typedef struct
{
   short real;
   short imag;
} complex16_t;

void
libvector_AccSquareNorm_ref (unsigned long long  *acc,
                             const complex16_t *x, unsigned len)
{
    for (unsigned i = 0; i < len; i++)
    {
        acc[i] +=
            ((unsigned long long)((int)x[i].real * x[i].real)) +
            ((unsigned long long)((int)x[i].imag * x[i].imag));
    }
}

Compiler the code with 
~/scratch/install-x86/bin/gcc tst.c -O2 -S -ftree-vectorize -fdump-tree-vect-details -std=c99

GCC generates unnecessary loop versioning because it cannot disambiguate mem accesses. 

tst.c:12:5: note: versioning for alias required: can't determine dependence between *_8 and _12->real
tst.c:12:5: note: mark for run-time aliasing test between *_8 and _12->real

This should be handled by TBAA info as acc & x clearly point to different data types. But unfortunately, TBAA doesn't handle Anti- & Output- dependencies.