Quirks when benchmarking allocations of StaticArrays in anonymous functions

General Usage
1

While benchmarking allocations I stumbled upon some strange behavior that I cannot explain. I reduced it to the following example.

using StaticArrays, BenchmarkTools
vec = [1, 2, 3]
svec = SA[1, 2, 3]

fnamed(x) = 0
fanon = x -> 0

@ballocations fnamed($vec)
# Output: 0
@ballocations fanon($vec)
# Output: 0

@ballocations fnamed($svec)
# Output: 0
@ballocations fanon($svec)
# Output: 1

I don’t understand why benchmarking shows that the last call allocates. Is there some reason this would be expected or could this be a bug in BenchmarkTools? The output of @code_lowered does not show any allocations

@code_lowered fanon(svec)
# Output:
# CodeInfo(
# 1 ─     return 0
# )
2

fanon is a (non-const) global variable:

julia> using StaticArrays, BenchmarkTools

julia> vec = [1, 2, 3]
3-element Vector{Int64}:
 1
 2
 3

julia> svec = SA[1, 2, 3]
3-element SVector{3, Int64} with indices SOneTo(3):
 1
 2
 3

julia> const c_fanon = x-> 0
#5 (generic function with 1 method)

julia> @ballocations cfanon($vec)
0

julia> @ballocations cfanon($svec)
0
3 Likes
3

Thanks, that explains it.