How can I multiply 64 bit operands and get 128 bit result portably?

As I understand the question, you want a portable pure C implementation of 64 bit multiplication, with output to a 128 bit value, stored in two 64 bit values. In which case this article purports to have what you need. That code is written for C++. It doesn't take much to turn it into C code:

void mult64to128(uint64_t op1, uint64_t op2, uint64_t *hi, uint64_t *lo)
{
    uint64_t u1 = (op1 & 0xffffffff);
    uint64_t v1 = (op2 & 0xffffffff);
    uint64_t t = (u1 * v1);
    uint64_t w3 = (t & 0xffffffff);
    uint64_t k = (t >> 32);

    op1 >>= 32;
    t = (op1 * v1) + k;
    k = (t & 0xffffffff);
    uint64_t w1 = (t >> 32);

    op2 >>= 32;
    t = (u1 * op2) + k;
    k = (t >> 32);

    *hi = (op1 * op2) + w1 + k;
    *lo = (t << 32) + w3;
}

The accepted solution isn't really the best solution, in my opinion.

• It is confusing to read.
• It has some funky carry handling.
• It doesn't take advantage of the fact that 64-bit arithmetic may be available.
• It displeases ARMv6, the God of Absolutely Ridiculous Multiplies. Whoever uses UMAAL shall not lag but have eternal 64-bit to 128-bit multiplies in 4 instructions.

Joking aside, it is much better to optimize for ARMv6 than any other platform because it will have the most benefit. x86 needs a complicated routine and it would be a dead end optimization.

The best way I have found (and used in xxHash3) is this, which takes advantage of multiple implementations using macros:

It is a tiny bit slower than mult64to128 on x86 (by 1-2 instructions), but a lot faster on ARMv6.

#include <stdint.h>
#ifdef _MSC_VER
#  include <intrin.h>
#endif

/* Prevents a partial vectorization from GCC. */
#if defined(__GNUC__) && !defined(__clang__) && defined(__i386__)
  __attribute__((__target__("no-sse")))
#endif
static uint64_t multiply64to128(uint64_t lhs, uint64_t rhs, uint64_t *high)
{
    /*
     * GCC and Clang usually provide __uint128_t on 64-bit targets,
     * although Clang also defines it on WASM despite having to use
     * builtins for most purposes - including multiplication.
     */
#if defined(__SIZEOF_INT128__) && !defined(__wasm__)
    __uint128_t product = (__uint128_t)lhs * (__uint128_t)rhs;
    *high = (uint64_t)(product >> 64);
    return (uint64_t)(product & 0xFFFFFFFFFFFFFFFF);

    /* Use the _umul128 intrinsic on MSVC x64 to hint for mulq. */
#elif defined(_MSC_VER) && defined(_M_IX64)
#   pragma intrinsic(_umul128)
    /* This intentionally has the same signature. */
    return _umul128(lhs, rhs, high);

#else
    /*
     * Fast yet simple grade school multiply that avoids
     * 64-bit carries with the properties of multiplying by 11
     * and takes advantage of UMAAL on ARMv6 to only need 4
     * calculations.
     */

    /* First calculate all of the cross products. */
    uint64_t lo_lo = (lhs & 0xFFFFFFFF) * (rhs & 0xFFFFFFFF);
    uint64_t hi_lo = (lhs >> 32)        * (rhs & 0xFFFFFFFF);
    uint64_t lo_hi = (lhs & 0xFFFFFFFF) * (rhs >> 32);
    uint64_t hi_hi = (lhs >> 32)        * (rhs >> 32);

    /* Now add the products together. These will never overflow. */
    uint64_t cross = (lo_lo >> 32) + (hi_lo & 0xFFFFFFFF) + lo_hi;
    uint64_t upper = (hi_lo >> 32) + (cross >> 32)        + hi_hi;

    *high = upper;
    return (cross << 32) | (lo_lo & 0xFFFFFFFF);
#endif /* portable */
}

On ARMv6, you can't get much better than this, at least on Clang:

multiply64to128:
        push    {r4, r5, r11, lr}
        umull   r12, r5, r2, r0
        umull   r2, r4, r2, r1
        umaal   r2, r5, r3, r0
        umaal   r4, r5, r3, r1
        ldr     r0, [sp, #16]
        mov     r1, r2
        strd    r4, r5, [r0]
        mov     r0, r12
        pop     {r4, r5, r11, pc}

The accepted solution generates a bunch of adds and adc, as well as an extra umull in Clang due to an instcombine bug.

I further explain the portable method in the link I posted.

Since you have gcc as a tag, note that you can just use gcc's 128-bit integer type:

typedef unsigned __int128 uint128_t;
// ...
uint64_t x, y;
// ...
uint128_t result = (uint128_t)x * y;
uint64_t lo = result;
uint64_t hi = result >> 64;