InternalsManual.rst

============================
"Clang" CFE Internals Manual
============================

.. contents::
   :local:

Introduction
============

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.

LLVM Support Library
====================

The LLVM ``libSupport`` library provides many underlying libraries and
`data-structures <http://llvm.org/docs/ProgrammersManual.html>`_, including
command line option processing, various containers and a system abstraction
layer, which is used for file system access.

The Clang "Basic" Library
=========================

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.

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.

We describe the roles of these classes in order of their dependencies.

The Diagnostics Subsystem
-------------------------

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, an English translation associated with it, a
:ref:`SourceLocation <SourceLocation>` to "put the caret", and a severity
(e.g., ``WARNING`` or ``ERROR``).  They can also optionally include a number of
arguments to the diagnostic (which fill in "%0"'s in the string) as well as a
number of source ranges that related to the diagnostic.

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

.. code-block:: text

  t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
  P = (P-42) + Gamma*4;
      ~~~~~~ ^ ~~~~~~~

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 :).

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.

The ``Diagnostic*Kinds.td`` files
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Diagnostics are created by adding an entry to one of the
``clang/Basic/Diagnostic*Kinds.td`` files, depending on what library will be
using it.  From this file, :program:`tblgen` generates the unique ID of the
diagnostic, the severity of the diagnostic and the English translation + format
string.

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.

The severity of the diagnostic comes from the set {``NOTE``, ``REMARK``,
``WARNING``,
``EXTENSION``, ``EXTWARN``, ``ERROR``}.  The ``ERROR`` 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 ``EXTENSION`` and ``EXTWARN`` 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 ``WARNING`` severity is used for
constructs that are valid in the currently selected source language but that
are dubious in some way.  The ``REMARK`` severity provides generic information
about the compilation that is not necessarily related to any dubious code.  The
``NOTE`` level is used to staple more information onto previous diagnostics.

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

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

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

The Format String
^^^^^^^^^^^^^^^^^

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:

.. code-block:: c++

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

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.

Arguments to the diagnostic are numbered according to how they are specified by
the C++ code that :ref:`produces them <internals-producing-diag>`, and are
referenced by ``%0`` .. ``%9``.  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 "``%1 %0``"
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.

Here are some "best practices" for writing the English format string:

* Keep the string short.  It should ideally fit in the 80 column limit of the
  ``DiagnosticKinds.td`` file.  This avoids the diagnostic wrapping when
  printed, and forces you to think about the important point you are conveying
  with the diagnostic.
* 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.
* Do not capitalize the diagnostic string, and do not end it with a period.
* If you need to quote something in the diagnostic string, use single quotes.

Diagnostics should never take random English strings as arguments: you
shouldn't use "``you have a problem with %0``" and pass in things like "``your
argument``" or "``your return value``" as arguments.  Doing this prevents
:ref:`translating <internals-diag-translation>` 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 (``/=``).
Note that things like "pointer" and "reference" are not keywords.  On the other
hand, you *can* 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.

Formatting a Diagnostic Argument
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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 :).

Here are the different diagnostic argument formats currently supported by
Clang:

**"s" format**

Example:
  ``"requires %1 parameter%s1"``
Class:
  Integers
Description:
  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
  ``"requires %1 parameter(s)"``.

**"select" format**

Example:
  ``"must be a %select{unary|binary|unary or binary}2 operator"``
Class:
  Integers
Description:
  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.  The selected string
  does undergo formatting.

**"plural" format**

Example:
  ``"you have %1 %plural{1:mouse|:mice}1 connected to your computer"``
Class:
  Integers
Description:
  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.

  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.

  * number: A simple decimal number matches if the argument is the same as the
    number.  Example: ``"%plural{1:mouse|:mice}4"``
  * range: A range in square brackets matches if the argument is within the
    range.  Then range is inclusive on both ends.  Example:
    ``"%plural{0:none|1:one|[2,5]:some|:many}2"``
  * 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:
    ``"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything else}1"``

  The parser is very unforgiving.  A syntax error, even whitespace, will abort,
  as will a failure to match the argument against any expression.

**"ordinal" format**

Example:
  ``"ambiguity in %ordinal0 argument"``
Class:
  Integers
Description:
  This is a formatter which represents the argument number as an ordinal: the
  value ``1`` becomes ``1st``, ``3`` becomes ``3rd``, and so on.  Values less
  than ``1`` are not supported.  This formatter is currently hard-coded to use
  English ordinals.

**"objcclass" format**

Example:
  ``"method %objcclass0 not found"``
Class:
  ``DeclarationName``
Description:
  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 "``+``".

**"objcinstance" format**

Example:
  ``"method %objcinstance0 not found"``
Class:
  ``DeclarationName``
Description:
  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 "``-``".

**"q" format**

Example:
  ``"candidate found by name lookup is %q0"``
Class:
  ``NamedDecl *``
Description:
  This formatter indicates that the fully-qualified name of the declaration
  should be printed, e.g., "``std::vector``" rather than "``vector``".

**"diff" format**

Example:
  ``"no known conversion %diff{from $ to $|from argument type to parameter type}1,2"``
Class:
  ``QualType``
Description:
  This formatter takes two ``QualType``\ s and attempts to print a template
  difference between the two.  If tree printing is off, the text inside the
  braces before the pipe is printed, with the formatted text replacing the $.
  If tree printing is on, the text after the pipe is printed and a type tree is
  printed after the diagnostic message.

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.

.. _internals-producing-diag:

Producing the Diagnostic
^^^^^^^^^^^^^^^^^^^^^^^^

Now that you've created the diagnostic in the ``Diagnostic*Kinds.td`` 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.

For example, the binary expression error comes from code like this:

.. code-block:: c++

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

This shows that use of the ``Diag`` method: it takes a location (a
:ref:`SourceLocation <SourceLocation>` object) and a diagnostic enum value
(which matches the name from ``Diagnostic*Kinds.td``).  If the diagnostic takes
arguments, they are specified with the ``<<`` operator: the first argument
becomes ``%0``, the second becomes ``%1``, etc.  The diagnostic interface
allows you to specify arguments of many different types, including ``int`` and
``unsigned`` for integer arguments, ``const char*`` and ``std::string`` for
string arguments, ``DeclarationName`` and ``const IdentifierInfo *`` for names,
``QualType`` for types, etc.  ``SourceRange``\ s are also specified with the
``<<`` operator, but do not have a specific ordering requirement.

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.

Fix-It Hints
^^^^^^^^^^^^

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

However, for these cases where the fix is obvious, the diagnostic can be
annotated with a hint (referred to as a "fix-it hint") that describes how to
change the code referenced by the diagnostic to fix the problem.  For example,
it might add the missing semicolon at the end of the statement or rewrite the
use of a deprecated construct into something more palatable.  Here is one such
example from the C++ front end, where we warn about the right-shift operator
changing meaning from C++98 to C++11:

.. code-block:: text

  test.cpp:3:7: warning: use of right-shift operator ('>>') in template argument
                         will require parentheses in C++11
  A<100 >> 2> *a;
        ^
    (       )

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

Fix-it hints on errors and warnings need to obey these rules:

* Since they are automatically applied if ``-Xclang -fixit`` is passed to the
  driver, they should only be used when it's very likely they match the user's
  intent.
* Clang must recover from errors as if the fix-it had been applied.

If a fix-it can't obey these rules, put the fix-it on a note.  Fix-its on notes
are not applied automatically.

All fix-it hints are described by the ``FixItHint`` class, instances of which
should be attached to the diagnostic using the ``<<`` operator in the same way
that highlighted source ranges and arguments are passed to the diagnostic.
Fix-it hints can be created with one of three constructors:

* ``FixItHint::CreateInsertion(Loc, Code)``

    Specifies that the given ``Code`` (a string) should be inserted before the
    source location ``Loc``.

* ``FixItHint::CreateRemoval(Range)``

    Specifies that the code in the given source ``Range`` should be removed.

* ``FixItHint::CreateReplacement(Range, Code)``

    Specifies that the code in the given source ``Range`` should be removed,
    and replaced with the given ``Code`` string.

.. _DiagnosticClient:

The ``DiagnosticClient`` Interface
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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 "``Ignore``") it invokes an object that implements the ``DiagnosticClient``
interface with the information.

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.

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.  Full
documentation for the ``-verify`` mode can be found in the Clang API
documentation for `VerifyDiagnosticConsumer
</doxygen/classclang_1_1VerifyDiagnosticConsumer.html#details>`_.

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.

.. _internals-diag-translation:

Adding Translations to Clang
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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.

.. _SourceLocation:
.. _SourceManager:

The ``SourceLocation`` and ``SourceManager`` classes
----------------------------------------------------

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

#. ``sizeof(SourceLocation)`` must be extremely small, as these are embedded
   into many AST nodes and are passed around often.  Currently it is 32 bits.
#. ``SourceLocation`` must be a simple value object that can be efficiently
   copied.
#. 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.
#. 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 ``#include``\ s active
   when the token was lexed.  This allows us to print the ``#include`` stack
   for a diagnostic.
#. ``SourceLocation`` must be able to describe macro expansions, capturing both
   the ultimate instantiation point and the source of the original character
   data.

In practice, the ``SourceLocation`` works together with the ``SourceManager``
class to encode two pieces of information about a location: its spelling
location and its expansion 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
expansion point or the location of the ``_Pragma`` itself).

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.

``SourceRange`` and ``CharSourceRange``
---------------------------------------

.. mostly taken from http://lists.llvm.org/pipermail/cfe-dev/2010-August/010595.html

Clang represents most source ranges by [first, last], where "first" and "last"
each point to the beginning of their respective tokens.  For example consider
the ``SourceRange`` of the following statement:

.. code-block:: text

  x = foo + bar;
  ^first    ^last

To map from this representation to a character-based representation, the "last"
location needs to be adjusted to point to (or past) the end of that token with
either ``Lexer::MeasureTokenLength()`` or ``Lexer::getLocForEndOfToken()``.  For
the rare cases where character-level source ranges information is needed we use
the ``CharSourceRange`` class.

The Driver Library
==================

The clang Driver and library are documented :doc:`here <DriverInternals>`.

Precompiled Headers
===================

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

The Frontend Library
====================

The Frontend library contains functionality useful for building tools on top of
the Clang libraries, for example several methods for outputting diagnostics.

The Lexer and Preprocessor Library
==================================

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 ``Preprocessor``
class.  It contains the various pieces of state that are required to coherently
read tokens out of a translation unit.

The core interface to the ``Preprocessor`` object (once it is set up) is the
``Preprocessor::Lex`` method, which returns the next :ref:`Token <Token>` 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
:ref:`Lexer <Lexer>` class) and a buffered token stream (provided by the
:ref:`TokenLexer <TokenLexer>` class).

.. _Token:

The Token class
---------------

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).

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`` matters.  On a
32-bit system, ``sizeof(Token)`` is currently 16 bytes.

Tokens occur in two forms: :ref:`annotation tokens <AnnotationToken>` 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:

* **A SourceLocation** --- This indicates the location of the start of the
  token.

* **A length** --- 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.

* **IdentifierInfo** --- 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``".

* **TokenKind** --- This indicates the kind of token as classified by the
  lexer.  This includes things like ``tok::starequal`` (for the "``*=``"
  operator), ``tok::ampamp`` for the "``&&``" token, and keyword values (e.g.,
  ``tok::kw_for``) 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
  "``&&``" operator.  In these cases, the kind value is set to ``tok::ampamp``,
  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.

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

  #. **StartOfLine** --- This was the first token that occurred on its input
     source line.
  #. **LeadingSpace** --- 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.
  #. **DisableExpand** --- 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.
  #. **NeedsCleaning** --- 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.

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".

.. _AnnotationToken:

Annotation Tokens
-----------------

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``" ``tok::identifier`` token is replaced with an
``tok::annot_typename``.  This is useful for a couple of reasons: 1) this makes
it easy to handle qualified type names (e.g., "``foo::bar::baz<42>::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.

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., "``a::b::c``" 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):

* **SourceLocation "Location"** --- 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.
* **SourceLocation "AnnotationEndLoc"** --- 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.
* **void* "AnnotationValue"** --- This contains an opaque object that the
  parser gets from ``Sema``.  The parser merely preserves the information for
  ``Sema`` to later interpret based on the annotation token kind.
* **TokenKind "Kind"** --- This indicates the kind of Annotation token this is.
  See below for the different valid kinds.

Annotation tokens currently come in three kinds:

#. **tok::annot_typename**: This annotation token represents a resolved
   typename token that is potentially qualified.  The ``AnnotationValue`` field
   contains the ``QualType`` returned by ``Sema::getTypeName()``, possibly with
   source location information attached.
#. **tok::annot_cxxscope**: 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 a ``NestedNameSpecifier *`` returned by the
   ``Sema::ActOnCXXGlobalScopeSpecifier`` and
   ``Sema::ActOnCXXNestedNameSpecifier`` callbacks.
#. **tok::annot_template_id**: This annotation token represents a C++
   template-id such as "``foo<int, 4>``", where "``foo``" is the name of a
   template.  The ``AnnotationValue`` pointer is a pointer to a ``malloc``'d
   ``TemplateIdAnnotation`` object.  Depending on the context, a parsed
   template-id that names a type might become a typename annotation token (if
   all we care about is the named type, e.g., because it occurs in a type
   specifier) or might remain a template-id token (if we want to retain more
   source location information or produce a new type, e.g., in a declaration of
   a class template specialization).  template-id annotation tokens that refer
   to a type can be "upgraded" to typename annotation tokens by the parser.

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.

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.

.. _Lexer:

The ``Lexer`` class
-------------------

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).

The lexer has a couple of interesting modal features:

* 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 "``#if 0``" block, for example.
* 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.
* The lexer can be in ``ParsingFilename`` mode, which happens when
  preprocessing after reading a ``#include`` directive.  This mode changes the
  parsing of "``<``" to return an "angled string" instead of a bunch of tokens
  for each thing within the filename.
* When parsing a preprocessor directive (after "``#``") the
  ``ParsingPreprocessorDirective`` mode is entered.  This changes the parser to
  return EOD at a newline.
* The ``Lexer`` uses a ``LangOptions`` object to know whether trigraphs are
  enabled, whether C++ or ObjC keywords are recognized, etc.

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:

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

.. _TokenLexer:

The ``TokenLexer`` class
------------------------

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.

.. _MultipleIncludeOpt:

The ``MultipleIncludeOpt`` class
--------------------------------

The ``MultipleIncludeOpt`` class implements a really simple little state
machine that is used to detect the standard "``#ifndef XX`` / ``#define XX``"
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.

.. _Parser:

The Parser Library
==================

This library contains a recursive-descent parser that polls tokens from the
preprocessor and notifies a client of the parsing progress.

Historically, the parser used to talk to an abstract ``Action`` interface that
had virtual methods for parse events, for example ``ActOnBinOp()``.  When Clang
grew C++ support, the parser stopped supporting general ``Action`` clients --
it now always talks to the :ref:`Sema library <Sema>`.  However, the Parser
still accesses AST objects only through opaque types like ``ExprResult`` and
``StmtResult``.  Only :ref:`Sema <Sema>` looks at the AST node contents of these
wrappers.

.. _AST:

The AST Library
===============

.. _Type:

The ``Type`` class and its subclasses
-------------------------------------

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 :ref:`QualType <QualType>`), and 2) they implicitly capture typedef
information.  Once created, types are immutable (unlike decls).

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:

.. code-block:: c++

  void func() {
    typedef int foo;
    foo X, *Y;
    typedef foo *bar;
    bar Z;
    *X; // error
    **Y; // error
    **Z; // error
  }

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:

.. code-block:: text

  test.c:6:1: error: indirection requires pointer operand ('foo' invalid)
    *X; // error
    ^~
  test.c:7:1: error: indirection requires pointer operand ('foo' invalid)
    **Y; // error
    ^~~
  test.c:8:1: error: indirection requires pointer operand ('foo' invalid)
    **Z; // error
    ^~~

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
"``std::string``" instead of "``std::basic_string<char, std:...``".  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``".

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
*actual structure* of a type, ignoring typedefs.  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.

Canonical Types
^^^^^^^^^^^^^^^

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

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 "``int*``" type).

Canonical types and typedef types bring up some complexities that must be
carefully managed.  Specifically, the ``isa``/``cast``/``dyn_cast`` 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 "``isa<PointerType>(SubExpr->getType())``", because
this predicate would fail if the subexpression had a typedef type.

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
"``SubExpr->getType()->isPointerType()``" to do the check.  This predicate will
return true if the *canonical type is a pointer*, which is true any time the
type is structurally a pointer type.  The only hard part here is remembering
not to use the ``isa``/``cast``/``dyn_cast`` operations.

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
"``foo*``", we could return that type as the result.  If the subexpression had
type "``bar``", we want to return "``foo*``" (note that we do *not* want
"``int*``").  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.

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

.. _QualType:

The ``QualType`` class
----------------------

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``, plus some
extended qualifiers required by language extensions) separately from the types
themselves.  ``QualType`` is conceptually a pair of "``Type*``" and the bits
for these type qualifiers.

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).

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 (:ref:`Type <Type>` does not even
contain qualifiers).

In practice, the two most common type qualifiers (``const`` and ``restrict``)
are stored in the low bits of the pointer to the ``Type`` object, together with
a flag indicating whether extended qualifiers are present (which must be
heap-allocated).  This means that ``QualType`` is exactly the same size as a
pointer.

.. _DeclarationName:

Declaration names
-----------------

The ``DeclarationName`` 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., "``f``" and "``x``" in
the function declaration ``f(int x)``.  In C++, declaration names can also name
class constructors ("``Class``" in ``struct Class { Class(); }``), class
destructors ("``~Class``"), overloaded operator names ("``operator+``"), and
conversion functions ("``operator void const *``").  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 *selector*, e.g.,
"``setWidth:height:``".  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 ``NamedDecl`` class,
``DeclarationName`` is designed to efficiently represent any kind of name.

Given a ``DeclarationName`` ``N``, ``N.getNameKind()`` will produce a value
that describes what kind of name ``N`` stores.  There are 10 options (all of
the names are inside the ``DeclarationName`` class).

``Identifier``

  The name is a simple identifier.  Use ``N.getAsIdentifierInfo()`` to retrieve
  the corresponding ``IdentifierInfo*`` pointing to the actual identifier.

``ObjCZeroArgSelector``, ``ObjCOneArgSelector``, ``ObjCMultiArgSelector``

  The name is an Objective-C selector, which can be retrieved as a ``Selector``
  instance via ``N.getObjCSelector()``.  The three possible name kinds for
  Objective-C reflect an optimization within the ``DeclarationName`` class:
  both zero- and one-argument selectors are stored as a masked
  ``IdentifierInfo`` 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).

``CXXConstructorName``

  The name is a C++ constructor name.  Use ``N.getCXXNameType()`` to retrieve
  the :ref:`type <QualType>` 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.

``CXXDestructorName``

  The name is a C++ destructor name.  Use ``N.getCXXNameType()`` to retrieve
  the :ref:`type <QualType>` whose destructor is being named.  This type is
  always a canonical type.