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

[Thomas Guillem]

Set LGTM. I don't feed the need for _Generic() either.

On Fri, Aug 7, 2020, at 19:55, Marvin Scholz wrote:
> Trying to use _Generic to ensure the expression has the expected type.


> How would you do the static assertion, it would lead to
> the exactly same issue as I am hitting here, no?

> On 7 Aug 2020, at 19:52, Rémi Denis-Courmont wrote:

>> 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];
>>>>>>    }
>>>>>>  -- 
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>>> 
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>> -- 
>> Envoyé de mon appareil Android avec Courriel K-9 Mail. Veuillez excuser ma brièveté.
>> 

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