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

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

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.
> 
> >>> 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 don't like the one in Linux because it's gcc specific (sizeof is
undefined for an empty structure).

But since people are worried about compiler using a variable sized
array, we should worry about this for QEMU_BUILD_BUG_ON too. I'll fix it
up.

-- 
MST