### 15.25 Defining target-specific uses of `__attribute__`

Target-specific attributes may be defined for functions, data and types. These are described using the following target hooks; they also need to be documented in `extend.texi`.

— Target Hook: const struct attribute_spec * TARGET_ATTRIBUTE_TABLE

> If defined, this target hook points to an array of ``struct attribute_spec`' (defined in `tree.h`) specifying the machine specific attributes for this target and some of the restrictions on the entities to which these attributes are applied and the arguments they take.

— Target Hook: int TARGET_COMP_TYPE_ATTRIBUTES (tree type1, tree type2)

> If defined, this target hook is a function which returns zero if the attributes on type1 and type2 are incompatible, one if they are compatible, and two if they are nearly compatible (which causes a warning to be generated). If this is not defined, machine-specific attributes are supposed always to be compatible.

— Target Hook: void TARGET_SET_DEFAULT_TYPE_ATTRIBUTES (tree type)

> If defined, this target hook is a function which assigns default attributes to newly defined type.

— Target Hook: tree TARGET_MERGE_TYPE_ATTRIBUTES (tree type1, tree type2)

> Define this target hook if the merging of type attributes needs special handling. If defined, the result is a list of the combined `TYPE_ATTRIBUTES` of type1 and type2. It is assumed that `comptypes` has already been called and returned 1. This function may call `merge_attributes` to handle machine-independent merging.

— Target Hook: tree TARGET_MERGE_DECL_ATTRIBUTES (tree olddecl, tree newdecl)

> Define this target hook if the merging of decl attributes needs special handling. If defined, the result is a list of the combined `DECL_ATTRIBUTES` of olddecl and newdecl. newdecl is a duplicate declaration of olddecl. Examples of when this is needed are when one attribute overrides another, or when an attribute is nullified by a subsequent definition. This function may call `merge_attributes` to handle machine-independent merging.

> If the only target-specific handling you require is ``dllimport`' for Microsoft Windows targets, you should define the macro `TARGET_DLLIMPORT_DECL_ATTRIBUTES` to `1`. The compiler will then define a function called `merge_dllimport_decl_attributes` which can then be defined as the expansion of `TARGET_MERGE_DECL_ATTRIBUTES`. You can also add `handle_dll_attribute` in the attribute table for your port to perform initial processing of the ``dllimport`' and ``dllexport`' attributes. This is done in `i386/cygwin.h` and `i386/i386.c`, for example.

— Target Hook: bool TARGET_VALID_DLLIMPORT_ATTRIBUTE_P (tree decl)

> decl is a variable or function with `__attribute__((dllimport))` specified. Use this hook if the target needs to add extra validation checks to `handle_dll_attribute`.

— Macro: TARGET_DECLSPEC

> Define this macro to a nonzero value if you want to treat `__declspec(X)` as equivalent to `__attribute((X))`. By default, this behavior is enabled only for targets that define `TARGET_DLLIMPORT_DECL_ATTRIBUTES`. The current implementation of `__declspec` is via a built-in macro, but you should not rely on this implementation detail.

— Target Hook: void TARGET_INSERT_ATTRIBUTES (tree node, tree *attr_ptr)

> Define this target hook if you want to be able to add attributes to a decl when it is being created. This is normally useful for back ends which wish to implement a pragma by using the attributes which correspond to the pragma's effect. The node argument is the decl which is being created. The attr_ptr argument is a pointer to the attribute list for this decl. The list itself should not be modified, since it may be shared with other decls, but attributes may be chained on the head of the list and `*`attr_ptr modified to point to the new attributes, or a copy of the list may be made if further changes are needed.

— Target Hook: bool TARGET_FUNCTION_ATTRIBUTE_INLINABLE_P (tree fndecl)

> This target hook returns `true` if it is ok to inline fndecl into the current function, despite its having target-specific attributes, `false` otherwise. By default, if a function has a target specific attribute attached to it, it will not be inlined.