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.
A C expression specifying which mode is required for saving nregs
of a pseudo-register in call-clobbered hard register regno. If
regno is unsuitable for caller save,
VOIDmode should be
returned. For most machines this macro need not be defined since GCC
will select the smallest suitable mode.