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If you enable it, GCC can save registers around function calls. Thismakes it possible to use call-clobbered registers to hold variables thatmust live across calls.
A C expression specifying which mode is required for savingnregsof a pseudo-register in call-clobbered hard registerregno. Ifregno is unsuitable for caller save,VOIDmode should bereturned. For most machines this macro need not be defined since GCCwill select the smallest suitable mode.