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/* C */ sint64_t A, B, Ah, Am, Al, Bh, Bm, Bl, Ch, Cl, /* etc */; ... Cm = Am + Bm + (Cl >> 32); Ch = Ah + Bh + (Cm >> 32); C = (Ch << 64) + ((Cm & 0xFFFFFFFF) << 32) + (Cl & 0xFFFFFFFF);