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

[Markus Armbruster]

"Michael S. Tsirkin" <address@hidden> writes:

> 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" <address@hidden> 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 <address@hidden>
>> >>> ---
>> >>>  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?

If we were talking about some obscure GCC extension, this would be a
valid question.  But we're talking about an ISO C feature that's pretty
central to how bit-fields work, and older than quite a few hackers.

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