# ReinterpretedArray Performance (even worse on 1.8)

**URL:** https://discourse.julialang.org/t/reinterpretedarray-performance-even-worse-on-1-8/80102
**Category:** Performance
**Tags:** performance
**Created:** [April 27, 2022, 3:31am UTC](https://discourse.julialang.org/t/reinterpretedarray-performance-even-worse-on-1-8/80102 "2022-04-27T03:31:02Z")
**Posts on this page:** 1
**Showing post:** 26

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### Author: ![cortner](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cortner/32/204_2.png) [@cortner](https://discourse.julialang.org/u/cortner)
#### Post date: [April 27, 2022, 5:56pm UTC](https://discourse.julialang.org/t/reinterpretedarray-performance-even-worse-on-1-8/80102/26 "2022-04-27T17:56:43Z")

</div>

Concretely the way I want to use this is to have temporary arrays for cases where I can predict their `eltype` at evaluation time, but not at the time I’m constructing the object where the temporary array is held. So very crudely, I would do something like this:

```julia
mutable struct A
    tmp::Vector{UInt8} 
    # other stuff
end 

function evaluate(a::A, x::TX)
    T = predict_eltype(TX)
    # resize a.tmp if necessary, or replace with larger array
    tmp = reinterpret(T, a.tmp)
    # ... do some computations in tmp 
    # return output
end

```

But then I noticed that the `reinterpret` sometimes causes a significant slow-down.

I should also acknowledge that the speedup I’m getting by not allocating is not at all clear, sometimes I seem to gain quite a bit, sometimes nothing. Maybe it depends on whether the GC can figure out that I want to reuse my arrays and keeps them around? (Basically that’s the functionality I’m trying to re-implement here…)

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