Avoid closures for custom operators

[simonbyrne]

To support custom operators for PowerPC and ARM, we will need to avoid using closures as these are not supported (JuliaLang/julia#34326 and JuliaLang/julia#32154). There might be performance advantages on other platforms as well.

We currently skip the relevant tests on these platforms (#353)

Some options:

• convert Op(f, T=Any) to a generated function
• write a macro that allows us to "declare" custom operator/type combinations, i.e.

  @Op f T

  would define a non-closure cfunction for Op(::typeof(f),::Type{T})

[sloede]

Is there any progress on this issue? I just ran into it on my ARM/M1 laptop and would like to help getting it fixed. Unfortunately, I do not have the necessary chops to go about this alone, but I would be willing to lend a hand, run tests on an M1 etc. if that would be useful.

[giordano]

May be relevant: https://discourse.julialang.org/t/ann-cclosure-jl-c-closures-like-what-c-does/81234

[simonbyrne]

For non-closures, we could use the same approach used by @yuyichao here https://github.com/JuliaMath/GSL.jl/pull/132/files#diff-ed5863a4bdd6189a79314f8c6cb0d3acefa0fe9235e7df221e6686fedb9efc08R40-R48
There is a bit of an explanation on how it works here:
JuliaMath/GSL.jl#130 (comment)

[simonbyrne]

Ah, I don't think that will work.

[vchuravy]

#871 partially fixes this. Still doesn't support user closures, but supports generic functions.