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
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)
If you don't define this macro, a default is used which is good on most
machines: 4 * calls < refs
.