GNU Compiler Collection (GCC) Internals

8.5 Declarations

This section covers the various kinds of declarations that appear in the internal representation, except for declarations of functions (represented by FUNCTION_DECL nodes), which are described in Functions.

Some macros can be used with any kind of declaration. These include:

DECL_NAME

This macro returns an IDENTIFIER_NODE giving the name of the entity.

TREE_TYPE

This macro returns the type of the entity declared.

TREE_FILENAME

This macro returns the name of the file in which the entity was declared, as a char*. For an entity declared implicitly by the compiler (like __builtin_memcpy), this will be the string "<internal>".

TREE_LINENO

This macro returns the line number at which the entity was declared, as an int.

DECL_ARTIFICIAL

This predicate holds if the declaration was implicitly generated by the compiler. For example, this predicate will hold of an implicitly declared member function, or of the TYPE_DECL implicitly generated for a class type. Recall that in C++ code like:

          struct S {};
     

is roughly equivalent to C code like:

          struct S {};
          typedef struct S S;
     

The implicitly generated typedef declaration is represented by a TYPE_DECL for which DECL_ARTIFICIAL holds.

DECL_NAMESPACE_SCOPE_P

This predicate holds if the entity was declared at a namespace scope.

DECL_CLASS_SCOPE_P

This predicate holds if the entity was declared at a class scope.

DECL_FUNCTION_SCOPE_P

This predicate holds if the entity was declared inside a function body.

The various kinds of declarations include:

LABEL_DECL

These nodes are used to represent labels in function bodies. For more information, see Functions. These nodes only appear in block scopes.

CONST_DECL

These nodes are used to represent enumeration constants. The value of the constant is given by DECL_INITIAL which will be an INTEGER_CST with the same type as the TREE_TYPE of the CONST_DECL, i.e., an ENUMERAL_TYPE.

RESULT_DECL

These nodes represent the value returned by a function. When a value is assigned to a RESULT_DECL, that indicates that the value should be returned, via bitwise copy, by the function. You can use DECL_SIZE and DECL_ALIGN on a RESULT_DECL, just as with a VAR_DECL.

TYPE_DECL

These nodes represent typedef declarations. The TREE_TYPE is the type declared to have the name given by DECL_NAME. In some cases, there is no associated name.

VAR_DECL

These nodes represent variables with namespace or block scope, as well as static data members. The DECL_SIZE and DECL_ALIGN are analogous to TYPE_SIZE and TYPE_ALIGN. For a declaration, you should always use the DECL_SIZE and DECL_ALIGN rather than the TYPE_SIZE and TYPE_ALIGN given by the TREE_TYPE, since special attributes may have been applied to the variable to give it a particular size and alignment. You may use the predicates DECL_THIS_STATIC or DECL_THIS_EXTERN to test whether the storage class specifiers static or extern were used to declare a variable.

If this variable is initialized (but does not require a constructor), the DECL_INITIAL will be an expression for the initializer. The initializer should be evaluated, and a bitwise copy into the variable performed. If the DECL_INITIAL is the error_mark_node, there is an initializer, but it is given by an explicit statement later in the code; no bitwise copy is required.

GCC provides an extension that allows either automatic variables, or global variables, to be placed in particular registers. This extension is being used for a particular VAR_DECL if DECL_REGISTER holds for the VAR_DECL, and if DECL_ASSEMBLER_NAME is not equal to DECL_NAME. In that case, DECL_ASSEMBLER_NAME is the name of the register into which the variable will be placed.

PARM_DECL

Used to represent a parameter to a function. Treat these nodes similarly to VAR_DECL nodes. These nodes only appear in the DECL_ARGUMENTS for a FUNCTION_DECL.

The DECL_ARG_TYPE for a PARM_DECL is the type that will actually be used when a value is passed to this function. It may be a wider type than the TREE_TYPE of the parameter; for example, the ordinary type might be short while the DECL_ARG_TYPE is int.

FIELD_DECL

These nodes represent non-static data members. The DECL_SIZE and DECL_ALIGN behave as for VAR_DECL nodes. The DECL_FIELD_BITPOS gives the first bit used for this field, as an INTEGER_CST. These values are indexed from zero, where zero indicates the first bit in the object.

If DECL_C_BIT_FIELD holds, this field is a bit-field.

NAMESPACE_DECL

See Namespaces.

TEMPLATE_DECL

These nodes are used to represent class, function, and variable (static data member) templates. The DECL_TEMPLATE_SPECIALIZATIONS are a TREE_LIST. The TREE_VALUE of each node in the list is a TEMPLATE_DECLs or FUNCTION_DECLs representing specializations (including instantiations) of this template. Back ends can safely ignore TEMPLATE_DECLs, but should examine FUNCTION_DECL nodes on the specializations list just as they would ordinary FUNCTION_DECL nodes.

For a class template, the DECL_TEMPLATE_INSTANTIATIONS list contains the instantiations. The TREE_VALUE of each node is an instantiation of the class. The DECL_TEMPLATE_SPECIALIZATIONS contains partial specializations of the class.

USING_DECL

Back ends can safely ignore these nodes.