Make _Py_ThreadId() work on PowerPC, IBM Z, etc.

[colesbury]

Feature or enhancement

Currently, _Py_ThreadId() is only defined in Py_GIL_DISABLED builds and does not compile on PowerPC or IBM Z. It would be convenient to make the functionality available in general: it will be useful to implement thread-safe recursive locks. (Currently our recursive lock implementations rely on the GIL for thread-safety.)

There are two parts to this:

1. We should use __builtin_thread_pointer() when available
2. We should fall back to a function that returns the address of a thread-local variable if no other fast implementation is available (instead of erroring out).

The fallback (2) will be a bit slower, but it provides a portable implementation.

Note that checking if __builtin_thread_pointer() is available is not trivial. __has_builtin(__builtin_thread_pointer) is not useful (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96952). You need some combination of platform and GCC/clang version check.

cc @vstinner @corona10

Linked PRs

• gh-112535: Update _Py_ThreadId() to support PowerPC, IBM Z, etc #112553
• gh-112535: Update _Py_ThreadId() to support PowerPC #112624
• gh-112535: Add test on _Py_ThreadId() #112709
• gh-112535: Add comment for ppc32/64 registers #112746
• gh-112535: Update _Py_ThreadId() to support s390/s390x #112751
• gh-112535: Update _Py_ThreadId() to support RISC-V #113084
• [WIP] gh-112535: Implement fallback implementation of _Py_ThreadId() #113094
• gh-112535: Implement fallback implementation of _Py_ThreadId() #113185

[corona10]

@colesbury Can I take a look at it?

[colesbury]

@corona10 - of course, thanks!

[vstinner]

Note that checking if __builtin_thread_pointer() is available is not trivial. __has_builtin(__builtin_thread_pointer) is not useful (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96952). You need some combination of platform and GCC/clang version check.

That's fine. We have many tests preprocessor checks like that, #if ....

[corona10]

@colesbury Do we need to support more platforms or we can close the issue?

[colesbury]

I think we still want the cross-platform fallback that returns the address of a thread-local variable. That will probably need to be implemented in a .c file because accessing thread-local variables across libraries can cause issues on some platforms.

[vstinner]

You may have a look at _PyThreadState_GET() which is implemented by reading a thread local variable. If Py_BUILD_CORE_MODULE macro is defined, the function is implemented with an opaque function call: implementation for libraries.

[corona10]

Got it, I will send a PR soon.

I am going to submit the PR early next week :)

[corona10]

Okay, let's close the issue since the fallback implementation is merged.
Thank you for all your supports and contribution @colesbury, @vstinner(Awesome test codes!) and @furkanonder (RISC-V)

Now, we can support free-threaded CPython for all platforms where the fallback implementation(it only requires thread-local feature) works!