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Re: [PATCH] Add a new type attribute always_alias (PR79671)
On 04/06/17 21:14, Richard Biener wrote:
> On April 6, 2017 7:39:14 PM GMT+02:00, Bernd Edlinger <bernd.edlinger@hotmail.de> wrote:
>> On 04/06/17 16:17, Florian Weimer wrote:
>>>> Here is what I want to write in the doc:
>>>>
>>>> @item typeless_storage
>>>> @cindex @code{typeless_storage} type attribute
>>>> A type declared with this attribute behaves like a character type
>>>> with respect to aliasing semantics.
>>>> This is attribute is similar to the @code{may_alias} attribute,
>>>> except that it is not restricted to pointers.
>>>
>>> As Jakub pointed out, this is not what we need here. An object of
>> type
>>> char does *not* have untyped storage. Accessing it as a different
>> type
>>> is still undefined.
>>>
>>
>> but, do you agree that this is valid in C11?
>>
>> typedef char char_a[4];
>>
>> int
>> main (void)
>> {
>> char_a a = {1,2,3,4};
>> short *b = (short *) &a;
>>
>> b[1] = 0;
>>
>> if (a[0] == 1 && a[1] == 2 && a[2] == 3 && a[3] == 4)
>> abort();
>>
>> exit(0);
>> }
>>
>>
>> all I want to do is replace "char" with a different type.
>
> Why?
- It feels more othogonal this way.
- Otherwise malloc would have magic power, in creating objects with no
declared type.
- And implementing something like malloc in plain C would actually be
forbidden, which is ridiculous, because I already have done it.
- It was easy to implement in the middle end.
- It feels useful in C and C++.
- Jason says :)
Bernd.
>
> Richard.