### 15.28 C++ ABI parameters

— Target Hook: tree TARGET_CXX_GUARD_TYPE (void)

> Define this hook to override the integer type used for guard variables. These are used to implement one-time construction of static objects. The default is long_long_integer_type_node.

— Target Hook: bool TARGET_CXX_GUARD_MASK_BIT (void)

> This hook determines how guard variables are used. It should return `false` (the default) if first byte should be used. A return value of `true` indicates the least significant bit should be used.

— Target Hook: tree TARGET_CXX_GET_COOKIE_SIZE (tree type)

> This hook returns the size of the cookie to use when allocating an array whose elements have the indicated type. Assumes that it is already known that a cookie is needed. The default is `max(sizeof (size_t), alignof(type))`, as defined in section 2.7 of the IA64/Generic C++ ABI.

— Target Hook: bool TARGET_CXX_COOKIE_HAS_SIZE (void)

> This hook should return `true` if the element size should be stored in array cookies. The default is to return `false`.

— Target Hook: int TARGET_CXX_IMPORT_EXPORT_CLASS (tree type, int import_export)

> If defined by a backend this hook allows the decision made to export class type to be overruled. Upon entry import_export will contain 1 if the class is going to be exported, −1 if it is going to be imported and 0 otherwise. This function should return the modified value and perform any other actions necessary to support the backend's targeted operating system.

— Target Hook: bool TARGET_CXX_CDTOR_RETURNS_THIS (void)

> This hook should return `true` if constructors and destructors return the address of the object created/destroyed. The default is to return `false`.

— Target Hook: bool TARGET_CXX_KEY_METHOD_MAY_BE_INLINE (void)

> This hook returns true if the key method for a class (i.e., the method which, if defined in the current translation unit, causes the virtual table to be emitted) may be an inline function. Under the standard Itanium C++ ABI the key method may be an inline function so long as the function is not declared inline in the class definition. Under some variants of the ABI, an inline function can never be the key method. The default is to return `true`.

— Target Hook: void TARGET_CXX_DETERMINE_CLASS_DATA_VISIBILITY (tree decl)

> decl is a virtual table, virtual table table, typeinfo object, or other similar implicit class data object that will be emitted with external linkage in this translation unit. No ELF visibility has been explicitly specified. If the target needs to specify a visibility other than that of the containing class, use this hook to set `DECL_VISIBILITY` and `DECL_VISIBILITY_SPECIFIED`.

— Target Hook: bool TARGET_CXX_CLASS_DATA_ALWAYS_COMDAT (void)

> This hook returns true (the default) if virtual tables and other similar implicit class data objects are always COMDAT if they have external linkage. If this hook returns false, then class data for classes whose virtual table will be emitted in only one translation unit will not be COMDAT.

— Target Hook: bool TARGET_CXX_USE_AEABI_ATEXIT (void)

> This hook returns true if `__aeabi_atexit` (as defined by the ARM EABI) should be used to register static destructors when `-fuse-cxa-atexit` is in effect. The default is to return false to use `__cxa_atexit`.

— Target Hook: bool TARGET_CXX_USE_ATEXIT_FOR_CXA_ATEXIT (void)

> This hook returns true if the target `atexit` function can be used in the same manner as `__cxa_atexit` to register C++ static destructors. This requires that `atexit`-registered functions in shared libraries are run in the correct order when the libraries are unloaded. The default is to return false.

— Target Hook: void TARGET_CXX_ADJUST_CLASS_AT_DEFINITION (tree type)

> type is a C++ class (i.e., RECORD_TYPE or UNION_TYPE) that has just been defined. Use this hook to make adjustments to the class (eg, tweak visibility or perform any other required target modifications).