# Closure which discards the return is slower?

**URL:** <https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583>\
**Category:** Performance\
**Created:** [October 20, 2017, 2:50pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583 "2017-10-20T14:50:34Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [October 20, 2017, 2:50pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583/1 "2017-10-20T14:50:34Z")

</div>

```julia
using FunctionWrappers

struct CallbackF64 f::FunctionWrappers.FunctionWrapper{Void,Tuple{Array{Float64}}} end
(cb::CallbackF64)(v) = cb.f(v)

f!(u) = (u.=u.^2)
f = CallbackF64(f!)
u = Float64[1,2,3]
f(u) # Errors

g!(u) = (u.=u.^2; nothing)
g = CallbackF64(g!)
g(u)
g!(u)

h! = (u) -> (f!(u);nothing)
h = CallbackF64(h!)
h(u)
h!(u)

using BenchmarkTools
@benchmark f!($u)

BenchmarkTools.Trial: 
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 10.949 ns (0.00% GC)
  median time: 12.083 ns (0.00% GC)
  mean time: 14.726 ns (0.00% GC)
  maximum time: 541.074 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 1000

@benchmark g!($u)

BenchmarkTools.Trial: 
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 10.572 ns (0.00% GC)
  median time: 11.328 ns (0.00% GC)
  mean time: 13.529 ns (0.00% GC)
  maximum time: 722.690 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 1000

@benchmark h!($u)

BenchmarkTools.Trial: 
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 26.053 ns (0.00% GC)
  median time: 28.318 ns (0.00% GC)
  mean time: 37.032 ns (0.00% GC)
  maximum time: 1.934 μs (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 1000

```

Why is `h` so much slower?

---

<div class="post-metadata">

**Author:** ![ohsonice](https://avatars.discourse-cdn.com/v4/letter/o/d6d6ee/32.png) [@ohsonice](https://discourse.julialang.org/u/ohsonice)\
**Post date:** [October 20, 2017, 6:08pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583/2 "2017-10-20T18:08:42Z")

</div>

Not sure if I’m misunderstanding you, but it doesn’t seem like that discarding the return is the reason for the performance hit but rather the difference between:  
`f(u) = u` and `f = (u) -> u`

Another example:

```julia
julia> f!(u) = (u.=u.^2)
f! (generic function with 1 method)

julia> g!(u) = (f!(u))
g! (generic function with 1 method)

julia> h! = (u) -> (f!(u))
(::#3) (generic function with 1 method)

julia> u = Float64[1,2,3]
3-element Array{Float64,1}:
 1.0
 2.0
 3.0

julia> using BenchmarkTools

julia> @benchmark f!($u)
BenchmarkTools.Trial:
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 10.922 ns (0.00% GC)
  median time: 11.223 ns (0.00% GC)
  mean time: 11.282 ns (0.00% GC)
  maximum time: 76.766 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 999

julia> @benchmark g!($u)
BenchmarkTools.Trial:
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 10.923 ns (0.00% GC)
  median time: 11.219 ns (0.00% GC)
  mean time: 11.348 ns (0.00% GC)
  maximum time: 61.750 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 999

julia> @benchmark h!($u)
BenchmarkTools.Trial:
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 29.161 ns (0.00% GC)
  median time: 29.177 ns (0.00% GC)
  mean time: 30.473 ns (0.00% GC)
  maximum time: 110.651 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 995

```

Also not sure why `FunctionWrappers` is necessary in your example, you call the in-place versions anyways.

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [October 20, 2017, 6:12pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583/3 "2017-10-20T18:12:57Z")

</div>

> [@ohsonice](#):
>
> Not sure if I’m misunderstanding you, but it doesn’t seem like that discarding the return is the reason for the performance hit but rather the difference between:
> 
> f(u) = u and f = (u) -\> u

Huh, I’m surprised that doesn’t inline the function call and end up as not cost… ?

> [@ohsonice](#):
>
> Also not sure why FunctionWrappers is necessary in your example, you call the in-place versions anyways.

I want to do this as an option to stop specialization on functions in DiffEq, but I cannot guarantee that a user’s mutating function will actually return `nothing` even if it doesn’t matter what they return, and the `FunctionWrapper` does care about that.

---

<div class="post-metadata">

**Author:** ![ohsonice](https://avatars.discourse-cdn.com/v4/letter/o/d6d6ee/32.png) [@ohsonice](https://discourse.julialang.org/u/ohsonice)\
**Post date:** [October 20, 2017, 6:29pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583/4 "2017-10-20T18:29:17Z")

</div>

Hm okay, so does this solve the issue for now? (a combination of your `g!` and `h!`)

```julia
julia> k!(u) = (f!(u);nothing)
k! (generic function with 1 method)

julia> @benchmark k!($u)
BenchmarkTools.Trial:
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 10.915 ns (0.00% GC)
  median time: 11.216 ns (0.00% GC)
  mean time: 11.518 ns (0.00% GC)
  maximum time: 50.934 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 999

```

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [October 20, 2017, 6:33pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583/5 "2017-10-20T18:33:27Z")

</div>

Hmm, interesting. I’ll need to see if that holds true in the scope of another function.

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [October 20, 2017, 6:51pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583/6 "2017-10-20T18:51:27Z")

</div>

> [@ohsonice](#):
>
> @benchmark h!($u)

`@benchmark $h!($u)`

---

<div class="post-metadata">

**Author:** ![cstjean](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cstjean/32/1444_2.png) [@cstjean](https://discourse.julialang.org/u/cstjean)\
**Post date:** [October 20, 2017, 7:30pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583/7 "2017-10-20T19:30:58Z")

</div>

> [@ChrisRackauckas](#):
>
> Huh, I’m surprised that doesn’t inline the function call and end up as not cost… ?

In your example, `f!` is a normal function, but `h!` is a global variable that could contain anything. You’re at least paying the cost of dynamic dispatch.

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [October 20, 2017, 7:34pm UTC](https://discourse.julialang.org/t/closure-which-discards-the-return-is-slower/6583/8 "2017-10-20T19:34:05Z")

</div>

> [@yuyichao](#):
>
> @benchmark $h!($u)

> [@cstjean](#):
>
> In your example, f! is a normal function, but h! is a global variable that could contain anything. You’re at least paying the cost of dynamic dispatch.

Yeah, that’s it. Anonymous functions aren’t global constants like named functions, so this was just a benchmarking mistake. Thanks!