When should we consider moving Halide to C++17?

[steven-johnson]

Currently, Halide assumes/requires C++11. When should we consider moving our baseline to C++17?

(FYI: Google recently moved to allow use of C++17 internally, so we'd be fine with this; I don't know how many other downstream users might be impacted by such a change. Would be nice to start a discussion.)

[abadams]

constexpr if and parameter pack folds would be very nice. However it would raise our required gcc version to 7, MSVC version to 2017, and clang version to 8. That seems fairly aggressive to me. Then again, we already require that you have clang 7 or greater just to compile the runtime.

[steven-johnson]

Since the current version of MSVC is 2019, requiring MSVC 2017 seems pretty reasonable to me. (Are we still running 2015 on the builtbots?) Ditto for gcc 7. But we should get widespread input to see if how this would impact existing users.

[benoitsteiner]

Debian stable only ships with llvm/clang 7, so it would make it harder to use Halide for folks. In my company we're using what looks like clang7 with some modification, so that would mean goodbye to Halide. It would be best to delay the upgrade for some time until the mainstream environments have caught up.

[slomp]

I'm OK with upgrading the compiler for the "internals", but at for the public interface, I think it's wise to hold and be a bit conservative with C++11 for now.

[steven-johnson]

@benoitsteiner I don't think anyone is in a rush (at least, I'm not); I mainly want to start thinking about timetables. Would you have a guesstimate as to when your folk would be able to use C++17 without causing any hardship?

[alexreinking]

Debian stable only ships with llvm/clang 7, so it would make it harder to use Halide for folks. [...]

We require LLVM/Clang 9+ to build Halide. I guess you're using binary releases?

[abadams]

Installing llvm 9 as a system toolchain is a bigger deal that just requiring that someone build llvm 9 as a library as part of building Halide.

[alexreinking]

This matrix might be helpful

Standard	Compiler	Supported since	Release date	Notes
C++11	GCC	4.8.1	May 31, 2013
	Clang	3.3	June 17, 2013
	MSVC	2015	July 20, 2015
C++14	GCC	5.1	April 22, 2015	default since v6.0
	Clang	3.4	Jan 2, 2014
	MSVC	2017 15.0	March 7, 2017
C++17	GCC	7.1	May 2, 2017	default since v11.0
	Clang	5	September 7, 2017
	MSVC	2017 15.7	May 07, 2018	DRs missing until 2019 16.4

AFAIK, our minimum supported compiler versions all permit C++14, though we would need to bump the version for GCC if we wanted to require C++17. See also #5154

References:

https://gcc.gnu.org/projects/cxx-status.html
https://gcc.gnu.org/releases.html
https://clang.llvm.org/cxx_status.html
https://releases.llvm.org/
https://docs.microsoft.com/en-us/cpp/overview/visual-cpp-language-conformance?view=vs-2019
https://docs.microsoft.com/en-us/visualstudio/releasenotes/vs2015-version-history
https://docs.microsoft.com/en-us/visualstudio/releasenotes/vs2017-relnotes-v15.0

[alexreinking]

Installing llvm 9 as a system toolchain is a bigger deal that just requiring that someone build llvm 9 as a library as part of building Halide.

Moot point since Clang supports C++17 since version 5, and @benoitsteiner is on version 7.

[abadams]

I claimed 8 because I was taking the max along the clang version column in https://clang.llvm.org/cxx_status.html, but actually there's a 9. I'm not sure how seriously to take the DRs.

[abadams]

Maybe the solution is to just make sure one of the buildbots is using our minimum supported clang/gcc version, and permit anything that it will accept?

[alexreinking]

I'm not sure how seriously to take the DRs. [...] Maybe the solution is to just make sure one of the buildbots is using our minimum supported clang/gcc version, and permit anything that it will accept?

I'm not either, but this is the correct solution no matter what minimum C++ version we take.

[steven-johnson]

AFAIK, our minimum supported compiler versions all permit C++14, though we would need to bump the version for GCC if we wanted to require C++17.

Do we actually explicitly specify minimum versions of gcc/clang anywhere? I'm not aware of that. I thought we just claimed "C++11 support".

[steven-johnson]

This matrix might be helpful

[snip]

IIRC, MSVC2015 "mostly" supported C++11 but had a few odd bugs and warts we had to work around. We absolutely should drop overt support for it as soon as no one complains.

[alexreinking]

Do we actually explicitly specify minimum versions of gcc/clang anywhere? I'm not aware of that. I thought we just claimed "C++11 support".

The versions on our buildbots are de facto minimums. I raised the issue of making things explicit in #5154