Re: [XEN PATCH v6] xen/include: avoid using a compiler extension for BUILD_BUG_ON_ZERO

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.07.2023 16:53, Nicola Vetrini wrote:
> 
> 
> On 04/07/23 16:24, Jan Beulich wrote:
>> On 26.06.2023 17:37, Nicola Vetrini wrote:
>>> Redefine BUILD_BUG_ON_ZERO to avoid using a compiler extension
>>> that gives an acceptable semantics to C99 undefined behavior 58
>>> ("A structure or union is defined as containing no named members
>>> (6.7.2.1)").
>>>
>>> The first definition includes an additional named field of type
>>> char.
>>>
>>> The chosen ill-formed construct for the second definition is a struct
>>> with a named bitfield of width 0 when the condition is true,
>>> which prevents the UB without using the compiler extension while keeping
>>> the semantic of the construct.
>>>
>>> The choice of the bitwise AND operation to bring the result to 0
>>> when cond is false boils down to possibly better portability.
>>>
>>> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
>>
>> The code change looks okay to me now, but I'd like to double check
>> towards the testing you did with this change: While it is clear that
>> you will have checked that the tree builds with the adjustments, I
>> expect that would have been with a compiler supporting _Static_assert.
>> Did you also check with an older compiler, using the alternative
>> implementation? Plus did you also check both constructs for actually
>> triggering when the supplied condition turns out to be true?
> 
> Besides using the build pipelines in gitlab, I checked just the macro on 
> Compiler Explorer using gcc 4.4.7 and clang 16 
> (https://godbolt.org/z/1d6vznxcW) and the construct is behaving as expected.

Great, thanks for confirming.

Jan