From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/2] UNREACHABLE() macro
Date: Fri, 11 Sep 2009 13:45:28 -0700
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To: Roland McGrath <roland@redhat.com>
Cc: torvalds@linux-foundation.org, jakub@redhat.com, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, x86@kernel.org

On Wed,  9 Sep 2009 19:00:31 -0700 (PDT)
Roland McGrath <roland@redhat.com> wrote:

> +/*
> + * __builtin_unreachable is available in GCC 4.5+ and
> + * also in Fedora/Red Hat GCC 4.4.1-10+.
> + */
> +#if (__GNUC_MINOR__ > 4					\
> +     || (__GNUC_MINOR__ == 4				\
> +	 && defined __GNUC_RH_RELEASE__			\
> +	 && (__GNUC_PATCHLEVEL__ > 1			\
> +	     || (__GNUC_PATCHLEVEL__ == 1		\
> +		 && __GNUC_RH_RELEASE__ >= 10))))
> +#define UNREACHABLE()		__builtin_unreachable()
> +#endif

That's a bit of a mouthful.  Did you consider a runtime probe with
scripts/Kbuild.include's try-run, cc-option, etc?