_Py_ThreadId() fails to compile with clang on Windows ARM64 (MSYS2 CLANGARM64)

Bug report

Bug description:

Building a free-threaded extension module for Windows ARM64 with MSYS2's CLANGARM64 (llvm-mingw) toolchain fails to compile Include/object.h:

object.h:200:11: error: call to undeclared function '__getReg'; ISO C99 and later do not
support implicit function declarations [-Wimplicit-function-declaration]
  200 |     tid = __getReg(18);
      |           ^

Example actions run where I tried to build jq.py.

Minimal repro

/* repro.c */
#include <Python.h>

uintptr_t repro(void) { return _Py_ThreadId(); }

$ gcc -c repro.c -O1 -Wall -DPy_GIL_DISABLED=1 -I <freethreaded-arm64-headers>
In file included from repro.c:1:
In file included from Python.h:81:
object.h:200:11: error: call to undeclared function '__getReg'; ISO C99 and later do not
support implicit function declarations [-Wimplicit-function-declaration]
  200 |     tid = __getReg(18);
      |           ^
1 error generated.

(gcc here is mingw-w64-clang-aarch64-gcc-compat, i.e. clang.)

_Py_ThreadId() reaches this branch, added in #124609 / GH-124663 for GCC:

#elif defined(__MINGW32__) && defined(_M_ARM64)
    tid = __getReg(18);

clang does not define _M_ARM64 but mingw-w64 does, in _mingw_mac.h:

#if defined(__aarch64__) && !defined(_M_ARM64)
#  define _M_ARM64 1

which is pulled in transitively by pyconfig.h. So the condition is true all aarch64 compilers. clang's predefines here are:

#define _WIN32 1
#define __GNUC__ 4
#define __MINGW32__ 1
#define __MINGW64__ 1
#define __aarch64__ 1
#define __clang__ 1

Why __getReg is unavailable to clang

• clang's intrin.h declares unsigned __int64 __getReg(int);, but only inside #ifdef _MSC_VER. For a *-windows-gnu target the header takes the #include_next <intrin.h> path and defers to mingw-w64.
• mingw-w64's intrin.h declares __getReg only via __MACHINEIA64 (Itanium), which expands to nothing unless __ia64__. There is no __MACHINEARM64 entry for it.
• It is also not a clang builtin.

Note that falling through to the __aarch64__ branch is not a valid fix:

#elif defined(__aarch64__)
    __asm__ ("mrs %0, tpidr_el0" : "=r" (tid));

On Windows ARM64 tpidr_el0 reads as 0 on every thread, whereas x18 holds the TEB and is correctly per-thread:

main x18=000000cc3736d000 tpidr_el0=0000000000000000 NtCurrentTeb=000000cc3736d000
thr1 x18=000000cc37371000 tpidr_el0=0000000000000000 NtCurrentTeb=000000cc37371000
thr2 x18=000000cc37373000 tpidr_el0=0000000000000000 NtCurrentTeb=000000cc37373000

A constant 0 would collide with _Py_UNOWNED_TID, so this would compile cleanly and then misbehave silently — the same failure mode discussed in #124609.

Suggested fix

Keep __getReg for GCC and give clang a dedicated branch reading x18:

#elif defined(__MINGW32__) && defined(_M_ARM64) && !defined(__clang__)
    tid = __getReg(18);
#elif defined(__MINGW32__) && defined(_M_ARM64)
    __asm__ ("mov %0, x18" : "=r" (tid));  // Windows/ARM64 keeps the TEB in x18

I have verified that this compiles and generates the expected read.

Versions tested

• CPython 3.14 (free-threaded)
• Windows 11 Pro 10.0.28000 ARM64
• clang 22.1.8 (MSYS2 CLANGARM64)

CPython versions tested on:

3.14

Operating systems tested on:

Windows

Linked PRs

• gh-157252
• gh-157253

Start with the _Py_ThreadId() platform branches in Include/object.h and reproduce the failure using the supplied repro.c example and compiler command. Review linked PRs gh-157252 and gh-157253 for work already underway. Done means the free-threaded Windows ARM64 CLANGARM64 repro compiles and reads a per-thread identifier correctly.