polymorphic vs monomorphic

[mcollina]

Jakob Kummerow wrote:

I can't reproduce (or explain) your results for "polymorphic functions" with a plain d8 binary. While it always has been and always will be true that polymorphism has a speed cost, it doesn't make sense that a single "obj.length" load amid all the other stuff (creating objects, branching, calling, looping) would cause a 2x slowdown overall, because all the other code (like the empty loop) is not affected by that one polymorphic property load.

How can we update the benchmark in a way that is meaningful? cc @bmeurer @fhinkel @hashseed @TimothyGu

[jakobkummerow]

function polymorphic() {
  var objects = [{a:1}, {b:1, a:2}, {c:1, b:2, a:3}, {d:1, c:2, b:3, a:4}];
  var sum = 0;
  for (var i = 0; i < 10000; i++) {
    var o = objects[i & 3];
    sum += o.a;
  }
  return sum;
}

function monomorphic() {
  var objects = [{a:1}, {a:2}, {a:3}, {a:4}];
  var sum = 0;
  for (var i = 0; i < 10000; i++) {
    var o = objects[i & 3];
    sum += o.a;
  }
  return sum;
}

(only var objects is different between the two)

The higher the degree of polymorphism, the bigger the performance cost. For two different shapes, I'm seeing about 10-30% more time in the polymorphic compared to the monomorphic version; for four different shapes the delta goes up to 60-100% (it's pretty flaky in repeated runs).

Note that all these numbers are very specific to the situation you're testing. If you swap the order of properties in the object -- it makes a difference. If some of the object shapes show up more frequently than others -- it makes a difference. For polymorphism degrees greater than four, even what the rest of your program is doing makes a (possibly huge!) difference. It's extremely difficult to draw simple, generally-true conclusions from this, or give general advice. Polymorphism can be anything from "nothing to worry about" to "the single biggest performance problem with this app".

[davidmarkclements]

Awesome thanks @jakobkummerow

I think the generalism we can make is stay away from polymorphism in hot paths because it could trigger a significant performance issue