title: clang osx march=native approximately 1/3rd speed of no optimisation
body: <p>I have a project where speed is paramount, so was experimenting with compiler flags to try and get some free performance. I have two builds that are identical except for the additional flag march=native in build 2.</p>
  
  <p>For completeness the flags are:</p>
  
  <p>A) -std=c++14 -g -fno-omit-frame-pointer -O3</p>
  
  <p>B) -std=c++14 -g -fno-omit-frame-pointer -O3 -march-native</p>
  
  <p>Running benchmarks on these builds yields a confusing result:</p>
  
  <p>A) 61s</p>
  
  <p>B) 160s</p>
  
  <p>What can possibly going on here?</p>