17.9.1 Basic Stack Layout

Here is the basic stack layout.

Macro: STACK_GROWS_DOWNWARD

Define this macro to be true if pushing a word onto the stack moves the stack pointer to a smaller address, and false otherwise.

Macro: STACK_PUSH_CODE

This macro defines the operation used when something is pushed on the stack. In RTL, a push operation will be (set (mem (STACK_PUSH_CODE (reg sp))) …)

The choices are PRE_DEC, POST_DEC, PRE_INC, and POST_INC. Which of these is correct depends on the stack direction and on whether the stack pointer points to the last item on the stack or whether it points to the space for the next item on the stack.

The default is PRE_DEC when STACK_GROWS_DOWNWARD is true, which is almost always right, and PRE_INC otherwise, which is often wrong.

Macro: FRAME_GROWS_DOWNWARD

Define this macro to nonzero value if the addresses of local variable slots are at negative offsets from the frame pointer.

Macro: ARGS_GROW_DOWNWARD

Define this macro if successive arguments to a function occupy decreasing addresses on the stack.

Macro: STARTING_FRAME_OFFSET

Offset from the frame pointer to the first local variable slot to be allocated.

If FRAME_GROWS_DOWNWARD, find the next slot’s offset by subtracting the first slot’s length from STARTING_FRAME_OFFSET. Otherwise, it is found by adding the length of the first slot to the value STARTING_FRAME_OFFSET.

Macro: STACK_ALIGNMENT_NEEDED

Define to zero to disable final alignment of the stack during reload. The nonzero default for this macro is suitable for most ports.

On ports where STARTING_FRAME_OFFSET is nonzero or where there is a register save block following the local block that doesn’t require alignment to STACK_BOUNDARY, it may be beneficial to disable stack alignment and do it in the backend.

Macro: STACK_POINTER_OFFSET

Offset from the stack pointer register to the first location at which outgoing arguments are placed. If not specified, the default value of zero is used. This is the proper value for most machines.

If ARGS_GROW_DOWNWARD, this is the offset to the location above the first location at which outgoing arguments are placed.

Macro: FIRST_PARM_OFFSET (fundecl)

Offset from the argument pointer register to the first argument’s address. On some machines it may depend on the data type of the function.

If ARGS_GROW_DOWNWARD, this is the offset to the location above the first argument’s address.

Macro: STACK_DYNAMIC_OFFSET (fundecl)

Offset from the stack pointer register to an item dynamically allocated on the stack, e.g., by alloca.

The default value for this macro is STACK_POINTER_OFFSET plus the length of the outgoing arguments. The default is correct for most machines. See function.c for details.

Macro: INITIAL_FRAME_ADDRESS_RTX

A C expression whose value is RTL representing the address of the initial stack frame. This address is passed to RETURN_ADDR_RTX and DYNAMIC_CHAIN_ADDRESS. If you don’t define this macro, a reasonable default value will be used. Define this macro in order to make frame pointer elimination work in the presence of __builtin_frame_address (count) and __builtin_return_address (count) for count not equal to zero.

Macro: DYNAMIC_CHAIN_ADDRESS (frameaddr)

A C expression whose value is RTL representing the address in a stack frame where the pointer to the caller’s frame is stored. Assume that frameaddr is an RTL expression for the address of the stack frame itself.

If you don’t define this macro, the default is to return the value of frameaddr—that is, the stack frame address is also the address of the stack word that points to the previous frame.

Macro: SETUP_FRAME_ADDRESSES

A C expression that produces the machine-specific code to setup the stack so that arbitrary frames can be accessed. For example, on the SPARC, we must flush all of the register windows to the stack before we can access arbitrary stack frames. You will seldom need to define this macro. The default is to do nothing.

Target Hook: rtx TARGET_BUILTIN_SETJMP_FRAME_VALUE (void)

This target hook should return an rtx that is used to store the address of the current frame into the built in setjmp buffer. The default value, virtual_stack_vars_rtx, is correct for most machines. One reason you may need to define this target hook is if hard_frame_pointer_rtx is the appropriate value on your machine.

Macro: FRAME_ADDR_RTX (frameaddr)

A C expression whose value is RTL representing the value of the frame address for the current frame. frameaddr is the frame pointer of the current frame. This is used for __builtin_frame_address. You need only define this macro if the frame address is not the same as the frame pointer. Most machines do not need to define it.

Macro: RETURN_ADDR_RTX (count, frameaddr)

A C expression whose value is RTL representing the value of the return address for the frame count steps up from the current frame, after the prologue. frameaddr is the frame pointer of the count frame, or the frame pointer of the count - 1 frame if RETURN_ADDR_IN_PREVIOUS_FRAME is nonzero.

The value of the expression must always be the correct address when count is zero, but may be NULL_RTX if there is no way to determine the return address of other frames.

Macro: RETURN_ADDR_IN_PREVIOUS_FRAME

Define this macro to nonzero value if the return address of a particular stack frame is accessed from the frame pointer of the previous stack frame. The zero default for this macro is suitable for most ports.

Macro: INCOMING_RETURN_ADDR_RTX

A C expression whose value is RTL representing the location of the incoming return address at the beginning of any function, before the prologue. This RTL is either a REG, indicating that the return value is saved in ‘REG’, or a MEM representing a location in the stack.

You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2.

If this RTL is a REG, you should also define DWARF_FRAME_RETURN_COLUMN to DWARF_FRAME_REGNUM (REGNO).

Macro: DWARF_ALT_FRAME_RETURN_COLUMN

A C expression whose value is an integer giving a DWARF 2 column number that may be used as an alternative return column. The column must not correspond to any gcc hard register (that is, it must not be in the range of DWARF_FRAME_REGNUM).

This macro can be useful if DWARF_FRAME_RETURN_COLUMN is set to a general register, but an alternative column needs to be used for signal frames. Some targets have also used different frame return columns over time.

Macro: DWARF_ZERO_REG

A C expression whose value is an integer giving a DWARF 2 register number that is considered to always have the value zero. This should only be defined if the target has an architected zero register, and someone decided it was a good idea to use that register number to terminate the stack backtrace. New ports should avoid this.

Target Hook: void TARGET_DWARF_HANDLE_FRAME_UNSPEC (const char *label, rtx pattern, int index)

This target hook allows the backend to emit frame-related insns that contain UNSPECs or UNSPEC_VOLATILEs. The DWARF 2 call frame debugging info engine will invoke it on insns of the form

(set (reg) (unspec […] UNSPEC_INDEX))

and

(set (reg) (unspec_volatile […] UNSPECV_INDEX)).

to let the backend emit the call frame instructions. label is the CFI label attached to the insn, pattern is the pattern of the insn and index is UNSPEC_INDEX or UNSPECV_INDEX.

Macro: INCOMING_FRAME_SP_OFFSET

A C expression whose value is an integer giving the offset, in bytes, from the value of the stack pointer register to the top of the stack frame at the beginning of any function, before the prologue. The top of the frame is defined to be the value of the stack pointer in the previous frame, just before the call instruction.

You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2.

Macro: ARG_POINTER_CFA_OFFSET (fundecl)

A C expression whose value is an integer giving the offset, in bytes, from the argument pointer to the canonical frame address (cfa). The final value should coincide with that calculated by INCOMING_FRAME_SP_OFFSET. Which is unfortunately not usable during virtual register instantiation.

The default value for this macro is FIRST_PARM_OFFSET (fundecl) + crtl->args.pretend_args_size, which is correct for most machines; in general, the arguments are found immediately before the stack frame. Note that this is not the case on some targets that save registers into the caller’s frame, such as SPARC and rs6000, and so such targets need to define this macro.

You only need to define this macro if the default is incorrect, and you want to support call frame debugging information like that provided by DWARF 2.

Macro: FRAME_POINTER_CFA_OFFSET (fundecl)

If defined, a C expression whose value is an integer giving the offset in bytes from the frame pointer to the canonical frame address (cfa). The final value should coincide with that calculated by INCOMING_FRAME_SP_OFFSET.

Normally the CFA is calculated as an offset from the argument pointer, via ARG_POINTER_CFA_OFFSET, but if the argument pointer is variable due to the ABI, this may not be possible. If this macro is defined, it implies that the virtual register instantiation should be based on the frame pointer instead of the argument pointer. Only one of FRAME_POINTER_CFA_OFFSET and ARG_POINTER_CFA_OFFSET should be defined.

Macro: CFA_FRAME_BASE_OFFSET (fundecl)

If defined, a C expression whose value is an integer giving the offset in bytes from the canonical frame address (cfa) to the frame base used in DWARF 2 debug information. The default is zero. A different value may reduce the size of debug information on some ports.