This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: [tree-ssa] mainline->branch merge status
Dale Johannesen <dalej@apple.com> writes:
> On Jan 29, 2004, at 3:12 AM, Andreas Schwab wrote:
>> This has the same problem.
>>
>> { \
>> *(void **) __o->next_free = (void *) datum; \
>> __o->next_free += sizeof (void *); \
>> } \
>
> True, cut and paste error. But it still has the problems I mentioned
> before.
Which problems do you mean?
>>> can happen. I believe the following is correct, performance neutral,
>>> and
>>> standard-conforming:
>>>
>>> {
>>> \
>>> void ***t = (void ***)&(__o->next_free);
>>> \
>>> **t = ((void *)datum);
>>> \
>>> (*t)++;
>>> \
>>> } \
>>
>> I think this will break strict aliasing.
>
> So does the other way; *(void **)__o->next_free = ... is exactly what you
> can't do.
I don't follow. __o->next_free is a character pointer, thus can alias all
other pointers. The problem with your version is, IIUC, that it converts
a (char **) into a (void ***), which cannot alias under the strict
aliasing rules, and the compiler can assume that (*t)++ does not modify
__o->next_free.
Andreas.
--
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux AG, Maxfeldstraße 5, 90409 Nürnberg, Germany
Key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5
"And now for something completely different."