GNU Compiler Collection (GCC) 4.2 Internals

15.4 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.