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Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUI
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON |
Date: |
Fri, 20 Jan 2017 19:50:16 +0200 |
On Fri, Jan 20, 2017 at 08:21:28AM +0100, Markus Armbruster wrote:
> "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.
Right I did that before reading this reply.
--
MST
- Re: [Qemu-devel] [PATCH v2 2/4] compiler: drop ; after BUILD_BUG_ON, (continued)
- [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Michael S. Tsirkin, 2017/01/18
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Markus Armbruster, 2017/01/19
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Paolo Bonzini, 2017/01/19
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Markus Armbruster, 2017/01/19
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Michael S. Tsirkin, 2017/01/19
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Eric Blake, 2017/01/19
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Michael S. Tsirkin, 2017/01/19
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Markus Armbruster, 2017/01/20
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Michael S. Tsirkin, 2017/01/19
[Qemu-devel] [PATCH v2 4/4] ARRAY_SIZE: check that argument is an array, Michael S. Tsirkin, 2017/01/18
Re: [Qemu-devel] [PATCH v2 0/4] virtio: ARRAY_SIZE fixups, no-reply, 2017/01/18