svn commit: r282907 - head/sys/sys

[Tijl Coosemans]

On Thu, 14 May 2015 15:49:49 +0000 (UTC) "Pedro F. Giffuni" <pfg at FreeBSD.org> wrote:
> Author: pfg
> Date: Thu May 14 15:49:48 2015
> New Revision: 282907
> URL: https://svnweb.freebsd.org/changeset/base/282907
> 
> Log:
>   Add new __unreachable() builtin
>   
>   This is one of the few post gcc4.2 builtins that has been implemented by
>   clang:
>   
>   __builtin_unreachable is used to indicate that a specific point in the
>   program cannot be reached, even if the compiler might otherwise think it
>   can. This is useful to improve optimization and eliminates certain
>   warnings.
>   
>   Hinted by:	NetBSD
>   Differential Revision:	https://reviews.freebsd.org/D2536
> 
> Modified:
>   head/sys/sys/cdefs.h
> 
> Modified: head/sys/sys/cdefs.h
> ==============================================================================
> --- head/sys/sys/cdefs.h	Thu May 14 15:14:03 2015	(r282906)
> +++ head/sys/sys/cdefs.h	Thu May 14 15:49:48 2015	(r282907)
> @@ -388,6 +388,12 @@
>  #define	__alloc_size(x)
>  #endif
>  
> +#if __has_builtin(__builtin_unreachable) || __GNUC_PREREQ__(4, 6)
> +#define	__unreachable()	__builtin_unreachable()
> +#else
> +#define	__unreachable()	do {} while (/*CONSTCOND*/0)

__builtin_unreachable() can be used in expressions so I think it's better
to replace do-while with ((void)0).  You can then do things like this:

#define assume(e)  ((e) ? (void)0 : __unreachable())
(like assert(e) but without error)