xref: /openbsd-src/gnu/llvm/clang/docs/InternalsManual.rst (revision 1a8dbaac879b9f3335ad7fb25429ce63ac1d6bac)

============================
"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 <https://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
<DiagnosticConsumer>` depending on how the ``DiagnosticConsumer`` 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 ``DiagnosticConsumer`` 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.

**"sub" format**

Example:
  Given the following record definition of type ``TextSubstitution``:

  .. code-block:: text

    def select_ovl_candidate : TextSubstitution<
      "%select{function|constructor}0%select{| template| %2}1">;

  which can be used as

  .. code-block:: text

    def note_ovl_candidate : Note<
      "candidate %sub{select_ovl_candidate}3,2,1 not viable">;

  and will act as if it was written
  ``"candidate %select{function|constructor}3%select{| template| %1}2 not viable"``.
Description:
  This format specifier is used to avoid repeating strings verbatim in multiple
  diagnostics. The argument to ``%sub`` must name a ``TextSubstitution`` tblgen
  record. The substitution must specify all arguments used by the substitution,
  and the modifier indexes in the substitution are re-numbered accordingly. The
  substituted text must itself be a valid format string before substitution.

.. _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
<DiagnosticConsumer>` 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.
* Fix-it hints on a warning must not change the meaning of the code.
  However, a hint may clarify the meaning as intentional, for example by adding
  parentheses when the precedence of operators isn't obvious.

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.

.. _DiagnosticConsumer:

The ``DiagnosticConsumer`` 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 ``DiagnosticConsumer``
interface with the information.

It is possible to implement this interface in many different ways.  For
example, the normal Clang ``DiagnosticConsumer`` (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 ``DiagnosticConsumer`` 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 https://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 precompiled headers (:doc:`PCH <PCHInternals>`), which  uses a
serialized representation of Clang's internal data structures, encoded with the
`LLVM bitstream format <https://llvm.org/docs/BitCodeFormat.html>`_.

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
===============

.. _ASTPhilosophy:

Design philosophy
-----------------

Immutability
^^^^^^^^^^^^

Clang AST nodes (types, declarations, statements, expressions, and so on) are
generally designed to be immutable once created. This provides a number of key
benefits:

  * Canonicalization of the "meaning" of nodes is possible as soon as the nodes
    are created, and is not invalidated by later addition of more information.
    For example, we :ref:`canonicalize types <CanonicalType>`, and use a
    canonicalized representation of expressions when determining whether two
    function template declarations involving dependent expressions declare the
    same entity.
  * AST nodes can be reused when they have the same meaning. For example, we
    reuse ``Type`` nodes when representing the same type (but maintain separate
    ``TypeLoc``\s for each instance where a type is written), and we reuse
    non-dependent ``Stmt`` and ``Expr`` nodes across instantiations of a
    template.
  * Serialization and deserialization of the AST to/from AST files is simpler:
    we do not need to track modifications made to AST nodes imported from AST
    files and serialize separate "update records".

There are unfortunately exceptions to this general approach, such as:

  * The first declaration of a redeclarable entity maintains a pointer to the
    most recent declaration of that entity, which naturally needs to change as
    more declarations are parsed.
  * Name lookup tables in declaration contexts change after the namespace
    declaration is formed.
  * We attempt to maintain only a single declaration for an instantiation of a
    template, rather than having distinct declarations for an instantiation of
    the declaration versus the definition, so template instantiation often
    updates parts of existing declarations.
  * Some parts of declarations are required to be instantiated separately (this
    includes default arguments and exception specifications), and such
    instantiations update the existing declaration.

These cases tend to be fragile; mutable AST state should be avoided where
possible.

As a consequence of this design principle, we typically do not provide setters
for AST state. (Some are provided for short-term modifications intended to be
used immediately after an AST node is created and before it's "published" as
part of the complete AST, or where language semantics require after-the-fact
updates.)

Faithfulness
^^^^^^^^^^^^

The AST intends to provide a representation of the program that is faithful to
the original source. We intend for it to be possible to write refactoring tools
using only information stored in, or easily reconstructible from, the Clang AST.
This means that the AST representation should either not desugar source-level
constructs to simpler forms, or -- where made necessary by language semantics
or a clear engineering tradeoff -- should desugar minimally and wrap the result
in a construct representing the original source form.

For example, ``CXXForRangeStmt`` directly represents the syntactic form of a
range-based for statement, but also holds a semantic representation of the
range declaration and iterator declarations. It does not contain a
fully-desugared ``ForStmt``, however.

Some AST nodes (for example, ``ParenExpr``) represent only syntax, and others
(for example, ``ImplicitCastExpr``) represent only semantics, but most nodes
will represent a combination of syntax and associated semantics. Inheritance
is typically used when representing different (but related) syntaxes for nodes
with the same or similar semantics.

.. _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.

.. _CanonicalType:

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.

``CXXConversionFunctionName``

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

``CXXOperatorName``

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

``CXXLiteralOperatorName``

  The name is a C++11 user defined literal operator.  User defined
  Literal operators are named according to the suffix they define,
  e.g., "``_foo``" for "``operator "" _foo``".  Use
  ``N.getCXXLiteralIdentifier()`` to retrieve the corresponding
  ``IdentifierInfo*`` pointing to the identifier.

``CXXUsingDirective``

  The name is a C++ using directive.  Using directives are not really
  NamedDecls, in that they all have the same name, but they are
  implemented as such in order to store them in DeclContext
  effectively.

``DeclarationName``\ 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 ``DeclarationName``\ s can be compared for
equality (``==``, ``!=``) using a simple bitwise comparison, can be ordered
with ``<``, ``>``, ``<=``, and ``>=`` (which provide a lexicographical ordering
for normal identifiers but an unspecified ordering for other kinds of names),
and can be placed into LLVM ``DenseMap``\ s and ``DenseSet``\ s.

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

.. _DeclContext:

Declaration contexts
--------------------

Every declaration in a program exists within some *declaration context*, such
as a translation unit, namespace, class, or function.  Declaration contexts in
Clang are represented by the ``DeclContext`` class, from which the various
declaration-context AST nodes (``TranslationUnitDecl``, ``NamespaceDecl``,
``RecordDecl``, ``FunctionDecl``, etc.) will derive.  The ``DeclContext`` class
provides several facilities common to each declaration context:

Source-centric vs. Semantics-centric View of Declarations

  ``DeclContext`` 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 :ref:`Redeclarations and Overloads
  <Redeclarations>`), while the semantics-centric view represents the program
  semantics.  The two views are kept synchronized by semantic analysis while
  the ASTs are being constructed.

Storage of declarations within that context

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

Lookup of declarations within that context

  The ``DeclContext`` structure provides efficient name lookup for names within
  that declaration context.  For example, if ``N`` is a namespace we can look
  for the name ``N::f`` using ``DeclContext::lookup``.  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.

Ownership of declarations

  The ``DeclContext`` 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.

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 ``DeclContext`` that contains a particular ``Decl`` using
``Decl::getDeclContext``.  However, see the section
:ref:`LexicalAndSemanticContexts` for more information about how to interpret
this context information.

.. _Redeclarations:

Redeclarations and Overloads
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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:

.. code-block:: c++

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

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

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``".

(Note that because ``f`` can be redeclared at block scope, or in a friend
declaration, etc. it is possible that the declaration of ``f`` found by name
lookup will not be the most recent one.)

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.,

.. code-block:: c++

  void g();
  void g(int);

the ``DeclContext::lookup`` operation will return a
``DeclContext::lookup_result`` that contains a range of iterators over
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.

.. _LexicalAndSemanticContexts:

Lexical and Semantic Contexts
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Each declaration has two potentially different declaration contexts: a
*lexical* context, which corresponds to the source-centric view of the
declaration context, and a *semantic* context, which corresponds to the
semantics-centric view.  The lexical context is accessible via
``Decl::getLexicalDeclContext`` while the semantic context is accessible via
``Decl::getDeclContext``, both of which return ``DeclContext`` pointers.  For
most declarations, the two contexts are identical.  For example:

.. code-block:: c++

  class X {
  public:
    void f(int x);
  };

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

.. code-block:: c++

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

This definition of "``f``" 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
``X``.  Thus, this declaration of ``X::f`` can be found by traversing the
declarations provided by [``decls_begin()``, ``decls_end()``) in the
translation unit.

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

Transparent Declaration Contexts
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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.,

.. code-block:: c++

  enum Color {
    Red,
    Green,
    Blue
  };

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

.. code-block:: c++

  Color c = Red;

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

.. code-block:: c++

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

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.

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
*transparent* declaration contexts (see
``DeclContext::isTransparentContext()``), 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 ``DeclContext`` 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).

The transparent ``DeclContext``\ s are:

* Enumerations (but not C++11 "scoped enumerations"):

  .. code-block:: c++

    enum Color {
      Red,
      Green,
      Blue
    };
    // Red, Green, and Blue are in scope

* C++ linkage specifications:

  .. code-block:: c++

    extern "C" {
      void f(int);
      void g(int);
    }
    // f and g are in scope

* Anonymous unions and structs:

  .. code-block:: c++

    struct LookupTable {
      bool IsVector;
      union {
        std::vector<Item> *Vector;
        std::set<Item> *Set;
      };
    };

    LookupTable LT;
    LT.Vector = 0; // Okay: finds Vector inside the unnamed union

* C++11 inline namespaces:

  .. code-block:: c++

    namespace mylib {
      inline namespace debug {
        class X;
      }
    }
    mylib::X *xp; // okay: mylib::X refers to mylib::debug::X

.. _MultiDeclContext:

Multiply-Defined Declaration Contexts
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

C++ namespaces have the interesting 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:

.. code-block:: c++

  // Snippet #1:
  namespace N {
    void f();
  }
  namespace N {
    void f(int);
  }

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

In Clang's representation, the source-centric view of declaration contexts will
actually have two separate ``NamespaceDecl`` 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
``N`` for "``f``" will return a ``DeclContext::lookup_result`` that contains a
range of iterators over declarations of "``f``".

``DeclContext`` manages multiply-defined declaration contexts internally.  The
function ``DeclContext::getPrimaryContext`` retrieves the "primary" context for
a given ``DeclContext`` instance, which is the ``DeclContext`` responsible for
maintaining the lookup table used for the semantics-centric view.  Given a
DeclContext, one can obtain the set of declaration contexts that are
semantically connected to this declaration context, in source order, including
this context (which will be the only result, for non-namespace contexts) via
``DeclContext::collectAllContexts``. Note that these functions are used
internally within the lookup and insertion methods of the ``DeclContext``, so
the vast majority of clients can ignore them.

Because the same entity can be defined multiple times in different modules,
it is also possible for there to be multiple definitions of (for instance)
a ``CXXRecordDecl``, all of which describe a definition of the same class.
In such a case, only one of those "definitions" is considered by Clang to be
the definiition of the class, and the others are treated as non-defining
declarations that happen to also contain member declarations. Corresponding
members in each definition of such multiply-defined classes are identified
either by redeclaration chains (if the members are ``Redeclarable``)
or by simply a pointer to the canonical declaration (if the declarations
are not ``Redeclarable`` -- in that case, a ``Mergeable`` base class is used
instead).

The ASTImporter
---------------

The ``ASTImporter`` class imports nodes of an ``ASTContext`` into another
``ASTContext``. Please refer to the document :doc:`ASTImporter: Merging Clang
ASTs <LibASTImporter>` for an introduction. And please read through the
high-level `description of the import algorithm
<LibASTImporter.html#algorithm-of-the-import>`_, this is essential for
understanding further implementation details of the importer.

.. _templated:

Abstract Syntax Graph
^^^^^^^^^^^^^^^^^^^^^

Despite the name, the Clang AST is not a tree. It is a directed graph with
cycles. One example of a cycle is the connection between a
``ClassTemplateDecl`` and its "templated" ``CXXRecordDecl``. The *templated*
``CXXRecordDecl`` represents all the fields and methods inside the class
template, while the ``ClassTemplateDecl`` holds the information which is
related to being a template, i.e. template arguments, etc. We can get the
*templated* class (the ``CXXRecordDecl``) of a ``ClassTemplateDecl`` with
``ClassTemplateDecl::getTemplatedDecl()``. And we can get back a pointer of the
"described" class template from the *templated* class:
``CXXRecordDecl::getDescribedTemplate()``. So, this is a cycle between two
nodes: between the *templated* and the *described* node. There may be various
other kinds of cycles in the AST especially in case of declarations.

.. _structural-eq:

Structural Equivalency
^^^^^^^^^^^^^^^^^^^^^^

Importing one AST node copies that node into the destination ``ASTContext``. To
copy one node means that we create a new node in the "to" context then we set
its properties to be equal to the properties of the source node. Before the
copy, we make sure that the source node is not *structurally equivalent* to any
existing node in the destination context. If it happens to be equivalent then
we skip the copy.

The informal definition of structural equivalency is the following:
Two nodes are **structurally equivalent** if they are

- builtin types and refer to the same type, e.g. ``int`` and ``int`` are
  structurally equivalent,
- function types and all their parameters have structurally equivalent types,
- record types and all their fields in order of their definition have the same
  identifier names and structurally equivalent types,
- variable or function declarations and they have the same identifier name and
  their types are structurally equivalent.

In C, two types are structurally equivalent if they are *compatible types*. For
a formal definition of *compatible types*, please refer to 6.2.7/1 in the C11
standard. However, there is no definition for *compatible types* in the C++
standard. Still, we extend the definition of structural equivalency to
templates and their instantiations similarly: besides checking the previously
mentioned properties, we have to check for equivalent template
parameters/arguments, etc.

The structural equivalent check can be and is used independently from the
ASTImporter, e.g. the ``clang::Sema`` class uses it also.

The equivalence of nodes may depend on the equivalency of other pairs of nodes.
Thus, the check is implemented as a parallel graph traversal. We traverse
through the nodes of both graphs at the same time. The actual implementation is
similar to breadth-first-search. Let's say we start the traverse with the <A,B>
pair of nodes. Whenever the traversal reaches a pair <X,Y> then the following
statements are true:

- A and X are nodes from the same ASTContext.
- B and Y are nodes from the same ASTContext.
- A and B may or may not be from the same ASTContext.
- if A == X and B == Y (pointer equivalency) then (there is a cycle during the
  traverse)

  - A and B are structurally equivalent if and only if

    - All dependent nodes on the path from <A,B> to <X,Y> are structurally
      equivalent.

When we compare two classes or enums and one of them is incomplete or has
unloaded external lexical declarations then we cannot descend to compare their
contained declarations. So in these cases they are considered equal if they
have the same names. This is the way how we compare forward declarations with
definitions.

.. TODO Should we elaborate the actual implementation of the graph traversal,
.. which is a very weird BFS traversal?

Redeclaration Chains
^^^^^^^^^^^^^^^^^^^^

The early version of the ``ASTImporter``'s merge mechanism squashed the
declarations, i.e. it aimed to have only one declaration instead of maintaining
a whole redeclaration chain. This early approach simply skipped importing a
function prototype, but it imported a definition. To demonstrate the problem
with this approach let's consider an empty "to" context and the following
``virtual`` function declarations of ``f`` in the "from" context:

.. code-block:: c++

  struct B { virtual void f(); };
  void B::f() {} // <-- let's import this definition

If we imported the definition with the "squashing" approach then we would
end-up having one declaration which is indeed a definition, but ``isVirtual()``
returns ``false`` for it. The reason is that the definition is indeed not
virtual, it is the property of the prototype!

Consequently, we must either set the virtual flag for the definition (but then
we create a malformed AST which the parser would never create), or we import
the whole redeclaration chain of the function. The most recent version of the
``ASTImporter`` uses the latter mechanism. We do import all function
declarations - regardless if they are definitions or prototypes - in the order
as they appear in the "from" context.

.. One definition

If we have an existing definition in the "to" context, then we cannot import
another definition, we will use the existing definition. However, we can import
prototype(s): we chain the newly imported prototype(s) to the existing
definition. Whenever we import a new prototype from a third context, that will
be added to the end of the redeclaration chain. This may result in long
redeclaration chains in certain cases, e.g. if we import from several
translation units which include the same header with the prototype.

.. Squashing prototypes

To mitigate the problem of long redeclaration chains of free functions, we
could compare prototypes to see if they have the same properties and if yes
then we could merge these prototypes. The implementation of squashing of
prototypes for free functions is future work.

.. Exception: Cannot have more than 1 prototype in-class

Chaining functions this way ensures that we do copy all information from the
source AST. Nonetheless, there is a problem with member functions: While we can
have many prototypes for free functions, we must have only one prototype for a
member function.

.. code-block:: c++

  void f(); // OK
  void f(); // OK

  struct X {
    void f(); // OK
    void f(); // ERROR
  };
  void X::f() {} // OK

Thus, prototypes of member functions must be squashed, we cannot just simply
attach a new prototype to the existing in-class prototype. Consider the
following contexts:

.. code-block:: c++

  // "to" context
  struct X {
    void f(); // D0
  };

.. code-block:: c++

  // "from" context
  struct X {
    void f(); // D1
  };
  void X::f() {} // D2

When we import the prototype and the definition of ``f`` from the "from"
context, then the resulting redecl chain will look like this ``D0 -> D2'``,
where ``D2'`` is the copy of ``D2`` in the "to" context.

.. Redecl chains of other declarations

Generally speaking, when we import declarations (like enums and classes) we do
attach the newly imported declaration to the existing redeclaration chain (if
there is structural equivalency). We do not import, however, the whole
redeclaration chain as we do in case of functions. Up till now, we haven't
found any essential property of forward declarations which is similar to the
case of the virtual flag in a member function prototype. In the future, this
may change, though.

Traversal during the Import
^^^^^^^^^^^^^^^^^^^^^^^^^^^

The node specific import mechanisms are implemented in
``ASTNodeImporter::VisitNode()`` functions, e.g. ``VisitFunctionDecl()``.
When we import a declaration then first we import everything which is needed to
call the constructor of that declaration node. Everything which can be set
later is set after the node is created. For example, in case of  a
``FunctionDecl`` we first import the declaration context in which the function
is declared, then we create the ``FunctionDecl`` and only then we import the
body of the function. This means there are implicit dependencies between AST
nodes. These dependencies determine the order in which we visit nodes in the
"from" context. As with the regular graph traversal algorithms like DFS, we
keep track which nodes we have already visited in
``ASTImporter::ImportedDecls``. Whenever we create a node then we immediately
add that to the ``ImportedDecls``. We must not start the import of any other
declarations before we keep track of the newly created one. This is essential,
otherwise, we would not be able to handle circular dependencies. To enforce
this, we wrap all constructor calls of all AST nodes in
``GetImportedOrCreateDecl()``. This wrapper ensures that all newly created
declarations are immediately marked as imported; also, if a declaration is
already marked as imported then we just return its counterpart in the "to"
context. Consequently, calling a declaration's ``::Create()`` function directly
would lead to errors, please don't do that!

Even with the use of ``GetImportedOrCreateDecl()`` there is still a
probability of having an infinite import recursion if things are imported from
each other in wrong way. Imagine that during the import of ``A``, the import of
``B`` is requested before we could create the node for ``A`` (the constructor
needs a reference to ``B``). And the same could be true for the import of ``B``
(``A`` is requested to be imported before we could create the node for ``B``).
In case of the :ref:`templated-described swing <templated>` we take
extra attention to break the cyclical dependency: we import and set the
described template only after the ``CXXRecordDecl`` is created. As a best
practice, before creating the node in the "to" context, avoid importing of
other nodes which are not needed for the constructor of node ``A``.

Error Handling
^^^^^^^^^^^^^^

Every import function returns with either an ``llvm::Error`` or an
``llvm::Expected<T>`` object. This enforces to check the return value of the
import functions. If there was an error during one import then we return with
that error. (Exception: when we import the members of a class, we collect the
individual errors with each member and we concatenate them in one Error
object.) We cache these errors in cases of declarations. During the next import
call if there is an existing error we just return with that. So, clients of the
library receive an Error object, which they must check.

During import of a specific declaration, it may happen that some AST nodes had
already been created before we recognize an error. In this case, we signal back
the error to the caller, but the "to" context remains polluted with those nodes
which had been created. Ideally, those nodes should not had been created, but
that time we did not know about the error, the error happened later. Since the
AST is immutable (most of the cases we can't remove existing nodes) we choose
to mark these nodes as erroneous.

We cache the errors associated with declarations in the "from" context in
``ASTImporter::ImportDeclErrors`` and the ones which are associated with the
"to" context in ``ASTImporterSharedState::ImportErrors``. Note that, there may
be several ASTImporter objects which import into the same "to" context but from
different "from" contexts; in this case, they have to share the associated
errors of the "to" context.

When an error happens, that propagates through the call stack, through all the
dependant nodes. However, in case of dependency cycles, this is not enough,
because we strive to mark the erroneous nodes so clients can act upon. In those
cases, we have to keep track of the errors for those nodes which are
intermediate nodes of a cycle.

An **import path** is the list of the AST nodes which we visit during an Import
call. If node ``A`` depends on node ``B`` then the path contains an ``A->B``
edge. From the call stack of the import functions, we can read the very same
path.

Now imagine the following AST, where the ``->`` represents dependency in terms
of the import (all nodes are declarations).

.. code-block:: text

  A->B->C->D
     `->E

We would like to import A.
The import behaves like a DFS, so we will visit the nodes in this order: ABCDE.
During the visitation we will have the following import paths:

.. code-block:: text

  A
  AB
  ABC
  ABCD
  ABC
  AB
  ABE
  AB
  A

If during the visit of E there is an error then we set an error for E, then as
the call stack shrinks for B, then for A:

.. code-block:: text

  A
  AB
  ABC
  ABCD
  ABC
  AB
  ABE // Error! Set an error to E
  AB  // Set an error to B
  A   // Set an error to A

However, during the import we could import C and D without any error and they
are independent of A,B and E. We must not set up an error for C and D. So, at
the end of the import we have an entry in ``ImportDeclErrors`` for A,B,E but
not for C,D.

Now, what happens if there is a cycle in the import path? Let's consider this
AST:

.. code-block:: text

  A->B->C->A
     `->E

During the visitation, we will have the below import paths and if during the
visit of E there is an error then we will set up an error for E,B,A. But what's
up with C?

.. code-block:: text

  A
  AB
  ABC
  ABCA
  ABC
  AB
  ABE // Error! Set an error to E
  AB  // Set an error to B
  A   // Set an error to A

This time we know that both B and C are dependent on A. This means we must set
up an error for C too. As the call stack reverses back we get to A and we must
set up an error to all nodes which depend on A (this includes C). But C is no
longer on the import path, it just had been previously. Such a situation can
happen only if during the visitation we had a cycle. If we didn't have any
cycle, then the normal way of passing an Error object through the call stack
could handle the situation. This is why we must track cycles during the import
process for each visited declaration.

Lookup Problems
^^^^^^^^^^^^^^^

When we import a declaration from the source context then we check whether we
already have a structurally equivalent node with the same name in the "to"
context. If the "from" node is a definition and the found one is also a
definition, then we do not create a new node, instead, we mark the found node
as the imported node. If the found definition and the one we want to import
have the same name but they are structurally in-equivalent, then we have an ODR
violation in case of C++. If the "from" node is not a definition then we add
that to the redeclaration chain of the found node. This behaviour is essential
when we merge ASTs from different translation units which include the same
header file(s). For example, we want to have only one definition for the class
template ``std::vector``, even if we included ``<vector>`` in several
translation units.

To find a structurally equivalent node we can use the regular C/C++ lookup
functions: ``DeclContext::noload_lookup()`` and
``DeclContext::localUncachedLookup()``. These functions do respect the C/C++
name hiding rules, thus you cannot find certain declarations in a given
declaration context. For instance, unnamed declarations (anonymous structs),
non-first ``friend`` declarations and template specializations are hidden. This
is a problem, because if we use the regular C/C++ lookup then we create
redundant AST nodes during the merge! Also, having two instances of the same
node could result in false :ref:`structural in-equivalencies <structural-eq>`
of other nodes which depend on the duplicated node. Because of these reasons,
we created a lookup class which has the sole purpose to register all
declarations, so later they can be looked up by subsequent import requests.
This is the ``ASTImporterLookupTable`` class. This lookup table should be
shared amongst the different ``ASTImporter`` instances if they happen to import
to the very same "to" context. This is why we can use the importer specific
lookup only via the ``ASTImporterSharedState`` class.

ExternalASTSource
~~~~~~~~~~~~~~~~~

The ``ExternalASTSource`` is an abstract interface associated with the
``ASTContext`` class. It provides the ability to read the declarations stored
within a declaration context either for iteration or for name lookup. A
declaration context with an external AST source may load its declarations
on-demand. This means that the list of declarations (represented as a linked
list, the head is ``DeclContext::FirstDecl``) could be empty. However, member
functions like ``DeclContext::lookup()`` may initiate a load.

Usually, external sources are associated with precompiled headers. For example,
when we load a class from a PCH then the members are loaded only if we do want
to look up something in the class' context.

In case of LLDB, an implementation of the ``ExternalASTSource`` interface is
attached to the AST context which is related to the parsed expression. This
implementation of the ``ExternalASTSource`` interface is realized with the help
of the ``ASTImporter`` class. This way, LLDB can reuse Clang's parsing
machinery while synthesizing the underlying AST from the debug data (e.g. from
DWARF). From the view of the ``ASTImporter`` this means both the "to" and the
"from" context may have declaration contexts with external lexical storage. If
a ``DeclContext`` in the "to" AST context has external lexical storage then we
must take extra attention to work only with the already loaded declarations!
Otherwise, we would end up with an uncontrolled import process. For instance,
if we used the regular ``DeclContext::lookup()`` to find the existing
declarations in the "to" context then the ``lookup()`` call itself would
initiate a new import while we are in the middle of importing a declaration!
(By the time we initiate the lookup we haven't registered yet that we already
started to import the node of the "from" context.) This is why we use
``DeclContext::noload_lookup()`` instead.

Class Template Instantiations
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Different translation units may have class template instantiations with the
same template arguments, but with a different set of instantiated
``MethodDecls`` and ``FieldDecls``. Consider the following files:

.. code-block:: c++

  // x.h
  template <typename T>
  struct X {
      int a{0}; // FieldDecl with InitListExpr
      X(char) : a(3) {}     // (1)
      X(int) {}             // (2)
  };

  // foo.cpp
  void foo() {
      // ClassTemplateSpec with ctor (1): FieldDecl without InitlistExpr
      X<char> xc('c');
  }

  // bar.cpp
  void bar() {
      // ClassTemplateSpec with ctor (2): FieldDecl WITH InitlistExpr
      X<char> xc(1);
  }

In ``foo.cpp`` we use the constructor with number ``(1)``, which explicitly
initializes the member ``a`` to ``3``, thus the ``InitListExpr`` ``{0}`` is not
used here and the AST node is not instantiated. However, in the case of
``bar.cpp`` we use the constructor with number ``(2)``, which does not
explicitly initialize the ``a`` member, so the default ``InitListExpr`` is
needed and thus instantiated. When we merge the AST of ``foo.cpp`` and
``bar.cpp`` we must create an AST node for the class template instantiation of
``X<char>`` which has all the required nodes. Therefore, when we find an
existing ``ClassTemplateSpecializationDecl`` then we merge the fields of the
``ClassTemplateSpecializationDecl`` in the "from" context in a way that the
``InitListExpr`` is copied if not existent yet. The same merge mechanism should
be done in the cases of instantiated default arguments and exception
specifications of functions.

.. _visibility:

Visibility of Declarations
^^^^^^^^^^^^^^^^^^^^^^^^^^

During import of a global variable with external visibility, the lookup will
find variables (with the same name) but with static visibility (linkage).
Clearly, we cannot put them into the same redeclaration chain. The same is true
the in case of functions. Also, we have to take care of other kinds of
declarations like enums, classes, etc. if they are in anonymous namespaces.
Therefore, we filter the lookup results and consider only those which have the
same visibility as the declaration we currently import.

We consider two declarations in two anonymous namespaces to have the same
visibility only if they are imported from the same AST context.

Strategies to Handle Conflicting Names
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

During the import we lookup existing declarations with the same name. We filter
the lookup results based on their :ref:`visibility <visibility>`. If any of the
found declarations are not structurally equivalent then we bumped to a name
conflict error (ODR violation in C++). In this case, we return with an
``Error`` and we set up the ``Error`` object for the declaration. However, some
clients of the ``ASTImporter`` may require a different, perhaps less
conservative and more liberal error handling strategy.

E.g. static analysis clients may benefit if the node is created even if there
is a name conflict. During the CTU analysis of certain projects, we recognized
that there are global declarations which collide with declarations from other
translation units, but they are not referenced outside from their translation
unit. These declarations should be in an unnamed namespace ideally. If we treat
these collisions liberally then CTU analysis can find more results. Note, the
feature be able to choose between name conflict handling strategies is still an
ongoing work.

.. _CFG:

The ``CFG`` class
-----------------

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

Basic Blocks
^^^^^^^^^^^^

Concretely, an instance of ``CFG`` is a collection of basic blocks.  Each basic
block is an instance of ``CFGBlock``, which simply contains an ordered sequence
of ``Stmt*`` (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.  :ref:`Conditional control-flow
<ConditionalControlFlow>` is represented using edges between basic blocks.  The
statements within a given ``CFGBlock`` can be traversed using the
``CFGBlock::*iterator`` interface.

A ``CFG`` object owns the instances of ``CFGBlock`` within the control-flow
graph it represents.  Each ``CFGBlock`` within a CFG is also uniquely numbered
(accessible via ``CFGBlock::getBlockID()``).  Currently the number is based on
the ordering the blocks were created, but no assumptions should be made on how
``CFGBlocks`` 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).

Entry and Exit Blocks
^^^^^^^^^^^^^^^^^^^^^

Each instance of ``CFG`` contains two special blocks: an *entry* block
(accessible via ``CFG::getEntry()``), which has no incoming edges, and an
*exit* block (accessible via ``CFG::getExit()``), 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.

.. _ConditionalControlFlow:

Conditional Control-Flow
^^^^^^^^^^^^^^^^^^^^^^^^

Conditional control-flow (such as those induced by if-statements and loops) is
represented as edges between ``CFGBlocks``.  Because different C language
constructs can induce control-flow, each ``CFGBlock`` also records an extra
``Stmt*`` that represents the *terminator* 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 ``IfStmt`` object in the
AST that represented the given branch.

To illustrate, consider the following code example:

.. code-block:: c++

  int foo(int x) {
    x = x + 1;
    if (x > 2)
      x++;
    else {
      x += 2;
      x *= 2;
    }

    return x;
  }

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

.. code-block:: c++

  Stmt *FooBody = ...
  std::unique_ptr<CFG> FooCFG = CFG::buildCFG(FooBody);

Along with providing an interface to iterate over its ``CFGBlocks``, the
``CFG`` class also provides methods that are useful for debugging and
visualizing CFGs.  For example, the method ``CFG::dump()`` 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
``FooCFG->dump()``:

.. code-block:: text

 [ B5 (ENTRY) ]
    Predecessors (0):
    Successors (1): B4

 [ B4 ]
    1: x = x + 1
    2: (x > 2)
    T: if [B4.2]
    Predecessors (1): B5
    Successors (2): B3 B2

 [ B3 ]
    1: x++
    Predecessors (1): B4
    Successors (1): B1

 [ B2 ]
    1: x += 2
    2: x *= 2
    Predecessors (1): B4
    Successors (1): B1

 [ B1 ]
    1: return x;
    Predecessors (2): B2 B3
    Successors (1): B0

 [ B0 (EXIT) ]
    Predecessors (1): B1
    Successors (0):

For each block, the pretty-printed output displays for each block the number of
*predecessor* blocks (blocks that have outgoing control-flow to the given
block) and *successor* 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.

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

The terminator of block B4 is a pointer to the ``IfStmt`` object in the AST.
The pretty-printer outputs ``if [B4.2]`` because the condition expression of
the if-statement has an actual place in the basic block, and thus the
terminator is essentially *referring* 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.

.. Implicit Control-Flow
.. ^^^^^^^^^^^^^^^^^^^^^

.. A key design principle of the ``CFG`` class was to not require any
.. transformations to the AST in order to represent control-flow.  Thus the
.. ``CFG`` 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.

Constant Folding in the Clang AST
---------------------------------

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.

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
``-pedantic-errors``.

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 unfortunate 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.

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.

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 have 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.

Implementation Approach
^^^^^^^^^^^^^^^^^^^^^^^

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 evaluation method (``Expr::Evaluate``), which is implemented
in ``AST/ExprConstant.cpp``.  Given an expression with "scalar" type (integer,
fp, complex, or pointer) this method returns the following information:

* 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.
* If the expression was computable in any way, this method returns the
  ``APValue`` for the result of the expression.
* 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 have ``ERROR`` type.
* 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 have ``EXTENSION`` type.

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 ``Sema::VerifyIntegerConstantExpression`` 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.

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

Extensions
^^^^^^^^^^

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

* ``__extension__``: The expression form of this extension causes any
  evaluatable subexpression to be accepted as an integer constant expression.
* ``__builtin_constant_p``: This returns true (as an integer constant
  expression) if the operand evaluates to either a numeric value (that is, not
  a pointer cast to integral type) of integral, enumeration, floating or
  complex type, or if it evaluates to the address of the first character of a
  string literal (possibly cast to some other type).  As a special case, if
  ``__builtin_constant_p`` 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.
* ``__builtin_choose_expr``: 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.
* ``__builtin_classify_type``: This always returns an integer constant
  expression.
* ``__builtin_inf, nan, ...``: These are treated just like a floating-point
  literal.
* ``__builtin_abs, copysign, ...``: These are constant folded as general
  constant expressions.
* ``__builtin_strlen`` and ``strlen``: These are constant folded as integer
  constant expressions if the argument is a string literal.

.. _Sema:

The Sema Library
================

This library is called by the :ref:`Parser library <Parser>` during parsing to
do semantic analysis of the input.  For valid programs, Sema builds an AST for
parsed constructs.

.. _CodeGen:

The CodeGen Library
===================

CodeGen takes an :ref:`AST <AST>` as input and produces `LLVM IR code
<//llvm.org/docs/LangRef.html>`_ from it.

How to change Clang
===================

How to add an attribute
-----------------------
Attributes are a form of metadata that can be attached to a program construct,
allowing the programmer to pass semantic information along to the compiler for
various uses. For example, attributes may be used to alter the code generation
for a program construct, or to provide extra semantic information for static
analysis. This document explains how to add a custom attribute to Clang.
Documentation on existing attributes can be found `here
<//clang.llvm.org/docs/AttributeReference.html>`_.

Attribute Basics
^^^^^^^^^^^^^^^^
Attributes in Clang are handled in three stages: parsing into a parsed attribute
representation, conversion from a parsed attribute into a semantic attribute,
and then the semantic handling of the attribute.

Parsing of the attribute is determined by the various syntactic forms attributes
can take, such as GNU, C++11, and Microsoft style attributes, as well as other
information provided by the table definition of the attribute. Ultimately, the
parsed representation of an attribute object is an ``ParsedAttr`` object.
These parsed attributes chain together as a list of parsed attributes attached
to a declarator or declaration specifier. The parsing of attributes is handled
automatically by Clang, except for attributes spelled as keywords. When
implementing a keyword attribute, the parsing of the keyword and creation of the
``ParsedAttr`` object must be done manually.

Eventually, ``Sema::ProcessDeclAttributeList()`` is called with a ``Decl`` and
an ``ParsedAttr``, at which point the parsed attribute can be transformed
into a semantic attribute. The process by which a parsed attribute is converted
into a semantic attribute depends on the attribute definition and semantic
requirements of the attribute. The end result, however, is that the semantic
attribute object is attached to the ``Decl`` object, and can be obtained by a
call to ``Decl::getAttr<T>()``.

The structure of the semantic attribute is also governed by the attribute
definition given in Attr.td. This definition is used to automatically generate
functionality used for the implementation of the attribute, such as a class
derived from ``clang::Attr``, information for the parser to use, automated
semantic checking for some attributes, etc.


``include/clang/Basic/Attr.td``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The first step to adding a new attribute to Clang is to add its definition to
`include/clang/Basic/Attr.td
<https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Basic/Attr.td>`_.
This tablegen definition must derive from the ``Attr`` (tablegen, not
semantic) type, or one of its derivatives. Most attributes will derive from the
``InheritableAttr`` type, which specifies that the attribute can be inherited by
later redeclarations of the ``Decl`` it is associated with.
``InheritableParamAttr`` is similar to ``InheritableAttr``, except that the
attribute is written on a parameter instead of a declaration. If the attribute
is intended to apply to a type instead of a declaration, such an attribute
should derive from ``TypeAttr``, and will generally not be given an AST
representation. (Note that this document does not cover the creation of type
attributes.) An attribute that inherits from ``IgnoredAttr`` is parsed, but will
generate an ignored attribute diagnostic when used, which may be useful when an
attribute is supported by another vendor but not supported by clang.

The definition will specify several key pieces of information, such as the
semantic name of the attribute, the spellings the attribute supports, the
arguments the attribute expects, and more. Most members of the ``Attr`` tablegen
type do not require definitions in the derived definition as the default
suffice. However, every attribute must specify at least a spelling list, a
subject list, and a documentation list.

Spellings
~~~~~~~~~
All attributes are required to specify a spelling list that denotes the ways in
which the attribute can be spelled. For instance, a single semantic attribute
may have a keyword spelling, as well as a C++11 spelling and a GNU spelling. An
empty spelling list is also permissible and may be useful for attributes which
are created implicitly. The following spellings are accepted:

  ============  ================================================================
  Spelling      Description
  ============  ================================================================
  ``GNU``       Spelled with a GNU-style ``__attribute__((attr))`` syntax and
                placement.
  ``CXX11``     Spelled with a C++-style ``[[attr]]`` syntax. If the attribute
                is meant to be used by Clang, it should set the namespace to
                ``"clang"``.
  ``Declspec``  Spelled with a Microsoft-style ``__declspec(attr)`` syntax.
  ``Keyword``   The attribute is spelled as a keyword, and required custom
                parsing.
  ``GCC``       Specifies two spellings: the first is a GNU-style spelling, and
                the second is a C++-style spelling with the ``gnu`` namespace.
                Attributes should only specify this spelling for attributes
                supported by GCC.
  ``Pragma``    The attribute is spelled as a ``#pragma``, and requires custom
                processing within the preprocessor. If the attribute is meant to
                be used by Clang, it should set the namespace to ``"clang"``.
                Note that this spelling is not used for declaration attributes.
  ============  ================================================================

Subjects
~~~~~~~~
Attributes appertain to one or more ``Decl`` subjects. If the attribute attempts
to attach to a subject that is not in the subject list, a diagnostic is issued
automatically. Whether the diagnostic is a warning or an error depends on how
the attribute's ``SubjectList`` is defined, but the default behavior is to warn.
The diagnostics displayed to the user are automatically determined based on the
subjects in the list, but a custom diagnostic parameter can also be specified in
the ``SubjectList``. The diagnostics generated for subject list violations are
either ``diag::warn_attribute_wrong_decl_type`` or
``diag::err_attribute_wrong_decl_type``, and the parameter enumeration is found
in `include/clang/Sema/ParsedAttr.h
<https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Sema/ParsedAttr.h>`_
If a previously unused Decl node is added to the ``SubjectList``, the logic used
to automatically determine the diagnostic parameter in `utils/TableGen/ClangAttrEmitter.cpp
<https://github.com/llvm/llvm-project/blob/master/clang/utils/TableGen/ClangAttrEmitter.cpp>`_
may need to be updated.

By default, all subjects in the SubjectList must either be a Decl node defined
in ``DeclNodes.td``, or a statement node defined in ``StmtNodes.td``. However,
more complex subjects can be created by creating a ``SubsetSubject`` object.
Each such object has a base subject which it appertains to (which must be a
Decl or Stmt node, and not a SubsetSubject node), and some custom code which is
called when determining whether an attribute appertains to the subject. For
instance, a ``NonBitField`` SubsetSubject appertains to a ``FieldDecl``, and
tests whether the given FieldDecl is a bit field. When a SubsetSubject is
specified in a SubjectList, a custom diagnostic parameter must also be provided.

Diagnostic checking for attribute subject lists is automated except when
``HasCustomParsing`` is set to ``1``.

Documentation
~~~~~~~~~~~~~
All attributes must have some form of documentation associated with them.
Documentation is table generated on the public web server by a server-side
process that runs daily. Generally, the documentation for an attribute is a
stand-alone definition in `include/clang/Basic/AttrDocs.td
<https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Basic/AttrDocs.td>`_
that is named after the attribute being documented.

If the attribute is not for public consumption, or is an implicitly-created
attribute that has no visible spelling, the documentation list can specify the
``Undocumented`` object. Otherwise, the attribute should have its documentation
added to AttrDocs.td.

Documentation derives from the ``Documentation`` tablegen type. All derived
types must specify a documentation category and the actual documentation itself.
Additionally, it can specify a custom heading for the attribute, though a
default heading will be chosen when possible.

There are four predefined documentation categories: ``DocCatFunction`` for
attributes that appertain to function-like subjects, ``DocCatVariable`` for
attributes that appertain to variable-like subjects, ``DocCatType`` for type
attributes, and ``DocCatStmt`` for statement attributes. A custom documentation
category should be used for groups of attributes with similar functionality.
Custom categories are good for providing overview information for the attributes
grouped under it. For instance, the consumed annotation attributes define a
custom category, ``DocCatConsumed``, that explains what consumed annotations are
at a high level.

Documentation content (whether it is for an attribute or a category) is written
using reStructuredText (RST) syntax.

After writing the documentation for the attribute, it should be locally tested
to ensure that there are no issues generating the documentation on the server.
Local testing requires a fresh build of clang-tblgen. To generate the attribute
documentation, execute the following command::

  clang-tblgen -gen-attr-docs -I /path/to/clang/include /path/to/clang/include/clang/Basic/Attr.td -o /path/to/clang/docs/AttributeReference.rst

When testing locally, *do not* commit changes to ``AttributeReference.rst``.
This file is generated by the server automatically, and any changes made to this
file will be overwritten.

Arguments
~~~~~~~~~
Attributes may optionally specify a list of arguments that can be passed to the
attribute. Attribute arguments specify both the parsed form and the semantic
form of the attribute. For example, if ``Args`` is
``[StringArgument<"Arg1">, IntArgument<"Arg2">]`` then
``__attribute__((myattribute("Hello", 3)))`` will be a valid use; it requires
two arguments while parsing, and the Attr subclass' constructor for the
semantic attribute will require a string and integer argument.

All arguments have a name and a flag that specifies whether the argument is
optional. The associated C++ type of the argument is determined by the argument
definition type. If the existing argument types are insufficient, new types can
be created, but it requires modifying `utils/TableGen/ClangAttrEmitter.cpp
<https://github.com/llvm/llvm-project/blob/master/clang/utils/TableGen/ClangAttrEmitter.cpp>`_
to properly support the type.

Other Properties
~~~~~~~~~~~~~~~~
The ``Attr`` definition has other members which control the behavior of the
attribute. Many of them are special-purpose and beyond the scope of this
document, however a few deserve mention.

If the parsed form of the attribute is more complex, or differs from the
semantic form, the ``HasCustomParsing`` bit can be set to ``1`` for the class,
and the parsing code in `Parser::ParseGNUAttributeArgs()
<https://github.com/llvm/llvm-project/blob/master/clang/lib/Parse/ParseDecl.cpp>`_
can be updated for the special case. Note that this only applies to arguments
with a GNU spelling -- attributes with a __declspec spelling currently ignore
this flag and are handled by ``Parser::ParseMicrosoftDeclSpec``.

Note that setting this member to 1 will opt out of common attribute semantic
handling, requiring extra implementation efforts to ensure the attribute
appertains to the appropriate subject, etc.

If the attribute should not be propagated from a template declaration to an
instantiation of the template, set the ``Clone`` member to 0. By default, all
attributes will be cloned to template instantiations.

Attributes that do not require an AST node should set the ``ASTNode`` field to
``0`` to avoid polluting the AST. Note that anything inheriting from
``TypeAttr`` or ``IgnoredAttr`` automatically do not generate an AST node. All
other attributes generate an AST node by default. The AST node is the semantic
representation of the attribute.

The ``LangOpts`` field specifies a list of language options required by the
attribute.  For instance, all of the CUDA-specific attributes specify ``[CUDA]``
for the ``LangOpts`` field, and when the CUDA language option is not enabled, an
"attribute ignored" warning diagnostic is emitted. Since language options are
not table generated nodes, new language options must be created manually and
should specify the spelling used by ``LangOptions`` class.

Custom accessors can be generated for an attribute based on the spelling list
for that attribute. For instance, if an attribute has two different spellings:
'Foo' and 'Bar', accessors can be created:
``[Accessor<"isFoo", [GNU<"Foo">]>, Accessor<"isBar", [GNU<"Bar">]>]``
These accessors will be generated on the semantic form of the attribute,
accepting no arguments and returning a ``bool``.

Attributes that do not require custom semantic handling should set the
``SemaHandler`` field to ``0``. Note that anything inheriting from
``IgnoredAttr`` automatically do not get a semantic handler. All other
attributes are assumed to use a semantic handler by default. Attributes
without a semantic handler are not given a parsed attribute ``Kind`` enumerator.

Target-specific attributes may share a spelling with other attributes in
different targets. For instance, the ARM and MSP430 targets both have an
attribute spelled ``GNU<"interrupt">``, but with different parsing and semantic
requirements. To support this feature, an attribute inheriting from
``TargetSpecificAttribute`` may specify a ``ParseKind`` field. This field
should be the same value between all arguments sharing a spelling, and
corresponds to the parsed attribute's ``Kind`` enumerator. This allows
attributes to share a parsed attribute kind, but have distinct semantic
attribute classes. For instance, ``ParsedAttr`` is the shared
parsed attribute kind, but ARMInterruptAttr and MSP430InterruptAttr are the
semantic attributes generated.

By default, attribute arguments are parsed in an evaluated context. If the
arguments for an attribute should be parsed in an unevaluated context (akin to
the way the argument to a ``sizeof`` expression is parsed), set
``ParseArgumentsAsUnevaluated`` to ``1``.

If additional functionality is desired for the semantic form of the attribute,
the ``AdditionalMembers`` field specifies code to be copied verbatim into the
semantic attribute class object, with ``public`` access.

Boilerplate
^^^^^^^^^^^
All semantic processing of declaration attributes happens in `lib/Sema/SemaDeclAttr.cpp
<https://github.com/llvm/llvm-project/blob/master/clang/lib/Sema/SemaDeclAttr.cpp>`_,
and generally starts in the ``ProcessDeclAttribute()`` function. If the
attribute is a "simple" attribute -- meaning that it requires no custom semantic
processing aside from what is automatically  provided, add a call to
``handleSimpleAttribute<YourAttr>(S, D, Attr);`` to the switch statement.
Otherwise, write a new ``handleYourAttr()`` function, and add that to the switch
statement. Please do not implement handling logic directly in the ``case`` for
the attribute.

Unless otherwise specified by the attribute definition, common semantic checking
of the parsed attribute is handled automatically. This includes diagnosing
parsed attributes that do not appertain to the given ``Decl``, ensuring the
correct minimum number of arguments are passed, etc.

If the attribute adds additional warnings, define a ``DiagGroup`` in
`include/clang/Basic/DiagnosticGroups.td
<https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Basic/DiagnosticGroups.td>`_
named after the attribute's ``Spelling`` with "_"s replaced by "-"s. If there
is only a single diagnostic, it is permissible to use ``InGroup<DiagGroup<"your-attribute">>``
directly in `DiagnosticSemaKinds.td
<https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Basic/DiagnosticSemaKinds.td>`_

All semantic diagnostics generated for your attribute, including automatically-
generated ones (such as subjects and argument counts), should have a
corresponding test case.

Semantic handling
^^^^^^^^^^^^^^^^^
Most attributes are implemented to have some effect on the compiler. For
instance, to modify the way code is generated, or to add extra semantic checks
for an analysis pass, etc. Having added the attribute definition and conversion
to the semantic representation for the attribute, what remains is to implement
the custom logic requiring use of the attribute.

The ``clang::Decl`` object can be queried for the presence or absence of an
attribute using ``hasAttr<T>()``. To obtain a pointer to the semantic
representation of the attribute, ``getAttr<T>`` may be used.

How to add an expression or statement
-------------------------------------

Expressions and statements are one of the most fundamental constructs within a
compiler, because they interact with many different parts of the AST, semantic
analysis, and IR generation.  Therefore, adding a new expression or statement
kind into Clang requires some care.  The following list details the various
places in Clang where an expression or statement needs to be introduced, along
with patterns to follow to ensure that the new expression or statement works
well across all of the C languages.  We focus on expressions, but statements
are similar.

#. Introduce parsing actions into the parser.  Recursive-descent parsing is
   mostly self-explanatory, but there are a few things that are worth keeping
   in mind:

   * Keep as much source location information as possible! You'll want it later
     to produce great diagnostics and support Clang's various features that map
     between source code and the AST.
   * Write tests for all of the "bad" parsing cases, to make sure your recovery
     is good.  If you have matched delimiters (e.g., parentheses, square
     brackets, etc.), use ``Parser::BalancedDelimiterTracker`` to give nice
     diagnostics when things go wrong.

#. Introduce semantic analysis actions into ``Sema``.  Semantic analysis should
   always involve two functions: an ``ActOnXXX`` function that will be called
   directly from the parser, and a ``BuildXXX`` function that performs the
   actual semantic analysis and will (eventually!) build the AST node.  It's
   fairly common for the ``ActOnCXX`` function to do very little (often just
   some minor translation from the parser's representation to ``Sema``'s
   representation of the same thing), but the separation is still important:
   C++ template instantiation, for example, should always call the ``BuildXXX``
   variant.  Several notes on semantic analysis before we get into construction
   of the AST:

   * Your expression probably involves some types and some subexpressions.
     Make sure to fully check that those types, and the types of those
     subexpressions, meet your expectations.  Add implicit conversions where
     necessary to make sure that all of the types line up exactly the way you
     want them.  Write extensive tests to check that you're getting good
     diagnostics for mistakes and that you can use various forms of
     subexpressions with your expression.
   * When type-checking a type or subexpression, make sure to first check
     whether the type is "dependent" (``Type::isDependentType()``) or whether a
     subexpression is type-dependent (``Expr::isTypeDependent()``).  If any of
     these return ``true``, then you're inside a template and you can't do much
     type-checking now.  That's normal, and your AST node (when you get there)
     will have to deal with this case.  At this point, you can write tests that
     use your expression within templates, but don't try to instantiate the
     templates.
   * For each subexpression, be sure to call ``Sema::CheckPlaceholderExpr()``
     to deal with "weird" expressions that don't behave well as subexpressions.
     Then, determine whether you need to perform lvalue-to-rvalue conversions
     (``Sema::DefaultLvalueConversions``) or the usual unary conversions
     (``Sema::UsualUnaryConversions``), for places where the subexpression is
     producing a value you intend to use.
   * Your ``BuildXXX`` function will probably just return ``ExprError()`` at
     this point, since you don't have an AST.  That's perfectly fine, and
     shouldn't impact your testing.

#. Introduce an AST node for your new expression.  This starts with declaring
   the node in ``include/Basic/StmtNodes.td`` and creating a new class for your
   expression in the appropriate ``include/AST/Expr*.h`` header.  It's best to
   look at the class for a similar expression to get ideas, and there are some
   specific things to watch for:

   * If you need to allocate memory, use the ``ASTContext`` allocator to
     allocate memory.  Never use raw ``malloc`` or ``new``, and never hold any
     resources in an AST node, because the destructor of an AST node is never
     called.
   * Make sure that ``getSourceRange()`` covers the exact source range of your
     expression.  This is needed for diagnostics and for IDE support.
   * Make sure that ``children()`` visits all of the subexpressions.  This is
     important for a number of features (e.g., IDE support, C++ variadic
     templates).  If you have sub-types, you'll also need to visit those
     sub-types in ``RecursiveASTVisitor``.
   * Add printing support (``StmtPrinter.cpp``) for your expression.
   * Add profiling support (``StmtProfile.cpp``) for your AST node, noting the
     distinguishing (non-source location) characteristics of an instance of
     your expression.  Omitting this step will lead to hard-to-diagnose
     failures regarding matching of template declarations.
   * Add serialization support (``ASTReaderStmt.cpp``, ``ASTWriterStmt.cpp``)
     for your AST node.

#. Teach semantic analysis to build your AST node.  At this point, you can wire
   up your ``Sema::BuildXXX`` function to actually create your AST.  A few
   things to check at this point:

   * If your expression can construct a new C++ class or return a new
     Objective-C object, be sure to update and then call
     ``Sema::MaybeBindToTemporary`` for your just-created AST node to be sure
     that the object gets properly destructed.  An easy way to test this is to
     return a C++ class with a private destructor: semantic analysis should
     flag an error here with the attempt to call the destructor.
   * Inspect the generated AST by printing it using ``clang -cc1 -ast-print``,
     to make sure you're capturing all of the important information about how
     the AST was written.
   * Inspect the generated AST under ``clang -cc1 -ast-dump`` to verify that
     all of the types in the generated AST line up the way you want them.
     Remember that clients of the AST should never have to "think" to
     understand what's going on.  For example, all implicit conversions should
     show up explicitly in the AST.
   * Write tests that use your expression as a subexpression of other,
     well-known expressions.  Can you call a function using your expression as
     an argument?  Can you use the ternary operator?

#. Teach code generation to create IR to your AST node.  This step is the first
   (and only) that requires knowledge of LLVM IR.  There are several things to
   keep in mind:

   * Code generation is separated into scalar/aggregate/complex and
     lvalue/rvalue paths, depending on what kind of result your expression
     produces.  On occasion, this requires some careful factoring of code to
     avoid duplication.
   * ``CodeGenFunction`` contains functions ``ConvertType`` and
     ``ConvertTypeForMem`` that convert Clang's types (``clang::Type*`` or
     ``clang::QualType``) to LLVM types.  Use the former for values, and the
     latter for memory locations: test with the C++ "``bool``" type to check
     this.  If you find that you are having to use LLVM bitcasts to make the
     subexpressions of your expression have the type that your expression
     expects, STOP!  Go fix semantic analysis and the AST so that you don't
     need these bitcasts.
   * The ``CodeGenFunction`` class has a number of helper functions to make
     certain operations easy, such as generating code to produce an lvalue or
     an rvalue, or to initialize a memory location with a given value.  Prefer
     to use these functions rather than directly writing loads and stores,
     because these functions take care of some of the tricky details for you
     (e.g., for exceptions).
   * If your expression requires some special behavior in the event of an
     exception, look at the ``push*Cleanup`` functions in ``CodeGenFunction``
     to introduce a cleanup.  You shouldn't have to deal with
     exception-handling directly.
   * Testing is extremely important in IR generation.  Use ``clang -cc1
     -emit-llvm`` and `FileCheck
     <https://llvm.org/docs/CommandGuide/FileCheck.html>`_ to verify that you're
     generating the right IR.

#. Teach template instantiation how to cope with your AST node, which requires
   some fairly simple code:

   * Make sure that your expression's constructor properly computes the flags
     for type dependence (i.e., the type your expression produces can change
     from one instantiation to the next), value dependence (i.e., the constant
     value your expression produces can change from one instantiation to the
     next), instantiation dependence (i.e., a template parameter occurs
     anywhere in your expression), and whether your expression contains a
     parameter pack (for variadic templates).  Often, computing these flags
     just means combining the results from the various types and
     subexpressions.
   * Add ``TransformXXX`` and ``RebuildXXX`` functions to the ``TreeTransform``
     class template in ``Sema``.  ``TransformXXX`` should (recursively)
     transform all of the subexpressions and types within your expression,
     using ``getDerived().TransformYYY``.  If all of the subexpressions and
     types transform without error, it will then call the ``RebuildXXX``
     function, which will in turn call ``getSema().BuildXXX`` to perform
     semantic analysis and build your expression.
   * To test template instantiation, take those tests you wrote to make sure
     that you were type checking with type-dependent expressions and dependent
     types (from step #2) and instantiate those templates with various types,
     some of which type-check and some that don't, and test the error messages
     in each case.

#. There are some "extras" that make other features work better.  It's worth
   handling these extras to give your expression complete integration into
   Clang:

   * Add code completion support for your expression in
     ``SemaCodeComplete.cpp``.
   * If your expression has types in it, or has any "interesting" features
     other than subexpressions, extend libclang's ``CursorVisitor`` to provide
     proper visitation for your expression, enabling various IDE features such
     as syntax highlighting, cross-referencing, and so on.  The
     ``c-index-test`` helper program can be used to test these features.