# Dangers of abusing multiple dispatch: missing optimization?

**URL:** <https://discourse.julialang.org/t/dangers-of-abusing-multiple-dispatch-missing-optimization/3221>\
**Category:** General Usage\
**Created:** [April 14, 2017, 5:16pm UTC](https://discourse.julialang.org/t/dangers-of-abusing-multiple-dispatch-missing-optimization/3221 "2017-04-14T17:16:37Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![aaowens](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aaowens/32/12101_2.png) [@aaowens](https://discourse.julialang.org/u/aaowens)\
**Post date:** [April 14, 2017, 5:16pm UTC](https://discourse.julialang.org/t/dangers-of-abusing-multiple-dispatch-missing-optimization/3221/1 "2017-04-14T17:16:37Z")

</div>

I was reading this section in the manual [https://docs.julialang.org/en/stable/manual/performance-tips/#the-dangers-of-abusing-multiple-dispatch-aka-more-on-types-with-values-as-parameters](https://docs.julialang.org/en/stable/manual/performance-tips/#the-dangers-of-abusing-multiple-dispatch-aka-more-on-types-with-values-as-parameters) , and I realized I’ve done this in some cases, as it is very convenient. I construct an instance of a type for dispatch purposes, but which type depends on some condition. Here’s two test examples

```julia
using BenchmarkTools
immutable Type1 end
immutable Type2 end
f(x, ::Type1) = 3.*x
f(x, ::Type2) = 5.*x
function test1(N)
  for i = 1:N
    r = rand()
    if r > .5
      a = Type1()
    else
      a = Type2()
    end
    f(r, a)
  end
end
function test2(N)
  for i = 1:N
    r = rand()
    if r > .5
      a = Type1()
      f(r, a)
    else
      a = Type2()
      f(r, a)
    end
  end
end
@benchmark test1(1000)
median time: 18.571 μs (0.00% GC)
@benchmark test2(1000)
  median time: 1.967 μs (0.00% GC)

```

`test1` is similar to what I’ve done in the past. I realized here that `test2` substantially faster. It appears that within the if block there is no type instability, so the dispatch is easy.

I wonder if some future optimization can make performance the same?

---

<div class="post-metadata">

**Author:** ![jameson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jameson/32/23_2.png) [@jameson](https://discourse.julialang.org/u/jameson)\
**Post date:** [April 14, 2017, 5:39pm UTC](https://discourse.julialang.org/t/dangers-of-abusing-multiple-dispatch-missing-optimization/3221/2 "2017-04-14T17:39:48Z")

</div>

yes. That optimization is known as “jump-threading” is pretty standard – although not yet implemented in Julia, it’s perhaps just waiting for someone to decide to dive into it 🙂.