| GNU Compiler Collection (GCC) Internals: Types |
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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.
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