Should the type of the ordinal in Teuchos::MpiComm ever be something other than int?

[jjramsey]

In looking at the example in https://docs.trilinos.org/dev/packages/tpetra/doc/html/Tpetra_Lesson01.html, I see that in this line of code

RCP<const Comm<int> > comm (new MpiComm<int> (yourComm));

the ordinal type used with MpiComm is int. If I look at the source for MpiComm, it looks like there are several member functions that depend on this ordinal, such as gather(), gatherAll(), reduceAll(), scan(), etc., but I don't know if they'd break if something other than int was used for the ordinal type?

Is there any particular rhyme or reason for this choice of ordinal type?

[ccober6]

@trilinos/tpetra

[cgcgcg]

This might be more of a Teuchos question @trilinos/teuchos.

[bartlettroscoe]

Is there any particular rhyme or reason for this choice of ordinal type?

The decision to template Teuchos::Comm on Ordinal goes back to at least 2005 (I think). It was the wrong decision. With MPI, it is hard coded to int and it never should have been templated (because the only production implementation in the last 20 years was MPI). Templating on Ordinal just complicates the code and allows for arbitrary incompatibilities and the need for silly conversion code for incompatible Ordinal types. This was an example of over exuberance on C++ templating in the early days of Trilinos. This is just one of those live and learn situations. It is much easier to add new template parameters than to remove an existing template parameter without breaking user code.

Not clear how to remove this template parameter without major impact to hundreds of thousands of lines of user code across thousands of users. (But I guess we could create a clang-tidy fixit to safely refactor Trilinos code and user code to remove the Ordinal template parameter?)