### 17.24 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: bool TARGET_ATTRIBUTE_TAKES_IDENTIFIER_P (const_tree name)

> If defined, this target hook is a function which returns true if the machine-specific attribute named name expects an identifier given as its first argument to be passed on as a plain identifier, not subjected to name lookup. If this is not defined, the default is false for all machine-specific attributes.

— Target Hook: int TARGET_COMP_TYPE_ATTRIBUTES (const_tree type1, const_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 the 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 (const_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 (const_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.

— Target Hook: bool TARGET_OPTION_VALID_ATTRIBUTE_P (tree fndecl, tree name, tree args, int flags)

> This hook is called to parse `attribute(target("..."))`, which allows setting target-specific options on individual functions. These function-specific options may differ from the options specified on the command line. The hook should return `true` if the options are valid.

> The hook should set the `DECL_FUNCTION_SPECIFIC_TARGET` field in the function declaration to hold a pointer to a target-specific `struct cl_target_option` structure.

— Target Hook: void TARGET_OPTION_SAVE (struct cl_target_option *ptr, struct gcc_options *opts)

> This hook is called to save any additional target-specific information in the `struct cl_target_option` structure for function-specific options from the `struct gcc_options` structure. See Option file format.

— Target Hook: void TARGET_OPTION_RESTORE (struct gcc_options *opts, struct cl_target_option *ptr)

> This hook is called to restore any additional target-specific information in the `struct cl_target_option` structure for function-specific options to the `struct gcc_options` structure.

— Target Hook: void TARGET_OPTION_POST_STREAM_IN (struct cl_target_option *ptr)

> This hook is called to update target-specific information in the `struct cl_target_option` structure after it is streamed in from LTO bytecode.

— Target Hook: void TARGET_OPTION_PRINT (FILE *file, int indent, struct cl_target_option *ptr)

> This hook is called to print any additional target-specific information in the `struct cl_target_option` structure for function-specific options.

— Target Hook: bool TARGET_OPTION_PRAGMA_PARSE (tree args, tree pop_target)

> This target hook parses the options for `#pragma GCC target`, which sets the target-specific options for functions that occur later in the input stream. The options accepted should be the same as those handled by the `TARGET_OPTION_VALID_ATTRIBUTE_P` hook.

— Target Hook: void TARGET_OPTION_OVERRIDE (void)

> Sometimes certain combinations of command options do not make sense on a particular target machine. You can override the hook `TARGET_OPTION_OVERRIDE` to take account of this. This hooks is called once just after all the command options have been parsed.

> Don't use this hook to turn on various extra optimizations for `-O`. That is what `TARGET_OPTION_OPTIMIZATION` is for.

> If you need to do something whenever the optimization level is changed via the optimize attribute or pragma, see `TARGET_OVERRIDE_OPTIONS_AFTER_CHANGE`

— Target Hook: bool TARGET_OPTION_FUNCTION_VERSIONS (tree decl1, tree decl2)

> This target hook returns `true` if DECL1 and DECL2 are versions of the same function. DECL1 and DECL2 are function versions if and only if they have the same function signature and different target specific attributes, that is, they are compiled for different target machines.

— Target Hook: bool TARGET_CAN_INLINE_P (tree caller, tree callee)

> This target hook returns `false` if the caller function cannot inline callee, based on target specific information. By default, inlining is not allowed if the callee function has function specific target options and the caller does not use the same options.