Fast multiplication of values in an array

Question

Is there a fast way to multiply values of a float array in C++, to optimize this function (where count is a multiple of 4):

void multiply(float* values, float factor, int count)
{
    for(int i=0; i < count; i++)
    {
        *value *= factor;
        value++;
    }
}

A solution must work on Mac OS X and Windows, Intel and non-Intel. Think SSE, vectorization, compiler (gcc vs. MSVC).

Answer 1

If you want your code to be cross-platform, then either you're going to have to write platform-independent code, or you're going to have to write a load of #ifdefs.

Have you tried some manual loop unrolling, and seeing if it makes any difference?

Answer 2

Since you know the count is a multiple of 4, you can unroll your loop...

void multiply(float* values, float factor, int count)
{
    count = count >> 2; // count / 4
    for(int i=0; i < count ; i++)
    {
        *value *= factor;
        *(value+1) *= factor;
        *(value+2) *= factor;
        *(value+3) *= factor;
        value += 4;
    }
}