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+.. _build_overview:
+
+=====================
+Build System Overview
+=====================
+
+This document provides an overview on how the build system works. It is
+targeted at people wanting to learn about internals of the build system.
+It is not meant for persons who casually interact with the build system.
+That being said, knowledge empowers, so consider reading on.
+
+The build system is composed of many different components working in
+harmony to build the source tree. We begin with a graphic overview.
+
+.. graphviz::
+
+   digraph build_components {
+      rankdir="LR";
+      "configure" -> "config.status" -> "build backend" -> "build output"
+   }
+
+Phase 1: Configuration
+======================
+
+Phase 1 centers around the ``configure`` script, which is a bash shell script.
+The file is generated from a file called ``configure.in`` which is written in M4
+and processed using Autoconf 2.13 to create the final configure script.
+You don't have to worry about how you obtain a ``configure`` file: the build
+system does this for you.
+
+The primary job of ``configure`` is to determine characteristics of the system
+and compiler, apply options passed into it, and validate everything looks OK to
+build. The primary output of the ``configure`` script is an executable file
+in the object directory called ``config.status``. ``configure`` also produces
+some additional files (like ``autoconf.mk``). However, the most important file
+in terms of architecture is ``config.status``.
+
+The existence of a ``config.status`` file may be familiar to those who have worked
+with Autoconf before. However, Mozilla's ``config.status`` is different from almost
+any other ``config.status`` you've ever seen: it's written in Python! Instead of
+having our ``configure`` script produce a shell script, we have it generating
+Python.
+
+Now is as good a time as any to mention that Python is prevalent in our build
+system. If we need to write code for the build system, we do it in Python.
+That's just how we roll. For more, see :ref:`python`.
+
+``config.status`` contains 2 parts: data structures representing the output of
+``configure`` and a command-line interface for preparing/configuring/generating
+an appropriate build backend. (A build backend is merely a tool used to build
+the tree - like GNU Make or Tup). These data structures essentially describe
+the current state of the system and what the existing build configuration looks
+like. For example, it defines which compiler to use, how to invoke it, which
+application features are enabled, etc. You are encouraged to open up
+``config.status`` to have a look for yourself!
+
+Once we have emitted a ``config.status`` file, we pass into the realm of
+phase 2.
+
+Phase 2: Build Backend Preparation and the Build Definition
+===========================================================
+
+Once ``configure`` has determined what the current build configuration is,
+we need to apply this to the source tree so we can actually build.
+
+What essentially happens is the automatically-produced ``config.status`` Python
+script is executed as soon as ``configure`` has generated it. ``config.status``
+is charged with the task of tell a tool how to build the tree. To do this,
+``config.status`` must first scan the build system definition.
+
+The build system definition consists of various ``moz.build`` files in the tree.
+There is roughly one ``moz.build`` file per directory or per set of related directories.
+Each ``moz.build`` files defines how its part of the build config works. For
+example it says *I want these C++ files compiled* or *look for additional
+information in these directories.* config.status starts with the ``moz.build``
+file from the root directory and then descends into referenced ``moz.build``
+files by following ``DIRS`` variables or similar.
+
+As the ``moz.build`` files are read, data structures describing the overall
+build system definition are emitted. These data structures are then fed into a
+build backend, which then performs actions, such as writing out files to
+be read by a build tool. e.g. a ``make`` backend will write a
+``Makefile``.
+
+When ``config.status`` runs, you'll see the following output::
+
+   Reticulating splines...
+   Finished reading 1096 moz.build files into 1276 descriptors in 2.40s
+   Backend executed in 2.39s
+   2188 total backend files. 0 created; 1 updated; 2187 unchanged
+   Total wall time: 5.03s; CPU time: 3.79s; Efficiency: 75%
+
+What this is saying is that a total of *1096* ``moz.build`` files were read.
+Altogether, *1276* data structures describing the build configuration were
+derived from them.  It took *2.40s* wall time to just read these files and
+produce the data structures.  The *1276* data structures were fed into the
+build backend which then determined it had to manage *2188* files derived
+from those data structures. Most of them already existed and didn't need
+changed. However, *1* was updated as a result of the new configuration.
+The whole process took *5.03s*. Although, only *3.79s* was in
+CPU time. That likely means we spent roughly *25%* of the time waiting on
+I/O.
+
+For more on how ``moz.build`` files work, see :ref:`mozbuild-files`.
+
+Phase 3: Invokation of the Build Backend
+========================================
+
+When most people think of the build system, they think of phase 3. This is
+where we take all the code in the tree and produce Firefox or whatever
+application you are creating. Phase 3 effectively takes whatever was
+generated by phase 2 and runs it. Since the dawn of Mozilla, this has been
+make consuming Makefiles. However, with the transition to moz.build files,
+you may soon see non-Make build backends, such as Tup or Visual Studio.
+
+When building the tree, most of the time is spent in phase 3. This is when
+header files are installed, C++ files are compiled, files are preprocessed, etc.