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 06:09:52PM +0100, Paolo Bonzini wrote:
> 
> 
> On 20/01/2017 17:57, Michael S. Tsirkin wrote:
> > 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.
> 
> Unnamed bitfields are in C89 and we definitely use unnamed unions.
> Maybe that was an unnamed struct or scalar.
> 
> Paolo

I don't think we use unnamed bitfields anywhere though. do we?


> >>> +#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)