Re: [Qemu-devel] [PATCH v3 2/4] compiler: rework BUG_ON using a struct

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 20, 2017 at 08:42:41AM +0100, Markus Armbruster wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > There are theoretical concerns that some compilers might not trigger
> > build failures on attempts to define an array of size -1 and make it a
> > variable sized array instead. Let rewrite using a struct with a negative
> > bit field size instead as there are no dynamic bit field sizes.  This is
> > similar to what Linux does.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  include/qemu/compiler.h | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> > index 7512082..c6f673e 100644
> > --- a/include/qemu/compiler.h
> > +++ b/include/qemu/compiler.h
> > @@ -85,9 +85,12 @@
> >  #define typeof_field(type, field) typeof(((type *)0)->field)
> >  #define type_check(t1,t2) ((t1*)0 - (t2*)0)
> >  
> > -#define QEMU_BUILD_BUG_ON(x) \
> > -    typedef char glue(qemu_build_bug_on__, __LINE__)[(x) ? -1 : 1] \
> > -        __attribute__((unused))
> > +#define QEMU_BUILD_BUG_ON_STRUCT(x) \
> > +    struct { \
> > +        int qemu_build_bug_on : (x) ? -1 : 1; \
> > +    }
> 
> The qemu_build_bug_on name space pollution is harmless, but quite
> unnecessary: the name can be simply omitted (unnamed bit-field).

I have concerns about it's portability though. I remember
we had to get rid of unnamed fields in some structs at some point
for the sake of some old compiler.

> > +#define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
> > +    glue(qemu_build_bug_on__, __LINE__) __attribute__((unused))
> >  
> >  #if defined __GNUC__
> >  # if !QEMU_GNUC_PREREQ(4, 4)