[Bug c/88867] New: -Waggressive-loop-optimizations doesn't warn when -faggressive-loop-optimizations is in play

[levon at movementarian dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88867

            Bug ID: 88867
           Summary: -Waggressive-loop-optimizations doesn't warn when
                    -faggressive-loop-optimizations is in play
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: levon at movementarian dot org
  Target Milestone: ---

Given how dangerous -faggressive-loop-optimizations has been proven to be, it's
unfortunate that -Waggressive-loop-optimizations is non-functional in even
simple cases. For example this test case:

#define NULL ((void *)0)

static char *arr[2] = { "nasal", "demons" };

long
func()
{
        int i;

        for (i = 0; i <= 2; i++) {
                if (arr[i] == NULL && i == 0)
                        return (0xbad);
        }

        return (0xfad);
}

Is optimized to "return 0xbad" with -faggressive-loop-optimizations, but it is
not possible to get GCC to warn about this. -Waggressive-loop-optimizations
should warn every single time it relies on out-of-bounds behaviour like this.
Otherwise, such code becomes impossible to find and fix.

Same with trunk gcc on godbolt:

https://godbolt.org/z/MN1beq