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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" />
<style type="text/css">
td {
vertical-align: top;
}
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</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="#Diagnostics">The Diagnostics Subsystem</a></li>
<li><a href="#SourceLocation">The SourceLocation and SourceManager
classes</a></li>
</ul>
</li>
<li><a href="#libdriver">The Driver Library</a>
<ul>
</ul>
</li>
<li><a href="#pch">Precompiled Headers</a>
<li><a href="#libfrontend">The Frontend Library</a>
<ul>
</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="#AnnotationToken">Annotation Tokens</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="#DeclContext">Declaration contexts</a>
<ul>
<li><a href="#Redeclarations">Redeclarations and Overloads</a></li>
<li><a href="#LexicalAndSemanticContexts">Lexical and Semantic
Contexts</a></li>
<li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li>
<li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li>
</ul>
</li>
<li><a href="#CFG">The CFG class</a></li>
<li><a href="#Constants">Constant Folding in the Clang AST</a></li>
</ul>
</li>
<li><a href="libIndex.html">The Index Library</a></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>
<p>In this example, you can see the English translation, the severity (error),
you can see the source location (the caret ("^") and file/line/column info),
the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex
float"). You'll have to believe me that there is a unique ID backing the
diagnostic :).</p>
<p>Getting all of this to happen has several steps and involves many moving
pieces, this section describes them and talks about best practices when adding
a new diagnostic.</p>
<!-- ============================== -->
<h4>The Diagnostic*Kinds.def files</h4>
<!-- ============================== -->
<p>Diagnostics are created by adding an entry to one of the <tt>
clang/Basic/Diagnostic*Kinds.def</tt> files, depending on what library will
be using it. This file encodes the unique ID of the
diagnostic (as an enum, the first argument), the severity of the diagnostic
(second argument) and the English translation + format string.</p>
<p>There is little sanity with the naming of the unique ID's right now. Some
start with err_, warn_, ext_ to encode the severity into the name. Since the
enum is referenced in the C++ code that produces the diagnostic, it is somewhat
useful for it to be reasonably short.</p>
<p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>,
<tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The
<tt>ERROR</tt> severity is used for diagnostics indicating the program is never
acceptable under any circumstances. When an error is emitted, the AST for the
input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt>
severities are used for extensions to the language that Clang accepts. This
means that Clang fully understands and can represent them in the AST, but we
produce diagnostics to tell the user their code is non-portable. The difference
is that the former are ignored by default, and the later warn by default. The
<tt>WARNING</tt> severity is used for constructs that are valid in the currently
selected source language but that are dubious in some way. The <tt>NOTE</tt>
level is used to staple more information onto previous diagnostics.</p>
<p>These <em>severities</em> are mapped into a smaller set (the
Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>,
<tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics
subsystem based on various configuration options. Clang internally supports a
fully fine grained mapping mechanism that allows you to map almost any
diagnostic to the output level that you want. The only diagnostics that cannot
be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously
emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to
<tt>Fatal</tt> (it is not possible to turn an error into a warning,
for example).</p>
<p>Diagnostic mappings are used in many ways. For example, if the user
specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if
they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is
used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc.
</p>
<p>
Mapping to <tt>Fatal</tt> should only be used for diagnostics that are
considered so severe that error recovery won't be able to recover sensibly from
them (thus spewing a ton of bogus errors). One example of this class of error
are failure to #include a file.
</p>
<!-- ================= -->
<h4>The Format String</h4>
<!-- ================= -->
<p>The format string for the diagnostic is very simple, but it has some power.
It takes the form of a string in English with markers that indicate where and
how arguments to the diagnostic are inserted and formatted. For example, here
are some simple format strings:</p>
<pre>
"binary integer literals are an extension"
"format string contains '\\0' within the string body"
"more '<b>%%</b>' conversions than data arguments"
"invalid operands to binary expression (<b>%0</b> and <b>%1</b>)"
"overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator"
" (has <b>%1</b> parameter<b>%s1</b>)"
</pre>
<p>These examples show some important points of format strings. You can use any
plain ASCII character in the diagnostic string except "%" without a problem,
but these are C strings, so you have to use and be aware of all the C escape
sequences (as in the second example). If you want to produce a "%" in the
output, use the "%%" escape sequence, like the third diagnostic. Finally,
Clang uses the "%...[digit]" sequences to specify where and how arguments to
the diagnostic are formatted.</p>
<p>Arguments to the diagnostic are numbered according to how they are specified
by the C++ code that <a href="#producingdiag">produces them</a>, and are
referenced by <tt>%0</tt> .. <tt>%9</tt>. If you have more than 10 arguments
to your diagnostic, you are doing something wrong :). Unlike printf, there
is no requirement that arguments to the diagnostic end up in the output in
the same order as they are specified, you could have a format string with
<tt>"%1 %0"</tt> that swaps them, for example. The text in between the
percent and digit are formatting instructions. If there are no instructions,
the argument is just turned into a string and substituted in.</p>
<p>Here are some "best practices" for writing the English format string:</p>
<ul>
<li>Keep the string short. It should ideally fit in the 80 column limit of the
<tt>DiagnosticKinds.def</tt> file. This avoids the diagnostic wrapping when
printed, and forces you to think about the important point you are conveying
with the diagnostic.</li>
<li>Take advantage of location information. The user will be able to see the
line and location of the caret, so you don't need to tell them that the
problem is with the 4th argument to the function: just point to it.</li>
<li>Do not capitalize the diagnostic string, and do not end it with a
period.</li>
<li>If you need to quote something in the diagnostic string, use single
quotes.</li>
</ul>
<p>Diagnostics should never take random English strings as arguments: you
shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like
<tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing
this prevents <a href="translation">translating</a> the Clang diagnostics to
other languages (because they'll get random English words in their otherwise
localized diagnostic). The exceptions to this are C/C++ language keywords
(e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note
that things like "pointer" and "reference" are not keywords. On the other
hand, you <em>can</em> include anything that comes from the user's source code,
including variable names, types, labels, etc. The 'select' format can be
used to achieve this sort of thing in a localizable way, see below.</p>
<!-- ==================================== -->
<h4>Formatting a Diagnostic Argument</a></h4>
<!-- ==================================== -->
<p>Arguments to diagnostics are fully typed internally, and come from a couple
different classes: integers, types, names, and random strings. Depending on
the class of the argument, it can be optionally formatted in different ways.
This gives the DiagnosticClient information about what the argument means
without requiring it to use a specific presentation (consider this MVC for
Clang :).</p>
<p>Here are the different diagnostic argument formats currently supported by
Clang:</p>
<table>
<tr><td colspan="2"><b>"s" format</b></td></tr>
<tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr>
<tr><td>Class:</td><td>Integers</td></tr>
<tr><td>Description:</td><td>This is a simple formatter for integers that is
useful when producing English diagnostics. When the integer is 1, it prints
as nothing. When the integer is not 1, it prints as "s". This allows some
simple grammatical forms to be to be handled correctly, and eliminates the
need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr>
<tr><td colspan="2"><b>"select" format</b></td></tr>
<tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2
operator"</tt></td></tr>
<tr><td>Class:</td><td>Integers</td></tr>
<tr><td>Description:</td><td><p>This format specifier is used to merge multiple
related diagnostics together into one common one, without requiring the
difference to be specified as an English string argument. Instead of
specifying the string, the diagnostic gets an integer argument and the
format string selects the numbered option. In this case, the "%2" value
must be an integer in the range [0..2]. If it is 0, it prints 'unary', if
it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This
allows other language translations to substitute reasonable words (or entire
phrases) based on the semantics of the diagnostic instead of having to do
things textually.</p>
<p>The selected string does undergo formatting.</p></td></tr>
<tr><td colspan="2"><b>"plural" format</b></td></tr>
<tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to
your computer"</tt></td></tr>
<tr><td>Class:</td><td>Integers</td></tr>
<tr><td>Description:</td><td><p>This is a formatter for complex plural forms.
It is designed to handle even the requirements of languages with very
complex plural forms, as many Baltic languages have. The argument consists
of a series of expression/form pairs, separated by ':', where the first form
whose expression evaluates to true is the result of the modifier.</p>
<p>An expression can be empty, in which case it is always true. See the
example at the top. Otherwise, it is a series of one or more numeric
conditions, separated by ','. If any condition matches, the expression
matches. Each numeric condition can take one of three forms.</p>
<ul>
<li>number: A simple decimal number matches if the argument is the same
as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li>
<li>range: A range in square brackets matches if the argument is within
the range. Then range is inclusive on both ends. Example:
<tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li>
<li>modulo: A modulo operator is followed by a number, and
equals sign and either a number or a range. The tests are the
same as for plain
numbers and ranges, but the argument is taken modulo the number first.
Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything
else}1"</tt></li>
</ul>
<p>The parser is very unforgiving. A syntax error, even whitespace, will
abort, as will a failure to match the argument against any
expression.</p></td></tr>
<tr><td colspan="2"><b>"ordinal" format</b></td></tr>
<tr><td>Example:</td><td><tt>"ambiguity in %ordinal0 argument"</tt></td></tr>
<tr><td>Class:</td><td>Integers</td></tr>
<tr><td>Description:</td><td><p>This is a formatter which represents the
argument number as an ordinal: the value <tt>1</tt> becomes <tt>1st</tt>,
<tt>3</tt> becomes <tt>3rd</tt>, and so on. Values less than <tt>1</tt>
are not supported.</p>
<p>This formatter is currently hard-coded to use English ordinals.</p></td></tr>
<tr><td colspan="2"><b>"objcclass" format</b></td></tr>
<tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr>
<tr><td>Class:</td><td>DeclarationName</td></tr>
<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
DeclarationName corresponds to an Objective-C class method selector. As
such, it prints the selector with a leading '+'.</p></td></tr>
<tr><td colspan="2"><b>"objcinstance" format</b></td></tr>
<tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr>
<tr><td>Class:</td><td>DeclarationName</td></tr>
<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
DeclarationName corresponds to an Objective-C instance method selector. As
such, it prints the selector with a leading '-'.</p></td></tr>
<tr><td colspan="2"><b>"q" format</b></td></tr>
<tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr>
<tr><td>Class:</td><td>NamedDecl*</td></tr>
<tr><td>Description</td><td><p>This formatter indicates that the fully-qualified name of the declaration should be printed, e.g., "std::vector" rather than "vector".</p></td></tr>
</table>
<p>It is really easy to add format specifiers to the Clang diagnostics system,
but they should be discussed before they are added. If you are creating a lot
of repetitive diagnostics and/or have an idea for a useful formatter, please
bring it up on the cfe-dev mailing list.</p>
<!-- ===================================================== -->
<h4><a name="#producingdiag">Producing the Diagnostic</a></h4>
<!-- ===================================================== -->
<p>Now that you've created the diagnostic in the DiagnosticKinds.def file, you
need to write the code that detects the condition in question and emits the
new diagnostic. Various components of Clang (e.g. the preprocessor, Sema,
etc) provide a helper function named "Diag". It creates a diagnostic and
accepts the arguments, ranges, and other information that goes along with
it.</p>
<p>For example, the binary expression error comes from code like this:</p>
<pre>
if (various things that are bad)
Diag(Loc, diag::err_typecheck_invalid_operands)
<< lex->getType() << rex->getType()
<< lex->getSourceRange() << rex->getSourceRange();
</pre>
<p>This shows that use of the Diag method: they take a location (a <a
href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value
(which matches the name from DiagnosticKinds.def). If the diagnostic takes
arguments, they are specified with the << operator: the first argument
becomes %0, the second becomes %1, etc. The diagnostic interface allows you to
specify arguments of many different types, including <tt>int</tt> and
<tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and
<tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and
<tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc.
SourceRanges are also specified with the << operator, but do not have a
specific ordering requirement.</p>
<p>As you can see, adding and producing a diagnostic is pretty straightforward.
The hard part is deciding exactly what you need to say to help the user, picking
a suitable wording, and providing the information needed to format it correctly.
The good news is that the call site that issues a diagnostic should be
completely independent of how the diagnostic is formatted and in what language
it is rendered.
</p>
<!-- ==================================================== -->
<h4 id="code-modification-hints">Code Modification Hints</h4>
<!-- ==================================================== -->
<p>In some cases, the front end emits diagnostics when it is clear
that some small change to the source code would fix the problem. For
example, a missing semicolon at the end of a statement or a use of
deprecated syntax that is easily rewritten into a more modern form.
Clang tries very hard to emit the diagnostic and recover gracefully
in these and other cases.</p>
<p>However, for these cases where the fix is obvious, the diagnostic
can be annotated with a code
modification "hint" that describes how to change the code referenced
by the diagnostic to fix the problem. For example, it might add the
missing semicolon at the end of the statement or rewrite the use of a
deprecated construct into something more palatable. Here is one such
example C++ front end, where we warn about the right-shift operator
changing meaning from C++98 to C++0x:</p>
<pre>
test.cpp:3:7: warning: use of right-shift operator ('>>') in template argument will require parentheses in C++0x
A<100 >> 2> *a;
^
( )
</pre>
<p>Here, the code modification hint is suggesting that parentheses be
added, and showing exactly where those parentheses would be inserted
into the source code. The code modification hints themselves describe
what changes to make to the source code in an abstract manner, which
the text diagnostic printer renders as a line of "insertions" below
the caret line. <a href="#DiagnosticClient">Other diagnostic
clients</a> might choose to render the code differently (e.g., as
markup inline) or even give the user the ability to automatically fix
the problem.</p>
<p>All code modification hints are described by the
<code>CodeModificationHint</code> class, instances of which should be
attached to the diagnostic using the << operator in the same way
that highlighted source ranges and arguments are passed to the
diagnostic. Code modification hints can be created with one of three
constructors:</p>
<dl>
<dt><code>CodeModificationHint::CreateInsertion(Loc, Code)</code></dt>
<dd>Specifies that the given <code>Code</code> (a string) should be inserted
before the source location <code>Loc</code>.</dd>
<dt><code>CodeModificationHint::CreateRemoval(Range)</code></dt>
<dd>Specifies that the code in the given source <code>Range</code>
should be removed.</dd>
<dt><code>CodeModificationHint::CreateReplacement(Range, Code)</code></dt>
<dd>Specifies that the code in the given source <code>Range</code>
should be removed, and replaced with the given <code>Code</code> string.</dd>
</dl>
<!-- ============================================================= -->
<h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4>
<!-- ============================================================= -->
<p>Once code generates a diagnostic with all of the arguments and the rest of
the relevant information, Clang needs to know what to do with it. As previously
mentioned, the diagnostic machinery goes through some filtering to map a
severity onto a diagnostic level, then (assuming the diagnostic is not mapped to
"<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient
interface with the information.</p>
<p>It is possible to implement this interface in many different ways. For
example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns
the arguments into strings (according to the various formatting rules), prints
out the file/line/column information and the string, then prints out the line of
code, the source ranges, and the caret. However, this behavior isn't required.
</p>
<p>Another implementation of the DiagnosticClient interface is the
'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode.
Instead of formatting and printing out the diagnostics, this implementation just
captures and remembers the diagnostics as they fly by. Then -verify compares
the list of produced diagnostics to the list of expected ones. If they disagree,
it prints out its own output.
</p>
<p>There are many other possible implementations of this interface, and this is
why we prefer diagnostics to pass down rich structured information in arguments.
For example, an HTML output might want declaration names be linkified to where
they come from in the source. Another example is that a GUI might let you click
on typedefs to expand them. This application would want to pass significantly
more information about types through to the GUI than a simple flat string. The
interface allows this to happen.</p>
<!-- ====================================================== -->
<h4><a name="translation">Adding Translations to Clang</a></h4>
<!-- ====================================================== -->
<p>Not possible yet! Diagnostic strings should be written in UTF-8, the client
can translate to the relevant code page if needed. Each translation completely
replaces the format string for the diagnostic.</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 spelling location
and it's instantiation 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>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 "spelling location" for the token.</p>
<!-- ======================================================================= -->
<h2 id="libdriver">The Driver Library</h2>
<!-- ======================================================================= -->
<p>The clang Driver and library are documented <a
href="DriverInternals.html">here<a>.<p>
<!-- ======================================================================= -->
<h2 id="pch">Precompiled Headers</h2>
<!-- ======================================================================= -->
<p>Clang supports two implementations of precompiled headers. The
default implementation, precompiled headers (<a
href="PCHInternals.html">PCH</a>) uses a serialized representation
of Clang's internal data structures, encoded with the <a
href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream
format</a>. Pretokenized headers (<a
href="PTHInternals.html">PTH</a>), on the other hand, contain a
serialized representation of the tokens encountered when
preprocessing a header (and anything that header includes).</p>
<!-- ======================================================================= -->
<h2 id="libfrontend">The Frontend Library</h2>
<!-- ======================================================================= -->
<p>The Frontend library contains functionality useful for building
tools on top of the clang libraries, for example several methods for
outputting diagnostics.</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 periodically needs 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 occur in two forms: "<a href="#AnnotationToken">Annotation
Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer,
annotation tokens represent semantic information and are produced by the parser,
replacing normal tokens in the token stream. Normal 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 normal 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 requires scope information
among other things). The parser can do this translation by replacing tokens
returned by the preprocessor with "Annotation Tokens".</p>
<!-- ======================================================================= -->
<h3 id="AnnotationToken">Annotation Tokens</h3>
<!-- ======================================================================= -->
<p>Annotation Tokens are tokens that are synthesized by the parser and injected
into the preprocessor's token stream (replacing existing tokens) to record
semantic information found by the parser. For example, if "foo" is found to be
a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an
<tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this
makes it easy to handle qualified type names (e.g. "foo::bar::baz<42>::t")
in C++ as a single "token" in the parser. 2) if the parser backtracks, the
reparse does not need to redo semantic analysis to determine whether a token
sequence is a variable, type, template, etc.</p>
<p>Annotation Tokens are created by the parser and reinjected into the parser's
token stream (when backtracking is enabled). Because they can only exist in
tokens that the preprocessor-proper is done with, it doesn't need to keep around
flags like "start of line" that the preprocessor uses to do its job.
Additionally, an annotation token may "cover" a sequence of preprocessor tokens
(e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields
of an annotation token are different than the fields for a normal token (but
they are multiplexed into the normal Token fields):</p>
<ul>
<li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation
token indicates the first token replaced by the annotation token. In the example
above, it would be the location of the "a" identifier.</li>
<li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the
last token replaced with the annotation token. In the example above, it would
be the location of the "c" identifier.</li>
<li><b>void* "AnnotationValue"</b> - This contains an opaque object that the
parser gets from Sema through an Actions module, it is passed around and Sema
intepretes it, based on the type of annotation token.</li>
<li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this
is. See below for the different valid kinds.</li>
</ul>
<p>Annotation tokens currently come in three kinds:</p>
<ol>
<li><b>tok::annot_typename</b>: This annotation token represents a
resolved typename token that is potentially qualified. The AnnotationValue
field contains a pointer returned by Action::getTypeName(). In the case of the
Sema actions module, this is a <tt>Decl*</tt> for the type.</li>
<li><b>tok::annot_cxxscope</b>: This annotation token represents a C++ scope
specifier, such as "A::B::". This corresponds to the grammar productions "::"
and ":: [opt] nested-name-specifier". The AnnotationValue pointer is returned
by the Action::ActOnCXXGlobalScopeSpecifier and
Action::ActOnCXXNestedNameSpecifier callbacks. In the case of Sema, this is a
<tt>DeclContext*</tt>.</li>
<li><b>tok::annot_template_id</b>: This annotation token represents a
C++ template-id such as "foo<int, 4>", where "foo" is the name
of a template. The AnnotationValue pointer is a pointer to a malloc'd
TemplateIdAnnotation object. Depending on the context, a parsed template-id that names a type might become a typename annotation token (if all we care about is the named type, e.g., because it occurs in a type specifier) or might remain a template-id token (if we want to retain more source location information or produce a new type, e.g., in a declaration of a class template specialization). template-id annotation tokens that refer to a type can be "upgraded" to typename annotation tokens by the parser.</li>
</ol>
<p>As mentioned above, annotation tokens are not returned by the preprocessor,
they are formed on demand by the parser. This means that the parser has to be
aware of cases where an annotation could occur and form it where appropriate.
This is somewhat similar to how the parser handles Translation Phase 6 of C99:
String Concatenation (see C99 5.1.1.2). In the case of string concatenation,
the preprocessor just returns distinct tok::string_literal and
tok::wide_string_literal tokens and the parser eats a sequence of them wherever
the grammar indicates that a string literal can occur.</p>
<p>In order to do this, whenever the parser expects a tok::identifier or
tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or
TryAnnotateCXXScopeToken methods to form the annotation token. These methods
will maximally form the specified annotation tokens and replace the current
token with them, if applicable. If the current tokens is not valid for an
annotation token, it will remain an identifier or :: token.</p>
<!-- ======================================================================= -->
<h3 id="Lexer">The Lexer class</h3>
<!-- ======================================================================= -->
<p>The Lexer class provides the mechanics of lexing tokens out of a source
buffer and deciding what they mean. The Lexer is complicated by the fact that
it operates on raw buffers that have not had spelling eliminated (this is a
necessity to get decent performance), but this is countered with careful coding
as well as standard performance techniques (for example, the comment handling
code is vectorized on X86 and PowerPC hosts).</p>
<p>The lexer has a couple of interesting modal features:</p>
<ul>
<li>The lexer can operate in 'raw' mode. This mode has several features that
make it possible to quickly lex the file (e.g. it stops identifier lookup,
doesn't specially handle preprocessor tokens, handles EOF differently, etc).
This mode is used for lexing within an "<tt>#if 0</tt>" block, for
example.</li>
<li>The lexer can capture and return comments as tokens. This is required to
support the -C preprocessor mode, which passes comments through, and is
used by the diagnostic checker to identifier expect-error annotations.</li>
<li>The lexer can be in ParsingFilename mode, which happens when preprocessing
after reading a #include directive. This mode changes the parsing of '<'
to return an "angled string" instead of a bunch of tokens for each thing
within the filename.</li>
<li>When parsing a preprocessor directive (after "<tt>#</tt>") the
ParsingPreprocessorDirective mode is entered. This changes the parser to
return EOM at a newline.</li>
<li>The Lexer uses a LangOptions object to know whether trigraphs are enabled,
whether C++ or ObjC keywords are recognized, etc.</li>
</ul>
<p>In addition to these modes, the lexer keeps track of a couple of other
features that are local to a lexed buffer, which change as the buffer is
lexed:</p>
<ul>
<li>The Lexer uses BufferPtr to keep track of the current character being
lexed.</li>
<li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token
will start with its "start of line" bit set.</li>
<li>The Lexer keeps track of the current #if directives that are active (which
can be nested).</li>
<li>The Lexer keeps track of an <a href="#MultipleIncludeOpt">
MultipleIncludeOpt</a> object, which is used to
detect whether the buffer uses the standard "<tt>#ifndef XX</tt> /
<tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does,
subsequent includes can be ignored if the XX macro is defined.</li>
</ul>
<!-- ======================================================================= -->
<h3 id="TokenLexer">The TokenLexer class</h3>
<!-- ======================================================================= -->
<p>The TokenLexer class is a token provider that returns tokens from a list
of tokens that came from somewhere else. It typically used for two things: 1)
returning tokens from a macro definition as it is being expanded 2) returning
tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and
will most likely be used to handle unbounded look-ahead for the C++ parser.</p>
<!-- ======================================================================= -->
<h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3>
<!-- ======================================================================= -->
<p>The MultipleIncludeOpt class implements a really simple little state machine
that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>"
idiom that people typically use to prevent multiple inclusion of headers. If a
buffer uses this idiom and is subsequently #include'd, the preprocessor can
simply check to see whether the guarding condition is defined or not. If so,
the preprocessor can completely ignore the include of the header.</p>
<!-- ======================================================================= -->
<h2 id="libparse">The Parser Library</h2>
<!-- ======================================================================= -->
<!-- ======================================================================= -->
<h2 id="libast">The AST Library</h2>
<!-- ======================================================================= -->
<!-- ======================================================================= -->
<h3 id="Type">The Type class and its subclasses</h3>
<!-- ======================================================================= -->
<p>The Type class (and its subclasses) are an important part of the AST. Types
are accessed through the ASTContext class, which implicitly creates and uniques
them as they are needed. Types have a couple of non-obvious features: 1) they
do not capture type qualifiers like const or volatile (See
<a href="#QualType">QualType</a>), and 2) they implicitly capture typedef
information. Once created, types are immutable (unlike decls).</p>
<p>Typedefs in C make semantic analysis a bit more complex than it would
be without them. The issue is that we want to capture typedef information
and represent it in the AST perfectly, but the semantics of operations need to
"see through" typedefs. For example, consider this code:</p>
<code>
void func() {<br>
typedef int foo;<br>
foo X, *Y;<br>
typedef foo* bar;<br>
bar Z;<br>
*X; <i>// error</i><br>
**Y; <i>// error</i><br>
**Z; <i>// error</i><br>
}<br>
</code>
<p>The code above is illegal, and thus we expect there to be diagnostics emitted
on the annotated lines. In this example, we expect to get:</p>
<pre>
<b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b>
*X; // error
<font color="blue">^~</font>
<b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b>
**Y; // error
<font color="blue">^~~</font>
<b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b>
**Z; // error
<font color="blue">^~~</font>
</pre>
<p>While this example is somewhat silly, it illustrates the point: we want to
retain typedef information where possible, so that we can emit errors about
"<tt>std::string</tt>" instead of "<tt>std::basic_string<char, std:...</tt>".
Doing this requires properly keeping typedef information (for example, the type
of "X" is "foo", not "int"), and requires properly propagating it through the
various operators (for example, the type of *Y is "foo", not "int"). In order
to retain this information, the type of these expressions is an instance of the
TypedefType class, which indicates that the type of these expressions is a
typedef for foo.
</p>
<p>Representing types like this is great for diagnostics, because the
user-specified type is always immediately available. There are two problems
with this: first, various semantic checks need to make judgements about the
<em>actual structure</em> of a type, ignoring typdefs. Second, we need an
efficient way to query whether two types are structurally identical to each
other, ignoring typedefs. The solution to both of these problems is the idea of
canonical types.</p>
<!-- =============== -->
<h4>Canonical Types</h4>
<!-- =============== -->
<p>Every instance of the Type class contains a canonical type pointer. For
simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>",
"<tt>int**</tt>"), the type just points to itself. For types that have a
typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>",
"<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their
structurally equivalent type without any typedefs (e.g. "<tt>int</tt>",
"<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p>
<p>This design provides a constant time operation (dereferencing the canonical
type pointer) that gives us access to the structure of types. For example,
we can trivially tell that "bar" and "foo*" are the same type by dereferencing
their canonical type pointers and doing a pointer comparison (they both point
to the single "<tt>int*</tt>" type).</p>
<p>Canonical types and typedef types bring up some complexities that must be
carefully managed. Specifically, the "isa/cast/dyncast" operators generally
shouldn't be used in code that is inspecting the AST. For example, when type
checking the indirection operator (unary '*' on a pointer), the type checker
must verify that the operand has a pointer type. It would not be correct to
check that with "<tt>isa<PointerType>(SubExpr->getType())</tt>",
because this predicate would fail if the subexpression had a typedef type.</p>
<p>The solution to this problem are a set of helper methods on Type, used to
check their properties. In this case, it would be correct to use
"<tt>SubExpr->getType()->isPointerType()</tt>" to do the check. This
predicate will return true if the <em>canonical type is a pointer</em>, which is
true any time the type is structurally a pointer type. The only hard part here
is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p>
<p>The second problem we face is how to get access to the pointer type once we
know it exists. To continue the example, the result type of the indirection
operator is the pointee type of the subexpression. In order to determine the
type, we need to get the instance of PointerType that best captures the typedef
information in the program. If the type of the expression is literally a
PointerType, we can return that, otherwise we have to dig through the
typedefs to find the pointer type. For example, if the subexpression had type
"<tt>foo*</tt>", we could return that type as the result. If the subexpression
had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do
<em>not</em> want "<tt>int*</tt>"). In order to provide all of this, Type has
a getAsPointerType() method that checks whether the type is structurally a
PointerType and, if so, returns the best one. If not, it returns a null
pointer.</p>
<p>This structure is somewhat mystical, but after meditating on it, it will
make sense to you :).</p>
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<h3 id="QualType">The QualType class</h3>
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<p>The QualType class is designed as a trivial value class that is small,
passed by-value and is efficient to query. The idea of QualType is that it
stores the type qualifiers (const, volatile, restrict) separately from the types
themselves: QualType is conceptually a pair of "Type*" and bits for the type
qualifiers.</p>
<p>By storing the type qualifiers as bits in the conceptual pair, it is
extremely efficient to get the set of qualifiers on a QualType (just return the
field of the pair), add a type qualifier (which is a trivial constant-time
operation that sets a bit), and remove one or more type qualifiers (just return
a QualType with the bitfield set to empty).</p>
<p>Further, because the bits are stored outside of the type itself, we do not
need to create duplicates of types with different sets of qualifiers (i.e. there
is only a single heap allocated "int" type: "const int" and "volatile const int"
both point to the same heap allocated "int" type). This reduces the heap size
used to represent bits and also means we do not have to consider qualifiers when
uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p>
<p>In practice, on hosts where it is safe, the 3 type qualifiers are stored in
the low bit of the pointer to the Type object. This means that QualType is
exactly the same size as a pointer, and this works fine on any system where
malloc'd objects are at least 8 byte aligned.</p>
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<h3 id="DeclarationName">Declaration names</h3>
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<p>The <tt>DeclarationName</tt> class represents the name of a
declaration in Clang. Declarations in the C family of languages can
take several different forms. Most declarations are named by
simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in
the function declaration <code>f(int x)</code>. In C++, declaration
names can also name class constructors ("<code>Class</code>"
in <code>struct Class { Class(); }</code>), class destructors
("<code>~Class</code>"), overloaded operator names ("operator+"),
and conversion functions ("<code>operator void const *</code>"). In
Objective-C, declaration names can refer to the names of Objective-C
methods, which involve the method name and the parameters,
collectively called a <i>selector</i>, e.g.,
"<code>setWidth:height:</code>". Since all of these kinds of
entities - variables, functions, Objective-C methods, C++
constructors, destructors, and operators - are represented as
subclasses of Clang's common <code>NamedDecl</code>
class, <code>DeclarationName</code> is designed to efficiently