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Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]5<title>"Clang" CFE Internals Manual</title>
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18<div id="content">
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]19
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]20<h1>"Clang" CFE Internals Manual</h1>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]21
22<ul>
23<li><a href="#intro">Introduction</a></li>
Peter Collingbourne967c1182011-10-15 16:59:24 +0000[diff] [blame]24<li><a href="#libsupport">LLVM Support Library</a></li>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]25<li><a href="#libbasic">The Clang 'Basic' Library</a>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]26 <ul>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]27 <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]28 <li><a href="#SourceLocation">The SourceLocation and SourceManager
29 classes</a></li>
Douglas Gregor715c92a2010-10-27 16:02:28 +0000[diff] [blame]30 <li><a href="#SourceRange">SourceRange and CharSourceRange</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]31 </ul>
32</li>
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]33<li><a href="#libdriver">The Driver Library</a>
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]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>
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]37</li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]38<li><a href="#liblex">The Lexer and Preprocessor Library</a>
39 <ul>
40 <li><a href="#Token">The Token class</a></li>
41 <li><a href="#Lexer">The Lexer class</a></li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]42 <li><a href="#AnnotationToken">Annotation Tokens</a></li>
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]43 <li><a href="#TokenLexer">The TokenLexer class</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]44 <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li>
45 </ul>
46</li>
47<li><a href="#libparse">The Parser Library</a>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]48</li>
49<li><a href="#libast">The AST Library</a>
50 <ul>
51 <li><a href="#Type">The Type class and its subclasses</a></li>
52 <li><a href="#QualType">The QualType class</a></li>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]53 <li><a href="#DeclarationName">Declaration names</a></li>
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]54 <li><a href="#DeclContext">Declaration contexts</a>
55 <ul>
56 <li><a href="#Redeclarations">Redeclarations and Overloads</a></li>
57 <li><a href="#LexicalAndSemanticContexts">Lexical and Semantic
58 Contexts</a></li>
59 <li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li>
60 <li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li>
61 </ul>
62 </li>
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]63 <li><a href="#CFG">The CFG class</a></li>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]64 <li><a href="#Constants">Constant Folding in the Clang AST</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]65 </ul>
66</li>
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]67<li><a href="#Howtos">Howto guides</a>
68 <ul>
69 <li><a href="#AddingAttributes">How to add an attribute</a></li>
Douglas Gregor1f634c62011-09-30 21:32:37 +0000[diff] [blame]70 <li><a href="#AddingExprStmt">How to add a new expression or statement</a></li>
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]71 </ul>
72</li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]73</ul>
74
75
76<!-- ======================================================================= -->
77<h2 id="intro">Introduction</h2>
78<!-- ======================================================================= -->
79
80<p>This document describes some of the more important APIs and internal design
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]81decisions made in the Clang C front-end. The purpose of this document is to
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]82both capture some of this high level information and also describe some of the
83design decisions behind it. This is meant for people interested in hacking on
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]84Clang, not for end-users. The description below is categorized by
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]85libraries, and does not describe any of the clients of the libraries.</p>
86
87<!-- ======================================================================= -->
Peter Collingbourne967c1182011-10-15 16:59:24 +0000[diff] [blame]88<h2 id="libsupport">LLVM Support Library</h2>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]89<!-- ======================================================================= -->
90
Peter Collingbourne967c1182011-10-15 16:59:24 +0000[diff] [blame]91<p>The LLVM libsupport library provides many underlying libraries and
92<a href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>,
93including command line option processing, various containers and a system
94abstraction layer, which is used for file system access.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]95
96<!-- ======================================================================= -->
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]97<h2 id="libbasic">The Clang 'Basic' Library</h2>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]98<!-- ======================================================================= -->
99
100<p>This library certainly needs a better name. The 'basic' library contains a
101number of low-level utilities for tracking and manipulating source buffers,
102locations within the source buffers, diagnostics, tokens, target abstraction,
103and information about the subset of the language being compiled for.</p>
104
105<p>Part of this infrastructure is specific to C (such as the TargetInfo class),
106other parts could be reused for other non-C-based languages (SourceLocation,
107SourceManager, Diagnostics, FileManager). When and if there is future demand
108we can figure out if it makes sense to introduce a new library, move the general
109classes somewhere else, or introduce some other solution.</p>
110
111<p>We describe the roles of these classes in order of their dependencies.</p>
112
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]113
114<!-- ======================================================================= -->
115<h3 id="Diagnostics">The Diagnostics Subsystem</h3>
116<!-- ======================================================================= -->
117
118<p>The Clang Diagnostics subsystem is an important part of how the compiler
119communicates with the human. Diagnostics are the warnings and errors produced
120when the code is incorrect or dubious. In Clang, each diagnostic produced has
Sebastian Redl9bc2a992010-07-07 23:42:27 +0000[diff] [blame]121(at the minimum) a unique ID, an English translation associated with it, a <a
122href="#SourceLocation">SourceLocation</a> to "put the caret", and a severity (e.g.
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]123<tt>WARNING</tt> or <tt>ERROR</tt>). They can also optionally include a number
124of arguments to the dianostic (which fill in "%0"'s in the string) as well as a
125number of source ranges that related to the diagnostic.</p>
126
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]127<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]128driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many
129different ways</a> depending on how the DiagnosticClient interface is
Sebastian Redl9bc2a992010-07-07 23:42:27 +0000[diff] [blame]130implemented. A representative example of a diagnostic is:</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]131
132<pre>
133t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]134 <span style="color:darkgreen">P = (P-42) + Gamma*4;</span>
135 <span style="color:blue">~~~~~~ ^ ~~~~~~~</span>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]136</pre>
137
138<p>In this example, you can see the English translation, the severity (error),
139you can see the source location (the caret ("^") and file/line/column info),
140the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex
141float"). You'll have to believe me that there is a unique ID backing the
142diagnostic :).</p>
143
144<p>Getting all of this to happen has several steps and involves many moving
145pieces, this section describes them and talks about best practices when adding
146a new diagnostic.</p>
147
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]148<!-- ============================= -->
149<h4>The Diagnostic*Kinds.td files</h4>
150<!-- ============================= -->
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]151
Chris Lattner4c50b692010-05-01 17:35:19 +0000[diff] [blame]152<p>Diagnostics are created by adding an entry to one of the <tt>
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]153clang/Basic/Diagnostic*Kinds.td</tt> files, depending on what library will
154be using it. From this file, tblgen generates the unique ID of the diagnostic,
155the severity of the diagnostic and the English translation + format string.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]156
157<p>There is little sanity with the naming of the unique ID's right now. Some
158start with err_, warn_, ext_ to encode the severity into the name. Since the
159enum is referenced in the C++ code that produces the diagnostic, it is somewhat
160useful for it to be reasonably short.</p>
161
162<p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>,
163<tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The
164<tt>ERROR</tt> severity is used for diagnostics indicating the program is never
165acceptable under any circumstances. When an error is emitted, the AST for the
166input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt>
167severities are used for extensions to the language that Clang accepts. This
168means that Clang fully understands and can represent them in the AST, but we
169produce diagnostics to tell the user their code is non-portable. The difference
170is that the former are ignored by default, and the later warn by default. The
171<tt>WARNING</tt> severity is used for constructs that are valid in the currently
172selected source language but that are dubious in some way. The <tt>NOTE</tt>
Daniel Dunbar426b8632009-02-17 15:49:03 +0000[diff] [blame]173level is used to staple more information onto previous diagnostics.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]174
175<p>These <em>severities</em> are mapped into a smaller set (the
176Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>,
Chris Lattner0aad2972009-02-05 22:49:08 +0000[diff] [blame]177<tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics
Chris Lattnera180fdd2009-02-17 07:07:29 +0000[diff] [blame]178subsystem based on various configuration options. Clang internally supports a
179fully fine grained mapping mechanism that allows you to map almost any
180diagnostic to the output level that you want. The only diagnostics that cannot
181be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously
182emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to
183<tt>Fatal</tt> (it is not possible to turn an error into a warning,
184for example).</p>
185
186<p>Diagnostic mappings are used in many ways. For example, if the user
187specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if
188they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is
189used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc.
190</p>
191
192<p>
193Mapping to <tt>Fatal</tt> should only be used for diagnostics that are
194considered so severe that error recovery won't be able to recover sensibly from
195them (thus spewing a ton of bogus errors). One example of this class of error
196are failure to #include a file.
197</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]198
199<!-- ================= -->
200<h4>The Format String</h4>
201<!-- ================= -->
202
203<p>The format string for the diagnostic is very simple, but it has some power.
204It takes the form of a string in English with markers that indicate where and
205how arguments to the diagnostic are inserted and formatted. For example, here
206are some simple format strings:</p>
207
208<pre>
209 "binary integer literals are an extension"
210 "format string contains '\\0' within the string body"
211 "more '<b>%%</b>' conversions than data arguments"
Chris Lattner545b3682008-11-23 20:27:13 +0000[diff] [blame]212 "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)"
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]213 "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator"
214 " (has <b>%1</b> parameter<b>%s1</b>)"
215</pre>
216
217<p>These examples show some important points of format strings. You can use any
218 plain ASCII character in the diagnostic string except "%" without a problem,
219 but these are C strings, so you have to use and be aware of all the C escape
220 sequences (as in the second example). If you want to produce a "%" in the
221 output, use the "%%" escape sequence, like the third diagnostic. Finally,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]222 Clang uses the "%...[digit]" sequences to specify where and how arguments to
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]223 the diagnostic are formatted.</p>
224
225<p>Arguments to the diagnostic are numbered according to how they are specified
226 by the C++ code that <a href="#producingdiag">produces them</a>, and are
227 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]228 to your diagnostic, you are doing something wrong :). Unlike printf, there
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]229 is no requirement that arguments to the diagnostic end up in the output in
230 the same order as they are specified, you could have a format string with
231 <tt>"%1 %0"</tt> that swaps them, for example. The text in between the
232 percent and digit are formatting instructions. If there are no instructions,
233 the argument is just turned into a string and substituted in.</p>
234
235<p>Here are some "best practices" for writing the English format string:</p>
236
237<ul>
238<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]239 <tt>DiagnosticKinds.td</tt> file. This avoids the diagnostic wrapping when
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]240 printed, and forces you to think about the important point you are conveying
241 with the diagnostic.</li>
242<li>Take advantage of location information. The user will be able to see the
243 line and location of the caret, so you don't need to tell them that the
244 problem is with the 4th argument to the function: just point to it.</li>
245<li>Do not capitalize the diagnostic string, and do not end it with a
246 period.</li>
247<li>If you need to quote something in the diagnostic string, use single
248 quotes.</li>
249</ul>
250
251<p>Diagnostics should never take random English strings as arguments: you
252shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like
253<tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]254this prevents <a href="#translation">translating</a> the Clang diagnostics to
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]255other languages (because they'll get random English words in their otherwise
256localized diagnostic). The exceptions to this are C/C++ language keywords
257(e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note
258that things like "pointer" and "reference" are not keywords. On the other
259hand, you <em>can</em> include anything that comes from the user's source code,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]260including variable names, types, labels, etc. The 'select' format can be
261used to achieve this sort of thing in a localizable way, see below.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]262
263<!-- ==================================== -->
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]264<h4>Formatting a Diagnostic Argument</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]265<!-- ==================================== -->
266
267<p>Arguments to diagnostics are fully typed internally, and come from a couple
268different classes: integers, types, names, and random strings. Depending on
269the class of the argument, it can be optionally formatted in different ways.
270This gives the DiagnosticClient information about what the argument means
271without requiring it to use a specific presentation (consider this MVC for
272Clang :).</p>
273
274<p>Here are the different diagnostic argument formats currently supported by
275Clang:</p>
276
277<table>
278<tr><td colspan="2"><b>"s" format</b></td></tr>
279<tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]280<tr><td>Class:</td><td>Integers</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]281<tr><td>Description:</td><td>This is a simple formatter for integers that is
282 useful when producing English diagnostics. When the integer is 1, it prints
283 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]284 simple grammatical forms to be to be handled correctly, and eliminates the
285 need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]286
287<tr><td colspan="2"><b>"select" format</b></td></tr>
288<tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2
289 operator"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]290<tr><td>Class:</td><td>Integers</td></tr>
John McCall3a47e232010-01-14 19:12:17 +0000[diff] [blame]291<tr><td>Description:</td><td><p>This format specifier is used to merge multiple
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]292 related diagnostics together into one common one, without requiring the
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]293 difference to be specified as an English string argument. Instead of
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]294 specifying the string, the diagnostic gets an integer argument and the
295 format string selects the numbered option. In this case, the "%2" value
296 must be an integer in the range [0..2]. If it is 0, it prints 'unary', if
297 it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This
298 allows other language translations to substitute reasonable words (or entire
299 phrases) based on the semantics of the diagnostic instead of having to do
John McCall3a47e232010-01-14 19:12:17 +0000[diff] [blame]300 things textually.</p>
301 <p>The selected string does undergo formatting.</p></td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]302
303<tr><td colspan="2"><b>"plural" format</b></td></tr>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]304<tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to
305 your computer"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]306<tr><td>Class:</td><td>Integers</td></tr>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]307<tr><td>Description:</td><td><p>This is a formatter for complex plural forms.
308 It is designed to handle even the requirements of languages with very
309 complex plural forms, as many Baltic languages have. The argument consists
310 of a series of expression/form pairs, separated by ':', where the first form
311 whose expression evaluates to true is the result of the modifier.</p>
312 <p>An expression can be empty, in which case it is always true. See the
313 example at the top. Otherwise, it is a series of one or more numeric
314 conditions, separated by ','. If any condition matches, the expression
315 matches. Each numeric condition can take one of three forms.</p>
316 <ul>
317 <li>number: A simple decimal number matches if the argument is the same
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]318 as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]319 <li>range: A range in square brackets matches if the argument is within
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]320 the range. Then range is inclusive on both ends. Example:
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]321 <tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li>
322 <li>modulo: A modulo operator is followed by a number, and
323 equals sign and either a number or a range. The tests are the
324 same as for plain
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]325 numbers and ranges, but the argument is taken modulo the number first.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]326 Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything
327 else}1"</tt></li>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]328 </ul>
329 <p>The parser is very unforgiving. A syntax error, even whitespace, will
330 abort, as will a failure to match the argument against any
331 expression.</p></td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]332
John McCall3a47e232010-01-14 19:12:17 +0000[diff] [blame]333<tr><td colspan="2"><b>"ordinal" format</b></td></tr>
334<tr><td>Example:</td><td><tt>"ambiguity in %ordinal0 argument"</tt></td></tr>
335<tr><td>Class:</td><td>Integers</td></tr>
336<tr><td>Description:</td><td><p>This is a formatter which represents the
337 argument number as an ordinal: the value <tt>1</tt> becomes <tt>1st</tt>,
338 <tt>3</tt> becomes <tt>3rd</tt>, and so on. Values less than <tt>1</tt>
339 are not supported.</p>
340 <p>This formatter is currently hard-coded to use English ordinals.</p></td></tr>
341
Chris Lattner077bf5e2008-11-24 03:33:13 +0000[diff] [blame]342<tr><td colspan="2"><b>"objcclass" format</b></td></tr>
343<tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr>
344<tr><td>Class:</td><td>DeclarationName</td></tr>
345<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
346 DeclarationName corresponds to an Objective-C class method selector. As
347 such, it prints the selector with a leading '+'.</p></td></tr>
348
349<tr><td colspan="2"><b>"objcinstance" format</b></td></tr>
350<tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr>
351<tr><td>Class:</td><td>DeclarationName</td></tr>
352<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
353 DeclarationName corresponds to an Objective-C instance method selector. As
354 such, it prints the selector with a leading '-'.</p></td></tr>
355
Douglas Gregor47b9a1c2009-02-04 17:27:36 +0000[diff] [blame]356<tr><td colspan="2"><b>"q" format</b></td></tr>
357<tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr>
358<tr><td>Class:</td><td>NamedDecl*</td></tr>
359<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>
360
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]361</table>
362
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]363<p>It is really easy to add format specifiers to the Clang diagnostics system,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]364but they should be discussed before they are added. If you are creating a lot
365of repetitive diagnostics and/or have an idea for a useful formatter, please
366bring it up on the cfe-dev mailing list.</p>
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]367
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]368<!-- ===================================================== -->
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]369<h4 id="producingdiag">Producing the Diagnostic</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]370<!-- ===================================================== -->
371
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]372<p>Now that you've created the diagnostic in the DiagnosticKinds.td file, you
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]373need to write the code that detects the condition in question and emits the
374new diagnostic. Various components of Clang (e.g. the preprocessor, Sema,
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]375etc) provide a helper function named "Diag". It creates a diagnostic and
376accepts the arguments, ranges, and other information that goes along with
377it.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]378
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]379<p>For example, the binary expression error comes from code like this:</p>
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]380
381<pre>
382 if (various things that are bad)
383 Diag(Loc, diag::err_typecheck_invalid_operands)
384 &lt;&lt; lex-&gt;getType() &lt;&lt; rex-&gt;getType()
385 &lt;&lt; lex-&gt;getSourceRange() &lt;&lt; rex-&gt;getSourceRange();
386</pre>
387
388<p>This shows that use of the Diag method: they take a location (a <a
389href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value
Chris Lattnercc2ac1e2011-02-14 06:42:50 +0000[diff] [blame]390(which matches the name from DiagnosticKinds.td). If the diagnostic takes
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]391arguments, they are specified with the &lt;&lt; operator: the first argument
392becomes %0, the second becomes %1, etc. The diagnostic interface allows you to
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]393specify arguments of many different types, including <tt>int</tt> and
394<tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and
395<tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and
396<tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc.
397SourceRanges are also specified with the &lt;&lt; operator, but do not have a
398specific ordering requirement.</p>
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]399
400<p>As you can see, adding and producing a diagnostic is pretty straightforward.
401The hard part is deciding exactly what you need to say to help the user, picking
402a suitable wording, and providing the information needed to format it correctly.
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]403The good news is that the call site that issues a diagnostic should be
404completely independent of how the diagnostic is formatted and in what language
405it is rendered.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]406</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]407
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]408<!-- ==================================================== -->
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]409<h4 id="fix-it-hints">Fix-It Hints</h4>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]410<!-- ==================================================== -->
411
412<p>In some cases, the front end emits diagnostics when it is clear
413that some small change to the source code would fix the problem. For
414example, a missing semicolon at the end of a statement or a use of
Chris Lattner34c05332009-02-27 19:31:12 +0000[diff] [blame]415deprecated syntax that is easily rewritten into a more modern form.
416Clang tries very hard to emit the diagnostic and recover gracefully
417in these and other cases.</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]418
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]419<p>However, for these cases where the fix is obvious, the diagnostic
420can be annotated with a hint (referred to as a "fix-it hint") that
421describes how to change the code referenced by the diagnostic to fix
422the problem. For example, it might add the missing semicolon at the
423end of the statement or rewrite the use of a deprecated construct
424into something more palatable. Here is one such example from the C++
425front end, where we warn about the right-shift operator changing
David Blaikie5090e9f2011-10-18 05:49:30 +0000[diff] [blame]426meaning from C++98 to C++11:</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]427
428<pre>
David Blaikie5090e9f2011-10-18 05:49:30 +0000[diff] [blame]429test.cpp:3:7: warning: use of right-shift operator ('&gt;&gt;') in template argument will require parentheses in C++11
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]430A&lt;100 &gt;&gt; 2&gt; *a;
431 ^
432 ( )
433</pre>
434
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]435<p>Here, the fix-it hint is suggesting that parentheses be added,
436and showing exactly where those parentheses would be inserted into the
437source code. The fix-it hints themselves describe what changes to make
438to the source code in an abstract manner, which the text diagnostic
439printer renders as a line of "insertions" below the caret line. <a
440href="#DiagnosticClient">Other diagnostic clients</a> might choose
441to render the code differently (e.g., as markup inline) or even give
442the user the ability to automatically fix the problem.</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]443
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]444<p>All fix-it hints are described by the <code>FixItHint</code> class,
445instances of which should be attached to the diagnostic using the
446&lt;&lt; operator in the same way that highlighted source ranges and
447arguments are passed to the diagnostic. Fix-it hints can be created
448with one of three constructors:</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]449
450<dl>
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]451 <dt><code>FixItHint::CreateInsertion(Loc, Code)</code></dt>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]452 <dd>Specifies that the given <code>Code</code> (a string) should be inserted
453 before the source location <code>Loc</code>.</dd>
454
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]455 <dt><code>FixItHint::CreateRemoval(Range)</code></dt>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]456 <dd>Specifies that the code in the given source <code>Range</code>
457 should be removed.</dd>
458
Peter Collingbourne38448d32011-03-21 01:45:18 +0000[diff] [blame]459 <dt><code>FixItHint::CreateReplacement(Range, Code)</code></dt>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]460 <dd>Specifies that the code in the given source <code>Range</code>
461 should be removed, and replaced with the given <code>Code</code> string.</dd>
462</dl>
463
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]464<!-- ============================================================= -->
465<h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4>
466<!-- ============================================================= -->
467
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]468<p>Once code generates a diagnostic with all of the arguments and the rest of
469the relevant information, Clang needs to know what to do with it. As previously
470mentioned, the diagnostic machinery goes through some filtering to map a
471severity onto a diagnostic level, then (assuming the diagnostic is not mapped to
472"<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient
473interface with the information.</p>
474
475<p>It is possible to implement this interface in many different ways. For
476example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns
477the arguments into strings (according to the various formatting rules), prints
478out the file/line/column information and the string, then prints out the line of
479code, the source ranges, and the caret. However, this behavior isn't required.
480</p>
481
482<p>Another implementation of the DiagnosticClient interface is the
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]483'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode.
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]484Instead of formatting and printing out the diagnostics, this implementation just
485captures and remembers the diagnostics as they fly by. Then -verify compares
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]486the list of produced diagnostics to the list of expected ones. If they disagree,
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]487it prints out its own output.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]488</p>
489
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]490<p>There are many other possible implementations of this interface, and this is
491why we prefer diagnostics to pass down rich structured information in arguments.
492For example, an HTML output might want declaration names be linkified to where
493they come from in the source. Another example is that a GUI might let you click
494on typedefs to expand them. This application would want to pass significantly
495more information about types through to the GUI than a simple flat string. The
496interface allows this to happen.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]497
498<!-- ====================================================== -->
499<h4><a name="translation">Adding Translations to Clang</a></h4>
500<!-- ====================================================== -->
501
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]502<p>Not possible yet! Diagnostic strings should be written in UTF-8, the client
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]503can translate to the relevant code page if needed. Each translation completely
504replaces the format string for the diagnostic.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]505
506
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]507<!-- ======================================================================= -->
508<h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3>
509<!-- ======================================================================= -->
510
511<p>Strangely enough, the SourceLocation class represents a location within the
512source code of the program. Important design points include:</p>
513
514<ol>
515<li>sizeof(SourceLocation) must be extremely small, as these are embedded into
516 many AST nodes and are passed around often. Currently it is 32 bits.</li>
517<li>SourceLocation must be a simple value object that can be efficiently
518 copied.</li>
519<li>We should be able to represent a source location for any byte of any input
520 file. This includes in the middle of tokens, in whitespace, in trigraphs,
521 etc.</li>
522<li>A SourceLocation must encode the current #include stack that was active when
523 the location was processed. For example, if the location corresponds to a
524 token, it should contain the set of #includes active when the token was
525 lexed. This allows us to print the #include stack for a diagnostic.</li>
526<li>SourceLocation must be able to describe macro expansions, capturing both
527 the ultimate instantiation point and the source of the original character
528 data.</li>
529</ol>
530
531<p>In practice, the SourceLocation works together with the SourceManager class
Nick Lewycky77561e52010-05-26 21:48:10 +0000[diff] [blame]532to encode two pieces of information about a location: its spelling location
533and its instantiation location. For most tokens, these will be the same.
534However, for a macro expansion (or tokens that came from a _Pragma directive)
535these will describe the location of the characters corresponding to the token
536and the location where the token was used (i.e. the macro instantiation point
537or the location of the _Pragma itself).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]538
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]539<p>The Clang front-end inherently depends on the location of a token being
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]540tracked correctly. If it is ever incorrect, the front-end may get confused and
541die. 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]542Clang depends on being able to find the original input characters for the token.
Chris Lattner18376dd2009-01-16 07:00:50 +0000[diff] [blame]543This concept maps directly to the "spelling location" for the token.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]544
Douglas Gregor715c92a2010-10-27 16:02:28 +0000[diff] [blame]545
546<!-- ======================================================================= -->
547<h3 id="SourceRange">SourceRange and CharSourceRange</h3>
548<!-- ======================================================================= -->
549<!-- mostly taken from
550 http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-August/010595.html -->
551
552<p>Clang represents most source ranges by [first, last], where first and last
553each point to the beginning of their respective tokens. For example
554consider the SourceRange of the following statement:</p>
555<pre>
556x = foo + bar;
557^first ^last
558</pre>
559
560<p>To map from this representation to a character-based
561representation, the 'last' location needs to be adjusted to point to
562(or past) the end of that token with either
563<code>Lexer::MeasureTokenLength()</code> or
Chris Lattner7ef5c272010-11-17 07:05:50 +0000[diff] [blame]564<code>Lexer::getLocForEndOfToken()</code>. For the rare cases
Douglas Gregor715c92a2010-10-27 16:02:28 +0000[diff] [blame]565where character-level source ranges information is needed we use
566the <code>CharSourceRange</code> class.</p>
567
568
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]569<!-- ======================================================================= -->
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]570<h2 id="libdriver">The Driver Library</h2>
571<!-- ======================================================================= -->
572
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]573<p>The clang Driver and library are documented <a
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]574href="DriverInternals.html">here</a>.<p>
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]575
576<!-- ======================================================================= -->
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame]577<h2 id="pch">Precompiled Headers</h2>
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]578<!-- ======================================================================= -->
579
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame]580<p>Clang supports two implementations of precompiled headers. The
581 default implementation, precompiled headers (<a
582 href="PCHInternals.html">PCH</a>) uses a serialized representation
583 of Clang's internal data structures, encoded with the <a
584 href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream
585 format</a>. Pretokenized headers (<a
586 href="PTHInternals.html">PTH</a>), on the other hand, contain a
587 serialized representation of the tokens encountered when
588 preprocessing a header (and anything that header includes).</p>
589
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]590
591<!-- ======================================================================= -->
592<h2 id="libfrontend">The Frontend Library</h2>
593<!-- ======================================================================= -->
594
595<p>The Frontend library contains functionality useful for building
596tools on top of the clang libraries, for example several methods for
597outputting diagnostics.</p>
598
599<!-- ======================================================================= -->
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]600<h2 id="liblex">The Lexer and Preprocessor Library</h2>
601<!-- ======================================================================= -->
602
603<p>The Lexer library contains several tightly-connected classes that are involved
604with the nasty process of lexing and preprocessing C source code. The main
605interface to this library for outside clients is the large <a
606href="#Preprocessor">Preprocessor</a> class.
607It contains the various pieces of state that are required to coherently read
608tokens out of a translation unit.</p>
609
610<p>The core interface to the Preprocessor object (once it is set up) is the
611Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from
612the preprocessor stream. There are two types of token providers that the
613preprocessor is capable of reading from: a buffer lexer (provided by the <a
614href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]615href="#TokenLexer">TokenLexer</a> class).
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]616
617
618<!-- ======================================================================= -->
619<h3 id="Token">The Token class</h3>
620<!-- ======================================================================= -->
621
622<p>The Token class is used to represent a single lexed token. Tokens are
623intended to be used by the lexer/preprocess and parser libraries, but are not
624intended to live beyond them (for example, they should not live in the ASTs).<p>
625
626<p>Tokens most often live on the stack (or some other location that is efficient
627to access) as the parser is running, but occasionally do get buffered up. For
628example, macro definitions are stored as a series of tokens, and the C++
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]629front-end periodically needs to buffer tokens up for tentative parsing and
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]630various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit
631system, sizeof(Token) is currently 16 bytes.</p>
632
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]633<p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation
634Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer,
635annotation tokens represent semantic information and are produced by the parser,
636replacing normal tokens in the token stream. Normal tokens contain the
637following information:</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]638
639<ul>
640<li><b>A SourceLocation</b> - This indicates the location of the start of the
641token.</li>
642
643<li><b>A length</b> - This stores the length of the token as stored in the
644SourceBuffer. For tokens that include them, this length includes trigraphs and
645escaped newlines which are ignored by later phases of the compiler. By pointing
646into the original source buffer, it is always possible to get the original
647spelling of a token completely accurately.</li>
648
649<li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if
650identifier lookup was enabled when the token was lexed (e.g. the lexer was not
651reading in 'raw' mode) this contains a pointer to the unique hash value for the
652identifier. Because the lookup happens before keyword identification, this
653field is set even for language keywords like 'for'.</li>
654
655<li><b>TokenKind</b> - This indicates the kind of token as classified by the
656lexer. This includes things like <tt>tok::starequal</tt> (for the "*="
657operator), <tt>tok::ampamp</tt> for the "&amp;&amp;" token, and keyword values
658(e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note
659that some tokens can be spelled multiple ways. For example, C++ supports
660"operator keywords", where things like "and" are treated exactly like the
661"&amp;&amp;" operator. In these cases, the kind value is set to
662<tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to
663consider both forms. For something that cares about which form is used (e.g.
664the preprocessor 'stringize' operator) the spelling indicates the original
665form.</li>
666
667<li><b>Flags</b> - There are currently four flags tracked by the
668lexer/preprocessor system on a per-token basis:
669
670 <ol>
671 <li><b>StartOfLine</b> - This was the first token that occurred on its input
672 source line.</li>
673 <li><b>LeadingSpace</b> - There was a space character either immediately
674 before the token or transitively before the token as it was expanded
675 through a macro. The definition of this flag is very closely defined by
676 the stringizing requirements of the preprocessor.</li>
677 <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to
678 represent identifier tokens which have macro expansion disabled. This
679 prevents them from being considered as candidates for macro expansion ever
680 in the future.</li>
681 <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the
682 token includes a trigraph or escaped newline. Since this is uncommon,
683 many pieces of code can fast-path on tokens that did not need cleaning.
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]684 </ol>
685</li>
686</ul>
687
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]688<p>One interesting (and somewhat unusual) aspect of normal tokens is that they
689don't contain any semantic information about the lexed value. For example, if
690the token was a pp-number token, we do not represent the value of the number
691that was lexed (this is left for later pieces of code to decide). Additionally,
692the lexer library has no notion of typedef names vs variable names: both are
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]693returned as identifiers, and the parser is left to decide whether a specific
694identifier is a typedef or a variable (tracking this requires scope information
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]695among other things). The parser can do this translation by replacing tokens
696returned by the preprocessor with "Annotation Tokens".</p>
697
698<!-- ======================================================================= -->
699<h3 id="AnnotationToken">Annotation Tokens</h3>
700<!-- ======================================================================= -->
701
702<p>Annotation Tokens are tokens that are synthesized by the parser and injected
703into the preprocessor's token stream (replacing existing tokens) to record
704semantic information found by the parser. For example, if "foo" is found to be
705a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an
706<tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this
707makes it easy to handle qualified type names (e.g. "foo::bar::baz&lt;42&gt;::t")
708in C++ as a single "token" in the parser. 2) if the parser backtracks, the
709reparse does not need to redo semantic analysis to determine whether a token
710sequence is a variable, type, template, etc.</p>
711
712<p>Annotation Tokens are created by the parser and reinjected into the parser's
713token stream (when backtracking is enabled). Because they can only exist in
714tokens that the preprocessor-proper is done with, it doesn't need to keep around
715flags like "start of line" that the preprocessor uses to do its job.
716Additionally, an annotation token may "cover" a sequence of preprocessor tokens
717(e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields
718of an annotation token are different than the fields for a normal token (but
719they are multiplexed into the normal Token fields):</p>
720
721<ul>
722<li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation
723token indicates the first token replaced by the annotation token. In the example
724above, it would be the location of the "a" identifier.</li>
725
726<li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the
727last token replaced with the annotation token. In the example above, it would
728be the location of the "c" identifier.</li>
729
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]730<li><b>void* "AnnotationValue"</b> - This contains an opaque object
731that the parser gets from Sema. The parser merely preserves the
732information for Sema to later interpret based on the annotation token
733kind.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]734
735<li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this
736is. See below for the different valid kinds.</li>
737</ul>
738
739<p>Annotation tokens currently come in three kinds:</p>
740
741<ol>
742<li><b>tok::annot_typename</b>: This annotation token represents a
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]743resolved typename token that is potentially qualified. The
744AnnotationValue field contains the <tt>QualType</tt> returned by
745Sema::getTypeName(), possibly with source location information
746attached.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]747
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]748<li><b>tok::annot_cxxscope</b>: This annotation token represents a C++
749scope specifier, such as "A::B::". This corresponds to the grammar
750productions "::" and ":: [opt] nested-name-specifier". The
751AnnotationValue pointer is a <tt>NestedNameSpecifier*</tt> returned by
752the Sema::ActOnCXXGlobalScopeSpecifier and
753Sema::ActOnCXXNestedNameSpecifier callbacks.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]754
Douglas Gregor39a8de12009-02-25 19:37:18 +0000[diff] [blame]755<li><b>tok::annot_template_id</b>: This annotation token represents a
756C++ template-id such as "foo&lt;int, 4&gt;", where "foo" is the name
757of a template. The AnnotationValue pointer is a pointer to a malloc'd
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]758TemplateIdAnnotation object. Depending on the context, a parsed
759template-id that names a type might become a typename annotation token
760(if all we care about is the named type, e.g., because it occurs in a
761type specifier) or might remain a template-id token (if we want to
762retain more source location information or produce a new type, e.g.,
763in a declaration of a class template specialization). template-id
764annotation tokens that refer to a type can be "upgraded" to typename
765annotation tokens by the parser.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]766
767</ol>
768
Cedric Venetda76b282009-01-06 16:22:54 +0000[diff] [blame]769<p>As mentioned above, annotation tokens are not returned by the preprocessor,
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]770they are formed on demand by the parser. This means that the parser has to be
771aware of cases where an annotation could occur and form it where appropriate.
772This is somewhat similar to how the parser handles Translation Phase 6 of C99:
773String Concatenation (see C99 5.1.1.2). In the case of string concatenation,
774the preprocessor just returns distinct tok::string_literal and
775tok::wide_string_literal tokens and the parser eats a sequence of them wherever
776the grammar indicates that a string literal can occur.</p>
777
778<p>In order to do this, whenever the parser expects a tok::identifier or
779tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or
780TryAnnotateCXXScopeToken methods to form the annotation token. These methods
781will maximally form the specified annotation tokens and replace the current
782token with them, if applicable. If the current tokens is not valid for an
783annotation token, it will remain an identifier or :: token.</p>
784
785
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]786
787<!-- ======================================================================= -->
788<h3 id="Lexer">The Lexer class</h3>
789<!-- ======================================================================= -->
790
791<p>The Lexer class provides the mechanics of lexing tokens out of a source
792buffer and deciding what they mean. The Lexer is complicated by the fact that
793it operates on raw buffers that have not had spelling eliminated (this is a
794necessity to get decent performance), but this is countered with careful coding
795as well as standard performance techniques (for example, the comment handling
796code is vectorized on X86 and PowerPC hosts).</p>
797
798<p>The lexer has a couple of interesting modal features:</p>
799
800<ul>
801<li>The lexer can operate in 'raw' mode. This mode has several features that
802 make it possible to quickly lex the file (e.g. it stops identifier lookup,
803 doesn't specially handle preprocessor tokens, handles EOF differently, etc).
804 This mode is used for lexing within an "<tt>#if 0</tt>" block, for
805 example.</li>
806<li>The lexer can capture and return comments as tokens. This is required to
807 support the -C preprocessor mode, which passes comments through, and is
808 used by the diagnostic checker to identifier expect-error annotations.</li>
809<li>The lexer can be in ParsingFilename mode, which happens when preprocessing
Chris Lattner84386242007-09-16 19:25:23 +0000[diff] [blame]810 after reading a #include directive. This mode changes the parsing of '&lt;'
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]811 to return an "angled string" instead of a bunch of tokens for each thing
812 within the filename.</li>
813<li>When parsing a preprocessor directive (after "<tt>#</tt>") the
814 ParsingPreprocessorDirective mode is entered. This changes the parser to
Peter Collingbourne84021552011-02-28 02:37:51 +0000[diff] [blame]815 return EOD at a newline.</li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]816<li>The Lexer uses a LangOptions object to know whether trigraphs are enabled,
817 whether C++ or ObjC keywords are recognized, etc.</li>
818</ul>
819
820<p>In addition to these modes, the lexer keeps track of a couple of other
821 features that are local to a lexed buffer, which change as the buffer is
822 lexed:</p>
823
824<ul>
825<li>The Lexer uses BufferPtr to keep track of the current character being
826 lexed.</li>
827<li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token
828 will start with its "start of line" bit set.</li>
829<li>The Lexer keeps track of the current #if directives that are active (which
830 can be nested).</li>
831<li>The Lexer keeps track of an <a href="#MultipleIncludeOpt">
832 MultipleIncludeOpt</a> object, which is used to
833 detect whether the buffer uses the standard "<tt>#ifndef XX</tt> /
834 <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does,
835 subsequent includes can be ignored if the XX macro is defined.</li>
836</ul>
837
838<!-- ======================================================================= -->
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]839<h3 id="TokenLexer">The TokenLexer class</h3>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]840<!-- ======================================================================= -->
841
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]842<p>The TokenLexer class is a token provider that returns tokens from a list
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]843of tokens that came from somewhere else. It typically used for two things: 1)
844returning tokens from a macro definition as it is being expanded 2) returning
845tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and
846will most likely be used to handle unbounded look-ahead for the C++ parser.</p>
847
848<!-- ======================================================================= -->
849<h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3>
850<!-- ======================================================================= -->
851
852<p>The MultipleIncludeOpt class implements a really simple little state machine
853that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>"
854idiom that people typically use to prevent multiple inclusion of headers. If a
855buffer uses this idiom and is subsequently #include'd, the preprocessor can
856simply check to see whether the guarding condition is defined or not. If so,
857the preprocessor can completely ignore the include of the header.</p>
858
859
860
861<!-- ======================================================================= -->
862<h2 id="libparse">The Parser Library</h2>
863<!-- ======================================================================= -->
864
865<!-- ======================================================================= -->
866<h2 id="libast">The AST Library</h2>
867<!-- ======================================================================= -->
868
869<!-- ======================================================================= -->
870<h3 id="Type">The Type class and its subclasses</h3>
871<!-- ======================================================================= -->
872
873<p>The Type class (and its subclasses) are an important part of the AST. Types
874are accessed through the ASTContext class, which implicitly creates and uniques
875them as they are needed. Types have a couple of non-obvious features: 1) they
876do not capture type qualifiers like const or volatile (See
877<a href="#QualType">QualType</a>), and 2) they implicitly capture typedef
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]878information. Once created, types are immutable (unlike decls).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]879
880<p>Typedefs in C make semantic analysis a bit more complex than it would
881be without them. The issue is that we want to capture typedef information
882and represent it in the AST perfectly, but the semantics of operations need to
883"see through" typedefs. For example, consider this code:</p>
884
885<code>
886void func() {<br>
Bill Wendling30d17752007-10-06 01:56:01 +0000[diff] [blame]887&nbsp;&nbsp;typedef int foo;<br>
888&nbsp;&nbsp;foo X, *Y;<br>
889&nbsp;&nbsp;typedef foo* bar;<br>
890&nbsp;&nbsp;bar Z;<br>
891&nbsp;&nbsp;*X; <i>// error</i><br>
892&nbsp;&nbsp;**Y; <i>// error</i><br>
893&nbsp;&nbsp;**Z; <i>// error</i><br>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]894}<br>
895</code>
896
897<p>The code above is illegal, and thus we expect there to be diagnostics emitted
898on the annotated lines. In this example, we expect to get:</p>
899
900<pre>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]901<b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]902*X; // error
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]903<span style="color:blue">^~</span>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]904<b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]905**Y; // error
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]906<span style="color:blue">^~~</span>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]907<b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b>
908**Z; // error
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]909<span style="color:blue">^~~</span>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]910</pre>
911
912<p>While this example is somewhat silly, it illustrates the point: we want to
913retain typedef information where possible, so that we can emit errors about
914"<tt>std::string</tt>" instead of "<tt>std::basic_string&lt;char, std:...</tt>".
915Doing this requires properly keeping typedef information (for example, the type
916of "X" is "foo", not "int"), and requires properly propagating it through the
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]917various operators (for example, the type of *Y is "foo", not "int"). In order
918to retain this information, the type of these expressions is an instance of the
919TypedefType class, which indicates that the type of these expressions is a
920typedef for foo.
921</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]922
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]923<p>Representing types like this is great for diagnostics, because the
924user-specified type is always immediately available. There are two problems
925with this: first, various semantic checks need to make judgements about the
Douglas Gregor2d1e21a2011-12-19 19:50:23 +0000[diff] [blame]926<em>actual structure</em> of a type, ignoring typedefs. Second, we need an
Chris Lattner33fc68a2007-07-31 18:54:50 +0000[diff] [blame]927efficient way to query whether two types are structurally identical to each
928other, ignoring typedefs. The solution to both of these problems is the idea of
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]929canonical types.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]930
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]931<!-- =============== -->
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]932<h4>Canonical Types</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]933<!-- =============== -->
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]934
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]935<p>Every instance of the Type class contains a canonical type pointer. For
936simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>",
937"<tt>int**</tt>"), the type just points to itself. For types that have a
938typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>",
939"<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their
940structurally equivalent type without any typedefs (e.g. "<tt>int</tt>",
941"<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]942
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]943<p>This design provides a constant time operation (dereferencing the canonical
944type pointer) that gives us access to the structure of types. For example,
945we can trivially tell that "bar" and "foo*" are the same type by dereferencing
946their canonical type pointers and doing a pointer comparison (they both point
947to the single "<tt>int*</tt>" type).</p>
948
949<p>Canonical types and typedef types bring up some complexities that must be
950carefully managed. Specifically, the "isa/cast/dyncast" operators generally
951shouldn't be used in code that is inspecting the AST. For example, when type
952checking the indirection operator (unary '*' on a pointer), the type checker
953must verify that the operand has a pointer type. It would not be correct to
954check that with "<tt>isa&lt;PointerType&gt;(SubExpr-&gt;getType())</tt>",
955because this predicate would fail if the subexpression had a typedef type.</p>
956
957<p>The solution to this problem are a set of helper methods on Type, used to
958check their properties. In this case, it would be correct to use
959"<tt>SubExpr-&gt;getType()-&gt;isPointerType()</tt>" to do the check. This
960predicate will return true if the <em>canonical type is a pointer</em>, which is
961true any time the type is structurally a pointer type. The only hard part here
962is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p>
963
964<p>The second problem we face is how to get access to the pointer type once we
965know it exists. To continue the example, the result type of the indirection
966operator is the pointee type of the subexpression. In order to determine the
967type, we need to get the instance of PointerType that best captures the typedef
968information in the program. If the type of the expression is literally a
969PointerType, we can return that, otherwise we have to dig through the
970typedefs to find the pointer type. For example, if the subexpression had type
971"<tt>foo*</tt>", we could return that type as the result. If the subexpression
972had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do
973<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]974a getAsPointerType() method that checks whether the type is structurally a
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]975PointerType and, if so, returns the best one. If not, it returns a null
976pointer.</p>
977
978<p>This structure is somewhat mystical, but after meditating on it, it will
979make sense to you :).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]980
981<!-- ======================================================================= -->
982<h3 id="QualType">The QualType class</h3>
983<!-- ======================================================================= -->
984
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]985<p>The QualType class is designed as a trivial value class that is
986small, passed by-value and is efficient to query. The idea of
987QualType is that it stores the type qualifiers (const, volatile,
988restrict, plus some extended qualifiers required by language
989extensions) separately from the types themselves. QualType is
990conceptually a pair of "Type*" and the bits for these type qualifiers.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]991
992<p>By storing the type qualifiers as bits in the conceptual pair, it is
993extremely efficient to get the set of qualifiers on a QualType (just return the
994field of the pair), add a type qualifier (which is a trivial constant-time
995operation that sets a bit), and remove one or more type qualifiers (just return
996a QualType with the bitfield set to empty).</p>
997
998<p>Further, because the bits are stored outside of the type itself, we do not
999need to create duplicates of types with different sets of qualifiers (i.e. there
1000is only a single heap allocated "int" type: "const int" and "volatile const int"
1001both point to the same heap allocated "int" type). This reduces the heap size
1002used to represent bits and also means we do not have to consider qualifiers when
1003uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p>
1004
John McCall027ac442010-09-03 05:07:55 +0000[diff] [blame]1005<p>In practice, the two most common type qualifiers (const and
1006restrict) are stored in the low bits of the pointer to the Type
1007object, together with a flag indicating whether extended qualifiers
1008are present (which must be heap-allocated). This means that QualType
1009is exactly the same size as a pointer.</p>
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1010
1011<!-- ======================================================================= -->
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1012<h3 id="DeclarationName">Declaration names</h3>
1013<!-- ======================================================================= -->
1014
1015<p>The <tt>DeclarationName</tt> class represents the name of a
1016 declaration in Clang. Declarations in the C family of languages can
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]1017 take several different forms. Most declarations are named by
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1018 simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in
1019 the function declaration <code>f(int x)</code>. In C++, declaration
1020 names can also name class constructors ("<code>Class</code>"
1021 in <code>struct Class { Class(); }</code>), class destructors
1022 ("<code>~Class</code>"), overloaded operator names ("operator+"),
1023 and conversion functions ("<code>operator void const *</code>"). In
1024 Objective-C, declaration names can refer to the names of Objective-C
1025 methods, which involve the method name and the parameters,
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]1026 collectively called a <i>selector</i>, e.g.,
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1027 "<code>setWidth:height:</code>". Since all of these kinds of
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]1028 entities - variables, functions, Objective-C methods, C++
1029 constructors, destructors, and operators - are represented as
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1030 subclasses of Clang's common <code>NamedDecl</code>
1031 class, <code>DeclarationName</code> is designed to efficiently
1032 represent any kind of name.</p>
1033
1034<p>Given
1035 a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code>
Douglas Gregor2def4832008-11-17 20:34:05 +0000[diff] [blame]1036 will produce a value that describes what kind of name <code>N</code>
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1037 stores. There are 8 options (all of the names are inside
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1038 the <code>DeclarationName</code> class)</p>
1039<dl>
1040 <dt>Identifier</dt>
1041 <dd>The name is a simple
1042 identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the
1043 corresponding <code>IdentifierInfo*</code> pointing to the actual
1044 identifier. Note that C++ overloaded operators (e.g.,
1045 "<code>operator+</code>") are represented as special kinds of
1046 identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code>
1047 function to determine whether an identifier is an overloaded
1048 operator name.</dd>
1049
1050 <dt>ObjCZeroArgSelector, ObjCOneArgSelector,
1051 ObjCMultiArgSelector</dt>
1052 <dd>The name is an Objective-C selector, which can be retrieved as a
1053 <code>Selector</code> instance
1054 via <code>N.getObjCSelector()</code>. The three possible name
1055 kinds for Objective-C reflect an optimization within
1056 the <code>DeclarationName</code> class: both zero- and
1057 one-argument selectors are stored as a
1058 masked <code>IdentifierInfo</code> pointer, and therefore require
1059 very little space, since zero- and one-argument selectors are far
1060 more common than multi-argument selectors (which use a different
1061 structure).</dd>
1062
1063 <dt>CXXConstructorName</dt>
1064 <dd>The name is a C++ constructor
1065 name. Use <code>N.getCXXNameType()</code> to retrieve
1066 the <a href="#QualType">type</a> that this constructor is meant to
1067 construct. The type is always the canonical type, since all
1068 constructors for a given type have the same name.</dd>
1069
1070 <dt>CXXDestructorName</dt>
1071 <dd>The name is a C++ destructor
1072 name. Use <code>N.getCXXNameType()</code> to retrieve
1073 the <a href="#QualType">type</a> whose destructor is being
1074 named. This type is always a canonical type.</dd>
1075
1076 <dt>CXXConversionFunctionName</dt>
1077 <dd>The name is a C++ conversion function. Conversion functions are
1078 named according to the type they convert to, e.g., "<code>operator void
1079 const *</code>". Use <code>N.getCXXNameType()</code> to retrieve
1080 the type that this conversion function converts to. This type is
1081 always a canonical type.</dd>
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1082
1083 <dt>CXXOperatorName</dt>
1084 <dd>The name is a C++ overloaded operator name. Overloaded operators
1085 are named according to their spelling, e.g.,
1086 "<code>operator+</code>" or "<code>operator new
1087 []</code>". Use <code>N.getCXXOverloadedOperator()</code> to
1088 retrieve the overloaded operator (a value of
1089 type <code>OverloadedOperatorKind</code>).</dd>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1090</dl>
1091
1092<p><code>DeclarationName</code>s are cheap to create, copy, and
1093 compare. They require only a single pointer's worth of storage in
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1094 the common cases (identifiers, zero-
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1095 and one-argument Objective-C selectors) and use dense, uniqued
1096 storage for the other kinds of
1097 names. Two <code>DeclarationName</code>s can be compared for
1098 equality (<code>==</code>, <code>!=</code>) using a simple bitwise
1099 comparison, can be ordered
1100 with <code>&lt;</code>, <code>&gt;</code>, <code>&lt;=</code>,
1101 and <code>&gt;=</code> (which provide a lexicographical ordering for
1102 normal identifiers but an unspecified ordering for other kinds of
1103 names), and can be placed into LLVM <code>DenseMap</code>s
1104 and <code>DenseSet</code>s.</p>
1105
1106<p><code>DeclarationName</code> instances can be created in different
1107 ways depending on what kind of name the instance will store. Normal
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1108 identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1109 (<code>Selector</code>) can be implicitly converted
1110 to <code>DeclarationName</code>s. Names for C++ constructors,
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1111 destructors, conversion functions, and overloaded operators can be retrieved from
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1112 the <code>DeclarationNameTable</code>, an instance of which is
1113 available as <code>ASTContext::DeclarationNames</code>. The member
1114 functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>,
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1115 <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively,
1116 return <code>DeclarationName</code> instances for the four kinds of
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1117 C++ special function names.</p>
1118
1119<!-- ======================================================================= -->
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1120<h3 id="DeclContext">Declaration contexts</h3>
1121<!-- ======================================================================= -->
1122<p>Every declaration in a program exists within some <i>declaration
1123 context</i>, such as a translation unit, namespace, class, or
1124 function. Declaration contexts in Clang are represented by
1125 the <code>DeclContext</code> class, from which the various
1126 declaration-context AST nodes
1127 (<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>,
1128 etc.) will derive. The <code>DeclContext</code> class provides
1129 several facilities common to each declaration context:</p>
1130<dl>
1131 <dt>Source-centric vs. Semantics-centric View of Declarations</dt>
1132 <dd><code>DeclContext</code> provides two views of the declarations
1133 stored within a declaration context. The source-centric view
1134 accurately represents the program source code as written, including
1135 multiple declarations of entities where present (see the
1136 section <a href="#Redeclarations">Redeclarations and
1137 Overloads</a>), while the semantics-centric view represents the
1138 program semantics. The two views are kept synchronized by semantic
1139 analysis while the ASTs are being constructed.</dd>
1140
1141 <dt>Storage of declarations within that context</dt>
1142 <dd>Every declaration context can contain some number of
1143 declarations. For example, a C++ class (represented
1144 by <code>RecordDecl</code>) contains various member functions,
1145 fields, nested types, and so on. All of these declarations will be
1146 stored within the <code>DeclContext</code>, and one can iterate
1147 over the declarations via
1148 [<code>DeclContext::decls_begin()</code>,
1149 <code>DeclContext::decls_end()</code>). This mechanism provides
1150 the source-centric view of declarations in the context.</dd>
1151
1152 <dt>Lookup of declarations within that context</dt>
1153 <dd>The <code>DeclContext</code> structure provides efficient name
1154 lookup for names within that declaration context. For example,
1155 if <code>N</code> is a namespace we can look for the
1156 name <code>N::f</code>
1157 using <code>DeclContext::lookup</code>. The lookup itself is
1158 based on a lazily-constructed array (for declaration contexts
1159 with a small number of declarations) or hash table (for
1160 declaration contexts with more declarations). The lookup
1161 operation provides the semantics-centric view of the declarations
1162 in the context.</dd>
1163
1164 <dt>Ownership of declarations</dt>
1165 <dd>The <code>DeclContext</code> owns all of the declarations that
1166 were declared within its declaration context, and is responsible
1167 for the management of their memory as well as their
1168 (de-)serialization.</dd>
1169</dl>
1170
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1171<p>All declarations are stored within a declaration context, and one
1172 can query
1173 information about the context in which each declaration lives. One
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1174 can retrieve the <code>DeclContext</code> that contains a
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1175 particular <code>Decl</code>
1176 using <code>Decl::getDeclContext</code>. However, see the
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1177 section <a href="#LexicalAndSemanticContexts">Lexical and Semantic
1178 Contexts</a> for more information about how to interpret this
1179 context information.</p>
1180
1181<h4 id="Redeclarations">Redeclarations and Overloads</h4>
1182<p>Within a translation unit, it is common for an entity to be
1183declared several times. For example, we might declare a function "f"
1184 and then later re-declare it as part of an inlined definition:</p>
1185
1186<pre>
1187void f(int x, int y, int z = 1);
1188
1189inline void f(int x, int y, int z) { /* ... */ }
1190</pre>
1191
1192<p>The representation of "f" differs in the source-centric and
1193 semantics-centric views of a declaration context. In the
1194 source-centric view, all redeclarations will be present, in the
1195 order they occurred in the source code, making
1196 this view suitable for clients that wish to see the structure of
1197 the source code. In the semantics-centric view, only the most recent "f"
1198 will be found by the lookup, since it effectively replaces the first
1199 declaration of "f".</p>
1200
1201<p>In the semantics-centric view, overloading of functions is
1202 represented explicitly. For example, given two declarations of a
1203 function "g" that are overloaded, e.g.,</p>
1204<pre>
1205void g();
1206void g(int);
1207</pre>
1208<p>the <code>DeclContext::lookup</code> operation will return
Jonathan D. Turnerd3224292011-07-06 18:12:36 +0000[diff] [blame]1209 a <code>DeclContext::lookup_result</code> that contains a range of iterators
1210 over declarations of "g". Clients that perform semantic analysis on a
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1211 program that is not concerned with the actual source code will
1212 primarily use this semantics-centric view.</p>
1213
1214<h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4>
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1215<p>Each declaration has two potentially different
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1216 declaration contexts: a <i>lexical</i> context, which corresponds to
1217 the source-centric view of the declaration context, and
1218 a <i>semantic</i> context, which corresponds to the
1219 semantics-centric view. The lexical context is accessible
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1220 via <code>Decl::getLexicalDeclContext</code> while the
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1221 semantic context is accessible
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1222 via <code>Decl::getDeclContext</code>, both of which return
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1223 <code>DeclContext</code> pointers. For most declarations, the two
1224 contexts are identical. For example:</p>
1225
1226<pre>
1227class X {
1228public:
1229 void f(int x);
1230};
1231</pre>
1232
1233<p>Here, the semantic and lexical contexts of <code>X::f</code> are
1234 the <code>DeclContext</code> associated with the
1235 class <code>X</code> (itself stored as a <code>RecordDecl</code> AST
1236 node). However, we can now define <code>X::f</code> out-of-line:</p>
1237
1238<pre>
1239void X::f(int x = 17) { /* ... */ }
1240</pre>
1241
1242<p>This definition of has different lexical and semantic
1243 contexts. The lexical context corresponds to the declaration
1244 context in which the actual declaration occurred in the source
1245 code, e.g., the translation unit containing <code>X</code>. Thus,
1246 this declaration of <code>X::f</code> can be found by traversing
1247 the declarations provided by
1248 [<code>decls_begin()</code>, <code>decls_end()</code>) in the
1249 translation unit.</p>
1250
1251<p>The semantic context of <code>X::f</code> corresponds to the
1252 class <code>X</code>, since this member function is (semantically) a
1253 member of <code>X</code>. Lookup of the name <code>f</code> into
1254 the <code>DeclContext</code> associated with <code>X</code> will
1255 then return the definition of <code>X::f</code> (including
1256 information about the default argument).</p>
1257
1258<h4 id="TransparentContexts">Transparent Declaration Contexts</h4>
1259<p>In C and C++, there are several contexts in which names that are
1260 logically declared inside another declaration will actually "leak"
1261 out into the enclosing scope from the perspective of name
1262 lookup. The most obvious instance of this behavior is in
1263 enumeration types, e.g.,</p>
1264<pre>
1265enum Color {
1266 Red,
1267 Green,
1268 Blue
1269};
1270</pre>
1271
1272<p>Here, <code>Color</code> is an enumeration, which is a declaration
1273 context that contains the
1274 enumerators <code>Red</code>, <code>Green</code>,
1275 and <code>Blue</code>. Thus, traversing the list of declarations
1276 contained in the enumeration <code>Color</code> will
1277 yield <code>Red</code>, <code>Green</code>,
1278 and <code>Blue</code>. However, outside of the scope
1279 of <code>Color</code> one can name the enumerator <code>Red</code>
1280 without qualifying the name, e.g.,</p>
1281
1282<pre>
1283Color c = Red;
1284</pre>
1285
1286<p>There are other entities in C++ that provide similar behavior. For
1287 example, linkage specifications that use curly braces:</p>
1288
1289<pre>
1290extern "C" {
1291 void f(int);
1292 void g(int);
1293}
1294// f and g are visible here
1295</pre>
1296
1297<p>For source-level accuracy, we treat the linkage specification and
1298 enumeration type as a
1299 declaration context in which its enclosed declarations ("Red",
1300 "Green", and "Blue"; "f" and "g")
1301 are declared. However, these declarations are visible outside of the
1302 scope of the declaration context.</p>
1303
1304<p>These language features (and several others, described below) have
1305 roughly the same set of
1306 requirements: declarations are declared within a particular lexical
1307 context, but the declarations are also found via name lookup in
1308 scopes enclosing the declaration itself. This feature is implemented
1309 via <i>transparent</i> declaration contexts
1310 (see <code>DeclContext::isTransparentContext()</code>), whose
1311 declarations are visible in the nearest enclosing non-transparent
1312 declaration context. This means that the lexical context of the
1313 declaration (e.g., an enumerator) will be the
1314 transparent <code>DeclContext</code> itself, as will the semantic
1315 context, but the declaration will be visible in every outer context
1316 up to and including the first non-transparent declaration context (since
1317 transparent declaration contexts can be nested).</p>
1318
1319<p>The transparent <code>DeclContexts</code> are:</p>
1320<ul>
David Blaikie5090e9f2011-10-18 05:49:30 +0000[diff] [blame]1321 <li>Enumerations (but not C++11 "scoped enumerations"):
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1322 <pre>
1323enum Color {
1324 Red,
1325 Green,
1326 Blue
1327};
1328// Red, Green, and Blue are in scope
1329 </pre></li>
1330 <li>C++ linkage specifications:
1331 <pre>
1332extern "C" {
1333 void f(int);
1334 void g(int);
1335}
1336// f and g are in scope
1337 </pre></li>
1338 <li>Anonymous unions and structs:
1339 <pre>
1340struct LookupTable {
1341 bool IsVector;
1342 union {
1343 std::vector&lt;Item&gt; *Vector;
1344 std::set&lt;Item&gt; *Set;
1345 };
1346};
1347
1348LookupTable LT;
1349LT.Vector = 0; // Okay: finds Vector inside the unnamed union
1350 </pre>
1351 </li>
David Blaikie5090e9f2011-10-18 05:49:30 +0000[diff] [blame]1352 <li>C++11 inline namespaces:
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1353<pre>
1354namespace mylib {
1355 inline namespace debug {
1356 class X;
1357 }
1358}
1359mylib::X *xp; // okay: mylib::X refers to mylib::debug::X
1360</pre>
1361</li>
1362</ul>
1363
1364
1365<h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4>
1366<p>C++ namespaces have the interesting--and, so far, unique--property that
1367the namespace can be defined multiple times, and the declarations
1368provided by each namespace definition are effectively merged (from
1369the semantic point of view). For example, the following two code
1370snippets are semantically indistinguishable:</p>
1371<pre>
1372// Snippet #1:
1373namespace N {
1374 void f();
1375}
1376namespace N {
1377 void f(int);
1378}
1379
1380// Snippet #2:
1381namespace N {
1382 void f();
1383 void f(int);
1384}
1385</pre>
1386
1387<p>In Clang's representation, the source-centric view of declaration
1388 contexts will actually have two separate <code>NamespaceDecl</code>
1389 nodes in Snippet #1, each of which is a declaration context that
1390 contains a single declaration of "f". However, the semantics-centric
1391 view provided by name lookup into the namespace <code>N</code> for
Jonathan D. Turnerd3224292011-07-06 18:12:36 +0000[diff] [blame]1392 "f" will return a <code>DeclContext::lookup_result</code> that contains
1393 a range of iterators over declarations of "f".</p>
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1394
1395<p><code>DeclContext</code> manages multiply-defined declaration
1396 contexts internally. The
1397 function <code>DeclContext::getPrimaryContext</code> retrieves the
1398 "primary" context for a given <code>DeclContext</code> instance,
1399 which is the <code>DeclContext</code> responsible for maintaining
1400 the lookup table used for the semantics-centric view. Given the
1401 primary context, one can follow the chain
1402 of <code>DeclContext</code> nodes that define additional
1403 declarations via <code>DeclContext::getNextContext</code>. Note that
1404 these functions are used internally within the lookup and insertion
1405 methods of the <code>DeclContext</code>, so the vast majority of
1406 clients can ignore them.</p>
1407
1408<!-- ======================================================================= -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1409<h3 id="CFG">The <tt>CFG</tt> class</h3>
1410<!-- ======================================================================= -->
1411
1412<p>The <tt>CFG</tt> class is designed to represent a source-level
1413control-flow graph for a single statement (<tt>Stmt*</tt>). Typically
1414instances of <tt>CFG</tt> are constructed for function bodies (usually
1415an instance of <tt>CompoundStmt</tt>), but can also be instantiated to
1416represent the control-flow of any class that subclasses <tt>Stmt</tt>,
1417which includes simple expressions. Control-flow graphs are especially
1418useful for performing
1419<a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow-
1420or path-sensitive</a> program analyses on a given function.</p>
1421
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1422<!-- ============ -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1423<h4>Basic Blocks</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1424<!-- ============ -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1425
1426<p>Concretely, an instance of <tt>CFG</tt> is a collection of basic
1427blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which
1428simply contains an ordered sequence of <tt>Stmt*</tt> (each referring
1429to statements in the AST). The ordering of statements within a block
1430indicates unconditional flow of control from one statement to the
1431next. <a href="#ConditionalControlFlow">Conditional control-flow</a>
1432is represented using edges between basic blocks. The statements
1433within a given <tt>CFGBlock</tt> can be traversed using
1434the <tt>CFGBlock::*iterator</tt> interface.</p>
1435
1436<p>
Ted Kremenek18e17e72007-10-18 22:50:52 +0000[diff] [blame]1437A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1438the control-flow graph it represents. Each <tt>CFGBlock</tt> within a
1439CFG is also uniquely numbered (accessible
1440via <tt>CFGBlock::getBlockID()</tt>). Currently the number is
1441based on the ordering the blocks were created, but no assumptions
1442should be made on how <tt>CFGBlock</tt>s are numbered other than their
1443numbers are unique and that they are numbered from 0..N-1 (where N is
1444the number of basic blocks in the CFG).</p>
1445
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1446<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1447<h4>Entry and Exit Blocks</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1448<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1449
1450Each instance of <tt>CFG</tt> contains two special blocks:
1451an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which
1452has no incoming edges, and an <i>exit</i> block (accessible
1453via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither
1454block contains any statements, and they serve the role of providing a
1455clear entrance and exit for a body of code such as a function body.
1456The presence of these empty blocks greatly simplifies the
1457implementation of many analyses built on top of CFGs.
1458
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1459<!-- ===================================================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1460<h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1461<!-- ===================================================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1462
1463<p>Conditional control-flow (such as those induced by if-statements
1464and loops) is represented as edges between <tt>CFGBlock</tt>s.
1465Because different C language constructs can induce control-flow,
1466each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that
1467represents the <i>terminator</i> of the block. A terminator is simply
1468the statement that caused the control-flow, and is used to identify
1469the nature of the conditional control-flow between blocks. For
1470example, in the case of an if-statement, the terminator refers to
1471the <tt>IfStmt</tt> object in the AST that represented the given
1472branch.</p>
1473
1474<p>To illustrate, consider the following code example:</p>
1475
1476<code>
1477int foo(int x) {<br>
1478&nbsp;&nbsp;x = x + 1;<br>
1479<br>
1480&nbsp;&nbsp;if (x > 2) x++;<br>
1481&nbsp;&nbsp;else {<br>
1482&nbsp;&nbsp;&nbsp;&nbsp;x += 2;<br>
1483&nbsp;&nbsp;&nbsp;&nbsp;x *= 2;<br>
1484&nbsp;&nbsp;}<br>
1485<br>
1486&nbsp;&nbsp;return x;<br>
1487}
1488</code>
1489
1490<p>After invoking the parser+semantic analyzer on this code fragment,
1491the AST of the body of <tt>foo</tt> is referenced by a
1492single <tt>Stmt*</tt>. We can then construct an instance
1493of <tt>CFG</tt> representing the control-flow graph of this function
1494body by single call to a static class method:</p>
1495
1496<code>
1497&nbsp;&nbsp;Stmt* FooBody = ...<br>
1498&nbsp;&nbsp;CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody);
1499</code>
1500
1501<p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt>
1502to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no
1503longer needed.</p>
1504
1505<p>Along with providing an interface to iterate over
1506its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods
1507that are useful for debugging and visualizing CFGs. For example, the
1508method
1509<tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to
1510standard error. This is especially useful when one is using a
1511debugger such as gdb. For example, here is the output
1512of <tt>FooCFG->dump()</tt>:</p>
1513
1514<code>
1515&nbsp;[ B5 (ENTRY) ]<br>
1516&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (0):<br>
1517&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B4<br>
1518<br>
1519&nbsp;[ B4 ]<br>
1520&nbsp;&nbsp;&nbsp;&nbsp;1: x = x + 1<br>
1521&nbsp;&nbsp;&nbsp;&nbsp;2: (x > 2)<br>
1522&nbsp;&nbsp;&nbsp;&nbsp;<b>T: if [B4.2]</b><br>
1523&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B5<br>
1524&nbsp;&nbsp;&nbsp;&nbsp;Successors (2): B3 B2<br>
1525<br>
1526&nbsp;[ B3 ]<br>
1527&nbsp;&nbsp;&nbsp;&nbsp;1: x++<br>
1528&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
1529&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
1530<br>
1531&nbsp;[ B2 ]<br>
1532&nbsp;&nbsp;&nbsp;&nbsp;1: x += 2<br>
1533&nbsp;&nbsp;&nbsp;&nbsp;2: x *= 2<br>
1534&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
1535&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
1536<br>
1537&nbsp;[ B1 ]<br>
1538&nbsp;&nbsp;&nbsp;&nbsp;1: return x;<br>
1539&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (2): B2 B3<br>
1540&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B0<br>
1541<br>
1542&nbsp;[ B0 (EXIT) ]<br>
1543&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B1<br>
1544&nbsp;&nbsp;&nbsp;&nbsp;Successors (0):
1545</code>
1546
1547<p>For each block, the pretty-printed output displays for each block
1548the number of <i>predecessor</i> blocks (blocks that have outgoing
1549control-flow to the given block) and <i>successor</i> blocks (blocks
1550that have control-flow that have incoming control-flow from the given
1551block). We can also clearly see the special entry and exit blocks at
1552the beginning and end of the pretty-printed output. For the entry
1553block (block B5), the number of predecessor blocks is 0, while for the
1554exit block (block B0) the number of successor blocks is 0.</p>
1555
1556<p>The most interesting block here is B4, whose outgoing control-flow
1557represents the branching caused by the sole if-statement
1558in <tt>foo</tt>. Of particular interest is the second statement in
1559the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed
1560as <b><tt>if [B4.2]</tt></b>. The second statement represents the
1561evaluation of the condition of the if-statement, which occurs before
1562the actual branching of control-flow. Within the <tt>CFGBlock</tt>
1563for B4, the <tt>Stmt*</tt> for the second statement refers to the
1564actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus
1565pointers to subclasses of <tt>Expr</tt> can appear in the list of
1566statements in a block, and not just subclasses of <tt>Stmt</tt> that
1567refer to proper C statements.</p>
1568
1569<p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt>
1570object in the AST. The pretty-printer outputs <b><tt>if
1571[B4.2]</tt></b> because the condition expression of the if-statement
1572has an actual place in the basic block, and thus the terminator is
1573essentially
1574<i>referring</i> to the expression that is the second statement of
1575block B4 (i.e., B4.2). In this manner, conditions for control-flow
1576(which also includes conditions for loops and switch statements) are
1577hoisted into the actual basic block.</p>
1578
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1579<!-- ===================== -->
1580<!-- <h4>Implicit Control-Flow</h4> -->
1581<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1582
1583<!--
1584<p>A key design principle of the <tt>CFG</tt> class was to not require
1585any transformations to the AST in order to represent control-flow.
1586Thus the <tt>CFG</tt> does not perform any "lowering" of the
1587statements in an AST: loops are not transformed into guarded gotos,
1588short-circuit operations are not converted to a set of if-statements,
1589and so on.</p>
1590-->
Ted Kremenek17a295d2008-06-11 06:19:49 +0000[diff] [blame]1591
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1592
1593<!-- ======================================================================= -->
1594<h3 id="Constants">Constant Folding in the Clang AST</h3>
1595<!-- ======================================================================= -->
1596
1597<p>There are several places where constants and constant folding matter a lot to
1598the Clang front-end. First, in general, we prefer the AST to retain the source
1599code as close to how the user wrote it as possible. This means that if they
1600wrote "5+4", we want to keep the addition and two constants in the AST, we don't
1601want to fold to "9". This means that constant folding in various ways turns
1602into a tree walk that needs to handle the various cases.</p>
1603
1604<p>However, there are places in both C and C++ that require constants to be
1605folded. For example, the C standard defines what an "integer constant
1606expression" (i-c-e) is with very precise and specific requirements. The
1607language then requires i-c-e's in a lot of places (for example, the size of a
1608bitfield, the value for a case statement, etc). For these, we have to be able
1609to constant fold the constants, to do semantic checks (e.g. verify bitfield size
1610is non-negative and that case statements aren't duplicated). We aim for Clang
1611to be very pedantic about this, diagnosing cases when the code does not use an
1612i-c-e where one is required, but accepting the code unless running with
1613<tt>-pedantic-errors</tt>.</p>
1614
1615<p>Things get a little bit more tricky when it comes to compatibility with
1616real-world source code. Specifically, GCC has historically accepted a huge
1617superset of expressions as i-c-e's, and a lot of real world code depends on this
1618unfortuate accident of history (including, e.g., the glibc system headers). GCC
1619accepts anything its "fold" optimizer is capable of reducing to an integer
1620constant, which means that the definition of what it accepts changes as its
1621optimizer does. One example is that GCC accepts things like "case X-X:" even
1622when X is a variable, because it can fold this to 0.</p>
1623
1624<p>Another issue are how constants interact with the extensions we support, such
1625as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99
1626obviously does not specify the semantics of any of these extensions, and the
1627definition of i-c-e does not include them. However, these extensions are often
1628used in real code, and we have to have a way to reason about them.</p>
1629
1630<p>Finally, this is not just a problem for semantic analysis. The code
1631generator and other clients have to be able to fold constants (e.g. to
1632initialize global variables) and has to handle a superset of what C99 allows.
1633Further, these clients can benefit from extended information. For example, we
1634know that "foo()||1" always evaluates to true, but we can't replace the
1635expression with true because it has side effects.</p>
1636
1637<!-- ======================= -->
1638<h4>Implementation Approach</h4>
1639<!-- ======================= -->
1640
1641<p>After trying several different approaches, we've finally converged on a
1642design (Note, at the time of this writing, not all of this has been implemented,
1643consider this a design goal!). Our basic approach is to define a single
1644recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is
1645implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar'
1646type (integer, fp, complex, or pointer) this method returns the following
1647information:</p>
1648
1649<ul>
1650<li>Whether the expression is an integer constant expression, a general
1651 constant that was folded but has no side effects, a general constant that
1652 was folded but that does have side effects, or an uncomputable/unfoldable
1653 value.
1654</li>
1655<li>If the expression was computable in any way, this method returns the APValue
1656 for the result of the expression.</li>
1657<li>If the expression is not evaluatable at all, this method returns
1658 information on one of the problems with the expression. This includes a
1659 SourceLocation for where the problem is, and a diagnostic ID that explains
1660 the problem. The diagnostic should be have ERROR type.</li>
1661<li>If the expression is not an integer constant expression, this method returns
1662 information on one of the problems with the expression. This includes a
1663 SourceLocation for where the problem is, and a diagnostic ID that explains
1664 the problem. The diagnostic should be have EXTENSION type.</li>
1665</ul>
1666
1667<p>This information gives various clients the flexibility that they want, and we
1668will eventually have some helper methods for various extensions. For example,
1669Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which
1670calls Evaluate on the expression. If the expression is not foldable, the error
1671is emitted, and it would return true. If the expression is not an i-c-e, the
1672EXTENSION diagnostic is emitted. Finally it would return false to indicate that
1673the AST is ok.</p>
1674
1675<p>Other clients can use the information in other ways, for example, codegen can
1676just use expressions that are foldable in any way.</p>
1677
1678<!-- ========== -->
1679<h4>Extensions</h4>
1680<!-- ========== -->
1681
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]1682<p>This section describes how some of the various extensions Clang supports
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1683interacts with constant evaluation:</p>
1684
1685<ul>
1686<li><b><tt>__extension__</tt></b>: The expression form of this extension causes
1687 any evaluatable subexpression to be accepted as an integer constant
1688 expression.</li>
1689<li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer
Richard Smith8a0f1552011-12-09 03:40:28 +0000[diff] [blame]1690 constant expression) if the operand evaluates to either a numeric value
1691 (that is, not a pointer cast to integral type) of integral, enumeration,
1692 floating or complex type, or if it evaluates to the address of the first
1693 character of a string literal (possibly cast to some other type). As a
Chris Lattner28daa532008-12-12 06:55:44 +0000[diff] [blame]1694 special case, if <tt>__builtin_constant_p</tt> is the (potentially
1695 parenthesized) condition of a conditional operator expression ("?:"), only
Chris Lattner42b83dd2008-12-12 18:00:51 +0000[diff] [blame]1696 the true side of the conditional operator is considered, and it is evaluated
1697 with full constant folding.</li>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1698<li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an
1699 integer constant expression, but we accept any constant as an "extension of
1700 an extension". This only evaluates one operand depending on which way the
1701 condition evaluates.</li>
1702<li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer
1703 constant expression.</li>
1704<li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a
1705 floating-point literal.</li>
1706<li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as
1707 general constant expressions.</li>
Richard Smith8a0f1552011-12-09 03:40:28 +0000[diff] [blame]1708<li><b><tt>__builtin_strlen</tt></b> and <b><tt>strlen</tt></b>: These are
1709 constant folded as integer constant expressions if the argument is a string
1710 literal.</li>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1711</ul>
1712
1713
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]1714<!-- ======================================================================= -->
1715<h2 id="Howtos">How to change Clang</h2>
1716<!-- ======================================================================= -->
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1717
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]1718<!-- ======================================================================= -->
1719<h3 id="AddingAttributes">How to add an attribute</h3>
1720<!-- ======================================================================= -->
1721
1722<p>To add an attribute, you'll have to add it to the list of attributes, add it
1723to the parsing phase, and look for it in the AST scan.
Benjamin Kramer665a8dc2012-01-15 15:26:07 +0000[diff] [blame]1724<a href="http://llvm.org/viewvc/llvm-project?view=rev&amp;revision=124217">r124217</a>
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]1725has a good example of adding a warning attribute.</p>
1726
1727<p>(Beware that this hasn't been reviewed/fixed by the people who designed the
1728attributes system yet.)</p>
1729
1730<h4><a
1731href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?view=markup">include/clang/Basic/Attr.td</a></h4>
1732
1733<p>Each attribute gets a <tt>def</tt> inheriting from <tt>Attr</tt> or one of
1734its subclasses. <tt>InheritableAttr</tt> means that the attribute also applies
1735to subsequent declarations of the same name.</p>
1736
1737<p><tt>Spellings</tt> lists the strings that can appear in
1738<tt>__attribute__((here))</tt> or <tt>[[here]]</tt>. All such strings
David Blaikie5090e9f2011-10-18 05:49:30 +0000[diff] [blame]1739will be synonymous. If you want to allow the <tt>[[]]</tt> C++11
Jeffrey Yasskin28dadd62011-01-28 23:41:54 +0000[diff] [blame]1740syntax, you have to define a list of <tt>Namespaces</tt>, which will
1741let users write <tt>[[namespace:spelling]]</tt>. Using the empty
1742string for a namespace will allow users to write just the spelling
1743with no "<tt>:</tt>".</p>
1744
1745<p><tt>Subjects</tt> restricts what kinds of AST node to which this attribute
1746can appertain (roughly, attach).</p>
1747
1748<p><tt>Args</tt> names the arguments the attribute takes, in order. If
1749<tt>Args</tt> is <tt>[StringArgument&lt;"Arg1">, IntArgument&lt;"Arg2">]</tt>
1750then <tt>__attribute__((myattribute("Hello", 3)))</tt> will be a valid use.</p>
1751
1752<h4>Boilerplate</h4>
1753
1754<p>Add an element to the <tt>AttributeList::Kind</tt> enum in <a
1755href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Sema/AttributeList.h?view=markup">include/clang/Sema/AttributeList.h</a>
1756named <tt>AT_lower_with_underscores</tt>. That is, a CamelCased
1757<tt>AttributeName</tt> in <tt>Attr.td</tt> name should become
1758<tt>AT_attribute_name</tt>.</p>
1759
1760<p>Add a case to the <tt>StringSwitch</tt> in <tt>AttributeList::getKind()</tt>
1761in <a
1762href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/AttributeList.cpp?view=markup">lib/Sema/AttributeList.cpp</a>
1763for each spelling of your attribute. Less common attributes should come toward
1764the end of that list.</p>
1765
1766<p>Write a new <tt>HandleYourAttr()</tt> function in <a
1767href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?view=markup">lib/Sema/SemaDeclAttr.cpp</a>,
1768and add a case to the switch in <tt>ProcessNonInheritableDeclAttr()</tt> or
1769<tt>ProcessInheritableDeclAttr()</tt> forwarding to it.</p>
1770
1771<p>If your attribute causes extra warnings to fire, define a <tt>DiagGroup</tt>
1772in <a
1773href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?view=markup">include/clang/Basic/DiagnosticGroups.td</a>
1774named after the attribute's <tt>Spelling</tt> with "_"s replaced by "-"s. If
1775you're only defining one diagnostic, you can skip <tt>DiagnosticGroups.td</tt>
1776and use <tt>InGroup&lt;DiagGroup&lt;"your-attribute">></tt> directly in <a
1777href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?view=markup">DiagnosticSemaKinds.td</a></p>
1778
1779<h4>The meat of your attribute</h4>
1780
1781<p>Find an appropriate place in Clang to do whatever your attribute needs to do.
1782Check for the attribute's presence using <tt>Decl::getAttr&lt;YourAttr>()</tt>.</p>
1783
1784<p>Update the <a href="LanguageExtensions.html">Clang Language Extensions</a>
1785document to describe your new attribute.</p>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1786
Douglas Gregor1f634c62011-09-30 21:32:37 +0000[diff] [blame]1787<!-- ======================================================================= -->
1788<h3 id="AddingExprStmt">How to add an expression or statement</h3>
1789<!-- ======================================================================= -->
1790
1791<p>Expressions and statements are one of the most fundamental constructs within a
1792compiler, because they interact with many different parts of the AST,
1793semantic analysis, and IR generation. Therefore, adding a new
1794expression or statement kind into Clang requires some care. The following list
1795details the various places in Clang where an expression or statement needs to be
1796introduced, along with patterns to follow to ensure that the new
1797expression or statement works well across all of the C languages. We
1798focus on expressions, but statements are similar.</p>
1799
1800<ol>
1801 <li>Introduce parsing actions into the parser. Recursive-descent
1802 parsing is mostly self-explanatory, but there are a few things that
1803 are worth keeping in mind:
1804 <ul>
1805 <li>Keep as much source location information as possible! You'll
1806 want it later to produce great diagnostics and support Clang's
1807 various features that map between source code and the AST.</li>
1808 <li>Write tests for all of the "bad" parsing cases, to make sure
1809 your recovery is good. If you have matched delimiters (e.g.,
1810 parentheses, square brackets, etc.), use
Douglas Gregor4a8dfb52011-10-12 16:37:45 +0000[diff] [blame]1811 <tt>Parser::BalancedDelimiterTracker</tt> to give nice diagnostics when
Douglas Gregor1f634c62011-09-30 21:32:37 +0000[diff] [blame]1812 things go wrong.</li>
1813 </ul>
1814 </li>
1815
1816 <li>Introduce semantic analysis actions into <tt>Sema</tt>. Semantic
1817 analysis should always involve two functions: an <tt>ActOnXXX</tt>
1818 function that will be called directly from the parser, and a
1819 <tt>BuildXXX</tt> function that performs the actual semantic
1820 analysis and will (eventually!) build the AST node. It's fairly
1821 common for the <tt>ActOnCXX</tt> function to do very little (often
1822 just some minor translation from the parser's representation to
1823 <tt>Sema</tt>'s representation of the same thing), but the separation
1824 is still important: C++ template instantiation, for example,
1825 should always call the <tt>BuildXXX</tt> variant. Several notes on
1826 semantic analysis before we get into construction of the AST:
1827 <ul>
1828 <li>Your expression probably involves some types and some
1829 subexpressions. Make sure to fully check that those types, and the
1830 types of those subexpressions, meet your expectations. Add
1831 implicit conversions where necessary to make sure that all of the
1832 types line up exactly the way you want them. Write extensive tests
1833 to check that you're getting good diagnostics for mistakes and
1834 that you can use various forms of subexpressions with your
1835 expression.</li>
1836 <li>When type-checking a type or subexpression, make sure to first
1837 check whether the type is "dependent"
1838 (<tt>Type::isDependentType()</tt>) or whether a subexpression is
1839 type-dependent (<tt>Expr::isTypeDependent()</tt>). If any of these
1840 return true, then you're inside a template and you can't do much
1841 type-checking now. That's normal, and your AST node (when you get
1842 there) will have to deal with this case. At this point, you can
1843 write tests that use your expression within templates, but don't
1844 try to instantiate the templates.</li>
1845 <li>For each subexpression, be sure to call
1846 <tt>Sema::CheckPlaceholderExpr()</tt> to deal with "weird"
1847 expressions that don't behave well as subexpressions. Then,
1848 determine whether you need to perform
1849 lvalue-to-rvalue conversions
1850 (<tt>Sema::DefaultLvalueConversion</tt>e) or
1851 the usual unary conversions
1852 (<tt>Sema::UsualUnaryConversions</tt>), for places where the
1853 subexpression is producing a value you intend to use.</li>
1854 <li>Your <tt>BuildXXX</tt> function will probably just return
1855 <tt>ExprError()</tt> at this point, since you don't have an AST.
1856 That's perfectly fine, and shouldn't impact your testing.</li>
1857 </ul>
1858 </li>
1859
1860 <li>Introduce an AST node for your new expression. This starts with
1861 declaring the node in <tt>include/Basic/StmtNodes.td</tt> and
1862 creating a new class for your expression in the appropriate
1863 <tt>include/AST/Expr*.h</tt> header. It's best to look at the class
1864 for a similar expression to get ideas, and there are some specific
1865 things to watch for:
1866 <ul>
1867 <li>If you need to allocate memory, use the <tt>ASTContext</tt>
1868 allocator to allocate memory. Never use raw <tt>malloc</tt> or
1869 <tt>new</tt>, and never hold any resources in an AST node, because
1870 the destructor of an AST node is never called.</li>
1871
1872 <li>Make sure that <tt>getSourceRange()</tt> covers the exact
1873 source range of your expression. This is needed for diagnostics
1874 and for IDE support.</li>
1875
1876 <li>Make sure that <tt>children()</tt> visits all of the
1877 subexpressions. This is important for a number of features (e.g., IDE
1878 support, C++ variadic templates). If you have sub-types, you'll
1879 also need to visit those sub-types in the
1880 <tt>RecursiveASTVisitor</tt>.</li>
1881
1882 <li>Add printing support (<tt>StmtPrinter.cpp</tt>) and dumping
1883 support (<tt>StmtDumper.cpp</tt>) for your expression.</li>
1884
1885 <li>Add profiling support (<tt>StmtProfile.cpp</tt>) for your AST
1886 node, noting the distinguishing (non-source location)
1887 characteristics of an instance of your expression. Omitting this
1888 step will lead to hard-to-diagnose failures regarding matching of
1889 template declarations.</li>
1890 </ul>
1891 </li>
1892
1893 <li>Teach semantic analysis to build your AST node! At this point,
1894 you can wire up your <tt>Sema::BuildXXX</tt> function to actually
1895 create your AST. A few things to check at this point:
1896 <ul>
1897 <li>If your expression can construct a new C++ class or return a
1898 new Objective-C object, be sure to update and then call
1899 <tt>Sema::MaybeBindToTemporary</tt> for your just-created AST node
1900 to be sure that the object gets properly destructed. An easy way
1901 to test this is to return a C++ class with a private destructor:
1902 semantic analysis should flag an error here with the attempt to
1903 call the destructor.</li>
1904 <li>Inspect the generated AST by printing it using <tt>clang -cc1
1905 -ast-print</tt>, to make sure you're capturing all of the
1906 important information about how the AST was written.</li>
1907 <li>Inspect the generated AST under <tt>clang -cc1 -ast-dump</tt>
1908 to verify that all of the types in the generated AST line up the
1909 way you want them. Remember that clients of the AST should never
1910 have to "think" to understand what's going on. For example, all
1911 implicit conversions should show up explicitly in the AST.</li>
1912 <li>Write tests that use your expression as a subexpression of
1913 other, well-known expressions. Can you call a function using your
1914 expression as an argument? Can you use the ternary operator?</li>
1915 </ul>
1916 </li>
1917
1918 <li>Teach code generation to create IR to your AST node. This step
1919 is the first (and only) that requires knowledge of LLVM IR. There
1920 are several things to keep in mind:
1921 <ul>
1922 <li>Code generation is separated into scalar/aggregate/complex and
1923 lvalue/rvalue paths, depending on what kind of result your
1924 expression produces. On occasion, this requires some careful
1925 factoring of code to avoid duplication.</li>
1926
1927 <li><tt>CodeGenFunction</tt> contains functions
1928 <tt>ConvertType</tt> and <tt>ConvertTypeForMem</tt> that convert
1929 Clang's types (<tt>clang::Type*</tt> or <tt>clang::QualType</tt>)
1930 to LLVM types.
1931 Use the former for values, and the later for memory locations:
1932 test with the C++ "bool" type to check this. If you find
1933 that you are having to use LLVM bitcasts to make
1934 the subexpressions of your expression have the type that your
1935 expression expects, STOP! Go fix semantic analysis and the AST so
1936 that you don't need these bitcasts.</li>
1937
1938 <li>The <tt>CodeGenFunction</tt> class has a number of helper
1939 functions to make certain operations easy, such as generating code
1940 to produce an lvalue or an rvalue, or to initialize a memory
1941 location with a given value. Prefer to use these functions rather
1942 than directly writing loads and stores, because these functions
1943 take care of some of the tricky details for you (e.g., for
1944 exceptions).</li>
1945
1946 <li>If your expression requires some special behavior in the event
1947 of an exception, look at the <tt>push*Cleanup</tt> functions in
1948 <tt>CodeGenFunction</tt> to introduce a cleanup. You shouldn't
1949 have to deal with exception-handling directly.</li>
1950
1951 <li>Testing is extremely important in IR generation. Use <tt>clang
1952 -cc1 -emit-llvm</tt> and <a
1953 href="http://llvm.org/cmds/FileCheck.html">FileCheck</a> to verify
1954 that you're generating the right IR.</li>
1955 </ul>
1956 </li>
1957
1958 <li>Teach template instantiation how to cope with your AST
1959 node, which requires some fairly simple code:
1960 <ul>
1961 <li>Make sure that your expression's constructor properly
1962 computes the flags for type dependence (i.e., the type your
1963 expression produces can change from one instantiation to the
1964 next), value dependence (i.e., the constant value your expression
1965 produces can change from one instantiation to the next),
Douglas Gregord1cb2dc2011-10-14 00:54:15 +0000[diff] [blame]1966 instantiation dependence (i.e., a template parameter occurs
Douglas Gregor1f634c62011-09-30 21:32:37 +0000[diff] [blame]1967 anywhere in your expression), and whether your expression contains
1968 a parameter pack (for variadic templates). Often, computing these
1969 flags just means combining the results from the various types and
1970 subexpressions.</li>
1971
1972 <li>Add <tt>TransformXXX</tt> and <tt>RebuildXXX</tt> functions to
1973 the
1974 <tt>TreeTransform</tt> class template in <tt>Sema</tt>.
1975 <tt>TransformXXX</tt> should (recursively) transform all of the
1976 subexpressions and types
1977 within your expression, using <tt>getDerived().TransformYYY</tt>.
1978 If all of the subexpressions and types transform without error, it
1979 will then call the <tt>RebuildXXX</tt> function, which will in
1980 turn call <tt>getSema().BuildXXX</tt> to perform semantic analysis
1981 and build your expression.</li>
1982
1983 <li>To test template instantiation, take those tests you wrote to
1984 make sure that you were type checking with type-dependent
1985 expressions and dependent types (from step #2) and instantiate
1986 those templates with various types, some of which type-check and
1987 some that don't, and test the error messages in each case.</li>
1988 </ul>
1989 </li>
1990
1991 <li>There are some "extras" that make other features work better.
1992 It's worth handling these extras to give your expression complete
1993 integration into Clang:
1994 <ul>
1995 <li>Add code completion support for your expression in
1996 <tt>SemaCodeComplete.cpp</tt>.</li>
1997
1998 <li>If your expression has types in it, or has any "interesting"
1999 features other than subexpressions, extend libclang's
2000 <tt>CursorVisitor</tt> to provide proper visitation for your
2001 expression, enabling various IDE features such as syntax
2002 highlighting, cross-referencing, and so on. The
2003 <tt>c-index-test</tt> helper program can be used to test these
2004 features.</li>
2005 </ul>
2006 </li>
2007</ol>
2008
Ted Kremenek17a295d2008-06-11 06:19:49 +0000[diff] [blame]2009</div>
2010</body>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]2011</html>