Re: [PATCH] Add a new type attribute always_alias (PR79671)

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

On 04/07/17 08:47, Richard Biener wrote:
> On Thu, 6 Apr 2017, Bernd Edlinger wrote:
>
>> On 04/06/17 21:05, Florian Weimer wrote:
>>> On 04/06/2017 08:49 PM, Bernd Edlinger wrote:
>>>
>>>> For instance how do you "declare an object without a declared type"?
>>>
>>> malloc and other allocation functions return pointers to objects without
>>> a declared type.
>>>
>>
>> Thanks Florian,
>>
>> this discussion is very helpful.
>>
>> How about this for the documentation:
>>
>> @item typeless_storage
>> @cindex @code{typeless_storage} type attribute
>> In the context of section 6.5 paragraph 6 of the C11 standard,
>> an object of this type behaves as if it has no declared type.
>> In the context of section 6.5 paragraph 7 of the C11 standard,
>> an object or a pointer if this type behaves as if it were a
>> character type.
>> This is attribute is similar to the @code{may_alias} attribute,
>> except that it is not restricted to pointers.
>>
>> Example of use:
>>
>> @smallexample
>> typedef int __attribute__((__typeless_storage__)) int_a;
>>
>> int
>> main (void)
>> @{
>>    int_a a = 0x12345678;
>>    short *b = (short *) &a;
>>
>>    b[1] = 0;
>>
>>    if (a == 0x12345678)
>>      abort();
>>
>>    exit(0);
>> @}
>> @end smallexample
>
> Seriously, do not suggest such broken case.  There's a union to
> do this example portably.
>

Well, it is just a mod of the other example above in
the documentation of may_alias:

typedef short __attribute__((__may_alias__)) short_a;

int
main (void)
@{
   int a = 0x12345678;
   short_a *b = (short_a *) &a;

   b[1] = 0;

   if (a == 0x12345678)
     abort();

   exit(0);
@}
@end smallexample

I just moved the attribute from "b" to "a", and that
is what the C++ people want to do as well, just they
call it a "class int_a { std::byte x[4]; }".

I personally like to have the symmetry between the
two concepts here, because it helps to understand the
differences.


Bernd.

> Richard.
>