Add int64 variant to package.py

[ebchin]

Summary

• This PR is a small addition to the axom spack recipe.
• It does the following:
  • Enables setting AXOM_USE_64BIT_INDEXTYPE through the spack variant int64
• It also enables AXOM_USE_64BIT_INDEXTYPE by default (see Issues related to axom::IndexType #718). As such axom::IndexType will default to std::int64_t instead of std::int32_t.

Context

Recently, we've been running large problems in Tribol which have needed this option. Exposing it to the spack recipe will simplify enabling this in our TPL builds.

[kennyweiss]

Thanks for adding this variant @ebchin

I wonder if there's a better name we could use for this though (but nothing comes immediately to mind). I checked the hypre package, and they use int64 for an equivalent option, so I suppose we should go with this.

See: https://github.com/spack/spack/blob/f0a7388496a5cb062d7ec6541273b6cc697ba051/var/spack/repos/builtin/packages/hypre/package.py#L65

[kennyweiss]

Side question for axom team -- we previously discussed setting the default IndexType to 64-bit.

Would this be a good time to do this? E.g. by making the default spack variant +int64?
See: #718

[ebchin]

Yeah, the int64 name came from hypre. Happy to change if there's a consensus on something better.

[rhornung67]

@kennyweiss I think changing the default index type in Axom to 64-bit int is a good idea. It would probably prevent some user issues and others can set it to a lower resolution type if they want/need to.

[kennyweiss]

Thanks @rhornung67.

In that case, @ebchin -- could you please change the default for int64 to True?

(we'll change the default CMake option on the axom side in a follow-up PR).

[kennyweiss]

@BradWhitlock -- this was failing w/ axom::IndexType == std::int64_t. Change the template to axom::IndexType fixed it and seems to be the right thing to do.