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16<div id="content">
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]17
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]18<h1>"Clang" CFE Internals Manual</h1>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]19
20<ul>
21<li><a href="#intro">Introduction</a></li>
22<li><a href="#libsystem">LLVM System and Support Libraries</a></li>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]23<li><a href="#libbasic">The Clang 'Basic' Library</a>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]24 <ul>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]25 <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]26 <li><a href="#SourceLocation">The SourceLocation and SourceManager
27 classes</a></li>
Douglas Gregor715c92a2010-10-27 16:02:28 +0000[diff] [blame]28 <li><a href="#SourceRange">SourceRange and CharSourceRange</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]29 </ul>
30</li>
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]31<li><a href="#libdriver">The Driver Library</a>
32 <ul>
33 </ul>
34</li>
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame]35<li><a href="#pch">Precompiled Headers</a>
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]36<li><a href="#libfrontend">The Frontend Library</a>
37 <ul>
38 </ul>
39</li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]40<li><a href="#liblex">The Lexer and Preprocessor Library</a>
41 <ul>
42 <li><a href="#Token">The Token class</a></li>
43 <li><a href="#Lexer">The Lexer class</a></li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]44 <li><a href="#AnnotationToken">Annotation Tokens</a></li>
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]45 <li><a href="#TokenLexer">The TokenLexer class</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]46 <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li>
47 </ul>
48</li>
49<li><a href="#libparse">The Parser Library</a>
50 <ul>
51 </ul>
52</li>
53<li><a href="#libast">The AST Library</a>
54 <ul>
55 <li><a href="#Type">The Type class and its subclasses</a></li>
56 <li><a href="#QualType">The QualType class</a></li>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]57 <li><a href="#DeclarationName">Declaration names</a></li>
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]58 <li><a href="#DeclContext">Declaration contexts</a>
59 <ul>
60 <li><a href="#Redeclarations">Redeclarations and Overloads</a></li>
61 <li><a href="#LexicalAndSemanticContexts">Lexical and Semantic
62 Contexts</a></li>
63 <li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li>
64 <li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li>
65 </ul>
66 </li>
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]67 <li><a href="#CFG">The CFG class</a></li>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]68 <li><a href="#Constants">Constant Folding in the Clang AST</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]69 </ul>
70</li>
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]71<li><a href="#Howtos">Howto guides</a>
72 <ul>
73 <li><a href="#AddingAttributes">How to add an attribute</a></li>
74 </ul>
75</li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]76</ul>
77
78
79<!-- ======================================================================= -->
80<h2 id="intro">Introduction</h2>
81<!-- ======================================================================= -->
82
83<p>This document describes some of the more important APIs and internal design
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]84decisions made in the Clang C front-end. The purpose of this document is to
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]85both capture some of this high level information and also describe some of the
86design decisions behind it. This is meant for people interested in hacking on
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]87Clang, not for end-users. The description below is categorized by
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]88libraries, and does not describe any of the clients of the libraries.</p>
89
90<!-- ======================================================================= -->
91<h2 id="libsystem">LLVM System and Support Libraries</h2>
92<!-- ======================================================================= -->
93
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]94<p>The LLVM libsystem library provides the basic Clang system abstraction layer,
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]95which is used for file system access. The LLVM libsupport library provides many
96underlying libraries and <a
97href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>,
98 including command line option
99processing and various containers.</p>
100
101<!-- ======================================================================= -->
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]102<h2 id="libbasic">The Clang 'Basic' Library</h2>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]103<!-- ======================================================================= -->
104
105<p>This library certainly needs a better name. The 'basic' library contains a
106number of low-level utilities for tracking and manipulating source buffers,
107locations within the source buffers, diagnostics, tokens, target abstraction,
108and information about the subset of the language being compiled for.</p>
109
110<p>Part of this infrastructure is specific to C (such as the TargetInfo class),
111other parts could be reused for other non-C-based languages (SourceLocation,
112SourceManager, Diagnostics, FileManager). When and if there is future demand
113we can figure out if it makes sense to introduce a new library, move the general
114classes somewhere else, or introduce some other solution.</p>
115
116<p>We describe the roles of these classes in order of their dependencies.</p>
117
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]118
119<!-- ======================================================================= -->
120<h3 id="Diagnostics">The Diagnostics Subsystem</h3>
121<!-- ======================================================================= -->
122
123<p>The Clang Diagnostics subsystem is an important part of how the compiler
124communicates with the human. Diagnostics are the warnings and errors produced
125when the code is incorrect or dubious. In Clang, each diagnostic produced has
Sebastian Redl9bc2a992010-07-07 23:42:27 +0000[diff] [blame]126(at the minimum) a unique ID, an English translation associated with it, a <a
127href="#SourceLocation">SourceLocation</a> to "put the caret", and a severity (e.g.
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]128<tt>WARNING</tt> or <tt>ERROR</tt>). They can also optionally include a number
129of arguments to the dianostic (which fill in "%0"'s in the string) as well as a
130number of source ranges that related to the diagnostic.</p>
131
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]132<p>In this section, we'll be giving examples produced by the Clang command line
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]133driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many
134different ways</a> depending on how the DiagnosticClient interface is
Sebastian Redl9bc2a992010-07-07 23:42:27 +0000[diff] [blame]135implemented. A representative example of a diagnostic is:</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]136
137<pre>
138t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
139 <font color="darkgreen">P = (P-42) + Gamma*4;</font>
140 <font color="blue">~~~~~~ ^ ~~~~~~~</font>
141</pre>
142
143<p>In this example, you can see the English translation, the severity (error),
144you can see the source location (the caret ("^") and file/line/column info),
145the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex
146float"). You'll have to believe me that there is a unique ID backing the
147diagnostic :).</p>
148
149<p>Getting all of this to happen has several steps and involves many moving
150pieces, this section describes them and talks about best practices when adding
151a new diagnostic.</p>
152
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]153<!-- ============================= -->
154<h4>The Diagnostic*Kinds.td files</h4>
155<!-- ============================= -->
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]156
Chris Lattner4c50b692010-05-01 17:35:19 +0000[diff] [blame]157<p>Diagnostics are created by adding an entry to one of the <tt>
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]158clang/Basic/Diagnostic*Kinds.td</tt> files, depending on what library will
159be using it. From this file, tblgen generates the unique ID of the diagnostic,
160the severity of the diagnostic and the English translation + format string.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]161
162<p>There is little sanity with the naming of the unique ID's right now. Some
163start with err_, warn_, ext_ to encode the severity into the name. Since the
164enum is referenced in the C++ code that produces the diagnostic, it is somewhat
165useful for it to be reasonably short.</p>
166
167<p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>,
168<tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The
169<tt>ERROR</tt> severity is used for diagnostics indicating the program is never
170acceptable under any circumstances. When an error is emitted, the AST for the
171input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt>
172severities are used for extensions to the language that Clang accepts. This
173means that Clang fully understands and can represent them in the AST, but we
174produce diagnostics to tell the user their code is non-portable. The difference
175is that the former are ignored by default, and the later warn by default. The
176<tt>WARNING</tt> severity is used for constructs that are valid in the currently
177selected source language but that are dubious in some way. The <tt>NOTE</tt>
Daniel Dunbar426b8632009-02-17 15:49:03 +0000[diff] [blame]178level is used to staple more information onto previous diagnostics.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]179
180<p>These <em>severities</em> are mapped into a smaller set (the
181Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>,
Chris Lattner0aad2972009-02-05 22:49:08 +0000[diff] [blame]182<tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics
Chris Lattnera180fdd2009-02-17 07:07:29 +0000[diff] [blame]183subsystem based on various configuration options. Clang internally supports a
184fully fine grained mapping mechanism that allows you to map almost any
185diagnostic to the output level that you want. The only diagnostics that cannot
186be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously
187emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to
188<tt>Fatal</tt> (it is not possible to turn an error into a warning,
189for example).</p>
190
191<p>Diagnostic mappings are used in many ways. For example, if the user
192specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if
193they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is
194used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc.
195</p>
196
197<p>
198Mapping to <tt>Fatal</tt> should only be used for diagnostics that are
199considered so severe that error recovery won't be able to recover sensibly from
200them (thus spewing a ton of bogus errors). One example of this class of error
201are failure to #include a file.
202</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]203
204<!-- ================= -->
205<h4>The Format String</h4>
206<!-- ================= -->
207
208<p>The format string for the diagnostic is very simple, but it has some power.
209It takes the form of a string in English with markers that indicate where and
210how arguments to the diagnostic are inserted and formatted. For example, here
211are some simple format strings:</p>
212
213<pre>
214 "binary integer literals are an extension"
215 "format string contains '\\0' within the string body"
216 "more '<b>%%</b>' conversions than data arguments"
Chris Lattner545b3682008-11-23 20:27:13 +0000[diff] [blame]217 "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)"
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]218 "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator"
219 " (has <b>%1</b> parameter<b>%s1</b>)"
220</pre>
221
222<p>These examples show some important points of format strings. You can use any
223 plain ASCII character in the diagnostic string except "%" without a problem,
224 but these are C strings, so you have to use and be aware of all the C escape
225 sequences (as in the second example). If you want to produce a "%" in the
226 output, use the "%%" escape sequence, like the third diagnostic. Finally,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]227 Clang uses the "%...[digit]" sequences to specify where and how arguments to
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]228 the diagnostic are formatted.</p>
229
230<p>Arguments to the diagnostic are numbered according to how they are specified
231 by the C++ code that <a href="#producingdiag">produces them</a>, and are
232 referenced by <tt>%0</tt> .. <tt>%9</tt>. If you have more than 10 arguments
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]233 to your diagnostic, you are doing something wrong :). Unlike printf, there
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]234 is no requirement that arguments to the diagnostic end up in the output in
235 the same order as they are specified, you could have a format string with
236 <tt>"%1 %0"</tt> that swaps them, for example. The text in between the
237 percent and digit are formatting instructions. If there are no instructions,
238 the argument is just turned into a string and substituted in.</p>
239
240<p>Here are some "best practices" for writing the English format string:</p>
241
242<ul>
243<li>Keep the string short. It should ideally fit in the 80 column limit of the
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]244 <tt>DiagnosticKinds.td</tt> file. This avoids the diagnostic wrapping when
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]245 printed, and forces you to think about the important point you are conveying
246 with the diagnostic.</li>
247<li>Take advantage of location information. The user will be able to see the
248 line and location of the caret, so you don't need to tell them that the
249 problem is with the 4th argument to the function: just point to it.</li>
250<li>Do not capitalize the diagnostic string, and do not end it with a
251 period.</li>
252<li>If you need to quote something in the diagnostic string, use single
253 quotes.</li>
254</ul>
255
256<p>Diagnostics should never take random English strings as arguments: you
257shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like
258<tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]259this prevents <a href="#translation">translating</a> the Clang diagnostics to
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]260other languages (because they'll get random English words in their otherwise
261localized diagnostic). The exceptions to this are C/C++ language keywords
262(e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note
263that things like "pointer" and "reference" are not keywords. On the other
264hand, you <em>can</em> include anything that comes from the user's source code,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]265including variable names, types, labels, etc. The 'select' format can be
266used to achieve this sort of thing in a localizable way, see below.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]267
268<!-- ==================================== -->
269<h4>Formatting a Diagnostic Argument</a></h4>
270<!-- ==================================== -->
271
272<p>Arguments to diagnostics are fully typed internally, and come from a couple
273different classes: integers, types, names, and random strings. Depending on
274the class of the argument, it can be optionally formatted in different ways.
275This gives the DiagnosticClient information about what the argument means
276without requiring it to use a specific presentation (consider this MVC for
277Clang :).</p>
278
279<p>Here are the different diagnostic argument formats currently supported by
280Clang:</p>
281
282<table>
283<tr><td colspan="2"><b>"s" format</b></td></tr>
284<tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]285<tr><td>Class:</td><td>Integers</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]286<tr><td>Description:</td><td>This is a simple formatter for integers that is
287 useful when producing English diagnostics. When the integer is 1, it prints
288 as nothing. When the integer is not 1, it prints as "s". This allows some
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]289 simple grammatical forms to be to be handled correctly, and eliminates the
290 need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]291
292<tr><td colspan="2"><b>"select" format</b></td></tr>
293<tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2
294 operator"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]295<tr><td>Class:</td><td>Integers</td></tr>
John McCall3a47e232010-01-14 19:12:17 +0000[diff] [blame]296<tr><td>Description:</td><td><p>This format specifier is used to merge multiple
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]297 related diagnostics together into one common one, without requiring the
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]298 difference to be specified as an English string argument. Instead of
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]299 specifying the string, the diagnostic gets an integer argument and the
300 format string selects the numbered option. In this case, the "%2" value
301 must be an integer in the range [0..2]. If it is 0, it prints 'unary', if
302 it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This
303 allows other language translations to substitute reasonable words (or entire
304 phrases) based on the semantics of the diagnostic instead of having to do
John McCall3a47e232010-01-14 19:12:17 +0000[diff] [blame]305 things textually.</p>
306 <p>The selected string does undergo formatting.</p></td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]307
308<tr><td colspan="2"><b>"plural" format</b></td></tr>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]309<tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to
310 your computer"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]311<tr><td>Class:</td><td>Integers</td></tr>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]312<tr><td>Description:</td><td><p>This is a formatter for complex plural forms.
313 It is designed to handle even the requirements of languages with very
314 complex plural forms, as many Baltic languages have. The argument consists
315 of a series of expression/form pairs, separated by ':', where the first form
316 whose expression evaluates to true is the result of the modifier.</p>
317 <p>An expression can be empty, in which case it is always true. See the
318 example at the top. Otherwise, it is a series of one or more numeric
319 conditions, separated by ','. If any condition matches, the expression
320 matches. Each numeric condition can take one of three forms.</p>
321 <ul>
322 <li>number: A simple decimal number matches if the argument is the same
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]323 as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]324 <li>range: A range in square brackets matches if the argument is within
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]325 the range. Then range is inclusive on both ends. Example:
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]326 <tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li>
327 <li>modulo: A modulo operator is followed by a number, and
328 equals sign and either a number or a range. The tests are the
329 same as for plain
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]330 numbers and ranges, but the argument is taken modulo the number first.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]331 Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything
332 else}1"</tt></li>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]333 </ul>
334 <p>The parser is very unforgiving. A syntax error, even whitespace, will
335 abort, as will a failure to match the argument against any
336 expression.</p></td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]337
John McCall3a47e232010-01-14 19:12:17 +0000[diff] [blame]338<tr><td colspan="2"><b>"ordinal" format</b></td></tr>
339<tr><td>Example:</td><td><tt>"ambiguity in %ordinal0 argument"</tt></td></tr>
340<tr><td>Class:</td><td>Integers</td></tr>
341<tr><td>Description:</td><td><p>This is a formatter which represents the
342 argument number as an ordinal: the value <tt>1</tt> becomes <tt>1st</tt>,
343 <tt>3</tt> becomes <tt>3rd</tt>, and so on. Values less than <tt>1</tt>
344 are not supported.</p>
345 <p>This formatter is currently hard-coded to use English ordinals.</p></td></tr>
346
Chris Lattner077bf5e2008-11-24 03:33:13 +0000[diff] [blame]347<tr><td colspan="2"><b>"objcclass" format</b></td></tr>
348<tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr>
349<tr><td>Class:</td><td>DeclarationName</td></tr>
350<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
351 DeclarationName corresponds to an Objective-C class method selector. As
352 such, it prints the selector with a leading '+'.</p></td></tr>
353
354<tr><td colspan="2"><b>"objcinstance" format</b></td></tr>
355<tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr>
356<tr><td>Class:</td><td>DeclarationName</td></tr>
357<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
358 DeclarationName corresponds to an Objective-C instance method selector. As
359 such, it prints the selector with a leading '-'.</p></td></tr>
360
Douglas Gregor47b9a1c2009-02-04 17:27:36 +0000[diff] [blame]361<tr><td colspan="2"><b>"q" format</b></td></tr>
362<tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr>
363<tr><td>Class:</td><td>NamedDecl*</td></tr>
364<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>
365
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]366</table>
367
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]368<p>It is really easy to add format specifiers to the Clang diagnostics system,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]369but they should be discussed before they are added. If you are creating a lot
370of repetitive diagnostics and/or have an idea for a useful formatter, please
371bring it up on the cfe-dev mailing list.</p>
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]372
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]373<!-- ===================================================== -->
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]374<h4 id="producingdiag">Producing the Diagnostic</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]375<!-- ===================================================== -->
376
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]377<p>Now that you've created the diagnostic in the DiagnosticKinds.td file, you
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]378need to write the code that detects the condition in question and emits the
379new diagnostic. Various components of Clang (e.g. the preprocessor, Sema,
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]380etc) provide a helper function named "Diag". It creates a diagnostic and
381accepts the arguments, ranges, and other information that goes along with
382it.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]383
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]384<p>For example, the binary expression error comes from code like this:</p>
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]385
386<pre>
387 if (various things that are bad)
388 Diag(Loc, diag::err_typecheck_invalid_operands)
389 &lt;&lt; lex-&gt;getType() &lt;&lt; rex-&gt;getType()
390 &lt;&lt; lex-&gt;getSourceRange() &lt;&lt; rex-&gt;getSourceRange();
391</pre>
392
393<p>This shows that use of the Diag method: they take a location (a <a
394href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]395(which matches the name from DiagnosticKinds.td). If the diagnostic takes
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]396arguments, they are specified with the &lt;&lt; operator: the first argument
397becomes %0, the second becomes %1, etc. The diagnostic interface allows you to
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]398specify arguments of many different types, including <tt>int</tt> and
399<tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and
400<tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and
401<tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc.
402SourceRanges are also specified with the &lt;&lt; operator, but do not have a
403specific ordering requirement.</p>
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]404
405<p>As you can see, adding and producing a diagnostic is pretty straightforward.
406The hard part is deciding exactly what you need to say to help the user, picking
407a suitable wording, and providing the information needed to format it correctly.
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]408The good news is that the call site that issues a diagnostic should be
409completely independent of how the diagnostic is formatted and in what language
410it is rendered.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]411</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]412
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]413<!-- ==================================================== -->
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]414<h4 id="fix-it-hints">Fix-It Hints</h4>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]415<!-- ==================================================== -->
416
417<p>In some cases, the front end emits diagnostics when it is clear
418that some small change to the source code would fix the problem. For
419example, a missing semicolon at the end of a statement or a use of
Chris Lattner34c05332009-02-27 19:31:12 +0000[diff] [blame]420deprecated syntax that is easily rewritten into a more modern form.
421Clang tries very hard to emit the diagnostic and recover gracefully
422in these and other cases.</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]423
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]424<p>However, for these cases where the fix is obvious, the diagnostic
425can be annotated with a hint (referred to as a "fix-it hint") that
426describes how to change the code referenced by the diagnostic to fix
427the problem. For example, it might add the missing semicolon at the
428end of the statement or rewrite the use of a deprecated construct
429into something more palatable. Here is one such example from the C++
430front end, where we warn about the right-shift operator changing
431meaning from C++98 to C++0x:</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]432
433<pre>
434test.cpp:3:7: warning: use of right-shift operator ('&gt;&gt;') in template argument will require parentheses in C++0x
435A&lt;100 &gt;&gt; 2&gt; *a;
436 ^
437 ( )
438</pre>
439
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]440<p>Here, the fix-it hint is suggesting that parentheses be added,
441and showing exactly where those parentheses would be inserted into the
442source code. The fix-it hints themselves describe what changes to make
443to the source code in an abstract manner, which the text diagnostic
444printer renders as a line of "insertions" below the caret line. <a
445href="#DiagnosticClient">Other diagnostic clients</a> might choose
446to render the code differently (e.g., as markup inline) or even give
447the user the ability to automatically fix the problem.</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]448
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]449<p>All fix-it hints are described by the <code>FixItHint</code> class,
450instances of which should be attached to the diagnostic using the
451&lt;&lt; operator in the same way that highlighted source ranges and
452arguments are passed to the diagnostic. Fix-it hints can be created
453with one of three constructors:</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]454
455<dl>
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]456 <dt><code>FixItHint::CreateInsertion(Loc, Code)</code></dt>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]457 <dd>Specifies that the given <code>Code</code> (a string) should be inserted
458 before the source location <code>Loc</code>.</dd>
459
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]460 <dt><code>FixItHint::CreateRemoval(Range)</code></dt>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]461 <dd>Specifies that the code in the given source <code>Range</code>
462 should be removed.</dd>
463
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]464 <dt><code>FixItHint::CreateReplacement(Range, Code)</code></dt>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]465 <dd>Specifies that the code in the given source <code>Range</code>
466 should be removed, and replaced with the given <code>Code</code> string.</dd>
467</dl>
468
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]469<!-- ============================================================= -->
470<h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4>
471<!-- ============================================================= -->
472
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]473<p>Once code generates a diagnostic with all of the arguments and the rest of
474the relevant information, Clang needs to know what to do with it. As previously
475mentioned, the diagnostic machinery goes through some filtering to map a
476severity onto a diagnostic level, then (assuming the diagnostic is not mapped to
477"<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient
478interface with the information.</p>
479
480<p>It is possible to implement this interface in many different ways. For
481example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns
482the arguments into strings (according to the various formatting rules), prints
483out the file/line/column information and the string, then prints out the line of
484code, the source ranges, and the caret. However, this behavior isn't required.
485</p>
486
487<p>Another implementation of the DiagnosticClient interface is the
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]488'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode.
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]489Instead of formatting and printing out the diagnostics, this implementation just
490captures and remembers the diagnostics as they fly by. Then -verify compares
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]491the list of produced diagnostics to the list of expected ones. If they disagree,
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]492it prints out its own output.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]493</p>
494
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]495<p>There are many other possible implementations of this interface, and this is
496why we prefer diagnostics to pass down rich structured information in arguments.
497For example, an HTML output might want declaration names be linkified to where
498they come from in the source. Another example is that a GUI might let you click
499on typedefs to expand them. This application would want to pass significantly
500more information about types through to the GUI than a simple flat string. The
501interface allows this to happen.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]502
503<!-- ====================================================== -->
504<h4><a name="translation">Adding Translations to Clang</a></h4>
505<!-- ====================================================== -->
506
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]507<p>Not possible yet! Diagnostic strings should be written in UTF-8, the client
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]508can translate to the relevant code page if needed. Each translation completely
509replaces the format string for the diagnostic.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]510
511
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]512<!-- ======================================================================= -->
513<h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3>
514<!-- ======================================================================= -->
515
516<p>Strangely enough, the SourceLocation class represents a location within the
517source code of the program. Important design points include:</p>
518
519<ol>
520<li>sizeof(SourceLocation) must be extremely small, as these are embedded into
521 many AST nodes and are passed around often. Currently it is 32 bits.</li>
522<li>SourceLocation must be a simple value object that can be efficiently
523 copied.</li>
524<li>We should be able to represent a source location for any byte of any input
525 file. This includes in the middle of tokens, in whitespace, in trigraphs,
526 etc.</li>
527<li>A SourceLocation must encode the current #include stack that was active when
528 the location was processed. For example, if the location corresponds to a
529 token, it should contain the set of #includes active when the token was
530 lexed. This allows us to print the #include stack for a diagnostic.</li>
531<li>SourceLocation must be able to describe macro expansions, capturing both
532 the ultimate instantiation point and the source of the original character
533 data.</li>
534</ol>
535
536<p>In practice, the SourceLocation works together with the SourceManager class
Nick Lewycky77561e52010-05-26 21:48:10 +0000[diff] [blame]537to encode two pieces of information about a location: its spelling location
538and its instantiation location. For most tokens, these will be the same.
539However, for a macro expansion (or tokens that came from a _Pragma directive)
540these will describe the location of the characters corresponding to the token
541and the location where the token was used (i.e. the macro instantiation point
542or the location of the _Pragma itself).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]543
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]544<p>The Clang front-end inherently depends on the location of a token being
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]545tracked correctly. If it is ever incorrect, the front-end may get confused and
546die. The reason for this is that the notion of the 'spelling' of a Token in
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]547Clang depends on being able to find the original input characters for the token.
Chris Lattner18376dd2009-01-16 07:00:50 +0000[diff] [blame]548This concept maps directly to the "spelling location" for the token.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]549
Douglas Gregor715c92a2010-10-27 16:02:28 +0000[diff] [blame]550
551<!-- ======================================================================= -->
552<h3 id="SourceRange">SourceRange and CharSourceRange</h3>
553<!-- ======================================================================= -->
554<!-- mostly taken from
555 http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-August/010595.html -->
556
557<p>Clang represents most source ranges by [first, last], where first and last
558each point to the beginning of their respective tokens. For example
559consider the SourceRange of the following statement:</p>
560<pre>
561x = foo + bar;
562^first ^last
563</pre>
564
565<p>To map from this representation to a character-based
566representation, the 'last' location needs to be adjusted to point to
567(or past) the end of that token with either
568<code>Lexer::MeasureTokenLength()</code> or
Chris Lattner7ef5c272010-11-17 07:05:50 +0000[diff] [blame]569<code>Lexer::getLocForEndOfToken()</code>. For the rare cases
Douglas Gregor715c92a2010-10-27 16:02:28 +0000[diff] [blame]570where character-level source ranges information is needed we use
571the <code>CharSourceRange</code> class.</p>
572
573
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]574<!-- ======================================================================= -->
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]575<h2 id="libdriver">The Driver Library</h2>
576<!-- ======================================================================= -->
577
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]578<p>The clang Driver and library are documented <a
579href="DriverInternals.html">here<a>.<p>
580
581<!-- ======================================================================= -->
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame]582<h2 id="pch">Precompiled Headers</h2>
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]583<!-- ======================================================================= -->
584
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame]585<p>Clang supports two implementations of precompiled headers. The
586 default implementation, precompiled headers (<a
587 href="PCHInternals.html">PCH</a>) uses a serialized representation
588 of Clang's internal data structures, encoded with the <a
589 href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream
590 format</a>. Pretokenized headers (<a
591 href="PTHInternals.html">PTH</a>), on the other hand, contain a
592 serialized representation of the tokens encountered when
593 preprocessing a header (and anything that header includes).</p>
594
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]595
596<!-- ======================================================================= -->
597<h2 id="libfrontend">The Frontend Library</h2>
598<!-- ======================================================================= -->
599
600<p>The Frontend library contains functionality useful for building
601tools on top of the clang libraries, for example several methods for
602outputting diagnostics.</p>
603
604<!-- ======================================================================= -->
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]605<h2 id="liblex">The Lexer and Preprocessor Library</h2>
606<!-- ======================================================================= -->
607
608<p>The Lexer library contains several tightly-connected classes that are involved
609with the nasty process of lexing and preprocessing C source code. The main
610interface to this library for outside clients is the large <a
611href="#Preprocessor">Preprocessor</a> class.
612It contains the various pieces of state that are required to coherently read
613tokens out of a translation unit.</p>
614
615<p>The core interface to the Preprocessor object (once it is set up) is the
616Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from
617the preprocessor stream. There are two types of token providers that the
618preprocessor is capable of reading from: a buffer lexer (provided by the <a
619href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]620href="#TokenLexer">TokenLexer</a> class).
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]621
622
623<!-- ======================================================================= -->
624<h3 id="Token">The Token class</h3>
625<!-- ======================================================================= -->
626
627<p>The Token class is used to represent a single lexed token. Tokens are
628intended to be used by the lexer/preprocess and parser libraries, but are not
629intended to live beyond them (for example, they should not live in the ASTs).<p>
630
631<p>Tokens most often live on the stack (or some other location that is efficient
632to access) as the parser is running, but occasionally do get buffered up. For
633example, macro definitions are stored as a series of tokens, and the C++
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]634front-end periodically needs to buffer tokens up for tentative parsing and
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]635various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit
636system, sizeof(Token) is currently 16 bytes.</p>
637
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]638<p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation
639Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer,
640annotation tokens represent semantic information and are produced by the parser,
641replacing normal tokens in the token stream. Normal tokens contain the
642following information:</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]643
644<ul>
645<li><b>A SourceLocation</b> - This indicates the location of the start of the
646token.</li>
647
648<li><b>A length</b> - This stores the length of the token as stored in the
649SourceBuffer. For tokens that include them, this length includes trigraphs and
650escaped newlines which are ignored by later phases of the compiler. By pointing
651into the original source buffer, it is always possible to get the original
652spelling of a token completely accurately.</li>
653
654<li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if
655identifier lookup was enabled when the token was lexed (e.g. the lexer was not
656reading in 'raw' mode) this contains a pointer to the unique hash value for the
657identifier. Because the lookup happens before keyword identification, this
658field is set even for language keywords like 'for'.</li>
659
660<li><b>TokenKind</b> - This indicates the kind of token as classified by the
661lexer. This includes things like <tt>tok::starequal</tt> (for the "*="
662operator), <tt>tok::ampamp</tt> for the "&amp;&amp;" token, and keyword values
663(e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note
664that some tokens can be spelled multiple ways. For example, C++ supports
665"operator keywords", where things like "and" are treated exactly like the
666"&amp;&amp;" operator. In these cases, the kind value is set to
667<tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to
668consider both forms. For something that cares about which form is used (e.g.
669the preprocessor 'stringize' operator) the spelling indicates the original
670form.</li>
671
672<li><b>Flags</b> - There are currently four flags tracked by the
673lexer/preprocessor system on a per-token basis:
674
675 <ol>
676 <li><b>StartOfLine</b> - This was the first token that occurred on its input
677 source line.</li>
678 <li><b>LeadingSpace</b> - There was a space character either immediately
679 before the token or transitively before the token as it was expanded
680 through a macro. The definition of this flag is very closely defined by
681 the stringizing requirements of the preprocessor.</li>
682 <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to
683 represent identifier tokens which have macro expansion disabled. This
684 prevents them from being considered as candidates for macro expansion ever
685 in the future.</li>
686 <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the
687 token includes a trigraph or escaped newline. Since this is uncommon,
688 many pieces of code can fast-path on tokens that did not need cleaning.
689 </p>
690 </ol>
691</li>
692</ul>
693
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]694<p>One interesting (and somewhat unusual) aspect of normal tokens is that they
695don't contain any semantic information about the lexed value. For example, if
696the token was a pp-number token, we do not represent the value of the number
697that was lexed (this is left for later pieces of code to decide). Additionally,
698the lexer library has no notion of typedef names vs variable names: both are
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]699returned as identifiers, and the parser is left to decide whether a specific
700identifier is a typedef or a variable (tracking this requires scope information
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]701among other things). The parser can do this translation by replacing tokens
702returned by the preprocessor with "Annotation Tokens".</p>
703
704<!-- ======================================================================= -->
705<h3 id="AnnotationToken">Annotation Tokens</h3>
706<!-- ======================================================================= -->
707
708<p>Annotation Tokens are tokens that are synthesized by the parser and injected
709into the preprocessor's token stream (replacing existing tokens) to record
710semantic information found by the parser. For example, if "foo" is found to be
711a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an
712<tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this
713makes it easy to handle qualified type names (e.g. "foo::bar::baz&lt;42&gt;::t")
714in C++ as a single "token" in the parser. 2) if the parser backtracks, the
715reparse does not need to redo semantic analysis to determine whether a token
716sequence is a variable, type, template, etc.</p>
717
718<p>Annotation Tokens are created by the parser and reinjected into the parser's
719token stream (when backtracking is enabled). Because they can only exist in
720tokens that the preprocessor-proper is done with, it doesn't need to keep around
721flags like "start of line" that the preprocessor uses to do its job.
722Additionally, an annotation token may "cover" a sequence of preprocessor tokens
723(e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields
724of an annotation token are different than the fields for a normal token (but
725they are multiplexed into the normal Token fields):</p>
726
727<ul>
728<li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation
729token indicates the first token replaced by the annotation token. In the example
730above, it would be the location of the "a" identifier.</li>
731
732<li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the
733last token replaced with the annotation token. In the example above, it would
734be the location of the "c" identifier.</li>
735
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]736<li><b>void* "AnnotationValue"</b> - This contains an opaque object
737that the parser gets from Sema. The parser merely preserves the
738information for Sema to later interpret based on the annotation token
739kind.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]740
741<li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this
742is. See below for the different valid kinds.</li>
743</ul>
744
745<p>Annotation tokens currently come in three kinds:</p>
746
747<ol>
748<li><b>tok::annot_typename</b>: This annotation token represents a
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]749resolved typename token that is potentially qualified. The
750AnnotationValue field contains the <tt>QualType</tt> returned by
751Sema::getTypeName(), possibly with source location information
752attached.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]753
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]754<li><b>tok::annot_cxxscope</b>: This annotation token represents a C++
755scope specifier, such as "A::B::". This corresponds to the grammar
756productions "::" and ":: [opt] nested-name-specifier". The
757AnnotationValue pointer is a <tt>NestedNameSpecifier*</tt> returned by
758the Sema::ActOnCXXGlobalScopeSpecifier and
759Sema::ActOnCXXNestedNameSpecifier callbacks.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]760
Douglas Gregor39a8de12009-02-25 19:37:18 +0000[diff] [blame]761<li><b>tok::annot_template_id</b>: This annotation token represents a
762C++ template-id such as "foo&lt;int, 4&gt;", where "foo" is the name
763of a template. The AnnotationValue pointer is a pointer to a malloc'd
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]764TemplateIdAnnotation object. Depending on the context, a parsed
765template-id that names a type might become a typename annotation token
766(if all we care about is the named type, e.g., because it occurs in a
767type specifier) or might remain a template-id token (if we want to
768retain more source location information or produce a new type, e.g.,
769in a declaration of a class template specialization). template-id
770annotation tokens that refer to a type can be "upgraded" to typename
771annotation tokens by the parser.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]772
773</ol>
774
Cedric Venetda76b282009-01-06 16:22:54 +0000[diff] [blame]775<p>As mentioned above, annotation tokens are not returned by the preprocessor,
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]776they are formed on demand by the parser. This means that the parser has to be
777aware of cases where an annotation could occur and form it where appropriate.
778This is somewhat similar to how the parser handles Translation Phase 6 of C99:
779String Concatenation (see C99 5.1.1.2). In the case of string concatenation,
780the preprocessor just returns distinct tok::string_literal and
781tok::wide_string_literal tokens and the parser eats a sequence of them wherever
782the grammar indicates that a string literal can occur.</p>
783
784<p>In order to do this, whenever the parser expects a tok::identifier or
785tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or
786TryAnnotateCXXScopeToken methods to form the annotation token. These methods
787will maximally form the specified annotation tokens and replace the current
788token with them, if applicable. If the current tokens is not valid for an
789annotation token, it will remain an identifier or :: token.</p>
790
791
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]792
793<!-- ======================================================================= -->
794<h3 id="Lexer">The Lexer class</h3>
795<!-- ======================================================================= -->
796
797<p>The Lexer class provides the mechanics of lexing tokens out of a source
798buffer and deciding what they mean. The Lexer is complicated by the fact that
799it operates on raw buffers that have not had spelling eliminated (this is a
800necessity to get decent performance), but this is countered with careful coding
801as well as standard performance techniques (for example, the comment handling
802code is vectorized on X86 and PowerPC hosts).</p>
803
804<p>The lexer has a couple of interesting modal features:</p>
805
806<ul>
807<li>The lexer can operate in 'raw' mode. This mode has several features that
808 make it possible to quickly lex the file (e.g. it stops identifier lookup,
809 doesn't specially handle preprocessor tokens, handles EOF differently, etc).
810 This mode is used for lexing within an "<tt>#if 0</tt>" block, for
811 example.</li>
812<li>The lexer can capture and return comments as tokens. This is required to
813 support the -C preprocessor mode, which passes comments through, and is
814 used by the diagnostic checker to identifier expect-error annotations.</li>
815<li>The lexer can be in ParsingFilename mode, which happens when preprocessing
Chris Lattner84386242007-09-16 19:25:23 +0000[diff] [blame]816 after reading a #include directive. This mode changes the parsing of '&lt;'
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]817 to return an "angled string" instead of a bunch of tokens for each thing
818 within the filename.</li>
819<li>When parsing a preprocessor directive (after "<tt>#</tt>") the
820 ParsingPreprocessorDirective mode is entered. This changes the parser to
Peter Collingbourne84021552011-02-28 02:37:51 +0000[diff] [blame]821 return EOD at a newline.</li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]822<li>The Lexer uses a LangOptions object to know whether trigraphs are enabled,
823 whether C++ or ObjC keywords are recognized, etc.</li>
824</ul>
825
826<p>In addition to these modes, the lexer keeps track of a couple of other
827 features that are local to a lexed buffer, which change as the buffer is
828 lexed:</p>
829
830<ul>
831<li>The Lexer uses BufferPtr to keep track of the current character being
832 lexed.</li>
833<li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token
834 will start with its "start of line" bit set.</li>
835<li>The Lexer keeps track of the current #if directives that are active (which
836 can be nested).</li>
837<li>The Lexer keeps track of an <a href="#MultipleIncludeOpt">
838 MultipleIncludeOpt</a> object, which is used to
839 detect whether the buffer uses the standard "<tt>#ifndef XX</tt> /
840 <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does,
841 subsequent includes can be ignored if the XX macro is defined.</li>
842</ul>
843
844<!-- ======================================================================= -->
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]845<h3 id="TokenLexer">The TokenLexer class</h3>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]846<!-- ======================================================================= -->
847
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]848<p>The TokenLexer class is a token provider that returns tokens from a list
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]849of tokens that came from somewhere else. It typically used for two things: 1)
850returning tokens from a macro definition as it is being expanded 2) returning
851tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and
852will most likely be used to handle unbounded look-ahead for the C++ parser.</p>
853
854<!-- ======================================================================= -->
855<h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3>
856<!-- ======================================================================= -->
857
858<p>The MultipleIncludeOpt class implements a really simple little state machine
859that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>"
860idiom that people typically use to prevent multiple inclusion of headers. If a
861buffer uses this idiom and is subsequently #include'd, the preprocessor can
862simply check to see whether the guarding condition is defined or not. If so,
863the preprocessor can completely ignore the include of the header.</p>
864
865
866
867<!-- ======================================================================= -->
868<h2 id="libparse">The Parser Library</h2>
869<!-- ======================================================================= -->
870
871<!-- ======================================================================= -->
872<h2 id="libast">The AST Library</h2>
873<!-- ======================================================================= -->
874
875<!-- ======================================================================= -->
876<h3 id="Type">The Type class and its subclasses</h3>
877<!-- ======================================================================= -->
878
879<p>The Type class (and its subclasses) are an important part of the AST. Types
880are accessed through the ASTContext class, which implicitly creates and uniques
881them as they are needed. Types have a couple of non-obvious features: 1) they
882do not capture type qualifiers like const or volatile (See
883<a href="#QualType">QualType</a>), and 2) they implicitly capture typedef
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]884information. Once created, types are immutable (unlike decls).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]885
886<p>Typedefs in C make semantic analysis a bit more complex than it would
887be without them. The issue is that we want to capture typedef information
888and represent it in the AST perfectly, but the semantics of operations need to
889"see through" typedefs. For example, consider this code:</p>
890
891<code>
892void func() {<br>
Bill Wendling30d17752007-10-06 01:56:01 +0000[diff] [blame]893&nbsp;&nbsp;typedef int foo;<br>
894&nbsp;&nbsp;foo X, *Y;<br>
895&nbsp;&nbsp;typedef foo* bar;<br>
896&nbsp;&nbsp;bar Z;<br>
897&nbsp;&nbsp;*X; <i>// error</i><br>
898&nbsp;&nbsp;**Y; <i>// error</i><br>
899&nbsp;&nbsp;**Z; <i>// error</i><br>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]900}<br>
901</code>
902
903<p>The code above is illegal, and thus we expect there to be diagnostics emitted
904on the annotated lines. In this example, we expect to get:</p>
905
906<pre>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]907<b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]908*X; // error
909<font color="blue">^~</font>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]910<b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]911**Y; // error
912<font color="blue">^~~</font>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]913<b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b>
914**Z; // error
915<font color="blue">^~~</font>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]916</pre>
917
918<p>While this example is somewhat silly, it illustrates the point: we want to
919retain typedef information where possible, so that we can emit errors about
920"<tt>std::string</tt>" instead of "<tt>std::basic_string&lt;char, std:...</tt>".
921Doing this requires properly keeping typedef information (for example, the type
922of "X" is "foo", not "int"), and requires properly propagating it through the
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]923various operators (for example, the type of *Y is "foo", not "int"). In order
924to retain this information, the type of these expressions is an instance of the
925TypedefType class, which indicates that the type of these expressions is a
926typedef for foo.
927</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]928
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]929<p>Representing types like this is great for diagnostics, because the
930user-specified type is always immediately available. There are two problems
931with this: first, various semantic checks need to make judgements about the
Chris Lattner33fc68a2007-07-31 18:54:50 +0000[diff] [blame]932<em>actual structure</em> of a type, ignoring typdefs. Second, we need an
933efficient way to query whether two types are structurally identical to each
934other, ignoring typedefs. The solution to both of these problems is the idea of
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]935canonical types.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]936
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]937<!-- =============== -->
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]938<h4>Canonical Types</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]939<!-- =============== -->
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]940
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]941<p>Every instance of the Type class contains a canonical type pointer. For
942simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>",
943"<tt>int**</tt>"), the type just points to itself. For types that have a
944typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>",
945"<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their
946structurally equivalent type without any typedefs (e.g. "<tt>int</tt>",
947"<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]948
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]949<p>This design provides a constant time operation (dereferencing the canonical
950type pointer) that gives us access to the structure of types. For example,
951we can trivially tell that "bar" and "foo*" are the same type by dereferencing
952their canonical type pointers and doing a pointer comparison (they both point
953to the single "<tt>int*</tt>" type).</p>
954
955<p>Canonical types and typedef types bring up some complexities that must be
956carefully managed. Specifically, the "isa/cast/dyncast" operators generally
957shouldn't be used in code that is inspecting the AST. For example, when type
958checking the indirection operator (unary '*' on a pointer), the type checker
959must verify that the operand has a pointer type. It would not be correct to
960check that with "<tt>isa&lt;PointerType&gt;(SubExpr-&gt;getType())</tt>",
961because this predicate would fail if the subexpression had a typedef type.</p>
962
963<p>The solution to this problem are a set of helper methods on Type, used to
964check their properties. In this case, it would be correct to use
965"<tt>SubExpr-&gt;getType()-&gt;isPointerType()</tt>" to do the check. This
966predicate will return true if the <em>canonical type is a pointer</em>, which is
967true any time the type is structurally a pointer type. The only hard part here
968is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p>
969
970<p>The second problem we face is how to get access to the pointer type once we
971know it exists. To continue the example, the result type of the indirection
972operator is the pointee type of the subexpression. In order to determine the
973type, we need to get the instance of PointerType that best captures the typedef
974information in the program. If the type of the expression is literally a
975PointerType, we can return that, otherwise we have to dig through the
976typedefs to find the pointer type. For example, if the subexpression had type
977"<tt>foo*</tt>", we could return that type as the result. If the subexpression
978had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do
979<em>not</em> want "<tt>int*</tt>"). In order to provide all of this, Type has
Chris Lattner11406c12007-07-31 16:50:51 +0000[diff] [blame]980a getAsPointerType() method that checks whether the type is structurally a
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]981PointerType and, if so, returns the best one. If not, it returns a null
982pointer.</p>
983
984<p>This structure is somewhat mystical, but after meditating on it, it will
985make sense to you :).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]986
987<!-- ======================================================================= -->
988<h3 id="QualType">The QualType class</h3>
989<!-- ======================================================================= -->
990
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]991<p>The QualType class is designed as a trivial value class that is
992small, passed by-value and is efficient to query. The idea of
993QualType is that it stores the type qualifiers (const, volatile,
994restrict, plus some extended qualifiers required by language
995extensions) separately from the types themselves. QualType is
996conceptually a pair of "Type*" and the bits for these type qualifiers.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]997
998<p>By storing the type qualifiers as bits in the conceptual pair, it is
999extremely efficient to get the set of qualifiers on a QualType (just return the
1000field of the pair), add a type qualifier (which is a trivial constant-time
1001operation that sets a bit), and remove one or more type qualifiers (just return
1002a QualType with the bitfield set to empty).</p>
1003
1004<p>Further, because the bits are stored outside of the type itself, we do not
1005need to create duplicates of types with different sets of qualifiers (i.e. there
1006is only a single heap allocated "int" type: "const int" and "volatile const int"
1007both point to the same heap allocated "int" type). This reduces the heap size
1008used to represent bits and also means we do not have to consider qualifiers when
1009uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p>
1010
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]1011<p>In practice, the two most common type qualifiers (const and
1012restrict) are stored in the low bits of the pointer to the Type
1013object, together with a flag indicating whether extended qualifiers
1014are present (which must be heap-allocated). This means that QualType
1015is exactly the same size as a pointer.</p>
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1016
1017<!-- ======================================================================= -->
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1018<h3 id="DeclarationName">Declaration names</h3>
1019<!-- ======================================================================= -->
1020
1021<p>The <tt>DeclarationName</tt> class represents the name of a
1022 declaration in Clang. Declarations in the C family of languages can
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]1023 take several different forms. Most declarations are named by
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1024 simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in
1025 the function declaration <code>f(int x)</code>. In C++, declaration
1026 names can also name class constructors ("<code>Class</code>"
1027 in <code>struct Class { Class(); }</code>), class destructors
1028 ("<code>~Class</code>"), overloaded operator names ("operator+"),
1029 and conversion functions ("<code>operator void const *</code>"). In
1030 Objective-C, declaration names can refer to the names of Objective-C
1031 methods, which involve the method name and the parameters,
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]1032 collectively called a <i>selector</i>, e.g.,
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1033 "<code>setWidth:height:</code>". Since all of these kinds of
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]1034 entities - variables, functions, Objective-C methods, C++
1035 constructors, destructors, and operators - are represented as
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1036 subclasses of Clang's common <code>NamedDecl</code>
1037 class, <code>DeclarationName</code> is designed to efficiently
1038 represent any kind of name.</p>
1039
1040<p>Given
1041 a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code>
Douglas Gregor2def4832008-11-17 20:34:05 +0000[diff] [blame]1042 will produce a value that describes what kind of name <code>N</code>
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1043 stores. There are 8 options (all of the names are inside
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1044 the <code>DeclarationName</code> class)</p>
1045<dl>
1046 <dt>Identifier</dt>
1047 <dd>The name is a simple
1048 identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the
1049 corresponding <code>IdentifierInfo*</code> pointing to the actual
1050 identifier. Note that C++ overloaded operators (e.g.,
1051 "<code>operator+</code>") are represented as special kinds of
1052 identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code>
1053 function to determine whether an identifier is an overloaded
1054 operator name.</dd>
1055
1056 <dt>ObjCZeroArgSelector, ObjCOneArgSelector,
1057 ObjCMultiArgSelector</dt>
1058 <dd>The name is an Objective-C selector, which can be retrieved as a
1059 <code>Selector</code> instance
1060 via <code>N.getObjCSelector()</code>. The three possible name
1061 kinds for Objective-C reflect an optimization within
1062 the <code>DeclarationName</code> class: both zero- and
1063 one-argument selectors are stored as a
1064 masked <code>IdentifierInfo</code> pointer, and therefore require
1065 very little space, since zero- and one-argument selectors are far
1066 more common than multi-argument selectors (which use a different
1067 structure).</dd>
1068
1069 <dt>CXXConstructorName</dt>
1070 <dd>The name is a C++ constructor
1071 name. Use <code>N.getCXXNameType()</code> to retrieve
1072 the <a href="#QualType">type</a> that this constructor is meant to
1073 construct. The type is always the canonical type, since all
1074 constructors for a given type have the same name.</dd>
1075
1076 <dt>CXXDestructorName</dt>
1077 <dd>The name is a C++ destructor
1078 name. Use <code>N.getCXXNameType()</code> to retrieve
1079 the <a href="#QualType">type</a> whose destructor is being
1080 named. This type is always a canonical type.</dd>
1081
1082 <dt>CXXConversionFunctionName</dt>
1083 <dd>The name is a C++ conversion function. Conversion functions are
1084 named according to the type they convert to, e.g., "<code>operator void
1085 const *</code>". Use <code>N.getCXXNameType()</code> to retrieve
1086 the type that this conversion function converts to. This type is
1087 always a canonical type.</dd>
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1088
1089 <dt>CXXOperatorName</dt>
1090 <dd>The name is a C++ overloaded operator name. Overloaded operators
1091 are named according to their spelling, e.g.,
1092 "<code>operator+</code>" or "<code>operator new
1093 []</code>". Use <code>N.getCXXOverloadedOperator()</code> to
1094 retrieve the overloaded operator (a value of
1095 type <code>OverloadedOperatorKind</code>).</dd>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1096</dl>
1097
1098<p><code>DeclarationName</code>s are cheap to create, copy, and
1099 compare. They require only a single pointer's worth of storage in
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1100 the common cases (identifiers, zero-
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1101 and one-argument Objective-C selectors) and use dense, uniqued
1102 storage for the other kinds of
1103 names. Two <code>DeclarationName</code>s can be compared for
1104 equality (<code>==</code>, <code>!=</code>) using a simple bitwise
1105 comparison, can be ordered
1106 with <code>&lt;</code>, <code>&gt;</code>, <code>&lt;=</code>,
1107 and <code>&gt;=</code> (which provide a lexicographical ordering for
1108 normal identifiers but an unspecified ordering for other kinds of
1109 names), and can be placed into LLVM <code>DenseMap</code>s
1110 and <code>DenseSet</code>s.</p>
1111
1112<p><code>DeclarationName</code> instances can be created in different
1113 ways depending on what kind of name the instance will store. Normal
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1114 identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1115 (<code>Selector</code>) can be implicitly converted
1116 to <code>DeclarationName</code>s. Names for C++ constructors,
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1117 destructors, conversion functions, and overloaded operators can be retrieved from
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1118 the <code>DeclarationNameTable</code>, an instance of which is
1119 available as <code>ASTContext::DeclarationNames</code>. The member
1120 functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>,
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1121 <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively,
1122 return <code>DeclarationName</code> instances for the four kinds of
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1123 C++ special function names.</p>
1124
1125<!-- ======================================================================= -->
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1126<h3 id="DeclContext">Declaration contexts</h3>
1127<!-- ======================================================================= -->
1128<p>Every declaration in a program exists within some <i>declaration
1129 context</i>, such as a translation unit, namespace, class, or
1130 function. Declaration contexts in Clang are represented by
1131 the <code>DeclContext</code> class, from which the various
1132 declaration-context AST nodes
1133 (<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>,
1134 etc.) will derive. The <code>DeclContext</code> class provides
1135 several facilities common to each declaration context:</p>
1136<dl>
1137 <dt>Source-centric vs. Semantics-centric View of Declarations</dt>
1138 <dd><code>DeclContext</code> provides two views of the declarations
1139 stored within a declaration context. The source-centric view
1140 accurately represents the program source code as written, including
1141 multiple declarations of entities where present (see the
1142 section <a href="#Redeclarations">Redeclarations and
1143 Overloads</a>), while the semantics-centric view represents the
1144 program semantics. The two views are kept synchronized by semantic
1145 analysis while the ASTs are being constructed.</dd>
1146
1147 <dt>Storage of declarations within that context</dt>
1148 <dd>Every declaration context can contain some number of
1149 declarations. For example, a C++ class (represented
1150 by <code>RecordDecl</code>) contains various member functions,
1151 fields, nested types, and so on. All of these declarations will be
1152 stored within the <code>DeclContext</code>, and one can iterate
1153 over the declarations via
1154 [<code>DeclContext::decls_begin()</code>,
1155 <code>DeclContext::decls_end()</code>). This mechanism provides
1156 the source-centric view of declarations in the context.</dd>
1157
1158 <dt>Lookup of declarations within that context</dt>
1159 <dd>The <code>DeclContext</code> structure provides efficient name
1160 lookup for names within that declaration context. For example,
1161 if <code>N</code> is a namespace we can look for the
1162 name <code>N::f</code>
1163 using <code>DeclContext::lookup</code>. The lookup itself is
1164 based on a lazily-constructed array (for declaration contexts
1165 with a small number of declarations) or hash table (for
1166 declaration contexts with more declarations). The lookup
1167 operation provides the semantics-centric view of the declarations
1168 in the context.</dd>
1169
1170 <dt>Ownership of declarations</dt>
1171 <dd>The <code>DeclContext</code> owns all of the declarations that
1172 were declared within its declaration context, and is responsible
1173 for the management of their memory as well as their
1174 (de-)serialization.</dd>
1175</dl>
1176
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1177<p>All declarations are stored within a declaration context, and one
1178 can query
1179 information about the context in which each declaration lives. One
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1180 can retrieve the <code>DeclContext</code> that contains a
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1181 particular <code>Decl</code>
1182 using <code>Decl::getDeclContext</code>. However, see the
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1183 section <a href="#LexicalAndSemanticContexts">Lexical and Semantic
1184 Contexts</a> for more information about how to interpret this
1185 context information.</p>
1186
1187<h4 id="Redeclarations">Redeclarations and Overloads</h4>
1188<p>Within a translation unit, it is common for an entity to be
1189declared several times. For example, we might declare a function "f"
1190 and then later re-declare it as part of an inlined definition:</p>
1191
1192<pre>
1193void f(int x, int y, int z = 1);
1194
1195inline void f(int x, int y, int z) { /* ... */ }
1196</pre>
1197
1198<p>The representation of "f" differs in the source-centric and
1199 semantics-centric views of a declaration context. In the
1200 source-centric view, all redeclarations will be present, in the
1201 order they occurred in the source code, making
1202 this view suitable for clients that wish to see the structure of
1203 the source code. In the semantics-centric view, only the most recent "f"
1204 will be found by the lookup, since it effectively replaces the first
1205 declaration of "f".</p>
1206
1207<p>In the semantics-centric view, overloading of functions is
1208 represented explicitly. For example, given two declarations of a
1209 function "g" that are overloaded, e.g.,</p>
1210<pre>
1211void g();
1212void g(int);
1213</pre>
1214<p>the <code>DeclContext::lookup</code> operation will return
Jonathan D. Turnerd3224292011-07-06 18:12:36 +0000[diff] [blame]1215 a <code>DeclContext::lookup_result</code> that contains a range of iterators
1216 over declarations of "g". Clients that perform semantic analysis on a
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1217 program that is not concerned with the actual source code will
1218 primarily use this semantics-centric view.</p>
1219
1220<h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4>
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1221<p>Each declaration has two potentially different
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1222 declaration contexts: a <i>lexical</i> context, which corresponds to
1223 the source-centric view of the declaration context, and
1224 a <i>semantic</i> context, which corresponds to the
1225 semantics-centric view. The lexical context is accessible
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1226 via <code>Decl::getLexicalDeclContext</code> while the
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1227 semantic context is accessible
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1228 via <code>Decl::getDeclContext</code>, both of which return
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1229 <code>DeclContext</code> pointers. For most declarations, the two
1230 contexts are identical. For example:</p>
1231
1232<pre>
1233class X {
1234public:
1235 void f(int x);
1236};
1237</pre>
1238
1239<p>Here, the semantic and lexical contexts of <code>X::f</code> are
1240 the <code>DeclContext</code> associated with the
1241 class <code>X</code> (itself stored as a <code>RecordDecl</code> AST
1242 node). However, we can now define <code>X::f</code> out-of-line:</p>
1243
1244<pre>
1245void X::f(int x = 17) { /* ... */ }
1246</pre>
1247
1248<p>This definition of has different lexical and semantic
1249 contexts. The lexical context corresponds to the declaration
1250 context in which the actual declaration occurred in the source
1251 code, e.g., the translation unit containing <code>X</code>. Thus,
1252 this declaration of <code>X::f</code> can be found by traversing
1253 the declarations provided by
1254 [<code>decls_begin()</code>, <code>decls_end()</code>) in the
1255 translation unit.</p>
1256
1257<p>The semantic context of <code>X::f</code> corresponds to the
1258 class <code>X</code>, since this member function is (semantically) a
1259 member of <code>X</code>. Lookup of the name <code>f</code> into
1260 the <code>DeclContext</code> associated with <code>X</code> will
1261 then return the definition of <code>X::f</code> (including
1262 information about the default argument).</p>
1263
1264<h4 id="TransparentContexts">Transparent Declaration Contexts</h4>
1265<p>In C and C++, there are several contexts in which names that are
1266 logically declared inside another declaration will actually "leak"
1267 out into the enclosing scope from the perspective of name
1268 lookup. The most obvious instance of this behavior is in
1269 enumeration types, e.g.,</p>
1270<pre>
1271enum Color {
1272 Red,
1273 Green,
1274 Blue
1275};
1276</pre>
1277
1278<p>Here, <code>Color</code> is an enumeration, which is a declaration
1279 context that contains the
1280 enumerators <code>Red</code>, <code>Green</code>,
1281 and <code>Blue</code>. Thus, traversing the list of declarations
1282 contained in the enumeration <code>Color</code> will
1283 yield <code>Red</code>, <code>Green</code>,
1284 and <code>Blue</code>. However, outside of the scope
1285 of <code>Color</code> one can name the enumerator <code>Red</code>
1286 without qualifying the name, e.g.,</p>
1287
1288<pre>
1289Color c = Red;
1290</pre>
1291
1292<p>There are other entities in C++ that provide similar behavior. For
1293 example, linkage specifications that use curly braces:</p>
1294
1295<pre>
1296extern "C" {
1297 void f(int);
1298 void g(int);
1299}
1300// f and g are visible here
1301</pre>
1302
1303<p>For source-level accuracy, we treat the linkage specification and
1304 enumeration type as a
1305 declaration context in which its enclosed declarations ("Red",
1306 "Green", and "Blue"; "f" and "g")
1307 are declared. However, these declarations are visible outside of the
1308 scope of the declaration context.</p>
1309
1310<p>These language features (and several others, described below) have
1311 roughly the same set of
1312 requirements: declarations are declared within a particular lexical
1313 context, but the declarations are also found via name lookup in
1314 scopes enclosing the declaration itself. This feature is implemented
1315 via <i>transparent</i> declaration contexts
1316 (see <code>DeclContext::isTransparentContext()</code>), whose
1317 declarations are visible in the nearest enclosing non-transparent
1318 declaration context. This means that the lexical context of the
1319 declaration (e.g., an enumerator) will be the
1320 transparent <code>DeclContext</code> itself, as will the semantic
1321 context, but the declaration will be visible in every outer context
1322 up to and including the first non-transparent declaration context (since
1323 transparent declaration contexts can be nested).</p>
1324
1325<p>The transparent <code>DeclContexts</code> are:</p>
1326<ul>
1327 <li>Enumerations (but not C++0x "scoped enumerations"):
1328 <pre>
1329enum Color {
1330 Red,
1331 Green,
1332 Blue
1333};
1334// Red, Green, and Blue are in scope
1335 </pre></li>
1336 <li>C++ linkage specifications:
1337 <pre>
1338extern "C" {
1339 void f(int);
1340 void g(int);
1341}
1342// f and g are in scope
1343 </pre></li>
1344 <li>Anonymous unions and structs:
1345 <pre>
1346struct LookupTable {
1347 bool IsVector;
1348 union {
1349 std::vector&lt;Item&gt; *Vector;
1350 std::set&lt;Item&gt; *Set;
1351 };
1352};
1353
1354LookupTable LT;
1355LT.Vector = 0; // Okay: finds Vector inside the unnamed union
1356 </pre>
1357 </li>
1358 <li>C++0x inline namespaces:
1359<pre>
1360namespace mylib {
1361 inline namespace debug {
1362 class X;
1363 }
1364}
1365mylib::X *xp; // okay: mylib::X refers to mylib::debug::X
1366</pre>
1367</li>
1368</ul>
1369
1370
1371<h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4>
1372<p>C++ namespaces have the interesting--and, so far, unique--property that
1373the namespace can be defined multiple times, and the declarations
1374provided by each namespace definition are effectively merged (from
1375the semantic point of view). For example, the following two code
1376snippets are semantically indistinguishable:</p>
1377<pre>
1378// Snippet #1:
1379namespace N {
1380 void f();
1381}
1382namespace N {
1383 void f(int);
1384}
1385
1386// Snippet #2:
1387namespace N {
1388 void f();
1389 void f(int);
1390}
1391</pre>
1392
1393<p>In Clang's representation, the source-centric view of declaration
1394 contexts will actually have two separate <code>NamespaceDecl</code>
1395 nodes in Snippet #1, each of which is a declaration context that
1396 contains a single declaration of "f". However, the semantics-centric
1397 view provided by name lookup into the namespace <code>N</code> for
Jonathan D. Turnerd3224292011-07-06 18:12:36 +0000[diff] [blame]1398 "f" will return a <code>DeclContext::lookup_result</code> that contains
1399 a range of iterators over declarations of "f".</p>
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1400
1401<p><code>DeclContext</code> manages multiply-defined declaration
1402 contexts internally. The
1403 function <code>DeclContext::getPrimaryContext</code> retrieves the
1404 "primary" context for a given <code>DeclContext</code> instance,
1405 which is the <code>DeclContext</code> responsible for maintaining
1406 the lookup table used for the semantics-centric view. Given the
1407 primary context, one can follow the chain
1408 of <code>DeclContext</code> nodes that define additional
1409 declarations via <code>DeclContext::getNextContext</code>. Note that
1410 these functions are used internally within the lookup and insertion
1411 methods of the <code>DeclContext</code>, so the vast majority of
1412 clients can ignore them.</p>
1413
1414<!-- ======================================================================= -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1415<h3 id="CFG">The <tt>CFG</tt> class</h3>
1416<!-- ======================================================================= -->
1417
1418<p>The <tt>CFG</tt> class is designed to represent a source-level
1419control-flow graph for a single statement (<tt>Stmt*</tt>). Typically
1420instances of <tt>CFG</tt> are constructed for function bodies (usually
1421an instance of <tt>CompoundStmt</tt>), but can also be instantiated to
1422represent the control-flow of any class that subclasses <tt>Stmt</tt>,
1423which includes simple expressions. Control-flow graphs are especially
1424useful for performing
1425<a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow-
1426or path-sensitive</a> program analyses on a given function.</p>
1427
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1428<!-- ============ -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1429<h4>Basic Blocks</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1430<!-- ============ -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1431
1432<p>Concretely, an instance of <tt>CFG</tt> is a collection of basic
1433blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which
1434simply contains an ordered sequence of <tt>Stmt*</tt> (each referring
1435to statements in the AST). The ordering of statements within a block
1436indicates unconditional flow of control from one statement to the
1437next. <a href="#ConditionalControlFlow">Conditional control-flow</a>
1438is represented using edges between basic blocks. The statements
1439within a given <tt>CFGBlock</tt> can be traversed using
1440the <tt>CFGBlock::*iterator</tt> interface.</p>
1441
1442<p>
Ted Kremenek18e17e72007-10-18 22:50:52 +0000[diff] [blame]1443A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1444the control-flow graph it represents. Each <tt>CFGBlock</tt> within a
1445CFG is also uniquely numbered (accessible
1446via <tt>CFGBlock::getBlockID()</tt>). Currently the number is
1447based on the ordering the blocks were created, but no assumptions
1448should be made on how <tt>CFGBlock</tt>s are numbered other than their
1449numbers are unique and that they are numbered from 0..N-1 (where N is
1450the number of basic blocks in the CFG).</p>
1451
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1452<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1453<h4>Entry and Exit Blocks</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1454<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1455
1456Each instance of <tt>CFG</tt> contains two special blocks:
1457an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which
1458has no incoming edges, and an <i>exit</i> block (accessible
1459via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither
1460block contains any statements, and they serve the role of providing a
1461clear entrance and exit for a body of code such as a function body.
1462The presence of these empty blocks greatly simplifies the
1463implementation of many analyses built on top of CFGs.
1464
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1465<!-- ===================================================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1466<h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1467<!-- ===================================================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1468
1469<p>Conditional control-flow (such as those induced by if-statements
1470and loops) is represented as edges between <tt>CFGBlock</tt>s.
1471Because different C language constructs can induce control-flow,
1472each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that
1473represents the <i>terminator</i> of the block. A terminator is simply
1474the statement that caused the control-flow, and is used to identify
1475the nature of the conditional control-flow between blocks. For
1476example, in the case of an if-statement, the terminator refers to
1477the <tt>IfStmt</tt> object in the AST that represented the given
1478branch.</p>
1479
1480<p>To illustrate, consider the following code example:</p>
1481
1482<code>
1483int foo(int x) {<br>
1484&nbsp;&nbsp;x = x + 1;<br>
1485<br>
1486&nbsp;&nbsp;if (x > 2) x++;<br>
1487&nbsp;&nbsp;else {<br>
1488&nbsp;&nbsp;&nbsp;&nbsp;x += 2;<br>
1489&nbsp;&nbsp;&nbsp;&nbsp;x *= 2;<br>
1490&nbsp;&nbsp;}<br>
1491<br>
1492&nbsp;&nbsp;return x;<br>
1493}
1494</code>
1495
1496<p>After invoking the parser+semantic analyzer on this code fragment,
1497the AST of the body of <tt>foo</tt> is referenced by a
1498single <tt>Stmt*</tt>. We can then construct an instance
1499of <tt>CFG</tt> representing the control-flow graph of this function
1500body by single call to a static class method:</p>
1501
1502<code>
1503&nbsp;&nbsp;Stmt* FooBody = ...<br>
1504&nbsp;&nbsp;CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody);
1505</code>
1506
1507<p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt>
1508to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no
1509longer needed.</p>
1510
1511<p>Along with providing an interface to iterate over
1512its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods
1513that are useful for debugging and visualizing CFGs. For example, the
1514method
1515<tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to
1516standard error. This is especially useful when one is using a
1517debugger such as gdb. For example, here is the output
1518of <tt>FooCFG->dump()</tt>:</p>
1519
1520<code>
1521&nbsp;[ B5 (ENTRY) ]<br>
1522&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (0):<br>
1523&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B4<br>
1524<br>
1525&nbsp;[ B4 ]<br>
1526&nbsp;&nbsp;&nbsp;&nbsp;1: x = x + 1<br>
1527&nbsp;&nbsp;&nbsp;&nbsp;2: (x > 2)<br>
1528&nbsp;&nbsp;&nbsp;&nbsp;<b>T: if [B4.2]</b><br>
1529&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B5<br>
1530&nbsp;&nbsp;&nbsp;&nbsp;Successors (2): B3 B2<br>
1531<br>
1532&nbsp;[ B3 ]<br>
1533&nbsp;&nbsp;&nbsp;&nbsp;1: x++<br>
1534&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
1535&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
1536<br>
1537&nbsp;[ B2 ]<br>
1538&nbsp;&nbsp;&nbsp;&nbsp;1: x += 2<br>
1539&nbsp;&nbsp;&nbsp;&nbsp;2: x *= 2<br>
1540&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
1541&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
1542<br>
1543&nbsp;[ B1 ]<br>
1544&nbsp;&nbsp;&nbsp;&nbsp;1: return x;<br>
1545&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (2): B2 B3<br>
1546&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B0<br>
1547<br>
1548&nbsp;[ B0 (EXIT) ]<br>
1549&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B1<br>
1550&nbsp;&nbsp;&nbsp;&nbsp;Successors (0):
1551</code>
1552
1553<p>For each block, the pretty-printed output displays for each block
1554the number of <i>predecessor</i> blocks (blocks that have outgoing
1555control-flow to the given block) and <i>successor</i> blocks (blocks
1556that have control-flow that have incoming control-flow from the given
1557block). We can also clearly see the special entry and exit blocks at
1558the beginning and end of the pretty-printed output. For the entry
1559block (block B5), the number of predecessor blocks is 0, while for the
1560exit block (block B0) the number of successor blocks is 0.</p>
1561
1562<p>The most interesting block here is B4, whose outgoing control-flow
1563represents the branching caused by the sole if-statement
1564in <tt>foo</tt>. Of particular interest is the second statement in
1565the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed
1566as <b><tt>if [B4.2]</tt></b>. The second statement represents the
1567evaluation of the condition of the if-statement, which occurs before
1568the actual branching of control-flow. Within the <tt>CFGBlock</tt>
1569for B4, the <tt>Stmt*</tt> for the second statement refers to the
1570actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus
1571pointers to subclasses of <tt>Expr</tt> can appear in the list of
1572statements in a block, and not just subclasses of <tt>Stmt</tt> that
1573refer to proper C statements.</p>
1574
1575<p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt>
1576object in the AST. The pretty-printer outputs <b><tt>if
1577[B4.2]</tt></b> because the condition expression of the if-statement
1578has an actual place in the basic block, and thus the terminator is
1579essentially
1580<i>referring</i> to the expression that is the second statement of
1581block B4 (i.e., B4.2). In this manner, conditions for control-flow
1582(which also includes conditions for loops and switch statements) are
1583hoisted into the actual basic block.</p>
1584
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1585<!-- ===================== -->
1586<!-- <h4>Implicit Control-Flow</h4> -->
1587<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1588
1589<!--
1590<p>A key design principle of the <tt>CFG</tt> class was to not require
1591any transformations to the AST in order to represent control-flow.
1592Thus the <tt>CFG</tt> does not perform any "lowering" of the
1593statements in an AST: loops are not transformed into guarded gotos,
1594short-circuit operations are not converted to a set of if-statements,
1595and so on.</p>
1596-->
Ted Kremenek17a295d2008-06-11 06:19:49 +0000[diff] [blame]1597
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1598
1599<!-- ======================================================================= -->
1600<h3 id="Constants">Constant Folding in the Clang AST</h3>
1601<!-- ======================================================================= -->
1602
1603<p>There are several places where constants and constant folding matter a lot to
1604the Clang front-end. First, in general, we prefer the AST to retain the source
1605code as close to how the user wrote it as possible. This means that if they
1606wrote "5+4", we want to keep the addition and two constants in the AST, we don't
1607want to fold to "9". This means that constant folding in various ways turns
1608into a tree walk that needs to handle the various cases.</p>
1609
1610<p>However, there are places in both C and C++ that require constants to be
1611folded. For example, the C standard defines what an "integer constant
1612expression" (i-c-e) is with very precise and specific requirements. The
1613language then requires i-c-e's in a lot of places (for example, the size of a
1614bitfield, the value for a case statement, etc). For these, we have to be able
1615to constant fold the constants, to do semantic checks (e.g. verify bitfield size
1616is non-negative and that case statements aren't duplicated). We aim for Clang
1617to be very pedantic about this, diagnosing cases when the code does not use an
1618i-c-e where one is required, but accepting the code unless running with
1619<tt>-pedantic-errors</tt>.</p>
1620
1621<p>Things get a little bit more tricky when it comes to compatibility with
1622real-world source code. Specifically, GCC has historically accepted a huge
1623superset of expressions as i-c-e's, and a lot of real world code depends on this
1624unfortuate accident of history (including, e.g., the glibc system headers). GCC
1625accepts anything its "fold" optimizer is capable of reducing to an integer
1626constant, which means that the definition of what it accepts changes as its
1627optimizer does. One example is that GCC accepts things like "case X-X:" even
1628when X is a variable, because it can fold this to 0.</p>
1629
1630<p>Another issue are how constants interact with the extensions we support, such
1631as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99
1632obviously does not specify the semantics of any of these extensions, and the
1633definition of i-c-e does not include them. However, these extensions are often
1634used in real code, and we have to have a way to reason about them.</p>
1635
1636<p>Finally, this is not just a problem for semantic analysis. The code
1637generator and other clients have to be able to fold constants (e.g. to
1638initialize global variables) and has to handle a superset of what C99 allows.
1639Further, these clients can benefit from extended information. For example, we
1640know that "foo()||1" always evaluates to true, but we can't replace the
1641expression with true because it has side effects.</p>
1642
1643<!-- ======================= -->
1644<h4>Implementation Approach</h4>
1645<!-- ======================= -->
1646
1647<p>After trying several different approaches, we've finally converged on a
1648design (Note, at the time of this writing, not all of this has been implemented,
1649consider this a design goal!). Our basic approach is to define a single
1650recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is
1651implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar'
1652type (integer, fp, complex, or pointer) this method returns the following
1653information:</p>
1654
1655<ul>
1656<li>Whether the expression is an integer constant expression, a general
1657 constant that was folded but has no side effects, a general constant that
1658 was folded but that does have side effects, or an uncomputable/unfoldable
1659 value.
1660</li>
1661<li>If the expression was computable in any way, this method returns the APValue
1662 for the result of the expression.</li>
1663<li>If the expression is not evaluatable at all, this method returns
1664 information on one of the problems with the expression. This includes a
1665 SourceLocation for where the problem is, and a diagnostic ID that explains
1666 the problem. The diagnostic should be have ERROR type.</li>
1667<li>If the expression is not an integer constant expression, this method returns
1668 information on one of the problems with the expression. This includes a
1669 SourceLocation for where the problem is, and a diagnostic ID that explains
1670 the problem. The diagnostic should be have EXTENSION type.</li>
1671</ul>
1672
1673<p>This information gives various clients the flexibility that they want, and we
1674will eventually have some helper methods for various extensions. For example,
1675Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which
1676calls Evaluate on the expression. If the expression is not foldable, the error
1677is emitted, and it would return true. If the expression is not an i-c-e, the
1678EXTENSION diagnostic is emitted. Finally it would return false to indicate that
1679the AST is ok.</p>
1680
1681<p>Other clients can use the information in other ways, for example, codegen can
1682just use expressions that are foldable in any way.</p>
1683
1684<!-- ========== -->
1685<h4>Extensions</h4>
1686<!-- ========== -->
1687
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]1688<p>This section describes how some of the various extensions Clang supports
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1689interacts with constant evaluation:</p>
1690
1691<ul>
1692<li><b><tt>__extension__</tt></b>: The expression form of this extension causes
1693 any evaluatable subexpression to be accepted as an integer constant
1694 expression.</li>
1695<li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer
Chris Lattner28daa532008-12-12 06:55:44 +0000[diff] [blame]1696 constant expression) if the operand is any evaluatable constant. As a
1697 special case, if <tt>__builtin_constant_p</tt> is the (potentially
1698 parenthesized) condition of a conditional operator expression ("?:"), only
Chris Lattner42b83dd2008-12-12 18:00:51 +0000[diff] [blame]1699 the true side of the conditional operator is considered, and it is evaluated
1700 with full constant folding.</li>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1701<li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an
1702 integer constant expression, but we accept any constant as an "extension of
1703 an extension". This only evaluates one operand depending on which way the
1704 condition evaluates.</li>
1705<li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer
1706 constant expression.</li>
1707<li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a
1708 floating-point literal.</li>
1709<li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as
1710 general constant expressions.</li>
Douglas Gregoreb661ed2010-09-11 18:08:34 +0000[diff] [blame]1711<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>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1712</ul>
1713
1714
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]1715<!-- ======================================================================= -->
1716<h2 id="Howtos">How to change Clang</h2>
1717<!-- ======================================================================= -->
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1718
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]1719<!-- ======================================================================= -->
1720<h3 id="AddingAttributes">How to add an attribute</h3>
1721<!-- ======================================================================= -->
1722
1723<p>To add an attribute, you'll have to add it to the list of attributes, add it
1724to the parsing phase, and look for it in the AST scan.
1725<a href="http://llvm.org/viewvc/llvm-project?view=rev&revision=124217">r124217</a>
1726has a good example of adding a warning attribute.</p>
1727
1728<p>(Beware that this hasn't been reviewed/fixed by the people who designed the
1729attributes system yet.)</p>
1730
1731<h4><a
1732href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?view=markup">include/clang/Basic/Attr.td</a></h4>
1733
1734<p>Each attribute gets a <tt>def</tt> inheriting from <tt>Attr</tt> or one of
1735its subclasses. <tt>InheritableAttr</tt> means that the attribute also applies
1736to subsequent declarations of the same name.</p>
1737
1738<p><tt>Spellings</tt> lists the strings that can appear in
1739<tt>__attribute__((here))</tt> or <tt>[[here]]</tt>. All such strings
1740will be synonymous. If you want to allow the <tt>[[]]</tt> C++0x
1741syntax, you have to define a list of <tt>Namespaces</tt>, which will
1742let users write <tt>[[namespace:spelling]]</tt>. Using the empty
1743string for a namespace will allow users to write just the spelling
1744with no "<tt>:</tt>".</p>
1745
1746<p><tt>Subjects</tt> restricts what kinds of AST node to which this attribute
1747can appertain (roughly, attach).</p>
1748
1749<p><tt>Args</tt> names the arguments the attribute takes, in order. If
1750<tt>Args</tt> is <tt>[StringArgument&lt;"Arg1">, IntArgument&lt;"Arg2">]</tt>
1751then <tt>__attribute__((myattribute("Hello", 3)))</tt> will be a valid use.</p>
1752
1753<h4>Boilerplate</h4>
1754
1755<p>Add an element to the <tt>AttributeList::Kind</tt> enum in <a
1756href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Sema/AttributeList.h?view=markup">include/clang/Sema/AttributeList.h</a>
1757named <tt>AT_lower_with_underscores</tt>. That is, a CamelCased
1758<tt>AttributeName</tt> in <tt>Attr.td</tt> name should become
1759<tt>AT_attribute_name</tt>.</p>
1760
1761<p>Add a case to the <tt>StringSwitch</tt> in <tt>AttributeList::getKind()</tt>
1762in <a
1763href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/AttributeList.cpp?view=markup">lib/Sema/AttributeList.cpp</a>
1764for each spelling of your attribute. Less common attributes should come toward
1765the end of that list.</p>
1766
1767<p>Write a new <tt>HandleYourAttr()</tt> function in <a
1768href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?view=markup">lib/Sema/SemaDeclAttr.cpp</a>,
1769and add a case to the switch in <tt>ProcessNonInheritableDeclAttr()</tt> or
1770<tt>ProcessInheritableDeclAttr()</tt> forwarding to it.</p>
1771
1772<p>If your attribute causes extra warnings to fire, define a <tt>DiagGroup</tt>
1773in <a
1774href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?view=markup">include/clang/Basic/DiagnosticGroups.td</a>
1775named after the attribute's <tt>Spelling</tt> with "_"s replaced by "-"s. If
1776you're only defining one diagnostic, you can skip <tt>DiagnosticGroups.td</tt>
1777and use <tt>InGroup&lt;DiagGroup&lt;"your-attribute">></tt> directly in <a
1778href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?view=markup">DiagnosticSemaKinds.td</a></p>
1779
1780<h4>The meat of your attribute</h4>
1781
1782<p>Find an appropriate place in Clang to do whatever your attribute needs to do.
1783Check for the attribute's presence using <tt>Decl::getAttr&lt;YourAttr>()</tt>.</p>
1784
1785<p>Update the <a href="LanguageExtensions.html">Clang Language Extensions</a>
1786document to describe your new attribute.</p>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1787
Ted Kremenek17a295d2008-06-11 06:19:49 +0000[diff] [blame]1788</div>
1789</body>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1790</html>