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-savesby default for all optimization levels. It has no effect for optimization levels 2 and higher, where
-fcaller-savesis the default.
If you don't define this macro, a default is used which is good on most
4 * calls
VOIDmodeshould be returned. For most machines this macro need not be defined since GCC will select the smallest suitable mode.