Am Montag, dem 26.08.2024 um 19:30 +0000 schrieb Qing Zhao:
> Hi, Martin,
>
> Looks like that there is some issue when I tried to use the _Generic for the
> testing cases, and then I narrowed down to a
> small testing case that shows the problem without any change to GCC.
>
> [opc@qinzhao-ol8u3-x86 gcc]$ cat t1.c
> struct annotated {
> char b;
> int c[];
> } *array_annotated;
> extern void * counted_by_ref (int *);
>
> int main(int argc, char *argv[])
> {
> typeof(counted_by_ref (array_annotated->c)) ret
> = counted_by_ref (array_annotated->c);
> _Generic (ret, void* : (void)0, default: *ret = 10);
>
> return 0;
> }
> [opc@qinzhao-ol8u3-x86 gcc]$ /home/opc/Install/latest/bin/gcc t1.c
> t1.c: In function ‘main’:
> t1.c:12:44: warning: dereferencing ‘void *’ pointer
> 12 | _Generic (ret, void* : (void)0, default: *ret = 10);
> | ^~~~
> t1.c:12:49: error: invalid use of void expression
> 12 | _Generic (ret, void* : (void)0, default: *ret = 10);
> | ^
>
> Actually, I debugged this issue into gcc’s C routine
> “c_parser_generic_selection”.
> And found that, the “default” branch of the _Generic is always parsed even
> though there is already
> a match in the previous conditions. Therefore, *ret = 10 is parsed even when
> ret is a void *, therefore the compilation error.
>
> So, I am not sure whether this is the correct behavior of the operator
> _Generic?
> Or is there any obvious error in the above small testing case?
> If So, then looks like that we cannot use the _Generic operator for this
> purpose.
>
> Any comments on this?
>
Ah, right. This is indeed the correct behavior for _Generic,
and I have overlooked this. One could work around it like this:
__auto_type ret = counted_by_ref (array_annotated->c);
*_Generic (ret, void*: &(int){ }, default: ret) = 10;
or, if one expects only specific types:
__auto_type ret = counted_by_ref (array_annotated->c);
_Generic (ret, void*: 0, int*: *(int*)ret = 10,
size_t*: *(size_t*)ret = 10);
But yes, a bit less elegant.
Martin
> Thanks a lot for your help.
>
> Qing
>
>
> > On Aug 21, 2024, at 11:43, Martin Uecker <[email protected]> wrote:
> >
> > Am Mittwoch, dem 21.08.2024 um 15:24 +0000 schrieb Qing Zhao:
> > > >
> > > > But if we changed it to return a void pointer, we could make this
> > > > a compile-time check:
> > > >
> > > > auto ret = __builtin_get_counted_by(__p->FAM);
> > > >
> > > > _Generic(ret, void*: (void)0, default: *ret = COUNT);
> > >
> > > Is there any benefit to return a void pointer than a SIZE_T pointer for
> > > the NULL pointer?
> >
> > Yes! You can test with _Generic (or __builtin_types_compatible_p)
> > at compile-time based on the type whether you can set *ret to COUNT
> > or not as in the example above.
> >
> > So it is not a weird run-time test which needs to be optimized
> > away.
> >
>
