[Bug c/66777] New: faggressive-loop-optimizations behavior.

[dongkyun.s at samsung dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66777

            Bug ID: 66777
           Summary: faggressive-loop-optimizations behavior.
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dongkyun.s at samsung dot com
  Target Milestone: ---

The following program seems to have optimized with -O2
(-faggressive-loop-optimizations).

#include <stdio.h>

int main(void)
{
        int a[5];
        int i;

        for (i = 0; a[i] && i < 5; i++)
                printf("%d\n", a[i]);

        return 0;
}

1) .s with -O2 option (-faggressive-loop-optimizations)
...
.L4
      r02     mo3w r0, #:lower16:.LC0
      movt    r0, #:upper16:.LC0
      bl      printf
      ldr     r1, [r4, #4]!
      cmp     r1, #0         // a[i]
      bne     .L4
...

2) .s without -On option (-fno-aggressive-loop-optimizations)
...
.L4
...
       ldr     r2, [fp, #-8]
       mvn     r3, #23
       mov     r2, r2, asl #2
       sub     r1, fp, #4
       add     r2, r2, r1
       add     r3, r2, r3
       ldr     r3, [r3, #0]
       cmp     r3, #0     // a[i]
       beq     .L3
       ldr     r3, [fp, #-8]
       cmp     r3, #4     // i < 5
       ble     .L4
.L3
...

< -faggressive-loop-optimizations >
This assumes that loop code does not invoke undefined behavior by for example
causing signed integer overflows or out-of-bound array accesses. The bounds for
the number of iterations of a loop are used to guide loop unrolling and peeling
and loop exit test optimizations. This option is enabled by default. 

The problem with 1) option (-faggressive-loop-optimizations) is that the
developer cannot recognize the program have bug which can cause NULL pointer
exception.

Is there some warning option releated to this optimization ?