From: Bernd Edlinger <bernd.edlinger@hotmail.de>
To: Richard Biener <rguenther@suse.de>
Cc: Florian Weimer <fweimer@redhat.com>,
Jakub Jelinek <jakub@redhat.com>,
Jonathan Wakely <jwakely@redhat.com>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Jason Merrill <jason@redhat.com>, Jeff Law <law@redhat.com>
Subject: Re: [PATCH] Add a new type attribute always_alias (PR79671)
Date: Fri, 07 Apr 2017 12:58:00 -0000 [thread overview]
Message-ID: <AM4PR0701MB21622AC24F50117986A58BD3E40C0@AM4PR0701MB2162.eurprd07.prod.outlook.com> (raw)
In-Reply-To: <alpine.LSU.2.20.1704070846470.30715@zhemvz.fhfr.qr>
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.
>
next prev parent reply other threads:[~2017-04-07 12:58 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-05 9:46 Bernd Edlinger
2017-04-05 13:28 ` Jakub Jelinek
2017-04-05 15:20 ` Richard Biener
2017-04-05 17:41 ` Bernd Edlinger
2017-04-05 20:18 ` Jason Merrill
2017-04-05 20:46 ` Bernd Edlinger
2017-04-05 22:54 ` Pedro Alves
2017-04-06 10:08 ` Jonathan Wakely
2017-04-06 7:23 ` Richard Biener
2017-04-05 15:27 ` Bernd Edlinger
2017-04-05 15:29 ` Jakub Jelinek
2017-04-05 14:50 ` Florian Weimer
2017-04-05 15:23 ` Richard Biener
2017-04-05 15:38 ` Bernd Edlinger
2017-04-05 16:03 ` Jonathan Wakely
2017-04-05 16:08 ` Jakub Jelinek
2017-04-05 17:23 ` Bernd Edlinger
2017-04-05 21:02 ` Bernd Edlinger
2017-04-05 23:17 ` Sandra Loosemore
2017-04-06 5:40 ` Jakub Jelinek
2017-04-06 7:47 ` Richard Biener
2017-04-06 7:51 ` Jakub Jelinek
2017-04-06 7:55 ` Richard Biener
2017-04-06 14:11 ` Bernd Edlinger
2017-04-06 14:17 ` Florian Weimer
2017-04-06 14:23 ` Richard Biener
2017-04-06 14:43 ` Jonathan Wakely
2017-04-06 14:51 ` Florian Weimer
2017-04-06 15:05 ` Jakub Jelinek
2017-04-06 15:10 ` Florian Weimer
2017-04-06 19:13 ` Richard Biener
2017-04-11 10:43 ` Florian Weimer
2017-04-11 10:48 ` Richard Biener
2017-04-06 17:39 ` Bernd Edlinger
2017-04-06 17:48 ` Florian Weimer
2017-04-06 18:12 ` Bernd Edlinger
2017-04-06 18:19 ` Florian Weimer
2017-04-06 18:49 ` Bernd Edlinger
2017-04-06 19:05 ` Florian Weimer
2017-04-06 19:20 ` Bernd Edlinger
2017-04-07 6:47 ` Richard Biener
2017-04-07 12:58 ` Bernd Edlinger [this message]
2017-04-06 19:16 ` Richard Biener
2017-04-07 6:56 ` Florian Weimer
2017-04-07 8:01 ` Richard Biener
2017-04-06 19:14 ` Richard Biener
2017-04-06 19:51 ` Bernd Edlinger
2017-04-06 14:22 ` Richard Biener
2017-04-06 21:00 ` Bernd Edlinger
2017-04-07 6:54 ` Richard Biener
2017-04-07 13:37 ` Bernd Edlinger
2017-04-07 15:10 ` Richard Biener
2017-04-07 15:33 ` Bernd Edlinger
2017-04-07 20:22 ` Jason Merrill
2017-04-10 12:50 ` Richard Biener
2017-04-10 14:41 ` Jason Merrill
2017-04-10 15:31 ` Richard Biener
2017-04-10 16:35 ` Jason Merrill
2017-04-11 10:32 ` Richard Biener
2017-04-11 11:53 ` Richard Biener
2017-04-11 13:35 ` Richard Biener
2017-04-11 18:47 ` Jason Merrill
2017-04-10 21:40 ` Pedro Alves
2017-04-11 7:37 ` Richard Biener
2017-04-06 20:20 ` Bernd Edlinger
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