1 <html> 2 <head> 3 <title>"Clang" CFE Internals Manual</title> 4 <link type="text/css" rel="stylesheet" href="../menu.css" /> 5 <link type="text/css" rel="stylesheet" href="../content.css" /> 6 <style type="text/css"> 7 td { 8 vertical-align: top; 9 } 10 </style> 11 </head> 12 <body> 13 14 <!--#include virtual="../menu.html.incl"--> 15 16 <div id="content"> 17 18 <h1>"Clang" CFE Internals Manual</h1> 19 20 <ul> 21 <li><a href="#intro">Introduction</a></li> 22 <li><a href="#libsupport">LLVM Support Library</a></li> 23 <li><a href="#libbasic">The Clang 'Basic' Library</a> 24 <ul> 25 <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li> 26 <li><a href="#SourceLocation">The SourceLocation and SourceManager 27 classes</a></li> 28 <li><a href="#SourceRange">SourceRange and CharSourceRange</a></li> 29 </ul> 30 </li> 31 <li><a href="#libdriver">The Driver Library</a> 32 <ul> 33 </ul> 34 </li> 35 <li><a href="#pch">Precompiled Headers</a> 36 <li><a href="#libfrontend">The Frontend Library</a> 37 <ul> 38 </ul> 39 </li> 40 <li><a href="#liblex">The Lexer and Preprocessor Library</a> 41 <ul> 42 <li><a href="#Token">The Token class</a></li> 43 <li><a href="#Lexer">The Lexer class</a></li> 44 <li><a href="#AnnotationToken">Annotation Tokens</a></li> 45 <li><a href="#TokenLexer">The TokenLexer class</a></li> 46 <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li> 47 </ul> 48 </li> 49 <li><a href="#libparse">The Parser Library</a> 50 <ul> 51 </ul> 52 </li> 53 <li><a href="#libast">The AST Library</a> 54 <ul> 55 <li><a href="#Type">The Type class and its subclasses</a></li> 56 <li><a href="#QualType">The QualType class</a></li> 57 <li><a href="#DeclarationName">Declaration names</a></li> 58 <li><a href="#DeclContext">Declaration contexts</a> 59 <ul> 60 <li><a href="#Redeclarations">Redeclarations and Overloads</a></li> 61 <li><a href="#LexicalAndSemanticContexts">Lexical and Semantic 62 Contexts</a></li> 63 <li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li> 64 <li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li> 65 </ul> 66 </li> 67 <li><a href="#CFG">The CFG class</a></li> 68 <li><a href="#Constants">Constant Folding in the Clang AST</a></li> 69 </ul> 70 </li> 71 <li><a href="#Howtos">Howto guides</a> 72 <ul> 73 <li><a href="#AddingAttributes">How to add an attribute</a></li> 74 <li><a href="#AddingExprStmt">How to add a new expression or statement</a></li> 75 </ul> 76 </li> 77 </ul> 78 79 80 <!-- ======================================================================= --> 81 <h2 id="intro">Introduction</h2> 82 <!-- ======================================================================= --> 83 84 <p>This document describes some of the more important APIs and internal design 85 decisions made in the Clang C front-end. The purpose of this document is to 86 both capture some of this high level information and also describe some of the 87 design decisions behind it. This is meant for people interested in hacking on 88 Clang, not for end-users. The description below is categorized by 89 libraries, and does not describe any of the clients of the libraries.</p> 90 91 <!-- ======================================================================= --> 92 <h2 id="libsupport">LLVM Support Library</h2> 93 <!-- ======================================================================= --> 94 95 <p>The LLVM libsupport library provides many underlying libraries and 96 <a href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>, 97 including command line option processing, various containers and a system 98 abstraction layer, which is used for file system access.</p> 99 100 <!-- ======================================================================= --> 101 <h2 id="libbasic">The Clang 'Basic' Library</h2> 102 <!-- ======================================================================= --> 103 104 <p>This library certainly needs a better name. The 'basic' library contains a 105 number of low-level utilities for tracking and manipulating source buffers, 106 locations within the source buffers, diagnostics, tokens, target abstraction, 107 and information about the subset of the language being compiled for.</p> 108 109 <p>Part of this infrastructure is specific to C (such as the TargetInfo class), 110 other parts could be reused for other non-C-based languages (SourceLocation, 111 SourceManager, Diagnostics, FileManager). When and if there is future demand 112 we can figure out if it makes sense to introduce a new library, move the general 113 classes somewhere else, or introduce some other solution.</p> 114 115 <p>We describe the roles of these classes in order of their dependencies.</p> 116 117 118 <!-- ======================================================================= --> 119 <h3 id="Diagnostics">The Diagnostics Subsystem</h3> 120 <!-- ======================================================================= --> 121 122 <p>The Clang Diagnostics subsystem is an important part of how the compiler 123 communicates with the human. Diagnostics are the warnings and errors produced 124 when the code is incorrect or dubious. In Clang, each diagnostic produced has 125 (at the minimum) a unique ID, an English translation associated with it, a <a 126 href="#SourceLocation">SourceLocation</a> to "put the caret", and a severity (e.g. 127 <tt>WARNING</tt> or <tt>ERROR</tt>). They can also optionally include a number 128 of arguments to the dianostic (which fill in "%0"'s in the string) as well as a 129 number of source ranges that related to the diagnostic.</p> 130 131 <p>In this section, we'll be giving examples produced by the Clang command line 132 driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many 133 different ways</a> depending on how the DiagnosticClient interface is 134 implemented. A representative example of a diagnostic is:</p> 135 136 <pre> 137 t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float') 138 <font color="darkgreen">P = (P-42) + Gamma*4;</font> 139 <font color="blue">~~~~~~ ^ ~~~~~~~</font> 140 </pre> 141 142 <p>In this example, you can see the English translation, the severity (error), 143 you can see the source location (the caret ("^") and file/line/column info), 144 the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex 145 float"). You'll have to believe me that there is a unique ID backing the 146 diagnostic :).</p> 147 148 <p>Getting all of this to happen has several steps and involves many moving 149 pieces, this section describes them and talks about best practices when adding 150 a new diagnostic.</p> 151 152 <!-- ============================= --> 153 <h4>The Diagnostic*Kinds.td files</h4> 154 <!-- ============================= --> 155 156 <p>Diagnostics are created by adding an entry to one of the <tt> 157 clang/Basic/Diagnostic*Kinds.td</tt> files, depending on what library will 158 be using it. From this file, tblgen generates the unique ID of the diagnostic, 159 the severity of the diagnostic and the English translation + format string.</p> 160 161 <p>There is little sanity with the naming of the unique ID's right now. Some 162 start with err_, warn_, ext_ to encode the severity into the name. Since the 163 enum is referenced in the C++ code that produces the diagnostic, it is somewhat 164 useful for it to be reasonably short.</p> 165 166 <p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>, 167 <tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The 168 <tt>ERROR</tt> severity is used for diagnostics indicating the program is never 169 acceptable under any circumstances. When an error is emitted, the AST for the 170 input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt> 171 severities are used for extensions to the language that Clang accepts. This 172 means that Clang fully understands and can represent them in the AST, but we 173 produce diagnostics to tell the user their code is non-portable. The difference 174 is that the former are ignored by default, and the later warn by default. The 175 <tt>WARNING</tt> severity is used for constructs that are valid in the currently 176 selected source language but that are dubious in some way. The <tt>NOTE</tt> 177 level is used to staple more information onto previous diagnostics.</p> 178 179 <p>These <em>severities</em> are mapped into a smaller set (the 180 Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>, 181 <tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics 182 subsystem based on various configuration options. Clang internally supports a 183 fully fine grained mapping mechanism that allows you to map almost any 184 diagnostic to the output level that you want. The only diagnostics that cannot 185 be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously 186 emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to 187 <tt>Fatal</tt> (it is not possible to turn an error into a warning, 188 for example).</p> 189 190 <p>Diagnostic mappings are used in many ways. For example, if the user 191 specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if 192 they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is 193 used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc. 194 </p> 195 196 <p> 197 Mapping to <tt>Fatal</tt> should only be used for diagnostics that are 198 considered so severe that error recovery won't be able to recover sensibly from 199 them (thus spewing a ton of bogus errors). One example of this class of error 200 are failure to #include a file. 201 </p> 202 203 <!-- ================= --> 204 <h4>The Format String</h4> 205 <!-- ================= --> 206 207 <p>The format string for the diagnostic is very simple, but it has some power. 208 It takes the form of a string in English with markers that indicate where and 209 how arguments to the diagnostic are inserted and formatted. For example, here 210 are some simple format strings:</p> 211 212 <pre> 213 "binary integer literals are an extension" 214 "format string contains '\\0' within the string body" 215 "more '<b>%%</b>' conversions than data arguments" 216 "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)" 217 "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator" 218 " (has <b>%1</b> parameter<b>%s1</b>)" 219 </pre> 220 221 <p>These examples show some important points of format strings. You can use any 222 plain ASCII character in the diagnostic string except "%" without a problem, 223 but these are C strings, so you have to use and be aware of all the C escape 224 sequences (as in the second example). If you want to produce a "%" in the 225 output, use the "%%" escape sequence, like the third diagnostic. Finally, 226 Clang uses the "%...[digit]" sequences to specify where and how arguments to 227 the diagnostic are formatted.</p> 228 229 <p>Arguments to the diagnostic are numbered according to how they are specified 230 by the C++ code that <a href="#producingdiag">produces them</a>, and are 231 referenced by <tt>%0</tt> .. <tt>%9</tt>. If you have more than 10 arguments 232 to your diagnostic, you are doing something wrong :). Unlike printf, there 233 is no requirement that arguments to the diagnostic end up in the output in 234 the same order as they are specified, you could have a format string with 235 <tt>"%1 %0"</tt> that swaps them, for example. The text in between the 236 percent and digit are formatting instructions. If there are no instructions, 237 the argument is just turned into a string and substituted in.</p> 238 239 <p>Here are some "best practices" for writing the English format string:</p> 240 241 <ul> 242 <li>Keep the string short. It should ideally fit in the 80 column limit of the 243 <tt>DiagnosticKinds.td</tt> file. This avoids the diagnostic wrapping when 244 printed, and forces you to think about the important point you are conveying 245 with the diagnostic.</li> 246 <li>Take advantage of location information. The user will be able to see the 247 line and location of the caret, so you don't need to tell them that the 248 problem is with the 4th argument to the function: just point to it.</li> 249 <li>Do not capitalize the diagnostic string, and do not end it with a 250 period.</li> 251 <li>If you need to quote something in the diagnostic string, use single 252 quotes.</li> 253 </ul> 254 255 <p>Diagnostics should never take random English strings as arguments: you 256 shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like 257 <tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing 258 this prevents <a href="#translation">translating</a> the Clang diagnostics to 259 other languages (because they'll get random English words in their otherwise 260 localized diagnostic). The exceptions to this are C/C++ language keywords 261 (e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note 262 that things like "pointer" and "reference" are not keywords. On the other 263 hand, you <em>can</em> include anything that comes from the user's source code, 264 including variable names, types, labels, etc. The 'select' format can be 265 used to achieve this sort of thing in a localizable way, see below.</p> 266 267 <!-- ==================================== --> 268 <h4>Formatting a Diagnostic Argument</a></h4> 269 <!-- ==================================== --> 270 271 <p>Arguments to diagnostics are fully typed internally, and come from a couple 272 different classes: integers, types, names, and random strings. Depending on 273 the class of the argument, it can be optionally formatted in different ways. 274 This gives the DiagnosticClient information about what the argument means 275 without requiring it to use a specific presentation (consider this MVC for 276 Clang :).</p> 277 278 <p>Here are the different diagnostic argument formats currently supported by 279 Clang:</p> 280 281 <table> 282 <tr><td colspan="2"><b>"s" format</b></td></tr> 283 <tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr> 284 <tr><td>Class:</td><td>Integers</td></tr> 285 <tr><td>Description:</td><td>This is a simple formatter for integers that is 286 useful when producing English diagnostics. When the integer is 1, it prints 287 as nothing. When the integer is not 1, it prints as "s". This allows some 288 simple grammatical forms to be to be handled correctly, and eliminates the 289 need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr> 290 291 <tr><td colspan="2"><b>"select" format</b></td></tr> 292 <tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2 293 operator"</tt></td></tr> 294 <tr><td>Class:</td><td>Integers</td></tr> 295 <tr><td>Description:</td><td><p>This format specifier is used to merge multiple 296 related diagnostics together into one common one, without requiring the 297 difference to be specified as an English string argument. Instead of 298 specifying the string, the diagnostic gets an integer argument and the 299 format string selects the numbered option. In this case, the "%2" value 300 must be an integer in the range [0..2]. If it is 0, it prints 'unary', if 301 it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This 302 allows other language translations to substitute reasonable words (or entire 303 phrases) based on the semantics of the diagnostic instead of having to do 304 things textually.</p> 305 <p>The selected string does undergo formatting.</p></td></tr> 306 307 <tr><td colspan="2"><b>"plural" format</b></td></tr> 308 <tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to 309 your computer"</tt></td></tr> 310 <tr><td>Class:</td><td>Integers</td></tr> 311 <tr><td>Description:</td><td><p>This is a formatter for complex plural forms. 312 It is designed to handle even the requirements of languages with very 313 complex plural forms, as many Baltic languages have. The argument consists 314 of a series of expression/form pairs, separated by ':', where the first form 315 whose expression evaluates to true is the result of the modifier.</p> 316 <p>An expression can be empty, in which case it is always true. See the 317 example at the top. Otherwise, it is a series of one or more numeric 318 conditions, separated by ','. If any condition matches, the expression 319 matches. Each numeric condition can take one of three forms.</p> 320 <ul> 321 <li>number: A simple decimal number matches if the argument is the same 322 as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li> 323 <li>range: A range in square brackets matches if the argument is within 324 the range. Then range is inclusive on both ends. Example: 325 <tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li> 326 <li>modulo: A modulo operator is followed by a number, and 327 equals sign and either a number or a range. The tests are the 328 same as for plain 329 numbers and ranges, but the argument is taken modulo the number first. 330 Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything 331 else}1"</tt></li> 332 </ul> 333 <p>The parser is very unforgiving. A syntax error, even whitespace, will 334 abort, as will a failure to match the argument against any 335 expression.</p></td></tr> 336 337 <tr><td colspan="2"><b>"ordinal" format</b></td></tr> 338 <tr><td>Example:</td><td><tt>"ambiguity in %ordinal0 argument"</tt></td></tr> 339 <tr><td>Class:</td><td>Integers</td></tr> 340 <tr><td>Description:</td><td><p>This is a formatter which represents the 341 argument number as an ordinal: the value <tt>1</tt> becomes <tt>1st</tt>, 342 <tt>3</tt> becomes <tt>3rd</tt>, and so on. Values less than <tt>1</tt> 343 are not supported.</p> 344 <p>This formatter is currently hard-coded to use English ordinals.</p></td></tr> 345 346 <tr><td colspan="2"><b>"objcclass" format</b></td></tr> 347 <tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr> 348 <tr><td>Class:</td><td>DeclarationName</td></tr> 349 <tr><td>Description:</td><td><p>This is a simple formatter that indicates the 350 DeclarationName corresponds to an Objective-C class method selector. As 351 such, it prints the selector with a leading '+'.</p></td></tr> 352 353 <tr><td colspan="2"><b>"objcinstance" format</b></td></tr> 354 <tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr> 355 <tr><td>Class:</td><td>DeclarationName</td></tr> 356 <tr><td>Description:</td><td><p>This is a simple formatter that indicates the 357 DeclarationName corresponds to an Objective-C instance method selector. As 358 such, it prints the selector with a leading '-'.</p></td></tr> 359 360 <tr><td colspan="2"><b>"q" format</b></td></tr> 361 <tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr> 362 <tr><td>Class:</td><td>NamedDecl*</td></tr> 363 <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> 364 365 </table> 366 367 <p>It is really easy to add format specifiers to the Clang diagnostics system, 368 but they should be discussed before they are added. If you are creating a lot 369 of repetitive diagnostics and/or have an idea for a useful formatter, please 370 bring it up on the cfe-dev mailing list.</p> 371 372 <!-- ===================================================== --> 373 <h4 id="producingdiag">Producing the Diagnostic</h4> 374 <!-- ===================================================== --> 375 376 <p>Now that you've created the diagnostic in the DiagnosticKinds.td file, you 377 need to write the code that detects the condition in question and emits the 378 new diagnostic. Various components of Clang (e.g. the preprocessor, Sema, 379 etc) provide a helper function named "Diag". It creates a diagnostic and 380 accepts the arguments, ranges, and other information that goes along with 381 it.</p> 382 383 <p>For example, the binary expression error comes from code like this:</p> 384 385 <pre> 386 if (various things that are bad) 387 Diag(Loc, diag::err_typecheck_invalid_operands) 388 << lex->getType() << rex->getType() 389 << lex->getSourceRange() << rex->getSourceRange(); 390 </pre> 391 392 <p>This shows that use of the Diag method: they take a location (a <a 393 href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value 394 (which matches the name from DiagnosticKinds.td). If the diagnostic takes 395 arguments, they are specified with the << operator: the first argument 396 becomes %0, the second becomes %1, etc. The diagnostic interface allows you to 397 specify arguments of many different types, including <tt>int</tt> and 398 <tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and 399 <tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and 400 <tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc. 401 SourceRanges are also specified with the << operator, but do not have a 402 specific ordering requirement.</p> 403 404 <p>As you can see, adding and producing a diagnostic is pretty straightforward. 405 The hard part is deciding exactly what you need to say to help the user, picking 406 a suitable wording, and providing the information needed to format it correctly. 407 The good news is that the call site that issues a diagnostic should be 408 completely independent of how the diagnostic is formatted and in what language 409 it is rendered. 410 </p> 411 412 <!-- ==================================================== --> 413 <h4 id="fix-it-hints">Fix-It Hints</h4> 414 <!-- ==================================================== --> 415 416 <p>In some cases, the front end emits diagnostics when it is clear 417 that some small change to the source code would fix the problem. For 418 example, a missing semicolon at the end of a statement or a use of 419 deprecated syntax that is easily rewritten into a more modern form. 420 Clang tries very hard to emit the diagnostic and recover gracefully 421 in these and other cases.</p> 422 423 <p>However, for these cases where the fix is obvious, the diagnostic 424 can be annotated with a hint (referred to as a "fix-it hint") that 425 describes how to change the code referenced by the diagnostic to fix 426 the problem. For example, it might add the missing semicolon at the 427 end of the statement or rewrite the use of a deprecated construct 428 into something more palatable. Here is one such example from the C++ 429 front end, where we warn about the right-shift operator changing 430 meaning from C++98 to C++11:</p> 431 432 <pre> 433 test.cpp:3:7: warning: use of right-shift operator ('>>') in template argument will require parentheses in C++11 434 A<100 >> 2> *a; 435 ^ 436 ( ) 437 </pre> 438 439 <p>Here, the fix-it hint is suggesting that parentheses be added, 440 and showing exactly where those parentheses would be inserted into the 441 source code. The fix-it hints themselves describe what changes to make 442 to the source code in an abstract manner, which the text diagnostic 443 printer renders as a line of "insertions" below the caret line. <a 444 href="#DiagnosticClient">Other diagnostic clients</a> might choose 445 to render the code differently (e.g., as markup inline) or even give 446 the user the ability to automatically fix the problem.</p> 447 448 <p>All fix-it hints are described by the <code>FixItHint</code> class, 449 instances of which should be attached to the diagnostic using the 450 << operator in the same way that highlighted source ranges and 451 arguments are passed to the diagnostic. Fix-it hints can be created 452 with one of three constructors:</p> 453 454 <dl> 455 <dt><code>FixItHint::CreateInsertion(Loc, Code)</code></dt> 456 <dd>Specifies that the given <code>Code</code> (a string) should be inserted 457 before the source location <code>Loc</code>.</dd> 458 459 <dt><code>FixItHint::CreateRemoval(Range)</code></dt> 460 <dd>Specifies that the code in the given source <code>Range</code> 461 should be removed.</dd> 462 463 <dt><code>FixItHint::CreateReplacement(Range, Code)</code></dt> 464 <dd>Specifies that the code in the given source <code>Range</code> 465 should be removed, and replaced with the given <code>Code</code> string.</dd> 466 </dl> 467 468 <!-- ============================================================= --> 469 <h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4> 470 <!-- ============================================================= --> 471 472 <p>Once code generates a diagnostic with all of the arguments and the rest of 473 the relevant information, Clang needs to know what to do with it. As previously 474 mentioned, the diagnostic machinery goes through some filtering to map a 475 severity onto a diagnostic level, then (assuming the diagnostic is not mapped to 476 "<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient 477 interface with the information.</p> 478 479 <p>It is possible to implement this interface in many different ways. For 480 example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns 481 the arguments into strings (according to the various formatting rules), prints 482 out the file/line/column information and the string, then prints out the line of 483 code, the source ranges, and the caret. However, this behavior isn't required. 484 </p> 485 486 <p>Another implementation of the DiagnosticClient interface is the 487 'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode. 488 Instead of formatting and printing out the diagnostics, this implementation just 489 captures and remembers the diagnostics as they fly by. Then -verify compares 490 the list of produced diagnostics to the list of expected ones. If they disagree, 491 it prints out its own output. 492 </p> 493 494 <p>There are many other possible implementations of this interface, and this is 495 why we prefer diagnostics to pass down rich structured information in arguments. 496 For example, an HTML output might want declaration names be linkified to where 497 they come from in the source. Another example is that a GUI might let you click 498 on typedefs to expand them. This application would want to pass significantly 499 more information about types through to the GUI than a simple flat string. The 500 interface allows this to happen.</p> 501 502 <!-- ====================================================== --> 503 <h4><a name="translation">Adding Translations to Clang</a></h4> 504 <!-- ====================================================== --> 505 506 <p>Not possible yet! Diagnostic strings should be written in UTF-8, the client 507 can translate to the relevant code page if needed. Each translation completely 508 replaces the format string for the diagnostic.</p> 509 510 511 <!-- ======================================================================= --> 512 <h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3> 513 <!-- ======================================================================= --> 514 515 <p>Strangely enough, the SourceLocation class represents a location within the 516 source code of the program. Important design points include:</p> 517 518 <ol> 519 <li>sizeof(SourceLocation) must be extremely small, as these are embedded into 520 many AST nodes and are passed around often. Currently it is 32 bits.</li> 521 <li>SourceLocation must be a simple value object that can be efficiently 522 copied.</li> 523 <li>We should be able to represent a source location for any byte of any input 524 file. This includes in the middle of tokens, in whitespace, in trigraphs, 525 etc.</li> 526 <li>A SourceLocation must encode the current #include stack that was active when 527 the location was processed. For example, if the location corresponds to a 528 token, it should contain the set of #includes active when the token was 529 lexed. This allows us to print the #include stack for a diagnostic.</li> 530 <li>SourceLocation must be able to describe macro expansions, capturing both 531 the ultimate instantiation point and the source of the original character 532 data.</li> 533 </ol> 534 535 <p>In practice, the SourceLocation works together with the SourceManager class 536 to encode two pieces of information about a location: its spelling location 537 and its instantiation location. For most tokens, these will be the same. 538 However, for a macro expansion (or tokens that came from a _Pragma directive) 539 these will describe the location of the characters corresponding to the token 540 and the location where the token was used (i.e. the macro instantiation point 541 or the location of the _Pragma itself).</p> 542 543 <p>The Clang front-end inherently depends on the location of a token being 544 tracked correctly. If it is ever incorrect, the front-end may get confused and 545 die. The reason for this is that the notion of the 'spelling' of a Token in 546 Clang depends on being able to find the original input characters for the token. 547 This concept maps directly to the "spelling location" for the token.</p> 548 549 550 <!-- ======================================================================= --> 551 <h3 id="SourceRange">SourceRange and CharSourceRange</h3> 552 <!-- ======================================================================= --> 553 <!-- mostly taken from 554 http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-August/010595.html --> 555 556 <p>Clang represents most source ranges by [first, last], where first and last 557 each point to the beginning of their respective tokens. For example 558 consider the SourceRange of the following statement:</p> 559 <pre> 560 x = foo + bar; 561 ^first ^last 562 </pre> 563 564 <p>To map from this representation to a character-based 565 representation, the 'last' location needs to be adjusted to point to 566 (or past) the end of that token with either 567 <code>Lexer::MeasureTokenLength()</code> or 568 <code>Lexer::getLocForEndOfToken()</code>. For the rare cases 569 where character-level source ranges information is needed we use 570 the <code>CharSourceRange</code> class.</p> 571 572 573 <!-- ======================================================================= --> 574 <h2 id="libdriver">The Driver Library</h2> 575 <!-- ======================================================================= --> 576 577 <p>The clang Driver and library are documented <a 578 href="DriverInternals.html">here<a>.<p> 579 580 <!-- ======================================================================= --> 581 <h2 id="pch">Precompiled Headers</h2> 582 <!-- ======================================================================= --> 583 584 <p>Clang supports two implementations of precompiled headers. The 585 default implementation, precompiled headers (<a 586 href="PCHInternals.html">PCH</a>) uses a serialized representation 587 of Clang's internal data structures, encoded with the <a 588 href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream 589 format</a>. Pretokenized headers (<a 590 href="PTHInternals.html">PTH</a>), on the other hand, contain a 591 serialized representation of the tokens encountered when 592 preprocessing a header (and anything that header includes).</p> 593 594 595 <!-- ======================================================================= --> 596 <h2 id="libfrontend">The Frontend Library</h2> 597 <!-- ======================================================================= --> 598 599 <p>The Frontend library contains functionality useful for building 600 tools on top of the clang libraries, for example several methods for 601 outputting diagnostics.</p> 602 603 <!-- ======================================================================= --> 604 <h2 id="liblex">The Lexer and Preprocessor Library</h2> 605 <!-- ======================================================================= --> 606 607 <p>The Lexer library contains several tightly-connected classes that are involved 608 with the nasty process of lexing and preprocessing C source code. The main 609 interface to this library for outside clients is the large <a 610 href="#Preprocessor">Preprocessor</a> class. 611 It contains the various pieces of state that are required to coherently read 612 tokens out of a translation unit.</p> 613 614 <p>The core interface to the Preprocessor object (once it is set up) is the 615 Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from 616 the preprocessor stream. There are two types of token providers that the 617 preprocessor is capable of reading from: a buffer lexer (provided by the <a 618 href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a 619 href="#TokenLexer">TokenLexer</a> class). 620 621 622 <!-- ======================================================================= --> 623 <h3 id="Token">The Token class</h3> 624 <!-- ======================================================================= --> 625 626 <p>The Token class is used to represent a single lexed token. Tokens are 627 intended to be used by the lexer/preprocess and parser libraries, but are not 628 intended to live beyond them (for example, they should not live in the ASTs).<p> 629 630 <p>Tokens most often live on the stack (or some other location that is efficient 631 to access) as the parser is running, but occasionally do get buffered up. For 632 example, macro definitions are stored as a series of tokens, and the C++ 633 front-end periodically needs to buffer tokens up for tentative parsing and 634 various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit 635 system, sizeof(Token) is currently 16 bytes.</p> 636 637 <p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation 638 Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer, 639 annotation tokens represent semantic information and are produced by the parser, 640 replacing normal tokens in the token stream. Normal tokens contain the 641 following information:</p> 642 643 <ul> 644 <li><b>A SourceLocation</b> - This indicates the location of the start of the 645 token.</li> 646 647 <li><b>A length</b> - This stores the length of the token as stored in the 648 SourceBuffer. For tokens that include them, this length includes trigraphs and 649 escaped newlines which are ignored by later phases of the compiler. By pointing 650 into the original source buffer, it is always possible to get the original 651 spelling of a token completely accurately.</li> 652 653 <li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if 654 identifier lookup was enabled when the token was lexed (e.g. the lexer was not 655 reading in 'raw' mode) this contains a pointer to the unique hash value for the 656 identifier. Because the lookup happens before keyword identification, this 657 field is set even for language keywords like 'for'.</li> 658 659 <li><b>TokenKind</b> - This indicates the kind of token as classified by the 660 lexer. This includes things like <tt>tok::starequal</tt> (for the "*=" 661 operator), <tt>tok::ampamp</tt> for the "&&" token, and keyword values 662 (e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note 663 that some tokens can be spelled multiple ways. For example, C++ supports 664 "operator keywords", where things like "and" are treated exactly like the 665 "&&" operator. In these cases, the kind value is set to 666 <tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to 667 consider both forms. For something that cares about which form is used (e.g. 668 the preprocessor 'stringize' operator) the spelling indicates the original 669 form.</li> 670 671 <li><b>Flags</b> - There are currently four flags tracked by the 672 lexer/preprocessor system on a per-token basis: 673 674 <ol> 675 <li><b>StartOfLine</b> - This was the first token that occurred on its input 676 source line.</li> 677 <li><b>LeadingSpace</b> - There was a space character either immediately 678 before the token or transitively before the token as it was expanded 679 through a macro. The definition of this flag is very closely defined by 680 the stringizing requirements of the preprocessor.</li> 681 <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to 682 represent identifier tokens which have macro expansion disabled. This 683 prevents them from being considered as candidates for macro expansion ever 684 in the future.</li> 685 <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the 686 token includes a trigraph or escaped newline. Since this is uncommon, 687 many pieces of code can fast-path on tokens that did not need cleaning. 688 </p> 689 </ol> 690 </li> 691 </ul> 692 693 <p>One interesting (and somewhat unusual) aspect of normal tokens is that they 694 don't contain any semantic information about the lexed value. For example, if 695 the token was a pp-number token, we do not represent the value of the number 696 that was lexed (this is left for later pieces of code to decide). Additionally, 697 the lexer library has no notion of typedef names vs variable names: both are 698 returned as identifiers, and the parser is left to decide whether a specific 699 identifier is a typedef or a variable (tracking this requires scope information 700 among other things). The parser can do this translation by replacing tokens 701 returned by the preprocessor with "Annotation Tokens".</p> 702 703 <!-- ======================================================================= --> 704 <h3 id="AnnotationToken">Annotation Tokens</h3> 705 <!-- ======================================================================= --> 706 707 <p>Annotation Tokens are tokens that are synthesized by the parser and injected 708 into the preprocessor's token stream (replacing existing tokens) to record 709 semantic information found by the parser. For example, if "foo" is found to be 710 a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an 711 <tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this 712 makes it easy to handle qualified type names (e.g. "foo::bar::baz<42>::t") 713 in C++ as a single "token" in the parser. 2) if the parser backtracks, the 714 reparse does not need to redo semantic analysis to determine whether a token 715 sequence is a variable, type, template, etc.</p> 716 717 <p>Annotation Tokens are created by the parser and reinjected into the parser's 718 token stream (when backtracking is enabled). Because they can only exist in 719 tokens that the preprocessor-proper is done with, it doesn't need to keep around 720 flags like "start of line" that the preprocessor uses to do its job. 721 Additionally, an annotation token may "cover" a sequence of preprocessor tokens 722 (e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields 723 of an annotation token are different than the fields for a normal token (but 724 they are multiplexed into the normal Token fields):</p> 725 726 <ul> 727 <li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation 728 token indicates the first token replaced by the annotation token. In the example 729 above, it would be the location of the "a" identifier.</li> 730 731 <li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the 732 last token replaced with the annotation token. In the example above, it would 733 be the location of the "c" identifier.</li> 734 735 <li><b>void* "AnnotationValue"</b> - This contains an opaque object 736 that the parser gets from Sema. The parser merely preserves the 737 information for Sema to later interpret based on the annotation token 738 kind.</li> 739 740 <li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this 741 is. See below for the different valid kinds.</li> 742 </ul> 743 744 <p>Annotation tokens currently come in three kinds:</p> 745 746 <ol> 747 <li><b>tok::annot_typename</b>: This annotation token represents a 748 resolved typename token that is potentially qualified. The 749 AnnotationValue field contains the <tt>QualType</tt> returned by 750 Sema::getTypeName(), possibly with source location information 751 attached.</li> 752 753 <li><b>tok::annot_cxxscope</b>: This annotation token represents a C++ 754 scope specifier, such as "A::B::". This corresponds to the grammar 755 productions "::" and ":: [opt] nested-name-specifier". The 756 AnnotationValue pointer is a <tt>NestedNameSpecifier*</tt> returned by 757 the Sema::ActOnCXXGlobalScopeSpecifier and 758 Sema::ActOnCXXNestedNameSpecifier callbacks.</li> 759 760 <li><b>tok::annot_template_id</b>: This annotation token represents a 761 C++ template-id such as "foo<int, 4>", where "foo" is the name 762 of a template. The AnnotationValue pointer is a pointer to a malloc'd 763 TemplateIdAnnotation object. Depending on the context, a parsed 764 template-id that names a type might become a typename annotation token 765 (if all we care about is the named type, e.g., because it occurs in a 766 type specifier) or might remain a template-id token (if we want to 767 retain more source location information or produce a new type, e.g., 768 in a declaration of a class template specialization). template-id 769 annotation tokens that refer to a type can be "upgraded" to typename 770 annotation tokens by the parser.</li> 771 772 </ol> 773 774 <p>As mentioned above, annotation tokens are not returned by the preprocessor, 775 they are formed on demand by the parser. This means that the parser has to be 776 aware of cases where an annotation could occur and form it where appropriate. 777 This is somewhat similar to how the parser handles Translation Phase 6 of C99: 778 String Concatenation (see C99 5.1.1.2). In the case of string concatenation, 779 the preprocessor just returns distinct tok::string_literal and 780 tok::wide_string_literal tokens and the parser eats a sequence of them wherever 781 the grammar indicates that a string literal can occur.</p> 782 783 <p>In order to do this, whenever the parser expects a tok::identifier or 784 tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or 785 TryAnnotateCXXScopeToken methods to form the annotation token. These methods 786 will maximally form the specified annotation tokens and replace the current 787 token with them, if applicable. If the current tokens is not valid for an 788 annotation token, it will remain an identifier or :: token.</p> 789 790 791 792 <!-- ======================================================================= --> 793 <h3 id="Lexer">The Lexer class</h3> 794 <!-- ======================================================================= --> 795 796 <p>The Lexer class provides the mechanics of lexing tokens out of a source 797 buffer and deciding what they mean. The Lexer is complicated by the fact that 798 it operates on raw buffers that have not had spelling eliminated (this is a 799 necessity to get decent performance), but this is countered with careful coding 800 as well as standard performance techniques (for example, the comment handling 801 code is vectorized on X86 and PowerPC hosts).</p> 802 803 <p>The lexer has a couple of interesting modal features:</p> 804 805 <ul> 806 <li>The lexer can operate in 'raw' mode. This mode has several features that 807 make it possible to quickly lex the file (e.g. it stops identifier lookup, 808 doesn't specially handle preprocessor tokens, handles EOF differently, etc). 809 This mode is used for lexing within an "<tt>#if 0</tt>" block, for 810 example.</li> 811 <li>The lexer can capture and return comments as tokens. This is required to 812 support the -C preprocessor mode, which passes comments through, and is 813 used by the diagnostic checker to identifier expect-error annotations.</li> 814 <li>The lexer can be in ParsingFilename mode, which happens when preprocessing 815 after reading a #include directive. This mode changes the parsing of '<' 816 to return an "angled string" instead of a bunch of tokens for each thing 817 within the filename.</li> 818 <li>When parsing a preprocessor directive (after "<tt>#</tt>") the 819 ParsingPreprocessorDirective mode is entered. This changes the parser to 820 return EOD at a newline.</li> 821 <li>The Lexer uses a LangOptions object to know whether trigraphs are enabled, 822 whether C++ or ObjC keywords are recognized, etc.</li> 823 </ul> 824 825 <p>In addition to these modes, the lexer keeps track of a couple of other 826 features that are local to a lexed buffer, which change as the buffer is 827 lexed:</p> 828 829 <ul> 830 <li>The Lexer uses BufferPtr to keep track of the current character being 831 lexed.</li> 832 <li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token 833 will start with its "start of line" bit set.</li> 834 <li>The Lexer keeps track of the current #if directives that are active (which 835 can be nested).</li> 836 <li>The Lexer keeps track of an <a href="#MultipleIncludeOpt"> 837 MultipleIncludeOpt</a> object, which is used to 838 detect whether the buffer uses the standard "<tt>#ifndef XX</tt> / 839 <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does, 840 subsequent includes can be ignored if the XX macro is defined.</li> 841 </ul> 842 843 <!-- ======================================================================= --> 844 <h3 id="TokenLexer">The TokenLexer class</h3> 845 <!-- ======================================================================= --> 846 847 <p>The TokenLexer class is a token provider that returns tokens from a list 848 of tokens that came from somewhere else. It typically used for two things: 1) 849 returning tokens from a macro definition as it is being expanded 2) returning 850 tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and 851 will most likely be used to handle unbounded look-ahead for the C++ parser.</p> 852 853 <!-- ======================================================================= --> 854 <h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3> 855 <!-- ======================================================================= --> 856 857 <p>The MultipleIncludeOpt class implements a really simple little state machine 858 that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>" 859 idiom that people typically use to prevent multiple inclusion of headers. If a 860 buffer uses this idiom and is subsequently #include'd, the preprocessor can 861 simply check to see whether the guarding condition is defined or not. If so, 862 the preprocessor can completely ignore the include of the header.</p> 863 864 865 866 <!-- ======================================================================= --> 867 <h2 id="libparse">The Parser Library</h2> 868 <!-- ======================================================================= --> 869 870 <!-- ======================================================================= --> 871 <h2 id="libast">The AST Library</h2> 872 <!-- ======================================================================= --> 873 874 <!-- ======================================================================= --> 875 <h3 id="Type">The Type class and its subclasses</h3> 876 <!-- ======================================================================= --> 877 878 <p>The Type class (and its subclasses) are an important part of the AST. Types 879 are accessed through the ASTContext class, which implicitly creates and uniques 880 them as they are needed. Types have a couple of non-obvious features: 1) they 881 do not capture type qualifiers like const or volatile (See 882 <a href="#QualType">QualType</a>), and 2) they implicitly capture typedef 883 information. Once created, types are immutable (unlike decls).</p> 884 885 <p>Typedefs in C make semantic analysis a bit more complex than it would 886 be without them. The issue is that we want to capture typedef information 887 and represent it in the AST perfectly, but the semantics of operations need to 888 "see through" typedefs. For example, consider this code:</p> 889 890 <code> 891 void func() {<br> 892 typedef int foo;<br> 893 foo X, *Y;<br> 894 typedef foo* bar;<br> 895 bar Z;<br> 896 *X; <i>// error</i><br> 897 **Y; <i>// error</i><br> 898 **Z; <i>// error</i><br> 899 }<br> 900 </code> 901 902 <p>The code above is illegal, and thus we expect there to be diagnostics emitted 903 on the annotated lines. In this example, we expect to get:</p> 904 905 <pre> 906 <b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b> 907 *X; // error 908 <font color="blue">^~</font> 909 <b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b> 910 **Y; // error 911 <font color="blue">^~~</font> 912 <b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b> 913 **Z; // error 914 <font color="blue">^~~</font> 915 </pre> 916 917 <p>While this example is somewhat silly, it illustrates the point: we want to 918 retain typedef information where possible, so that we can emit errors about 919 "<tt>std::string</tt>" instead of "<tt>std::basic_string<char, std:...</tt>". 920 Doing this requires properly keeping typedef information (for example, the type 921 of "X" is "foo", not "int"), and requires properly propagating it through the 922 various operators (for example, the type of *Y is "foo", not "int"). In order 923 to retain this information, the type of these expressions is an instance of the 924 TypedefType class, which indicates that the type of these expressions is a 925 typedef for foo. 926 </p> 927 928 <p>Representing types like this is great for diagnostics, because the 929 user-specified type is always immediately available. There are two problems 930 with this: first, various semantic checks need to make judgements about the 931 <em>actual structure</em> of a type, ignoring typdefs. Second, we need an 932 efficient way to query whether two types are structurally identical to each 933 other, ignoring typedefs. The solution to both of these problems is the idea of 934 canonical types.</p> 935 936 <!-- =============== --> 937 <h4>Canonical Types</h4> 938 <!-- =============== --> 939 940 <p>Every instance of the Type class contains a canonical type pointer. For 941 simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>", 942 "<tt>int**</tt>"), the type just points to itself. For types that have a 943 typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>", 944 "<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their 945 structurally equivalent type without any typedefs (e.g. "<tt>int</tt>", 946 "<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p> 947 948 <p>This design provides a constant time operation (dereferencing the canonical 949 type pointer) that gives us access to the structure of types. For example, 950 we can trivially tell that "bar" and "foo*" are the same type by dereferencing 951 their canonical type pointers and doing a pointer comparison (they both point 952 to the single "<tt>int*</tt>" type).</p> 953 954 <p>Canonical types and typedef types bring up some complexities that must be 955 carefully managed. Specifically, the "isa/cast/dyncast" operators generally 956 shouldn't be used in code that is inspecting the AST. For example, when type 957 checking the indirection operator (unary '*' on a pointer), the type checker 958 must verify that the operand has a pointer type. It would not be correct to 959 check that with "<tt>isa<PointerType>(SubExpr->getType())</tt>", 960 because this predicate would fail if the subexpression had a typedef type.</p> 961 962 <p>The solution to this problem are a set of helper methods on Type, used to 963 check their properties. In this case, it would be correct to use 964 "<tt>SubExpr->getType()->isPointerType()</tt>" to do the check. This 965 predicate will return true if the <em>canonical type is a pointer</em>, which is 966 true any time the type is structurally a pointer type. The only hard part here 967 is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p> 968 969 <p>The second problem we face is how to get access to the pointer type once we 970 know it exists. To continue the example, the result type of the indirection 971 operator is the pointee type of the subexpression. In order to determine the 972 type, we need to get the instance of PointerType that best captures the typedef 973 information in the program. If the type of the expression is literally a 974 PointerType, we can return that, otherwise we have to dig through the 975 typedefs to find the pointer type. For example, if the subexpression had type 976 "<tt>foo*</tt>", we could return that type as the result. If the subexpression 977 had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do 978 <em>not</em> want "<tt>int*</tt>"). In order to provide all of this, Type has 979 a getAsPointerType() method that checks whether the type is structurally a 980 PointerType and, if so, returns the best one. If not, it returns a null 981 pointer.</p> 982 983 <p>This structure is somewhat mystical, but after meditating on it, it will 984 make sense to you :).</p> 985 986 <!-- ======================================================================= --> 987 <h3 id="QualType">The QualType class</h3> 988 <!-- ======================================================================= --> 989 990 <p>The QualType class is designed as a trivial value class that is 991 small, passed by-value and is efficient to query. The idea of 992 QualType is that it stores the type qualifiers (const, volatile, 993 restrict, plus some extended qualifiers required by language 994 extensions) separately from the types themselves. QualType is 995 conceptually a pair of "Type*" and the bits for these type qualifiers.</p> 996 997 <p>By storing the type qualifiers as bits in the conceptual pair, it is 998 extremely efficient to get the set of qualifiers on a QualType (just return the 999 field of the pair), add a type qualifier (which is a trivial constant-time 1000 operation that sets a bit), and remove one or more type qualifiers (just return 1001 a QualType with the bitfield set to empty).</p> 1002 1003 <p>Further, because the bits are stored outside of the type itself, we do not 1004 need to create duplicates of types with different sets of qualifiers (i.e. there 1005 is only a single heap allocated "int" type: "const int" and "volatile const int" 1006 both point to the same heap allocated "int" type). This reduces the heap size 1007 used to represent bits and also means we do not have to consider qualifiers when 1008 uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p> 1009 1010 <p>In practice, the two most common type qualifiers (const and 1011 restrict) are stored in the low bits of the pointer to the Type 1012 object, together with a flag indicating whether extended qualifiers 1013 are present (which must be heap-allocated). This means that QualType 1014 is exactly the same size as a pointer.</p> 1015 1016 <!-- ======================================================================= --> 1017 <h3 id="DeclarationName">Declaration names</h3> 1018 <!-- ======================================================================= --> 1019 1020 <p>The <tt>DeclarationName</tt> class represents the name of a 1021 declaration in Clang. Declarations in the C family of languages can 1022 take several different forms. Most declarations are named by 1023 simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in 1024 the function declaration <code>f(int x)</code>. In C++, declaration 1025 names can also name class constructors ("<code>Class</code>" 1026 in <code>struct Class { Class(); }</code>), class destructors 1027 ("<code>~Class</code>"), overloaded operator names ("operator+"), 1028 and conversion functions ("<code>operator void const *</code>"). In 1029 Objective-C, declaration names can refer to the names of Objective-C 1030 methods, which involve the method name and the parameters, 1031 collectively called a <i>selector</i>, e.g., 1032 "<code>setWidth:height:</code>". Since all of these kinds of 1033 entities - variables, functions, Objective-C methods, C++ 1034 constructors, destructors, and operators - are represented as 1035 subclasses of Clang's common <code>NamedDecl</code> 1036 class, <code>DeclarationName</code> is designed to efficiently 1037 represent any kind of name.</p> 1038 1039 <p>Given 1040 a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code> 1041 will produce a value that describes what kind of name <code>N</code> 1042 stores. There are 8 options (all of the names are inside 1043 the <code>DeclarationName</code> class)</p> 1044 <dl> 1045 <dt>Identifier</dt> 1046 <dd>The name is a simple 1047 identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the 1048 corresponding <code>IdentifierInfo*</code> pointing to the actual 1049 identifier. Note that C++ overloaded operators (e.g., 1050 "<code>operator+</code>") are represented as special kinds of 1051 identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code> 1052 function to determine whether an identifier is an overloaded 1053 operator name.</dd> 1054 1055 <dt>ObjCZeroArgSelector, ObjCOneArgSelector, 1056 ObjCMultiArgSelector</dt> 1057 <dd>The name is an Objective-C selector, which can be retrieved as a 1058 <code>Selector</code> instance 1059 via <code>N.getObjCSelector()</code>. The three possible name 1060 kinds for Objective-C reflect an optimization within 1061 the <code>DeclarationName</code> class: both zero- and 1062 one-argument selectors are stored as a 1063 masked <code>IdentifierInfo</code> pointer, and therefore require 1064 very little space, since zero- and one-argument selectors are far 1065 more common than multi-argument selectors (which use a different 1066 structure).</dd> 1067 1068 <dt>CXXConstructorName</dt> 1069 <dd>The name is a C++ constructor 1070 name. Use <code>N.getCXXNameType()</code> to retrieve 1071 the <a href="#QualType">type</a> that this constructor is meant to 1072 construct. The type is always the canonical type, since all 1073 constructors for a given type have the same name.</dd> 1074 1075 <dt>CXXDestructorName</dt> 1076 <dd>The name is a C++ destructor 1077 name. Use <code>N.getCXXNameType()</code> to retrieve 1078 the <a href="#QualType">type</a> whose destructor is being 1079 named. This type is always a canonical type.</dd> 1080 1081 <dt>CXXConversionFunctionName</dt> 1082 <dd>The name is a C++ conversion function. Conversion functions are 1083 named according to the type they convert to, e.g., "<code>operator void 1084 const *</code>". Use <code>N.getCXXNameType()</code> to retrieve 1085 the type that this conversion function converts to. This type is 1086 always a canonical type.</dd> 1087 1088 <dt>CXXOperatorName</dt> 1089 <dd>The name is a C++ overloaded operator name. Overloaded operators 1090 are named according to their spelling, e.g., 1091 "<code>operator+</code>" or "<code>operator new 1092 []</code>". Use <code>N.getCXXOverloadedOperator()</code> to 1093 retrieve the overloaded operator (a value of 1094 type <code>OverloadedOperatorKind</code>).</dd> 1095 </dl> 1096 1097 <p><code>DeclarationName</code>s are cheap to create, copy, and 1098 compare. They require only a single pointer's worth of storage in 1099 the common cases (identifiers, zero- 1100 and one-argument Objective-C selectors) and use dense, uniqued 1101 storage for the other kinds of 1102 names. Two <code>DeclarationName</code>s can be compared for 1103 equality (<code>==</code>, <code>!=</code>) using a simple bitwise 1104 comparison, can be ordered 1105 with <code><</code>, <code>></code>, <code><=</code>, 1106 and <code>>=</code> (which provide a lexicographical ordering for 1107 normal identifiers but an unspecified ordering for other kinds of 1108 names), and can be placed into LLVM <code>DenseMap</code>s 1109 and <code>DenseSet</code>s.</p> 1110 1111 <p><code>DeclarationName</code> instances can be created in different 1112 ways depending on what kind of name the instance will store. Normal 1113 identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors 1114 (<code>Selector</code>) can be implicitly converted 1115 to <code>DeclarationName</code>s. Names for C++ constructors, 1116 destructors, conversion functions, and overloaded operators can be retrieved from 1117 the <code>DeclarationNameTable</code>, an instance of which is 1118 available as <code>ASTContext::DeclarationNames</code>. The member 1119 functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>, 1120 <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively, 1121 return <code>DeclarationName</code> instances for the four kinds of 1122 C++ special function names.</p> 1123 1124 <!-- ======================================================================= --> 1125 <h3 id="DeclContext">Declaration contexts</h3> 1126 <!-- ======================================================================= --> 1127 <p>Every declaration in a program exists within some <i>declaration 1128 context</i>, such as a translation unit, namespace, class, or 1129 function. Declaration contexts in Clang are represented by 1130 the <code>DeclContext</code> class, from which the various 1131 declaration-context AST nodes 1132 (<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>, 1133 etc.) will derive. The <code>DeclContext</code> class provides 1134 several facilities common to each declaration context:</p> 1135 <dl> 1136 <dt>Source-centric vs. Semantics-centric View of Declarations</dt> 1137 <dd><code>DeclContext</code> provides two views of the declarations 1138 stored within a declaration context. The source-centric view 1139 accurately represents the program source code as written, including 1140 multiple declarations of entities where present (see the 1141 section <a href="#Redeclarations">Redeclarations and 1142 Overloads</a>), while the semantics-centric view represents the 1143 program semantics. The two views are kept synchronized by semantic 1144 analysis while the ASTs are being constructed.</dd> 1145 1146 <dt>Storage of declarations within that context</dt> 1147 <dd>Every declaration context can contain some number of 1148 declarations. For example, a C++ class (represented 1149 by <code>RecordDecl</code>) contains various member functions, 1150 fields, nested types, and so on. All of these declarations will be 1151 stored within the <code>DeclContext</code>, and one can iterate 1152 over the declarations via 1153 [<code>DeclContext::decls_begin()</code>, 1154 <code>DeclContext::decls_end()</code>). This mechanism provides 1155 the source-centric view of declarations in the context.</dd> 1156 1157 <dt>Lookup of declarations within that context</dt> 1158 <dd>The <code>DeclContext</code> structure provides efficient name 1159 lookup for names within that declaration context. For example, 1160 if <code>N</code> is a namespace we can look for the 1161 name <code>N::f</code> 1162 using <code>DeclContext::lookup</code>. The lookup itself is 1163 based on a lazily-constructed array (for declaration contexts 1164 with a small number of declarations) or hash table (for 1165 declaration contexts with more declarations). The lookup 1166 operation provides the semantics-centric view of the declarations 1167 in the context.</dd> 1168 1169 <dt>Ownership of declarations</dt> 1170 <dd>The <code>DeclContext</code> owns all of the declarations that 1171 were declared within its declaration context, and is responsible 1172 for the management of their memory as well as their 1173 (de-)serialization.</dd> 1174 </dl> 1175 1176 <p>All declarations are stored within a declaration context, and one 1177 can query 1178 information about the context in which each declaration lives. One 1179 can retrieve the <code>DeclContext</code> that contains a 1180 particular <code>Decl</code> 1181 using <code>Decl::getDeclContext</code>. However, see the 1182 section <a href="#LexicalAndSemanticContexts">Lexical and Semantic 1183 Contexts</a> for more information about how to interpret this 1184 context information.</p> 1185 1186 <h4 id="Redeclarations">Redeclarations and Overloads</h4> 1187 <p>Within a translation unit, it is common for an entity to be 1188 declared several times. For example, we might declare a function "f" 1189 and then later re-declare it as part of an inlined definition:</p> 1190 1191 <pre> 1192 void f(int x, int y, int z = 1); 1193 1194 inline void f(int x, int y, int z) { /* ... */ } 1195 </pre> 1196 1197 <p>The representation of "f" differs in the source-centric and 1198 semantics-centric views of a declaration context. In the 1199 source-centric view, all redeclarations will be present, in the 1200 order they occurred in the source code, making 1201 this view suitable for clients that wish to see the structure of 1202 the source code. In the semantics-centric view, only the most recent "f" 1203 will be found by the lookup, since it effectively replaces the first 1204 declaration of "f".</p> 1205 1206 <p>In the semantics-centric view, overloading of functions is 1207 represented explicitly. For example, given two declarations of a 1208 function "g" that are overloaded, e.g.,</p> 1209 <pre> 1210 void g(); 1211 void g(int); 1212 </pre> 1213 <p>the <code>DeclContext::lookup</code> operation will return 1214 a <code>DeclContext::lookup_result</code> that contains a range of iterators 1215 over declarations of "g". Clients that perform semantic analysis on a 1216 program that is not concerned with the actual source code will 1217 primarily use this semantics-centric view.</p> 1218 1219 <h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4> 1220 <p>Each declaration has two potentially different 1221 declaration contexts: a <i>lexical</i> context, which corresponds to 1222 the source-centric view of the declaration context, and 1223 a <i>semantic</i> context, which corresponds to the 1224 semantics-centric view. The lexical context is accessible 1225 via <code>Decl::getLexicalDeclContext</code> while the 1226 semantic context is accessible 1227 via <code>Decl::getDeclContext</code>, both of which return 1228 <code>DeclContext</code> pointers. For most declarations, the two 1229 contexts are identical. For example:</p> 1230 1231 <pre> 1232 class X { 1233 public: 1234 void f(int x); 1235 }; 1236 </pre> 1237 1238 <p>Here, the semantic and lexical contexts of <code>X::f</code> are 1239 the <code>DeclContext</code> associated with the 1240 class <code>X</code> (itself stored as a <code>RecordDecl</code> AST 1241 node). However, we can now define <code>X::f</code> out-of-line:</p> 1242 1243 <pre> 1244 void X::f(int x = 17) { /* ... */ } 1245 </pre> 1246 1247 <p>This definition of has different lexical and semantic 1248 contexts. The lexical context corresponds to the declaration 1249 context in which the actual declaration occurred in the source 1250 code, e.g., the translation unit containing <code>X</code>. Thus, 1251 this declaration of <code>X::f</code> can be found by traversing 1252 the declarations provided by 1253 [<code>decls_begin()</code>, <code>decls_end()</code>) in the 1254 translation unit.</p> 1255 1256 <p>The semantic context of <code>X::f</code> corresponds to the 1257 class <code>X</code>, since this member function is (semantically) a 1258 member of <code>X</code>. Lookup of the name <code>f</code> into 1259 the <code>DeclContext</code> associated with <code>X</code> will 1260 then return the definition of <code>X::f</code> (including 1261 information about the default argument).</p> 1262 1263 <h4 id="TransparentContexts">Transparent Declaration Contexts</h4> 1264 <p>In C and C++, there are several contexts in which names that are 1265 logically declared inside another declaration will actually "leak" 1266 out into the enclosing scope from the perspective of name 1267 lookup. The most obvious instance of this behavior is in 1268 enumeration types, e.g.,</p> 1269 <pre> 1270 enum Color { 1271 Red, 1272 Green, 1273 Blue 1274 }; 1275 </pre> 1276 1277 <p>Here, <code>Color</code> is an enumeration, which is a declaration 1278 context that contains the 1279 enumerators <code>Red</code>, <code>Green</code>, 1280 and <code>Blue</code>. Thus, traversing the list of declarations 1281 contained in the enumeration <code>Color</code> will 1282 yield <code>Red</code>, <code>Green</code>, 1283 and <code>Blue</code>. However, outside of the scope 1284 of <code>Color</code> one can name the enumerator <code>Red</code> 1285 without qualifying the name, e.g.,</p> 1286 1287 <pre> 1288 Color c = Red; 1289 </pre> 1290 1291 <p>There are other entities in C++ that provide similar behavior. For 1292 example, linkage specifications that use curly braces:</p> 1293 1294 <pre> 1295 extern "C" { 1296 void f(int); 1297 void g(int); 1298 } 1299 // f and g are visible here 1300 </pre> 1301 1302 <p>For source-level accuracy, we treat the linkage specification and 1303 enumeration type as a 1304 declaration context in which its enclosed declarations ("Red", 1305 "Green", and "Blue"; "f" and "g") 1306 are declared. However, these declarations are visible outside of the 1307 scope of the declaration context.</p> 1308 1309 <p>These language features (and several others, described below) have 1310 roughly the same set of 1311 requirements: declarations are declared within a particular lexical 1312 context, but the declarations are also found via name lookup in 1313 scopes enclosing the declaration itself. This feature is implemented 1314 via <i>transparent</i> declaration contexts 1315 (see <code>DeclContext::isTransparentContext()</code>), whose 1316 declarations are visible in the nearest enclosing non-transparent 1317 declaration context. This means that the lexical context of the 1318 declaration (e.g., an enumerator) will be the 1319 transparent <code>DeclContext</code> itself, as will the semantic 1320 context, but the declaration will be visible in every outer context 1321 up to and including the first non-transparent declaration context (since 1322 transparent declaration contexts can be nested).</p> 1323 1324 <p>The transparent <code>DeclContexts</code> are:</p> 1325 <ul> 1326 <li>Enumerations (but not C++11 "scoped enumerations"): 1327 <pre> 1328 enum Color { 1329 Red, 1330 Green, 1331 Blue 1332 }; 1333 // Red, Green, and Blue are in scope 1334 </pre></li> 1335 <li>C++ linkage specifications: 1336 <pre> 1337 extern "C" { 1338 void f(int); 1339 void g(int); 1340 } 1341 // f and g are in scope 1342 </pre></li> 1343 <li>Anonymous unions and structs: 1344 <pre> 1345 struct LookupTable { 1346 bool IsVector; 1347 union { 1348 std::vector<Item> *Vector; 1349 std::set<Item> *Set; 1350 }; 1351 }; 1352 1353 LookupTable LT; 1354 LT.Vector = 0; // Okay: finds Vector inside the unnamed union 1355 </pre> 1356 </li> 1357 <li>C++11 inline namespaces: 1358 <pre> 1359 namespace mylib { 1360 inline namespace debug { 1361 class X; 1362 } 1363 } 1364 mylib::X *xp; // okay: mylib::X refers to mylib::debug::X 1365 </pre> 1366 </li> 1367 </ul> 1368 1369 1370 <h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4> 1371 <p>C++ namespaces have the interesting--and, so far, unique--property that 1372 the namespace can be defined multiple times, and the declarations 1373 provided by each namespace definition are effectively merged (from 1374 the semantic point of view). For example, the following two code 1375 snippets are semantically indistinguishable:</p> 1376 <pre> 1377 // Snippet #1: 1378 namespace N { 1379 void f(); 1380 } 1381 namespace N { 1382 void f(int); 1383 } 1384 1385 // Snippet #2: 1386 namespace N { 1387 void f(); 1388 void f(int); 1389 } 1390 </pre> 1391 1392 <p>In Clang's representation, the source-centric view of declaration 1393 contexts will actually have two separate <code>NamespaceDecl</code> 1394 nodes in Snippet #1, each of which is a declaration context that 1395 contains a single declaration of "f". However, the semantics-centric 1396 view provided by name lookup into the namespace <code>N</code> for 1397 "f" will return a <code>DeclContext::lookup_result</code> that contains 1398 a range of iterators over declarations of "f".</p> 1399 1400 <p><code>DeclContext</code> manages multiply-defined declaration 1401 contexts internally. The 1402 function <code>DeclContext::getPrimaryContext</code> retrieves the 1403 "primary" context for a given <code>DeclContext</code> instance, 1404 which is the <code>DeclContext</code> responsible for maintaining 1405 the lookup table used for the semantics-centric view. Given the 1406 primary context, one can follow the chain 1407 of <code>DeclContext</code> nodes that define additional 1408 declarations via <code>DeclContext::getNextContext</code>. Note that 1409 these functions are used internally within the lookup and insertion 1410 methods of the <code>DeclContext</code>, so the vast majority of 1411 clients can ignore them.</p> 1412 1413 <!-- ======================================================================= --> 1414 <h3 id="CFG">The <tt>CFG</tt> class</h3> 1415 <!-- ======================================================================= --> 1416 1417 <p>The <tt>CFG</tt> class is designed to represent a source-level 1418 control-flow graph for a single statement (<tt>Stmt*</tt>). Typically 1419 instances of <tt>CFG</tt> are constructed for function bodies (usually 1420 an instance of <tt>CompoundStmt</tt>), but can also be instantiated to 1421 represent the control-flow of any class that subclasses <tt>Stmt</tt>, 1422 which includes simple expressions. Control-flow graphs are especially 1423 useful for performing 1424 <a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow- 1425 or path-sensitive</a> program analyses on a given function.</p> 1426 1427 <!-- ============ --> 1428 <h4>Basic Blocks</h4> 1429 <!-- ============ --> 1430 1431 <p>Concretely, an instance of <tt>CFG</tt> is a collection of basic 1432 blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which 1433 simply contains an ordered sequence of <tt>Stmt*</tt> (each referring 1434 to statements in the AST). The ordering of statements within a block 1435 indicates unconditional flow of control from one statement to the 1436 next. <a href="#ConditionalControlFlow">Conditional control-flow</a> 1437 is represented using edges between basic blocks. The statements 1438 within a given <tt>CFGBlock</tt> can be traversed using 1439 the <tt>CFGBlock::*iterator</tt> interface.</p> 1440 1441 <p> 1442 A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within 1443 the control-flow graph it represents. Each <tt>CFGBlock</tt> within a 1444 CFG is also uniquely numbered (accessible 1445 via <tt>CFGBlock::getBlockID()</tt>). Currently the number is 1446 based on the ordering the blocks were created, but no assumptions 1447 should be made on how <tt>CFGBlock</tt>s are numbered other than their 1448 numbers are unique and that they are numbered from 0..N-1 (where N is 1449 the number of basic blocks in the CFG).</p> 1450 1451 <!-- ===================== --> 1452 <h4>Entry and Exit Blocks</h4> 1453 <!-- ===================== --> 1454 1455 Each instance of <tt>CFG</tt> contains two special blocks: 1456 an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which 1457 has no incoming edges, and an <i>exit</i> block (accessible 1458 via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither 1459 block contains any statements, and they serve the role of providing a 1460 clear entrance and exit for a body of code such as a function body. 1461 The presence of these empty blocks greatly simplifies the 1462 implementation of many analyses built on top of CFGs. 1463 1464 <!-- ===================================================== --> 1465 <h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4> 1466 <!-- ===================================================== --> 1467 1468 <p>Conditional control-flow (such as those induced by if-statements 1469 and loops) is represented as edges between <tt>CFGBlock</tt>s. 1470 Because different C language constructs can induce control-flow, 1471 each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that 1472 represents the <i>terminator</i> of the block. A terminator is simply 1473 the statement that caused the control-flow, and is used to identify 1474 the nature of the conditional control-flow between blocks. For 1475 example, in the case of an if-statement, the terminator refers to 1476 the <tt>IfStmt</tt> object in the AST that represented the given 1477 branch.</p> 1478 1479 <p>To illustrate, consider the following code example:</p> 1480 1481 <code> 1482 int foo(int x) {<br> 1483 x = x + 1;<br> 1484 <br> 1485 if (x > 2) x++;<br> 1486 else {<br> 1487 x += 2;<br> 1488 x *= 2;<br> 1489 }<br> 1490 <br> 1491 return x;<br> 1492 } 1493 </code> 1494 1495 <p>After invoking the parser+semantic analyzer on this code fragment, 1496 the AST of the body of <tt>foo</tt> is referenced by a 1497 single <tt>Stmt*</tt>. We can then construct an instance 1498 of <tt>CFG</tt> representing the control-flow graph of this function 1499 body by single call to a static class method:</p> 1500 1501 <code> 1502 Stmt* FooBody = ...<br> 1503 CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody); 1504 </code> 1505 1506 <p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt> 1507 to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no 1508 longer needed.</p> 1509 1510 <p>Along with providing an interface to iterate over 1511 its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods 1512 that are useful for debugging and visualizing CFGs. For example, the 1513 method 1514 <tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to 1515 standard error. This is especially useful when one is using a 1516 debugger such as gdb. For example, here is the output 1517 of <tt>FooCFG->dump()</tt>:</p> 1518 1519 <code> 1520 [ B5 (ENTRY) ]<br> 1521 Predecessors (0):<br> 1522 Successors (1): B4<br> 1523 <br> 1524 [ B4 ]<br> 1525 1: x = x + 1<br> 1526 2: (x > 2)<br> 1527 <b>T: if [B4.2]</b><br> 1528 Predecessors (1): B5<br> 1529 Successors (2): B3 B2<br> 1530 <br> 1531 [ B3 ]<br> 1532 1: x++<br> 1533 Predecessors (1): B4<br> 1534 Successors (1): B1<br> 1535 <br> 1536 [ B2 ]<br> 1537 1: x += 2<br> 1538 2: x *= 2<br> 1539 Predecessors (1): B4<br> 1540 Successors (1): B1<br> 1541 <br> 1542 [ B1 ]<br> 1543 1: return x;<br> 1544 Predecessors (2): B2 B3<br> 1545 Successors (1): B0<br> 1546 <br> 1547 [ B0 (EXIT) ]<br> 1548 Predecessors (1): B1<br> 1549 Successors (0): 1550 </code> 1551 1552 <p>For each block, the pretty-printed output displays for each block 1553 the number of <i>predecessor</i> blocks (blocks that have outgoing 1554 control-flow to the given block) and <i>successor</i> blocks (blocks 1555 that have control-flow that have incoming control-flow from the given 1556 block). We can also clearly see the special entry and exit blocks at 1557 the beginning and end of the pretty-printed output. For the entry 1558 block (block B5), the number of predecessor blocks is 0, while for the 1559 exit block (block B0) the number of successor blocks is 0.</p> 1560 1561 <p>The most interesting block here is B4, whose outgoing control-flow 1562 represents the branching caused by the sole if-statement 1563 in <tt>foo</tt>. Of particular interest is the second statement in 1564 the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed 1565 as <b><tt>if [B4.2]</tt></b>. The second statement represents the 1566 evaluation of the condition of the if-statement, which occurs before 1567 the actual branching of control-flow. Within the <tt>CFGBlock</tt> 1568 for B4, the <tt>Stmt*</tt> for the second statement refers to the 1569 actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus 1570 pointers to subclasses of <tt>Expr</tt> can appear in the list of 1571 statements in a block, and not just subclasses of <tt>Stmt</tt> that 1572 refer to proper C statements.</p> 1573 1574 <p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt> 1575 object in the AST. The pretty-printer outputs <b><tt>if 1576 [B4.2]</tt></b> because the condition expression of the if-statement 1577 has an actual place in the basic block, and thus the terminator is 1578 essentially 1579 <i>referring</i> to the expression that is the second statement of 1580 block B4 (i.e., B4.2). In this manner, conditions for control-flow 1581 (which also includes conditions for loops and switch statements) are 1582 hoisted into the actual basic block.</p> 1583 1584 <!-- ===================== --> 1585 <!-- <h4>Implicit Control-Flow</h4> --> 1586 <!-- ===================== --> 1587 1588 <!-- 1589 <p>A key design principle of the <tt>CFG</tt> class was to not require 1590 any transformations to the AST in order to represent control-flow. 1591 Thus the <tt>CFG</tt> does not perform any "lowering" of the 1592 statements in an AST: loops are not transformed into guarded gotos, 1593 short-circuit operations are not converted to a set of if-statements, 1594 and so on.</p> 1595 --> 1596 1597 1598 <!-- ======================================================================= --> 1599 <h3 id="Constants">Constant Folding in the Clang AST</h3> 1600 <!-- ======================================================================= --> 1601 1602 <p>There are several places where constants and constant folding matter a lot to 1603 the Clang front-end. First, in general, we prefer the AST to retain the source 1604 code as close to how the user wrote it as possible. This means that if they 1605 wrote "5+4", we want to keep the addition and two constants in the AST, we don't 1606 want to fold to "9". This means that constant folding in various ways turns 1607 into a tree walk that needs to handle the various cases.</p> 1608 1609 <p>However, there are places in both C and C++ that require constants to be 1610 folded. For example, the C standard defines what an "integer constant 1611 expression" (i-c-e) is with very precise and specific requirements. The 1612 language then requires i-c-e's in a lot of places (for example, the size of a 1613 bitfield, the value for a case statement, etc). For these, we have to be able 1614 to constant fold the constants, to do semantic checks (e.g. verify bitfield size 1615 is non-negative and that case statements aren't duplicated). We aim for Clang 1616 to be very pedantic about this, diagnosing cases when the code does not use an 1617 i-c-e where one is required, but accepting the code unless running with 1618 <tt>-pedantic-errors</tt>.</p> 1619 1620 <p>Things get a little bit more tricky when it comes to compatibility with 1621 real-world source code. Specifically, GCC has historically accepted a huge 1622 superset of expressions as i-c-e's, and a lot of real world code depends on this 1623 unfortuate accident of history (including, e.g., the glibc system headers). GCC 1624 accepts anything its "fold" optimizer is capable of reducing to an integer 1625 constant, which means that the definition of what it accepts changes as its 1626 optimizer does. One example is that GCC accepts things like "case X-X:" even 1627 when X is a variable, because it can fold this to 0.</p> 1628 1629 <p>Another issue are how constants interact with the extensions we support, such 1630 as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99 1631 obviously does not specify the semantics of any of these extensions, and the 1632 definition of i-c-e does not include them. However, these extensions are often 1633 used in real code, and we have to have a way to reason about them.</p> 1634 1635 <p>Finally, this is not just a problem for semantic analysis. The code 1636 generator and other clients have to be able to fold constants (e.g. to 1637 initialize global variables) and has to handle a superset of what C99 allows. 1638 Further, these clients can benefit from extended information. For example, we 1639 know that "foo()||1" always evaluates to true, but we can't replace the 1640 expression with true because it has side effects.</p> 1641 1642 <!-- ======================= --> 1643 <h4>Implementation Approach</h4> 1644 <!-- ======================= --> 1645 1646 <p>After trying several different approaches, we've finally converged on a 1647 design (Note, at the time of this writing, not all of this has been implemented, 1648 consider this a design goal!). Our basic approach is to define a single 1649 recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is 1650 implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar' 1651 type (integer, fp, complex, or pointer) this method returns the following 1652 information:</p> 1653 1654 <ul> 1655 <li>Whether the expression is an integer constant expression, a general 1656 constant that was folded but has no side effects, a general constant that 1657 was folded but that does have side effects, or an uncomputable/unfoldable 1658 value. 1659 </li> 1660 <li>If the expression was computable in any way, this method returns the APValue 1661 for the result of the expression.</li> 1662 <li>If the expression is not evaluatable at all, this method returns 1663 information on one of the problems with the expression. This includes a 1664 SourceLocation for where the problem is, and a diagnostic ID that explains 1665 the problem. The diagnostic should be have ERROR type.</li> 1666 <li>If the expression is not an integer constant expression, this method returns 1667 information on one of the problems with the expression. This includes a 1668 SourceLocation for where the problem is, and a diagnostic ID that explains 1669 the problem. The diagnostic should be have EXTENSION type.</li> 1670 </ul> 1671 1672 <p>This information gives various clients the flexibility that they want, and we 1673 will eventually have some helper methods for various extensions. For example, 1674 Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which 1675 calls Evaluate on the expression. If the expression is not foldable, the error 1676 is emitted, and it would return true. If the expression is not an i-c-e, the 1677 EXTENSION diagnostic is emitted. Finally it would return false to indicate that 1678 the AST is ok.</p> 1679 1680 <p>Other clients can use the information in other ways, for example, codegen can 1681 just use expressions that are foldable in any way.</p> 1682 1683 <!-- ========== --> 1684 <h4>Extensions</h4> 1685 <!-- ========== --> 1686 1687 <p>This section describes how some of the various extensions Clang supports 1688 interacts with constant evaluation:</p> 1689 1690 <ul> 1691 <li><b><tt>__extension__</tt></b>: The expression form of this extension causes 1692 any evaluatable subexpression to be accepted as an integer constant 1693 expression.</li> 1694 <li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer 1695 constant expression) if the operand is any evaluatable constant. As a 1696 special case, if <tt>__builtin_constant_p</tt> is the (potentially 1697 parenthesized) condition of a conditional operator expression ("?:"), only 1698 the true side of the conditional operator is considered, and it is evaluated 1699 with full constant folding.</li> 1700 <li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an 1701 integer constant expression, but we accept any constant as an "extension of 1702 an extension". This only evaluates one operand depending on which way the 1703 condition evaluates.</li> 1704 <li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer 1705 constant expression.</li> 1706 <li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a 1707 floating-point literal.</li> 1708 <li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as 1709 general constant expressions.</li> 1710 <li><b><tt>__builtin_strlen</tt></b> and <b><tt>strlen</tt></b>: These are constant folded as integer constant expressions if the argument is a string literal.</li> 1711 </ul> 1712 1713 1714 <!-- ======================================================================= --> 1715 <h2 id="Howtos">How to change Clang</h2> 1716 <!-- ======================================================================= --> 1717 1718 <!-- ======================================================================= --> 1719 <h3 id="AddingAttributes">How to add an attribute</h3> 1720 <!-- ======================================================================= --> 1721 1722 <p>To add an attribute, you'll have to add it to the list of attributes, add it 1723 to the parsing phase, and look for it in the AST scan. 1724 <a href="http://llvm.org/viewvc/llvm-project?view=rev&revision=124217">r124217</a> 1725 has a good example of adding a warning attribute.</p> 1726 1727 <p>(Beware that this hasn't been reviewed/fixed by the people who designed the 1728 attributes system yet.)</p> 1729 1730 <h4><a 1731 href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?view=markup">include/clang/Basic/Attr.td</a></h4> 1732 1733 <p>Each attribute gets a <tt>def</tt> inheriting from <tt>Attr</tt> or one of 1734 its subclasses. <tt>InheritableAttr</tt> means that the attribute also applies 1735 to subsequent declarations of the same name.</p> 1736 1737 <p><tt>Spellings</tt> lists the strings that can appear in 1738 <tt>__attribute__((here))</tt> or <tt>[[here]]</tt>. All such strings 1739 will be synonymous. If you want to allow the <tt>[[]]</tt> C++11 1740 syntax, you have to define a list of <tt>Namespaces</tt>, which will 1741 let users write <tt>[[namespace:spelling]]</tt>. Using the empty 1742 string for a namespace will allow users to write just the spelling 1743 with no "<tt>:</tt>".</p> 1744 1745 <p><tt>Subjects</tt> restricts what kinds of AST node to which this attribute 1746 can appertain (roughly, attach).</p> 1747 1748 <p><tt>Args</tt> names the arguments the attribute takes, in order. If 1749 <tt>Args</tt> is <tt>[StringArgument<"Arg1">, IntArgument<"Arg2">]</tt> 1750 then <tt>__attribute__((myattribute("Hello", 3)))</tt> will be a valid use.</p> 1751 1752 <h4>Boilerplate</h4> 1753 1754 <p>Add an element to the <tt>AttributeList::Kind</tt> enum in <a 1755 href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Sema/AttributeList.h?view=markup">include/clang/Sema/AttributeList.h</a> 1756 named <tt>AT_lower_with_underscores</tt>. That is, a CamelCased 1757 <tt>AttributeName</tt> in <tt>Attr.td</tt> name should become 1758 <tt>AT_attribute_name</tt>.</p> 1759 1760 <p>Add a case to the <tt>StringSwitch</tt> in <tt>AttributeList::getKind()</tt> 1761 in <a 1762 href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/AttributeList.cpp?view=markup">lib/Sema/AttributeList.cpp</a> 1763 for each spelling of your attribute. Less common attributes should come toward 1764 the end of that list.</p> 1765 1766 <p>Write a new <tt>HandleYourAttr()</tt> function in <a 1767 href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?view=markup">lib/Sema/SemaDeclAttr.cpp</a>, 1768 and add a case to the switch in <tt>ProcessNonInheritableDeclAttr()</tt> or 1769 <tt>ProcessInheritableDeclAttr()</tt> forwarding to it.</p> 1770 1771 <p>If your attribute causes extra warnings to fire, define a <tt>DiagGroup</tt> 1772 in <a 1773 href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?view=markup">include/clang/Basic/DiagnosticGroups.td</a> 1774 named after the attribute's <tt>Spelling</tt> with "_"s replaced by "-"s. If 1775 you're only defining one diagnostic, you can skip <tt>DiagnosticGroups.td</tt> 1776 and use <tt>InGroup<DiagGroup<"your-attribute">></tt> directly in <a 1777 href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?view=markup">DiagnosticSemaKinds.td</a></p> 1778 1779 <h4>The meat of your attribute</h4> 1780 1781 <p>Find an appropriate place in Clang to do whatever your attribute needs to do. 1782 Check for the attribute's presence using <tt>Decl::getAttr<YourAttr>()</tt>.</p> 1783 1784 <p>Update the <a href="LanguageExtensions.html">Clang Language Extensions</a> 1785 document to describe your new attribute.</p> 1786 1787 <!-- ======================================================================= --> 1788 <h3 id="AddingExprStmt">How to add an expression or statement</h3> 1789 <!-- ======================================================================= --> 1790 1791 <p>Expressions and statements are one of the most fundamental constructs within a 1792 compiler, because they interact with many different parts of the AST, 1793 semantic analysis, and IR generation. Therefore, adding a new 1794 expression or statement kind into Clang requires some care. The following list 1795 details the various places in Clang where an expression or statement needs to be 1796 introduced, along with patterns to follow to ensure that the new 1797 expression or statement works well across all of the C languages. We 1798 focus on expressions, but statements are similar.</p> 1799 1800 <ol> 1801 <li>Introduce parsing actions into the parser. Recursive-descent 1802 parsing is mostly self-explanatory, but there are a few things that 1803 are worth keeping in mind: 1804 <ul> 1805 <li>Keep as much source location information as possible! You'll 1806 want it later to produce great diagnostics and support Clang's 1807 various features that map between source code and the AST.</li> 1808 <li>Write tests for all of the "bad" parsing cases, to make sure 1809 your recovery is good. If you have matched delimiters (e.g., 1810 parentheses, square brackets, etc.), use 1811 <tt>Parser::BalancedDelimiterTracker</tt> to give nice diagnostics when 1812 things go wrong.</li> 1813 </ul> 1814 </li> 1815 1816 <li>Introduce semantic analysis actions into <tt>Sema</tt>. Semantic 1817 analysis should always involve two functions: an <tt>ActOnXXX</tt> 1818 function that will be called directly from the parser, and a 1819 <tt>BuildXXX</tt> function that performs the actual semantic 1820 analysis and will (eventually!) build the AST node. It's fairly 1821 common for the <tt>ActOnCXX</tt> function to do very little (often 1822 just some minor translation from the parser's representation to 1823 <tt>Sema</tt>'s representation of the same thing), but the separation 1824 is still important: C++ template instantiation, for example, 1825 should always call the <tt>BuildXXX</tt> variant. Several notes on 1826 semantic analysis before we get into construction of the AST: 1827 <ul> 1828 <li>Your expression probably involves some types and some 1829 subexpressions. Make sure to fully check that those types, and the 1830 types of those subexpressions, meet your expectations. Add 1831 implicit conversions where necessary to make sure that all of the 1832 types line up exactly the way you want them. Write extensive tests 1833 to check that you're getting good diagnostics for mistakes and 1834 that you can use various forms of subexpressions with your 1835 expression.</li> 1836 <li>When type-checking a type or subexpression, make sure to first 1837 check whether the type is "dependent" 1838 (<tt>Type::isDependentType()</tt>) or whether a subexpression is 1839 type-dependent (<tt>Expr::isTypeDependent()</tt>). If any of these 1840 return true, then you're inside a template and you can't do much 1841 type-checking now. That's normal, and your AST node (when you get 1842 there) will have to deal with this case. At this point, you can 1843 write tests that use your expression within templates, but don't 1844 try to instantiate the templates.</li> 1845 <li>For each subexpression, be sure to call 1846 <tt>Sema::CheckPlaceholderExpr()</tt> to deal with "weird" 1847 expressions that don't behave well as subexpressions. Then, 1848 determine whether you need to perform 1849 lvalue-to-rvalue conversions 1850 (<tt>Sema::DefaultLvalueConversion</tt>e) or 1851 the usual unary conversions 1852 (<tt>Sema::UsualUnaryConversions</tt>), for places where the 1853 subexpression is producing a value you intend to use.</li> 1854 <li>Your <tt>BuildXXX</tt> function will probably just return 1855 <tt>ExprError()</tt> at this point, since you don't have an AST. 1856 That's perfectly fine, and shouldn't impact your testing.</li> 1857 </ul> 1858 </li> 1859 1860 <li>Introduce an AST node for your new expression. This starts with 1861 declaring the node in <tt>include/Basic/StmtNodes.td</tt> and 1862 creating a new class for your expression in the appropriate 1863 <tt>include/AST/Expr*.h</tt> header. It's best to look at the class 1864 for a similar expression to get ideas, and there are some specific 1865 things to watch for: 1866 <ul> 1867 <li>If you need to allocate memory, use the <tt>ASTContext</tt> 1868 allocator to allocate memory. Never use raw <tt>malloc</tt> or 1869 <tt>new</tt>, and never hold any resources in an AST node, because 1870 the destructor of an AST node is never called.</li> 1871 1872 <li>Make sure that <tt>getSourceRange()</tt> covers the exact 1873 source range of your expression. This is needed for diagnostics 1874 and for IDE support.</li> 1875 1876 <li>Make sure that <tt>children()</tt> visits all of the 1877 subexpressions. This is important for a number of features (e.g., IDE 1878 support, C++ variadic templates). If you have sub-types, you'll 1879 also need to visit those sub-types in the 1880 <tt>RecursiveASTVisitor</tt>.</li> 1881 1882 <li>Add printing support (<tt>StmtPrinter.cpp</tt>) and dumping 1883 support (<tt>StmtDumper.cpp</tt>) for your expression.</li> 1884 1885 <li>Add profiling support (<tt>StmtProfile.cpp</tt>) for your AST 1886 node, noting the distinguishing (non-source location) 1887 characteristics of an instance of your expression. Omitting this 1888 step will lead to hard-to-diagnose failures regarding matching of 1889 template declarations.</li> 1890 </ul> 1891 </li> 1892 1893 <li>Teach semantic analysis to build your AST node! At this point, 1894 you can wire up your <tt>Sema::BuildXXX</tt> function to actually 1895 create your AST. A few things to check at this point: 1896 <ul> 1897 <li>If your expression can construct a new C++ class or return a 1898 new Objective-C object, be sure to update and then call 1899 <tt>Sema::MaybeBindToTemporary</tt> for your just-created AST node 1900 to be sure that the object gets properly destructed. An easy way 1901 to test this is to return a C++ class with a private destructor: 1902 semantic analysis should flag an error here with the attempt to 1903 call the destructor.</li> 1904 <li>Inspect the generated AST by printing it using <tt>clang -cc1 1905 -ast-print</tt>, to make sure you're capturing all of the 1906 important information about how the AST was written.</li> 1907 <li>Inspect the generated AST under <tt>clang -cc1 -ast-dump</tt> 1908 to verify that all of the types in the generated AST line up the 1909 way you want them. Remember that clients of the AST should never 1910 have to "think" to understand what's going on. For example, all 1911 implicit conversions should show up explicitly in the AST.</li> 1912 <li>Write tests that use your expression as a subexpression of 1913 other, well-known expressions. Can you call a function using your 1914 expression as an argument? Can you use the ternary operator?</li> 1915 </ul> 1916 </li> 1917 1918 <li>Teach code generation to create IR to your AST node. This step 1919 is the first (and only) that requires knowledge of LLVM IR. There 1920 are several things to keep in mind: 1921 <ul> 1922 <li>Code generation is separated into scalar/aggregate/complex and 1923 lvalue/rvalue paths, depending on what kind of result your 1924 expression produces. On occasion, this requires some careful 1925 factoring of code to avoid duplication.</li> 1926 1927 <li><tt>CodeGenFunction</tt> contains functions 1928 <tt>ConvertType</tt> and <tt>ConvertTypeForMem</tt> that convert 1929 Clang's types (<tt>clang::Type*</tt> or <tt>clang::QualType</tt>) 1930 to LLVM types. 1931 Use the former for values, and the later for memory locations: 1932 test with the C++ "bool" type to check this. If you find 1933 that you are having to use LLVM bitcasts to make 1934 the subexpressions of your expression have the type that your 1935 expression expects, STOP! Go fix semantic analysis and the AST so 1936 that you don't need these bitcasts.</li> 1937 1938 <li>The <tt>CodeGenFunction</tt> class has a number of helper 1939 functions to make certain operations easy, such as generating code 1940 to produce an lvalue or an rvalue, or to initialize a memory 1941 location with a given value. Prefer to use these functions rather 1942 than directly writing loads and stores, because these functions 1943 take care of some of the tricky details for you (e.g., for 1944 exceptions).</li> 1945 1946 <li>If your expression requires some special behavior in the event 1947 of an exception, look at the <tt>push*Cleanup</tt> functions in 1948 <tt>CodeGenFunction</tt> to introduce a cleanup. You shouldn't 1949 have to deal with exception-handling directly.</li> 1950 1951 <li>Testing is extremely important in IR generation. Use <tt>clang 1952 -cc1 -emit-llvm</tt> and <a 1953 href="http://llvm.org/cmds/FileCheck.html">FileCheck</a> to verify 1954 that you're generating the right IR.</li> 1955 </ul> 1956 </li> 1957 1958 <li>Teach template instantiation how to cope with your AST 1959 node, which requires some fairly simple code: 1960 <ul> 1961 <li>Make sure that your expression's constructor properly 1962 computes the flags for type dependence (i.e., the type your 1963 expression produces can change from one instantiation to the 1964 next), value dependence (i.e., the constant value your expression 1965 produces can change from one instantiation to the next), 1966 instantiation dependence (i.e., a template parameter occurs 1967 anywhere in your expression), and whether your expression contains 1968 a parameter pack (for variadic templates). Often, computing these 1969 flags just means combining the results from the various types and 1970 subexpressions.</li> 1971 1972 <li>Add <tt>TransformXXX</tt> and <tt>RebuildXXX</tt> functions to 1973 the 1974 <tt>TreeTransform</tt> class template in <tt>Sema</tt>. 1975 <tt>TransformXXX</tt> should (recursively) transform all of the 1976 subexpressions and types 1977 within your expression, using <tt>getDerived().TransformYYY</tt>. 1978 If all of the subexpressions and types transform without error, it 1979 will then call the <tt>RebuildXXX</tt> function, which will in 1980 turn call <tt>getSema().BuildXXX</tt> to perform semantic analysis 1981 and build your expression.</li> 1982 1983 <li>To test template instantiation, take those tests you wrote to 1984 make sure that you were type checking with type-dependent 1985 expressions and dependent types (from step #2) and instantiate 1986 those templates with various types, some of which type-check and 1987 some that don't, and test the error messages in each case.</li> 1988 </ul> 1989 </li> 1990 1991 <li>There are some "extras" that make other features work better. 1992 It's worth handling these extras to give your expression complete 1993 integration into Clang: 1994 <ul> 1995 <li>Add code completion support for your expression in 1996 <tt>SemaCodeComplete.cpp</tt>.</li> 1997 1998 <li>If your expression has types in it, or has any "interesting" 1999 features other than subexpressions, extend libclang's 2000 <tt>CursorVisitor</tt> to provide proper visitation for your 2001 expression, enabling various IDE features such as syntax 2002 highlighting, cross-referencing, and so on. The 2003 <tt>c-index-test</tt> helper program can be used to test these 2004 features.</li> 2005 </ul> 2006 </li> 2007 </ol> 2008 2009 </div> 2010 </body> 2011 </html> 2012
