@tgamblin: After looking over the spack.architecture code a bit, it looks like the problems related to multi-architecture handling stem from the fact that all new architectures are constructing using the current host architecture as a base. I can think of two ways to fix this:

1. Store all of the information about the host architecture (instead of just the platform, platform os, and target) in the Spack database when installing packages and then use this information to create new architecture objects when reading specs from the database.
2. Continue using the host architecture as the base and simply update the operating system and target of architecture object created for each package based on its associated platform_os and target database fields (which assumes that the host platform is the same as that used for all installed packages).

The first method is more general, but requires updating the database fields for the benefit of allowing a single Spack instance to handle installs that span different platforms (which I'm not sure is something that would be done very often if at all). With that being said, which of these methods seems preferable to you (or do you have a better alternative in mind)? Please let me know whenever you get a chance!

@xjrc: I don't think we need to be as general as (1), as the platform metadata isn't needed unless you are actually building on the host. The way we've separated things out for other parts of Spack, like compilers, is to have a CompilerSpec object that's just a descriptor, then a separate Compiler object that you can look up using the Spec. The Compiler objects are dependent on things like local compiler paths, but that is (intentionally) not stored in the DB -- only build provenance is stored in the DB and Specs describe the DAG and contain build provenance.

So the way I would do this is to pull an ArchSpec class out of the current Arch class, and move all the Spec methods (satisfies, to_dict, etc.) over to ArchSpec. That way Spec i just a descriptor that can live wherever. The other stuff -- like making an actual platform object, establishing the default OS for this platform, picking a target, etc, can stay where it is, and can be lazily evaluated when needed -- does that make sense?

@tgamblin: The concept makes sense, and I can certainly envision how this could be used to replace the current functionality for generating architecture information from the Spack database. I'm not entirely sure how to go about implementing ArchSpec, but I think that looking a bit more at how CompilerSpec is implemented and how it's used with Compiler will give me some ideas. Thanks!

@tgamblin: I finished implementing a preliminary version of the spack.spec.ArchSpec class. Would you mind taking a quick look at it and letting me know whether or not I'm on the right track? I just want to make sure I'm not doing anything too unusual before I start integrating this into the spack.spec.Spec class and other parts of the code.

@xjrc: this is actually looking really good. The fact that it also adapts the compilers could also fix another current problem: the compilers can't currently differentiate from each other by target. We see this on the ppc64le systems when they share a home dir with Linux machines.

I will take a more detailed look tonight, but it looks close to merge-able to me.

@xjrc Can you check the box that lets us push to your PR?

@becker33: It looks to me like the box is already checked. Are you having problems pushing?

@xjrc I get 403 Forbidden error when attempting to push to this branch, I would assume that's from that setting being messed up but I'm sure it could come from something else.

@becker33: you need to use the git URL for the remote to be able to authenticate with ssh and push to branches like this one.

@tgamblin I only switched to https because I got a similar fatal error when trying to push to the same branch through ssh.

Pushing issues resolved. I'll resolve the merge conflicts.

@tgamblin, @becker33: Are there any other adjustments that need to be made to this PR before it can be merged? If so, I'd be happy to help out if I can!