Initialize array with "static" changes run time? - c

Question

I noticed that two different ways of initializing an array in C seems to result in very different running time after compiling with O3 optimization. Here is a minimum (albeit meaningless) example to replicate such difference:
#include <stdio.h>
#include <time.h>
int main(void) {
int i, j, k;
int size=10000;
int a[size];
clock_t time1 = clock();
for (i=0; i<size; i++) {
for (j=0; j<300000; j++) {
for (k=0; k<700000; k++) {
a[i] = j+k;
}
}
}
clock_t time2 = clock();
double time = (double)(time2-time1)/CLOCKS_PER_SEC*1000.0;
printf("%f\n", time);
getchar();
return 0;
}
Compile this program with gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609 with O3 optimization turned on. This program takes about 0.02s to finish on my computer.
Now, change the array initialization from "int a[size];" to "static int a[10000];" and keep everything else the same. Again compile with the same environment and O3 optimization. This time, the program runs for about 0.001s.
Can anyone explain why there is such a different? Thanks!

Answer 1

I think this largely depends on compiler. My GCC 5.4 completely removes the loop when static is present, probly because it can figure out that computations have no side-effects ("dead code elimination"). For some reason it fails to do so when VLA is present (that's a missing optimization).
As a side note, to reliably measure performance you need to prevent compiler from optimizing too much. In your case I'd suggest to separate array creation and computations e.g. like
void __attribute__((noinline, noclone)) benchmark(int *a, int size) {
for (i=0; i<size; i++)
for (j=0; j<300000; j++)
for (k=0; k<700000; k++)
a[i] = j+k;
}
int main(void) {
int i, j, k;
int size=10000;
int a[size];
clock_t time1 = clock();
benchmark(a, size);
clock_t time2 = clock();
double time = (double)(time2-time1)/CLOCKS_PER_SEC*1000.0;
printf("%f\n", time);
getchar();
return 0;
}