The problem is described here: https://bugs.gentoo.org/show_bug.cgi?id=505026
The code to trigger comes from a launchpad bug: https://bugs.launchpad.net/ubuntu/+source/gcc-defaults/+bug/1072650

As you may notice, it's acros distros and affects many compiler versions (confirmed for 4.6.3 on ubuntu, 4.7.3 and 4.8.2 on Gentoo).

Following code (AFAICT) violates strict aliasing rules:
#include <stdio.h>

struct psuedo_hdr
{
  int saddr;
  int daddr;
  char zero;
  char protocol;
  short len;
} __attribute__((packed));

int main()
{
  unsigned int i;
  unsigned int sum = 0;
  struct psuedo_hdr hdr;

  hdr.saddr = 0xaabbccdd;
  hdr.daddr = 0x11223344;
  hdr.zero = 0;
  hdr.protocol = 6;
  hdr.len = 2;
  for (i = 0; i < sizeof(hdr); i += 2)
    sum += *(short *)((char *)(&hdr) + i);
  printf("0x%x\n", sum);
  return 0;
}

however, '-O2 -Wall' doesn't result in the strict aliasing warning.