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+build-clang.py
+==============
+
+A script to build clang from source.
+
+```
+usage: build-clang.py [-h] -c CONFIG [--clean]
+
+optional arguments:
+  -h, --help            show this help message and exit
+  -c CONFIG, --config CONFIG
+                        Clang configuration file
+  --clean               Clean the build directory
+```
+
+Pre-requisites
+--------------
+* Working build toolchain.
+* git
+* CMake
+* Ninja
+* Python 2.7 and 3
+
+Please use the latest available CMake for your platform to avoid surprises.
+
+Config file format
+------------------
+
+build-clang.py accepts a JSON config format with the following fields:
+
+* stages: Use 1, 2, 3 or 4 to select different compiler stages.  The default is 2.
+* cc: Path to the bootsraping C Compiler.
+* cxx: Path to the bootsraping C++ Compiler.
+* as: Path to the assembler tool.
+* ar: Path to the library archiver tool.
+* ranlib: Path to the ranlib tool (optional).
+* libtool: Path to the libtool tool (optional).
+* ld: Path to the linker.
+* patches: Optional list of patches to apply.
+* build_type: The type of build to make.  Supported types: Release, Debug, RelWithDebInfo or MinSizeRel.
+* targets: The targets supported by the final stage LLVM/clang.
+* build_clang_tidy: Whether to build clang-tidy with the Mozilla checks imported.  The default is false.
+* osx_cross_compile: Whether to invoke CMake for OS X cross compile builds.
+* assertions: Whether to enable LLVM assertions.  The default is false.
+* pgo: Whether to build with PGO (requires stages == 4).  The default is false.
+
+The revisions are defined in taskcluster/ci/fetch/toolchains.yml. They are usually commit sha1s corresponding to upstream tags.
+
+Environment Variables
+---------------------
+
+The following environment variables are used for cross-compile builds targeting OS X on Linux.
+
+* CROSS_CCTOOLS_PATH: Path to the cctools directory where the cross compiler toolchain is located.
+* CROSS_SYSROOT: Path to the OS X SDK directory for cross compile builds.