MSVC emits warnings in non-debug builds

[chris-eibl]

Since #130398, the below warning is emitted several times in case of MSVC release (or PGO) builds:

Include\internal\pycore_ceval.h(209): warning C4172: returning address of local variable or temporary : here

So the comment in the code

cpython/Include/internal/pycore_ceval.h

Lines 202 to 211 in 55815a6

	static inline uintptr_t
	_Py_get_machine_stack_pointer(void) {
	#if _Py__has_builtin(__builtin_frame_address)
	return (uintptr_t)__builtin_frame_address(0);
	#else
	char here;
	/* Avoid compiler warning about returning stack address */
	return return_pointer_as_int(&here);
	#endif
	}

only works for debug builds. This is because in case of optimizing, MSVC is inlining return_pointer_as_int

cpython/Include/internal/pycore_ceval.h

Lines 196 to 200 in 55815a6

	#if !_Py__has_builtin(__builtin_frame_address)
	static uintptr_t return_pointer_as_int(char* p) {
	return (uintptr_t)p;
	}
	#endif

and then is "smart" enough to realize and warn about it.

See here https://godbolt.org/z/ooK5Poxo3 (btw gcc or clang won't do that https://godbolt.org/z/459f6hj9G).

__declspec(noinline) would help, but IMHO is an unneeded performance penalty.

I suggest to use #pragma warning(disable:4172) to silence the warning.
Since this is a code generation warning (4700-4999), we have to guard the whole
_Py_get_machine_stack_pointer and cannot just place it around line 209.

@colesbury had a much better idea: use _AddressOfReturnAddress which even gets rid of UB: see https://godbolt.org/z/7c9Gq9jzM and https://github.com/llvm/llvm-project/blob/llvmorg-10.0.0-rc1/clang/lib/Basic/Stack.cpp#L24.

Linked PRs

• GH-131288: use _AddressOfReturnAddress in _Py_get_machine_stack_pointer to silence MSVC warning in pycore_ceval.h for release builds #131289