speed up (de)serialization of Base bits types in abstract containers

[JeffBezanson]

helps #30148

Before:
Serialize array of Union{Missing,Int}:

  7.327029 seconds (100.00 M allocations: 1.990 GiB, 4.15% gc time)

Deserialize that:

 54.608122 seconds (400.37 M allocations: 8.810 GiB, 1.18% gc time)

Serialize array of Union{Missing,UInt}:

 22.448258 seconds (200.19 M allocations: 3.493 GiB, 3.03% gc time)

Deserialize that:

 62.783948 seconds (399.90 M allocations: 8.787 GiB, 0.94% gc time)

Same tests with this PR:

  5.504755 seconds (100.00 M allocations: 1.990 GiB, 12.72% gc time)
  6.396924 seconds (199.90 M allocations: 4.317 GiB, 5.81% gc time)
  4.939101 seconds (100.22 M allocations: 2.001 GiB, 8.17% gc time)
  6.633005 seconds (200.56 M allocations: 4.349 GiB, 4.78% gc time)

The times are a bit variable but the basic story is clear. I tested both Int and UInt, since Int already had some special-case code. Other Base number types (Int8, UInt8, ...) should have roughly the same performance characteristics as UInt.

[vtjnash]

Isn't this breaking though (because of the new list of special objects), so can only be partially backported?

[KristofferC]

I thought we gave no guarantees about being able to deserialize data from any previous version of Julia?

[JeffBezanson]

No this should be 100% backwards compatible. It just uses if statements instead of dispatch, and additional specialized methods. There were already special tags for those types.

[nalimilan]

Maybe do the same for Nothing?

[JeffBezanson]

nothing already has a special representation; in a future version I'll add one for missing and then this code can be removed.

[vtjnash]

should we just do isdefined(t, :instance) && return t.instance here? Since deserialize is required to return that object, I don't think there's anything very meaningful that a deserializing implementation would be permitted to do after dispatch

[JeffBezanson]

Agreed, but I would consider that slightly breaking so I'll do it separately.

[JeffBezanson]

I'm also inclined to say that custom deserialize methods aren't called for primitive types (and possibly more things). Of course that's even more breaking, but it's hard to get performance otherwise (e.g. #14678).

[evveric]

Hello
The example and the fix seems to be only focusing on vector{union{int, missing}}
How about other primitive types. Eg vector {union {float64,missing}} or float32 float16 bool char?
Thank you