mfikes/negative-benchmark.md

## negative-benchmark.md

      
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              negative-benchmark.md
            
          
    If you run a trivial benchmark that can be heavily optimized, such as
(simple-benchmark [] 1 1)
You can sometimes see negative benchmarks under :advanced such as
[], 1, 1 runs, -2 msecs

In this case, you end up with heavily elided JavaScript code, as if
it were effectively produced from ClojureScript like the following
(measuring the elapsed time to do nothing):
(let [start (.getTime (js/Date.))
      end   (.getTime (js/Date.))]
    (- end start))
Ignoring simple-benchmark and what :advanced does with the code generated
for it, let's say you actually start with the above code. This gets transpiled
by ClojureScript to the following, which is fine:
(function() {
  var start = new Date().getTime();
  var end = new Date().getTime();
  return end - start;
})();
But then Closure :advanced performs an overly aggressive inlining optimization
of the date expressions, producing
(new Date).getTime()-(new Date).getTime()
Given the left-to-right evaluation order, the number on the right can be larger
(if, for example, evaluated a couple of milliseconds later), thus producing a negative
number.
I've written google/closure-compiler#3079