128 by 64 bit integer division for gcc and clang

div128.h

#include <stdint.h>

// 128 by 64 bit integer division for gcc and clang
inline uint64_t udiv128_64(unsigned __int128 a, uint64_t b)
{
    // We don't want 128 bit software division
    uint64_t alo = (uint64_t)a;
    uint64_t ahi = (a >> 64);
    uint64_t d, e;
    __asm__("divq %[b]"
        : "=a"(d), "=d"(e)
        : [b] "r"(b), "a"(alo), "d"(ahi)
    );
    return d;
}

inline int64_t idiv128_64(__int128 a, int64_t b)
{
    // We don't want 128 bit software division
    uint64_t alo = (uint64_t)a;
    uint64_t ahi = ((unsigned __int128)a >> 64);
    int64_t d, e;
    __asm__("idivq %[b]"
        : "=a"(d), "=d"(e)
        : [b] "r"(b), "a"(alo), "d"(ahi)
    );
    return d;
}