RFC: Add rowval index type to SparseMatrixCSC

[dkarrasch]

This allows to have distinct "index types" for the nonzeros on the one hand, and the rowvals on the other hand. In general, this may not be a dramatic reduction in memory usage, but in some cases this may push limits, see #38790. This should be fully backward compatible, and basically all existing tests pass up to minor obvious/inavoidable changes.

TODO (if we agree to go in this direction):

• new tests
• update documentation
• mention in NEWS

Closes #38790.

[fredrikekre]

This should be fully backward compatible

All the changes you had to do, e.g. Symmetric{T,SparseMatrixCSC{T,SuiteSparse_long}} -> Symmetric{T,<:SparseMatrixCSC{T,SuiteSparse_long}} suggest it is not though.

[dkarrasch]

I see, thanks for clarifying. I had in mind the call to constructors, the assumption that Ti == Tr and Ti == Int by default, and that the new Tr is the trailing parameter which doesn't require specification... except for the wrapper cases, as you point out.

[ViralBShah]

It is possible this could be quite breaking. We should run pkgeval. Also, does it increase compile time noticeably?

[ViralBShah]

@nanosoldier runtests(ALL, vs = ":master")

[nanosoldier]

Your package evaluation job has completed - possible new issues were detected. A full report can be found here. cc @maleadt

[dkarrasch]

I checked a few, and the test failures are related to things that match my modification pattern Fredrik pointed out. So I believe these 33- packages (some fail for unknown reasons) could be changed in a way that doesn't require version-checking and alike, and then should work also with previous Julia versions. Is this something we want to consider?

[fredrikekre]

There is also the cases of, for example,

struct X
    m::SparseMatrixCSC{Float64,Int}
end

that will now be abstractly typed.

It seems like a very niche usecase (?) so perhaps it could just live in a package? After #33039 it might not even require much more code than simply

struct SparseMatrixCSC2{Tv,Ti<:Integer,Tr<:Integer} <: SparseArrays.AbstractSparseMatrixCSC{Tv,Ti}
    m::Int
    n::Int
    colptr::Vector{Ti}
    rowval::Vector{Tr}
    nzval::Vector{Tv}
end

[dkarrasch]

@msekino Could you check if Fredrik's suggestion above works for you? If it can be done without much effort in an extra package (or just as part of your package), then I now agree that this is the way to go, since the cost of adaptation throughout the ecosystem is rather high for such a change here.

[msekino]

@dkarrasch @fredrikekre
Thank you for your consideration!
I think that the suggestion is a simple and good solution and will work for us without much effort.