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 DiagnosticClient information about what the argument means without requiring it to use a specific presentation (consider this MVC for Clang :).

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

“s” format

Example:

"requires %1 parameter%s1"

Class:

Integers

Description:

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

“select” format

Example:

"must be a %select{unary|binary|unary or binary}2 operator"

Class:

Integers

Description:

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

“plural” format

Example:

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

Class:

Integers

Description:

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

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

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

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

“ordinal” format

Example:

"ambiguity in %ordinal0 argument"

Class:

Integers

Description:

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

“objcclass” format

Example:

"method %objcclass0 not found"

Class:

DeclarationName

Description:

This is a simple formatter that indicates the DeclarationName corresponds to an Objective-C class method selector. As such, it prints the selector with a leading “+”.

“objcinstance” format

Example:

"method %objcinstance0 not found"

Class:

DeclarationName

Description:

This is a simple formatter that indicates the DeclarationName corresponds to an Objective-C instance method selector. As such, it prints the selector with a leading “-”.

“q” format

Example:

"candidate found by name lookup is %q0"

Class:

NamedDecl *

Description:

This formatter indicates that the fully-qualified name of the declaration should be printed, e.g., “std::vector” rather than “vector”.

“diff” format

Example:

"no known conversion %diff{from $ to $|from argument type to parameter type}1,2"

Class:

QualType

Description:

This formatter takes two 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.

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.

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 DiagnosticClient Interface¶

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

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

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

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.

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 :).

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:

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.,

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.

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:

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:

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.,

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.,

Color c = Red;

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

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 DeclContexts are:

• Enumerations (but not C++11 “scoped enumerations”):

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

• C++ linkage specifications:

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

• Anonymous unions and structs:

  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:

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

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:

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

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

In Clang’s representation, the source-centric view of declaration contexts will actually have two separate NamespaceDecl nodes in Snippet #1, each of which is a declaration context that contains a single declaration of “f”. However, the semantics-centric view provided by name lookup into the namespace N for “f” will return a DeclContext::lookup_result that contains a range of iterators over declarations of “f”.

DeclContext manages multiply-defined declaration contexts internally. The function DeclContext::getPrimaryContext retrieves the “primary” context for a given DeclContext instance, which is the DeclContext responsible for maintaining the lookup table used for the semantics-centric view. Given the primary context, one can follow the chain of DeclContext nodes that define additional declarations via DeclContext::getNextContext. Note that these functions are used internally within the lookup and insertion methods of the DeclContext, so the vast majority of clients can ignore them.

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. Conditional control-flow 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.

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:

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:

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():

[ 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.

Implementation Approach¶

After trying several different approaches, we’ve finally converged on a design (Note, at the time of this writing, not all of this has been implemented, consider this a design goal!). Our basic approach is to define a single recursive method evaluation method (Expr::Evaluate), which is implemented in AST/ExprConstant.cpp. Given an expression with “scalar” type (integer, fp, complex, or pointer) this method returns the following information:

• Whether the expression is an integer constant expression, a general constant that was folded but has no side effects, a general constant that was folded but that does have side effects, or an uncomputable/unfoldable value.
• If the expression was computable in any way, this method returns the APValue for the result of the expression.
• If the expression is not evaluatable at all, this method returns information on one of the problems with the expression. This includes a SourceLocation for where the problem is, and a diagnostic ID that explains the problem. The diagnostic should have ERROR type.
• If the expression is not an integer constant expression, this method returns information on one of the problems with the expression. This includes a SourceLocation for where the problem is, and a diagnostic ID that explains the problem. The diagnostic should have EXTENSION type.

This information gives various clients the flexibility that they want, and we will eventually have some helper methods for various extensions. For example, Sema should have a Sema::VerifyIntegerConstantExpression method, which calls Evaluate on the expression. If the expression is not foldable, the error is emitted, and it would return true. If the expression is not an i-c-e, the EXTENSION diagnostic is emitted. Finally it would return false to indicate that the AST is OK.

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

Extensions¶

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

• __extension__: The expression form of this extension causes any evaluatable subexpression to be accepted as an integer constant expression.
• __builtin_constant_p: This returns true (as an integer constant expression) if the operand evaluates to either a numeric value (that is, not a pointer cast to integral type) of integral, enumeration, floating or complex type, or if it evaluates to the address of the first character of a string literal (possibly cast to some other type). As a special case, if __builtin_constant_p is the (potentially parenthesized) condition of a conditional operator expression (“?:”), only the true side of the conditional operator is considered, and it is evaluated with full constant folding.
• __builtin_choose_expr: The condition is required to be an integer constant expression, but we accept any constant as an “extension of an extension”. This only evaluates one operand depending on which way the condition evaluates.
• __builtin_classify_type: This always returns an integer constant expression.
• __builtin_inf, nan, ...: These are treated just like a floating-point literal.
• __builtin_abs, copysign, ...: These are constant folded as general constant expressions.
• __builtin_strlen and strlen: These are constant folded as integer constant expressions if the argument is a string literal.

Attribute Basics¶

Attributes in clang come in two forms: parsed form, and semantic form. Both forms are represented via a tablegen definition of the attribute, specified in Attr.td.

include/clang/Basic/Attr.td¶

First, add your attribute to the include/clang/Basic/Attr.td file.

Each attribute gets a def inheriting from Attr or one of its subclasses. InheritableAttr means that the attribute also applies to subsequent declarations of the same name. InheritableParamAttr is similar to InheritableAttr, except that the attribute is written on a parameter instead of a declaration, type or statement. Attributes inheriting from TypeAttr are pure type attributes which generally are not given a representation in the AST. Attributes inheriting from TargetSpecificAttr are attributes specific to one or more target architectures. An attribute that inherits from IgnoredAttr is parsed, but will generate an ignored attribute diagnostic when used. The attribute type may be useful when an attribute is supported by another vendor, but not supported by clang.

Spellings lists the strings that can appear in __attribute__((here)) or [[here]]. All such strings will be synonymous. Possible Spellings are: GNU (for use with GNU-style __attribute__ spellings), Declspec (for use with Microsoft Visual Studio-style __declspec spellings), CXX11` (for use with C++11-style [[foo]] and [[foo::bar]] spellings), and ``Keyword (for use with attributes that are implemented as keywords, like C++11’s override or final). If you want to allow the [[]] C++11 syntax, you have to define a list of Namespaces, which will let users write [[namespace::spelling]]. Using the empty string for a namespace will allow users to write just the spelling with no “::”. Attributes which g++-4.8 or later accepts should also have a CXX11<"gnu", "spelling"> spelling.

Subjects restricts what kinds of AST node to which this attribute can appertain (roughly, attach). The subjects are specified via a SubjectList, which specify the list of subjects. Additionally, subject-related diagnostics can be specified to be warnings or errors, with the default being a warning. 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/AttributeList.h If you add new Decl nodes to the SubjectList, you may need to update the logic used to automatically determine the diagnostic parameter in utils/TableGen/ClangAttrEmitter.cpp.

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

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.

Args names the arguments the attribute takes, in order. If Args is [StringArgument<"Arg1">, IntArgument<"Arg2">] then __attribute__((myattribute("Hello", 3))) will be a valid use. Attribute arguments specify both the parsed form and the semantic form of the attribute. The previous example shows an attribute which requires two attributes while parsing, and the Attr subclass’ constructor for the attribute will require a string and integer argument.

Diagnostic checking for argument counts is automated except when HasCustomParsing is set to 1, or when the attribute uses an optional or variadic argument. Diagnostic checking for argument semantics is not automated.

If the parsed form of the attribute is more complex, or differs from the semantic form, the HasCustomParsing bit can be set to 1 for the class, and the parsing code in Parser::ParseGNUAttributeArgs can be updated for the special case. Note that this only applies to arguments with a GNU spelling – attributes with a __declspec spelling currently ignore this flag and are handled by Parser::ParseMicrosoftDeclSpec.

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

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

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

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

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

By default, when declarations are merging attributes, an attribute will not be duplicated. However, if an attribute can be duplicated during this merging stage, set DuplicatesAllowedWhileMerging to 1, and the attribute will be merged.

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

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

All attributes must have one or more form of documentation, which is provided in the Documentation list. Generally, the documentation for an attribute is a stand-alone definition in include/clang/Basic/AttrDocs.td that is named after the attribute being documented. Each documentation element is given a Category (variable, function, or type) and Content. A single attribute may contain multiple documentation elements for distinct categories. For instance, an attribute which can appertain to both function and types (such as a calling convention attribute), should contain two documentation elements. The Content for an attribute uses reStructuredText (RST) syntax.

If an attribute is used internally by the compiler, but is not written by users (such as attributes with an empty spelling list), it can use the Undocumented documentation element.

Boilerplate¶

All semantic processing of declaration attributes happens in lib/Sema/SemaDeclAttr.cpp, and generally starts in the ProcessDeclAttribute function. If your attribute is a “simple” attribute – meaning that it requires no custom semantic processing aside from what is automatically provided for you, you can add a call to handleSimpleAttribute<YourAttr>(S, D, Attr); to the switch statement. Otherwise, write a new handleYourAttr() function, and add that to the switch statement.

If your attribute causes extra warnings to fire, define a DiagGroup in include/clang/Basic/DiagnosticGroups.td named after the attribute’s Spelling with “_”s replaced by “-“s. If you’re only defining one diagnostic, you can skip DiagnosticGroups.td and use InGroup<DiagGroup<"your-attribute">> directly in DiagnosticSemaKinds.td

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

The meat of your attribute¶

Find an appropriate place in Clang to do whatever your attribute needs to do. Check for the attribute’s presence using Decl::getAttr<YourAttr>().

Update the Clang Language Extensions document to describe your new attribute.