/* C */
sint64_t A, B, Ah, Am, Al, Bh, Bm, Bl, Ch, Cl, /* etc */;
sint128 C;
Al = A & 0xFFFFFFFF;
Am = A >> 32;
Ah = A >> 64;
Bl = B & 0xFFFFFFFF;
Bm = A >> 32;
Bh = A >> 64;
Cl = Al + Bl;
Cm = Am + Bm + (Cl >> 32);
Ch = Ah + Bh + (Cm >> 32);
C = (Ch << 64) + ((Cm & 0xFFFFFFFF) << 32)  + (Cl & 0xFFFFFFFF);