How come my array index is faster than pointer

Question

Why the array index is faster than pointer? Isn\'t pointer supposed to be faster than array index?

** i used time.h clock_t to tested two functions, each loop 2 mil

Answer 1

This is a very hard thing to time, because compilers are very good at optimising these things. Still it's better to give the compiler as much information as possible, that's why in this case I'd advise using std::fill, and let the compiler choose.

But... If you want to get into the detail

a) CPU's normally give pointer+value for free, like : mov r1, r2(r3).
b) This means an index operation requires just : mul r3,r1,size
This is just one cycle extra, per loop.
c) CPU's often provide stall/delay slots, meaning you can often hide single-cycle operations.

All in all, even if your loops are very large, the cost of the access is nothing compared to the cost of even a few cache-misses. You are best advised to optimise your structures before you care about loop costs. Try for example, packing your structures to reduce the memory footprint first