title: ARM Neon: Store n-th position(s) of non-zero byte(s) in a 8-byte vector lane
body: <p>I want to convert a Neon 64-bit vector lane to get the n-th position(s) of non-zero (aka. 0xFF) 8-bit value(s), and then fill the rest of the vector with zeros. Here are some examples:</p>
  
  <pre><code>    0  1  2  3  4  5  6  7
  
  d0: 00 FF 00 FF 00 00 00 FF
  d1: 1  3  7  0  0  0  0  0
  
  d0: 00 FF FF FF 00 00 FF 00
  d1: 1  2  3  6  0  0  0  0
  
  d0: FF FF FF FF FF FF FF FF
  d1: 0  1  2  3  4  5  6  7
  
  d0: FF 00 00 00 00 00 00 00
  d1: 0  0  0  0  0  0  0  0
  
  d0: 00 00 00 00 00 00 00 00
  d1: 0  0  0  0  0  0  0  0
  </code></pre>
  
  <p>I have the feeling that it's probably one or two bit-shift Neon instructions with another "good" vector. How can I do that?</p>