The second loop will enable the gcc compiler to use auto-vectorization

looop_vect_v2.c

//gcc -O3 -fopt-info-vec-missed=loop_vect_v2.miss loop_vect_v2.c -o loop_vect_v2
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define MAX_ARRAY_SIZE 1000

int main() {

int* array1 = calloc(MAX_ARRAY_SIZE,sizeof(int));
int* array2 = calloc(MAX_ARRAY_SIZE,sizeof(int));
long* array3 = calloc(MAX_ARRAY_SIZE,sizeof(long));
long arraySum = 0;

srand(time(NULL));

for (int i = 0; i < MAX_ARRAY_SIZE; i++) {
	array1[i] = (rand()% 2001) - 1000;
 	array2[i] = (rand()% 2001) - 1000;
}

for (int i = 0; i < MAX_ARRAY_SIZE; i++) {
	array3[i] = array1[i] + array2[i];
	arraySum += array3[i];
}	
	printf("The total array sum is: %li\n", arraySum);
	
free(array1);
free(array2);
free(array3);

return 0;
}

loop_vect_v2.miss

loop_vect_v2.c:22:1: note: step unknown.
loop_vect_v2.c:22:1: note: Unknown misalignment, naturally aligned
loop_vect_v2.c:22:1: note: Unknown misalignment, naturally aligned
loop_vect_v2.c:22:1: note: Unknown misalignment, naturally aligned
loop_vect_v2.c:22:1: note: not ssa-name.
loop_vect_v2.c:22:1: note: use not simple.
loop_vect_v2.c:22:1: note: not ssa-name.
loop_vect_v2.c:22:1: note: use not simple.
loop_vect_v2.c:22:1: note: not ssa-name.
loop_vect_v2.c:22:1: note: use not simple.
loop_vect_v2.c:22:1: note: not ssa-name.
loop_vect_v2.c:22:1: note: use not simple.
loop_vect_v2.c:16:1: note: not vectorized: loop contains function calls or data references that cannot be analyzed
loop_vect_v2.c:14:1: note: not vectorized: not enough data-refs in basic block.
loop_vect_v2.c:19:16: note: not vectorized: not enough data-refs in basic block.
loop_vect_v2.c:16:1: note: not vectorized: not enough data-refs in basic block.
loop_vect_v2.c:16:1: note: not vectorized: not enough data-refs in basic block.
loop_vect_v2.c:22:38: note: not consecutive access *_28 = _27;
loop_vect_v2.c:22:38: note: not consecutive access _54 = *_55;
loop_vect_v2.c:22:38: note: not consecutive access _52 = *_53;
loop_vect_v2.c:22:38: note: not vectorized: no grouped stores in basic block.
loop_vect_v2.c:22:38: note: not vectorized: not enough data-refs in basic block.
loop_vect_v2.c:24:12: note: not vectorized: no vectype for stmt: vect__16.13_124 = MEM[(int *)vectp_array1.11_122];
 scalar_type: vector(4) int
loop_vect_v2.c:24:12: note: not vectorized: no vectype for stmt: vect__19.16_129 = MEM[(int *)vectp_array2.14_127];
 scalar_type: vector(4) int
loop_vect_v2.c:24:12: note: not vectorized: no vectype for stmt: MEM[(long int *)vectp_array3.19_135] = vect__23.18_131;
 scalar_type: vector(2) long int
loop_vect_v2.c:24:12: note: not vectorized: no vectype for stmt: MEM[(long int *)vectp_array3.19_138] = vect__23.18_132;
 scalar_type: vector(2) long int
loop_vect_v2.c:24:12: note: not vectorized: no grouped stores in basic block.
loop_vect_v2.c:24:12: note: not vectorized: not enough data-refs in basic block.
loop_vect_v2.c:22:1: note: not consecutive access *_20 = _21;
loop_vect_v2.c:22:1: note: not consecutive access _14 = *_13;
loop_vect_v2.c:22:1: note: not consecutive access _16 = *_15;
loop_vect_v2.c:22:1: note: not vectorized: no grouped stores in basic block.
loop_vect_v2.c:22:1: note: not consecutive access *_152 = _153;
loop_vect_v2.c:22:1: note: not consecutive access _147 = *_42;
loop_vect_v2.c:22:1: note: not consecutive access _149 = *_148;
loop_vect_v2.c:22:1: note: not vectorized: no grouped stores in basic block.
loop_vect_v2.c:22:38: note: not consecutive access *_105 = _106;
loop_vect_v2.c:22:38: note: not consecutive access _100 = *_99;
loop_vect_v2.c:22:38: note: not consecutive access _102 = *_101;
loop_vect_v2.c:22:38: note: not vectorized: no grouped stores in basic block.
loop_vect_v2.c:32:1: note: not vectorized: not enough data-refs in basic block.

loop_vect_v2.opt

loop_vect_v2.c:22:1: note: loop vectorized
loop_vect_v2.c:22:1: note: loop turned into non-loop; it never loops.
loop_vect_v2.c:22:1: note: loop with 3 iterations completely unrolled
loop_vect_v2.c:7:5: note: loop turned into non-loop; it never loops