
      * 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**.  To improve speed, try running a
        parallel build. That's done by default in Ninja; for ``make``, use the
        option ``-j NN``, where ``NN`` is the number of parallel jobs, e.g. the
        number of available CPUs.
 
-   * For more information see `CMake <CMake.html>`__
+   * A basic CMake and build/test invocation which only builds LLVM and no other
+     subprojects:
+
+     ``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 and
+     run LLVM tests.
+
+   * For more detailed information on CMake options, see `CMake <CMake.html>`__
 
-   * If you get an "internal compiler error (ICE)" or test failures, see
-     `below`_.
+   * If you get build or test failures, see `below`_.
 
 Consult the `Getting Started with LLVM`_ section for detailed information on
 configuring and compiling LLVM.  Go to `Directory Layout`_ to learn about the
 layout of the source code tree.
 
 Stand-alone Builds
 ------------------
 

 +-------------------------+----------------------------------------------------+
 | 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``.                |
-|                         |                                                    |
+|                         | 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 |