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   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. _elimination:
   7
   8 Eliminating Frame Pointer and Arg Pointer
   9 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  10
  11 .. prevent bad page break with this line
  12
  13 This is about eliminating the frame pointer and arg pointer.
  14
  15 .. include:: ../tm.rst.in
  16   :start-after: [TARGET_FRAME_POINTER_REQUIRED]
  17   :end-before: [TARGET_FRAME_POINTER_REQUIRED]
  18
  19
  20 .. c:macro:: ELIMINABLE_REGS
  21
  22   This macro specifies a table of register pairs used to eliminate
  23   unneeded registers that point into the stack frame.
  24
  25   The definition of this macro is a list of structure initializations, each
  26   of which specifies an original and replacement register.
  27
  28   On some machines, the position of the argument pointer is not known until
  29   the compilation is completed.  In such a case, a separate hard register
  30   must be used for the argument pointer.  This register can be eliminated by
  31   replacing it with either the frame pointer or the argument pointer,
  32   depending on whether or not the frame pointer has been eliminated.
  33
  34   In this case, you might specify:
  35
  36   .. code-block:: c++
  37
  38     #define ELIMINABLE_REGS  \
  39     {{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \
  40      {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \
  41      {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}
  42
  43   Note that the elimination of the argument pointer with the stack pointer is
  44   specified first since that is the preferred elimination.
  45
  46 .. include:: ../tm.rst.in
  47   :start-after: [TARGET_CAN_ELIMINATE]
  48   :end-before: [TARGET_CAN_ELIMINATE]
  49
  50
  51 .. c:macro:: INITIAL_ELIMINATION_OFFSET (from_reg, to_reg, offset_var)
  52
  53   This macro returns the initial difference between the specified pair
  54   of registers.  The value would be computed from information
  55   such as the result of ``get_frame_size ()`` and the tables of
  56   registers ``df_regs_ever_live_p`` and ``call_used_regs``.
  57
  58 .. include:: ../tm.rst.in
  59   :start-after: [TARGET_COMPUTE_FRAME_LAYOUT]
  60   :end-before: [TARGET_COMPUTE_FRAME_LAYOUT]