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
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
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
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
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
If this function pointer is non-
NULLit 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 functionstructure. It is intended that this would be used to initialize the
machineof that structure.
struct machine_functionstructures are expected to be freed by GC. Generally, any memory that they reference must be allocated by using
ggc_alloc, including the structure itself.