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diff --git a/docs/InternalsManual.html b/docs/InternalsManual.html deleted file mode 100644 index 57f0631..0000000 --- a/docs/InternalsManual.html +++ /dev/null @@ -1,2019 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> -<head> -<title>"Clang" CFE Internals Manual</title> -<link type="text/css" rel="stylesheet" href="../menu.css"> -<link type="text/css" rel="stylesheet" href="../content.css"> -<style type="text/css"> -td { - vertical-align: top; -} -</style> -</head> -<body> - -<!--#include virtual="../menu.html.incl"--> - -<div id="content"> - -<h1>"Clang" CFE Internals Manual</h1> - -<ul> -<li><a href="#intro">Introduction</a></li> -<li><a href="#libsupport">LLVM Support Library</a></li> -<li><a href="#libbasic">The Clang 'Basic' Library</a> - <ul> - <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li> - <li><a href="#SourceLocation">The SourceLocation and SourceManager - classes</a></li> - <li><a href="#SourceRange">SourceRange and CharSourceRange</a></li> - </ul> -</li> -<li><a href="#libdriver">The Driver Library</a> -</li> -<li><a href="#pch">Precompiled Headers</a> -<li><a href="#libfrontend">The Frontend Library</a> -</li> -<li><a href="#liblex">The Lexer and Preprocessor Library</a> - <ul> - <li><a href="#Token">The Token class</a></li> - <li><a href="#Lexer">The Lexer class</a></li> - <li><a href="#AnnotationToken">Annotation Tokens</a></li> - <li><a href="#TokenLexer">The TokenLexer class</a></li> - <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li> - </ul> -</li> -<li><a href="#libparse">The Parser Library</a> -</li> -<li><a href="#libast">The AST Library</a> - <ul> - <li><a href="#Type">The Type class and its subclasses</a></li> - <li><a href="#QualType">The QualType class</a></li> - <li><a href="#DeclarationName">Declaration names</a></li> - <li><a href="#DeclContext">Declaration contexts</a> - <ul> - <li><a href="#Redeclarations">Redeclarations and Overloads</a></li> - <li><a href="#LexicalAndSemanticContexts">Lexical and Semantic - Contexts</a></li> - <li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li> - <li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li> - </ul> - </li> - <li><a href="#CFG">The CFG class</a></li> - <li><a href="#Constants">Constant Folding in the Clang AST</a></li> - </ul> -</li> -<li><a href="#Howtos">Howto guides</a> - <ul> - <li><a href="#AddingAttributes">How to add an attribute</a></li> - <li><a href="#AddingExprStmt">How to add a new expression or statement</a></li> - </ul> -</li> -</ul> - - -<!-- ======================================================================= --> -<h2 id="intro">Introduction</h2> -<!-- ======================================================================= --> - -<p>This document describes some of the more important APIs and internal design -decisions made in the Clang C front-end. The purpose of this document is to -both capture some of this high level information and also describe some of the -design decisions behind it. This is meant for people interested in hacking on -Clang, not for end-users. The description below is categorized by -libraries, and does not describe any of the clients of the libraries.</p> - -<!-- ======================================================================= --> -<h2 id="libsupport">LLVM Support Library</h2> -<!-- ======================================================================= --> - -<p>The LLVM libsupport library provides many underlying libraries and -<a href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>, -including command line option processing, various containers and a system -abstraction layer, which is used for file system access.</p> - -<!-- ======================================================================= --> -<h2 id="libbasic">The Clang 'Basic' Library</h2> -<!-- ======================================================================= --> - -<p>This library certainly needs a better name. The 'basic' library contains a -number of low-level utilities for tracking and manipulating source buffers, -locations within the source buffers, diagnostics, tokens, target abstraction, -and information about the subset of the language being compiled for.</p> - -<p>Part of this infrastructure is specific to C (such as the TargetInfo class), -other parts could be reused for other non-C-based languages (SourceLocation, -SourceManager, Diagnostics, FileManager). When and if there is future demand -we can figure out if it makes sense to introduce a new library, move the general -classes somewhere else, or introduce some other solution.</p> - -<p>We describe the roles of these classes in order of their dependencies.</p> - - -<!-- ======================================================================= --> -<h3 id="Diagnostics">The Diagnostics Subsystem</h3> -<!-- ======================================================================= --> - -<p>The Clang Diagnostics subsystem is an important part of how the compiler -communicates with the human. Diagnostics are the warnings and errors produced -when the code is incorrect or dubious. In Clang, each diagnostic produced has -(at the minimum) a unique ID, an English translation associated with it, a <a -href="#SourceLocation">SourceLocation</a> to "put the caret", and a severity (e.g. -<tt>WARNING</tt> or <tt>ERROR</tt>). They can also optionally include a number -of arguments to the dianostic (which fill in "%0"'s in the string) as well as a -number of source ranges that related to the diagnostic.</p> - -<p>In this section, we'll be giving examples produced by the Clang command line -driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many -different ways</a> depending on how the DiagnosticClient interface is -implemented. A representative example of a diagnostic is:</p> - -<pre> -t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float') - <span style="color:darkgreen">P = (P-42) + Gamma*4;</span> - <span style="color:blue">~~~~~~ ^ ~~~~~~~</span> -</pre> - -<p>In this example, you can see the English translation, the severity (error), -you can see the source location (the caret ("^") and file/line/column info), -the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex -float"). You'll have to believe me that there is a unique ID backing the -diagnostic :).</p> - -<p>Getting all of this to happen has several steps and involves many moving -pieces, this section describes them and talks about best practices when adding -a new diagnostic.</p> - -<!-- ============================= --> -<h4>The Diagnostic*Kinds.td files</h4> -<!-- ============================= --> - -<p>Diagnostics are created by adding an entry to one of the <tt> -clang/Basic/Diagnostic*Kinds.td</tt> files, depending on what library will -be using it. From this file, tblgen generates the unique ID of the diagnostic, -the severity of the diagnostic and the English translation + format string.</p> - -<p>There is little sanity with the naming of the unique ID's right now. Some -start with err_, warn_, ext_ to encode the severity into the name. Since the -enum is referenced in the C++ code that produces the diagnostic, it is somewhat -useful for it to be reasonably short.</p> - -<p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>, -<tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The -<tt>ERROR</tt> severity is used for diagnostics indicating the program is never -acceptable under any circumstances. When an error is emitted, the AST for the -input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt> -severities are used for extensions to the language that Clang accepts. This -means that Clang fully understands and can represent them in the AST, but we -produce diagnostics to tell the user their code is non-portable. The difference -is that the former are ignored by default, and the later warn by default. The -<tt>WARNING</tt> severity is used for constructs that are valid in the currently -selected source language but that are dubious in some way. The <tt>NOTE</tt> -level is used to staple more information onto previous diagnostics.</p> - -<p>These <em>severities</em> are mapped into a smaller set (the -Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>, -<tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics -subsystem based on various configuration options. Clang internally supports a -fully fine grained mapping mechanism that allows you to map almost any -diagnostic to the output level that you want. The only diagnostics that cannot -be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously -emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to -<tt>Fatal</tt> (it is not possible to turn an error into a warning, -for example).</p> - -<p>Diagnostic mappings are used in many ways. For example, if the user -specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if -they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is -used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc. -</p> - -<p> -Mapping to <tt>Fatal</tt> should only be used for diagnostics that are -considered so severe that error recovery won't be able to recover sensibly from -them (thus spewing a ton of bogus errors). One example of this class of error -are failure to #include a file. -</p> - -<!-- ================= --> -<h4>The Format String</h4> -<!-- ================= --> - -<p>The format string for the diagnostic is very simple, but it has some power. -It takes the form of a string in English with markers that indicate where and -how arguments to the diagnostic are inserted and formatted. For example, here -are some simple format strings:</p> - -<pre> - "binary integer literals are an extension" - "format string contains '\\0' within the string body" - "more '<b>%%</b>' conversions than data arguments" - "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)" - "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator" - " (has <b>%1</b> parameter<b>%s1</b>)" -</pre> - -<p>These examples show some important points of format strings. You can use any - plain ASCII character in the diagnostic string except "%" without a problem, - but these are C strings, so you have to use and be aware of all the C escape - sequences (as in the second example). If you want to produce a "%" in the - output, use the "%%" escape sequence, like the third diagnostic. Finally, - Clang uses the "%...[digit]" sequences to specify where and how arguments to - the diagnostic are formatted.</p> - -<p>Arguments to the diagnostic are numbered according to how they are specified - by the C++ code that <a href="#producingdiag">produces them</a>, and are - referenced by <tt>%0</tt> .. <tt>%9</tt>. If you have more than 10 arguments - to your diagnostic, you are doing something wrong :). Unlike printf, there - is no requirement that arguments to the diagnostic end up in the output in - the same order as they are specified, you could have a format string with - <tt>"%1 %0"</tt> that swaps them, for example. The text in between the - percent and digit are formatting instructions. If there are no instructions, - the argument is just turned into a string and substituted in.</p> - -<p>Here are some "best practices" for writing the English format string:</p> - -<ul> -<li>Keep the string short. It should ideally fit in the 80 column limit of the - <tt>DiagnosticKinds.td</tt> file. This avoids the diagnostic wrapping when - printed, and forces you to think about the important point you are conveying - with the diagnostic.</li> -<li>Take advantage of location information. The user will be able to see the - line and location of the caret, so you don't need to tell them that the - problem is with the 4th argument to the function: just point to it.</li> -<li>Do not capitalize the diagnostic string, and do not end it with a - period.</li> -<li>If you need to quote something in the diagnostic string, use single - quotes.</li> -</ul> - -<p>Diagnostics should never take random English strings as arguments: you -shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like -<tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing -this prevents <a href="#translation">translating</a> the Clang diagnostics to -other languages (because they'll get random English words in their otherwise -localized diagnostic). The exceptions to this are C/C++ language keywords -(e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note -that things like "pointer" and "reference" are not keywords. On the other -hand, you <em>can</em> include anything that comes from the user's source code, -including variable names, types, labels, etc. The 'select' format can be -used to achieve this sort of thing in a localizable way, see below.</p> - -<!-- ==================================== --> -<h4>Formatting a Diagnostic Argument</h4> -<!-- ==================================== --> - -<p>Arguments to diagnostics are fully typed internally, and come from a couple -different classes: integers, types, names, and random strings. Depending on -the class of the argument, it can be optionally formatted in different ways. -This gives the DiagnosticClient information about what the argument means -without requiring it to use a specific presentation (consider this MVC for -Clang :).</p> - -<p>Here are the different diagnostic argument formats currently supported by -Clang:</p> - -<table> -<tr><td colspan="2"><b>"s" format</b></td></tr> -<tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr> -<tr><td>Class:</td><td>Integers</td></tr> -<tr><td>Description:</td><td>This is a simple formatter for integers that is - useful when producing English diagnostics. When the integer is 1, it prints - as nothing. When the integer is not 1, it prints as "s". This allows some - simple grammatical forms to be to be handled correctly, and eliminates the - need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr> - -<tr><td colspan="2"><b>"select" format</b></td></tr> -<tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2 - operator"</tt></td></tr> -<tr><td>Class:</td><td>Integers</td></tr> -<tr><td>Description:</td><td><p>This format specifier is used to merge multiple - related diagnostics together into one common one, without requiring the - difference to be specified as an English string argument. Instead of - specifying the string, the diagnostic gets an integer argument and the - format string selects the numbered option. In this case, the "%2" value - must be an integer in the range [0..2]. If it is 0, it prints 'unary', if - it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This - allows other language translations to substitute reasonable words (or entire - phrases) based on the semantics of the diagnostic instead of having to do - things textually.</p> - <p>The selected string does undergo formatting.</p></td></tr> - -<tr><td colspan="2"><b>"plural" format</b></td></tr> -<tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to - your computer"</tt></td></tr> -<tr><td>Class:</td><td>Integers</td></tr> -<tr><td>Description:</td><td><p>This is a formatter for complex plural forms. - It is designed to handle even the requirements of languages with very - complex plural forms, as many Baltic languages have. The argument consists - of a series of expression/form pairs, separated by ':', where the first form - whose expression evaluates to true is the result of the modifier.</p> - <p>An expression can be empty, in which case it is always true. See the - example at the top. Otherwise, it is a series of one or more numeric - conditions, separated by ','. If any condition matches, the expression - matches. Each numeric condition can take one of three forms.</p> - <ul> - <li>number: A simple decimal number matches if the argument is the same - as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li> - <li>range: A range in square brackets matches if the argument is within - the range. Then range is inclusive on both ends. Example: - <tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li> - <li>modulo: A modulo operator is followed by a number, and - equals sign and either a number or a range. The tests are the - same as for plain - numbers and ranges, but the argument is taken modulo the number first. - Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything - else}1"</tt></li> - </ul> - <p>The parser is very unforgiving. A syntax error, even whitespace, will - abort, as will a failure to match the argument against any - expression.</p></td></tr> - -<tr><td colspan="2"><b>"ordinal" format</b></td></tr> -<tr><td>Example:</td><td><tt>"ambiguity in %ordinal0 argument"</tt></td></tr> -<tr><td>Class:</td><td>Integers</td></tr> -<tr><td>Description:</td><td><p>This is a formatter which represents the - argument number as an ordinal: the value <tt>1</tt> becomes <tt>1st</tt>, - <tt>3</tt> becomes <tt>3rd</tt>, and so on. Values less than <tt>1</tt> - are not supported.</p> - <p>This formatter is currently hard-coded to use English ordinals.</p></td></tr> - -<tr><td colspan="2"><b>"objcclass" format</b></td></tr> -<tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr> -<tr><td>Class:</td><td>DeclarationName</td></tr> -<tr><td>Description:</td><td><p>This is a simple formatter that indicates the - DeclarationName corresponds to an Objective-C class method selector. As - such, it prints the selector with a leading '+'.</p></td></tr> - -<tr><td colspan="2"><b>"objcinstance" format</b></td></tr> -<tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr> -<tr><td>Class:</td><td>DeclarationName</td></tr> -<tr><td>Description:</td><td><p>This is a simple formatter that indicates the - DeclarationName corresponds to an Objective-C instance method selector. As - such, it prints the selector with a leading '-'.</p></td></tr> - -<tr><td colspan="2"><b>"q" format</b></td></tr> -<tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr> -<tr><td>Class:</td><td>NamedDecl*</td></tr> -<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> - -<tr><td colspan="2"><b>"diff" format</b></td></tr> -<tr><td>Example:</td><td><tt>"no known conversion %diff{from | to | }1,2"</tt></td></tr> -<tr><td>Class:</td><td>QualType</td></tr> -<tr><td>Description</td><td><p>This formatter takes two QualTypes and attempts to print a template difference between the two. If tree printing is off, the text inside the braces before the pipe is printed, with the formatted text replacing the $. If tree printing is on, the text after the pipe is printed and a type tree is printed after the diagnostic message. -</p></td></tr> - -</table> - -<p>It is really easy to add format specifiers to the Clang diagnostics system, -but they should be discussed before they are added. If you are creating a lot -of repetitive diagnostics and/or have an idea for a useful formatter, please -bring it up on the cfe-dev mailing list.</p> - -<!-- ===================================================== --> -<h4 id="producingdiag">Producing the Diagnostic</h4> -<!-- ===================================================== --> - -<p>Now that you've created the diagnostic in the DiagnosticKinds.td file, you -need to write the code that detects the condition in question and emits the -new diagnostic. Various components of Clang (e.g. the preprocessor, Sema, -etc) provide a helper function named "Diag". It creates a diagnostic and -accepts the arguments, ranges, and other information that goes along with -it.</p> - -<p>For example, the binary expression error comes from code like this:</p> - -<pre> - if (various things that are bad) - Diag(Loc, diag::err_typecheck_invalid_operands) - << lex->getType() << rex->getType() - << lex->getSourceRange() << rex->getSourceRange(); -</pre> - -<p>This shows that use of the Diag method: they take a location (a <a -href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value -(which matches the name from DiagnosticKinds.td). If the diagnostic takes -arguments, they are specified with the << operator: the first argument -becomes %0, the second becomes %1, etc. The diagnostic interface allows you to -specify arguments of many different types, including <tt>int</tt> and -<tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and -<tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and -<tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc. -SourceRanges are also specified with the << operator, but do not have a -specific ordering requirement.</p> - -<p>As you can see, adding and producing a diagnostic is pretty straightforward. -The hard part is deciding exactly what you need to say to help the user, picking -a suitable wording, and providing the information needed to format it correctly. -The good news is that the call site that issues a diagnostic should be -completely independent of how the diagnostic is formatted and in what language -it is rendered. -</p> - -<!-- ==================================================== --> -<h4 id="fix-it-hints">Fix-It Hints</h4> -<!-- ==================================================== --> - -<p>In some cases, the front end emits diagnostics when it is clear -that some small change to the source code would fix the problem. For -example, a missing semicolon at the end of a statement or a use of -deprecated syntax that is easily rewritten into a more modern form. -Clang tries very hard to emit the diagnostic and recover gracefully -in these and other cases.</p> - -<p>However, for these cases where the fix is obvious, the diagnostic -can be annotated with a hint (referred to as a "fix-it hint") that -describes how to change the code referenced by the diagnostic to fix -the problem. For example, it might add the missing semicolon at the -end of the statement or rewrite the use of a deprecated construct -into something more palatable. Here is one such example from the C++ -front end, where we warn about the right-shift operator changing -meaning from C++98 to C++11:</p> - -<pre> -test.cpp:3:7: warning: use of right-shift operator ('>>') in template argument will require parentheses in C++11 -A<100 >> 2> *a; - ^ - ( ) -</pre> - -<p>Here, the fix-it hint is suggesting that parentheses be added, -and showing exactly where those parentheses would be inserted into the -source code. The fix-it hints themselves describe what changes to make -to the source code in an abstract manner, which the text diagnostic -printer renders as a line of "insertions" below the caret line. <a -href="#DiagnosticClient">Other diagnostic clients</a> might choose -to render the code differently (e.g., as markup inline) or even give -the user the ability to automatically fix the problem.</p> - -<p>Fix-it hints on errors and warnings need to obey these rules:</p> - -<ul> -<li>Since they are automatically applied if <code>-Xclang -fixit</code> -is passed to the driver, they should only be used when it's very likely they -match the user's intent.</li> -<li>Clang must recover from errors as if the fix-it had been applied.</li> -</ul> - -<p>If a fix-it can't obey these rules, put the fix-it on a note. Fix-its on -notes are not applied automatically.</p> - -<p>All fix-it hints are described by the <code>FixItHint</code> class, -instances of which should be attached to the diagnostic using the -<< operator in the same way that highlighted source ranges and -arguments are passed to the diagnostic. Fix-it hints can be created -with one of three constructors:</p> - -<dl> - <dt><code>FixItHint::CreateInsertion(Loc, Code)</code></dt> - <dd>Specifies that the given <code>Code</code> (a string) should be inserted - before the source location <code>Loc</code>.</dd> - - <dt><code>FixItHint::CreateRemoval(Range)</code></dt> - <dd>Specifies that the code in the given source <code>Range</code> - should be removed.</dd> - - <dt><code>FixItHint::CreateReplacement(Range, Code)</code></dt> - <dd>Specifies that the code in the given source <code>Range</code> - should be removed, and replaced with the given <code>Code</code> string.</dd> -</dl> - -<!-- ============================================================= --> -<h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4> -<!-- ============================================================= --> - -<p>Once code generates a diagnostic with all of the arguments and the rest of -the relevant information, Clang needs to know what to do with it. As previously -mentioned, the diagnostic machinery goes through some filtering to map a -severity onto a diagnostic level, then (assuming the diagnostic is not mapped to -"<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient -interface with the information.</p> - -<p>It is possible to implement this interface in many different ways. For -example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns -the arguments into strings (according to the various formatting rules), prints -out the file/line/column information and the string, then prints out the line of -code, the source ranges, and the caret. However, this behavior isn't required. -</p> - -<p>Another implementation of the DiagnosticClient interface is the -'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode. -Instead of formatting and printing out the diagnostics, this implementation just -captures and remembers the diagnostics as they fly by. Then -verify compares -the list of produced diagnostics to the list of expected ones. If they disagree, -it prints out its own output. Full documentation for the -verify mode can be -found in the Clang API documentation for VerifyDiagnosticConsumer, <a -href="/doxygen/classclang_1_1VerifyDiagnosticConsumer.html#details">here</a>. -</p> - -<p>There are many other possible implementations of this interface, and this is -why we prefer diagnostics to pass down rich structured information in arguments. -For example, an HTML output might want declaration names be linkified to where -they come from in the source. Another example is that a GUI might let you click -on typedefs to expand them. This application would want to pass significantly -more information about types through to the GUI than a simple flat string. The -interface allows this to happen.</p> - -<!-- ====================================================== --> -<h4><a name="translation">Adding Translations to Clang</a></h4> -<!-- ====================================================== --> - -<p>Not possible yet! Diagnostic strings should be written in UTF-8, the client -can translate to the relevant code page if needed. Each translation completely -replaces the format string for the diagnostic.</p> - - -<!-- ======================================================================= --> -<h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3> -<!-- ======================================================================= --> - -<p>Strangely enough, the SourceLocation class represents a location within the -source code of the program. Important design points include:</p> - -<ol> -<li>sizeof(SourceLocation) must be extremely small, as these are embedded into - many AST nodes and are passed around often. Currently it is 32 bits.</li> -<li>SourceLocation must be a simple value object that can be efficiently - copied.</li> -<li>We should be able to represent a source location for any byte of any input - file. This includes in the middle of tokens, in whitespace, in trigraphs, - etc.</li> -<li>A SourceLocation must encode the current #include stack that was active when - the location was processed. For example, if the location corresponds to a - token, it should contain the set of #includes active when the token was - lexed. This allows us to print the #include stack for a diagnostic.</li> -<li>SourceLocation must be able to describe macro expansions, capturing both - the ultimate instantiation point and the source of the original character - data.</li> -</ol> - -<p>In practice, the SourceLocation works together with the SourceManager class -to encode two pieces of information about a location: its spelling location -and its instantiation location. For most tokens, these will be the same. -However, for a macro expansion (or tokens that came from a _Pragma directive) -these will describe the location of the characters corresponding to the token -and the location where the token was used (i.e. the macro instantiation point -or the location of the _Pragma itself).</p> - -<p>The Clang front-end inherently depends on the location of a token being -tracked correctly. If it is ever incorrect, the front-end may get confused and -die. The reason for this is that the notion of the 'spelling' of a Token in -Clang depends on being able to find the original input characters for the token. -This concept maps directly to the "spelling location" for the token.</p> - - -<!-- ======================================================================= --> -<h3 id="SourceRange">SourceRange and CharSourceRange</h3> -<!-- ======================================================================= --> -<!-- mostly taken from - http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-August/010595.html --> - -<p>Clang represents most source ranges by [first, last], where first and last -each point to the beginning of their respective tokens. For example -consider the SourceRange of the following statement:</p> -<pre> -x = foo + bar; -^first ^last -</pre> - -<p>To map from this representation to a character-based -representation, the 'last' location needs to be adjusted to point to -(or past) the end of that token with either -<code>Lexer::MeasureTokenLength()</code> or -<code>Lexer::getLocForEndOfToken()</code>. For the rare cases -where character-level source ranges information is needed we use -the <code>CharSourceRange</code> class.</p> - - -<!-- ======================================================================= --> -<h2 id="libdriver">The Driver Library</h2> -<!-- ======================================================================= --> - -<p>The clang Driver and library are documented <a -href="DriverInternals.html">here</a>.<p> - -<!-- ======================================================================= --> -<h2 id="pch">Precompiled Headers</h2> -<!-- ======================================================================= --> - -<p>Clang supports two implementations of precompiled headers. The - default implementation, precompiled headers (<a - href="PCHInternals.html">PCH</a>) uses a serialized representation - of Clang's internal data structures, encoded with the <a - href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream - format</a>. Pretokenized headers (<a - href="PTHInternals.html">PTH</a>), on the other hand, contain a - serialized representation of the tokens encountered when - preprocessing a header (and anything that header includes).</p> - - -<!-- ======================================================================= --> -<h2 id="libfrontend">The Frontend Library</h2> -<!-- ======================================================================= --> - -<p>The Frontend library contains functionality useful for building -tools on top of the clang libraries, for example several methods for -outputting diagnostics.</p> - -<!-- ======================================================================= --> -<h2 id="liblex">The Lexer and Preprocessor Library</h2> -<!-- ======================================================================= --> - -<p>The Lexer library contains several tightly-connected classes that are involved -with the nasty process of lexing and preprocessing C source code. The main -interface to this library for outside clients is the large <a -href="#Preprocessor">Preprocessor</a> class. -It contains the various pieces of state that are required to coherently read -tokens out of a translation unit.</p> - -<p>The core interface to the Preprocessor object (once it is set up) is the -Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from -the preprocessor stream. There are two types of token providers that the -preprocessor is capable of reading from: a buffer lexer (provided by the <a -href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a -href="#TokenLexer">TokenLexer</a> class). - - -<!-- ======================================================================= --> -<h3 id="Token">The Token class</h3> -<!-- ======================================================================= --> - -<p>The Token class is used to represent a single lexed token. Tokens are -intended to be used by the lexer/preprocess and parser libraries, but are not -intended to live beyond them (for example, they should not live in the ASTs).<p> - -<p>Tokens most often live on the stack (or some other location that is efficient -to access) as the parser is running, but occasionally do get buffered up. For -example, macro definitions are stored as a series of tokens, and the C++ -front-end periodically needs to buffer tokens up for tentative parsing and -various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit -system, sizeof(Token) is currently 16 bytes.</p> - -<p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation -Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer, -annotation tokens represent semantic information and are produced by the parser, -replacing normal tokens in the token stream. Normal tokens contain the -following information:</p> - -<ul> -<li><b>A SourceLocation</b> - This indicates the location of the start of the -token.</li> - -<li><b>A length</b> - This stores the length of the token as stored in the -SourceBuffer. For tokens that include them, this length includes trigraphs and -escaped newlines which are ignored by later phases of the compiler. By pointing -into the original source buffer, it is always possible to get the original -spelling of a token completely accurately.</li> - -<li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if -identifier lookup was enabled when the token was lexed (e.g. the lexer was not -reading in 'raw' mode) this contains a pointer to the unique hash value for the -identifier. Because the lookup happens before keyword identification, this -field is set even for language keywords like 'for'.</li> - -<li><b>TokenKind</b> - This indicates the kind of token as classified by the -lexer. This includes things like <tt>tok::starequal</tt> (for the "*=" -operator), <tt>tok::ampamp</tt> for the "&&" token, and keyword values -(e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note -that some tokens can be spelled multiple ways. For example, C++ supports -"operator keywords", where things like "and" are treated exactly like the -"&&" operator. In these cases, the kind value is set to -<tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to -consider both forms. For something that cares about which form is used (e.g. -the preprocessor 'stringize' operator) the spelling indicates the original -form.</li> - -<li><b>Flags</b> - There are currently four flags tracked by the -lexer/preprocessor system on a per-token basis: - - <ol> - <li><b>StartOfLine</b> - This was the first token that occurred on its input - source line.</li> - <li><b>LeadingSpace</b> - There was a space character either immediately - before the token or transitively before the token as it was expanded - through a macro. The definition of this flag is very closely defined by - the stringizing requirements of the preprocessor.</li> - <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to - represent identifier tokens which have macro expansion disabled. This - prevents them from being considered as candidates for macro expansion ever - in the future.</li> - <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the - token includes a trigraph or escaped newline. Since this is uncommon, - many pieces of code can fast-path on tokens that did not need cleaning. - </ol> -</li> -</ul> - -<p>One interesting (and somewhat unusual) aspect of normal tokens is that they -don't contain any semantic information about the lexed value. For example, if -the token was a pp-number token, we do not represent the value of the number -that was lexed (this is left for later pieces of code to decide). Additionally, -the lexer library has no notion of typedef names vs variable names: both are -returned as identifiers, and the parser is left to decide whether a specific -identifier is a typedef or a variable (tracking this requires scope information -among other things). The parser can do this translation by replacing tokens -returned by the preprocessor with "Annotation Tokens".</p> - -<!-- ======================================================================= --> -<h3 id="AnnotationToken">Annotation Tokens</h3> -<!-- ======================================================================= --> - -<p>Annotation Tokens are tokens that are synthesized by the parser and injected -into the preprocessor's token stream (replacing existing tokens) to record -semantic information found by the parser. For example, if "foo" is found to be -a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an -<tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this -makes it easy to handle qualified type names (e.g. "foo::bar::baz<42>::t") -in C++ as a single "token" in the parser. 2) if the parser backtracks, the -reparse does not need to redo semantic analysis to determine whether a token -sequence is a variable, type, template, etc.</p> - -<p>Annotation Tokens are created by the parser and reinjected into the parser's -token stream (when backtracking is enabled). Because they can only exist in -tokens that the preprocessor-proper is done with, it doesn't need to keep around -flags like "start of line" that the preprocessor uses to do its job. -Additionally, an annotation token may "cover" a sequence of preprocessor tokens -(e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields -of an annotation token are different than the fields for a normal token (but -they are multiplexed into the normal Token fields):</p> - -<ul> -<li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation -token indicates the first token replaced by the annotation token. In the example -above, it would be the location of the "a" identifier.</li> - -<li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the -last token replaced with the annotation token. In the example above, it would -be the location of the "c" identifier.</li> - -<li><b>void* "AnnotationValue"</b> - This contains an opaque object -that the parser gets from Sema. The parser merely preserves the -information for Sema to later interpret based on the annotation token -kind.</li> - -<li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this -is. See below for the different valid kinds.</li> -</ul> - -<p>Annotation tokens currently come in three kinds:</p> - -<ol> -<li><b>tok::annot_typename</b>: This annotation token represents a -resolved typename token that is potentially qualified. The -AnnotationValue field contains the <tt>QualType</tt> returned by -Sema::getTypeName(), possibly with source location information -attached.</li> - -<li><b>tok::annot_cxxscope</b>: This annotation token represents a C++ -scope specifier, such as "A::B::". This corresponds to the grammar -productions "::" and ":: [opt] nested-name-specifier". The -AnnotationValue pointer is a <tt>NestedNameSpecifier*</tt> returned by -the Sema::ActOnCXXGlobalScopeSpecifier and -Sema::ActOnCXXNestedNameSpecifier callbacks.</li> - -<li><b>tok::annot_template_id</b>: This annotation token represents a -C++ template-id such as "foo<int, 4>", where "foo" is the name -of a template. The AnnotationValue pointer is a pointer to a malloc'd -TemplateIdAnnotation object. Depending on the context, a parsed -template-id that names a type might become a typename annotation token -(if all we care about is the named type, e.g., because it occurs in a -type specifier) or might remain a template-id token (if we want to -retain more source location information or produce a new type, e.g., -in a declaration of a class template specialization). template-id -annotation tokens that refer to a type can be "upgraded" to typename -annotation tokens by the parser.</li> - -</ol> - -<p>As mentioned above, annotation tokens are not returned by the preprocessor, -they are formed on demand by the parser. This means that the parser has to be -aware of cases where an annotation could occur and form it where appropriate. -This is somewhat similar to how the parser handles Translation Phase 6 of C99: -String Concatenation (see C99 5.1.1.2). In the case of string concatenation, -the preprocessor just returns distinct tok::string_literal and -tok::wide_string_literal tokens and the parser eats a sequence of them wherever -the grammar indicates that a string literal can occur.</p> - -<p>In order to do this, whenever the parser expects a tok::identifier or -tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or -TryAnnotateCXXScopeToken methods to form the annotation token. These methods -will maximally form the specified annotation tokens and replace the current -token with them, if applicable. If the current tokens is not valid for an -annotation token, it will remain an identifier or :: token.</p> - - - -<!-- ======================================================================= --> -<h3 id="Lexer">The Lexer class</h3> -<!-- ======================================================================= --> - -<p>The Lexer class provides the mechanics of lexing tokens out of a source -buffer and deciding what they mean. The Lexer is complicated by the fact that -it operates on raw buffers that have not had spelling eliminated (this is a -necessity to get decent performance), but this is countered with careful coding -as well as standard performance techniques (for example, the comment handling -code is vectorized on X86 and PowerPC hosts).</p> - -<p>The lexer has a couple of interesting modal features:</p> - -<ul> -<li>The lexer can operate in 'raw' mode. This mode has several features that - make it possible to quickly lex the file (e.g. it stops identifier lookup, - doesn't specially handle preprocessor tokens, handles EOF differently, etc). - This mode is used for lexing within an "<tt>#if 0</tt>" block, for - example.</li> -<li>The lexer can capture and return comments as tokens. This is required to - support the -C preprocessor mode, which passes comments through, and is - used by the diagnostic checker to identifier expect-error annotations.</li> -<li>The lexer can be in ParsingFilename mode, which happens when preprocessing - after reading a #include directive. This mode changes the parsing of '<' - to return an "angled string" instead of a bunch of tokens for each thing - within the filename.</li> -<li>When parsing a preprocessor directive (after "<tt>#</tt>") the - ParsingPreprocessorDirective mode is entered. This changes the parser to - return EOD at a newline.</li> -<li>The Lexer uses a LangOptions object to know whether trigraphs are enabled, - whether C++ or ObjC keywords are recognized, etc.</li> -</ul> - -<p>In addition to these modes, the lexer keeps track of a couple of other - features that are local to a lexed buffer, which change as the buffer is - lexed:</p> - -<ul> -<li>The Lexer uses BufferPtr to keep track of the current character being - lexed.</li> -<li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token - will start with its "start of line" bit set.</li> -<li>The Lexer keeps track of the current #if directives that are active (which - can be nested).</li> -<li>The Lexer keeps track of an <a href="#MultipleIncludeOpt"> - MultipleIncludeOpt</a> object, which is used to - detect whether the buffer uses the standard "<tt>#ifndef XX</tt> / - <tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does, - subsequent includes can be ignored if the XX macro is defined.</li> -</ul> - -<!-- ======================================================================= --> -<h3 id="TokenLexer">The TokenLexer class</h3> -<!-- ======================================================================= --> - -<p>The TokenLexer class is a token provider that returns tokens from a list -of tokens that came from somewhere else. It typically used for two things: 1) -returning tokens from a macro definition as it is being expanded 2) returning -tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and -will most likely be used to handle unbounded look-ahead for the C++ parser.</p> - -<!-- ======================================================================= --> -<h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3> -<!-- ======================================================================= --> - -<p>The MultipleIncludeOpt class implements a really simple little state machine -that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>" -idiom that people typically use to prevent multiple inclusion of headers. If a -buffer uses this idiom and is subsequently #include'd, the preprocessor can -simply check to see whether the guarding condition is defined or not. If so, -the preprocessor can completely ignore the include of the header.</p> - - - -<!-- ======================================================================= --> -<h2 id="libparse">The Parser Library</h2> -<!-- ======================================================================= --> - -<!-- ======================================================================= --> -<h2 id="libast">The AST Library</h2> -<!-- ======================================================================= --> - -<!-- ======================================================================= --> -<h3 id="Type">The Type class and its subclasses</h3> -<!-- ======================================================================= --> - -<p>The Type class (and its subclasses) are an important part of the AST. Types -are accessed through the ASTContext class, which implicitly creates and uniques -them as they are needed. Types have a couple of non-obvious features: 1) they -do not capture type qualifiers like const or volatile (See -<a href="#QualType">QualType</a>), and 2) they implicitly capture typedef -information. Once created, types are immutable (unlike decls).</p> - -<p>Typedefs in C make semantic analysis a bit more complex than it would -be without them. The issue is that we want to capture typedef information -and represent it in the AST perfectly, but the semantics of operations need to -"see through" typedefs. For example, consider this code:</p> - -<code> -void func() {<br> - typedef int foo;<br> - foo X, *Y;<br> - typedef foo* bar;<br> - bar Z;<br> - *X; <i>// error</i><br> - **Y; <i>// error</i><br> - **Z; <i>// error</i><br> -}<br> -</code> - -<p>The code above is illegal, and thus we expect there to be diagnostics emitted -on the annotated lines. In this example, we expect to get:</p> - -<pre> -<b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b> -*X; // error -<span style="color:blue">^~</span> -<b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b> -**Y; // error -<span style="color:blue">^~~</span> -<b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b> -**Z; // error -<span style="color:blue">^~~</span> -</pre> - -<p>While this example is somewhat silly, it illustrates the point: we want to -retain typedef information where possible, so that we can emit errors about -"<tt>std::string</tt>" instead of "<tt>std::basic_string<char, std:...</tt>". -Doing this requires properly keeping typedef information (for example, the type -of "X" is "foo", not "int"), and requires properly propagating it through the -various operators (for example, the type of *Y is "foo", not "int"). In order -to retain this information, the type of these expressions is an instance of the -TypedefType class, which indicates that the type of these expressions is a -typedef for foo. -</p> - -<p>Representing types like this is great for diagnostics, because the -user-specified type is always immediately available. There are two problems -with this: first, various semantic checks need to make judgements about the -<em>actual structure</em> of a type, ignoring typedefs. Second, we need an -efficient way to query whether two types are structurally identical to each -other, ignoring typedefs. The solution to both of these problems is the idea of -canonical types.</p> - -<!-- =============== --> -<h4>Canonical Types</h4> -<!-- =============== --> - -<p>Every instance of the Type class contains a canonical type pointer. For -simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>", -"<tt>int**</tt>"), the type just points to itself. For types that have a -typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>", -"<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their -structurally equivalent type without any typedefs (e.g. "<tt>int</tt>", -"<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p> - -<p>This design provides a constant time operation (dereferencing the canonical -type pointer) that gives us access to the structure of types. For example, -we can trivially tell that "bar" and "foo*" are the same type by dereferencing -their canonical type pointers and doing a pointer comparison (they both point -to the single "<tt>int*</tt>" type).</p> - -<p>Canonical types and typedef types bring up some complexities that must be -carefully managed. Specifically, the "isa/cast/dyncast" operators generally -shouldn't be used in code that is inspecting the AST. For example, when type -checking the indirection operator (unary '*' on a pointer), the type checker -must verify that the operand has a pointer type. It would not be correct to -check that with "<tt>isa<PointerType>(SubExpr->getType())</tt>", -because this predicate would fail if the subexpression had a typedef type.</p> - -<p>The solution to this problem are a set of helper methods on Type, used to -check their properties. In this case, it would be correct to use -"<tt>SubExpr->getType()->isPointerType()</tt>" to do the check. This -predicate will return true if the <em>canonical type is a pointer</em>, which is -true any time the type is structurally a pointer type. The only hard part here -is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p> - -<p>The second problem we face is how to get access to the pointer type once we -know it exists. To continue the example, the result type of the indirection -operator is the pointee type of the subexpression. In order to determine the -type, we need to get the instance of PointerType that best captures the typedef -information in the program. If the type of the expression is literally a -PointerType, we can return that, otherwise we have to dig through the -typedefs to find the pointer type. For example, if the subexpression had type -"<tt>foo*</tt>", we could return that type as the result. If the subexpression -had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do -<em>not</em> want "<tt>int*</tt>"). In order to provide all of this, Type has -a getAsPointerType() method that checks whether the type is structurally a -PointerType and, if so, returns the best one. If not, it returns a null -pointer.</p> - -<p>This structure is somewhat mystical, but after meditating on it, it will -make sense to you :).</p> - -<!-- ======================================================================= --> -<h3 id="QualType">The QualType class</h3> -<!-- ======================================================================= --> - -<p>The QualType class is designed as a trivial value class that is -small, passed by-value and is efficient to query. The idea of -QualType is that it stores the type qualifiers (const, volatile, -restrict, plus some extended qualifiers required by language -extensions) separately from the types themselves. QualType is -conceptually a pair of "Type*" and the bits for these type qualifiers.</p> - -<p>By storing the type qualifiers as bits in the conceptual pair, it is -extremely efficient to get the set of qualifiers on a QualType (just return the -field of the pair), add a type qualifier (which is a trivial constant-time -operation that sets a bit), and remove one or more type qualifiers (just return -a QualType with the bitfield set to empty).</p> - -<p>Further, because the bits are stored outside of the type itself, we do not -need to create duplicates of types with different sets of qualifiers (i.e. there -is only a single heap allocated "int" type: "const int" and "volatile const int" -both point to the same heap allocated "int" type). This reduces the heap size -used to represent bits and also means we do not have to consider qualifiers when -uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p> - -<p>In practice, the two most common type qualifiers (const and -restrict) are stored in the low bits of the pointer to the Type -object, together with a flag indicating whether extended qualifiers -are present (which must be heap-allocated). This means that QualType -is exactly the same size as a pointer.</p> - -<!-- ======================================================================= --> -<h3 id="DeclarationName">Declaration names</h3> -<!-- ======================================================================= --> - -<p>The <tt>DeclarationName</tt> class represents the name of a - declaration in Clang. Declarations in the C family of languages can - take several different forms. Most declarations are named by - simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in - the function declaration <code>f(int x)</code>. In C++, declaration - names can also name class constructors ("<code>Class</code>" - in <code>struct Class { Class(); }</code>), class destructors - ("<code>~Class</code>"), overloaded operator names ("operator+"), - and conversion functions ("<code>operator void const *</code>"). In - Objective-C, declaration names can refer to the names of Objective-C - methods, which involve the method name and the parameters, - collectively called a <i>selector</i>, e.g., - "<code>setWidth:height:</code>". Since all of these kinds of - entities - variables, functions, Objective-C methods, C++ - constructors, destructors, and operators - are represented as - subclasses of Clang's common <code>NamedDecl</code> - class, <code>DeclarationName</code> is designed to efficiently - represent any kind of name.</p> - -<p>Given - a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code> - will produce a value that describes what kind of name <code>N</code> - stores. There are 8 options (all of the names are inside - the <code>DeclarationName</code> class)</p> -<dl> - <dt>Identifier</dt> - <dd>The name is a simple - identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the - corresponding <code>IdentifierInfo*</code> pointing to the actual - identifier. Note that C++ overloaded operators (e.g., - "<code>operator+</code>") are represented as special kinds of - identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code> - function to determine whether an identifier is an overloaded - operator name.</dd> - - <dt>ObjCZeroArgSelector, ObjCOneArgSelector, - ObjCMultiArgSelector</dt> - <dd>The name is an Objective-C selector, which can be retrieved as a - <code>Selector</code> instance - via <code>N.getObjCSelector()</code>. The three possible name - kinds for Objective-C reflect an optimization within - the <code>DeclarationName</code> class: both zero- and - one-argument selectors are stored as a - masked <code>IdentifierInfo</code> pointer, and therefore require - very little space, since zero- and one-argument selectors are far - more common than multi-argument selectors (which use a different - structure).</dd> - - <dt>CXXConstructorName</dt> - <dd>The name is a C++ constructor - name. Use <code>N.getCXXNameType()</code> to retrieve - the <a href="#QualType">type</a> that this constructor is meant to - construct. The type is always the canonical type, since all - constructors for a given type have the same name.</dd> - - <dt>CXXDestructorName</dt> - <dd>The name is a C++ destructor - name. Use <code>N.getCXXNameType()</code> to retrieve - the <a href="#QualType">type</a> whose destructor is being - named. This type is always a canonical type.</dd> - - <dt>CXXConversionFunctionName</dt> - <dd>The name is a C++ conversion function. Conversion functions are - named according to the type they convert to, e.g., "<code>operator void - const *</code>". Use <code>N.getCXXNameType()</code> to retrieve - the type that this conversion function converts to. This type is - always a canonical type.</dd> - - <dt>CXXOperatorName</dt> - <dd>The name is a C++ overloaded operator name. Overloaded operators - are named according to their spelling, e.g., - "<code>operator+</code>" or "<code>operator new - []</code>". Use <code>N.getCXXOverloadedOperator()</code> to - retrieve the overloaded operator (a value of - type <code>OverloadedOperatorKind</code>).</dd> -</dl> - -<p><code>DeclarationName</code>s are cheap to create, copy, and - compare. They require only a single pointer's worth of storage in - the common cases (identifiers, zero- - and one-argument Objective-C selectors) and use dense, uniqued - storage for the other kinds of - names. Two <code>DeclarationName</code>s can be compared for - equality (<code>==</code>, <code>!=</code>) using a simple bitwise - comparison, can be ordered - with <code><</code>, <code>></code>, <code><=</code>, - and <code>>=</code> (which provide a lexicographical ordering for - normal identifiers but an unspecified ordering for other kinds of - names), and can be placed into LLVM <code>DenseMap</code>s - and <code>DenseSet</code>s.</p> - -<p><code>DeclarationName</code> instances can be created in different - ways depending on what kind of name the instance will store. Normal - identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors - (<code>Selector</code>) can be implicitly converted - to <code>DeclarationName</code>s. Names for C++ constructors, - destructors, conversion functions, and overloaded operators can be retrieved from - the <code>DeclarationNameTable</code>, an instance of which is - available as <code>ASTContext::DeclarationNames</code>. The member - functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>, - <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively, - return <code>DeclarationName</code> instances for the four kinds of - C++ special function names.</p> - -<!-- ======================================================================= --> -<h3 id="DeclContext">Declaration contexts</h3> -<!-- ======================================================================= --> -<p>Every declaration in a program exists within some <i>declaration - context</i>, such as a translation unit, namespace, class, or - function. Declaration contexts in Clang are represented by - the <code>DeclContext</code> class, from which the various - declaration-context AST nodes - (<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>, - etc.) will derive. The <code>DeclContext</code> class provides - several facilities common to each declaration context:</p> -<dl> - <dt>Source-centric vs. Semantics-centric View of Declarations</dt> - <dd><code>DeclContext</code> provides two views of the declarations - stored within a declaration context. The source-centric view - accurately represents the program source code as written, including - multiple declarations of entities where present (see the - section <a href="#Redeclarations">Redeclarations and - Overloads</a>), while the semantics-centric view represents the - program semantics. The two views are kept synchronized by semantic - analysis while the ASTs are being constructed.</dd> - - <dt>Storage of declarations within that context</dt> - <dd>Every declaration context can contain some number of - declarations. For example, a C++ class (represented - by <code>RecordDecl</code>) contains various member functions, - fields, nested types, and so on. All of these declarations will be - stored within the <code>DeclContext</code>, and one can iterate - over the declarations via - [<code>DeclContext::decls_begin()</code>, - <code>DeclContext::decls_end()</code>). This mechanism provides - the source-centric view of declarations in the context.</dd> - - <dt>Lookup of declarations within that context</dt> - <dd>The <code>DeclContext</code> structure provides efficient name - lookup for names within that declaration context. For example, - if <code>N</code> is a namespace we can look for the - name <code>N::f</code> - using <code>DeclContext::lookup</code>. The lookup itself is - based on a lazily-constructed array (for declaration contexts - with a small number of declarations) or hash table (for - declaration contexts with more declarations). The lookup - operation provides the semantics-centric view of the declarations - in the context.</dd> - - <dt>Ownership of declarations</dt> - <dd>The <code>DeclContext</code> owns all of the declarations that - were declared within its declaration context, and is responsible - for the management of their memory as well as their - (de-)serialization.</dd> -</dl> - -<p>All declarations are stored within a declaration context, and one - can query - information about the context in which each declaration lives. One - can retrieve the <code>DeclContext</code> that contains a - particular <code>Decl</code> - using <code>Decl::getDeclContext</code>. However, see the - section <a href="#LexicalAndSemanticContexts">Lexical and Semantic - Contexts</a> for more information about how to interpret this - context information.</p> - -<h4 id="Redeclarations">Redeclarations and Overloads</h4> -<p>Within a translation unit, it is common for an entity to be -declared several times. For example, we might declare a function "f" - and then later re-declare it as part of an inlined definition:</p> - -<pre> -void f(int x, int y, int z = 1); - -inline void f(int x, int y, int z) { /* ... */ } -</pre> - -<p>The representation of "f" differs in the source-centric and - semantics-centric views of a declaration context. In the - source-centric view, all redeclarations will be present, in the - order they occurred in the source code, making - this view suitable for clients that wish to see the structure of - the source code. In the semantics-centric view, only the most recent "f" - will be found by the lookup, since it effectively replaces the first - declaration of "f".</p> - -<p>In the semantics-centric view, overloading of functions is - represented explicitly. For example, given two declarations of a - function "g" that are overloaded, e.g.,</p> -<pre> -void g(); -void g(int); -</pre> -<p>the <code>DeclContext::lookup</code> operation will return - a <code>DeclContext::lookup_result</code> that contains a range of iterators - over declarations of "g". Clients that perform semantic analysis on a - program that is not concerned with the actual source code will - primarily use this semantics-centric view.</p> - -<h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4> -<p>Each declaration has two potentially different - declaration contexts: a <i>lexical</i> context, which corresponds to - the source-centric view of the declaration context, and - a <i>semantic</i> context, which corresponds to the - semantics-centric view. The lexical context is accessible - via <code>Decl::getLexicalDeclContext</code> while the - semantic context is accessible - via <code>Decl::getDeclContext</code>, both of which return - <code>DeclContext</code> pointers. For most declarations, the two - contexts are identical. For example:</p> - -<pre> -class X { -public: - void f(int x); -}; -</pre> - -<p>Here, the semantic and lexical contexts of <code>X::f</code> are - the <code>DeclContext</code> associated with the - class <code>X</code> (itself stored as a <code>RecordDecl</code> AST - node). However, we can now define <code>X::f</code> out-of-line:</p> - -<pre> -void X::f(int x = 17) { /* ... */ } -</pre> - -<p>This definition of has different lexical and semantic - contexts. The lexical context corresponds to the declaration - context in which the actual declaration occurred in the source - code, e.g., the translation unit containing <code>X</code>. Thus, - this declaration of <code>X::f</code> can be found by traversing - the declarations provided by - [<code>decls_begin()</code>, <code>decls_end()</code>) in the - translation unit.</p> - -<p>The semantic context of <code>X::f</code> corresponds to the - class <code>X</code>, since this member function is (semantically) a - member of <code>X</code>. Lookup of the name <code>f</code> into - the <code>DeclContext</code> associated with <code>X</code> will - then return the definition of <code>X::f</code> (including - information about the default argument).</p> - -<h4 id="TransparentContexts">Transparent Declaration Contexts</h4> -<p>In C and C++, there are several contexts in which names that are - logically declared inside another declaration will actually "leak" - out into the enclosing scope from the perspective of name - lookup. The most obvious instance of this behavior is in - enumeration types, e.g.,</p> -<pre> -enum Color { - Red, - Green, - Blue -}; -</pre> - -<p>Here, <code>Color</code> is an enumeration, which is a declaration - context that contains the - enumerators <code>Red</code>, <code>Green</code>, - and <code>Blue</code>. Thus, traversing the list of declarations - contained in the enumeration <code>Color</code> will - yield <code>Red</code>, <code>Green</code>, - and <code>Blue</code>. However, outside of the scope - of <code>Color</code> one can name the enumerator <code>Red</code> - without qualifying the name, e.g.,</p> - -<pre> -Color c = Red; -</pre> - -<p>There are other entities in C++ that provide similar behavior. For - example, linkage specifications that use curly braces:</p> - -<pre> -extern "C" { - void f(int); - void g(int); -} -// f and g are visible here -</pre> - -<p>For source-level accuracy, we treat the linkage specification and - enumeration type as a - declaration context in which its enclosed declarations ("Red", - "Green", and "Blue"; "f" and "g") - are declared. However, these declarations are visible outside of the - scope of the declaration context.</p> - -<p>These language features (and several others, described below) have - roughly the same set of - requirements: declarations are declared within a particular lexical - context, but the declarations are also found via name lookup in - scopes enclosing the declaration itself. This feature is implemented - via <i>transparent</i> declaration contexts - (see <code>DeclContext::isTransparentContext()</code>), whose - declarations are visible in the nearest enclosing non-transparent - declaration context. This means that the lexical context of the - declaration (e.g., an enumerator) will be the - transparent <code>DeclContext</code> itself, as will the semantic - context, but the declaration will be visible in every outer context - up to and including the first non-transparent declaration context (since - transparent declaration contexts can be nested).</p> - -<p>The transparent <code>DeclContexts</code> are:</p> -<ul> - <li>Enumerations (but not C++11 "scoped enumerations"): - <pre> -enum Color { - Red, - Green, - Blue -}; -// Red, Green, and Blue are in scope - </pre></li> - <li>C++ linkage specifications: - <pre> -extern "C" { - void f(int); - void g(int); -} -// f and g are in scope - </pre></li> - <li>Anonymous unions and structs: - <pre> -struct LookupTable { - bool IsVector; - union { - std::vector<Item> *Vector; - std::set<Item> *Set; - }; -}; - -LookupTable LT; -LT.Vector = 0; // Okay: finds Vector inside the unnamed union - </pre> - </li> - <li>C++11 inline namespaces: -<pre> -namespace mylib { - inline namespace debug { - class X; - } -} -mylib::X *xp; // okay: mylib::X refers to mylib::debug::X -</pre> -</li> -</ul> - - -<h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4> -<p>C++ namespaces have the interesting--and, so far, unique--property that -the namespace can be defined multiple times, and the declarations -provided by each namespace definition are effectively merged (from -the semantic point of view). For example, the following two code -snippets are semantically indistinguishable:</p> -<pre> -// Snippet #1: -namespace N { - void f(); -} -namespace N { - void f(int); -} - -// Snippet #2: -namespace N { - void f(); - void f(int); -} -</pre> - -<p>In Clang's representation, the source-centric view of declaration - contexts will actually have two separate <code>NamespaceDecl</code> - nodes in Snippet #1, each of which is a declaration context that - contains a single declaration of "f". However, the semantics-centric - view provided by name lookup into the namespace <code>N</code> for - "f" will return a <code>DeclContext::lookup_result</code> that contains - a range of iterators over declarations of "f".</p> - -<p><code>DeclContext</code> manages multiply-defined declaration - contexts internally. The - function <code>DeclContext::getPrimaryContext</code> retrieves the - "primary" context for a given <code>DeclContext</code> instance, - which is the <code>DeclContext</code> responsible for maintaining - the lookup table used for the semantics-centric view. Given the - primary context, one can follow the chain - of <code>DeclContext</code> nodes that define additional - declarations via <code>DeclContext::getNextContext</code>. Note that - these functions are used internally within the lookup and insertion - methods of the <code>DeclContext</code>, so the vast majority of - clients can ignore them.</p> - -<!-- ======================================================================= --> -<h3 id="CFG">The <tt>CFG</tt> class</h3> -<!-- ======================================================================= --> - -<p>The <tt>CFG</tt> class is designed to represent a source-level -control-flow graph for a single statement (<tt>Stmt*</tt>). Typically -instances of <tt>CFG</tt> are constructed for function bodies (usually -an instance of <tt>CompoundStmt</tt>), but can also be instantiated to -represent the control-flow of any class that subclasses <tt>Stmt</tt>, -which includes simple expressions. Control-flow graphs are especially -useful for performing -<a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow- -or path-sensitive</a> program analyses on a given function.</p> - -<!-- ============ --> -<h4>Basic Blocks</h4> -<!-- ============ --> - -<p>Concretely, an instance of <tt>CFG</tt> is a collection of basic -blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which -simply contains an ordered sequence of <tt>Stmt*</tt> (each referring -to statements in the AST). The ordering of statements within a block -indicates unconditional flow of control from one statement to the -next. <a href="#ConditionalControlFlow">Conditional control-flow</a> -is represented using edges between basic blocks. The statements -within a given <tt>CFGBlock</tt> can be traversed using -the <tt>CFGBlock::*iterator</tt> interface.</p> - -<p> -A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within -the control-flow graph it represents. Each <tt>CFGBlock</tt> within a -CFG is also uniquely numbered (accessible -via <tt>CFGBlock::getBlockID()</tt>). Currently the number is -based on the ordering the blocks were created, but no assumptions -should be made on how <tt>CFGBlock</tt>s are numbered other than their -numbers are unique and that they are numbered from 0..N-1 (where N is -the number of basic blocks in the CFG).</p> - -<!-- ===================== --> -<h4>Entry and Exit Blocks</h4> -<!-- ===================== --> - -Each instance of <tt>CFG</tt> contains two special blocks: -an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which -has no incoming edges, and an <i>exit</i> block (accessible -via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither -block contains any statements, and they serve the role of providing a -clear entrance and exit for a body of code such as a function body. -The presence of these empty blocks greatly simplifies the -implementation of many analyses built on top of CFGs. - -<!-- ===================================================== --> -<h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4> -<!-- ===================================================== --> - -<p>Conditional control-flow (such as those induced by if-statements -and loops) is represented as edges between <tt>CFGBlock</tt>s. -Because different C language constructs can induce control-flow, -each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that -represents the <i>terminator</i> of the block. A terminator is simply -the statement that caused the control-flow, and is used to identify -the nature of the conditional control-flow between blocks. For -example, in the case of an if-statement, the terminator refers to -the <tt>IfStmt</tt> object in the AST that represented the given -branch.</p> - -<p>To illustrate, consider the following code example:</p> - -<code> -int foo(int x) {<br> - x = x + 1;<br> -<br> - if (x > 2) x++;<br> - else {<br> - x += 2;<br> - x *= 2;<br> - }<br> -<br> - return x;<br> -} -</code> - -<p>After invoking the parser+semantic analyzer on this code fragment, -the AST of the body of <tt>foo</tt> is referenced by a -single <tt>Stmt*</tt>. We can then construct an instance -of <tt>CFG</tt> representing the control-flow graph of this function -body by single call to a static class method:</p> - -<code> - Stmt* FooBody = ...<br> - CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody); -</code> - -<p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt> -to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no -longer needed.</p> - -<p>Along with providing an interface to iterate over -its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods -that are useful for debugging and visualizing CFGs. For example, the -method -<tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to -standard error. This is especially useful when one is using a -debugger such as gdb. For example, here is the output -of <tt>FooCFG->dump()</tt>:</p> - -<code> - [ B5 (ENTRY) ]<br> - Predecessors (0):<br> - Successors (1): B4<br> -<br> - [ B4 ]<br> - 1: x = x + 1<br> - 2: (x > 2)<br> - <b>T: if [B4.2]</b><br> - Predecessors (1): B5<br> - Successors (2): B3 B2<br> -<br> - [ B3 ]<br> - 1: x++<br> - Predecessors (1): B4<br> - Successors (1): B1<br> -<br> - [ B2 ]<br> - 1: x += 2<br> - 2: x *= 2<br> - Predecessors (1): B4<br> - Successors (1): B1<br> -<br> - [ B1 ]<br> - 1: return x;<br> - Predecessors (2): B2 B3<br> - Successors (1): B0<br> -<br> - [ B0 (EXIT) ]<br> - Predecessors (1): B1<br> - Successors (0): -</code> - -<p>For each block, the pretty-printed output displays for each block -the number of <i>predecessor</i> blocks (blocks that have outgoing -control-flow to the given block) and <i>successor</i> blocks (blocks -that have control-flow that have incoming control-flow from the given -block). We can also clearly see the special entry and exit blocks at -the beginning and end of the pretty-printed output. For the entry -block (block B5), the number of predecessor blocks is 0, while for the -exit block (block B0) the number of successor blocks is 0.</p> - -<p>The most interesting block here is B4, whose outgoing control-flow -represents the branching caused by the sole if-statement -in <tt>foo</tt>. Of particular interest is the second statement in -the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed -as <b><tt>if [B4.2]</tt></b>. The second statement represents the -evaluation of the condition of the if-statement, which occurs before -the actual branching of control-flow. Within the <tt>CFGBlock</tt> -for B4, the <tt>Stmt*</tt> for the second statement refers to the -actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus -pointers to subclasses of <tt>Expr</tt> can appear in the list of -statements in a block, and not just subclasses of <tt>Stmt</tt> that -refer to proper C statements.</p> - -<p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt> -object in the AST. The pretty-printer outputs <b><tt>if -[B4.2]</tt></b> because the condition expression of the if-statement -has an actual place in the basic block, and thus the terminator is -essentially -<i>referring</i> to the expression that is the second statement of -block B4 (i.e., B4.2). In this manner, conditions for control-flow -(which also includes conditions for loops and switch statements) are -hoisted into the actual basic block.</p> - -<!-- ===================== --> -<!-- <h4>Implicit Control-Flow</h4> --> -<!-- ===================== --> - -<!-- -<p>A key design principle of the <tt>CFG</tt> class was to not require -any transformations to the AST in order to represent control-flow. -Thus the <tt>CFG</tt> does not perform any "lowering" of the -statements in an AST: loops are not transformed into guarded gotos, -short-circuit operations are not converted to a set of if-statements, -and so on.</p> ---> - - -<!-- ======================================================================= --> -<h3 id="Constants">Constant Folding in the Clang AST</h3> -<!-- ======================================================================= --> - -<p>There are several places where constants and constant folding matter a lot to -the Clang front-end. First, in general, we prefer the AST to retain the source -code as close to how the user wrote it as possible. This means that if they -wrote "5+4", we want to keep the addition and two constants in the AST, we don't -want to fold to "9". This means that constant folding in various ways turns -into a tree walk that needs to handle the various cases.</p> - -<p>However, there are places in both C and C++ that require constants to be -folded. For example, the C standard defines what an "integer constant -expression" (i-c-e) is with very precise and specific requirements. The -language then requires i-c-e's in a lot of places (for example, the size of a -bitfield, the value for a case statement, etc). For these, we have to be able -to constant fold the constants, to do semantic checks (e.g. verify bitfield size -is non-negative and that case statements aren't duplicated). We aim for Clang -to be very pedantic about this, diagnosing cases when the code does not use an -i-c-e where one is required, but accepting the code unless running with -<tt>-pedantic-errors</tt>.</p> - -<p>Things get a little bit more tricky when it comes to compatibility with -real-world source code. Specifically, GCC has historically accepted a huge -superset of expressions as i-c-e's, and a lot of real world code depends on this -unfortuate accident of history (including, e.g., the glibc system headers). GCC -accepts anything its "fold" optimizer is capable of reducing to an integer -constant, which means that the definition of what it accepts changes as its -optimizer does. One example is that GCC accepts things like "case X-X:" even -when X is a variable, because it can fold this to 0.</p> - -<p>Another issue are how constants interact with the extensions we support, such -as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99 -obviously does not specify the semantics of any of these extensions, and the -definition of i-c-e does not include them. However, these extensions are often -used in real code, and we have to have a way to reason about them.</p> - -<p>Finally, this is not just a problem for semantic analysis. The code -generator and other clients have to be able to fold constants (e.g. to -initialize global variables) and has to handle a superset of what C99 allows. -Further, these clients can benefit from extended information. For example, we -know that "foo()||1" always evaluates to true, but we can't replace the -expression with true because it has side effects.</p> - -<!-- ======================= --> -<h4>Implementation Approach</h4> -<!-- ======================= --> - -<p>After trying several different approaches, we've finally converged on a -design (Note, at the time of this writing, not all of this has been implemented, -consider this a design goal!). Our basic approach is to define a single -recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is -implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar' -type (integer, fp, complex, or pointer) this method returns the following -information:</p> - -<ul> -<li>Whether the expression is an integer constant expression, a general - constant that was folded but has no side effects, a general constant that - was folded but that does have side effects, or an uncomputable/unfoldable - value. -</li> -<li>If the expression was computable in any way, this method returns the APValue - for the result of the expression.</li> -<li>If the expression is not evaluatable at all, this method returns - information on one of the problems with the expression. This includes a - SourceLocation for where the problem is, and a diagnostic ID that explains - the problem. The diagnostic should be have ERROR type.</li> -<li>If the expression is not an integer constant expression, this method returns - information on one of the problems with the expression. This includes a - SourceLocation for where the problem is, and a diagnostic ID that explains - the problem. The diagnostic should be have EXTENSION type.</li> -</ul> - -<p>This information gives various clients the flexibility that they want, and we -will eventually have some helper methods for various extensions. For example, -Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which -calls Evaluate on the expression. If the expression is not foldable, the error -is emitted, and it would return true. If the expression is not an i-c-e, the -EXTENSION diagnostic is emitted. Finally it would return false to indicate that -the AST is ok.</p> - -<p>Other clients can use the information in other ways, for example, codegen can -just use expressions that are foldable in any way.</p> - -<!-- ========== --> -<h4>Extensions</h4> -<!-- ========== --> - -<p>This section describes how some of the various extensions Clang supports -interacts with constant evaluation:</p> - -<ul> -<li><b><tt>__extension__</tt></b>: The expression form of this extension causes - any evaluatable subexpression to be accepted as an integer constant - expression.</li> -<li><b><tt>__builtin_constant_p</tt></b>: This returns true (as an integer - constant expression) if the operand evaluates to either a numeric value - (that is, not a pointer cast to integral type) of integral, enumeration, - floating or complex type, or if it evaluates to the address of the first - character of a string literal (possibly cast to some other type). As a - special case, if <tt>__builtin_constant_p</tt> is the (potentially - parenthesized) condition of a conditional operator expression ("?:"), only - the true side of the conditional operator is considered, and it is evaluated - with full constant folding.</li> -<li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an - integer constant expression, but we accept any constant as an "extension of - an extension". This only evaluates one operand depending on which way the - condition evaluates.</li> -<li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer - constant expression.</li> -<li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a - floating-point literal.</li> -<li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as - general constant expressions.</li> -<li><b><tt>__builtin_strlen</tt></b> and <b><tt>strlen</tt></b>: These are - constant folded as integer constant expressions if the argument is a string - literal.</li> -</ul> - - -<!-- ======================================================================= --> -<h2 id="Howtos">How to change Clang</h2> -<!-- ======================================================================= --> - -<!-- ======================================================================= --> -<h3 id="AddingAttributes">How to add an attribute</h3> -<!-- ======================================================================= --> - -<p>To add an attribute, you'll have to add it to the list of attributes, add it -to the parsing phase, and look for it in the AST scan. -<a href="http://llvm.org/viewvc/llvm-project?view=rev&revision=124217">r124217</a> -has a good example of adding a warning attribute.</p> - -<p>(Beware that this hasn't been reviewed/fixed by the people who designed the -attributes system yet.)</p> - -<h4><a -href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?view=markup">include/clang/Basic/Attr.td</a></h4> - -<p>Each attribute gets a <tt>def</tt> inheriting from <tt>Attr</tt> or one of -its subclasses. <tt>InheritableAttr</tt> means that the attribute also applies -to subsequent declarations of the same name.</p> - -<p><tt>Spellings</tt> lists the strings that can appear in -<tt>__attribute__((here))</tt> or <tt>[[here]]</tt>. All such strings -will be synonymous. If you want to allow the <tt>[[]]</tt> C++11 -syntax, you have to define a list of <tt>Namespaces</tt>, which will -let users write <tt>[[namespace:spelling]]</tt>. Using the empty -string for a namespace will allow users to write just the spelling -with no "<tt>:</tt>".</p> - -<p><tt>Subjects</tt> restricts what kinds of AST node to which this attribute -can appertain (roughly, attach).</p> - -<p><tt>Args</tt> names the arguments the attribute takes, in order. If -<tt>Args</tt> is <tt>[StringArgument<"Arg1">, IntArgument<"Arg2">]</tt> -then <tt>__attribute__((myattribute("Hello", 3)))</tt> will be a valid use.</p> - -<h4>Boilerplate</h4> - -<p>Write a new <tt>HandleYourAttr()</tt> function in <a -href="http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?view=markup">lib/Sema/SemaDeclAttr.cpp</a>, -and add a case to the switch in <tt>ProcessNonInheritableDeclAttr()</tt> or -<tt>ProcessInheritableDeclAttr()</tt> forwarding to it.</p> - -<p>If your attribute causes extra warnings to fire, define a <tt>DiagGroup</tt> -in <a -href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?view=markup">include/clang/Basic/DiagnosticGroups.td</a> -named after the attribute's <tt>Spelling</tt> with "_"s replaced by "-"s. If -you're only defining one diagnostic, you can skip <tt>DiagnosticGroups.td</tt> -and use <tt>InGroup<DiagGroup<"your-attribute">></tt> directly in <a -href="http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?view=markup">DiagnosticSemaKinds.td</a></p> - -<h4>The meat of your attribute</h4> - -<p>Find an appropriate place in Clang to do whatever your attribute needs to do. -Check for the attribute's presence using <tt>Decl::getAttr<YourAttr>()</tt>.</p> - -<p>Update the <a href="LanguageExtensions.html">Clang Language Extensions</a> -document to describe your new attribute.</p> - -<!-- ======================================================================= --> -<h3 id="AddingExprStmt">How to add an expression or statement</h3> -<!-- ======================================================================= --> - -<p>Expressions and statements are one of the most fundamental constructs within a -compiler, because they interact with many different parts of the AST, -semantic analysis, and IR generation. Therefore, adding a new -expression or statement kind into Clang requires some care. The following list -details the various places in Clang where an expression or statement needs to be -introduced, along with patterns to follow to ensure that the new -expression or statement works well across all of the C languages. We -focus on expressions, but statements are similar.</p> - -<ol> - <li>Introduce parsing actions into the parser. Recursive-descent - parsing is mostly self-explanatory, but there are a few things that - are worth keeping in mind: - <ul> - <li>Keep as much source location information as possible! You'll - want it later to produce great diagnostics and support Clang's - various features that map between source code and the AST.</li> - <li>Write tests for all of the "bad" parsing cases, to make sure - your recovery is good. If you have matched delimiters (e.g., - parentheses, square brackets, etc.), use - <tt>Parser::BalancedDelimiterTracker</tt> to give nice diagnostics when - things go wrong.</li> - </ul> - </li> - - <li>Introduce semantic analysis actions into <tt>Sema</tt>. Semantic - analysis should always involve two functions: an <tt>ActOnXXX</tt> - function that will be called directly from the parser, and a - <tt>BuildXXX</tt> function that performs the actual semantic - analysis and will (eventually!) build the AST node. It's fairly - common for the <tt>ActOnCXX</tt> function to do very little (often - just some minor translation from the parser's representation to - <tt>Sema</tt>'s representation of the same thing), but the separation - is still important: C++ template instantiation, for example, - should always call the <tt>BuildXXX</tt> variant. Several notes on - semantic analysis before we get into construction of the AST: - <ul> - <li>Your expression probably involves some types and some - subexpressions. Make sure to fully check that those types, and the - types of those subexpressions, meet your expectations. Add - implicit conversions where necessary to make sure that all of the - types line up exactly the way you want them. Write extensive tests - to check that you're getting good diagnostics for mistakes and - that you can use various forms of subexpressions with your - expression.</li> - <li>When type-checking a type or subexpression, make sure to first - check whether the type is "dependent" - (<tt>Type::isDependentType()</tt>) or whether a subexpression is - type-dependent (<tt>Expr::isTypeDependent()</tt>). If any of these - return true, then you're inside a template and you can't do much - type-checking now. That's normal, and your AST node (when you get - there) will have to deal with this case. At this point, you can - write tests that use your expression within templates, but don't - try to instantiate the templates.</li> - <li>For each subexpression, be sure to call - <tt>Sema::CheckPlaceholderExpr()</tt> to deal with "weird" - expressions that don't behave well as subexpressions. Then, - determine whether you need to perform - lvalue-to-rvalue conversions - (<tt>Sema::DefaultLvalueConversion</tt>e) or - the usual unary conversions - (<tt>Sema::UsualUnaryConversions</tt>), for places where the - subexpression is producing a value you intend to use.</li> - <li>Your <tt>BuildXXX</tt> function will probably just return - <tt>ExprError()</tt> at this point, since you don't have an AST. - That's perfectly fine, and shouldn't impact your testing.</li> - </ul> - </li> - - <li>Introduce an AST node for your new expression. This starts with - declaring the node in <tt>include/Basic/StmtNodes.td</tt> and - creating a new class for your expression in the appropriate - <tt>include/AST/Expr*.h</tt> header. It's best to look at the class - for a similar expression to get ideas, and there are some specific - things to watch for: - <ul> - <li>If you need to allocate memory, use the <tt>ASTContext</tt> - allocator to allocate memory. Never use raw <tt>malloc</tt> or - <tt>new</tt>, and never hold any resources in an AST node, because - the destructor of an AST node is never called.</li> - - <li>Make sure that <tt>getSourceRange()</tt> covers the exact - source range of your expression. This is needed for diagnostics - and for IDE support.</li> - - <li>Make sure that <tt>children()</tt> visits all of the - subexpressions. This is important for a number of features (e.g., IDE - support, C++ variadic templates). If you have sub-types, you'll - also need to visit those sub-types in the - <tt>RecursiveASTVisitor</tt>.</li> - - <li>Add printing support (<tt>StmtPrinter.cpp</tt>) and dumping - support (<tt>StmtDumper.cpp</tt>) for your expression.</li> - - <li>Add profiling support (<tt>StmtProfile.cpp</tt>) for your AST - node, noting the distinguishing (non-source location) - characteristics of an instance of your expression. Omitting this - step will lead to hard-to-diagnose failures regarding matching of - template declarations.</li> - </ul> - </li> - - <li>Teach semantic analysis to build your AST node! At this point, - you can wire up your <tt>Sema::BuildXXX</tt> function to actually - create your AST. A few things to check at this point: - <ul> - <li>If your expression can construct a new C++ class or return a - new Objective-C object, be sure to update and then call - <tt>Sema::MaybeBindToTemporary</tt> for your just-created AST node - to be sure that the object gets properly destructed. An easy way - to test this is to return a C++ class with a private destructor: - semantic analysis should flag an error here with the attempt to - call the destructor.</li> - <li>Inspect the generated AST by printing it using <tt>clang -cc1 - -ast-print</tt>, to make sure you're capturing all of the - important information about how the AST was written.</li> - <li>Inspect the generated AST under <tt>clang -cc1 -ast-dump</tt> - to verify that all of the types in the generated AST line up the - way you want them. Remember that clients of the AST should never - have to "think" to understand what's going on. For example, all - implicit conversions should show up explicitly in the AST.</li> - <li>Write tests that use your expression as a subexpression of - other, well-known expressions. Can you call a function using your - expression as an argument? Can you use the ternary operator?</li> - </ul> - </li> - - <li>Teach code generation to create IR to your AST node. This step - is the first (and only) that requires knowledge of LLVM IR. There - are several things to keep in mind: - <ul> - <li>Code generation is separated into scalar/aggregate/complex and - lvalue/rvalue paths, depending on what kind of result your - expression produces. On occasion, this requires some careful - factoring of code to avoid duplication.</li> - - <li><tt>CodeGenFunction</tt> contains functions - <tt>ConvertType</tt> and <tt>ConvertTypeForMem</tt> that convert - Clang's types (<tt>clang::Type*</tt> or <tt>clang::QualType</tt>) - to LLVM types. - Use the former for values, and the later for memory locations: - test with the C++ "bool" type to check this. If you find - that you are having to use LLVM bitcasts to make - the subexpressions of your expression have the type that your - expression expects, STOP! Go fix semantic analysis and the AST so - that you don't need these bitcasts.</li> - - <li>The <tt>CodeGenFunction</tt> class has a number of helper - functions to make certain operations easy, such as generating code - to produce an lvalue or an rvalue, or to initialize a memory - location with a given value. Prefer to use these functions rather - than directly writing loads and stores, because these functions - take care of some of the tricky details for you (e.g., for - exceptions).</li> - - <li>If your expression requires some special behavior in the event - of an exception, look at the <tt>push*Cleanup</tt> functions in - <tt>CodeGenFunction</tt> to introduce a cleanup. You shouldn't - have to deal with exception-handling directly.</li> - - <li>Testing is extremely important in IR generation. Use <tt>clang - -cc1 -emit-llvm</tt> and <a - href="http://llvm.org/cmds/FileCheck.html">FileCheck</a> to verify - that you're generating the right IR.</li> - </ul> - </li> - - <li>Teach template instantiation how to cope with your AST - node, which requires some fairly simple code: - <ul> - <li>Make sure that your expression's constructor properly - computes the flags for type dependence (i.e., the type your - expression produces can change from one instantiation to the - next), value dependence (i.e., the constant value your expression - produces can change from one instantiation to the next), - instantiation dependence (i.e., a template parameter occurs - anywhere in your expression), and whether your expression contains - a parameter pack (for variadic templates). Often, computing these - flags just means combining the results from the various types and - subexpressions.</li> - - <li>Add <tt>TransformXXX</tt> and <tt>RebuildXXX</tt> functions to - the - <tt>TreeTransform</tt> class template in <tt>Sema</tt>. - <tt>TransformXXX</tt> should (recursively) transform all of the - subexpressions and types - within your expression, using <tt>getDerived().TransformYYY</tt>. - If all of the subexpressions and types transform without error, it - will then call the <tt>RebuildXXX</tt> function, which will in - turn call <tt>getSema().BuildXXX</tt> to perform semantic analysis - and build your expression.</li> - - <li>To test template instantiation, take those tests you wrote to - make sure that you were type checking with type-dependent - expressions and dependent types (from step #2) and instantiate - those templates with various types, some of which type-check and - some that don't, and test the error messages in each case.</li> - </ul> - </li> - - <li>There are some "extras" that make other features work better. - It's worth handling these extras to give your expression complete - integration into Clang: - <ul> - <li>Add code completion support for your expression in - <tt>SemaCodeComplete.cpp</tt>.</li> - - <li>If your expression has types in it, or has any "interesting" - features other than subexpressions, extend libclang's - <tt>CursorVisitor</tt> to provide proper visitation for your - expression, enabling various IDE features such as syntax - highlighting, cross-referencing, and so on. The - <tt>c-index-test</tt> helper program can be used to test these - features.</li> - </ul> - </li> -</ol> - -</div> -</body> -</html> |
