Re: [Qemu-devel] [PATCH] QEMU_BUILD_BUG_ON: use __COUNTER__

[Michael S. Tsirkin]

On Tue, Jan 31, 2017 at 04:26:13PM +0100, Markus Armbruster wrote:
> "Michael S. Tsirkin" <address@hidden> writes:
> 
> > Some headers use QEMU_BUILD_BUG_ON. This causes a problem
> > if the C file including that header happens to have
> > QEMU_BUILD_BUG_ON at the same line number.
> >
> > Fix using a widely available extension: __COUNTER__.
> > If unavailable, provide a stub.
> >
> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > ---
> >  include/qemu/compiler.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> > index e0fb18b..bad25a9 100644
> > --- a/include/qemu/compiler.h
> > +++ b/include/qemu/compiler.h
> > @@ -89,8 +89,12 @@
> >      struct { \
> >          int:(x) ? -1 : 1; \
> >      }
> > +#ifdef __COUNTER__
> >  #define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
> > -    glue(qemu_build_bug_on__, __LINE__) __attribute__((unused))
> > +    glue(qemu_build_bug_on__, __COUNTER__) __attribute__((unused))
> > +#else
> > +#define QEMU_BUILD_BUG_ON(x)
> > +#endif
> 
> __COUNTER__ was added to GNU cpp in 2007.  Good.  What about clang?
> "clang -dM -E -x c /dev/null" comes up empty on my machine (3.8.0).
> 
> Can we use an extern declaration instead?  You can have any number of
> these, as long as they match.  Have a look at the appended sketch.  It
> compiles without -DBUGGY, and errors out with -DBUGGY, as it should.

I tried that first thing.
This generates lots of warnings if you have multiple users
within a function: gcc is unhappy about the redundant extern
declarations.


> >  
> >  #define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)) - \
> >                                     sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)))
> 
> By the way, QEMU_BUILD_BUG_ON_ZERO() could use a comment explaining its
> value.
> 
> 
> 
> #define QEMU_BUILD_BUG_ON_STRUCT(x) \
>     struct { \
>         int:(x) ? -1 : 1; \
>     }
> 
> #define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)) - \
>                                    sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)))
> 
> #define QEMU_BUILD_BUG_ON(x) \
>     extern char qemu_build_bug_on_[QEMU_BUILD_BUG_ON_ZERO((x))]
> 
> QEMU_BUILD_BUG_ON(0);
> QEMU_BUILD_BUG_ON(0);
> #ifdef BUGGY
> QEMU_BUILD_BUG_ON(1);
> #endif