[vlc-devel] [PATCH 1/2] posix/wait: do not use alignof on expression

[Rémi Denis-Courmont]

I don't know what you're trying to do there, and that does not look like a static assertion to me.

Le 7 août 2020 16:37:47 GMT+03:00, Marvin Scholz <epirat07 at gmail.com> a écrit :
>
>
>On 7 Aug 2020, at 14:42, Rémi Denis-Courmont wrote:
>
>> Hi,
>>
>> You can use _Generic() if you want to match types. No need for 
>> compiler-dependent ifdef'ery.
>>
>> Might be overkill here though.
>
>I thought about that too but the following does not work:
>
># define VLC_ALIGNOF(t, expr) \
>   (_Generic(expr, t: _Alignof(t)))
>
>atomic_uint *addr;
>VLC_ALIGNOFT(atomic_uint, *addr);
>
>This fails to compile with:
>error: '_Generic' selector of type 'unsigned int' is not compatible
>with 
>any association
>
>>
>> Le 7 août 2020 07:55:35 GMT+03:00, Steve Lhomme <robux4 at ycbcr.xyz> a 
>> écrit :
>>> OK for both. Although it would be nice to have a safety check in
>case
>>> the underlying type changes and the alignof is not valid anymore.
>>>
>>> May a static_assert when compiling with GCC, like
>>> #if defined(__GNUC__) && !defined(__llvm__) &&
>>> !defined(__INTEL_COMPILER)
>>> static_assert(alignof(*addr) == alignof(atomic_uint), "type 
>>> mismatch");
>>> #endif
>>>
>>>
>>> On 2020-08-07 6:44, Marvin Scholz wrote:
>>>> Using alignof not on a type but an expression is a GNU extension
>and
>>>> causes the following warning:
>>>>
>>>>     warning: '_Alignof' applied to an expression is a GNU extension
>>>> ---
>>>>   src/posix/wait.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/src/posix/wait.c b/src/posix/wait.c
>>>> index 584cc493b1a..3fa6890193a 100644
>>>> --- a/src/posix/wait.c
>>>> +++ b/src/posix/wait.c
>>>> @@ -52,7 +52,7 @@ static struct wait_bucket
>>>>   static struct wait_bucket *wait_bucket_get(atomic_uint *addr)
>>>>   {
>>>>       uintptr_t u = (uintptr_t)addr;
>>>> -    size_t idx = (u / alignof (*addr)) % ARRAY_SIZE(wait_buckets);
>>>> +    size_t idx = (u / alignof (atomic_uint)) %
>>> ARRAY_SIZE(wait_buckets);
>>>>
>>>>       return &wait_buckets[idx];
>>>>   }
>>>> -- 
>>>> 2.24.3 (Apple Git-128)
>>>>
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>> -- 
>> Envoyé de mon appareil Android avec Courriel K-9 Mail. Veuillez 
>> excuser ma brièveté.
>
>
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-- 
Envoyé de mon appareil Android avec Courriel K-9 Mail. Veuillez excuser ma brièveté.
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