[issue28855] Compiler warnings in _PyObject_CallArg1()

[STINNER Victor]

New submission from STINNER Victor:

_PyObject_CallArg1() is the following macro:
---
#define _PyObject_CallArg1(func, arg) \
    _PyObject_FastCall((func), &(arg), 1)
---

It works well in most cases, but my change 8f258245c391 or change b9c9691c72c5 added compiler warnings, because an argument which is not directly a PyObject* type is passed as "arg".

I tried to cast in the caller: _PyObject_CallArg1(func, (PyObject*)arg), but sadly it doesn't work :-( I get a compiler error.

Another option is to cast after "&" in the macro:
---
 #define _PyObject_CallArg1(func, arg) \
-    _PyObject_FastCall((func), &(arg), 1)
+    _PyObject_FastCall((func), (PyObject **)&(arg), 1)
---

This option may hide real bugs, so I dislike it.

A better option is to stop using ugly C macros and use a modern static inline function: see attached static_inline.patch. This patch casts to PyObject* before calling _PyObject_CallArg1() to fix warnings.

The question is if compilers are able to emit efficient code for static inline functions using "&" on an argument. I wrote the macro to implement the "&" optimization: convert a PyObject* to a stack of arguments: C array of PyObject* objects.

----------
files: static_inline.patch
keywords: patch
messages: 282212
nosy: benjamin.peterson, haypo, serhiy.storchaka
priority: normal
severity: normal
status: open
title: Compiler warnings in _PyObject_CallArg1()
versions: Python 3.7
Added file: http://bugs.python.org/file45729/static_inline.patch

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