RE: Still fails with strict-volatile-bitfields

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

On, Fri, 10 Jan 2014 09:41:06, Richard Earnshaw wrote:
>
> On 10/01/14 08:49, Bernd Edlinger wrote:
>> On Thu, 9 Jan 2014 16:22:33, Richard Earnshaw wrote:
>>>
>>> On 09/01/14 08:26, Bernd Edlinger wrote:
>>>> Hi,
>>>>
>>>> On Thu, 9 Jan 2014 15:01:54, Yoey Ye wrote:
>>>>>
>>>>> Sandra, Bernd,
>>>>>
>>>>> Can you take a look at
>>>>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59734
>>>>>
>>>>> It seems a siimple case still doesn't work as expected. Did I miss anything?
>>>>>
>>>>> Thanks,
>>>>> Joey
>>>>
>>>> Yes,
>>>>
>>>> this is a major case where the C++ memory model is
>>>> in conflict with AAPCS.
>>>>
>>>
>>> Does the compiler warn about this? And if so, is the warning on by
>>> default? I think it's essential that we don't have quiet changes in
>>> behaviour without such warnings.
>>>
>>> R.
>>
>> No. This example was working in 4.6 and broken in 4.7 and 4.8.
>> Well, 4.7 should have warned about that.
>>
>> 4.9 does simply not change that, because it would violate the C++
>> memory model. Maybe that should go into release notes.
>
> That's no excuse for not generating a warning at compile time when the
> situation is encountered. Users of this feature are experiencing a
> quiet change of behaviour and that's unacceptable, even if the compiler
> is doing what it was intended to do; that doesn't necessarily match the
> user's expectations. There should always be a way to rewrite the C11
> intended semantics in a way that doesn't violate the strict volatile
> bitfields semantics.
>

Hmm...
You mean we should have a (ugly) warning enabled by default in 4.9 (at expmed.c)
if a bit-field access would be perfectly aligned and so, but is in conflict with the
C++ memory model, and -fstrict-volatile-bitfields is in effect.
Maybe only once per compilation?


>>
>> At the same time we had all kinds of invalid code generation,
>> starting at 4.6, especially with packed structures in C and Ada(!),
>> (writing not all bits, and using unaligned accesses)
>> and that is fixed in 4.9.
>>
>
> I'm not worried about packed structures. You can't sensibly implement
> the strict volatile bitfields rules when things aren't aligned.
>
> R.
>
>>
>> Bernd.
>>
>>>
>>>> You can get the expected code, by changing the struct
>>>> like this:
>>>>
>>>> struct str {
>>>> volatile unsigned f1: 8;
>>>> unsigned dummy:24;
>>>> };
>>>>
>>>> If it is written this way the C++ memory model allows
>>>> QImode, HImode, SImode. And -fstrict-volatile-bitfields
>>>> demands SImode, so the conflict is resolved. This dummy
>>>> member makes only a difference on the C level, and is
>>>> completely invisible in the generated code.
>>>>
>>>> If -fstrict-volatile-bitfields is now given, we use SImode,
>>>> if -fno-strict-volatile-bitfields is given, we give GCC the
>>>> freedom to choose the access mode, maybe QImode if that is
>>>> faster.
>>>>
>>>> In the _very_ difficult process to find an solution
>>>> that seems to be acceptable to all maintainers, we came to
>>>> the solution, that we need to adhere to the C++ memory
>>>> model by default. And we may not change the default
>>>> setting of -fstruct-volatile-bitfields at the same time!
>>>>
>>>> As a future extension we discussed the possibility
>>>> to add a new option -fstrict-volatile-bitfields=aapcs
>>>> that explicitly allows us to break the C++ memory model.
>>>>
>>>> But I did not yet try to implement this, as I feel that
>>>> would certainly not be accepted as we are in Phase3 now.
>>>>
>>>> As another future extension there was the discussion
>>>> about the -Wportable-volatility warning, which I see now
>>>> as a warning that analyzes the structure layout and
>>>> warns about any structures that are not "well-formed",
>>>> in the sense, that a bit-field fails to define all
>>>> bits of the container.
>>>>
>>>> Those people that do use bit-fields to access device-registers
>>>> do always define all bits, and of course in the same mode.
>>>>
>>>> It would be good to have a warning, when some bits are missing.
>>>> They currently have to use great care to check their structures
>>>> manually.
>>>>
>>>> I had a proposal for that warning but that concentrated
>>>> only on the volatile attribute, but I will have to re-write
>>>> that completely so that it can be done in stor-layout.c:
>>>>
>>>> It should warn independent of optimization levels or actual
>>>> bitfield member references, thus, be implemented entirely at
>>>> the time we lay out the structure. The well-formed-ness of
>>>> a bit-field makes that possible.
>>>>
>>>> But that will come too late for Phase3 as well.
>>>>
>>>>
>>>> Regards
>>>> Bernd.
>>>>
>>>>
>>>
>>>
>>
>
>