/// halves-optimized, 32-bit using bswap
unsigned rbH4 (unsigned arg)
{
  __asm
  {
    mov eax, arg

    lea edx, [eax+eax]
    shr eax, 1
    and eax,  55555555H
    and edx, 0AAAAAAAAH
    or eax, edx

    lea edx, [eax+eax]
    shr eax, 2
    add edx, edx
    and eax,  33333333H
    and edx, 0CCCCCCCCH
    or eax, edx

    mov edx, eax
    shr eax, 4
    shl edx, 4
    and eax,  0F0F0F0FH
    and edx, 0F0F0F0F0H
    or eax, edx

    bswap eax     // reverse bytes
  }
  // return eax
}