[RFC] An ABI lowering library for LLVM

[nikic]

@aengelke

Re type system duplication: I’d consider using a traits class to wrap existing type representations to reduce the runtime costs. The design should allow for covering Clang’s representation as well as a more simple design, which could be provided by the library (and exposed as C API). Clang is already rather slow, so if a type conversion turns out to have a somewhat substantial cost, I’d rather like to avoid that.

Interesting idea. I don’t think we can use traits in the sense LLVM usually uses that word, because those require a templated, header-only implementation, and I don’t think that’s feasible here. We could use virtual inheritance though, with some caveats (for example, to make the memory management work out, we’d want sub-type iteration to be callback-based rather than iterator-based).

My general inclination here is to try the separate type system first, and fall back to this variant if it turns out to add significant cost.

@rnk

Thanks for proposing this, it sounds promising. I wouldn’t worry about the cost of the representation shift as long as the function prototype representations are short-lived and not stored in some kind of global, lives-forever context like LLVMContext of ASTContext.

The function prototype itself would be short-lived, but my assumption was that the argument/return types would use the “usual” uniqued type system representation, with a cache on the translation from Clang to ABI types. If we make the types ephemeral, we wouldn’t be able to cache the translation, which may be problematic for complex record types.