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<html>
<head>
<title>"Clang" CFE Internals Manual</title>
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<link type="text/css" rel="stylesheet" href="../content.css" />
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<body>

<!--#include virtual="../menu.html.incl"-->

<div id="content">

<h1>"Clang" CFE Internals Manual</h1>

<ul>
<li><a href="#intro">Introduction</a></li>
<li><a href="#libsystem">LLVM System and Support Libraries</a></li>
<li><a href="#libbasic">The Clang 'Basic' Library</a>
  <ul>
  <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li>
  <li><a href="#SourceLocation">The SourceLocation and SourceManager
      classes</a></li>
  </ul>
</li>
<li><a href="#liblex">The Lexer and Preprocessor Library</a>
  <ul>
  <li><a href="#Token">The Token class</a></li>
  <li><a href="#Lexer">The Lexer class</a></li>
  <li><a href="#TokenLexer">The TokenLexer class</a></li>
  <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li>
  </ul>
</li>
<li><a href="#libparse">The Parser Library</a>
  <ul>
  </ul>
</li>
<li><a href="#libast">The AST Library</a>
  <ul>
  <li><a href="#Type">The Type class and its subclasses</a></li>
  <li><a href="#QualType">The QualType class</a></li>
  <li><a href="#DeclarationName">Declaration names</a></li>
  <li><a href="#CFG">The CFG class</a></li>
  <li><a href="#Constants">Constant Folding in the Clang AST</a></li>
  </ul>
</li>
</ul>


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

<p>This document describes some of the more important APIs and internal design
decisions made in the Clang C front-end.  The purpose of this document is to
both capture some of this high level information and also describe some of the
design decisions behind it.  This is meant for people interested in hacking on
Clang, not for end-users.  The description below is categorized by
libraries, and does not describe any of the clients of the libraries.</p>

<!-- ======================================================================= -->
<h2 id="libsystem">LLVM System and Support Libraries</h2>
<!-- ======================================================================= -->

<p>The LLVM libsystem library provides the basic Clang system abstraction layer,
which is used for file system access.  The LLVM libsupport library provides many
underlying libraries and <a 
href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>,
 including command line option
processing and various containers.</p>

<!-- ======================================================================= -->
<h2 id="libbasic">The Clang 'Basic' Library</h2>
<!-- ======================================================================= -->

<p>This library certainly needs a better name.  The 'basic' library contains a
number of low-level utilities for tracking and manipulating source buffers,
locations within the source buffers, diagnostics, tokens, target abstraction,
and information about the subset of the language being compiled for.</p>

<p>Part of this infrastructure is specific to C (such as the TargetInfo class),
other parts could be reused for other non-C-based languages (SourceLocation,
SourceManager, Diagnostics, FileManager).  When and if there is future demand
we can figure out if it makes sense to introduce a new library, move the general
classes somewhere else, or introduce some other solution.</p>

<p>We describe the roles of these classes in order of their dependencies.</p>


<!-- ======================================================================= -->
<h3 id="Diagnostics">The Diagnostics Subsystem</h3>
<!-- ======================================================================= -->

<p>The Clang Diagnostics subsystem is an important part of how the compiler
communicates with the human.  Diagnostics are the warnings and errors produced
when the code is incorrect or dubious.  In Clang, each diagnostic produced has
(at the minimum) a unique ID, a <a href="#SourceLocation">SourceLocation</a> to
"put the caret", an English translation associated with it, and a severity (e.g.
<tt>WARNING</tt> or <tt>ERROR</tt>).  They can also optionally include a number
of arguments to the dianostic (which fill in "%0"'s in the string) as well as a
number of source ranges that related to the diagnostic.</p>

<p>In this section, we'll be giving examples produced by the Clang command line
driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many
different ways</a> depending on how the DiagnosticClient interface is
implemented.  A representative example of a diagonstic is:</p>

<pre>
t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
   <font color="darkgreen">P = (P-42) + Gamma*4;</font>
       <font color="blue">~~~~~~ ^ ~~~~~~~</font>
</pre>