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Chris Lattner	86920d3	2007-07-31 05:42:17 +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]	12
				13	<h1>"clang" CFE Internals Manual</h1>
				14
				15	<ul>
				16	<li><a href="#intro">Introduction</a></li>
				17	<li><a href="#libsystem">LLVM System and Support Libraries</a></li>
				18	<li><a href="#libbasic">The clang 'Basic' Library</a>
				19	<ul>
				20	<li><a href="#SourceLocation">The SourceLocation and SourceManager
				21	classes</a></li>
				22	</ul>
				23	</li>
				24	<li><a href="#liblex">The Lexer and Preprocessor Library</a>
				25	<ul>
				26	<li><a href="#Token">The Token class</a></li>
				27	<li><a href="#Lexer">The Lexer class</a></li>
Chris Lattner	7928125	2008-03-09 02:27:26 +0000	[diff] [blame]	28	<li><a href="#TokenLexer">The TokenLexer class</a></li>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	29	<li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li>
				30	</ul>
				31	</li>
				32	<li><a href="#libparse">The Parser Library</a>
				33	<ul>
				34	</ul>
				35	</li>
				36	<li><a href="#libast">The AST Library</a>
				37	<ul>
				38	<li><a href="#Type">The Type class and its subclasses</a></li>
				39	<li><a href="#QualType">The QualType class</a></li>
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	40	<li><a href="#DeclarationName">Declaration names</a></li>
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	41	<li><a href="#CFG">The CFG class</a></li>
Chris Lattner	7bad199	2008-11-16 21:48:07 +0000	[diff] [blame]	42	<li><a href="#Constants">Constant Folding in the Clang AST</a></li>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	43	</ul>
				44	</li>
				45	</ul>
				46
				47
				48	<!-- ======================================================================= -->
				49	<h2 id="intro">Introduction</h2>
				50	<!-- ======================================================================= -->
				51
				52	<p>This document describes some of the more important APIs and internal design
				53	decisions made in the clang C front-end. The purpose of this document is to
				54	both capture some of this high level information and also describe some of the
				55	design decisions behind it. This is meant for people interested in hacking on
				56	clang, not for end-users. The description below is categorized by
				57	libraries, and does not describe any of the clients of the libraries.</p>
				58
				59	<!-- ======================================================================= -->
				60	<h2 id="libsystem">LLVM System and Support Libraries</h2>
				61	<!-- ======================================================================= -->
				62
				63	<p>The LLVM libsystem library provides the basic clang system abstraction layer,
				64	which is used for file system access. The LLVM libsupport library provides many
				65	underlying libraries and <a
				66	href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>,
				67	including command line option
				68	processing and various containers.</p>
				69
				70	<!-- ======================================================================= -->
				71	<h2 id="libbasic">The clang 'Basic' Library</h2>
				72	<!-- ======================================================================= -->
				73
				74	<p>This library certainly needs a better name. The 'basic' library contains a
				75	number of low-level utilities for tracking and manipulating source buffers,
				76	locations within the source buffers, diagnostics, tokens, target abstraction,
				77	and information about the subset of the language being compiled for.</p>
				78
				79	<p>Part of this infrastructure is specific to C (such as the TargetInfo class),
				80	other parts could be reused for other non-C-based languages (SourceLocation,
				81	SourceManager, Diagnostics, FileManager). When and if there is future demand
				82	we can figure out if it makes sense to introduce a new library, move the general
				83	classes somewhere else, or introduce some other solution.</p>
				84
				85	<p>We describe the roles of these classes in order of their dependencies.</p>
				86
				87	<!-- ======================================================================= -->
				88	<h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3>
				89	<!-- ======================================================================= -->
				90
				91	<p>Strangely enough, the SourceLocation class represents a location within the
				92	source code of the program. Important design points include:</p>
				93
				94	<ol>
				95	<li>sizeof(SourceLocation) must be extremely small, as these are embedded into
				96	many AST nodes and are passed around often. Currently it is 32 bits.</li>
				97	<li>SourceLocation must be a simple value object that can be efficiently
				98	copied.</li>
				99	<li>We should be able to represent a source location for any byte of any input
				100	file. This includes in the middle of tokens, in whitespace, in trigraphs,
				101	etc.</li>
				102	<li>A SourceLocation must encode the current #include stack that was active when
				103	the location was processed. For example, if the location corresponds to a
				104	token, it should contain the set of #includes active when the token was
				105	lexed. This allows us to print the #include stack for a diagnostic.</li>
				106	<li>SourceLocation must be able to describe macro expansions, capturing both
				107	the ultimate instantiation point and the source of the original character
				108	data.</li>
				109	</ol>
				110
				111	<p>In practice, the SourceLocation works together with the SourceManager class
				112	to encode two pieces of information about a location: it's physical location
				113	and it's virtual location. For most tokens, these will be the same. However,
				114	for a macro expansion (or tokens that came from a _Pragma directive) these will
				115	describe the location of the characters corresponding to the token and the
				116	location where the token was used (i.e. the macro instantiation point or the
				117	location of the _Pragma itself).</p>
				118
				119	<p>For efficiency, we only track one level of macro instantions: if a token was
				120	produced by multiple instantiations, we only track the source and ultimate
				121	destination. Though we could track the intermediate instantiation points, this
				122	would require extra bookkeeping and no known client would benefit substantially
				123	from this.</p>
				124
				125	<p>The clang front-end inherently depends on the location of a token being
				126	tracked correctly. If it is ever incorrect, the front-end may get confused and
				127	die. The reason for this is that the notion of the 'spelling' of a Token in
				128	clang depends on being able to find the original input characters for the token.
				129	This concept maps directly to the "physical" location for the token.</p>
				130
				131	<!-- ======================================================================= -->
				132	<h2 id="liblex">The Lexer and Preprocessor Library</h2>
				133	<!-- ======================================================================= -->
				134
				135	<p>The Lexer library contains several tightly-connected classes that are involved
				136	with the nasty process of lexing and preprocessing C source code. The main
				137	interface to this library for outside clients is the large <a
				138	href="#Preprocessor">Preprocessor</a> class.
				139	It contains the various pieces of state that are required to coherently read
				140	tokens out of a translation unit.</p>
				141
				142	<p>The core interface to the Preprocessor object (once it is set up) is the
				143	Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from
				144	the preprocessor stream. There are two types of token providers that the
				145	preprocessor is capable of reading from: a buffer lexer (provided by the <a
				146	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]	147	href="#TokenLexer">TokenLexer</a> class).
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	148
				149
				150	<!-- ======================================================================= -->
				151	<h3 id="Token">The Token class</h3>
				152	<!-- ======================================================================= -->
				153
				154	<p>The Token class is used to represent a single lexed token. Tokens are
				155	intended to be used by the lexer/preprocess and parser libraries, but are not
				156	intended to live beyond them (for example, they should not live in the ASTs).<p>
				157
				158	<p>Tokens most often live on the stack (or some other location that is efficient
				159	to access) as the parser is running, but occasionally do get buffered up. For
				160	example, macro definitions are stored as a series of tokens, and the C++
				161	front-end will eventually need to buffer tokens up for tentative parsing and
				162	various pieces of look-ahead. As such, the size of a Token matter. On a 32-bit
				163	system, sizeof(Token) is currently 16 bytes.</p>
				164
				165	<p>Tokens contain the following information:</p>
				166
				167	<ul>
				168	<li><b>A SourceLocation</b> - This indicates the location of the start of the
				169	token.</li>
				170
				171	<li><b>A length</b> - This stores the length of the token as stored in the
				172	SourceBuffer. For tokens that include them, this length includes trigraphs and
				173	escaped newlines which are ignored by later phases of the compiler. By pointing
				174	into the original source buffer, it is always possible to get the original
				175	spelling of a token completely accurately.</li>
				176
				177	<li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if
				178	identifier lookup was enabled when the token was lexed (e.g. the lexer was not
				179	reading in 'raw' mode) this contains a pointer to the unique hash value for the
				180	identifier. Because the lookup happens before keyword identification, this
				181	field is set even for language keywords like 'for'.</li>
				182
				183	<li><b>TokenKind</b> - This indicates the kind of token as classified by the
				184	lexer. This includes things like <tt>tok::starequal</tt> (for the "*="
				185	operator), <tt>tok::ampamp</tt> for the "&&" token, and keyword values
				186	(e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords. Note
				187	that some tokens can be spelled multiple ways. For example, C++ supports
				188	"operator keywords", where things like "and" are treated exactly like the
				189	"&&" operator. In these cases, the kind value is set to
				190	<tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to
				191	consider both forms. For something that cares about which form is used (e.g.
				192	the preprocessor 'stringize' operator) the spelling indicates the original
				193	form.</li>
				194
				195	<li><b>Flags</b> - There are currently four flags tracked by the
				196	lexer/preprocessor system on a per-token basis:
				197
				198	<ol>
				199	<li><b>StartOfLine</b> - This was the first token that occurred on its input
				200	source line.</li>
				201	<li><b>LeadingSpace</b> - There was a space character either immediately
				202	before the token or transitively before the token as it was expanded
				203	through a macro. The definition of this flag is very closely defined by
				204	the stringizing requirements of the preprocessor.</li>
				205	<li><b>DisableExpand</b> - This flag is used internally to the preprocessor to
				206	represent identifier tokens which have macro expansion disabled. This
				207	prevents them from being considered as candidates for macro expansion ever
				208	in the future.</li>
				209	<li><b>NeedsCleaning</b> - This flag is set if the original spelling for the
				210	token includes a trigraph or escaped newline. Since this is uncommon,
				211	many pieces of code can fast-path on tokens that did not need cleaning.
				212	</p>
				213	</ol>
				214	</li>
				215	</ul>
				216
				217	<p>One interesting (and somewhat unusual) aspect of tokens is that they don't
				218	contain any semantic information about the lexed value. For example, if the
				219	token was a pp-number token, we do not represent the value of the number that
				220	was lexed (this is left for later pieces of code to decide). Additionally, the
				221	lexer library has no notion of typedef names vs variable names: both are
				222	returned as identifiers, and the parser is left to decide whether a specific
				223	identifier is a typedef or a variable (tracking this requires scope information
				224	among other things).</p>
				225
				226	<!-- ======================================================================= -->
				227	<h3 id="Lexer">The Lexer class</h3>
				228	<!-- ======================================================================= -->
				229
				230	<p>The Lexer class provides the mechanics of lexing tokens out of a source
				231	buffer and deciding what they mean. The Lexer is complicated by the fact that
				232	it operates on raw buffers that have not had spelling eliminated (this is a
				233	necessity to get decent performance), but this is countered with careful coding
				234	as well as standard performance techniques (for example, the comment handling
				235	code is vectorized on X86 and PowerPC hosts).</p>
				236
				237	<p>The lexer has a couple of interesting modal features:</p>
				238
				239	<ul>
				240	<li>The lexer can operate in 'raw' mode. This mode has several features that
				241	make it possible to quickly lex the file (e.g. it stops identifier lookup,
				242	doesn't specially handle preprocessor tokens, handles EOF differently, etc).
				243	This mode is used for lexing within an "<tt>#if 0</tt>" block, for
				244	example.</li>
				245	<li>The lexer can capture and return comments as tokens. This is required to
				246	support the -C preprocessor mode, which passes comments through, and is
				247	used by the diagnostic checker to identifier expect-error annotations.</li>
				248	<li>The lexer can be in ParsingFilename mode, which happens when preprocessing
Chris Lattner	8438624	2007-09-16 19:25:23 +0000	[diff] [blame]	249	after reading a #include directive. This mode changes the parsing of '<'
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	250	to return an "angled string" instead of a bunch of tokens for each thing
				251	within the filename.</li>
				252	<li>When parsing a preprocessor directive (after "<tt>#</tt>") the
				253	ParsingPreprocessorDirective mode is entered. This changes the parser to
				254	return EOM at a newline.</li>
				255	<li>The Lexer uses a LangOptions object to know whether trigraphs are enabled,
				256	whether C++ or ObjC keywords are recognized, etc.</li>
				257	</ul>
				258
				259	<p>In addition to these modes, the lexer keeps track of a couple of other
				260	features that are local to a lexed buffer, which change as the buffer is
				261	lexed:</p>
				262
				263	<ul>
				264	<li>The Lexer uses BufferPtr to keep track of the current character being
				265	lexed.</li>
				266	<li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token
				267	will start with its "start of line" bit set.</li>
				268	<li>The Lexer keeps track of the current #if directives that are active (which
				269	can be nested).</li>
				270	<li>The Lexer keeps track of an <a href="#MultipleIncludeOpt">
				271	MultipleIncludeOpt</a> object, which is used to
				272	detect whether the buffer uses the standard "<tt>#ifndef XX</tt> /
				273	<tt>#define XX</tt>" idiom to prevent multiple inclusion. If a buffer does,
				274	subsequent includes can be ignored if the XX macro is defined.</li>
				275	</ul>
				276
				277	<!-- ======================================================================= -->
Chris Lattner	7928125	2008-03-09 02:27:26 +0000	[diff] [blame]	278	<h3 id="TokenLexer">The TokenLexer class</h3>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	279	<!-- ======================================================================= -->
				280
Chris Lattner	7928125	2008-03-09 02:27:26 +0000	[diff] [blame]	281	<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]	282	of tokens that came from somewhere else. It typically used for two things: 1)
				283	returning tokens from a macro definition as it is being expanded 2) returning
				284	tokens from an arbitrary buffer of tokens. The later use is used by _Pragma and
				285	will most likely be used to handle unbounded look-ahead for the C++ parser.</p>
				286
				287	<!-- ======================================================================= -->
				288	<h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3>
				289	<!-- ======================================================================= -->
				290
				291	<p>The MultipleIncludeOpt class implements a really simple little state machine
				292	that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>"
				293	idiom that people typically use to prevent multiple inclusion of headers. If a
				294	buffer uses this idiom and is subsequently #include'd, the preprocessor can
				295	simply check to see whether the guarding condition is defined or not. If so,
				296	the preprocessor can completely ignore the include of the header.</p>
				297
				298
				299
				300	<!-- ======================================================================= -->
				301	<h2 id="libparse">The Parser Library</h2>
				302	<!-- ======================================================================= -->
				303
				304	<!-- ======================================================================= -->
				305	<h2 id="libast">The AST Library</h2>
				306	<!-- ======================================================================= -->
				307
				308	<!-- ======================================================================= -->
				309	<h3 id="Type">The Type class and its subclasses</h3>
				310	<!-- ======================================================================= -->
				311
				312	<p>The Type class (and its subclasses) are an important part of the AST. Types
				313	are accessed through the ASTContext class, which implicitly creates and uniques
				314	them as they are needed. Types have a couple of non-obvious features: 1) they
				315	do not capture type qualifiers like const or volatile (See
				316	<a href="#QualType">QualType</a>), and 2) they implicitly capture typedef
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	317	information. Once created, types are immutable (unlike decls).</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	318
				319	<p>Typedefs in C make semantic analysis a bit more complex than it would
				320	be without them. The issue is that we want to capture typedef information
				321	and represent it in the AST perfectly, but the semantics of operations need to
				322	"see through" typedefs. For example, consider this code:</p>
				323
				324	<code>
				325	void func() {<br>
Bill Wendling	30d1775	2007-10-06 01:56:01 +0000	[diff] [blame]	326	typedef int foo;<br>
				327	foo X, *Y;<br>
				328	typedef foo* bar;<br>
				329	bar Z;<br>
				330	*X; <i>// error</i><br>
				331	**Y; <i>// error</i><br>
				332	**Z; <i>// error</i><br>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	333	}<br>
				334	</code>
				335
				336	<p>The code above is illegal, and thus we expect there to be diagnostics emitted
				337	on the annotated lines. In this example, we expect to get:</p>
				338
				339	<pre>
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	340	<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]	341	*X; // error
				342	<font color="blue">^~</font>
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	343	<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]	344	**Y; // error
				345	<font color="blue">^~~</font>
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	346	<b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b>
				347	**Z; // error
				348	<font color="blue">^~~</font>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	349	</pre>
				350
				351	<p>While this example is somewhat silly, it illustrates the point: we want to
				352	retain typedef information where possible, so that we can emit errors about
				353	"<tt>std::string</tt>" instead of "<tt>std::basic_string<char, std:...</tt>".
				354	Doing this requires properly keeping typedef information (for example, the type
				355	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]	356	various operators (for example, the type of *Y is "foo", not "int"). In order
				357	to retain this information, the type of these expressions is an instance of the
				358	TypedefType class, which indicates that the type of these expressions is a
				359	typedef for foo.
				360	</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	361
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	362	<p>Representing types like this is great for diagnostics, because the
				363	user-specified type is always immediately available. There are two problems
				364	with this: first, various semantic checks need to make judgements about the
Chris Lattner	33fc68a	2007-07-31 18:54:50 +0000	[diff] [blame]	365	<em>actual structure</em> of a type, ignoring typdefs. Second, we need an
				366	efficient way to query whether two types are structurally identical to each
				367	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]	368	canonical types.</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	369
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	370	<h4>Canonical Types</h4>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	371
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	372	<p>Every instance of the Type class contains a canonical type pointer. For
				373	simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>",
				374	"<tt>int**</tt>"), the type just points to itself. For types that have a
				375	typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>",
				376	"<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their
				377	structurally equivalent type without any typedefs (e.g. "<tt>int</tt>",
				378	"<tt>int</tt>", "<tt>int</tt>", and "<tt>int</tt>" respectively).</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	379
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	380	<p>This design provides a constant time operation (dereferencing the canonical
				381	type pointer) that gives us access to the structure of types. For example,
				382	we can trivially tell that "bar" and "foo*" are the same type by dereferencing
				383	their canonical type pointers and doing a pointer comparison (they both point
				384	to the single "<tt>int*</tt>" type).</p>
				385
				386	<p>Canonical types and typedef types bring up some complexities that must be
				387	carefully managed. Specifically, the "isa/cast/dyncast" operators generally
				388	shouldn't be used in code that is inspecting the AST. For example, when type
				389	checking the indirection operator (unary '*' on a pointer), the type checker
				390	must verify that the operand has a pointer type. It would not be correct to
				391	check that with "<tt>isa<PointerType>(SubExpr->getType())</tt>",
				392	because this predicate would fail if the subexpression had a typedef type.</p>
				393
				394	<p>The solution to this problem are a set of helper methods on Type, used to
				395	check their properties. In this case, it would be correct to use
				396	"<tt>SubExpr->getType()->isPointerType()</tt>" to do the check. This
				397	predicate will return true if the <em>canonical type is a pointer</em>, which is
				398	true any time the type is structurally a pointer type. The only hard part here
				399	is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p>
				400
				401	<p>The second problem we face is how to get access to the pointer type once we
				402	know it exists. To continue the example, the result type of the indirection
				403	operator is the pointee type of the subexpression. In order to determine the
				404	type, we need to get the instance of PointerType that best captures the typedef
				405	information in the program. If the type of the expression is literally a
				406	PointerType, we can return that, otherwise we have to dig through the
				407	typedefs to find the pointer type. For example, if the subexpression had type
				408	"<tt>foo*</tt>", we could return that type as the result. If the subexpression
				409	had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do
				410	<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]	411	a getAsPointerType() method that checks whether the type is structurally a
Chris Lattner	8a2bc62	2007-07-31 06:37:39 +0000	[diff] [blame]	412	PointerType and, if so, returns the best one. If not, it returns a null
				413	pointer.</p>
				414
				415	<p>This structure is somewhat mystical, but after meditating on it, it will
				416	make sense to you :).</p>
Chris Lattner	86920d3	2007-07-31 05:42:17 +0000	[diff] [blame]	417
				418	<!-- ======================================================================= -->
				419	<h3 id="QualType">The QualType class</h3>
				420	<!-- ======================================================================= -->
				421
				422	<p>The QualType class is designed as a trivial value class that is small,
				423	passed by-value and is efficient to query. The idea of QualType is that it
				424	stores the type qualifiers (const, volatile, restrict) separately from the types
				425	themselves: QualType is conceptually a pair of "Type*" and bits for the type
				426	qualifiers.</p>
				427
				428	<p>By storing the type qualifiers as bits in the conceptual pair, it is
				429	extremely efficient to get the set of qualifiers on a QualType (just return the
				430	field of the pair), add a type qualifier (which is a trivial constant-time
				431	operation that sets a bit), and remove one or more type qualifiers (just return
				432	a QualType with the bitfield set to empty).</p>
				433
				434	<p>Further, because the bits are stored outside of the type itself, we do not
				435	need to create duplicates of types with different sets of qualifiers (i.e. there
				436	is only a single heap allocated "int" type: "const int" and "volatile const int"
				437	both point to the same heap allocated "int" type). This reduces the heap size
				438	used to represent bits and also means we do not have to consider qualifiers when
				439	uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p>
				440
				441	<p>In practice, on hosts where it is safe, the 3 type qualifiers are stored in
				442	the low bit of the pointer to the Type object. This means that QualType is
				443	exactly the same size as a pointer, and this works fine on any system where
				444	malloc'd objects are at least 8 byte aligned.</p>
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	445
				446	<!-- ======================================================================= -->
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	447	<h3 id="DeclarationName">Declaration names</h3>
				448	<!-- ======================================================================= -->
				449
				450	<p>The <tt>DeclarationName</tt> class represents the name of a
				451	declaration in Clang. Declarations in the C family of languages can
				452	take several different forms. Most declarations are named by are
				453	simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in
				454	the function declaration <code>f(int x)</code>. In C++, declaration
				455	names can also name class constructors ("<code>Class</code>"
				456	in <code>struct Class { Class(); }</code>), class destructors
				457	("<code>~Class</code>"), overloaded operator names ("operator+"),
				458	and conversion functions ("<code>operator void const *</code>"). In
				459	Objective-C, declaration names can refer to the names of Objective-C
				460	methods, which involve the method name and the parameters,
				461	collectively called a <i>selector</i>, e.g..,
				462	"<code>setWidth:height:</code>". Since all of these kinds of
				463	entities--variables, functions, Objective-C methods, C++
				464	constructors, destructors, and operators---are represented as
				465	subclasses of Clang's common <code>NamedDecl</code>
				466	class, <code>DeclarationName</code> is designed to efficiently
				467	represent any kind of name.</p>
				468
				469	<p>Given
				470	a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code>
Douglas Gregor	2def483	2008-11-17 20:34:05 +0000	[diff] [blame^]	471	will produce a value that describes what kind of name <code>N</code>
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	472	stores. There are 7 options (all of the names are inside
				473	the <code>DeclarationName</code> class)</p>
				474	<dl>
				475	<dt>Identifier</dt>
				476	<dd>The name is a simple
				477	identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the
				478	corresponding <code>IdentifierInfo*</code> pointing to the actual
				479	identifier. Note that C++ overloaded operators (e.g.,
				480	"<code>operator+</code>") are represented as special kinds of
				481	identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code>
				482	function to determine whether an identifier is an overloaded
				483	operator name.</dd>
				484
				485	<dt>ObjCZeroArgSelector, ObjCOneArgSelector,
				486	ObjCMultiArgSelector</dt>
				487	<dd>The name is an Objective-C selector, which can be retrieved as a
				488	<code>Selector</code> instance
				489	via <code>N.getObjCSelector()</code>. The three possible name
				490	kinds for Objective-C reflect an optimization within
				491	the <code>DeclarationName</code> class: both zero- and
				492	one-argument selectors are stored as a
				493	masked <code>IdentifierInfo</code> pointer, and therefore require
				494	very little space, since zero- and one-argument selectors are far
				495	more common than multi-argument selectors (which use a different
				496	structure).</dd>
				497
				498	<dt>CXXConstructorName</dt>
				499	<dd>The name is a C++ constructor
				500	name. Use <code>N.getCXXNameType()</code> to retrieve
				501	the <a href="#QualType">type</a> that this constructor is meant to
				502	construct. The type is always the canonical type, since all
				503	constructors for a given type have the same name.</dd>
				504
				505	<dt>CXXDestructorName</dt>
				506	<dd>The name is a C++ destructor
				507	name. Use <code>N.getCXXNameType()</code> to retrieve
				508	the <a href="#QualType">type</a> whose destructor is being
				509	named. This type is always a canonical type.</dd>
				510
				511	<dt>CXXConversionFunctionName</dt>
				512	<dd>The name is a C++ conversion function. Conversion functions are
				513	named according to the type they convert to, e.g., "<code>operator void
				514	const *</code>". Use <code>N.getCXXNameType()</code> to retrieve
				515	the type that this conversion function converts to. This type is
				516	always a canonical type.</dd>
				517	</dl>
				518
				519	<p><code>DeclarationName</code>s are cheap to create, copy, and
				520	compare. They require only a single pointer's worth of storage in
				521	the common cases (identifiers, C++ overloaded operator names, zero-
				522	and one-argument Objective-C selectors) and use dense, uniqued
				523	storage for the other kinds of
				524	names. Two <code>DeclarationName</code>s can be compared for
				525	equality (<code>==</code>, <code>!=</code>) using a simple bitwise
				526	comparison, can be ordered
				527	with <code><</code>, <code>></code>, <code><=</code>,
				528	and <code>>=</code> (which provide a lexicographical ordering for
				529	normal identifiers but an unspecified ordering for other kinds of
				530	names), and can be placed into LLVM <code>DenseMap</code>s
				531	and <code>DenseSet</code>s.</p>
				532
				533	<p><code>DeclarationName</code> instances can be created in different
				534	ways depending on what kind of name the instance will store. Normal
				535	identifiers (<code>IdentifierInfo</code> pointers), including
				536	overloaded operator names, and Objective-C selectors
				537	(<code>Selector</code>) can be implicitly converted
				538	to <code>DeclarationName</code>s. Names for C++ constructors,
				539	destructors, and conversion functions can be retrieved from
				540	the <code>DeclarationNameTable</code>, an instance of which is
				541	available as <code>ASTContext::DeclarationNames</code>. The member
				542	functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>,
				543	and <code>getCXXConversionFunctionName</code>, respectively,
				544	return <code>DeclarationName</code> instances for the three kinds of
				545	C++ special function names.</p>
				546
				547	<!-- ======================================================================= -->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	548	<h3 id="CFG">The <tt>CFG</tt> class</h3>
				549	<!-- ======================================================================= -->
				550
				551	<p>The <tt>CFG</tt> class is designed to represent a source-level
				552	control-flow graph for a single statement (<tt>Stmt*</tt>). Typically
				553	instances of <tt>CFG</tt> are constructed for function bodies (usually
				554	an instance of <tt>CompoundStmt</tt>), but can also be instantiated to
				555	represent the control-flow of any class that subclasses <tt>Stmt</tt>,
				556	which includes simple expressions. Control-flow graphs are especially
				557	useful for performing
				558	<a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow-
				559	or path-sensitive</a> program analyses on a given function.</p>
				560
				561	<h4>Basic Blocks</h4>
				562
				563	<p>Concretely, an instance of <tt>CFG</tt> is a collection of basic
				564	blocks. Each basic block is an instance of <tt>CFGBlock</tt>, which
				565	simply contains an ordered sequence of <tt>Stmt*</tt> (each referring
				566	to statements in the AST). The ordering of statements within a block
				567	indicates unconditional flow of control from one statement to the
				568	next. <a href="#ConditionalControlFlow">Conditional control-flow</a>
				569	is represented using edges between basic blocks. The statements
				570	within a given <tt>CFGBlock</tt> can be traversed using
				571	the <tt>CFGBlock::*iterator</tt> interface.</p>
				572
				573	<p>
Ted Kremenek	18e17e7	2007-10-18 22:50:52 +0000	[diff] [blame]	574	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]	575	the control-flow graph it represents. Each <tt>CFGBlock</tt> within a
				576	CFG is also uniquely numbered (accessible
				577	via <tt>CFGBlock::getBlockID()</tt>). Currently the number is
				578	based on the ordering the blocks were created, but no assumptions
				579	should be made on how <tt>CFGBlock</tt>s are numbered other than their
				580	numbers are unique and that they are numbered from 0..N-1 (where N is
				581	the number of basic blocks in the CFG).</p>
				582
				583	<h4>Entry and Exit Blocks</h4>
				584
				585	Each instance of <tt>CFG</tt> contains two special blocks:
				586	an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which
				587	has no incoming edges, and an <i>exit</i> block (accessible
				588	via <tt>CFG::getExit()</tt>), which has no outgoing edges. Neither
				589	block contains any statements, and they serve the role of providing a
				590	clear entrance and exit for a body of code such as a function body.
				591	The presence of these empty blocks greatly simplifies the
				592	implementation of many analyses built on top of CFGs.
				593
				594	<h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4>
				595
				596	<p>Conditional control-flow (such as those induced by if-statements
				597	and loops) is represented as edges between <tt>CFGBlock</tt>s.
				598	Because different C language constructs can induce control-flow,
				599	each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that
				600	represents the <i>terminator</i> of the block. A terminator is simply
				601	the statement that caused the control-flow, and is used to identify
				602	the nature of the conditional control-flow between blocks. For
				603	example, in the case of an if-statement, the terminator refers to
				604	the <tt>IfStmt</tt> object in the AST that represented the given
				605	branch.</p>
				606
				607	<p>To illustrate, consider the following code example:</p>
				608
				609	<code>
				610	int foo(int x) {<br>
				611	x = x + 1;<br>
				612	<br>
				613	if (x > 2) x++;<br>
				614	else {<br>
				615	x += 2;<br>
				616	x *= 2;<br>
				617	}<br>
				618	<br>
				619	return x;<br>
				620	}
				621	</code>
				622
				623	<p>After invoking the parser+semantic analyzer on this code fragment,
				624	the AST of the body of <tt>foo</tt> is referenced by a
				625	single <tt>Stmt*</tt>. We can then construct an instance
				626	of <tt>CFG</tt> representing the control-flow graph of this function
				627	body by single call to a static class method:</p>
				628
				629	<code>
				630	Stmt* FooBody = ...<br>
				631	CFG* FooCFG = <b>CFG::buildCFG</b>(FooBody);
				632	</code>
				633
				634	<p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt>
				635	to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no
				636	longer needed.</p>
				637
				638	<p>Along with providing an interface to iterate over
				639	its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods
				640	that are useful for debugging and visualizing CFGs. For example, the
				641	method
				642	<tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to
				643	standard error. This is especially useful when one is using a
				644	debugger such as gdb. For example, here is the output
				645	of <tt>FooCFG->dump()</tt>:</p>
				646
				647	<code>
				648	[ B5 (ENTRY) ]<br>
				649	Predecessors (0):<br>
				650	Successors (1): B4<br>
				651	<br>
				652	[ B4 ]<br>
				653	1: x = x + 1<br>
				654	2: (x > 2)<br>
				655	<b>T: if [B4.2]</b><br>
				656	Predecessors (1): B5<br>
				657	Successors (2): B3 B2<br>
				658	<br>
				659	[ B3 ]<br>
				660	1: x++<br>
				661	Predecessors (1): B4<br>
				662	Successors (1): B1<br>
				663	<br>
				664	[ B2 ]<br>
				665	1: x += 2<br>
				666	2: x *= 2<br>
				667	Predecessors (1): B4<br>
				668	Successors (1): B1<br>
				669	<br>
				670	[ B1 ]<br>
				671	1: return x;<br>
				672	Predecessors (2): B2 B3<br>
				673	Successors (1): B0<br>
				674	<br>
				675	[ B0 (EXIT) ]<br>
				676	Predecessors (1): B1<br>
				677	Successors (0):
				678	</code>
				679
				680	<p>For each block, the pretty-printed output displays for each block
				681	the number of <i>predecessor</i> blocks (blocks that have outgoing
				682	control-flow to the given block) and <i>successor</i> blocks (blocks
				683	that have control-flow that have incoming control-flow from the given
				684	block). We can also clearly see the special entry and exit blocks at
				685	the beginning and end of the pretty-printed output. For the entry
				686	block (block B5), the number of predecessor blocks is 0, while for the
				687	exit block (block B0) the number of successor blocks is 0.</p>
				688
				689	<p>The most interesting block here is B4, whose outgoing control-flow
				690	represents the branching caused by the sole if-statement
				691	in <tt>foo</tt>. Of particular interest is the second statement in
				692	the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed
				693	as <b><tt>if [B4.2]</tt></b>. The second statement represents the
				694	evaluation of the condition of the if-statement, which occurs before
				695	the actual branching of control-flow. Within the <tt>CFGBlock</tt>
				696	for B4, the <tt>Stmt*</tt> for the second statement refers to the
				697	actual expression in the AST for <b><tt>(x > 2)</tt></b>. Thus
				698	pointers to subclasses of <tt>Expr</tt> can appear in the list of
				699	statements in a block, and not just subclasses of <tt>Stmt</tt> that
				700	refer to proper C statements.</p>
				701
				702	<p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt>
				703	object in the AST. The pretty-printer outputs <b><tt>if
				704	[B4.2]</tt></b> because the condition expression of the if-statement
				705	has an actual place in the basic block, and thus the terminator is
				706	essentially
				707	<i>referring</i> to the expression that is the second statement of
				708	block B4 (i.e., B4.2). In this manner, conditions for control-flow
				709	(which also includes conditions for loops and switch statements) are
				710	hoisted into the actual basic block.</p>
				711
Ted Kremenek	98f19b6	2007-10-10 23:22:00 +0000	[diff] [blame]	712	<!--
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	713	<h4>Implicit Control-Flow</h4>
Ted Kremenek	98f19b6	2007-10-10 23:22:00 +0000	[diff] [blame]	714	-->
Ted Kremenek	8bc0571	2007-10-10 23:01:43 +0000	[diff] [blame]	715
				716	<!--
				717	<p>A key design principle of the <tt>CFG</tt> class was to not require
				718	any transformations to the AST in order to represent control-flow.
				719	Thus the <tt>CFG</tt> does not perform any "lowering" of the
				720	statements in an AST: loops are not transformed into guarded gotos,
				721	short-circuit operations are not converted to a set of if-statements,
				722	and so on.</p>
				723	-->
Ted Kremenek	17a295d	2008-06-11 06:19:49 +0000	[diff] [blame]	724
Chris Lattner	7bad199	2008-11-16 21:48:07 +0000	[diff] [blame]	725
				726	<!-- ======================================================================= -->
				727	<h3 id="Constants">Constant Folding in the Clang AST</h3>
				728	<!-- ======================================================================= -->
				729
				730	<p>There are several places where constants and constant folding matter a lot to
				731	the Clang front-end. First, in general, we prefer the AST to retain the source
				732	code as close to how the user wrote it as possible. This means that if they
				733	wrote "5+4", we want to keep the addition and two constants in the AST, we don't
				734	want to fold to "9". This means that constant folding in various ways turns
				735	into a tree walk that needs to handle the various cases.</p>
				736
				737	<p>However, there are places in both C and C++ that require constants to be
				738	folded. For example, the C standard defines what an "integer constant
				739	expression" (i-c-e) is with very precise and specific requirements. The
				740	language then requires i-c-e's in a lot of places (for example, the size of a
				741	bitfield, the value for a case statement, etc). For these, we have to be able
				742	to constant fold the constants, to do semantic checks (e.g. verify bitfield size
				743	is non-negative and that case statements aren't duplicated). We aim for Clang
				744	to be very pedantic about this, diagnosing cases when the code does not use an
				745	i-c-e where one is required, but accepting the code unless running with
				746	<tt>-pedantic-errors</tt>.</p>
				747
				748	<p>Things get a little bit more tricky when it comes to compatibility with
				749	real-world source code. Specifically, GCC has historically accepted a huge
				750	superset of expressions as i-c-e's, and a lot of real world code depends on this
				751	unfortuate accident of history (including, e.g., the glibc system headers). GCC
				752	accepts anything its "fold" optimizer is capable of reducing to an integer
				753	constant, which means that the definition of what it accepts changes as its
				754	optimizer does. One example is that GCC accepts things like "case X-X:" even
				755	when X is a variable, because it can fold this to 0.</p>
				756
				757	<p>Another issue are how constants interact with the extensions we support, such
				758	as __builtin_constant_p, __builtin_inf, __extension__ and many others. C99
				759	obviously does not specify the semantics of any of these extensions, and the
				760	definition of i-c-e does not include them. However, these extensions are often
				761	used in real code, and we have to have a way to reason about them.</p>
				762
				763	<p>Finally, this is not just a problem for semantic analysis. The code
				764	generator and other clients have to be able to fold constants (e.g. to
				765	initialize global variables) and has to handle a superset of what C99 allows.
				766	Further, these clients can benefit from extended information. For example, we
				767	know that "foo()\|\|1" always evaluates to true, but we can't replace the
				768	expression with true because it has side effects.</p>
				769
				770	<!-- ======================= -->
				771	<h4>Implementation Approach</h4>
				772	<!-- ======================= -->
				773
				774	<p>After trying several different approaches, we've finally converged on a
				775	design (Note, at the time of this writing, not all of this has been implemented,
				776	consider this a design goal!). Our basic approach is to define a single
				777	recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is
				778	implemented in <tt>AST/ExprConstant.cpp</tt>. Given an expression with 'scalar'
				779	type (integer, fp, complex, or pointer) this method returns the following
				780	information:</p>
				781
				782	<ul>
				783	<li>Whether the expression is an integer constant expression, a general
				784	constant that was folded but has no side effects, a general constant that
				785	was folded but that does have side effects, or an uncomputable/unfoldable
				786	value.
				787	</li>
				788	<li>If the expression was computable in any way, this method returns the APValue
				789	for the result of the expression.</li>
				790	<li>If the expression is not evaluatable at all, this method returns
				791	information on one of the problems with the expression. This includes a
				792	SourceLocation for where the problem is, and a diagnostic ID that explains
				793	the problem. The diagnostic should be have ERROR type.</li>
				794	<li>If the expression is not an integer constant expression, this method returns
				795	information on one of the problems with the expression. This includes a
				796	SourceLocation for where the problem is, and a diagnostic ID that explains
				797	the problem. The diagnostic should be have EXTENSION type.</li>
				798	</ul>
				799
				800	<p>This information gives various clients the flexibility that they want, and we
				801	will eventually have some helper methods for various extensions. For example,
				802	Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which
				803	calls Evaluate on the expression. If the expression is not foldable, the error
				804	is emitted, and it would return true. If the expression is not an i-c-e, the
				805	EXTENSION diagnostic is emitted. Finally it would return false to indicate that
				806	the AST is ok.</p>
				807
				808	<p>Other clients can use the information in other ways, for example, codegen can
				809	just use expressions that are foldable in any way.</p>
				810
				811	<!-- ========== -->
				812	<h4>Extensions</h4>
				813	<!-- ========== -->
				814
				815	<p>This section describes how some of the various extensions clang supports
				816	interacts with constant evaluation:</p>
				817
				818	<ul>
				819	<li><b><tt>__extension__</tt></b>: The expression form of this extension causes
				820	any evaluatable subexpression to be accepted as an integer constant
				821	expression.</li>
				822	<li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer
				823	constant expression) if the operand is any evaluatable constant.</li>
				824	<li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an
				825	integer constant expression, but we accept any constant as an "extension of
				826	an extension". This only evaluates one operand depending on which way the
				827	condition evaluates.</li>
				828	<li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer
				829	constant expression.</li>
				830	<li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a
				831	floating-point literal.</li>
				832	<li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as
				833	general constant expressions.</li>
				834	</ul>
				835
				836
				837
				838
Ted Kremenek	17a295d	2008-06-11 06:19:49 +0000	[diff] [blame]	839	</div>
				840	</body>
Douglas Gregor	2e1cd42	2008-11-17 14:58:09 +0000	[diff] [blame]	841	</html>