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 <h1>"Clang" CFE Internals Manual</h1>

 <ul>
 <li><a href="#intro">Introduction</a></li>
 <li><a href="#libsystem">LLVM System and Support Libraries</a></li>
 <li><a href="#libbasic">The Clang 'Basic' Library</a>
   <ul>
   <li><a href="#Diagnostics">The Diagnostics Subsystem</a></li>
   <li><a href="#SourceLocation">The SourceLocation and SourceManager
       classes</a></li>
   </ul>
 </li>
 <li><a href="#libdriver">The Driver Library</a>
   <ul>
   </ul>
 </li>
 <li><a href="#pch">Precompiled Headers</a>
 <li><a href="#libfrontend">The Frontend Library</a>
   <ul>
   </ul>
 </li>
 <li><a href="#liblex">The Lexer and Preprocessor Library</a>
   <ul>
   <li><a href="#Token">The Token class</a></li>
   <li><a href="#Lexer">The Lexer class</a></li>
   <li><a href="#AnnotationToken">Annotation Tokens</a></li>
   <li><a href="#TokenLexer">The TokenLexer class</a></li>
   <li><a href="#MultipleIncludeOpt">The MultipleIncludeOpt class</a></li>
   </ul>
 </li>
 <li><a href="#libparse">The Parser Library</a>
   <ul>
   </ul>
 </li>
 <li><a href="#libast">The AST Library</a>
   <ul>
   <li><a href="#Type">The Type class and its subclasses</a></li>
   <li><a href="#QualType">The QualType class</a></li>
   <li><a href="#DeclarationName">Declaration names</a></li>
   <li><a href="#DeclContext">Declaration contexts</a>
     <ul>
       <li><a href="#Redeclarations">Redeclarations and Overloads</a></li>
       <li><a href="#LexicalAndSemanticContexts">Lexical and Semantic
       Contexts</a></li>
       <li><a href="#TransparentContexts">Transparent Declaration Contexts</a></li>
       <li><a href="#MultiDeclContext">Multiply-Defined Declaration Contexts</a></li>
     </ul>
   </li>
   <li><a href="#CFG">The CFG class</a></li>
   <li><a href="#Constants">Constant Folding in the Clang AST</a></li>
   </ul>
 </li>
 <li><a href="libIndex.html">The Index Library</a></li>
 </ul>


 <!-- ======================================================================= -->
 <h2 id="intro">Introduction</h2>
 <!-- ======================================================================= -->

 <p>This document describes some of the more important APIs and internal design
 decisions made in the Clang C front-end.  The purpose of this document is to
 both capture some of this high level information and also describe some of the
 design decisions behind it.  This is meant for people interested in hacking on
 Clang, not for end-users.  The description below is categorized by
 libraries, and does not describe any of the clients of the libraries.</p>

 <!-- ======================================================================= -->
 <h2 id="libsystem">LLVM System and Support Libraries</h2>
 <!-- ======================================================================= -->

 <p>The LLVM libsystem library provides the basic Clang system abstraction layer,
 which is used for file system access.  The LLVM libsupport library provides many
 underlying libraries and <a
 href="http://llvm.org/docs/ProgrammersManual.html">data-structures</a>,
  including command line option
 processing and various containers.</p>

 <!-- ======================================================================= -->
 <h2 id="libbasic">The Clang 'Basic' Library</h2>
 <!-- ======================================================================= -->

 <p>This library certainly needs a better name.  The 'basic' library contains a
 number of low-level utilities for tracking and manipulating source buffers,
 locations within the source buffers, diagnostics, tokens, target abstraction,
 and information about the subset of the language being compiled for.</p>

 <p>Part of this infrastructure is specific to C (such as the TargetInfo class),
 other parts could be reused for other non-C-based languages (SourceLocation,
 SourceManager, Diagnostics, FileManager).  When and if there is future demand
 we can figure out if it makes sense to introduce a new library, move the general
 classes somewhere else, or introduce some other solution.</p>

 <p>We describe the roles of these classes in order of their dependencies.</p>


 <!-- ======================================================================= -->
 <h3 id="Diagnostics">The Diagnostics Subsystem</h3>
 <!-- ======================================================================= -->

 <p>The Clang Diagnostics subsystem is an important part of how the compiler
 communicates with the human.  Diagnostics are the warnings and errors produced
 when the code is incorrect or dubious.  In Clang, each diagnostic produced has
 (at the minimum) a unique ID, a <a href="#SourceLocation">SourceLocation</a> to
 "put the caret", an English translation associated with it, and a severity (e.g.
 <tt>WARNING</tt> or <tt>ERROR</tt>).  They can also optionally include a number
 of arguments to the dianostic (which fill in "%0"'s in the string) as well as a
 number of source ranges that related to the diagnostic.</p>

 <p>In this section, we'll be giving examples produced by the Clang command line
 driver, but diagnostics can be <a href="#DiagnosticClient">rendered in many
 different ways</a> depending on how the DiagnosticClient interface is
 implemented.  A representative example of a diagonstic is:</p>

 <pre>
 t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
    <font color="darkgreen">P = (P-42) + Gamma*4;</font>
        <font color="blue">~~~~~~ ^ ~~~~~~~</font>
 </pre>

 <p>In this example, you can see the English translation, the severity (error),
 you can see the source location (the caret ("^") and file/line/column info),
 the source ranges "~~~~", arguments to the diagnostic ("int*" and "_Complex
 float").  You'll have to believe me that there is a unique ID backing the
 diagnostic :).</p>

 <p>Getting all of this to happen has several steps and involves many moving
 pieces, this section describes them and talks about best practices when adding
 a new diagnostic.</p>

 <!-- ============================== -->
 <h4>The Diagnostic*Kinds.def files</h4>
 <!-- ============================== -->

 <p>Diagnostics are created by adding an entry to one of the <tt>
 clang/Basic/Diagnostic*Kinds.def</tt> files, depending on what library will
 be using it.  This file encodes the unique ID of the
 diagnostic (as an enum, the first argument), the severity of the diagnostic
 (second argument) and the English translation + format string.</p>

 <p>There is little sanity with the naming of the unique ID's right now.  Some
 start with err_, warn_, ext_ to encode the severity into the name.  Since the
 enum is referenced in the C++ code that produces the diagnostic, it is somewhat
 useful for it to be reasonably short.</p>

 <p>The severity of the diagnostic comes from the set {<tt>NOTE</tt>,
 <tt>WARNING</tt>, <tt>EXTENSION</tt>, <tt>EXTWARN</tt>, <tt>ERROR</tt>}.  The
 <tt>ERROR</tt> severity is used for diagnostics indicating the program is never
 acceptable under any circumstances.  When an error is emitted, the AST for the
 input code may not be fully built.  The <tt>EXTENSION</tt> and <tt>EXTWARN</tt>
 severities are used for extensions to the language that Clang accepts.  This
 means that Clang fully understands and can represent them in the AST, but we
 produce diagnostics to tell the user their code is non-portable.  The difference
 is that the former are ignored by default, and the later warn by default.  The
 <tt>WARNING</tt> severity is used for constructs that are valid in the currently
 selected source language but that are dubious in some way.  The <tt>NOTE</tt>
 level is used to staple more information onto previous diagnostics.</p>

 <p>These <em>severities</em> are mapped into a smaller set (the
 Diagnostic::Level enum, {<tt>Ignored</tt>, <tt>Note</tt>, <tt>Warning</tt>,
 <tt>Error</tt>, <tt>Fatal</tt> }) of output <em>levels</em> by the diagnostics
 subsystem based on various configuration options.  Clang internally supports a
 fully fine grained mapping mechanism that allows you to map almost any
 diagnostic to the output level that you want.  The only diagnostics that cannot
 be mapped are <tt>NOTE</tt>s, which always follow the severity of the previously
 emitted diagnostic and <tt>ERROR</tt>s, which can only be mapped to
 <tt>Fatal</tt> (it is not possible to turn an error into a warning,
 for example).</p>

 <p>Diagnostic mappings are used in many ways.  For example, if the user
 specifies <tt>-pedantic</tt>, <tt>EXTENSION</tt> maps to <tt>Warning</tt>, if
 they specify <tt>-pedantic-errors</tt>, it turns into <tt>Error</tt>.  This is
 used to implement options like <tt>-Wunused_macros</tt>, <tt>-Wundef</tt> etc.
 </p>

 <p>
 Mapping to <tt>Fatal</tt> should only be used for diagnostics that are
 considered so severe that error recovery won't be able to recover sensibly from
 them (thus spewing a ton of bogus errors).  One example of this class of error
 are failure to #include a file.
 </p>

 <!-- ================= -->
 <h4>The Format String</h4>
 <!-- ================= -->

 <p>The format string for the diagnostic is very simple, but it has some power.
 It takes the form of a string in English with markers that indicate where and
 how arguments to the diagnostic are inserted and formatted.  For example, here
 are some simple format strings:</p>

 <pre>
   "binary integer literals are an extension"
   "format string contains '\\0' within the string body"
   "more '<b>%%</b>' conversions than data arguments"
   "invalid operands to binary expression (<b>%0</b> and <b>%1</b>)"
   "overloaded '<b>%0</b>' must be a <b>%select{unary|binary|unary or binary}2</b> operator"
        " (has <b>%1</b> parameter<b>%s1</b>)"
 </pre>

 <p>These examples show some important points of format strings.  You can use any
    plain ASCII character in the diagnostic string except "%" without a problem,
    but these are C strings, so you have to use and be aware of all the C escape
    sequences (as in the second example).  If you want to produce a "%" in the
    output, use the "%%" escape sequence, like the third diagnostic.  Finally,
    Clang uses the "%...[digit]" sequences to specify where and how arguments to
    the diagnostic are formatted.</p>

 <p>Arguments to the diagnostic are numbered according to how they are specified
    by the C++ code that <a href="#producingdiag">produces them</a>, and are
    referenced by <tt>%0</tt> .. <tt>%9</tt>.  If you have more than 10 arguments
    to your diagnostic, you are doing something wrong :).  Unlike printf, there
    is no requirement that arguments to the diagnostic end up in the output in
    the same order as they are specified, you could have a format string with
    <tt>"%1 %0"</tt> that swaps them, for example.  The text in between the
    percent and digit are formatting instructions.  If there are no instructions,
    the argument is just turned into a string and substituted in.</p>

 <p>Here are some "best practices" for writing the English format string:</p>

 <ul>
 <li>Keep the string short.  It should ideally fit in the 80 column limit of the
     <tt>DiagnosticKinds.def</tt> file.  This avoids the diagnostic wrapping when
     printed, and forces you to think about the important point you are conveying
     with the diagnostic.</li>
 <li>Take advantage of location information.  The user will be able to see the
     line and location of the caret, so you don't need to tell them that the
     problem is with the 4th argument to the function: just point to it.</li>
 <li>Do not capitalize the diagnostic string, and do not end it with a
     period.</li>
 <li>If you need to quote something in the diagnostic string, use single
     quotes.</li>
 </ul>

 <p>Diagnostics should never take random English strings as arguments: you
 shouldn't use <tt>"you have a problem with %0"</tt> and pass in things like
 <tt>"your argument"</tt> or <tt>"your return value"</tt> as arguments. Doing
 this prevents <a href="translation">translating</a> the Clang diagnostics to
 other languages (because they'll get random English words in their otherwise
 localized diagnostic).  The exceptions to this are C/C++ language keywords
 (e.g. auto, const, mutable, etc) and C/C++ operators (<tt>/=</tt>).  Note
 that things like "pointer" and "reference" are not keywords.  On the other
 hand, you <em>can</em> include anything that comes from the user's source code,
 including variable names, types, labels, etc.  The 'select' format can be
 used to achieve this sort of thing in a localizable way, see below.</p>

 <!-- ==================================== -->
 <h4>Formatting a Diagnostic Argument</a></h4>
 <!-- ==================================== -->

 <p>Arguments to diagnostics are fully typed internally, and come from a couple
 different classes: integers, types, names, and random strings.  Depending on
 the class of the argument, it can be optionally formatted in different ways.
 This gives the DiagnosticClient information about what the argument means
 without requiring it to use a specific presentation (consider this MVC for
 Clang :).</p>

 <p>Here are the different diagnostic argument formats currently supported by
 Clang:</p>

 <table>
 <tr><td colspan="2"><b>"s" format</b></td></tr>
 <tr><td>Example:</td><td><tt>"requires %1 parameter%s1"</tt></td></tr>
 <tr><td>Class:</td><td>Integers</td></tr>
 <tr><td>Description:</td><td>This is a simple formatter for integers that is
     useful when producing English diagnostics.  When the integer is 1, it prints
     as nothing.  When the integer is not 1, it prints as "s".  This allows some
     simple grammatical forms to be to be handled correctly, and eliminates the
     need to use gross things like <tt>"requires %1 parameter(s)"</tt>.</td></tr>

 <tr><td colspan="2"><b>"select" format</b></td></tr>
 <tr><td>Example:</td><td><tt>"must be a %select{unary|binary|unary or binary}2
      operator"</tt></td></tr>
 <tr><td>Class:</td><td>Integers</td></tr>
 <tr><td>Description:</td><td><p>This format specifier is used to merge multiple
     related diagnostics together into one common one, without requiring the
     difference to be specified as an English string argument.  Instead of
     specifying the string, the diagnostic gets an integer argument and the
     format string selects the numbered option.  In this case, the "%2" value
     must be an integer in the range [0..2].  If it is 0, it prints 'unary', if
     it is 1 it prints 'binary' if it is 2, it prints 'unary or binary'.  This
     allows other language translations to substitute reasonable words (or entire
     phrases) based on the semantics of the diagnostic instead of having to do
     things textually.</p>
     <p>The selected string does undergo formatting.</p></td></tr>

 <tr><td colspan="2"><b>"plural" format</b></td></tr>
 <tr><td>Example:</td><td><tt>"you have %1 %plural{1:mouse|:mice}1 connected to
     your computer"</tt></td></tr>
 <tr><td>Class:</td><td>Integers</td></tr>
 <tr><td>Description:</td><td><p>This is a formatter for complex plural forms.
     It is designed to handle even the requirements of languages with very
 	complex plural forms, as many Baltic languages have. The argument consists
 	of a series of expression/form pairs, separated by ':', where the first form
 	whose expression evaluates to true is the result of the modifier.</p>
 	<p>An expression can be empty, in which case it is always true. See the
 	example at the top. Otherwise, it is a series of one or more numeric
 	conditions, separated by ','. If any condition matches, the expression
 	matches. Each numeric condition can take one of three forms.</p>
 	<ul>
 	    <li>number: A simple decimal number matches if the argument is the same
 		as the number. Example: <tt>"%plural{1:mouse|:mice}4"</tt></li>
 		<li>range: A range in square brackets matches if the argument is within
 		the range. Then range is inclusive on both ends. Example:
 		<tt>"%plural{0:none|1:one|[2,5]:some|:many}2"</tt></li>
 		<li>modulo: A modulo operator is followed by a number, and
                 equals sign and either a number or a range. The tests are the
                 same as for plain
 		numbers and ranges, but the argument is taken modulo the number first.
 		Example: <tt>"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything
 		else}1"</tt></li>
 	</ul>
 	<p>The parser is very unforgiving. A syntax error, even whitespace, will
 	abort, as will a failure to match the argument against any
 	expression.</p></td></tr>

 <tr><td colspan="2"><b>"ordinal" format</b></td></tr>
 <tr><td>Example:</td><td><tt>"ambiguity in %ordinal0 argument"</tt></td></tr>
 <tr><td>Class:</td><td>Integers</td></tr>
 <tr><td>Description:</td><td><p>This is a formatter which represents the
     argument number as an ordinal:  the value <tt>1</tt> becomes <tt>1st</tt>,
     <tt>3</tt> becomes <tt>3rd</tt>, and so on.  Values less than <tt>1</tt>
     are not supported.</p>
     <p>This formatter is currently hard-coded to use English ordinals.</p></td></tr>

 <tr><td colspan="2"><b>"objcclass" format</b></td></tr>
 <tr><td>Example:</td><td><tt>"method %objcclass0 not found"</tt></td></tr>
 <tr><td>Class:</td><td>DeclarationName</td></tr>
 <tr><td>Description:</td><td><p>This is a simple formatter that indicates the
     DeclarationName corresponds to an Objective-C class method selector.  As
     such, it prints the selector with a leading '+'.</p></td></tr>

 <tr><td colspan="2"><b>"objcinstance" format</b></td></tr>
 <tr><td>Example:</td><td><tt>"method %objcinstance0 not found"</tt></td></tr>
 <tr><td>Class:</td><td>DeclarationName</td></tr>
 <tr><td>Description:</td><td><p>This is a simple formatter that indicates the
     DeclarationName corresponds to an Objective-C instance method selector.  As
     such, it prints the selector with a leading '-'.</p></td></tr>

 <tr><td colspan="2"><b>"q" format</b></td></tr>
 <tr><td>Example:</td><td><tt>"candidate found by name lookup is %q0"</tt></td></tr>
 <tr><td>Class:</td><td>NamedDecl*</td></tr>
 <tr><td>Description</td><td><p>This formatter indicates that the fully-qualified name of the declaration should be printed, e.g., "std::vector" rather than "vector".</p></td></tr>

 </table>

 <p>It is really easy to add format specifiers to the Clang diagnostics system,
 but they should be discussed before they are added.  If you are creating a lot
 of repetitive diagnostics and/or have an idea for a useful formatter, please
 bring it up on the cfe-dev mailing list.</p>

 <!-- ===================================================== -->
 <h4><a name="#producingdiag">Producing the Diagnostic</a></h4>
 <!-- ===================================================== -->

 <p>Now that you've created the diagnostic in the DiagnosticKinds.def file, you
 need to write the code that detects the condition in question and emits the
 new diagnostic.  Various components of Clang (e.g. the preprocessor, Sema,
 etc) provide a helper function named "Diag".  It creates a diagnostic and
 accepts the arguments, ranges, and other information that goes along with
 it.</p>

 <p>For example, the binary expression error comes from code like this:</p>

 <pre>
   if (various things that are bad)
     Diag(Loc, diag::err_typecheck_invalid_operands)
       &lt;&lt; lex-&gt;getType() &lt;&lt; rex-&gt;getType()
       &lt;&lt; lex-&gt;getSourceRange() &lt;&lt; rex-&gt;getSourceRange();
 </pre>

 <p>This shows that use of the Diag method: they take a location (a <a
 href="#SourceLocation">SourceLocation</a> object) and a diagnostic enum value
 (which matches the name from DiagnosticKinds.def).  If the diagnostic takes
 arguments, they are specified with the &lt;&lt; operator: the first argument
 becomes %0, the second becomes %1, etc.  The diagnostic interface allows you to
 specify arguments of many different types, including <tt>int</tt> and
 <tt>unsigned</tt> for integer arguments, <tt>const char*</tt> and
 <tt>std::string</tt> for string arguments, <tt>DeclarationName</tt> and
 <tt>const IdentifierInfo*</tt> for names, <tt>QualType</tt> for types, etc.
 SourceRanges are also specified with the &lt;&lt; operator, but do not have a
 specific ordering requirement.</p>

 <p>As you can see, adding and producing a diagnostic is pretty straightforward.
 The hard part is deciding exactly what you need to say to help the user, picking
 a suitable wording, and providing the information needed to format it correctly.
 The good news is that the call site that issues a diagnostic should be
 completely independent of how the diagnostic is formatted and in what language
 it is rendered.
 </p>

 <!-- ==================================================== -->
 <h4 id="code-modification-hints">Code Modification Hints</h4>
 <!-- ==================================================== -->

 <p>In some cases, the front end emits diagnostics when it is clear
 that some small change to the source code would fix the problem. For
 example, a missing semicolon at the end of a statement or a use of
 deprecated syntax that is easily rewritten into a more modern form.
 Clang tries very hard to emit the diagnostic and recover gracefully
 in these and other cases.</p>

 <p>However, for these cases where the fix is obvious, the diagnostic
 can be annotated with a code
 modification "hint" that describes how to change the code referenced
 by the diagnostic to fix the problem. For example, it might add the
 missing semicolon at the end of the statement or rewrite the use of a
 deprecated construct into something more palatable. Here is one such
 example C++ front end, where we warn about the right-shift operator
 changing meaning from C++98 to C++0x:</p>

 <pre>
 test.cpp:3:7: warning: use of right-shift operator ('&gt;&gt;') in template argument will require parentheses in C++0x
 A&lt;100 &gt;&gt; 2&gt; *a;
       ^
   (       )
 </pre>

 <p>Here, the code modification hint is suggesting that parentheses be
 added, and showing exactly where those parentheses would be inserted
 into the source code. The code modification hints themselves describe
 what changes to make to the source code in an abstract manner, which
 the text diagnostic printer renders as a line of "insertions" below
 the caret line. <a href="#DiagnosticClient">Other diagnostic
 clients</a> might choose to render the code differently (e.g., as
 markup inline) or even give the user the ability to automatically fix
 the problem.</p>

 <p>All code modification hints are described by the
 <code>CodeModificationHint</code> class, instances of which should be
 attached to the diagnostic using the &lt;&lt; operator in the same way
 that highlighted source ranges and arguments are passed to the
 diagnostic. Code modification hints can be created with one of three
 constructors:</p>

 <dl>
   <dt><code>CodeModificationHint::CreateInsertion(Loc, Code)</code></dt>
   <dd>Specifies that the given <code>Code</code> (a string) should be inserted
   before the source location <code>Loc</code>.</dd>

   <dt><code>CodeModificationHint::CreateRemoval(Range)</code></dt>
   <dd>Specifies that the code in the given source <code>Range</code>
   should be removed.</dd>

   <dt><code>CodeModificationHint::CreateReplacement(Range, Code)</code></dt>
   <dd>Specifies that the code in the given source <code>Range</code>
   should be removed, and replaced with the given <code>Code</code> string.</dd>
 </dl>

 <!-- ============================================================= -->
 <h4><a name="DiagnosticClient">The DiagnosticClient Interface</a></h4>
 <!-- ============================================================= -->

 <p>Once code generates a diagnostic with all of the arguments and the rest of
 the relevant information, Clang needs to know what to do with it.  As previously
 mentioned, the diagnostic machinery goes through some filtering to map a
 severity onto a diagnostic level, then (assuming the diagnostic is not mapped to
 "<tt>Ignore</tt>") it invokes an object that implements the DiagnosticClient
 interface with the information.</p>

 <p>It is possible to implement this interface in many different ways.  For
 example, the normal Clang DiagnosticClient (named 'TextDiagnosticPrinter') turns
 the arguments into strings (according to the various formatting rules), prints
 out the file/line/column information and the string, then prints out the line of
 code, the source ranges, and the caret.  However, this behavior isn't required.
 </p>

 <p>Another implementation of the DiagnosticClient interface is the
 'TextDiagnosticBuffer' class, which is used when Clang is in -verify mode.
 Instead of formatting and printing out the diagnostics, this implementation just
 captures and remembers the diagnostics as they fly by.  Then -verify compares
 the list of produced diagnostics to the list of expected ones.  If they disagree,
 it prints out its own output.
 </p>

 <p>There are many other possible implementations of this interface, and this is
 why we prefer diagnostics to pass down rich structured information in arguments.
 For example, an HTML output might want declaration names be linkified to where
 they come from in the source.  Another example is that a GUI might let you click
 on typedefs to expand them.  This application would want to pass significantly
 more information about types through to the GUI than a simple flat string.  The
 interface allows this to happen.</p>

 <!-- ====================================================== -->
 <h4><a name="translation">Adding Translations to Clang</a></h4>
 <!-- ====================================================== -->

 <p>Not possible yet!  Diagnostic strings should be written in UTF-8, the client
 can translate to the relevant code page if needed.  Each translation completely
 replaces the format string for the diagnostic.</p>


 <!-- ======================================================================= -->
 <h3 id="SourceLocation">The SourceLocation and SourceManager classes</h3>
 <!-- ======================================================================= -->

 <p>Strangely enough, the SourceLocation class represents a location within the
 source code of the program.  Important design points include:</p>

 <ol>
 <li>sizeof(SourceLocation) must be extremely small, as these are embedded into
     many AST nodes and are passed around often.  Currently it is 32 bits.</li>
 <li>SourceLocation must be a simple value object that can be efficiently
     copied.</li>
 <li>We should be able to represent a source location for any byte of any input
     file.  This includes in the middle of tokens, in whitespace, in trigraphs,
     etc.</li>
 <li>A SourceLocation must encode the current #include stack that was active when
     the location was processed.  For example, if the location corresponds to a
     token, it should contain the set of #includes active when the token was
     lexed.  This allows us to print the #include stack for a diagnostic.</li>
 <li>SourceLocation must be able to describe macro expansions, capturing both
     the ultimate instantiation point and the source of the original character
     data.</li>
 </ol>

 <p>In practice, the SourceLocation works together with the SourceManager class
 to encode two pieces of information about a location: its spelling location
 and its instantiation location.  For most tokens, these will be the same.
 However, for a macro expansion (or tokens that came from a _Pragma directive)
 these will describe the location of the characters corresponding to the token
 and the location where the token was used (i.e. the macro instantiation point
 or the location of the _Pragma itself).</p>

 <p>The Clang front-end inherently depends on the location of a token being
 tracked correctly.  If it is ever incorrect, the front-end may get confused and
 die.  The reason for this is that the notion of the 'spelling' of a Token in
 Clang depends on being able to find the original input characters for the token.
 This concept maps directly to the "spelling location" for the token.</p>

 <!-- ======================================================================= -->
 <h2 id="libdriver">The Driver Library</h2>
 <!-- ======================================================================= -->

 <p>The clang Driver and library are documented <a
 href="DriverInternals.html">here<a>.<p>

 <!-- ======================================================================= -->
 <h2 id="pch">Precompiled Headers</h2>
 <!-- ======================================================================= -->

 <p>Clang supports two implementations of precompiled headers. The
    default implementation, precompiled headers (<a
     href="PCHInternals.html">PCH</a>) uses a serialized representation
    of Clang's internal data structures, encoded with the <a
     href="http://llvm.org/docs/BitCodeFormat.html">LLVM bitstream
    format</a>. Pretokenized headers (<a
     href="PTHInternals.html">PTH</a>), on the other hand, contain a
    serialized representation of the tokens encountered when
    preprocessing a header (and anything that header includes).</p>


 <!-- ======================================================================= -->
 <h2 id="libfrontend">The Frontend Library</h2>
 <!-- ======================================================================= -->

 <p>The Frontend library contains functionality useful for building
 tools on top of the clang libraries, for example several methods for
 outputting diagnostics.</p>

 <!-- ======================================================================= -->
 <h2 id="liblex">The Lexer and Preprocessor Library</h2>
 <!-- ======================================================================= -->

 <p>The Lexer library contains several tightly-connected classes that are involved
 with the nasty process of lexing and preprocessing C source code.  The main
 interface to this library for outside clients is the large <a
 href="#Preprocessor">Preprocessor</a> class.
 It contains the various pieces of state that are required to coherently read
 tokens out of a translation unit.</p>

 <p>The core interface to the Preprocessor object (once it is set up) is the
 Preprocessor::Lex method, which returns the next <a href="#Token">Token</a> from
 the preprocessor stream.  There are two types of token providers that the
 preprocessor is capable of reading from: a buffer lexer (provided by the <a
 href="#Lexer">Lexer</a> class) and a buffered token stream (provided by the <a
 href="#TokenLexer">TokenLexer</a> class).


 <!-- ======================================================================= -->
 <h3 id="Token">The Token class</h3>
 <!-- ======================================================================= -->

 <p>The Token class is used to represent a single lexed token.  Tokens are
 intended to be used by the lexer/preprocess and parser libraries, but are not
 intended to live beyond them (for example, they should not live in the ASTs).<p>

 <p>Tokens most often live on the stack (or some other location that is efficient
 to access) as the parser is running, but occasionally do get buffered up.  For
 example, macro definitions are stored as a series of tokens, and the C++
 front-end periodically needs to buffer tokens up for tentative parsing and
 various pieces of look-ahead.  As such, the size of a Token matter.  On a 32-bit
 system, sizeof(Token) is currently 16 bytes.</p>

 <p>Tokens occur in two forms: "<a href="#AnnotationToken">Annotation
 Tokens</a>" and normal tokens.  Normal tokens are those returned by the lexer,
 annotation tokens represent semantic information and are produced by the parser,
 replacing normal tokens in the token stream.  Normal tokens contain the
 following information:</p>

 <ul>
 <li><b>A SourceLocation</b> - This indicates the location of the start of the
 token.</li>

 <li><b>A length</b> - This stores the length of the token as stored in the
 SourceBuffer.  For tokens that include them, this length includes trigraphs and
 escaped newlines which are ignored by later phases of the compiler.  By pointing
 into the original source buffer, it is always possible to get the original
 spelling of a token completely accurately.</li>

 <li><b>IdentifierInfo</b> - If a token takes the form of an identifier, and if
 identifier lookup was enabled when the token was lexed (e.g. the lexer was not
 reading in 'raw' mode) this contains a pointer to the unique hash value for the
 identifier.  Because the lookup happens before keyword identification, this
 field is set even for language keywords like 'for'.</li>

 <li><b>TokenKind</b> - This indicates the kind of token as classified by the
 lexer.  This includes things like <tt>tok::starequal</tt> (for the "*="
 operator), <tt>tok::ampamp</tt> for the "&amp;&amp;" token, and keyword values
 (e.g. <tt>tok::kw_for</tt>) for identifiers that correspond to keywords.  Note
 that some tokens can be spelled multiple ways.  For example, C++ supports
 "operator keywords", where things like "and" are treated exactly like the
 "&amp;&amp;" operator.  In these cases, the kind value is set to
 <tt>tok::ampamp</tt>, which is good for the parser, which doesn't have to
 consider both forms.  For something that cares about which form is used (e.g.
 the preprocessor 'stringize' operator) the spelling indicates the original
 form.</li>

 <li><b>Flags</b> - There are currently four flags tracked by the
 lexer/preprocessor system on a per-token basis:

   <ol>
   <li><b>StartOfLine</b> - This was the first token that occurred on its input
        source line.</li>
   <li><b>LeadingSpace</b> - There was a space character either immediately
        before the token or transitively before the token as it was expanded
        through a macro.  The definition of this flag is very closely defined by
        the stringizing requirements of the preprocessor.</li>
   <li><b>DisableExpand</b> - This flag is used internally to the preprocessor to
       represent identifier tokens which have macro expansion disabled.  This
       prevents them from being considered as candidates for macro expansion ever
       in the future.</li>
   <li><b>NeedsCleaning</b> - This flag is set if the original spelling for the
       token includes a trigraph or escaped newline.  Since this is uncommon,
       many pieces of code can fast-path on tokens that did not need cleaning.
       </p>
    </ol>
 </li>
 </ul>

 <p>One interesting (and somewhat unusual) aspect of normal tokens is that they
 don't contain any semantic information about the lexed value.  For example, if
 the token was a pp-number token, we do not represent the value of the number
 that was lexed (this is left for later pieces of code to decide).  Additionally,
 the lexer library has no notion of typedef names vs variable names: both are
 returned as identifiers, and the parser is left to decide whether a specific
 identifier is a typedef or a variable (tracking this requires scope information
 among other things).  The parser can do this translation by replacing tokens
 returned by the preprocessor with "Annotation Tokens".</p>

 <!-- ======================================================================= -->
 <h3 id="AnnotationToken">Annotation Tokens</h3>
 <!-- ======================================================================= -->

 <p>Annotation Tokens are tokens that are synthesized by the parser and injected
 into the preprocessor's token stream (replacing existing tokens) to record
 semantic information found by the parser.  For example, if "foo" is found to be
 a typedef, the "foo" <tt>tok::identifier</tt> token is replaced with an
 <tt>tok::annot_typename</tt>.  This is useful for a couple of reasons: 1) this
 makes it easy to handle qualified type names (e.g. "foo::bar::baz&lt;42&gt;::t")
 in C++ as a single "token" in the parser. 2) if the parser backtracks, the
 reparse does not need to redo semantic analysis to determine whether a token
 sequence is a variable, type, template, etc.</p>

 <p>Annotation Tokens are created by the parser and reinjected into the parser's
 token stream (when backtracking is enabled).  Because they can only exist in
 tokens that the preprocessor-proper is done with, it doesn't need to keep around
 flags like "start of line" that the preprocessor uses to do its job.
 Additionally, an annotation token may "cover" a sequence of preprocessor tokens
 (e.g. <tt>a::b::c</tt> is five preprocessor tokens).  As such, the valid fields
 of an annotation token are different than the fields for a normal token (but
 they are multiplexed into the normal Token fields):</p>

 <ul>
 <li><b>SourceLocation "Location"</b> - The SourceLocation for the annotation
 token indicates the first token replaced by the annotation token. In the example
 above, it would be the location of the "a" identifier.</li>

 <li><b>SourceLocation "AnnotationEndLoc"</b> - This holds the location of the
 last token replaced with the annotation token.  In the example above, it would
 be the location of the "c" identifier.</li>

 <li><b>void* "AnnotationValue"</b> - This contains an opaque object that the
 parser gets from Sema through an Actions module, it is passed around and Sema
 interprets it, based on the type of annotation token.</li>

 <li><b>TokenKind "Kind"</b> - This indicates the kind of Annotation token this
 is.  See below for the different valid kinds.</li>
 </ul>

 <p>Annotation tokens currently come in three kinds:</p>

 <ol>
 <li><b>tok::annot_typename</b>: This annotation token represents a
 resolved typename token that is potentially qualified.  The AnnotationValue
 field contains a pointer returned by Action::getTypeName().  In the case of the
 Sema actions module, this is a <tt>Decl*</tt> for the type.</li>

 <li><b>tok::annot_cxxscope</b>: This annotation token represents a C++ scope
 specifier, such as "A::B::".  This corresponds to the grammar productions "::"
 and ":: [opt] nested-name-specifier".  The AnnotationValue pointer is returned
 by the Action::ActOnCXXGlobalScopeSpecifier and
 Action::ActOnCXXNestedNameSpecifier callbacks.  In the case of Sema, this is a
 <tt>DeclContext*</tt>.</li>

 <li><b>tok::annot_template_id</b>: This annotation token represents a
 C++ template-id such as "foo&lt;int, 4&gt;", where "foo" is the name
 of a template. The AnnotationValue pointer is a pointer to a malloc'd
 TemplateIdAnnotation object. Depending on the context, a parsed template-id that names a type might become a typename annotation token (if all we care about is the named type, e.g., because it occurs in a type specifier) or might remain a template-id token (if we want to retain more source location information or produce a new type, e.g., in a declaration of a class template specialization). template-id annotation tokens that refer to a type can be "upgraded" to typename annotation tokens by the parser.</li>

 </ol>

 <p>As mentioned above, annotation tokens are not returned by the preprocessor,
 they are formed on demand by the parser.  This means that the parser has to be
 aware of cases where an annotation could occur and form it where appropriate.
 This is somewhat similar to how the parser handles Translation Phase 6 of C99:
 String Concatenation (see C99 5.1.1.2).  In the case of string concatenation,
 the preprocessor just returns distinct tok::string_literal and
 tok::wide_string_literal tokens and the parser eats a sequence of them wherever
 the grammar indicates that a string literal can occur.</p>

 <p>In order to do this, whenever the parser expects a tok::identifier or
 tok::coloncolon, it should call the TryAnnotateTypeOrScopeToken or
 TryAnnotateCXXScopeToken methods to form the annotation token.  These methods
 will maximally form the specified annotation tokens and replace the current
 token with them, if applicable.  If the current tokens is not valid for an
 annotation token, it will remain an identifier or :: token.</p>


 <!-- ======================================================================= -->
 <h3 id="Lexer">The Lexer class</h3>
 <!-- ======================================================================= -->

 <p>The Lexer class provides the mechanics of lexing tokens out of a source
 buffer and deciding what they mean.  The Lexer is complicated by the fact that
 it operates on raw buffers that have not had spelling eliminated (this is a
 necessity to get decent performance), but this is countered with careful coding
 as well as standard performance techniques (for example, the comment handling
 code is vectorized on X86 and PowerPC hosts).</p>

 <p>The lexer has a couple of interesting modal features:</p>

 <ul>
 <li>The lexer can operate in 'raw' mode.  This mode has several features that
     make it possible to quickly lex the file (e.g. it stops identifier lookup,
     doesn't specially handle preprocessor tokens, handles EOF differently, etc).
     This mode is used for lexing within an "<tt>#if 0</tt>" block, for
     example.</li>
 <li>The lexer can capture and return comments as tokens.  This is required to
     support the -C preprocessor mode, which passes comments through, and is
     used by the diagnostic checker to identifier expect-error annotations.</li>
 <li>The lexer can be in ParsingFilename mode, which happens when preprocessing
     after reading a #include directive.  This mode changes the parsing of '&lt;'
     to return an "angled string" instead of a bunch of tokens for each thing
     within the filename.</li>
 <li>When parsing a preprocessor directive (after "<tt>#</tt>") the
     ParsingPreprocessorDirective mode is entered.  This changes the parser to
     return EOM at a newline.</li>
 <li>The Lexer uses a LangOptions object to know whether trigraphs are enabled,
     whether C++ or ObjC keywords are recognized, etc.</li>
 </ul>

 <p>In addition to these modes, the lexer keeps track of a couple of other
    features that are local to a lexed buffer, which change as the buffer is
    lexed:</p>

 <ul>
 <li>The Lexer uses BufferPtr to keep track of the current character being
     lexed.</li>
 <li>The Lexer uses IsAtStartOfLine to keep track of whether the next lexed token
     will start with its "start of line" bit set.</li>
 <li>The Lexer keeps track of the current #if directives that are active (which
     can be nested).</li>
 <li>The Lexer keeps track of an <a href="#MultipleIncludeOpt">
     MultipleIncludeOpt</a> object, which is used to
     detect whether the buffer uses the standard "<tt>#ifndef XX</tt> /
     <tt>#define XX</tt>" idiom to prevent multiple inclusion.  If a buffer does,
     subsequent includes can be ignored if the XX macro is defined.</li>
 </ul>

 <!-- ======================================================================= -->
 <h3 id="TokenLexer">The TokenLexer class</h3>
 <!-- ======================================================================= -->

 <p>The TokenLexer class is a token provider that returns tokens from a list
 of tokens that came from somewhere else.  It typically used for two things: 1)
 returning tokens from a macro definition as it is being expanded 2) returning
 tokens from an arbitrary buffer of tokens.  The later use is used by _Pragma and
 will most likely be used to handle unbounded look-ahead for the C++ parser.</p>

 <!-- ======================================================================= -->
 <h3 id="MultipleIncludeOpt">The MultipleIncludeOpt class</h3>
 <!-- ======================================================================= -->

 <p>The MultipleIncludeOpt class implements a really simple little state machine
 that is used to detect the standard "<tt>#ifndef XX</tt> / <tt>#define XX</tt>"
 idiom that people typically use to prevent multiple inclusion of headers.  If a
 buffer uses this idiom and is subsequently #include'd, the preprocessor can
 simply check to see whether the guarding condition is defined or not.  If so,
 the preprocessor can completely ignore the include of the header.</p>


 <!-- ======================================================================= -->
 <h2 id="libparse">The Parser Library</h2>
 <!-- ======================================================================= -->

 <!-- ======================================================================= -->
 <h2 id="libast">The AST Library</h2>
 <!-- ======================================================================= -->

 <!-- ======================================================================= -->
 <h3 id="Type">The Type class and its subclasses</h3>
 <!-- ======================================================================= -->

 <p>The Type class (and its subclasses) are an important part of the AST.  Types
 are accessed through the ASTContext class, which implicitly creates and uniques
 them as they are needed.  Types have a couple of non-obvious features: 1) they
 do not capture type qualifiers like const or volatile (See
 <a href="#QualType">QualType</a>), and 2) they implicitly capture typedef
 information.  Once created, types are immutable (unlike decls).</p>

 <p>Typedefs in C make semantic analysis a bit more complex than it would
 be without them.  The issue is that we want to capture typedef information
 and represent it in the AST perfectly, but the semantics of operations need to
 "see through" typedefs.  For example, consider this code:</p>

 <code>
 void func() {<br>
 &nbsp;&nbsp;typedef int foo;<br>
 &nbsp;&nbsp;foo X, *Y;<br>
 &nbsp;&nbsp;typedef foo* bar;<br>
 &nbsp;&nbsp;bar Z;<br>
 &nbsp;&nbsp;*X;   <i>// error</i><br>
 &nbsp;&nbsp;**Y;  <i>// error</i><br>
 &nbsp;&nbsp;**Z;  <i>// error</i><br>
 }<br>
 </code>

 <p>The code above is illegal, and thus we expect there to be diagnostics emitted
 on the annotated lines.  In this example, we expect to get:</p>

 <pre>
 <b>test.c:6:1: error: indirection requires pointer operand ('foo' invalid)</b>
 *X; // error
 <font color="blue">^~</font>
 <b>test.c:7:1: error: indirection requires pointer operand ('foo' invalid)</b>
 **Y; // error
 <font color="blue">^~~</font>
 <b>test.c:8:1: error: indirection requires pointer operand ('foo' invalid)</b>
 **Z; // error
 <font color="blue">^~~</font>
 </pre>

 <p>While this example is somewhat silly, it illustrates the point: we want to
 retain typedef information where possible, so that we can emit errors about
 "<tt>std::string</tt>" instead of "<tt>std::basic_string&lt;char, std:...</tt>".
 Doing this requires properly keeping typedef information (for example, the type
 of "X" is "foo", not "int"), and requires properly propagating it through the
 various operators (for example, the type of *Y is "foo", not "int").  In order
 to retain this information, the type of these expressions is an instance of the
 TypedefType class, which indicates that the type of these expressions is a
 typedef for foo.
 </p>

 <p>Representing types like this is great for diagnostics, because the
 user-specified type is always immediately available.  There are two problems
 with this: first, various semantic checks need to make judgements about the
 <em>actual structure</em> of a type, ignoring typdefs.  Second, we need an
 efficient way to query whether two types are structurally identical to each
 other, ignoring typedefs.  The solution to both of these problems is the idea of
 canonical types.</p>

 <!-- =============== -->
 <h4>Canonical Types</h4>
 <!-- =============== -->

 <p>Every instance of the Type class contains a canonical type pointer.  For
 simple types with no typedefs involved (e.g. "<tt>int</tt>", "<tt>int*</tt>",
 "<tt>int**</tt>"), the type just points to itself.  For types that have a
 typedef somewhere in their structure (e.g. "<tt>foo</tt>", "<tt>foo*</tt>",
 "<tt>foo**</tt>", "<tt>bar</tt>"), the canonical type pointer points to their
 structurally equivalent type without any typedefs (e.g. "<tt>int</tt>",
 "<tt>int*</tt>", "<tt>int**</tt>", and "<tt>int*</tt>" respectively).</p>

 <p>This design provides a constant time operation (dereferencing the canonical
 type pointer) that gives us access to the structure of types.  For example,
 we can trivially tell that "bar" and "foo*" are the same type by dereferencing
 their canonical type pointers and doing a pointer comparison (they both point
 to the single "<tt>int*</tt>" type).</p>

 <p>Canonical types and typedef types bring up some complexities that must be
 carefully managed.  Specifically, the "isa/cast/dyncast" operators generally
 shouldn't be used in code that is inspecting the AST.  For example, when type
 checking the indirection operator (unary '*' on a pointer), the type checker
 must verify that the operand has a pointer type.  It would not be correct to
 check that with "<tt>isa&lt;PointerType&gt;(SubExpr-&gt;getType())</tt>",
 because this predicate would fail if the subexpression had a typedef type.</p>

 <p>The solution to this problem are a set of helper methods on Type, used to
 check their properties.  In this case, it would be correct to use
 "<tt>SubExpr-&gt;getType()-&gt;isPointerType()</tt>" to do the check.  This
 predicate will return true if the <em>canonical type is a pointer</em>, which is
 true any time the type is structurally a pointer type.  The only hard part here
 is remembering not to use the <tt>isa/cast/dyncast</tt> operations.</p>

 <p>The second problem we face is how to get access to the pointer type once we
 know it exists.  To continue the example, the result type of the indirection
 operator is the pointee type of the subexpression.  In order to determine the
 type, we need to get the instance of PointerType that best captures the typedef
 information in the program.  If the type of the expression is literally a
 PointerType, we can return that, otherwise we have to dig through the
 typedefs to find the pointer type.  For example, if the subexpression had type
 "<tt>foo*</tt>", we could return that type as the result.  If the subexpression
 had type "<tt>bar</tt>", we want to return "<tt>foo*</tt>" (note that we do
 <em>not</em> want "<tt>int*</tt>").  In order to provide all of this, Type has
 a getAsPointerType() method that checks whether the type is structurally a
 PointerType and, if so, returns the best one.  If not, it returns a null
 pointer.</p>

 <p>This structure is somewhat mystical, but after meditating on it, it will
 make sense to you :).</p>

 <!-- ======================================================================= -->
 <h3 id="QualType">The QualType class</h3>
 <!-- ======================================================================= -->

 <p>The QualType class is designed as a trivial value class that is small,
 passed by-value and is efficient to query.  The idea of QualType is that it
 stores the type qualifiers (const, volatile, restrict) separately from the types
 themselves: QualType is conceptually a pair of "Type*" and bits for the type
 qualifiers.</p>

 <p>By storing the type qualifiers as bits in the conceptual pair, it is
 extremely efficient to get the set of qualifiers on a QualType (just return the
 field of the pair), add a type qualifier (which is a trivial constant-time
 operation that sets a bit), and remove one or more type qualifiers (just return
 a QualType with the bitfield set to empty).</p>

 <p>Further, because the bits are stored outside of the type itself, we do not
 need to create duplicates of types with different sets of qualifiers (i.e. there
 is only a single heap allocated "int" type: "const int" and "volatile const int"
 both point to the same heap allocated "int" type).  This reduces the heap size
 used to represent bits and also means we do not have to consider qualifiers when
 uniquing types (<a href="#Type">Type</a> does not even contain qualifiers).</p>

 <p>In practice, on hosts where it is safe, the 3 type qualifiers are stored in
 the low bit of the pointer to the Type object.  This means that QualType is
 exactly the same size as a pointer, and this works fine on any system where
 malloc'd objects are at least 8 byte aligned.</p>

 <!-- ======================================================================= -->
 <h3 id="DeclarationName">Declaration names</h3>
 <!-- ======================================================================= -->

 <p>The <tt>DeclarationName</tt> class represents the name of a
   declaration in Clang. Declarations in the C family of languages can
   take several different forms. Most declarations are named by
   simple identifiers, e.g., "<code>f</code>" and "<code>x</code>" in
   the function declaration <code>f(int x)</code>. In C++, declaration
   names can also name class constructors ("<code>Class</code>"
   in <code>struct Class { Class(); }</code>), class destructors
   ("<code>~Class</code>"), overloaded operator names ("operator+"),
   and conversion functions ("<code>operator void const *</code>"). In
   Objective-C, declaration names can refer to the names of Objective-C
   methods, which involve the method name and the parameters,
   collectively called a <i>selector</i>, e.g.,
   "<code>setWidth:height:</code>". Since all of these kinds of
   entities - variables, functions, Objective-C methods, C++
   constructors, destructors, and operators - are represented as
   subclasses of Clang's common <code>NamedDecl</code>
   class, <code>DeclarationName</code> is designed to efficiently
   represent any kind of name.</p>

 <p>Given
   a <code>DeclarationName</code> <code>N</code>, <code>N.getNameKind()</code>
   will produce a value that describes what kind of name <code>N</code>
   stores. There are 8 options (all of the names are inside
   the <code>DeclarationName</code> class)</p>
 <dl>
   <dt>Identifier</dt>
   <dd>The name is a simple
   identifier. Use <code>N.getAsIdentifierInfo()</code> to retrieve the
   corresponding <code>IdentifierInfo*</code> pointing to the actual
   identifier. Note that C++ overloaded operators (e.g.,
   "<code>operator+</code>") are represented as special kinds of
   identifiers. Use <code>IdentifierInfo</code>'s <code>getOverloadedOperatorID</code>
   function to determine whether an identifier is an overloaded
   operator name.</dd>

   <dt>ObjCZeroArgSelector, ObjCOneArgSelector,
   ObjCMultiArgSelector</dt>
   <dd>The name is an Objective-C selector, which can be retrieved as a
     <code>Selector</code> instance
     via <code>N.getObjCSelector()</code>. The three possible name
     kinds for Objective-C reflect an optimization within
     the <code>DeclarationName</code> class: both zero- and
     one-argument selectors are stored as a
     masked <code>IdentifierInfo</code> pointer, and therefore require
     very little space, since zero- and one-argument selectors are far
     more common than multi-argument selectors (which use a different
     structure).</dd>

   <dt>CXXConstructorName</dt>
   <dd>The name is a C++ constructor
     name. Use <code>N.getCXXNameType()</code> to retrieve
     the <a href="#QualType">type</a> that this constructor is meant to
     construct. The type is always the canonical type, since all
     constructors for a given type have the same name.</dd>

   <dt>CXXDestructorName</dt>
   <dd>The name is a C++ destructor
     name. Use <code>N.getCXXNameType()</code> to retrieve
     the <a href="#QualType">type</a> whose destructor is being
     named. This type is always a canonical type.</dd>

   <dt>CXXConversionFunctionName</dt>
   <dd>The name is a C++ conversion function. Conversion functions are
   named according to the type they convert to, e.g., "<code>operator void
       const *</code>". Use <code>N.getCXXNameType()</code> to retrieve
   the type that this conversion function converts to. This type is
     always a canonical type.</dd>

   <dt>CXXOperatorName</dt>
   <dd>The name is a C++ overloaded operator name. Overloaded operators
   are named according to their spelling, e.g.,
   "<code>operator+</code>" or "<code>operator new
   []</code>". Use <code>N.getCXXOverloadedOperator()</code> to
   retrieve the overloaded operator (a value of
     type <code>OverloadedOperatorKind</code>).</dd>
 </dl>

 <p><code>DeclarationName</code>s are cheap to create, copy, and
   compare. They require only a single pointer's worth of storage in
   the common cases (identifiers, zero-
   and one-argument Objective-C selectors) and use dense, uniqued
   storage for the other kinds of
   names. Two <code>DeclarationName</code>s can be compared for
   equality (<code>==</code>, <code>!=</code>) using a simple bitwise
   comparison, can be ordered
   with <code>&lt;</code>, <code>&gt;</code>, <code>&lt;=</code>,
   and <code>&gt;=</code> (which provide a lexicographical ordering for
   normal identifiers but an unspecified ordering for other kinds of
   names), and can be placed into LLVM <code>DenseMap</code>s
   and <code>DenseSet</code>s.</p>

 <p><code>DeclarationName</code> instances can be created in different
   ways depending on what kind of name the instance will store. Normal
   identifiers (<code>IdentifierInfo</code> pointers) and Objective-C selectors
   (<code>Selector</code>) can be implicitly converted
   to <code>DeclarationName</code>s. Names for C++ constructors,
   destructors, conversion functions, and overloaded operators can be retrieved from
   the <code>DeclarationNameTable</code>, an instance of which is
   available as <code>ASTContext::DeclarationNames</code>. The member
   functions <code>getCXXConstructorName</code>, <code>getCXXDestructorName</code>,
   <code>getCXXConversionFunctionName</code>, and <code>getCXXOperatorName</code>, respectively,
   return <code>DeclarationName</code> instances for the four kinds of
   C++ special function names.</p>

 <!-- ======================================================================= -->
 <h3 id="DeclContext">Declaration contexts</h3>
 <!-- ======================================================================= -->
 <p>Every declaration in a program exists within some <i>declaration
     context</i>, such as a translation unit, namespace, class, or
     function. Declaration contexts in Clang are represented by
     the <code>DeclContext</code> class, from which the various
   declaration-context AST nodes
   (<code>TranslationUnitDecl</code>, <code>NamespaceDecl</code>, <code>RecordDecl</code>, <code>FunctionDecl</code>,
   etc.) will derive. The <code>DeclContext</code> class provides
   several facilities common to each declaration context:</p>
 <dl>
   <dt>Source-centric vs. Semantics-centric View of Declarations</dt>
   <dd><code>DeclContext</code> provides two views of the declarations
   stored within a declaration context. The source-centric view
   accurately represents the program source code as written, including
   multiple declarations of entities where present (see the
     section <a href="#Redeclarations">Redeclarations and
   Overloads</a>), while the semantics-centric view represents the
   program semantics. The two views are kept synchronized by semantic
   analysis while the ASTs are being constructed.</dd>

   <dt>Storage of declarations within that context</dt>
   <dd>Every declaration context can contain some number of
     declarations. For example, a C++ class (represented
     by <code>RecordDecl</code>) contains various member functions,
     fields, nested types, and so on. All of these declarations will be
     stored within the <code>DeclContext</code>, and one can iterate
     over the declarations via
     [<code>DeclContext::decls_begin()</code>,
     <code>DeclContext::decls_end()</code>). This mechanism provides
     the source-centric view of declarations in the context.</dd>

   <dt>Lookup of declarations within that context</dt>
   <dd>The <code>DeclContext</code> structure provides efficient name
     lookup for names within that declaration context. For example,
     if <code>N</code> is a namespace we can look for the
     name <code>N::f</code>
     using <code>DeclContext::lookup</code>. The lookup itself is
     based on a lazily-constructed array (for declaration contexts
     with a small number of declarations) or hash table (for
     declaration contexts with more declarations). The lookup
     operation provides the semantics-centric view of the declarations
     in the context.</dd>

   <dt>Ownership of declarations</dt>
   <dd>The <code>DeclContext</code> owns all of the declarations that
   were declared within its declaration context, and is responsible
   for the management of their memory as well as their
   (de-)serialization.</dd>
 </dl>

 <p>All declarations are stored within a declaration context, and one
   can query
   information about the context in which each declaration lives. One
   can retrieve the <code>DeclContext</code> that contains a
   particular <code>Decl</code>
   using <code>Decl::getDeclContext</code>. However, see the
   section <a href="#LexicalAndSemanticContexts">Lexical and Semantic
   Contexts</a> for more information about how to interpret this
   context information.</p>

 <h4 id="Redeclarations">Redeclarations and Overloads</h4>
 <p>Within a translation unit, it is common for an entity to be
 declared several times. For example, we might declare a function "f"
   and then later re-declare it as part of an inlined definition:</p>

 <pre>
 void f(int x, int y, int z = 1);

 inline void f(int x, int y, int z) { /* ... */ }
 </pre>

 <p>The representation of "f" differs in the source-centric and
   semantics-centric views of a declaration context. In the
   source-centric view, all redeclarations will be present, in the
   order they occurred in the source code, making
     this view suitable for clients that wish to see the structure of
     the source code. In the semantics-centric view, only the most recent "f"
   will be found by the lookup, since it effectively replaces the first
   declaration of "f".</p>

 <p>In the semantics-centric view, overloading of functions is
   represented explicitly. For example, given two declarations of a
   function "g" that are overloaded, e.g.,</p>
 <pre>
 void g();
 void g(int);
 </pre>
 <p>the <code>DeclContext::lookup</code> operation will return
   an <code>OverloadedFunctionDecl</code> that contains both
   declarations of "g". Clients that perform semantic analysis on a
   program that is not concerned with the actual source code will
   primarily use this semantics-centric view.</p>

 <h4 id="LexicalAndSemanticContexts">Lexical and Semantic Contexts</h4>
 <p>Each declaration has two potentially different
   declaration contexts: a <i>lexical</i> context, which corresponds to
   the source-centric view of the declaration context, and
   a <i>semantic</i> context, which corresponds to the
   semantics-centric view. The lexical context is accessible
   via <code>Decl::getLexicalDeclContext</code> while the
   semantic context is accessible
   via <code>Decl::getDeclContext</code>, both of which return
   <code>DeclContext</code> pointers. For most declarations, the two
   contexts are identical. For example:</p>

 <pre>
 class X {
 public:
   void f(int x);
 };
 </pre>

 <p>Here, the semantic and lexical contexts of <code>X::f</code> are
   the <code>DeclContext</code> associated with the
   class <code>X</code> (itself stored as a <code>RecordDecl</code> AST
   node). However, we can now define <code>X::f</code> out-of-line:</p>

 <pre>
 void X::f(int x = 17) { /* ... */ }
 </pre>

 <p>This definition of has different lexical and semantic
   contexts. The lexical context corresponds to the declaration
   context in which the actual declaration occurred in the source
   code, e.g., the translation unit containing <code>X</code>. Thus,
   this declaration of <code>X::f</code> can be found by traversing
   the declarations provided by
   [<code>decls_begin()</code>, <code>decls_end()</code>) in the
   translation unit.</p>

 <p>The semantic context of <code>X::f</code> corresponds to the
   class <code>X</code>, since this member function is (semantically) a
   member of <code>X</code>. Lookup of the name <code>f</code> into
   the <code>DeclContext</code> associated with <code>X</code> will
   then return the definition of <code>X::f</code> (including
   information about the default argument).</p>

 <h4 id="TransparentContexts">Transparent Declaration Contexts</h4>
 <p>In C and C++, there are several contexts in which names that are
   logically declared inside another declaration will actually "leak"
   out into the enclosing scope from the perspective of name
   lookup. The most obvious instance of this behavior is in
   enumeration types, e.g.,</p>
 <pre>
 enum Color {
   Red,
   Green,
   Blue
 };
 </pre>

 <p>Here, <code>Color</code> is an enumeration, which is a declaration
   context that contains the
   enumerators <code>Red</code>, <code>Green</code>,
   and <code>Blue</code>. Thus, traversing the list of declarations
   contained in the enumeration <code>Color</code> will
   yield <code>Red</code>, <code>Green</code>,
   and <code>Blue</code>. However, outside of the scope
   of <code>Color</code> one can name the enumerator <code>Red</code>
   without qualifying the name, e.g.,</p>

 <pre>
 Color c = Red;
 </pre>

 <p>There are other entities in C++ that provide similar behavior. For
   example, linkage specifications that use curly braces:</p>

 <pre>
 extern "C" {
   void f(int);
   void g(int);
 }
 // f and g are visible here
 </pre>

 <p>For source-level accuracy, we treat the linkage specification and
   enumeration type as a
   declaration context in which its enclosed declarations ("Red",
   "Green", and "Blue"; "f" and "g")
   are declared. However, these declarations are visible outside of the
   scope of the declaration context.</p>

 <p>These language features (and several others, described below) have
   roughly the same set of
   requirements: declarations are declared within a particular lexical
   context, but the declarations are also found via name lookup in
   scopes enclosing the declaration itself. This feature is implemented
   via <i>transparent</i> declaration contexts
   (see <code>DeclContext::isTransparentContext()</code>), whose
   declarations are visible in the nearest enclosing non-transparent
   declaration context. This means that the lexical context of the
   declaration (e.g., an enumerator) will be the
   transparent <code>DeclContext</code> itself, as will the semantic
   context, but the declaration will be visible in every outer context
   up to and including the first non-transparent declaration context (since
   transparent declaration contexts can be nested).</p>

 <p>The transparent <code>DeclContexts</code> are:</p>
 <ul>
   <li>Enumerations (but not C++0x "scoped enumerations"):
     <pre>
 enum Color {
   Red,
   Green,
   Blue
 };
 // Red, Green, and Blue are in scope
   </pre></li>
   <li>C++ linkage specifications:
   <pre>
 extern "C" {
   void f(int);
   void g(int);
 }
 // f and g are in scope
   </pre></li>
   <li>Anonymous unions and structs:
     <pre>
 struct LookupTable {
   bool IsVector;
   union {
     std::vector&lt;Item&gt; *Vector;
     std::set&lt;Item&gt; *Set;
   };
 };

 LookupTable LT;
 LT.Vector = 0; // Okay: finds Vector inside the unnamed union
     </pre>
   </li>
   <li>C++0x inline namespaces:
 <pre>
 namespace mylib {
   inline namespace debug {
     class X;
   }
 }
 mylib::X *xp; // okay: mylib::X refers to mylib::debug::X
 </pre>
 </li>
 </ul>


 <h4 id="MultiDeclContext">Multiply-Defined Declaration Contexts</h4>
 <p>C++ namespaces have the interesting--and, so far, unique--property that
 the namespace can be defined multiple times, and the declarations
 provided by each namespace definition are effectively merged (from
 the semantic point of view). For example, the following two code
 snippets are semantically indistinguishable:</p>
 <pre>
 // Snippet #1:
 namespace N {
   void f();
 }
 namespace N {
   void f(int);
 }

 // Snippet #2:
 namespace N {
   void f();
   void f(int);
 }
 </pre>

 <p>In Clang's representation, the source-centric view of declaration
   contexts will actually have two separate <code>NamespaceDecl</code>
   nodes in Snippet #1, each of which is a declaration context that
   contains a single declaration of "f". However, the semantics-centric
   view provided by name lookup into the namespace <code>N</code> for
   "f" will return an <code>OverloadedFunctionDecl</code> that contains
   both declarations of "f".</p>

 <p><code>DeclContext</code> manages multiply-defined declaration
   contexts internally. The
   function <code>DeclContext::getPrimaryContext</code> retrieves the
   "primary" context for a given <code>DeclContext</code> instance,
   which is the <code>DeclContext</code> responsible for maintaining
   the lookup table used for the semantics-centric view. Given the
   primary context, one can follow the chain
   of <code>DeclContext</code> nodes that define additional
   declarations via <code>DeclContext::getNextContext</code>. Note that
   these functions are used internally within the lookup and insertion
   methods of the <code>DeclContext</code>, so the vast majority of
   clients can ignore them.</p>

 <!-- ======================================================================= -->
 <h3 id="CFG">The <tt>CFG</tt> class</h3>
 <!-- ======================================================================= -->

 <p>The <tt>CFG</tt> class is designed to represent a source-level
 control-flow graph for a single statement (<tt>Stmt*</tt>).  Typically
 instances of <tt>CFG</tt> are constructed for function bodies (usually
 an instance of <tt>CompoundStmt</tt>), but can also be instantiated to
 represent the control-flow of any class that subclasses <tt>Stmt</tt>,
 which includes simple expressions.  Control-flow graphs are especially
 useful for performing
 <a href="http://en.wikipedia.org/wiki/Data_flow_analysis#Sensitivities">flow-
 or path-sensitive</a> program analyses on a given function.</p>

 <!-- ============ -->
 <h4>Basic Blocks</h4>
 <!-- ============ -->

 <p>Concretely, an instance of <tt>CFG</tt> is a collection of basic
 blocks.  Each basic block is an instance of <tt>CFGBlock</tt>, which
 simply contains an ordered sequence of <tt>Stmt*</tt> (each referring
 to statements in the AST).  The ordering of statements within a block
 indicates unconditional flow of control from one statement to the
 next.  <a href="#ConditionalControlFlow">Conditional control-flow</a>
 is represented using edges between basic blocks.  The statements
 within a given <tt>CFGBlock</tt> can be traversed using
 the <tt>CFGBlock::*iterator</tt> interface.</p>

 <p>
 A <tt>CFG</tt> object owns the instances of <tt>CFGBlock</tt> within
 the control-flow graph it represents.  Each <tt>CFGBlock</tt> within a
 CFG is also uniquely numbered (accessible
 via <tt>CFGBlock::getBlockID()</tt>).  Currently the number is
 based on the ordering the blocks were created, but no assumptions
 should be made on how <tt>CFGBlock</tt>s are numbered other than their
 numbers are unique and that they are numbered from 0..N-1 (where N is
 the number of basic blocks in the CFG).</p>

 <!-- ===================== -->
 <h4>Entry and Exit Blocks</h4>
 <!-- ===================== -->

 Each instance of <tt>CFG</tt> contains two special blocks:
 an <i>entry</i> block (accessible via <tt>CFG::getEntry()</tt>), which
 has no incoming edges, and an <i>exit</i> block (accessible
 via <tt>CFG::getExit()</tt>), which has no outgoing edges.  Neither
 block contains any statements, and they serve the role of providing a
 clear entrance and exit for a body of code such as a function body.
 The presence of these empty blocks greatly simplifies the
 implementation of many analyses built on top of CFGs.

 <!-- ===================================================== -->
 <h4 id ="ConditionalControlFlow">Conditional Control-Flow</h4>
 <!-- ===================================================== -->

 <p>Conditional control-flow (such as those induced by if-statements
 and loops) is represented as edges between <tt>CFGBlock</tt>s.
 Because different C language constructs can induce control-flow,
 each <tt>CFGBlock</tt> also records an extra <tt>Stmt*</tt> that
 represents the <i>terminator</i> of the block.  A terminator is simply
 the statement that caused the control-flow, and is used to identify
 the nature of the conditional control-flow between blocks.  For
 example, in the case of an if-statement, the terminator refers to
 the <tt>IfStmt</tt> object in the AST that represented the given
 branch.</p>

 <p>To illustrate, consider the following code example:</p>

 <code>
 int foo(int x) {<br>
 &nbsp;&nbsp;x = x + 1;<br>
 <br>
 &nbsp;&nbsp;if (x > 2) x++;<br>
 &nbsp;&nbsp;else {<br>
 &nbsp;&nbsp;&nbsp;&nbsp;x += 2;<br>
 &nbsp;&nbsp;&nbsp;&nbsp;x *= 2;<br>
 &nbsp;&nbsp;}<br>
 <br>
 &nbsp;&nbsp;return x;<br>
 }
 </code>

 <p>After invoking the parser+semantic analyzer on this code fragment,
 the AST of the body of <tt>foo</tt> is referenced by a
 single <tt>Stmt*</tt>.  We can then construct an instance
 of <tt>CFG</tt> representing the control-flow graph of this function
 body by single call to a static class method:</p>

 <code>
 &nbsp;&nbsp;Stmt* FooBody = ...<br>
 &nbsp;&nbsp;CFG*  FooCFG = <b>CFG::buildCFG</b>(FooBody);
 </code>

 <p>It is the responsibility of the caller of <tt>CFG::buildCFG</tt>
 to <tt>delete</tt> the returned <tt>CFG*</tt> when the CFG is no
 longer needed.</p>

 <p>Along with providing an interface to iterate over
 its <tt>CFGBlock</tt>s, the <tt>CFG</tt> class also provides methods
 that are useful for debugging and visualizing CFGs.  For example, the
 method
 <tt>CFG::dump()</tt> dumps a pretty-printed version of the CFG to
 standard error.  This is especially useful when one is using a
 debugger such as gdb.  For example, here is the output
 of <tt>FooCFG->dump()</tt>:</p>

 <code>
 &nbsp;[ B5 (ENTRY) ]<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Predecessors (0):<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B4<br>
 <br>
 &nbsp;[ B4 ]<br>
 &nbsp;&nbsp;&nbsp;&nbsp;1: x = x + 1<br>
 &nbsp;&nbsp;&nbsp;&nbsp;2: (x > 2)<br>
 &nbsp;&nbsp;&nbsp;&nbsp;<b>T: if [B4.2]</b><br>
 &nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B5<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Successors (2): B3 B2<br>
 <br>
 &nbsp;[ B3 ]<br>
 &nbsp;&nbsp;&nbsp;&nbsp;1: x++<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
 <br>
 &nbsp;[ B2 ]<br>
 &nbsp;&nbsp;&nbsp;&nbsp;1: x += 2<br>
 &nbsp;&nbsp;&nbsp;&nbsp;2: x *= 2<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B4<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B1<br>
 <br>
 &nbsp;[ B1 ]<br>
 &nbsp;&nbsp;&nbsp;&nbsp;1: return x;<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Predecessors (2): B2 B3<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Successors (1): B0<br>
 <br>
 &nbsp;[ B0 (EXIT) ]<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Predecessors (1): B1<br>
 &nbsp;&nbsp;&nbsp;&nbsp;Successors (0):
 </code>

 <p>For each block, the pretty-printed output displays for each block
 the number of <i>predecessor</i> blocks (blocks that have outgoing
 control-flow to the given block) and <i>successor</i> blocks (blocks
 that have control-flow that have incoming control-flow from the given
 block).  We can also clearly see the special entry and exit blocks at
 the beginning and end of the pretty-printed output.  For the entry
 block (block B5), the number of predecessor blocks is 0, while for the
 exit block (block B0) the number of successor blocks is 0.</p>

 <p>The most interesting block here is B4, whose outgoing control-flow
 represents the branching caused by the sole if-statement
 in <tt>foo</tt>.  Of particular interest is the second statement in
 the block, <b><tt>(x > 2)</tt></b>, and the terminator, printed
 as <b><tt>if [B4.2]</tt></b>.  The second statement represents the
 evaluation of the condition of the if-statement, which occurs before
 the actual branching of control-flow.  Within the <tt>CFGBlock</tt>
 for B4, the <tt>Stmt*</tt> for the second statement refers to the
 actual expression in the AST for <b><tt>(x > 2)</tt></b>.  Thus
 pointers to subclasses of <tt>Expr</tt> can appear in the list of
 statements in a block, and not just subclasses of <tt>Stmt</tt> that
 refer to proper C statements.</p>

 <p>The terminator of block B4 is a pointer to the <tt>IfStmt</tt>
 object in the AST.  The pretty-printer outputs <b><tt>if
 [B4.2]</tt></b> because the condition expression of the if-statement
 has an actual place in the basic block, and thus the terminator is
 essentially
 <i>referring</i> to the expression that is the second statement of
 block B4 (i.e., B4.2).  In this manner, conditions for control-flow
 (which also includes conditions for loops and switch statements) are
 hoisted into the actual basic block.</p>

 <!-- ===================== -->
 <!-- <h4>Implicit Control-Flow</h4> -->
 <!-- ===================== -->

 <!--
 <p>A key design principle of the <tt>CFG</tt> class was to not require
 any transformations to the AST in order to represent control-flow.
 Thus the <tt>CFG</tt> does not perform any "lowering" of the
 statements in an AST: loops are not transformed into guarded gotos,
 short-circuit operations are not converted to a set of if-statements,
 and so on.</p>
 -->


 <!-- ======================================================================= -->
 <h3 id="Constants">Constant Folding in the Clang AST</h3>
 <!-- ======================================================================= -->

 <p>There are several places where constants and constant folding matter a lot to
 the Clang front-end.  First, in general, we prefer the AST to retain the source
 code as close to how the user wrote it as possible.  This means that if they
 wrote "5+4", we want to keep the addition and two constants in the AST, we don't
 want to fold to "9".  This means that constant folding in various ways turns
 into a tree walk that needs to handle the various cases.</p>

 <p>However, there are places in both C and C++ that require constants to be
 folded.  For example, the C standard defines what an "integer constant
 expression" (i-c-e) is with very precise and specific requirements.  The
 language then requires i-c-e's in a lot of places (for example, the size of a
 bitfield, the value for a case statement, etc).  For these, we have to be able
 to constant fold the constants, to do semantic checks (e.g. verify bitfield size
 is non-negative and that case statements aren't duplicated).  We aim for Clang
 to be very pedantic about this, diagnosing cases when the code does not use an
 i-c-e where one is required, but accepting the code unless running with
 <tt>-pedantic-errors</tt>.</p>

 <p>Things get a little bit more tricky when it comes to compatibility with
 real-world source code.  Specifically, GCC has historically accepted a huge
 superset of expressions as i-c-e's, and a lot of real world code depends on this
 unfortuate accident of history (including, e.g., the glibc system headers).  GCC
 accepts anything its "fold" optimizer is capable of reducing to an integer
 constant, which means that the definition of what it accepts changes as its
 optimizer does.  One example is that GCC accepts things like "case X-X:" even
 when X is a variable, because it can fold this to 0.</p>

 <p>Another issue are how constants interact with the extensions we support, such
 as __builtin_constant_p, __builtin_inf, __extension__ and many others.  C99
 obviously does not specify the semantics of any of these extensions, and the
 definition of i-c-e does not include them.  However, these extensions are often
 used in real code, and we have to have a way to reason about them.</p>

 <p>Finally, this is not just a problem for semantic analysis.  The code
 generator and other clients have to be able to fold constants (e.g. to
 initialize global variables) and has to handle a superset of what C99 allows.
 Further, these clients can benefit from extended information.  For example, we
 know that "foo()||1" always evaluates to true, but we can't replace the
 expression with true because it has side effects.</p>

 <!-- ======================= -->
 <h4>Implementation Approach</h4>
 <!-- ======================= -->

 <p>After trying several different approaches, we've finally converged on a
 design (Note, at the time of this writing, not all of this has been implemented,
 consider this a design goal!).  Our basic approach is to define a single
 recursive method evaluation method (<tt>Expr::Evaluate</tt>), which is
 implemented in <tt>AST/ExprConstant.cpp</tt>.  Given an expression with 'scalar'
 type (integer, fp, complex, or pointer) this method returns the following
 information:</p>

 <ul>
 <li>Whether the expression is an integer constant expression, a general
     constant that was folded but has no side effects, a general constant that
     was folded but that does have side effects, or an uncomputable/unfoldable
     value.
 </li>
 <li>If the expression was computable in any way, this method returns the APValue
     for the result of the expression.</li>
 <li>If the expression is not evaluatable at all, this method returns
     information on one of the problems with the expression.  This includes a
     SourceLocation for where the problem is, and a diagnostic ID that explains
     the problem.  The diagnostic should be have ERROR type.</li>
 <li>If the expression is not an integer constant expression, this method returns
     information on one of the problems with the expression.  This includes a
     SourceLocation for where the problem is, and a diagnostic ID that explains
     the problem.  The diagnostic should be have EXTENSION type.</li>
 </ul>

 <p>This information gives various clients the flexibility that they want, and we
 will eventually have some helper methods for various extensions.  For example,
 Sema should have a <tt>Sema::VerifyIntegerConstantExpression</tt> method, which
 calls Evaluate on the expression.  If the expression is not foldable, the error
 is emitted, and it would return true.  If the expression is not an i-c-e, the
 EXTENSION diagnostic is emitted.  Finally it would return false to indicate that
 the AST is ok.</p>

 <p>Other clients can use the information in other ways, for example, codegen can
 just use expressions that are foldable in any way.</p>

 <!-- ========== -->
 <h4>Extensions</h4>
 <!-- ========== -->

 <p>This section describes how some of the various extensions Clang supports
 interacts with constant evaluation:</p>

 <ul>
 <li><b><tt>__extension__</tt></b>: The expression form of this extension causes
     any evaluatable subexpression to be accepted as an integer constant
     expression.</li>
 <li><b><tt>__builtin_constant_p</tt></b>: This returns true (as a integer
     constant expression) if the operand is any evaluatable constant.  As a
     special case, if <tt>__builtin_constant_p</tt> is the (potentially
     parenthesized) condition of a conditional operator expression ("?:"), only
     the true side of the conditional operator is considered, and it is evaluated
     with full constant folding.</li>
 <li><b><tt>__builtin_choose_expr</tt></b>: The condition is required to be an
     integer constant expression, but we accept any constant as an "extension of
     an extension".  This only evaluates one operand depending on which way the
     condition evaluates.</li>
 <li><b><tt>__builtin_classify_type</tt></b>: This always returns an integer
     constant expression.</li>
 <li><b><tt>__builtin_inf,nan,..</tt></b>: These are treated just like a
     floating-point literal.</li>
 <li><b><tt>__builtin_abs,copysign,..</tt></b>: These are constant folded as
     general constant expressions.</li>
 </ul>


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