Bump min cxx standard to 17

[oandreeva-nv]

This PR is one of the series PRs to update Triton to C++17 standard.

Here if TRITON_MIN_CXX_STANDARD is not specified in cache, it will be set to 17.

The main discussion point is if we even need THIRD_PARTY_CMAKE_CXX_STANDARD or we can simply use TRITON_MIN_CXX_STANDARD instead.

Current implementation:

Then if CMAKE_CXX_STANDARD is not set, THIRD_PARTY_CMAKE_CXX_STANDARD is set to TRITON_MIN_CXX_STANDARD, otherwise, THIRD_PARTY_CMAKE_CXX_STANDARD is set to CMAKE_CXX_STANDARD.

Note, that by the time third party is going to be build, TRITON_MIN_CXX_STANDARD and CMAKE_CXX_STANDARD should already be set in CMakeCache (server or core)

[GuanLuo]

Should we still check CMAKE_CXX_STANDARD if we are introducing a new variable? I think CMAKE_CXX_STANDARD is set in server CMakeLists and can be replaced by setting the new variable directly?

[oandreeva-nv]

I don't think we set CMAKE_CXX_STANDARD anywhere. I can remove check for CMAKE_CXX_STANDARD and go with TRITON_MIN_CXX_STANDARD only

[Tabrizian]

The main discussion point is if we even need THIRD_PARTY_CMAKE_CXX_STANDARD or we can simply use TRITON_MIN_CXX_STANDARD instead.

I feel like TRITON_MIN_CXX_STANDARD and THIRD_PARTY_CMAKE_CXX_STANDARD serve different purposes but I agree that we should get rid of the THIRD_PARTY_CMAKE_CXX_STANDARD. I think we can probably just use CMAKE_CXX_STANDARD and make sure to provide a default value when using build.py script.

[Tabrizian]

I think we should be propagating CMAKE_CXX_STANDARD rather than TRITON_MIN_CXX_STANDARD.

[oandreeva-nv]

Could you please elaborate? Since for this set of PRs, I introduce TRITON_MIN_CXX_STANDARD, so I use it to set CMAKE_CXX_STANDARD for other third_party packages (note: not all of them take it).

[Tabrizian]

My understanding was that TRITON_MIN_CXX_STANDARD determines the minimum required standard when compiling the project (i.e., since we'll start using C++ X version features). CMAKE_CXX_STANDARD determines the version that the server is actually compiled with. Based on this understanding, I think we should propagate CMAKE_CXX_STANDARD and not TRITON_MIN_CXX_STANDARD. Feel free to correct me if I'm wrong.

[oandreeva-nv]

You are correct. However, there are multiple ways how to specify c++ standard for compilation. In third_party library I set each library's CMAKE_CXX_STANDARD to be equal to TRITON_MIN_CXX_STANDARD, since TRITON_MIN_CXX_STANDARD is used throughout our triton libraries.

I am not sure if it makes sense to explicitly set CMAKE_CXX_STANDARD here, since we do not compile any custom code, and third parties will use provided CMAKE_CXX_STANDARD through -DCMAKE_CXX_STANDARD:STRING=${TRITON_MIN_CXX_STANDARD}.

In other repositories, target_compile_features(... PRIVATE cxx_std_${TRITON_MIN_CXX_STANDARD}) will make sure that libraries are compiled at least with the provided standard, and if newer standard is needed, then compiler will add appropriate flag, eg -std=gnu++20.

Since I don't introduce hard set of CMAKE_CXX_STANDARD throughout Triton's server, core, backend, common libraries, I didn't see the reason why we would need to set it here. And my logic was, since Triton requires at least TRITON_MIN_CXX_STANDARD standard, third parties should be compiled with the same standard as minimum required. Note, that some third-parties do not take CMAKE_CXX_STANDARD.

[Tabrizian]

I'll be approving the PR but still to me it seems like the correct thing would be to propagate CMAKE_CXX_STANDARD. I think the interface for specifying the CMAKE standard is CMAKE_CXX_STANDARD. IMO, TRITON_MIN_CXX_STANDARD is an internal variable that determines the minimum required standard but CMAKE_CXX_STANDARD should be greater than or equal to TRITON_MIN_CXX_STANDARD.