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    SubjectRE: detecting integer constant expressions in macros
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    From: Uecker, Martin
    > Sent: 21 March 2018 10:22
    > 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
    > > > <Martin.Uecker@med.uni-goettingen.de> 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.

    False positives with old compilers don't matter when max() is being used
    for an on-stack array.
    The compilations with a new compiler will detect real VLA, the old compiler
    will generate valid code with a constant sized VLA.

    OTOH these horrid:
    long buf[max(sizeof (struct foo), sizeof (struct bar)) + 7 / 8];
    would be better replaced with:
    union buf { struct foo foo; struct bar bar; };

    David

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