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2114 lines
101 KiB
ReStructuredText
2114 lines
101 KiB
ReStructuredText
============================
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"Clang" CFE Internals Manual
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============================
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.. contents::
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:local:
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Introduction
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============
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This document describes some of the more important APIs and internal design
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decisions made in the Clang C front-end. The purpose of this document is to
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both capture some of this high level information and also describe some of the
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design decisions behind it. This is meant for people interested in hacking on
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Clang, not for end-users. The description below is categorized by libraries,
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and does not describe any of the clients of the libraries.
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LLVM Support Library
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====================
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The LLVM ``libSupport`` library provides many underlying libraries and
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`data-structures <https://llvm.org/docs/ProgrammersManual.html>`_, including
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command line option processing, various containers and a system abstraction
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layer, which is used for file system access.
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The Clang "Basic" Library
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=========================
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This library certainly needs a better name. The "basic" library contains a
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number of low-level utilities for tracking and manipulating source buffers,
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locations within the source buffers, diagnostics, tokens, target abstraction,
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and information about the subset of the language being compiled for.
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Part of this infrastructure is specific to C (such as the ``TargetInfo``
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class), other parts could be reused for other non-C-based languages
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(``SourceLocation``, ``SourceManager``, ``Diagnostics``, ``FileManager``).
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When and if there is future demand we can figure out if it makes sense to
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introduce a new library, move the general classes somewhere else, or introduce
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some other solution.
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We describe the roles of these classes in order of their dependencies.
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The Diagnostics Subsystem
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-------------------------
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The Clang Diagnostics subsystem is an important part of how the compiler
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communicates with the human. Diagnostics are the warnings and errors produced
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when the code is incorrect or dubious. In Clang, each diagnostic produced has
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(at the minimum) a unique ID, an English translation associated with it, a
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:ref:`SourceLocation <SourceLocation>` to "put the caret", and a severity
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(e.g., ``WARNING`` or ``ERROR``). They can also optionally include a number of
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arguments to the diagnostic (which fill in "%0"'s in the string) as well as a
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number of source ranges that related to the diagnostic.
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In this section, we'll be giving examples produced by the Clang command line
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driver, but diagnostics can be :ref:`rendered in many different ways
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<DiagnosticConsumer>` depending on how the ``DiagnosticConsumer`` interface is
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implemented. A representative example of a diagnostic is:
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.. code-block:: text
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t.c:38:15: error: invalid operands to binary expression ('int *' and '_Complex float')
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P = (P-42) + Gamma*4;
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~~~~~~ ^ ~~~~~~~
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In this example, you can see the English translation, the severity (error), you
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can see the source location (the caret ("``^``") and file/line/column info),
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the source ranges "``~~~~``", arguments to the diagnostic ("``int*``" and
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"``_Complex float``"). You'll have to believe me that there is a unique ID
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backing the diagnostic :).
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Getting all of this to happen has several steps and involves many moving
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pieces, this section describes them and talks about best practices when adding
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a new diagnostic.
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The ``Diagnostic*Kinds.td`` files
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Diagnostics are created by adding an entry to one of the
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``clang/Basic/Diagnostic*Kinds.td`` files, depending on what library will be
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using it. From this file, :program:`tblgen` generates the unique ID of the
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diagnostic, the severity of the diagnostic and the English translation + format
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string.
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There is little sanity with the naming of the unique ID's right now. Some
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start with ``err_``, ``warn_``, ``ext_`` to encode the severity into the name.
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Since the enum is referenced in the C++ code that produces the diagnostic, it
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is somewhat useful for it to be reasonably short.
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The severity of the diagnostic comes from the set {``NOTE``, ``REMARK``,
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``WARNING``,
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``EXTENSION``, ``EXTWARN``, ``ERROR``}. The ``ERROR`` severity is used for
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diagnostics indicating the program is never acceptable under any circumstances.
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When an error is emitted, the AST for the input code may not be fully built.
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The ``EXTENSION`` and ``EXTWARN`` severities are used for extensions to the
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language that Clang accepts. This means that Clang fully understands and can
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represent them in the AST, but we produce diagnostics to tell the user their
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code is non-portable. The difference is that the former are ignored by
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default, and the later warn by default. The ``WARNING`` severity is used for
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constructs that are valid in the currently selected source language but that
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are dubious in some way. The ``REMARK`` severity provides generic information
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about the compilation that is not necessarily related to any dubious code. The
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``NOTE`` level is used to staple more information onto previous diagnostics.
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These *severities* are mapped into a smaller set (the ``Diagnostic::Level``
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enum, {``Ignored``, ``Note``, ``Remark``, ``Warning``, ``Error``, ``Fatal``}) of
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output
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*levels* by the diagnostics subsystem based on various configuration options.
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Clang internally supports a fully fine grained mapping mechanism that allows
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you to map almost any diagnostic to the output level that you want. The only
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diagnostics that cannot be mapped are ``NOTE``\ s, which always follow the
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severity of the previously emitted diagnostic and ``ERROR``\ s, which can only
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be mapped to ``Fatal`` (it is not possible to turn an error into a warning, for
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example).
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Diagnostic mappings are used in many ways. For example, if the user specifies
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``-pedantic``, ``EXTENSION`` maps to ``Warning``, if they specify
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``-pedantic-errors``, it turns into ``Error``. This is used to implement
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options like ``-Wunused_macros``, ``-Wundef`` etc.
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Mapping to ``Fatal`` should only be used for diagnostics that are considered so
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severe that error recovery won't be able to recover sensibly from them (thus
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spewing a ton of bogus errors). One example of this class of error are failure
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to ``#include`` a file.
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The Format String
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^^^^^^^^^^^^^^^^^
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The format string for the diagnostic is very simple, but it has some power. It
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takes the form of a string in English with markers that indicate where and how
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arguments to the diagnostic are inserted and formatted. For example, here are
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some simple format strings:
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.. code-block:: c++
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"binary integer literals are an extension"
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"format string contains '\\0' within the string body"
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"more '%%' conversions than data arguments"
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"invalid operands to binary expression (%0 and %1)"
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"overloaded '%0' must be a %select{unary|binary|unary or binary}2 operator"
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" (has %1 parameter%s1)"
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These examples show some important points of format strings. You can use any
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plain ASCII character in the diagnostic string except "``%``" without a
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problem, but these are C strings, so you have to use and be aware of all the C
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escape sequences (as in the second example). If you want to produce a "``%``"
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in the output, use the "``%%``" escape sequence, like the third diagnostic.
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Finally, Clang uses the "``%...[digit]``" sequences to specify where and how
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arguments to the diagnostic are formatted.
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Arguments to the diagnostic are numbered according to how they are specified by
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the C++ code that :ref:`produces them <internals-producing-diag>`, and are
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referenced by ``%0`` .. ``%9``. If you have more than 10 arguments to your
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diagnostic, you are doing something wrong :). Unlike ``printf``, there is no
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requirement that arguments to the diagnostic end up in the output in the same
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order as they are specified, you could have a format string with "``%1 %0``"
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that swaps them, for example. The text in between the percent and digit are
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formatting instructions. If there are no instructions, the argument is just
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turned into a string and substituted in.
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Here are some "best practices" for writing the English format string:
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* Keep the string short. It should ideally fit in the 80 column limit of the
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``DiagnosticKinds.td`` file. This avoids the diagnostic wrapping when
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printed, and forces you to think about the important point you are conveying
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with the diagnostic.
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* Take advantage of location information. The user will be able to see the
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line and location of the caret, so you don't need to tell them that the
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problem is with the 4th argument to the function: just point to it.
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* Do not capitalize the diagnostic string, and do not end it with a period.
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* If you need to quote something in the diagnostic string, use single quotes.
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Diagnostics should never take random English strings as arguments: you
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shouldn't use "``you have a problem with %0``" and pass in things like "``your
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argument``" or "``your return value``" as arguments. Doing this prevents
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:ref:`translating <internals-diag-translation>` the Clang diagnostics to other
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languages (because they'll get random English words in their otherwise
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localized diagnostic). The exceptions to this are C/C++ language keywords
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(e.g., ``auto``, ``const``, ``mutable``, etc) and C/C++ operators (``/=``).
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Note that things like "pointer" and "reference" are not keywords. On the other
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hand, you *can* include anything that comes from the user's source code,
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including variable names, types, labels, etc. The "``select``" format can be
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used to achieve this sort of thing in a localizable way, see below.
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Formatting a Diagnostic Argument
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Arguments to diagnostics are fully typed internally, and come from a couple
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different classes: integers, types, names, and random strings. Depending on
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the class of the argument, it can be optionally formatted in different ways.
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This gives the ``DiagnosticConsumer`` information about what the argument means
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without requiring it to use a specific presentation (consider this MVC for
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Clang :).
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Here are the different diagnostic argument formats currently supported by
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Clang:
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**"s" format**
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Example:
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``"requires %1 parameter%s1"``
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Class:
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Integers
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Description:
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This is a simple formatter for integers that is useful when producing English
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diagnostics. When the integer is 1, it prints as nothing. When the integer
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is not 1, it prints as "``s``". This allows some simple grammatical forms to
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be to be handled correctly, and eliminates the need to use gross things like
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``"requires %1 parameter(s)"``.
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**"select" format**
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Example:
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``"must be a %select{unary|binary|unary or binary}2 operator"``
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Class:
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Integers
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Description:
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This format specifier is used to merge multiple related diagnostics together
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into one common one, without requiring the difference to be specified as an
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English string argument. Instead of specifying the string, the diagnostic
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gets an integer argument and the format string selects the numbered option.
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In this case, the "``%2``" value must be an integer in the range [0..2]. If
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it is 0, it prints "unary", if it is 1 it prints "binary" if it is 2, it
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prints "unary or binary". This allows other language translations to
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substitute reasonable words (or entire phrases) based on the semantics of the
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diagnostic instead of having to do things textually. The selected string
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does undergo formatting.
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**"plural" format**
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Example:
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``"you have %1 %plural{1:mouse|:mice}1 connected to your computer"``
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Class:
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Integers
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Description:
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This is a formatter for complex plural forms. It is designed to handle even
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the requirements of languages with very complex plural forms, as many Baltic
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languages have. The argument consists of a series of expression/form pairs,
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separated by ":", where the first form whose expression evaluates to true is
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the result of the modifier.
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An expression can be empty, in which case it is always true. See the example
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at the top. Otherwise, it is a series of one or more numeric conditions,
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separated by ",". If any condition matches, the expression matches. Each
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numeric condition can take one of three forms.
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* number: A simple decimal number matches if the argument is the same as the
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number. Example: ``"%plural{1:mouse|:mice}4"``
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* range: A range in square brackets matches if the argument is within the
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range. Then range is inclusive on both ends. Example:
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``"%plural{0:none|1:one|[2,5]:some|:many}2"``
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* modulo: A modulo operator is followed by a number, and equals sign and
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either a number or a range. The tests are the same as for plain numbers
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and ranges, but the argument is taken modulo the number first. Example:
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``"%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything else}1"``
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The parser is very unforgiving. A syntax error, even whitespace, will abort,
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as will a failure to match the argument against any expression.
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**"ordinal" format**
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Example:
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``"ambiguity in %ordinal0 argument"``
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Class:
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Integers
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Description:
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This is a formatter which represents the argument number as an ordinal: the
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value ``1`` becomes ``1st``, ``3`` becomes ``3rd``, and so on. Values less
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than ``1`` are not supported. This formatter is currently hard-coded to use
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English ordinals.
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**"objcclass" format**
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Example:
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``"method %objcclass0 not found"``
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Class:
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``DeclarationName``
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Description:
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This is a simple formatter that indicates the ``DeclarationName`` corresponds
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to an Objective-C class method selector. As such, it prints the selector
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with a leading "``+``".
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**"objcinstance" format**
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Example:
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``"method %objcinstance0 not found"``
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Class:
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``DeclarationName``
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Description:
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This is a simple formatter that indicates the ``DeclarationName`` corresponds
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to an Objective-C instance method selector. As such, it prints the selector
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with a leading "``-``".
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**"q" format**
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Example:
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``"candidate found by name lookup is %q0"``
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Class:
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``NamedDecl *``
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Description:
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This formatter indicates that the fully-qualified name of the declaration
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should be printed, e.g., "``std::vector``" rather than "``vector``".
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**"diff" format**
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Example:
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``"no known conversion %diff{from $ to $|from argument type to parameter type}1,2"``
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Class:
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``QualType``
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Description:
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This formatter takes two ``QualType``\ s and attempts to print a template
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difference between the two. If tree printing is off, the text inside the
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braces before the pipe is printed, with the formatted text replacing the $.
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If tree printing is on, the text after the pipe is printed and a type tree is
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printed after the diagnostic message.
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It is really easy to add format specifiers to the Clang diagnostics system, but
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they should be discussed before they are added. If you are creating a lot of
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repetitive diagnostics and/or have an idea for a useful formatter, please bring
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it up on the cfe-dev mailing list.
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**"sub" format**
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Example:
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Given the following record definition of type ``TextSubstitution``:
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.. code-block:: text
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def select_ovl_candidate : TextSubstitution<
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"%select{function|constructor}0%select{| template| %2}1">;
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which can be used as
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.. code-block:: text
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def note_ovl_candidate : Note<
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"candidate %sub{select_ovl_candidate}3,2,1 not viable">;
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and will act as if it was written
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``"candidate %select{function|constructor}3%select{| template| %1}2 not viable"``.
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Description:
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This format specifier is used to avoid repeating strings verbatim in multiple
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diagnostics. The argument to ``%sub`` must name a ``TextSubstitution`` tblgen
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record. The substitution must specify all arguments used by the substitution,
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and the modifier indexes in the substitution are re-numbered accordingly. The
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substituted text must itself be a valid format string before substitution.
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.. _internals-producing-diag:
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Producing the Diagnostic
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^^^^^^^^^^^^^^^^^^^^^^^^
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Now that you've created the diagnostic in the ``Diagnostic*Kinds.td`` file, you
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need to write the code that detects the condition in question and emits the new
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diagnostic. Various components of Clang (e.g., the preprocessor, ``Sema``,
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etc.) provide a helper function named "``Diag``". It creates a diagnostic and
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accepts the arguments, ranges, and other information that goes along with it.
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For example, the binary expression error comes from code like this:
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.. code-block:: c++
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if (various things that are bad)
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Diag(Loc, diag::err_typecheck_invalid_operands)
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<< lex->getType() << rex->getType()
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<< lex->getSourceRange() << rex->getSourceRange();
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This shows that use of the ``Diag`` method: it takes a location (a
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:ref:`SourceLocation <SourceLocation>` object) and a diagnostic enum value
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(which matches the name from ``Diagnostic*Kinds.td``). If the diagnostic takes
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arguments, they are specified with the ``<<`` operator: the first argument
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becomes ``%0``, the second becomes ``%1``, etc. The diagnostic interface
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allows you to specify arguments of many different types, including ``int`` and
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``unsigned`` for integer arguments, ``const char*`` and ``std::string`` for
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string arguments, ``DeclarationName`` and ``const IdentifierInfo *`` for names,
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``QualType`` for types, etc. ``SourceRange``\ s are also specified with the
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``<<`` operator, but do not have a specific ordering requirement.
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As you can see, adding and producing a diagnostic is pretty straightforward.
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The hard part is deciding exactly what you need to say to help the user,
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picking a suitable wording, and providing the information needed to format it
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correctly. The good news is that the call site that issues a diagnostic should
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be completely independent of how the diagnostic is formatted and in what
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language it is rendered.
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Fix-It Hints
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^^^^^^^^^^^^
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In some cases, the front end emits diagnostics when it is clear that some small
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change to the source code would fix the problem. For example, a missing
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semicolon at the end of a statement or a use of deprecated syntax that is
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easily rewritten into a more modern form. Clang tries very hard to emit the
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diagnostic and recover gracefully in these and other cases.
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However, for these cases where the fix is obvious, the diagnostic can be
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annotated with a hint (referred to as a "fix-it hint") that describes how to
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change the code referenced by the diagnostic to fix the problem. For example,
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it might add the missing semicolon at the end of the statement or rewrite the
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use of a deprecated construct into something more palatable. Here is one such
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example from the C++ front end, where we warn about the right-shift operator
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changing meaning from C++98 to C++11:
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.. code-block:: text
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test.cpp:3:7: warning: use of right-shift operator ('>>') in template argument
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will require parentheses in C++11
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A<100 >> 2> *a;
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^
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( )
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Here, the fix-it hint is suggesting that parentheses be added, and showing
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exactly where those parentheses would be inserted into the source code. The
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fix-it hints themselves describe what changes to make to the source code in an
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abstract manner, which the text diagnostic printer renders as a line of
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"insertions" below the caret line. :ref:`Other diagnostic clients
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<DiagnosticConsumer>` might choose to render the code differently (e.g., as
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markup inline) or even give the user the ability to automatically fix the
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problem.
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Fix-it hints on errors and warnings need to obey these rules:
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* Since they are automatically applied if ``-Xclang -fixit`` is passed to the
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driver, they should only be used when it's very likely they match the user's
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intent.
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* Clang must recover from errors as if the fix-it had been applied.
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If a fix-it can't obey these rules, put the fix-it on a note. Fix-its on notes
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are not applied automatically.
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All fix-it hints are described by the ``FixItHint`` class, instances of which
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should be attached to the diagnostic using the ``<<`` operator in the same way
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that highlighted source ranges and arguments are passed to the diagnostic.
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Fix-it hints can be created with one of three constructors:
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* ``FixItHint::CreateInsertion(Loc, Code)``
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Specifies that the given ``Code`` (a string) should be inserted before the
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source location ``Loc``.
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* ``FixItHint::CreateRemoval(Range)``
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Specifies that the code in the given source ``Range`` should be removed.
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* ``FixItHint::CreateReplacement(Range, Code)``
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Specifies that the code in the given source ``Range`` should be removed,
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and replaced with the given ``Code`` string.
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.. _DiagnosticConsumer:
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The ``DiagnosticConsumer`` Interface
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Once code generates a diagnostic with all of the arguments and the rest of the
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relevant information, Clang needs to know what to do with it. As previously
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mentioned, the diagnostic machinery goes through some filtering to map a
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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
|
|
===============
|
|
|
|
.. _Type:
|
|
|
|
The ``Type`` class and its subclasses
|
|
-------------------------------------
|
|
|
|
The ``Type`` class (and its subclasses) are an important part of the AST.
|
|
Types are accessed through the ``ASTContext`` class, which implicitly creates
|
|
and uniques them as they are needed. Types have a couple of non-obvious
|
|
features: 1) they do not capture type qualifiers like ``const`` or ``volatile``
|
|
(see :ref:`QualType <QualType>`), and 2) they implicitly capture typedef
|
|
information. Once created, types are immutable (unlike decls).
|
|
|
|
Typedefs in C make semantic analysis a bit more complex than it would be without
|
|
them. The issue is that we want to capture typedef information and represent it
|
|
in the AST perfectly, but the semantics of operations need to "see through"
|
|
typedefs. For example, consider this code:
|
|
|
|
.. code-block:: c++
|
|
|
|
void func() {
|
|
typedef int foo;
|
|
foo X, *Y;
|
|
typedef foo *bar;
|
|
bar Z;
|
|
*X; // error
|
|
**Y; // error
|
|
**Z; // error
|
|
}
|
|
|
|
The code above is illegal, and thus we expect there to be diagnostics emitted
|
|
on the annotated lines. In this example, we expect to get:
|
|
|
|
.. code-block:: text
|
|
|
|
test.c:6:1: error: indirection requires pointer operand ('foo' invalid)
|
|
*X; // error
|
|
^~
|
|
test.c:7:1: error: indirection requires pointer operand ('foo' invalid)
|
|
**Y; // error
|
|
^~~
|
|
test.c:8:1: error: indirection requires pointer operand ('foo' invalid)
|
|
**Z; // error
|
|
^~~
|
|
|
|
While this example is somewhat silly, it illustrates the point: we want to
|
|
retain typedef information where possible, so that we can emit errors about
|
|
"``std::string``" instead of "``std::basic_string<char, std:...``". Doing this
|
|
requires properly keeping typedef information (for example, the type of ``X``
|
|
is "``foo``", not "``int``"), and requires properly propagating it through the
|
|
various operators (for example, the type of ``*Y`` is "``foo``", not
|
|
"``int``"). In order to retain this information, the type of these expressions
|
|
is an instance of the ``TypedefType`` class, which indicates that the type of
|
|
these expressions is a typedef for "``foo``".
|
|
|
|
Representing types like this is great for diagnostics, because the
|
|
user-specified type is always immediately available. There are two problems
|
|
with this: first, various semantic checks need to make judgements about the
|
|
*actual structure* of a type, ignoring typedefs. Second, we need an efficient
|
|
way to query whether two types are structurally identical to each other,
|
|
ignoring typedefs. The solution to both of these problems is the idea of
|
|
canonical types.
|
|
|
|
Canonical Types
|
|
^^^^^^^^^^^^^^^
|
|
|
|
Every instance of the ``Type`` class contains a canonical type pointer. For
|
|
simple types with no typedefs involved (e.g., "``int``", "``int*``",
|
|
"``int**``"), the type just points to itself. For types that have a typedef
|
|
somewhere in their structure (e.g., "``foo``", "``foo*``", "``foo**``",
|
|
"``bar``"), the canonical type pointer points to their structurally equivalent
|
|
type without any typedefs (e.g., "``int``", "``int*``", "``int**``", and
|
|
"``int*``" respectively).
|
|
|
|
This design provides a constant time operation (dereferencing the canonical type
|
|
pointer) that gives us access to the structure of types. For example, we can
|
|
trivially tell that "``bar``" and "``foo*``" are the same type by dereferencing
|
|
their canonical type pointers and doing a pointer comparison (they both point
|
|
to the single "``int*``" type).
|
|
|
|
Canonical types and typedef types bring up some complexities that must be
|
|
carefully managed. Specifically, the ``isa``/``cast``/``dyn_cast`` operators
|
|
generally shouldn't be used in code that is inspecting the AST. For example,
|
|
when type checking the indirection operator (unary "``*``" on a pointer), the
|
|
type checker must verify that the operand has a pointer type. It would not be
|
|
correct to check that with "``isa<PointerType>(SubExpr->getType())``", because
|
|
this predicate would fail if the subexpression had a typedef type.
|
|
|
|
The solution to this problem are a set of helper methods on ``Type``, used to
|
|
check their properties. In this case, it would be correct to use
|
|
"``SubExpr->getType()->isPointerType()``" to do the check. This predicate will
|
|
return true if the *canonical type is a pointer*, which is true any time the
|
|
type is structurally a pointer type. The only hard part here is remembering
|
|
not to use the ``isa``/``cast``/``dyn_cast`` operations.
|
|
|
|
The second problem we face is how to get access to the pointer type once we
|
|
know it exists. To continue the example, the result type of the indirection
|
|
operator is the pointee type of the subexpression. In order to determine the
|
|
type, we need to get the instance of ``PointerType`` that best captures the
|
|
typedef information in the program. If the type of the expression is literally
|
|
a ``PointerType``, we can return that, otherwise we have to dig through the
|
|
typedefs to find the pointer type. For example, if the subexpression had type
|
|
"``foo*``", we could return that type as the result. If the subexpression had
|
|
type "``bar``", we want to return "``foo*``" (note that we do *not* want
|
|
"``int*``"). In order to provide all of this, ``Type`` has a
|
|
``getAsPointerType()`` method that checks whether the type is structurally a
|
|
``PointerType`` and, if so, returns the best one. If not, it returns a null
|
|
pointer.
|
|
|
|
This structure is somewhat mystical, but after meditating on it, it will make
|
|
sense to you :).
|
|
|
|
.. _QualType:
|
|
|
|
The ``QualType`` class
|
|
----------------------
|
|
|
|
The ``QualType`` class is designed as a trivial value class that is small,
|
|
passed by-value and is efficient to query. The idea of ``QualType`` is that it
|
|
stores the type qualifiers (``const``, ``volatile``, ``restrict``, plus some
|
|
extended qualifiers required by language extensions) separately from the types
|
|
themselves. ``QualType`` is conceptually a pair of "``Type*``" and the bits
|
|
for these type qualifiers.
|
|
|
|
By storing the type qualifiers as bits in the conceptual pair, it is extremely
|
|
efficient to get the set of qualifiers on a ``QualType`` (just return the field
|
|
of the pair), add a type qualifier (which is a trivial constant-time operation
|
|
that sets a bit), and remove one or more type qualifiers (just return a
|
|
``QualType`` with the bitfield set to empty).
|
|
|
|
Further, because the bits are stored outside of the type itself, we do not need
|
|
to create duplicates of types with different sets of qualifiers (i.e. there is
|
|
only a single heap allocated "``int``" type: "``const int``" and "``volatile
|
|
const int``" both point to the same heap allocated "``int``" type). This
|
|
reduces the heap size used to represent bits and also means we do not have to
|
|
consider qualifiers when uniquing types (:ref:`Type <Type>` does not even
|
|
contain qualifiers).
|
|
|
|
In practice, the two most common type qualifiers (``const`` and ``restrict``)
|
|
are stored in the low bits of the pointer to the ``Type`` object, together with
|
|
a flag indicating whether extended qualifiers are present (which must be
|
|
heap-allocated). This means that ``QualType`` is exactly the same size as a
|
|
pointer.
|
|
|
|
.. _DeclarationName:
|
|
|
|
Declaration names
|
|
-----------------
|
|
|
|
The ``DeclarationName`` class represents the name of a declaration in Clang.
|
|
Declarations in the C family of languages can take several different forms.
|
|
Most declarations are named by simple identifiers, e.g., "``f``" and "``x``" in
|
|
the function declaration ``f(int x)``. In C++, declaration names can also name
|
|
class constructors ("``Class``" in ``struct Class { Class(); }``), class
|
|
destructors ("``~Class``"), overloaded operator names ("``operator+``"), and
|
|
conversion functions ("``operator void const *``"). In Objective-C,
|
|
declaration names can refer to the names of Objective-C methods, which involve
|
|
the method name and the parameters, collectively called a *selector*, e.g.,
|
|
"``setWidth:height:``". Since all of these kinds of entities --- variables,
|
|
functions, Objective-C methods, C++ constructors, destructors, and operators
|
|
--- are represented as subclasses of Clang's common ``NamedDecl`` class,
|
|
``DeclarationName`` is designed to efficiently represent any kind of name.
|
|
|
|
Given a ``DeclarationName`` ``N``, ``N.getNameKind()`` will produce a value
|
|
that describes what kind of name ``N`` stores. There are 10 options (all of
|
|
the names are inside the ``DeclarationName`` class).
|
|
|
|
``Identifier``
|
|
|
|
The name is a simple identifier. Use ``N.getAsIdentifierInfo()`` to retrieve
|
|
the corresponding ``IdentifierInfo*`` pointing to the actual identifier.
|
|
|
|
``ObjCZeroArgSelector``, ``ObjCOneArgSelector``, ``ObjCMultiArgSelector``
|
|
|
|
The name is an Objective-C selector, which can be retrieved as a ``Selector``
|
|
instance via ``N.getObjCSelector()``. The three possible name kinds for
|
|
Objective-C reflect an optimization within the ``DeclarationName`` class:
|
|
both zero- and one-argument selectors are stored as a masked
|
|
``IdentifierInfo`` pointer, and therefore require very little space, since
|
|
zero- and one-argument selectors are far more common than multi-argument
|
|
selectors (which use a different structure).
|
|
|
|
``CXXConstructorName``
|
|
|
|
The name is a C++ constructor name. Use ``N.getCXXNameType()`` to retrieve
|
|
the :ref:`type <QualType>` that this constructor is meant to construct. The
|
|
type is always the canonical type, since all constructors for a given type
|
|
have the same name.
|
|
|
|
``CXXDestructorName``
|
|
|
|
The name is a C++ destructor name. Use ``N.getCXXNameType()`` to retrieve
|
|
the :ref:`type <QualType>` whose destructor is being named. This type is
|
|
always a canonical type.
|
|
|
|
``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``".
|
|
|
|
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 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.
|
|
|
|
.. _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 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 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
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When testing locally, *do not* commit changes to ``AttributeReference.rst``.
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This file is generated by the server automatically, and any changes made to this
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file will be overwritten.
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Arguments
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~~~~~~~~~
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Attributes may optionally specify a list of arguments that can be passed to the
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attribute. Attribute arguments specify both the parsed form and the semantic
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form of the attribute. For example, if ``Args`` is
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``[StringArgument<"Arg1">, IntArgument<"Arg2">]`` then
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``__attribute__((myattribute("Hello", 3)))`` will be a valid use; it requires
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two arguments while parsing, and the Attr subclass' constructor for the
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semantic attribute will require a string and integer argument.
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All arguments have a name and a flag that specifies whether the argument is
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optional. The associated C++ type of the argument is determined by the argument
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definition type. If the existing argument types are insufficient, new types can
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be created, but it requires modifying `utils/TableGen/ClangAttrEmitter.cpp
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<https://github.com/llvm/llvm-project/blob/master/clang/utils/TableGen/ClangAttrEmitter.cpp>`_
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to properly support the type.
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Other Properties
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~~~~~~~~~~~~~~~~
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The ``Attr`` definition has other members which control the behavior of the
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attribute. Many of them are special-purpose and beyond the scope of this
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document, however a few deserve mention.
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If the parsed form of the attribute is more complex, or differs from the
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semantic form, the ``HasCustomParsing`` bit can be set to ``1`` for the class,
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and the parsing code in `Parser::ParseGNUAttributeArgs()
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<https://github.com/llvm/llvm-project/blob/master/clang/lib/Parse/ParseDecl.cpp>`_
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can be updated for the special case. Note that this only applies to arguments
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with a GNU spelling -- attributes with a __declspec spelling currently ignore
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this flag and are handled by ``Parser::ParseMicrosoftDeclSpec``.
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Note that setting this member to 1 will opt out of common attribute semantic
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handling, requiring extra implementation efforts to ensure the attribute
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appertains to the appropriate subject, etc.
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If the attribute should not be propagated from a template declaration to an
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instantiation of the template, set the ``Clone`` member to 0. By default, all
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attributes will be cloned to template instantiations.
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Attributes that do not require an AST node should set the ``ASTNode`` field to
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``0`` to avoid polluting the AST. Note that anything inheriting from
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``TypeAttr`` or ``IgnoredAttr`` automatically do not generate an AST node. All
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other attributes generate an AST node by default. The AST node is the semantic
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representation of the attribute.
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The ``LangOpts`` field specifies a list of language options required by the
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attribute. For instance, all of the CUDA-specific attributes specify ``[CUDA]``
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for the ``LangOpts`` field, and when the CUDA language option is not enabled, an
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"attribute ignored" warning diagnostic is emitted. Since language options are
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not table generated nodes, new language options must be created manually and
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should specify the spelling used by ``LangOptions`` class.
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Custom accessors can be generated for an attribute based on the spelling list
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for that attribute. For instance, if an attribute has two different spellings:
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'Foo' and 'Bar', accessors can be created:
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``[Accessor<"isFoo", [GNU<"Foo">]>, Accessor<"isBar", [GNU<"Bar">]>]``
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These accessors will be generated on the semantic form of the attribute,
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accepting no arguments and returning a ``bool``.
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Attributes that do not require custom semantic handling should set the
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``SemaHandler`` field to ``0``. Note that anything inheriting from
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``IgnoredAttr`` automatically do not get a semantic handler. All other
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attributes are assumed to use a semantic handler by default. Attributes
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without a semantic handler are not given a parsed attribute ``Kind`` enumerator.
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Target-specific attributes may share a spelling with other attributes in
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different targets. For instance, the ARM and MSP430 targets both have an
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attribute spelled ``GNU<"interrupt">``, but with different parsing and semantic
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requirements. To support this feature, an attribute inheriting from
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``TargetSpecificAttribute`` may specify a ``ParseKind`` field. This field
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should be the same value between all arguments sharing a spelling, and
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corresponds to the parsed attribute's ``Kind`` enumerator. This allows
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attributes to share a parsed attribute kind, but have distinct semantic
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attribute classes. For instance, ``ParsedAttr`` is the shared
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parsed attribute kind, but ARMInterruptAttr and MSP430InterruptAttr are the
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semantic attributes generated.
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By default, attribute arguments are parsed in an evaluated context. If the
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arguments for an attribute should be parsed in an unevaluated context (akin to
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the way the argument to a ``sizeof`` expression is parsed), set
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``ParseArgumentsAsUnevaluated`` to ``1``.
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If additional functionality is desired for the semantic form of the attribute,
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the ``AdditionalMembers`` field specifies code to be copied verbatim into the
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semantic attribute class object, with ``public`` access.
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Boilerplate
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^^^^^^^^^^^
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All semantic processing of declaration attributes happens in `lib/Sema/SemaDeclAttr.cpp
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<https://github.com/llvm/llvm-project/blob/master/clang/lib/Sema/SemaDeclAttr.cpp>`_,
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and generally starts in the ``ProcessDeclAttribute()`` function. If the
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attribute is a "simple" attribute -- meaning that it requires no custom semantic
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processing aside from what is automatically provided, add a call to
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``handleSimpleAttribute<YourAttr>(S, D, Attr);`` to the switch statement.
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Otherwise, write a new ``handleYourAttr()`` function, and add that to the switch
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statement. Please do not implement handling logic directly in the ``case`` for
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the attribute.
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Unless otherwise specified by the attribute definition, common semantic checking
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of the parsed attribute is handled automatically. This includes diagnosing
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parsed attributes that do not appertain to the given ``Decl``, ensuring the
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correct minimum number of arguments are passed, etc.
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If the attribute adds additional warnings, define a ``DiagGroup`` in
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`include/clang/Basic/DiagnosticGroups.td
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<https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Basic/DiagnosticGroups.td>`_
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named after the attribute's ``Spelling`` with "_"s replaced by "-"s. If there
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is only a single diagnostic, it is permissible to use ``InGroup<DiagGroup<"your-attribute">>``
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directly in `DiagnosticSemaKinds.td
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<https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Basic/DiagnosticSemaKinds.td>`_
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All semantic diagnostics generated for your attribute, including automatically-
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generated ones (such as subjects and argument counts), should have a
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corresponding test case.
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Semantic handling
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^^^^^^^^^^^^^^^^^
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Most attributes are implemented to have some effect on the compiler. For
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instance, to modify the way code is generated, or to add extra semantic checks
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for an analysis pass, etc. Having added the attribute definition and conversion
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to the semantic representation for the attribute, what remains is to implement
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the custom logic requiring use of the attribute.
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The ``clang::Decl`` object can be queried for the presence or absence of an
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attribute using ``hasAttr<T>()``. To obtain a pointer to the semantic
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representation of the attribute, ``getAttr<T>`` may be used.
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How to add an expression or statement
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-------------------------------------
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Expressions and statements are one of the most fundamental constructs within a
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compiler, because they interact with many different parts of the AST, semantic
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analysis, and IR generation. Therefore, adding a new expression or statement
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kind into Clang requires some care. The following list details the various
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places in Clang where an expression or statement needs to be introduced, along
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with patterns to follow to ensure that the new expression or statement works
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well across all of the C languages. We focus on expressions, but statements
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are similar.
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#. Introduce parsing actions into the parser. Recursive-descent parsing is
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mostly self-explanatory, but there are a few things that are worth keeping
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in mind:
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* Keep as much source location information as possible! You'll want it later
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to produce great diagnostics and support Clang's various features that map
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between source code and the AST.
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* Write tests for all of the "bad" parsing cases, to make sure your recovery
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is good. If you have matched delimiters (e.g., parentheses, square
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brackets, etc.), use ``Parser::BalancedDelimiterTracker`` to give nice
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diagnostics when things go wrong.
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#. Introduce semantic analysis actions into ``Sema``. Semantic analysis should
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always involve two functions: an ``ActOnXXX`` function that will be called
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directly from the parser, and a ``BuildXXX`` function that performs the
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actual semantic analysis and will (eventually!) build the AST node. It's
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fairly common for the ``ActOnCXX`` function to do very little (often just
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some minor translation from the parser's representation to ``Sema``'s
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representation of the same thing), but the separation is still important:
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C++ template instantiation, for example, should always call the ``BuildXXX``
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variant. Several notes on semantic analysis before we get into construction
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of the AST:
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* Your expression probably involves some types and some subexpressions.
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Make sure to fully check that those types, and the types of those
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subexpressions, meet your expectations. Add implicit conversions where
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necessary to make sure that all of the types line up exactly the way you
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want them. Write extensive tests to check that you're getting good
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diagnostics for mistakes and that you can use various forms of
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subexpressions with your expression.
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* When type-checking a type or subexpression, make sure to first check
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whether the type is "dependent" (``Type::isDependentType()``) or whether a
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subexpression is type-dependent (``Expr::isTypeDependent()``). If any of
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these return ``true``, then you're inside a template and you can't do much
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type-checking now. That's normal, and your AST node (when you get there)
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will have to deal with this case. At this point, you can write tests that
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use your expression within templates, but don't try to instantiate the
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templates.
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* For each subexpression, be sure to call ``Sema::CheckPlaceholderExpr()``
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to deal with "weird" expressions that don't behave well as subexpressions.
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Then, determine whether you need to perform lvalue-to-rvalue conversions
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(``Sema::DefaultLvalueConversions``) or the usual unary conversions
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(``Sema::UsualUnaryConversions``), for places where the subexpression is
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producing a value you intend to use.
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* Your ``BuildXXX`` function will probably just return ``ExprError()`` at
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this point, since you don't have an AST. That's perfectly fine, and
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shouldn't impact your testing.
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#. Introduce an AST node for your new expression. This starts with declaring
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the node in ``include/Basic/StmtNodes.td`` and creating a new class for your
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expression in the appropriate ``include/AST/Expr*.h`` header. It's best to
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look at the class for a similar expression to get ideas, and there are some
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specific things to watch for:
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* If you need to allocate memory, use the ``ASTContext`` allocator to
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allocate memory. Never use raw ``malloc`` or ``new``, and never hold any
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resources in an AST node, because the destructor of an AST node is never
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called.
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* Make sure that ``getSourceRange()`` covers the exact source range of your
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expression. This is needed for diagnostics and for IDE support.
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* Make sure that ``children()`` visits all of the subexpressions. This is
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important for a number of features (e.g., IDE support, C++ variadic
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templates). If you have sub-types, you'll also need to visit those
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sub-types in ``RecursiveASTVisitor``.
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* Add printing support (``StmtPrinter.cpp``) for your expression.
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* Add profiling support (``StmtProfile.cpp``) for your AST node, noting the
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distinguishing (non-source location) characteristics of an instance of
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your expression. Omitting this step will lead to hard-to-diagnose
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failures regarding matching of template declarations.
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* Add serialization support (``ASTReaderStmt.cpp``, ``ASTWriterStmt.cpp``)
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for your AST node.
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#. Teach semantic analysis to build your AST node. At this point, you can wire
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up your ``Sema::BuildXXX`` function to actually create your AST. A few
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things to check at this point:
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* If your expression can construct a new C++ class or return a new
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Objective-C object, be sure to update and then call
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``Sema::MaybeBindToTemporary`` for your just-created AST node to be sure
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that the object gets properly destructed. An easy way to test this is to
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return a C++ class with a private destructor: semantic analysis should
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flag an error here with the attempt to call the destructor.
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* Inspect the generated AST by printing it using ``clang -cc1 -ast-print``,
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to make sure you're capturing all of the important information about how
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the AST was written.
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* Inspect the generated AST under ``clang -cc1 -ast-dump`` to verify that
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all of the types in the generated AST line up the way you want them.
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Remember that clients of the AST should never have to "think" to
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understand what's going on. For example, all implicit conversions should
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show up explicitly in the AST.
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* Write tests that use your expression as a subexpression of other,
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well-known expressions. Can you call a function using your expression as
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an argument? Can you use the ternary operator?
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#. Teach code generation to create IR to your AST node. This step is the first
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(and only) that requires knowledge of LLVM IR. There are several things to
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keep in mind:
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* Code generation is separated into scalar/aggregate/complex and
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lvalue/rvalue paths, depending on what kind of result your expression
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produces. On occasion, this requires some careful factoring of code to
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avoid duplication.
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* ``CodeGenFunction`` contains functions ``ConvertType`` and
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``ConvertTypeForMem`` that convert Clang's types (``clang::Type*`` or
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``clang::QualType``) to LLVM types. Use the former for values, and the
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latter for memory locations: test with the C++ "``bool``" type to check
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this. If you find that you are having to use LLVM bitcasts to make the
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subexpressions of your expression have the type that your expression
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expects, STOP! Go fix semantic analysis and the AST so that you don't
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need these bitcasts.
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* The ``CodeGenFunction`` class has a number of helper functions to make
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certain operations easy, such as generating code to produce an lvalue or
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an rvalue, or to initialize a memory location with a given value. Prefer
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to use these functions rather than directly writing loads and stores,
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because these functions take care of some of the tricky details for you
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(e.g., for exceptions).
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* If your expression requires some special behavior in the event of an
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exception, look at the ``push*Cleanup`` functions in ``CodeGenFunction``
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to introduce a cleanup. You shouldn't have to deal with
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exception-handling directly.
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* Testing is extremely important in IR generation. Use ``clang -cc1
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-emit-llvm`` and `FileCheck
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<https://llvm.org/docs/CommandGuide/FileCheck.html>`_ to verify that you're
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generating the right IR.
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#. Teach template instantiation how to cope with your AST node, which requires
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some fairly simple code:
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* Make sure that your expression's constructor properly computes the flags
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for type dependence (i.e., the type your expression produces can change
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from one instantiation to the next), value dependence (i.e., the constant
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value your expression produces can change from one instantiation to the
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next), instantiation dependence (i.e., a template parameter occurs
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anywhere in your expression), and whether your expression contains a
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parameter pack (for variadic templates). Often, computing these flags
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just means combining the results from the various types and
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subexpressions.
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* Add ``TransformXXX`` and ``RebuildXXX`` functions to the ``TreeTransform``
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class template in ``Sema``. ``TransformXXX`` should (recursively)
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transform all of the subexpressions and types within your expression,
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using ``getDerived().TransformYYY``. If all of the subexpressions and
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types transform without error, it will then call the ``RebuildXXX``
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function, which will in turn call ``getSema().BuildXXX`` to perform
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semantic analysis and build your expression.
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* To test template instantiation, take those tests you wrote to make sure
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that you were type checking with type-dependent expressions and dependent
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types (from step #2) and instantiate those templates with various types,
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some of which type-check and some that don't, and test the error messages
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in each case.
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#. There are some "extras" that make other features work better. It's worth
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handling these extras to give your expression complete integration into
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Clang:
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* Add code completion support for your expression in
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``SemaCodeComplete.cpp``.
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* If your expression has types in it, or has any "interesting" features
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other than subexpressions, extend libclang's ``CursorVisitor`` to provide
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proper visitation for your expression, enabling various IDE features such
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as syntax highlighting, cross-referencing, and so on. The
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``c-index-test`` helper program can be used to test these features.
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