uint16 gcc 5.1 bug?

// gcc -O0 -m32 -march=i586 -mtune=i586 -o foo foo.c

#include <stdint.h>

static volatile uint16_t foo = 6;
static uint16_t start = 4;

uint16_t get_foo(void)
{
  return foo;
}

int main(void)
{
  uint16_t diff = (get_foo() - start) + 1;

  return 0;
}




// objdump -d foo
080483cb <get_foo>:
 80483cb:   55                      push   %ebp
 80483cc:   89 e5                   mov    %esp,%ebp
 80483ce:   66 a1 a8 96 04 08       mov    0x80496a8,%ax
 80483d4:   5d                      pop    %ebp
 80483d5:   c3                      ret

080483d6 <main>:
 80483d6:   55                      push   %ebp
 80483d7:   89 e5                   mov    %esp,%ebp
 80483d9:   83 ec 10                sub    $0x10,%esp
 80483dc:   e8 ea ff ff ff          call   80483cb <get_foo>
 80483e1:   89 c2                   mov    %eax,%edx
 80483e3:   66 a1 aa 96 04 08       mov    0x80496aa,%ax
 80483e9:   29 c2                   sub    %eax,%edx
 80483eb:   89 d0                   mov    %edx,%eax
 80483ed:   40                      inc    %eax
 80483ee:   66 89 45 fe             mov    %ax,-0x2(%ebp)
 80483f2:   b8 00 00 00 00          mov    $0x0,%eax
 80483f7:   c9                      leave
 80483f8:   c3                      ret
 80483f9:   66 90                   xchg   %ax,%ax
 80483fb:   66 90                   xchg   %ax,%ax
 80483fd:   66 90                   xchg   %ax,%ax
 80483ff:   90                      nop