### 17.4 Defining data structures for per-function information.

If the target needs to store information on a per-function basis, GCC provides a macro and a couple of variables to allow this. Note, just using statics to store the information is a bad idea, since GCC supports nested functions, so you can be halfway through encoding one function when another one comes along.

GCC defines a data structure called `struct function` which contains all of the data specific to an individual function. This structure contains a field called `machine` whose type is `struct machine_function *`, which can be used by targets to point to their own specific data.

If a target needs per-function specific data it should define the type `struct machine_function` and also the macro `INIT_EXPANDERS`. This macro should be used to initialize the function pointer `init_machine_status`. This pointer is explained below.

One typical use of per-function, target specific data is to create an RTX to hold the register containing the function's return address. This RTX can then be used to implement the `__builtin_return_address` function, for level 0.

Note—earlier implementations of GCC used a single data area to hold all of the per-function information. Thus when processing of a nested function began the old per-function data had to be pushed onto a stack, and when the processing was finished, it had to be popped off the stack. GCC used to provide function pointers called `save_machine_status` and `restore_machine_status` to handle the saving and restoring of the target specific information. Since the single data area approach is no longer used, these pointers are no longer supported.

— Macro: INIT_EXPANDERS

> Macro called to initialize any target specific information. This macro is called once per function, before generation of any RTL has begun. The intention of this macro is to allow the initialization of the function pointer `init_machine_status`.

— Variable: void (*)(struct function *) init_machine_status

> If this function pointer is non-`NULL` it will be called once per function, before function compilation starts, in order to allow the target to perform any target specific initialization of the `struct function` structure. It is intended that this would be used to initialize the `machine` of that structure.

> `struct machine_function` structures are expected to be freed by GC. Generally, any memory that they reference must be allocated by using GC allocation, including the structure itself.