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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);