# Reinterpret UInt64 as SVector{8,UInt8}

**URL:** https://discourse.julialang.org/t/reinterpret-uint64-as-svector-8-uint8/18652
**Category:** Performance
**Created:** [December 14, 2018, 10:18am UTC](https://discourse.julialang.org/t/reinterpret-uint64-as-svector-8-uint8/18652 "2018-12-14T10:18:03Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![zsoerenm](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/zsoerenm/32/664_2.png) [@zsoerenm](https://discourse.julialang.org/u/zsoerenm)
#### Post date: [December 14, 2018, 10:18am UTC](https://discourse.julialang.org/t/reinterpret-uint64-as-svector-8-uint8/18652/1 "2018-12-14T10:18:03Z")

</div>

I’d like to reinterpret a UInt64 as a SVector{8, UInt8}:

```julia
julia> reinterpret(SVector{8, UInt8}, UInt64(1235189))
ERROR: bitcast: target type not a leaf primitive type
Stacktrace:
 [1] reinterpret(::Type{SArray{Tuple{8},UInt8,1,8}}, ::UInt64) at ./essentials.jl:381
 [2] top-level scope at none:0

```

It’s not working the other way round either:

```julia
julia> reinterpret(UInt64, SVector{8, UInt8}(1,1,1,1,1,1,1,1))
Error showing value of type Base.ReinterpretArray{UInt64,1,UInt8,SArray{Tuple{8},UInt8,1,8}}:
ERROR: MethodError: no method matching SOneTo{8}(::UnitRange{Int64})

```

Is there something that I am missing?

---

<div class="post-metadata">

### Author: ![fabiangans](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fabiangans/32/2624_2.png) [@fabiangans](https://discourse.julialang.org/u/fabiangans)
#### Post date: [December 14, 2018, 11:41am UTC](https://discourse.julialang.org/t/reinterpret-uint64-as-svector-8-uint8/18652/2 "2018-12-14T11:41:21Z")

</div>

reinterpret for non-primitive types works only on Arrays, so this works:

```julia
julia> reinterpret(SVector{8, UInt8}, [UInt64(1235189)])
1-element reinterpret(SArray{Tuple{8},UInt8,1,8}, ::Array{UInt64,1}):
 [0xf5, 0xd8, 0x12, 0x00, 0x00, 0x00, 0x00, 0x00]

```

For further reading the are some issues about this: [Extend reinterpret to immutable isbits() type · Issue #13701 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/13701) [Reinterpret immutables · Issue #10140 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/10140)

---

<div class="post-metadata">

### Author: ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)
#### Post date: [December 14, 2018, 11:53am UTC](https://discourse.julialang.org/t/reinterpret-uint64-as-svector-8-uint8/18652/3 "2018-12-14T11:53:50Z")

</div>

```julia
julia> using StaticArrays
julia> function force_reinterpret(dst_typ, src)
       M=MVector(src)
       GC.@preserve M begin 
       p = convert(Ptr{dst_typ}, pointer_from_objref(M))
       return unsafe_load(p)
       end
       end
julia> t = SVector{8, UInt8}; u=rand(UInt)
0x7789306ff6a51882
julia> force_reinterpret(t, u)
8-element SArray{Tuple{8},UInt8,1,8}:
 0x82
 0x18
 0xa5
 0xf6
 0x6f
 0x30
 0x89
 0x77

```

No allocation, but `@code_native` is nevertheless disgusting. Same kind of disgusting as `@code_native getindex(reinterpret(t, [u]),1)`, which is the perennial issue of the new `ReinterpretArray`: Bytes are loaded one by one.