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

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

On Fri, Jan 20, 2017 at 08:21:28AM +0100, Markus Armbruster wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Thu, Jan 19, 2017 at 02:33:40PM +0100, Markus Armbruster wrote:
> >> Paolo Bonzini <pbonzini@redhat.com> writes:
> >> 
> >> > On 19/01/2017 09:12, Markus Armbruster wrote:
> >> >> "Michael S. Tsirkin" <mst@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 <mst@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.

Right I did that before reading this reply.

-- 
MST