Fast multiplication/division by 2 for floats and doubles (C/C++)

Question

In the software I'm writing, I'm doing millions of multiplication or division by 2 (or powers of 2) of my values. I would really like these values to be int so that I could access the bitshift operators

int a = 1;
int b = a<<24

However, I cannot, and I have to stick with doubles.

My question is : as there is a standard representation of doubles (sign, exponent, mantissa), is there a way to play with the exponent to get fast multiplications/divisions by a power of 2?

I can even assume that the number of bits is going to be fixed (the software will work on machines that will always have 64 bits long doubles)

P.S : And yes, the algorithm mostly does these operations only. This is the bottleneck (it's already multithreaded).

Edit : Or am I completely mistaken and clever compilers already optimize things for me?

---

Temporary results (with Qt to measure time, overkill, but I don't care):

#include <QtCore/QCoreApplication>
#include <QtCore/QElapsedTimer>
#include <QtCore/QDebug>

#include <iostream>
#include <math.h>

using namespace std;

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);

while(true)
{
    QElapsedTimer timer;
    timer.start();

    int n=100000000;
    volatile double d=12.4;
    volatile double D;
    for(unsigned int i=0; i<n; ++i)
    {
        //D = d*32;      // 200 ms
        //D = d*(1<<5);  // 200 ms
        D = ldexp (d,5); // 6000 ms
    }

    qDebug() << "The operation took" << timer.elapsed() << "milliseconds";
}

return a.exec();
}

Runs suggest that D = d*(1<<5); and D = d*32; run in the same time (200 ms) whereas D = ldexp (d,5); is much slower (6000 ms). I know that this is a micro benchmark, and that suddenly, my RAM has exploded because Chrome has suddenly asked to compute Pi in my back every single time I run ldexp(), so this benchmark is worth nothing. But I'll keep it nevertheless.

On the other had, I'm having trouble doing reinterpret_cast<uint64_t *> because there's a const violation (seems the volatile keyword interferes)

Answer 1

This is one of those highly-application specific things. It may help in some cases and not in others. (In the vast majority of cases, a straight-forward multiplication is still best.)

The "intuitive" way of doing this is just to extract the bits into a 64-bit integer and add the shift value directly into the exponent. (this will work as long as you don't hit NAN or INF)

So something like this:

union{
    uint64 i;
    double f;
};

f = 123.;
i += 0x0010000000000000ull;

//  Check for zero. And if it matters, denormals as well.

Note that this code is not C-compliant in any way, and is shown just to illustrate the idea. Any attempt to implement this should be done directly in assembly or SSE intrinsics.

However, in most cases the overhead of moving the data from the FP unit to the integer unit (and back) will cost much more than just doing a multiplication outright. This is especially the case for pre-SSE era where the value needs to be stored from the x87 FPU into memory and then read back into the integer registers.

In the SSE era, the Integer SSE and FP SSE use the same ISA registers (though they still have separate register files). According the Agner Fog, there's a 1 to 2 cycle penalty for moving data between the Integer SSE and FP SSE execution units. So the cost is much better than the x87 era, but it's still there.

All-in-all, it will depend on what else you have on your pipeline. But in most cases, multiplying will still be faster. I've run into this exact same problem before so I'm speaking from first-hand experience.

Now with 256-bit AVX instructions that only support FP instructions, there's even less of an incentive to play tricks like this.