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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 .. _caller-saves:
   7
   8 Caller-Saves Register Allocation
   9 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  10
  11 If you enable it, GCC can save registers around function calls.  This
  12 makes it possible to use call-clobbered registers to hold variables that
  13 must live across calls.
  14
  15 .. c:macro:: HARD_REGNO_CALLER_SAVE_MODE (regno, nregs)
  16
  17   A C expression specifying which mode is required for saving :samp:`{nregs}`
  18   of a pseudo-register in call-clobbered hard register :samp:`{regno}`.  If
  19   :samp:`{regno}` is unsuitable for caller save, ``VOIDmode`` should be
  20   returned.  For most machines this macro need not be defined since GCC
  21   will select the smallest suitable mode.