Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON

[Markus Armbruster]

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

> On Thu, Jan 19, 2017 at 02:33:40PM +0100, Markus Armbruster wrote:
>> Paolo Bonzini <address@hidden> writes:
>> 
>> > On 19/01/2017 09:12, Markus Armbruster wrote:
>> >> "Michael S. Tsirkin" <address@hidden> 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 <address@hidden>
>> >>> ---
>> >>>  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.