Using and Porting GNU CC
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.
CALL_USED_REGISTERShas 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
4 * calls < refs.