when to use registers in C?

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

In gcc, register is definitely not ignored, unless you specify optimization options. Testing your code with something like this

unsigned int array[10];

int n;

#define REG register

int main()
{
    REG unsigned int a, b, c;

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

you obtain (depending on whether REG is defined or empty)

http://picasaweb.google.com/lh/photo/v2hBpl6D-soIdBXUOmAeMw?feat=directlink

On the left is shown the result of using registers.