title: Fastest way to set __m256 value to all ONE bits
body: <p>How can I set a value of 1 to all bits in an <code>__m256</code> value?
  Using either AVX or AVX2 intrinsics?</p>
  <p>To get all zeros, you can use <code>_mm256_setzero_si256()</code>.</p>
  <p>To get all ones, I'm currently using <code>_mm256_set1_epi64x(-1)</code>, but I suspect that this is slower than the all-zero case. Is there memory access or Scalar/SSE/AVX switching involved here?</p>
  <p>And I can't seem to find a simple bitwise NOT operation in AVX?
  If that was available, I could simply use the setzero, followed by a vector NOT.</p>