GNU Compiler Collection (GCC) Internals: Working with declarations

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10.4.1 Working with declarations

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.

EXPR_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>".

EXPR_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.

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.

DEBUG_EXPR_DECL

Used to represent an anonymous debug-information temporary created to hold an expression as it is optimized away, so that its value can be referenced in debug bind statements.

FIELD_DECL

These nodes represent non-static data members. The DECL_SIZE and DECL_ALIGN behave as for VAR_DECL nodes. The position of the field within the parent record is specified by a combination of three attributes. DECL_FIELD_OFFSET is the position, counting in bytes, of the DECL_OFFSET_ALIGN-bit sized word containing the bit of the field closest to the beginning of the structure. DECL_FIELD_BIT_OFFSET is the bit offset of the first bit of the field within this word; this may be nonzero even for fields that are not bit-fields, since DECL_OFFSET_ALIGN may be greater than the natural alignment of the field’s type.

If DECL_C_BIT_FIELD holds, this field is a bit-field. In a bit-field, DECL_BIT_FIELD_TYPE also contains the type that was originally specified for it, while DECL_TYPE may be a modified type with lesser precision, according to the size of the bit field.

NAMESPACE_DECL

Namespaces provide a name hierarchy for other declarations. They appear in the DECL_CONTEXT of other _DECL nodes.

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