vec_type_hint() does not work for me

trying to disable auto-vectorization, I used vec_type_hint(char) as below.  But checking the built assembly code, I see the kernel is still compiled as SIMD32.  Any advice?

__kernel __attribute__((vec_type_hint(uchar)))  void modulate_v1_uchar(global const uchar *pSrc, global  uchar *pDst) 
//__kernel void modulate_v1_uchar(global const uchar *pSrc, global  uchar *pDst) 
{ 

    uint idx = get_global_id(0);

    uchar src = pSrc[idx];
    src >>= MODULATE_SHIFT_FACTOR;
    pDst[idx] = src;
}