[Bug tree-optimization/87540] New: Missed inner loop hoist if the loop does not depend on outer loop

[david.bolvansky at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87540

            Bug ID: 87540
           Summary: Missed inner loop hoist if the loop does not depend on
                    outer loop
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david.bolvansky at gmail dot com
  Target Milestone: ---

Not sure how often this happens in the real world apps but anyway idea is..

int foo(void)
{
    double  *array = calloc(ARRAY_SIZE, sizeof(double));
    double  sum = 0;
    int     i;
    for (i = 0; i < N_TIMES; i++) {
        // lot of code
        // well, this loop does not even depend on "i", hoist it?
        for (int j = 0; j < ARRAY_SIZE; j += 8) {
            sum += array[j] + array[j+1] + array[j+2] + array[j+3] + array[j+4]
+ array[j+5] +  array[j+6] + array[j+7];
        }
    }

    return sum;
}

Let's say we have the big outer loop with many inner loops. GCC should detect
if these loops really depend on the outer loop and if not, hoist them out of
this loop.