gh-157212: Fix _Py_ThreadId() on Windows ARM64 with MinGW (gh-157253)

Author: christianaurichzm

Include/cpython/object.h

@@ -523,7 +523,8 @@ _Py_ThreadId(void)
 #elif defined(__MINGW32__) && defined(_M_IX86)
     tid = __readfsdword(24);
 #elif defined(__MINGW32__) && defined(_M_ARM64)
-    tid = __getReg(18);
+    // x18 is the Windows ARM64 platform register and points to the TEB.
+    __asm__ ("mov %0, x18" : "=r" (tid));
 #elif defined(__i386__)
     __asm__("{movl %%gs:0, %0|mov %0, dword ptr gs:[0]}" : "=r" (tid));  // 32-bit always uses GS
 #elif defined(__MACH__) && defined(__x86_64__)

Misc/NEWS.d/next/Windows/2026-09-10-02-00-00.gh-issue-157212.x18TEB.rst

@@ -0,0 +1,3 @@
+Fix ``_Py_ThreadId()`` compilation on Windows ARM64 with MinGW toolchains,
+where ``__getReg()`` is unavailable. The TEB address is now read directly
+from ``x18``. Patch by Christian Aurich.