pyatomic_gcc.h with GCC discards const qualifier

[ndparker]

Bug report

Bug description:

Hi,

including Python.h with gcc and -Wcast-qual raises the following issue (Regression from 3.12):

    /usr/include/python3.13/cpython/pyatomic_gcc.h: In function '_Py_atomic_load_ptr':
    /usr/include/python3.13/cpython/pyatomic_gcc.h:300:34: error: cast discards 'const' qualifier from pointer target type [-Werror=cast-qual]
      300 | { return (void *)__atomic_load_n((void **)obj, __ATOMIC_SEQ_CST); }
          |                                  ^
    /usr/include/python3.13/cpython/pyatomic_gcc.h: In function '_Py_atomic_load_ptr_relaxed':
    /usr/include/python3.13/cpython/pyatomic_gcc.h:359:34: error: cast discards 'const' qualifier from pointer target xt (line 8))

$ python3.13 -V
Python 3.13.0b1

This is a follow-up to #120293

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-120593: Fix const qualifier in pyatomic.h #121052
• gh-120593: Fix const qualifier in _PyLong_CompactValue() #121053
• gh-120593: Fix const qualifier in pyatomic.h #121055
• [3.13] gh-120593: Fix const qualifier in pyatomic.h (GH-121055) #121057
• gh-120593: Check -Wcast-qual flag in test_cext #121081
• gh-120593: Make _PyLong_CompactValue() parameter const again #122367

[picnixz]

For the record, this is not the only place where const qualifiers are lost. If you compile cpython with make CFLAGS=-Wcast-qual -j12 you end up with a lot of warnings. For the line 359, the fix would be:

// pyatomic_gcc.h
static inline void *
_Py_atomic_load_ptr_relaxed(const void *obj)
-{ return (void *)__atomic_load_n((void **)obj, __ATOMIC_RELAXED); }
+{ return (void *)__atomic_load_n((void * const *)obj, __ATOMIC_RELAXED); }

As you can see you lose constness twice, namely when calling __atomic_load_n and when returning the type. Note that we have

// pyatomic_std.h
static inline void *
_Py_atomic_load_ptr_relaxed(const void *obj)
{
    _Py_USING_STD;
    return atomic_load_explicit((const _Atomic(void*)*)obj,
                                memory_order_relaxed);
}

As you can see, the cast here is essentially using a const void **, but still the return type should be a const as well (in the case of __atomic_load_n, the signature is type __atomic_load_n(type *ptr, int memorder);).

EDIT: I edited my comment in order to reflect the real signature of _Py_atomic_load_ptr_relaxed in cpython. The return type is a void * and not a const void * but the inner cast fix remains correct.

[ndparker]

Yes, I'd like to clarify that this is specifically not about compiling CPython, it's about including Python.h only.

[picnixz]

Yes, I understood that from the other issue but since you only mentioned 2 lines, I wanted to 1) give a way to fix it 2) say that it's not just 2 lines that are affected but a lot of calls.

[vstinner]

The warnings were fixed by 9cd2dcb and e51e880.

I also added a check in test_cext for check for non-regression: b7a95df.

Thanks @ndparker for your bug report. It's now fixed, I close the issue.

[markshannon]

I don't think weakening the type of calls that should be const is the correct thing to do here.
Where there is a conflict in types we should be strengthening the type, not weakening it.
Why not strengthen the types in the callees, rather than weakening them in the callers?

[vstinner]

@markshannon:

I don't think weakening the type of calls that should be const is the correct thing to do here.

If you're talking about _PyLong_CompactValue(): I wrote PR gh-122367 to restore the const modifier.

If you're talking about something else, please elaborate which function/code you are referring to.