Fast method to multiply integer by proper fraction without floats or overflow

Tolerate imprecision and use the 16 MSBits of n,d,x

Algorithm
while (|n| > 0xffff) n/2, sh++
while (|x| > 0xffff) x/2, sh++
while (|d| > 0xffff) d/2, sh--
r = n*x/d  // A 16x16 to 32 multiply followed by a 32/16-bit divide.
shift r by sh.

When 64 bit divide is expensive, the pre/post processing here may be worth to do a 32-bit divide - which will certainly be the big chunk of CPU.

If the compiler cannot be coaxed into doing a 32-bit/16-bit divide, then skip the while (|d| > 0xffff) d/2, sh-- step and do a 32/32 divide.

Use unsigned math as possible.

I've now benchmarked several possible solutions, including weird/clever ones from other sources like combining 32-bit div & mod & add or using peasant math, and here are my conclusions:

First, if you are only targeting Windows and using VSC++, just use MulDiv(). It is quite fast (faster than directly using 64-bit variables in my tests) while still being just as accurate and rounding the result for you. I could not find any superior method to do this kind of thing on Windows with VSC++, even taking into account restrictions like unsigned-only and N <= D.

However, in my case having a function with deterministic results even across platforms is even more important than speed. On another platform I was using as a test, the 64-bit divide is much, much slower than the 32-bit one when using the 32-bit libraries, and there is no MulDiv() to use. The 64-bit divide on this platform takes ~26x as long as a 32-bit divide (yet the 64-bit multiply is just as fast as the 32-bit version...).

So if you have a case like me, I will share the best results I got, which turned out to be just optimizations of chux's answer.

Both of the methods I will share below make use of the following function (though the compiler-specific intrinsics only actually helped in speed with MSVC in Windows):

inline u32 bitsRequired(u32 val)
{
    #ifdef _MSC_VER
        DWORD r = 0;
        _BitScanReverse(&r, val | 1);
        return r+1;
    #elif defined(__GNUC__) || defined(__clang__)
        return 32 - __builtin_clz(val | 1);
    #else
        int r = 1;
        while (val >>= 1) ++r;
        return r;
    #endif
}

Now, if x is a constant that's 16-bit in size or smaller and you can pre-compute the bits required, I found the best results in speed and accuracy from this function:

u32 multConstByPropFrac(u32 x, u32 nMaxBits, u32 n, u32 d)
{
    //assert(nMaxBits == 32 - bitsRequired(x));
    //assert(n <= d);
    const int bitShift = bitsRequired(n) - nMaxBits;
    if( bitShift > 0 )
    {
        n >>= bitShift;
        d >>= bitShift;
    }

    // Remove the + d/2 part if don't need rounding
    return (x * n + d/2) / d;
}

On the platform with the slow 64-bit divide, the above function ran ~16.75x as fast as return ((u64)x * n + d/2) / d; and with an average 99.999981% accuracy (comparing difference in return value from expected to range of x, i.e. returning +/-1 from expected when x is 2048 would be 100 - (1/2048 * 100) = 99.95% accurate) when testing it with a million or so randomized inputs where roughly half of them would normally have been an overflow. Worst-case accuracy was 99.951172%.

For the general use case, I found the best results from the following (and without needing to restrict N <= D to boot!):

u32 scaleToFraction(u32 x, u32 n, u32 d)
{
    u32 bits = bitsRequired(x);
    int bitShift = bits - 16;
    if( bitShift < 0 ) bitShift = 0;
    int sh = bitShift;
    x >>= bitShift;

    bits = bitsRequired(n);
    bitShift = bits - 16;
    if( bitShift < 0 ) bitShift = 0;
    sh += bitShift;
    n >>= bitShift;

    bits = bitsRequired(d);
    bitShift = bits - 16;
    if( bitShift < 0 ) bitShift = 0;
    sh -= bitShift;
    d >>= bitShift;

    // Remove the + d/2 part if don't need rounding
    u32 r = (x * n + d/2) / d;
    if( sh < 0 )
        r >>= (-sh);
    else //if( sh > 0 )
        r <<= sh;

    return r;
}

On the platform with the slow 64-bit divide, the above function ran ~18.5x as fast as using 64-bit variables and with 99.999426% average and 99.947479% worst-case accuracy.

I was able to get more speed or more accuracy by messing with the shifting, such as trying to not shift all the way down to 16-bit if it wasn't strictly necessary, but any increase in speed came at a high cost in accuracy and vice versa.

None of the other methods I tested came even close to the same speed or accuracy, most being slower than just using the 64-bit method or having huge loss in precision, so not worth going into.

Obviously, no guarantee that anyone else will get similar results on other platforms!

EDIT: Replaced some bit-twiddling hacks with plain code that actually ran faster anyway by letting the compiler do its job.

The basic correct approach to this is simply (uint64_t)x*n/d. That's optimal assuming d is variable and unpredictable. But if d is constant or changes infrequently, you can pre-generate constants such that exact division by d can be performed as a multiplication followed by a bitshift. A good description of the algorithm, which is roughly what GCC uses internally to transform division by a constant into multiplication, is here:

http://ridiculousfish.com/blog/posts/labor-of-division-episode-iii.html

I'm not sure how easy it is to make it work for a "64/32" division (i.e. dividing the result of (uint64_t)x*n), but you should be able to just break it up into high and low parts if nothing else.

Note that these algorithms are also available as libdivide.