# Type-instability because of @threads boxing variables

**URL:** <https://discourse.julialang.org/t/type-instability-because-of-threads-boxing-variables/78395>\
**Category:** Performance\
**Tags:** parallel\
**Created:** [March 24, 2022, 2:17pm UTC](https://discourse.julialang.org/t/type-instability-because-of-threads-boxing-variables/78395 "2022-03-24T14:17:45Z")\
**Posts on this page:** 1\
**Showing post:** 12

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [March 25, 2022, 12:42am UTC](https://discourse.julialang.org/t/type-instability-because-of-threads-boxing-variables/78395/12 "2022-03-25T00:42:03Z")

</div>

So, I think I have a MWE (and it is a strange type-instability):

Copy and past the code below, and you will see that running:

```julia-auto
run()

```

will result in a type instability, or not, depending on these lines of the `test` function:

```julia-auto
    reduce!(list, list_threaded) # type stable
    #list = reduce!(list, list_threaded) # type unstable

```

The problem is inferring the type of `list`. Because it is mutable, and `reduce` also returns `list`, both calls above should be equivalent (and they are except if using `@threads`).

Thus, removing the reassignment of `list` on the return of `reduce ` solves the problem. Except that the reason I am using that is that by doing that I can use the same function to operate on mutable or immutable `list` variables. If I remove the reassignment, I cannot use the same interface to operate on immutables. In any case, seems an issue the fact that this is affected by the use of `@threads` in some other part of the code.

## The example:

```julia-auto
function reduce!(list,list_threaded)
    list .= list[1]
    for i in 2:length(list_threaded)
        list .+= list_threaded[i]
    end
    return list
end

function test(list,list_threaded)
    #for it in 1:Threads.nthreads() # type stable
    Threads.@threads for it in 1:Threads.nthreads()
        for i in it:Threads.nthreads():length(list)
            list_threaded[it][i] += rand()
        end
    end
    #reduce!(list, list_threaded) # type stable
    list = reduce!(list, list_threaded) # type unstable
    return list
end

function run()
    list = zeros(10^4)
    list_threaded = [copy(list) for _ in 1:Threads.nthreads()]
    @code_warntype test(list, list_threaded)
end

```

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