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