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