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Re: [XEN PATCH v3] xen/include: avoid using a compiler extension for BUILD_BUG_ON_ZERO.
- To: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 22 Jun 2023 10:56:59 +0200
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- Delivery-date: Thu, 22 Jun 2023 08:57:21 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.06.2023 10:24, Nicola Vetrini wrote:
> --- a/xen/include/xen/lib.h
> +++ b/xen/include/xen/lib.h
> @@ -51,9 +51,10 @@
> e.g. in a structure initializer (or where-ever else comma expressions
> aren't permitted). */
> #define BUILD_BUG_ON_ZERO(cond) \
> - sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
> + (sizeof(struct { char c; _Static_assert(!(cond), "!(" #cond ")"); }) &
> 0U)
> #else
> -#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
> +#define BUILD_BUG_ON_ZERO(cond) \
> + (sizeof(struct { unsigned u : (cond) ? -1 : 1; }) & 0U)
Given your remark on named bitfields not being allowed to be zero length
(which hopefully holds universally, i.e. isn't subject to compiler
extensions), how about
#define BUILD_BUG_ON_ZERO(cond) (sizeof(struct { int _:!(cond); }) & 0)
? (Implicitly, as said before, I question the value of the U suffixes here.
Even better might be to make sure the expression is explicitly not of
unsigned type, to avoid surprises if someone used ~BUILD_BUG_ON_ZERO()
somewhere.)
Jan
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