Unexpected allocations when acessing field of struct

Hello,

I don’t understand why any of the function calls underneath allocate memory on the heap.
My reasoning is that these are unmutable structs, so when the functions are called, the type of field ´a´ is known to be a Float64 and floats don’t need to be allocated on the heap.
Where am i wrong in my reasoning? And why does the only function call where ´@code_warntype´ wouldn’t know the type, allocate no memory?

using BenchmarkTools

struct untyped
    a
end

struct typed{T}
    a::T
end

function f(x)
    x.a
end

function f2(x)
    x.a::Float64
end

unt = untyped(5.0)
t = typed(5.0)

@btime f(unt)  #18.837 ns (0 allocations: 0 bytes)
@btime f(t) #21.342 ns (1 allocation: 16 bytes)
@btime f2(unt) #22.067 ns (1 allocation: 16 bytes)
@btime f2(t) #21.406 ns (1 allocation: 16 bytes)

3 posts - 2 participants

Read full topic