зеркало из https://github.com/microsoft/clang.git
1794 строки
86 KiB
HTML
1794 строки
86 KiB
HTML
<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;
|
|
}
|
|
</style>
|
|
</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>
|
|
<li><a href="#SourceRange">SourceRange and CharSourceRange</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>
|
|
<li><a href="#Howtos">Howto guides</a>
|
|
<ul>
|
|
<li><a href="#AddingAttributes">How to add an attribute</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, an English translation associated with it, a <a
|
|
href="#SourceLocation">SourceLocation</a> to "put the caret", 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 diagnostic 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.td files</h4>
|
|
<!-- ============================= -->
|
|
|
|
<p>Diagnostics are created by adding an entry to one of the <tt>
|
|
clang/Basic/Diagnostic*Kinds.td</tt> files, depending on what library will
|
|
be using it. From this file, tblgen generates the unique ID of the diagnostic,
|
|
the severity of the diagnostic 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.td</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 id="producingdiag">Producing the Diagnostic</h4>
|
|
<!-- ===================================================== -->
|
|
|
|
<p>Now that you've created the diagnostic in the DiagnosticKinds.td 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.td). 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: its spelling location
|
|
and its 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>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="SourceRange">SourceRange and CharSourceRange</h3>
|
|
<!-- ======================================================================= -->
|
|
<!-- mostly taken from
|
|
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-August/010595.html -->
|
|
|
|
<p>Clang represents most source ranges by [first, last], where first and last
|
|
each point to the beginning of their respective tokens. For example
|
|
consider the SourceRange of the following statement:</p>
|
|
<pre>
|
|
x = foo + bar;
|
|
^first ^last
|
|
</pre>
|
|
|
|
<p>To map from this representation to a character-based
|
|
representation, the 'last' location needs to be adjusted to point to
|
|
(or past) the end of that token with either
|
|
<code>Lexer::MeasureTokenLength()</code> or
|
|
<code>Lexer::getLocForEndOfToken()</code>. For the rare cases
|
|
where character-level source ranges information is needed we use
|
|
the <code>CharSourceRange</code> class.</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. The parser merely preserves the
|
|
information for Sema to later interpret based on the annotation token
|
|
kind.</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 the <tt>QualType</tt> returned by
|
|
Sema::getTypeName(), possibly with source location information
|
|
attached.</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 a <tt>NestedNameSpecifier*</tt> returned by
|
|
the Sema::ActOnCXXGlobalScopeSpecifier and
|
|
Sema::ActOnCXXNestedNameSpecifier callbacks.</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 EOD 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>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="QualType">The QualType class</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<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, plus some extended qualifiers required by language
|
|
extensions) separately from the types themselves. QualType is
|
|
conceptually a pair of "Type*" and the bits for these 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, the two most common type qualifiers (const and
|
|
restrict) are stored in the low bits of the pointer to the Type
|
|
object, together with a flag indicating whether extended qualifiers
|
|
are present (which must be heap-allocated). This means that QualType
|
|
is exactly the same size as a pointer.</p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="DeclarationName">Declaration names</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<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
|
|
represent any kind of name.</p>
|
|
|
|
<p>Given
|
|
a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code>
|
|
will produce a value that describes what kind of name <code>N</code>
|
|
stores. There are 8 options (all of the names are inside
|
|
the <code>DeclarationName</code> class)</p>
|
|
<dl>
|
|
<dt>Identifier</dt>
|
|
<dd>The name is a simple
|
|
identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the
|
|
corresponding <code>IdentifierInfo*</code> pointing to the actual
|
|
identifier. Note that C++ overloaded operators (e.g.,
|
|
"<code>operator+</code>") are represented as special kinds of
|
|
identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code>
|
|
function to determine whether an identifier is an overloaded
|
|
operator name.</dd>
|
|
|
|
<dt>ObjCZeroArgSelector, ObjCOneArgSelector,
|
|
ObjCMultiArgSelector</dt>
|
|
<dd>The name is an Objective-C selector, which can be retrieved as a
|
|
<code>Selector</code> instance
|
|
via <code>N.getObjCSelector()</code>. The three possible name
|
|
kinds for Objective-C reflect an optimization within
|
|
the <code>DeclarationName</code> class: both zero- and
|
|
one-argument selectors are stored as a
|
|
masked <code>IdentifierInfo</code> pointer, and therefore require
|
|
very little space, since zero- and one-argument selectors are far
|
|
more common than multi-argument selectors (which use a different
|
|
structure).</dd>
|
|
|
|
<dt>CXXConstructorName</dt>
|
|
<dd>The name is a C++ constructor
|
|
name. Use <code>N.getCXXNameType()</code> to retrieve
|
|
the <a href="#QualType">type</a> that this constructor is meant to
|
|
construct. The type is always the canonical type, since all
|
|
constructors for a given type have the same name.</dd>
|
|
|
|
<dt>CXXDestructorName</dt>
|
|
<dd>The name is a C++ destructor
|
|
name. Use <code>N.getCXXNameType()</code> to retrieve
|
|
the <a href="#QualType">type</a> whose destructor is being
|
|
named. This type is always a canonical type.</dd>
|
|
|
|
<dt>CXXConversionFunctionName</dt>
|
|
<dd>The name is a C++ conversion function. Conversion functions are
|
|
named according to the type they convert to, e.g., "<code>operator void
|
|
const *</code>". Use <code>N.getCXXNameType()</code> to retrieve
|
|
the type that this conversion function converts to. This type is
|
|
always a canonical type.</dd>
|
|
|
|
<dt>CXXOperatorName</dt>
|
|
<dd>The name is a C++ overloaded operator name. Overloaded operators
|
|
are named according to their spelling, e.g.,
|
|
"<code>operator+</code>" or "<code>operator new
|
|
[]</code>". Use <code>N.getCXXOverloadedOperator()</code> to
|
|
retrieve the overloaded operator (a value of
|
|
type <code>OverloadedOperatorKind</code>).</dd>
|
|
</dl>
|
|
|
|
<p><code>DeclarationName</code>s are cheap to create, copy, and
|
|
compare. They require only a single pointer's worth of storage in
|
|
the common cases (identifiers, zero-
|
|
and one-argument Objective-C selectors) and use dense, uniqued
|
|
storage for the other kinds of
|
|
names. Two <code>DeclarationName</code>s can be compared for
|
|
equality (<code>==</code>, <code>!=</code>) using a simple bitwise
|
|
comparison, can be ordered
|
|
with <code><</code>, <code>></code>, <code><=</code>,
|
|
and <code>>=</code> (which provide a lexicographical ordering for
|
|
normal identifiers but an unspecified ordering for other kinds of
|
|
names), and can be placed into LLVM <code>DenseMap</code>s
|
|
and <code>DenseSet</code>s.</p>
|
|
|
|
<p><code>DeclarationName</code> instances can be created in different
|
|
ways depending on what kind of name the instance will store. Normal
|
|
identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors
|
|
(<code>Selector</code>) can be implicitly converted
|
|
to <code>DeclarationName</code>s. Names for C++ constructors,
|
|
destructors, conversion functions, and overloaded operators can be retrieved from
|
|
the <code>DeclarationNameTable</code>, an instance of which is
|
|
available as <code>ASTContext::DeclarationNames</code>. The member
|
|
functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>,
|
|
<code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively,
|
|
return <code>DeclarationName</code> instances for the four kinds of
|
|
C++ special function names.</p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="DeclContext">Declaration contexts</h3>
|
|
<!-- ======================================================================= -->
|
|
<p>Every declaration in a program exists within some <i>declaration
|
|
context</i>, such as a translation unit, namespace, class, or
|
|
function. Declaration contexts in Clang are represented by
|
|
the <code>DeclContext</code> class, from which the various
|
|
declaration-context AST nodes
|
|
(<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>,
|
|
etc.) will derive. The <code>DeclContext</code> class provides
|
|
several facilities common to each declaration context:</p>
|
|
<dl>
|
|
<dt>Source-centric vs. Semantics-centric View of Declarations</dt>
|
|
<dd><code>DeclContext</code> provides two views of the declarations
|
|
stored within a declaration context. The source-centric view
|
|
accurately represents the program source code as written, including
|
|
multiple declarations of entities where present (see the
|
|
section <a href="#Redeclarations">Redeclarations and
|
|
Overloads</a>), while the semantics-centric view represents the
|
|
program semantics. The two views are kept synchronized by semantic
|
|
analysis while the ASTs are being constructed.</dd>
|
|
|
|
<dt>Storage of declarations within that context</dt>
|
|
<dd>Every declaration context can contain some number of
|
|
declarations. For example, a C++ class (represented
|
|
by <code>RecordDecl</code>) contains various member functions,
|
|
fields, nested types, and so on. All of these declarations will be
|
|
stored within the <code>DeclContext</code>, and one can iterate
|
|
over the declarations via
|
|
[<code>DeclContext::decls_begin()</code>,
|
|
<code>DeclContext::decls_end()</code>). This mechanism provides
|
|
the source-centric view of declarations in the context.</dd>
|
|
|
|
<dt>Lookup of declarations within that context</dt>
|
|
<dd>The <code>DeclContext</code> structure provides efficient name
|
|
lookup for names within that declaration context. For example,
|
|
if <code>N</code> is a namespace we can look for the
|
|
name <code>N::f</code>
|
|
using <code>DeclContext::lookup</code>. The lookup itself is
|
|
based on a lazily-constructed array (for declaration contexts
|
|
with a small number of declarations) or hash table (for
|
|
declaration contexts with more declarations). The lookup
|
|
operation provides the semantics-centric view of the declarations
|
|
in the context.</dd>
|
|
|
|
<dt>Ownership of declarations</dt>
|
|
<dd>The <code>DeclContext</code> owns all of the declarations that
|
|
were declared within its declaration context, and is responsible
|
|
for the management of their memory as well as their
|
|
(de-)serialization.</dd>
|
|
</dl>
|
|
|
|
<p>All declarations are stored within a declaration context, and one
|
|
can query
|
|
information about the context in which each declaration lives. One
|
|
can retrieve the <code>DeclContext</code> that contains a
|
|
particular <code>Decl</code>
|
|
using <code>Decl::getDeclContext</code>. However, see the
|
|
section <a href="#LexicalAndSemanticContexts">Lexical and Semantic
|
|
Contexts</a> for more information about how to interpret this
|
|
context information.</p>
|
|
|
|
<h4 id="Redeclarations">Redeclarations and Overloads</h4>
|
|
<p>Within a translation unit, it is common for an entity to be
|
|
declared several times. For example, we might declare a function "f"
|
|
and then later re-declare it as part of an inlined definition:</p>
|
|
|
|
<pre>
|
|
void f(int x, int y, int z = 1);
|
|
|
|
inline void f(int x, int y, int z) { /* ... */ }
|
|
</pre>
|
|
|
|
<p>The representation of "f" differs in the source-centric and
|
|
semantics-centric views of a declaration context. In the
|
|
source-centric view, all redeclarations will be present, in the
|
|
order they occurred in the source code, making
|
|
this view suitable for clients that wish to see the structure of
|
|
the source code. In the semantics-centric view, only the most recent "f"
|
|
will be found by the lookup, since it effectively replaces the first
|
|
declaration of "f".</p>
|
|
|
|
<p>In the semantics-centric view, overloading of functions is
|
|
represented explicitly. For example, given two declarations of a
|
|
function "g" that are overloaded, e.g.,</p>
|
|
<pre>
|
|
void g();
|
|
void g(int);
|
|
</pre>
|
|
<p>the <code>DeclContext::lookup</code> operation will return
|
|
an <code>OverloadedFunctionDecl</code> that contains both
|
|
declarations of "g". Clients that perform semantic analysis on a
|
|
program that is not concerned with the actual source code will
|
|
primarily use this semantics-centric view.</p>
|
|
|
|
<h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4>
|
|
<p>Each declaration has two potentially different
|
|
declaration contexts: a <i>lexical</i> context, which corresponds to
|
|
the source-centric view of the declaration context, and
|
|
a <i>semantic</i> context, which corresponds to the
|
|
semantics-centric view. The lexical context is accessible
|
|
via <code>Decl::getLexicalDeclContext</code> while the
|
|
semantic context is accessible
|
|
via <code>Decl::getDeclContext</code>, both of which return
|
|
<code>DeclContext</code> pointers. For most declarations, the two
|
|
contexts are identical. For example:</p>
|
|
|
|
<pre>
|
|
class X {
|
|
public:
|
|
void f(int x);
|
|
};
|
|
</pre>
|
|
|
|
<p>Here, the semantic and lexical contexts of <code>X::f</code> are
|
|
the <code>DeclContext</code> associated with the
|
|
class <code>X</code> (itself stored as a <code>RecordDecl</code> AST
|
|
node). However, we can now define <code>X::f</code> out-of-line:</p>
|
|
|
|
<pre>
|
|
void X::f(int x = 17) { /* ... */ }
|
|
</pre>
|
|
|
|
<p>This definition of has different lexical and semantic
|
|
contexts. The lexical context corresponds to the declaration
|
|
context in which the actual declaration occurred in the source
|
|
code, e.g., the translation unit containing <code>X</code>. Thus,
|
|
this declaration of <code>X::f</code> can be found by traversing
|
|
the declarations provided by
|
|
[<code>decls_begin()</code>, <code>decls_end()</code>) in the
|
|
translation unit.</p>
|
|
|
|
<p>The semantic context of <code>X::f</code> corresponds to the
|
|
class <code>X</code>, since this member function is (semantically) a
|
|
member of <code>X</code>. Lookup of the name <code>f</code> into
|
|
the <code>DeclContext</code> associated with <code>X</code> will
|
|
then return the definition of <code>X::f</code> (including
|
|
information about the default argument).</p>
|
|
|
|
<h4 id="TransparentContexts">Transparent Declaration Contexts</h4>
|
|
<p>In C and C++, there are several contexts in which names that are
|
|
logically declared inside another declaration will actually "leak"
|
|
out into the enclosing scope from the perspective of name
|
|
lookup. The most obvious instance of this behavior is in
|
|
enumeration types, e.g.,</p>
|
|
<pre>
|
|
enum Color {
|
|
Red,
|
|
Green,
|
|
Blue
|
|
};
|
|
</pre>
|
|
|
|
<p>Here, <code>Color</code> is an enumeration, which is a declaration
|
|
context that contains the
|
|
enumerators <code>Red</code>, <code>Green</code>,
|
|
and <code>Blue</code>. Thus, traversing the list of declarations
|
|
contained in the enumeration <code>Color</code> will
|
|
yield <code>Red</code>, <code>Green</code>,
|
|
and <code>Blue</code>. However, outside of the scope
|
|
of <code>Color</code> one can name the enumerator <code>Red</code>
|
|
without qualifying the name, e.g.,</p>
|
|
|
|
<pre>
|
|
Color c = Red;
|
|
</pre>
|
|
|
|
<p>There are other entities in C++ that provide similar behavior. For
|
|
example, linkage specifications that use curly braces:</p>
|
|
|
|
<pre>
|
|
extern "C" {
|
|
void f(int);
|
|
void g(int);
|
|
}
|
|
// f and g are visible here
|
|
</pre>
|
|
|
|
<p>For source-level accuracy, we treat the linkage specification and
|
|
enumeration type as a
|
|
declaration context in which its enclosed declarations ("Red",
|
|
"Green", and "Blue"; "f" and "g")
|
|
are declared. However, these declarations are visible outside of the
|
|
scope of the declaration context.</p>
|
|
|
|
<p>These language features (and several others, described below) have
|
|
roughly the same set of
|
|
requirements: declarations are declared within a particular lexical
|
|
context, but the declarations are also found via name lookup in
|
|
scopes enclosing the declaration itself. This feature is implemented
|
|
via <i>transparent</i> declaration contexts
|
|
(see <code>DeclContext::isTransparentContext()</code>), whose
|
|
declarations are visible in the nearest enclosing non-transparent
|
|
declaration context. This means that the lexical context of the
|
|
declaration (e.g., an enumerator) will be the
|
|
transparent <code>DeclContext</code> itself, as will the semantic
|
|
context, but the declaration will be visible in every outer context
|
|
up to and including the first non-transparent declaration context (since
|
|
transparent declaration contexts can be nested).</p>
|
|
|
|
<p>The transparent <code>DeclContexts</code> are:</p>
|
|
<ul>
|
|
<li>Enumerations (but not C++0x "scoped enumerations"):
|
|
<pre>
|
|
enum Color {
|
|
Red,
|
|
Green,
|
|
Blue
|
|
};
|
|
// Red, Green, and Blue are in scope
|
|
</pre></li>
|
|
<li>C++ linkage specifications:
|
|
<pre>
|
|
extern "C" {
|
|
void f(int);
|
|
void g(int);
|
|
}
|
|
// f and g are in scope
|
|
</pre></li>
|
|
<li>Anonymous unions and structs:
|
|
<pre>
|
|
struct LookupTable {
|
|
bool IsVector;
|
|
union {
|
|
std::vector<Item> *Vector;
|
|
std::set<Item> *Set;
|
|
};
|
|
};
|
|
|
|
LookupTable LT;
|
|
LT.Vector = 0; // Okay: finds Vector inside the unnamed union
|
|
</pre>
|
|
</li>
|
|
<li>C++0x inline namespaces:
|
|
<pre>
|
|
namespace mylib {
|
|
inline namespace debug {
|
|
class X;
|
|
}
|
|
}
|
|
mylib::X *xp; // okay: mylib::X refers to mylib::debug::X
|
|
</pre>
|
|
</li>
|
|
</ul>
|
|
|
|
|
|
<h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4>
|
|
<p>C++ namespaces have the interesting--and, so far, unique--property that
|
|
the namespace can be defined multiple times, and the declarations
|
|
provided by each namespace definition are effectively merged (from
|
|
the semantic point of view). For example, the following two code
|
|
snippets are semantically indistinguishable:</p>
|
|
<pre>
|
|
// Snippet #1:
|
|
namespace N {
|
|
void f();
|
|
}
|
|
namespace N {
|
|
void f(int);
|
|
}
|
|
|
|
// Snippet #2:
|
|
namespace N {
|
|
void f();
|
|
void f(int);
|
|
}
|
|
</pre>
|
|
|
|
<p>In Clang's representation, the source-centric view of declaration
|
|
contexts will actually have two separate <code>NamespaceDecl</code>
|
|
nodes in Snippet #1, each of which is a declaration context that
|
|
contains a single declaration of "f". However, the semantics-centric
|
|
view provided by name lookup into the namespace <code>N</code> for
|
|
"f" will return an <code>OverloadedFunctionDecl</code> that contains
|
|
both declarations of "f".</p>
|
|
|
|
<p><code>DeclContext</code> manages multiply-defined declaration
|
|
contexts internally. The
|
|
function <code>DeclContext::getPrimaryContext</code> retrieves the
|
|
"primary" context for a given <code>DeclContext</code> instance,
|
|
which is the <code>DeclContext</code> responsible for maintaining
|
|
the lookup table used for the semantics-centric view. Given the
|
|
primary context, one can follow the chain
|
|
of <code>DeclContext</code> nodes that define additional
|
|
declarations via <code>DeclContext::getNextContext</code>. Note that
|
|
these functions are used internally within the lookup and insertion
|
|
methods of the <code>DeclContext</code>, so the vast majority of
|
|
clients can ignore them.</p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="CFG">The <tt>CFG</tt> class</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>The <tt>CFG</tt> class is designed to represent a source-level
|
|
control-flow graph for a single statement (<tt>Stmt*</tt>). Typically
|
|
instances of <tt>CFG</tt> are constructed for function bodies (usually
|
|
an instance of <tt>CompoundStmt</tt>), but can also be instantiated to
|
|
represent the control-flow of any class that subclasses <tt>Stmt</tt>,
|
|
which includes simple expressions. Control-flow graphs are especially
|
|
useful for performing
|
|
<a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow-
|
|
or path-sensitive</a> program analyses on a given function.</p>
|
|
|
|
<!-- ============ -->
|
|
<h4>Basic Blocks</h4>
|
|
<!-- ============ -->
|
|
|
|
<p>Concretely, an instance of <tt>CFG</tt> is a collection of basic
|
|
blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which
|
|
simply contains an ordered sequence of <tt>Stmt*</tt> (each referring
|
|
to statements in the AST). The ordering of statements within a block
|
|
indicates unconditional flow of control from one statement to the
|
|
next. <a href="#ConditionalControlFlow">Conditional control-flow</a>
|
|
is represented using edges between basic blocks. The statements
|
|
within a given <tt>CFGBlock</tt> can be traversed using
|
|
the <tt>CFGBlock::*iterator</tt> interface.</p>
|
|
|
|
<p>
|
|
A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within
|
|
the control-flow graph it represents. Each <tt>CFGBlock</tt> within a
|
|
CFG is also uniquely numbered (accessible
|
|
via <tt>CFGBlock::getBlockID()</tt>). Currently the number is
|
|
based on the ordering the blocks were created, but no assumptions
|
|
should be made on how <tt>CFGBlock</tt>s are numbered other than their
|
|
numbers are unique and that they are numbered from 0..N-1 (where N is
|
|
the number of basic blocks in the CFG).</p>
|
|
|
|
<!-- ===================== -->
|
|
<h4>Entry and Exit Blocks</h4>
|
|
<!-- ===================== -->
|
|
|
|
Each instance of <tt>CFG</tt> contains two special blocks:
|
|
an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which
|
|
has no incoming edges, and an <i>exit</i> block (accessible
|
|
via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither
|
|
block contains any statements, and they serve the role of providing a
|
|
clear entrance and exit for a body of code such as a function body.
|
|
The presence of these empty blocks greatly simplifies the
|
|
implementation of many analyses built on top of CFGs.
|
|
|
|
<!-- ===================================================== -->
|
|
<h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4>
|
|
<!-- ===================================================== -->
|
|
|
|
<p>Conditional control-flow (such as those induced by if-statements
|
|
and loops) is represented as edges between <tt>CFGBlock</tt>s.
|
|
Because different C language constructs can induce control-flow,
|
|
each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that
|
|
represents the <i>terminator</i> of the block. A terminator is simply
|
|
the statement that caused the control-flow, and is used to identify
|
|
the nature of the conditional control-flow between blocks. For
|
|
example, in the case of an if-statement, the terminator refers to
|
|
the <tt>IfStmt</tt> object in the AST that represented the given
|
|
branch.</p>
|
|
|
|
<p>To illustrate, consider the following code example:</p>
|
|
|
|
<code>
|
|
int foo(int x) {<br>
|
|
x = x + 1;<br>
|
|
<br>
|
|
if (x > 2) x++;<br>
|
|
else {<br>
|
|
x += 2;<br>
|
|
x *= 2;<br>
|
|
}<br>
|
|
<br>
|
|
return x;<br>
|
|
}
|
|
</code>
|
|
|
|
<p>After invoking the parser+semantic analyzer on this code fragment,
|
|
the AST of the body of <tt>foo</tt> is referenced by a
|
|
single <tt>Stmt*</tt>. We can then construct an instance
|
|
of <tt>CFG</tt> representing the control-flow graph of this function
|
|
body by single call to a static class method:</p>
|
|
|
|
<code>
|
|
Stmt* FooBody = ...<br>
|
|
CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody);
|
|
</code>
|
|
|
|
<p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt>
|
|
to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no
|
|
longer needed.</p>
|
|
|
|
<p>Along with providing an interface to iterate over
|
|
its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods
|
|
that are useful for debugging and visualizing CFGs. For example, the
|
|
method
|
|
<tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to
|
|
standard error. This is especially useful when one is using a
|
|
debugger such as gdb. For example, here is the output
|
|
of <tt>FooCFG->dump()</tt>:</p>
|
|
|
|
<code>
|
|
[ B5 (ENTRY) ]<br>
|
|
Predecessors (0):<br>
|
|
Successors (1): B4<br>
|
|
<br>
|
|
[ B4 ]<br>
|
|
1: x = x + 1<br>
|
|
2: (x > 2)<br>
|
|
<b>T: if [B4.2]</b><br>
|
|
Predecessors (1): B5<br>
|
|
Successors (2): B3 B2<br>
|
|
<br>
|
|
[ B3 ]<br>
|
|
1: x++<br>
|
|
Predecessors (1): B4<br>
|
|
Successors (1): B1<br>
|
|
<br>
|
|
[ B2 ]<br>
|
|
1: x += 2<br>
|
|
2: x *= 2<br>
|
|
Predecessors (1): B4<br>
|
|
Successors (1): B1<br>
|
|
<br>
|
|
[ B1 ]<br>
|
|
1: return x;<br>
|
|
Predecessors (2): B2 B3<br>
|
|
Successors (1): B0<br>
|
|
<br>
|
|
[ B0 (EXIT) ]<br>
|
|
Predecessors (1): B1<br>
|
|
Successors (0):
|
|
</code>
|
|
|
|
<p>For each block, the pretty-printed output displays for each block
|
|
the number of <i>predecessor</i> blocks (blocks that have outgoing
|
|
control-flow to the given block) and <i>successor</i> blocks (blocks
|
|
that have control-flow that have incoming control-flow from the given
|
|
block). We can also clearly see the special entry and exit blocks at
|
|
the beginning and end of the pretty-printed output. For the entry
|
|
block (block B5), the number of predecessor blocks is 0, while for the
|
|
exit block (block B0) the number of successor blocks is 0.</p>
|
|
|
|
<p>The most interesting block here is B4, whose outgoing control-flow
|
|
represents the branching caused by the sole if-statement
|
|
in <tt>foo</tt>. Of particular interest is the second statement in
|
|
the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed
|
|
as <b><tt>if [B4.2]</tt></b>. The second statement represents the
|
|
evaluation of the condition of the if-statement, which occurs before
|
|
the actual branching of control-flow. Within the <tt>CFGBlock</tt>
|
|
for B4, the <tt>Stmt*</tt> for the second statement refers to the
|
|
actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus
|
|
pointers to subclasses of <tt>Expr</tt> can appear in the list of
|
|
statements in a block, and not just subclasses of <tt>Stmt</tt> that
|
|
refer to proper C statements.</p>
|
|
|
|
<p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt>
|
|
object in the AST. The pretty-printer outputs <b><tt>if
|
|
[B4.2]</tt></b> because the condition expression of the if-statement
|
|
has an actual place in the basic block, and thus the terminator is
|
|
essentially
|
|
<i>referring</i> to the expression that is the second statement of
|
|
block B4 (i.e., B4.2). In this manner, conditions for control-flow
|
|
(which also includes conditions for loops and switch statements) are
|
|
hoisted into the actual basic block.</p>
|
|
|
|
<!-- ===================== -->
|
|
<!-- <h4>Implicit Control-Flow</h4> -->
|
|
<!-- ===================== -->
|
|
|
|
<!--
|
|
<p>A key design principle of the <tt>CFG</tt> class was to not require
|
|
any transformations to the AST in order to represent control-flow.
|
|
Thus the <tt>CFG</tt> does not perform any "lowering" of the
|
|
statements in an AST: loops are not transformed into guarded gotos,
|
|
short-circuit operations are not converted to a set of if-statements,
|
|
and so on.</p>
|
|
-->
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="Constants">Constant Folding in the Clang AST</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>There are several places where constants and constant folding matter a lot to
|
|
the Clang front-end. First, in general, we prefer the AST to retain the source
|
|
code as close to how the user wrote it as possible. This means that if they
|
|
wrote "5+4", we want to keep the addition and two constants in the AST, we don't
|
|
want to fold to "9". This means that constant folding in various ways turns
|
|
into a tree walk that needs to handle the various cases.</p>
|
|
|
|
<p>However, there are places in both C and C++ that require constants to be
|
|
folded. For example, the C standard defines what an "integer constant
|
|
expression" (i-c-e) is with very precise and specific requirements. The
|
|
language then requires i-c-e's in a lot of places (for example, the size of a
|
|
bitfield, the value for a case statement, etc). For these, we have to be able
|
|
to constant fold the constants, to do semantic checks (e.g. verify bitfield size
|
|
is non-negative and that case statements aren't duplicated). We aim for Clang
|
|
to be very pedantic about this, diagnosing cases when the code does not use an
|
|
i-c-e where one is required, but accepting the code unless running with
|
|
<tt>-pedantic-errors</tt>.</p>
|
|
|
|
<p>Things get a little bit more tricky when it comes to compatibility with
|
|
real-world source code. Specifically, GCC has historically accepted a huge
|
|
superset of expressions as i-c-e's, and a lot of real world code depends on this
|
|
unfortuate accident of history (including, e.g., the glibc system headers). GCC
|
|
accepts anything its "fold" optimizer is capable of reducing to an integer
|
|
constant, which means that the definition of what it accepts changes as its
|
|
optimizer does. One example is that GCC accepts things like "case X-X:" even
|
|
when X is a variable, because it can fold this to 0.</p>
|
|
|
|
<p>Another issue are how constants interact with the extensions we support, such
|
|
as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99
|
|
obviously does not specify the semantics of any of these extensions, and the
|
|
definition of i-c-e does not include them. However, these extensions are often
|
|
used in real code, and we have to have a way to reason about them.</p>
|
|
|
|
<p>Finally, this is not just a problem for semantic analysis. The code
|
|
generator and other clients have to be able to fold constants (e.g. to
|
|
initialize global variables) and has to handle a superset of what C99 allows.
|
|
Further, these clients can benefit from extended information. For example, we
|
|
know that "foo()||1" always evaluates to true, but we can't replace the
|
|
expression with true because it has side effects.</p>
|
|
|
|
<!-- ======================= -->
|
|
<h4>Implementation Approach</h4>
|
|
<!-- ======================= -->
|
|
|
|
<p>After trying several different approaches, we've finally converged on a
|
|
design (Note, at the time of this writing, not all of this has been implemented,
|
|
consider this a design goal!). Our basic approach is to define a single
|
|
recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is
|
|
implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar'
|
|
type (integer, fp, complex, or pointer) this method returns the following
|
|
information:</p>
|
|
|
|
<ul>
|
|
<li>Whether the expression is an integer constant expression, a general
|
|
constant that was folded but has no side effects, a general constant that
|
|
was folded but that does have side effects, or an uncomputable/unfoldable
|
|
value.
|
|
</li>
|
|
<li>If the expression was computable in any way, this method returns the APValue
|
|
for the result of the expression.</li>
|
|
<li>If the expression is not evaluatable at all, this method returns
|
|
information on one of the problems with the expression. This includes a
|
|
SourceLocation for where the problem is, and a diagnostic ID that explains
|
|
the problem. The diagnostic should be have ERROR type.</li>
|
|
<li>If the expression is not an integer constant expression, this method returns
|
|
information on one of the problems with the expression. This includes a
|
|
SourceLocation for where the problem is, and a diagnostic ID that explains
|
|
the problem. The diagnostic should be have EXTENSION type.</li>
|
|
</ul>
|
|
|
|
<p>This information gives various clients the flexibility that they want, and we
|
|
will eventually have some helper methods for various extensions. For example,
|
|
Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which
|
|
calls Evaluate on the expression. If the expression is not foldable, the error
|
|
is emitted, and it would return true. If the expression is not an i-c-e, the
|
|
EXTENSION diagnostic is emitted. Finally it would return false to indicate that
|
|
the AST is ok.</p>
|
|
|
|
<p>Other clients can use the information in other ways, for example, codegen can
|
|
just use expressions that are foldable in any way.</p>
|
|
|
|
<!-- ========== -->
|
|
<h4>Extensions</h4>
|
|
<!-- ========== -->
|
|
|
|
<p>This section describes how some of the various extensions Clang supports
|
|
interacts with constant evaluation:</p>
|
|
|
|
<ul>
|
|
<li><b><tt>__extension__</tt></b>: The expression form of this extension causes
|
|
any evaluatable subexpression to be accepted as an integer constant
|
|
expression.</li>
|
|
<li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer
|
|
constant expression) if the operand is any evaluatable constant. As a
|
|
special case, if <tt>__builtin_constant_p</tt> is the (potentially
|
|
parenthesized) condition of a conditional operator expression ("?:"), only
|
|
the true side of the conditional operator is considered, and it is evaluated
|
|
with full constant folding.</li>
|
|
<li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an
|
|
integer constant expression, but we accept any constant as an "extension of
|
|
an extension". This only evaluates one operand depending on which way the
|
|
condition evaluates.</li>
|
|
<li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer
|
|
constant expression.</li>
|
|
<li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a
|
|
floating-point literal.</li>
|
|
<li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as
|
|
general constant expressions.</li>
|
|
<li><b><tt>__builtin_strlen</tt></b> and <b><tt>strlen</tt></b>: These are constant folded as integer constant expressions if the argument is a string literal.</li>
|
|
</ul>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2 id="Howtos">How to change Clang</h2>
|
|
<!-- ======================================================================= -->
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="AddingAttributes">How to add an attribute</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>To add an attribute, you'll have to add it to the list of attributes, add it
|
|
to the parsing phase, and look for it in the AST scan.
|
|
<a href="http://llvm.org/viewvc/llvm-project?view=rev&revision=124217">r124217</a>
|
|
has a good example of adding a warning attribute.</p>
|
|
|
|
<p>(Beware that this hasn't been reviewed/fixed by the people who designed the
|
|
attributes system yet.)</p>
|
|
|
|
<h4><a
|
|
href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?view=markup">include/clang/Basic/Attr.td</a></h4>
|
|
|
|
<p>Each attribute gets a <tt>def</tt> inheriting from <tt>Attr</tt> or one of
|
|
its subclasses. <tt>InheritableAttr</tt> means that the attribute also applies
|
|
to subsequent declarations of the same name.</p>
|
|
|
|
<p><tt>Spellings</tt> lists the strings that can appear in
|
|
<tt>__attribute__((here))</tt> or <tt>[[here]]</tt>. All such strings
|
|
will be synonymous. If you want to allow the <tt>[[]]</tt> C++0x
|
|
syntax, you have to define a list of <tt>Namespaces</tt>, which will
|
|
let users write <tt>[[namespace:spelling]]</tt>. Using the empty
|
|
string for a namespace will allow users to write just the spelling
|
|
with no "<tt>:</tt>".</p>
|
|
|
|
<p><tt>Subjects</tt> restricts what kinds of AST node to which this attribute
|
|
can appertain (roughly, attach).</p>
|
|
|
|
<p><tt>Args</tt> names the arguments the attribute takes, in order. If
|
|
<tt>Args</tt> is <tt>[StringArgument<"Arg1">, IntArgument<"Arg2">]</tt>
|
|
then <tt>__attribute__((myattribute("Hello", 3)))</tt> will be a valid use.</p>
|
|
|
|
<h4>Boilerplate</h4>
|
|
|
|
<p>Add an element to the <tt>AttributeList::Kind</tt> enum in <a
|
|
href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Sema/AttributeList.h?view=markup">include/clang/Sema/AttributeList.h</a>
|
|
named <tt>AT_lower_with_underscores</tt>. That is, a CamelCased
|
|
<tt>AttributeName</tt> in <tt>Attr.td</tt> name should become
|
|
<tt>AT_attribute_name</tt>.</p>
|
|
|
|
<p>Add a case to the <tt>StringSwitch</tt> in <tt>AttributeList::getKind()</tt>
|
|
in <a
|
|
href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/AttributeList.cpp?view=markup">lib/Sema/AttributeList.cpp</a>
|
|
for each spelling of your attribute. Less common attributes should come toward
|
|
the end of that list.</p>
|
|
|
|
<p>Write a new <tt>HandleYourAttr()</tt> function in <a
|
|
href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?view=markup">lib/Sema/SemaDeclAttr.cpp</a>,
|
|
and add a case to the switch in <tt>ProcessNonInheritableDeclAttr()</tt> or
|
|
<tt>ProcessInheritableDeclAttr()</tt> forwarding to it.</p>
|
|
|
|
<p>If your attribute causes extra warnings to fire, define a <tt>DiagGroup</tt>
|
|
in <a
|
|
href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?view=markup">include/clang/Basic/DiagnosticGroups.td</a>
|
|
named after the attribute's <tt>Spelling</tt> with "_"s replaced by "-"s. If
|
|
you're only defining one diagnostic, you can skip <tt>DiagnosticGroups.td</tt>
|
|
and use <tt>InGroup<DiagGroup<"your-attribute">></tt> directly in <a
|
|
href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?view=markup">DiagnosticSemaKinds.td</a></p>
|
|
|
|
<h4>The meat of your attribute</h4>
|
|
|
|
<p>Find an appropriate place in Clang to do whatever your attribute needs to do.
|
|
Check for the attribute's presence using <tt>Decl::getAttr<YourAttr>()</tt>.</p>
|
|
|
|
<p>Update the <a href="LanguageExtensions.html">Clang Language Extensions</a>
|
|
document to describe your new attribute.</p>
|
|
|
|
</div>
|
|
</body>
|
|
</html>
|