Caller-Saves Register Allocation
--------------------------------

   If you enable it, GNU CC can save registers around function calls.
This makes it possible to use call-clobbered registers to hold
variables that must live across calls.

`DEFAULT_CALLER_SAVES'
     Define this macro if function calls on the target machine do not
     preserve any registers; in other words, if `CALL_USED_REGISTERS'
     has 1 for all registers.  This macro enables `-fcaller-saves' by
     default.  Eventually that option will be enabled by default on all
     machines and both the option and this macro will be eliminated.

`CALLER_SAVE_PROFITABLE (REFS, CALLS)'
     A C expression to determine whether it is worthwhile to consider
     placing a pseudo-register in a call-clobbered hard register and
     saving and restoring it around each function call.  The expression
     should be 1 when this is worth doing, and 0 otherwise.

     If you don't define this macro, a default is used which is good on
     most machines: `4 * CALLS < REFS'.