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<title>Clang Driver Manual</title>

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<h1>Driver Design & Internals</h1>

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<ul>

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<li><a href="#intro">Introduction</a></li>

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<li><a href="#features">Features and Goals</a></li>

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<ul>

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<li><a href="#gcccompat">GCC Compatibility</a></li>

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<li><a href="#components">Flexible</a></li>

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<li><a href="#performance">Low Overhead</a></li>

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<li><a href="#simple">Simple</a></li>

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</ul>

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<li><a href="#design">Design</a></li>

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<ul>

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<li><a href="#int_intro">Internals Introduction</a></li>

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<li><a href="#int_overview">Design Overview</a></li>

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<li><a href="#int_notes">Additional Notes</a></li>

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<ul>

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<li><a href="#int_compilation">The Compilation Object</a></li>

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<li><a href="#int_unified_parsing">Unified Parsing & Pipelining</a></li>

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<li><a href="#int_toolchain_translation">ToolChain Argument Translation</a></li>

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<li><a href="#int_unused_warnings">Unused Argument Warnings</a></li>

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</ul>

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<li><a href="#int_gcc_concepts">Relation to GCC Driver Concepts</a></li>

</ul>

</ul>

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<h2 id="intro">Introduction</h2>

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<p>This document describes the Clang driver. The purpose of this

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document is to describe both the motivation and design goals

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for the driver, as well as details of the internal

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implementation.</p>

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<h2 id="features">Features and Goals</h2>

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<p>The Clang driver is intended to be a production quality

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compiler driver providing access to the Clang compiler and

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tools, with a command line interface which is compatible with

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the gcc driver.</p>

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<p>Although the driver is part of and driven by the Clang

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project, it is logically a separate tool which shares many of

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the same goals as Clang:</p>

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<p><b>Features</b>:</p>

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<ul>

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<li><a href="#gcccompat">GCC Compatibility</a></li>

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<li><a href="#components">Flexible</a></li>

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<li><a href="#performance">Low Overhead</a></li>

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<li><a href="#simple">Simple</a></li>

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</ul>

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<h3 id="gcccompat">GCC Compatibility</h3>

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<p>The number one goal of the driver is to ease the adoption of

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Clang by allowing users to drop Clang into a build system

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which was designed to call GCC. Although this makes the driver

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much more complicated than might otherwise be necessary, we

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decided that being very compatible with the gcc command line

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interface was worth it in order to allow users to quickly test

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clang on their projects.</p>

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<h3 id="components">Flexible</h3>

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<p>The driver was designed to be flexible and easily accommodate

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new uses as we grow the clang and LLVM infrastructure. As one

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example, the driver can easily support the introduction of

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tools which have an integrated assembler; something we hope to

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add to LLVM in the future.</p>

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<p>Similarly, most of the driver functionality is kept in a

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library which can be used to build other tools which want to

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implement or accept a gcc like interface. </p>

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<h3 id="performance">Low Overhead</h3>

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<p>The driver should have as little overhead as possible. In

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practice, we found that the gcc driver by itself incurred a

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small but meaningful overhead when compiling many small

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files. The driver doesn't do much work compared to a

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compilation, but we have tried to keep it as efficient as

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possible by following a few simple principles:</p>

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<ul>

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<li>Avoid memory allocation and string copying when

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possible.</li>

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<li>Don't parse arguments more than once.</li>

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<li>Provide a few simple interfaces for efficiently searching

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arguments.</li>

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<h3 id="simple">Simple</h3>

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<p>Finally, the driver was designed to be "as simple as

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possible", given the other goals. Notably, trying to be

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completely compatible with the gcc driver adds a significant

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amount of complexity. However, the design of the driver

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attempts to mitigate this complexity by dividing the process

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into a number of independent stages instead of a single

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monolithic task.</p>

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<h2 id="design">Internal Design and Implementation</h2>

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<ul>

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<li><a href="#int_intro">Internals Introduction</a></li>

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<li><a href="#int_overview">Design Overview</a></li>

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<li><a href="#int_notes">Additional Notes</a></li>

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<li><a href="#int_gcc_concepts">Relation to GCC Driver Concepts</a></li>

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</ul>

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<h3><a name="int_intro">Internals Introduction</a></h3>

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<p>In order to satisfy the stated goals, the driver was designed

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to completely subsume the functionality of the gcc executable;

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that is, the driver should not need to delegate to gcc to

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perform subtasks. On Darwin, this implies that the Clang

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driver also subsumes the gcc driver-driver, which is used to

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implement support for building universal images (binaries and

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object files). This also implies that the driver should be

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able to call the language specific compilers (e.g. cc1)

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directly, which means that it must have enough information to

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forward command line arguments to child processes

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correctly.</p>

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<h3><a name="int_overview">Design Overview</a></h3>

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<p>The diagram below shows the significant components of the

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driver architecture and how they relate to one another. The

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orange components represent concrete data structures built by

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the driver, the green components indicate conceptually

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distinct stages which manipulate these data structures, and

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the blue components are important helper classes. </p>

</a>

</center>

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<h3><a name="int_stages">Driver Stages</a></h3>

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<p>The driver functionality is conceptually divided into five stages:</p>

<ol>

<li>

<b>Parse: Option Parsing</b>

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<p>The command line argument strings are decomposed into

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arguments (<tt>Arg</tt> instances). The driver expects to

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understand all available options, although there is some

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facility for just passing certain classes of options

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through (like <tt>-Wl,</tt>).</p>

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<p>Each argument corresponds to exactly one

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abstract <tt>Option</tt> definition, which describes how

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the option is parsed along with some additional

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metadata. The Arg instances themselves are lightweight and

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merely contain enough information for clients to determine

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which option they correspond to and their values (if they

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have additional parameters).</p>

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<p>For example, a command line like "-Ifoo -I foo" would

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parse to two Arg instances (a JoinedArg and a SeparateArg

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instance), but each would refer to the same Option.</p>

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<p>Options are lazily created in order to avoid populating

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all Option classes when the driver is loaded. Most of the

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driver code only needs to deal with options by their

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unique ID (e.g., <tt>options::OPT_I</tt>),</p>

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<p>Arg instances themselves do not generally store the

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values of parameters. In many cases, this would

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simply result in creating unnecessary string

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copies. Instead, Arg instances are always embedded inside

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an ArgList structure, which contains the original vector

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of argument strings. Each Arg itself only needs to contain

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an index into this vector instead of storing its values

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directly.</p>

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<p>The clang driver can dump the results of this

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stage using the <tt>-ccc-print-options</tt> flag (which

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must precede any actual command line arguments). For

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example:</p>

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<pre>

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$ <b>clang -ccc-print-options -Xarch_i386 -fomit-frame-pointer -Wa,-fast -Ifoo -I foo t.c</b>

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Option 0 - Name: "-Xarch_", Values: {"i386", "-fomit-frame-pointer"}

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Option 1 - Name: "-Wa,", Values: {"-fast"}

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Option 2 - Name: "-I", Values: {"foo"}

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Option 3 - Name: "-I", Values: {"foo"}

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Option 4 - Name: "<input>", Values: {"t.c"}

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</pre>

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<p>After this stage is complete the command line should be

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broken down into well defined option objects with their

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appropriate parameters. Subsequent stages should rarely,

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if ever, need to do any string processing.</p>

</li>

<li>

<b>Pipeline: Compilation Job Construction</b>

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<p>Once the arguments are parsed, the tree of subprocess

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jobs needed for the desired compilation sequence are

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constructed. This involves determining the input files and

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their types, what work is to be done on them (preprocess,

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compile, assemble, link, etc.), and constructing a list of

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Action instances for each task. The result is a list of

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one or more top-level actions, each of which generally

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corresponds to a single output (for example, an object or

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linked executable).</p>

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<p>The majority of Actions correspond to actual tasks,

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however there are two special Actions. The first is

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InputAction, which simply serves to adapt an input

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argument for use as an input to other Actions. The second

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is BindArchAction, which conceptually alters the

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architecture to be used for all of its input Actions.</p>

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<p>The clang driver can dump the results of this

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stage using the <tt>-ccc-print-phases</tt> flag. For

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example:</p>

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<pre>

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$ <b>clang -ccc-print-phases -x c t.c -x assembler t.s</b>

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0: input, "t.c", c

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1: preprocessor, {0}, cpp-output

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2: compiler, {1}, assembler

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3: assembler, {2}, object

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4: input, "t.s", assembler

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5: assembler, {4}, object

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6: linker, {3, 5}, image

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</pre>

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<p>Here the driver is constructing seven distinct actions,

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four to compile the "t.c" input into an object file, two to

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assemble the "t.s" input, and one to link them together.</p>

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<p>A rather different compilation pipeline is shown here; in

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this example there are two top level actions to compile

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the input files into two separate object files, where each

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object file is built using <tt>lipo</tt> to merge results

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built for two separate architectures.</p>

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<pre>

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$ <b>clang -ccc-print-phases -c -arch i386 -arch x86_64 t0.c t1.c</b>

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0: input, "t0.c", c

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1: preprocessor, {0}, cpp-output

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2: compiler, {1}, assembler

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3: assembler, {2}, object

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4: bind-arch, "i386", {3}, object

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5: bind-arch, "x86_64", {3}, object

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6: lipo, {4, 5}, object

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7: input, "t1.c", c

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8: preprocessor, {7}, cpp-output

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9: compiler, {8}, assembler

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10: assembler, {9}, object

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11: bind-arch, "i386", {10}, object

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12: bind-arch, "x86_64", {10}, object

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13: lipo, {11, 12}, object

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</pre>

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<p>After this stage is complete the compilation process is

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divided into a simple set of actions which need to be

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performed to produce intermediate or final outputs (in

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some cases, like <tt>-fsyntax-only</tt>, there is no

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"real" final output). Phases are well known compilation

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steps, such as "preprocess", "compile", "assemble",

"link", etc.</p>

</li>

<li>

<b>Bind: Tool & Filename Selection</b>

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<p>This stage (in conjunction with the Translate stage)

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turns the tree of Actions into a list of actual subprocess

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to run. Conceptually, the driver performs a top down

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matching to assign Action(s) to Tools. The ToolChain is

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responsible for selecting the tool to perform a particular

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action; once selected the driver interacts with the tool

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to see if it can match additional actions (for example, by

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having an integrated preprocessor).

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<p>Once Tools have been selected for all actions, the driver

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determines how the tools should be connected (for example,

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using an inprocess module, pipes, temporary files, or user

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provided filenames). If an output file is required, the

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driver also computes the appropriate file name (the suffix

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and file location depend on the input types and options

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such as <tt>-save-temps</tt>).

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<p>The driver interacts with a ToolChain to perform the Tool

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bindings. Each ToolChain contains information about all

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the tools needed for compilation for a particular

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architecture, platform, and operating system. A single

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driver invocation may query multiple ToolChains during one

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compilation in order to interact with tools for separate

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architectures.</p>

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<p>The results of this stage are not computed directly, but

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the driver can print the results via

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the <tt>-ccc-print-bindings</tt> option. For example:</p>

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<pre>

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$ <b>clang -ccc-print-bindings -arch i386 -arch ppc t0.c</b>

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# "i386-apple-darwin9" - "clang", inputs: ["t0.c"], output: "/tmp/cc-Sn4RKF.s"

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# "i386-apple-darwin9" - "darwin::Assemble", inputs: ["/tmp/cc-Sn4RKF.s"], output: "/tmp/cc-gvSnbS.o"

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# "i386-apple-darwin9" - "darwin::Link", inputs: ["/tmp/cc-gvSnbS.o"], output: "/tmp/cc-jgHQxi.out"

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# "ppc-apple-darwin9" - "gcc::Compile", inputs: ["t0.c"], output: "/tmp/cc-Q0bTox.s"

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# "ppc-apple-darwin9" - "gcc::Assemble", inputs: ["/tmp/cc-Q0bTox.s"], output: "/tmp/cc-WCdicw.o"

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# "ppc-apple-darwin9" - "gcc::Link", inputs: ["/tmp/cc-WCdicw.o"], output: "/tmp/cc-HHBEBh.out"

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# "i386-apple-darwin9" - "darwin::Lipo", inputs: ["/tmp/cc-jgHQxi.out", "/tmp/cc-HHBEBh.out"], output: "a.out"

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</pre>

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<p>This shows the tool chain, tool, inputs and outputs which

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have been bound for this compilation sequence. Here clang

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is being used to compile t0.c on the i386 architecture and

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darwin specific versions of the tools are being used to

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assemble and link the result, but generic gcc versions of

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the tools are being used on PowerPC.</p>

</li>

<li>

<b>Translate: Tool Specific Argument Translation</b>

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<p>Once a Tool has been selected to perform a particular

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Action, the Tool must construct concrete Jobs which will be

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executed during compilation. The main work is in translating

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from the gcc style command line options to whatever options

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the subprocess expects.</p>

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<p>Some tools, such as the assembler, only interact with a

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handful of arguments and just determine the path of the

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executable to call and pass on their input and output

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arguments. Others, like the compiler or the linker, may

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translate a large number of arguments in addition.</p>

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<p>The ArgList class provides a number of simple helper

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methods to assist with translating arguments; for example,

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to pass on only the last of arguments corresponding to some

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option, or all arguments for an option.</p>

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<p>The result of this stage is a list of Jobs (executable

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paths and argument strings) to execute.</p>

</li>

<li>

<b>Execute</b>

<p>Finally, the compilation pipeline is executed. This is

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mostly straightforward, although there is some interaction

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with options

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like <tt>-pipe</tt>, <tt>-pass-exit-codes</tt>

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and <tt>-time</tt>.</p>

</li>

</ol>

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<h3><a name="int_notes">Additional Notes</a></h3>

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<h4 id="int_compilation">The Compilation Object</h4>

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<p>The driver constructs a Compilation object for each set of

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command line arguments. The Driver itself is intended to be

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invariant during construction of a Compilation; an IDE should be

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able to construct a single long lived driver instance to use

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for an entire build, for example.</p>

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<p>The Compilation object holds information that is particular

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to each compilation sequence. For example, the list of used

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temporary files (which must be removed once compilation is

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finished) and result files (which should be removed if

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compilation fails).</p>

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<h4 id="int_unified_parsing">Unified Parsing & Pipelining</h4>

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<p>Parsing and pipelining both occur without reference to a

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Compilation instance. This is by design; the driver expects that

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both of these phases are platform neutral, with a few very well

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defined exceptions such as whether the platform uses a driver

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driver.</p>

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<h4 id="int_toolchain_translation">ToolChain Argument Translation</h4>

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<p>In order to match gcc very closely, the clang driver

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currently allows tool chains to perform their own translation of

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the argument list (into a new ArgList data structure). Although

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this allows the clang driver to match gcc easily, it also makes

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the driver operation much harder to understand (since the Tools

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stop seeing some arguments the user provided, and see new ones

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instead).</p>

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<p>For example, on Darwin <tt>-gfull</tt> gets translated into two

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separate arguments, <tt>-g</tt>

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and <tt>-fno-eliminate-unused-debug-symbols</tt>. Trying to write Tool

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logic to do something with <tt>-gfull</tt> will not work, because Tool

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argument translation is done after the arguments have been

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translated.</p>

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<p>A long term goal is to remove this tool chain specific

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translation, and instead force each tool to change its own logic

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to do the right thing on the untranslated original arguments.</p>

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<h4 id="int_unused_warnings">Unused Argument Warnings</h4>

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<p>The driver operates by parsing all arguments but giving Tools

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the opportunity to choose which arguments to pass on. One

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downside of this infrastructure is that if the user misspells

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some option, or is confused about which options to use, some

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command line arguments the user really cared about may go

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unused. This problem is particularly important when using

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clang as a compiler, since the clang compiler does not support

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anywhere near all the options that gcc does, and we want to make

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sure users know which ones are being used.</p>

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<p>To support this, the driver maintains a bit associated with

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each argument of whether it has been used (at all) during the

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compilation. This bit usually doesn't need to be set by hand,

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as the key ArgList accessors will set it automatically.</p>

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<p>When a compilation is successful (there are no errors), the

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driver checks the bit and emits an "unused argument" warning for

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any arguments which were never accessed. This is conservative

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(the argument may not have been used to do what the user wanted)

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but still catches the most obvious cases.</p>

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<h3><a name="int_gcc_concepts">Relation to GCC Driver Concepts</a></h3>

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<p>For those familiar with the gcc driver, this section provides

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a brief overview of how things from the gcc driver map to the

clang driver.</p>

<ul>

<li>

<b>Driver Driver</b>

<p>The driver driver is fully integrated into the clang

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driver. The driver simply constructs additional Actions to

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bind the architecture during the <i>Pipeline</i>

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phase. The tool chain specific argument translation is

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responsible for handling <tt>-Xarch_</tt>.</p>

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<p>The one caveat is that this approach

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requires <tt>-Xarch_</tt> not be used to alter the

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compilation itself (for example, one cannot

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provide <tt>-S</tt> as an <tt>-Xarch_</tt> argument). The

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driver attempts to reject such invocations, and overall

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there isn't a good reason to abuse <tt>-Xarch_</tt> to

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that end in practice.</p>

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<p>The upside is that the clang driver is more efficient and

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does little extra work to support universal builds. It also

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provides better error reporting and UI consistency.</p>

</li>

<li>

<b>Specs</b>

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<p>The clang driver has no direct correspondent for

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