11.3 Types

All types have corresponding tree nodes. However, you should not assume that there is exactly one tree node corresponding to each type. There are often multiple nodes corresponding to the same type.

For the most part, different kinds of types have different tree codes. (For example, pointer types use a POINTER_TYPE code while arrays use an ARRAY_TYPE code.) However, pointers to member functions use the RECORD_TYPE code. Therefore, when writing a switch statement that depends on the code associated with a particular type, you should take care to handle pointers to member functions under the RECORD_TYPE case label.

The following functions and macros deal with cv-qualification of types:

TYPE_MAIN_VARIANT

This macro returns the unqualified version of a type. It may be applied to an unqualified type, but it is not always the identity function in that case.

A few other macros and functions are usable with all types:

TYPE_SIZE

The number of bits required to represent the type, represented as an INTEGER_CST. For an incomplete type, TYPE_SIZE will be NULL_TREE.

TYPE_ALIGN

The alignment of the type, in bits, represented as an int.

TYPE_NAME

This macro returns a declaration (in the form of a TYPE_DECL) for the type. (Note this macro does not return an IDENTIFIER_NODE, as you might expect, given its name!) You can look at the DECL_NAME of the TYPE_DECL to obtain the actual name of the type. The TYPE_NAME will be NULL_TREE for a type that is not a built-in type, the result of a typedef, or a named class type.

TYPE_CANONICAL

This macro returns the “canonical” type for the given type node. Canonical types are used to improve performance in the C++ and Objective-C++ front ends by allowing efficient comparison between two type nodes in same_type_p: if the TYPE_CANONICAL values of the types are equal, the types are equivalent; otherwise, the types are not equivalent. The notion of equivalence for canonical types is the same as the notion of type equivalence in the language itself. For instance,

When TYPE_CANONICAL is NULL_TREE, there is no canonical type for the given type node. In this case, comparison between this type and any other type requires the compiler to perform a deep, “structural” comparison to see if the two type nodes have the same form and properties.

The canonical type for a node is always the most fundamental type in the equivalence class of types. For instance, int is its own canonical type. A typedef I of int will have int as its canonical type. Similarly, I* and a typedef IP (defined to I*) will has int* as their canonical type. When building a new type node, be sure to set TYPE_CANONICAL to the appropriate canonical type. If the new type is a compound type (built from other types), and any of those other types require structural equality, use SET_TYPE_STRUCTURAL_EQUALITY to ensure that the new type also requires structural equality. Finally, if for some reason you cannot guarantee that TYPE_CANONICAL will point to the canonical type, use SET_TYPE_STRUCTURAL_EQUALITY to make sure that the new type–and any type constructed based on it–requires structural equality. If you suspect that the canonical type system is miscomparing types, pass --param verify-canonical-types=1 to the compiler or configure with --enable-checking to force the compiler to verify its canonical-type comparisons against the structural comparisons; the compiler will then print any warnings if the canonical types miscompare.

TYPE_STRUCTURAL_EQUALITY_P

This predicate holds when the node requires structural equality checks, e.g., when TYPE_CANONICAL is NULL_TREE.

SET_TYPE_STRUCTURAL_EQUALITY

This macro states that the type node it is given requires structural equality checks, e.g., it sets TYPE_CANONICAL to NULL_TREE.

same_type_p

This predicate takes two types as input, and holds if they are the same type. For example, if one type is a typedef for the other, or both are typedefs for the same type. This predicate also holds if the two trees given as input are simply copies of one another; i.e., there is no difference between them at the source level, but, for whatever reason, a duplicate has been made in the representation. You should never use == (pointer equality) to compare types; always use same_type_p instead.

Detailed below are the various kinds of types, and the macros that can be used to access them. Although other kinds of types are used elsewhere in G++, the types described here are the only ones that you will encounter while examining the intermediate representation.

VOID_TYPE

Used to represent the void type.

INTEGER_TYPE

Used to represent the various integral types, including char, short, int, long, and long long. This code is not used for enumeration types, nor for the bool type. The TYPE_PRECISION is the number of bits used in the representation, represented as an unsigned int. (Note that in the general case this is not the same value as TYPE_SIZE; suppose that there were a 24-bit integer type, but that alignment requirements for the ABI required 32-bit alignment. Then, TYPE_SIZE would be an INTEGER_CST for 32, while TYPE_PRECISION would be 24.) The integer type is unsigned if TYPE_UNSIGNED holds; otherwise, it is signed.

The TYPE_MIN_VALUE is an INTEGER_CST for the smallest integer that may be represented by this type. Similarly, the TYPE_MAX_VALUE is an INTEGER_CST for the largest integer that may be represented by this type.

REAL_TYPE

Used to represent the float, double, and long double types. The number of bits in the floating-point representation is given by TYPE_PRECISION, as in the INTEGER_TYPE case.

FIXED_POINT_TYPE

Used to represent the short _Fract, _Fract, long _Fract, long long _Fract, short _Accum, _Accum, long _Accum, and long long _Accum types. The number of bits in the fixed-point representation is given by TYPE_PRECISION, as in the INTEGER_TYPE case. There may be padding bits, fractional bits and integral bits. The number of fractional bits is given by TYPE_FBIT, and the number of integral bits is given by TYPE_IBIT. The fixed-point type is unsigned if TYPE_UNSIGNED holds; otherwise, it is signed. The fixed-point type is saturating if TYPE_SATURATING holds; otherwise, it is not saturating.

COMPLEX_TYPE

Used to represent GCC built-in __complex__ data types. The TREE_TYPE is the type of the real and imaginary parts.

ENUMERAL_TYPE

Used to represent an enumeration type. The TYPE_PRECISION gives (as an int), the number of bits used to represent the type. If there are no negative enumeration constants, TYPE_UNSIGNED will hold. The minimum and maximum enumeration constants may be obtained with TYPE_MIN_VALUE and TYPE_MAX_VALUE, respectively; each of these macros returns an INTEGER_CST.

The actual enumeration constants themselves may be obtained by looking at the TYPE_VALUES. This macro will return a TREE_LIST, containing the constants. The TREE_PURPOSE of each node will be an IDENTIFIER_NODE giving the name of the constant; the TREE_VALUE will be an INTEGER_CST giving the value assigned to that constant. These constants will appear in the order in which they were declared. The TREE_TYPE of each of these constants will be the type of enumeration type itself.

BOOLEAN_TYPE

Used to represent the bool type.

POINTER_TYPE

Used to represent pointer types, and pointer to data member types. The TREE_TYPE gives the type to which this type points.

REFERENCE_TYPE

Used to represent reference types. The TREE_TYPE gives the type to which this type refers.

FUNCTION_TYPE

Used to represent the type of non-member functions and of static member functions. The TREE_TYPE gives the return type of the function. The TYPE_ARG_TYPES are a TREE_LIST of the argument types. The TREE_VALUE of each node in this list is the type of the corresponding argument; the TREE_PURPOSE is an expression for the default argument value, if any. If the last node in the list is void_list_node (a TREE_LIST node whose TREE_VALUE is the void_type_node), then functions of this type do not take variable arguments. Otherwise, they do take a variable number of arguments.

Note that in C (but not in C++) a function declared like void f() is an unprototyped function taking a variable number of arguments; the TYPE_ARG_TYPES of such a function will be NULL.

METHOD_TYPE

Used to represent the type of a non-static member function. Like a FUNCTION_TYPE, the return type is given by the TREE_TYPE. The type of *this, i.e., the class of which functions of this type are a member, is given by the TYPE_METHOD_BASETYPE. The TYPE_ARG_TYPES is the parameter list, as for a FUNCTION_TYPE, and includes the this argument.

ARRAY_TYPE

Used to represent array types. The TREE_TYPE gives the type of the elements in the array. If the array-bound is present in the type, the TYPE_DOMAIN is an INTEGER_TYPE whose TYPE_MIN_VALUE and TYPE_MAX_VALUE will be the lower and upper bounds of the array, respectively. The TYPE_MIN_VALUE will always be an INTEGER_CST for zero, while the TYPE_MAX_VALUE will be one less than the number of elements in the array, i.e., the highest value which may be used to index an element in the array.

RECORD_TYPE

Used to represent struct and class types, as well as pointers to member functions and similar constructs in other languages. TYPE_FIELDS contains the items contained in this type, each of which can be a FIELD_DECL, VAR_DECL, CONST_DECL, or TYPE_DECL. You may not make any assumptions about the ordering of the fields in the type or whether one or more of them overlap.

UNION_TYPE

Used to represent union types. Similar to RECORD_TYPE except that all FIELD_DECL nodes in TYPE_FIELD start at bit position zero.

QUAL_UNION_TYPE

Used to represent part of a variant record in Ada. Similar to UNION_TYPE except that each FIELD_DECL has a DECL_QUALIFIER field, which contains a boolean expression that indicates whether the field is present in the object. The type will only have one field, so each field’s DECL_QUALIFIER is only evaluated if none of the expressions in the previous fields in TYPE_FIELDS are nonzero. Normally these expressions will reference a field in the outer object using a PLACEHOLDER_EXPR.

LANG_TYPE

This node is used to represent a language-specific type. The front end must handle it.

OFFSET_TYPE

This node is used to represent a pointer-to-data member. For a data member X::m the TYPE_OFFSET_BASETYPE is X and the TREE_TYPE is the type of m.

There are variables whose values represent some of the basic types. These include:

void_type_node

A node for void.

integer_type_node

A node for int.

unsigned_type_node.

A node for unsigned int.

char_type_node.

A node for char.

It may sometimes be useful to compare one of these variables with a type in hand, using same_type_p.