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

loop_vect_v1.c

//gcc -O3 -fopt-info-vec-missed=loop_vect_v1.miss loop_vect_v1.c -o loop_vect_v1
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main() {
  
int array1[1000];
int array2[1000];
long array3[1000];
long arraySum = 0;

srand(time(NULL));

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

for (int i = 0; i < 1000; i++) {
		array3[i] = array1[i] + array2[i];
		arraySum += array3[i];
}
        printf("The total array sum is: %li\n", arraySum);

return 0;
}

loop_vect_v1.miss

loop_vect_v1.c:20:1: note: step unknown.
loop_vect_v1.c:20:1: note: misalign = 0 bytes of ref array1[i_41]
loop_vect_v1.c:20:1: note: misalign = 0 bytes of ref array2[i_41]
loop_vect_v1.c:20:1: note: num. args = 4 (not unary/binary/ternary op).
loop_vect_v1.c:20:1: note: not ssa-name.
loop_vect_v1.c:20:1: note: use not simple.
loop_vect_v1.c:20:1: note: num. args = 4 (not unary/binary/ternary op).
loop_vect_v1.c:20:1: note: not ssa-name.
loop_vect_v1.c:20:1: note: use not simple.
loop_vect_v1.c:14:1: note: not vectorized: loop contains function calls or data references that cannot be analyzed
loop_vect_v1.c:12:1: note: not vectorized: not enough data-refs in basic block.
loop_vect_v1.c:16:22: note: not vectorized: not enough data-refs in basic block.
loop_vect_v1.c:14:1: note: not vectorized: not enough data-refs in basic block.
loop_vect_v1.c:21:33: note: not vectorized: no vectype for stmt: vect__9.5_17 = MEM[(int *)vectp_array1.3_32];
 scalar_type: vector(4) int
loop_vect_v1.c:21:33: note: not vectorized: no vectype for stmt: vect__10.8_14 = MEM[(int *)vectp_array2.6_16];
 scalar_type: vector(4) int
loop_vect_v1.c:21:33: note: not vectorized: no grouped stores in basic block.
loop_vect_v1.c:24:9: note: not vectorized: not enough data-refs in basic block.
loop_vect_v1.c:5:5: note: not vectorized: not enough data-refs in basic block.

loop_vect_v1.opt

loop_vect_v1.c:20:1: note: loop vectorized