This is happening because of compiler optimisation technique called Loop Invariant Code Motion [0], which means that the author is not measuring what he thinks

This is happening because of compiler optimisation technique called Loop Invariant Code Motion [0], which means that the author is not measuring what he thinks he's measuring (and this should be obvious from the numbers really), so the result is meaningless. 0. https://en.wikipedia.org/wiki/Loop-invariant_code_motion https://en.wikipedia.org/wiki/Loop-invariant_code_motion