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