[Bug c/66090] New: Wrong loop code generation with -O2 on ARM

["christian dot prochaska at genode-labs dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66090

            Bug ID: 66090
           Summary: Wrong loop code generation with -O2 on ARM
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: christian.prochaska@genode-labs.com
  Target Milestone: ---

test.c:

void func()
{
    unsigned int i;

    unsigned int *ptr = (unsigned int*)0xfffff000;

    for (i = 0; i < 1024; i++)
        *(ptr++) = 0;
}

Compiled with GCC 4.9.2 from the Xubuntu 15.04 repository:

arm-linux-gnueabihf-gcc-4.9 -marm -c -O2 test.c

Generated code:

00000000 <func>:
func():
   0:   e3a03a0f        mov     r3, #61440      ; 0xf000
   4:   e3a02000        mov     r2, #0
   8:   e34f3fff        movt    r3, #65535      ; 0xffff
   c:   e4832004        str     r2, [r3], #4
  10:   eafffffd        b       c <func+0xc>

There is no check of the loop exit condition.

When compiling with -O1, it is there:

00000000 <func>:
func():
   0:   e3a03a0f        mov     r3, #61440      ; 0xf000
   4:   e34f3fff        movt    r3, #65535      ; 0xffff
   8:   e3a02000        mov     r2, #0
   c:   e4832004        str     r2, [r3], #4
  10:   e3530000        cmp     r3, #0
  14:   1afffffc        bne     c <func+0xc>
  18:   e12fff1e        bx      lr

The problem does not occur with GCC 4.8.4.