# Unexpected Memory Allocation of Broadcasting Copy
**URL:** https://discourse.julialang.org/t/unexpected-memory-allocation-of-broadcasting-copy/115634
**Category:** Performance
**Tags:** question
**Created:** [June 14, 2024, 4:04am UTC](https://discourse.julialang.org/t/unexpected-memory-allocation-of-broadcasting-copy/115634 "2024-06-14T04:04:32Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@0samuraiE](https://discourse.julialang.org/u/0samuraiE)
#### Post date: [June 14, 2024, 4:04am UTC](https://discourse.julialang.org/t/unexpected-memory-allocation-of-broadcasting-copy/115634/1 "2024-06-14T04:04:32Z")
Hi all. I encounted the problem as topic saids.
```julia
julia> a = zeros(4, 4, 3)
julia> @btime @views $a[:, :, 2] .= $a[:, :, 2]
38.862 ns (1 allocation: 192 bytes)
julia> @btime @views for i in eachindex($a[:, :, 2])
$a[:, :, 2][i] = $a[:, :, 2][i]
end
7.744 ns (0 allocations: 0 bytes)
```
I thought these two benchmarks do the exactly same thing.
Is this problem a bug or a specification or my wrong?
Can someone please tell me what’s going on?
---
### Author:  [@screw\_dog](https://discourse.julialang.org/u/screw_dog)
#### Post date: [June 14, 2024, 4:10am UTC](https://discourse.julialang.org/t/unexpected-memory-allocation-of-broadcasting-copy/115634/2 "2024-06-14T04:10:27Z")
I believe the allocations are occurring because `@views` recognises that you are assigning from `a` to `a` which refer to the same part of memory.
For example:
```julia
a = zeros(4, 4, 3)
b = zeros(4, 4, 3)
f(a, b) = (@views a[:, :, 2] .= b[:, :, 2]; return nothing)
@btime f(a, b) # no allocations
@btime f(a, a) # 1 allocation
```
---
### Author:  [@0samuraiE](https://discourse.julialang.org/u/0samuraiE)
#### Post date: [June 14, 2024, 4:15am UTC](https://discourse.julialang.org/t/unexpected-memory-allocation-of-broadcasting-copy/115634/3 "2024-06-14T04:15:40Z")
Thank you for fast answering.
> [screw\_dog](https://discourse.julialang.org/u/screw_dog)
> you are assigning from a to a which refer to the same part of memory.
I agree with you. But my second benchmark also assigns by the same way.
I wonder what the difference is.
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [June 14, 2024, 4:22pm UTC](https://discourse.julialang.org/t/unexpected-memory-allocation-of-broadcasting-copy/115634/4 "2024-06-14T16:22:00Z")
IIRC, broadcasting can conservatively insert a copy if the source & target arrays alias, but I’m only like 85% sure on that.
---
### Author:  [@0samuraiE](https://discourse.julialang.org/u/0samuraiE)
#### Post date: [June 15, 2024, 2:59am UTC](https://discourse.julialang.org/t/unexpected-memory-allocation-of-broadcasting-copy/115634/5 "2024-06-15T02:59:47Z")
Thus, this is a specification.
Thank you for answering.