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

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Message-ID: <2375c9f90909092142i58595fedod6780a7c41190f5f@mail.gmail.com>
Date:	Thu, 10 Sep 2009 12:42:26 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jakub Jelinek <jakub@...hat.com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 0/2] __builtin_unreachable

On Thu, Sep 10, 2009 at 9:59 AM, Roland McGrath <roland@...hat.com> 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.
>
> There are numerous matches from "git grep -n 'for *(;;) *;'" but it takes
> someone who knows each bit of code to know where that means UNREACHABLE()
> and where it really wants an infinite loop.

Looks good!

Reviewed-by: WANG Cong <xiyou.wangcong@...il.com>

Just curious, what different asm code will gcc generate for this? Comparing it
to for(;;) ? I am sorry that I don't have gcc 4.5 on hand.

Thanks!
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