The following code is not constant-time.

#include <stdint.h>
#include <stdio.h>
#include <string.h>

typedef int(* issue_t)(const uint64_t*, const uint64_t*);

#define THE_SIZE 9

int comparator(const uint64_t *a, const uint64_t *b) {
  return (memcmp(a, b, sizeof(uint64_t) * THE_SIZE) == 0);
}

int issue(const uint64_t *a, const uint64_t *b) {
  int ret = 0;
  for (int i = 2;i < 3;i += 2) { 
    ret |= comparator(a,b);
  }
  return ret;
}

int main(int argc, char *argv[]){
  uint64_t a[THE_SIZE] = {0}, b[THE_SIZE] = {0}, c[THE_SIZE] = {0};
  // c is our secret value containing array, we read its content from the 
  // environment so the compiler cannot play tricks on us by inlining
  if (argc >= 2) {
    memset(c, (uint64_t) argv[1][0], THE_SIZE*sizeof(uint64_t));
  }
  else {
    fprintf(stderr, "USAGE: %s hex_value\n", argv[0]);
    return 1;
  }
  // declarations and markup here

  volatile issue_t func = issue;
  printf("%d\n", func(a, b));
  printf("%d\n", func(a, c));

  return 0;
}

See the compilation results with different optimization levels on GodBolt.