Allocation depending on value of typed field in struct

New to Julia
, ,
1

I’m seeing some behavior with accessing data members of structs that I don’t understand - depending on the value of typed field in a struct, I’m getting an allocation with getfield.

struct MyStruct
   v::Int
end


ms0 = MyStruct(0)
ms512 = MyStruct(512)

@btime getfield(ms0, :v)
  15.415 ns (0 allocations: 0 bytes)

@btime getfield(ms512, :v)
  20.361 ns (1 allocation: 16 bytes)

It seems that for values less than 512, there is no allocation. I also do not see any allocations for any size number if I instead define the field v without a type. Is there a reason for this?

Also, why does accessing the member directly with . cause more allocations and why is it so much slower?

@btime ms0.v
  43.088 ns (1 allocation: 16 bytes)

@btime ms512.v
  51.874 ns (3 allocations: 48 bytes)
2

You’re using non constant global variable

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3

Try

These should both be expected to use zero time.

1 Like
4

Interesting:

  1. Making them const is basically free (0.001 ns and no allocations).
  2. Putting the creation and getfield into a function also made it free
  3. Trying @DNF’s suggestion, resulted in a time of 1.100 ns with 0 allocations

I’m assuming (1) and (2) are due to the whole thing being compiled out and essentially returning the value? How/why does (3) behave differently than these?

5

It’s probably just benchmarking artefacts. It’s difficult to get good, accurate timings of operations that are essentially free. In statically inferred code this should not take any time at all.

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