Am Mittwoch, den 21.03.2018, 10:51 +0100 schrieb Martin Uecker:
>
> Am Dienstag, den 20.03.2018, 17:30 -0700 schrieb Linus Torvalds:
> > On Tue, Mar 20, 2018 at 5:10 PM, Uecker, Martin
> > <[email protected]> wrote:
> >
> > > But one could also use __builtin_types_compatible_p instead.
> >
> > That might be the right approach, even if I like how it only used
> > standard C (although _disgusting_ standard C) without it apart from
> > the small issue of sizeof(void)
> >
> > So something like
> >
> > #define __is_constant(a) \
> > __builtin_types_compatible_p(int *, typeof(1 ? ((void*)((a)
> > *
> > 0l)) : (int*)1 ) )
> >
> > if I counted the parentheses right..
>
> This seems to work fine on all recent compilers. Sadly, it
> produces false positives on 4.4.7 and earlier when
> tested on godbolt.org
>
> Surprisingly, the MAX macro as defined below still seems
> to do the right thing with respect to avoiding the VLA
> even on the old compilers.
>
> I am probably missing something... or there are two
> compiler bugs cancelling out, or the __builting_choose_expr
> changes things.
Nevermind, of course it avoids the VLA if it produces a false
positive and uses the simple version. So it is unsafe to use
on very old compilers.
Martin
> Martin
>
> My test code:
>
> #define ICE_P(x) (__builtin_types_compatible_p(int*, __typeof__(1 ?
> ((void*)((x) * 0l)) : (int*)1)))
>
> #define SIMPLE_MAX(a, b) ((a) > (b) ? (a) : (b))
> #define SAFE_MAX(a, b) ({ __typeof(a) _a = (a); __typeof(b) _b = (b);
> SIMPLE_MAX(_a, _b); })
> #define MAX(a, b) (__builtin_choose_expr(ICE_P(a) && ICE_P(b),
> SIMPLE_MAX(a, b), SAFE_MAX(a, b)))
>
>
>
> int foo(int x)
> {
> int a[MAX(3, 4)];
> //int a[MAX(3, x)];
> //int a[SAFE_MAX(3, 4)];
> //return ICE_P(MAX(3, 4));
> return ICE_P(MAX(3, x));
> }
