Question

What I’m trying to do is avoid the following:

if(*ptr > 128) {
   number = 5;
}

Such code performs poorly when there’s no clear pattern as to which way the branch will go. What I came up with is this:

int arr[] = { number, 5 };
int cond = *ptr > 128;
number = arr[cond];

Based on my testing, that runs more than twice as fast as doing the conditional when the input is random. What I’m wondering is if there’s a more clever way to do this, perhaps using bitwise operators.

Answer 1

A clever compiler should definitely compile this to a conditional move with the right optimization settings; check the disassembly to be sure.

There is this branchless solution:

int mask = -(*ptr > 128);
number = (number & mask) | (5 & ~mask);

The last line can also be

number = ((mask & (number ^ 5)) ^ 5);

if you’re looking to use one less operation. But, caveat emptor, the compiler won’t be able to optimize either of these nearly as well. You are best leaving this particular optimization for the compiler to worry about, unless you specifically know that the compiler is unable to make the optimization (in that case, you may want to check your compiler version or flags).