gh-112535: Implement fallback implementation of _Py_ThreadId()

[corona10]

• Issue: Make _Py_ThreadId() work on PowerPC, IBM Z, etc. #112535

[corona10]

I reopened the PR, since there was some accident with #113094
As same as before, I checked that the fallback logic works correctly on my local machine.

diff --git a/Include/object.h b/Include/object.h
index 6afdc1688e..d31df80db0 100644
--- a/Include/object.h+++ b/Include/object.h@@ -245,54 +245,7 @@ static inline uintptr_t
_Py_ThreadId(void)
{
uintptr_t tid;
-#if defined(_MSC_VER) && defined(_M_X64)- tid = __readgsqword(48);-#elif defined(_MSC_VER) && defined(_M_IX86)- tid = __readfsdword(24);-#elif defined(_MSC_VER) && defined(_M_ARM64)- tid = __getReg(18);-#elif defined(__i386__)- __asm__("movl %%gs:0, %0" : "=r" (tid)); // 32-bit always uses GS-#elif defined(__MACH__) && defined(__x86_64__)- __asm__("movq %%gs:0, %0" : "=r" (tid)); // x86_64 macOSX uses GS-#elif defined(__x86_64__)- __asm__("movq %%fs:0, %0" : "=r" (tid)); // x86_64 Linux, BSD uses FS-#elif defined(__arm__)- __asm__ ("mrc p15, 0, %0, c13, c0, 3\nbic %0, %0, #3" : "=r" (tid));-#elif defined(__aarch64__) && defined(__APPLE__)- __asm__ ("mrs %0, tpidrro_el0" : "=r" (tid));-#elif defined(__aarch64__)- __asm__ ("mrs %0, tpidr_el0" : "=r" (tid));-#elif defined(__powerpc64__)- #if defined(__clang__) && _Py__has_builtin(__builtin_thread_pointer)- tid = (uintptr_t)__builtin_thread_pointer();- #else- // r13 is reserved for use as system thread ID by the Power 64-bit ABI.- register uintptr_t tp __asm__ ("r13");- __asm__("" : "=r" (tp));- tid = tp;- #endif-#elif defined(__powerpc__)- #if defined(__clang__) && _Py__has_builtin(__builtin_thread_pointer)- tid = (uintptr_t)__builtin_thread_pointer();- #else- // r2 is reserved for use as system thread ID by the Power 32-bit ABI.- register uintptr_t tp __asm__ ("r2");- __asm__ ("" : "=r" (tp));- tid = tp;- #endif-#elif defined(__s390__) && defined(__GNUC__)- // Both GCC and Clang have supported __builtin_thread_pointer- // for s390 from long time ago.- tid = (uintptr_t)__builtin_thread_pointer();-#elif defined(__riscv)- #if defined(__clang__) && _Py__has_builtin(__builtin_thread_pointer)- tid = (uintptr_t)__builtin_thread_pointer();- #else- // tp is Thread Pointer provided by the RISC-V ABI.- __asm__ ("mv %0, tp" : "=r" (tid));- #endif-#elif defined(thread_local) || defined(__GNUC__)+#if defined(thread_local) || defined(__GNUC__)
// gh-112535: Using characteristics of TLS address mapping.
// The address of the thread-local variable is not equal with the actual thread pointer,
// However, it has the property of being determined by loader at runtime, with no duplication of values

[colesbury]

Some suggestions on code comments below. I think we should put any detailed explanations of _Py_GetThreadLocal_Addr near the implementation (and not where it's called in _Py_ThreadId().

I don't think there's a lot we can say generally about the implementation of &_Py_tss_tstate -- it depends on the platform, but adding a constant offset is generally not expensive. There are two common costs:

1. Calling _Py_GetThreadLocal_Addr(), which mostly can't be inlined.
2. Depending on the implementation of TLS, computing the address may or may not require another function call.

[colesbury]

I think this should just be an #else. We're not using thread_local or depending on GNU specific features here in object.h.