# Are broadcast-assignment and \`copyto!\` safe with aliasing?

**URL:** <https://discourse.julialang.org/t/are-broadcast-assignment-and-copyto-safe-with-aliasing/111903>\
**Category:** New to Julia\
**Tags:** question, aliasing\
**Created:** [March 20, 2024, 9:03pm UTC](https://discourse.julialang.org/t/are-broadcast-assignment-and-copyto-safe-with-aliasing/111903 "2024-03-20T21:03:10Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![goerz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goerz/32/3269_2.png) [@goerz](https://discourse.julialang.org/u/goerz)\
**Post date:** [March 20, 2024, 9:03pm UTC](https://discourse.julialang.org/t/are-broadcast-assignment-and-copyto-safe-with-aliasing/111903/1 "2024-03-20T21:03:10Z")

</div>

Are broadcast-assignment and `copyto!` between two `AbstractVector` objects guaranteed to be safe if the two vectors alias each other (in a non-crazy way)? I’m especially interested if copying from an array into itself is guaranteed to be a no-op:

```julia
a .= a
copyto!(a, a)

```

Or do I have to prefix that with an explicit check?

```julia
if a !== b
    copyto(a, b)
end

```

Would the compiler be able to figure out that `copyto!(a, a)` is _exactly_ a no-op? That is, should I use the check in my code for performance reasons, even if the `copyto!` itself is safe?

In my application, I probably don’t need other types of aliasing than the vectors being identical, so anything beyond that is more of a curiosity. I’m definitely not interested in “crazy” aliasing, e.g. if `b` is an AbstractVector that is some permuted view of `a`. I think what I’m asking is whether I can assume that broadcast-assignment and `copyto!` work equivalently to

```julia
function copyto!(a, b)
    for i in eachindex(b)
        a[i] = b[i]
    end
end

```

(ignoring that `a` and `b` might have different indices; you get the point).

---

<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [March 20, 2024, 9:54pm UTC](https://discourse.julialang.org/t/are-broadcast-assignment-and-copyto-safe-with-aliasing/111903/2 "2024-03-20T21:54:13Z")

</div>

There’s two questions here:

1. Will aliasing cause _semantic_ differences in the values it returns?
2. Will aliasing cause _performance_ differences in the runtime?

The answer to (1) will depend on the exact method. On Julia v1.11, most `!` functions have explicit warnings about unexpected behaviors if the output aliases with any of the inputs — and `copyto!` is no exception. Broadcasted assignments do attempt to avoid introducing unexpected behaviors when the destination aliases any of the inputs, but they do so by making preventative copies _at the expense of performance_.

So, yes, aliasing can cause performance differences — but often to your detriment.