Global Variables performance effect (c, c++)

It really depends on your compiler, platform, and other details. However, I can describe one scenario where global variables are faster.

In many cases, a global variable is at a fixed offset. This allows the generated instructions to simply use that address directly. (Something along the lines of MOV AX,[MyVar].)

However, if you have a variable that's relative to the current stack pointer or a member of a class or array, some math is required to take the address of the array and determine the address of the actual variable.

Obviously, if you need to place some sort of mutex on your global variable in order to keep it thread-safe, then you'll almost certainly more than lose any performance gain.

Creating local variables can be literally free if they are POD types. You likely are overflowing a cache line with too many stack variables or other similar alignment-based causes which are very specific to your piece of code. I usually find that non-local variables significantly decrease performance.

It's hard to beat static allocation for speed, and while the 10% is a pretty small difference, it could be due to address calculation.

But if you're looking for speed, your example in a comment while(p<end)stats[*p++]++; is an obvious candidate for unrolling, such as:

static int stats[M];static int index_array[N];int *p = index_array, *pend = p+N;// ... initialize the arrays ...while (p < pend-8){  stats[p[0]]++;  stats[p[1]]++;  stats[p[2]]++;  stats[p[3]]++;  stats[p[4]]++;  stats[p[5]]++;  stats[p[6]]++;  stats[p[7]]++;  p += 8;}while(p<pend) stats[*p++]++;

Don't count on the compiler to do it for you. It might or might not be able to figure it out.

Other possible optimizations come to mind, but they depend on what you're actually trying to do.