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Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]17
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]18<h1>"Clang" CFE Internals Manual</h1>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]19
20<ul>
21<li><a href="#intro">Introduction</a></li>
22<li><a href="#libsystem">LLVM System and Support Libraries</a></li>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]23<li><a href="#libbasic">The Clang 'Basic' Library</a>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]24 <ul>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]25 <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]26 <li><a href="#SourceLocation">The SourceLocation and SourceManager
27 classes</a></li>
28 </ul>
29</li>
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]30<li><a href="#libdriver">The Driver Library</a>
31 <ul>
32 </ul>
33</li>
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame]34<li><a href="#pch">Precompiled Headers</a>
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]35<li><a href="#libfrontend">The Frontend Library</a>
36 <ul>
37 </ul>
38</li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]39<li><a href="#liblex">The Lexer and Preprocessor Library</a>
40 <ul>
41 <li><a href="#Token">The Token class</a></li>
42 <li><a href="#Lexer">The Lexer class</a></li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]43 <li><a href="#AnnotationToken">Annotation Tokens</a></li>
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]44 <li><a href="#TokenLexer">The TokenLexer class</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]45 <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li>
46 </ul>
47</li>
48<li><a href="#libparse">The Parser Library</a>
49 <ul>
50 </ul>
51</li>
52<li><a href="#libast">The AST Library</a>
53 <ul>
54 <li><a href="#Type">The Type class and its subclasses</a></li>
55 <li><a href="#QualType">The QualType class</a></li>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]56 <li><a href="#DeclarationName">Declaration names</a></li>
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]57 <li><a href="#DeclContext">Declaration contexts</a>
58 <ul>
59 <li><a href="#Redeclarations">Redeclarations and Overloads</a></li>
60 <li><a href="#LexicalAndSemanticContexts">Lexical and Semantic
61 Contexts</a></li>
62 <li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li>
63 <li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li>
64 </ul>
65 </li>
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]66 <li><a href="#CFG">The CFG class</a></li>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]67 <li><a href="#Constants">Constant Folding in the Clang AST</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]68 </ul>
69</li>
Argyrios Kyrtzidis7240d772009-07-10 03:41:36 +0000[diff] [blame]70<li><a href="libIndex.html">The Index Library</a></li>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]71</ul>
72
73
74<!-- ======================================================================= -->
75<h2 id="intro">Introduction</h2>
76<!-- ======================================================================= -->
77
78<p>This document describes some of the more important APIs and internal design
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]79decisions made in the Clang C front-end. The purpose of this document is to
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]80both capture some of this high level information and also describe some of the
81design decisions behind it. This is meant for people interested in hacking on
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]82Clang, not for end-users. The description below is categorized by
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]83libraries, and does not describe any of the clients of the libraries.</p>
84
85<!-- ======================================================================= -->
86<h2 id="libsystem">LLVM System and Support Libraries</h2>
87<!-- ======================================================================= -->
88
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]89<p>The LLVM libsystem library provides the basic Clang system abstraction layer,
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]90which is used for file system access. The LLVM libsupport library provides many
91underlying libraries and <a
92href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>,
93 including command line option
94processing and various containers.</p>
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
121(at the minimum) a unique ID, a <a href="#SourceLocation">SourceLocation</a> to
122"put the caret", an English translation associated with it, and a severity (e.g.
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
130implemented. A representative example of a diagonstic is:</p>
131
132<pre>
133t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
134 <font color="darkgreen">P = (P-42) + Gamma*4;</font>
135 <font color="blue">~~~~~~ ^ ~~~~~~~</font>
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
148<!-- ============================ -->
149<h4>The DiagnosticKinds.def file</h4>
150<!-- ============================ -->
151
152<p>Diagnostics are created by adding an entry to the <tt><a
153href="http://llvm.org/svn/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticKinds.def"
154>DiagnosticKinds.def</a></tt> file. This file encodes the unique ID of the
155diagnostic (as an enum, the first argument), the severity of the diagnostic
156(second argument) and the English translation + format string.</p>
157
158<p>There is little sanity with the naming of the unique ID's right now. Some
159start with err_, warn_, ext_ to encode the severity into the name. Since the
160enum is referenced in the C++ code that produces the diagnostic, it is somewhat
161useful for it to be reasonably short.</p>
162
163<p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>,
164<tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The
165<tt>ERROR</tt> severity is used for diagnostics indicating the program is never
166acceptable under any circumstances. When an error is emitted, the AST for the
167input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt>
168severities are used for extensions to the language that Clang accepts. This
169means that Clang fully understands and can represent them in the AST, but we
170produce diagnostics to tell the user their code is non-portable. The difference
171is that the former are ignored by default, and the later warn by default. The
172<tt>WARNING</tt> severity is used for constructs that are valid in the currently
173selected source language but that are dubious in some way. The <tt>NOTE</tt>
Daniel Dunbar426b8632009-02-17 15:49:03 +0000[diff] [blame]174level is used to staple more information onto previous diagnostics.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]175
176<p>These <em>severities</em> are mapped into a smaller set (the
177Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>,
Chris Lattner0aad2972009-02-05 22:49:08 +0000[diff] [blame]178<tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics
Chris Lattnera180fdd2009-02-17 07:07:29 +0000[diff] [blame]179subsystem based on various configuration options. Clang internally supports a
180fully fine grained mapping mechanism that allows you to map almost any
181diagnostic to the output level that you want. The only diagnostics that cannot
182be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously
183emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to
184<tt>Fatal</tt> (it is not possible to turn an error into a warning,
185for example).</p>
186
187<p>Diagnostic mappings are used in many ways. For example, if the user
188specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if
189they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is
190used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc.
191</p>
192
193<p>
194Mapping to <tt>Fatal</tt> should only be used for diagnostics that are
195considered so severe that error recovery won't be able to recover sensibly from
196them (thus spewing a ton of bogus errors). One example of this class of error
197are failure to #include a file.
198</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]199
200<!-- ================= -->
201<h4>The Format String</h4>
202<!-- ================= -->
203
204<p>The format string for the diagnostic is very simple, but it has some power.
205It takes the form of a string in English with markers that indicate where and
206how arguments to the diagnostic are inserted and formatted. For example, here
207are some simple format strings:</p>
208
209<pre>
210 "binary integer literals are an extension"
211 "format string contains '\\0' within the string body"
212 "more '<b>%%</b>' conversions than data arguments"
Chris Lattner545b3682008-11-23 20:27:13 +0000[diff] [blame]213 "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)"
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]214 "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator"
215 " (has <b>%1</b> parameter<b>%s1</b>)"
216</pre>
217
218<p>These examples show some important points of format strings. You can use any
219 plain ASCII character in the diagnostic string except "%" without a problem,
220 but these are C strings, so you have to use and be aware of all the C escape
221 sequences (as in the second example). If you want to produce a "%" in the
222 output, use the "%%" escape sequence, like the third diagnostic. Finally,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]223 Clang uses the "%...[digit]" sequences to specify where and how arguments to
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]224 the diagnostic are formatted.</p>
225
226<p>Arguments to the diagnostic are numbered according to how they are specified
227 by the C++ code that <a href="#producingdiag">produces them</a>, and are
228 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]229 to your diagnostic, you are doing something wrong :). Unlike printf, there
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]230 is no requirement that arguments to the diagnostic end up in the output in
231 the same order as they are specified, you could have a format string with
232 <tt>"%1 %0"</tt> that swaps them, for example. The text in between the
233 percent and digit are formatting instructions. If there are no instructions,
234 the argument is just turned into a string and substituted in.</p>
235
236<p>Here are some "best practices" for writing the English format string:</p>
237
238<ul>
239<li>Keep the string short. It should ideally fit in the 80 column limit of the
240 <tt>DiagnosticKinds.def</tt> file. This avoids the diagnostic wrapping when
241 printed, and forces you to think about the important point you are conveying
242 with the diagnostic.</li>
243<li>Take advantage of location information. The user will be able to see the
244 line and location of the caret, so you don't need to tell them that the
245 problem is with the 4th argument to the function: just point to it.</li>
246<li>Do not capitalize the diagnostic string, and do not end it with a
247 period.</li>
248<li>If you need to quote something in the diagnostic string, use single
249 quotes.</li>
250</ul>
251
252<p>Diagnostics should never take random English strings as arguments: you
253shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like
254<tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing
255this prevents <a href="translation">translating</a> the Clang diagnostics to
256other languages (because they'll get random English words in their otherwise
257localized diagnostic). The exceptions to this are C/C++ language keywords
258(e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note
259that things like "pointer" and "reference" are not keywords. On the other
260hand, you <em>can</em> include anything that comes from the user's source code,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]261including variable names, types, labels, etc. The 'select' format can be
262used to achieve this sort of thing in a localizable way, see below.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]263
264<!-- ==================================== -->
265<h4>Formatting a Diagnostic Argument</a></h4>
266<!-- ==================================== -->
267
268<p>Arguments to diagnostics are fully typed internally, and come from a couple
269different classes: integers, types, names, and random strings. Depending on
270the class of the argument, it can be optionally formatted in different ways.
271This gives the DiagnosticClient information about what the argument means
272without requiring it to use a specific presentation (consider this MVC for
273Clang :).</p>
274
275<p>Here are the different diagnostic argument formats currently supported by
276Clang:</p>
277
278<table>
279<tr><td colspan="2"><b>"s" format</b></td></tr>
280<tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]281<tr><td>Class:</td><td>Integers</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]282<tr><td>Description:</td><td>This is a simple formatter for integers that is
283 useful when producing English diagnostics. When the integer is 1, it prints
284 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]285 simple grammatical forms to be to be handled correctly, and eliminates the
286 need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]287
288<tr><td colspan="2"><b>"select" format</b></td></tr>
289<tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2
290 operator"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]291<tr><td>Class:</td><td>Integers</td></tr>
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]292<tr><td>Description:</td><td>This format specifier is used to merge multiple
293 related diagnostics together into one common one, without requiring the
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]294 difference to be specified as an English string argument. Instead of
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]295 specifying the string, the diagnostic gets an integer argument and the
296 format string selects the numbered option. In this case, the "%2" value
297 must be an integer in the range [0..2]. If it is 0, it prints 'unary', if
298 it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This
299 allows other language translations to substitute reasonable words (or entire
300 phrases) based on the semantics of the diagnostic instead of having to do
301 things textually.</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
Chris Lattner077bf5e2008-11-24 03:33:13 +0000[diff] [blame]333<tr><td colspan="2"><b>"objcclass" format</b></td></tr>
334<tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr>
335<tr><td>Class:</td><td>DeclarationName</td></tr>
336<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
337 DeclarationName corresponds to an Objective-C class method selector. As
338 such, it prints the selector with a leading '+'.</p></td></tr>
339
340<tr><td colspan="2"><b>"objcinstance" format</b></td></tr>
341<tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr>
342<tr><td>Class:</td><td>DeclarationName</td></tr>
343<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
344 DeclarationName corresponds to an Objective-C instance method selector. As
345 such, it prints the selector with a leading '-'.</p></td></tr>
346
Douglas Gregor47b9a1c2009-02-04 17:27:36 +0000[diff] [blame]347<tr><td colspan="2"><b>"q" format</b></td></tr>
348<tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr>
349<tr><td>Class:</td><td>NamedDecl*</td></tr>
350<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>
351
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]352</table>
353
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]354<p>It is really easy to add format specifiers to the Clang diagnostics system,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]355but they should be discussed before they are added. If you are creating a lot
356of repetitive diagnostics and/or have an idea for a useful formatter, please
357bring it up on the cfe-dev mailing list.</p>
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]358
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]359<!-- ===================================================== -->
360<h4><a name="#producingdiag">Producing the Diagnostic</a></h4>
361<!-- ===================================================== -->
362
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]363<p>Now that you've created the diagnostic in the DiagnosticKinds.def file, you
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]364need to write the code that detects the condition in question and emits the
365new diagnostic. Various components of Clang (e.g. the preprocessor, Sema,
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]366etc) provide a helper function named "Diag". It creates a diagnostic and
367accepts the arguments, ranges, and other information that goes along with
368it.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]369
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]370<p>For example, the binary expression error comes from code like this:</p>
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]371
372<pre>
373 if (various things that are bad)
374 Diag(Loc, diag::err_typecheck_invalid_operands)
375 &lt;&lt; lex-&gt;getType() &lt;&lt; rex-&gt;getType()
376 &lt;&lt; lex-&gt;getSourceRange() &lt;&lt; rex-&gt;getSourceRange();
377</pre>
378
379<p>This shows that use of the Diag method: they take a location (a <a
380href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value
381(which matches the name from DiagnosticKinds.def). If the diagnostic takes
382arguments, they are specified with the &lt;&lt; operator: the first argument
383becomes %0, the second becomes %1, etc. The diagnostic interface allows you to
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]384specify arguments of many different types, including <tt>int</tt> and
385<tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and
386<tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and
387<tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc.
388SourceRanges are also specified with the &lt;&lt; operator, but do not have a
389specific ordering requirement.</p>
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]390
391<p>As you can see, adding and producing a diagnostic is pretty straightforward.
392The hard part is deciding exactly what you need to say to help the user, picking
393a suitable wording, and providing the information needed to format it correctly.
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]394The good news is that the call site that issues a diagnostic should be
395completely independent of how the diagnostic is formatted and in what language
396it is rendered.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]397</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]398
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]399<!-- ==================================================== -->
400<h4 id="code-modification-hints">Code Modification Hints</h4>
401<!-- ==================================================== -->
402
403<p>In some cases, the front end emits diagnostics when it is clear
404that some small change to the source code would fix the problem. For
405example, a missing semicolon at the end of a statement or a use of
Chris Lattner34c05332009-02-27 19:31:12 +0000[diff] [blame]406deprecated syntax that is easily rewritten into a more modern form.
407Clang tries very hard to emit the diagnostic and recover gracefully
408in these and other cases.</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]409
Chris Lattner34c05332009-02-27 19:31:12 +0000[diff] [blame]410<p>However, for these cases where the fix is obvious, the diagnostic
411can be annotated with a code
412modification "hint" that describes how to change the code referenced
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]413by the diagnostic to fix the problem. For example, it might add the
414missing semicolon at the end of the statement or rewrite the use of a
415deprecated construct into something more palatable. Here is one such
416example C++ front end, where we warn about the right-shift operator
417changing meaning from C++98 to C++0x:</p>
418
419<pre>
420test.cpp:3:7: warning: use of right-shift operator ('&gt;&gt;') in template argument will require parentheses in C++0x
421A&lt;100 &gt;&gt; 2&gt; *a;
422 ^
423 ( )
424</pre>
425
426<p>Here, the code modification hint is suggesting that parentheses be
427added, and showing exactly where those parentheses would be inserted
428into the source code. The code modification hints themselves describe
429what changes to make to the source code in an abstract manner, which
430the text diagnostic printer renders as a line of "insertions" below
431the caret line. <a href="#DiagnosticClient">Other diagnostic
432clients</a> might choose to render the code differently (e.g., as
433markup inline) or even give the user the ability to automatically fix
434the problem.</p>
435
436<p>All code modification hints are described by the
437<code>CodeModificationHint</code> class, instances of which should be
438attached to the diagnostic using the &lt;&lt; operator in the same way
439that highlighted source ranges and arguments are passed to the
440diagnostic. Code modification hints can be created with one of three
441constructors:</p>
442
443<dl>
444 <dt><code>CodeModificationHint::CreateInsertion(Loc, Code)</code></dt>
445 <dd>Specifies that the given <code>Code</code> (a string) should be inserted
446 before the source location <code>Loc</code>.</dd>
447
448 <dt><code>CodeModificationHint::CreateRemoval(Range)</code></dt>
449 <dd>Specifies that the code in the given source <code>Range</code>
450 should be removed.</dd>
451
452 <dt><code>CodeModificationHint::CreateReplacement(Range, Code)</code></dt>
453 <dd>Specifies that the code in the given source <code>Range</code>
454 should be removed, and replaced with the given <code>Code</code> string.</dd>
455</dl>
456
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]457<!-- ============================================================= -->
458<h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4>
459<!-- ============================================================= -->
460
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]461<p>Once code generates a diagnostic with all of the arguments and the rest of
462the relevant information, Clang needs to know what to do with it. As previously
463mentioned, the diagnostic machinery goes through some filtering to map a
464severity onto a diagnostic level, then (assuming the diagnostic is not mapped to
465"<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient
466interface with the information.</p>
467
468<p>It is possible to implement this interface in many different ways. For
469example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns
470the arguments into strings (according to the various formatting rules), prints
471out the file/line/column information and the string, then prints out the line of
472code, the source ranges, and the caret. However, this behavior isn't required.
473</p>
474
475<p>Another implementation of the DiagnosticClient interface is the
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]476'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode.
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]477Instead of formatting and printing out the diagnostics, this implementation just
478captures and remembers the diagnostics as they fly by. Then -verify compares
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]479the list of produced diagnostics to the list of expected ones. If they disagree,
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]480it prints out its own output.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]481</p>
482
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]483<p>There are many other possible implementations of this interface, and this is
484why we prefer diagnostics to pass down rich structured information in arguments.
485For example, an HTML output might want declaration names be linkified to where
486they come from in the source. Another example is that a GUI might let you click
487on typedefs to expand them. This application would want to pass significantly
488more information about types through to the GUI than a simple flat string. The
489interface allows this to happen.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]490
491<!-- ====================================================== -->
492<h4><a name="translation">Adding Translations to Clang</a></h4>
493<!-- ====================================================== -->
494
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]495<p>Not possible yet! Diagnostic strings should be written in UTF-8, the client
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]496can translate to the relevant code page if needed. Each translation completely
497replaces the format string for the diagnostic.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]498
499
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]500<!-- ======================================================================= -->
501<h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3>
502<!-- ======================================================================= -->
503
504<p>Strangely enough, the SourceLocation class represents a location within the
505source code of the program. Important design points include:</p>
506
507<ol>
508<li>sizeof(SourceLocation) must be extremely small, as these are embedded into
509 many AST nodes and are passed around often. Currently it is 32 bits.</li>
510<li>SourceLocation must be a simple value object that can be efficiently
511 copied.</li>
512<li>We should be able to represent a source location for any byte of any input
513 file. This includes in the middle of tokens, in whitespace, in trigraphs,
514 etc.</li>
515<li>A SourceLocation must encode the current #include stack that was active when
516 the location was processed. For example, if the location corresponds to a
517 token, it should contain the set of #includes active when the token was
518 lexed. This allows us to print the #include stack for a diagnostic.</li>
519<li>SourceLocation must be able to describe macro expansions, capturing both
520 the ultimate instantiation point and the source of the original character
521 data.</li>
522</ol>
523
524<p>In practice, the SourceLocation works together with the SourceManager class
Chris Lattner18376dd2009-01-16 07:00:50 +0000[diff] [blame]525to encode two pieces of information about a location: it's spelling location
Chris Lattner88054de2009-01-16 07:15:35 +0000[diff] [blame]526and it's instantiation location. For most tokens, these will be the same. However,
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]527for a macro expansion (or tokens that came from a _Pragma directive) these will
528describe the location of the characters corresponding to the token and the
529location where the token was used (i.e. the macro instantiation point or the
530location of the _Pragma itself).</p>
531
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]532<p>For efficiency, we only track one level of macro instantiations: if a token was
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]533produced by multiple instantiations, we only track the source and ultimate
534destination. Though we could track the intermediate instantiation points, this
535would require extra bookkeeping and no known client would benefit substantially
536from this.</p>
537
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]538<p>The Clang front-end inherently depends on the location of a token being
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]539tracked correctly. If it is ever incorrect, the front-end may get confused and
540die. 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]541Clang depends on being able to find the original input characters for the token.
Chris Lattner18376dd2009-01-16 07:00:50 +0000[diff] [blame]542This concept maps directly to the "spelling location" for the token.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]543
544<!-- ======================================================================= -->
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]545<h2 id="libdriver">The Driver Library</h2>
546<!-- ======================================================================= -->
547
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]548<p>The clang Driver and library are documented <a
549href="DriverInternals.html">here<a>.<p>
550
551<!-- ======================================================================= -->
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame]552<h2 id="pch">Precompiled Headers</h2>
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]553<!-- ======================================================================= -->
554
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame]555<p>Clang supports two implementations of precompiled headers. The
556 default implementation, precompiled headers (<a
557 href="PCHInternals.html">PCH</a>) uses a serialized representation
558 of Clang's internal data structures, encoded with the <a
559 href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream
560 format</a>. Pretokenized headers (<a
561 href="PTHInternals.html">PTH</a>), on the other hand, contain a
562 serialized representation of the tokens encountered when
563 preprocessing a header (and anything that header includes).</p>
564
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]565
566<!-- ======================================================================= -->
567<h2 id="libfrontend">The Frontend Library</h2>
568<!-- ======================================================================= -->
569
570<p>The Frontend library contains functionality useful for building
571tools on top of the clang libraries, for example several methods for
572outputting diagnostics.</p>
573
574<!-- ======================================================================= -->
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]575<h2 id="liblex">The Lexer and Preprocessor Library</h2>
576<!-- ======================================================================= -->
577
578<p>The Lexer library contains several tightly-connected classes that are involved
579with the nasty process of lexing and preprocessing C source code. The main
580interface to this library for outside clients is the large <a
581href="#Preprocessor">Preprocessor</a> class.
582It contains the various pieces of state that are required to coherently read
583tokens out of a translation unit.</p>
584
585<p>The core interface to the Preprocessor object (once it is set up) is the
586Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from
587the preprocessor stream. There are two types of token providers that the
588preprocessor is capable of reading from: a buffer lexer (provided by the <a
589href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]590href="#TokenLexer">TokenLexer</a> class).
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]591
592
593<!-- ======================================================================= -->
594<h3 id="Token">The Token class</h3>
595<!-- ======================================================================= -->
596
597<p>The Token class is used to represent a single lexed token. Tokens are
598intended to be used by the lexer/preprocess and parser libraries, but are not
599intended to live beyond them (for example, they should not live in the ASTs).<p>
600
601<p>Tokens most often live on the stack (or some other location that is efficient
602to access) as the parser is running, but occasionally do get buffered up. For
603example, macro definitions are stored as a series of tokens, and the C++
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]604front-end periodically needs to buffer tokens up for tentative parsing and
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]605various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit
606system, sizeof(Token) is currently 16 bytes.</p>
607
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]608<p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation
609Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer,
610annotation tokens represent semantic information and are produced by the parser,
611replacing normal tokens in the token stream. Normal tokens contain the
612following information:</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]613
614<ul>
615<li><b>A SourceLocation</b> - This indicates the location of the start of the
616token.</li>
617
618<li><b>A length</b> - This stores the length of the token as stored in the
619SourceBuffer. For tokens that include them, this length includes trigraphs and
620escaped newlines which are ignored by later phases of the compiler. By pointing
621into the original source buffer, it is always possible to get the original
622spelling of a token completely accurately.</li>
623
624<li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if
625identifier lookup was enabled when the token was lexed (e.g. the lexer was not
626reading in 'raw' mode) this contains a pointer to the unique hash value for the
627identifier. Because the lookup happens before keyword identification, this
628field is set even for language keywords like 'for'.</li>
629
630<li><b>TokenKind</b> - This indicates the kind of token as classified by the
631lexer. This includes things like <tt>tok::starequal</tt> (for the "*="
632operator), <tt>tok::ampamp</tt> for the "&amp;&amp;" token, and keyword values
633(e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note
634that some tokens can be spelled multiple ways. For example, C++ supports
635"operator keywords", where things like "and" are treated exactly like the
636"&amp;&amp;" operator. In these cases, the kind value is set to
637<tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to
638consider both forms. For something that cares about which form is used (e.g.
639the preprocessor 'stringize' operator) the spelling indicates the original
640form.</li>
641
642<li><b>Flags</b> - There are currently four flags tracked by the
643lexer/preprocessor system on a per-token basis:
644
645 <ol>
646 <li><b>StartOfLine</b> - This was the first token that occurred on its input
647 source line.</li>
648 <li><b>LeadingSpace</b> - There was a space character either immediately
649 before the token or transitively before the token as it was expanded
650 through a macro. The definition of this flag is very closely defined by
651 the stringizing requirements of the preprocessor.</li>
652 <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to
653 represent identifier tokens which have macro expansion disabled. This
654 prevents them from being considered as candidates for macro expansion ever
655 in the future.</li>
656 <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the
657 token includes a trigraph or escaped newline. Since this is uncommon,
658 many pieces of code can fast-path on tokens that did not need cleaning.
659 </p>
660 </ol>
661</li>
662</ul>
663
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]664<p>One interesting (and somewhat unusual) aspect of normal tokens is that they
665don't contain any semantic information about the lexed value. For example, if
666the token was a pp-number token, we do not represent the value of the number
667that was lexed (this is left for later pieces of code to decide). Additionally,
668the lexer library has no notion of typedef names vs variable names: both are
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]669returned as identifiers, and the parser is left to decide whether a specific
670identifier is a typedef or a variable (tracking this requires scope information
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]671among other things). The parser can do this translation by replacing tokens
672returned by the preprocessor with "Annotation Tokens".</p>
673
674<!-- ======================================================================= -->
675<h3 id="AnnotationToken">Annotation Tokens</h3>
676<!-- ======================================================================= -->
677
678<p>Annotation Tokens are tokens that are synthesized by the parser and injected
679into the preprocessor's token stream (replacing existing tokens) to record
680semantic information found by the parser. For example, if "foo" is found to be
681a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an
682<tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this
683makes it easy to handle qualified type names (e.g. "foo::bar::baz&lt;42&gt;::t")
684in C++ as a single "token" in the parser. 2) if the parser backtracks, the
685reparse does not need to redo semantic analysis to determine whether a token
686sequence is a variable, type, template, etc.</p>
687
688<p>Annotation Tokens are created by the parser and reinjected into the parser's
689token stream (when backtracking is enabled). Because they can only exist in
690tokens that the preprocessor-proper is done with, it doesn't need to keep around
691flags like "start of line" that the preprocessor uses to do its job.
692Additionally, an annotation token may "cover" a sequence of preprocessor tokens
693(e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields
694of an annotation token are different than the fields for a normal token (but
695they are multiplexed into the normal Token fields):</p>
696
697<ul>
698<li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation
699token indicates the first token replaced by the annotation token. In the example
700above, it would be the location of the "a" identifier.</li>
701
702<li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the
703last token replaced with the annotation token. In the example above, it would
704be the location of the "c" identifier.</li>
705
706<li><b>void* "AnnotationValue"</b> - This contains an opaque object that the
707parser gets from Sema through an Actions module, it is passed around and Sema
708intepretes it, based on the type of annotation token.</li>
709
710<li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this
711is. See below for the different valid kinds.</li>
712</ul>
713
714<p>Annotation tokens currently come in three kinds:</p>
715
716<ol>
717<li><b>tok::annot_typename</b>: This annotation token represents a
718resolved typename token that is potentially qualified. The AnnotationValue
Steve Naroffb43a50f2009-01-28 19:39:02 +0000[diff] [blame]719field contains a pointer returned by Action::getTypeName(). In the case of the
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]720Sema actions module, this is a <tt>Decl*</tt> for the type.</li>
721
722<li><b>tok::annot_cxxscope</b>: This annotation token represents a C++ scope
723specifier, such as "A::B::". This corresponds to the grammar productions "::"
724and ":: [opt] nested-name-specifier". The AnnotationValue pointer is returned
725by the Action::ActOnCXXGlobalScopeSpecifier and
726Action::ActOnCXXNestedNameSpecifier callbacks. In the case of Sema, this is a
727<tt>DeclContext*</tt>.</li>
728
Douglas Gregor39a8de12009-02-25 19:37:18 +0000[diff] [blame]729<li><b>tok::annot_template_id</b>: This annotation token represents a
730C++ template-id such as "foo&lt;int, 4&gt;", where "foo" is the name
731of a template. The AnnotationValue pointer is a pointer to a malloc'd
732TemplateIdAnnotation object. Depending on the context, a parsed template-id that names a type might become a typename annotation token (if all we care about is the named type, e.g., because it occurs in a type specifier) or might remain a template-id token (if we want to retain more source location information or produce a new type, e.g., in a declaration of a class template specialization). template-id annotation tokens that refer to a type can be "upgraded" to typename annotation tokens by the parser.</li>
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]733
734</ol>
735
Cedric Venetda76b282009-01-06 16:22:54 +0000[diff] [blame]736<p>As mentioned above, annotation tokens are not returned by the preprocessor,
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]737they are formed on demand by the parser. This means that the parser has to be
738aware of cases where an annotation could occur and form it where appropriate.
739This is somewhat similar to how the parser handles Translation Phase 6 of C99:
740String Concatenation (see C99 5.1.1.2). In the case of string concatenation,
741the preprocessor just returns distinct tok::string_literal and
742tok::wide_string_literal tokens and the parser eats a sequence of them wherever
743the grammar indicates that a string literal can occur.</p>
744
745<p>In order to do this, whenever the parser expects a tok::identifier or
746tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or
747TryAnnotateCXXScopeToken methods to form the annotation token. These methods
748will maximally form the specified annotation tokens and replace the current
749token with them, if applicable. If the current tokens is not valid for an
750annotation token, it will remain an identifier or :: token.</p>
751
752
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]753
754<!-- ======================================================================= -->
755<h3 id="Lexer">The Lexer class</h3>
756<!-- ======================================================================= -->
757
758<p>The Lexer class provides the mechanics of lexing tokens out of a source
759buffer and deciding what they mean. The Lexer is complicated by the fact that
760it operates on raw buffers that have not had spelling eliminated (this is a
761necessity to get decent performance), but this is countered with careful coding
762as well as standard performance techniques (for example, the comment handling
763code is vectorized on X86 and PowerPC hosts).</p>
764
765<p>The lexer has a couple of interesting modal features:</p>
766
767<ul>
768<li>The lexer can operate in 'raw' mode. This mode has several features that
769 make it possible to quickly lex the file (e.g. it stops identifier lookup,
770 doesn't specially handle preprocessor tokens, handles EOF differently, etc).
771 This mode is used for lexing within an "<tt>#if 0</tt>" block, for
772 example.</li>
773<li>The lexer can capture and return comments as tokens. This is required to
774 support the -C preprocessor mode, which passes comments through, and is
775 used by the diagnostic checker to identifier expect-error annotations.</li>
776<li>The lexer can be in ParsingFilename mode, which happens when preprocessing
Chris Lattner84386242007-09-16 19:25:23 +0000[diff] [blame]777 after reading a #include directive. This mode changes the parsing of '&lt;'
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]778 to return an "angled string" instead of a bunch of tokens for each thing
779 within the filename.</li>
780<li>When parsing a preprocessor directive (after "<tt>#</tt>") the
781 ParsingPreprocessorDirective mode is entered. This changes the parser to
782 return EOM at a newline.</li>
783<li>The Lexer uses a LangOptions object to know whether trigraphs are enabled,
784 whether C++ or ObjC keywords are recognized, etc.</li>
785</ul>
786
787<p>In addition to these modes, the lexer keeps track of a couple of other
788 features that are local to a lexed buffer, which change as the buffer is
789 lexed:</p>
790
791<ul>
792<li>The Lexer uses BufferPtr to keep track of the current character being
793 lexed.</li>
794<li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token
795 will start with its "start of line" bit set.</li>
796<li>The Lexer keeps track of the current #if directives that are active (which
797 can be nested).</li>
798<li>The Lexer keeps track of an <a href="#MultipleIncludeOpt">
799 MultipleIncludeOpt</a> object, which is used to
800 detect whether the buffer uses the standard "<tt>#ifndef XX</tt> /
801 <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does,
802 subsequent includes can be ignored if the XX macro is defined.</li>
803</ul>
804
805<!-- ======================================================================= -->
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]806<h3 id="TokenLexer">The TokenLexer class</h3>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]807<!-- ======================================================================= -->
808
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]809<p>The TokenLexer class is a token provider that returns tokens from a list
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]810of tokens that came from somewhere else. It typically used for two things: 1)
811returning tokens from a macro definition as it is being expanded 2) returning
812tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and
813will most likely be used to handle unbounded look-ahead for the C++ parser.</p>
814
815<!-- ======================================================================= -->
816<h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3>
817<!-- ======================================================================= -->
818
819<p>The MultipleIncludeOpt class implements a really simple little state machine
820that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>"
821idiom that people typically use to prevent multiple inclusion of headers. If a
822buffer uses this idiom and is subsequently #include'd, the preprocessor can
823simply check to see whether the guarding condition is defined or not. If so,
824the preprocessor can completely ignore the include of the header.</p>
825
826
827
828<!-- ======================================================================= -->
829<h2 id="libparse">The Parser Library</h2>
830<!-- ======================================================================= -->
831
832<!-- ======================================================================= -->
833<h2 id="libast">The AST Library</h2>
834<!-- ======================================================================= -->
835
836<!-- ======================================================================= -->
837<h3 id="Type">The Type class and its subclasses</h3>
838<!-- ======================================================================= -->
839
840<p>The Type class (and its subclasses) are an important part of the AST. Types
841are accessed through the ASTContext class, which implicitly creates and uniques
842them as they are needed. Types have a couple of non-obvious features: 1) they
843do not capture type qualifiers like const or volatile (See
844<a href="#QualType">QualType</a>), and 2) they implicitly capture typedef
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]845information. Once created, types are immutable (unlike decls).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]846
847<p>Typedefs in C make semantic analysis a bit more complex than it would
848be without them. The issue is that we want to capture typedef information
849and represent it in the AST perfectly, but the semantics of operations need to
850"see through" typedefs. For example, consider this code:</p>
851
852<code>
853void func() {<br>
Bill Wendling30d17752007-10-06 01:56:01 +0000[diff] [blame]854&nbsp;&nbsp;typedef int foo;<br>
855&nbsp;&nbsp;foo X, *Y;<br>
856&nbsp;&nbsp;typedef foo* bar;<br>
857&nbsp;&nbsp;bar Z;<br>
858&nbsp;&nbsp;*X; <i>// error</i><br>
859&nbsp;&nbsp;**Y; <i>// error</i><br>
860&nbsp;&nbsp;**Z; <i>// error</i><br>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]861}<br>
862</code>
863
864<p>The code above is illegal, and thus we expect there to be diagnostics emitted
865on the annotated lines. In this example, we expect to get:</p>
866
867<pre>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]868<b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]869*X; // error
870<font color="blue">^~</font>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]871<b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]872**Y; // error
873<font color="blue">^~~</font>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]874<b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b>
875**Z; // error
876<font color="blue">^~~</font>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]877</pre>
878
879<p>While this example is somewhat silly, it illustrates the point: we want to
880retain typedef information where possible, so that we can emit errors about
881"<tt>std::string</tt>" instead of "<tt>std::basic_string&lt;char, std:...</tt>".
882Doing this requires properly keeping typedef information (for example, the type
883of "X" is "foo", not "int"), and requires properly propagating it through the
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]884various operators (for example, the type of *Y is "foo", not "int"). In order
885to retain this information, the type of these expressions is an instance of the
886TypedefType class, which indicates that the type of these expressions is a
887typedef for foo.
888</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]889
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]890<p>Representing types like this is great for diagnostics, because the
891user-specified type is always immediately available. There are two problems
892with this: first, various semantic checks need to make judgements about the
Chris Lattner33fc68a2007-07-31 18:54:50 +0000[diff] [blame]893<em>actual structure</em> of a type, ignoring typdefs. Second, we need an
894efficient way to query whether two types are structurally identical to each
895other, ignoring typedefs. The solution to both of these problems is the idea of
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]896canonical types.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]897
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]898<!-- =============== -->
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]899<h4>Canonical Types</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]900<!-- =============== -->
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]901
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]902<p>Every instance of the Type class contains a canonical type pointer. For
903simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>",
904"<tt>int**</tt>"), the type just points to itself. For types that have a
905typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>",
906"<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their
907structurally equivalent type without any typedefs (e.g. "<tt>int</tt>",
908"<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]909
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]910<p>This design provides a constant time operation (dereferencing the canonical
911type pointer) that gives us access to the structure of types. For example,
912we can trivially tell that "bar" and "foo*" are the same type by dereferencing
913their canonical type pointers and doing a pointer comparison (they both point
914to the single "<tt>int*</tt>" type).</p>
915
916<p>Canonical types and typedef types bring up some complexities that must be
917carefully managed. Specifically, the "isa/cast/dyncast" operators generally
918shouldn't be used in code that is inspecting the AST. For example, when type
919checking the indirection operator (unary '*' on a pointer), the type checker
920must verify that the operand has a pointer type. It would not be correct to
921check that with "<tt>isa&lt;PointerType&gt;(SubExpr-&gt;getType())</tt>",
922because this predicate would fail if the subexpression had a typedef type.</p>
923
924<p>The solution to this problem are a set of helper methods on Type, used to
925check their properties. In this case, it would be correct to use
926"<tt>SubExpr-&gt;getType()-&gt;isPointerType()</tt>" to do the check. This
927predicate will return true if the <em>canonical type is a pointer</em>, which is
928true any time the type is structurally a pointer type. The only hard part here
929is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p>
930
931<p>The second problem we face is how to get access to the pointer type once we
932know it exists. To continue the example, the result type of the indirection
933operator is the pointee type of the subexpression. In order to determine the
934type, we need to get the instance of PointerType that best captures the typedef
935information in the program. If the type of the expression is literally a
936PointerType, we can return that, otherwise we have to dig through the
937typedefs to find the pointer type. For example, if the subexpression had type
938"<tt>foo*</tt>", we could return that type as the result. If the subexpression
939had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do
940<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]941a getAsPointerType() method that checks whether the type is structurally a
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]942PointerType and, if so, returns the best one. If not, it returns a null
943pointer.</p>
944
945<p>This structure is somewhat mystical, but after meditating on it, it will
946make sense to you :).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]947
948<!-- ======================================================================= -->
949<h3 id="QualType">The QualType class</h3>
950<!-- ======================================================================= -->
951
952<p>The QualType class is designed as a trivial value class that is small,
953passed by-value and is efficient to query. The idea of QualType is that it
954stores the type qualifiers (const, volatile, restrict) separately from the types
955themselves: QualType is conceptually a pair of "Type*" and bits for the type
956qualifiers.</p>
957
958<p>By storing the type qualifiers as bits in the conceptual pair, it is
959extremely efficient to get the set of qualifiers on a QualType (just return the
960field of the pair), add a type qualifier (which is a trivial constant-time
961operation that sets a bit), and remove one or more type qualifiers (just return
962a QualType with the bitfield set to empty).</p>
963
964<p>Further, because the bits are stored outside of the type itself, we do not
965need to create duplicates of types with different sets of qualifiers (i.e. there
966is only a single heap allocated "int" type: "const int" and "volatile const int"
967both point to the same heap allocated "int" type). This reduces the heap size
968used to represent bits and also means we do not have to consider qualifiers when
969uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p>
970
971<p>In practice, on hosts where it is safe, the 3 type qualifiers are stored in
972the low bit of the pointer to the Type object. This means that QualType is
973exactly the same size as a pointer, and this works fine on any system where
974malloc'd objects are at least 8 byte aligned.</p>
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]975
976<!-- ======================================================================= -->
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]977<h3 id="DeclarationName">Declaration names</h3>
978<!-- ======================================================================= -->
979
980<p>The <tt>DeclarationName</tt> class represents the name of a
981 declaration in Clang. Declarations in the C family of languages can
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]982 take several different forms. Most declarations are named by
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]983 simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in
984 the function declaration <code>f(int x)</code>. In C++, declaration
985 names can also name class constructors ("<code>Class</code>"
986 in <code>struct Class { Class(); }</code>), class destructors
987 ("<code>~Class</code>"), overloaded operator names ("operator+"),
988 and conversion functions ("<code>operator void const *</code>"). In
989 Objective-C, declaration names can refer to the names of Objective-C
990 methods, which involve the method name and the parameters,
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]991 collectively called a <i>selector</i>, e.g.,
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]992 "<code>setWidth:height:</code>". Since all of these kinds of
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]993 entities - variables, functions, Objective-C methods, C++
994 constructors, destructors, and operators - are represented as
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]995 subclasses of Clang's common <code>NamedDecl</code>
996 class, <code>DeclarationName</code> is designed to efficiently
997 represent any kind of name.</p>
998
999<p>Given
1000 a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code>
Douglas Gregor2def4832008-11-17 20:34:05 +0000[diff] [blame]1001 will produce a value that describes what kind of name <code>N</code>
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1002 stores. There are 8 options (all of the names are inside
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1003 the <code>DeclarationName</code> class)</p>
1004<dl>
1005 <dt>Identifier</dt>
1006 <dd>The name is a simple
1007 identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the
1008 corresponding <code>IdentifierInfo*</code> pointing to the actual
1009 identifier. Note that C++ overloaded operators (e.g.,
1010 "<code>operator+</code>") are represented as special kinds of
1011 identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code>
1012 function to determine whether an identifier is an overloaded
1013 operator name.</dd>
1014
1015 <dt>ObjCZeroArgSelector, ObjCOneArgSelector,
1016 ObjCMultiArgSelector</dt>
1017 <dd>The name is an Objective-C selector, which can be retrieved as a
1018 <code>Selector</code> instance
1019 via <code>N.getObjCSelector()</code>. The three possible name
1020 kinds for Objective-C reflect an optimization within
1021 the <code>DeclarationName</code> class: both zero- and
1022 one-argument selectors are stored as a
1023 masked <code>IdentifierInfo</code> pointer, and therefore require
1024 very little space, since zero- and one-argument selectors are far
1025 more common than multi-argument selectors (which use a different
1026 structure).</dd>
1027
1028 <dt>CXXConstructorName</dt>
1029 <dd>The name is a C++ constructor
1030 name. Use <code>N.getCXXNameType()</code> to retrieve
1031 the <a href="#QualType">type</a> that this constructor is meant to
1032 construct. The type is always the canonical type, since all
1033 constructors for a given type have the same name.</dd>
1034
1035 <dt>CXXDestructorName</dt>
1036 <dd>The name is a C++ destructor
1037 name. Use <code>N.getCXXNameType()</code> to retrieve
1038 the <a href="#QualType">type</a> whose destructor is being
1039 named. This type is always a canonical type.</dd>
1040
1041 <dt>CXXConversionFunctionName</dt>
1042 <dd>The name is a C++ conversion function. Conversion functions are
1043 named according to the type they convert to, e.g., "<code>operator void
1044 const *</code>". Use <code>N.getCXXNameType()</code> to retrieve
1045 the type that this conversion function converts to. This type is
1046 always a canonical type.</dd>
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1047
1048 <dt>CXXOperatorName</dt>
1049 <dd>The name is a C++ overloaded operator name. Overloaded operators
1050 are named according to their spelling, e.g.,
1051 "<code>operator+</code>" or "<code>operator new
1052 []</code>". Use <code>N.getCXXOverloadedOperator()</code> to
1053 retrieve the overloaded operator (a value of
1054 type <code>OverloadedOperatorKind</code>).</dd>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1055</dl>
1056
1057<p><code>DeclarationName</code>s are cheap to create, copy, and
1058 compare. They require only a single pointer's worth of storage in
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1059 the common cases (identifiers, zero-
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1060 and one-argument Objective-C selectors) and use dense, uniqued
1061 storage for the other kinds of
1062 names. Two <code>DeclarationName</code>s can be compared for
1063 equality (<code>==</code>, <code>!=</code>) using a simple bitwise
1064 comparison, can be ordered
1065 with <code>&lt;</code>, <code>&gt;</code>, <code>&lt;=</code>,
1066 and <code>&gt;=</code> (which provide a lexicographical ordering for
1067 normal identifiers but an unspecified ordering for other kinds of
1068 names), and can be placed into LLVM <code>DenseMap</code>s
1069 and <code>DenseSet</code>s.</p>
1070
1071<p><code>DeclarationName</code> instances can be created in different
1072 ways depending on what kind of name the instance will store. Normal
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1073 identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1074 (<code>Selector</code>) can be implicitly converted
1075 to <code>DeclarationName</code>s. Names for C++ constructors,
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1076 destructors, conversion functions, and overloaded operators can be retrieved from
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1077 the <code>DeclarationNameTable</code>, an instance of which is
1078 available as <code>ASTContext::DeclarationNames</code>. The member
1079 functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>,
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1080 <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively,
1081 return <code>DeclarationName</code> instances for the four kinds of
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1082 C++ special function names.</p>
1083
1084<!-- ======================================================================= -->
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1085<h3 id="DeclContext">Declaration contexts</h3>
1086<!-- ======================================================================= -->
1087<p>Every declaration in a program exists within some <i>declaration
1088 context</i>, such as a translation unit, namespace, class, or
1089 function. Declaration contexts in Clang are represented by
1090 the <code>DeclContext</code> class, from which the various
1091 declaration-context AST nodes
1092 (<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>,
1093 etc.) will derive. The <code>DeclContext</code> class provides
1094 several facilities common to each declaration context:</p>
1095<dl>
1096 <dt>Source-centric vs. Semantics-centric View of Declarations</dt>
1097 <dd><code>DeclContext</code> provides two views of the declarations
1098 stored within a declaration context. The source-centric view
1099 accurately represents the program source code as written, including
1100 multiple declarations of entities where present (see the
1101 section <a href="#Redeclarations">Redeclarations and
1102 Overloads</a>), while the semantics-centric view represents the
1103 program semantics. The two views are kept synchronized by semantic
1104 analysis while the ASTs are being constructed.</dd>
1105
1106 <dt>Storage of declarations within that context</dt>
1107 <dd>Every declaration context can contain some number of
1108 declarations. For example, a C++ class (represented
1109 by <code>RecordDecl</code>) contains various member functions,
1110 fields, nested types, and so on. All of these declarations will be
1111 stored within the <code>DeclContext</code>, and one can iterate
1112 over the declarations via
1113 [<code>DeclContext::decls_begin()</code>,
1114 <code>DeclContext::decls_end()</code>). This mechanism provides
1115 the source-centric view of declarations in the context.</dd>
1116
1117 <dt>Lookup of declarations within that context</dt>
1118 <dd>The <code>DeclContext</code> structure provides efficient name
1119 lookup for names within that declaration context. For example,
1120 if <code>N</code> is a namespace we can look for the
1121 name <code>N::f</code>
1122 using <code>DeclContext::lookup</code>. The lookup itself is
1123 based on a lazily-constructed array (for declaration contexts
1124 with a small number of declarations) or hash table (for
1125 declaration contexts with more declarations). The lookup
1126 operation provides the semantics-centric view of the declarations
1127 in the context.</dd>
1128
1129 <dt>Ownership of declarations</dt>
1130 <dd>The <code>DeclContext</code> owns all of the declarations that
1131 were declared within its declaration context, and is responsible
1132 for the management of their memory as well as their
1133 (de-)serialization.</dd>
1134</dl>
1135
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1136<p>All declarations are stored within a declaration context, and one
1137 can query
1138 information about the context in which each declaration lives. One
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1139 can retrieve the <code>DeclContext</code> that contains a
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1140 particular <code>Decl</code>
1141 using <code>Decl::getDeclContext</code>. However, see the
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1142 section <a href="#LexicalAndSemanticContexts">Lexical and Semantic
1143 Contexts</a> for more information about how to interpret this
1144 context information.</p>
1145
1146<h4 id="Redeclarations">Redeclarations and Overloads</h4>
1147<p>Within a translation unit, it is common for an entity to be
1148declared several times. For example, we might declare a function "f"
1149 and then later re-declare it as part of an inlined definition:</p>
1150
1151<pre>
1152void f(int x, int y, int z = 1);
1153
1154inline void f(int x, int y, int z) { /* ... */ }
1155</pre>
1156
1157<p>The representation of "f" differs in the source-centric and
1158 semantics-centric views of a declaration context. In the
1159 source-centric view, all redeclarations will be present, in the
1160 order they occurred in the source code, making
1161 this view suitable for clients that wish to see the structure of
1162 the source code. In the semantics-centric view, only the most recent "f"
1163 will be found by the lookup, since it effectively replaces the first
1164 declaration of "f".</p>
1165
1166<p>In the semantics-centric view, overloading of functions is
1167 represented explicitly. For example, given two declarations of a
1168 function "g" that are overloaded, e.g.,</p>
1169<pre>
1170void g();
1171void g(int);
1172</pre>
1173<p>the <code>DeclContext::lookup</code> operation will return
1174 an <code>OverloadedFunctionDecl</code> that contains both
1175 declarations of "g". Clients that perform semantic analysis on a
1176 program that is not concerned with the actual source code will
1177 primarily use this semantics-centric view.</p>
1178
1179<h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4>
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1180<p>Each declaration has two potentially different
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1181 declaration contexts: a <i>lexical</i> context, which corresponds to
1182 the source-centric view of the declaration context, and
1183 a <i>semantic</i> context, which corresponds to the
1184 semantics-centric view. The lexical context is accessible
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1185 via <code>Decl::getLexicalDeclContext</code> while the
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1186 semantic context is accessible
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1187 via <code>Decl::getDeclContext</code>, both of which return
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1188 <code>DeclContext</code> pointers. For most declarations, the two
1189 contexts are identical. For example:</p>
1190
1191<pre>
1192class X {
1193public:
1194 void f(int x);
1195};
1196</pre>
1197
1198<p>Here, the semantic and lexical contexts of <code>X::f</code> are
1199 the <code>DeclContext</code> associated with the
1200 class <code>X</code> (itself stored as a <code>RecordDecl</code> AST
1201 node). However, we can now define <code>X::f</code> out-of-line:</p>
1202
1203<pre>
1204void X::f(int x = 17) { /* ... */ }
1205</pre>
1206
1207<p>This definition of has different lexical and semantic
1208 contexts. The lexical context corresponds to the declaration
1209 context in which the actual declaration occurred in the source
1210 code, e.g., the translation unit containing <code>X</code>. Thus,
1211 this declaration of <code>X::f</code> can be found by traversing
1212 the declarations provided by
1213 [<code>decls_begin()</code>, <code>decls_end()</code>) in the
1214 translation unit.</p>
1215
1216<p>The semantic context of <code>X::f</code> corresponds to the
1217 class <code>X</code>, since this member function is (semantically) a
1218 member of <code>X</code>. Lookup of the name <code>f</code> into
1219 the <code>DeclContext</code> associated with <code>X</code> will
1220 then return the definition of <code>X::f</code> (including
1221 information about the default argument).</p>
1222
1223<h4 id="TransparentContexts">Transparent Declaration Contexts</h4>
1224<p>In C and C++, there are several contexts in which names that are
1225 logically declared inside another declaration will actually "leak"
1226 out into the enclosing scope from the perspective of name
1227 lookup. The most obvious instance of this behavior is in
1228 enumeration types, e.g.,</p>
1229<pre>
1230enum Color {
1231 Red,
1232 Green,
1233 Blue
1234};
1235</pre>
1236
1237<p>Here, <code>Color</code> is an enumeration, which is a declaration
1238 context that contains the
1239 enumerators <code>Red</code>, <code>Green</code>,
1240 and <code>Blue</code>. Thus, traversing the list of declarations
1241 contained in the enumeration <code>Color</code> will
1242 yield <code>Red</code>, <code>Green</code>,
1243 and <code>Blue</code>. However, outside of the scope
1244 of <code>Color</code> one can name the enumerator <code>Red</code>
1245 without qualifying the name, e.g.,</p>
1246
1247<pre>
1248Color c = Red;
1249</pre>
1250
1251<p>There are other entities in C++ that provide similar behavior. For
1252 example, linkage specifications that use curly braces:</p>
1253
1254<pre>
1255extern "C" {
1256 void f(int);
1257 void g(int);
1258}
1259// f and g are visible here
1260</pre>
1261
1262<p>For source-level accuracy, we treat the linkage specification and
1263 enumeration type as a
1264 declaration context in which its enclosed declarations ("Red",
1265 "Green", and "Blue"; "f" and "g")
1266 are declared. However, these declarations are visible outside of the
1267 scope of the declaration context.</p>
1268
1269<p>These language features (and several others, described below) have
1270 roughly the same set of
1271 requirements: declarations are declared within a particular lexical
1272 context, but the declarations are also found via name lookup in
1273 scopes enclosing the declaration itself. This feature is implemented
1274 via <i>transparent</i> declaration contexts
1275 (see <code>DeclContext::isTransparentContext()</code>), whose
1276 declarations are visible in the nearest enclosing non-transparent
1277 declaration context. This means that the lexical context of the
1278 declaration (e.g., an enumerator) will be the
1279 transparent <code>DeclContext</code> itself, as will the semantic
1280 context, but the declaration will be visible in every outer context
1281 up to and including the first non-transparent declaration context (since
1282 transparent declaration contexts can be nested).</p>
1283
1284<p>The transparent <code>DeclContexts</code> are:</p>
1285<ul>
1286 <li>Enumerations (but not C++0x "scoped enumerations"):
1287 <pre>
1288enum Color {
1289 Red,
1290 Green,
1291 Blue
1292};
1293// Red, Green, and Blue are in scope
1294 </pre></li>
1295 <li>C++ linkage specifications:
1296 <pre>
1297extern "C" {
1298 void f(int);
1299 void g(int);
1300}
1301// f and g are in scope
1302 </pre></li>
1303 <li>Anonymous unions and structs:
1304 <pre>
1305struct LookupTable {
1306 bool IsVector;
1307 union {
1308 std::vector&lt;Item&gt; *Vector;
1309 std::set&lt;Item&gt; *Set;
1310 };
1311};
1312
1313LookupTable LT;
1314LT.Vector = 0; // Okay: finds Vector inside the unnamed union
1315 </pre>
1316 </li>
1317 <li>C++0x inline namespaces:
1318<pre>
1319namespace mylib {
1320 inline namespace debug {
1321 class X;
1322 }
1323}
1324mylib::X *xp; // okay: mylib::X refers to mylib::debug::X
1325</pre>
1326</li>
1327</ul>
1328
1329
1330<h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4>
1331<p>C++ namespaces have the interesting--and, so far, unique--property that
1332the namespace can be defined multiple times, and the declarations
1333provided by each namespace definition are effectively merged (from
1334the semantic point of view). For example, the following two code
1335snippets are semantically indistinguishable:</p>
1336<pre>
1337// Snippet #1:
1338namespace N {
1339 void f();
1340}
1341namespace N {
1342 void f(int);
1343}
1344
1345// Snippet #2:
1346namespace N {
1347 void f();
1348 void f(int);
1349}
1350</pre>
1351
1352<p>In Clang's representation, the source-centric view of declaration
1353 contexts will actually have two separate <code>NamespaceDecl</code>
1354 nodes in Snippet #1, each of which is a declaration context that
1355 contains a single declaration of "f". However, the semantics-centric
1356 view provided by name lookup into the namespace <code>N</code> for
1357 "f" will return an <code>OverloadedFunctionDecl</code> that contains
1358 both declarations of "f".</p>
1359
1360<p><code>DeclContext</code> manages multiply-defined declaration
1361 contexts internally. The
1362 function <code>DeclContext::getPrimaryContext</code> retrieves the
1363 "primary" context for a given <code>DeclContext</code> instance,
1364 which is the <code>DeclContext</code> responsible for maintaining
1365 the lookup table used for the semantics-centric view. Given the
1366 primary context, one can follow the chain
1367 of <code>DeclContext</code> nodes that define additional
1368 declarations via <code>DeclContext::getNextContext</code>. Note that
1369 these functions are used internally within the lookup and insertion
1370 methods of the <code>DeclContext</code>, so the vast majority of
1371 clients can ignore them.</p>
1372
1373<!-- ======================================================================= -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1374<h3 id="CFG">The <tt>CFG</tt> class</h3>
1375<!-- ======================================================================= -->
1376
1377<p>The <tt>CFG</tt> class is designed to represent a source-level
1378control-flow graph for a single statement (<tt>Stmt*</tt>). Typically
1379instances of <tt>CFG</tt> are constructed for function bodies (usually
1380an instance of <tt>CompoundStmt</tt>), but can also be instantiated to
1381represent the control-flow of any class that subclasses <tt>Stmt</tt>,
1382which includes simple expressions. Control-flow graphs are especially
1383useful for performing
1384<a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow-
1385or path-sensitive</a> program analyses on a given function.</p>
1386
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1387<!-- ============ -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1388<h4>Basic Blocks</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1389<!-- ============ -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1390
1391<p>Concretely, an instance of <tt>CFG</tt> is a collection of basic
1392blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which
1393simply contains an ordered sequence of <tt>Stmt*</tt> (each referring
1394to statements in the AST). The ordering of statements within a block
1395indicates unconditional flow of control from one statement to the
1396next. <a href="#ConditionalControlFlow">Conditional control-flow</a>
1397is represented using edges between basic blocks. The statements
1398within a given <tt>CFGBlock</tt> can be traversed using
1399the <tt>CFGBlock::*iterator</tt> interface.</p>
1400
1401<p>
Ted Kremenek18e17e72007-10-18 22:50:52 +0000[diff] [blame]1402A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1403the control-flow graph it represents. Each <tt>CFGBlock</tt> within a
1404CFG is also uniquely numbered (accessible
1405via <tt>CFGBlock::getBlockID()</tt>). Currently the number is
1406based on the ordering the blocks were created, but no assumptions
1407should be made on how <tt>CFGBlock</tt>s are numbered other than their
1408numbers are unique and that they are numbered from 0..N-1 (where N is
1409the number of basic blocks in the CFG).</p>
1410
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1411<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1412<h4>Entry and Exit Blocks</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1413<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1414
1415Each instance of <tt>CFG</tt> contains two special blocks:
1416an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which
1417has no incoming edges, and an <i>exit</i> block (accessible
1418via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither
1419block contains any statements, and they serve the role of providing a
1420clear entrance and exit for a body of code such as a function body.
1421The presence of these empty blocks greatly simplifies the
1422implementation of many analyses built on top of CFGs.
1423
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1424<!-- ===================================================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1425<h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1426<!-- ===================================================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1427
1428<p>Conditional control-flow (such as those induced by if-statements
1429and loops) is represented as edges between <tt>CFGBlock</tt>s.
1430Because different C language constructs can induce control-flow,
1431each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that
1432represents the <i>terminator</i> of the block. A terminator is simply
1433the statement that caused the control-flow, and is used to identify
1434the nature of the conditional control-flow between blocks. For
1435example, in the case of an if-statement, the terminator refers to
1436the <tt>IfStmt</tt> object in the AST that represented the given
1437branch.</p>
1438
1439<p>To illustrate, consider the following code example:</p>
1440
1441<code>
1442int foo(int x) {<br>
1443&nbsp;&nbsp;x = x + 1;<br>
1444<br>
1445&nbsp;&nbsp;if (x > 2) x++;<br>
1446&nbsp;&nbsp;else {<br>
1447&nbsp;&nbsp;&nbsp;&nbsp;x += 2;<br>
1448&nbsp;&nbsp;&nbsp;&nbsp;x *= 2;<br>
1449&nbsp;&nbsp;}<br>
1450<br>
1451&nbsp;&nbsp;return x;<br>
1452}
1453</code>
1454
1455<p>After invoking the parser+semantic analyzer on this code fragment,
1456the AST of the body of <tt>foo</tt> is referenced by a
1457single <tt>Stmt*</tt>. We can then construct an instance
1458of <tt>CFG</tt> representing the control-flow graph of this function
1459body by single call to a static class method:</p>
1460
1461<code>
1462&nbsp;&nbsp;Stmt* FooBody = ...<br>
1463&nbsp;&nbsp;CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody);
1464</code>
1465
1466<p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt>
1467to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no
1468longer needed.</p>
1469
1470<p>Along with providing an interface to iterate over
1471its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods
1472that are useful for debugging and visualizing CFGs. For example, the
1473method
1474<tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to
1475standard error. This is especially useful when one is using a
1476debugger such as gdb. For example, here is the output
1477of <tt>FooCFG->dump()</tt>:</p>
1478
1479<code>
1480&nbsp;[ B5 (ENTRY) ]<br>
1481&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (0):<br>
1482&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B4<br>
1483<br>
1484&nbsp;[ B4 ]<br>
1485&nbsp;&nbsp;&nbsp;&nbsp;1: x = x + 1<br>
1486&nbsp;&nbsp;&nbsp;&nbsp;2: (x > 2)<br>
1487&nbsp;&nbsp;&nbsp;&nbsp;<b>T: if [B4.2]</b><br>
1488&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B5<br>
1489&nbsp;&nbsp;&nbsp;&nbsp;Successors (2): B3 B2<br>
1490<br>
1491&nbsp;[ B3 ]<br>
1492&nbsp;&nbsp;&nbsp;&nbsp;1: x++<br>
1493&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
1494&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
1495<br>
1496&nbsp;[ B2 ]<br>
1497&nbsp;&nbsp;&nbsp;&nbsp;1: x += 2<br>
1498&nbsp;&nbsp;&nbsp;&nbsp;2: x *= 2<br>
1499&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
1500&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
1501<br>
1502&nbsp;[ B1 ]<br>
1503&nbsp;&nbsp;&nbsp;&nbsp;1: return x;<br>
1504&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (2): B2 B3<br>
1505&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B0<br>
1506<br>
1507&nbsp;[ B0 (EXIT) ]<br>
1508&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B1<br>
1509&nbsp;&nbsp;&nbsp;&nbsp;Successors (0):
1510</code>
1511
1512<p>For each block, the pretty-printed output displays for each block
1513the number of <i>predecessor</i> blocks (blocks that have outgoing
1514control-flow to the given block) and <i>successor</i> blocks (blocks
1515that have control-flow that have incoming control-flow from the given
1516block). We can also clearly see the special entry and exit blocks at
1517the beginning and end of the pretty-printed output. For the entry
1518block (block B5), the number of predecessor blocks is 0, while for the
1519exit block (block B0) the number of successor blocks is 0.</p>
1520
1521<p>The most interesting block here is B4, whose outgoing control-flow
1522represents the branching caused by the sole if-statement
1523in <tt>foo</tt>. Of particular interest is the second statement in
1524the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed
1525as <b><tt>if [B4.2]</tt></b>. The second statement represents the
1526evaluation of the condition of the if-statement, which occurs before
1527the actual branching of control-flow. Within the <tt>CFGBlock</tt>
1528for B4, the <tt>Stmt*</tt> for the second statement refers to the
1529actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus
1530pointers to subclasses of <tt>Expr</tt> can appear in the list of
1531statements in a block, and not just subclasses of <tt>Stmt</tt> that
1532refer to proper C statements.</p>
1533
1534<p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt>
1535object in the AST. The pretty-printer outputs <b><tt>if
1536[B4.2]</tt></b> because the condition expression of the if-statement
1537has an actual place in the basic block, and thus the terminator is
1538essentially
1539<i>referring</i> to the expression that is the second statement of
1540block B4 (i.e., B4.2). In this manner, conditions for control-flow
1541(which also includes conditions for loops and switch statements) are
1542hoisted into the actual basic block.</p>
1543
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1544<!-- ===================== -->
1545<!-- <h4>Implicit Control-Flow</h4> -->
1546<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1547
1548<!--
1549<p>A key design principle of the <tt>CFG</tt> class was to not require
1550any transformations to the AST in order to represent control-flow.
1551Thus the <tt>CFG</tt> does not perform any "lowering" of the
1552statements in an AST: loops are not transformed into guarded gotos,
1553short-circuit operations are not converted to a set of if-statements,
1554and so on.</p>
1555-->
Ted Kremenek17a295d2008-06-11 06:19:49 +0000[diff] [blame]1556
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1557
1558<!-- ======================================================================= -->
1559<h3 id="Constants">Constant Folding in the Clang AST</h3>
1560<!-- ======================================================================= -->
1561
1562<p>There are several places where constants and constant folding matter a lot to
1563the Clang front-end. First, in general, we prefer the AST to retain the source
1564code as close to how the user wrote it as possible. This means that if they
1565wrote "5+4", we want to keep the addition and two constants in the AST, we don't
1566want to fold to "9". This means that constant folding in various ways turns
1567into a tree walk that needs to handle the various cases.</p>
1568
1569<p>However, there are places in both C and C++ that require constants to be
1570folded. For example, the C standard defines what an "integer constant
1571expression" (i-c-e) is with very precise and specific requirements. The
1572language then requires i-c-e's in a lot of places (for example, the size of a
1573bitfield, the value for a case statement, etc). For these, we have to be able
1574to constant fold the constants, to do semantic checks (e.g. verify bitfield size
1575is non-negative and that case statements aren't duplicated). We aim for Clang
1576to be very pedantic about this, diagnosing cases when the code does not use an
1577i-c-e where one is required, but accepting the code unless running with
1578<tt>-pedantic-errors</tt>.</p>
1579
1580<p>Things get a little bit more tricky when it comes to compatibility with
1581real-world source code. Specifically, GCC has historically accepted a huge
1582superset of expressions as i-c-e's, and a lot of real world code depends on this
1583unfortuate accident of history (including, e.g., the glibc system headers). GCC
1584accepts anything its "fold" optimizer is capable of reducing to an integer
1585constant, which means that the definition of what it accepts changes as its
1586optimizer does. One example is that GCC accepts things like "case X-X:" even
1587when X is a variable, because it can fold this to 0.</p>
1588
1589<p>Another issue are how constants interact with the extensions we support, such
1590as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99
1591obviously does not specify the semantics of any of these extensions, and the
1592definition of i-c-e does not include them. However, these extensions are often
1593used in real code, and we have to have a way to reason about them.</p>
1594
1595<p>Finally, this is not just a problem for semantic analysis. The code
1596generator and other clients have to be able to fold constants (e.g. to
1597initialize global variables) and has to handle a superset of what C99 allows.
1598Further, these clients can benefit from extended information. For example, we
1599know that "foo()||1" always evaluates to true, but we can't replace the
1600expression with true because it has side effects.</p>
1601
1602<!-- ======================= -->
1603<h4>Implementation Approach</h4>
1604<!-- ======================= -->
1605
1606<p>After trying several different approaches, we've finally converged on a
1607design (Note, at the time of this writing, not all of this has been implemented,
1608consider this a design goal!). Our basic approach is to define a single
1609recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is
1610implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar'
1611type (integer, fp, complex, or pointer) this method returns the following
1612information:</p>
1613
1614<ul>
1615<li>Whether the expression is an integer constant expression, a general
1616 constant that was folded but has no side effects, a general constant that
1617 was folded but that does have side effects, or an uncomputable/unfoldable
1618 value.
1619</li>
1620<li>If the expression was computable in any way, this method returns the APValue
1621 for the result of the expression.</li>
1622<li>If the expression is not evaluatable at all, this method returns
1623 information on one of the problems with the expression. This includes a
1624 SourceLocation for where the problem is, and a diagnostic ID that explains
1625 the problem. The diagnostic should be have ERROR type.</li>
1626<li>If the expression is not an integer constant expression, this method returns
1627 information on one of the problems with the expression. This includes a
1628 SourceLocation for where the problem is, and a diagnostic ID that explains
1629 the problem. The diagnostic should be have EXTENSION type.</li>
1630</ul>
1631
1632<p>This information gives various clients the flexibility that they want, and we
1633will eventually have some helper methods for various extensions. For example,
1634Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which
1635calls Evaluate on the expression. If the expression is not foldable, the error
1636is emitted, and it would return true. If the expression is not an i-c-e, the
1637EXTENSION diagnostic is emitted. Finally it would return false to indicate that
1638the AST is ok.</p>
1639
1640<p>Other clients can use the information in other ways, for example, codegen can
1641just use expressions that are foldable in any way.</p>
1642
1643<!-- ========== -->
1644<h4>Extensions</h4>
1645<!-- ========== -->
1646
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]1647<p>This section describes how some of the various extensions Clang supports
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1648interacts with constant evaluation:</p>
1649
1650<ul>
1651<li><b><tt>__extension__</tt></b>: The expression form of this extension causes
1652 any evaluatable subexpression to be accepted as an integer constant
1653 expression.</li>
1654<li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer
Chris Lattner28daa532008-12-12 06:55:44 +0000[diff] [blame]1655 constant expression) if the operand is any evaluatable constant. As a
1656 special case, if <tt>__builtin_constant_p</tt> is the (potentially
1657 parenthesized) condition of a conditional operator expression ("?:"), only
Chris Lattner42b83dd2008-12-12 18:00:51 +0000[diff] [blame]1658 the true side of the conditional operator is considered, and it is evaluated
1659 with full constant folding.</li>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1660<li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an
1661 integer constant expression, but we accept any constant as an "extension of
1662 an extension". This only evaluates one operand depending on which way the
1663 condition evaluates.</li>
1664<li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer
1665 constant expression.</li>
1666<li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a
1667 floating-point literal.</li>
1668<li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as
1669 general constant expressions.</li>
1670</ul>
1671
1672
1673
1674
Ted Kremenek17a295d2008-06-11 06:19:49 +0000[diff] [blame]1675</div>
1676</body>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1677</html>