C++ Power Calculation – Fastest Way to Compute Power of 2

c++math

What would be the quickest way to find the power of 2, that a certain number (that is a power of two) used?

I'm not very skilled at mathematics, so I'm not sure how best to describe it. But the function would look similar to x = 2^y where y is the output, and x is the input. Here's a truth table of what it'd look like if that helps explain it. 

0 = f(1)
1 = f(2)
2 = f(4)
3 = f(8)
...
8 = f(256)
9 = f(512)

I've made a function that does this, but I fear it's not very efficient (or elegant for that matter). Would there be a simpler and more efficient way of doing this? I'm using this to compute what area of a texture is used to buffer how drawing is done, so it's called at least once for every drawn object. Here's the function I've made so far:

uint32 getThePowerOfTwo(uint32 value){
    for(uint32 n = 0; n < 32; ++n){
        if(value <= (1 << n)){
            return n;
        }
    }
    return 32; // should never be called
}

Best Answer

Building on woolstar's answer - I wonder if a binary search of a lookup table would be slightly faster? (and much nicer looking)...

int getThePowerOfTwo(int value) {
    static constexpr int twos[] = {
        1<<0,  1<<1,  1<<2,  1<<3,  1<<4,  1<<5,  1<<6,  1<<7,
        1<<8,  1<<9,  1<<10, 1<<11, 1<<12, 1<<13, 1<<14, 1<<15,
        1<<16, 1<<17, 1<<18, 1<<19, 1<<20, 1<<21, 1<<22, 1<<23,
        1<<24, 1<<25, 1<<26, 1<<27, 1<<28, 1<<29, 1<<30, 1<<31
    };

    return std::lower_bound(std::begin(twos), std::end(twos), value) - std::begin(twos);
}
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