[PATCH] D79744: clang: Add address space to indirect abi info and use it for kernels

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

In D79744#2035109 <https://reviews.llvm.org/D79744#2035109>, @arsenm wrote:

> In D79744#2030710 <https://reviews.llvm.org/D79744#2030710>, @rjmccall wrote:
>
> > Okay.  So the only real ABI here is the layout of the memory that the arguments are actually written into?  And that memory needs to be treated as constant?
>
>
> Yes, the actual kernel ABI is supposed to invisible.
>
> > Unfortunately, I think `byval` just doesn't match what you want because of the mutability — the frontend *has* to have a copy into a local to get IR with correct semantics, because `byval` is assumed to be locally mutable by both IR-generation and (potentially) LLVM optimization.  And I don't think you really want non-`byval` indirect.  So I guess the question is what we can do in the frontend to get the optimizer behavior you need.
>
> You are allowed to have readonly on a byval pointer argument, in which case optimizations wouldn't be allowed to write into it. Is just adding readonly parameter attributes sufficient? It would be somewhat contrived, but could also define byval as constant if it's not in the alloca address space.


`byval` is fundamentally about expressing that something is passed on the stack.  If you want an indirect readonly noalias argument, you can make one; however, to convince IRGen to do it, you really need to think of this as a new kind of argument-passing, because "pass the address of an immutable object that the callee can't modify" is not something that we normally need to do in calling-convention lowering.  That feels like a lot of complexity to solve a rather narrow problem.

A completely different approach: OpenMP has to solve some very similar problems and just lowers them completely in the frontend; have you considered just doing that?  Kernels need a ton of special-case handling anyway, and IIUC you can never optimize over the boundary anyway.


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