GH-131278: Support building using "computed gotos" for clang-cl on Windows

[chris-eibl]

For details please see #131278.

• Issue: Support building using "computed gotos" for clang-cl on Windows #131278

[picnixz]

Some readability comments. Do you want to actually add a What's New as well? (I don't know how wide is the audience)

[Fidget-Spinner]

I don't remember if we had a Whats New for the original clang-cl issue. We should definitely mention in the whats new that clang cl building on Windows is now possible, and say PGO too.

[Fidget-Spinner]

Woops sorry pressed the wrong button!

[picnixz]

@chris-eibl How do you plan to address:

We should definitely mention in the whats new that clang cl building on Windows is now possible, and say PGO too.

Are there more PRs for those kind of optimizations on Windows? if so, we can delay everything until they are done. Otherwise, let's mention that in the What's New entry

[chris-eibl]

I don't remember if we had a Whats New for the original clang-cl issue. We should definitely mention in the whats new that clang cl building on Windows is now possible, and say PGO too.

Building with clang-cl has been possible for a very long time: #101352.

The only fixes I am aware of are just these two (and just main and current 3.14 alphas are/have been affected):

• _GENERATE_DEBUG_SECTION_WINDOWS https://github.com/python/cpython/pull/130040/files#r1981801677 already fixed by you
• blake2module.c needs to be compiled with libhacl SIMD flags #130213: close to being fixed

So maybe just mention PGO in the Whats New?
Maybe this here and -flto=thin, too?

@zooba treats clang-cl as quite optional in #129907 (comment)
but I think a Whats New entry would not hurt.

Shall I give it a go here and mention those three things?

[chris-eibl]

@Fidget-Spinner : your tail call for Windows #130040 (comment) came without What's new, too. Shall I add an entry, too?

Are there more PRs for those kind of optimizations on Windows?

I'd like to work on the many warnings clang-cl emits, but IMHO that won't go into What's new. Regarding optimizations, I think that's it atm, we can amend later, anyway.

[Fidget-Spinner]

@Fidget-Spinner : your tail call for Windows #130040 (comment) came without What's new, too. Shall I add an entry, too?

We don't need it, as there's already a whats new for tail call in general.

[chris-eibl]

IMHO "computed gotos and/or tail calls" would be better here, but up to @Fidget-Spinner 's liking :)

[zooba]

I think we want to be careful putting too much into What's New, since we still aren't technically supporting clang-cl on Windows yet, we're really just unblocking it. No doubt if @chris-eibl sticks around it'll remain unblocked, but also, if Clang one day breaks down and MSVC is fine then we're still going to release.

A simple entry (which should already be there?) stating that build.bat now has a clang-cl specific option (previously you needed to set PlatformToolset but that was the only difference) and perhaps calling out that this enables some clang-specific optimisations, referencing PCbuild\readme.txt for more info seems to hit the balance. Let's avoid using the word "support" in regards to our source code.

[chris-eibl]

previously you needed to set PlatformToolset but that was the only difference

This is still true, we do not have a "clang" switch for build.bat, yet.

I am fine with entirely removing the whatsnew if that makes it easier :)

[zooba]

PCbuild changes look good, just suggesting some clarifications to the docs.

[zooba]

Suggested change

	Add optimizing flag ``WITH_COMPUTED_GOTOS`` to support such builds using
	clang-cl on Windows. Patch by Chris Eibl.
	Add optimizing flag ``WITH_COMPUTED_GOTOS`` to Windows builds for when
	using a compiler that supports it (currently clang-cl). Patch by Chris Eibl.

[zooba]

Suggested change

	* building with clang-cl on Windows now supports PGO (profile guided
	* clang-cl builds on Windows now work with ``--pgo`` (profile guided

[zooba]

Suggested change

	optimization), uses ``-flto=thin`` and can be configured to use
	optimization), and can be configured to use

Is using -flto=thin important to call out? It shouldn't change the ABI of the build output at all, and it's not configurable.

[zooba]

This is still true, we do not have a "clang" switch for build.bat, yet.

(Seen after submitting my review.) Oh don't we? Okay, yeah, let's treat all clang-cl specific changes as undocumented then. The day it gets an option is when we'd start putting notes in the Build section of NEWS, and give the option a brief mention in What's New.