Question

I have something like this

register unsigned int a, b, c;
int n;
for (n = 0; n < 10; ++n){
c = a + b
b = a
a = c
array[n] = c;
}

what it does, it doesn’t matter. The code runs quickly the way it is now, slower if the register keyword is removed. However, when I add in register before int n, it actually runs slower than now, but faster than if no registers is used.

Can someone explain this to me? Thanks.

Answer 1

register gives the compiler a hint to place the variable in a register instead of memory/stack space. In some cases, there won’t be enough registers for every variable you place this keyword on so placing it on too many variables can force some of the others out of registers again.

This is just a hint, though, and the compiler doesn’t have to take it.