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If you enable it, GCC 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. When defined, this macro enables `-fcaller-saves' by default for all optimization levels. It has no effect for optimization levels 2 and higher, where `-fcaller-saves' is the default.
CALLER_SAVE_PROFITABLE (refs, calls)
If you don't define this macro, a default is used which is good on most
4 * calls < refs.
HARD_REGNO_CALLER_SAVE_MODE (regno, nregs)
VOIDmodeshould be returned. For most machines this macro need not be defined since GCC will select the smallest suitable mode.