title: Summing 3 lanes in a NEON float32x4_t
body: <p>I'm vectorizing an inner loop with ARM NEON intrinsics (llvm, iOS). I'm generally using <code>float32x4_t</code>s. My computation finishes with the need to sum three of the four floats in this vector. </p>
  
  <p>I can drop back to C floats at this point and <code>vst1q_f32</code> to get the four values out and add up the three I need. But I figure it may be more effective if there's a way to do it directly with the vector in an instruction or two, and then just grab a single lane result, but I couldn't figure out any clear path to doing this.</p>
  
  <p>I'm new to NEON programming, and the existing "documentation" is pretty horrific. Any ideas? Thanks!</p>