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(gcc-300.info)Per-Function Data

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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 initialise some or all of the function
pointers `init_machine_status', `free_machine_status' and
`mark_machine_status'.  These pointers are 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.

   The macro and function pointers are described below.

`INIT_EXPANDERS'
     Macro called to initialise 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
     initialisation of the function pointers below.

`init_machine_status'
     This is a `void (*)(struct function *)' function pointer.  If this
     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 initialisation of the `struct
     function' structure.  It is intended that this would be used to
     initialise the `machine' of that structure.

`free_machine_status'
     This is a `void (*)(struct function *)' function pointer.  If this
     pointer is non-`NULL' it will be called once per function, after
     the function has been compiled, in order to allow any memory
     allocated during the `init_machine_status' function call to be
     freed.

`mark_machine_status'
     This is a `void (*)(struct function *)' function pointer.  If this
     pointer is non-`NULL' it will be called once per function in order
     to mark any data items in the `struct machine_function' structure
     which need garbage collection.