[Bug c/86341] New: Aggressive loop unrolling in gcc 7.x produces out-of-bounds index

[tomek.bury at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86341

            Bug ID: 86341
           Summary: Aggressive loop unrolling in gcc 7.x produces
                    out-of-bounds index
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tomek.bury at gmail dot com
  Target Milestone: ---

Created attachment 44331
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44331&action=edit
test case, compile with 7.x gcc -Wall -O3

Compiling the below code produces a warning in gcc 7.x series:

test.c:
---
struct test
{
    unsigned num_items;
    int items[3];
};

struct test t = {2, {1, 2}};

int oopsie(void)
{
    int foo = 0;
    for(unsigned i = 0; i < t.num_items; i++)
    {
        const int *item_i = &t.items[i];
        foo += *item_i;
        for(unsigned j = i + 1; j < t.num_items; j++)
        {
            const int *item_j = &t.items[j];
            foo += *item_j;
        }
    }
    return foo;
}
---

$ gcc -Wall -o test.o -c test.c -O3
test.c: In function ‘oopsie’:
test.c:18:41: warning: array subscript is above array bounds [-Warray-bounds]
             const int *item_j = &t.items[j];
                                  ~~~~~~~^~~

Copying the t.num_items to a local variable and using a local copy in any of
the loops makes the problem disappear. Also disabling aggressive loop unrolling
make the warning disappear. The gcc 6.x and 8.x seem to be all right.