Why do those two high(64bx64b) functions give different results?

Question

static __inline__ uint64_t mulhilo64(uint64_t a, uint64_t b, uint64_t* hip) {
        __uint128_t product = ((__uint128_t)a)*((__uint128_t)b);
        *hip = product>>64;
        return (uint64_t)product;
}

I am trying to write following above using MULX intrinsics on AVX2 (more specifically BMI2). But they do not give the same results.

static __inline__ uint64_t mulhilo64(uint64_t  a, uint64_t b, uint64_t *c){
     return _mulx_u64(a, b, &c);
}

Answer 1

It looks like this function could be wrong:

static __inline__ uint64_t mulhilo64(uint64_t  a, uint64_t b, uint64_t *c){
     return _mulx_u64(a, b, &c);
}

It should probably be:

static __inline__ uint64_t mulhilo64(uint64_t  a, uint64_t b, uint64_t *c){
     return _mulx_u64(a, b, c);
}                        // ^

Note that compiling with warnings enabled (e.g. gcc -Wall ...) helps to catch simple mistakes like this.