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1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" |
| 2 | "http://www.w3.org/TR/html4/strict.dtd"> | |
| 3 | <html> | |
| 4 | <head> | |
| 5 | <title>"Clang" CFE Internals Manual</title> | |
| 6 | <link type="text/css" rel="stylesheet" href="../menu.css"> | |
| 7 | <link type="text/css" rel="stylesheet" href="../content.css"> | |
| 8 | <style type="text/css"> | |
| 9 | td { | |
| 10 | vertical-align: top; | |
| 11 | } | |
| 12 | </style> | |
| 13 | </head> | |
| 14 | <body> | |
| 15 | ||
| 16 | <!--#include virtual="../menu.html.incl"--> | |
| 17 | ||
| 18 | <div id="content"> | |
| 19 | ||
| 20 | <h1>"Clang" CFE Internals Manual</h1> | |
| 21 | ||
| 22 | <ul> | |
| 23 | <li><a href="#intro">Introduction</a></li> | |
| 24 | <li><a href="#libsupport">LLVM Support Library</a></li> | |
| 25 | <li><a href="#libbasic">The Clang 'Basic' Library</a> | |
| 26 | <ul> | |
| 27 | <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li> | |
| 28 | <li><a href="#SourceLocation">The SourceLocation and SourceManager | |
| 29 | classes</a></li> | |
| 30 | <li><a href="#SourceRange">SourceRange and CharSourceRange</a></li> | |
| 31 | </ul> | |
| 32 | </li> | |
| 33 | <li><a href="#libdriver">The Driver Library</a> | |
| 34 | </li> | |
| 35 | <li><a href="#pch">Precompiled Headers</a> | |
| 36 | <li><a href="#libfrontend">The Frontend Library</a> | |
| 37 | </li> | |
| 38 | <li><a href="#liblex">The Lexer and Preprocessor Library</a> | |
| 39 | <ul> | |
| 40 | <li><a href="#Token">The Token class</a></li> | |
| 41 | <li><a href="#Lexer">The Lexer class</a></li> | |
| 42 | <li><a href="#AnnotationToken">Annotation Tokens</a></li> | |
| 43 | <li><a href="#TokenLexer">The TokenLexer class</a></li> | |
| 44 | <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li> | |
| 45 | </ul> | |
| 46 | </li> | |
| 47 | <li><a href="#libparse">The Parser Library</a> | |
| 48 | </li> | |
| 49 | <li><a href="#libast">The AST Library</a> | |
| 50 | <ul> | |
| 51 | <li><a href="#Type">The Type class and its subclasses</a></li> | |
| 52 | <li><a href="#QualType">The QualType class</a></li> | |
| 53 | <li><a href="#DeclarationName">Declaration names</a></li> | |
| 54 | <li><a href="#DeclContext">Declaration contexts</a> | |
| 55 | <ul> | |
| 56 | <li><a href="#Redeclarations">Redeclarations and Overloads</a></li> | |
| 57 | <li><a href="#LexicalAndSemanticContexts">Lexical and Semantic | |
| 58 | Contexts</a></li> | |
| 59 | <li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li> | |
| 60 | <li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li> | |
| 61 | </ul> | |
| 62 | </li> | |
| 63 | <li><a href="#CFG">The CFG class</a></li> | |
| 64 | <li><a href="#Constants">Constant Folding in the Clang AST</a></li> | |
| 65 | </ul> | |
| 66 | </li> | |
| 67 | <li><a href="#Howtos">Howto guides</a> | |
| 68 | <ul> | |
| 69 | <li><a href="#AddingAttributes">How to add an attribute</a></li> | |
| 70 | <li><a href="#AddingExprStmt">How to add a new expression or statement</a></li> | |
| 71 | </ul> | |
| 72 | </li> | |
| 73 | </ul> | |
| 74 | ||
| 75 | ||
| 76 | <!-- ======================================================================= --> | |
| 77 | <h2 id="intro">Introduction</h2> | |
| 78 | <!-- ======================================================================= --> | |
| 79 | ||
| 80 | <p>This document describes some of the more important APIs and internal design | |
| 81 | decisions made in the Clang C front-end. The purpose of this document is to | |
| 82 | both capture some of this high level information and also describe some of the | |
| 83 | design decisions behind it. This is meant for people interested in hacking on | |
| 84 | Clang, not for end-users. The description below is categorized by | |
| 85 | libraries, and does not describe any of the clients of the libraries.</p> | |
| 86 | ||
| 87 | <!-- ======================================================================= --> | |
| 88 | <h2 id="libsupport">LLVM Support Library</h2> | |
| 89 | <!-- ======================================================================= --> | |
| 90 | ||
| 91 | <p>The LLVM libsupport library provides many underlying libraries and | |
| 92 | <a href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>, | |
| 93 | including command line option processing, various containers and a system | |
| 94 | abstraction layer, which is used for file system access.</p> | |
| 95 | ||
| 96 | <!-- ======================================================================= --> | |
| 97 | <h2 id="libbasic">The Clang 'Basic' Library</h2> | |
| 98 | <!-- ======================================================================= --> | |
| 99 | ||
| 100 | <p>This library certainly needs a better name. The 'basic' library contains a | |
| 101 | number of low-level utilities for tracking and manipulating source buffers, | |
| 102 | locations within the source buffers, diagnostics, tokens, target abstraction, | |
| 103 | and information about the subset of the language being compiled for.</p> | |
| 104 | ||
| 105 | <p>Part of this infrastructure is specific to C (such as the TargetInfo class), | |
| 106 | other parts could be reused for other non-C-based languages (SourceLocation, | |
| 107 | SourceManager, Diagnostics, FileManager). When and if there is future demand | |
| 108 | we can figure out if it makes sense to introduce a new library, move the general | |
| 109 | classes somewhere else, or introduce some other solution.</p> | |
| 110 | ||
| 111 | <p>We describe the roles of these classes in order of their dependencies.</p> | |
| 112 | ||
| 113 | ||
| 114 | <!-- ======================================================================= --> | |
| 115 | <h3 id="Diagnostics">The Diagnostics Subsystem</h3> | |
| 116 | <!-- ======================================================================= --> | |
| 117 | ||
| 118 | <p>The Clang Diagnostics subsystem is an important part of how the compiler | |
| 119 | communicates with the human. Diagnostics are the warnings and errors produced | |
| 120 | when the code is incorrect or dubious. In Clang, each diagnostic produced has | |
| 121 | (at the minimum) a unique ID, an English translation associated with it, a <a | |
| 122 | href="#SourceLocation">SourceLocation</a> to "put the caret", and a severity (e.g. | |
| 123 | <tt>WARNING</tt> or <tt>ERROR</tt>). They can also optionally include a number | |
| 124 | of arguments to the dianostic (which fill in "%0"'s in the string) as well as a | |
| 125 | number of source ranges that related to the diagnostic.</p> | |
| 126 | ||
| 127 | <p>In this section, we'll be giving examples produced by the Clang command line | |
| 128 | driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many | |
| 129 | different ways</a> depending on how the DiagnosticClient interface is | |
| 130 | implemented. A representative example of a diagnostic is:</p> | |
| 131 | ||
| 132 | <pre> | |
| 133 | t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float') | |
| 134 | <span style="color:darkgreen">P = (P-42) + Gamma*4;</span> | |
| 135 | <span style="color:blue">~~~~~~ ^ ~~~~~~~</span> | |
| 136 | </pre> | |
| 137 | ||
| 138 | <p>In this example, you can see the English translation, the severity (error), | |
| 139 | you can see the source location (the caret ("^") and file/line/column info), | |
| 140 | the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex | |
| 141 | float"). You'll have to believe me that there is a unique ID backing the | |
| 142 | diagnostic :).</p> | |
| 143 | ||
| 144 | <p>Getting all of this to happen has several steps and involves many moving | |
| 145 | pieces, this section describes them and talks about best practices when adding | |
| 146 | a new diagnostic.</p> | |
| 147 | ||
| 148 | <!-- ============================= --> | |
| 149 | <h4>The Diagnostic*Kinds.td files</h4> | |
| 150 | <!-- ============================= --> | |
| 151 | ||
| 152 | <p>Diagnostics are created by adding an entry to one of the <tt> | |
| 153 | clang/Basic/Diagnostic*Kinds.td</tt> files, depending on what library will | |
| 154 | be using it. From this file, tblgen generates the unique ID of the diagnostic, | |
| 155 | the severity of the diagnostic and the English translation + format string.</p> | |
| 156 | ||
| 157 | <p>There is little sanity with the naming of the unique ID's right now. Some | |
| 158 | start with err_, warn_, ext_ to encode the severity into the name. Since the | |
| 159 | enum is referenced in the C++ code that produces the diagnostic, it is somewhat | |
| 160 | useful for it to be reasonably short.</p> | |
| 161 | ||
| 162 | <p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>, | |
| 163 | <tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The | |
| 164 | <tt>ERROR</tt> severity is used for diagnostics indicating the program is never | |
| 165 | acceptable under any circumstances. When an error is emitted, the AST for the | |
| 166 | input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt> | |
| 167 | severities are used for extensions to the language that Clang accepts. This | |
| 168 | means that Clang fully understands and can represent them in the AST, but we | |
| 169 | produce diagnostics to tell the user their code is non-portable. The difference | |
| 170 | is that the former are ignored by default, and the later warn by default. The | |
| 171 | <tt>WARNING</tt> severity is used for constructs that are valid in the currently | |
| 172 | selected source language but that are dubious in some way. The <tt>NOTE</tt> | |
| 173 | level is used to staple more information onto previous diagnostics.</p> | |
| 174 | ||
| 175 | <p>These <em>severities</em> are mapped into a smaller set (the | |
| 176 | Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>, | |
| 177 | <tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics | |
| 178 | subsystem based on various configuration options. Clang internally supports a | |
| 179 | fully fine grained mapping mechanism that allows you to map almost any | |
| 180 | diagnostic to the output level that you want. The only diagnostics that cannot | |
| 181 | be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously | |
| 182 | emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to | |
| 183 | <tt>Fatal</tt> (it is not possible to turn an error into a warning, | |
| 184 | for example).</p> | |
| 185 | ||
| 186 | <p>Diagnostic mappings are used in many ways. For example, if the user | |
| 187 | specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if | |
| 188 | they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is | |
| 189 | used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc. | |
| 190 | </p> | |
| 191 | ||
| 192 | <p> | |
| 193 | Mapping to <tt>Fatal</tt> should only be used for diagnostics that are | |
| 194 | considered so severe that error recovery won't be able to recover sensibly from | |
| 195 | them (thus spewing a ton of bogus errors). One example of this class of error | |
| 196 | are failure to #include a file. | |
| 197 | </p> | |
| 198 | ||
| 199 | <!-- ================= --> | |
| 200 | <h4>The Format String</h4> | |
| 201 | <!-- ================= --> | |
| 202 | ||
| 203 | <p>The format string for the diagnostic is very simple, but it has some power. | |
| 204 | It takes the form of a string in English with markers that indicate where and | |
| 205 | how arguments to the diagnostic are inserted and formatted. For example, here | |
| 206 | are some simple format strings:</p> | |
| 207 | ||
| 208 | <pre> | |
| 209 | "binary integer literals are an extension" | |
| 210 | "format string contains '\\0' within the string body" | |
| 211 | "more '<b>%%</b>' conversions than data arguments" | |
| 212 | "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)" | |
| 213 | "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator" | |
| 214 | " (has <b>%1</b> parameter<b>%s1</b>)" | |
| 215 | </pre> | |
| 216 | ||
| 217 | <p>These examples show some important points of format strings. You can use any | |
| 218 | plain ASCII character in the diagnostic string except "%" without a problem, | |
| 219 | but these are C strings, so you have to use and be aware of all the C escape | |
| 220 | sequences (as in the second example). If you want to produce a "%" in the | |
| 221 | output, use the "%%" escape sequence, like the third diagnostic. Finally, | |
| 222 | Clang uses the "%...[digit]" sequences to specify where and how arguments to | |
| 223 | the diagnostic are formatted.</p> | |
| 224 | ||
| 225 | <p>Arguments to the diagnostic are numbered according to how they are specified | |
| 226 | by the C++ code that <a href="#producingdiag">produces them</a>, and are | |
| 227 | referenced by <tt>%0</tt> .. <tt>%9</tt>. If you have more than 10 arguments | |
| 228 | to your diagnostic, you are doing something wrong :). Unlike printf, there | |
| 229 | is no requirement that arguments to the diagnostic end up in the output in | |
| 230 | the same order as they are specified, you could have a format string with | |
| 231 | <tt>"%1 %0"</tt> that swaps them, for example. The text in between the | |
| 232 | percent and digit are formatting instructions. If there are no instructions, | |
| 233 | the argument is just turned into a string and substituted in.</p> | |
| 234 | ||
| 235 | <p>Here are some "best practices" for writing the English format string:</p> | |
| 236 | ||
| 237 | <ul> | |
| 238 | <li>Keep the string short. It should ideally fit in the 80 column limit of the | |
| 239 | <tt>DiagnosticKinds.td</tt> file. This avoids the diagnostic wrapping when | |
| 240 | printed, and forces you to think about the important point you are conveying | |
| 241 | with the diagnostic.</li> | |
| 242 | <li>Take advantage of location information. The user will be able to see the | |
| 243 | line and location of the caret, so you don't need to tell them that the | |
| 244 | problem is with the 4th argument to the function: just point to it.</li> | |
| 245 | <li>Do not capitalize the diagnostic string, and do not end it with a | |
| 246 | period.</li> | |
| 247 | <li>If you need to quote something in the diagnostic string, use single | |
| 248 | quotes.</li> | |
| 249 | </ul> | |
| 250 | ||
| 251 | <p>Diagnostics should never take random English strings as arguments: you | |
| 252 | shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like | |
| 253 | <tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing | |
| 254 | this prevents <a href="#translation">translating</a> the Clang diagnostics to | |
| 255 | other languages (because they'll get random English words in their otherwise | |
| 256 | localized diagnostic). The exceptions to this are C/C++ language keywords | |
| 257 | (e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note | |
| 258 | that things like "pointer" and "reference" are not keywords. On the other | |
| 259 | hand, you <em>can</em> include anything that comes from the user's source code, | |
| 260 | including variable names, types, labels, etc. The 'select' format can be | |
| 261 | used to achieve this sort of thing in a localizable way, see below.</p> | |
| 262 | ||
| 263 | <!-- ==================================== --> | |
| 264 | <h4>Formatting a Diagnostic Argument</h4> | |
| 265 | <!-- ==================================== --> | |
| 266 | ||
| 267 | <p>Arguments to diagnostics are fully typed internally, and come from a couple | |
| 268 | different classes: integers, types, names, and random strings. Depending on | |
| 269 | the class of the argument, it can be optionally formatted in different ways. | |
| 270 | This gives the DiagnosticClient information about what the argument means | |
| 271 | without requiring it to use a specific presentation (consider this MVC for | |
| 272 | Clang :).</p> | |
| 273 | ||
| 274 | <p>Here are the different diagnostic argument formats currently supported by | |
| 275 | Clang:</p> | |
| 276 | ||
| 277 | <table> | |
| 278 | <tr><td colspan="2"><b>"s" format</b></td></tr> | |
| 279 | <tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr> | |
| 280 | <tr><td>Class:</td><td>Integers</td></tr> | |
| 281 | <tr><td>Description:</td><td>This is a simple formatter for integers that is | |
| 282 | useful when producing English diagnostics. When the integer is 1, it prints | |
| 283 | as nothing. When the integer is not 1, it prints as "s". This allows some | |
| 284 | simple grammatical forms to be to be handled correctly, and eliminates the | |
| 285 | need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr> | |
| 286 | ||
| 287 | <tr><td colspan="2"><b>"select" format</b></td></tr> | |
| 288 | <tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2 | |
| 289 | operator"</tt></td></tr> | |
| 290 | <tr><td>Class:</td><td>Integers</td></tr> | |
| 291 | <tr><td>Description:</td><td><p>This format specifier is used to merge multiple | |
| 292 | related diagnostics together into one common one, without requiring the | |
| 293 | difference to be specified as an English string argument. Instead of | |
| 294 | specifying the string, the diagnostic gets an integer argument and the | |
| 295 | format string selects the numbered option. In this case, the "%2" value | |
| 296 | must be an integer in the range [0..2]. If it is 0, it prints 'unary', if | |
| 297 | it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This | |
| 298 | allows other language translations to substitute reasonable words (or entire | |
| 299 | phrases) based on the semantics of the diagnostic instead of having to do | |
| 300 | things textually.</p> | |
| 301 | <p>The selected string does undergo formatting.</p></td></tr> | |
| 302 | ||
| 303 | <tr><td colspan="2"><b>"plural" format</b></td></tr> | |
| 304 | <tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to | |
| 305 | your computer"</tt></td></tr> | |
| 306 | <tr><td>Class:</td><td>Integers</td></tr> | |
| 307 | <tr><td>Description:</td><td><p>This is a formatter for complex plural forms. | |
| 308 | It is designed to handle even the requirements of languages with very | |
| 309 | complex plural forms, as many Baltic languages have. The argument consists | |
| 310 | of a series of expression/form pairs, separated by ':', where the first form | |
| 311 | whose expression evaluates to true is the result of the modifier.</p> | |
| 312 | <p>An expression can be empty, in which case it is always true. See the | |
| 313 | example at the top. Otherwise, it is a series of one or more numeric | |
| 314 | conditions, separated by ','. If any condition matches, the expression | |
| 315 | matches. Each numeric condition can take one of three forms.</p> | |
| 316 | <ul> | |
| 317 | <li>number: A simple decimal number matches if the argument is the same | |
| 318 | as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li> | |
| 319 | <li>range: A range in square brackets matches if the argument is within | |
| 320 | the range. Then range is inclusive on both ends. Example: | |
| 321 | <tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li> | |
| 322 | <li>modulo: A modulo operator is followed by a number, and | |
| 323 | equals sign and either a number or a range. The tests are the | |
| 324 | same as for plain | |
| 325 | numbers and ranges, but the argument is taken modulo the number first. | |
| 326 | Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything | |
| 327 | else}1"</tt></li> | |
| 328 | </ul> | |
| 329 | <p>The parser is very unforgiving. A syntax error, even whitespace, will | |
| 330 | abort, as will a failure to match the argument against any | |
| 331 | expression.</p></td></tr> | |
| 332 | ||
| 333 | <tr><td colspan="2"><b>"ordinal" format</b></td></tr> | |
| 334 | <tr><td>Example:</td><td><tt>"ambiguity in %ordinal0 argument"</tt></td></tr> | |
| 335 | <tr><td>Class:</td><td>Integers</td></tr> | |
| 336 | <tr><td>Description:</td><td><p>This is a formatter which represents the | |
| 337 | argument number as an ordinal: the value <tt>1</tt> becomes <tt>1st</tt>, | |
| 338 | <tt>3</tt> becomes <tt>3rd</tt>, and so on. Values less than <tt>1</tt> | |
| 339 | are not supported.</p> | |
| 340 | <p>This formatter is currently hard-coded to use English ordinals.</p></td></tr> | |
| 341 | ||
| 342 | <tr><td colspan="2"><b>"objcclass" format</b></td></tr> | |
| 343 | <tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr> | |
| 344 | <tr><td>Class:</td><td>DeclarationName</td></tr> | |
| 345 | <tr><td>Description:</td><td><p>This is a simple formatter that indicates the | |
| 346 | DeclarationName corresponds to an Objective-C class method selector. As | |
| 347 | such, it prints the selector with a leading '+'.</p></td></tr> | |
| 348 | ||
| 349 | <tr><td colspan="2"><b>"objcinstance" format</b></td></tr> | |
| 350 | <tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr> | |
| 351 | <tr><td>Class:</td><td>DeclarationName</td></tr> | |
| 352 | <tr><td>Description:</td><td><p>This is a simple formatter that indicates the | |
| 353 | DeclarationName corresponds to an Objective-C instance method selector. As | |
| 354 | such, it prints the selector with a leading '-'.</p></td></tr> | |
| 355 | ||
| 356 | <tr><td colspan="2"><b>"q" format</b></td></tr> | |
| 357 | <tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr> | |
| 358 | <tr><td>Class:</td><td>NamedDecl*</td></tr> | |
| 359 | <tr><td>Description</td><td><p>This formatter indicates that the fully-qualified name of the declaration should be printed, e.g., "std::vector" rather than "vector".</p></td></tr> | |
| 360 | ||
| 361 | <tr><td colspan="2"><b>"diff" format</b></td></tr> | |
| 362 | <tr><td>Example:</td><td><tt>"no known conversion %diff{from | to | }1,2"</tt></td></tr> | |
| 363 | <tr><td>Class:</td><td>QualType</td></tr> | |
| 364 | <tr><td>Description</td><td><p>This formatter takes two QualTypes and attempts to print a template difference between the two. If tree printing is off, the text inside the braces before the pipe is printed, with the formatted text replacing the $. If tree printing is on, the text after the pipe is printed and a type tree is printed after the diagnostic message. | |
| 365 | </p></td></tr> | |
| 366 | ||
| 367 | </table> | |
| 368 | ||
| 369 | <p>It is really easy to add format specifiers to the Clang diagnostics system, | |
| 370 | but they should be discussed before they are added. If you are creating a lot | |
| 371 | of repetitive diagnostics and/or have an idea for a useful formatter, please | |
| 372 | bring it up on the cfe-dev mailing list.</p> | |
| 373 | ||
| 374 | <!-- ===================================================== --> | |
| 375 | <h4 id="producingdiag">Producing the Diagnostic</h4> | |
| 376 | <!-- ===================================================== --> | |
| 377 | ||
| 378 | <p>Now that you've created the diagnostic in the DiagnosticKinds.td file, you | |
| 379 | need to write the code that detects the condition in question and emits the | |
| 380 | new diagnostic. Various components of Clang (e.g. the preprocessor, Sema, | |
| 381 | etc) provide a helper function named "Diag". It creates a diagnostic and | |
| 382 | accepts the arguments, ranges, and other information that goes along with | |
| 383 | it.</p> | |
| 384 | ||
| 385 | <p>For example, the binary expression error comes from code like this:</p> | |
| 386 | ||
| 387 | <pre> | |
| 388 | if (various things that are bad) | |
| 389 | Diag(Loc, diag::err_typecheck_invalid_operands) | |
| 390 | << lex->getType() << rex->getType() | |
| 391 | << lex->getSourceRange() << rex->getSourceRange(); | |
| 392 | </pre> | |
| 393 | ||
| 394 | <p>This shows that use of the Diag method: they take a location (a <a | |
| 395 | href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value | |
| 396 | (which matches the name from DiagnosticKinds.td). If the diagnostic takes | |
| 397 | arguments, they are specified with the << operator: the first argument | |
| 398 | becomes %0, the second becomes %1, etc. The diagnostic interface allows you to | |
| 399 | specify arguments of many different types, including <tt>int</tt> and | |
| 400 | <tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and | |
| 401 | <tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and | |
| 402 | <tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc. | |
| 403 | SourceRanges are also specified with the << operator, but do not have a | |
| 404 | specific ordering requirement.</p> | |
| 405 | ||
| 406 | <p>As you can see, adding and producing a diagnostic is pretty straightforward. | |
| 407 | The hard part is deciding exactly what you need to say to help the user, picking | |
| 408 | a suitable wording, and providing the information needed to format it correctly. | |
| 409 | The good news is that the call site that issues a diagnostic should be | |
| 410 | completely independent of how the diagnostic is formatted and in what language | |
| 411 | it is rendered. | |
| 412 | </p> | |
| 413 | ||
| 414 | <!-- ==================================================== --> | |
| 415 | <h4 id="fix-it-hints">Fix-It Hints</h4> | |
| 416 | <!-- ==================================================== --> | |
| 417 | ||
| 418 | <p>In some cases, the front end emits diagnostics when it is clear | |
| 419 | that some small change to the source code would fix the problem. For | |
| 420 | example, a missing semicolon at the end of a statement or a use of | |
| 421 | deprecated syntax that is easily rewritten into a more modern form. | |
| 422 | Clang tries very hard to emit the diagnostic and recover gracefully | |
| 423 | in these and other cases.</p> | |
| 424 | ||
| 425 | <p>However, for these cases where the fix is obvious, the diagnostic | |
| 426 | can be annotated with a hint (referred to as a "fix-it hint") that | |
| 427 | describes how to change the code referenced by the diagnostic to fix | |
| 428 | the problem. For example, it might add the missing semicolon at the | |
| 429 | end of the statement or rewrite the use of a deprecated construct | |
| 430 | into something more palatable. Here is one such example from the C++ | |
| 431 | front end, where we warn about the right-shift operator changing | |
| 432 | meaning from C++98 to C++11:</p> | |
| 433 | ||
| 434 | <pre> | |
| 435 | test.cpp:3:7: warning: use of right-shift operator ('>>') in template argument will require parentheses in C++11 | |
| 436 | A<100 >> 2> *a; | |
| 437 | ^ | |
| 438 | ( ) | |
| 439 | </pre> | |
| 440 | ||
| 441 | <p>Here, the fix-it hint is suggesting that parentheses be added, | |
| 442 | and showing exactly where those parentheses would be inserted into the | |
| 443 | source code. The fix-it hints themselves describe what changes to make | |
| 444 | to the source code in an abstract manner, which the text diagnostic | |
| 445 | printer renders as a line of "insertions" below the caret line. <a | |
| 446 | href="#DiagnosticClient">Other diagnostic clients</a> might choose | |
| 447 | to render the code differently (e.g., as markup inline) or even give | |
| 448 | the user the ability to automatically fix the problem.</p> | |
| 449 | ||
| 450 | <p>Fix-it hints on errors and warnings need to obey these rules:</p> | |
| 451 | ||
| 452 | <ul> | |
| 453 | <li>Since they are automatically applied if <code>-Xclang -fixit</code> | |
| 454 | is passed to the driver, they should only be used when it's very likely they | |
| 455 | match the user's intent.</li> | |
| 456 | <li>Clang must recover from errors as if the fix-it had been applied.</li> | |
| 457 | </ul> | |
| 458 | ||
| 459 | <p>If a fix-it can't obey these rules, put the fix-it on a note. Fix-its on | |
| 460 | notes are not applied automatically.</p> | |
| 461 | ||
| 462 | <p>All fix-it hints are described by the <code>FixItHint</code> class, | |
| 463 | instances of which should be attached to the diagnostic using the | |
| 464 | << operator in the same way that highlighted source ranges and | |
| 465 | arguments are passed to the diagnostic. Fix-it hints can be created | |
| 466 | with one of three constructors:</p> | |
| 467 | ||
| 468 | <dl> | |
| 469 | <dt><code>FixItHint::CreateInsertion(Loc, Code)</code></dt> | |
| 470 | <dd>Specifies that the given <code>Code</code> (a string) should be inserted | |
| 471 | before the source location <code>Loc</code>.</dd> | |
| 472 | ||
| 473 | <dt><code>FixItHint::CreateRemoval(Range)</code></dt> | |
| 474 | <dd>Specifies that the code in the given source <code>Range</code> | |
| 475 | should be removed.</dd> | |
| 476 | ||
| 477 | <dt><code>FixItHint::CreateReplacement(Range, Code)</code></dt> | |
| 478 | <dd>Specifies that the code in the given source <code>Range</code> | |
| 479 | should be removed, and replaced with the given <code>Code</code> string.</dd> | |
| 480 | </dl> | |
| 481 | ||
| 482 | <!-- ============================================================= --> | |
| 483 | <h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4> | |
| 484 | <!-- ============================================================= --> | |
| 485 | ||
| 486 | <p>Once code generates a diagnostic with all of the arguments and the rest of | |
| 487 | the relevant information, Clang needs to know what to do with it. As previously | |
| 488 | mentioned, the diagnostic machinery goes through some filtering to map a | |
| 489 | severity onto a diagnostic level, then (assuming the diagnostic is not mapped to | |
| 490 | "<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient | |
| 491 | interface with the information.</p> | |
| 492 | ||
| 493 | <p>It is possible to implement this interface in many different ways. For | |
| 494 | example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns | |
| 495 | the arguments into strings (according to the various formatting rules), prints | |
| 496 | out the file/line/column information and the string, then prints out the line of | |
| 497 | code, the source ranges, and the caret. However, this behavior isn't required. | |
| 498 | </p> | |
| 499 | ||
| 500 | <p>Another implementation of the DiagnosticClient interface is the | |
| 501 | 'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode. | |
| 502 | Instead of formatting and printing out the diagnostics, this implementation just | |
| 503 | captures and remembers the diagnostics as they fly by. Then -verify compares | |
| 504 | the list of produced diagnostics to the list of expected ones. If they disagree, | |
| 505 | it prints out its own output. | |
| 506 | </p> | |
| 507 | ||
| 508 | <p>There are many other possible implementations of this interface, and this is | |
| 509 | why we prefer diagnostics to pass down rich structured information in arguments. | |
| 510 | For example, an HTML output might want declaration names be linkified to where | |
| 511 | they come from in the source. Another example is that a GUI might let you click | |
| 512 | on typedefs to expand them. This application would want to pass significantly | |
| 513 | more information about types through to the GUI than a simple flat string. The | |
| 514 | interface allows this to happen.</p> | |
| 515 | ||
| 516 | <!-- ====================================================== --> | |
| 517 | <h4><a name="translation">Adding Translations to Clang</a></h4> | |
| 518 | <!-- ====================================================== --> | |
| 519 | ||
| 520 | <p>Not possible yet! Diagnostic strings should be written in UTF-8, the client | |
| 521 | can translate to the relevant code page if needed. Each translation completely | |
| 522 | replaces the format string for the diagnostic.</p> | |
| 523 | ||
| 524 | ||
| 525 | <!-- ======================================================================= --> | |
| 526 | <h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3> | |
| 527 | <!-- ======================================================================= --> | |
| 528 | ||
| 529 | <p>Strangely enough, the SourceLocation class represents a location within the | |
| 530 | source code of the program. Important design points include:</p> | |
| 531 | ||
| 532 | <ol> | |
| 533 | <li>sizeof(SourceLocation) must be extremely small, as these are embedded into | |
| 534 | many AST nodes and are passed around often. Currently it is 32 bits.</li> | |
| 535 | <li>SourceLocation must be a simple value object that can be efficiently | |
| 536 | copied.</li> | |
| 537 | <li>We should be able to represent a source location for any byte of any input | |
| 538 | file. This includes in the middle of tokens, in whitespace, in trigraphs, | |
| 539 | etc.</li> | |
| 540 | <li>A SourceLocation must encode the current #include stack that was active when | |
| 541 | the location was processed. For example, if the location corresponds to a | |
| 542 | token, it should contain the set of #includes active when the token was | |
| 543 | lexed. This allows us to print the #include stack for a diagnostic.</li> | |
| 544 | <li>SourceLocation must be able to describe macro expansions, capturing both | |
| 545 | the ultimate instantiation point and the source of the original character | |
| 546 | data.</li> | |
| 547 | </ol> | |
| 548 | ||
| 549 | <p>In practice, the SourceLocation works together with the SourceManager class | |
| 550 | to encode two pieces of information about a location: its spelling location | |
| 551 | and its instantiation location. For most tokens, these will be the same. | |
| 552 | However, for a macro expansion (or tokens that came from a _Pragma directive) | |
| 553 | these will describe the location of the characters corresponding to the token | |
| 554 | and the location where the token was used (i.e. the macro instantiation point | |
| 555 | or the location of the _Pragma itself).</p> | |
| 556 | ||
| 557 | <p>The Clang front-end inherently depends on the location of a token being | |
| 558 | tracked correctly. If it is ever incorrect, the front-end may get confused and | |
| 559 | die. The reason for this is that the notion of the 'spelling' of a Token in | |
| 560 | Clang depends on being able to find the original input characters for the token. | |
| 561 | This concept maps directly to the "spelling location" for the token.</p> | |
| 562 | ||
| 563 | ||
| 564 | <!-- ======================================================================= --> | |
| 565 | <h3 id="SourceRange">SourceRange and CharSourceRange</h3> | |
| 566 | <!-- ======================================================================= --> | |
| 567 | <!-- mostly taken from | |
| 568 | http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-August/010595.html --> | |
| 569 | ||
| 570 | <p>Clang represents most source ranges by [first, last], where first and last | |
| 571 | each point to the beginning of their respective tokens. For example | |
| 572 | consider the SourceRange of the following statement:</p> | |
| 573 | <pre> | |
| 574 | x = foo + bar; | |
| 575 | ^first ^last | |
| 576 | </pre> | |
| 577 | ||
| 578 | <p>To map from this representation to a character-based | |
| 579 | representation, the 'last' location needs to be adjusted to point to | |
| 580 | (or past) the end of that token with either | |
| 581 | <code>Lexer::MeasureTokenLength()</code> or | |
| 582 | <code>Lexer::getLocForEndOfToken()</code>. For the rare cases | |
| 583 | where character-level source ranges information is needed we use | |
| 584 | the <code>CharSourceRange</code> class.</p> | |
| 585 | ||
| 586 | ||
| 587 | <!-- ======================================================================= --> | |
| 588 | <h2 id="libdriver">The Driver Library</h2> | |
| 589 | <!-- ======================================================================= --> | |
| 590 | ||
| 591 | <p>The clang Driver and library are documented <a | |
| 592 | href="DriverInternals.html">here</a>.<p> | |
| 593 | ||
| 594 | <!-- ======================================================================= --> | |
| 595 | <h2 id="pch">Precompiled Headers</h2> | |
| 596 | <!-- ======================================================================= --> | |
| 597 | ||
| 598 | <p>Clang supports two implementations of precompiled headers. The | |
| 599 | default implementation, precompiled headers (<a | |
| 600 | href="PCHInternals.html">PCH</a>) uses a serialized representation | |
| 601 | of Clang's internal data structures, encoded with the <a | |
| 602 | href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream | |
| 603 | format</a>. Pretokenized headers (<a | |
| 604 | href="PTHInternals.html">PTH</a>), on the other hand, contain a | |
| 605 | serialized representation of the tokens encountered when | |
| 606 | preprocessing a header (and anything that header includes).</p> | |
| 607 | ||
| 608 | ||
| 609 | <!-- ======================================================================= --> | |
| 610 | <h2 id="libfrontend">The Frontend Library</h2> | |
| 611 | <!-- ======================================================================= --> | |
| 612 | ||
| 613 | <p>The Frontend library contains functionality useful for building | |
| 614 | tools on top of the clang libraries, for example several methods for | |
| 615 | outputting diagnostics.</p> | |
| 616 | ||
| 617 | <!-- ======================================================================= --> | |
| 618 | <h2 id="liblex">The Lexer and Preprocessor Library</h2> | |
| 619 | <!-- ======================================================================= --> | |
| 620 | ||
| 621 | <p>The Lexer library contains several tightly-connected classes that are involved | |
| 622 | with the nasty process of lexing and preprocessing C source code. The main | |
| 623 | interface to this library for outside clients is the large <a | |
| 624 | href="#Preprocessor">Preprocessor</a> class. | |
| 625 | It contains the various pieces of state that are required to coherently read | |
| 626 | tokens out of a translation unit.</p> | |
| 627 | ||
| 628 | <p>The core interface to the Preprocessor object (once it is set up) is the | |
| 629 | Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from | |
| 630 | the preprocessor stream. There are two types of token providers that the | |
| 631 | preprocessor is capable of reading from: a buffer lexer (provided by the <a | |
| 632 | href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a | |
| 633 | href="#TokenLexer">TokenLexer</a> class). | |
| 634 | ||
| 635 | ||
| 636 | <!-- ======================================================================= --> | |
| 637 | <h3 id="Token">The Token class</h3> | |
| 638 | <!-- ======================================================================= --> | |
| 639 | ||
| 640 | <p>The Token class is used to represent a single lexed token. Tokens are | |
| 641 | intended to be used by the lexer/preprocess and parser libraries, but are not | |
| 642 | intended to live beyond them (for example, they should not live in the ASTs).<p> | |
| 643 | ||
| 644 | <p>Tokens most often live on the stack (or some other location that is efficient | |
| 645 | to access) as the parser is running, but occasionally do get buffered up. For | |
| 646 | example, macro definitions are stored as a series of tokens, and the C++ | |
| 647 | front-end periodically needs to buffer tokens up for tentative parsing and | |
| 648 | various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit | |
| 649 | system, sizeof(Token) is currently 16 bytes.</p> | |
| 650 | ||
| 651 | <p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation | |
| 652 | Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer, | |
| 653 | annotation tokens represent semantic information and are produced by the parser, | |
| 654 | replacing normal tokens in the token stream. Normal tokens contain the | |
| 655 | following information:</p> | |
| 656 | ||
| 657 | <ul> | |
| 658 | <li><b>A SourceLocation</b> - This indicates the location of the start of the | |
| 659 | token.</li> | |
| 660 | ||
| 661 | <li><b>A length</b> - This stores the length of the token as stored in the | |
| 662 | SourceBuffer. For tokens that include them, this length includes trigraphs and | |
| 663 | escaped newlines which are ignored by later phases of the compiler. By pointing | |
| 664 | into the original source buffer, it is always possible to get the original | |
| 665 | spelling of a token completely accurately.</li> | |
| 666 | ||
| 667 | <li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if | |
| 668 | identifier lookup was enabled when the token was lexed (e.g. the lexer was not | |
| 669 | reading in 'raw' mode) this contains a pointer to the unique hash value for the | |
| 670 | identifier. Because the lookup happens before keyword identification, this | |
| 671 | field is set even for language keywords like 'for'.</li> | |
| 672 | ||
| 673 | <li><b>TokenKind</b> - This indicates the kind of token as classified by the | |
| 674 | lexer. This includes things like <tt>tok::starequal</tt> (for the "*=" | |
| 675 | operator), <tt>tok::ampamp</tt> for the "&&" token, and keyword values | |
| 676 | (e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note | |
| 677 | that some tokens can be spelled multiple ways. For example, C++ supports | |
| 678 | "operator keywords", where things like "and" are treated exactly like the | |
| 679 | "&&" operator. In these cases, the kind value is set to | |
| 680 | <tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to | |
| 681 | consider both forms. For something that cares about which form is used (e.g. | |
| 682 | the preprocessor 'stringize' operator) the spelling indicates the original | |
| 683 | form.</li> | |
| 684 | ||
| 685 | <li><b>Flags</b> - There are currently four flags tracked by the | |
| 686 | lexer/preprocessor system on a per-token basis: | |
| 687 | ||
| 688 | <ol> | |
| 689 | <li><b>StartOfLine</b> - This was the first token that occurred on its input | |
| 690 | source line.</li> | |
| 691 | <li><b>LeadingSpace</b> - There was a space character either immediately | |
| 692 | before the token or transitively before the token as it was expanded | |
| 693 | through a macro. The definition of this flag is very closely defined by | |
| 694 | the stringizing requirements of the preprocessor.</li> | |
| 695 | <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to | |
| 696 | represent identifier tokens which have macro expansion disabled. This | |
| 697 | prevents them from being considered as candidates for macro expansion ever | |
| 698 | in the future.</li> | |
| 699 | <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the | |
| 700 | token includes a trigraph or escaped newline. Since this is uncommon, | |
| 701 | many pieces of code can fast-path on tokens that did not need cleaning. | |
| 702 | </ol> | |
| 703 | </li> | |
| 704 | </ul> | |
| 705 | ||
| 706 | <p>One interesting (and somewhat unusual) aspect of normal tokens is that they | |
| 707 | don't contain any semantic information about the lexed value. For example, if | |
| 708 | the token was a pp-number token, we do not represent the value of the number | |
| 709 | that was lexed (this is left for later pieces of code to decide). Additionally, | |
| 710 | the lexer library has no notion of typedef names vs variable names: both are | |
| 711 | returned as identifiers, and the parser is left to decide whether a specific | |
| 712 | identifier is a typedef or a variable (tracking this requires scope information | |
| 713 | among other things). The parser can do this translation by replacing tokens | |
| 714 | returned by the preprocessor with "Annotation Tokens".</p> | |
| 715 | ||
| 716 | <!-- ======================================================================= --> | |
| 717 | <h3 id="AnnotationToken">Annotation Tokens</h3> | |
| 718 | <!-- ======================================================================= --> | |
| 719 | ||
| 720 | <p>Annotation Tokens are tokens that are synthesized by the parser and injected | |
| 721 | into the preprocessor's token stream (replacing existing tokens) to record | |
| 722 | semantic information found by the parser. For example, if "foo" is found to be | |
| 723 | a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an | |
| 724 | <tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this | |
| 725 | makes it easy to handle qualified type names (e.g. "foo::bar::baz<42>::t") | |
| 726 | in C++ as a single "token" in the parser. 2) if the parser backtracks, the | |
| 727 | reparse does not need to redo semantic analysis to determine whether a token | |
| 728 | sequence is a variable, type, template, etc.</p> | |
| 729 | ||
| 730 | <p>Annotation Tokens are created by the parser and reinjected into the parser's | |
| 731 | token stream (when backtracking is enabled). Because they can only exist in | |
| 732 | tokens that the preprocessor-proper is done with, it doesn't need to keep around | |
| 733 | flags like "start of line" that the preprocessor uses to do its job. | |
| 734 | Additionally, an annotation token may "cover" a sequence of preprocessor tokens | |
| 735 | (e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields | |
| 736 | of an annotation token are different than the fields for a normal token (but | |
| 737 | they are multiplexed into the normal Token fields):</p> | |
| 738 | ||
| 739 | <ul> | |
| 740 | <li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation | |
| 741 | token indicates the first token replaced by the annotation token. In the example | |
| 742 | above, it would be the location of the "a" identifier.</li> | |
| 743 | ||
| 744 | <li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the | |
| 745 | last token replaced with the annotation token. In the example above, it would | |
| 746 | be the location of the "c" identifier.</li> | |
| 747 | ||
| 748 | <li><b>void* "AnnotationValue"</b> - This contains an opaque object | |
| 749 | that the parser gets from Sema. The parser merely preserves the | |
| 750 | information for Sema to later interpret based on the annotation token | |
| 751 | kind.</li> | |
| 752 | ||
| 753 | <li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this | |
| 754 | is. See below for the different valid kinds.</li> | |
| 755 | </ul> | |
| 756 | ||
| 757 | <p>Annotation tokens currently come in three kinds:</p> | |
| 758 | ||
| 759 | <ol> | |
| 760 | <li><b>tok::annot_typename</b>: This annotation token represents a | |
| 761 | resolved typename token that is potentially qualified. The | |
| 762 | AnnotationValue field contains the <tt>QualType</tt> returned by | |
| 763 | Sema::getTypeName(), possibly with source location information | |
| 764 | attached.</li> | |
| 765 | ||
| 766 | <li><b>tok::annot_cxxscope</b>: This annotation token represents a C++ | |
| 767 | scope specifier, such as "A::B::". This corresponds to the grammar | |
| 768 | productions "::" and ":: [opt] nested-name-specifier". The | |
| 769 | AnnotationValue pointer is a <tt>NestedNameSpecifier*</tt> returned by | |
| 770 | the Sema::ActOnCXXGlobalScopeSpecifier and | |
| 771 | Sema::ActOnCXXNestedNameSpecifier callbacks.</li> | |
| 772 | ||
| 773 | <li><b>tok::annot_template_id</b>: This annotation token represents a | |
| 774 | C++ template-id such as "foo<int, 4>", where "foo" is the name | |
| 775 | of a template. The AnnotationValue pointer is a pointer to a malloc'd | |
| 776 | TemplateIdAnnotation object. Depending on the context, a parsed | |
| 777 | template-id that names a type might become a typename annotation token | |
| 778 | (if all we care about is the named type, e.g., because it occurs in a | |
| 779 | type specifier) or might remain a template-id token (if we want to | |
| 780 | retain more source location information or produce a new type, e.g., | |
| 781 | in a declaration of a class template specialization). template-id | |
| 782 | annotation tokens that refer to a type can be "upgraded" to typename | |
| 783 | annotation tokens by the parser.</li> | |
| 784 | ||
| 785 | </ol> | |
| 786 | ||
| 787 | <p>As mentioned above, annotation tokens are not returned by the preprocessor, | |
| 788 | they are formed on demand by the parser. This means that the parser has to be | |
| 789 | aware of cases where an annotation could occur and form it where appropriate. | |
| 790 | This is somewhat similar to how the parser handles Translation Phase 6 of C99: | |
| 791 | String Concatenation (see C99 5.1.1.2). In the case of string concatenation, | |
| 792 | the preprocessor just returns distinct tok::string_literal and | |
| 793 | tok::wide_string_literal tokens and the parser eats a sequence of them wherever | |
| 794 | the grammar indicates that a string literal can occur.</p> | |
| 795 | ||
| 796 | <p>In order to do this, whenever the parser expects a tok::identifier or | |
| 797 | tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or | |
| 798 | TryAnnotateCXXScopeToken methods to form the annotation token. These methods | |
| 799 | will maximally form the specified annotation tokens and replace the current | |
| 800 | token with them, if applicable. If the current tokens is not valid for an | |
| 801 | annotation token, it will remain an identifier or :: token.</p> | |
| 802 | ||
| 803 | ||
| 804 | ||
| 805 | <!-- ======================================================================= --> | |
| 806 | <h3 id="Lexer">The Lexer class</h3> | |
| 807 | <!-- ======================================================================= --> | |
| 808 | ||
| 809 | <p>The Lexer class provides the mechanics of lexing tokens out of a source | |
| 810 | buffer and deciding what they mean. The Lexer is complicated by the fact that | |
| 811 | it operates on raw buffers that have not had spelling eliminated (this is a | |
| 812 | necessity to get decent performance), but this is countered with careful coding | |
| 813 | as well as standard performance techniques (for example, the comment handling | |
| 814 | code is vectorized on X86 and PowerPC hosts).</p> | |
| 815 | ||
| 816 | <p>The lexer has a couple of interesting modal features:</p> | |
| 817 | ||
| 818 | <ul> | |
| 819 | <li>The lexer can operate in 'raw' mode. This mode has several features that | |
| 820 | make it possible to quickly lex the file (e.g. it stops identifier lookup, | |
| 821 | doesn't specially handle preprocessor tokens, handles EOF differently, etc). | |
| 822 | This mode is used for lexing within an "<tt>#if 0</tt>" block, for | |
| 823 | example.</li> | |
| 824 | <li>The lexer can capture and return comments as tokens. This is required to | |
| 825 | support the -C preprocessor mode, which passes comments through, and is | |
| 826 | used by the diagnostic checker to identifier expect-error annotations.</li> | |
| 827 | <li>The lexer can be in ParsingFilename mode, which happens when preprocessing | |
| 828 | after reading a #include directive. This mode changes the parsing of '<' | |
| 829 | to return an "angled string" instead of a bunch of tokens for each thing | |
| 830 | within the filename.</li> | |
| 831 | <li>When parsing a preprocessor directive (after "<tt>#</tt>") the | |
| 832 | ParsingPreprocessorDirective mode is entered. This changes the parser to | |
| 833 | return EOD at a newline.</li> | |
| 834 | <li>The Lexer uses a LangOptions object to know whether trigraphs are enabled, | |
| 835 | whether C++ or ObjC keywords are recognized, etc.</li> | |
| 836 | </ul> | |
| 837 | ||
| 838 | <p>In addition to these modes, the lexer keeps track of a couple of other | |
| 839 | features that are local to a lexed buffer, which change as the buffer is | |
| 840 | lexed:</p> | |
| 841 | ||
| 842 | <ul> | |
| 843 | <li>The Lexer uses BufferPtr to keep track of the current character being | |
| 844 | lexed.</li> | |
| 845 | <li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token | |
| 846 | will start with its "start of line" bit set.</li> | |
| 847 | <li>The Lexer keeps track of the current #if directives that are active (which | |
| 848 | can be nested).</li> | |
| 849 | <li>The Lexer keeps track of an <a href="#MultipleIncludeOpt"> | |
| 850 | MultipleIncludeOpt</a> object, which is used to | |
| 851 | detect whether the buffer uses the standard "<tt>#ifndef XX</tt> / | |
| 852 | <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does, | |
| 853 | subsequent includes can be ignored if the XX macro is defined.</li> | |
| 854 | </ul> | |
| 855 | ||
| 856 | <!-- ======================================================================= --> | |
| 857 | <h3 id="TokenLexer">The TokenLexer class</h3> | |
| 858 | <!-- ======================================================================= --> | |
| 859 | ||
| 860 | <p>The TokenLexer class is a token provider that returns tokens from a list | |
| 861 | of tokens that came from somewhere else. It typically used for two things: 1) | |
| 862 | returning tokens from a macro definition as it is being expanded 2) returning | |
| 863 | tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and | |
| 864 | will most likely be used to handle unbounded look-ahead for the C++ parser.</p> | |
| 865 | ||
| 866 | <!-- ======================================================================= --> | |
| 867 | <h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3> | |
| 868 | <!-- ======================================================================= --> | |
| 869 | ||
| 870 | <p>The MultipleIncludeOpt class implements a really simple little state machine | |
| 871 | that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>" | |
| 872 | idiom that people typically use to prevent multiple inclusion of headers. If a | |
| 873 | buffer uses this idiom and is subsequently #include'd, the preprocessor can | |
| 874 | simply check to see whether the guarding condition is defined or not. If so, | |
| 875 | the preprocessor can completely ignore the include of the header.</p> | |
| 876 | ||
| 877 | ||
| 878 | ||
| 879 | <!-- ======================================================================= --> | |
| 880 | <h2 id="libparse">The Parser Library</h2> | |
| 881 | <!-- ======================================================================= --> | |
| 882 | ||
| 883 | <!-- ======================================================================= --> | |
| 884 | <h2 id="libast">The AST Library</h2> | |
| 885 | <!-- ======================================================================= --> | |
| 886 | ||
| 887 | <!-- ======================================================================= --> | |
| 888 | <h3 id="Type">The Type class and its subclasses</h3> | |
| 889 | <!-- ======================================================================= --> | |
| 890 | ||
| 891 | <p>The Type class (and its subclasses) are an important part of the AST. Types | |
| 892 | are accessed through the ASTContext class, which implicitly creates and uniques | |
| 893 | them as they are needed. Types have a couple of non-obvious features: 1) they | |
| 894 | do not capture type qualifiers like const or volatile (See | |
| 895 | <a href="#QualType">QualType</a>), and 2) they implicitly capture typedef | |
| 896 | information. Once created, types are immutable (unlike decls).</p> | |
| 897 | ||
| 898 | <p>Typedefs in C make semantic analysis a bit more complex than it would | |
| 899 | be without them. The issue is that we want to capture typedef information | |
| 900 | and represent it in the AST perfectly, but the semantics of operations need to | |
| 901 | "see through" typedefs. For example, consider this code:</p> | |
| 902 | ||
| 903 | <code> | |
| 904 | void func() {<br> | |
| 905 | typedef int foo;<br> | |
| 906 | foo X, *Y;<br> | |
| 907 | typedef foo* bar;<br> | |
| 908 | bar Z;<br> | |
| 909 | *X; <i>// error</i><br> | |
| 910 | **Y; <i>// error</i><br> | |
| 911 | **Z; <i>// error</i><br> | |
| 912 | }<br> | |
| 913 | </code> | |
| 914 | ||
| 915 | <p>The code above is illegal, and thus we expect there to be diagnostics emitted | |
| 916 | on the annotated lines. In this example, we expect to get:</p> | |
| 917 | ||
| 918 | <pre> | |
| 919 | <b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b> | |
| 920 | *X; // error | |
| 921 | <span style="color:blue">^~</span> | |
| 922 | <b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b> | |
| 923 | **Y; // error | |
| 924 | <span style="color:blue">^~~</span> | |
| 925 | <b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b> | |
| 926 | **Z; // error | |
| 927 | <span style="color:blue">^~~</span> | |
| 928 | </pre> | |
| 929 | ||
| 930 | <p>While this example is somewhat silly, it illustrates the point: we want to | |
| 931 | retain typedef information where possible, so that we can emit errors about | |
| 932 | "<tt>std::string</tt>" instead of "<tt>std::basic_string<char, std:...</tt>". | |
| 933 | Doing this requires properly keeping typedef information (for example, the type | |
| 934 | of "X" is "foo", not "int"), and requires properly propagating it through the | |
| 935 | various operators (for example, the type of *Y is "foo", not "int"). In order | |
| 936 | to retain this information, the type of these expressions is an instance of the | |
| 937 | TypedefType class, which indicates that the type of these expressions is a | |
| 938 | typedef for foo. | |
| 939 | </p> | |
| 940 | ||
| 941 | <p>Representing types like this is great for diagnostics, because the | |
| 942 | user-specified type is always immediately available. There are two problems | |
| 943 | with this: first, various semantic checks need to make judgements about the | |
| 944 | <em>actual structure</em> of a type, ignoring typedefs. Second, we need an | |
| 945 | efficient way to query whether two types are structurally identical to each | |
| 946 | other, ignoring typedefs. The solution to both of these problems is the idea of | |
| 947 | canonical types.</p> | |
| 948 | ||
| 949 | <!-- =============== --> | |
| 950 | <h4>Canonical Types</h4> | |
| 951 | <!-- =============== --> | |
| 952 | ||
| 953 | <p>Every instance of the Type class contains a canonical type pointer. For | |
| 954 | simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>", | |
| 955 | "<tt>int**</tt>"), the type just points to itself. For types that have a | |
| 956 | typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>", | |
| 957 | "<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their | |
| 958 | structurally equivalent type without any typedefs (e.g. "<tt>int</tt>", | |
| 959 | "<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p> | |
| 960 | ||
| 961 | <p>This design provides a constant time operation (dereferencing the canonical | |
| 962 | type pointer) that gives us access to the structure of types. For example, | |
| 963 | we can trivially tell that "bar" and "foo*" are the same type by dereferencing | |
| 964 | their canonical type pointers and doing a pointer comparison (they both point | |
| 965 | to the single "<tt>int*</tt>" type).</p> | |
| 966 | ||
| 967 | <p>Canonical types and typedef types bring up some complexities that must be | |
| 968 | carefully managed. Specifically, the "isa/cast/dyncast" operators generally | |
| 969 | shouldn't be used in code that is inspecting the AST. For example, when type | |
| 970 | checking the indirection operator (unary '*' on a pointer), the type checker | |
| 971 | must verify that the operand has a pointer type. It would not be correct to | |
| 972 | check that with "<tt>isa<PointerType>(SubExpr->getType())</tt>", | |
| 973 | because this predicate would fail if the subexpression had a typedef type.</p> | |
| 974 | ||
| 975 | <p>The solution to this problem are a set of helper methods on Type, used to | |
| 976 | check their properties. In this case, it would be correct to use | |
| 977 | "<tt>SubExpr->getType()->isPointerType()</tt>" to do the check. This | |
| 978 | predicate will return true if the <em>canonical type is a pointer</em>, which is | |
| 979 | true any time the type is structurally a pointer type. The only hard part here | |
| 980 | is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p> | |
| 981 | ||
| 982 | <p>The second problem we face is how to get access to the pointer type once we | |
| 983 | know it exists. To continue the example, the result type of the indirection | |
| 984 | operator is the pointee type of the subexpression. In order to determine the | |
| 985 | type, we need to get the instance of PointerType that best captures the typedef | |
| 986 | information in the program. If the type of the expression is literally a | |
| 987 | PointerType, we can return that, otherwise we have to dig through the | |
| 988 | typedefs to find the pointer type. For example, if the subexpression had type | |
| 989 | "<tt>foo*</tt>", we could return that type as the result. If the subexpression | |
| 990 | had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do | |
| 991 | <em>not</em> want "<tt>int*</tt>"). In order to provide all of this, Type has | |
| 992 | a getAsPointerType() method that checks whether the type is structurally a | |
| 993 | PointerType and, if so, returns the best one. If not, it returns a null | |
| 994 | pointer.</p> | |
| 995 | ||
| 996 | <p>This structure is somewhat mystical, but after meditating on it, it will | |
| 997 | make sense to you :).</p> | |
| 998 | ||
| 999 | <!-- ======================================================================= --> | |
| 1000 | <h3 id="QualType">The QualType class</h3> | |
| 1001 | <!-- ======================================================================= --> | |
| 1002 | ||
| 1003 | <p>The QualType class is designed as a trivial value class that is | |
| 1004 | small, passed by-value and is efficient to query. The idea of | |
| 1005 | QualType is that it stores the type qualifiers (const, volatile, | |
| 1006 | restrict, plus some extended qualifiers required by language | |
| 1007 | extensions) separately from the types themselves. QualType is | |
| 1008 | conceptually a pair of "Type*" and the bits for these type qualifiers.</p> | |
| 1009 | ||
| 1010 | <p>By storing the type qualifiers as bits in the conceptual pair, it is | |
| 1011 | extremely efficient to get the set of qualifiers on a QualType (just return the | |
| 1012 | field of the pair), add a type qualifier (which is a trivial constant-time | |
| 1013 | operation that sets a bit), and remove one or more type qualifiers (just return | |
| 1014 | a QualType with the bitfield set to empty).</p> | |
| 1015 | ||
| 1016 | <p>Further, because the bits are stored outside of the type itself, we do not | |
| 1017 | need to create duplicates of types with different sets of qualifiers (i.e. there | |
| 1018 | is only a single heap allocated "int" type: "const int" and "volatile const int" | |
| 1019 | both point to the same heap allocated "int" type). This reduces the heap size | |
| 1020 | used to represent bits and also means we do not have to consider qualifiers when | |
| 1021 | uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p> | |
| 1022 | ||
| 1023 | <p>In practice, the two most common type qualifiers (const and | |
| 1024 | restrict) are stored in the low bits of the pointer to the Type | |
| 1025 | object, together with a flag indicating whether extended qualifiers | |
| 1026 | are present (which must be heap-allocated). This means that QualType | |
| 1027 | is exactly the same size as a pointer.</p> | |
| 1028 | ||
| 1029 | <!-- ======================================================================= --> | |
| 1030 | <h3 id="DeclarationName">Declaration names</h3> | |
| 1031 | <!-- ======================================================================= --> | |
| 1032 | ||
| 1033 | <p>The <tt>DeclarationName</tt> class represents the name of a | |
| 1034 | declaration in Clang. Declarations in the C family of languages can | |
| 1035 | take several different forms. Most declarations are named by | |
| 1036 | simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in | |
| 1037 | the function declaration <code>f(int x)</code>. In C++, declaration | |
| 1038 | names can also name class constructors ("<code>Class</code>" | |
| 1039 | in <code>struct Class { Class(); }</code>), class destructors | |
| 1040 | ("<code>~Class</code>"), overloaded operator names ("operator+"), | |
| 1041 | and conversion functions ("<code>operator void const *</code>"). In | |
| 1042 | Objective-C, declaration names can refer to the names of Objective-C | |
| 1043 | methods, which involve the method name and the parameters, | |
| 1044 | collectively called a <i>selector</i>, e.g., | |
| 1045 | "<code>setWidth:height:</code>". Since all of these kinds of | |
| 1046 | entities - variables, functions, Objective-C methods, C++ | |
| 1047 | constructors, destructors, and operators - are represented as | |
| 1048 | subclasses of Clang's common <code>NamedDecl</code> | |
| 1049 | class, <code>DeclarationName</code> is designed to efficiently | |
| 1050 | represent any kind of name.</p> | |
| 1051 | ||
| 1052 | <p>Given | |
| 1053 | a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code> | |
| 1054 | will produce a value that describes what kind of name <code>N</code> | |
| 1055 | stores. There are 8 options (all of the names are inside | |
| 1056 | the <code>DeclarationName</code> class)</p> | |
| 1057 | <dl> | |
| 1058 | <dt>Identifier</dt> | |
| 1059 | <dd>The name is a simple | |
| 1060 | identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the | |
| 1061 | corresponding <code>IdentifierInfo*</code> pointing to the actual | |
| 1062 | identifier. Note that C++ overloaded operators (e.g., | |
| 1063 | "<code>operator+</code>") are represented as special kinds of | |
| 1064 | identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code> | |
| 1065 | function to determine whether an identifier is an overloaded | |
| 1066 | operator name.</dd> | |
| 1067 | ||
| 1068 | <dt>ObjCZeroArgSelector, ObjCOneArgSelector, | |
| 1069 | ObjCMultiArgSelector</dt> | |
| 1070 | <dd>The name is an Objective-C selector, which can be retrieved as a | |
| 1071 | <code>Selector</code> instance | |
| 1072 | via <code>N.getObjCSelector()</code>. The three possible name | |
| 1073 | kinds for Objective-C reflect an optimization within | |
| 1074 | the <code>DeclarationName</code> class: both zero- and | |
| 1075 | one-argument selectors are stored as a | |
| 1076 | masked <code>IdentifierInfo</code> pointer, and therefore require | |
| 1077 | very little space, since zero- and one-argument selectors are far | |
| 1078 | more common than multi-argument selectors (which use a different | |
| 1079 | structure).</dd> | |
| 1080 | ||
| 1081 | <dt>CXXConstructorName</dt> | |
| 1082 | <dd>The name is a C++ constructor | |
| 1083 | name. Use <code>N.getCXXNameType()</code> to retrieve | |
| 1084 | the <a href="#QualType">type</a> that this constructor is meant to | |
| 1085 | construct. The type is always the canonical type, since all | |
| 1086 | constructors for a given type have the same name.</dd> | |
| 1087 | ||
| 1088 | <dt>CXXDestructorName</dt> | |
| 1089 | <dd>The name is a C++ destructor | |
| 1090 | name. Use <code>N.getCXXNameType()</code> to retrieve | |
| 1091 | the <a href="#QualType">type</a> whose destructor is being | |
| 1092 | named. This type is always a canonical type.</dd> | |
| 1093 | ||
| 1094 | <dt>CXXConversionFunctionName</dt> | |
| 1095 | <dd>The name is a C++ conversion function. Conversion functions are | |
| 1096 | named according to the type they convert to, e.g., "<code>operator void | |
| 1097 | const *</code>". Use <code>N.getCXXNameType()</code> to retrieve | |
| 1098 | the type that this conversion function converts to. This type is | |
| 1099 | always a canonical type.</dd> | |
| 1100 | ||
| 1101 | <dt>CXXOperatorName</dt> | |
| 1102 | <dd>The name is a C++ overloaded operator name. Overloaded operators | |
| 1103 | are named according to their spelling, e.g., | |
| 1104 | "<code>operator+</code>" or "<code>operator new | |
| 1105 | []</code>". Use <code>N.getCXXOverloadedOperator()</code> to | |
| 1106 | retrieve the overloaded operator (a value of | |
| 1107 | type <code>OverloadedOperatorKind</code>).</dd> | |
| 1108 | </dl> | |
| 1109 | ||
| 1110 | <p><code>DeclarationName</code>s are cheap to create, copy, and | |
| 1111 | compare. They require only a single pointer's worth of storage in | |
| 1112 | the common cases (identifiers, zero- | |
| 1113 | and one-argument Objective-C selectors) and use dense, uniqued | |
| 1114 | storage for the other kinds of | |
| 1115 | names. Two <code>DeclarationName</code>s can be compared for | |
| 1116 | equality (<code>==</code>, <code>!=</code>) using a simple bitwise | |
| 1117 | comparison, can be ordered | |
| 1118 | with <code><</code>, <code>></code>, <code><=</code>, | |
| 1119 | and <code>>=</code> (which provide a lexicographical ordering for | |
| 1120 | normal identifiers but an unspecified ordering for other kinds of | |
| 1121 | names), and can be placed into LLVM <code>DenseMap</code>s | |
| 1122 | and <code>DenseSet</code>s.</p> | |
| 1123 | ||
| 1124 | <p><code>DeclarationName</code> instances can be created in different | |
| 1125 | ways depending on what kind of name the instance will store. Normal | |
| 1126 | identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors | |
| 1127 | (<code>Selector</code>) can be implicitly converted | |
| 1128 | to <code>DeclarationName</code>s. Names for C++ constructors, | |
| 1129 | destructors, conversion functions, and overloaded operators can be retrieved from | |
| 1130 | the <code>DeclarationNameTable</code>, an instance of which is | |
| 1131 | available as <code>ASTContext::DeclarationNames</code>. The member | |
| 1132 | functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>, | |
| 1133 | <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively, | |
| 1134 | return <code>DeclarationName</code> instances for the four kinds of | |
| 1135 | C++ special function names.</p> | |
| 1136 | ||
| 1137 | <!-- ======================================================================= --> | |
| 1138 | <h3 id="DeclContext">Declaration contexts</h3> | |
| 1139 | <!-- ======================================================================= --> | |
| 1140 | <p>Every declaration in a program exists within some <i>declaration | |
| 1141 | context</i>, such as a translation unit, namespace, class, or | |
| 1142 | function. Declaration contexts in Clang are represented by | |
| 1143 | the <code>DeclContext</code> class, from which the various | |
| 1144 | declaration-context AST nodes | |
| 1145 | (<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>, | |
| 1146 | etc.) will derive. The <code>DeclContext</code> class provides | |
| 1147 | several facilities common to each declaration context:</p> | |
| 1148 | <dl> | |
| 1149 | <dt>Source-centric vs. Semantics-centric View of Declarations</dt> | |
| 1150 | <dd><code>DeclContext</code> provides two views of the declarations | |
| 1151 | stored within a declaration context. The source-centric view | |
| 1152 | accurately represents the program source code as written, including | |
| 1153 | multiple declarations of entities where present (see the | |
| 1154 | section <a href="#Redeclarations">Redeclarations and | |
| 1155 | Overloads</a>), while the semantics-centric view represents the | |
| 1156 | program semantics. The two views are kept synchronized by semantic | |
| 1157 | analysis while the ASTs are being constructed.</dd> | |
| 1158 | ||
| 1159 | <dt>Storage of declarations within that context</dt> | |
| 1160 | <dd>Every declaration context can contain some number of | |
| 1161 | declarations. For example, a C++ class (represented | |
| 1162 | by <code>RecordDecl</code>) contains various member functions, | |
| 1163 | fields, nested types, and so on. All of these declarations will be | |
| 1164 | stored within the <code>DeclContext</code>, and one can iterate | |
| 1165 | over the declarations via | |
| 1166 | [<code>DeclContext::decls_begin()</code>, | |
| 1167 | <code>DeclContext::decls_end()</code>). This mechanism provides | |
| 1168 | the source-centric view of declarations in the context.</dd> | |
| 1169 | ||
| 1170 | <dt>Lookup of declarations within that context</dt> | |
| 1171 | <dd>The <code>DeclContext</code> structure provides efficient name | |
| 1172 | lookup for names within that declaration context. For example, | |
| 1173 | if <code>N</code> is a namespace we can look for the | |
| 1174 | name <code>N::f</code> | |
| 1175 | using <code>DeclContext::lookup</code>. The lookup itself is | |
| 1176 | based on a lazily-constructed array (for declaration contexts | |
| 1177 | with a small number of declarations) or hash table (for | |
| 1178 | declaration contexts with more declarations). The lookup | |
| 1179 | operation provides the semantics-centric view of the declarations | |
| 1180 | in the context.</dd> | |
| 1181 | ||
| 1182 | <dt>Ownership of declarations</dt> | |
| 1183 | <dd>The <code>DeclContext</code> owns all of the declarations that | |
| 1184 | were declared within its declaration context, and is responsible | |
| 1185 | for the management of their memory as well as their | |
| 1186 | (de-)serialization.</dd> | |
| 1187 | </dl> | |
| 1188 | ||
| 1189 | <p>All declarations are stored within a declaration context, and one | |
| 1190 | can query | |
| 1191 | information about the context in which each declaration lives. One | |
| 1192 | can retrieve the <code>DeclContext</code> that contains a | |
| 1193 | particular <code>Decl</code> | |
| 1194 | using <code>Decl::getDeclContext</code>. However, see the | |
| 1195 | section <a href="#LexicalAndSemanticContexts">Lexical and Semantic | |
| 1196 | Contexts</a> for more information about how to interpret this | |
| 1197 | context information.</p> | |
| 1198 | ||
| 1199 | <h4 id="Redeclarations">Redeclarations and Overloads</h4> | |
| 1200 | <p>Within a translation unit, it is common for an entity to be | |
| 1201 | declared several times. For example, we might declare a function "f" | |
| 1202 | and then later re-declare it as part of an inlined definition:</p> | |
| 1203 | ||
| 1204 | <pre> | |
| 1205 | void f(int x, int y, int z = 1); | |
| 1206 | ||
| 1207 | inline void f(int x, int y, int z) { /* ... */ } | |
| 1208 | </pre> | |
| 1209 | ||
| 1210 | <p>The representation of "f" differs in the source-centric and | |
| 1211 | semantics-centric views of a declaration context. In the | |
| 1212 | source-centric view, all redeclarations will be present, in the | |
| 1213 | order they occurred in the source code, making | |
| 1214 | this view suitable for clients that wish to see the structure of | |
| 1215 | the source code. In the semantics-centric view, only the most recent "f" | |
| 1216 | will be found by the lookup, since it effectively replaces the first | |
| 1217 | declaration of "f".</p> | |
| 1218 | ||
| 1219 | <p>In the semantics-centric view, overloading of functions is | |
| 1220 | represented explicitly. For example, given two declarations of a | |
| 1221 | function "g" that are overloaded, e.g.,</p> | |
| 1222 | <pre> | |
| 1223 | void g(); | |
| 1224 | void g(int); | |
| 1225 | </pre> | |
| 1226 | <p>the <code>DeclContext::lookup</code> operation will return | |
| 1227 | a <code>DeclContext::lookup_result</code> that contains a range of iterators | |
| 1228 | over declarations of "g". Clients that perform semantic analysis on a | |
| 1229 | program that is not concerned with the actual source code will | |
| 1230 | primarily use this semantics-centric view.</p> | |
| 1231 | ||
| 1232 | <h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4> | |
| 1233 | <p>Each declaration has two potentially different | |
| 1234 | declaration contexts: a <i>lexical</i> context, which corresponds to | |
| 1235 | the source-centric view of the declaration context, and | |
| 1236 | a <i>semantic</i> context, which corresponds to the | |
| 1237 | semantics-centric view. The lexical context is accessible | |
| 1238 | via <code>Decl::getLexicalDeclContext</code> while the | |
| 1239 | semantic context is accessible | |
| 1240 | via <code>Decl::getDeclContext</code>, both of which return | |
| 1241 | <code>DeclContext</code> pointers. For most declarations, the two | |
| 1242 | contexts are identical. For example:</p> | |
| 1243 | ||
| 1244 | <pre> | |
| 1245 | class X { | |
| 1246 | public: | |
| 1247 | void f(int x); | |
| 1248 | }; | |
| 1249 | </pre> | |
| 1250 | ||
| 1251 | <p>Here, the semantic and lexical contexts of <code>X::f</code> are | |
| 1252 | the <code>DeclContext</code> associated with the | |
| 1253 | class <code>X</code> (itself stored as a <code>RecordDecl</code> AST | |
| 1254 | node). However, we can now define <code>X::f</code> out-of-line:</p> | |
| 1255 | ||
| 1256 | <pre> | |
| 1257 | void X::f(int x = 17) { /* ... */ } | |
| 1258 | </pre> | |
| 1259 | ||
| 1260 | <p>This definition of has different lexical and semantic | |
| 1261 | contexts. The lexical context corresponds to the declaration | |
| 1262 | context in which the actual declaration occurred in the source | |
| 1263 | code, e.g., the translation unit containing <code>X</code>. Thus, | |
| 1264 | this declaration of <code>X::f</code> can be found by traversing | |
| 1265 | the declarations provided by | |
| 1266 | [<code>decls_begin()</code>, <code>decls_end()</code>) in the | |
| 1267 | translation unit.</p> | |
| 1268 | ||
| 1269 | <p>The semantic context of <code>X::f</code> corresponds to the | |
| 1270 | class <code>X</code>, since this member function is (semantically) a | |
| 1271 | member of <code>X</code>. Lookup of the name <code>f</code> into | |
| 1272 | the <code>DeclContext</code> associated with <code>X</code> will | |
| 1273 | then return the definition of <code>X::f</code> (including | |
| 1274 | information about the default argument).</p> | |
| 1275 | ||
| 1276 | <h4 id="TransparentContexts">Transparent Declaration Contexts</h4> | |
| 1277 | <p>In C and C++, there are several contexts in which names that are | |
| 1278 | logically declared inside another declaration will actually "leak" | |
| 1279 | out into the enclosing scope from the perspective of name | |
| 1280 | lookup. The most obvious instance of this behavior is in | |
| 1281 | enumeration types, e.g.,</p> | |
| 1282 | <pre> | |
| 1283 | enum Color { | |
| 1284 | Red, | |
| 1285 | Green, | |
| 1286 | Blue | |
| 1287 | }; | |
| 1288 | </pre> | |
| 1289 | ||
| 1290 | <p>Here, <code>Color</code> is an enumeration, which is a declaration | |
| 1291 | context that contains the | |
| 1292 | enumerators <code>Red</code>, <code>Green</code>, | |
| 1293 | and <code>Blue</code>. Thus, traversing the list of declarations | |
| 1294 | contained in the enumeration <code>Color</code> will | |
| 1295 | yield <code>Red</code>, <code>Green</code>, | |
| 1296 | and <code>Blue</code>. However, outside of the scope | |
| 1297 | of <code>Color</code> one can name the enumerator <code>Red</code> | |
| 1298 | without qualifying the name, e.g.,</p> | |
| 1299 | ||
| 1300 | <pre> | |
| 1301 | Color c = Red; | |
| 1302 | </pre> | |
| 1303 | ||
| 1304 | <p>There are other entities in C++ that provide similar behavior. For | |
| 1305 | example, linkage specifications that use curly braces:</p> | |
| 1306 | ||
| 1307 | <pre> | |
| 1308 | extern "C" { | |
| 1309 | void f(int); | |
| 1310 | void g(int); | |
| 1311 | } | |
| 1312 | // f and g are visible here | |
| 1313 | </pre> | |
| 1314 | ||
| 1315 | <p>For source-level accuracy, we treat the linkage specification and | |
| 1316 | enumeration type as a | |
| 1317 | declaration context in which its enclosed declarations ("Red", | |
| 1318 | "Green", and "Blue"; "f" and "g") | |
| 1319 | are declared. However, these declarations are visible outside of the | |
| 1320 | scope of the declaration context.</p> | |
| 1321 | ||
| 1322 | <p>These language features (and several others, described below) have | |
| 1323 | roughly the same set of | |
| 1324 | requirements: declarations are declared within a particular lexical | |
| 1325 | context, but the declarations are also found via name lookup in | |
| 1326 | scopes enclosing the declaration itself. This feature is implemented | |
| 1327 | via <i>transparent</i> declaration contexts | |
| 1328 | (see <code>DeclContext::isTransparentContext()</code>), whose | |
| 1329 | declarations are visible in the nearest enclosing non-transparent | |
| 1330 | declaration context. This means that the lexical context of the | |
| 1331 | declaration (e.g., an enumerator) will be the | |
| 1332 | transparent <code>DeclContext</code> itself, as will the semantic | |
| 1333 | context, but the declaration will be visible in every outer context | |
| 1334 | up to and including the first non-transparent declaration context (since | |
| 1335 | transparent declaration contexts can be nested).</p> | |
| 1336 | ||
| 1337 | <p>The transparent <code>DeclContexts</code> are:</p> | |
| 1338 | <ul> | |
| 1339 | <li>Enumerations (but not C++11 "scoped enumerations"): | |
| 1340 | <pre> | |
| 1341 | enum Color { | |
| 1342 | Red, | |
| 1343 | Green, | |
| 1344 | Blue | |
| 1345 | }; | |
| 1346 | // Red, Green, and Blue are in scope | |
| 1347 | </pre></li> | |
| 1348 | <li>C++ linkage specifications: | |
| 1349 | <pre> | |
| 1350 | extern "C" { | |
| 1351 | void f(int); | |
| 1352 | void g(int); | |
| 1353 | } | |
| 1354 | // f and g are in scope | |
| 1355 | </pre></li> | |
| 1356 | <li>Anonymous unions and structs: | |
| 1357 | <pre> | |
| 1358 | struct LookupTable { | |
| 1359 | bool IsVector; | |
| 1360 | union { | |
| 1361 | std::vector<Item> *Vector; | |
| 1362 | std::set<Item> *Set; | |
| 1363 | }; | |
| 1364 | }; | |
| 1365 | ||
| 1366 | LookupTable LT; | |
| 1367 | LT.Vector = 0; // Okay: finds Vector inside the unnamed union | |
| 1368 | </pre> | |
| 1369 | </li> | |
| 1370 | <li>C++11 inline namespaces: | |
| 1371 | <pre> | |
| 1372 | namespace mylib { | |
| 1373 | inline namespace debug { | |
| 1374 | class X; | |
| 1375 | } | |
| 1376 | } | |
| 1377 | mylib::X *xp; // okay: mylib::X refers to mylib::debug::X | |
| 1378 | </pre> | |
| 1379 | </li> | |
| 1380 | </ul> | |
| 1381 | ||
| 1382 | ||
| 1383 | <h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4> | |
| 1384 | <p>C++ namespaces have the interesting--and, so far, unique--property that | |
| 1385 | the namespace can be defined multiple times, and the declarations | |
| 1386 | provided by each namespace definition are effectively merged (from | |
| 1387 | the semantic point of view). For example, the following two code | |
| 1388 | snippets are semantically indistinguishable:</p> | |
| 1389 | <pre> | |
| 1390 | // Snippet #1: | |
| 1391 | namespace N { | |
| 1392 | void f(); | |
| 1393 | } | |
| 1394 | namespace N { | |
| 1395 | void f(int); | |
| 1396 | } | |
| 1397 | ||
| 1398 | // Snippet #2: | |
| 1399 | namespace N { | |
| 1400 | void f(); | |
| 1401 | void f(int); | |
| 1402 | } | |
| 1403 | </pre> | |
| 1404 | ||
| 1405 | <p>In Clang's representation, the source-centric view of declaration | |
| 1406 | contexts will actually have two separate <code>NamespaceDecl</code> | |
| 1407 | nodes in Snippet #1, each of which is a declaration context that | |
| 1408 | contains a single declaration of "f". However, the semantics-centric | |
| 1409 | view provided by name lookup into the namespace <code>N</code> for | |
| 1410 | "f" will return a <code>DeclContext::lookup_result</code> that contains | |
| 1411 | a range of iterators over declarations of "f".</p> | |
| 1412 | ||
| 1413 | <p><code>DeclContext</code> manages multiply-defined declaration | |
| 1414 | contexts internally. The | |
| 1415 | function <code>DeclContext::getPrimaryContext</code> retrieves the | |
| 1416 | "primary" context for a given <code>DeclContext</code> instance, | |
| 1417 | which is the <code>DeclContext</code> responsible for maintaining | |
| 1418 | the lookup table used for the semantics-centric view. Given the | |
| 1419 | primary context, one can follow the chain | |
| 1420 | of <code>DeclContext</code> nodes that define additional | |
| 1421 | declarations via <code>DeclContext::getNextContext</code>. Note that | |
| 1422 | these functions are used internally within the lookup and insertion | |
| 1423 | methods of the <code>DeclContext</code>, so the vast majority of | |
| 1424 | clients can ignore them.</p> | |
| 1425 | ||
| 1426 | <!-- ======================================================================= --> | |
| 1427 | <h3 id="CFG">The <tt>CFG</tt> class</h3> | |
| 1428 | <!-- ======================================================================= --> | |
| 1429 | ||
| 1430 | <p>The <tt>CFG</tt> class is designed to represent a source-level | |
| 1431 | control-flow graph for a single statement (<tt>Stmt*</tt>). Typically | |
| 1432 | instances of <tt>CFG</tt> are constructed for function bodies (usually | |
| 1433 | an instance of <tt>CompoundStmt</tt>), but can also be instantiated to | |
| 1434 | represent the control-flow of any class that subclasses <tt>Stmt</tt>, | |
| 1435 | which includes simple expressions. Control-flow graphs are especially | |
| 1436 | useful for performing | |
| 1437 | <a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow- | |
| 1438 | or path-sensitive</a> program analyses on a given function.</p> | |
| 1439 | ||
| 1440 | <!-- ============ --> | |
| 1441 | <h4>Basic Blocks</h4> | |
| 1442 | <!-- ============ --> | |
| 1443 | ||
| 1444 | <p>Concretely, an instance of <tt>CFG</tt> is a collection of basic | |
| 1445 | blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which | |
| 1446 | simply contains an ordered sequence of <tt>Stmt*</tt> (each referring | |
| 1447 | to statements in the AST). The ordering of statements within a block | |
| 1448 | indicates unconditional flow of control from one statement to the | |
| 1449 | next. <a href="#ConditionalControlFlow">Conditional control-flow</a> | |
| 1450 | is represented using edges between basic blocks. The statements | |
| 1451 | within a given <tt>CFGBlock</tt> can be traversed using | |
| 1452 | the <tt>CFGBlock::*iterator</tt> interface.</p> | |
| 1453 | ||
| 1454 | <p> | |
| 1455 | A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within | |
| 1456 | the control-flow graph it represents. Each <tt>CFGBlock</tt> within a | |
| 1457 | CFG is also uniquely numbered (accessible | |
| 1458 | via <tt>CFGBlock::getBlockID()</tt>). Currently the number is | |
| 1459 | based on the ordering the blocks were created, but no assumptions | |
| 1460 | should be made on how <tt>CFGBlock</tt>s are numbered other than their | |
| 1461 | numbers are unique and that they are numbered from 0..N-1 (where N is | |
| 1462 | the number of basic blocks in the CFG).</p> | |
| 1463 | ||
| 1464 | <!-- ===================== --> | |
| 1465 | <h4>Entry and Exit Blocks</h4> | |
| 1466 | <!-- ===================== --> | |
| 1467 | ||
| 1468 | Each instance of <tt>CFG</tt> contains two special blocks: | |
| 1469 | an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which | |
| 1470 | has no incoming edges, and an <i>exit</i> block (accessible | |
| 1471 | via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither | |
| 1472 | block contains any statements, and they serve the role of providing a | |
| 1473 | clear entrance and exit for a body of code such as a function body. | |
| 1474 | The presence of these empty blocks greatly simplifies the | |
| 1475 | implementation of many analyses built on top of CFGs. | |
| 1476 | ||
| 1477 | <!-- ===================================================== --> | |
| 1478 | <h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4> | |
| 1479 | <!-- ===================================================== --> | |
| 1480 | ||
| 1481 | <p>Conditional control-flow (such as those induced by if-statements | |
| 1482 | and loops) is represented as edges between <tt>CFGBlock</tt>s. | |
| 1483 | Because different C language constructs can induce control-flow, | |
| 1484 | each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that | |
| 1485 | represents the <i>terminator</i> of the block. A terminator is simply | |
| 1486 | the statement that caused the control-flow, and is used to identify | |
| 1487 | the nature of the conditional control-flow between blocks. For | |
| 1488 | example, in the case of an if-statement, the terminator refers to | |
| 1489 | the <tt>IfStmt</tt> object in the AST that represented the given | |
| 1490 | branch.</p> | |
| 1491 | ||
| 1492 | <p>To illustrate, consider the following code example:</p> | |
| 1493 | ||
| 1494 | <code> | |
| 1495 | int foo(int x) {<br> | |
| 1496 | x = x + 1;<br> | |
| 1497 | <br> | |
| 1498 | if (x > 2) x++;<br> | |
| 1499 | else {<br> | |
| 1500 | x += 2;<br> | |
| 1501 | x *= 2;<br> | |
| 1502 | }<br> | |
| 1503 | <br> | |
| 1504 | return x;<br> | |
| 1505 | } | |
| 1506 | </code> | |
| 1507 | ||
| 1508 | <p>After invoking the parser+semantic analyzer on this code fragment, | |
| 1509 | the AST of the body of <tt>foo</tt> is referenced by a | |
| 1510 | single <tt>Stmt*</tt>. We can then construct an instance | |
| 1511 | of <tt>CFG</tt> representing the control-flow graph of this function | |
| 1512 | body by single call to a static class method:</p> | |
| 1513 | ||
| 1514 | <code> | |
| 1515 | Stmt* FooBody = ...<br> | |
| 1516 | CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody); | |
| 1517 | </code> | |
| 1518 | ||
| 1519 | <p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt> | |
| 1520 | to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no | |
| 1521 | longer needed.</p> | |
| 1522 | ||
| 1523 | <p>Along with providing an interface to iterate over | |
| 1524 | its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods | |
| 1525 | that are useful for debugging and visualizing CFGs. For example, the | |
| 1526 | method | |
| 1527 | <tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to | |
| 1528 | standard error. This is especially useful when one is using a | |
| 1529 | debugger such as gdb. For example, here is the output | |
| 1530 | of <tt>FooCFG->dump()</tt>:</p> | |
| 1531 | ||
| 1532 | <code> | |
| 1533 | [ B5 (ENTRY) ]<br> | |
| 1534 | Predecessors (0):<br> | |
| 1535 | Successors (1): B4<br> | |
| 1536 | <br> | |
| 1537 | [ B4 ]<br> | |
| 1538 | 1: x = x + 1<br> | |
| 1539 | 2: (x > 2)<br> | |
| 1540 | <b>T: if [B4.2]</b><br> | |
| 1541 | Predecessors (1): B5<br> | |
| 1542 | Successors (2): B3 B2<br> | |
| 1543 | <br> | |
| 1544 | [ B3 ]<br> | |
| 1545 | 1: x++<br> | |
| 1546 | Predecessors (1): B4<br> | |
| 1547 | Successors (1): B1<br> | |
| 1548 | <br> | |
| 1549 | [ B2 ]<br> | |
| 1550 | 1: x += 2<br> | |
| 1551 | 2: x *= 2<br> | |
| 1552 | Predecessors (1): B4<br> | |
| 1553 | Successors (1): B1<br> | |
| 1554 | <br> | |
| 1555 | [ B1 ]<br> | |
| 1556 | 1: return x;<br> | |
| 1557 | Predecessors (2): B2 B3<br> | |
| 1558 | Successors (1): B0<br> | |
| 1559 | <br> | |
| 1560 | [ B0 (EXIT) ]<br> | |
| 1561 | Predecessors (1): B1<br> | |
| 1562 | Successors (0): | |
| 1563 | </code> | |
| 1564 | ||
| 1565 | <p>For each block, the pretty-printed output displays for each block | |
| 1566 | the number of <i>predecessor</i> blocks (blocks that have outgoing | |
| 1567 | control-flow to the given block) and <i>successor</i> blocks (blocks | |
| 1568 | that have control-flow that have incoming control-flow from the given | |
| 1569 | block). We can also clearly see the special entry and exit blocks at | |
| 1570 | the beginning and end of the pretty-printed output. For the entry | |
| 1571 | block (block B5), the number of predecessor blocks is 0, while for the | |
| 1572 | exit block (block B0) the number of successor blocks is 0.</p> | |
| 1573 | ||
| 1574 | <p>The most interesting block here is B4, whose outgoing control-flow | |
| 1575 | represents the branching caused by the sole if-statement | |
| 1576 | in <tt>foo</tt>. Of particular interest is the second statement in | |
| 1577 | the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed | |
| 1578 | as <b><tt>if [B4.2]</tt></b>. The second statement represents the | |
| 1579 | evaluation of the condition of the if-statement, which occurs before | |
| 1580 | the actual branching of control-flow. Within the <tt>CFGBlock</tt> | |
| 1581 | for B4, the <tt>Stmt*</tt> for the second statement refers to the | |
| 1582 | actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus | |
| 1583 | pointers to subclasses of <tt>Expr</tt> can appear in the list of | |
| 1584 | statements in a block, and not just subclasses of <tt>Stmt</tt> that | |
| 1585 | refer to proper C statements.</p> | |
| 1586 | ||
| 1587 | <p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt> | |
| 1588 | object in the AST. The pretty-printer outputs <b><tt>if | |
| 1589 | [B4.2]</tt></b> because the condition expression of the if-statement | |
| 1590 | has an actual place in the basic block, and thus the terminator is | |
| 1591 | essentially | |
| 1592 | <i>referring</i> to the expression that is the second statement of | |
| 1593 | block B4 (i.e., B4.2). In this manner, conditions for control-flow | |
| 1594 | (which also includes conditions for loops and switch statements) are | |
| 1595 | hoisted into the actual basic block.</p> | |
| 1596 | ||
| 1597 | <!-- ===================== --> | |
| 1598 | <!-- <h4>Implicit Control-Flow</h4> --> | |
| 1599 | <!-- ===================== --> | |
| 1600 | ||
| 1601 | <!-- | |
| 1602 | <p>A key design principle of the <tt>CFG</tt> class was to not require | |
| 1603 | any transformations to the AST in order to represent control-flow. | |
| 1604 | Thus the <tt>CFG</tt> does not perform any "lowering" of the | |
| 1605 | statements in an AST: loops are not transformed into guarded gotos, | |
| 1606 | short-circuit operations are not converted to a set of if-statements, | |
| 1607 | and so on.</p> | |
| 1608 | --> | |
| 1609 | ||
| 1610 | ||
| 1611 | <!-- ======================================================================= --> | |
| 1612 | <h3 id="Constants">Constant Folding in the Clang AST</h3> | |
| 1613 | <!-- ======================================================================= --> | |
| 1614 | ||
| 1615 | <p>There are several places where constants and constant folding matter a lot to | |
| 1616 | the Clang front-end. First, in general, we prefer the AST to retain the source | |
| 1617 | code as close to how the user wrote it as possible. This means that if they | |
| 1618 | wrote "5+4", we want to keep the addition and two constants in the AST, we don't | |
| 1619 | want to fold to "9". This means that constant folding in various ways turns | |
| 1620 | into a tree walk that needs to handle the various cases.</p> | |
| 1621 | ||
| 1622 | <p>However, there are places in both C and C++ that require constants to be | |
| 1623 | folded. For example, the C standard defines what an "integer constant | |
| 1624 | expression" (i-c-e) is with very precise and specific requirements. The | |
| 1625 | language then requires i-c-e's in a lot of places (for example, the size of a | |
| 1626 | bitfield, the value for a case statement, etc). For these, we have to be able | |
| 1627 | to constant fold the constants, to do semantic checks (e.g. verify bitfield size | |
| 1628 | is non-negative and that case statements aren't duplicated). We aim for Clang | |
| 1629 | to be very pedantic about this, diagnosing cases when the code does not use an | |
| 1630 | i-c-e where one is required, but accepting the code unless running with | |
| 1631 | <tt>-pedantic-errors</tt>.</p> | |
| 1632 | ||
| 1633 | <p>Things get a little bit more tricky when it comes to compatibility with | |
| 1634 | real-world source code. Specifically, GCC has historically accepted a huge | |
| 1635 | superset of expressions as i-c-e's, and a lot of real world code depends on this | |
| 1636 | unfortuate accident of history (including, e.g., the glibc system headers). GCC | |
| 1637 | accepts anything its "fold" optimizer is capable of reducing to an integer | |
| 1638 | constant, which means that the definition of what it accepts changes as its | |
| 1639 | optimizer does. One example is that GCC accepts things like "case X-X:" even | |
| 1640 | when X is a variable, because it can fold this to 0.</p> | |
| 1641 | ||
| 1642 | <p>Another issue are how constants interact with the extensions we support, such | |
| 1643 | as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99 | |
| 1644 | obviously does not specify the semantics of any of these extensions, and the | |
| 1645 | definition of i-c-e does not include them. However, these extensions are often | |
| 1646 | used in real code, and we have to have a way to reason about them.</p> | |
| 1647 | ||
| 1648 | <p>Finally, this is not just a problem for semantic analysis. The code | |
| 1649 | generator and other clients have to be able to fold constants (e.g. to | |
| 1650 | initialize global variables) and has to handle a superset of what C99 allows. | |
| 1651 | Further, these clients can benefit from extended information. For example, we | |
| 1652 | know that "foo()||1" always evaluates to true, but we can't replace the | |
| 1653 | expression with true because it has side effects.</p> | |
| 1654 | ||
| 1655 | <!-- ======================= --> | |
| 1656 | <h4>Implementation Approach</h4> | |
| 1657 | <!-- ======================= --> | |
| 1658 | ||
| 1659 | <p>After trying several different approaches, we've finally converged on a | |
| 1660 | design (Note, at the time of this writing, not all of this has been implemented, | |
| 1661 | consider this a design goal!). Our basic approach is to define a single | |
| 1662 | recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is | |
| 1663 | implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar' | |
| 1664 | type (integer, fp, complex, or pointer) this method returns the following | |
| 1665 | information:</p> | |
| 1666 | ||
| 1667 | <ul> | |
| 1668 | <li>Whether the expression is an integer constant expression, a general | |
| 1669 | constant that was folded but has no side effects, a general constant that | |
| 1670 | was folded but that does have side effects, or an uncomputable/unfoldable | |
| 1671 | value. | |
| 1672 | </li> | |
| 1673 | <li>If the expression was computable in any way, this method returns the APValue | |
| 1674 | for the result of the expression.</li> | |
| 1675 | <li>If the expression is not evaluatable at all, this method returns | |
| 1676 | information on one of the problems with the expression. This includes a | |
| 1677 | SourceLocation for where the problem is, and a diagnostic ID that explains | |
| 1678 | the problem. The diagnostic should be have ERROR type.</li> | |
| 1679 | <li>If the expression is not an integer constant expression, this method returns | |
| 1680 | information on one of the problems with the expression. This includes a | |
| 1681 | SourceLocation for where the problem is, and a diagnostic ID that explains | |
| 1682 | the problem. The diagnostic should be have EXTENSION type.</li> | |
| 1683 | </ul> | |
| 1684 | ||
| 1685 | <p>This information gives various clients the flexibility that they want, and we | |
| 1686 | will eventually have some helper methods for various extensions. For example, | |
| 1687 | Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which | |
| 1688 | calls Evaluate on the expression. If the expression is not foldable, the error | |
| 1689 | is emitted, and it would return true. If the expression is not an i-c-e, the | |
| 1690 | EXTENSION diagnostic is emitted. Finally it would return false to indicate that | |
| 1691 | the AST is ok.</p> | |
| 1692 | ||
| 1693 | <p>Other clients can use the information in other ways, for example, codegen can | |
| 1694 | just use expressions that are foldable in any way.</p> | |
| 1695 | ||
| 1696 | <!-- ========== --> | |
| 1697 | <h4>Extensions</h4> | |
| 1698 | <!-- ========== --> | |
| 1699 | ||
| 1700 | <p>This section describes how some of the various extensions Clang supports | |
| 1701 | interacts with constant evaluation:</p> | |
| 1702 | ||
| 1703 | <ul> | |
| 1704 | <li><b><tt>__extension__</tt></b>: The expression form of this extension causes | |
| 1705 | any evaluatable subexpression to be accepted as an integer constant | |
| 1706 | expression.</li> | |
| 1707 | <li><b><tt>__builtin_constant_p</tt></b>: This returns true (as an integer | |
| 1708 | constant expression) if the operand evaluates to either a numeric value | |
| 1709 | (that is, not a pointer cast to integral type) of integral, enumeration, | |
| 1710 | floating or complex type, or if it evaluates to the address of the first | |
| 1711 | character of a string literal (possibly cast to some other type). As a | |
| 1712 | special case, if <tt>__builtin_constant_p</tt> is the (potentially | |
| 1713 | parenthesized) condition of a conditional operator expression ("?:"), only | |
| 1714 | the true side of the conditional operator is considered, and it is evaluated | |
| 1715 | with full constant folding.</li> | |
| 1716 | <li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an | |
| 1717 | integer constant expression, but we accept any constant as an "extension of | |
| 1718 | an extension". This only evaluates one operand depending on which way the | |
| 1719 | condition evaluates.</li> | |
| 1720 | <li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer | |
| 1721 | constant expression.</li> | |
| 1722 | <li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a | |
| 1723 | floating-point literal.</li> | |
| 1724 | <li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as | |
| 1725 | general constant expressions.</li> | |
| 1726 | <li><b><tt>__builtin_strlen</tt></b> and <b><tt>strlen</tt></b>: These are | |
| 1727 | constant folded as integer constant expressions if the argument is a string | |
| 1728 | literal.</li> | |
| 1729 | </ul> | |
| 1730 | ||
| 1731 | ||
| 1732 | <!-- ======================================================================= --> | |
| 1733 | <h2 id="Howtos">How to change Clang</h2> | |
| 1734 | <!-- ======================================================================= --> | |
| 1735 | ||
| 1736 | <!-- ======================================================================= --> | |
| 1737 | <h3 id="AddingAttributes">How to add an attribute</h3> | |
| 1738 | <!-- ======================================================================= --> | |
| 1739 | ||
| 1740 | <p>To add an attribute, you'll have to add it to the list of attributes, add it | |
| 1741 | to the parsing phase, and look for it in the AST scan. | |
| 1742 | <a href="http://llvm.org/viewvc/llvm-project?view=rev&revision=124217">r124217</a> | |
| 1743 | has a good example of adding a warning attribute.</p> | |
| 1744 | ||
| 1745 | <p>(Beware that this hasn't been reviewed/fixed by the people who designed the | |
| 1746 | attributes system yet.)</p> | |
| 1747 | ||
| 1748 | <h4><a | |
| 1749 | href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?view=markup">include/clang/Basic/Attr.td</a></h4> | |
| 1750 | ||
| 1751 | <p>Each attribute gets a <tt>def</tt> inheriting from <tt>Attr</tt> or one of | |
| 1752 | its subclasses. <tt>InheritableAttr</tt> means that the attribute also applies | |
| 1753 | to subsequent declarations of the same name.</p> | |
| 1754 | ||
| 1755 | <p><tt>Spellings</tt> lists the strings that can appear in | |
| 1756 | <tt>__attribute__((here))</tt> or <tt>[[here]]</tt>. All such strings | |
| 1757 | will be synonymous. If you want to allow the <tt>[[]]</tt> C++11 | |
| 1758 | syntax, you have to define a list of <tt>Namespaces</tt>, which will | |
| 1759 | let users write <tt>[[namespace:spelling]]</tt>. Using the empty | |
| 1760 | string for a namespace will allow users to write just the spelling | |
| 1761 | with no "<tt>:</tt>".</p> | |
| 1762 | ||
| 1763 | <p><tt>Subjects</tt> restricts what kinds of AST node to which this attribute | |
| 1764 | can appertain (roughly, attach).</p> | |
| 1765 | ||
| 1766 | <p><tt>Args</tt> names the arguments the attribute takes, in order. If | |
| 1767 | <tt>Args</tt> is <tt>[StringArgument<"Arg1">, IntArgument<"Arg2">]</tt> | |
| 1768 | then <tt>__attribute__((myattribute("Hello", 3)))</tt> will be a valid use.</p> | |
| 1769 | ||
| 1770 | <h4>Boilerplate</h4> | |
| 1771 | ||
| 1772 | <p>Write a new <tt>HandleYourAttr()</tt> function in <a | |
| 1773 | href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?view=markup">lib/Sema/SemaDeclAttr.cpp</a>, | |
| 1774 | and add a case to the switch in <tt>ProcessNonInheritableDeclAttr()</tt> or | |
| 1775 | <tt>ProcessInheritableDeclAttr()</tt> forwarding to it.</p> | |
| 1776 | ||
| 1777 | <p>If your attribute causes extra warnings to fire, define a <tt>DiagGroup</tt> | |
| 1778 | in <a | |
| 1779 | href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?view=markup">include/clang/Basic/DiagnosticGroups.td</a> | |
| 1780 | named after the attribute's <tt>Spelling</tt> with "_"s replaced by "-"s. If | |
| 1781 | you're only defining one diagnostic, you can skip <tt>DiagnosticGroups.td</tt> | |
| 1782 | and use <tt>InGroup<DiagGroup<"your-attribute">></tt> directly in <a | |
| 1783 | href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?view=markup">DiagnosticSemaKinds.td</a></p> | |
| 1784 | ||
| 1785 | <h4>The meat of your attribute</h4> | |
| 1786 | ||
| 1787 | <p>Find an appropriate place in Clang to do whatever your attribute needs to do. | |
| 1788 | Check for the attribute's presence using <tt>Decl::getAttr<YourAttr>()</tt>.</p> | |
| 1789 | ||
| 1790 | <p>Update the <a href="LanguageExtensions.html">Clang Language Extensions</a> | |
| 1791 | document to describe your new attribute.</p> | |
| 1792 | ||
| 1793 | <!-- ======================================================================= --> | |
| 1794 | <h3 id="AddingExprStmt">How to add an expression or statement</h3> | |
| 1795 | <!-- ======================================================================= --> | |
| 1796 | ||
| 1797 | <p>Expressions and statements are one of the most fundamental constructs within a | |
| 1798 | compiler, because they interact with many different parts of the AST, | |
| 1799 | semantic analysis, and IR generation. Therefore, adding a new | |
| 1800 | expression or statement kind into Clang requires some care. The following list | |
| 1801 | details the various places in Clang where an expression or statement needs to be | |
| 1802 | introduced, along with patterns to follow to ensure that the new | |
| 1803 | expression or statement works well across all of the C languages. We | |
| 1804 | focus on expressions, but statements are similar.</p> | |
| 1805 | ||
| 1806 | <ol> | |
| 1807 | <li>Introduce parsing actions into the parser. Recursive-descent | |
| 1808 | parsing is mostly self-explanatory, but there are a few things that | |
| 1809 | are worth keeping in mind: | |
| 1810 | <ul> | |
| 1811 | <li>Keep as much source location information as possible! You'll | |
| 1812 | want it later to produce great diagnostics and support Clang's | |
| 1813 | various features that map between source code and the AST.</li> | |
| 1814 | <li>Write tests for all of the "bad" parsing cases, to make sure | |
| 1815 | your recovery is good. If you have matched delimiters (e.g., | |
| 1816 | parentheses, square brackets, etc.), use | |
| 1817 | <tt>Parser::BalancedDelimiterTracker</tt> to give nice diagnostics when | |
| 1818 | things go wrong.</li> | |
| 1819 | </ul> | |
| 1820 | </li> | |
| 1821 | ||
| 1822 | <li>Introduce semantic analysis actions into <tt>Sema</tt>. Semantic | |
| 1823 | analysis should always involve two functions: an <tt>ActOnXXX</tt> | |
| 1824 | function that will be called directly from the parser, and a | |
| 1825 | <tt>BuildXXX</tt> function that performs the actual semantic | |
| 1826 | analysis and will (eventually!) build the AST node. It's fairly | |
| 1827 | common for the <tt>ActOnCXX</tt> function to do very little (often | |
| 1828 | just some minor translation from the parser's representation to | |
| 1829 | <tt>Sema</tt>'s representation of the same thing), but the separation | |
| 1830 | is still important: C++ template instantiation, for example, | |
| 1831 | should always call the <tt>BuildXXX</tt> variant. Several notes on | |
| 1832 | semantic analysis before we get into construction of the AST: | |
| 1833 | <ul> | |
| 1834 | <li>Your expression probably involves some types and some | |
| 1835 | subexpressions. Make sure to fully check that those types, and the | |
| 1836 | types of those subexpressions, meet your expectations. Add | |
| 1837 | implicit conversions where necessary to make sure that all of the | |
| 1838 | types line up exactly the way you want them. Write extensive tests | |
| 1839 | to check that you're getting good diagnostics for mistakes and | |
| 1840 | that you can use various forms of subexpressions with your | |
| 1841 | expression.</li> | |
| 1842 | <li>When type-checking a type or subexpression, make sure to first | |
| 1843 | check whether the type is "dependent" | |
| 1844 | (<tt>Type::isDependentType()</tt>) or whether a subexpression is | |
| 1845 | type-dependent (<tt>Expr::isTypeDependent()</tt>). If any of these | |
| 1846 | return true, then you're inside a template and you can't do much | |
| 1847 | type-checking now. That's normal, and your AST node (when you get | |
| 1848 | there) will have to deal with this case. At this point, you can | |
| 1849 | write tests that use your expression within templates, but don't | |
| 1850 | try to instantiate the templates.</li> | |
| 1851 | <li>For each subexpression, be sure to call | |
| 1852 | <tt>Sema::CheckPlaceholderExpr()</tt> to deal with "weird" | |
| 1853 | expressions that don't behave well as subexpressions. Then, | |
| 1854 | determine whether you need to perform | |
| 1855 | lvalue-to-rvalue conversions | |
| 1856 | (<tt>Sema::DefaultLvalueConversion</tt>e) or | |
| 1857 | the usual unary conversions | |
| 1858 | (<tt>Sema::UsualUnaryConversions</tt>), for places where the | |
| 1859 | subexpression is producing a value you intend to use.</li> | |
| 1860 | <li>Your <tt>BuildXXX</tt> function will probably just return | |
| 1861 | <tt>ExprError()</tt> at this point, since you don't have an AST. | |
| 1862 | That's perfectly fine, and shouldn't impact your testing.</li> | |
| 1863 | </ul> | |
| 1864 | </li> | |
| 1865 | ||
| 1866 | <li>Introduce an AST node for your new expression. This starts with | |
| 1867 | declaring the node in <tt>include/Basic/StmtNodes.td</tt> and | |
| 1868 | creating a new class for your expression in the appropriate | |
| 1869 | <tt>include/AST/Expr*.h</tt> header. It's best to look at the class | |
| 1870 | for a similar expression to get ideas, and there are some specific | |
| 1871 | things to watch for: | |
| 1872 | <ul> | |
| 1873 | <li>If you need to allocate memory, use the <tt>ASTContext</tt> | |
| 1874 | allocator to allocate memory. Never use raw <tt>malloc</tt> or | |
| 1875 | <tt>new</tt>, and never hold any resources in an AST node, because | |
| 1876 | the destructor of an AST node is never called.</li> | |
| 1877 | ||
| 1878 | <li>Make sure that <tt>getSourceRange()</tt> covers the exact | |
| 1879 | source range of your expression. This is needed for diagnostics | |
| 1880 | and for IDE support.</li> | |
| 1881 | ||
| 1882 | <li>Make sure that <tt>children()</tt> visits all of the | |
| 1883 | subexpressions. This is important for a number of features (e.g., IDE | |
| 1884 | support, C++ variadic templates). If you have sub-types, you'll | |
| 1885 | also need to visit those sub-types in the | |
| 1886 | <tt>RecursiveASTVisitor</tt>.</li> | |
| 1887 | ||
| 1888 | <li>Add printing support (<tt>StmtPrinter.cpp</tt>) and dumping | |
| 1889 | support (<tt>StmtDumper.cpp</tt>) for your expression.</li> | |
| 1890 | ||
| 1891 | <li>Add profiling support (<tt>StmtProfile.cpp</tt>) for your AST | |
| 1892 | node, noting the distinguishing (non-source location) | |
| 1893 | characteristics of an instance of your expression. Omitting this | |
| 1894 | step will lead to hard-to-diagnose failures regarding matching of | |
| 1895 | template declarations.</li> | |
| 1896 | </ul> | |
| 1897 | </li> | |
| 1898 | ||
| 1899 | <li>Teach semantic analysis to build your AST node! At this point, | |
| 1900 | you can wire up your <tt>Sema::BuildXXX</tt> function to actually | |
| 1901 | create your AST. A few things to check at this point: | |
| 1902 | <ul> | |
| 1903 | <li>If your expression can construct a new C++ class or return a | |
| 1904 | new Objective-C object, be sure to update and then call | |
| 1905 | <tt>Sema::MaybeBindToTemporary</tt> for your just-created AST node | |
| 1906 | to be sure that the object gets properly destructed. An easy way | |
| 1907 | to test this is to return a C++ class with a private destructor: | |
| 1908 | semantic analysis should flag an error here with the attempt to | |
| 1909 | call the destructor.</li> | |
| 1910 | <li>Inspect the generated AST by printing it using <tt>clang -cc1 | |
| 1911 | -ast-print</tt>, to make sure you're capturing all of the | |
| 1912 | important information about how the AST was written.</li> | |
| 1913 | <li>Inspect the generated AST under <tt>clang -cc1 -ast-dump</tt> | |
| 1914 | to verify that all of the types in the generated AST line up the | |
| 1915 | way you want them. Remember that clients of the AST should never | |
| 1916 | have to "think" to understand what's going on. For example, all | |
| 1917 | implicit conversions should show up explicitly in the AST.</li> | |
| 1918 | <li>Write tests that use your expression as a subexpression of | |
| 1919 | other, well-known expressions. Can you call a function using your | |
| 1920 | expression as an argument? Can you use the ternary operator?</li> | |
| 1921 | </ul> | |
| 1922 | </li> | |
| 1923 | ||
| 1924 | <li>Teach code generation to create IR to your AST node. This step | |
| 1925 | is the first (and only) that requires knowledge of LLVM IR. There | |
| 1926 | are several things to keep in mind: | |
| 1927 | <ul> | |
| 1928 | <li>Code generation is separated into scalar/aggregate/complex and | |
| 1929 | lvalue/rvalue paths, depending on what kind of result your | |
| 1930 | expression produces. On occasion, this requires some careful | |
| 1931 | factoring of code to avoid duplication.</li> | |
| 1932 | ||
| 1933 | <li><tt>CodeGenFunction</tt> contains functions | |
| 1934 | <tt>ConvertType</tt> and <tt>ConvertTypeForMem</tt> that convert | |
| 1935 | Clang's types (<tt>clang::Type*</tt> or <tt>clang::QualType</tt>) | |
| 1936 | to LLVM types. | |
| 1937 | Use the former for values, and the later for memory locations: | |
| 1938 | test with the C++ "bool" type to check this. If you find | |
| 1939 | that you are having to use LLVM bitcasts to make | |
| 1940 | the subexpressions of your expression have the type that your | |
| 1941 | expression expects, STOP! Go fix semantic analysis and the AST so | |
| 1942 | that you don't need these bitcasts.</li> | |
| 1943 | ||
| 1944 | <li>The <tt>CodeGenFunction</tt> class has a number of helper | |
| 1945 | functions to make certain operations easy, such as generating code | |
| 1946 | to produce an lvalue or an rvalue, or to initialize a memory | |
| 1947 | location with a given value. Prefer to use these functions rather | |
| 1948 | than directly writing loads and stores, because these functions | |
| 1949 | take care of some of the tricky details for you (e.g., for | |
| 1950 | exceptions).</li> | |
| 1951 | ||
| 1952 | <li>If your expression requires some special behavior in the event | |
| 1953 | of an exception, look at the <tt>push*Cleanup</tt> functions in | |
| 1954 | <tt>CodeGenFunction</tt> to introduce a cleanup. You shouldn't | |
| 1955 | have to deal with exception-handling directly.</li> | |
| 1956 | ||
| 1957 | <li>Testing is extremely important in IR generation. Use <tt>clang | |
| 1958 | -cc1 -emit-llvm</tt> and <a | |
| 1959 | href="http://llvm.org/cmds/FileCheck.html">FileCheck</a> to verify | |
| 1960 | that you're generating the right IR.</li> | |
| 1961 | </ul> | |
| 1962 | </li> | |
| 1963 | ||
| 1964 | <li>Teach template instantiation how to cope with your AST | |
| 1965 | node, which requires some fairly simple code: | |
| 1966 | <ul> | |
| 1967 | <li>Make sure that your expression's constructor properly | |
| 1968 | computes the flags for type dependence (i.e., the type your | |
| 1969 | expression produces can change from one instantiation to the | |
| 1970 | next), value dependence (i.e., the constant value your expression | |
| 1971 | produces can change from one instantiation to the next), | |
| 1972 | instantiation dependence (i.e., a template parameter occurs | |
| 1973 | anywhere in your expression), and whether your expression contains | |
| 1974 | a parameter pack (for variadic templates). Often, computing these | |
| 1975 | flags just means combining the results from the various types and | |
| 1976 | subexpressions.</li> | |
| 1977 | ||
| 1978 | <li>Add <tt>TransformXXX</tt> and <tt>RebuildXXX</tt> functions to | |
| 1979 | the | |
| 1980 | <tt>TreeTransform</tt> class template in <tt>Sema</tt>. | |
| 1981 | <tt>TransformXXX</tt> should (recursively) transform all of the | |
| 1982 | subexpressions and types | |
| 1983 | within your expression, using <tt>getDerived().TransformYYY</tt>. | |
| 1984 | If all of the subexpressions and types transform without error, it | |
| 1985 | will then call the <tt>RebuildXXX</tt> function, which will in | |
| 1986 | turn call <tt>getSema().BuildXXX</tt> to perform semantic analysis | |
| 1987 | and build your expression.</li> | |
| 1988 | ||
| 1989 | <li>To test template instantiation, take those tests you wrote to | |
| 1990 | make sure that you were type checking with type-dependent | |
| 1991 | expressions and dependent types (from step #2) and instantiate | |
| 1992 | those templates with various types, some of which type-check and | |
| 1993 | some that don't, and test the error messages in each case.</li> | |
| 1994 | </ul> | |
| 1995 | </li> | |
| 1996 | ||
| 1997 | <li>There are some "extras" that make other features work better. | |
| 1998 | It's worth handling these extras to give your expression complete | |
| 1999 | integration into Clang: | |
| 2000 | <ul> | |
| 2001 | <li>Add code completion support for your expression in | |
| 2002 | <tt>SemaCodeComplete.cpp</tt>.</li> | |
| 2003 | ||
| 2004 | <li>If your expression has types in it, or has any "interesting" | |
| 2005 | features other than subexpressions, extend libclang's | |
| 2006 | <tt>CursorVisitor</tt> to provide proper visitation for your | |
| 2007 | expression, enabling various IDE features such as syntax | |
| 2008 | highlighting, cross-referencing, and so on. The | |
| 2009 | <tt>c-index-test</tt> helper program can be used to test these | |
| 2010 | features.</li> | |
| 2011 | </ul> | |
| 2012 | </li> | |
| 2013 | </ol> | |
| 2014 | ||
| 2015 | </div> | |
| 2016 | </body> | |
| 2017 | </html> |