[clang] [llvm] [SPIRV][RFC] Rework / extend support for memory scopes (PR #106429)

[Vyacheslav Levytskyy via cfe-commits]

VyacheslavLevytskyy wrote:

Thank you for the PR! I'd like to better understand motivation and justification of SPIR-V BE-related changes though. The goal would be to understand whether AllSvmDevices is indeed a better choice (for whom?) than Device as a default mem scope value in SPIR-V BE.
 
1) Questions to the description of the PR.
 
> "These were previously unconditionally lowered to Device scope, which is can be too conservative and possibly incorrect."
The claim is not justified by any docs/specs. Why Device scope is incorrect as a default? In my opinion, it's AllSvmDevices that looks like a conservative choice that may lead to performance degradation in general case when we change the default without notifying customers. Or, we may say that potential performance changes may depend on a vendor-specific behavior in this case.
 
> "Furthermore, the default / implicit scope is changed from Device (an OpenCL assumption) to AllSvmDevices (aka System), since the SPIR-V BE is not OpenCL specific / can ingest IR coming from other language front-ends."
What I know without additional references to other docs/specs is that Device is default by OpenCL spec (https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_C.html#atomic-functions). It would help if you can provide references where AllSvmDevices is a preferable choice, so that we are able to compare and figure out the best default for the Computational flavor of SPIR-V. For sure, SPIR-V BE is not OpenCL (=Device) specific, and it's also not specific to any particular vendor or computational framework. I've seen usages of AllSvmDevices as default in the code base (for example, in https://github.com/llvm/llvm-project/blob/319e8cd201e6744199da377fba237dd276063e49/clang/lib/CodeGen/Targets/AMDGPU.cpp#L537), but it seems not enough to flip the default over.
 
> "OpenCL defaulting to Device scope is now reflected in the front-end handling of atomic ops, which seems preferable."
Changes in clang part looks really good to me. However, when we add to it changes in SPIR-V part of the code base, things look less optimistic, because what this PR means by "the front-end handling of atomic ops" is the upstream clang only, whereas actual choices of a front-end are more versatile, and users coming to SPIR-V by other paths would get a sudden change of behavior in the worst case (e.g., MLIR input for the GenAI domain).

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2) If it's acceptable to split this PR into two separate PR's (clang and SPIR-V), I'd gladly support changes in clang part, it makes sense for me. At the moment, however, I have objections against SPIR-V Backend changes as they are represented in the PR:
 
* This PR looks like a breaking change that would flip over the default value of mem scope for all environments except for OpenCL and may have a potentially negative impact on an unknown number of projects/customers. I'd guess that OpenCL would not notice the difference, because path that goes via upstream clang front-end redefines default mem scope as Device. All other toolchains just get a breaking change in the form of the AllSvmDevices default. clang-related changes do not help to smooth this, because SPIRV BE should remain agnostic towards front-ends, frameworks, etc.
 
* A technical comment is that the proposed implementation in SPIR-V part is less efficient that existing. It compares strings rather than integers and fills in scope names on each call to the getMemScope() function, whereas existing implementation does it just once per a machine function.

* A terminology (the choice of syncscope names) is debatable. The closest thing in specs that I see is https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#_scope_id. I don't see any references to "singlethread" in the specs. Name "workitem" (spelling precisely as "work-item") is used at least in the official Khronos documents (see for example https://registry.khronos.org/SPIR-V/specs/1.0/SPIR-V-execution-and-memory-model.pdf). "all_svm_devices" is not mentioned in the specs at all (there is only the "CrossDevice" term).
 
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For now, I'd rather see an eventual solution in the form of further classification of the computational flavor of SPIR-V (not just Compute vs. Vulkan but breaking Compute part further where this is required) -- comparing to this sudden change of the default in favor of any incarnation of Compute targets. As the first approach, all SPIR-V-related changes may require just a short snippet of the kind "if TheTriple is XXX-specific then use CrossDevice instead of Device" and minor rename of syncscope names ("subgroup", for example, indeed makes more sense than "sub_group"). This would probably require a description in the SPIRVUsage doc as well to avoid confusion among customers. Anyway, I'd be glad to talk out a reasonable way forward to get a better solution than we have now, if needed.

https://github.com/llvm/llvm-project/pull/106429