# Inference and speeding in an interpolation

**URL:** https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027
**Category:** Performance
**Tags:** inference
**Created:** [March 28, 2018, 10:13am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027 "2018-03-28T10:13:04Z")
**Posts on this page:** 9
**Page:** 1

<div class="post-metadata">

### Author: ![amrods](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/amrods/32/2543_2.png) [@amrods](https://discourse.julialang.org/u/amrods)
#### Post date: [March 28, 2018, 10:13am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/1 "2018-03-28T10:13:04Z")

</div>

Out of convenience I’m wrapping a scaled interpolation from `Interpolations.jl`:

```julia
grid_x = 0:0.1:100.0
grid_y = 0:0.1:100.0

f(x, y) = (x + y)^2

farr = [f(x, y) for x in grid_x, y in grid_y]

using Interpolations

function interp1(grid_x, grid_y, farr)
    itp = interpolate(farr, BSpline(Cubic(Line())), OnGrid())
    sitp = scale(itp, grid_x, grid_y)
    return sitp
end

interp(grid_x, grid_y, xy) = interp1(grid_x, grid_y, farr)[xy...]

xy = [0.01, 0.02]

@code_warntype interp(grid_x, grid_y, xy)
Variables:
  #self# <optimized out>
  grid_x::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}
  grid_y::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}
  xy::Array{Float64,1}
  itp::Interpolations.BSplineInterpolation{_,_,_,Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,_} where _ where _ where _ where _
  sitp::Any

Body:
  begin 
      SSAValue(0) = Main.farr
      $(Expr(:inbounds, false))
      # meta: location In[4] interp1 3
      itp::Interpolations.BSplineInterpolation{_,_,_,Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,_} where _ where _ where _ where _ = (Main.interpolate)(SSAValue(0), $(QuoteNode(Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}}())), $(QuoteNode(Interpolations.OnGrid())))::Interpolations.BSplineInterpolation{_,_,_,Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,_} where _ where _ where _ where _ # line 4:
      sitp::Any = (Main.scale)(itp::Interpolations.BSplineInterpolation{_,_,_,Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,_} where _ where _ where _ where _, grid_x::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, grid_y::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}})::Any
      # meta: pop location
      $(Expr(:inbounds, :pop))
      return (Core._apply)(Main.getindex, (Core.tuple)(sitp::Any)::Tuple{Any}, xy::Array{Float64,1})::Any
  end::Any

```

How can I improve inference to get rid of the `Any`s? I tried annotating the output and arguments of `interp1` and `interp` but I still get some `Any`s in the body.

---

<div class="post-metadata">

### Author: ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [March 28, 2018, 10:27am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/2 "2018-03-28T10:27:19Z")

</div>

Inference fails because `farr` is [global](https://docs.julialang.org/en/latest/manual/performance-tips/#Avoid-global-variables-1), and the compiler can’t ensure that you will not change it to something else (with a different type). Make it a parameter, eg

```julia
interp(grid_x, grid_y, farr, xy) = interp1(grid_x, grid_y, farr)[xy...]

```

---

<div class="post-metadata">

### Author: ![amrods](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/amrods/32/2543_2.png) [@amrods](https://discourse.julialang.org/u/amrods)
#### Post date: [March 28, 2018, 11:07am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/3 "2018-03-28T11:07:27Z")

</div>

I tried with making `farr` a `const`, and making it a parameter like you suggest, but I still get some `Any`s:

```julia
@code_warntype interp(grid_x, grid_y, farr, xy)
Variables:
  #self# <optimized out>
  grid_x::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}
  grid_y::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}
  farr::Array{Float64,2}
  xy::Array{Float64,1}
  itp::Interpolations.BSplineInterpolation{Float64,2,Array{Float64,2},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,1}
  sitp::Interpolations.ScaledInterpolation{Float64,2,Interpolations.BSplineInterpolation{Float64,2,Array{Float64,2},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,1},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,Tuple{StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}},StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}}}

Body:
  begin 
      $(Expr(:inbounds, false))
      # meta: location In[7] interp1 3
      itp::Interpolations.BSplineInterpolation{Float64,2,Array{Float64,2},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,1} = $(Expr(:invoke, MethodInstance for interpolate(::Type{Float64}, ::Type{Float64}, ::Array{Float64,2}, ::Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}}, ::Interpolations.OnGrid), :(Interpolations.interpolate), Float64, Float64, :(farr), :($(QuoteNode(Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}}()))), :($(QuoteNode(Interpolations.OnGrid()))))) # line 4:
      sitp::Interpolations.ScaledInterpolation{Float64,2,Interpolations.BSplineInterpolation{Float64,2,Array{Float64,2},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,1},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,Tuple{StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}},StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}}} = $(Expr(:invoke, MethodInstance for scale(::Interpolations.BSplineInterpolation{Float64,2,Array{Float64,2},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,1}, ::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, ::Vararg{StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}},N} where N), :(Main.scale), :(itp), :(grid_x), :(grid_y)))
      # meta: pop location
      $(Expr(:inbounds, :pop))
      return (Core._apply)(Main.getindex, (Core.tuple)(sitp::Interpolations.ScaledInterpolation{Float64,2,Interpolations.BSplineInterpolation{Float64,2,Array{Float64,2},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,1},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,Tuple{StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}},StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}}})::Tuple{Interpolations.ScaledInterpolation{Float64,2,Interpolations.BSplineInterpolation{Float64,2,Array{Float64,2},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,1},Interpolations.BSpline{Interpolations.Cubic{Interpolations.Line}},Interpolations.OnGrid,Tuple{StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}},StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}}}}, xy::Array{Float64,1})::Any
  end::Any

```

---

<div class="post-metadata">

### Author: ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [March 28, 2018, 11:15am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/4 "2018-03-28T11:15:39Z")

</div>

Don’t pass `xy` as a `Vector`, as the type itself does not determine the number of elements. The following is inferred correctly:

```julia
using Interpolations
grid_x = 0:0.1:100.0
grid_y = 0:0.1:100.0
f(x, y) = (x + y)^2
farr = [f(x, y) for x in grid_x, y in grid_y]

function interp1(grid_x, grid_y, farr)
    itp = interpolate(farr, BSpline(Cubic(Line())), OnGrid())
    scale(itp, grid_x, grid_y)
end

interp(grid_x, grid_y, farr, x, y) = interp1(grid_x, grid_y, farr)[x, y]

@code_warntype interp1(grid_x, grid_y, farr) # OK
@code_warntype interp(grid_x, grid_y, farr, 0.01, 0.02) # OK

```

---

<div class="post-metadata">

### Author: ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [March 28, 2018, 11:16am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/5 "2018-03-28T11:16:59Z")

</div>

Also, I would suggest just saving the result of `inter1p` and working with that, not calling `interpolate` and `scale` each time you want an interpolation.

---

<div class="post-metadata">

### Author: ![amrods](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/amrods/32/2543_2.png) [@amrods](https://discourse.julialang.org/u/amrods)
#### Post date: [March 28, 2018, 11:23am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/6 "2018-03-28T11:23:57Z")

</div>

Yes, this works. So here the problem is that the compiler doesn’t know the number of elements? It would be really annoying to have to have to define the function for a large number of components of `xy`.

---

<div class="post-metadata">

### Author: ![amrods](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/amrods/32/2543_2.png) [@amrods](https://discourse.julialang.org/u/amrods)
#### Post date: [March 28, 2018, 11:26am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/7 "2018-03-28T11:26:34Z")

</div>

Yes, I can work like that, but eventually I will have to modify `farr` in the course of an algorithm. That was the whole idea of wrapping `interpolate` in a function.

---

<div class="post-metadata">

### Author: ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [March 28, 2018, 11:30am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/8 "2018-03-28T11:30:16Z")

</div>

Depending on how large the number of elements is, you can try something inferrable, eg

> **[GitHub - JuliaArrays/StaticArrays.jl: Statically sized arrays for Julia](https://github.com/JuliaArrays/StaticArrays.jl)**
>
> Statically sized arrays for Julia. Contribute to JuliaArrays/StaticArrays.jl development by creating an account on GitHub.

---

<div class="post-metadata">

### Author: ![amrods](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/amrods/32/2543_2.png) [@amrods](https://discourse.julialang.org/u/amrods)
#### Post date: [March 28, 2018, 11:40am UTC](https://discourse.julialang.org/t/inference-and-speeding-in-an-interpolation/10027/9 "2018-03-28T11:40:18Z")

</div>

Thanks, I’ll play more with this, but simply calling `interp` on `SVector(0.01, 0.02)` still breaks inference.