[PATCH] D36580: [OpenCL] Support variable memory scope in atomic builtins

John McCall via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Aug 14 11:21:25 PDT 2017

rjmccall added inline comments.

Comment at: include/clang/Basic/SyncScope.h:92
+/// \brief Defines the synch scope ABI for OpenCL.
+class AtomicScopeOpenCLABI : public AtomicScopeABI {
yaxunl wrote:
> rjmccall wrote:
> > Please correct me if I'm wrong, but I believe you don't get to define the "ABI for OpenCL" any more than I get to define the "ABI for C".  You're defining the ABI for your specific OpenCL implementation, but Clang might reasonably support other implementations with their own ABIs.  So this name is inappropriate.
> > 
> > Now, if I understand how SPIR works, SPIR does require some sort of stable lowering of these atomic operations in order to ensure that the resulting LLVM IR can be ingested into an arbitrary implementation.  (Ot at least that was true prior to SPIR V?)  Therefore, SPIR needs to specify a lowering for these atomic operations, and that probably includes ABI values for specific sync-scopes.  But the appropriate name for that would be the "SPIR ABI", not the "OpenCL ABI".  And, of course, you would need to be sure that you're implementing the lowering that SPIR actually wants.
> The ABI is not intended for a specific OpenCL implementation, but for extending to other languages. For example, we have a downstream C++ based language called HCC, which may support atomic scope with an ABI different than OpenCL atomic scope ABI.
By "ABI" you mean that it might present a different model of synchronization scopes to the source language?  Okay, that's great, and it explains a lot of things that were very murky about some of our previous conversations.

In that case, I think the appropriate design for these builtins is:

  - They don't need to be in the __builtin_opencl namespace.
  - They *do* need to be enabled only in language modes with well-defined sync-scope models, which for now just means OpenCL. 
  - The language mode's sync-scope model should determine everything about the scope argument, including its type.  Sema-level validation requires first determining the language's sync-scope model.
  - There is no meaningful way to "default" the scope argument on the non-scoped builtins the way that we are now.  Instead, the scope argument should only exist for the scoped builtins.

Alternatively, if you potentially want the opencl-model builtins to be usable from non-opencl languages, the right design is for HCC to use its own set of builtins with their own type-checking rules.

In either case, I don't think it's a good idea to call this "ABI", which is associated too strongly with target-lowering concepts.  You're really talking about a source-language concept.


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