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<h1>Precompiled Header and Modules Internals</h1>

  <p>This document describes the design and implementation of Clang's
  precompiled headers (PCH) and modules. If you are interested in the end-user
  view, please see the <a
   href="UsersManual.html#precompiledheaders">User's Manual</a>.</p>

  <p><b>Table of Contents</b></p>
  <ul>
    <li><a href="#usage">Using Precompiled Headers with
    <tt>clang</tt></a></li>
    <li><a href="#philosophy">Design Philosophy</a></li>
    <li><a href="#contents">Serialized AST File Contents</a>
      <ul>
        <li><a href="#metadata">Metadata Block</a></li>
        <li><a href="#sourcemgr">Source Manager Block</a></li>
        <li><a href="#preprocessor">Preprocessor Block</a></li>
        <li><a href="#types">Types Block</a></li>
        <li><a href="#decls">Declarations Block</a></li>
        <li><a href="#stmt">Statements and Expressions</a></li>
        <li><a href="#idtable">Identifier Table Block</a></li>
        <li><a href="#method-pool">Method Pool Block</a></li>
      </ul>
    </li>
    <li><a href="#tendrils">AST Reader Integration Points</a></li>
    <li><a href="#chained">Chained precompiled headers</a></li>
    <li><a href="#modules">Modules</a></li>
</ul>
    
<h2 id="usage">Using Precompiled Headers with <tt>clang</tt></h2>

<p>The Clang compiler frontend, <tt>clang -cc1</tt>, supports two command line
options for generating and using PCH files.<p>

<p>To generate PCH files using <tt>clang -cc1</tt>, use the option
<b><tt>-emit-pch</tt></b>:

<pre> $ clang -cc1 test.h -emit-pch -o test.h.pch </pre>

<p>This option is transparently used by <tt>clang</tt> when generating
PCH files. The resulting PCH file contains the serialized form of the
compiler's internal representation after it has completed parsing and
semantic analysis. The PCH file can then be used as a prefix header
with the <b><tt>-include-pch</tt></b> option:</p>

<pre>
  $ clang -cc1 -include-pch test.h.pch test.c -o test.s
</pre>

<h2 id="philosophy">Design Philosophy</h2>
  
<p>Precompiled headers are meant to improve overall compile times for
  projects, so the design of precompiled headers is entirely driven by
  performance concerns. The use case for precompiled headers is
  relatively simple: when there is a common set of headers that is
  included in nearly every source file in the project, we
  <i>precompile</i> that bundle of headers into a single precompiled
  header (PCH file). Then, when compiling the source files in the
  project, we load the PCH file first (as a prefix header), which acts
  as a stand-in for that bundle of headers.</p>

<p>A precompiled header implementation improves performance when:</p>
<ul>
  <li>Loading the PCH file is significantly faster than re-parsing the
  bundle of headers stored within the PCH file. Thus, a precompiled
  header design attempts to minimize the cost of reading the PCH
  file. Ideally, this cost should not vary with the size of the
  precompiled header file.</li>
  
  <li>The cost of generating the PCH file initially is not so large
  that it counters the per-source-file performance improvement due to
  eliminating the need to parse the bundled headers in the first
  place. This is particularly important on multi-core systems, because
  PCH file generation serializes the build when all compilations
  require the PCH file to be up-to-date.</li>
</ul>

<p>Modules, as implemented in Clang, use the same mechanisms as
precompiled headers to save a serialized AST file (one per module) and
use those AST modules. From an implementation standpoint, modules are
a generalization of precompiled headers, lifting a number of
restrictions placed on precompiled headers. In particular, there can
only be one precompiled header and it must be included at the
beginning of the translation unit. The extensions to the AST file
format required for modules are discussed in the section on <a href="#modules">modules</a>.</p>

<p>Clang's AST files are designed with a compact on-disk
representation, which minimizes both creation time and the time
required to initially load the AST file. The AST file itself contains
a serialized representation of Clang's abstract syntax trees and
supporting data structures, stored using the same compressed bitstream
as <a href="http://llvm.org/docs/BitCodeFormat.html">LLVM's bitcode
file format</a>.</p>

<p>Clang's AST files are loaded "lazily" from disk. When an
AST file is initially loaded, Clang reads only a small amount of data
from the AST file to establish where certain important data structures
are stored. The amount of data read in this initial load is
independent of the size of the AST file, such that a larger AST file
does not lead to longer AST load times. The actual header data in the
AST file--macros, functions, variables, types, etc.--is loaded only
when it is referenced from the user's code, at which point only that
entity (and those entities it depends on) are deserialized from the
AST file. With this approach, the cost of using an AST file
for a translation unit is proportional to the amount of code actually
used from the AST file, rather than being proportional to the size of
the AST file itself.</p> 

<p>When given the <code>-print-stats</code> option, Clang produces
statistics describing how much of the AST file was actually
loaded from disk. For a simple "Hello, World!" program that includes
the Apple <code>Cocoa.h</code> header (which is built as a precompiled
header), this option illustrates how little of the actual precompiled
header is required:</p>

<pre>
*** PCH Statistics:
  933 stat cache hits
  4 stat cache misses
  895/39981 source location entries read (2.238563%)
  19/15315 types read (0.124061%)
  20/82685 declarations read (0.024188%)
  154/58070 identifiers read (0.265197%)
  0/7260 selectors read (0.000000%)
  0/30842 statements read (0.000000%)
  4/8400 macros read (0.047619%)
  1/4995 lexical declcontexts read (0.020020%)
  0/4413 visible declcontexts read (0.000000%)
  0/7230 method pool entries read (0.000000%)
  0 method pool misses
</pre>

<p>For this small program, only a tiny fraction of the source
locations, types, declarations, identifiers, and macros were actually
deserialized from the precompiled header. These statistics can be
useful to determine whether the AST file implementation can
be improved by making more of the implementation lazy.</p>

<p>Precompiled headers can be chained. When you create a PCH while
including an existing PCH, Clang can create the new PCH by referencing
the original file and only writing the new data to the new file. For
example, you could create a PCH out of all the headers that are very
commonly used throughout your project, and then create a PCH for every
single source file in the project that includes the code that is
specific to that file, so that recompiling the file itself is very fast,
without duplicating the data from the common headers for every
file. The mechanisms behind chained precompiled headers are discussed
in a <a href="#chained">later section</a>.

<h2 id="contents">AST File Contents</h2>

<img src="PCHLayout.png" style="float:right" alt="Precompiled header layout">

<p>Clang's AST files are organized into several different
blocks, each of which contains the serialized representation of a part
of Clang's internal representation. Each of the blocks corresponds to
either a block or a record within <a
 href="http://llvm.org/docs/BitCodeFormat.html">LLVM's bitstream
format</a>. The contents of each of these logical blocks are described
below.</p>

<p>For a given AST file, the <a
href="http://llvm.org/cmds/llvm-bcanalyzer.html"><code>llvm-bcanalyzer</code></a>
utility can be used to examine the actual structure of the bitstream
for the AST file. This information can be used both to help
understand the structure of the AST file and to isolate
areas where AST files can still be optimized, e.g., through
the introduction of abbreviations.</p>

<h3 id="metadata">Metadata Block</h3>

<p>The metadata block contains several records that provide
information about how the AST file was built. This metadata
is primarily used to validate the use of an AST file. For
example, a precompiled header built for a 32-bit x86 target cannot be used
when compiling for a 64-bit x86 target. The metadata block contains
information about:</p>

<dl>
  <dt>Language options</dt>
  <dd>Describes the particular language dialect used to compile the
AST file, including major options (e.g., Objective-C support) and more
minor options (e.g., support for "//" comments). The contents of this
record correspond to the <code>LangOptions</code> class.</dd>
  
  <dt>Target architecture</dt>
  <dd>The target triple that describes the architecture, platform, and
ABI for which the AST file was generated, e.g.,
<code>i386-apple-darwin9</code>.</dd>
  
  <dt>AST version</dt>
  <dd>The major and minor version numbers of the AST file
format. Changes in the minor version number should not affect backward
compatibility, while changes in the major version number imply that a
newer compiler cannot read an older precompiled header (and
vice-versa).</dd>

  <dt>Original file name</dt>
  <dd>The full path of the header that was used to generate the
AST file.</dd>

  <dt>Predefines buffer</dt>
  <dd>Although not explicitly stored as part of the metadata, the
predefines buffer is used in the validation of the AST file.
The predefines buffer itself contains code generated by the compiler
to initialize the preprocessor state according to the current target,
platform, and command-line options. For example, the predefines buffer
will contain "<code>#define __STDC__ 1</code>" when we are compiling C
without Microsoft extensions. The predefines buffer itself is stored
within the <a href="#sourcemgr">source manager block</a>, but its
contents are verified along with the rest of the metadata.</dd>

</dl>

<p>A chained PCH file (that is, one that references another PCH) and a
module (which may import other modules) have additional metadata
containing the list of all AST files that this AST file depends
on. Each of those files will be loaded along with this AST file.</p>

<p>For chained precompiled headers, the language options, target
architecture and predefines buffer data is taken from the end of the
chain, since they have to match anyway.</p>

<h3 id="sourcemgr">Source Manager Block</h3>

<p>The source manager block contains the serialized representation of
Clang's <a
 href="InternalsManual.html#SourceLocation">SourceManager</a> class,
which handles the mapping from source locations (as represented in
Clang's abstract syntax tree) into actual column/line positions within
a source file or macro instantiation. The AST file's
representation of the source manager also includes information about
all of the headers that were (transitively) included when building the
AST file.</p>

<p>The bulk of the source manager block is dedicated to information
about the various files, buffers, and macro instantiations into which
a source location can refer. Each of these is referenced by a numeric
"file ID", which is a unique number (allocated starting at 1) stored
in the source location. Clang serializes the information for each kind
of file ID, along with an index that maps file IDs to the position
within the AST file where the information about that file ID is
stored. The data associated with a file ID is loaded only when
required by the front end, e.g., to emit a diagnostic that includes a
macro instantiation history inside the header itself.</p>

<p>The source manager block also contains information about all of the
headers that were included when building the AST file. This
includes information about the controlling macro for the header (e.g.,
when the preprocessor identified that the contents of the header
dependent on a macro like <code>LLVM_CLANG_SOURCEMANAGER_H</code>)
along with a cached version of the results of the <code>stat()</code>
system calls performed when building the AST file. The
latter is particularly useful in reducing system time when searching
for include files.</p>

<h3 id="preprocessor">Preprocessor Block</h3>

<p>The preprocessor block contains the serialized representation of
the preprocessor. Specifically, it contains all of the macros that
have been defined by the end of the header used to build the
AST file, along with the token sequences that comprise each
macro. The macro definitions are only read from the AST file when the
name of the macro first occurs in the program. This lazy loading of
macro definitions is triggered by lookups into the <a
 href="#idtable">identifier table</a>.</p>

<h3 id="types">Types Block</h3>

<p>The types block contains the serialized representation of all of
the types referenced in the translation unit. Each Clang type node
(<code>PointerType</code>, <code>FunctionProtoType</code>, etc.) has a
corresponding record type in the AST file. When types are deserialized
from the AST file, the data within the record is used to
reconstruct the appropriate type node using the AST context.</p>

<p>Each type has a unique type ID, which is an integer that uniquely
identifies that type. Type ID 0 represents the NULL type, type IDs
less than <code>NUM_PREDEF_TYPE_IDS</code> represent predefined types
(<code>void</code>, <code>float</code>, etc.), while other
"user-defined" type IDs are assigned consecutively from
<code>NUM_PREDEF_TYPE_IDS</code> upward as the types are encountered.
The AST file has an associated mapping from the user-defined types
block to the location within the types block where the serialized
representation of that type resides, enabling lazy deserialization of
types. When a type is referenced from within the AST file, that
reference is encoded using the type ID shifted left by 3 bits. The
lower three bits are used to represent the <code>const</code>,
<code>volatile</code>, and <code>restrict</code> qualifiers, as in
Clang's <a
 href="http://clang.llvm.org/docs/InternalsManual.html#Type">QualType</a>
class.</p>

<h3 id="decls">Declarations Block</h3>

<p>The declarations block contains the serialized representation of
all of the declarations referenced in the translation unit. Each Clang
declaration node (<code>VarDecl</code>, <code>FunctionDecl</code>,
etc.) has a corresponding record type in the AST file. When
declarations are deserialized from the AST file, the data
within the record is used to build and populate a new instance of the
corresponding <code>Decl</code> node. As with types, each declaration
node has a numeric ID that is used to refer to that declaration within
the AST file. In addition, a lookup table provides a mapping from that
numeric ID to the offset within the precompiled header where that
declaration is described.</p>

<p>Declarations in Clang's abstract syntax trees are stored
hierarchically. At the top of the hierarchy is the translation unit
(<code>TranslationUnitDecl</code>), which contains all of the
declarations in the translation unit but is not actually written as a
specific declaration node. Its child declarations (such as
functions or struct types) may also contain other declarations inside
them, and so on. Within Clang, each declaration is stored within a <a
href="http://clang.llvm.org/docs/InternalsManual.html#DeclContext">declaration
context</a>, as represented by the <code>DeclContext</code> class.
Declaration contexts provide the mechanism to perform name lookup
within a given declaration (e.g., find the member named <code>x</code>
in a structure) and iterate over the declarations stored within a
context (e.g., iterate over all of the fields of a structure for
structure layout).</p>

<p>In Clang's AST file format, deserializing a declaration
that is a <code>DeclContext</code> is a separate operation from
deserializing all of the declarations stored within that declaration
context. Therefore, Clang will deserialize the translation unit
declaration without deserializing the declarations within that
translation unit. When required, the declarations stored within a
declaration context will be deserialized. There are two representations
of the declarations within a declaration context, which correspond to
the name-lookup and iteration behavior described above:</p>

<ul>
  <li>When the front end performs name lookup to find a name
  <code>x</code> within a given declaration context (for example,
  during semantic analysis of the expression <code>p-&gt;x</code>,
  where <code>p</code>'s type is defined in the precompiled header),
  Clang refers to an on-disk hash table that maps from the names
  within that declaration context to the declaration IDs that
  represent each visible declaration with that name. The actual
  declarations will then be deserialized to provide the results of
  name lookup.</li>

  <li>When the front end performs iteration over all of the
  declarations within a declaration context, all of those declarations
  are immediately de-serialized. For large declaration contexts (e.g.,
  the translation unit), this operation is expensive; however, large
  declaration contexts are not traversed in normal compilation, since
  such a traversal is unnecessary. However, it is common for the code
  generator and semantic analysis to traverse declaration contexts for
  structs, classes, unions, and enumerations, although those contexts
  contain relatively few declarations in the common case.</li>
</ul>

<h3 id="stmt">Statements and Expressions</h3>

<p>Statements and expressions are stored in the AST file in
both the <a href="#types">types</a> and the <a
 href="#decls">declarations</a> blocks, because every statement or
expression will be associated with either a type or declaration. The
actual statement and expression records are stored immediately
following the declaration or type that owns the statement or
expression. For example, the statement representing the body of a
function will be stored directly following the declaration of the
function.</p>

<p>As with types and declarations, each statement and expression kind
in Clang's abstract syntax tree (<code>ForStmt</code>,
<code>CallExpr</code>, etc.) has a corresponding record type in the
AST file, which contains the serialized representation of
that statement or expression. Each substatement or subexpression
within an expression is stored as a separate record (which keeps most
records to a fixed size). Within the AST file, the
subexpressions of an expression are stored, in reverse order, prior to the expression
that owns those expression, using a form of <a
href="http://en.wikipedia.org/wiki/Reverse_Polish_notation">Reverse
Polish Notation</a>. For example, an expression <code>3 - 4 + 5</code>
would be represented as follows:</p>

<table border="1">
  <tr><td><code>IntegerLiteral(5)</code></td></tr>
  <tr><td><code>IntegerLiteral(4)</code></td></tr>
  <tr><td><code>IntegerLiteral(3)</code></td></tr>
  <tr><td><code>BinaryOperator(-)</code></td></tr>
  <tr><td><code>BinaryOperator(+)</code></td></tr>
  <tr><td>STOP</td></tr>
</table>

<p>When reading this representation, Clang evaluates each expression
record it encounters, builds the appropriate abstract syntax tree node,
and then pushes that expression on to a stack. When a record contains <i>N</i>
subexpressions--<code>BinaryOperator</code> has two of them--those
expressions are popped from the top of the stack. The special STOP
code indicates that we have reached the end of a serialized expression
or statement; other expression or statement records may follow, but
they are part of a different expression.</p>

<h3 id="idtable">Identifier Table Block</h3>

<p>The identifier table block contains an on-disk hash table that maps
each identifier mentioned within the AST file to the
serialized representation of the identifier's information (e.g, the
<code>IdentifierInfo</code> structure). The serialized representation
contains:</p>

<ul>
  <li>The actual identifier string.</li>
  <li>Flags that describe whether this identifier is the name of a
  built-in, a poisoned identifier, an extension token, or a
  macro.</li>
  <li>If the identifier names a macro, the offset of the macro
  definition within the <a href="#preprocessor">preprocessor
  block</a>.</li>
  <li>If the identifier names one or more declarations visible from
  translation unit scope, the <a href="#decls">declaration IDs</a> of these
  declarations.</li>
</ul>

<p>When an AST file is loaded, the AST file reader
mechanism introduces itself into the identifier table as an external
lookup source. Thus, when the user program refers to an identifier
that has not yet been seen, Clang will perform a lookup into the
identifier table. If an identifier is found, its contents (macro 
definitions, flags, top-level declarations, etc.) will be
deserialized, at which point the corresponding
<code>IdentifierInfo</code> structure will have the same contents it
would have after parsing the headers in the AST file.</p>

<p>Within the AST file, the identifiers used to name declarations are represented with an integral value. A separate table provides a mapping from this integral value (the identifier ID) to the location within the on-disk
hash table where that identifier is stored. This mapping is used when
deserializing the name of a declaration, the identifier of a token, or
any other construct in the AST file that refers to a name.</p>

<h3 id="method-pool">Method Pool Block</h3>

<p>The method pool block is represented as an on-disk hash table that
serves two purposes: it provides a mapping from the names of
Objective-C selectors to the set of Objective-C instance and class
methods that have that particular selector (which is required for
semantic analysis in Objective-C) and also stores all of the selectors
used by entities within the AST file. The design of the
method pool is similar to that of the <a href="#idtable">identifier
table</a>: the first time a particular selector is formed during the
compilation of the program, Clang will search in the on-disk hash
table of selectors; if found, Clang will read the Objective-C methods
associated with that selector into the appropriate front-end data
structure (<code>Sema::InstanceMethodPool</code> and
<code>Sema::FactoryMethodPool</code> for instance and class methods,
respectively).</p>

<p>As with identifiers, selectors are represented by numeric values
within the AST file. A separate index maps these numeric selector
values to the offset of the selector within the on-disk hash table,
and will be used when de-serializing an Objective-C method declaration
(or other Objective-C construct) that refers to the selector.</p>

<h2 id="tendrils">AST Reader Integration Points</h2>

<p>The "lazy" deserialization behavior of AST files requires
their integration into several completely different submodules of
Clang. For example, lazily deserializing the declarations during name
lookup requires that the name-lookup routines be able to query the
AST file to find entities stored there.</p>

<p>For each Clang data structure that requires direct interaction with
the AST reader logic, there is an abstract class that provides
the interface between the two modules. The <code>ASTReader</code>
class, which handles the loading of an AST file, inherits
from all of these abstract classes to provide lazy deserialization of
Clang's data structures. <code>ASTReader</code> implements the
following abstract classes:</p>

<dl>
  <dt><code>StatSysCallCache</code></dt>
  <dd>This abstract interface is associated with the
    <code>FileManager</code> class, and is used whenever the file
    manager is going to perform a <code>stat()</code> system call.</dd>
    
  <dt><code>ExternalSLocEntrySource</code></dt>
  <dd>This abstract interface is associated with the
    <code>SourceManager</code> class, and is used whenever the
    <a href="#sourcemgr">source manager</a> needs to load the details
    of a file, buffer, or macro instantiation.</dd>

  <dt><code>IdentifierInfoLookup</code></dt>
  <dd>This abstract interface is associated with the
    <code>IdentifierTable</code> class, and is used whenever the
    program source refers to an identifier that has not yet been seen.
    In this case, the AST reader searches for
    this identifier within its <a href="#idtable">identifier table</a>
    to load any top-level declarations or macros associated with that
    identifier.</dd>

  <dt><code>ExternalASTSource</code></dt>
  <dd>This abstract interface is associated with the
    <code>ASTContext</code> class, and is used whenever the abstract
    syntax tree nodes need to loaded from the AST file. It
    provides the ability to de-serialize declarations and types
    identified by their numeric values, read the bodies of functions
    when required, and read the declarations stored within a
    declaration context (either for iteration or for name lookup).</dd>
    
  <dt><code>ExternalSemaSource</code></dt>
  <dd>This abstract interface is associated with the <code>Sema</code>
    class, and is used whenever semantic analysis needs to read
    information from the <a href="#methodpool">global method
    pool</a>.</dd>
</dl>

<h2 id="chained">Chained precompiled headers</h2>

<p>Chained precompiled headers were initially intended to improve the
performance of IDE-centric operations such as syntax highlighting and
code completion while a particular source file is being edited by the
user. To minimize the amount of reparsing required after a change to
the file, a form of precompiled header--called a precompiled
<i>preamble</i>--is automatically generated by parsing all of the
headers in the source file, up to and including the last
#include. When only the source file changes (and none of the headers
it depends on), reparsing of that source file can use the precompiled
preamble and start parsing after the #includes, so parsing time is
proportional to the size of the source file (rather than all of its
includes). However, the compilation of that translation unit
may already use a precompiled header: in this case, Clang will create
the precompiled preamble as a chained precompiled header that refers
to the original precompiled header. This drastically reduces the time
needed to serialize the precompiled preamble for use in reparsing.</p>

<p>Chained precompiled headers get their name because each precompiled header
can depend on one other precompiled header, forming a chain of
dependencies. A translation unit will then include the precompiled
header that starts the chain (i.e., nothing depends on it). This
linearity of dependencies is important for the semantic model of
chained precompiled headers, because the most-recent precompiled
header can provide information that overrides the information provided
by the precompiled headers it depends on, just like a header file
<code>B.h</code> that includes another header <code>A.h</code> can
modify the state produced by parsing <code>A.h</code>, e.g., by
<code>#undef</code>'ing a macro defined in <code>A.h</code>.</p>

<p>There are several ways in which chained precompiled headers
generalize the AST file model:</p>

<dl>
  <dt>Numbering of IDs</dt>
  <dd>Many different kinds of entities--identifiers, declarations,
  types, etc.---have ID numbers that start at 1 or some other
  predefined constant and grow upward. Each precompiled header records
  the maximum ID number it has assigned in each category. Then, when a
  new precompiled header is generated that depends on (chains to)
  another precompiled header, it will start counting at the next
  available ID number. This way, one can determine, given an ID
  number, which AST file actually contains the entity.</dd>

  <dt>Name lookup</dt>
  <dd>When writing a chained precompiled header, Clang attempts to
  write only information that has changed from the precompiled header
  on which it is based. This changes the lookup algorithm for the
  various tables, such as the <a href="#idtable">identifier table</a>:
  the search starts at the most-recent precompiled header. If no entry
  is found, lookup then proceeds to the identifier table in the
  precompiled header it depends on, and so one. Once a lookup
  succeeds, that result is considered definitive, overriding any
  results from earlier precompiled headers.</dd>

  <dt>Update records</dt>
  <dd>There are various ways in which a later precompiled header can
  modify the entities described in an earlier precompiled header. For
  example, later precompiled headers can add entries into the various
  name-lookup tables for the translation unit or namespaces, or add
  new categories to an Objective-C class. Each of these updates is
  captured in an "update record" that is stored in the chained
  precompiled header file and will be loaded along with the original
  entity.</dd>
</dl>

<h2 id="modules">Modules</h2>

<p>Modules generalize the chained precompiled header model yet
further, from a linear chain of precompiled headers to an arbitrary
directed acyclic graph (DAG) of AST files. All of the same techniques
used to make chained precompiled headers work---ID number, name
lookup, update records---are shared with modules. However, the DAG
nature of modules introduce a number of additional complications to
the model:

<dl>
  <dt>Numbering of IDs</dt>
  <dd>The simple, linear numbering scheme used in chained precompiled
  headers falls apart with the module DAG, because different modules
  may end up with different numbering schemes for entities they
  imported from common shared modules. To account for this, each
  module file provides information about which modules it depends on
  and which ID numbers it assigned to the entities in those modules,
  as well as which ID numbers it took for its own new entities. The
  AST reader then maps these "local" ID numbers into a "global" ID
  number space for the current translation unit, providing a 1-1
  mapping between entities (in whatever AST file they inhabit) and
  global ID numbers. If that translation unit is then serialized into
  an AST file, this mapping will be stored for use when the AST file
  is imported.</dd>

  <dt>Declaration merging</dt>
  <dd>It is possible for a given entity (from the language's
  perspective) to be declared multiple times in different places. For
  example, two different headers can have the declaration of
  <tt>printf</tt> or could forward-declare <tt>struct stat</tt>. If
  each of those headers is included in a module, and some third party
  imports both of those modules, there is a potentially serious
  problem: name lookup for <tt>printf</tt> or <tt>struct stat</tt> will
  find both declarations, but the AST nodes are unrelated. This would
  result in a compilation error, due to an ambiguity in name
  lookup. Therefore, the AST reader performs declaration merging
  according to the appropriate language semantics, ensuring that the
  two disjoint declarations are merged into a single redeclaration
  chain (with a common canonical declaration), so that it is as if one
  of the headers had been included before the other.</dd>

  <dt>Name Visibility</dt>
  <dd>Modules allow certain names that occur during module creation to
  be "hidden", so that they are not part of the public interface of
  the module and are not visible to its clients. The AST reader
  maintains a "visible" bit on various AST nodes (declarations, macros,
  etc.) to indicate whether that particular AST node is currently
  visible; the various name lookup mechanisms in Clang inspect the
  visible bit to determine whether that entity, which is still in the
  AST (because other, visible AST nodes may depend on it), can
  actually be found by name lookup. When a new (sub)module is
  imported, it may make existing, non-visible, already-deserialized
  AST nodes visible; it is the responsibility of the AST reader to
  find and update these AST nodes when it is notified of the import.</dd>
    
</dl>
  
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