最小化C ++中的分支-如果值不为零，则递增

Question

I have the following nested for-loop:

for(k = 0; k < n; ++k) {
    for(m = 0; m < n; ++m) {
        /* other logic altering a */
        if(a[index] != 0) count++;
    }
}

where a contains uint32_t . Since n can be quite large (but constant), and this is the only branch (besides comparing k and m with n ), I would like to optimize this away.

The distribution of zero and non-zero in a can be considered uniformly random.

My first approach was

count += a[index] & 1;

but then count would only be incremented for all odd numbers.

In addition: I also have a case where a contains bool , but according to C++ Conditionals true and false are defined as non-zero and zero, which basically are equivalent to the above problem.

Answer 1

As stated in the comments for the question if(a[index] != 0) count++; does not produce a branch (in this case), which was somewhat verified in the assembly.

For the sake of the completeness an equivalent to the mentioned statement are count += a[index] != 0; (according to standard §4.7 [conv.integral])