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#### 17.9.4 Registers That Address the Stack Frame

This discusses registers that address the stack frame.

Macro: STACK_POINTER_REGNUM

  The register number of the stack pointer register, which must also be a fixed register according to `FIXED_REGISTERS`. On most machines, the hardware determines which register this is.

Macro: FRAME_POINTER_REGNUM

  The register number of the frame pointer register, which is used to access automatic variables in the stack frame. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose.

Macro: HARD_FRAME_POINTER_REGNUM

  On some machines the offset between the frame pointer and starting offset of the automatic variables is not known until after register allocation has been done (for example, because the saved registers are between these two locations). On those machines, define `FRAME_POINTER_REGNUM` the number of a special, fixed register to be used internally until the offset is known, and define `HARD_FRAME_POINTER_REGNUM` to be the actual hard register number used for the frame pointer.

  You should define this macro only in the very rare circumstances when it is not possible to calculate the offset between the frame pointer and the automatic variables until after register allocation has been completed. When this macro is defined, you must also indicate in your definition of `ELIMINABLE_REGS` how to eliminate `FRAME_POINTER_REGNUM` into either `HARD_FRAME_POINTER_REGNUM` or `STACK_POINTER_REGNUM`.

  Do not define this macro if it would be the same as `FRAME_POINTER_REGNUM`.

Macro: ARG_POINTER_REGNUM

  The register number of the arg pointer register, which is used to access the function’s argument list. On some machines, this is the same as the frame pointer register. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose. If this is not the same register as the frame pointer register, then you must mark it as a fixed register according to `FIXED_REGISTERS`, or arrange to be able to eliminate it (see Elimination).

Macro: HARD_FRAME_POINTER_IS_FRAME_POINTER

  Define this to a preprocessor constant that is nonzero if `hard_frame_pointer_rtx` and `frame_pointer_rtx` should be the same. The default definition is ‘`(HARD_FRAME_POINTER_REGNUM == FRAME_POINTER_REGNUM)`’; you only need to define this macro if that definition is not suitable for use in preprocessor conditionals.

Macro: HARD_FRAME_POINTER_IS_ARG_POINTER

  Define this to a preprocessor constant that is nonzero if `hard_frame_pointer_rtx` and `arg_pointer_rtx` should be the same. The default definition is ‘`(HARD_FRAME_POINTER_REGNUM == ARG_POINTER_REGNUM)`’; you only need to define this macro if that definition is not suitable for use in preprocessor conditionals.

Macro: RETURN_ADDRESS_POINTER_REGNUM

  The register number of the return address pointer register, which is used to access the current function’s return address from the stack. On some machines, the return address is not at a fixed offset from the frame pointer or stack pointer or argument pointer. This register can be defined to point to the return address on the stack, and then be converted by `ELIMINABLE_REGS` into either the frame pointer or stack pointer.

  Do not define this macro unless there is no other way to get the return address from the stack.

Macro: STATIC_CHAIN_REGNUM
Macro: STATIC_CHAIN_INCOMING_REGNUM

  Register numbers used for passing a function’s static chain pointer. If register windows are used, the register number as seen by the called function is `STATIC_CHAIN_INCOMING_REGNUM`, while the register number as seen by the calling function is `STATIC_CHAIN_REGNUM`. If these registers are the same, `STATIC_CHAIN_INCOMING_REGNUM` need not be defined.

  The static chain register need not be a fixed register.

  If the static chain is passed in memory, these macros should not be defined; instead, the `TARGET_STATIC_CHAIN` hook should be used.

Target Hook: rtx TARGET_STATIC_CHAIN (const_tree fndecl_or_type, bool incoming_p)

  This hook replaces the use of `STATIC_CHAIN_REGNUM` et al for targets that may use different static chain locations for different nested functions. This may be required if the target has function attributes that affect the calling conventions of the function and those calling conventions use different static chain locations.

  The default version of this hook uses `STATIC_CHAIN_REGNUM` et al.

  If the static chain is passed in memory, this hook should be used to provide rtx giving `mem` expressions that denote where they are stored. Often the `mem` expression as seen by the caller will be at an offset from the stack pointer and the `mem` expression as seen by the callee will be at an offset from the frame pointer. The variables `stack_pointer_rtx`, `frame_pointer_rtx`, and `arg_pointer_rtx` will have been initialized and should be used to refer to those items.

Macro: DWARF_FRAME_REGISTERS

  This macro specifies the maximum number of hard registers that can be saved in a call frame. This is used to size data structures used in DWARF2 exception handling.

  Prior to GCC 3.0, this macro was needed in order to establish a stable exception handling ABI in the face of adding new hard registers for ISA extensions. In GCC 3.0 and later, the EH ABI is insulated from changes in the number of hard registers. Nevertheless, this macro can still be used to reduce the runtime memory requirements of the exception handling routines, which can be substantial if the ISA contains a lot of registers that are not call-saved.

  If this macro is not defined, it defaults to `FIRST_PSEUDO_REGISTER`.

Macro: PRE_GCC3_DWARF_FRAME_REGISTERS

  This macro is similar to `DWARF_FRAME_REGISTERS`, but is provided for backward compatibility in pre GCC 3.0 compiled code.

  If this macro is not defined, it defaults to `DWARF_FRAME_REGISTERS`.

Macro: DWARF_REG_TO_UNWIND_COLUMN (regno)

  Define this macro if the target’s representation for dwarf registers is different than the internal representation for unwind column. Given a dwarf register, this macro should return the internal unwind column number to use instead.

  See the PowerPC’s SPE target for an example.

Macro: DWARF_FRAME_REGNUM (regno)

  Define this macro if the target’s representation for dwarf registers used in .eh_frame or .debug_frame is different from that used in other debug info sections. Given a GCC hard register number, this macro should return the .eh_frame register number. The default is `DBX_REGISTER_NUMBER (regno)`.

Macro: DWARF2_FRAME_REG_OUT (regno, for_eh)

  Define this macro to map register numbers held in the call frame info that GCC has collected using `DWARF_FRAME_REGNUM` to those that should be output in .debug_frame (`for_eh` is zero) and .eh_frame (`for_eh` is nonzero). The default is to return `regno`.

Macro: REG_VALUE_IN_UNWIND_CONTEXT

  Define this macro if the target stores register values as `_Unwind_Word` type in unwind context. It should be defined if target register size is larger than the size of `void *`. The default is to store register values as `void *` type.

Macro: ASSUME_EXTENDED_UNWIND_CONTEXT

  Define this macro to be 1 if the target always uses extended unwind context with version, args_size and by_value fields. If it is undefined, it will be defined to 1 when `REG_VALUE_IN_UNWIND_CONTEXT` is defined and 0 otherwise.

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