Add beginning of LLVM's GettingStarted to GitHub readme

Reviewed and approved by chandlerc.

As GitHub is the canonical LLVM repository now, embrace GitHub's way of displaying basic build instructions in the top-level readme.md.

1 files changed, 99 insertions, 0 deletions

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 This directory and its subdirectories contain source code for LLVM,
 a toolkit for the construction of highly optimized compilers,
 optimizers, and runtime environments.
+
+## Getting Started with the LLVM System
+
+Taken from https://llvm.org/docs/GettingStarted.html.
+
+### Overview
+
+Welcome to the LLVM project!
+
+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 converts it into
+object files.  Tools include an assembler, disassembler, bitcode analyzer, and
+bitcode optimizer.  It also contains basic regression tests.
+
+C-like languages use the [Clang](http://clang.llvm.org/) front end.  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+The LLVM Getting Started documentation may be out of date.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 workflow and configuration to get and build the LLVM source:
+
+1. Checkout LLVM (including related subprojects 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``
+
+     * ``mkdir build``
+
+     * ``cd build``
+
+     * ``cmake -G <generator> [options] ../llvm``
+
+        Some common 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, libcxx, libcxxabi, libunwind, lldb, compiler-rt, lld,
+          polly, or debuginfo-tests.
+
+          For example, to build LLVM, Clang, libcxx, and libcxxabi, use
+          ``-DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi"``.
+
+        * ``-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. ``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 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
+          ``make -j NNN`` (NNN is the number of parallel jobs, use e.g. number of
+          CPUs you have.)
+
+      * For more information see [CMake](https://llvm.org/docs/CMake.html)
+
+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.