-mabi=altivec in CFLAGS

Hi all,

AFAIK, some packages (e.g., dev-cpp/eigen) can use AltiVec vectorization
only when -mabi=altivec is specified in GCC flags.

Is it safe to use -mabi=altivec in global CFLAGS in make.conf?
Does anybody know if the system will perform better or worse in average?

-- 
Regards,
Konstantin