# How to wrap a vector so that it does simd?

**URL:** <https://discourse.julialang.org/t/how-to-wrap-a-vector-so-that-it-does-simd/74242>\
**Category:** Performance\
**Created:** [January 8, 2022, 2:40pm UTC](https://discourse.julialang.org/t/how-to-wrap-a-vector-so-that-it-does-simd/74242 "2022-01-08T14:40:14Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![abulak](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/abulak/32/28314_2.png) [@abulak](https://discourse.julialang.org/u/abulak)\
**Post date:** [January 8, 2022, 2:40pm UTC](https://discourse.julialang.org/t/how-to-wrap-a-vector-so-that-it-does-simd/74242/1 "2022-01-08T14:40:14Z")

</div>

Hey,

I’ve been recently hit with this issue: I have a very thin wrapper around `Vector{T}`, let’s say it’s

```julia
struct MyVector{T} <: AbstractVector{T}
    data::Vector{T}
end

# Array Interface
Base.size(v::MyVector) = size(v.data)
Base.IndexStyle(::Type{<:MyVector}) = IndexLinear()
Base.@propagate_inbounds Base.getindex(w::MyVector, n) = w.data[n]

```

It’s very common in my code to do a tight loops where I loop over `mv = MyVector(...)`. recently I found that looping over `mv.data` is **much** faster, and by **much** I mean 5×faster:

```julia
function testf(w::AbstractVector{<:Unsigned})
    k = zero(UInt)
    for i in w
        isodd(i) && continue
        k += i^2
    end
    return k
end

v = MyVector{UInt8}(rand(1:100, 1000));
using BenchmarkTools
@btime testf($v) # 768.257 ns (0 allocations: 0 bytes)
@btime testf($(v.data)) # 165.248 ns (0 allocations: 0 bytes)

```

Of course by inspecting `@code_llvm` / `@code_native` it’s clear that this is due to the fact that the latter call vectorizes. So here’s my question:

> Is there an easy way to nudge llvm to emit vector instructions for loops over `MyVector`?

E.g. when I redefine

```julia
function Base.iterate(v::MyVector, s=0)
    # (ab)using that eachindex(v) = Base.OneTo(1000) 
    s == length(v) && return nothing
    return @inbounds v[s], s+1
end

```

I can recover

```julia
julia> @btime testf($v); # was: 768.257 ns (0 allocations: 0 bytes)
  161.286 ns (0 allocations: 0 bytes)

julia> @btime testf($(v.data)); # was: 165.248 ns (0 allocations: 0 bytes)
  162.222 ns (0 allocations: 0 bytes)

```

Is this the correct way to do so? My feel is that there should be a more generic way (but the default provided by `Base` inhibits vectorization)…