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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="#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>
</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 DiagnosticKinds.def file</h4>
<!-- ============================ -->

<p>Diagnostics are created by adding an entry to the <tt><a
href="http://llvm.org/svn/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticKinds.def"
>DiagnosticKinds.def</a></tt> file.  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 a 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.  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>.  Clang also internally
supports a fully fine grained mapping mechanism that allows you to map any
diagnostic that doesn't have <tt>ERRROR</tt> severity to any output level that
you want.  This is used to implement options like <tt>-Wunused_macros</tt>,
<tt>-Wundef</tt> etc.  Fatal errors are considered so severe that diagnostics
that occur after them are supressed as "almost certainly useless".</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>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.</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>"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><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: it's spelling location
and it's 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>For efficiency, we only track one level of macro instantiations: if a token was
produced by multiple instantiations, we only track the source and ultimate
destination.  Though we could track the intermediate instantiation points, this
would require extra bookkeeping and no known client would benefit substantially
from this.</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="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
intepretes 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;", which may refer to a function or type
depending on whether foo is a function template or class template.  The
AnnotationValue pointer is a pointer to a malloc'd TemplateIdAnnotation object.
FIXME: I don't think the parsing logic is right for this.  Shouldn't type
templates be turned into annot_typename??</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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