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+============================
+"Clang" CFE Internals Manual
+============================
+
+.. contents::
+ :local:
+
+Introduction
+============
+
+This document describes some of the more important APIs and internal design
+decisions made in the Clang C front-end. The purpose of this document is to
+both capture some of this high level information and also describe some of the
+design decisions behind it. This is meant for people interested in hacking on
+Clang, not for end-users. The description below is categorized by libraries,
+and does not describe any of the clients of the libraries.
+
+LLVM Support Library
+====================
+
+The LLVM ``libSupport`` library provides many underlying libraries and
+`data-structures <http://llvm.org/docs/ProgrammersManual.html>`_, including
+command line option processing, various containers and a system abstraction
+layer, which is used for file system access.
+
+The Clang "Basic" Library
+=========================
+
+This library certainly needs a better name. The "basic" library contains a
+number of low-level utilities for tracking and manipulating source buffers,
+locations within the source buffers, diagnostics, tokens, target abstraction,
+and information about the subset of the language being compiled for.
+
+Part of this infrastructure is specific to C (such as the ``TargetInfo``
+class), other parts could be reused for other non-C-based languages
+(``SourceLocation``, ``SourceManager``, ``Diagnostics``, ``FileManager``).
+When and if there is future demand we can figure out if it makes sense to
+introduce a new library, move the general classes somewhere else, or introduce
+some other solution.
+
+We describe the roles of these classes in order of their dependencies.
+
+The Diagnostics Subsystem
+-------------------------
+
+The Clang Diagnostics subsystem is an important part of how the compiler
+communicates with the human. Diagnostics are the warnings and errors produced
+when the code is incorrect or dubious. In Clang, each diagnostic produced has
+(at the minimum) a unique ID, an English translation associated with it, a
+:ref:`SourceLocation <SourceLocation>` to "put the caret", and a severity
+(e.g., ``WARNING`` or ``ERROR``). They can also optionally include a number of
+arguments to the dianostic (which fill in "%0"'s in the string) as well as a
+number of source ranges that related to the diagnostic.
+
+In this section, we'll be giving examples produced by the Clang command line
+driver, but diagnostics can be :ref:`rendered in many different ways
+<DiagnosticClient>` depending on how the ``DiagnosticClient`` interface is
+implemented. A representative example of a diagnostic is:
+
+.. code-block:: c++
+
+ 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``, ``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 ``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``, ``Warning``, ``Error``, ``Fatal``}) of output
+*levels* by the diagnostics subsystem based on various configuration options.
+Clang internally supports a fully fine grained mapping mechanism that allows
+you to map almost any diagnostic to the output level that you want. The only
+diagnostics that cannot be mapped are ``NOTE``\ s, which always follow the
+severity of the previously emitted diagnostic and ``ERROR``\ s, which can only
+be mapped to ``Fatal`` (it is not possible to turn an error into a warning, for
+example).
+
+Diagnostic mappings are used in many ways. For example, if the user specifies
+``-pedantic``, ``EXTENSION`` maps to ``Warning``, if they specify
+``-pedantic-errors``, it turns into ``Error``. This is used to implement
+options like ``-Wunused_macros``, ``-Wundef`` etc.
+
+Mapping to ``Fatal`` should only be used for diagnostics that are considered so
+severe that error recovery won't be able to recover sensibly from them (thus
+spewing a ton of bogus errors). One example of this class of error are failure
+to ``#include`` a file.
+
+The Format String
+^^^^^^^^^^^^^^^^^
+
+The format string for the diagnostic is very simple, but it has some power. It
+takes the form of a string in English with markers that indicate where and how
+arguments to the diagnostic are inserted and formatted. For example, here are
+some simple format strings:
+
+.. code-block:: c++
+
+ "binary integer literals are an extension"
+ "format string contains '\\0' within the string body"
+ "more '%%' conversions than data arguments"
+ "invalid operands to binary expression (%0 and %1)"
+ "overloaded '%0' must be a %select{unary|binary|unary or binary}2 operator"
+ " (has %1 parameter%s1)"
+
+These examples show some important points of format strings. You can use any
+plain ASCII character in the diagnostic string except "``%``" without a
+problem, but these are C strings, so you have to use and be aware of all the C
+escape sequences (as in the second example). If you want to produce a "``%``"
+in the output, use the "``%%``" escape sequence, like the third diagnostic.
+Finally, Clang uses the "``%...[digit]``" sequences to specify where and how
+arguments to the diagnostic are formatted.
+
+Arguments to the diagnostic are numbered according to how they are specified by
+the C++ code that :ref:`produces them <internals-producing-diag>`, and are
+referenced by ``%0`` .. ``%9``. If you have more than 10 arguments to your
+diagnostic, you are doing something wrong :). Unlike ``printf``, there is no
+requirement that arguments to the diagnostic end up in the output in the same
+order as they are specified, you could have a format string with "``%1 %0``"
+that swaps them, for example. The text in between the percent and digit are
+formatting instructions. If there are no instructions, the argument is just
+turned into a string and substituted in.
+
+Here are some "best practices" for writing the English format string:
+
+* Keep the string short. It should ideally fit in the 80 column limit of the
+ ``DiagnosticKinds.td`` file. This avoids the diagnostic wrapping when
+ printed, and forces you to think about the important point you are conveying
+ with the diagnostic.
+* Take advantage of location information. The user will be able to see the
+ line and location of the caret, so you don't need to tell them that the
+ problem is with the 4th argument to the function: just point to it.
+* Do not capitalize the diagnostic string, and do not end it with a period.
+* If you need to quote something in the diagnostic string, use single quotes.
+
+Diagnostics should never take random English strings as arguments: you
+shouldn't use "``you have a problem with %0``" and pass in things like "``your
+argument``" or "``your return value``" as arguments. Doing this prevents
+:ref:`translating <internals-diag-translation>` the Clang diagnostics to other
+languages (because they'll get random English words in their otherwise
+localized diagnostic). The exceptions to this are C/C++ language keywords
+(e.g., ``auto``, ``const``, ``mutable``, etc) and C/C++ operators (``/=``).
+Note that things like "pointer" and "reference" are not keywords. On the other
+hand, you *can* include anything that comes from the user's source code,
+including variable names, types, labels, etc. The "``select``" format can be
+used to achieve this sort of thing in a localizable way, see below.
+
+Formatting a Diagnostic Argument
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Arguments to diagnostics are fully typed internally, and come from a couple
+different classes: integers, types, names, and random strings. Depending on
+the class of the argument, it can be optionally formatted in different ways.
+This gives the ``DiagnosticClient`` information about what the argument means
+without requiring it to use a specific presentation (consider this MVC for
+Clang :).
+
+Here are the different diagnostic argument formats currently supported by
+Clang:
+
+**"s" format**
+
+Example:
+ ``"requires %1 parameter%s1"``
+Class:
+ Integers
+Description:
+ This is a simple formatter for integers that is useful when producing English
+ diagnostics. When the integer is 1, it prints as nothing. When the integer
+ is not 1, it prints as "``s``". This allows some simple grammatical forms to
+ be to be handled correctly, and eliminates the need to use gross things like
+ ``"requires %1 parameter(s)"``.
+
+**"select" format**
+
+Example:
+ ``"must be a %select{unary|binary|unary or binary}2 operator"``
+Class:
+ Integers
+Description:
+ This format specifier is used to merge multiple related diagnostics together
+ into one common one, without requiring the difference to be specified as an
+ English string argument. Instead of specifying the string, the diagnostic
+ gets an integer argument and the format string selects the numbered option.
+ In this case, the "``%2``" value must be an integer in the range [0..2]. If
+ it is 0, it prints "unary", if it is 1 it prints "binary" if it is 2, it
+ prints "unary or binary". This allows other language translations to
+ substitute reasonable words (or entire phrases) based on the semantics of the
+ diagnostic instead of having to do things textually. The selected string
+ does undergo formatting.
+
+**"plural" format**
+
+Example:
+ ``"you have %1 %plural{1:mouse|:mice}1 connected to your computer"``
+Class:
+ Integers
+Description:
+ This is a formatter for complex plural forms. It is designed to handle even
+ the requirements of languages with very complex plural forms, as many Baltic
+ languages have. The argument consists of a series of expression/form pairs,
+ separated by ":", where the first form whose expression evaluates to true is
+ the result of the modifier.
+
+ An expression can be empty, in which case it is always true. See the example
+ at the top. Otherwise, it is a series of one or more numeric conditions,
+ separated by ",". If any condition matches, the expression matches. Each
+ numeric condition can take one of three forms.
+
+ * number: A simple decimal number matches if the argument is the same as the
+ number. Example: ``"%plural{1:mouse|:mice}4"``
+ * range: A range in square brackets matches if the argument is within the
+ range. Then range is inclusive on both ends. Example:
+ ``"%plural{0:none|1:one|[2,5]:some|:many}2"``
+ * modulo: A modulo operator is followed by a number, and equals sign and
+ either a number or a range. The tests are the same as for plain numbers
+ and ranges, but the argument is taken modulo the number first. Example:
+ ``"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything else}1"``
+
+ The parser is very unforgiving. A syntax error, even whitespace, will abort,
+ as will a failure to match the argument against any expression.
+
+**"ordinal" format**
+
+Example:
+ ``"ambiguity in %ordinal0 argument"``
+Class:
+ Integers
+Description:
+ This is a formatter which represents the argument number as an ordinal: the
+ value ``1`` becomes ``1st``, ``3`` becomes ``3rd``, and so on. Values less
+ than ``1`` are not supported. This formatter is currently hard-coded to use
+ English ordinals.
+
+**"objcclass" format**
+
+Example:
+ ``"method %objcclass0 not found"``
+Class:
+ ``DeclarationName``
+Description:
+ This is a simple formatter that indicates the ``DeclarationName`` corresponds
+ to an Objective-C class method selector. As such, it prints the selector
+ with a leading "``+``".
+
+**"objcinstance" format**
+
+Example:
+ ``"method %objcinstance0 not found"``
+Class:
+ ``DeclarationName``
+Description:
+ This is a simple formatter that indicates the ``DeclarationName`` corresponds
+ to an Objective-C instance method selector. As such, it prints the selector
+ with a leading "``-``".
+
+**"q" format**
+
+Example:
+ ``"candidate found by name lookup is %q0"``
+Class:
+ ``NamedDecl *``
+Description:
+ This formatter indicates that the fully-qualified name of the declaration
+ should be printed, e.g., "``std::vector``" rather than "``vector``".
+
+**"diff" format**
+
+Example:
+ ``"no known conversion %diff{from $ to $|from argument type to parameter type}1,2"``
+Class:
+ ``QualType``
+Description:
+ This formatter takes two ``QualType``\ s and attempts to print a template
+ difference between the two. If tree printing is off, the text inside the
+ braces before the pipe is printed, with the formatted text replacing the $.
+ If tree printing is on, the text after the pipe is printed and a type tree is
+ printed after the diagnostic message.
+
+It is really easy to add format specifiers to the Clang diagnostics system, but
+they should be discussed before they are added. If you are creating a lot of
+repetitive diagnostics and/or have an idea for a useful formatter, please bring
+it up on the cfe-dev mailing list.
+
+.. _internals-producing-diag:
+
+Producing the Diagnostic
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Now that you've created the diagnostic in the ``Diagnostic*Kinds.td`` file, you
+need to write the code that detects the condition in question and emits the new
+diagnostic. Various components of Clang (e.g., the preprocessor, ``Sema``,
+etc.) provide a helper function named "``Diag``". It creates a diagnostic and
+accepts the arguments, ranges, and other information that goes along with it.
+
+For example, the binary expression error comes from code like this:
+
+.. code-block:: c++
+
+ if (various things that are bad)
+ Diag(Loc, diag::err_typecheck_invalid_operands)
+ << lex->getType() << rex->getType()
+ << lex->getSourceRange() << rex->getSourceRange();
+
+This shows that use of the ``Diag`` method: it takes a location (a
+:ref:`SourceLocation <SourceLocation>` object) and a diagnostic enum value
+(which matches the name from ``Diagnostic*Kinds.td``). If the diagnostic takes
+arguments, they are specified with the ``<<`` operator: the first argument
+becomes ``%0``, the second becomes ``%1``, etc. The diagnostic interface
+allows you to specify arguments of many different types, including ``int`` and
+``unsigned`` for integer arguments, ``const char*`` and ``std::string`` for
+string arguments, ``DeclarationName`` and ``const IdentifierInfo *`` for names,
+``QualType`` for types, etc. ``SourceRange``\ s are also specified with the
+``<<`` operator, but do not have a specific ordering requirement.
+
+As you can see, adding and producing a diagnostic is pretty straightforward.
+The hard part is deciding exactly what you need to say to help the user,
+picking a suitable wording, and providing the information needed to format it
+correctly. The good news is that the call site that issues a diagnostic should
+be completely independent of how the diagnostic is formatted and in what
+language it is rendered.
+
+Fix-It Hints
+^^^^^^^^^^^^
+
+In some cases, the front end emits diagnostics when it is clear that some small
+change to the source code would fix the problem. For example, a missing
+semicolon at the end of a statement or a use of deprecated syntax that is
+easily rewritten into a more modern form. Clang tries very hard to emit the
+diagnostic and recover gracefully in these and other cases.
+
+However, for these cases where the fix is obvious, the diagnostic can be
+annotated with a hint (referred to as a "fix-it hint") that describes how to
+change the code referenced by the diagnostic to fix the problem. For example,
+it might add the missing semicolon at the end of the statement or rewrite the
+use of a deprecated construct into something more palatable. Here is one such
+example from the C++ front end, where we warn about the right-shift operator
+changing meaning from C++98 to C++11:
+
+.. code-block:: c++
+
+ test.cpp:3:7: warning: use of right-shift operator ('>>') in template argument
+ will require parentheses in C++11
+ A<100 >> 2> *a;
+ ^
+ ( )
+
+Here, the fix-it hint is suggesting that parentheses be added, and showing
+exactly where those parentheses would be inserted into the source code. The
+fix-it hints themselves describe what changes to make to the source code in an
+abstract manner, which the text diagnostic printer renders as a line of
+"insertions" below the caret line. :ref:`Other diagnostic clients
+<DiagnosticClient>` might choose to render the code differently (e.g., as
+markup inline) or even give the user the ability to automatically fix the
+problem.
+
+Fix-it hints on errors and warnings need to obey these rules:
+
+* Since they are automatically applied if ``-Xclang -fixit`` is passed to the
+ driver, they should only be used when it's very likely they match the user's
+ intent.
+* Clang must recover from errors as if the fix-it had been applied.
+
+If a fix-it can't obey these rules, put the fix-it on a note. Fix-its on notes
+are not applied automatically.
+
+All fix-it hints are described by the ``FixItHint`` class, instances of which
+should be attached to the diagnostic using the ``<<`` operator in the same way
+that highlighted source ranges and arguments are passed to the diagnostic.
+Fix-it hints can be created with one of three constructors:
+
+* ``FixItHint::CreateInsertion(Loc, Code)``
+
+ Specifies that the given ``Code`` (a string) should be inserted before the
+ source location ``Loc``.
+
+* ``FixItHint::CreateRemoval(Range)``
+
+ Specifies that the code in the given source ``Range`` should be removed.
+
+* ``FixItHint::CreateReplacement(Range, Code)``
+
+ Specifies that the code in the given source ``Range`` should be removed,
+ and replaced with the given ``Code`` string.
+
+.. _DiagnosticClient:
+
+The ``DiagnosticClient`` Interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Once code generates a diagnostic with all of the arguments and the rest of the
+relevant information, Clang needs to know what to do with it. As previously
+mentioned, the diagnostic machinery goes through some filtering to map a
+severity onto a diagnostic level, then (assuming the diagnostic is not mapped
+to "``Ignore``") it invokes an object that implements the ``DiagnosticClient``
+interface with the information.
+
+It is possible to implement this interface in many different ways. For
+example, the normal Clang ``DiagnosticClient`` (named
+``TextDiagnosticPrinter``) turns the arguments into strings (according to the
+various formatting rules), prints out the file/line/column information and the
+string, then prints out the line of code, the source ranges, and the caret.
+However, this behavior isn't required.
+
+Another implementation of the ``DiagnosticClient`` interface is the
+``TextDiagnosticBuffer`` class, which is used when Clang is in ``-verify``
+mode. Instead of formatting and printing out the diagnostics, this
+implementation just captures and remembers the diagnostics as they fly by.
+Then ``-verify`` compares the list of produced diagnostics to the list of
+expected ones. If they disagree, it prints out its own output. Full
+documentation for the ``-verify`` mode can be found in the Clang API
+documentation for `VerifyDiagnosticConsumer
+</doxygen/classclang_1_1VerifyDiagnosticConsumer.html#details>`_.
+
+There are many other possible implementations of this interface, and this is
+why we prefer diagnostics to pass down rich structured information in
+arguments. For example, an HTML output might want declaration names be
+linkified to where they come from in the source. Another example is that a GUI
+might let you click on typedefs to expand them. This application would want to
+pass significantly more information about types through to the GUI than a
+simple flat string. The interface allows this to happen.
+
+.. _internals-diag-translation:
+
+Adding Translations to Clang
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Not possible yet! Diagnostic strings should be written in UTF-8, the client can
+translate to the relevant code page if needed. Each translation completely
+replaces the format string for the diagnostic.
+
+.. _SourceLocation:
+.. _SourceManager:
+
+The ``SourceLocation`` and ``SourceManager`` classes
+----------------------------------------------------
+
+Strangely enough, the ``SourceLocation`` class represents a location within the
+source code of the program. Important design points include:
+
+#. ``sizeof(SourceLocation)`` must be extremely small, as these are embedded
+ into many AST nodes and are passed around often. Currently it is 32 bits.
+#. ``SourceLocation`` must be a simple value object that can be efficiently
+ copied.
+#. We should be able to represent a source location for any byte of any input
+ file. This includes in the middle of tokens, in whitespace, in trigraphs,
+ etc.
+#. A ``SourceLocation`` must encode the current ``#include`` stack that was
+ active when the location was processed. For example, if the location
+ corresponds to a token, it should contain the set of ``#include``\ s active
+ when the token was lexed. This allows us to print the ``#include`` stack
+ for a diagnostic.
+#. ``SourceLocation`` must be able to describe macro expansions, capturing both
+ the ultimate instantiation point and the source of the original character
+ data.
+
+In practice, the ``SourceLocation`` works together with the ``SourceManager``
+class to encode two pieces of information about a location: its spelling
+location and its instantiation location. For most tokens, these will be the
+same. However, for a macro expansion (or tokens that came from a ``_Pragma``
+directive) these will describe the location of the characters corresponding to
+the token and the location where the token was used (i.e., the macro
+instantiation point or the location of the ``_Pragma`` itself).
+
+The Clang front-end inherently depends on the location of a token being tracked
+correctly. If it is ever incorrect, the front-end may get confused and die.
+The reason for this is that the notion of the "spelling" of a ``Token`` in
+Clang depends on being able to find the original input characters for the
+token. This concept maps directly to the "spelling location" for the token.
+
+``SourceRange`` and ``CharSourceRange``
+---------------------------------------
+
+.. mostly taken from http://lists.cs.uiuc.edu/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:: c++
+
+ 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 `here <DriverInternals.html>`_.
+
+Precompiled Headers
+===================
+
+Clang supports two implementations of precompiled headers. The default
+implementation, precompiled headers (:doc:`PCH <PCHInternals>`) uses a
+serialized representation of Clang's internal data structures, encoded with the
+`LLVM bitstream format <http://llvm.org/docs/BitCodeFormat.html>`_.
+Pretokenized headers (`PTH <PTHInternals.html>`_), on the other hand, contain a
+serialized representation of the tokens encountered when preprocessing a header
+(and anything that header includes).
+
+The Frontend Library
+====================
+
+The Frontend library contains functionality useful for building tools on top of
+the Clang libraries, for example several methods for outputting diagnostics.
+
+The Lexer and Preprocessor Library
+==================================
+
+The Lexer library contains several tightly-connected classes that are involved
+with the nasty process of lexing and preprocessing C source code. The main
+interface to this library for outside clients is the large ``Preprocessor``
+class. It contains the various pieces of state that are required to coherently
+read tokens out of a translation unit.
+
+The core interface to the ``Preprocessor`` object (once it is set up) is the
+``Preprocessor::Lex`` method, which returns the next :ref:`Token <Token>` from
+the preprocessor stream. There are two types of token providers that the
+preprocessor is capable of reading from: a buffer lexer (provided by the
+:ref:`Lexer <Lexer>` class) and a buffered token stream (provided by the
+:ref:`TokenLexer <TokenLexer>` class).
+
+.. _Token:
+
+The Token class
+---------------
+
+The ``Token`` class is used to represent a single lexed token. Tokens are
+intended to be used by the lexer/preprocess and parser libraries, but are not
+intended to live beyond them (for example, they should not live in the ASTs).
+
+Tokens most often live on the stack (or some other location that is efficient
+to access) as the parser is running, but occasionally do get buffered up. For
+example, macro definitions are stored as a series of tokens, and the C++
+front-end periodically needs to buffer tokens up for tentative parsing and
+various pieces of look-ahead. As such, the size of a ``Token`` matters. On a
+32-bit system, ``sizeof(Token)`` is currently 16 bytes.
+
+Tokens occur in two forms: :ref:`annotation tokens <AnnotationToken>` and
+normal tokens. Normal tokens are those returned by the lexer, annotation
+tokens represent semantic information and are produced by the parser, replacing
+normal tokens in the token stream. Normal tokens contain the following
+information:
+
+* **A SourceLocation** --- This indicates the location of the start of the
+ token.
+
+* **A length** --- This stores the length of the token as stored in the
+ ``SourceBuffer``. For tokens that include them, this length includes
+ trigraphs and escaped newlines which are ignored by later phases of the
+ compiler. By pointing into the original source buffer, it is always possible
+ to get the original spelling of a token completely accurately.
+
+* **IdentifierInfo** --- If a token takes the form of an identifier, and if
+ identifier lookup was enabled when the token was lexed (e.g., the lexer was
+ not reading in "raw" mode) this contains a pointer to the unique hash value
+ for the identifier. Because the lookup happens before keyword
+ identification, this field is set even for language keywords like "``for``".
+
+* **TokenKind** --- This indicates the kind of token as classified by the
+ lexer. This includes things like ``tok::starequal`` (for the "``*=``"
+ operator), ``tok::ampamp`` for the "``&&``" token, and keyword values (e.g.,
+ ``tok::kw_for``) for identifiers that correspond to keywords. Note that
+ some tokens can be spelled multiple ways. For example, C++ supports
+ "operator keywords", where things like "``and``" are treated exactly like the
+ "``&&``" operator. In these cases, the kind value is set to ``tok::ampamp``,
+ which is good for the parser, which doesn't have to consider both forms. For
+ something that cares about which form is used (e.g., the preprocessor
+ "stringize" operator) the spelling indicates the original form.
+
+* **Flags** --- There are currently four flags tracked by the
+ lexer/preprocessor system on a per-token basis:
+
+ #. **StartOfLine** --- This was the first token that occurred on its input
+ source line.
+ #. **LeadingSpace** --- There was a space character either immediately before
+ the token or transitively before the token as it was expanded through a
+ macro. The definition of this flag is very closely defined by the
+ stringizing requirements of the preprocessor.
+ #. **DisableExpand** --- This flag is used internally to the preprocessor to
+ represent identifier tokens which have macro expansion disabled. This
+ prevents them from being considered as candidates for macro expansion ever
+ in the future.
+ #. **NeedsCleaning** --- This flag is set if the original spelling for the
+ token includes a trigraph or escaped newline. Since this is uncommon,
+ many pieces of code can fast-path on tokens that did not need cleaning.
+
+One interesting (and somewhat unusual) aspect of normal tokens is that they
+don't contain any semantic information about the lexed value. For example, if
+the token was a pp-number token, we do not represent the value of the number
+that was lexed (this is left for later pieces of code to decide).
+Additionally, the lexer library has no notion of typedef names vs variable
+names: both are returned as identifiers, and the parser is left to decide
+whether a specific identifier is a typedef or a variable (tracking this
+requires scope information among other things). The parser can do this
+translation by replacing tokens returned by the preprocessor with "Annotation
+Tokens".
+
+.. _AnnotationToken:
+
+Annotation Tokens
+-----------------
+
+Annotation tokens are tokens that are synthesized by the parser and injected
+into the preprocessor's token stream (replacing existing tokens) to record
+semantic information found by the parser. For example, if "``foo``" is found
+to be a typedef, the "``foo``" ``tok::identifier`` token is replaced with an
+``tok::annot_typename``. This is useful for a couple of reasons: 1) this makes
+it easy to handle qualified type names (e.g., "``foo::bar::baz<42>::t``") in
+C++ as a single "token" in the parser. 2) if the parser backtracks, the
+reparse does not need to redo semantic analysis to determine whether a token
+sequence is a variable, type, template, etc.
+
+Annotation tokens are created by the parser and reinjected into the parser's
+token stream (when backtracking is enabled). Because they can only exist in
+tokens that the preprocessor-proper is done with, it doesn't need to keep
+around flags like "start of line" that the preprocessor uses to do its job.
+Additionally, an annotation token may "cover" a sequence of preprocessor tokens
+(e.g., "``a::b::c``" is five preprocessor tokens). As such, the valid fields
+of an annotation token are different than the fields for a normal token (but
+they are multiplexed into the normal ``Token`` fields):
+
+* **SourceLocation "Location"** --- The ``SourceLocation`` for the annotation
+ token indicates the first token replaced by the annotation token. In the
+ example above, it would be the location of the "``a``" identifier.
+* **SourceLocation "AnnotationEndLoc"** --- This holds the location of the last
+ token replaced with the annotation token. In the example above, it would be
+ the location of the "``c``" identifier.
+* **void* "AnnotationValue"** --- This contains an opaque object that the
+ parser gets from ``Sema``. The parser merely preserves the information for
+ ``Sema`` to later interpret based on the annotation token kind.
+* **TokenKind "Kind"** --- This indicates the kind of Annotation token this is.
+ See below for the different valid kinds.
+
+Annotation tokens currently come in three kinds:
+
+#. **tok::annot_typename**: This annotation token represents a resolved
+ typename token that is potentially qualified. The ``AnnotationValue`` field
+ contains the ``QualType`` returned by ``Sema::getTypeName()``, possibly with
+ source location information attached.
+#. **tok::annot_cxxscope**: This annotation token represents a C++ scope
+ specifier, such as "``A::B::``". This corresponds to the grammar
+ productions "*::*" and "*:: [opt] nested-name-specifier*". The
+ ``AnnotationValue`` pointer is a ``NestedNameSpecifier *`` returned by the
+ ``Sema::ActOnCXXGlobalScopeSpecifier`` and
+ ``Sema::ActOnCXXNestedNameSpecifier`` callbacks.
+#. **tok::annot_template_id**: This annotation token represents a C++
+ template-id such as "``foo<int, 4>``", where "``foo``" is the name of a
+ template. The ``AnnotationValue`` pointer is a pointer to a ``malloc``'d
+ ``TemplateIdAnnotation`` object. Depending on the context, a parsed
+ template-id that names a type might become a typename annotation token (if
+ all we care about is the named type, e.g., because it occurs in a type
+ specifier) or might remain a template-id token (if we want to retain more
+ source location information or produce a new type, e.g., in a declaration of
+ a class template specialization). template-id annotation tokens that refer
+ to a type can be "upgraded" to typename annotation tokens by the parser.
+
+As mentioned above, annotation tokens are not returned by the preprocessor,
+they are formed on demand by the parser. This means that the parser has to be
+aware of cases where an annotation could occur and form it where appropriate.
+This is somewhat similar to how the parser handles Translation Phase 6 of C99:
+String Concatenation (see C99 5.1.1.2). In the case of string concatenation,
+the preprocessor just returns distinct ``tok::string_literal`` and
+``tok::wide_string_literal`` tokens and the parser eats a sequence of them
+wherever the grammar indicates that a string literal can occur.
+
+In order to do this, whenever the parser expects a ``tok::identifier`` or
+``tok::coloncolon``, it should call the ``TryAnnotateTypeOrScopeToken`` or
+``TryAnnotateCXXScopeToken`` methods to form the annotation token. These
+methods will maximally form the specified annotation tokens and replace the
+current token with them, if applicable. If the current tokens is not valid for
+an annotation token, it will remain an identifier or "``::``" token.
+
+.. _Lexer:
+
+The ``Lexer`` class
+-------------------
+
+The ``Lexer`` class provides the mechanics of lexing tokens out of a source
+buffer and deciding what they mean. The ``Lexer`` is complicated by the fact
+that it operates on raw buffers that have not had spelling eliminated (this is
+a necessity to get decent performance), but this is countered with careful
+coding as well as standard performance techniques (for example, the comment
+handling code is vectorized on X86 and PowerPC hosts).
+
+The lexer has a couple of interesting modal features:
+
+* The lexer can operate in "raw" mode. This mode has several features that
+ make it possible to quickly lex the file (e.g., it stops identifier lookup,
+ doesn't specially handle preprocessor tokens, handles EOF differently, etc).
+ This mode is used for lexing within an "``#if 0``" block, for example.
+* The lexer can capture and return comments as tokens. This is required to
+ support the ``-C`` preprocessor mode, which passes comments through, and is
+ used by the diagnostic checker to identifier expect-error annotations.
+* The lexer can be in ``ParsingFilename`` mode, which happens when
+ preprocessing after reading a ``#include`` directive. This mode changes the
+ parsing of "``<``" to return an "angled string" instead of a bunch of tokens
+ for each thing within the filename.
+* When parsing a preprocessor directive (after "``#``") the
+ ``ParsingPreprocessorDirective`` mode is entered. This changes the parser to
+ return EOD at a newline.
+* The ``Lexer`` uses a ``LangOptions`` object to know whether trigraphs are
+ enabled, whether C++ or ObjC keywords are recognized, etc.
+
+In addition to these modes, the lexer keeps track of a couple of other features
+that are local to a lexed buffer, which change as the buffer is lexed:
+
+* The ``Lexer`` uses ``BufferPtr`` to keep track of the current character being
+ lexed.
+* The ``Lexer`` uses ``IsAtStartOfLine`` to keep track of whether the next
+ lexed token will start with its "start of line" bit set.
+* The ``Lexer`` keeps track of the current "``#if``" directives that are active
+ (which can be nested).
+* The ``Lexer`` keeps track of an :ref:`MultipleIncludeOpt
+ <MultipleIncludeOpt>` object, which is used to detect whether the buffer uses
+ the standard "``#ifndef XX`` / ``#define XX``" idiom to prevent multiple
+ inclusion. If a buffer does, subsequent includes can be ignored if the
+ "``XX``" macro is defined.
+
+.. _TokenLexer:
+
+The ``TokenLexer`` class
+------------------------
+
+The ``TokenLexer`` class is a token provider that returns tokens from a list of
+tokens that came from somewhere else. It typically used for two things: 1)
+returning tokens from a macro definition as it is being expanded 2) returning
+tokens from an arbitrary buffer of tokens. The later use is used by
+``_Pragma`` and will most likely be used to handle unbounded look-ahead for the
+C++ parser.
+
+.. _MultipleIncludeOpt:
+
+The ``MultipleIncludeOpt`` class
+--------------------------------
+
+The ``MultipleIncludeOpt`` class implements a really simple little state
+machine that is used to detect the standard "``#ifndef XX`` / ``#define XX``"
+idiom that people typically use to prevent multiple inclusion of headers. If a
+buffer uses this idiom and is subsequently ``#include``'d, the preprocessor can
+simply check to see whether the guarding condition is defined or not. If so,
+the preprocessor can completely ignore the include of the header.
+
+The Parser Library
+==================
+
+The AST Library
+===============
+
+.. _Type:
+
+The ``Type`` class and its subclasses
+-------------------------------------
+
+The ``Type`` class (and its subclasses) are an important part of the AST.
+Types are accessed through the ``ASTContext`` class, which implicitly creates
+and uniques them as they are needed. Types have a couple of non-obvious
+features: 1) they do not capture type qualifiers like ``const`` or ``volatile``
+(see :ref:`QualType <QualType>`), and 2) they implicitly capture typedef
+information. Once created, types are immutable (unlike decls).
+
+Typedefs in C make semantic analysis a bit more complex than it would be without
+them. The issue is that we want to capture typedef information and represent it
+in the AST perfectly, but the semantics of operations need to "see through"
+typedefs. For example, consider this code:
+
+.. code-block:: c++
+
+ void func() {
+ typedef int foo;
+ foo X, *Y;
+ typedef foo *bar;
+ bar Z;
+ *X; // error
+ **Y; // error
+ **Z; // error
+ }
+
+The code above is illegal, and thus we expect there to be diagnostics emitted
+on the annotated lines. In this example, we expect to get:
+
+.. code-block:: c++
+
+ test.c:6:1: error: indirection requires pointer operand ('foo' invalid)
+ *X; // error
+ ^~
+ test.c:7:1: error: indirection requires pointer operand ('foo' invalid)
+ **Y; // error
+ ^~~
+ test.c:8:1: error: indirection requires pointer operand ('foo' invalid)
+ **Z; // error
+ ^~~
+
+While this example is somewhat silly, it illustrates the point: we want to
+retain typedef information where possible, so that we can emit errors about
+"``std::string``" instead of "``std::basic_string<char, std:...``". Doing this
+requires properly keeping typedef information (for example, the type of ``X``
+is "``foo``", not "``int``"), and requires properly propagating it through the
+various operators (for example, the type of ``*Y`` is "``foo``", not
+"``int``"). In order to retain this information, the type of these expressions
+is an instance of the ``TypedefType`` class, which indicates that the type of
+these expressions is a typedef for "``foo``".
+
+Representing types like this is great for diagnostics, because the
+user-specified type is always immediately available. There are two problems
+with this: first, various semantic checks need to make judgements about the
+*actual structure* of a type, ignoring typedefs. Second, we need an efficient
+way to query whether two types are structurally identical to each other,
+ignoring typedefs. The solution to both of these problems is the idea of
+canonical types.
+
+Canonical Types
+^^^^^^^^^^^^^^^
+
+Every instance of the ``Type`` class contains a canonical type pointer. For
+simple types with no typedefs involved (e.g., "``int``", "``int*``",
+"``int**``"), the type just points to itself. For types that have a typedef
+somewhere in their structure (e.g., "``foo``", "``foo*``", "``foo**``",
+"``bar``"), the canonical type pointer points to their structurally equivalent
+type without any typedefs (e.g., "``int``", "``int*``", "``int**``", and
+"``int*``" respectively).
+
+This design provides a constant time operation (dereferencing the canonical type
+pointer) that gives us access to the structure of types. For example, we can
+trivially tell that "``bar``" and "``foo*``" are the same type by dereferencing
+their canonical type pointers and doing a pointer comparison (they both point
+to the single "``int*``" type).
+
+Canonical types and typedef types bring up some complexities that must be
+carefully managed. Specifically, the ``isa``/``cast``/``dyn_cast`` operators
+generally shouldn't be used in code that is inspecting the AST. For example,
+when type checking the indirection operator (unary "``*``" on a pointer), the
+type checker must verify that the operand has a pointer type. It would not be
+correct to check that with "``isa<PointerType>(SubExpr->getType())``", because
+this predicate would fail if the subexpression had a typedef type.
+
+The solution to this problem are a set of helper methods on ``Type``, used to
+check their properties. In this case, it would be correct to use
+"``SubExpr->getType()->isPointerType()``" to do the check. This predicate will
+return true if the *canonical type is a pointer*, which is true any time the
+type is structurally a pointer type. The only hard part here is remembering
+not to use the ``isa``/``cast``/``dyn_cast`` operations.
+
+The second problem we face is how to get access to the pointer type once we
+know it exists. To continue the example, the result type of the indirection
+operator is the pointee type of the subexpression. In order to determine the
+type, we need to get the instance of ``PointerType`` that best captures the
+typedef information in the program. If the type of the expression is literally
+a ``PointerType``, we can return that, otherwise we have to dig through the
+typedefs to find the pointer type. For example, if the subexpression had type
+"``foo*``", we could return that type as the result. If the subexpression had
+type "``bar``", we want to return "``foo*``" (note that we do *not* want
+"``int*``"). In order to provide all of this, ``Type`` has a
+``getAsPointerType()`` method that checks whether the type is structurally a
+``PointerType`` and, if so, returns the best one. If not, it returns a null
+pointer.
+
+This structure is somewhat mystical, but after meditating on it, it will make
+sense to you :).
+
+.. _QualType:
+
+The ``QualType`` class
+----------------------
+
+The ``QualType`` class is designed as a trivial value class that is small,
+passed by-value and is efficient to query. The idea of ``QualType`` is that it
+stores the type qualifiers (``const``, ``volatile``, ``restrict``, plus some
+extended qualifiers required by language extensions) separately from the types
+themselves. ``QualType`` is conceptually a pair of "``Type*``" and the bits
+for these type qualifiers.
+
+By storing the type qualifiers as bits in the conceptual pair, it is extremely
+efficient to get the set of qualifiers on a ``QualType`` (just return the field
+of the pair), add a type qualifier (which is a trivial constant-time operation
+that sets a bit), and remove one or more type qualifiers (just return a
+``QualType`` with the bitfield set to empty).
+
+Further, because the bits are stored outside of the type itself, we do not need
+to create duplicates of types with different sets of qualifiers (i.e. there is
+only a single heap allocated "``int``" type: "``const int``" and "``volatile
+const int``" both point to the same heap allocated "``int``" type). This
+reduces the heap size used to represent bits and also means we do not have to
+consider qualifiers when uniquing types (:ref:`Type <Type>` does not even
+contain qualifiers).
+
+In practice, the two most common type qualifiers (``const`` and ``restrict``)
+are stored in the low bits of the pointer to the ``Type`` object, together with
+a flag indicating whether extended qualifiers are present (which must be
+heap-allocated). This means that ``QualType`` is exactly the same size as a
+pointer.
+
+.. _DeclarationName:
+
+Declaration names
+-----------------
+
+The ``DeclarationName`` class represents the name of a declaration in Clang.
+Declarations in the C family of languages can take several different forms.
+Most declarations are named by simple identifiers, e.g., "``f``" and "``x``" in
+the function declaration ``f(int x)``. In C++, declaration names can also name
+class constructors ("``Class``" in ``struct Class { Class(); }``), class
+destructors ("``~Class``"), overloaded operator names ("``operator+``"), and
+conversion functions ("``operator void const *``"). In Objective-C,
+declaration names can refer to the names of Objective-C methods, which involve
+the method name and the parameters, collectively called a *selector*, e.g.,
+"``setWidth:height:``". Since all of these kinds of entities --- variables,
+functions, Objective-C methods, C++ constructors, destructors, and operators
+--- are represented as subclasses of Clang's common ``NamedDecl`` class,
+``DeclarationName`` is designed to efficiently represent any kind of name.
+
+Given a ``DeclarationName`` ``N``, ``N.getNameKind()`` will produce a value
+that describes what kind of name ``N`` stores. There are 8 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.
+ Note that C++ overloaded operators (e.g., "``operator+``") are represented as
+ special kinds of identifiers. Use ``IdentifierInfo``'s
+ ``getOverloadedOperatorID`` function to determine whether an identifier is an
+ overloaded operator name.
+
+``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``).
+
+``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``".
+
+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 --- and, so far, unique --- property that
+the namespace can be defined multiple times, and the declarations provided by
+each namespace definition are effectively merged (from the semantic point of
+view). For example, the following two code snippets are semantically
+indistinguishable:
+
+.. 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 the
+primary context, one can follow the chain of ``DeclContext`` nodes that define
+additional declarations via ``DeclContext::getNextContext``. 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.
+
+.. _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
+<http://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 = ...
+ CFG *FooCFG = CFG::buildCFG(FooBody);
+
+It is the responsibility of the caller of ``CFG::buildCFG`` to ``delete`` the
+returned ``CFG*`` when the CFG is no longer needed.
+
+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:: c++
+
+ [ 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 unfortuate accident of history (including, e.g., the glibc system
+headers). GCC accepts anything its "fold" optimizer is capable of reducing to
+an integer constant, which means that the definition of what it accepts changes
+as its optimizer does. One example is that GCC accepts things like "``case
+X-X:``" even when ``X`` is a variable, because it can fold this to 0.
+
+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 has to handle a superset of what C99 allows. Further, these
+clients can benefit from extended information. For example, we know that
+"``foo() || 1``" always evaluates to ``true``, but we can't replace the
+expression with ``true`` because it has side effects.
+
+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 method 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.
+
+How to change Clang
+===================
+
+How to add an attribute
+-----------------------
+
+To add an attribute, you'll have to add it to the list of attributes, add it to
+the parsing phase, and look for it in the AST scan.
+`r124217 <http://llvm.org/viewvc/llvm-project?view=rev&revision=124217>`_
+has a good example of adding a warning attribute.
+
+(Beware that this hasn't been reviewed/fixed by the people who designed the
+attributes system yet.)
+
+
+``include/clang/Basic/Attr.td``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+First, add your attribute to the `include/clang/Basic/Attr.td file
+<http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?view=markup>`_.
+
+Each attribute gets a ``def`` inheriting from ``Attr`` or one of its
+subclasses. ``InheritableAttr`` means that the attribute also applies to
+subsequent declarations of the same name.
+
+``Spellings`` lists the strings that can appear in ``__attribute__((here))`` or
+``[[here]]``. All such strings will be synonymous. If you want to allow the
+``[[]]`` C++11 syntax, you have to define a list of ``Namespaces``, which will
+let users write ``[[namespace::spelling]]``. Using the empty string for a
+namespace will allow users to write just the spelling with no "``::``".
+
+``Subjects`` restricts what kinds of AST node to which this attribute can
+appertain (roughly, attach).
+
+``Args`` names the arguments the attribute takes, in order. If ``Args`` is
+``[StringArgument<"Arg1">, IntArgument<"Arg2">]`` then
+``__attribute__((myattribute("Hello", 3)))`` will be a valid use.
+
+Boilerplate
+^^^^^^^^^^^
+
+Write a new ``HandleYourAttr()`` function in `lib/Sema/SemaDeclAttr.cpp
+<http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?view=markup>`_,
+and add a case to the switch in ``ProcessNonInheritableDeclAttr()`` or
+``ProcessInheritableDeclAttr()`` forwarding to it.
+
+If your attribute causes extra warnings to fire, define a ``DiagGroup`` in
+`include/clang/Basic/DiagnosticGroups.td
+<http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?view=markup>`_
+named after the attribute's ``Spelling`` with "_"s replaced by "-"s. If you're
+only defining one diagnostic, you can skip ``DiagnosticGroups.td`` and use
+``InGroup<DiagGroup<"your-attribute">>`` directly in `DiagnosticSemaKinds.td
+<http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?view=markup>`_
+
+The meat of your attribute
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Find an appropriate place in Clang to do whatever your attribute needs to do.
+Check for the attribute's presence using ``Decl::getAttr<YourAttr>()``.
+
+Update the :doc:`LanguageExtensions` document to describe your new attribute.
+
+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 the ``RecursiveASTVisitor``.
+ * Add printing support (``StmtPrinter.cpp``) and dumping support
+ (``StmtDumper.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.
+
+#. 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
+ later 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
+ <http://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.
+