FRAME_POINTER_REQUIRED
The expression can in principle examine the current function and decide according to the facts, but on most machines the constant 0 or the constant 1 suffices. Use 0 when the machine allows code to be generated with no frame pointer, and doing so saves some time or space. Use 1 when there is no possible advantage to avoiding a frame pointer.
In certain cases, the compiler does not know how to produce valid code
without a frame pointer. The compiler recognizes those cases and
automatically gives the function a frame pointer regardless of what
FRAME_POINTER_REQUIRED
says. You don't need to worry about
them.
In a function that does not require a frame pointer, the frame pointer
register can be allocated for ordinary usage, unless you mark it as a
fixed register. See FIXED_REGISTERS
for more information.
INITIAL_FRAME_POINTER_OFFSET (depth-var)
get_frame_size ()
and the tables of
registers regs_ever_live
and call_used_regs
.
If ELIMINABLE_REGS
is defined, this macro will be not be used and
need not be defined. Otherwise, it must be defined even if
FRAME_POINTER_REQUIRED
is defined to always be true; in that
case, you may set depth-var to anything.
ELIMINABLE_REGS
The definition of this macro is a list of structure initializations, each of which specifies an original and replacement register.
On some machines, the position of the argument pointer is not known until the compilation is completed. In such a case, a separate hard register must be used for the argument pointer. This register can be eliminated by replacing it with either the frame pointer or the argument pointer, depending on whether or not the frame pointer has been eliminated.
In this case, you might specify:
#define ELIMINABLE_REGS \ {{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \ {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \ {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}
Note that the elimination of the argument pointer with the stack pointer is specified first since that is the preferred elimination.
CAN_ELIMINATE (from-reg, to-reg)
ELIMINABLE_REGS
is defined, and will usually be the constant 1, since most of the cases
preventing register elimination are things that the compiler already
knows about.
INITIAL_ELIMINATION_OFFSET (from-reg, to-reg, offset-var)
INITIAL_FRAME_POINTER_OFFSET
. It
specifies the initial difference between the specified pair of
registers. This macro must be defined if ELIMINABLE_REGS
is
defined.
LONGJMP_RESTORE_FROM_STACK
longjmp
function restores registers from
the stack frames, rather than from those saved specifically by
setjmp
. Certain quantities must not be kept in registers across
a call to setjmp
on such machines.