# What's the benefit of inlining?

**URL:** <https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [December 13, 2017, 3:13am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727 "2017-12-13T03:13:32Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![chobbes](https://avatars.discourse-cdn.com/v4/letter/c/848f3c/32.png) [@chobbes](https://discourse.julialang.org/u/chobbes)\
**Post date:** [December 13, 2017, 3:13am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727/1 "2017-12-13T03:13:32Z")

</div>

[https://docs.julialang.org/en/stable/stdlib/base/#Base.@inline](https://docs.julialang.org/en/stable/stdlib/base/#Base.@inline)

I wonder why we want to inline a function by @inline or tacitly allow the compiler to do so automatically? Any benefit of having a function inlined? Is there any difference between an inlined version of a function and its un-inlined version from user point of view? Thanks.

---

<div class="post-metadata">

**Author:** ![Elrod](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elrod/32/22461_2.png) [@Elrod](https://discourse.julialang.org/u/Elrod)\
**Post date:** [December 13, 2017, 6:53am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727/2 "2017-12-13T06:53:01Z")

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You can see an example [here](https://discourse.julialang.org/t/immutables-with-reference-fields-why-boxed/7706/7). Copy and pasting foobar\_lv2’s example:

```julia
@inline foo_in(n) = (n, Vector{Int}(n))
@noinline foo_ni(n) = (n, Vector{Int}(n))

function ft_in(n)
       s= 0
       for i= 1:n
        jj,v = foo_in(i)
       s+=sum(v)
       end
       s
end

function ft_ni(n)
       s= 0
       for i= 1:n
        jj,v = foo_ni(i)
       s+=sum(v)
       end
       s
end

@time ft_in(1000)
  0.001948 seconds (1.00 k allocations: 3.962 MiB)

@time ft_ni(1000)
  0.002083 seconds (2.00 k allocations: 3.992 MiB)

```

That example was to demonstrate allocations (which will hopefully be fixed). If you want an example for runtime:

```julia
julia> using BenchmarkTools

julia> @noinline ni(a, b, c) = a * b + c
ni (generic function with 1 method)

julia> @inline fin(a, b, c) = a * b + c
in (generic function with 1 method)

julia> function muladd_fni(va, vb, vc)
           out = zero(eltype(va))
           @assert length(va) == length(vb) == length(vc)
           @inbounds @simd for i ∈ eachindex(va)
               out += ni(va[i], vb[i], vc[i])
           end
           out
       end
muladd_ni (generic function with 1 method)

julia> function muladd_fin(va, vb, vc)
           out = zero(eltype(va))
           @assert length(va) == length(vb) == length(vc)
           @inbounds @simd for i ∈ eachindex(va)
               out += fin(va[i], vb[i], vc[i])
           end
           out
       end
muladd_in (generic function with 1 method)

julia> muladd_fin(va, vb, vc)
155.9768456827954

julia> muladd_fni(va, vb, vc)
155.97684568279539

julia> @benchmark muladd_fni($va, $vb, $vc)
BenchmarkTools.Trial: 
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 898.750 ns (0.00% GC)
  median time: 905.795 ns (0.00% GC)
  mean time: 925.898 ns (0.00% GC)
  maximum time: 2.154 μs (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 44

julia> @benchmark muladd_fin($va, $vb, $vc)
BenchmarkTools.Trial: 
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 43.738 ns (0.00% GC)
  median time: 44.548 ns (0.00% GC)
  mean time: 46.434 ns (0.00% GC)
  maximum time: 79.827 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 990

```

`@simd` requires inlining.

---

<div class="post-metadata">

**Author:** ![chobbes](https://avatars.discourse-cdn.com/v4/letter/c/848f3c/32.png) [@chobbes](https://discourse.julialang.org/u/chobbes)\
**Post date:** [December 13, 2017, 7:06am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727/3 "2017-12-13T07:06:53Z")

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Thanks for the examples! I guess it should **in** `(va[i], vb[i], vc[i])` in `muladd_in`?

If so, `muladd_in` and `muladd_ni` are not much different. Then I wonder why the benchmark statistics are different.

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<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [December 13, 2017, 7:32am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727/4 "2017-12-13T07:32:00Z")

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This has good info [Inline expansion - Wikipedia](https://en.wikipedia.org/wiki/Inline_expansion#Effect_on_performance)

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<div class="post-metadata">

**Author:** ![chobbes](https://avatars.discourse-cdn.com/v4/letter/c/848f3c/32.png) [@chobbes](https://discourse.julialang.org/u/chobbes)\
**Post date:** [December 13, 2017, 7:37am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727/5 "2017-12-13T07:37:08Z")

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Thanks for the pointer!

---

<div class="post-metadata">

**Author:** ![Elrod](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elrod/32/22461_2.png) [@Elrod](https://discourse.julialang.org/u/Elrod)\
**Post date:** [December 13, 2017, 7:47am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727/6 "2017-12-13T07:47:19Z")

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Oops. I have no idea what was going on there @\_@.  
Because of my mistake (now fixed!), it should have been the exact same code.

The difference is now dramatic.

---

<div class="post-metadata">

**Author:** ![chobbes](https://avatars.discourse-cdn.com/v4/letter/c/848f3c/32.png) [@chobbes](https://discourse.julialang.org/u/chobbes)\
**Post date:** [December 13, 2017, 7:50am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727/7 "2017-12-13T07:50:51Z")

</div>

Thanks, buddy. Now I’m impressed by inline.

---

<div class="post-metadata">

**Author:** ![Elrod](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elrod/32/22461_2.png) [@Elrod](https://discourse.julialang.org/u/Elrod)\
**Post date:** [December 13, 2017, 8:03am UTC](https://discourse.julialang.org/t/whats-the-benefit-of-inlining/7727/8 "2017-12-13T08:03:12Z")

</div>

For some reason something odd seems to be going on on my machine, where the first defined instance of a function is sometimes slower than subsequent instances. I’m tired, so I’ll look into that tomorrow.  
I add that disclaimer because I updated the times to reflect the redefinition.  
(The `@code_lowered` looks exactly the same.)

I make this comment to point out that you don’t need to write `@inline` everywhere:

```julia
julia> let_julia_decide(a, b, c) = a * b + c
let_julia_decide (generic function with 1 method)

julia> function muladd_ljd(va, vb, vc)
           out = zero(eltype(va))
           @assert length(va) == length(vb) == length(vc)
           @inbounds @simd for i ∈ eachindex(va)
               out += let_julia_decide(va[i], vb[i], vc[i])
           end
           out
       end
muladd_ljd (generic function with 1 method)

julia> muladd_ljd(va, vb, vc)
155.72913245066408

julia> @benchmark muladd_ljd($va, $vb, $vc)
BenchmarkTools.Trial: 
  memory estimate: 0 bytes
  allocs estimate: 0
  --------------
  minimum time: 43.738 ns (0.00% GC)
  median time: 44.275 ns (0.00% GC)
  mean time: 45.947 ns (0.00% GC)
  maximum time: 72.803 ns (0.00% GC)
  --------------
  samples: 10000
  evals/sample: 990

```

Julia will inline small functions automatically.