“Clang” CFE Internals Manual¶

Introduction
LLVM Support Library
The Clang “Basic” Library
The Driver Library
Precompiled Headers
The Frontend Library
The Lexer and Preprocessor Library
The Parser Library
The AST Library
The Sema Library
The CodeGen Library
How to change Clang
- How to add an attribute
- How to add an expression or statement

Introduction ¶

This document describes some of the more important APIs and internal design decisions made in the Clang C front-end. The purpose of this document is to both capture some of this high level information and also describe some of the design decisions behind it. This is meant for people interested in hacking on Clang, not for end-users. The description below is categorized by libraries, and does not describe any of the clients of the libraries.

LLVM Support Library ¶

The LLVM libSupport library provides many underlying libraries and data-structures, including command line option processing, various containers and a system abstraction layer, which is used for file system access.

The Clang “Basic” Library ¶

This library certainly needs a better name. The “basic” library contains a number of low-level utilities for tracking and manipulating source buffers, locations within the source buffers, diagnostics, tokens, target abstraction, and information about the subset of the language being compiled for.

Part of this infrastructure is specific to C (such as the TargetInfo class), other parts could be reused for other non-C-based languages (SourceLocation, SourceManager, Diagnostics, FileManager). When and if there is future demand we can figure out if it makes sense to introduce a new library, move the general classes somewhere else, or introduce some other solution.

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

The Diagnostics Subsystem ¶

The Clang Diagnostics subsystem is an important part of how the compiler communicates with the human. Diagnostics are the warnings and errors produced when the code is incorrect or dubious. In Clang, each diagnostic produced has (at the minimum) a unique ID, an English translation associated with it, a SourceLocation to “put the caret”, and a severity (e.g., WARNING or ERROR). They can also optionally include a number of arguments to the diagnostic (which fill in “%0“‘s in the string) as well as a number of source ranges that related to the diagnostic.

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

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

In this example, you can see the English translation, the severity (error), you can see the source location (the caret (”^”) and file/line/column info), the source ranges “~~~~”, arguments to the diagnostic (”int*” and “_Complex float”). You’ll have to believe me that there is a unique ID backing the diagnostic :).

Getting all of this to happen has several steps and involves many moving pieces, this section describes them and talks about best practices when adding a new diagnostic.

The `Diagnostic*Kinds.td` files ¶

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

There is little sanity with the naming of the unique ID’s right now. Some start with err_, warn_, ext_ to encode the severity into the name. Since the enum is referenced in the C++ code that produces the diagnostic, it is somewhat useful for it to be reasonably short.

The severity of the diagnostic comes from the set {NOTE, REMARK, WARNING, EXTENSION, EXTWARN, ERROR}. The ERROR severity is used for diagnostics indicating the program is never acceptable under any circumstances. When an error is emitted, the AST for the input code may not be fully built. The EXTENSION and EXTWARN severities are used for extensions to the language that Clang accepts. This means that Clang fully understands and can represent them in the AST, but we produce diagnostics to tell the user their code is non-portable. The difference is that the former are ignored by default, and the later warn by default. The WARNING severity is used for constructs that are valid in the currently selected source language but that are dubious in some way. The REMARK severity provides generic information about the compilation that is not necessarily related to any dubious code. The NOTE level is used to staple more information onto previous diagnostics.

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

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

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

The Format String ¶

The format string for the diagnostic is very simple, but it has some power. It takes the form of a string in English with markers that indicate where and how arguments to the diagnostic are inserted and formatted. For example, here are some simple format strings:

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

These examples show some important points of format strings. You can use any plain ASCII character in the diagnostic string except “%” without a problem, but these are C strings, so you have to use and be aware of all the C escape sequences (as in the second example). If you want to produce a “%” in the output, use the “%%” escape sequence, like the third diagnostic. Finally, Clang uses the “%...[digit]” sequences to specify where and how arguments to the diagnostic are formatted.

Arguments to the diagnostic are numbered according to how they are specified by the C++ code that produces them, and are referenced by %0 .. %9. If you have more than 10 arguments to your diagnostic, you are doing something wrong :). Unlike printf, there is no requirement that arguments to the diagnostic end up in the output in the same order as they are specified, you could have a format string with “%1 %0” that swaps them, for example. The text in between the percent and digit are formatting instructions. If there are no instructions, the argument is just turned into a string and substituted in.

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

Keep the string short. It should ideally fit in the 80 column limit of the DiagnosticKinds.td file. This avoids the diagnostic wrapping when printed, and forces you to think about the important point you are conveying with the diagnostic.
Take advantage of location information. The user will be able to see the line and location of the caret, so you don’t need to tell them that the problem is with the 4th argument to the function: just point to it.
Do not capitalize the diagnostic string, and do not end it with a period.
If you need to quote something in the diagnostic string, use single quotes.

Diagnostics should never take random English strings as arguments: you shouldn’t use “you have a problem with %0” and pass in things like “your argument” or “your return value” as arguments. Doing this prevents translating the Clang diagnostics to other languages (because they’ll get random English words in their otherwise localized diagnostic). The exceptions to this are C/C++ language keywords (e.g., auto, const, mutable, etc) and C/C++ operators (/=). Note that things like “pointer” and “reference” are not keywords. On the other hand, you can include anything that comes from the user’s source code, including variable names, types, labels, etc. The “select” format can be used to achieve this sort of thing in a localizable way, see below.

Formatting a Diagnostic Argument ¶

Arguments to diagnostics are fully typed internally, and come from a couple different classes: integers, types, names, and random strings. Depending on the class of the argument, it can be optionally formatted in different ways. This gives the DiagnosticConsumer information about what the argument means without requiring it to use a specific presentation (consider this MVC for Clang :).

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

“s” format

Example:: "requires %1 parameter%s1"
Class:: Integers
Description:: This is a simple formatter for integers that is useful when producing English diagnostics. When the integer is 1, it prints as nothing. When the integer is not 1, it prints as “s”. This allows some simple grammatical forms to be to be handled correctly, and eliminates the need to use gross things like "requires %1 parameter(s)".

“select” format

Example:: "must be a %select{unary|binary|unary or binary}2 operator"
Class:: Integers
Description:: This format specifier is used to merge multiple related diagnostics together into one common one, without requiring the difference to be specified as an English string argument. Instead of specifying the string, the diagnostic gets an integer argument and the format string selects the numbered option. In this case, the “%2” value must be an integer in the range [0..2]. If it is 0, it prints “unary”, if it is 1 it prints “binary” if it is 2, it prints “unary or binary”. This allows other language translations to substitute reasonable words (or entire phrases) based on the semantics of the diagnostic instead of having to do things textually. The selected string does undergo formatting.

“plural” format

Example:

"you have %1 %plural{1:mouse|:mice}1 connected to your computer"

Class:

Integers

Description:

This is a formatter for complex plural forms. It is designed to handle even the requirements of languages with very complex plural forms, as many Baltic languages have. The argument consists of a series of expression/form pairs, separated by “:”, where the first form whose expression evaluates to true is the result of the modifier.

An expression can be empty, in which case it is always true. See the example at the top. Otherwise, it is a series of one or more numeric conditions, separated by “,”. If any condition matches, the expression matches. Each numeric condition can take one of three forms.

number: A simple decimal number matches if the argument is the same as the number. Example: "%plural{1:mouse|:mice}4"
range: A range in square brackets matches if the argument is within the range. Then range is inclusive on both ends. Example: "%plural{0:none|1:one|[2,5]:some|:many}2"
modulo: A modulo operator is followed by a number, and equals sign and either a number or a range. The tests are the same as for plain numbers and ranges, but the argument is taken modulo the number first. Example: "%plural{%100=0:even hundred|%100=[1,50]:lower half|:everything else}1"

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

“ordinal” format

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

“objcclass” format

Example:: "method %objcclass0 not found"
Class:: DeclarationName
Description:: This is a simple formatter that indicates the DeclarationName corresponds to an Objective-C class method selector. As such, it prints the selector with a leading “+”.

“objcinstance” format

Example:: "method %objcinstance0 not found"
Class:: DeclarationName
Description:: This is a simple formatter that indicates the DeclarationName corresponds to an Objective-C instance method selector. As such, it prints the selector with a leading “-“.

“q” format

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

“diff” format

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

It is really easy to add format specifiers to the Clang diagnostics system, but they should be discussed before they are added. If you are creating a lot of repetitive diagnostics and/or have an idea for a useful formatter, please bring it up on the cfe-dev mailing list.

“sub” format

Example:

Given the following record definition of type TextSubstitution:

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

which can be used as

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

and will act as if it was written "candidate %select{function|constructor}3%select{| template| %1}2 not viable".

Description:

This format specifier is used to avoid repeating strings verbatim in multiple diagnostics. The argument to %sub must name a TextSubstitution tblgen record. The substitution must specify all arguments used by the substitution, and the modifier indexes in the substitution are re-numbered accordingly. The substituted text must itself be a valid format string before substitution.

Producing the Diagnostic ¶

Now that you’ve created the diagnostic in the Diagnostic*Kinds.td file, you need to write the code that detects the condition in question and emits the new diagnostic. Various components of Clang (e.g., the preprocessor, Sema, etc.) provide a helper function named “Diag”. It creates a diagnostic and accepts the arguments, ranges, and other information that goes along with it.

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

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

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

As you can see, adding and producing a diagnostic is pretty straightforward. The hard part is deciding exactly what you need to say to help the user, picking a suitable wording, and providing the information needed to format it correctly. The good news is that the call site that issues a diagnostic should be completely independent of how the diagnostic is formatted and in what language it is rendered.

Fix-It Hints ¶

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

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

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

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

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

Since they are automatically applied if -Xclang -fixit is passed to the driver, they should only be used when it’s very likely they match the user’s intent.
Clang must recover from errors as if the fix-it had been applied.
Fix-it hints on a warning must not change the meaning of the code. However, a hint may clarify the meaning as intentional, for example by adding parentheses when the precedence of operators isn’t obvious.

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

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

FixItHint::CreateInsertion(Loc, Code)

Specifies that the given Code (a string) should be inserted before the source location Loc.
FixItHint::CreateRemoval(Range)

Specifies that the code in the given source Range should be removed.
FixItHint::CreateReplacement(Range, Code)

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

The `DiagnosticConsumer` Interface ¶

Once code generates a diagnostic with all of the arguments and the rest of the relevant information, Clang needs to know what to do with it. As previously mentioned, the diagnostic machinery goes through some filtering to map a severity onto a diagnostic level, then (assuming the diagnostic is not mapped to “Ignore”) it invokes an object that implements the DiagnosticConsumer interface with the information.

It is possible to implement this interface in many different ways. For example, the normal Clang DiagnosticConsumer (named TextDiagnosticPrinter) turns the arguments into strings (according to the various formatting rules), prints out the file/line/column information and the string, then prints out the line of code, the source ranges, and the caret. However, this behavior isn’t required.

Another implementation of the DiagnosticConsumer interface is the TextDiagnosticBuffer class, which is used when Clang is in -verify mode. Instead of formatting and printing out the diagnostics, this implementation just captures and remembers the diagnostics as they fly by. Then -verify compares the list of produced diagnostics to the list of expected ones. If they disagree, it prints out its own output. Full documentation for the -verify mode can be found in the Clang API documentation for VerifyDiagnosticConsumer.

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.

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.

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 #includes 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`¶

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:

x = foo + bar;
^first    ^last

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

The Driver Library ¶

The clang Driver and library are documented here.

Precompiled Headers ¶

Clang supports precompiled headers (PCH), which uses a serialized representation of Clang’s internal data structures, encoded with the LLVM bitstream format.

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.

Compiler Invocation ¶

One of the classes provided by the Frontend library is CompilerInvocation, which holds information that describe current invocation of the Clang -cc1 frontend. The information typically comes from the command line constructed by the Clang driver or from clients performing custom initialization. The data structure is split into logical units used by different parts of the compiler, for example PreprocessorOptions, LanguageOptions or CodeGenOptions.

Command Line Interface ¶

The command line interface of the Clang -cc1 frontend is defined alongside the driver options in clang/Driver/Options.td. The information making up an option definition includes its prefix and name (for example -std=), form and position of the option value, help text, aliases and more. Each option may belong to a certain group and can be marked with zero or more flags. Options accepted by the -cc1 frontend are marked with the CC1Option flag.

Command Line Parsing ¶

Option definitions are processed by the -gen-opt-parser-defs tablegen backend during early stages of the build. Options are then used for querying an instance llvm::opt::ArgList, a wrapper around the command line arguments. This is done in the Clang driver to construct individual jobs based on the driver arguments and also in the CompilerInvocation::CreateFromArgs function that parses the -cc1 frontend arguments.

Command Line Generation ¶

Any valid CompilerInvocation created from a -cc1 command line can be also serialized back into semantically equivalent command line in a deterministic manner. This enables features such as implicitly discovered, explicitly built modules.

Adding new Command Line Option ¶

When adding a new command line option, the first place of interest is the header file declaring the corresponding options class (e.g. CodeGenOptions.h for command line option that affects the code generation). Create new member variable for the option value:

  class CodeGenOptions : public CodeGenOptionsBase {

+   /// List of dynamic shared object files to be loaded as pass plugins.
+   std::vector<std::string> PassPlugins;

  }

Next, declare the command line interface of the option in the tablegen file clang/include/clang/Driver/Options.td. This is done by instantiating the Option class (defined in llvm/include/llvm/Option/OptParser.td). The instance is typically created through one of the helper classes that encode the acceptable ways to specify the option value on the command line:

Flag - the option does not accept any value,
Joined - the value must immediately follow the option name within the same argument,
Separate - the value must follow the option name in the next command line argument,
JoinedOrSeparate - the value can be specified either as Joined or Separate,
CommaJoined - the values are comma-separated and must immediately follow the option name within the same argument (see Wl, for an example).

The helper classes take a list of acceptable prefixes of the option (e.g. "-", "--" or "/") and the option name:

  // Options.td

+ def fpass_plugin_EQ : Joined<["-"], "fpass-plugin=">;

Then, specify additional attributes via mix-ins:

HelpText holds the text that will be printed besides the option name when the user requests help (e.g. via clang --help).
Group specifies the “category” of options this option belongs to. This is used by various tools to filter certain options of interest.
Flags may contain a number of “tags” associated with the option. This enables more granular filtering than the Group attribute.
Alias denotes that the option is an alias of another option. This may be combined with AliasArgs that holds the implied value.

  // Options.td

  def fpass_plugin_EQ : Joined<["-"], "fpass-plugin=">,
+   Group<f_Group>, Flags<[CC1Option]>,
+   HelpText<"Load pass plugin from a dynamic shared object file.">;

New options are recognized by the Clang driver unless marked with the NoDriverOption flag. On the other hand, options intended for the -cc1 frontend must be explicitly marked with the CC1Option flag.

Next, parse (or manufacture) the command line arguments in the Clang driver and use them to construct the -cc1 job:

  void Clang::ConstructJob(const ArgList &Args /*...*/) const {
    ArgStringList CmdArgs;
    // ...

+   for (const Arg *A : Args.filtered(OPT_fpass_plugin_EQ)) {
+     CmdArgs.push_back(Args.MakeArgString(Twine("-fpass-plugin=") + A->getValue()));
+     A->claim();
+   }
  }

The last step is implementing the -cc1 command line argument parsing/generation that initializes/serializes the option class (in our case CodeGenOptions) stored within CompilerInvocation. This can be done automatically by using the marshalling annotations on the option definition:

  // Options.td

  def fpass_plugin_EQ : Joined<["-"], "fpass-plugin=">,
    Group<f_Group>, Flags<[CC1Option]>,
    HelpText<"Load pass plugin from a dynamic shared object file.">,
+   MarshallingInfoStringVector<CodeGenOpts<"PassPlugins">>;

Inner workings of the system are introduced in the marshalling infrastructure section and the available annotations are listed here.

In case the marshalling infrastructure does not support the desired semantics, consider simplifying it to fit the existing model. This makes the command line more uniform and reduces the amount of custom, manually written code. Remember that the -cc1 command line interface is intended only for Clang developers, meaning it does not need to mirror the driver interface, maintain backward compatibility or be compatible with GCC.

If the option semantics cannot be encoded via marshalling annotations, you can resort to parsing/serializing the command line arguments manually:

  // CompilerInvocation.cpp

  static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args /*...*/) {
    // ...

+   Opts.PassPlugins = Args.getAllArgValues(OPT_fpass_plugin_EQ);
  }

  static void GenerateCodeGenArgs(const CodeGenOptions &Opts,
                                  SmallVectorImpl<const char *> &Args,
                                  CompilerInvocation::StringAllocator SA /*...*/) {
    // ...

+   for (const std::string &PassPlugin : Opts.PassPlugins)
+     GenerateArg(Args, OPT_fpass_plugin_EQ, PassPlugin, SA);
  }

Finally, you can specify the argument on the command line: clang -fpass-plugin=a -fpass-plugin=b and use the new member variable as desired.

  void EmitAssemblyHelper::EmitAssemblyWithNewPassManager(/*...*/) {
    // ...
+   for (auto &PluginFN : CodeGenOpts.PassPlugins)
+     if (auto PassPlugin = PassPlugin::Load(PluginFN))
+        PassPlugin->registerPassBuilderCallbacks(PB);
  }

Option Marshalling Infrastructure ¶

The option marshalling infrastructure automates the parsing of the Clang -cc1 frontend command line arguments into CompilerInvocation and their generation from CompilerInvocation. The system replaces lots of repetitive C++ code with simple, declarative tablegen annotations and it’s being used for the majority of the -cc1 command line interface. This section provides an overview of the system.

Note: The marshalling infrastructure is not intended for driver-only options. Only options of the -cc1 frontend need to be marshalled to/from CompilerInvocation instance.

To read and modify contents of CompilerInvocation, the marshalling system uses key paths, which are declared in two steps. First, a tablegen definition for the CompilerInvocation member is created by inheriting from KeyPathAndMacro:

// Options.td

class LangOpts<string field> : KeyPathAndMacro<"LangOpts->", field, "LANG_"> {}
//                   CompilerInvocation member  ^^^^^^^^^^
//                                    OPTION_WITH_MARSHALLING prefix ^^^^^

The first argument to the parent class is the beginning of the key path that references the CompilerInvocation member. This argument ends with -> if the member is a pointer type or with . if it’s a value type. The child class takes a single parameter field that is forwarded as the second argument to the base class. The child class can then be used like so: LangOpts<"IgnoreExceptions">, constructing a key path to the field LangOpts->IgnoreExceptions. The third argument passed to the parent class is a string that the tablegen backend uses as a prefix to the OPTION_WITH_MARSHALLING macro. Using the key path as a mix-in on an Option instance instructs the backend to generate the following code:

// Options.inc

#ifdef LANG_OPTION_WITH_MARSHALLING
LANG_OPTION_WITH_MARSHALLING([...], LangOpts->IgnoreExceptions, [...])
#endif // LANG_OPTION_WITH_MARSHALLING

Such definition can be used used in the function for parsing and generating command line:

// clang/lib/Frontend/CompilerInvoation.cpp

bool CompilerInvocation::ParseLangArgs(LangOptions *LangOpts, ArgList &Args,
                                       DiagnosticsEngine &Diags) {
  bool Success = true;

#define LANG_OPTION_WITH_MARSHALLING(                                          \
    PREFIX_TYPE, NAME, ID, KIND, GROUP, ALIAS, ALIASARGS, FLAGS, PARAM,        \
    HELPTEXT, METAVAR, VALUES, SPELLING, SHOULD_PARSE, ALWAYS_EMIT, KEYPATH,   \
    DEFAULT_VALUE, IMPLIED_CHECK, IMPLIED_VALUE, NORMALIZER, DENORMALIZER,     \
    MERGER, EXTRACTOR, TABLE_INDEX)                                            \
  PARSE_OPTION_WITH_MARSHALLING(Args, Diags, Success, ID, FLAGS, PARAM,        \
                                SHOULD_PARSE, KEYPATH, DEFAULT_VALUE,          \
                                IMPLIED_CHECK, IMPLIED_VALUE, NORMALIZER,      \
                                MERGER, TABLE_INDEX)
#include "clang/Driver/Options.inc"
#undef LANG_OPTION_WITH_MARSHALLING

  // ...

  return Success;
}

void CompilerInvocation::GenerateLangArgs(LangOptions *LangOpts,
                                          SmallVectorImpl<const char *> &Args,
                                          StringAllocator SA) {
#define LANG_OPTION_WITH_MARSHALLING(                                          \
    PREFIX_TYPE, NAME, ID, KIND, GROUP, ALIAS, ALIASARGS, FLAGS, PARAM,        \
    HELPTEXT, METAVAR, VALUES, SPELLING, SHOULD_PARSE, ALWAYS_EMIT, KEYPATH,   \
    DEFAULT_VALUE, IMPLIED_CHECK, IMPLIED_VALUE, NORMALIZER, DENORMALIZER,     \
    MERGER, EXTRACTOR, TABLE_INDEX)                                            \
  GENERATE_OPTION_WITH_MARSHALLING(                                            \
      Args, SA, KIND, FLAGS, SPELLING, ALWAYS_EMIT, KEYPATH, DEFAULT_VALUE,    \
      IMPLIED_CHECK, IMPLIED_VALUE, DENORMALIZER, EXTRACTOR, TABLE_INDEX)
#include "clang/Driver/Options.inc"
#undef LANG_OPTION_WITH_MARSHALLING

  // ...
}

The PARSE_OPTION_WITH_MARSHALLING and GENERATE_OPTION_WITH_MARSHALLING macros are defined in CompilerInvocation.cpp and they implement the generic algorithm for parsing and generating command line arguments.

Option Marshalling Annotations ¶

How does the tablegen backend know what to put in place of [...] in the generated Options.inc? This is specified by the Marshalling utilities described below. All of them take a key path argument and possibly other information required for parsing or generating the command line argument.

Note: The marshalling infrastructure is not intended for driver-only options. Only options of the -cc1 frontend need to be marshalled to/from CompilerInvocation instance.

Positive Flag

The key path defaults to false and is set to true when the flag is present on command line.

def fignore_exceptions : Flag<["-"], "fignore-exceptions">, Flags<[CC1Option]>,
  MarshallingInfoFlag<LangOpts<"IgnoreExceptions">>;

Negative Flag

The key path defaults to true and is set to false when the flag is present on command line.

def fno_verbose_asm : Flag<["-"], "fno-verbose-asm">, Flags<[CC1Option]>,
  MarshallingInfoNegativeFlag<CodeGenOpts<"AsmVerbose">>;

Negative and Positive Flag

The key path defaults to the specified value (false, true or some boolean value that’s statically unknown in the tablegen file). Then, the key path is set to the value associated with the flag that appears last on command line.

defm legacy_pass_manager : BoolOption<"f", "legacy-pass-manager",
  CodeGenOpts<"LegacyPassManager">, DefaultFalse,
  PosFlag<SetTrue, [], "Use the legacy pass manager in LLVM">,
  NegFlag<SetFalse, [], "Use the new pass manager in LLVM">,
  BothFlags<[CC1Option]>>;

With most such pair of flags, the -cc1 frontend accepts only the flag that changes the default key path value. The Clang driver is responsible for accepting both and either forwarding the changing flag or discarding the flag that would just set the key path to its default.

The first argument to BoolOption is a prefix that is used to construct the full names of both flags. The positive flag would then be named flegacy-pass-manager and the negative fno-legacy-pass-manager. BoolOption also implies the - prefix for both flags. It’s also possible to use BoolFOption that implies the "f" prefix and Group<f_Group>. The PosFlag and NegFlag classes hold the associated boolean value, an array of elements passed to the Flag class and the help text. The optional BothFlags class holds an array of Flag elements that are common for both the positive and negative flag and their common help text suffix.

String

The key path defaults to the specified string, or an empty one, if omitted. When the option appears on the command line, the argument value is simply copied.

def isysroot : JoinedOrSeparate<["-"], "isysroot">, Flags<[CC1Option]>,
  MarshallingInfoString<HeaderSearchOpts<"Sysroot">, [{"/"}]>;

List of Strings

The key path defaults to an empty std::vector<std::string>. Values specified with each appearance of the option on the command line are appended to the vector.

def frewrite_map_file : Separate<["-"], "frewrite-map-file">, Flags<[CC1Option]>,
  MarshallingInfoStringVector<CodeGenOpts<"RewriteMapFiles">>;

Integer

The key path defaults to the specified integer value, or 0 if omitted. When the option appears on the command line, its value gets parsed by llvm::APInt and the result is assigned to the key path on success.

def mstack_probe_size : Joined<["-"], "mstack-probe-size=">, Flags<[CC1Option]>,
  MarshallingInfoInt<CodeGenOpts<"StackProbeSize">, "4096">;

Enumeration

The key path defaults to the value specified in MarshallingInfoEnum prefixed by the contents of NormalizedValuesScope and ::. This ensures correct reference to an enum case is formed even if the enum resides in different namespace or is an enum class. If the value present on command line does not match any of the comma-separated values from Values, an error diagnostics is issued. Otherwise, the corresponding element from NormalizedValues at the same index is assigned to the key path (also correctly scoped). The number of comma-separated string values and elements of the array within NormalizedValues must match.

def mthread_model : Separate<["-"], "mthread-model">, Flags<[CC1Option]>,
  Values<"posix,single">, NormalizedValues<["POSIX", "Single"]>,
  NormalizedValuesScope<"LangOptions::ThreadModelKind">,
  MarshallingInfoEnum<LangOpts<"ThreadModel">, "POSIX">;

It is also possible to define relationships between options.

Implication

The key path defaults to the default value from the primary Marshalling annotation. Then, if any of the elements of ImpliedByAnyOf evaluate to true, the key path value is changed to the specified value or true if missing. Finally, the command line is parsed according to the primary annotation.

def fms_extensions : Flag<["-"], "fms-extensions">, Flags<[CC1Option]>,
  MarshallingInfoFlag<LangOpts<"MicrosoftExt">>,
  ImpliedByAnyOf<[fms_compatibility.KeyPath], "true">;

Condition

The option is parsed only if the expression in ShouldParseIf evaluates to true.

def fopenmp_enable_irbuilder : Flag<["-"], "fopenmp-enable-irbuilder">, Flags<[CC1Option]>,
  MarshallingInfoFlag<LangOpts<"OpenMPIRBuilder">>,
  ShouldParseIf<fopenmp.KeyPath>;

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 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 Lexer class) and a buffered token stream (provided by the TokenLexer class).

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: annotation tokens 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:
1. StartOfLine — This was the first token that occurred on its input source line.
2. 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.
3. 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.
4. 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”.

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.

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 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.

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.

The `MultipleIncludeOpt` class ¶

The MultipleIncludeOpt class implements a really simple little state machine that is used to detect the standard “#ifndef XX / #define XX” idiom that people typically use to prevent multiple inclusion of headers. If a buffer uses this idiom and is subsequently #include’d, the preprocessor can simply check to see whether the guarding condition is defined or not. If so, the preprocessor can completely ignore the include of the header.

The Parser Library ¶

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 Sema library. However, the Parser still accesses AST objects only through opaque types like ExprResult and StmtResult. Only Sema looks at the AST node contents of these wrappers.

The AST Library ¶

Design philosophy ¶

Immutability ¶

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

Canonicalization of the “meaning” of nodes is possible as soon as the nodes are created, and is not invalidated by later addition of more information. For example, we canonicalize types, and use a canonicalized representation of expressions when determining whether two function template declarations involving dependent expressions declare the same entity.

AST nodes can be reused when they have the same meaning. For example, we reuse Type nodes when representing the same type (but maintain separate TypeLocs for each instance where a type is written), and we reuse non-dependent Stmt and Expr nodes across instantiations of a template.

Serialization and deserialization of the AST to/from AST files is simpler: we do not need to track modifications made to AST nodes imported from AST files and serialize separate “update records”.

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

The first declaration of a redeclarable entity maintains a pointer to the most recent declaration of that entity, which naturally needs to change as more declarations are parsed.

Name lookup tables in declaration contexts change after the namespace declaration is formed.

We attempt to maintain only a single declaration for an instantiation of a template, rather than having distinct declarations for an instantiation of the declaration versus the definition, so template instantiation often updates parts of existing declarations.

Some parts of declarations are required to be instantiated separately (this includes default arguments and exception specifications), and such instantiations update the existing declaration.

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

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

Faithfulness ¶

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

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

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

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 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:

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:

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 :).

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 (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.

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 type 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.