title: Inverse square root intrinsics
body: <p>Are there compiler intrinsics for the inverse square root, single and double-precision floating-point, on a scalar argument ?</p>
  
  <p>I can find these for packed SIMD arguments (SSE and AVX) but not for scalars.</p>
  
  <p>Anything faster than division by the <code>&lt;math.h&gt;</code> square-root is welcome as well.</p>