Unexpected `sizeof` struct with long StaticArrays

General Usage
1

Consider this:

using StaticArrays

struct Dummy
    B::SVector{34,UInt8}
end
@assert sizeof(Dummy) == sum(sizeof.(fieldtypes(Dummy)))

struct Dummy32
    A::Float32
    B::SVector{32,UInt8}
end
@assert sizeof(Dummy32) == sum(sizeof.(fieldtypes(Dummy32)))

struct Dummy34
    A::Float32
    B::SVector{34,UInt8}
end
@assert sizeof(Dummy34) == sum(sizeof.(fieldtypes(Dummy34)))

The last assertion fails, sizeof(Dummy34) return 40, 2 more bytes than expected. The behavior switched based on number of elements surprised me.

This was a problem, I needed to read some binary into a large struct quick, and it didn’t lined up…

open(...) do fp
    dummy = Ref{Dummy34}()
    read!(fp, dummy)
    position(fp) # --> 40
end

I guess I needed to do it explicitly

import Base.read
function read(fp::IOStream, ::Type{Dummy34})
    elem = [
        read(fp, f)
        for f in fieldtypes(Dummy34)
    ]
    return Dummy34(elm...)   
end

Any comments or suggestions would be nice, thank you.

Julia Version 1.6.3
Commit ae8452a9e0 (2021-09-23 17:34 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i5-8365U CPU @ 1.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)

[90137ffa] StaticArrays v1.4.1
2

The following seems to work for me

using StaticArrays

struct Dummy
    B::SVector{34,UInt8}
end
@assert sizeof(Dummy) == sum(sizeof.(fieldtypes(Dummy)))

struct Dummy32
    A::Float32
    B::SVector{32,UInt8}
end
@assert sizeof(Dummy32) == sum(sizeof.(fieldtypes(Dummy32)))

struct Dummy34
    A::Float32
    B::SVector{34,UInt8}
end

dummy1 = Dummy34(0.0, SVector((0 for _ in 1:34)...))
open("test.dat", "w") do fp
    write(fp, Ref(dummy1))
    @show position(fp) # --> 40
end

open("test.dat", "r") do fp
    dummy2 = Ref{Dummy34}()
    read!(fp, dummy2)
    @show position(fp) # --> 40
    @assert dummy2[] == dummy1
end

Regarding the size of Dummy34: I could imagine some kind of 32 bit alignment.