Re: Invalid _Static_assert expanded from HASH_CALLBACKS_CHECK

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.05.2021 11:59, Roberto Bagnara wrote:
> On 25/05/21 10:58, Jan Beulich wrote:
>> On 24.05.2021 06:29, Roberto Bagnara wrote:
>>> I stumbled upon parsing errors due to invalid uses of
>>> _Static_assert expanded from HASH_CALLBACKS_CHECK where
>>> the tested expression is not constant, as mandated by
>>> the C standard.
>>>
>>> Judging from the following comment, there is partial awareness
>>> of the fact this is an issue:
>>>
>>> #ifndef __clang__ /* At least some versions dislike some of the uses. */
>>> #define HASH_CALLBACKS_CHECK(mask) \
>>>       BUILD_BUG_ON((mask) > (1U << ARRAY_SIZE(callbacks)) - 1)
>>>
>>> Indeed, this is not a fault of Clang: the point is that some
>>> of the expansions of this macro are not C.  Moreover,
>>> the fact that GCC sometimes accepts them is not
>>> something we can rely upon:
>>>
>>> $ cat p.c
>>> void f() {
>>> static const int x = 3;
>>> _Static_assert(x < 4, "");
>>> }
>>> $ gcc -c -O p.c
>>> $ gcc -c p.c
>>> p.c: In function ‘f’:
>>> p.c:3:20: error: expression in static assertion is not constant
>>> 3 | _Static_assert(x < 4, "");
>>> | ~^~
>>> $
>>
>> I'd nevertheless like to stick to this as long as not proven
>> otherwise by future gcc.
> 
> Just two observations:
> 
> 1) Violating the C standard makes MISRA complicance significantly
>     more difficult.  In addition, it complicates also compiler
>     qualification, for those who are required to do it.
> 
> 2) GCC is already proving otherwise: if you try compiling
>     without optimization, compilation fails.

I'm afraid we have other issues when building without optimization.

In any event - feel free to contribute a patch. As said, I'm not
the maintainer of that piece of code, and you may well find him
agreeing with such a change. He didn't reply yet on the earlier
mail, which would be a prereq to me possibly making a patch
myself.

Jan