Question

If multiplication is slower than addition instead of doing

7 * 8

Will this theoretically improve performance ?

for(int i =0; i < 8 ; i++){
temp += 7
}

Or else do i just need to do

7 + 7 + 7 + 7 + 7 + 7 + 7 + 7

Answer 1

Have you tried it and timed it?

On nearly every single modern machine today, there is fast hardware support for multiplication. So unless it’s simple multiplication by 2, no, it will not be faster.

To give some hard numbers on the current Intel machines:

add/sub   1 cycle latency
mul/imul  3 cycle latency

Taken from Agner Fog’s manuals.

Although it’s actually a lot more complicated than this, the point is still: No, you’re not going to get any benefit trying to replace multiplications with additions.

In the few cases where it is better (such as multiplication by a power of two – using shifts), the compiler will make that optimization for you, if it knows the value at compile-time.

On x86, the compiler can also play with the lea instruction to do fast multiplication by 3, 5, 10, etc…