[docs] Update README and GettingStarted

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).

Reviewed By: MaskRay, hans

Differential Revision: https://reviews.llvm.org/D145413

1 files changed, 14 insertions, 97 deletions

diff --git a/README.md b/README.md
index 1273ba17c2fa..eb8d624d75ce 100644
--- a/README.md
+++ b/README.md
@@ -1,27 +1,16 @@
 # 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
@@ -31,92 +20,20 @@ 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-The LLVM Getting Started documentation may be out of date.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.