clang/docs/DriverInternals.rst - llvm-project - Git at Google

 =========================
 Driver Design & Internals
 =========================

 .. contents::
    :local:

 Introduction
 ============

 This document describes the Clang driver. The purpose of this document
 is to describe both the motivation and design goals for the driver, as
 well as details of the internal implementation.

 Features and Goals
 ==================

 The Clang driver is intended to be a production quality compiler driver
 providing access to the Clang compiler and tools, with a command line
 interface which is compatible with the gcc driver.

 Although the driver is part of and driven by the Clang project, it is
 logically a separate tool which shares many of the same goals as Clang:

 .. contents:: Features
    :local:

 GCC Compatibility
 -----------------

 The number one goal of the driver is to ease the adoption of Clang by
 allowing users to drop Clang into a build system which was designed to
 call GCC. Although this makes the driver much more complicated than
 might otherwise be necessary, we decided that being very compatible with
 the gcc command line interface was worth it in order to allow users to
 quickly test clang on their projects.

 Flexible
 --------

 The driver was designed to be flexible and easily accommodate new uses
 as we grow the clang and LLVM infrastructure. As one example, the driver
 can easily support the introduction of tools which have an integrated
 assembler; something we hope to add to LLVM in the future.

 Similarly, most of the driver functionality is kept in a library which
 can be used to build other tools which want to implement or accept a gcc
 like interface.

 Low Overhead
 ------------

 The driver should have as little overhead as possible. In practice, we
 found that the gcc driver by itself incurred a small but meaningful
 overhead when compiling many small files. The driver doesn't do much
 work compared to a compilation, but we have tried to keep it as
 efficient as possible by following a few simple principles:

 -  Avoid memory allocation and string copying when possible.
 -  Don't parse arguments more than once.
 -  Provide a few simple interfaces for efficiently searching arguments.

 Simple
 ------

 Finally, the driver was designed to be "as simple as possible", given
 the other goals. Notably, trying to be completely compatible with the
 gcc driver adds a significant amount of complexity. However, the design
 of the driver attempts to mitigate this complexity by dividing the
 process into a number of independent stages instead of a single
 monolithic task.

 Internal Design and Implementation
 ==================================

 .. contents::
    :local:
    :depth: 1

 Internals Introduction
 ----------------------

 In order to satisfy the stated goals, the driver was designed to
 completely subsume the functionality of the gcc executable; that is, the
 driver should not need to delegate to gcc to perform subtasks. On
 Darwin, this implies that the Clang driver also subsumes the gcc
 driver-driver, which is used to implement support for building universal
 images (binaries and object files). This also implies that the driver
 should be able to call the language specific compilers (e.g. cc1)
 directly, which means that it must have enough information to forward
 command line arguments to child processes correctly.

 Design Overview
 ---------------

 The diagram below shows the significant components of the driver
 architecture and how they relate to one another. The orange components
 represent concrete data structures built by the driver, the green
 components indicate conceptually distinct stages which manipulate these
 data structures, and the blue components are important helper classes.

 .. image:: DriverArchitecture.png
    :align: center
    :alt: Driver Architecture Diagram

 Driver Stages
 -------------

 The driver functionality is conceptually divided into five stages:

 #. **Parse: Option Parsing**

    The command line argument strings are decomposed into arguments
    (``Arg`` instances). The driver expects to understand all available
    options, although there is some facility for just passing certain
    classes of options through (like ``-Wl,``).

    Each argument corresponds to exactly one abstract ``Option``
    definition, which describes how the option is parsed along with some
    additional metadata. The Arg instances themselves are lightweight and
    merely contain enough information for clients to determine which
    option they correspond to and their values (if they have additional
    parameters).

    For example, a command line like "-Ifoo -I foo" would parse to two
    Arg instances (a JoinedArg and a SeparateArg instance), but each
    would refer to the same Option.

    Options are lazily created in order to avoid populating all Option
    classes when the driver is loaded. Most of the driver code only needs
    to deal with options by their unique ID (e.g., ``options::OPT_I``),

    Arg instances themselves do not generally store the values of
    parameters. In many cases, this would simply result in creating
    unnecessary string copies. Instead, Arg instances are always embedded
    inside an ArgList structure, which contains the original vector of
    argument strings. Each Arg itself only needs to contain an index into
    this vector instead of storing its values directly.

    The clang driver can dump the results of this stage using the
    ``-###`` flag (which must precede any actual command
    line arguments). For example:

    .. code-block:: console

       $ clang -### -Xarch_i386 -fomit-frame-pointer -Wa,-fast -Ifoo -I foo t.c
       Option 0 - Name: "-Xarch_", Values: {"i386", "-fomit-frame-pointer"}
       Option 1 - Name: "-Wa,", Values: {"-fast"}
       Option 2 - Name: "-I", Values: {"foo"}
       Option 3 - Name: "-I", Values: {"foo"}
       Option 4 - Name: "<input>", Values: {"t.c"}

    After this stage is complete the command line should be broken down
    into well defined option objects with their appropriate parameters.
    Subsequent stages should rarely, if ever, need to do any string
    processing.

 #. **Pipeline: Compilation Action Construction**

    Once the arguments are parsed, the tree of subprocess jobs needed for
    the desired compilation sequence are constructed. This involves
    determining the input files and their types, what work is to be done
    on them (preprocess, compile, assemble, link, etc.), and constructing
    a list of Action instances for each task. The result is a list of one
    or more top-level actions, each of which generally corresponds to a
    single output (for example, an object or linked executable).

    The majority of Actions correspond to actual tasks, however there are
    two special Actions. The first is InputAction, which simply serves to
    adapt an input argument for use as an input to other Actions. The
    second is BindArchAction, which conceptually alters the architecture
    to be used for all of its input Actions.

    The clang driver can dump the results of this stage using the
    ``-ccc-print-phases`` flag. For example:

    .. code-block:: console

       $ clang -ccc-print-phases -x c t.c -x assembler t.s
       0: input, "t.c", c
       1: preprocessor, {0}, cpp-output
       2: compiler, {1}, assembler
       3: assembler, {2}, object
       4: input, "t.s", assembler
       5: assembler, {4}, object
       6: linker, {3, 5}, image

    Here the driver is constructing seven distinct actions, four to
    compile the "t.c" input into an object file, two to assemble the
    "t.s" input, and one to link them together.

    A rather different compilation pipeline is shown here; in this
    example there are two top level actions to compile the input files
    into two separate object files, where each object file is built using
    ``lipo`` to merge results built for two separate architectures.

    .. code-block:: console

       $ clang -ccc-print-phases -c -arch i386 -arch x86_64 t0.c t1.c
       0: input, "t0.c", c
       1: preprocessor, {0}, cpp-output
       2: compiler, {1}, assembler
       3: assembler, {2}, object
       4: bind-arch, "i386", {3}, object
       5: bind-arch, "x86_64", {3}, object
       6: lipo, {4, 5}, object
       7: input, "t1.c", c
       8: preprocessor, {7}, cpp-output
       9: compiler, {8}, assembler
       10: assembler, {9}, object
       11: bind-arch, "i386", {10}, object
       12: bind-arch, "x86_64", {10}, object
       13: lipo, {11, 12}, object

    After this stage is complete the compilation process is divided into
    a simple set of actions which need to be performed to produce
    intermediate or final outputs (in some cases, like ``-fsyntax-only``,
    there is no "real" final output). Phases are well known compilation
    steps, such as "preprocess", "compile", "assemble", "link", etc.

 #. **Bind: Tool & Filename Selection**

    This stage (in conjunction with the Translate stage) turns the tree
    of Actions into a list of actual subprocess to run. Conceptually, the
    driver performs a top down matching to assign Action(s) to Tools. The
    ToolChain is responsible for selecting the tool to perform a
    particular action; once selected the driver interacts with the tool
    to see if it can match additional actions (for example, by having an
    integrated preprocessor).

    Once Tools have been selected for all actions, the driver determines
    how the tools should be connected (for example, using an inprocess
    module, pipes, temporary files, or user provided filenames). If an
    output file is required, the driver also computes the appropriate
    file name (the suffix and file location depend on the input types and
    options such as ``-save-temps``).

    The driver interacts with a ToolChain to perform the Tool bindings.
    Each ToolChain contains information about all the tools needed for
    compilation for a particular architecture, platform, and operating
    system. A single driver invocation may query multiple ToolChains
    during one compilation in order to interact with tools for separate
    architectures.

    The results of this stage are not computed directly, but the driver
    can print the results via the ``-ccc-print-bindings`` option. For
    example:

    .. code-block:: console

       $ clang -ccc-print-bindings -arch i386 -arch ppc t0.c
       # "i386-apple-darwin9" - "clang", inputs: ["t0.c"], output: "/tmp/cc-Sn4RKF.s"
       # "i386-apple-darwin9" - "darwin::Assemble", inputs: ["/tmp/cc-Sn4RKF.s"], output: "/tmp/cc-gvSnbS.o"
       # "i386-apple-darwin9" - "darwin::Link", inputs: ["/tmp/cc-gvSnbS.o"], output: "/tmp/cc-jgHQxi.out"
       # "ppc-apple-darwin9" - "gcc::Compile", inputs: ["t0.c"], output: "/tmp/cc-Q0bTox.s"
       # "ppc-apple-darwin9" - "gcc::Assemble", inputs: ["/tmp/cc-Q0bTox.s"], output: "/tmp/cc-WCdicw.o"
       # "ppc-apple-darwin9" - "gcc::Link", inputs: ["/tmp/cc-WCdicw.o"], output: "/tmp/cc-HHBEBh.out"
       # "i386-apple-darwin9" - "darwin::Lipo", inputs: ["/tmp/cc-jgHQxi.out", "/tmp/cc-HHBEBh.out"], output: "a.out"

    This shows the tool chain, tool, inputs and outputs which have been
    bound for this compilation sequence. Here clang is being used to
    compile t0.c on the i386 architecture and darwin specific versions of
    the tools are being used to assemble and link the result, but generic
    gcc versions of the tools are being used on PowerPC.

 #. **Translate: Tool Specific Argument Translation**

    Once a Tool has been selected to perform a particular Action, the
    Tool must construct concrete Commands which will be executed during
    compilation. The main work is in translating from the gcc style
    command line options to whatever options the subprocess expects.

    Some tools, such as the assembler, only interact with a handful of
    arguments and just determine the path of the executable to call and
    pass on their input and output arguments. Others, like the compiler
    or the linker, may translate a large number of arguments in addition.

    The ArgList class provides a number of simple helper methods to
    assist with translating arguments; for example, to pass on only the
    last of arguments corresponding to some option, or all arguments for
    an option.

    The result of this stage is a list of Commands (executable paths and
    argument strings) to execute.

 #. **Execute**

    Finally, the compilation pipeline is executed. This is mostly
    straightforward, although there is some interaction with options like
    ``-pipe``, ``-pass-exit-codes`` and ``-time``.

 Additional Notes
 ----------------

 The Compilation Object
 ^^^^^^^^^^^^^^^^^^^^^^

 The driver constructs a Compilation object for each set of command line
 arguments. The Driver itself is intended to be invariant during
 construction of a Compilation; an IDE should be able to construct a
 single long lived driver instance to use for an entire build, for
 example.

 The Compilation object holds information that is particular to each
 compilation sequence. For example, the list of used temporary files
 (which must be removed once compilation is finished) and result files
 (which should be removed if compilation fails).

 Unified Parsing & Pipelining
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Parsing and pipelining both occur without reference to a Compilation
 instance. This is by design; the driver expects that both of these
 phases are platform neutral, with a few very well defined exceptions
 such as whether the platform uses a driver driver.

 ToolChain Argument Translation
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 In order to match gcc very closely, the clang driver currently allows
 tool chains to perform their own translation of the argument list (into
 a new ArgList data structure). Although this allows the clang driver to
 match gcc easily, it also makes the driver operation much harder to
 understand (since the Tools stop seeing some arguments the user
 provided, and see new ones instead).

 For example, on Darwin ``-gfull`` gets translated into two separate
 arguments, ``-g`` and ``-fno-eliminate-unused-debug-symbols``. Trying to
 write Tool logic to do something with ``-gfull`` will not work, because
 Tool argument translation is done after the arguments have been
 translated.

 A long term goal is to remove this tool chain specific translation, and
 instead force each tool to change its own logic to do the right thing on
 the untranslated original arguments.

 Unused Argument Warnings
 ^^^^^^^^^^^^^^^^^^^^^^^^

 The driver operates by parsing all arguments but giving Tools the
 opportunity to choose which arguments to pass on. One downside of this
 infrastructure is that if the user misspells some option, or is confused
 about which options to use, some command line arguments the user really
 cared about may go unused. This problem is particularly important when
 using clang as a compiler, since the clang compiler does not support
 anywhere near all the options that gcc does, and we want to make sure
 users know which ones are being used.

 To support this, the driver maintains a bit associated with each
 argument of whether it has been used (at all) during the compilation.
 This bit usually doesn't need to be set by hand, as the key ArgList
 accessors will set it automatically.

 When a compilation is successful (there are no errors), the driver
 checks the bit and emits an "unused argument" warning for any arguments
 which were never accessed. This is conservative (the argument may not
 have been used to do what the user wanted) but still catches the most
 obvious cases.

 Relation to GCC Driver Concepts
 -------------------------------

 For those familiar with the gcc driver, this section provides a brief
 overview of how things from the gcc driver map to the clang driver.

 -  **Driver Driver**

    The driver driver is fully integrated into the clang driver. The
    driver simply constructs additional Actions to bind the architecture
    during the *Pipeline* phase. The tool chain specific argument
    translation is responsible for handling ``-Xarch_``.

    The one caveat is that this approach requires ``-Xarch_`` not be used
    to alter the compilation itself (for example, one cannot provide
    ``-S`` as an ``-Xarch_`` argument). The driver attempts to reject
    such invocations, and overall there isn't a good reason to abuse
    ``-Xarch_`` to that end in practice.

    The upside is that the clang driver is more efficient and does little
    extra work to support universal builds. It also provides better error
    reporting and UI consistency.

 -  **Specs**

    The clang driver has no direct correspondent for "specs". The
    majority of the functionality that is embedded in specs is in the
    Tool specific argument translation routines. The parts of specs which
    control the compilation pipeline are generally part of the *Pipeline*
    stage.

 -  **Toolchains**

    The gcc driver has no direct understanding of tool chains. Each gcc
    binary roughly corresponds to the information which is embedded
    inside a single ToolChain.

    The clang driver is intended to be portable and support complex
    compilation environments. All platform and tool chain specific code
    should be protected behind either abstract or well defined interfaces
    (such as whether the platform supports use as a driver driver).
	=========================
	Driver Design & Internals
	=========================

	.. contents::
	:local:

	Introduction
	============

	This document describes the Clang driver. The purpose of this document
	is to describe both the motivation and design goals for the driver, as
	well as details of the internal implementation.

	Features and Goals
	==================

	The Clang driver is intended to be a production quality compiler driver
	providing access to the Clang compiler and tools, with a command line
	interface which is compatible with the gcc driver.

	Although the driver is part of and driven by the Clang project, it is
	logically a separate tool which shares many of the same goals as Clang:

	.. contents:: Features
	:local:

	GCC Compatibility
	-----------------

	The number one goal of the driver is to ease the adoption of Clang by
	allowing users to drop Clang into a build system which was designed to
	call GCC. Although this makes the driver much more complicated than
	might otherwise be necessary, we decided that being very compatible with
	the gcc command line interface was worth it in order to allow users to
	quickly test clang on their projects.

	Flexible
	--------

	The driver was designed to be flexible and easily accommodate new uses
	as we grow the clang and LLVM infrastructure. As one example, the driver
	can easily support the introduction of tools which have an integrated
	assembler; something we hope to add to LLVM in the future.

	Similarly, most of the driver functionality is kept in a library which
	can be used to build other tools which want to implement or accept a gcc
	like interface.

	Low Overhead
	------------

	The driver should have as little overhead as possible. In practice, we
	found that the gcc driver by itself incurred a small but meaningful
	overhead when compiling many small files. The driver doesn't do much
	work compared to a compilation, but we have tried to keep it as
	efficient as possible by following a few simple principles:

	- Avoid memory allocation and string copying when possible.
	- Don't parse arguments more than once.
	- Provide a few simple interfaces for efficiently searching arguments.

	Simple
	------

	Finally, the driver was designed to be "as simple as possible", given
	the other goals. Notably, trying to be completely compatible with the
	gcc driver adds a significant amount of complexity. However, the design
	of the driver attempts to mitigate this complexity by dividing the
	process into a number of independent stages instead of a single
	monolithic task.

	Internal Design and Implementation
	==================================

	.. contents::
	:local:
	:depth: 1

	Internals Introduction
	----------------------

	In order to satisfy the stated goals, the driver was designed to
	completely subsume the functionality of the gcc executable; that is, the
	driver should not need to delegate to gcc to perform subtasks. On
	Darwin, this implies that the Clang driver also subsumes the gcc
	driver-driver, which is used to implement support for building universal
	images (binaries and object files). This also implies that the driver
	should be able to call the language specific compilers (e.g. cc1)
	directly, which means that it must have enough information to forward
	command line arguments to child processes correctly.

	Design Overview
	---------------

	The diagram below shows the significant components of the driver
	architecture and how they relate to one another. The orange components
	represent concrete data structures built by the driver, the green
	components indicate conceptually distinct stages which manipulate these
	data structures, and the blue components are important helper classes.

	.. image:: DriverArchitecture.png
	:align: center
	:alt: Driver Architecture Diagram

	Driver Stages
	-------------

	The driver functionality is conceptually divided into five stages:

	#. Parse: Option Parsing

	The command line argument strings are decomposed into arguments
	(``Arg`` instances). The driver expects to understand all available
	options, although there is some facility for just passing certain
	classes of options through (like ``-Wl,``).

	Each argument corresponds to exactly one abstract ``Option``
	definition, which describes how the option is parsed along with some
	additional metadata. The Arg instances themselves are lightweight and
	merely contain enough information for clients to determine which
	option they correspond to and their values (if they have additional
	parameters).

	For example, a command line like "-Ifoo -I foo" would parse to two
	Arg instances (a JoinedArg and a SeparateArg instance), but each
	would refer to the same Option.

	Options are lazily created in order to avoid populating all Option
	classes when the driver is loaded. Most of the driver code only needs
	to deal with options by their unique ID (e.g., ``options::OPT_I``),

	Arg instances themselves do not generally store the values of
	parameters. In many cases, this would simply result in creating
	unnecessary string copies. Instead, Arg instances are always embedded
	inside an ArgList structure, which contains the original vector of
	argument strings. Each Arg itself only needs to contain an index into
	this vector instead of storing its values directly.

	The clang driver can dump the results of this stage using the
	``-###`` flag (which must precede any actual command
	line arguments). For example:

	.. code-block:: console

	$ clang -### -Xarch_i386 -fomit-frame-pointer -Wa,-fast -Ifoo -I foo t.c
	Option 0 - Name: "-Xarch_", Values: {"i386", "-fomit-frame-pointer"}
	Option 1 - Name: "-Wa,", Values: {"-fast"}
	Option 2 - Name: "-I", Values: {"foo"}
	Option 3 - Name: "-I", Values: {"foo"}
	Option 4 - Name: "<input>", Values: {"t.c"}

	After this stage is complete the command line should be broken down
	into well defined option objects with their appropriate parameters.
	Subsequent stages should rarely, if ever, need to do any string
	processing.

	#. Pipeline: Compilation Action Construction

	Once the arguments are parsed, the tree of subprocess jobs needed for
	the desired compilation sequence are constructed. This involves
	determining the input files and their types, what work is to be done
	on them (preprocess, compile, assemble, link, etc.), and constructing
	a list of Action instances for each task. The result is a list of one
	or more top-level actions, each of which generally corresponds to a
	single output (for example, an object or linked executable).

	The majority of Actions correspond to actual tasks, however there are
	two special Actions. The first is InputAction, which simply serves to
	adapt an input argument for use as an input to other Actions. The
	second is BindArchAction, which conceptually alters the architecture
	to be used for all of its input Actions.

	The clang driver can dump the results of this stage using the
	``-ccc-print-phases`` flag. For example:

	.. code-block:: console

	$ clang -ccc-print-phases -x c t.c -x assembler t.s
	0: input, "t.c", c
	1: preprocessor, {0}, cpp-output
	2: compiler, {1}, assembler
	3: assembler, {2}, object
	4: input, "t.s", assembler
	5: assembler, {4}, object
	6: linker, {3, 5}, image

	Here the driver is constructing seven distinct actions, four to
	compile the "t.c" input into an object file, two to assemble the
	"t.s" input, and one to link them together.

	A rather different compilation pipeline is shown here; in this
	example there are two top level actions to compile the input files
	into two separate object files, where each object file is built using
	``lipo`` to merge results built for two separate architectures.

	.. code-block:: console

	$ clang -ccc-print-phases -c -arch i386 -arch x86_64 t0.c t1.c
	0: input, "t0.c", c
	1: preprocessor, {0}, cpp-output
	2: compiler, {1}, assembler
	3: assembler, {2}, object
	4: bind-arch, "i386", {3}, object
	5: bind-arch, "x86_64", {3}, object
	6: lipo, {4, 5}, object
	7: input, "t1.c", c
	8: preprocessor, {7}, cpp-output
	9: compiler, {8}, assembler
	10: assembler, {9}, object
	11: bind-arch, "i386", {10}, object
	12: bind-arch, "x86_64", {10}, object
	13: lipo, {11, 12}, object

	After this stage is complete the compilation process is divided into
	a simple set of actions which need to be performed to produce
	intermediate or final outputs (in some cases, like ``-fsyntax-only``,
	there is no "real" final output). Phases are well known compilation
	steps, such as "preprocess", "compile", "assemble", "link", etc.

	#. Bind: Tool & Filename Selection

	This stage (in conjunction with the Translate stage) turns the tree
	of Actions into a list of actual subprocess to run. Conceptually, the
	driver performs a top down matching to assign Action(s) to Tools. The
	ToolChain is responsible for selecting the tool to perform a
	particular action; once selected the driver interacts with the tool
	to see if it can match additional actions (for example, by having an
	integrated preprocessor).

	Once Tools have been selected for all actions, the driver determines
	how the tools should be connected (for example, using an inprocess
	module, pipes, temporary files, or user provided filenames). If an
	output file is required, the driver also computes the appropriate
	file name (the suffix and file location depend on the input types and
	options such as ``-save-temps``).

	The driver interacts with a ToolChain to perform the Tool bindings.
	Each ToolChain contains information about all the tools needed for
	compilation for a particular architecture, platform, and operating
	system. A single driver invocation may query multiple ToolChains
	during one compilation in order to interact with tools for separate
	architectures.

	The results of this stage are not computed directly, but the driver
	can print the results via the ``-ccc-print-bindings`` option. For
	example:

	.. code-block:: console

	$ clang -ccc-print-bindings -arch i386 -arch ppc t0.c
	# "i386-apple-darwin9" - "clang", inputs: ["t0.c"], output: "/tmp/cc-Sn4RKF.s"
	# "i386-apple-darwin9" - "darwin::Assemble", inputs: ["/tmp/cc-Sn4RKF.s"], output: "/tmp/cc-gvSnbS.o"
	# "i386-apple-darwin9" - "darwin::Link", inputs: ["/tmp/cc-gvSnbS.o"], output: "/tmp/cc-jgHQxi.out"
	# "ppc-apple-darwin9" - "gcc::Compile", inputs: ["t0.c"], output: "/tmp/cc-Q0bTox.s"
	# "ppc-apple-darwin9" - "gcc::Assemble", inputs: ["/tmp/cc-Q0bTox.s"], output: "/tmp/cc-WCdicw.o"
	# "ppc-apple-darwin9" - "gcc::Link", inputs: ["/tmp/cc-WCdicw.o"], output: "/tmp/cc-HHBEBh.out"
	# "i386-apple-darwin9" - "darwin::Lipo", inputs: ["/tmp/cc-jgHQxi.out", "/tmp/cc-HHBEBh.out"], output: "a.out"

	This shows the tool chain, tool, inputs and outputs which have been
	bound for this compilation sequence. Here clang is being used to
	compile t0.c on the i386 architecture and darwin specific versions of
	the tools are being used to assemble and link the result, but generic
	gcc versions of the tools are being used on PowerPC.

	#. Translate: Tool Specific Argument Translation

	Once a Tool has been selected to perform a particular Action, the
	Tool must construct concrete Commands which will be executed during
	compilation. The main work is in translating from the gcc style
	command line options to whatever options the subprocess expects.

	Some tools, such as the assembler, only interact with a handful of
	arguments and just determine the path of the executable to call and
	pass on their input and output arguments. Others, like the compiler
	or the linker, may translate a large number of arguments in addition.

	The ArgList class provides a number of simple helper methods to
	assist with translating arguments; for example, to pass on only the
	last of arguments corresponding to some option, or all arguments for
	an option.

	The result of this stage is a list of Commands (executable paths and
	argument strings) to execute.

	#. Execute

	Finally, the compilation pipeline is executed. This is mostly
	straightforward, although there is some interaction with options like
	``-pipe``, ``-pass-exit-codes`` and ``-time``.

	Additional Notes
	----------------

	The Compilation Object
	^^^^^^^^^^^^^^^^^^^^^^

	The driver constructs a Compilation object for each set of command line
	arguments. The Driver itself is intended to be invariant during
	construction of a Compilation; an IDE should be able to construct a
	single long lived driver instance to use for an entire build, for
	example.

	The Compilation object holds information that is particular to each
	compilation sequence. For example, the list of used temporary files
	(which must be removed once compilation is finished) and result files
	(which should be removed if compilation fails).

	Unified Parsing & Pipelining
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^

	Parsing and pipelining both occur without reference to a Compilation
	instance. This is by design; the driver expects that both of these
	phases are platform neutral, with a few very well defined exceptions
	such as whether the platform uses a driver driver.

	ToolChain Argument Translation
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

	In order to match gcc very closely, the clang driver currently allows
	tool chains to perform their own translation of the argument list (into
	a new ArgList data structure). Although this allows the clang driver to
	match gcc easily, it also makes the driver operation much harder to
	understand (since the Tools stop seeing some arguments the user
	provided, and see new ones instead).

	For example, on Darwin ``-gfull`` gets translated into two separate
	arguments, ``-g`` and ``-fno-eliminate-unused-debug-symbols``. Trying to
	write Tool logic to do something with ``-gfull`` will not work, because
	Tool argument translation is done after the arguments have been
	translated.

	A long term goal is to remove this tool chain specific translation, and
	instead force each tool to change its own logic to do the right thing on
	the untranslated original arguments.

	Unused Argument Warnings
	^^^^^^^^^^^^^^^^^^^^^^^^

	The driver operates by parsing all arguments but giving Tools the
	opportunity to choose which arguments to pass on. One downside of this
	infrastructure is that if the user misspells some option, or is confused
	about which options to use, some command line arguments the user really
	cared about may go unused. This problem is particularly important when
	using clang as a compiler, since the clang compiler does not support
	anywhere near all the options that gcc does, and we want to make sure
	users know which ones are being used.

	To support this, the driver maintains a bit associated with each
	argument of whether it has been used (at all) during the compilation.
	This bit usually doesn't need to be set by hand, as the key ArgList
	accessors will set it automatically.

	When a compilation is successful (there are no errors), the driver
	checks the bit and emits an "unused argument" warning for any arguments
	which were never accessed. This is conservative (the argument may not
	have been used to do what the user wanted) but still catches the most
	obvious cases.

	Relation to GCC Driver Concepts
	-------------------------------

	For those familiar with the gcc driver, this section provides a brief
	overview of how things from the gcc driver map to the clang driver.

	- Driver Driver

	The driver driver is fully integrated into the clang driver. The
	driver simply constructs additional Actions to bind the architecture
	during the Pipeline phase. The tool chain specific argument
	translation is responsible for handling ``-Xarch_``.

	The one caveat is that this approach requires ``-Xarch_`` not be used
	to alter the compilation itself (for example, one cannot provide
	``-S`` as an ``-Xarch_`` argument). The driver attempts to reject
	such invocations, and overall there isn't a good reason to abuse
	``-Xarch_`` to that end in practice.

	The upside is that the clang driver is more efficient and does little
	extra work to support universal builds. It also provides better error
	reporting and UI consistency.

	- Specs

	The clang driver has no direct correspondent for "specs". The
	majority of the functionality that is embedded in specs is in the
	Tool specific argument translation routines. The parts of specs which
	control the compilation pipeline are generally part of the Pipeline
	stage.

	- Toolchains

	The gcc driver has no direct understanding of tool chains. Each gcc
	binary roughly corresponds to the information which is embedded
	inside a single ToolChain.

	The clang driver is intended to be portable and support complex
	compilation environments. All platform and tool chain specific code
	should be protected behind either abstract or well defined interfaces
	(such as whether the platform supports use as a driver driver).