linux-kernel - Re: [PATCH 0/2] __builtin

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Date:	Thu, 10 Sep 2009 08:13:47 +0200
From:	Jakub Jelinek <jakub@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH 0/2] __builtin_unreachable

On Wed, Sep 09, 2009 at 06:59:23PM -0700, Roland McGrath wrote:
> The latest GCC has a better way than "for (;;)" to indicate that a code
> path cannot be reached due to reasons the compiler doesn't understand (such
> as code in an asm).  These patches provide UNREACHABLE() as a macro to hide
> the details of this, and then use it for the BUG() macro on x86, saving
> some dead code otherwise generated.
> 
> Other arch's BUG() may want to this too instead of "for (;;)" or __builtin_trap.

Just instead of for (;;) (or any place where an asm never returns). 
__builtin_trap () shouldn't be replaced - __builtin_unreachable () just says
that the location is never reachable, so everything after it not reachable
through other ways can be optimized out - while __builtin_trap () emits an
instruction that traps and then assumes anything after it is unreachable.

Also, I wonder if Fedora/RHEL specific GCC backport is desirable to be
handled in upstream kernel (i.e. whether compiler-gcc4.h shouldn't enable it
just for GCC 4.5+) and Fedora/RHEL should patch it for our backport.

	Jakub
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