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