JawnV6 JawnV6 - 2 months ago 9
C Question

What is the fastest way to swap values in C?

I want to swap two integers, and I want to know which of these two implementations will be faster:
The obvious way with a temp variable:

void swap(int* a, int* b)
{
int temp = *a;
*a = *b;
*b = temp;
}


Or the xor version that I'm sure most people have seen:

void swap(int* a, int* b)
{
*a ^= *b;
*b ^= *a;
*a ^= *b;
}


It seems like the first uses an extra register, but the second one is doing three loads and stores while the first only does two of each. Can someone tell me which is faster and why? The why being more important.

Ant Ant
Answer Source

The XOR method fails if a and b point to the same address. The first XOR will clear all of the bits at the memory address pointed to by both variables, so once the function returns (*a == *b == 0), regardless of the initial value.

More info on the Wiki page: XOR swap algorithm

Although it's not likely that this issue would come up, I'd always prefer to use the method that's guaranteed to work, not the clever method that fails at unexpected moments.