GNU Compiler Collection (GCC) Internals: Types

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### 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 `typedef`s 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.

`OPAQUE_TYPE`

  Used for things that have a `MODE_OPAQUE` mode class in the backend. Opaque types have a size and precision, and can be held in memory or registers. They are used when we do not want the compiler to make assumptions about the availability of other operations as would happen with integer types.

`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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