# Segfault with SIMD.jl and BenchmarkTools.jl
**URL:** https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352
**Category:** Performance
**Tags:** package
**Created:** [August 9, 2019, 1:31pm UTC](https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352 "2019-08-09T13:31:51Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@LaurentPlagne](https://discourse.julialang.org/u/LaurentPlagne)
#### Post date: [August 9, 2019, 1:31pm UTC](https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352/1 "2019-08-09T13:31:51Z")
Hi,
I try to learn how to use SIMD.jl and the following snippet produces a segmentation fault when I use @btime.
```julia
using BenchmarkTools
using SIMD
const SBS=8
function microaxpy_simd!(ys::Vector{T}, a::T, xs::Vector{T}) where {T}
@assert length(ys) == length(xs)
lane = VecRange{SBS}(0)
ys[lane+1] += a*xs[lane+1]
end
function test()
x=ones(SBS)
y=ones(SBS)
a=2.0
microaxpy_simd!(y,a,x) #Seems OK
@show y
# @btime microaxpy_simd!($y,$a,$x) #produce s segfault
end
```
Any hints ?
---
### Author:  [@tkoolen](https://discourse.julialang.org/u/tkoolen)
#### Post date: [August 9, 2019, 2:24pm UTC](https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352/2 "2019-08-09T14:24:25Z")
> [@LaurentPlagne](#):
>
> lane = VecRange{SBS}(0)
Shouldn’t this be `VecRange{SBS}(1)`, in accordance with
>
?
---
### Author:  [@LaurentPlagne](https://discourse.julialang.org/u/LaurentPlagne)
#### Post date: [August 9, 2019, 3:10pm UTC](https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352/3 "2019-08-09T15:10:04Z")
Thank you for the suggestion but when I try
```julia
lane = VecRange{SBS}(1)
ys[lane+0] += a*xs[lane+0]
```
I have the same segfault crash as with
```julia
lane = VecRange{SBS}(0)
ys[lane+1] += a*xs[lane+1]
```
which I copied from the SIMD.jl markdown README (I could not find a more detailed doc).
What is strange is that if you try the top MWE, the result `y` looks fine (`[3.0,...3.0]`).
---
### Author:  [@tkoolen](https://discourse.julialang.org/u/tkoolen)
#### Post date: [August 9, 2019, 7:56pm UTC](https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352/4 "2019-08-09T19:56:45Z")
Ah OK, yeah. I think it might have to do with returning the `Vec{8, Float64}` from `microaxpy_simd!`. If you return `ys` or `nothing`, it’s fine. Not sure why.
---
### Author:  [@LaurentPlagne](https://discourse.julialang.org/u/LaurentPlagne)
#### Post date: [August 9, 2019, 10:16pm UTC](https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352/5 "2019-08-09T22:16:58Z")
Argh, not the first time that I forget that returning nothing must be explicit 😊
Thank you very much for your help !
---
### Author:  [@tkoolen](https://discourse.julialang.org/u/tkoolen)
#### Post date: [August 9, 2019, 11:49pm UTC](https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352/6 "2019-08-09T23:49:06Z")
Still, if anyone knows why returning a `SIMD.Vec` (a glorified `NTuple` of `Core.VecElement`s) causes a segfault, I’d like to know.
---
### Author:  [@Elrod](https://discourse.julialang.org/u/Elrod)
#### Post date: [August 10, 2019, 1:59am UTC](https://discourse.julialang.org/t/segfault-with-simd-jl-and-benchmarktools-jl/27352/7 "2019-08-10T01:59:08Z")
[Here](https://github.com/JuliaLang/julia/issues/30426) is a GitHub issue (from last December).
If you return a `NTuple{W,Core.VecElement{T}}` wrapped in a struct (eg, a `SIMD.Vec` or a tuple), you’re likely to get a segfault.