17.9.5 Eliminating Frame Pointer and Arg Pointer

This is about eliminating the frame pointer and arg pointer.

Target Hook: bool TARGET_FRAME_POINTER_REQUIRED (void)

This target hook should return true if a function must have and use a frame pointer. This target hook is called in the reload pass. If its return value is true the function will have a frame pointer.

This target hook can in principle examine the current function and decide according to the facts, but on most machines the constant false or the constant true suffices. Use false when the machine allows code to be generated with no frame pointer, and doing so saves some time or space. Use true 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 TARGET_FRAME_POINTER_REQUIRED returns. 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.

Default return value is false.

Macro: INITIAL_FRAME_POINTER_OFFSET (depth-var)

A C statement to store in the variable depth-var the difference between the frame pointer and the stack pointer values immediately after the function prologue. The value would be computed from information such as the result of 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 TARGET_FRAME_POINTER_REQUIRED always returns true; in that case, you may set depth-var to anything.

Macro: ELIMINABLE_REGS

If defined, this macro specifies a table of register pairs used to eliminate unneeded registers that point into the stack frame. If it is not defined, the only elimination attempted by the compiler is to replace references to the frame pointer with references to the stack pointer.

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.

Target Hook: bool TARGET_CAN_ELIMINATE (const int from_reg, const int to_reg)

This target hook should returns true if the compiler is allowed to try to replace register number from_reg with register number to_reg. This target hook need only be defined if ELIMINABLE_REGS is defined, and will usually be true, since most of the cases preventing register elimination are things that the compiler already knows about.

Default return value is true.

Macro: INITIAL_ELIMINATION_OFFSET (from-reg, to-reg, offset-var)

This macro is similar to 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.