| Ted Kremenek | 17a295d | 2008-06-11 06:19:49 +0000 | [diff] [blame] | 1 | <html> |
| 2 | <head> |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 3 | <title>"Clang" CFE Internals Manual</title> |
| Ted Kremenek | 17a295d | 2008-06-11 06:19:49 +0000 | [diff] [blame] | 4 | <link type="text/css" rel="stylesheet" href="../menu.css" /> |
| 5 | <link type="text/css" rel="stylesheet" href="../content.css" /> |
| Sebastian Redl | 6816856 | 2008-11-22 22:16:45 +0000 | [diff] [blame] | 6 | <style type="text/css"> |
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| Ted Kremenek | 17a295d | 2008-06-11 06:19:49 +0000 | [diff] [blame] | 11 | </head> |
| 12 | <body> |
| 13 | |
| 14 | <!--#include virtual="../menu.html.incl"--> |
| 15 | |
| 16 | <div id="content"> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 17 | |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 18 | <h1>"Clang" CFE Internals Manual</h1> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 19 | |
| 20 | <ul> |
| 21 | <li><a href="#intro">Introduction</a></li> |
| 22 | <li><a href="#libsystem">LLVM System and Support Libraries</a></li> |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 23 | <li><a href="#libbasic">The Clang 'Basic' Library</a> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 24 | <ul> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 25 | <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 26 | <li><a href="#SourceLocation">The SourceLocation and SourceManager |
| 27 | classes</a></li> |
| 28 | </ul> |
| 29 | </li> |
| 30 | <li><a href="#liblex">The Lexer and Preprocessor Library</a> |
| 31 | <ul> |
| 32 | <li><a href="#Token">The Token class</a></li> |
| 33 | <li><a href="#Lexer">The Lexer class</a></li> |
| Chris Lattner | 7928125 | 2008-03-09 02:27:26 +0000 | [diff] [blame] | 34 | <li><a href="#TokenLexer">The TokenLexer class</a></li> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 35 | <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li> |
| 36 | </ul> |
| 37 | </li> |
| 38 | <li><a href="#libparse">The Parser Library</a> |
| 39 | <ul> |
| 40 | </ul> |
| 41 | </li> |
| 42 | <li><a href="#libast">The AST Library</a> |
| 43 | <ul> |
| 44 | <li><a href="#Type">The Type class and its subclasses</a></li> |
| 45 | <li><a href="#QualType">The QualType class</a></li> |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 46 | <li><a href="#DeclarationName">Declaration names</a></li> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 47 | <li><a href="#CFG">The CFG class</a></li> |
| Chris Lattner | 7bad199 | 2008-11-16 21:48:07 +0000 | [diff] [blame] | 48 | <li><a href="#Constants">Constant Folding in the Clang AST</a></li> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 49 | </ul> |
| 50 | </li> |
| 51 | </ul> |
| 52 | |
| 53 | |
| 54 | <!-- ======================================================================= --> |
| 55 | <h2 id="intro">Introduction</h2> |
| 56 | <!-- ======================================================================= --> |
| 57 | |
| 58 | <p>This document describes some of the more important APIs and internal design |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 59 | decisions made in the Clang C front-end. The purpose of this document is to |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 60 | both capture some of this high level information and also describe some of the |
| 61 | design decisions behind it. This is meant for people interested in hacking on |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 62 | Clang, not for end-users. The description below is categorized by |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 63 | libraries, and does not describe any of the clients of the libraries.</p> |
| 64 | |
| 65 | <!-- ======================================================================= --> |
| 66 | <h2 id="libsystem">LLVM System and Support Libraries</h2> |
| 67 | <!-- ======================================================================= --> |
| 68 | |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 69 | <p>The LLVM libsystem library provides the basic Clang system abstraction layer, |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 70 | which is used for file system access. The LLVM libsupport library provides many |
| 71 | underlying libraries and <a |
| 72 | href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>, |
| 73 | including command line option |
| 74 | processing and various containers.</p> |
| 75 | |
| 76 | <!-- ======================================================================= --> |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 77 | <h2 id="libbasic">The Clang 'Basic' Library</h2> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 78 | <!-- ======================================================================= --> |
| 79 | |
| 80 | <p>This library certainly needs a better name. The 'basic' library contains a |
| 81 | number of low-level utilities for tracking and manipulating source buffers, |
| 82 | locations within the source buffers, diagnostics, tokens, target abstraction, |
| 83 | and information about the subset of the language being compiled for.</p> |
| 84 | |
| 85 | <p>Part of this infrastructure is specific to C (such as the TargetInfo class), |
| 86 | other parts could be reused for other non-C-based languages (SourceLocation, |
| 87 | SourceManager, Diagnostics, FileManager). When and if there is future demand |
| 88 | we can figure out if it makes sense to introduce a new library, move the general |
| 89 | classes somewhere else, or introduce some other solution.</p> |
| 90 | |
| 91 | <p>We describe the roles of these classes in order of their dependencies.</p> |
| 92 | |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 93 | |
| 94 | <!-- ======================================================================= --> |
| 95 | <h3 id="Diagnostics">The Diagnostics Subsystem</h3> |
| 96 | <!-- ======================================================================= --> |
| 97 | |
| 98 | <p>The Clang Diagnostics subsystem is an important part of how the compiler |
| 99 | communicates with the human. Diagnostics are the warnings and errors produced |
| 100 | when the code is incorrect or dubious. In Clang, each diagnostic produced has |
| 101 | (at the minimum) a unique ID, a <a href="#SourceLocation">SourceLocation</a> to |
| 102 | "put the caret", an English translation associated with it, and a severity (e.g. |
| 103 | <tt>WARNING</tt> or <tt>ERROR</tt>). They can also optionally include a number |
| 104 | of arguments to the dianostic (which fill in "%0"'s in the string) as well as a |
| 105 | number of source ranges that related to the diagnostic.</p> |
| 106 | |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 107 | <p>In this section, we'll be giving examples produced by the Clang command line |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 108 | driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many |
| 109 | different ways</a> depending on how the DiagnosticClient interface is |
| 110 | implemented. A representative example of a diagonstic is:</p> |
| 111 | |
| 112 | <pre> |
| 113 | t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float') |
| 114 | <font color="darkgreen">P = (P-42) + Gamma*4;</font> |
| 115 | <font color="blue">~~~~~~ ^ ~~~~~~~</font> |
| 116 | </pre> |
| 117 | |
| 118 | <p>In this example, you can see the English translation, the severity (error), |
| 119 | you can see the source location (the caret ("^") and file/line/column info), |
| 120 | the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex |
| 121 | float"). You'll have to believe me that there is a unique ID backing the |
| 122 | diagnostic :).</p> |
| 123 | |
| 124 | <p>Getting all of this to happen has several steps and involves many moving |
| 125 | pieces, this section describes them and talks about best practices when adding |
| 126 | a new diagnostic.</p> |
| 127 | |
| 128 | <!-- ============================ --> |
| 129 | <h4>The DiagnosticKinds.def file</h4> |
| 130 | <!-- ============================ --> |
| 131 | |
| 132 | <p>Diagnostics are created by adding an entry to the <tt><a |
| 133 | href="http://llvm.org/svn/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticKinds.def" |
| 134 | >DiagnosticKinds.def</a></tt> file. This file encodes the unique ID of the |
| 135 | diagnostic (as an enum, the first argument), the severity of the diagnostic |
| 136 | (second argument) and the English translation + format string.</p> |
| 137 | |
| 138 | <p>There is little sanity with the naming of the unique ID's right now. Some |
| 139 | start with err_, warn_, ext_ to encode the severity into the name. Since the |
| 140 | enum is referenced in the C++ code that produces the diagnostic, it is somewhat |
| 141 | useful for it to be reasonably short.</p> |
| 142 | |
| 143 | <p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>, |
| 144 | <tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The |
| 145 | <tt>ERROR</tt> severity is used for diagnostics indicating the program is never |
| 146 | acceptable under any circumstances. When an error is emitted, the AST for the |
| 147 | input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt> |
| 148 | severities are used for extensions to the language that Clang accepts. This |
| 149 | means that Clang fully understands and can represent them in the AST, but we |
| 150 | produce diagnostics to tell the user their code is non-portable. The difference |
| 151 | is that the former are ignored by default, and the later warn by default. The |
| 152 | <tt>WARNING</tt> severity is used for constructs that are valid in the currently |
| 153 | selected source language but that are dubious in some way. The <tt>NOTE</tt> |
| 154 | level is used to staple more information onto a previous diagnostics.</p> |
| 155 | |
| 156 | <p>These <em>severities</em> are mapped into a smaller set (the |
| 157 | Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>, |
| 158 | <tt>Error</tt> }) of output <em>levels</em> by the diagnostics subsystem based |
| 159 | on various configuration options. For example, if the user specifies |
| 160 | <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if they specify |
| 161 | <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. Clang also internally |
| 162 | supports a fully fine grained mapping mechanism that allows you to map any |
| 163 | diagnostic that doesn't have <tt>ERRROR</tt> severity to any output level that |
| 164 | you want. This is used to implement options like <tt>-Wunused_macros</tt>, |
| 165 | <tt>-Wundef</tt> etc.</p> |
| 166 | |
| 167 | <!-- ================= --> |
| 168 | <h4>The Format String</h4> |
| 169 | <!-- ================= --> |
| 170 | |
| 171 | <p>The format string for the diagnostic is very simple, but it has some power. |
| 172 | It takes the form of a string in English with markers that indicate where and |
| 173 | how arguments to the diagnostic are inserted and formatted. For example, here |
| 174 | are some simple format strings:</p> |
| 175 | |
| 176 | <pre> |
| 177 | "binary integer literals are an extension" |
| 178 | "format string contains '\\0' within the string body" |
| 179 | "more '<b>%%</b>' conversions than data arguments" |
| Chris Lattner | 545b368 | 2008-11-23 20:27:13 +0000 | [diff] [blame] | 180 | "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)" |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 181 | "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator" |
| 182 | " (has <b>%1</b> parameter<b>%s1</b>)" |
| 183 | </pre> |
| 184 | |
| 185 | <p>These examples show some important points of format strings. You can use any |
| 186 | plain ASCII character in the diagnostic string except "%" without a problem, |
| 187 | but these are C strings, so you have to use and be aware of all the C escape |
| 188 | sequences (as in the second example). If you want to produce a "%" in the |
| 189 | output, use the "%%" escape sequence, like the third diagnostic. Finally, |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 190 | Clang uses the "%...[digit]" sequences to specify where and how arguments to |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 191 | the diagnostic are formatted.</p> |
| 192 | |
| 193 | <p>Arguments to the diagnostic are numbered according to how they are specified |
| 194 | by the C++ code that <a href="#producingdiag">produces them</a>, and are |
| 195 | referenced by <tt>%0</tt> .. <tt>%9</tt>. If you have more than 10 arguments |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 196 | to your diagnostic, you are doing something wrong :). Unlike printf, there |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 197 | is no requirement that arguments to the diagnostic end up in the output in |
| 198 | the same order as they are specified, you could have a format string with |
| 199 | <tt>"%1 %0"</tt> that swaps them, for example. The text in between the |
| 200 | percent and digit are formatting instructions. If there are no instructions, |
| 201 | the argument is just turned into a string and substituted in.</p> |
| 202 | |
| 203 | <p>Here are some "best practices" for writing the English format string:</p> |
| 204 | |
| 205 | <ul> |
| 206 | <li>Keep the string short. It should ideally fit in the 80 column limit of the |
| 207 | <tt>DiagnosticKinds.def</tt> file. This avoids the diagnostic wrapping when |
| 208 | printed, and forces you to think about the important point you are conveying |
| 209 | with the diagnostic.</li> |
| 210 | <li>Take advantage of location information. The user will be able to see the |
| 211 | line and location of the caret, so you don't need to tell them that the |
| 212 | problem is with the 4th argument to the function: just point to it.</li> |
| 213 | <li>Do not capitalize the diagnostic string, and do not end it with a |
| 214 | period.</li> |
| 215 | <li>If you need to quote something in the diagnostic string, use single |
| 216 | quotes.</li> |
| 217 | </ul> |
| 218 | |
| 219 | <p>Diagnostics should never take random English strings as arguments: you |
| 220 | shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like |
| 221 | <tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing |
| 222 | this prevents <a href="translation">translating</a> the Clang diagnostics to |
| 223 | other languages (because they'll get random English words in their otherwise |
| 224 | localized diagnostic). The exceptions to this are C/C++ language keywords |
| 225 | (e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note |
| 226 | that things like "pointer" and "reference" are not keywords. On the other |
| 227 | hand, you <em>can</em> include anything that comes from the user's source code, |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 228 | including variable names, types, labels, etc. The 'select' format can be |
| 229 | used to achieve this sort of thing in a localizable way, see below.</p> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 230 | |
| 231 | <!-- ==================================== --> |
| 232 | <h4>Formatting a Diagnostic Argument</a></h4> |
| 233 | <!-- ==================================== --> |
| 234 | |
| 235 | <p>Arguments to diagnostics are fully typed internally, and come from a couple |
| 236 | different classes: integers, types, names, and random strings. Depending on |
| 237 | the class of the argument, it can be optionally formatted in different ways. |
| 238 | This gives the DiagnosticClient information about what the argument means |
| 239 | without requiring it to use a specific presentation (consider this MVC for |
| 240 | Clang :).</p> |
| 241 | |
| 242 | <p>Here are the different diagnostic argument formats currently supported by |
| 243 | Clang:</p> |
| 244 | |
| 245 | <table> |
| 246 | <tr><td colspan="2"><b>"s" format</b></td></tr> |
| 247 | <tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr> |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 248 | <tr><td>Class:</td><td>Integers</td></tr> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 249 | <tr><td>Description:</td><td>This is a simple formatter for integers that is |
| 250 | useful when producing English diagnostics. When the integer is 1, it prints |
| 251 | as nothing. When the integer is not 1, it prints as "s". This allows some |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 252 | simple grammatical forms to be to be handled correctly, and eliminates the |
| 253 | need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 254 | |
| 255 | <tr><td colspan="2"><b>"select" format</b></td></tr> |
| 256 | <tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2 |
| 257 | operator"</tt></td></tr> |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 258 | <tr><td>Class:</td><td>Integers</td></tr> |
| Chris Lattner | cc54334 | 2008-11-22 23:50:47 +0000 | [diff] [blame] | 259 | <tr><td>Description:</td><td>This format specifier is used to merge multiple |
| 260 | related diagnostics together into one common one, without requiring the |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 261 | difference to be specified as an English string argument. Instead of |
| Chris Lattner | cc54334 | 2008-11-22 23:50:47 +0000 | [diff] [blame] | 262 | specifying the string, the diagnostic gets an integer argument and the |
| 263 | format string selects the numbered option. In this case, the "%2" value |
| 264 | must be an integer in the range [0..2]. If it is 0, it prints 'unary', if |
| 265 | it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This |
| 266 | allows other language translations to substitute reasonable words (or entire |
| 267 | phrases) based on the semantics of the diagnostic instead of having to do |
| 268 | things textually.</td></tr> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 269 | |
| 270 | <tr><td colspan="2"><b>"plural" format</b></td></tr> |
| Sebastian Redl | 6816856 | 2008-11-22 22:16:45 +0000 | [diff] [blame] | 271 | <tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to |
| 272 | your computer"</tt></td></tr> |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 273 | <tr><td>Class:</td><td>Integers</td></tr> |
| Sebastian Redl | 6816856 | 2008-11-22 22:16:45 +0000 | [diff] [blame] | 274 | <tr><td>Description:</td><td><p>This is a formatter for complex plural forms. |
| 275 | It is designed to handle even the requirements of languages with very |
| 276 | complex plural forms, as many Baltic languages have. The argument consists |
| 277 | of a series of expression/form pairs, separated by ':', where the first form |
| 278 | whose expression evaluates to true is the result of the modifier.</p> |
| 279 | <p>An expression can be empty, in which case it is always true. See the |
| 280 | example at the top. Otherwise, it is a series of one or more numeric |
| 281 | conditions, separated by ','. If any condition matches, the expression |
| 282 | matches. Each numeric condition can take one of three forms.</p> |
| 283 | <ul> |
| 284 | <li>number: A simple decimal number matches if the argument is the same |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 285 | as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li> |
| Sebastian Redl | 6816856 | 2008-11-22 22:16:45 +0000 | [diff] [blame] | 286 | <li>range: A range in square brackets matches if the argument is within |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 287 | the range. Then range is inclusive on both ends. Example: |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 288 | <tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li> |
| 289 | <li>modulo: A modulo operator is followed by a number, and |
| 290 | equals sign and either a number or a range. The tests are the |
| 291 | same as for plain |
| Sebastian Redl | 6816856 | 2008-11-22 22:16:45 +0000 | [diff] [blame] | 292 | numbers and ranges, but the argument is taken modulo the number first. |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 293 | Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything |
| 294 | else}1"</tt></li> |
| Sebastian Redl | 6816856 | 2008-11-22 22:16:45 +0000 | [diff] [blame] | 295 | </ul> |
| 296 | <p>The parser is very unforgiving. A syntax error, even whitespace, will |
| 297 | abort, as will a failure to match the argument against any |
| 298 | expression.</p></td></tr> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 299 | |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 300 | </table> |
| 301 | |
| Chris Lattner | cc54334 | 2008-11-22 23:50:47 +0000 | [diff] [blame] | 302 | <p>It is really easy to add format specifiers to the Clang diagnostics system, |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 303 | but they should be discussed before they are added. If you are creating a lot |
| 304 | of repetitive diagnostics and/or have an idea for a useful formatter, please |
| 305 | bring it up on the cfe-dev mailing list.</p> |
| Chris Lattner | cc54334 | 2008-11-22 23:50:47 +0000 | [diff] [blame] | 306 | |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 307 | <!-- ===================================================== --> |
| 308 | <h4><a name="#producingdiag">Producing the Diagnostic</a></h4> |
| 309 | <!-- ===================================================== --> |
| 310 | |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 311 | <p>Now that you've created the diagnostic in the DiagnosticKinds.def file, you |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 312 | need to write the code that detects the condition in question and emits the |
| 313 | new diagnostic. Various components of Clang (e.g. the preprocessor, Sema, |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 314 | etc) provide a helper function named "Diag". It creates a diagnostic and |
| 315 | accepts the arguments, ranges, and other information that goes along with |
| 316 | it.</p> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 317 | |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 318 | <p>For example, the binary expression error comes from code like this:</p> |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 319 | |
| 320 | <pre> |
| 321 | if (various things that are bad) |
| 322 | Diag(Loc, diag::err_typecheck_invalid_operands) |
| 323 | << lex->getType() << rex->getType() |
| 324 | << lex->getSourceRange() << rex->getSourceRange(); |
| 325 | </pre> |
| 326 | |
| 327 | <p>This shows that use of the Diag method: they take a location (a <a |
| 328 | href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value |
| 329 | (which matches the name from DiagnosticKinds.def). If the diagnostic takes |
| 330 | arguments, they are specified with the << operator: the first argument |
| 331 | becomes %0, the second becomes %1, etc. The diagnostic interface allows you to |
| Chris Lattner | fd408ea | 2008-11-23 00:42:53 +0000 | [diff] [blame] | 332 | specify arguments of many different types, including <tt>int</tt> and |
| 333 | <tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and |
| 334 | <tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and |
| 335 | <tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc. |
| 336 | SourceRanges are also specified with the << operator, but do not have a |
| 337 | specific ordering requirement.</p> |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 338 | |
| 339 | <p>As you can see, adding and producing a diagnostic is pretty straightforward. |
| 340 | The hard part is deciding exactly what you need to say to help the user, picking |
| 341 | a suitable wording, and providing the information needed to format it correctly. |
| Chris Lattner | fd408ea | 2008-11-23 00:42:53 +0000 | [diff] [blame] | 342 | The good news is that the call site that issues a diagnostic should be |
| 343 | completely independent of how the diagnostic is formatted and in what language |
| 344 | it is rendered. |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 345 | </p> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 346 | |
| 347 | <!-- ============================================================= --> |
| 348 | <h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4> |
| 349 | <!-- ============================================================= --> |
| 350 | |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 351 | <p>Once code generates a diagnostic with all of the arguments and the rest of |
| 352 | the relevant information, Clang needs to know what to do with it. As previously |
| 353 | mentioned, the diagnostic machinery goes through some filtering to map a |
| 354 | severity onto a diagnostic level, then (assuming the diagnostic is not mapped to |
| 355 | "<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient |
| 356 | interface with the information.</p> |
| 357 | |
| 358 | <p>It is possible to implement this interface in many different ways. For |
| 359 | example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns |
| 360 | the arguments into strings (according to the various formatting rules), prints |
| 361 | out the file/line/column information and the string, then prints out the line of |
| 362 | code, the source ranges, and the caret. However, this behavior isn't required. |
| 363 | </p> |
| 364 | |
| 365 | <p>Another implementation of the DiagnosticClient interface is the |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 366 | 'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode. |
| Chris Lattner | fd408ea | 2008-11-23 00:42:53 +0000 | [diff] [blame] | 367 | Instead of formatting and printing out the diagnostics, this implementation just |
| 368 | captures and remembers the diagnostics as they fly by. Then -verify compares |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 369 | the list of produced diagnostics to the list of expected ones. If they disagree, |
| Chris Lattner | fd408ea | 2008-11-23 00:42:53 +0000 | [diff] [blame] | 370 | it prints out its own output. |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 371 | </p> |
| 372 | |
| Chris Lattner | fd408ea | 2008-11-23 00:42:53 +0000 | [diff] [blame] | 373 | <p>There are many other possible implementations of this interface, and this is |
| 374 | why we prefer diagnostics to pass down rich structured information in arguments. |
| 375 | For example, an HTML output might want declaration names be linkified to where |
| 376 | they come from in the source. Another example is that a GUI might let you click |
| 377 | on typedefs to expand them. This application would want to pass significantly |
| 378 | more information about types through to the GUI than a simple flat string. The |
| 379 | interface allows this to happen.</p> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 380 | |
| 381 | <!-- ====================================================== --> |
| 382 | <h4><a name="translation">Adding Translations to Clang</a></h4> |
| 383 | <!-- ====================================================== --> |
| 384 | |
| Chris Lattner | 627b705 | 2008-11-23 00:28:33 +0000 | [diff] [blame] | 385 | <p>Not possible yet! Diagnostic strings should be written in UTF-8, the client |
| Chris Lattner | fd408ea | 2008-11-23 00:42:53 +0000 | [diff] [blame] | 386 | can translate to the relevant code page if needed. Each translation completely |
| 387 | replaces the format string for the diagnostic.</p> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 388 | |
| 389 | |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 390 | <!-- ======================================================================= --> |
| 391 | <h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3> |
| 392 | <!-- ======================================================================= --> |
| 393 | |
| 394 | <p>Strangely enough, the SourceLocation class represents a location within the |
| 395 | source code of the program. Important design points include:</p> |
| 396 | |
| 397 | <ol> |
| 398 | <li>sizeof(SourceLocation) must be extremely small, as these are embedded into |
| 399 | many AST nodes and are passed around often. Currently it is 32 bits.</li> |
| 400 | <li>SourceLocation must be a simple value object that can be efficiently |
| 401 | copied.</li> |
| 402 | <li>We should be able to represent a source location for any byte of any input |
| 403 | file. This includes in the middle of tokens, in whitespace, in trigraphs, |
| 404 | etc.</li> |
| 405 | <li>A SourceLocation must encode the current #include stack that was active when |
| 406 | the location was processed. For example, if the location corresponds to a |
| 407 | token, it should contain the set of #includes active when the token was |
| 408 | lexed. This allows us to print the #include stack for a diagnostic.</li> |
| 409 | <li>SourceLocation must be able to describe macro expansions, capturing both |
| 410 | the ultimate instantiation point and the source of the original character |
| 411 | data.</li> |
| 412 | </ol> |
| 413 | |
| 414 | <p>In practice, the SourceLocation works together with the SourceManager class |
| 415 | to encode two pieces of information about a location: it's physical location |
| 416 | and it's virtual location. For most tokens, these will be the same. However, |
| 417 | for a macro expansion (or tokens that came from a _Pragma directive) these will |
| 418 | describe the location of the characters corresponding to the token and the |
| 419 | location where the token was used (i.e. the macro instantiation point or the |
| 420 | location of the _Pragma itself).</p> |
| 421 | |
| Chris Lattner | 3fcbb89 | 2008-11-23 08:32:53 +0000 | [diff] [blame] | 422 | <p>For efficiency, we only track one level of macro instantiations: if a token was |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 423 | produced by multiple instantiations, we only track the source and ultimate |
| 424 | destination. Though we could track the intermediate instantiation points, this |
| 425 | would require extra bookkeeping and no known client would benefit substantially |
| 426 | from this.</p> |
| 427 | |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 428 | <p>The Clang front-end inherently depends on the location of a token being |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 429 | tracked correctly. If it is ever incorrect, the front-end may get confused and |
| 430 | die. The reason for this is that the notion of the 'spelling' of a Token in |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 431 | Clang depends on being able to find the original input characters for the token. |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 432 | This concept maps directly to the "physical" location for the token.</p> |
| 433 | |
| 434 | <!-- ======================================================================= --> |
| 435 | <h2 id="liblex">The Lexer and Preprocessor Library</h2> |
| 436 | <!-- ======================================================================= --> |
| 437 | |
| 438 | <p>The Lexer library contains several tightly-connected classes that are involved |
| 439 | with the nasty process of lexing and preprocessing C source code. The main |
| 440 | interface to this library for outside clients is the large <a |
| 441 | href="#Preprocessor">Preprocessor</a> class. |
| 442 | It contains the various pieces of state that are required to coherently read |
| 443 | tokens out of a translation unit.</p> |
| 444 | |
| 445 | <p>The core interface to the Preprocessor object (once it is set up) is the |
| 446 | Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from |
| 447 | the preprocessor stream. There are two types of token providers that the |
| 448 | preprocessor is capable of reading from: a buffer lexer (provided by the <a |
| 449 | href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a |
| Chris Lattner | 7928125 | 2008-03-09 02:27:26 +0000 | [diff] [blame] | 450 | href="#TokenLexer">TokenLexer</a> class). |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 451 | |
| 452 | |
| 453 | <!-- ======================================================================= --> |
| 454 | <h3 id="Token">The Token class</h3> |
| 455 | <!-- ======================================================================= --> |
| 456 | |
| 457 | <p>The Token class is used to represent a single lexed token. Tokens are |
| 458 | intended to be used by the lexer/preprocess and parser libraries, but are not |
| 459 | intended to live beyond them (for example, they should not live in the ASTs).<p> |
| 460 | |
| 461 | <p>Tokens most often live on the stack (or some other location that is efficient |
| 462 | to access) as the parser is running, but occasionally do get buffered up. For |
| 463 | example, macro definitions are stored as a series of tokens, and the C++ |
| Chris Lattner | 3fcbb89 | 2008-11-23 08:32:53 +0000 | [diff] [blame] | 464 | front-end periodically needs to buffer tokens up for tentative parsing and |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 465 | various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit |
| 466 | system, sizeof(Token) is currently 16 bytes.</p> |
| 467 | |
| 468 | <p>Tokens contain the following information:</p> |
| 469 | |
| 470 | <ul> |
| 471 | <li><b>A SourceLocation</b> - This indicates the location of the start of the |
| 472 | token.</li> |
| 473 | |
| 474 | <li><b>A length</b> - This stores the length of the token as stored in the |
| 475 | SourceBuffer. For tokens that include them, this length includes trigraphs and |
| 476 | escaped newlines which are ignored by later phases of the compiler. By pointing |
| 477 | into the original source buffer, it is always possible to get the original |
| 478 | spelling of a token completely accurately.</li> |
| 479 | |
| 480 | <li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if |
| 481 | identifier lookup was enabled when the token was lexed (e.g. the lexer was not |
| 482 | reading in 'raw' mode) this contains a pointer to the unique hash value for the |
| 483 | identifier. Because the lookup happens before keyword identification, this |
| 484 | field is set even for language keywords like 'for'.</li> |
| 485 | |
| 486 | <li><b>TokenKind</b> - This indicates the kind of token as classified by the |
| 487 | lexer. This includes things like <tt>tok::starequal</tt> (for the "*=" |
| 488 | operator), <tt>tok::ampamp</tt> for the "&&" token, and keyword values |
| 489 | (e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note |
| 490 | that some tokens can be spelled multiple ways. For example, C++ supports |
| 491 | "operator keywords", where things like "and" are treated exactly like the |
| 492 | "&&" operator. In these cases, the kind value is set to |
| 493 | <tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to |
| 494 | consider both forms. For something that cares about which form is used (e.g. |
| 495 | the preprocessor 'stringize' operator) the spelling indicates the original |
| 496 | form.</li> |
| 497 | |
| 498 | <li><b>Flags</b> - There are currently four flags tracked by the |
| 499 | lexer/preprocessor system on a per-token basis: |
| 500 | |
| 501 | <ol> |
| 502 | <li><b>StartOfLine</b> - This was the first token that occurred on its input |
| 503 | source line.</li> |
| 504 | <li><b>LeadingSpace</b> - There was a space character either immediately |
| 505 | before the token or transitively before the token as it was expanded |
| 506 | through a macro. The definition of this flag is very closely defined by |
| 507 | the stringizing requirements of the preprocessor.</li> |
| 508 | <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to |
| 509 | represent identifier tokens which have macro expansion disabled. This |
| 510 | prevents them from being considered as candidates for macro expansion ever |
| 511 | in the future.</li> |
| 512 | <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the |
| 513 | token includes a trigraph or escaped newline. Since this is uncommon, |
| 514 | many pieces of code can fast-path on tokens that did not need cleaning. |
| 515 | </p> |
| 516 | </ol> |
| 517 | </li> |
| 518 | </ul> |
| 519 | |
| 520 | <p>One interesting (and somewhat unusual) aspect of tokens is that they don't |
| 521 | contain any semantic information about the lexed value. For example, if the |
| 522 | token was a pp-number token, we do not represent the value of the number that |
| 523 | was lexed (this is left for later pieces of code to decide). Additionally, the |
| 524 | lexer library has no notion of typedef names vs variable names: both are |
| 525 | returned as identifiers, and the parser is left to decide whether a specific |
| 526 | identifier is a typedef or a variable (tracking this requires scope information |
| 527 | among other things).</p> |
| 528 | |
| 529 | <!-- ======================================================================= --> |
| 530 | <h3 id="Lexer">The Lexer class</h3> |
| 531 | <!-- ======================================================================= --> |
| 532 | |
| 533 | <p>The Lexer class provides the mechanics of lexing tokens out of a source |
| 534 | buffer and deciding what they mean. The Lexer is complicated by the fact that |
| 535 | it operates on raw buffers that have not had spelling eliminated (this is a |
| 536 | necessity to get decent performance), but this is countered with careful coding |
| 537 | as well as standard performance techniques (for example, the comment handling |
| 538 | code is vectorized on X86 and PowerPC hosts).</p> |
| 539 | |
| 540 | <p>The lexer has a couple of interesting modal features:</p> |
| 541 | |
| 542 | <ul> |
| 543 | <li>The lexer can operate in 'raw' mode. This mode has several features that |
| 544 | make it possible to quickly lex the file (e.g. it stops identifier lookup, |
| 545 | doesn't specially handle preprocessor tokens, handles EOF differently, etc). |
| 546 | This mode is used for lexing within an "<tt>#if 0</tt>" block, for |
| 547 | example.</li> |
| 548 | <li>The lexer can capture and return comments as tokens. This is required to |
| 549 | support the -C preprocessor mode, which passes comments through, and is |
| 550 | used by the diagnostic checker to identifier expect-error annotations.</li> |
| 551 | <li>The lexer can be in ParsingFilename mode, which happens when preprocessing |
| Chris Lattner | 8438624 | 2007-09-16 19:25:23 +0000 | [diff] [blame] | 552 | after reading a #include directive. This mode changes the parsing of '<' |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 553 | to return an "angled string" instead of a bunch of tokens for each thing |
| 554 | within the filename.</li> |
| 555 | <li>When parsing a preprocessor directive (after "<tt>#</tt>") the |
| 556 | ParsingPreprocessorDirective mode is entered. This changes the parser to |
| 557 | return EOM at a newline.</li> |
| 558 | <li>The Lexer uses a LangOptions object to know whether trigraphs are enabled, |
| 559 | whether C++ or ObjC keywords are recognized, etc.</li> |
| 560 | </ul> |
| 561 | |
| 562 | <p>In addition to these modes, the lexer keeps track of a couple of other |
| 563 | features that are local to a lexed buffer, which change as the buffer is |
| 564 | lexed:</p> |
| 565 | |
| 566 | <ul> |
| 567 | <li>The Lexer uses BufferPtr to keep track of the current character being |
| 568 | lexed.</li> |
| 569 | <li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token |
| 570 | will start with its "start of line" bit set.</li> |
| 571 | <li>The Lexer keeps track of the current #if directives that are active (which |
| 572 | can be nested).</li> |
| 573 | <li>The Lexer keeps track of an <a href="#MultipleIncludeOpt"> |
| 574 | MultipleIncludeOpt</a> object, which is used to |
| 575 | detect whether the buffer uses the standard "<tt>#ifndef XX</tt> / |
| 576 | <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does, |
| 577 | subsequent includes can be ignored if the XX macro is defined.</li> |
| 578 | </ul> |
| 579 | |
| 580 | <!-- ======================================================================= --> |
| Chris Lattner | 7928125 | 2008-03-09 02:27:26 +0000 | [diff] [blame] | 581 | <h3 id="TokenLexer">The TokenLexer class</h3> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 582 | <!-- ======================================================================= --> |
| 583 | |
| Chris Lattner | 7928125 | 2008-03-09 02:27:26 +0000 | [diff] [blame] | 584 | <p>The TokenLexer class is a token provider that returns tokens from a list |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 585 | of tokens that came from somewhere else. It typically used for two things: 1) |
| 586 | returning tokens from a macro definition as it is being expanded 2) returning |
| 587 | tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and |
| 588 | will most likely be used to handle unbounded look-ahead for the C++ parser.</p> |
| 589 | |
| 590 | <!-- ======================================================================= --> |
| 591 | <h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3> |
| 592 | <!-- ======================================================================= --> |
| 593 | |
| 594 | <p>The MultipleIncludeOpt class implements a really simple little state machine |
| 595 | that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>" |
| 596 | idiom that people typically use to prevent multiple inclusion of headers. If a |
| 597 | buffer uses this idiom and is subsequently #include'd, the preprocessor can |
| 598 | simply check to see whether the guarding condition is defined or not. If so, |
| 599 | the preprocessor can completely ignore the include of the header.</p> |
| 600 | |
| 601 | |
| 602 | |
| 603 | <!-- ======================================================================= --> |
| 604 | <h2 id="libparse">The Parser Library</h2> |
| 605 | <!-- ======================================================================= --> |
| 606 | |
| 607 | <!-- ======================================================================= --> |
| 608 | <h2 id="libast">The AST Library</h2> |
| 609 | <!-- ======================================================================= --> |
| 610 | |
| 611 | <!-- ======================================================================= --> |
| 612 | <h3 id="Type">The Type class and its subclasses</h3> |
| 613 | <!-- ======================================================================= --> |
| 614 | |
| 615 | <p>The Type class (and its subclasses) are an important part of the AST. Types |
| 616 | are accessed through the ASTContext class, which implicitly creates and uniques |
| 617 | them as they are needed. Types have a couple of non-obvious features: 1) they |
| 618 | do not capture type qualifiers like const or volatile (See |
| 619 | <a href="#QualType">QualType</a>), and 2) they implicitly capture typedef |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 620 | information. Once created, types are immutable (unlike decls).</p> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 621 | |
| 622 | <p>Typedefs in C make semantic analysis a bit more complex than it would |
| 623 | be without them. The issue is that we want to capture typedef information |
| 624 | and represent it in the AST perfectly, but the semantics of operations need to |
| 625 | "see through" typedefs. For example, consider this code:</p> |
| 626 | |
| 627 | <code> |
| 628 | void func() {<br> |
| Bill Wendling | 30d1775 | 2007-10-06 01:56:01 +0000 | [diff] [blame] | 629 | typedef int foo;<br> |
| 630 | foo X, *Y;<br> |
| 631 | typedef foo* bar;<br> |
| 632 | bar Z;<br> |
| 633 | *X; <i>// error</i><br> |
| 634 | **Y; <i>// error</i><br> |
| 635 | **Z; <i>// error</i><br> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 636 | }<br> |
| 637 | </code> |
| 638 | |
| 639 | <p>The code above is illegal, and thus we expect there to be diagnostics emitted |
| 640 | on the annotated lines. In this example, we expect to get:</p> |
| 641 | |
| 642 | <pre> |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 643 | <b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 644 | *X; // error |
| 645 | <font color="blue">^~</font> |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 646 | <b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 647 | **Y; // error |
| 648 | <font color="blue">^~~</font> |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 649 | <b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b> |
| 650 | **Z; // error |
| 651 | <font color="blue">^~~</font> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 652 | </pre> |
| 653 | |
| 654 | <p>While this example is somewhat silly, it illustrates the point: we want to |
| 655 | retain typedef information where possible, so that we can emit errors about |
| 656 | "<tt>std::string</tt>" instead of "<tt>std::basic_string<char, std:...</tt>". |
| 657 | Doing this requires properly keeping typedef information (for example, the type |
| 658 | of "X" is "foo", not "int"), and requires properly propagating it through the |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 659 | various operators (for example, the type of *Y is "foo", not "int"). In order |
| 660 | to retain this information, the type of these expressions is an instance of the |
| 661 | TypedefType class, which indicates that the type of these expressions is a |
| 662 | typedef for foo. |
| 663 | </p> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 664 | |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 665 | <p>Representing types like this is great for diagnostics, because the |
| 666 | user-specified type is always immediately available. There are two problems |
| 667 | with this: first, various semantic checks need to make judgements about the |
| Chris Lattner | 33fc68a | 2007-07-31 18:54:50 +0000 | [diff] [blame] | 668 | <em>actual structure</em> of a type, ignoring typdefs. Second, we need an |
| 669 | efficient way to query whether two types are structurally identical to each |
| 670 | other, ignoring typedefs. The solution to both of these problems is the idea of |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 671 | canonical types.</p> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 672 | |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 673 | <!-- =============== --> |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 674 | <h4>Canonical Types</h4> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 675 | <!-- =============== --> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 676 | |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 677 | <p>Every instance of the Type class contains a canonical type pointer. For |
| 678 | simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>", |
| 679 | "<tt>int**</tt>"), the type just points to itself. For types that have a |
| 680 | typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>", |
| 681 | "<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their |
| 682 | structurally equivalent type without any typedefs (e.g. "<tt>int</tt>", |
| 683 | "<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 684 | |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 685 | <p>This design provides a constant time operation (dereferencing the canonical |
| 686 | type pointer) that gives us access to the structure of types. For example, |
| 687 | we can trivially tell that "bar" and "foo*" are the same type by dereferencing |
| 688 | their canonical type pointers and doing a pointer comparison (they both point |
| 689 | to the single "<tt>int*</tt>" type).</p> |
| 690 | |
| 691 | <p>Canonical types and typedef types bring up some complexities that must be |
| 692 | carefully managed. Specifically, the "isa/cast/dyncast" operators generally |
| 693 | shouldn't be used in code that is inspecting the AST. For example, when type |
| 694 | checking the indirection operator (unary '*' on a pointer), the type checker |
| 695 | must verify that the operand has a pointer type. It would not be correct to |
| 696 | check that with "<tt>isa<PointerType>(SubExpr->getType())</tt>", |
| 697 | because this predicate would fail if the subexpression had a typedef type.</p> |
| 698 | |
| 699 | <p>The solution to this problem are a set of helper methods on Type, used to |
| 700 | check their properties. In this case, it would be correct to use |
| 701 | "<tt>SubExpr->getType()->isPointerType()</tt>" to do the check. This |
| 702 | predicate will return true if the <em>canonical type is a pointer</em>, which is |
| 703 | true any time the type is structurally a pointer type. The only hard part here |
| 704 | is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p> |
| 705 | |
| 706 | <p>The second problem we face is how to get access to the pointer type once we |
| 707 | know it exists. To continue the example, the result type of the indirection |
| 708 | operator is the pointee type of the subexpression. In order to determine the |
| 709 | type, we need to get the instance of PointerType that best captures the typedef |
| 710 | information in the program. If the type of the expression is literally a |
| 711 | PointerType, we can return that, otherwise we have to dig through the |
| 712 | typedefs to find the pointer type. For example, if the subexpression had type |
| 713 | "<tt>foo*</tt>", we could return that type as the result. If the subexpression |
| 714 | had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do |
| 715 | <em>not</em> want "<tt>int*</tt>"). In order to provide all of this, Type has |
| Chris Lattner | 11406c1 | 2007-07-31 16:50:51 +0000 | [diff] [blame] | 716 | a getAsPointerType() method that checks whether the type is structurally a |
| Chris Lattner | 8a2bc62 | 2007-07-31 06:37:39 +0000 | [diff] [blame] | 717 | PointerType and, if so, returns the best one. If not, it returns a null |
| 718 | pointer.</p> |
| 719 | |
| 720 | <p>This structure is somewhat mystical, but after meditating on it, it will |
| 721 | make sense to you :).</p> |
| Chris Lattner | 86920d3 | 2007-07-31 05:42:17 +0000 | [diff] [blame] | 722 | |
| 723 | <!-- ======================================================================= --> |
| 724 | <h3 id="QualType">The QualType class</h3> |
| 725 | <!-- ======================================================================= --> |
| 726 | |
| 727 | <p>The QualType class is designed as a trivial value class that is small, |
| 728 | passed by-value and is efficient to query. The idea of QualType is that it |
| 729 | stores the type qualifiers (const, volatile, restrict) separately from the types |
| 730 | themselves: QualType is conceptually a pair of "Type*" and bits for the type |
| 731 | qualifiers.</p> |
| 732 | |
| 733 | <p>By storing the type qualifiers as bits in the conceptual pair, it is |
| 734 | extremely efficient to get the set of qualifiers on a QualType (just return the |
| 735 | field of the pair), add a type qualifier (which is a trivial constant-time |
| 736 | operation that sets a bit), and remove one or more type qualifiers (just return |
| 737 | a QualType with the bitfield set to empty).</p> |
| 738 | |
| 739 | <p>Further, because the bits are stored outside of the type itself, we do not |
| 740 | need to create duplicates of types with different sets of qualifiers (i.e. there |
| 741 | is only a single heap allocated "int" type: "const int" and "volatile const int" |
| 742 | both point to the same heap allocated "int" type). This reduces the heap size |
| 743 | used to represent bits and also means we do not have to consider qualifiers when |
| 744 | uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p> |
| 745 | |
| 746 | <p>In practice, on hosts where it is safe, the 3 type qualifiers are stored in |
| 747 | the low bit of the pointer to the Type object. This means that QualType is |
| 748 | exactly the same size as a pointer, and this works fine on any system where |
| 749 | malloc'd objects are at least 8 byte aligned.</p> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 750 | |
| 751 | <!-- ======================================================================= --> |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 752 | <h3 id="DeclarationName">Declaration names</h3> |
| 753 | <!-- ======================================================================= --> |
| 754 | |
| 755 | <p>The <tt>DeclarationName</tt> class represents the name of a |
| 756 | declaration in Clang. Declarations in the C family of languages can |
| Chris Lattner | 3fcbb89 | 2008-11-23 08:32:53 +0000 | [diff] [blame] | 757 | take several different forms. Most declarations are named by |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 758 | simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in |
| 759 | the function declaration <code>f(int x)</code>. In C++, declaration |
| 760 | names can also name class constructors ("<code>Class</code>" |
| 761 | in <code>struct Class { Class(); }</code>), class destructors |
| 762 | ("<code>~Class</code>"), overloaded operator names ("operator+"), |
| 763 | and conversion functions ("<code>operator void const *</code>"). In |
| 764 | Objective-C, declaration names can refer to the names of Objective-C |
| 765 | methods, which involve the method name and the parameters, |
| Chris Lattner | 3fcbb89 | 2008-11-23 08:32:53 +0000 | [diff] [blame] | 766 | collectively called a <i>selector</i>, e.g., |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 767 | "<code>setWidth:height:</code>". Since all of these kinds of |
| Chris Lattner | 3fcbb89 | 2008-11-23 08:32:53 +0000 | [diff] [blame] | 768 | entities - variables, functions, Objective-C methods, C++ |
| 769 | constructors, destructors, and operators - are represented as |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 770 | subclasses of Clang's common <code>NamedDecl</code> |
| 771 | class, <code>DeclarationName</code> is designed to efficiently |
| 772 | represent any kind of name.</p> |
| 773 | |
| 774 | <p>Given |
| 775 | a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code> |
| Douglas Gregor | 2def483 | 2008-11-17 20:34:05 +0000 | [diff] [blame] | 776 | will produce a value that describes what kind of name <code>N</code> |
| Douglas Gregor | e94ca9e4 | 2008-11-18 14:39:36 +0000 | [diff] [blame] | 777 | stores. There are 8 options (all of the names are inside |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 778 | the <code>DeclarationName</code> class)</p> |
| 779 | <dl> |
| 780 | <dt>Identifier</dt> |
| 781 | <dd>The name is a simple |
| 782 | identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the |
| 783 | corresponding <code>IdentifierInfo*</code> pointing to the actual |
| 784 | identifier. Note that C++ overloaded operators (e.g., |
| 785 | "<code>operator+</code>") are represented as special kinds of |
| 786 | identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code> |
| 787 | function to determine whether an identifier is an overloaded |
| 788 | operator name.</dd> |
| 789 | |
| 790 | <dt>ObjCZeroArgSelector, ObjCOneArgSelector, |
| 791 | ObjCMultiArgSelector</dt> |
| 792 | <dd>The name is an Objective-C selector, which can be retrieved as a |
| 793 | <code>Selector</code> instance |
| 794 | via <code>N.getObjCSelector()</code>. The three possible name |
| 795 | kinds for Objective-C reflect an optimization within |
| 796 | the <code>DeclarationName</code> class: both zero- and |
| 797 | one-argument selectors are stored as a |
| 798 | masked <code>IdentifierInfo</code> pointer, and therefore require |
| 799 | very little space, since zero- and one-argument selectors are far |
| 800 | more common than multi-argument selectors (which use a different |
| 801 | structure).</dd> |
| 802 | |
| 803 | <dt>CXXConstructorName</dt> |
| 804 | <dd>The name is a C++ constructor |
| 805 | name. Use <code>N.getCXXNameType()</code> to retrieve |
| 806 | the <a href="#QualType">type</a> that this constructor is meant to |
| 807 | construct. The type is always the canonical type, since all |
| 808 | constructors for a given type have the same name.</dd> |
| 809 | |
| 810 | <dt>CXXDestructorName</dt> |
| 811 | <dd>The name is a C++ destructor |
| 812 | name. Use <code>N.getCXXNameType()</code> to retrieve |
| 813 | the <a href="#QualType">type</a> whose destructor is being |
| 814 | named. This type is always a canonical type.</dd> |
| 815 | |
| 816 | <dt>CXXConversionFunctionName</dt> |
| 817 | <dd>The name is a C++ conversion function. Conversion functions are |
| 818 | named according to the type they convert to, e.g., "<code>operator void |
| 819 | const *</code>". Use <code>N.getCXXNameType()</code> to retrieve |
| 820 | the type that this conversion function converts to. This type is |
| 821 | always a canonical type.</dd> |
| Douglas Gregor | e94ca9e4 | 2008-11-18 14:39:36 +0000 | [diff] [blame] | 822 | |
| 823 | <dt>CXXOperatorName</dt> |
| 824 | <dd>The name is a C++ overloaded operator name. Overloaded operators |
| 825 | are named according to their spelling, e.g., |
| 826 | "<code>operator+</code>" or "<code>operator new |
| 827 | []</code>". Use <code>N.getCXXOverloadedOperator()</code> to |
| 828 | retrieve the overloaded operator (a value of |
| 829 | type <code>OverloadedOperatorKind</code>).</dd> |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 830 | </dl> |
| 831 | |
| 832 | <p><code>DeclarationName</code>s are cheap to create, copy, and |
| 833 | compare. They require only a single pointer's worth of storage in |
| Douglas Gregor | e94ca9e4 | 2008-11-18 14:39:36 +0000 | [diff] [blame] | 834 | the common cases (identifiers, zero- |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 835 | and one-argument Objective-C selectors) and use dense, uniqued |
| 836 | storage for the other kinds of |
| 837 | names. Two <code>DeclarationName</code>s can be compared for |
| 838 | equality (<code>==</code>, <code>!=</code>) using a simple bitwise |
| 839 | comparison, can be ordered |
| 840 | with <code><</code>, <code>></code>, <code><=</code>, |
| 841 | and <code>>=</code> (which provide a lexicographical ordering for |
| 842 | normal identifiers but an unspecified ordering for other kinds of |
| 843 | names), and can be placed into LLVM <code>DenseMap</code>s |
| 844 | and <code>DenseSet</code>s.</p> |
| 845 | |
| 846 | <p><code>DeclarationName</code> instances can be created in different |
| 847 | ways depending on what kind of name the instance will store. Normal |
| Douglas Gregor | e94ca9e4 | 2008-11-18 14:39:36 +0000 | [diff] [blame] | 848 | identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 849 | (<code>Selector</code>) can be implicitly converted |
| 850 | to <code>DeclarationName</code>s. Names for C++ constructors, |
| Douglas Gregor | e94ca9e4 | 2008-11-18 14:39:36 +0000 | [diff] [blame] | 851 | destructors, conversion functions, and overloaded operators can be retrieved from |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 852 | the <code>DeclarationNameTable</code>, an instance of which is |
| 853 | available as <code>ASTContext::DeclarationNames</code>. The member |
| 854 | functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>, |
| Douglas Gregor | e94ca9e4 | 2008-11-18 14:39:36 +0000 | [diff] [blame] | 855 | <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively, |
| 856 | return <code>DeclarationName</code> instances for the four kinds of |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 857 | C++ special function names.</p> |
| 858 | |
| 859 | <!-- ======================================================================= --> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 860 | <h3 id="CFG">The <tt>CFG</tt> class</h3> |
| 861 | <!-- ======================================================================= --> |
| 862 | |
| 863 | <p>The <tt>CFG</tt> class is designed to represent a source-level |
| 864 | control-flow graph for a single statement (<tt>Stmt*</tt>). Typically |
| 865 | instances of <tt>CFG</tt> are constructed for function bodies (usually |
| 866 | an instance of <tt>CompoundStmt</tt>), but can also be instantiated to |
| 867 | represent the control-flow of any class that subclasses <tt>Stmt</tt>, |
| 868 | which includes simple expressions. Control-flow graphs are especially |
| 869 | useful for performing |
| 870 | <a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow- |
| 871 | or path-sensitive</a> program analyses on a given function.</p> |
| 872 | |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 873 | <!-- ============ --> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 874 | <h4>Basic Blocks</h4> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 875 | <!-- ============ --> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 876 | |
| 877 | <p>Concretely, an instance of <tt>CFG</tt> is a collection of basic |
| 878 | blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which |
| 879 | simply contains an ordered sequence of <tt>Stmt*</tt> (each referring |
| 880 | to statements in the AST). The ordering of statements within a block |
| 881 | indicates unconditional flow of control from one statement to the |
| 882 | next. <a href="#ConditionalControlFlow">Conditional control-flow</a> |
| 883 | is represented using edges between basic blocks. The statements |
| 884 | within a given <tt>CFGBlock</tt> can be traversed using |
| 885 | the <tt>CFGBlock::*iterator</tt> interface.</p> |
| 886 | |
| 887 | <p> |
| Ted Kremenek | 18e17e7 | 2007-10-18 22:50:52 +0000 | [diff] [blame] | 888 | A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 889 | the control-flow graph it represents. Each <tt>CFGBlock</tt> within a |
| 890 | CFG is also uniquely numbered (accessible |
| 891 | via <tt>CFGBlock::getBlockID()</tt>). Currently the number is |
| 892 | based on the ordering the blocks were created, but no assumptions |
| 893 | should be made on how <tt>CFGBlock</tt>s are numbered other than their |
| 894 | numbers are unique and that they are numbered from 0..N-1 (where N is |
| 895 | the number of basic blocks in the CFG).</p> |
| 896 | |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 897 | <!-- ===================== --> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 898 | <h4>Entry and Exit Blocks</h4> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 899 | <!-- ===================== --> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 900 | |
| 901 | Each instance of <tt>CFG</tt> contains two special blocks: |
| 902 | an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which |
| 903 | has no incoming edges, and an <i>exit</i> block (accessible |
| 904 | via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither |
| 905 | block contains any statements, and they serve the role of providing a |
| 906 | clear entrance and exit for a body of code such as a function body. |
| 907 | The presence of these empty blocks greatly simplifies the |
| 908 | implementation of many analyses built on top of CFGs. |
| 909 | |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 910 | <!-- ===================================================== --> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 911 | <h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4> |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 912 | <!-- ===================================================== --> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 913 | |
| 914 | <p>Conditional control-flow (such as those induced by if-statements |
| 915 | and loops) is represented as edges between <tt>CFGBlock</tt>s. |
| 916 | Because different C language constructs can induce control-flow, |
| 917 | each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that |
| 918 | represents the <i>terminator</i> of the block. A terminator is simply |
| 919 | the statement that caused the control-flow, and is used to identify |
| 920 | the nature of the conditional control-flow between blocks. For |
| 921 | example, in the case of an if-statement, the terminator refers to |
| 922 | the <tt>IfStmt</tt> object in the AST that represented the given |
| 923 | branch.</p> |
| 924 | |
| 925 | <p>To illustrate, consider the following code example:</p> |
| 926 | |
| 927 | <code> |
| 928 | int foo(int x) {<br> |
| 929 | x = x + 1;<br> |
| 930 | <br> |
| 931 | if (x > 2) x++;<br> |
| 932 | else {<br> |
| 933 | x += 2;<br> |
| 934 | x *= 2;<br> |
| 935 | }<br> |
| 936 | <br> |
| 937 | return x;<br> |
| 938 | } |
| 939 | </code> |
| 940 | |
| 941 | <p>After invoking the parser+semantic analyzer on this code fragment, |
| 942 | the AST of the body of <tt>foo</tt> is referenced by a |
| 943 | single <tt>Stmt*</tt>. We can then construct an instance |
| 944 | of <tt>CFG</tt> representing the control-flow graph of this function |
| 945 | body by single call to a static class method:</p> |
| 946 | |
| 947 | <code> |
| 948 | Stmt* FooBody = ...<br> |
| 949 | CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody); |
| 950 | </code> |
| 951 | |
| 952 | <p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt> |
| 953 | to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no |
| 954 | longer needed.</p> |
| 955 | |
| 956 | <p>Along with providing an interface to iterate over |
| 957 | its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods |
| 958 | that are useful for debugging and visualizing CFGs. For example, the |
| 959 | method |
| 960 | <tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to |
| 961 | standard error. This is especially useful when one is using a |
| 962 | debugger such as gdb. For example, here is the output |
| 963 | of <tt>FooCFG->dump()</tt>:</p> |
| 964 | |
| 965 | <code> |
| 966 | [ B5 (ENTRY) ]<br> |
| 967 | Predecessors (0):<br> |
| 968 | Successors (1): B4<br> |
| 969 | <br> |
| 970 | [ B4 ]<br> |
| 971 | 1: x = x + 1<br> |
| 972 | 2: (x > 2)<br> |
| 973 | <b>T: if [B4.2]</b><br> |
| 974 | Predecessors (1): B5<br> |
| 975 | Successors (2): B3 B2<br> |
| 976 | <br> |
| 977 | [ B3 ]<br> |
| 978 | 1: x++<br> |
| 979 | Predecessors (1): B4<br> |
| 980 | Successors (1): B1<br> |
| 981 | <br> |
| 982 | [ B2 ]<br> |
| 983 | 1: x += 2<br> |
| 984 | 2: x *= 2<br> |
| 985 | Predecessors (1): B4<br> |
| 986 | Successors (1): B1<br> |
| 987 | <br> |
| 988 | [ B1 ]<br> |
| 989 | 1: return x;<br> |
| 990 | Predecessors (2): B2 B3<br> |
| 991 | Successors (1): B0<br> |
| 992 | <br> |
| 993 | [ B0 (EXIT) ]<br> |
| 994 | Predecessors (1): B1<br> |
| 995 | Successors (0): |
| 996 | </code> |
| 997 | |
| 998 | <p>For each block, the pretty-printed output displays for each block |
| 999 | the number of <i>predecessor</i> blocks (blocks that have outgoing |
| 1000 | control-flow to the given block) and <i>successor</i> blocks (blocks |
| 1001 | that have control-flow that have incoming control-flow from the given |
| 1002 | block). We can also clearly see the special entry and exit blocks at |
| 1003 | the beginning and end of the pretty-printed output. For the entry |
| 1004 | block (block B5), the number of predecessor blocks is 0, while for the |
| 1005 | exit block (block B0) the number of successor blocks is 0.</p> |
| 1006 | |
| 1007 | <p>The most interesting block here is B4, whose outgoing control-flow |
| 1008 | represents the branching caused by the sole if-statement |
| 1009 | in <tt>foo</tt>. Of particular interest is the second statement in |
| 1010 | the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed |
| 1011 | as <b><tt>if [B4.2]</tt></b>. The second statement represents the |
| 1012 | evaluation of the condition of the if-statement, which occurs before |
| 1013 | the actual branching of control-flow. Within the <tt>CFGBlock</tt> |
| 1014 | for B4, the <tt>Stmt*</tt> for the second statement refers to the |
| 1015 | actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus |
| 1016 | pointers to subclasses of <tt>Expr</tt> can appear in the list of |
| 1017 | statements in a block, and not just subclasses of <tt>Stmt</tt> that |
| 1018 | refer to proper C statements.</p> |
| 1019 | |
| 1020 | <p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt> |
| 1021 | object in the AST. The pretty-printer outputs <b><tt>if |
| 1022 | [B4.2]</tt></b> because the condition expression of the if-statement |
| 1023 | has an actual place in the basic block, and thus the terminator is |
| 1024 | essentially |
| 1025 | <i>referring</i> to the expression that is the second statement of |
| 1026 | block B4 (i.e., B4.2). In this manner, conditions for control-flow |
| 1027 | (which also includes conditions for loops and switch statements) are |
| 1028 | hoisted into the actual basic block.</p> |
| 1029 | |
| Chris Lattner | 62fd278 | 2008-11-22 21:41:31 +0000 | [diff] [blame] | 1030 | <!-- ===================== --> |
| 1031 | <!-- <h4>Implicit Control-Flow</h4> --> |
| 1032 | <!-- ===================== --> |
| Ted Kremenek | 8bc0571 | 2007-10-10 23:01:43 +0000 | [diff] [blame] | 1033 | |
| 1034 | <!-- |
| 1035 | <p>A key design principle of the <tt>CFG</tt> class was to not require |
| 1036 | any transformations to the AST in order to represent control-flow. |
| 1037 | Thus the <tt>CFG</tt> does not perform any "lowering" of the |
| 1038 | statements in an AST: loops are not transformed into guarded gotos, |
| 1039 | short-circuit operations are not converted to a set of if-statements, |
| 1040 | and so on.</p> |
| 1041 | --> |
| Ted Kremenek | 17a295d | 2008-06-11 06:19:49 +0000 | [diff] [blame] | 1042 | |
| Chris Lattner | 7bad199 | 2008-11-16 21:48:07 +0000 | [diff] [blame] | 1043 | |
| 1044 | <!-- ======================================================================= --> |
| 1045 | <h3 id="Constants">Constant Folding in the Clang AST</h3> |
| 1046 | <!-- ======================================================================= --> |
| 1047 | |
| 1048 | <p>There are several places where constants and constant folding matter a lot to |
| 1049 | the Clang front-end. First, in general, we prefer the AST to retain the source |
| 1050 | code as close to how the user wrote it as possible. This means that if they |
| 1051 | wrote "5+4", we want to keep the addition and two constants in the AST, we don't |
| 1052 | want to fold to "9". This means that constant folding in various ways turns |
| 1053 | into a tree walk that needs to handle the various cases.</p> |
| 1054 | |
| 1055 | <p>However, there are places in both C and C++ that require constants to be |
| 1056 | folded. For example, the C standard defines what an "integer constant |
| 1057 | expression" (i-c-e) is with very precise and specific requirements. The |
| 1058 | language then requires i-c-e's in a lot of places (for example, the size of a |
| 1059 | bitfield, the value for a case statement, etc). For these, we have to be able |
| 1060 | to constant fold the constants, to do semantic checks (e.g. verify bitfield size |
| 1061 | is non-negative and that case statements aren't duplicated). We aim for Clang |
| 1062 | to be very pedantic about this, diagnosing cases when the code does not use an |
| 1063 | i-c-e where one is required, but accepting the code unless running with |
| 1064 | <tt>-pedantic-errors</tt>.</p> |
| 1065 | |
| 1066 | <p>Things get a little bit more tricky when it comes to compatibility with |
| 1067 | real-world source code. Specifically, GCC has historically accepted a huge |
| 1068 | superset of expressions as i-c-e's, and a lot of real world code depends on this |
| 1069 | unfortuate accident of history (including, e.g., the glibc system headers). GCC |
| 1070 | accepts anything its "fold" optimizer is capable of reducing to an integer |
| 1071 | constant, which means that the definition of what it accepts changes as its |
| 1072 | optimizer does. One example is that GCC accepts things like "case X-X:" even |
| 1073 | when X is a variable, because it can fold this to 0.</p> |
| 1074 | |
| 1075 | <p>Another issue are how constants interact with the extensions we support, such |
| 1076 | as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99 |
| 1077 | obviously does not specify the semantics of any of these extensions, and the |
| 1078 | definition of i-c-e does not include them. However, these extensions are often |
| 1079 | used in real code, and we have to have a way to reason about them.</p> |
| 1080 | |
| 1081 | <p>Finally, this is not just a problem for semantic analysis. The code |
| 1082 | generator and other clients have to be able to fold constants (e.g. to |
| 1083 | initialize global variables) and has to handle a superset of what C99 allows. |
| 1084 | Further, these clients can benefit from extended information. For example, we |
| 1085 | know that "foo()||1" always evaluates to true, but we can't replace the |
| 1086 | expression with true because it has side effects.</p> |
| 1087 | |
| 1088 | <!-- ======================= --> |
| 1089 | <h4>Implementation Approach</h4> |
| 1090 | <!-- ======================= --> |
| 1091 | |
| 1092 | <p>After trying several different approaches, we've finally converged on a |
| 1093 | design (Note, at the time of this writing, not all of this has been implemented, |
| 1094 | consider this a design goal!). Our basic approach is to define a single |
| 1095 | recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is |
| 1096 | implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar' |
| 1097 | type (integer, fp, complex, or pointer) this method returns the following |
| 1098 | information:</p> |
| 1099 | |
| 1100 | <ul> |
| 1101 | <li>Whether the expression is an integer constant expression, a general |
| 1102 | constant that was folded but has no side effects, a general constant that |
| 1103 | was folded but that does have side effects, or an uncomputable/unfoldable |
| 1104 | value. |
| 1105 | </li> |
| 1106 | <li>If the expression was computable in any way, this method returns the APValue |
| 1107 | for the result of the expression.</li> |
| 1108 | <li>If the expression is not evaluatable at all, this method returns |
| 1109 | information on one of the problems with the expression. This includes a |
| 1110 | SourceLocation for where the problem is, and a diagnostic ID that explains |
| 1111 | the problem. The diagnostic should be have ERROR type.</li> |
| 1112 | <li>If the expression is not an integer constant expression, this method returns |
| 1113 | information on one of the problems with the expression. This includes a |
| 1114 | SourceLocation for where the problem is, and a diagnostic ID that explains |
| 1115 | the problem. The diagnostic should be have EXTENSION type.</li> |
| 1116 | </ul> |
| 1117 | |
| 1118 | <p>This information gives various clients the flexibility that they want, and we |
| 1119 | will eventually have some helper methods for various extensions. For example, |
| 1120 | Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which |
| 1121 | calls Evaluate on the expression. If the expression is not foldable, the error |
| 1122 | is emitted, and it would return true. If the expression is not an i-c-e, the |
| 1123 | EXTENSION diagnostic is emitted. Finally it would return false to indicate that |
| 1124 | the AST is ok.</p> |
| 1125 | |
| 1126 | <p>Other clients can use the information in other ways, for example, codegen can |
| 1127 | just use expressions that are foldable in any way.</p> |
| 1128 | |
| 1129 | <!-- ========== --> |
| 1130 | <h4>Extensions</h4> |
| 1131 | <!-- ========== --> |
| 1132 | |
| Chris Lattner | 552de0a | 2008-11-23 08:16:56 +0000 | [diff] [blame] | 1133 | <p>This section describes how some of the various extensions Clang supports |
| Chris Lattner | 7bad199 | 2008-11-16 21:48:07 +0000 | [diff] [blame] | 1134 | interacts with constant evaluation:</p> |
| 1135 | |
| 1136 | <ul> |
| 1137 | <li><b><tt>__extension__</tt></b>: The expression form of this extension causes |
| 1138 | any evaluatable subexpression to be accepted as an integer constant |
| 1139 | expression.</li> |
| 1140 | <li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer |
| 1141 | constant expression) if the operand is any evaluatable constant.</li> |
| 1142 | <li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an |
| 1143 | integer constant expression, but we accept any constant as an "extension of |
| 1144 | an extension". This only evaluates one operand depending on which way the |
| 1145 | condition evaluates.</li> |
| 1146 | <li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer |
| 1147 | constant expression.</li> |
| 1148 | <li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a |
| 1149 | floating-point literal.</li> |
| 1150 | <li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as |
| 1151 | general constant expressions.</li> |
| 1152 | </ul> |
| 1153 | |
| 1154 | |
| 1155 | |
| 1156 | |
| Ted Kremenek | 17a295d | 2008-06-11 06:19:49 +0000 | [diff] [blame] | 1157 | </div> |
| 1158 | </body> |
| Douglas Gregor | 2e1cd42 | 2008-11-17 14:58:09 +0000 | [diff] [blame] | 1159 | </html> |