# Broadcasting Heisen-allocations

**URL:** <https://discourse.julialang.org/t/broadcasting-heisen-allocations/101572>\
**Category:** Performance\
**Tags:** question, bug\
**Created:** [July 13, 2023, 1:08pm UTC](https://discourse.julialang.org/t/broadcasting-heisen-allocations/101572 "2023-07-13T13:08:38Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![pablosanjose](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pablosanjose/32/7006_2.png) [@pablosanjose](https://discourse.julialang.org/u/pablosanjose)\
**Post date:** [July 13, 2023, 1:08pm UTC](https://discourse.julialang.org/t/broadcasting-heisen-allocations/101572/1 "2023-07-13T13:08:39Z")

</div>

I have a weird code snippet that was reduced from a real codebase that was mysteriously allocating where it shouldn’t, for all I could tell:

```julia

using BenchmarkTools
using StaticArrays

test(z::SVector{N}) where {N} = problematic(test(Val(N)), first(z))

test(::Val{N}) where {N} = SVector(ntuple(identity, Val(N)))

function problematic(d::SVector{N}, α) where {N}
    αp = ntuple(Returns(α), Val(N))
    d´ = d .* SVector(αp)
    return d´
end

v = SA[1.0, 0, 0, 0]; vs = SVector(v,v,v,v);
@btime test.($vs);

```

The result on my computer (Apple Silicon), all the way from Julia v1.7 to master, is the following:

```julia
julia> @btime test.($vs);
  342.905 ns (5 allocations: 720 bytes)

```

If I run the whole snippet again, I get the same 5 allocations and slow runtime. However, if I now re-run only from `function problematic...` till the end (without changing _anything_) I get

```julia
julia> function problematic(d::SVector{N}, α) where {N}
           αp = ntuple(Returns(α), Val(N))
           d´ = d .* SVector(αp)
           return d´
       end
problematic (generic function with 1 method)

julia> v = SA[1.0, 0, 0, 0]; vs = SVector(v,v,v,v);

julia> @btime test.($vs);
  2.041 ns (0 allocations: 0 bytes)

```

So merely repeating the definition of `problematic` appears to free something inside the guts of the compiler, that is then able to digest through the code without introducing allocations. At this point, if I rerun everything again, I remain fast and allocation-free.

[By the way, the actual, problematic line seems to be `d´ = d .* SVector(αp)` in the `problematic` function. If I do `d´ = SVector(Tuple(d) .* αp)` instead, things remain allocation free from the beginning. EDIT: which might mean that the bug is ultimately originating in StaticArrays]

This has driven me crazy for quite a bit, because with Revise, sometimes touching a function in some absolutely trivial way that should not change its behavior was suddenly changing my allocations and performance. I suspect there is some kind of compiler bug hiding here? Any clue as to why this is happening?

---

<div class="post-metadata">

**Author:** ![pablosanjose](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pablosanjose/32/7006_2.png) [@pablosanjose](https://discourse.julialang.org/u/pablosanjose)\
**Post date:** [July 13, 2023, 1:57pm UTC](https://discourse.julialang.org/t/broadcasting-heisen-allocations/101572/2 "2023-07-13T13:57:55Z")

</div>

A further reduced MWE

```julia
using BenchmarkTools
using StaticArrays

test(f::SVector) = f .* f'

v = SA[1, 0]; vs = SVector(v,v);
@btime test.($vs);

```

```julia
julia> @btime test.($vs);
  288.321 ns (8 allocations: 416 bytes)

```

---

<div class="post-metadata">

**Author:** ![pablosanjose](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pablosanjose/32/7006_2.png) [@pablosanjose](https://discourse.julialang.org/u/pablosanjose)\
**Post date:** [July 13, 2023, 2:14pm UTC](https://discourse.julialang.org/t/broadcasting-heisen-allocations/101572/3 "2023-07-13T14:14:48Z")

</div>

filed: [Allocations in broadcast of broadcast · Issue #1178 · JuliaArrays/StaticArrays.jl · GitHub](https://github.com/JuliaArrays/StaticArrays.jl/issues/1178)