Blame - docs/InternalsManual.html - platform/external/clang

blob: 2497cc67a6180935c25764a253c9ed40664f3442 [file] [log] [blame]

Ted Kremenek	17a295d	2008-06-11 06:19:49 +0000	[diff] [blame]	1	<html>
				2	<head>
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	3	<title>"Clang" CFE Internals Manual</title>
Ted Kremenek	17a295d	2008-06-11 06:19:49 +0000	[diff] [blame]	4	<link type="text/css" rel="stylesheet" href="../menu.css" />
				5	<link type="text/css" rel="stylesheet" href="../content.css" />
Sebastian Redl	6816856	2008-11-22 22:16:45 +0000	[diff] [blame]	6	<style type="text/css">
				7	td {
				8	vertical-align: top;
				9	}
				10	</style>
Ted Kremenek	17a295d	2008-06-11 06:19:49 +0000	[diff] [blame]	11	</head>
				12	<body>
				13
				14	<!--#include virtual="../menu.html.incl"-->
				15
				16	<div id="content">
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	17
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	18	<h1>"Clang" CFE Internals Manual</h1>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	19
				20	<ul>
				21	<li><a href="#intro">Introduction</a></li>
				22	<li><a href="#libsystem">LLVM System and Support Libraries</a></li>
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	23	<li><a href="#libbasic">The Clang 'Basic' Library</a>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	24	<ul>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	25	<li><a href="#Diagnostics">The Diagnostics Subsystem</a></li>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	26	<li><a href="#SourceLocation">The SourceLocation and SourceManager
				27	classes</a></li>
				28	</ul>
				29	</li>
				30	<li><a href="#liblex">The Lexer and Preprocessor Library</a>
				31	<ul>
				32	<li><a href="#Token">The Token class</a></li>
				33	<li><a href="#Lexer">The Lexer class</a></li>
Chris Lattner	3932fe0	2009-01-06 06:02:08 +0000	[diff] [blame]	34	<li><a href="#AnnotationToken">Annotation Tokens</a></li>
Chris Lattner	7928125	2008-03-09 02:27:26 +0000	[diff] [blame]	35	<li><a href="#TokenLexer">The TokenLexer class</a></li>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	36	<li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li>
				37	</ul>
				38	</li>
				39	<li><a href="#libparse">The Parser Library</a>
				40	<ul>
				41	</ul>
				42	</li>
				43	<li><a href="#libast">The AST Library</a>
				44	<ul>
				45	<li><a href="#Type">The Type class and its subclasses</a></li>
				46	<li><a href="#QualType">The QualType class</a></li>
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	47	<li><a href="#DeclarationName">Declaration names</a></li>
Douglas Gregor	074149e	2009-01-05 19:45:36 +0000	[diff] [blame]	48	<li><a href="#DeclContext">Declaration contexts</a>
				49	<ul>
				50	<li><a href="#Redeclarations">Redeclarations and Overloads</a></li>
				51	<li><a href="#LexicalAndSemanticContexts">Lexical and Semantic
				52	Contexts</a></li>
				53	<li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li>
				54	<li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li>
				55	</ul>
				56	</li>
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	57	<li><a href="#CFG">The CFG class</a></li>
Chris Lattner	7bad199	2008-11-16 21:48:07 +0000	[diff] [blame]	58	<li><a href="#Constants">Constant Folding in the Clang AST</a></li>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	59	</ul>
				60	</li>
				61	</ul>
				62
				63
				64	<!-- ======================================================================= -->
				65	<h2 id="intro">Introduction</h2>
				66	<!-- ======================================================================= -->
				67
				68	<p>This document describes some of the more important APIs and internal design
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	69	decisions made in the Clang C front-end. The purpose of this document is to
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	70	both capture some of this high level information and also describe some of the
				71	design decisions behind it. This is meant for people interested in hacking on
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	72	Clang, not for end-users. The description below is categorized by
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	73	libraries, and does not describe any of the clients of the libraries.</p>
				74
				75	<!-- ======================================================================= -->
				76	<h2 id="libsystem">LLVM System and Support Libraries</h2>
				77	<!-- ======================================================================= -->
				78
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	79	<p>The LLVM libsystem library provides the basic Clang system abstraction layer,
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	80	which is used for file system access. The LLVM libsupport library provides many
				81	underlying libraries and <a
				82	href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>,
				83	including command line option
				84	processing and various containers.</p>
				85
				86	<!-- ======================================================================= -->
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	87	<h2 id="libbasic">The Clang 'Basic' Library</h2>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	88	<!-- ======================================================================= -->
				89
				90	<p>This library certainly needs a better name. The 'basic' library contains a
				91	number of low-level utilities for tracking and manipulating source buffers,
				92	locations within the source buffers, diagnostics, tokens, target abstraction,
				93	and information about the subset of the language being compiled for.</p>
				94
				95	<p>Part of this infrastructure is specific to C (such as the TargetInfo class),
				96	other parts could be reused for other non-C-based languages (SourceLocation,
				97	SourceManager, Diagnostics, FileManager). When and if there is future demand
				98	we can figure out if it makes sense to introduce a new library, move the general
				99	classes somewhere else, or introduce some other solution.</p>
				100
				101	<p>We describe the roles of these classes in order of their dependencies.</p>
				102
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	103
				104	<!-- ======================================================================= -->
				105	<h3 id="Diagnostics">The Diagnostics Subsystem</h3>
				106	<!-- ======================================================================= -->
				107
				108	<p>The Clang Diagnostics subsystem is an important part of how the compiler
				109	communicates with the human. Diagnostics are the warnings and errors produced
				110	when the code is incorrect or dubious. In Clang, each diagnostic produced has
				111	(at the minimum) a unique ID, a <a href="#SourceLocation">SourceLocation</a> to
				112	"put the caret", an English translation associated with it, and a severity (e.g.
				113	<tt>WARNING</tt> or <tt>ERROR</tt>). They can also optionally include a number
				114	of arguments to the dianostic (which fill in "%0"'s in the string) as well as a
				115	number of source ranges that related to the diagnostic.</p>
				116
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	117	<p>In this section, we'll be giving examples produced by the Clang command line
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	118	driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many
				119	different ways</a> depending on how the DiagnosticClient interface is
				120	implemented. A representative example of a diagonstic is:</p>
				121
				122	<pre>
				123	t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
				124	<font color="darkgreen">P = (P-42) + Gamma*4;</font>
				125	<font color="blue">~~~~~~ ^ ~~~~~~~</font>
				126	</pre>
				127
				128	<p>In this example, you can see the English translation, the severity (error),
				129	you can see the source location (the caret ("^") and file/line/column info),
				130	the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex
				131	float"). You'll have to believe me that there is a unique ID backing the
				132	diagnostic :).</p>
				133
				134	<p>Getting all of this to happen has several steps and involves many moving
				135	pieces, this section describes them and talks about best practices when adding
				136	a new diagnostic.</p>
				137
				138	<!-- ============================ -->
				139	<h4>The DiagnosticKinds.def file</h4>
				140	<!-- ============================ -->
				141
				142	<p>Diagnostics are created by adding an entry to the <tt><a
				143	href="http://llvm.org/svn/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticKinds.def"
				144	>DiagnosticKinds.def</a></tt> file. This file encodes the unique ID of the
				145	diagnostic (as an enum, the first argument), the severity of the diagnostic
				146	(second argument) and the English translation + format string.</p>
				147
				148	<p>There is little sanity with the naming of the unique ID's right now. Some
				149	start with err_, warn_, ext_ to encode the severity into the name. Since the
				150	enum is referenced in the C++ code that produces the diagnostic, it is somewhat
				151	useful for it to be reasonably short.</p>
				152
				153	<p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>,
				154	<tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}. The
				155	<tt>ERROR</tt> severity is used for diagnostics indicating the program is never
				156	acceptable under any circumstances. When an error is emitted, the AST for the
				157	input code may not be fully built. The <tt>EXTENSION</tt> and <tt>EXTWARN</tt>
				158	severities are used for extensions to the language that Clang accepts. This
				159	means that Clang fully understands and can represent them in the AST, but we
				160	produce diagnostics to tell the user their code is non-portable. The difference
				161	is that the former are ignored by default, and the later warn by default. The
				162	<tt>WARNING</tt> severity is used for constructs that are valid in the currently
				163	selected source language but that are dubious in some way. The <tt>NOTE</tt>
Chris Lattner	a180fdd	2009-02-17 07:07:29 +0000	[diff] [blame^]	164	level is used to staple more information onto a previous diagnostics.
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	165
				166	<p>These <em>severities</em> are mapped into a smaller set (the
				167	Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>,
Chris Lattner	0aad297	2009-02-05 22:49:08 +0000	[diff] [blame]	168	<tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics
Chris Lattner	a180fdd	2009-02-17 07:07:29 +0000	[diff] [blame^]	169	subsystem based on various configuration options. Clang internally supports a
				170	fully fine grained mapping mechanism that allows you to map almost any
				171	diagnostic to the output level that you want. The only diagnostics that cannot
				172	be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously
				173	emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to
				174	<tt>Fatal</tt> (it is not possible to turn an error into a warning,
				175	for example).</p>
				176
				177	<p>Diagnostic mappings are used in many ways. For example, if the user
				178	specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if
				179	they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>. This is
				180	used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc.
				181	</p>
				182
				183	<p>
				184	Mapping to <tt>Fatal</tt> should only be used for diagnostics that are
				185	considered so severe that error recovery won't be able to recover sensibly from
				186	them (thus spewing a ton of bogus errors). One example of this class of error
				187	are failure to #include a file.
				188	</p>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	189
				190	<!-- ================= -->
				191	<h4>The Format String</h4>
				192	<!-- ================= -->
				193
				194	<p>The format string for the diagnostic is very simple, but it has some power.
				195	It takes the form of a string in English with markers that indicate where and
				196	how arguments to the diagnostic are inserted and formatted. For example, here
				197	are some simple format strings:</p>
				198
				199	<pre>
				200	"binary integer literals are an extension"
				201	"format string contains '\\0' within the string body"
				202	"more '<b>%%</b>' conversions than data arguments"
Chris Lattner	545b368	2008-11-23 20:27:13 +0000	[diff] [blame]	203	"invalid operands to binary expression (<b>%0</b> and <b>%1</b>)"
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	204	"overloaded '<b>%0</b>' must be a <b>%select{unary\|binary\|unary or binary}2</b> operator"
				205	" (has <b>%1</b> parameter<b>%s1</b>)"
				206	</pre>
				207
				208	<p>These examples show some important points of format strings. You can use any
				209	plain ASCII character in the diagnostic string except "%" without a problem,
				210	but these are C strings, so you have to use and be aware of all the C escape
				211	sequences (as in the second example). If you want to produce a "%" in the
				212	output, use the "%%" escape sequence, like the third diagnostic. Finally,
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	213	Clang uses the "%...[digit]" sequences to specify where and how arguments to
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	214	the diagnostic are formatted.</p>
				215
				216	<p>Arguments to the diagnostic are numbered according to how they are specified
				217	by the C++ code that <a href="#producingdiag">produces them</a>, and are
				218	referenced by <tt>%0</tt> .. <tt>%9</tt>. If you have more than 10 arguments
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	219	to your diagnostic, you are doing something wrong :). Unlike printf, there
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	220	is no requirement that arguments to the diagnostic end up in the output in
				221	the same order as they are specified, you could have a format string with
				222	<tt>"%1 %0"</tt> that swaps them, for example. The text in between the
				223	percent and digit are formatting instructions. If there are no instructions,
				224	the argument is just turned into a string and substituted in.</p>
				225
				226	<p>Here are some "best practices" for writing the English format string:</p>
				227
				228	<ul>
				229	<li>Keep the string short. It should ideally fit in the 80 column limit of the
				230	<tt>DiagnosticKinds.def</tt> file. This avoids the diagnostic wrapping when
				231	printed, and forces you to think about the important point you are conveying
				232	with the diagnostic.</li>
				233	<li>Take advantage of location information. The user will be able to see the
				234	line and location of the caret, so you don't need to tell them that the
				235	problem is with the 4th argument to the function: just point to it.</li>
				236	<li>Do not capitalize the diagnostic string, and do not end it with a
				237	period.</li>
				238	<li>If you need to quote something in the diagnostic string, use single
				239	quotes.</li>
				240	</ul>
				241
				242	<p>Diagnostics should never take random English strings as arguments: you
				243	shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like
				244	<tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing
				245	this prevents <a href="translation">translating</a> the Clang diagnostics to
				246	other languages (because they'll get random English words in their otherwise
				247	localized diagnostic). The exceptions to this are C/C++ language keywords
				248	(e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>). Note
				249	that things like "pointer" and "reference" are not keywords. On the other
				250	hand, you <em>can</em> include anything that comes from the user's source code,
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	251	including variable names, types, labels, etc. The 'select' format can be
				252	used to achieve this sort of thing in a localizable way, see below.</p>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	253
				254	<!-- ==================================== -->
				255	<h4>Formatting a Diagnostic Argument</a></h4>
				256	<!-- ==================================== -->
				257
				258	<p>Arguments to diagnostics are fully typed internally, and come from a couple
				259	different classes: integers, types, names, and random strings. Depending on
				260	the class of the argument, it can be optionally formatted in different ways.
				261	This gives the DiagnosticClient information about what the argument means
				262	without requiring it to use a specific presentation (consider this MVC for
				263	Clang :).</p>
				264
				265	<p>Here are the different diagnostic argument formats currently supported by
				266	Clang:</p>
				267
				268	<table>
				269	<tr><td colspan="2"><b>"s" format</b></td></tr>
				270	<tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr>
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	271	<tr><td>Class:</td><td>Integers</td></tr>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	272	<tr><td>Description:</td><td>This is a simple formatter for integers that is
				273	useful when producing English diagnostics. When the integer is 1, it prints
				274	as nothing. When the integer is not 1, it prints as "s". This allows some
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	275	simple grammatical forms to be to be handled correctly, and eliminates the
				276	need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	277
				278	<tr><td colspan="2"><b>"select" format</b></td></tr>
				279	<tr><td>Example:</td><td><tt>"must be a %select{unary\|binary\|unary or binary}2
				280	operator"</tt></td></tr>
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	281	<tr><td>Class:</td><td>Integers</td></tr>
Chris Lattner	cc54334	2008-11-22 23:50:47 +0000	[diff] [blame]	282	<tr><td>Description:</td><td>This format specifier is used to merge multiple
				283	related diagnostics together into one common one, without requiring the
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	284	difference to be specified as an English string argument. Instead of
Chris Lattner	cc54334	2008-11-22 23:50:47 +0000	[diff] [blame]	285	specifying the string, the diagnostic gets an integer argument and the
				286	format string selects the numbered option. In this case, the "%2" value
				287	must be an integer in the range [0..2]. If it is 0, it prints 'unary', if
				288	it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'. This
				289	allows other language translations to substitute reasonable words (or entire
				290	phrases) based on the semantics of the diagnostic instead of having to do
				291	things textually.</td></tr>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	292
				293	<tr><td colspan="2"><b>"plural" format</b></td></tr>
Sebastian Redl	6816856	2008-11-22 22:16:45 +0000	[diff] [blame]	294	<tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse\|:mice}1 connected to
				295	your computer"</tt></td></tr>
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	296	<tr><td>Class:</td><td>Integers</td></tr>
Sebastian Redl	6816856	2008-11-22 22:16:45 +0000	[diff] [blame]	297	<tr><td>Description:</td><td><p>This is a formatter for complex plural forms.
				298	It is designed to handle even the requirements of languages with very
				299	complex plural forms, as many Baltic languages have. The argument consists
				300	of a series of expression/form pairs, separated by ':', where the first form
				301	whose expression evaluates to true is the result of the modifier.</p>
				302	<p>An expression can be empty, in which case it is always true. See the
				303	example at the top. Otherwise, it is a series of one or more numeric
				304	conditions, separated by ','. If any condition matches, the expression
				305	matches. Each numeric condition can take one of three forms.</p>
				306	<ul>
				307	<li>number: A simple decimal number matches if the argument is the same
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	308	as the number. Example: <tt>"%plural{1:mouse\|:mice}4"</tt></li>
Sebastian Redl	6816856	2008-11-22 22:16:45 +0000	[diff] [blame]	309	<li>range: A range in square brackets matches if the argument is within
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	310	the range. Then range is inclusive on both ends. Example:
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	311	<tt>"%plural{0:none\|1:one\|[2,5]:some\|:many}2"</tt></li>
				312	<li>modulo: A modulo operator is followed by a number, and
				313	equals sign and either a number or a range. The tests are the
				314	same as for plain
Sebastian Redl	6816856	2008-11-22 22:16:45 +0000	[diff] [blame]	315	numbers and ranges, but the argument is taken modulo the number first.
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	316	Example: <tt>"%plural{%100=0:even hundred\|%100=[1,50]:lower half\|:everything
				317	else}1"</tt></li>
Sebastian Redl	6816856	2008-11-22 22:16:45 +0000	[diff] [blame]	318	</ul>
				319	<p>The parser is very unforgiving. A syntax error, even whitespace, will
				320	abort, as will a failure to match the argument against any
				321	expression.</p></td></tr>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	322
Chris Lattner	077bf5e	2008-11-24 03:33:13 +0000	[diff] [blame]	323	<tr><td colspan="2"><b>"objcclass" format</b></td></tr>
				324	<tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr>
				325	<tr><td>Class:</td><td>DeclarationName</td></tr>
				326	<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
				327	DeclarationName corresponds to an Objective-C class method selector. As
				328	such, it prints the selector with a leading '+'.</p></td></tr>
				329
				330	<tr><td colspan="2"><b>"objcinstance" format</b></td></tr>
				331	<tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr>
				332	<tr><td>Class:</td><td>DeclarationName</td></tr>
				333	<tr><td>Description:</td><td><p>This is a simple formatter that indicates the
				334	DeclarationName corresponds to an Objective-C instance method selector. As
				335	such, it prints the selector with a leading '-'.</p></td></tr>
				336
Douglas Gregor	47b9a1c	2009-02-04 17:27:36 +0000	[diff] [blame]	337	<tr><td colspan="2"><b>"q" format</b></td></tr>
				338	<tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr>
				339	<tr><td>Class:</td><td>NamedDecl*</td></tr>
				340	<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>
				341
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	342	</table>
				343
Chris Lattner	cc54334	2008-11-22 23:50:47 +0000	[diff] [blame]	344	<p>It is really easy to add format specifiers to the Clang diagnostics system,
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	345	but they should be discussed before they are added. If you are creating a lot
				346	of repetitive diagnostics and/or have an idea for a useful formatter, please
				347	bring it up on the cfe-dev mailing list.</p>
Chris Lattner	cc54334	2008-11-22 23:50:47 +0000	[diff] [blame]	348
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	349	<!-- ===================================================== -->
				350	<h4><a name="#producingdiag">Producing the Diagnostic</a></h4>
				351	<!-- ===================================================== -->
				352
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	353	<p>Now that you've created the diagnostic in the DiagnosticKinds.def file, you
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	354	need to write the code that detects the condition in question and emits the
				355	new diagnostic. Various components of Clang (e.g. the preprocessor, Sema,
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	356	etc) provide a helper function named "Diag". It creates a diagnostic and
				357	accepts the arguments, ranges, and other information that goes along with
				358	it.</p>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	359
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	360	<p>For example, the binary expression error comes from code like this:</p>
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	361
				362	<pre>
				363	if (various things that are bad)
				364	Diag(Loc, diag::err_typecheck_invalid_operands)
				365	<< lex->getType() << rex->getType()
				366	<< lex->getSourceRange() << rex->getSourceRange();
				367	</pre>
				368
				369	<p>This shows that use of the Diag method: they take a location (a <a
				370	href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value
				371	(which matches the name from DiagnosticKinds.def). If the diagnostic takes
				372	arguments, they are specified with the << operator: the first argument
				373	becomes %0, the second becomes %1, etc. The diagnostic interface allows you to
Chris Lattner	fd408ea	2008-11-23 00:42:53 +0000	[diff] [blame]	374	specify arguments of many different types, including <tt>int</tt> and
				375	<tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and
				376	<tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and
				377	<tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc.
				378	SourceRanges are also specified with the << operator, but do not have a
				379	specific ordering requirement.</p>
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	380
				381	<p>As you can see, adding and producing a diagnostic is pretty straightforward.
				382	The hard part is deciding exactly what you need to say to help the user, picking
				383	a suitable wording, and providing the information needed to format it correctly.
Chris Lattner	fd408ea	2008-11-23 00:42:53 +0000	[diff] [blame]	384	The good news is that the call site that issues a diagnostic should be
				385	completely independent of how the diagnostic is formatted and in what language
				386	it is rendered.
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	387	</p>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	388
				389	<!-- ============================================================= -->
				390	<h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4>
				391	<!-- ============================================================= -->
				392
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	393	<p>Once code generates a diagnostic with all of the arguments and the rest of
				394	the relevant information, Clang needs to know what to do with it. As previously
				395	mentioned, the diagnostic machinery goes through some filtering to map a
				396	severity onto a diagnostic level, then (assuming the diagnostic is not mapped to
				397	"<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient
				398	interface with the information.</p>
				399
				400	<p>It is possible to implement this interface in many different ways. For
				401	example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns
				402	the arguments into strings (according to the various formatting rules), prints
				403	out the file/line/column information and the string, then prints out the line of
				404	code, the source ranges, and the caret. However, this behavior isn't required.
				405	</p>
				406
				407	<p>Another implementation of the DiagnosticClient interface is the
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	408	'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode.
Chris Lattner	fd408ea	2008-11-23 00:42:53 +0000	[diff] [blame]	409	Instead of formatting and printing out the diagnostics, this implementation just
				410	captures and remembers the diagnostics as they fly by. Then -verify compares
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	411	the list of produced diagnostics to the list of expected ones. If they disagree,
Chris Lattner	fd408ea	2008-11-23 00:42:53 +0000	[diff] [blame]	412	it prints out its own output.
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	413	</p>
				414
Chris Lattner	fd408ea	2008-11-23 00:42:53 +0000	[diff] [blame]	415	<p>There are many other possible implementations of this interface, and this is
				416	why we prefer diagnostics to pass down rich structured information in arguments.
				417	For example, an HTML output might want declaration names be linkified to where
				418	they come from in the source. Another example is that a GUI might let you click
				419	on typedefs to expand them. This application would want to pass significantly
				420	more information about types through to the GUI than a simple flat string. The
				421	interface allows this to happen.</p>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	422
				423	<!-- ====================================================== -->
				424	<h4><a name="translation">Adding Translations to Clang</a></h4>
				425	<!-- ====================================================== -->
				426
Chris Lattner	627b705	2008-11-23 00:28:33 +0000	[diff] [blame]	427	<p>Not possible yet! Diagnostic strings should be written in UTF-8, the client
Chris Lattner	fd408ea	2008-11-23 00:42:53 +0000	[diff] [blame]	428	can translate to the relevant code page if needed. Each translation completely
				429	replaces the format string for the diagnostic.</p>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	430
				431
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	432	<!-- ======================================================================= -->
				433	<h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3>
				434	<!-- ======================================================================= -->
				435
				436	<p>Strangely enough, the SourceLocation class represents a location within the
				437	source code of the program. Important design points include:</p>
				438
				439	<ol>
				440	<li>sizeof(SourceLocation) must be extremely small, as these are embedded into
				441	many AST nodes and are passed around often. Currently it is 32 bits.</li>
				442	<li>SourceLocation must be a simple value object that can be efficiently
				443	copied.</li>
				444	<li>We should be able to represent a source location for any byte of any input
				445	file. This includes in the middle of tokens, in whitespace, in trigraphs,
				446	etc.</li>
				447	<li>A SourceLocation must encode the current #include stack that was active when
				448	the location was processed. For example, if the location corresponds to a
				449	token, it should contain the set of #includes active when the token was
				450	lexed. This allows us to print the #include stack for a diagnostic.</li>
				451	<li>SourceLocation must be able to describe macro expansions, capturing both
				452	the ultimate instantiation point and the source of the original character
				453	data.</li>
				454	</ol>
				455
				456	<p>In practice, the SourceLocation works together with the SourceManager class
Chris Lattner	18376dd	2009-01-16 07:00:50 +0000	[diff] [blame]	457	to encode two pieces of information about a location: it's spelling location
Chris Lattner	88054de	2009-01-16 07:15:35 +0000	[diff] [blame]	458	and it's instantiation location. For most tokens, these will be the same. However,
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	459	for a macro expansion (or tokens that came from a _Pragma directive) these will
				460	describe the location of the characters corresponding to the token and the
				461	location where the token was used (i.e. the macro instantiation point or the
				462	location of the _Pragma itself).</p>
				463
Chris Lattner	3fcbb89	2008-11-23 08:32:53 +0000	[diff] [blame]	464	<p>For efficiency, we only track one level of macro instantiations: if a token was
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	465	produced by multiple instantiations, we only track the source and ultimate
				466	destination. Though we could track the intermediate instantiation points, this
				467	would require extra bookkeeping and no known client would benefit substantially
				468	from this.</p>
				469
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	470	<p>The Clang front-end inherently depends on the location of a token being
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	471	tracked correctly. If it is ever incorrect, the front-end may get confused and
				472	die. The reason for this is that the notion of the 'spelling' of a Token in
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	473	Clang depends on being able to find the original input characters for the token.
Chris Lattner	18376dd	2009-01-16 07:00:50 +0000	[diff] [blame]	474	This concept maps directly to the "spelling location" for the token.</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	475
				476	<!-- ======================================================================= -->
				477	<h2 id="liblex">The Lexer and Preprocessor Library</h2>
				478	<!-- ======================================================================= -->
				479
				480	<p>The Lexer library contains several tightly-connected classes that are involved
				481	with the nasty process of lexing and preprocessing C source code. The main
				482	interface to this library for outside clients is the large <a
				483	href="#Preprocessor">Preprocessor</a> class.
				484	It contains the various pieces of state that are required to coherently read
				485	tokens out of a translation unit.</p>
				486
				487	<p>The core interface to the Preprocessor object (once it is set up) is the
				488	Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from
				489	the preprocessor stream. There are two types of token providers that the
				490	preprocessor is capable of reading from: a buffer lexer (provided by the <a
				491	href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a
Chris Lattner	7928125	2008-03-09 02:27:26 +0000	[diff] [blame]	492	href="#TokenLexer">TokenLexer</a> class).
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	493
				494
				495	<!-- ======================================================================= -->
				496	<h3 id="Token">The Token class</h3>
				497	<!-- ======================================================================= -->
				498
				499	<p>The Token class is used to represent a single lexed token. Tokens are
				500	intended to be used by the lexer/preprocess and parser libraries, but are not
				501	intended to live beyond them (for example, they should not live in the ASTs).<p>
				502
				503	<p>Tokens most often live on the stack (or some other location that is efficient
				504	to access) as the parser is running, but occasionally do get buffered up. For
				505	example, macro definitions are stored as a series of tokens, and the C++
Chris Lattner	3fcbb89	2008-11-23 08:32:53 +0000	[diff] [blame]	506	front-end periodically needs to buffer tokens up for tentative parsing and
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	507	various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit
				508	system, sizeof(Token) is currently 16 bytes.</p>
				509
Chris Lattner	3932fe0	2009-01-06 06:02:08 +0000	[diff] [blame]	510	<p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation
				511	Tokens</a>" and normal tokens. Normal tokens are those returned by the lexer,
				512	annotation tokens represent semantic information and are produced by the parser,
				513	replacing normal tokens in the token stream. Normal tokens contain the
				514	following information:</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	515
				516	<ul>
				517	<li><b>A SourceLocation</b> - This indicates the location of the start of the
				518	token.</li>
				519
				520	<li><b>A length</b> - This stores the length of the token as stored in the
				521	SourceBuffer. For tokens that include them, this length includes trigraphs and
				522	escaped newlines which are ignored by later phases of the compiler. By pointing
				523	into the original source buffer, it is always possible to get the original
				524	spelling of a token completely accurately.</li>
				525
				526	<li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if
				527	identifier lookup was enabled when the token was lexed (e.g. the lexer was not
				528	reading in 'raw' mode) this contains a pointer to the unique hash value for the
				529	identifier. Because the lookup happens before keyword identification, this
				530	field is set even for language keywords like 'for'.</li>
				531
				532	<li><b>TokenKind</b> - This indicates the kind of token as classified by the
				533	lexer. This includes things like <tt>tok::starequal</tt> (for the "*="
				534	operator), <tt>tok::ampamp</tt> for the "&&" token, and keyword values
				535	(e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note
				536	that some tokens can be spelled multiple ways. For example, C++ supports
				537	"operator keywords", where things like "and" are treated exactly like the
				538	"&&" operator. In these cases, the kind value is set to
				539	<tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to
				540	consider both forms. For something that cares about which form is used (e.g.
				541	the preprocessor 'stringize' operator) the spelling indicates the original
				542	form.</li>
				543
				544	<li><b>Flags</b> - There are currently four flags tracked by the
				545	lexer/preprocessor system on a per-token basis:
				546
				547	<ol>
				548	<li><b>StartOfLine</b> - This was the first token that occurred on its input
				549	source line.</li>
				550	<li><b>LeadingSpace</b> - There was a space character either immediately
				551	before the token or transitively before the token as it was expanded
				552	through a macro. The definition of this flag is very closely defined by
				553	the stringizing requirements of the preprocessor.</li>
				554	<li><b>DisableExpand</b> - This flag is used internally to the preprocessor to
				555	represent identifier tokens which have macro expansion disabled. This
				556	prevents them from being considered as candidates for macro expansion ever
				557	in the future.</li>
				558	<li><b>NeedsCleaning</b> - This flag is set if the original spelling for the
				559	token includes a trigraph or escaped newline. Since this is uncommon,
				560	many pieces of code can fast-path on tokens that did not need cleaning.
				561	</p>
				562	</ol>
				563	</li>
				564	</ul>
				565
Chris Lattner	3932fe0	2009-01-06 06:02:08 +0000	[diff] [blame]	566	<p>One interesting (and somewhat unusual) aspect of normal tokens is that they
				567	don't contain any semantic information about the lexed value. For example, if
				568	the token was a pp-number token, we do not represent the value of the number
				569	that was lexed (this is left for later pieces of code to decide). Additionally,
				570	the lexer library has no notion of typedef names vs variable names: both are
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	571	returned as identifiers, and the parser is left to decide whether a specific
				572	identifier is a typedef or a variable (tracking this requires scope information
Chris Lattner	3932fe0	2009-01-06 06:02:08 +0000	[diff] [blame]	573	among other things). The parser can do this translation by replacing tokens
				574	returned by the preprocessor with "Annotation Tokens".</p>
				575
				576	<!-- ======================================================================= -->
				577	<h3 id="AnnotationToken">Annotation Tokens</h3>
				578	<!-- ======================================================================= -->
				579
				580	<p>Annotation Tokens are tokens that are synthesized by the parser and injected
				581	into the preprocessor's token stream (replacing existing tokens) to record
				582	semantic information found by the parser. For example, if "foo" is found to be
				583	a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an
				584	<tt>tok::annot_typename</tt>. This is useful for a couple of reasons: 1) this
				585	makes it easy to handle qualified type names (e.g. "foo::bar::baz<42>::t")
				586	in C++ as a single "token" in the parser. 2) if the parser backtracks, the
				587	reparse does not need to redo semantic analysis to determine whether a token
				588	sequence is a variable, type, template, etc.</p>
				589
				590	<p>Annotation Tokens are created by the parser and reinjected into the parser's
				591	token stream (when backtracking is enabled). Because they can only exist in
				592	tokens that the preprocessor-proper is done with, it doesn't need to keep around
				593	flags like "start of line" that the preprocessor uses to do its job.
				594	Additionally, an annotation token may "cover" a sequence of preprocessor tokens
				595	(e.g. <tt>a::b::c</tt> is five preprocessor tokens). As such, the valid fields
				596	of an annotation token are different than the fields for a normal token (but
				597	they are multiplexed into the normal Token fields):</p>
				598
				599	<ul>
				600	<li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation
				601	token indicates the first token replaced by the annotation token. In the example
				602	above, it would be the location of the "a" identifier.</li>
				603
				604	<li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the
				605	last token replaced with the annotation token. In the example above, it would
				606	be the location of the "c" identifier.</li>
				607
				608	<li><b>void* "AnnotationValue"</b> - This contains an opaque object that the
				609	parser gets from Sema through an Actions module, it is passed around and Sema
				610	intepretes it, based on the type of annotation token.</li>
				611
				612	<li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this
				613	is. See below for the different valid kinds.</li>
				614	</ul>
				615
				616	<p>Annotation tokens currently come in three kinds:</p>
				617
				618	<ol>
				619	<li><b>tok::annot_typename</b>: This annotation token represents a
				620	resolved typename token that is potentially qualified. The AnnotationValue
Steve Naroff	b43a50f	2009-01-28 19:39:02 +0000	[diff] [blame]	621	field contains a pointer returned by Action::getTypeName(). In the case of the
Chris Lattner	3932fe0	2009-01-06 06:02:08 +0000	[diff] [blame]	622	Sema actions module, this is a <tt>Decl*</tt> for the type.</li>
				623
				624	<li><b>tok::annot_cxxscope</b>: This annotation token represents a C++ scope
				625	specifier, such as "A::B::". This corresponds to the grammar productions "::"
				626	and ":: [opt] nested-name-specifier". The AnnotationValue pointer is returned
				627	by the Action::ActOnCXXGlobalScopeSpecifier and
				628	Action::ActOnCXXNestedNameSpecifier callbacks. In the case of Sema, this is a
				629	<tt>DeclContext*</tt>.</li>
				630
				631	<li><b>tok::annot_template_id</b>: This annotation token represents a C++
				632	template-id such as "foo<int, 4>", which may refer to a function or type
				633	depending on whether foo is a function template or class template. The
				634	AnnotationValue pointer is a pointer to a malloc'd TemplateIdAnnotation object.
				635	FIXME: I don't think the parsing logic is right for this. Shouldn't type
				636	templates be turned into annot_typename??</li>
				637
				638	</ol>
				639
Cedric Venet	da76b28	2009-01-06 16:22:54 +0000	[diff] [blame]	640	<p>As mentioned above, annotation tokens are not returned by the preprocessor,
Chris Lattner	3932fe0	2009-01-06 06:02:08 +0000	[diff] [blame]	641	they are formed on demand by the parser. This means that the parser has to be
				642	aware of cases where an annotation could occur and form it where appropriate.
				643	This is somewhat similar to how the parser handles Translation Phase 6 of C99:
				644	String Concatenation (see C99 5.1.1.2). In the case of string concatenation,
				645	the preprocessor just returns distinct tok::string_literal and
				646	tok::wide_string_literal tokens and the parser eats a sequence of them wherever
				647	the grammar indicates that a string literal can occur.</p>
				648
				649	<p>In order to do this, whenever the parser expects a tok::identifier or
				650	tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or
				651	TryAnnotateCXXScopeToken methods to form the annotation token. These methods
				652	will maximally form the specified annotation tokens and replace the current
				653	token with them, if applicable. If the current tokens is not valid for an
				654	annotation token, it will remain an identifier or :: token.</p>
				655
				656
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	657
				658	<!-- ======================================================================= -->
				659	<h3 id="Lexer">The Lexer class</h3>
				660	<!-- ======================================================================= -->
				661
				662	<p>The Lexer class provides the mechanics of lexing tokens out of a source
				663	buffer and deciding what they mean. The Lexer is complicated by the fact that
				664	it operates on raw buffers that have not had spelling eliminated (this is a
				665	necessity to get decent performance), but this is countered with careful coding
				666	as well as standard performance techniques (for example, the comment handling
				667	code is vectorized on X86 and PowerPC hosts).</p>
				668
				669	<p>The lexer has a couple of interesting modal features:</p>
				670
				671	<ul>
				672	<li>The lexer can operate in 'raw' mode. This mode has several features that
				673	make it possible to quickly lex the file (e.g. it stops identifier lookup,
				674	doesn't specially handle preprocessor tokens, handles EOF differently, etc).
				675	This mode is used for lexing within an "<tt>#if 0</tt>" block, for
				676	example.</li>
				677	<li>The lexer can capture and return comments as tokens. This is required to
				678	support the -C preprocessor mode, which passes comments through, and is
				679	used by the diagnostic checker to identifier expect-error annotations.</li>
				680	<li>The lexer can be in ParsingFilename mode, which happens when preprocessing
Chris Lattner	8438624	2007-09-16 19:25:23 +0000	[diff] [blame]	681	after reading a #include directive. This mode changes the parsing of '<'
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	682	to return an "angled string" instead of a bunch of tokens for each thing
				683	within the filename.</li>
				684	<li>When parsing a preprocessor directive (after "<tt>#</tt>") the
				685	ParsingPreprocessorDirective mode is entered. This changes the parser to
				686	return EOM at a newline.</li>
				687	<li>The Lexer uses a LangOptions object to know whether trigraphs are enabled,
				688	whether C++ or ObjC keywords are recognized, etc.</li>
				689	</ul>
				690
				691	<p>In addition to these modes, the lexer keeps track of a couple of other
				692	features that are local to a lexed buffer, which change as the buffer is
				693	lexed:</p>
				694
				695	<ul>
				696	<li>The Lexer uses BufferPtr to keep track of the current character being
				697	lexed.</li>
				698	<li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token
				699	will start with its "start of line" bit set.</li>
				700	<li>The Lexer keeps track of the current #if directives that are active (which
				701	can be nested).</li>
				702	<li>The Lexer keeps track of an <a href="#MultipleIncludeOpt">
				703	MultipleIncludeOpt</a> object, which is used to
				704	detect whether the buffer uses the standard "<tt>#ifndef XX</tt> /
				705	<tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does,
				706	subsequent includes can be ignored if the XX macro is defined.</li>
				707	</ul>
				708
				709	<!-- ======================================================================= -->
Chris Lattner	7928125	2008-03-09 02:27:26 +0000	[diff] [blame]	710	<h3 id="TokenLexer">The TokenLexer class</h3>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	711	<!-- ======================================================================= -->
				712
Chris Lattner	7928125	2008-03-09 02:27:26 +0000	[diff] [blame]	713	<p>The TokenLexer class is a token provider that returns tokens from a list
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	714	of tokens that came from somewhere else. It typically used for two things: 1)
				715	returning tokens from a macro definition as it is being expanded 2) returning
				716	tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and
				717	will most likely be used to handle unbounded look-ahead for the C++ parser.</p>
				718
				719	<!-- ======================================================================= -->
				720	<h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3>
				721	<!-- ======================================================================= -->
				722
				723	<p>The MultipleIncludeOpt class implements a really simple little state machine
				724	that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>"
				725	idiom that people typically use to prevent multiple inclusion of headers. If a
				726	buffer uses this idiom and is subsequently #include'd, the preprocessor can
				727	simply check to see whether the guarding condition is defined or not. If so,
				728	the preprocessor can completely ignore the include of the header.</p>
				729
				730
				731
				732	<!-- ======================================================================= -->
				733	<h2 id="libparse">The Parser Library</h2>
				734	<!-- ======================================================================= -->
				735
				736	<!-- ======================================================================= -->
				737	<h2 id="libast">The AST Library</h2>
				738	<!-- ======================================================================= -->
				739
				740	<!-- ======================================================================= -->
				741	<h3 id="Type">The Type class and its subclasses</h3>
				742	<!-- ======================================================================= -->
				743
				744	<p>The Type class (and its subclasses) are an important part of the AST. Types
				745	are accessed through the ASTContext class, which implicitly creates and uniques
				746	them as they are needed. Types have a couple of non-obvious features: 1) they
				747	do not capture type qualifiers like const or volatile (See
				748	<a href="#QualType">QualType</a>), and 2) they implicitly capture typedef
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	749	information. Once created, types are immutable (unlike decls).</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	750
				751	<p>Typedefs in C make semantic analysis a bit more complex than it would
				752	be without them. The issue is that we want to capture typedef information
				753	and represent it in the AST perfectly, but the semantics of operations need to
				754	"see through" typedefs. For example, consider this code:</p>
				755
				756	<code>
				757	void func() {<br>
Bill Wendling	30d1775	2007-10-06 01:56:01 +0000	[diff] [blame]	758	typedef int foo;<br>
				759	foo X, *Y;<br>
				760	typedef foo* bar;<br>
				761	bar Z;<br>
				762	*X; <i>// error</i><br>
				763	**Y; <i>// error</i><br>
				764	**Z; <i>// error</i><br>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	765	}<br>
				766	</code>
				767
				768	<p>The code above is illegal, and thus we expect there to be diagnostics emitted
				769	on the annotated lines. In this example, we expect to get:</p>
				770
				771	<pre>
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	772	<b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	773	*X; // error
				774	<font color="blue">^~</font>
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	775	<b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	776	**Y; // error
				777	<font color="blue">^~~</font>
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	778	<b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b>
				779	**Z; // error
				780	<font color="blue">^~~</font>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	781	</pre>
				782
				783	<p>While this example is somewhat silly, it illustrates the point: we want to
				784	retain typedef information where possible, so that we can emit errors about
				785	"<tt>std::string</tt>" instead of "<tt>std::basic_string<char, std:...</tt>".
				786	Doing this requires properly keeping typedef information (for example, the type
				787	of "X" is "foo", not "int"), and requires properly propagating it through the
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	788	various operators (for example, the type of *Y is "foo", not "int"). In order
				789	to retain this information, the type of these expressions is an instance of the
				790	TypedefType class, which indicates that the type of these expressions is a
				791	typedef for foo.
				792	</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	793
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	794	<p>Representing types like this is great for diagnostics, because the
				795	user-specified type is always immediately available. There are two problems
				796	with this: first, various semantic checks need to make judgements about the
Chris Lattner	33fc68a	2007-07-31 18:54:50 +0000	[diff] [blame]	797	<em>actual structure</em> of a type, ignoring typdefs. Second, we need an
				798	efficient way to query whether two types are structurally identical to each
				799	other, ignoring typedefs. The solution to both of these problems is the idea of
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	800	canonical types.</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	801
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	802	<!-- =============== -->
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	803	<h4>Canonical Types</h4>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	804	<!-- =============== -->
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	805
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	806	<p>Every instance of the Type class contains a canonical type pointer. For
				807	simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>",
				808	"<tt>int**</tt>"), the type just points to itself. For types that have a
				809	typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>",
				810	"<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their
				811	structurally equivalent type without any typedefs (e.g. "<tt>int</tt>",
				812	"<tt>int</tt>", "<tt>int</tt>", and "<tt>int</tt>" respectively).</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	813
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	814	<p>This design provides a constant time operation (dereferencing the canonical
				815	type pointer) that gives us access to the structure of types. For example,
				816	we can trivially tell that "bar" and "foo*" are the same type by dereferencing
				817	their canonical type pointers and doing a pointer comparison (they both point
				818	to the single "<tt>int*</tt>" type).</p>
				819
				820	<p>Canonical types and typedef types bring up some complexities that must be
				821	carefully managed. Specifically, the "isa/cast/dyncast" operators generally
				822	shouldn't be used in code that is inspecting the AST. For example, when type
				823	checking the indirection operator (unary '*' on a pointer), the type checker
				824	must verify that the operand has a pointer type. It would not be correct to
				825	check that with "<tt>isa<PointerType>(SubExpr->getType())</tt>",
				826	because this predicate would fail if the subexpression had a typedef type.</p>
				827
				828	<p>The solution to this problem are a set of helper methods on Type, used to
				829	check their properties. In this case, it would be correct to use
				830	"<tt>SubExpr->getType()->isPointerType()</tt>" to do the check. This
				831	predicate will return true if the <em>canonical type is a pointer</em>, which is
				832	true any time the type is structurally a pointer type. The only hard part here
				833	is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p>
				834
				835	<p>The second problem we face is how to get access to the pointer type once we
				836	know it exists. To continue the example, the result type of the indirection
				837	operator is the pointee type of the subexpression. In order to determine the
				838	type, we need to get the instance of PointerType that best captures the typedef
				839	information in the program. If the type of the expression is literally a
				840	PointerType, we can return that, otherwise we have to dig through the
				841	typedefs to find the pointer type. For example, if the subexpression had type
				842	"<tt>foo*</tt>", we could return that type as the result. If the subexpression
				843	had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do
				844	<em>not</em> want "<tt>int*</tt>"). In order to provide all of this, Type has
Chris Lattner	11406c1	2007-07-31 16:50:51 +0000	[diff] [blame]	845	a getAsPointerType() method that checks whether the type is structurally a
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	846	PointerType and, if so, returns the best one. If not, it returns a null
				847	pointer.</p>
				848
				849	<p>This structure is somewhat mystical, but after meditating on it, it will
				850	make sense to you :).</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	851
				852	<!-- ======================================================================= -->
				853	<h3 id="QualType">The QualType class</h3>
				854	<!-- ======================================================================= -->
				855
				856	<p>The QualType class is designed as a trivial value class that is small,
				857	passed by-value and is efficient to query. The idea of QualType is that it
				858	stores the type qualifiers (const, volatile, restrict) separately from the types
				859	themselves: QualType is conceptually a pair of "Type*" and bits for the type
				860	qualifiers.</p>
				861
				862	<p>By storing the type qualifiers as bits in the conceptual pair, it is
				863	extremely efficient to get the set of qualifiers on a QualType (just return the
				864	field of the pair), add a type qualifier (which is a trivial constant-time
				865	operation that sets a bit), and remove one or more type qualifiers (just return
				866	a QualType with the bitfield set to empty).</p>
				867
				868	<p>Further, because the bits are stored outside of the type itself, we do not
				869	need to create duplicates of types with different sets of qualifiers (i.e. there
				870	is only a single heap allocated "int" type: "const int" and "volatile const int"
				871	both point to the same heap allocated "int" type). This reduces the heap size
				872	used to represent bits and also means we do not have to consider qualifiers when
				873	uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p>
				874
				875	<p>In practice, on hosts where it is safe, the 3 type qualifiers are stored in
				876	the low bit of the pointer to the Type object. This means that QualType is
				877	exactly the same size as a pointer, and this works fine on any system where
				878	malloc'd objects are at least 8 byte aligned.</p>
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	879
				880	<!-- ======================================================================= -->
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	881	<h3 id="DeclarationName">Declaration names</h3>
				882	<!-- ======================================================================= -->
				883
				884	<p>The <tt>DeclarationName</tt> class represents the name of a
				885	declaration in Clang. Declarations in the C family of languages can
Chris Lattner	3fcbb89	2008-11-23 08:32:53 +0000	[diff] [blame]	886	take several different forms. Most declarations are named by
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	887	simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in
				888	the function declaration <code>f(int x)</code>. In C++, declaration
				889	names can also name class constructors ("<code>Class</code>"
				890	in <code>struct Class { Class(); }</code>), class destructors
				891	("<code>~Class</code>"), overloaded operator names ("operator+"),
				892	and conversion functions ("<code>operator void const *</code>"). In
				893	Objective-C, declaration names can refer to the names of Objective-C
				894	methods, which involve the method name and the parameters,
Chris Lattner	3fcbb89	2008-11-23 08:32:53 +0000	[diff] [blame]	895	collectively called a <i>selector</i>, e.g.,
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	896	"<code>setWidth:height:</code>". Since all of these kinds of
Chris Lattner	3fcbb89	2008-11-23 08:32:53 +0000	[diff] [blame]	897	entities - variables, functions, Objective-C methods, C++
				898	constructors, destructors, and operators - are represented as
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	899	subclasses of Clang's common <code>NamedDecl</code>
				900	class, <code>DeclarationName</code> is designed to efficiently
				901	represent any kind of name.</p>
				902
				903	<p>Given
				904	a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code>
Douglas Gregor	2def483	2008-11-17 20:34:05 +0000	[diff] [blame]	905	will produce a value that describes what kind of name <code>N</code>
Douglas Gregor	e94ca9e4	2008-11-18 14:39:36 +0000	[diff] [blame]	906	stores. There are 8 options (all of the names are inside
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	907	the <code>DeclarationName</code> class)</p>
				908	<dl>
				909	<dt>Identifier</dt>
				910	<dd>The name is a simple
				911	identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the
				912	corresponding <code>IdentifierInfo*</code> pointing to the actual
				913	identifier. Note that C++ overloaded operators (e.g.,
				914	"<code>operator+</code>") are represented as special kinds of
				915	identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code>
				916	function to determine whether an identifier is an overloaded
				917	operator name.</dd>
				918
				919	<dt>ObjCZeroArgSelector, ObjCOneArgSelector,
				920	ObjCMultiArgSelector</dt>
				921	<dd>The name is an Objective-C selector, which can be retrieved as a
				922	<code>Selector</code> instance
				923	via <code>N.getObjCSelector()</code>. The three possible name
				924	kinds for Objective-C reflect an optimization within
				925	the <code>DeclarationName</code> class: both zero- and
				926	one-argument selectors are stored as a
				927	masked <code>IdentifierInfo</code> pointer, and therefore require
				928	very little space, since zero- and one-argument selectors are far
				929	more common than multi-argument selectors (which use a different
				930	structure).</dd>
				931
				932	<dt>CXXConstructorName</dt>
				933	<dd>The name is a C++ constructor
				934	name. Use <code>N.getCXXNameType()</code> to retrieve
				935	the <a href="#QualType">type</a> that this constructor is meant to
				936	construct. The type is always the canonical type, since all
				937	constructors for a given type have the same name.</dd>
				938
				939	<dt>CXXDestructorName</dt>
				940	<dd>The name is a C++ destructor
				941	name. Use <code>N.getCXXNameType()</code> to retrieve
				942	the <a href="#QualType">type</a> whose destructor is being
				943	named. This type is always a canonical type.</dd>
				944
				945	<dt>CXXConversionFunctionName</dt>
				946	<dd>The name is a C++ conversion function. Conversion functions are
				947	named according to the type they convert to, e.g., "<code>operator void
				948	const *</code>". Use <code>N.getCXXNameType()</code> to retrieve
				949	the type that this conversion function converts to. This type is
				950	always a canonical type.</dd>
Douglas Gregor	e94ca9e4	2008-11-18 14:39:36 +0000	[diff] [blame]	951
				952	<dt>CXXOperatorName</dt>
				953	<dd>The name is a C++ overloaded operator name. Overloaded operators
				954	are named according to their spelling, e.g.,
				955	"<code>operator+</code>" or "<code>operator new
				956	[]</code>". Use <code>N.getCXXOverloadedOperator()</code> to
				957	retrieve the overloaded operator (a value of
				958	type <code>OverloadedOperatorKind</code>).</dd>
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	959	</dl>
				960
				961	<p><code>DeclarationName</code>s are cheap to create, copy, and
				962	compare. They require only a single pointer's worth of storage in
Douglas Gregor	e94ca9e4	2008-11-18 14:39:36 +0000	[diff] [blame]	963	the common cases (identifiers, zero-
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	964	and one-argument Objective-C selectors) and use dense, uniqued
				965	storage for the other kinds of
				966	names. Two <code>DeclarationName</code>s can be compared for
				967	equality (<code>==</code>, <code>!=</code>) using a simple bitwise
				968	comparison, can be ordered
				969	with <code><</code>, <code>></code>, <code><=</code>,
				970	and <code>>=</code> (which provide a lexicographical ordering for
				971	normal identifiers but an unspecified ordering for other kinds of
				972	names), and can be placed into LLVM <code>DenseMap</code>s
				973	and <code>DenseSet</code>s.</p>
				974
				975	<p><code>DeclarationName</code> instances can be created in different
				976	ways depending on what kind of name the instance will store. Normal
Douglas Gregor	e94ca9e4	2008-11-18 14:39:36 +0000	[diff] [blame]	977	identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	978	(<code>Selector</code>) can be implicitly converted
				979	to <code>DeclarationName</code>s. Names for C++ constructors,
Douglas Gregor	e94ca9e4	2008-11-18 14:39:36 +0000	[diff] [blame]	980	destructors, conversion functions, and overloaded operators can be retrieved from
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	981	the <code>DeclarationNameTable</code>, an instance of which is
				982	available as <code>ASTContext::DeclarationNames</code>. The member
				983	functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>,
Douglas Gregor	e94ca9e4	2008-11-18 14:39:36 +0000	[diff] [blame]	984	<code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively,
				985	return <code>DeclarationName</code> instances for the four kinds of
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	986	C++ special function names.</p>
				987
				988	<!-- ======================================================================= -->
Douglas Gregor	074149e	2009-01-05 19:45:36 +0000	[diff] [blame]	989	<h3 id="DeclContext">Declaration contexts</h3>
				990	<!-- ======================================================================= -->
				991	<p>Every declaration in a program exists within some <i>declaration
				992	context</i>, such as a translation unit, namespace, class, or
				993	function. Declaration contexts in Clang are represented by
				994	the <code>DeclContext</code> class, from which the various
				995	declaration-context AST nodes
				996	(<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>,
				997	etc.) will derive. The <code>DeclContext</code> class provides
				998	several facilities common to each declaration context:</p>
				999	<dl>
				1000	<dt>Source-centric vs. Semantics-centric View of Declarations</dt>
				1001	<dd><code>DeclContext</code> provides two views of the declarations
				1002	stored within a declaration context. The source-centric view
				1003	accurately represents the program source code as written, including
				1004	multiple declarations of entities where present (see the
				1005	section <a href="#Redeclarations">Redeclarations and
				1006	Overloads</a>), while the semantics-centric view represents the
				1007	program semantics. The two views are kept synchronized by semantic
				1008	analysis while the ASTs are being constructed.</dd>
				1009
				1010	<dt>Storage of declarations within that context</dt>
				1011	<dd>Every declaration context can contain some number of
				1012	declarations. For example, a C++ class (represented
				1013	by <code>RecordDecl</code>) contains various member functions,
				1014	fields, nested types, and so on. All of these declarations will be
				1015	stored within the <code>DeclContext</code>, and one can iterate
				1016	over the declarations via
				1017	[<code>DeclContext::decls_begin()</code>,
				1018	<code>DeclContext::decls_end()</code>). This mechanism provides
				1019	the source-centric view of declarations in the context.</dd>
				1020
				1021	<dt>Lookup of declarations within that context</dt>
				1022	<dd>The <code>DeclContext</code> structure provides efficient name
				1023	lookup for names within that declaration context. For example,
				1024	if <code>N</code> is a namespace we can look for the
				1025	name <code>N::f</code>
				1026	using <code>DeclContext::lookup</code>. The lookup itself is
				1027	based on a lazily-constructed array (for declaration contexts
				1028	with a small number of declarations) or hash table (for
				1029	declaration contexts with more declarations). The lookup
				1030	operation provides the semantics-centric view of the declarations
				1031	in the context.</dd>
				1032
				1033	<dt>Ownership of declarations</dt>
				1034	<dd>The <code>DeclContext</code> owns all of the declarations that
				1035	were declared within its declaration context, and is responsible
				1036	for the management of their memory as well as their
				1037	(de-)serialization.</dd>
				1038	</dl>
				1039
Douglas Gregor	4afa39d	2009-01-20 01:17:11 +0000	[diff] [blame]	1040	<p>All declarations are stored within a declaration context, and one
				1041	can query
				1042	information about the context in which each declaration lives. One
Douglas Gregor	074149e	2009-01-05 19:45:36 +0000	[diff] [blame]	1043	can retrieve the <code>DeclContext</code> that contains a
Douglas Gregor	4afa39d	2009-01-20 01:17:11 +0000	[diff] [blame]	1044	particular <code>Decl</code>
				1045	using <code>Decl::getDeclContext</code>. However, see the
Douglas Gregor	074149e	2009-01-05 19:45:36 +0000	[diff] [blame]	1046	section <a href="#LexicalAndSemanticContexts">Lexical and Semantic
				1047	Contexts</a> for more information about how to interpret this
				1048	context information.</p>
				1049
				1050	<h4 id="Redeclarations">Redeclarations and Overloads</h4>
				1051	<p>Within a translation unit, it is common for an entity to be
				1052	declared several times. For example, we might declare a function "f"
				1053	and then later re-declare it as part of an inlined definition:</p>
				1054
				1055	<pre>
				1056	void f(int x, int y, int z = 1);
				1057
				1058	inline void f(int x, int y, int z) { /* ... */ }
				1059	</pre>
				1060
				1061	<p>The representation of "f" differs in the source-centric and
				1062	semantics-centric views of a declaration context. In the
				1063	source-centric view, all redeclarations will be present, in the
				1064	order they occurred in the source code, making
				1065	this view suitable for clients that wish to see the structure of
				1066	the source code. In the semantics-centric view, only the most recent "f"
				1067	will be found by the lookup, since it effectively replaces the first
				1068	declaration of "f".</p>
				1069
				1070	<p>In the semantics-centric view, overloading of functions is
				1071	represented explicitly. For example, given two declarations of a
				1072	function "g" that are overloaded, e.g.,</p>
				1073	<pre>
				1074	void g();
				1075	void g(int);
				1076	</pre>
				1077	<p>the <code>DeclContext::lookup</code> operation will return
				1078	an <code>OverloadedFunctionDecl</code> that contains both
				1079	declarations of "g". Clients that perform semantic analysis on a
				1080	program that is not concerned with the actual source code will
				1081	primarily use this semantics-centric view.</p>
				1082
				1083	<h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4>
Douglas Gregor	4afa39d	2009-01-20 01:17:11 +0000	[diff] [blame]	1084	<p>Each declaration has two potentially different
Douglas Gregor	074149e	2009-01-05 19:45:36 +0000	[diff] [blame]	1085	declaration contexts: a <i>lexical</i> context, which corresponds to
				1086	the source-centric view of the declaration context, and
				1087	a <i>semantic</i> context, which corresponds to the
				1088	semantics-centric view. The lexical context is accessible
Douglas Gregor	4afa39d	2009-01-20 01:17:11 +0000	[diff] [blame]	1089	via <code>Decl::getLexicalDeclContext</code> while the
Douglas Gregor	074149e	2009-01-05 19:45:36 +0000	[diff] [blame]	1090	semantic context is accessible
Douglas Gregor	4afa39d	2009-01-20 01:17:11 +0000	[diff] [blame]	1091	via <code>Decl::getDeclContext</code>, both of which return
Douglas Gregor	074149e	2009-01-05 19:45:36 +0000	[diff] [blame]	1092	<code>DeclContext</code> pointers. For most declarations, the two
				1093	contexts are identical. For example:</p>
				1094
				1095	<pre>
				1096	class X {
				1097	public:
				1098	void f(int x);
				1099	};
				1100	</pre>
				1101
				1102	<p>Here, the semantic and lexical contexts of <code>X::f</code> are
				1103	the <code>DeclContext</code> associated with the
				1104	class <code>X</code> (itself stored as a <code>RecordDecl</code> AST
				1105	node). However, we can now define <code>X::f</code> out-of-line:</p>
				1106
				1107	<pre>
				1108	void X::f(int x = 17) { /* ... */ }
				1109	</pre>
				1110
				1111	<p>This definition of has different lexical and semantic
				1112	contexts. The lexical context corresponds to the declaration
				1113	context in which the actual declaration occurred in the source
				1114	code, e.g., the translation unit containing <code>X</code>. Thus,
				1115	this declaration of <code>X::f</code> can be found by traversing
				1116	the declarations provided by
				1117	[<code>decls_begin()</code>, <code>decls_end()</code>) in the
				1118	translation unit.</p>
				1119
				1120	<p>The semantic context of <code>X::f</code> corresponds to the
				1121	class <code>X</code>, since this member function is (semantically) a
				1122	member of <code>X</code>. Lookup of the name <code>f</code> into
				1123	the <code>DeclContext</code> associated with <code>X</code> will
				1124	then return the definition of <code>X::f</code> (including
				1125	information about the default argument).</p>
				1126
				1127	<h4 id="TransparentContexts">Transparent Declaration Contexts</h4>
				1128	<p>In C and C++, there are several contexts in which names that are
				1129	logically declared inside another declaration will actually "leak"
				1130	out into the enclosing scope from the perspective of name
				1131	lookup. The most obvious instance of this behavior is in
				1132	enumeration types, e.g.,</p>
				1133	<pre>
				1134	enum Color {
				1135	Red,
				1136	Green,
				1137	Blue
				1138	};
				1139	</pre>
				1140
				1141	<p>Here, <code>Color</code> is an enumeration, which is a declaration
				1142	context that contains the
				1143	enumerators <code>Red</code>, <code>Green</code>,
				1144	and <code>Blue</code>. Thus, traversing the list of declarations
				1145	contained in the enumeration <code>Color</code> will
				1146	yield <code>Red</code>, <code>Green</code>,
				1147	and <code>Blue</code>. However, outside of the scope
				1148	of <code>Color</code> one can name the enumerator <code>Red</code>
				1149	without qualifying the name, e.g.,</p>
				1150
				1151	<pre>
				1152	Color c = Red;
				1153	</pre>
				1154
				1155	<p>There are other entities in C++ that provide similar behavior. For
				1156	example, linkage specifications that use curly braces:</p>
				1157
				1158	<pre>
				1159	extern "C" {
				1160	void f(int);
				1161	void g(int);
				1162	}
				1163	// f and g are visible here
				1164	</pre>
				1165
				1166	<p>For source-level accuracy, we treat the linkage specification and
				1167	enumeration type as a
				1168	declaration context in which its enclosed declarations ("Red",
				1169	"Green", and "Blue"; "f" and "g")
				1170	are declared. However, these declarations are visible outside of the
				1171	scope of the declaration context.</p>
				1172
				1173	<p>These language features (and several others, described below) have
				1174	roughly the same set of
				1175	requirements: declarations are declared within a particular lexical
				1176	context, but the declarations are also found via name lookup in
				1177	scopes enclosing the declaration itself. This feature is implemented
				1178	via <i>transparent</i> declaration contexts
				1179	(see <code>DeclContext::isTransparentContext()</code>), whose
				1180	declarations are visible in the nearest enclosing non-transparent
				1181	declaration context. This means that the lexical context of the
				1182	declaration (e.g., an enumerator) will be the
				1183	transparent <code>DeclContext</code> itself, as will the semantic
				1184	context, but the declaration will be visible in every outer context
				1185	up to and including the first non-transparent declaration context (since
				1186	transparent declaration contexts can be nested).</p>
				1187
				1188	<p>The transparent <code>DeclContexts</code> are:</p>
				1189	<ul>
				1190	<li>Enumerations (but not C++0x "scoped enumerations"):
				1191	<pre>
				1192	enum Color {
				1193	Red,
				1194	Green,
				1195	Blue
				1196	};
				1197	// Red, Green, and Blue are in scope
				1198	</pre></li>
				1199	<li>C++ linkage specifications:
				1200	<pre>
				1201	extern "C" {
				1202	void f(int);
				1203	void g(int);
				1204	}
				1205	// f and g are in scope
				1206	</pre></li>
				1207	<li>Anonymous unions and structs:
				1208	<pre>
				1209	struct LookupTable {
				1210	bool IsVector;
				1211	union {
				1212	std::vector<Item> *Vector;
				1213	std::set<Item> *Set;
				1214	};
				1215	};
				1216
				1217	LookupTable LT;
				1218	LT.Vector = 0; // Okay: finds Vector inside the unnamed union
				1219	</pre>
				1220	</li>
				1221	<li>C++0x inline namespaces:
				1222	<pre>
				1223	namespace mylib {
				1224	inline namespace debug {
				1225	class X;
				1226	}
				1227	}
				1228	mylib::X *xp; // okay: mylib::X refers to mylib::debug::X
				1229	</pre>
				1230	</li>
				1231	</ul>
				1232
				1233
				1234	<h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4>
				1235	<p>C++ namespaces have the interesting--and, so far, unique--property that
				1236	the namespace can be defined multiple times, and the declarations
				1237	provided by each namespace definition are effectively merged (from
				1238	the semantic point of view). For example, the following two code
				1239	snippets are semantically indistinguishable:</p>
				1240	<pre>
				1241	// Snippet #1:
				1242	namespace N {
				1243	void f();
				1244	}
				1245	namespace N {
				1246	void f(int);
				1247	}
				1248
				1249	// Snippet #2:
				1250	namespace N {
				1251	void f();
				1252	void f(int);
				1253	}
				1254	</pre>
				1255
				1256	<p>In Clang's representation, the source-centric view of declaration
				1257	contexts will actually have two separate <code>NamespaceDecl</code>
				1258	nodes in Snippet #1, each of which is a declaration context that
				1259	contains a single declaration of "f". However, the semantics-centric
				1260	view provided by name lookup into the namespace <code>N</code> for
				1261	"f" will return an <code>OverloadedFunctionDecl</code> that contains
				1262	both declarations of "f".</p>
				1263
				1264	<p><code>DeclContext</code> manages multiply-defined declaration
				1265	contexts internally. The
				1266	function <code>DeclContext::getPrimaryContext</code> retrieves the
				1267	"primary" context for a given <code>DeclContext</code> instance,
				1268	which is the <code>DeclContext</code> responsible for maintaining
				1269	the lookup table used for the semantics-centric view. Given the
				1270	primary context, one can follow the chain
				1271	of <code>DeclContext</code> nodes that define additional
				1272	declarations via <code>DeclContext::getNextContext</code>. Note that
				1273	these functions are used internally within the lookup and insertion
				1274	methods of the <code>DeclContext</code>, so the vast majority of
				1275	clients can ignore them.</p>
				1276
				1277	<!-- ======================================================================= -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1278	<h3 id="CFG">The <tt>CFG</tt> class</h3>
				1279	<!-- ======================================================================= -->
				1280
				1281	<p>The <tt>CFG</tt> class is designed to represent a source-level
				1282	control-flow graph for a single statement (<tt>Stmt*</tt>). Typically
				1283	instances of <tt>CFG</tt> are constructed for function bodies (usually
				1284	an instance of <tt>CompoundStmt</tt>), but can also be instantiated to
				1285	represent the control-flow of any class that subclasses <tt>Stmt</tt>,
				1286	which includes simple expressions. Control-flow graphs are especially
				1287	useful for performing
				1288	<a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow-
				1289	or path-sensitive</a> program analyses on a given function.</p>
				1290
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	1291	<!-- ============ -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1292	<h4>Basic Blocks</h4>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	1293	<!-- ============ -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1294
				1295	<p>Concretely, an instance of <tt>CFG</tt> is a collection of basic
				1296	blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which
				1297	simply contains an ordered sequence of <tt>Stmt*</tt> (each referring
				1298	to statements in the AST). The ordering of statements within a block
				1299	indicates unconditional flow of control from one statement to the
				1300	next. <a href="#ConditionalControlFlow">Conditional control-flow</a>
				1301	is represented using edges between basic blocks. The statements
				1302	within a given <tt>CFGBlock</tt> can be traversed using
				1303	the <tt>CFGBlock::*iterator</tt> interface.</p>
				1304
				1305	<p>
Ted Kremenek	18e17e7	2007-10-18 22:50:52 +0000	[diff] [blame]	1306	A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1307	the control-flow graph it represents. Each <tt>CFGBlock</tt> within a
				1308	CFG is also uniquely numbered (accessible
				1309	via <tt>CFGBlock::getBlockID()</tt>). Currently the number is
				1310	based on the ordering the blocks were created, but no assumptions
				1311	should be made on how <tt>CFGBlock</tt>s are numbered other than their
				1312	numbers are unique and that they are numbered from 0..N-1 (where N is
				1313	the number of basic blocks in the CFG).</p>
				1314
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	1315	<!-- ===================== -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1316	<h4>Entry and Exit Blocks</h4>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	1317	<!-- ===================== -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1318
				1319	Each instance of <tt>CFG</tt> contains two special blocks:
				1320	an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which
				1321	has no incoming edges, and an <i>exit</i> block (accessible
				1322	via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither
				1323	block contains any statements, and they serve the role of providing a
				1324	clear entrance and exit for a body of code such as a function body.
				1325	The presence of these empty blocks greatly simplifies the
				1326	implementation of many analyses built on top of CFGs.
				1327
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	1328	<!-- ===================================================== -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1329	<h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4>
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	1330	<!-- ===================================================== -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1331
				1332	<p>Conditional control-flow (such as those induced by if-statements
				1333	and loops) is represented as edges between <tt>CFGBlock</tt>s.
				1334	Because different C language constructs can induce control-flow,
				1335	each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that
				1336	represents the <i>terminator</i> of the block. A terminator is simply
				1337	the statement that caused the control-flow, and is used to identify
				1338	the nature of the conditional control-flow between blocks. For
				1339	example, in the case of an if-statement, the terminator refers to
				1340	the <tt>IfStmt</tt> object in the AST that represented the given
				1341	branch.</p>
				1342
				1343	<p>To illustrate, consider the following code example:</p>
				1344
				1345	<code>
				1346	int foo(int x) {<br>
				1347	x = x + 1;<br>
				1348	<br>
				1349	if (x > 2) x++;<br>
				1350	else {<br>
				1351	x += 2;<br>
				1352	x *= 2;<br>
				1353	}<br>
				1354	<br>
				1355	return x;<br>
				1356	}
				1357	</code>
				1358
				1359	<p>After invoking the parser+semantic analyzer on this code fragment,
				1360	the AST of the body of <tt>foo</tt> is referenced by a
				1361	single <tt>Stmt*</tt>. We can then construct an instance
				1362	of <tt>CFG</tt> representing the control-flow graph of this function
				1363	body by single call to a static class method:</p>
				1364
				1365	<code>
				1366	Stmt* FooBody = ...<br>
				1367	CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody);
				1368	</code>
				1369
				1370	<p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt>
				1371	to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no
				1372	longer needed.</p>
				1373
				1374	<p>Along with providing an interface to iterate over
				1375	its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods
				1376	that are useful for debugging and visualizing CFGs. For example, the
				1377	method
				1378	<tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to
				1379	standard error. This is especially useful when one is using a
				1380	debugger such as gdb. For example, here is the output
				1381	of <tt>FooCFG->dump()</tt>:</p>
				1382
				1383	<code>
				1384	[ B5 (ENTRY) ]<br>
				1385	Predecessors (0):<br>
				1386	Successors (1): B4<br>
				1387	<br>
				1388	[ B4 ]<br>
				1389	1: x = x + 1<br>
				1390	2: (x > 2)<br>
				1391	<b>T: if [B4.2]</b><br>
				1392	Predecessors (1): B5<br>
				1393	Successors (2): B3 B2<br>
				1394	<br>
				1395	[ B3 ]<br>
				1396	1: x++<br>
				1397	Predecessors (1): B4<br>
				1398	Successors (1): B1<br>
				1399	<br>
				1400	[ B2 ]<br>
				1401	1: x += 2<br>
				1402	2: x *= 2<br>
				1403	Predecessors (1): B4<br>
				1404	Successors (1): B1<br>
				1405	<br>
				1406	[ B1 ]<br>
				1407	1: return x;<br>
				1408	Predecessors (2): B2 B3<br>
				1409	Successors (1): B0<br>
				1410	<br>
				1411	[ B0 (EXIT) ]<br>
				1412	Predecessors (1): B1<br>
				1413	Successors (0):
				1414	</code>
				1415
				1416	<p>For each block, the pretty-printed output displays for each block
				1417	the number of <i>predecessor</i> blocks (blocks that have outgoing
				1418	control-flow to the given block) and <i>successor</i> blocks (blocks
				1419	that have control-flow that have incoming control-flow from the given
				1420	block). We can also clearly see the special entry and exit blocks at
				1421	the beginning and end of the pretty-printed output. For the entry
				1422	block (block B5), the number of predecessor blocks is 0, while for the
				1423	exit block (block B0) the number of successor blocks is 0.</p>
				1424
				1425	<p>The most interesting block here is B4, whose outgoing control-flow
				1426	represents the branching caused by the sole if-statement
				1427	in <tt>foo</tt>. Of particular interest is the second statement in
				1428	the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed
				1429	as <b><tt>if [B4.2]</tt></b>. The second statement represents the
				1430	evaluation of the condition of the if-statement, which occurs before
				1431	the actual branching of control-flow. Within the <tt>CFGBlock</tt>
				1432	for B4, the <tt>Stmt*</tt> for the second statement refers to the
				1433	actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus
				1434	pointers to subclasses of <tt>Expr</tt> can appear in the list of
				1435	statements in a block, and not just subclasses of <tt>Stmt</tt> that
				1436	refer to proper C statements.</p>
				1437
				1438	<p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt>
				1439	object in the AST. The pretty-printer outputs <b><tt>if
				1440	[B4.2]</tt></b> because the condition expression of the if-statement
				1441	has an actual place in the basic block, and thus the terminator is
				1442	essentially
				1443	<i>referring</i> to the expression that is the second statement of
				1444	block B4 (i.e., B4.2). In this manner, conditions for control-flow
				1445	(which also includes conditions for loops and switch statements) are
				1446	hoisted into the actual basic block.</p>
				1447
Chris Lattner	62fd278	2008-11-22 21:41:31 +0000	[diff] [blame]	1448	<!-- ===================== -->
				1449	<!-- <h4>Implicit Control-Flow</h4> -->
				1450	<!-- ===================== -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	1451
				1452	<!--
				1453	<p>A key design principle of the <tt>CFG</tt> class was to not require
				1454	any transformations to the AST in order to represent control-flow.
				1455	Thus the <tt>CFG</tt> does not perform any "lowering" of the
				1456	statements in an AST: loops are not transformed into guarded gotos,
				1457	short-circuit operations are not converted to a set of if-statements,
				1458	and so on.</p>
				1459	-->
Ted Kremenek	17a295d	2008-06-11 06:19:49 +0000	[diff] [blame]	1460
Chris Lattner	7bad199	2008-11-16 21:48:07 +0000	[diff] [blame]	1461
				1462	<!-- ======================================================================= -->
				1463	<h3 id="Constants">Constant Folding in the Clang AST</h3>
				1464	<!-- ======================================================================= -->
				1465
				1466	<p>There are several places where constants and constant folding matter a lot to
				1467	the Clang front-end. First, in general, we prefer the AST to retain the source
				1468	code as close to how the user wrote it as possible. This means that if they
				1469	wrote "5+4", we want to keep the addition and two constants in the AST, we don't
				1470	want to fold to "9". This means that constant folding in various ways turns
				1471	into a tree walk that needs to handle the various cases.</p>
				1472
				1473	<p>However, there are places in both C and C++ that require constants to be
				1474	folded. For example, the C standard defines what an "integer constant
				1475	expression" (i-c-e) is with very precise and specific requirements. The
				1476	language then requires i-c-e's in a lot of places (for example, the size of a
				1477	bitfield, the value for a case statement, etc). For these, we have to be able
				1478	to constant fold the constants, to do semantic checks (e.g. verify bitfield size
				1479	is non-negative and that case statements aren't duplicated). We aim for Clang
				1480	to be very pedantic about this, diagnosing cases when the code does not use an
				1481	i-c-e where one is required, but accepting the code unless running with
				1482	<tt>-pedantic-errors</tt>.</p>
				1483
				1484	<p>Things get a little bit more tricky when it comes to compatibility with
				1485	real-world source code. Specifically, GCC has historically accepted a huge
				1486	superset of expressions as i-c-e's, and a lot of real world code depends on this
				1487	unfortuate accident of history (including, e.g., the glibc system headers). GCC
				1488	accepts anything its "fold" optimizer is capable of reducing to an integer
				1489	constant, which means that the definition of what it accepts changes as its
				1490	optimizer does. One example is that GCC accepts things like "case X-X:" even
				1491	when X is a variable, because it can fold this to 0.</p>
				1492
				1493	<p>Another issue are how constants interact with the extensions we support, such
				1494	as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99
				1495	obviously does not specify the semantics of any of these extensions, and the
				1496	definition of i-c-e does not include them. However, these extensions are often
				1497	used in real code, and we have to have a way to reason about them.</p>
				1498
				1499	<p>Finally, this is not just a problem for semantic analysis. The code
				1500	generator and other clients have to be able to fold constants (e.g. to
				1501	initialize global variables) and has to handle a superset of what C99 allows.
				1502	Further, these clients can benefit from extended information. For example, we
				1503	know that "foo()\|\|1" always evaluates to true, but we can't replace the
				1504	expression with true because it has side effects.</p>
				1505
				1506	<!-- ======================= -->
				1507	<h4>Implementation Approach</h4>
				1508	<!-- ======================= -->
				1509
				1510	<p>After trying several different approaches, we've finally converged on a
				1511	design (Note, at the time of this writing, not all of this has been implemented,
				1512	consider this a design goal!). Our basic approach is to define a single
				1513	recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is
				1514	implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar'
				1515	type (integer, fp, complex, or pointer) this method returns the following
				1516	information:</p>
				1517
				1518	<ul>
				1519	<li>Whether the expression is an integer constant expression, a general
				1520	constant that was folded but has no side effects, a general constant that
				1521	was folded but that does have side effects, or an uncomputable/unfoldable
				1522	value.
				1523	</li>
				1524	<li>If the expression was computable in any way, this method returns the APValue
				1525	for the result of the expression.</li>
				1526	<li>If the expression is not evaluatable at all, this method returns
				1527	information on one of the problems with the expression. This includes a
				1528	SourceLocation for where the problem is, and a diagnostic ID that explains
				1529	the problem. The diagnostic should be have ERROR type.</li>
				1530	<li>If the expression is not an integer constant expression, this method returns
				1531	information on one of the problems with the expression. This includes a
				1532	SourceLocation for where the problem is, and a diagnostic ID that explains
				1533	the problem. The diagnostic should be have EXTENSION type.</li>
				1534	</ul>
				1535
				1536	<p>This information gives various clients the flexibility that they want, and we
				1537	will eventually have some helper methods for various extensions. For example,
				1538	Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which
				1539	calls Evaluate on the expression. If the expression is not foldable, the error
				1540	is emitted, and it would return true. If the expression is not an i-c-e, the
				1541	EXTENSION diagnostic is emitted. Finally it would return false to indicate that
				1542	the AST is ok.</p>
				1543
				1544	<p>Other clients can use the information in other ways, for example, codegen can
				1545	just use expressions that are foldable in any way.</p>
				1546
				1547	<!-- ========== -->
				1548	<h4>Extensions</h4>
				1549	<!-- ========== -->
				1550
Chris Lattner	552de0a	2008-11-23 08:16:56 +0000	[diff] [blame]	1551	<p>This section describes how some of the various extensions Clang supports
Chris Lattner	7bad199	2008-11-16 21:48:07 +0000	[diff] [blame]	1552	interacts with constant evaluation:</p>
				1553
				1554	<ul>
				1555	<li><b><tt>__extension__</tt></b>: The expression form of this extension causes
				1556	any evaluatable subexpression to be accepted as an integer constant
				1557	expression.</li>
				1558	<li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer
Chris Lattner	28daa53	2008-12-12 06:55:44 +0000	[diff] [blame]	1559	constant expression) if the operand is any evaluatable constant. As a
				1560	special case, if <tt>__builtin_constant_p</tt> is the (potentially
				1561	parenthesized) condition of a conditional operator expression ("?:"), only
Chris Lattner	42b83dd	2008-12-12 18:00:51 +0000	[diff] [blame]	1562	the true side of the conditional operator is considered, and it is evaluated
				1563	with full constant folding.</li>
Chris Lattner	7bad199	2008-11-16 21:48:07 +0000	[diff] [blame]	1564	<li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an
				1565	integer constant expression, but we accept any constant as an "extension of
				1566	an extension". This only evaluates one operand depending on which way the
				1567	condition evaluates.</li>
				1568	<li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer
				1569	constant expression.</li>
				1570	<li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a
				1571	floating-point literal.</li>
				1572	<li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as
				1573	general constant expressions.</li>
				1574	</ul>
				1575
				1576
				1577
				1578
Ted Kremenek	17a295d	2008-06-11 06:19:49 +0000	[diff] [blame]	1579	</div>
				1580	</body>
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	1581	</html>