"Michael S. Tsirkin" <m...@redhat.com> writes:
> On Thu, Jan 19, 2017 at 02:33:40PM +0100, Markus Armbruster wrote:
>> Paolo Bonzini <pbonz...@redhat.com> writes:
>>
>> > On 19/01/2017 09:12, Markus Armbruster wrote:
>> >> "Michael S. Tsirkin" <m...@redhat.com> writes:
>> >>
>> >>> QEMU_BUILD_BUG_ON uses a typedef in order to be safe
>> >>> to use outside functions, but sometimes it's useful
>> >>> to have a version that can be used within an expression.
>> >>> Following what Linux does, introduce QEMU_BUILD_BUG_ON_ZERO
>> >>> that return zero after checking condition at build time.
>> >>
>> >> Following Linux's example makes sense, but I can't help but wonder
>> >> whether we need both QEMU_BUILD_BUG_ON_ZERO() and QEMU_BUILD_BUG_ON().
>> >
>> > I think so, most notably QEMU_BUILD_BUG_ON was added to C11 as
>> > _Static_assert but QEMU_BUILD_BUG_ON_ZERO wasn't.
>>
>> Okay.
>>
>> > But we can indeed redefine QEMU_BUILD_BUG_ON to
>> > (void)QEMU_BUILD_BUG_ON_ZERO(x) like Linux does, until we add optional
>> > support for _Static_assert.
>>
>> Yes, please.
>
>
> I don't think we can because we use QEMU_BUILD_BUG_ON outside
> any functions. I don't think you can put 0 there.
Point taken. Still, we should be able to factor out a common core of
the bug condition.
>> >>> Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
>> >>> ---
>> >>> include/qemu/compiler.h | 2 ++
>> >>> 1 file changed, 2 insertions(+)
>> >>>
>> >>> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
>> >>> index 2882470..f4cf13b 100644
>> >>> --- a/include/qemu/compiler.h
>> >>> +++ b/include/qemu/compiler.h
>> >>> @@ -89,6 +89,8 @@
>> >>> typedef char glue(qemu_build_bug_on__,__LINE__)[(x) ? -1 : 1] \
>> >>> __attribute__((unused))
>> >>>
>> >>> +#define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(int[(x) ? -1 : 1]) -
>> >>> sizeof(int))
>> >
>> > Linux here uses:
>> >
>> > #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
>> >
>> > and the issue is that sizeof(int[(x) ? -1 : 1]) could be
>> > runtime-evaluated (the type is a variable-length array).
>>
>> Let's copy both macros from Linux.
>
> I prefer our variant, I don't think it's portable to assume that
> sizeof(struct {int:0}) is 0. Besides, Linux code is GPLv2 only and this
> file is 2 or later.
Use (sizeof(struct { int: X }) - sizeof(struct { int: 0 })) if you care
for portability to lesser compilers. I don't, because we use common
extensions supported by both GCC and Clang all over the place.
The idea to use bitfield size is not copyrightable, only expressions of
the idea.
