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<html>
<head>
<title>"clang" CFE Internals Manual</title>
<link type="text/css" rel="stylesheet" href="../menu.css" />
<link type="text/css" rel="stylesheet" href="../content.css" />
</head>
<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="#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="SourceLocation">The SourceLocation and SourceManager classes</h3>
<!-- ======================================================================= -->
<p>Strangely enough, the SourceLocation class represents a location within the
source code of the program. Important design points include:</p>
<ol>
<li>sizeof(SourceLocation) must be extremely small, as these are embedded into
many AST nodes and are passed around often. Currently it is 32 bits.</li>
<li>SourceLocation must be a simple value object that can be efficiently
copied.</li>
<li>We should be able to represent a source location for any byte of any input
file. This includes in the middle of tokens, in whitespace, in trigraphs,
etc.</li>
<li>A SourceLocation must encode the current #include stack that was active when
the location was processed. For example, if the location corresponds to a
token, it should contain the set of #includes active when the token was
lexed. This allows us to print the #include stack for a diagnostic.</li>
<li>SourceLocation must be able to describe macro expansions, capturing both
the ultimate instantiation point and the source of the original character
data.</li>
</ol>
<p>In practice, the SourceLocation works together with the SourceManager class
to encode two pieces of information about a location: it's physical location
and it's virtual location. For most tokens, these will be the same. However,
for a macro expansion (or tokens that came from a _Pragma directive) these will
describe the location of the characters corresponding to the token and the
location where the token was used (i.e. the macro instantiation point or the
location of the _Pragma itself).</p>
<p>For efficiency, we only track one level of macro instantions: if a token was
produced by multiple instantiations, we only track the source and ultimate
destination. Though we could track the intermediate instantiation points, this
would require extra bookkeeping and no known client would benefit substantially
from this.</p>
<p>The clang front-end inherently depends on the location of a token being
tracked correctly. If it is ever incorrect, the front-end may get confused and
die. The reason for this is that the notion of the 'spelling' of a Token in
clang depends on being able to find the original input characters for the token.
This concept maps directly to the "physical" location for the token.</p>
<!-- ======================================================================= -->
<h2 id="liblex">The Lexer and Preprocessor Library</h2>
<!-- ======================================================================= -->
<p>The Lexer library contains several tightly-connected classes that are involved
with the nasty process of lexing and preprocessing C source code. The main
interface to this library for outside clients is the large <a
href="#Preprocessor">Preprocessor</a> class.
It contains the various pieces of state that are required to coherently read
tokens out of a translation unit.</p>
<p>The core interface to the Preprocessor object (once it is set up) is the
Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from
the preprocessor stream. There are two types of token providers that the
preprocessor is capable of reading from: a buffer lexer (provided by the <a
href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a
href="#TokenLexer">TokenLexer</a> class).
<!-- ======================================================================= -->
<h3 id="Token">The Token class</h3>
<!-- ======================================================================= -->
<p>The Token class is used to represent a single lexed token. Tokens are
intended to be used by the lexer/preprocess and parser libraries, but are not
intended to live beyond them (for example, they should not live in the ASTs).<p>
<p>Tokens most often live on the stack (or some other location that is efficient
to access) as the parser is running, but occasionally do get buffered up. For
example, macro definitions are stored as a series of tokens, and the C++
front-end will eventually need to buffer tokens up for tentative parsing and
various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit
system, sizeof(Token) is currently 16 bytes.</p>
<p>Tokens contain the following information:</p>
<ul>
<li><b>A SourceLocation</b> - This indicates the location of the start of the
token.</li>
<li><b>A length</b> - This stores the length of the token as stored in the
SourceBuffer. For tokens that include them, this length includes trigraphs and
escaped newlines which are ignored by later phases of the compiler. By pointing
into the original source buffer, it is always possible to get the original
spelling of a token completely accurately.</li>
<li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if
identifier lookup was enabled when the token was lexed (e.g. the lexer was not
reading in 'raw' mode) this contains a pointer to the unique hash value for the
identifier. Because the lookup happens before keyword identification, this
field is set even for language keywords like 'for'.</li>
<li><b>TokenKind</b> - This indicates the kind of token as classified by the
lexer. This includes things like <tt>tok::starequal</tt> (for the "*="
operator), <tt>tok::ampamp</tt> for the "&&" token, and keyword values
(e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note
that some tokens can be spelled multiple ways. For example, C++ supports
"operator keywords", where things like "and" are treated exactly like the
"&&" operator. In these cases, the kind value is set to
<tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to
consider both forms. For something that cares about which form is used (e.g.
the preprocessor 'stringize' operator) the spelling indicates the original
form.</li>
<li><b>Flags</b> - There are currently four flags tracked by the
lexer/preprocessor system on a per-token basis:
<ol>
<li><b>StartOfLine</b> - This was the first token that occurred on its input
source line.</li>
<li><b>LeadingSpace</b> - There was a space character either immediately
before the token or transitively before the token as it was expanded
through a macro. The definition of this flag is very closely defined by
the stringizing requirements of the preprocessor.</li>
<li><b>DisableExpand</b> - This flag is used internally to the preprocessor to
represent identifier tokens which have macro expansion disabled. This
prevents them from being considered as candidates for macro expansion ever
in the future.</li>
<li><b>NeedsCleaning</b> - This flag is set if the original spelling for the
token includes a trigraph or escaped newline. Since this is uncommon,
many pieces of code can fast-path on tokens that did not need cleaning.
</p>
</ol>
</li>
</ul>
<p>One interesting (and somewhat unusual) aspect of tokens is that they don't
contain any semantic information about the lexed value. For example, if the
token was a pp-number token, we do not represent the value of the number that
was lexed (this is left for later pieces of code to decide). Additionally, the
lexer library has no notion of typedef names vs variable names: both are
returned as identifiers, and the parser is left to decide whether a specific
identifier is a typedef or a variable (tracking this requ
|
