# Compile-time for large SizedArray (StaticArrays.jl)

**URL:** <https://discourse.julialang.org/t/compile-time-for-large-sizedarray-staticarrays-jl/28945>\
**Category:** General Usage\
**Tags:** question\
**Created:** [September 19, 2019, 1:20pm UTC](https://discourse.julialang.org/t/compile-time-for-large-sizedarray-staticarrays-jl/28945 "2019-09-19T13:20:16Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![josuagrw](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/josuagrw/32/1015_2.png) [@josuagrw](https://discourse.julialang.org/u/josuagrw)\
**Post date:** [September 19, 2019, 1:20pm UTC](https://discourse.julialang.org/t/compile-time-for-large-sizedarray-staticarrays-jl/28945/1 "2019-09-19T13:20:16Z")

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I noticed compile time rapidly increases for large `SizedArray`s.

For example:

```julia
julia> using StaticArrays

julia> bench(a) = @time maximum(a)
bench (generic function with 1 method)

julia> bench(randn(1_000, 1_000))
  0.000811 seconds
4.8615467274882045

julia> n = 10
10

julia> bench(Size(n,n)(randn(n, n)))
  0.270037 seconds (241.64 k allocations: 12.865 MiB, 2.54% gc time)
2.3638881215887215

julia> n = 20
20

julia> bench(Size(n,n)(randn(n, n)))
  3.662963 seconds (993.21 k allocations: 51.066 MiB, 0.58% gc time)
3.099424860659397

julia> bench(Size(n,n)(randn(n, n)))
  0.000010 seconds (1 allocation: 16 bytes)
2.8169198602554775

```

I understand that each `SizedArray` of a new `Size` is a different type which requires further compilation. However, why does the compile time depend on the exact size of the array? Why isn’t it constant? Is there a way to make it constant?

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**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [September 19, 2019, 1:31pm UTC](https://discourse.julialang.org/t/compile-time-for-large-sizedarray-staticarrays-jl/28945/2 "2019-09-19T13:31:54Z")

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StaticArrays has its own implementation of `maximum` which unrolls the loop over the elements of the array (using an `@generated` function). That results in an amount of code being generated which is proportional to the number of elements in the array. You can see the custom `mapreduce` implementation that is used under the hood here: [https://github.com/JuliaArrays/StaticArrays.jl/blob/master/src/mapreduce.jl#L79-L91](https://github.com/JuliaArrays/StaticArrays.jl/blob/master/src/mapreduce.jl#L79-L91)

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<div class="post-metadata">

**Author:** ![josuagrw](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/josuagrw/32/1015_2.png) [@josuagrw](https://discourse.julialang.org/u/josuagrw)\
**Post date:** [September 19, 2019, 1:45pm UTC](https://discourse.julialang.org/t/compile-time-for-large-sizedarray-staticarrays-jl/28945/3 "2019-09-19T13:45:16Z")

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Ok, I assume this is also true for `map`, `broadcast`, etc? I guess that means, `SizedArray` really shouldn’t be used for more than 100-element arrays.

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**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:** [September 20, 2019, 1:22am UTC](https://discourse.julialang.org/t/compile-time-for-large-sizedarray-staticarrays-jl/28945/4 "2019-09-20T01:22:14Z")

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This isn’t inherent to the approach.  
If you just add a check on the length inside the generated function, you could return a loop over a certain size, and an unrolled expression below it. The matrix multiplication code does something like this (with `MMatrix` ultimately dispathching to BLAS):  
[https://github.com/JuliaArrays/StaticArrays.jl/blob/master/src/matrix\_multiply.jl#L95](https://github.com/JuliaArrays/StaticArrays.jl/blob/master/src/matrix_multiply.jl#L95)