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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>
70</ul>
71
72
73<!-- ======================================================================= -->
74<h2 id="intro">Introduction</h2>
75<!-- ======================================================================= -->
76
77<p>This document describes some of the more important APIs and internal design
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]78decisions made in the Clang C front-end. The purpose of this document is to
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]79both capture some of this high level information and also describe some of the
80design decisions behind it. This is meant for people interested in hacking on
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]81Clang, not for end-users. The description below is categorized by
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]82libraries, and does not describe any of the clients of the libraries.</p>
83
84<!-- ======================================================================= -->
85<h2 id="libsystem">LLVM System and Support Libraries</h2>
86<!-- ======================================================================= -->
87
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]88<p>The LLVM libsystem library provides the basic Clang system abstraction layer,
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]89which is used for file system access. The LLVM libsupport library provides many
90underlying libraries and <a
91href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>,
92 including command line option
93processing and various containers.</p>
94
95<!-- ======================================================================= -->
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]96<h2 id="libbasic">The Clang 'Basic' Library</h2>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]97<!-- ======================================================================= -->
98
99<p>This library certainly needs a better name. The 'basic' library contains a
100number of low-level utilities for tracking and manipulating source buffers,
101locations within the source buffers, diagnostics, tokens, target abstraction,
102and information about the subset of the language being compiled for.</p>
103
104<p>Part of this infrastructure is specific to C (such as the TargetInfo class),
105other parts could be reused for other non-C-based languages (SourceLocation,
106SourceManager, Diagnostics, FileManager). When and if there is future demand
107we can figure out if it makes sense to introduce a new library, move the general
108classes somewhere else, or introduce some other solution.</p>
109
110<p>We describe the roles of these classes in order of their dependencies.</p>
111
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]112
113<!-- ======================================================================= -->
114<h3 id="Diagnostics">The Diagnostics Subsystem</h3>
115<!-- ======================================================================= -->
116
117<p>The Clang Diagnostics subsystem is an important part of how the compiler
118communicates with the human. Diagnostics are the warnings and errors produced
119when the code is incorrect or dubious. In Clang, each diagnostic produced has
120(at the minimum) a unique ID, a <a href="#SourceLocation">SourceLocation</a> to
121"put the caret", an English translation associated with it, and a severity (e.g.
122<tt>WARNING</tt> or <tt>ERROR</tt>). They can also optionally include a number
123of arguments to the dianostic (which fill in "%0"'s in the string) as well as a
124number of source ranges that related to the diagnostic.</p>
125
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]126<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]127driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many
128different ways</a> depending on how the DiagnosticClient interface is
129implemented. A representative example of a diagonstic is:</p>
130
131<pre>
132t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
133 <font color="darkgreen">P = (P-42) + Gamma*4;</font>
134 <font color="blue">~~~~~~ ^ ~~~~~~~</font>
135</pre>
136
137<p>In this example, you can see the English translation, the severity (error),
138you can see the source location (the caret ("^") and file/line/column info),
139the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex
140float"). You'll have to believe me that there is a unique ID backing the
141diagnostic :).</p>
142
143<p>Getting all of this to happen has several steps and involves many moving
144pieces, this section describes them and talks about best practices when adding
145a new diagnostic.</p>
146
147<!-- ============================ -->
148<h4>The DiagnosticKinds.def file</h4>
149<!-- ============================ -->
150
151<p>Diagnostics are created by adding an entry to the <tt><a
152href="http://llvm.org/svn/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticKinds.def"
153>DiagnosticKinds.def</a></tt> file. This file encodes the unique ID of the
154diagnostic (as an enum, the first argument), the severity of the diagnostic
155(second argument) and the English translation + format string.</p>
156
157<p>There is little sanity with the naming of the unique ID's right now. Some
158start with err_, warn_, ext_ to encode the severity into the name. Since the
159enum is referenced in the C++ code that produces the diagnostic, it is somewhat
160useful for it to be reasonably short.</p>
161
162<p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>,
163<tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The
164<tt>ERROR</tt> severity is used for diagnostics indicating the program is never
165acceptable under any circumstances. When an error is emitted, the AST for the
166input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt>
167severities are used for extensions to the language that Clang accepts. This
168means that Clang fully understands and can represent them in the AST, but we
169produce diagnostics to tell the user their code is non-portable. The difference
170is that the former are ignored by default, and the later warn by default. The
171<tt>WARNING</tt> severity is used for constructs that are valid in the currently
172selected source language but that are dubious in some way. The <tt>NOTE</tt>
Daniel Dunbar426b8632009-02-17 15:49:03 +0000[diff] [blame]173level is used to staple more information onto previous diagnostics.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]174
175<p>These <em>severities</em> are mapped into a smaller set (the
176Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>,
Chris Lattner0aad2972009-02-05 22:49:08 +0000[diff] [blame]177<tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics
Chris Lattnera180fdd2009-02-17 07:07:29 +0000[diff] [blame]178subsystem based on various configuration options. Clang internally supports a
179fully fine grained mapping mechanism that allows you to map almost any
180diagnostic to the output level that you want. The only diagnostics that cannot
181be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously
182emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to
183<tt>Fatal</tt> (it is not possible to turn an error into a warning,
184for example).</p>
185
186<p>Diagnostic mappings are used in many ways. For example, if the user
187specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if
188they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is
189used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc.
190</p>
191
192<p>
193Mapping to <tt>Fatal</tt> should only be used for diagnostics that are
194considered so severe that error recovery won't be able to recover sensibly from
195them (thus spewing a ton of bogus errors). One example of this class of error
196are failure to #include a file.
197</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]198
199<!-- ================= -->
200<h4>The Format String</h4>
201<!-- ================= -->
202
203<p>The format string for the diagnostic is very simple, but it has some power.
204It takes the form of a string in English with markers that indicate where and
205how arguments to the diagnostic are inserted and formatted. For example, here
206are some simple format strings:</p>
207
208<pre>
209 "binary integer literals are an extension"
210 "format string contains '\\0' within the string body"
211 "more '<b>%%</b>' conversions than data arguments"
Chris Lattner545b3682008-11-23 20:27:13 +0000[diff] [blame]212 "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)"
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]213 "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator"
214 " (has <b>%1</b> parameter<b>%s1</b>)"
215</pre>
216
217<p>These examples show some important points of format strings. You can use any
218 plain ASCII character in the diagnostic string except "%" without a problem,
219 but these are C strings, so you have to use and be aware of all the C escape
220 sequences (as in the second example). If you want to produce a "%" in the
221 output, use the "%%" escape sequence, like the third diagnostic. Finally,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]222 Clang uses the "%...[digit]" sequences to specify where and how arguments to
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]223 the diagnostic are formatted.</p>
224
225<p>Arguments to the diagnostic are numbered according to how they are specified
226 by the C++ code that <a href="#producingdiag">produces them</a>, and are
227 referenced by <tt>%0</tt> .. <tt>%9</tt>. If you have more than 10 arguments
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]228 to your diagnostic, you are doing something wrong :). Unlike printf, there
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]229 is no requirement that arguments to the diagnostic end up in the output in
230 the same order as they are specified, you could have a format string with
231 <tt>"%1 %0"</tt> that swaps them, for example. The text in between the
232 percent and digit are formatting instructions. If there are no instructions,
233 the argument is just turned into a string and substituted in.</p>
234
235<p>Here are some "best practices" for writing the English format string:</p>
236
237<ul>
238<li>Keep the string short. It should ideally fit in the 80 column limit of the
239 <tt>DiagnosticKinds.def</tt> file. This avoids the diagnostic wrapping when
240 printed, and forces you to think about the important point you are conveying
241 with the diagnostic.</li>
242<li>Take advantage of location information. The user will be able to see the
243 line and location of the caret, so you don't need to tell them that the
244 problem is with the 4th argument to the function: just point to it.</li>
245<li>Do not capitalize the diagnostic string, and do not end it with a
246 period.</li>
247<li>If you need to quote something in the diagnostic string, use single
248 quotes.</li>
249</ul>
250
251<p>Diagnostics should never take random English strings as arguments: you
252shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like
253<tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing
254this prevents <a href="translation">translating</a> the Clang diagnostics to
255other languages (because they'll get random English words in their otherwise
256localized diagnostic). The exceptions to this are C/C++ language keywords
257(e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note
258that things like "pointer" and "reference" are not keywords. On the other
259hand, you <em>can</em> include anything that comes from the user's source code,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]260including variable names, types, labels, etc. The 'select' format can be
261used to achieve this sort of thing in a localizable way, see below.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]262
263<!-- ==================================== -->
264<h4>Formatting a Diagnostic Argument</a></h4>
265<!-- ==================================== -->
266
267<p>Arguments to diagnostics are fully typed internally, and come from a couple
268different classes: integers, types, names, and random strings. Depending on
269the class of the argument, it can be optionally formatted in different ways.
270This gives the DiagnosticClient information about what the argument means
271without requiring it to use a specific presentation (consider this MVC for
272Clang :).</p>
273
274<p>Here are the different diagnostic argument formats currently supported by
275Clang:</p>
276
277<table>
278<tr><td colspan="2"><b>"s" format</b></td></tr>
279<tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]280<tr><td>Class:</td><td>Integers</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]281<tr><td>Description:</td><td>This is a simple formatter for integers that is
282 useful when producing English diagnostics. When the integer is 1, it prints
283 as nothing. When the integer is not 1, it prints as "s". This allows some
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]284 simple grammatical forms to be to be handled correctly, and eliminates the
285 need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]286
287<tr><td colspan="2"><b>"select" format</b></td></tr>
288<tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2
289 operator"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]290<tr><td>Class:</td><td>Integers</td></tr>
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]291<tr><td>Description:</td><td>This format specifier is used to merge multiple
292 related diagnostics together into one common one, without requiring the
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]293 difference to be specified as an English string argument. Instead of
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]294 specifying the string, the diagnostic gets an integer argument and the
295 format string selects the numbered option. In this case, the "%2" value
296 must be an integer in the range [0..2]. If it is 0, it prints 'unary', if
297 it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This
298 allows other language translations to substitute reasonable words (or entire
299 phrases) based on the semantics of the diagnostic instead of having to do
300 things textually.</td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]301
302<tr><td colspan="2"><b>"plural" format</b></td></tr>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]303<tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to
304 your computer"</tt></td></tr>
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]305<tr><td>Class:</td><td>Integers</td></tr>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]306<tr><td>Description:</td><td><p>This is a formatter for complex plural forms.
307 It is designed to handle even the requirements of languages with very
308 complex plural forms, as many Baltic languages have. The argument consists
309 of a series of expression/form pairs, separated by ':', where the first form
310 whose expression evaluates to true is the result of the modifier.</p>
311 <p>An expression can be empty, in which case it is always true. See the
312 example at the top. Otherwise, it is a series of one or more numeric
313 conditions, separated by ','. If any condition matches, the expression
314 matches. Each numeric condition can take one of three forms.</p>
315 <ul>
316 <li>number: A simple decimal number matches if the argument is the same
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]317 as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]318 <li>range: A range in square brackets matches if the argument is within
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]319 the range. Then range is inclusive on both ends. Example:
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]320 <tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li>
321 <li>modulo: A modulo operator is followed by a number, and
322 equals sign and either a number or a range. The tests are the
323 same as for plain
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]324 numbers and ranges, but the argument is taken modulo the number first.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]325 Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything
326 else}1"</tt></li>
Sebastian Redl68168562008-11-22 22:16:45 +0000[diff] [blame]327 </ul>
328 <p>The parser is very unforgiving. A syntax error, even whitespace, will
329 abort, as will a failure to match the argument against any
330 expression.</p></td></tr>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]331
Chris Lattner077bf5e2008-11-24 03:33:13 +0000[diff] [blame]332<tr><td colspan="2"><b>"objcclass" format</b></td></tr>
333<tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr>
334<tr><td>Class:</td><td>DeclarationName</td></tr>
335<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
336 DeclarationName corresponds to an Objective-C class method selector. As
337 such, it prints the selector with a leading '+'.</p></td></tr>
338
339<tr><td colspan="2"><b>"objcinstance" format</b></td></tr>
340<tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr>
341<tr><td>Class:</td><td>DeclarationName</td></tr>
342<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
343 DeclarationName corresponds to an Objective-C instance method selector. As
344 such, it prints the selector with a leading '-'.</p></td></tr>
345
Douglas Gregor47b9a1c2009-02-04 17:27:36 +0000[diff] [blame]346<tr><td colspan="2"><b>"q" format</b></td></tr>
347<tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr>
348<tr><td>Class:</td><td>NamedDecl*</td></tr>
349<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>
350
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]351</table>
352
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]353<p>It is really easy to add format specifiers to the Clang diagnostics system,
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]354but they should be discussed before they are added. If you are creating a lot
355of repetitive diagnostics and/or have an idea for a useful formatter, please
356bring it up on the cfe-dev mailing list.</p>
Chris Lattnercc543342008-11-22 23:50:47 +0000[diff] [blame]357
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]358<!-- ===================================================== -->
359<h4><a name="#producingdiag">Producing the Diagnostic</a></h4>
360<!-- ===================================================== -->
361
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]362<p>Now that you've created the diagnostic in the DiagnosticKinds.def file, you
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]363need to write the code that detects the condition in question and emits the
364new diagnostic. Various components of Clang (e.g. the preprocessor, Sema,
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]365etc) provide a helper function named "Diag". It creates a diagnostic and
366accepts the arguments, ranges, and other information that goes along with
367it.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]368
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]369<p>For example, the binary expression error comes from code like this:</p>
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]370
371<pre>
372 if (various things that are bad)
373 Diag(Loc, diag::err_typecheck_invalid_operands)
374 &lt;&lt; lex-&gt;getType() &lt;&lt; rex-&gt;getType()
375 &lt;&lt; lex-&gt;getSourceRange() &lt;&lt; rex-&gt;getSourceRange();
376</pre>
377
378<p>This shows that use of the Diag method: they take a location (a <a
379href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value
380(which matches the name from DiagnosticKinds.def). If the diagnostic takes
381arguments, they are specified with the &lt;&lt; operator: the first argument
382becomes %0, the second becomes %1, etc. The diagnostic interface allows you to
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]383specify arguments of many different types, including <tt>int</tt> and
384<tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and
385<tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and
386<tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc.
387SourceRanges are also specified with the &lt;&lt; operator, but do not have a
388specific ordering requirement.</p>
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]389
390<p>As you can see, adding and producing a diagnostic is pretty straightforward.
391The hard part is deciding exactly what you need to say to help the user, picking
392a suitable wording, and providing the information needed to format it correctly.
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]393The good news is that the call site that issues a diagnostic should be
394completely independent of how the diagnostic is formatted and in what language
395it is rendered.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]396</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]397
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]398<!-- ==================================================== -->
399<h4 id="code-modification-hints">Code Modification Hints</h4>
400<!-- ==================================================== -->
401
402<p>In some cases, the front end emits diagnostics when it is clear
403that some small change to the source code would fix the problem. For
404example, a missing semicolon at the end of a statement or a use of
Chris Lattner34c05332009-02-27 19:31:12 +0000[diff] [blame]405deprecated syntax that is easily rewritten into a more modern form.
406Clang tries very hard to emit the diagnostic and recover gracefully
407in these and other cases.</p>
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]408
Chris Lattner34c05332009-02-27 19:31:12 +0000[diff] [blame]409<p>However, for these cases where the fix is obvious, the diagnostic
410can be annotated with a code
411modification "hint" that describes how to change the code referenced
Douglas Gregorb2fb6de2009-02-27 17:53:17 +0000[diff] [blame]412by the diagnostic to fix the problem. For example, it might add the
413missing semicolon at the end of the statement or rewrite the use of a
414deprecated construct into something more palatable. Here is one such
415example C++ front end, where we warn about the right-shift operator
416changing meaning from C++98 to C++0x:</p>
417
418<pre>
419test.cpp:3:7: warning: use of right-shift operator ('&gt;&gt;') in template argument will require parentheses in C++0x
420A&lt;100 &gt;&gt; 2&gt; *a;
421 ^
422 ( )
423</pre>
424
425<p>Here, the code modification hint is suggesting that parentheses be
426added, and showing exactly where those parentheses would be inserted
427into the source code. The code modification hints themselves describe
428what changes to make to the source code in an abstract manner, which
429the text diagnostic printer renders as a line of "insertions" below
430the caret line. <a href="#DiagnosticClient">Other diagnostic
431clients</a> might choose to render the code differently (e.g., as
432markup inline) or even give the user the ability to automatically fix
433the problem.</p>
434
435<p>All code modification hints are described by the
436<code>CodeModificationHint</code> class, instances of which should be
437attached to the diagnostic using the &lt;&lt; operator in the same way
438that highlighted source ranges and arguments are passed to the
439diagnostic. Code modification hints can be created with one of three
440constructors:</p>
441
442<dl>
443 <dt><code>CodeModificationHint::CreateInsertion(Loc, Code)</code></dt>
444 <dd>Specifies that the given <code>Code</code> (a string) should be inserted
445 before the source location <code>Loc</code>.</dd>
446
447 <dt><code>CodeModificationHint::CreateRemoval(Range)</code></dt>
448 <dd>Specifies that the code in the given source <code>Range</code>
449 should be removed.</dd>
450
451 <dt><code>CodeModificationHint::CreateReplacement(Range, Code)</code></dt>
452 <dd>Specifies that the code in the given source <code>Range</code>
453 should be removed, and replaced with the given <code>Code</code> string.</dd>
454</dl>
455
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]456<!-- ============================================================= -->
457<h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4>
458<!-- ============================================================= -->
459
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]460<p>Once code generates a diagnostic with all of the arguments and the rest of
461the relevant information, Clang needs to know what to do with it. As previously
462mentioned, the diagnostic machinery goes through some filtering to map a
463severity onto a diagnostic level, then (assuming the diagnostic is not mapped to
464"<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient
465interface with the information.</p>
466
467<p>It is possible to implement this interface in many different ways. For
468example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns
469the arguments into strings (according to the various formatting rules), prints
470out the file/line/column information and the string, then prints out the line of
471code, the source ranges, and the caret. However, this behavior isn't required.
472</p>
473
474<p>Another implementation of the DiagnosticClient interface is the
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]475'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode.
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]476Instead of formatting and printing out the diagnostics, this implementation just
477captures and remembers the diagnostics as they fly by. Then -verify compares
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]478the list of produced diagnostics to the list of expected ones. If they disagree,
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]479it prints out its own output.
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]480</p>
481
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]482<p>There are many other possible implementations of this interface, and this is
483why we prefer diagnostics to pass down rich structured information in arguments.
484For example, an HTML output might want declaration names be linkified to where
485they come from in the source. Another example is that a GUI might let you click
486on typedefs to expand them. This application would want to pass significantly
487more information about types through to the GUI than a simple flat string. The
488interface allows this to happen.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]489
490<!-- ====================================================== -->
491<h4><a name="translation">Adding Translations to Clang</a></h4>
492<!-- ====================================================== -->
493
Chris Lattner627b7052008-11-23 00:28:33 +0000[diff] [blame]494<p>Not possible yet! Diagnostic strings should be written in UTF-8, the client
Chris Lattnerfd408ea2008-11-23 00:42:53 +0000[diff] [blame]495can translate to the relevant code page if needed. Each translation completely
496replaces the format string for the diagnostic.</p>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]497
498
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]499<!-- ======================================================================= -->
500<h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3>
501<!-- ======================================================================= -->
502
503<p>Strangely enough, the SourceLocation class represents a location within the
504source code of the program. Important design points include:</p>
505
506<ol>
507<li>sizeof(SourceLocation) must be extremely small, as these are embedded into
508 many AST nodes and are passed around often. Currently it is 32 bits.</li>
509<li>SourceLocation must be a simple value object that can be efficiently
510 copied.</li>
511<li>We should be able to represent a source location for any byte of any input
512 file. This includes in the middle of tokens, in whitespace, in trigraphs,
513 etc.</li>
514<li>A SourceLocation must encode the current #include stack that was active when
515 the location was processed. For example, if the location corresponds to a
516 token, it should contain the set of #includes active when the token was
517 lexed. This allows us to print the #include stack for a diagnostic.</li>
518<li>SourceLocation must be able to describe macro expansions, capturing both
519 the ultimate instantiation point and the source of the original character
520 data.</li>
521</ol>
522
523<p>In practice, the SourceLocation works together with the SourceManager class
Chris Lattner18376dd2009-01-16 07:00:50 +0000[diff] [blame]524to encode two pieces of information about a location: it's spelling location
Chris Lattner88054de2009-01-16 07:15:35 +0000[diff] [blame]525and it's instantiation location. For most tokens, these will be the same. However,
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]526for a macro expansion (or tokens that came from a _Pragma directive) these will
527describe the location of the characters corresponding to the token and the
528location where the token was used (i.e. the macro instantiation point or the
529location of the _Pragma itself).</p>
530
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]531<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]532produced by multiple instantiations, we only track the source and ultimate
533destination. Though we could track the intermediate instantiation points, this
534would require extra bookkeeping and no known client would benefit substantially
535from this.</p>
536
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]537<p>The Clang front-end inherently depends on the location of a token being
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]538tracked correctly. If it is ever incorrect, the front-end may get confused and
539die. 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]540Clang depends on being able to find the original input characters for the token.
Chris Lattner18376dd2009-01-16 07:00:50 +0000[diff] [blame]541This concept maps directly to the "spelling location" for the token.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]542
543<!-- ======================================================================= -->
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]544<h2 id="libdriver">The Driver Library</h2>
545<!-- ======================================================================= -->
546
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]547<p>The clang Driver and library are documented <a
548href="DriverInternals.html">here<a>.<p>
549
550<!-- ======================================================================= -->
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame^]551<h2 id="pch">Precompiled Headers</h2>
Ted Kremenekcfa8d572009-04-09 18:08:18 +0000[diff] [blame]552<!-- ======================================================================= -->
553
Douglas Gregor32110df2009-05-20 00:16:32 +0000[diff] [blame^]554<p>Clang supports two implementations of precompiled headers. The
555 default implementation, precompiled headers (<a
556 href="PCHInternals.html">PCH</a>) uses a serialized representation
557 of Clang's internal data structures, encoded with the <a
558 href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream
559 format</a>. Pretokenized headers (<a
560 href="PTHInternals.html">PTH</a>), on the other hand, contain a
561 serialized representation of the tokens encountered when
562 preprocessing a header (and anything that header includes).</p>
563
Daniel Dunbar27d9e9f2009-03-30 06:50:01 +0000[diff] [blame]564
565<!-- ======================================================================= -->
566<h2 id="libfrontend">The Frontend Library</h2>
567<!-- ======================================================================= -->
568
569<p>The Frontend library contains functionality useful for building
570tools on top of the clang libraries, for example several methods for
571outputting diagnostics.</p>
572
573<!-- ======================================================================= -->
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]574<h2 id="liblex">The Lexer and Preprocessor Library</h2>
575<!-- ======================================================================= -->
576
577<p>The Lexer library contains several tightly-connected classes that are involved
578with the nasty process of lexing and preprocessing C source code. The main
579interface to this library for outside clients is the large <a
580href="#Preprocessor">Preprocessor</a> class.
581It contains the various pieces of state that are required to coherently read
582tokens out of a translation unit.</p>
583
584<p>The core interface to the Preprocessor object (once it is set up) is the
585Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from
586the preprocessor stream. There are two types of token providers that the
587preprocessor is capable of reading from: a buffer lexer (provided by the <a
588href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]589href="#TokenLexer">TokenLexer</a> class).
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]590
591
592<!-- ======================================================================= -->
593<h3 id="Token">The Token class</h3>
594<!-- ======================================================================= -->
595
596<p>The Token class is used to represent a single lexed token. Tokens are
597intended to be used by the lexer/preprocess and parser libraries, but are not
598intended to live beyond them (for example, they should not live in the ASTs).<p>
599
600<p>Tokens most often live on the stack (or some other location that is efficient
601to access) as the parser is running, but occasionally do get buffered up. For
602example, macro definitions are stored as a series of tokens, and the C++
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]603front-end periodically needs to buffer tokens up for tentative parsing and
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]604various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit
605system, sizeof(Token) is currently 16 bytes.</p>
606
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]607<p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation
608Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer,
609annotation tokens represent semantic information and are produced by the parser,
610replacing normal tokens in the token stream. Normal tokens contain the
611following information:</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]612
613<ul>
614<li><b>A SourceLocation</b> - This indicates the location of the start of the
615token.</li>
616
617<li><b>A length</b> - This stores the length of the token as stored in the
618SourceBuffer. For tokens that include them, this length includes trigraphs and
619escaped newlines which are ignored by later phases of the compiler. By pointing
620into the original source buffer, it is always possible to get the original
621spelling of a token completely accurately.</li>
622
623<li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if
624identifier lookup was enabled when the token was lexed (e.g. the lexer was not
625reading in 'raw' mode) this contains a pointer to the unique hash value for the
626identifier. Because the lookup happens before keyword identification, this
627field is set even for language keywords like 'for'.</li>
628
629<li><b>TokenKind</b> - This indicates the kind of token as classified by the
630lexer. This includes things like <tt>tok::starequal</tt> (for the "*="
631operator), <tt>tok::ampamp</tt> for the "&amp;&amp;" token, and keyword values
632(e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note
633that some tokens can be spelled multiple ways. For example, C++ supports
634"operator keywords", where things like "and" are treated exactly like the
635"&amp;&amp;" operator. In these cases, the kind value is set to
636<tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to
637consider both forms. For something that cares about which form is used (e.g.
638the preprocessor 'stringize' operator) the spelling indicates the original
639form.</li>
640
641<li><b>Flags</b> - There are currently four flags tracked by the
642lexer/preprocessor system on a per-token basis:
643
644 <ol>
645 <li><b>StartOfLine</b> - This was the first token that occurred on its input
646 source line.</li>
647 <li><b>LeadingSpace</b> - There was a space character either immediately
648 before the token or transitively before the token as it was expanded
649 through a macro. The definition of this flag is very closely defined by
650 the stringizing requirements of the preprocessor.</li>
651 <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to
652 represent identifier tokens which have macro expansion disabled. This
653 prevents them from being considered as candidates for macro expansion ever
654 in the future.</li>
655 <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the
656 token includes a trigraph or escaped newline. Since this is uncommon,
657 many pieces of code can fast-path on tokens that did not need cleaning.
658 </p>
659 </ol>
660</li>
661</ul>
662
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]663<p>One interesting (and somewhat unusual) aspect of normal tokens is that they
664don't contain any semantic information about the lexed value. For example, if
665the token was a pp-number token, we do not represent the value of the number
666that was lexed (this is left for later pieces of code to decide). Additionally,
667the lexer library has no notion of typedef names vs variable names: both are
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]668returned as identifiers, and the parser is left to decide whether a specific
669identifier is a typedef or a variable (tracking this requires scope information
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]670among other things). The parser can do this translation by replacing tokens
671returned by the preprocessor with "Annotation Tokens".</p>
672
673<!-- ======================================================================= -->
674<h3 id="AnnotationToken">Annotation Tokens</h3>
675<!-- ======================================================================= -->
676
677<p>Annotation Tokens are tokens that are synthesized by the parser and injected
678into the preprocessor's token stream (replacing existing tokens) to record
679semantic information found by the parser. For example, if "foo" is found to be
680a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an
681<tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this
682makes it easy to handle qualified type names (e.g. "foo::bar::baz&lt;42&gt;::t")
683in C++ as a single "token" in the parser. 2) if the parser backtracks, the
684reparse does not need to redo semantic analysis to determine whether a token
685sequence is a variable, type, template, etc.</p>
686
687<p>Annotation Tokens are created by the parser and reinjected into the parser's
688token stream (when backtracking is enabled). Because they can only exist in
689tokens that the preprocessor-proper is done with, it doesn't need to keep around
690flags like "start of line" that the preprocessor uses to do its job.
691Additionally, an annotation token may "cover" a sequence of preprocessor tokens
692(e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields
693of an annotation token are different than the fields for a normal token (but
694they are multiplexed into the normal Token fields):</p>
695
696<ul>
697<li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation
698token indicates the first token replaced by the annotation token. In the example
699above, it would be the location of the "a" identifier.</li>
700
701<li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the
702last token replaced with the annotation token. In the example above, it would
703be the location of the "c" identifier.</li>
704
705<li><b>void* "AnnotationValue"</b> - This contains an opaque object that the
706parser gets from Sema through an Actions module, it is passed around and Sema
707intepretes it, based on the type of annotation token.</li>
708
709<li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this
710is. See below for the different valid kinds.</li>
711</ul>
712
713<p>Annotation tokens currently come in three kinds:</p>
714
715<ol>
716<li><b>tok::annot_typename</b>: This annotation token represents a
717resolved typename token that is potentially qualified. The AnnotationValue
Steve Naroffb43a50f2009-01-28 19:39:02 +0000[diff] [blame]718field contains a pointer returned by Action::getTypeName(). In the case of the
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]719Sema actions module, this is a <tt>Decl*</tt> for the type.</li>
720
721<li><b>tok::annot_cxxscope</b>: This annotation token represents a C++ scope
722specifier, such as "A::B::". This corresponds to the grammar productions "::"
723and ":: [opt] nested-name-specifier". The AnnotationValue pointer is returned
724by the Action::ActOnCXXGlobalScopeSpecifier and
725Action::ActOnCXXNestedNameSpecifier callbacks. In the case of Sema, this is a
726<tt>DeclContext*</tt>.</li>
727
Douglas Gregor39a8de12009-02-25 19:37:18 +0000[diff] [blame]728<li><b>tok::annot_template_id</b>: This annotation token represents a
729C++ template-id such as "foo&lt;int, 4&gt;", where "foo" is the name
730of a template. The AnnotationValue pointer is a pointer to a malloc'd
731TemplateIdAnnotation 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]732
733</ol>
734
Cedric Venetda76b282009-01-06 16:22:54 +0000[diff] [blame]735<p>As mentioned above, annotation tokens are not returned by the preprocessor,
Chris Lattner3932fe02009-01-06 06:02:08 +0000[diff] [blame]736they are formed on demand by the parser. This means that the parser has to be
737aware of cases where an annotation could occur and form it where appropriate.
738This is somewhat similar to how the parser handles Translation Phase 6 of C99:
739String Concatenation (see C99 5.1.1.2). In the case of string concatenation,
740the preprocessor just returns distinct tok::string_literal and
741tok::wide_string_literal tokens and the parser eats a sequence of them wherever
742the grammar indicates that a string literal can occur.</p>
743
744<p>In order to do this, whenever the parser expects a tok::identifier or
745tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or
746TryAnnotateCXXScopeToken methods to form the annotation token. These methods
747will maximally form the specified annotation tokens and replace the current
748token with them, if applicable. If the current tokens is not valid for an
749annotation token, it will remain an identifier or :: token.</p>
750
751
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]752
753<!-- ======================================================================= -->
754<h3 id="Lexer">The Lexer class</h3>
755<!-- ======================================================================= -->
756
757<p>The Lexer class provides the mechanics of lexing tokens out of a source
758buffer and deciding what they mean. The Lexer is complicated by the fact that
759it operates on raw buffers that have not had spelling eliminated (this is a
760necessity to get decent performance), but this is countered with careful coding
761as well as standard performance techniques (for example, the comment handling
762code is vectorized on X86 and PowerPC hosts).</p>
763
764<p>The lexer has a couple of interesting modal features:</p>
765
766<ul>
767<li>The lexer can operate in 'raw' mode. This mode has several features that
768 make it possible to quickly lex the file (e.g. it stops identifier lookup,
769 doesn't specially handle preprocessor tokens, handles EOF differently, etc).
770 This mode is used for lexing within an "<tt>#if 0</tt>" block, for
771 example.</li>
772<li>The lexer can capture and return comments as tokens. This is required to
773 support the -C preprocessor mode, which passes comments through, and is
774 used by the diagnostic checker to identifier expect-error annotations.</li>
775<li>The lexer can be in ParsingFilename mode, which happens when preprocessing
Chris Lattner84386242007-09-16 19:25:23 +0000[diff] [blame]776 after reading a #include directive. This mode changes the parsing of '&lt;'
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]777 to return an "angled string" instead of a bunch of tokens for each thing
778 within the filename.</li>
779<li>When parsing a preprocessor directive (after "<tt>#</tt>") the
780 ParsingPreprocessorDirective mode is entered. This changes the parser to
781 return EOM at a newline.</li>
782<li>The Lexer uses a LangOptions object to know whether trigraphs are enabled,
783 whether C++ or ObjC keywords are recognized, etc.</li>
784</ul>
785
786<p>In addition to these modes, the lexer keeps track of a couple of other
787 features that are local to a lexed buffer, which change as the buffer is
788 lexed:</p>
789
790<ul>
791<li>The Lexer uses BufferPtr to keep track of the current character being
792 lexed.</li>
793<li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token
794 will start with its "start of line" bit set.</li>
795<li>The Lexer keeps track of the current #if directives that are active (which
796 can be nested).</li>
797<li>The Lexer keeps track of an <a href="#MultipleIncludeOpt">
798 MultipleIncludeOpt</a> object, which is used to
799 detect whether the buffer uses the standard "<tt>#ifndef XX</tt> /
800 <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does,
801 subsequent includes can be ignored if the XX macro is defined.</li>
802</ul>
803
804<!-- ======================================================================= -->
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]805<h3 id="TokenLexer">The TokenLexer class</h3>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]806<!-- ======================================================================= -->
807
Chris Lattner79281252008-03-09 02:27:26 +0000[diff] [blame]808<p>The TokenLexer class is a token provider that returns tokens from a list
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]809of tokens that came from somewhere else. It typically used for two things: 1)
810returning tokens from a macro definition as it is being expanded 2) returning
811tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and
812will most likely be used to handle unbounded look-ahead for the C++ parser.</p>
813
814<!-- ======================================================================= -->
815<h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3>
816<!-- ======================================================================= -->
817
818<p>The MultipleIncludeOpt class implements a really simple little state machine
819that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>"
820idiom that people typically use to prevent multiple inclusion of headers. If a
821buffer uses this idiom and is subsequently #include'd, the preprocessor can
822simply check to see whether the guarding condition is defined or not. If so,
823the preprocessor can completely ignore the include of the header.</p>
824
825
826
827<!-- ======================================================================= -->
828<h2 id="libparse">The Parser Library</h2>
829<!-- ======================================================================= -->
830
831<!-- ======================================================================= -->
832<h2 id="libast">The AST Library</h2>
833<!-- ======================================================================= -->
834
835<!-- ======================================================================= -->
836<h3 id="Type">The Type class and its subclasses</h3>
837<!-- ======================================================================= -->
838
839<p>The Type class (and its subclasses) are an important part of the AST. Types
840are accessed through the ASTContext class, which implicitly creates and uniques
841them as they are needed. Types have a couple of non-obvious features: 1) they
842do not capture type qualifiers like const or volatile (See
843<a href="#QualType">QualType</a>), and 2) they implicitly capture typedef
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]844information. Once created, types are immutable (unlike decls).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]845
846<p>Typedefs in C make semantic analysis a bit more complex than it would
847be without them. The issue is that we want to capture typedef information
848and represent it in the AST perfectly, but the semantics of operations need to
849"see through" typedefs. For example, consider this code:</p>
850
851<code>
852void func() {<br>
Bill Wendling30d17752007-10-06 01:56:01 +0000[diff] [blame]853&nbsp;&nbsp;typedef int foo;<br>
854&nbsp;&nbsp;foo X, *Y;<br>
855&nbsp;&nbsp;typedef foo* bar;<br>
856&nbsp;&nbsp;bar Z;<br>
857&nbsp;&nbsp;*X; <i>// error</i><br>
858&nbsp;&nbsp;**Y; <i>// error</i><br>
859&nbsp;&nbsp;**Z; <i>// error</i><br>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]860}<br>
861</code>
862
863<p>The code above is illegal, and thus we expect there to be diagnostics emitted
864on the annotated lines. In this example, we expect to get:</p>
865
866<pre>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]867<b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]868*X; // error
869<font color="blue">^~</font>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]870<b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]871**Y; // error
872<font color="blue">^~~</font>
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]873<b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b>
874**Z; // error
875<font color="blue">^~~</font>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]876</pre>
877
878<p>While this example is somewhat silly, it illustrates the point: we want to
879retain typedef information where possible, so that we can emit errors about
880"<tt>std::string</tt>" instead of "<tt>std::basic_string&lt;char, std:...</tt>".
881Doing this requires properly keeping typedef information (for example, the type
882of "X" is "foo", not "int"), and requires properly propagating it through the
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]883various operators (for example, the type of *Y is "foo", not "int"). In order
884to retain this information, the type of these expressions is an instance of the
885TypedefType class, which indicates that the type of these expressions is a
886typedef for foo.
887</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]888
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]889<p>Representing types like this is great for diagnostics, because the
890user-specified type is always immediately available. There are two problems
891with this: first, various semantic checks need to make judgements about the
Chris Lattner33fc68a2007-07-31 18:54:50 +0000[diff] [blame]892<em>actual structure</em> of a type, ignoring typdefs. Second, we need an
893efficient way to query whether two types are structurally identical to each
894other, ignoring typedefs. The solution to both of these problems is the idea of
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]895canonical types.</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]896
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]897<!-- =============== -->
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]898<h4>Canonical Types</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]899<!-- =============== -->
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]900
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]901<p>Every instance of the Type class contains a canonical type pointer. For
902simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>",
903"<tt>int**</tt>"), the type just points to itself. For types that have a
904typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>",
905"<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their
906structurally equivalent type without any typedefs (e.g. "<tt>int</tt>",
907"<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]908
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]909<p>This design provides a constant time operation (dereferencing the canonical
910type pointer) that gives us access to the structure of types. For example,
911we can trivially tell that "bar" and "foo*" are the same type by dereferencing
912their canonical type pointers and doing a pointer comparison (they both point
913to the single "<tt>int*</tt>" type).</p>
914
915<p>Canonical types and typedef types bring up some complexities that must be
916carefully managed. Specifically, the "isa/cast/dyncast" operators generally
917shouldn't be used in code that is inspecting the AST. For example, when type
918checking the indirection operator (unary '*' on a pointer), the type checker
919must verify that the operand has a pointer type. It would not be correct to
920check that with "<tt>isa&lt;PointerType&gt;(SubExpr-&gt;getType())</tt>",
921because this predicate would fail if the subexpression had a typedef type.</p>
922
923<p>The solution to this problem are a set of helper methods on Type, used to
924check their properties. In this case, it would be correct to use
925"<tt>SubExpr-&gt;getType()-&gt;isPointerType()</tt>" to do the check. This
926predicate will return true if the <em>canonical type is a pointer</em>, which is
927true any time the type is structurally a pointer type. The only hard part here
928is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p>
929
930<p>The second problem we face is how to get access to the pointer type once we
931know it exists. To continue the example, the result type of the indirection
932operator is the pointee type of the subexpression. In order to determine the
933type, we need to get the instance of PointerType that best captures the typedef
934information in the program. If the type of the expression is literally a
935PointerType, we can return that, otherwise we have to dig through the
936typedefs to find the pointer type. For example, if the subexpression had type
937"<tt>foo*</tt>", we could return that type as the result. If the subexpression
938had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do
939<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]940a getAsPointerType() method that checks whether the type is structurally a
Chris Lattner8a2bc622007-07-31 06:37:39 +0000[diff] [blame]941PointerType and, if so, returns the best one. If not, it returns a null
942pointer.</p>
943
944<p>This structure is somewhat mystical, but after meditating on it, it will
945make sense to you :).</p>
Chris Lattner86920d32007-07-31 05:42:17 +0000[diff] [blame]946
947<!-- ======================================================================= -->
948<h3 id="QualType">The QualType class</h3>
949<!-- ======================================================================= -->
950
951<p>The QualType class is designed as a trivial value class that is small,
952passed by-value and is efficient to query. The idea of QualType is that it
953stores the type qualifiers (const, volatile, restrict) separately from the types
954themselves: QualType is conceptually a pair of "Type*" and bits for the type
955qualifiers.</p>
956
957<p>By storing the type qualifiers as bits in the conceptual pair, it is
958extremely efficient to get the set of qualifiers on a QualType (just return the
959field of the pair), add a type qualifier (which is a trivial constant-time
960operation that sets a bit), and remove one or more type qualifiers (just return
961a QualType with the bitfield set to empty).</p>
962
963<p>Further, because the bits are stored outside of the type itself, we do not
964need to create duplicates of types with different sets of qualifiers (i.e. there
965is only a single heap allocated "int" type: "const int" and "volatile const int"
966both point to the same heap allocated "int" type). This reduces the heap size
967used to represent bits and also means we do not have to consider qualifiers when
968uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p>
969
970<p>In practice, on hosts where it is safe, the 3 type qualifiers are stored in
971the low bit of the pointer to the Type object. This means that QualType is
972exactly the same size as a pointer, and this works fine on any system where
973malloc'd objects are at least 8 byte aligned.</p>
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]974
975<!-- ======================================================================= -->
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]976<h3 id="DeclarationName">Declaration names</h3>
977<!-- ======================================================================= -->
978
979<p>The <tt>DeclarationName</tt> class represents the name of a
980 declaration in Clang. Declarations in the C family of languages can
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]981 take several different forms. Most declarations are named by
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]982 simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in
983 the function declaration <code>f(int x)</code>. In C++, declaration
984 names can also name class constructors ("<code>Class</code>"
985 in <code>struct Class { Class(); }</code>), class destructors
986 ("<code>~Class</code>"), overloaded operator names ("operator+"),
987 and conversion functions ("<code>operator void const *</code>"). In
988 Objective-C, declaration names can refer to the names of Objective-C
989 methods, which involve the method name and the parameters,
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]990 collectively called a <i>selector</i>, e.g.,
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]991 "<code>setWidth:height:</code>". Since all of these kinds of
Chris Lattner3fcbb892008-11-23 08:32:53 +0000[diff] [blame]992 entities - variables, functions, Objective-C methods, C++
993 constructors, destructors, and operators - are represented as
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]994 subclasses of Clang's common <code>NamedDecl</code>
995 class, <code>DeclarationName</code> is designed to efficiently
996 represent any kind of name.</p>
997
998<p>Given
999 a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code>
Douglas Gregor2def4832008-11-17 20:34:05 +0000[diff] [blame]1000 will produce a value that describes what kind of name <code>N</code>
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1001 stores. There are 8 options (all of the names are inside
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1002 the <code>DeclarationName</code> class)</p>
1003<dl>
1004 <dt>Identifier</dt>
1005 <dd>The name is a simple
1006 identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the
1007 corresponding <code>IdentifierInfo*</code> pointing to the actual
1008 identifier. Note that C++ overloaded operators (e.g.,
1009 "<code>operator+</code>") are represented as special kinds of
1010 identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code>
1011 function to determine whether an identifier is an overloaded
1012 operator name.</dd>
1013
1014 <dt>ObjCZeroArgSelector, ObjCOneArgSelector,
1015 ObjCMultiArgSelector</dt>
1016 <dd>The name is an Objective-C selector, which can be retrieved as a
1017 <code>Selector</code> instance
1018 via <code>N.getObjCSelector()</code>. The three possible name
1019 kinds for Objective-C reflect an optimization within
1020 the <code>DeclarationName</code> class: both zero- and
1021 one-argument selectors are stored as a
1022 masked <code>IdentifierInfo</code> pointer, and therefore require
1023 very little space, since zero- and one-argument selectors are far
1024 more common than multi-argument selectors (which use a different
1025 structure).</dd>
1026
1027 <dt>CXXConstructorName</dt>
1028 <dd>The name is a C++ constructor
1029 name. Use <code>N.getCXXNameType()</code> to retrieve
1030 the <a href="#QualType">type</a> that this constructor is meant to
1031 construct. The type is always the canonical type, since all
1032 constructors for a given type have the same name.</dd>
1033
1034 <dt>CXXDestructorName</dt>
1035 <dd>The name is a C++ destructor
1036 name. Use <code>N.getCXXNameType()</code> to retrieve
1037 the <a href="#QualType">type</a> whose destructor is being
1038 named. This type is always a canonical type.</dd>
1039
1040 <dt>CXXConversionFunctionName</dt>
1041 <dd>The name is a C++ conversion function. Conversion functions are
1042 named according to the type they convert to, e.g., "<code>operator void
1043 const *</code>". Use <code>N.getCXXNameType()</code> to retrieve
1044 the type that this conversion function converts to. This type is
1045 always a canonical type.</dd>
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1046
1047 <dt>CXXOperatorName</dt>
1048 <dd>The name is a C++ overloaded operator name. Overloaded operators
1049 are named according to their spelling, e.g.,
1050 "<code>operator+</code>" or "<code>operator new
1051 []</code>". Use <code>N.getCXXOverloadedOperator()</code> to
1052 retrieve the overloaded operator (a value of
1053 type <code>OverloadedOperatorKind</code>).</dd>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1054</dl>
1055
1056<p><code>DeclarationName</code>s are cheap to create, copy, and
1057 compare. They require only a single pointer's worth of storage in
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1058 the common cases (identifiers, zero-
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1059 and one-argument Objective-C selectors) and use dense, uniqued
1060 storage for the other kinds of
1061 names. Two <code>DeclarationName</code>s can be compared for
1062 equality (<code>==</code>, <code>!=</code>) using a simple bitwise
1063 comparison, can be ordered
1064 with <code>&lt;</code>, <code>&gt;</code>, <code>&lt;=</code>,
1065 and <code>&gt;=</code> (which provide a lexicographical ordering for
1066 normal identifiers but an unspecified ordering for other kinds of
1067 names), and can be placed into LLVM <code>DenseMap</code>s
1068 and <code>DenseSet</code>s.</p>
1069
1070<p><code>DeclarationName</code> instances can be created in different
1071 ways depending on what kind of name the instance will store. Normal
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1072 identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1073 (<code>Selector</code>) can be implicitly converted
1074 to <code>DeclarationName</code>s. Names for C++ constructors,
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1075 destructors, conversion functions, and overloaded operators can be retrieved from
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1076 the <code>DeclarationNameTable</code>, an instance of which is
1077 available as <code>ASTContext::DeclarationNames</code>. The member
1078 functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>,
Douglas Gregore94ca9e42008-11-18 14:39:36 +0000[diff] [blame]1079 <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively,
1080 return <code>DeclarationName</code> instances for the four kinds of
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1081 C++ special function names.</p>
1082
1083<!-- ======================================================================= -->
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1084<h3 id="DeclContext">Declaration contexts</h3>
1085<!-- ======================================================================= -->
1086<p>Every declaration in a program exists within some <i>declaration
1087 context</i>, such as a translation unit, namespace, class, or
1088 function. Declaration contexts in Clang are represented by
1089 the <code>DeclContext</code> class, from which the various
1090 declaration-context AST nodes
1091 (<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>,
1092 etc.) will derive. The <code>DeclContext</code> class provides
1093 several facilities common to each declaration context:</p>
1094<dl>
1095 <dt>Source-centric vs. Semantics-centric View of Declarations</dt>
1096 <dd><code>DeclContext</code> provides two views of the declarations
1097 stored within a declaration context. The source-centric view
1098 accurately represents the program source code as written, including
1099 multiple declarations of entities where present (see the
1100 section <a href="#Redeclarations">Redeclarations and
1101 Overloads</a>), while the semantics-centric view represents the
1102 program semantics. The two views are kept synchronized by semantic
1103 analysis while the ASTs are being constructed.</dd>
1104
1105 <dt>Storage of declarations within that context</dt>
1106 <dd>Every declaration context can contain some number of
1107 declarations. For example, a C++ class (represented
1108 by <code>RecordDecl</code>) contains various member functions,
1109 fields, nested types, and so on. All of these declarations will be
1110 stored within the <code>DeclContext</code>, and one can iterate
1111 over the declarations via
1112 [<code>DeclContext::decls_begin()</code>,
1113 <code>DeclContext::decls_end()</code>). This mechanism provides
1114 the source-centric view of declarations in the context.</dd>
1115
1116 <dt>Lookup of declarations within that context</dt>
1117 <dd>The <code>DeclContext</code> structure provides efficient name
1118 lookup for names within that declaration context. For example,
1119 if <code>N</code> is a namespace we can look for the
1120 name <code>N::f</code>
1121 using <code>DeclContext::lookup</code>. The lookup itself is
1122 based on a lazily-constructed array (for declaration contexts
1123 with a small number of declarations) or hash table (for
1124 declaration contexts with more declarations). The lookup
1125 operation provides the semantics-centric view of the declarations
1126 in the context.</dd>
1127
1128 <dt>Ownership of declarations</dt>
1129 <dd>The <code>DeclContext</code> owns all of the declarations that
1130 were declared within its declaration context, and is responsible
1131 for the management of their memory as well as their
1132 (de-)serialization.</dd>
1133</dl>
1134
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1135<p>All declarations are stored within a declaration context, and one
1136 can query
1137 information about the context in which each declaration lives. One
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1138 can retrieve the <code>DeclContext</code> that contains a
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1139 particular <code>Decl</code>
1140 using <code>Decl::getDeclContext</code>. However, see the
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1141 section <a href="#LexicalAndSemanticContexts">Lexical and Semantic
1142 Contexts</a> for more information about how to interpret this
1143 context information.</p>
1144
1145<h4 id="Redeclarations">Redeclarations and Overloads</h4>
1146<p>Within a translation unit, it is common for an entity to be
1147declared several times. For example, we might declare a function "f"
1148 and then later re-declare it as part of an inlined definition:</p>
1149
1150<pre>
1151void f(int x, int y, int z = 1);
1152
1153inline void f(int x, int y, int z) { /* ... */ }
1154</pre>
1155
1156<p>The representation of "f" differs in the source-centric and
1157 semantics-centric views of a declaration context. In the
1158 source-centric view, all redeclarations will be present, in the
1159 order they occurred in the source code, making
1160 this view suitable for clients that wish to see the structure of
1161 the source code. In the semantics-centric view, only the most recent "f"
1162 will be found by the lookup, since it effectively replaces the first
1163 declaration of "f".</p>
1164
1165<p>In the semantics-centric view, overloading of functions is
1166 represented explicitly. For example, given two declarations of a
1167 function "g" that are overloaded, e.g.,</p>
1168<pre>
1169void g();
1170void g(int);
1171</pre>
1172<p>the <code>DeclContext::lookup</code> operation will return
1173 an <code>OverloadedFunctionDecl</code> that contains both
1174 declarations of "g". Clients that perform semantic analysis on a
1175 program that is not concerned with the actual source code will
1176 primarily use this semantics-centric view.</p>
1177
1178<h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4>
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1179<p>Each declaration has two potentially different
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1180 declaration contexts: a <i>lexical</i> context, which corresponds to
1181 the source-centric view of the declaration context, and
1182 a <i>semantic</i> context, which corresponds to the
1183 semantics-centric view. The lexical context is accessible
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1184 via <code>Decl::getLexicalDeclContext</code> while the
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1185 semantic context is accessible
Douglas Gregor4afa39d2009-01-20 01:17:11 +0000[diff] [blame]1186 via <code>Decl::getDeclContext</code>, both of which return
Douglas Gregor074149e2009-01-05 19:45:36 +0000[diff] [blame]1187 <code>DeclContext</code> pointers. For most declarations, the two
1188 contexts are identical. For example:</p>
1189
1190<pre>
1191class X {
1192public:
1193 void f(int x);
1194};
1195</pre>
1196
1197<p>Here, the semantic and lexical contexts of <code>X::f</code> are
1198 the <code>DeclContext</code> associated with the
1199 class <code>X</code> (itself stored as a <code>RecordDecl</code> AST
1200 node). However, we can now define <code>X::f</code> out-of-line:</p>
1201
1202<pre>
1203void X::f(int x = 17) { /* ... */ }
1204</pre>
1205
1206<p>This definition of has different lexical and semantic
1207 contexts. The lexical context corresponds to the declaration
1208 context in which the actual declaration occurred in the source
1209 code, e.g., the translation unit containing <code>X</code>. Thus,
1210 this declaration of <code>X::f</code> can be found by traversing
1211 the declarations provided by
1212 [<code>decls_begin()</code>, <code>decls_end()</code>) in the
1213 translation unit.</p>
1214
1215<p>The semantic context of <code>X::f</code> corresponds to the
1216 class <code>X</code>, since this member function is (semantically) a
1217 member of <code>X</code>. Lookup of the name <code>f</code> into
1218 the <code>DeclContext</code> associated with <code>X</code> will
1219 then return the definition of <code>X::f</code> (including
1220 information about the default argument).</p>
1221
1222<h4 id="TransparentContexts">Transparent Declaration Contexts</h4>
1223<p>In C and C++, there are several contexts in which names that are
1224 logically declared inside another declaration will actually "leak"
1225 out into the enclosing scope from the perspective of name
1226 lookup. The most obvious instance of this behavior is in
1227 enumeration types, e.g.,</p>
1228<pre>
1229enum Color {
1230 Red,
1231 Green,
1232 Blue
1233};
1234</pre>
1235
1236<p>Here, <code>Color</code> is an enumeration, which is a declaration
1237 context that contains the
1238 enumerators <code>Red</code>, <code>Green</code>,
1239 and <code>Blue</code>. Thus, traversing the list of declarations
1240 contained in the enumeration <code>Color</code> will
1241 yield <code>Red</code>, <code>Green</code>,
1242 and <code>Blue</code>. However, outside of the scope
1243 of <code>Color</code> one can name the enumerator <code>Red</code>
1244 without qualifying the name, e.g.,</p>
1245
1246<pre>
1247Color c = Red;
1248</pre>
1249
1250<p>There are other entities in C++ that provide similar behavior. For
1251 example, linkage specifications that use curly braces:</p>
1252
1253<pre>
1254extern "C" {
1255 void f(int);
1256 void g(int);
1257}
1258// f and g are visible here
1259</pre>
1260
1261<p>For source-level accuracy, we treat the linkage specification and
1262 enumeration type as a
1263 declaration context in which its enclosed declarations ("Red",
1264 "Green", and "Blue"; "f" and "g")
1265 are declared. However, these declarations are visible outside of the
1266 scope of the declaration context.</p>
1267
1268<p>These language features (and several others, described below) have
1269 roughly the same set of
1270 requirements: declarations are declared within a particular lexical
1271 context, but the declarations are also found via name lookup in
1272 scopes enclosing the declaration itself. This feature is implemented
1273 via <i>transparent</i> declaration contexts
1274 (see <code>DeclContext::isTransparentContext()</code>), whose
1275 declarations are visible in the nearest enclosing non-transparent
1276 declaration context. This means that the lexical context of the
1277 declaration (e.g., an enumerator) will be the
1278 transparent <code>DeclContext</code> itself, as will the semantic
1279 context, but the declaration will be visible in every outer context
1280 up to and including the first non-transparent declaration context (since
1281 transparent declaration contexts can be nested).</p>
1282
1283<p>The transparent <code>DeclContexts</code> are:</p>
1284<ul>
1285 <li>Enumerations (but not C++0x "scoped enumerations"):
1286 <pre>
1287enum Color {
1288 Red,
1289 Green,
1290 Blue
1291};
1292// Red, Green, and Blue are in scope
1293 </pre></li>
1294 <li>C++ linkage specifications:
1295 <pre>
1296extern "C" {
1297 void f(int);
1298 void g(int);
1299}
1300// f and g are in scope
1301 </pre></li>
1302 <li>Anonymous unions and structs:
1303 <pre>
1304struct LookupTable {
1305 bool IsVector;
1306 union {
1307 std::vector&lt;Item&gt; *Vector;
1308 std::set&lt;Item&gt; *Set;
1309 };
1310};
1311
1312LookupTable LT;
1313LT.Vector = 0; // Okay: finds Vector inside the unnamed union
1314 </pre>
1315 </li>
1316 <li>C++0x inline namespaces:
1317<pre>
1318namespace mylib {
1319 inline namespace debug {
1320 class X;
1321 }
1322}
1323mylib::X *xp; // okay: mylib::X refers to mylib::debug::X
1324</pre>
1325</li>
1326</ul>
1327
1328
1329<h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4>
1330<p>C++ namespaces have the interesting--and, so far, unique--property that
1331the namespace can be defined multiple times, and the declarations
1332provided by each namespace definition are effectively merged (from
1333the semantic point of view). For example, the following two code
1334snippets are semantically indistinguishable:</p>
1335<pre>
1336// Snippet #1:
1337namespace N {
1338 void f();
1339}
1340namespace N {
1341 void f(int);
1342}
1343
1344// Snippet #2:
1345namespace N {
1346 void f();
1347 void f(int);
1348}
1349</pre>
1350
1351<p>In Clang's representation, the source-centric view of declaration
1352 contexts will actually have two separate <code>NamespaceDecl</code>
1353 nodes in Snippet #1, each of which is a declaration context that
1354 contains a single declaration of "f". However, the semantics-centric
1355 view provided by name lookup into the namespace <code>N</code> for
1356 "f" will return an <code>OverloadedFunctionDecl</code> that contains
1357 both declarations of "f".</p>
1358
1359<p><code>DeclContext</code> manages multiply-defined declaration
1360 contexts internally. The
1361 function <code>DeclContext::getPrimaryContext</code> retrieves the
1362 "primary" context for a given <code>DeclContext</code> instance,
1363 which is the <code>DeclContext</code> responsible for maintaining
1364 the lookup table used for the semantics-centric view. Given the
1365 primary context, one can follow the chain
1366 of <code>DeclContext</code> nodes that define additional
1367 declarations via <code>DeclContext::getNextContext</code>. Note that
1368 these functions are used internally within the lookup and insertion
1369 methods of the <code>DeclContext</code>, so the vast majority of
1370 clients can ignore them.</p>
1371
1372<!-- ======================================================================= -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1373<h3 id="CFG">The <tt>CFG</tt> class</h3>
1374<!-- ======================================================================= -->
1375
1376<p>The <tt>CFG</tt> class is designed to represent a source-level
1377control-flow graph for a single statement (<tt>Stmt*</tt>). Typically
1378instances of <tt>CFG</tt> are constructed for function bodies (usually
1379an instance of <tt>CompoundStmt</tt>), but can also be instantiated to
1380represent the control-flow of any class that subclasses <tt>Stmt</tt>,
1381which includes simple expressions. Control-flow graphs are especially
1382useful for performing
1383<a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow-
1384or path-sensitive</a> program analyses on a given function.</p>
1385
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1386<!-- ============ -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1387<h4>Basic Blocks</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1388<!-- ============ -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1389
1390<p>Concretely, an instance of <tt>CFG</tt> is a collection of basic
1391blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which
1392simply contains an ordered sequence of <tt>Stmt*</tt> (each referring
1393to statements in the AST). The ordering of statements within a block
1394indicates unconditional flow of control from one statement to the
1395next. <a href="#ConditionalControlFlow">Conditional control-flow</a>
1396is represented using edges between basic blocks. The statements
1397within a given <tt>CFGBlock</tt> can be traversed using
1398the <tt>CFGBlock::*iterator</tt> interface.</p>
1399
1400<p>
Ted Kremenek18e17e72007-10-18 22:50:52 +0000[diff] [blame]1401A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1402the control-flow graph it represents. Each <tt>CFGBlock</tt> within a
1403CFG is also uniquely numbered (accessible
1404via <tt>CFGBlock::getBlockID()</tt>). Currently the number is
1405based on the ordering the blocks were created, but no assumptions
1406should be made on how <tt>CFGBlock</tt>s are numbered other than their
1407numbers are unique and that they are numbered from 0..N-1 (where N is
1408the number of basic blocks in the CFG).</p>
1409
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1410<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1411<h4>Entry and Exit Blocks</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1412<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1413
1414Each instance of <tt>CFG</tt> contains two special blocks:
1415an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which
1416has no incoming edges, and an <i>exit</i> block (accessible
1417via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither
1418block contains any statements, and they serve the role of providing a
1419clear entrance and exit for a body of code such as a function body.
1420The presence of these empty blocks greatly simplifies the
1421implementation of many analyses built on top of CFGs.
1422
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1423<!-- ===================================================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1424<h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4>
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1425<!-- ===================================================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1426
1427<p>Conditional control-flow (such as those induced by if-statements
1428and loops) is represented as edges between <tt>CFGBlock</tt>s.
1429Because different C language constructs can induce control-flow,
1430each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that
1431represents the <i>terminator</i> of the block. A terminator is simply
1432the statement that caused the control-flow, and is used to identify
1433the nature of the conditional control-flow between blocks. For
1434example, in the case of an if-statement, the terminator refers to
1435the <tt>IfStmt</tt> object in the AST that represented the given
1436branch.</p>
1437
1438<p>To illustrate, consider the following code example:</p>
1439
1440<code>
1441int foo(int x) {<br>
1442&nbsp;&nbsp;x = x + 1;<br>
1443<br>
1444&nbsp;&nbsp;if (x > 2) x++;<br>
1445&nbsp;&nbsp;else {<br>
1446&nbsp;&nbsp;&nbsp;&nbsp;x += 2;<br>
1447&nbsp;&nbsp;&nbsp;&nbsp;x *= 2;<br>
1448&nbsp;&nbsp;}<br>
1449<br>
1450&nbsp;&nbsp;return x;<br>
1451}
1452</code>
1453
1454<p>After invoking the parser+semantic analyzer on this code fragment,
1455the AST of the body of <tt>foo</tt> is referenced by a
1456single <tt>Stmt*</tt>. We can then construct an instance
1457of <tt>CFG</tt> representing the control-flow graph of this function
1458body by single call to a static class method:</p>
1459
1460<code>
1461&nbsp;&nbsp;Stmt* FooBody = ...<br>
1462&nbsp;&nbsp;CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody);
1463</code>
1464
1465<p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt>
1466to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no
1467longer needed.</p>
1468
1469<p>Along with providing an interface to iterate over
1470its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods
1471that are useful for debugging and visualizing CFGs. For example, the
1472method
1473<tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to
1474standard error. This is especially useful when one is using a
1475debugger such as gdb. For example, here is the output
1476of <tt>FooCFG->dump()</tt>:</p>
1477
1478<code>
1479&nbsp;[ B5 (ENTRY) ]<br>
1480&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (0):<br>
1481&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B4<br>
1482<br>
1483&nbsp;[ B4 ]<br>
1484&nbsp;&nbsp;&nbsp;&nbsp;1: x = x + 1<br>
1485&nbsp;&nbsp;&nbsp;&nbsp;2: (x > 2)<br>
1486&nbsp;&nbsp;&nbsp;&nbsp;<b>T: if [B4.2]</b><br>
1487&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B5<br>
1488&nbsp;&nbsp;&nbsp;&nbsp;Successors (2): B3 B2<br>
1489<br>
1490&nbsp;[ B3 ]<br>
1491&nbsp;&nbsp;&nbsp;&nbsp;1: x++<br>
1492&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
1493&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
1494<br>
1495&nbsp;[ B2 ]<br>
1496&nbsp;&nbsp;&nbsp;&nbsp;1: x += 2<br>
1497&nbsp;&nbsp;&nbsp;&nbsp;2: x *= 2<br>
1498&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
1499&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
1500<br>
1501&nbsp;[ B1 ]<br>
1502&nbsp;&nbsp;&nbsp;&nbsp;1: return x;<br>
1503&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (2): B2 B3<br>
1504&nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B0<br>
1505<br>
1506&nbsp;[ B0 (EXIT) ]<br>
1507&nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B1<br>
1508&nbsp;&nbsp;&nbsp;&nbsp;Successors (0):
1509</code>
1510
1511<p>For each block, the pretty-printed output displays for each block
1512the number of <i>predecessor</i> blocks (blocks that have outgoing
1513control-flow to the given block) and <i>successor</i> blocks (blocks
1514that have control-flow that have incoming control-flow from the given
1515block). We can also clearly see the special entry and exit blocks at
1516the beginning and end of the pretty-printed output. For the entry
1517block (block B5), the number of predecessor blocks is 0, while for the
1518exit block (block B0) the number of successor blocks is 0.</p>
1519
1520<p>The most interesting block here is B4, whose outgoing control-flow
1521represents the branching caused by the sole if-statement
1522in <tt>foo</tt>. Of particular interest is the second statement in
1523the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed
1524as <b><tt>if [B4.2]</tt></b>. The second statement represents the
1525evaluation of the condition of the if-statement, which occurs before
1526the actual branching of control-flow. Within the <tt>CFGBlock</tt>
1527for B4, the <tt>Stmt*</tt> for the second statement refers to the
1528actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus
1529pointers to subclasses of <tt>Expr</tt> can appear in the list of
1530statements in a block, and not just subclasses of <tt>Stmt</tt> that
1531refer to proper C statements.</p>
1532
1533<p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt>
1534object in the AST. The pretty-printer outputs <b><tt>if
1535[B4.2]</tt></b> because the condition expression of the if-statement
1536has an actual place in the basic block, and thus the terminator is
1537essentially
1538<i>referring</i> to the expression that is the second statement of
1539block B4 (i.e., B4.2). In this manner, conditions for control-flow
1540(which also includes conditions for loops and switch statements) are
1541hoisted into the actual basic block.</p>
1542
Chris Lattner62fd2782008-11-22 21:41:31 +0000[diff] [blame]1543<!-- ===================== -->
1544<!-- <h4>Implicit Control-Flow</h4> -->
1545<!-- ===================== -->
Ted Kremenek8bc05712007-10-10 23:01:43 +0000[diff] [blame]1546
1547<!--
1548<p>A key design principle of the <tt>CFG</tt> class was to not require
1549any transformations to the AST in order to represent control-flow.
1550Thus the <tt>CFG</tt> does not perform any "lowering" of the
1551statements in an AST: loops are not transformed into guarded gotos,
1552short-circuit operations are not converted to a set of if-statements,
1553and so on.</p>
1554-->
Ted Kremenek17a295d2008-06-11 06:19:49 +0000[diff] [blame]1555
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1556
1557<!-- ======================================================================= -->
1558<h3 id="Constants">Constant Folding in the Clang AST</h3>
1559<!-- ======================================================================= -->
1560
1561<p>There are several places where constants and constant folding matter a lot to
1562the Clang front-end. First, in general, we prefer the AST to retain the source
1563code as close to how the user wrote it as possible. This means that if they
1564wrote "5+4", we want to keep the addition and two constants in the AST, we don't
1565want to fold to "9". This means that constant folding in various ways turns
1566into a tree walk that needs to handle the various cases.</p>
1567
1568<p>However, there are places in both C and C++ that require constants to be
1569folded. For example, the C standard defines what an "integer constant
1570expression" (i-c-e) is with very precise and specific requirements. The
1571language then requires i-c-e's in a lot of places (for example, the size of a
1572bitfield, the value for a case statement, etc). For these, we have to be able
1573to constant fold the constants, to do semantic checks (e.g. verify bitfield size
1574is non-negative and that case statements aren't duplicated). We aim for Clang
1575to be very pedantic about this, diagnosing cases when the code does not use an
1576i-c-e where one is required, but accepting the code unless running with
1577<tt>-pedantic-errors</tt>.</p>
1578
1579<p>Things get a little bit more tricky when it comes to compatibility with
1580real-world source code. Specifically, GCC has historically accepted a huge
1581superset of expressions as i-c-e's, and a lot of real world code depends on this
1582unfortuate accident of history (including, e.g., the glibc system headers). GCC
1583accepts anything its "fold" optimizer is capable of reducing to an integer
1584constant, which means that the definition of what it accepts changes as its
1585optimizer does. One example is that GCC accepts things like "case X-X:" even
1586when X is a variable, because it can fold this to 0.</p>
1587
1588<p>Another issue are how constants interact with the extensions we support, such
1589as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99
1590obviously does not specify the semantics of any of these extensions, and the
1591definition of i-c-e does not include them. However, these extensions are often
1592used in real code, and we have to have a way to reason about them.</p>
1593
1594<p>Finally, this is not just a problem for semantic analysis. The code
1595generator and other clients have to be able to fold constants (e.g. to
1596initialize global variables) and has to handle a superset of what C99 allows.
1597Further, these clients can benefit from extended information. For example, we
1598know that "foo()||1" always evaluates to true, but we can't replace the
1599expression with true because it has side effects.</p>
1600
1601<!-- ======================= -->
1602<h4>Implementation Approach</h4>
1603<!-- ======================= -->
1604
1605<p>After trying several different approaches, we've finally converged on a
1606design (Note, at the time of this writing, not all of this has been implemented,
1607consider this a design goal!). Our basic approach is to define a single
1608recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is
1609implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar'
1610type (integer, fp, complex, or pointer) this method returns the following
1611information:</p>
1612
1613<ul>
1614<li>Whether the expression is an integer constant expression, a general
1615 constant that was folded but has no side effects, a general constant that
1616 was folded but that does have side effects, or an uncomputable/unfoldable
1617 value.
1618</li>
1619<li>If the expression was computable in any way, this method returns the APValue
1620 for the result of the expression.</li>
1621<li>If the expression is not evaluatable at all, this method returns
1622 information on one of the problems with the expression. This includes a
1623 SourceLocation for where the problem is, and a diagnostic ID that explains
1624 the problem. The diagnostic should be have ERROR type.</li>
1625<li>If the expression is not an integer constant expression, this method returns
1626 information on one of the problems with the expression. This includes a
1627 SourceLocation for where the problem is, and a diagnostic ID that explains
1628 the problem. The diagnostic should be have EXTENSION type.</li>
1629</ul>
1630
1631<p>This information gives various clients the flexibility that they want, and we
1632will eventually have some helper methods for various extensions. For example,
1633Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which
1634calls Evaluate on the expression. If the expression is not foldable, the error
1635is emitted, and it would return true. If the expression is not an i-c-e, the
1636EXTENSION diagnostic is emitted. Finally it would return false to indicate that
1637the AST is ok.</p>
1638
1639<p>Other clients can use the information in other ways, for example, codegen can
1640just use expressions that are foldable in any way.</p>
1641
1642<!-- ========== -->
1643<h4>Extensions</h4>
1644<!-- ========== -->
1645
Chris Lattner552de0a2008-11-23 08:16:56 +0000[diff] [blame]1646<p>This section describes how some of the various extensions Clang supports
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1647interacts with constant evaluation:</p>
1648
1649<ul>
1650<li><b><tt>__extension__</tt></b>: The expression form of this extension causes
1651 any evaluatable subexpression to be accepted as an integer constant
1652 expression.</li>
1653<li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer
Chris Lattner28daa532008-12-12 06:55:44 +0000[diff] [blame]1654 constant expression) if the operand is any evaluatable constant. As a
1655 special case, if <tt>__builtin_constant_p</tt> is the (potentially
1656 parenthesized) condition of a conditional operator expression ("?:"), only
Chris Lattner42b83dd2008-12-12 18:00:51 +0000[diff] [blame]1657 the true side of the conditional operator is considered, and it is evaluated
1658 with full constant folding.</li>
Chris Lattner7bad1992008-11-16 21:48:07 +0000[diff] [blame]1659<li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an
1660 integer constant expression, but we accept any constant as an "extension of
1661 an extension". This only evaluates one operand depending on which way the
1662 condition evaluates.</li>
1663<li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer
1664 constant expression.</li>
1665<li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a
1666 floating-point literal.</li>
1667<li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as
1668 general constant expressions.</li>
1669</ul>
1670
1671
1672
1673
Ted Kremenek17a295d2008-06-11 06:19:49 +0000[diff] [blame]1674</div>
1675</body>
Douglas Gregor2e1cd422008-11-17 14:58:09 +0000[diff] [blame]1676</html>