linux-kernel - Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

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Message-ID: <1395505786.2143.14.camel@dabdike.int.hansenpartnership.com>
Date:	Sat, 22 Mar 2014 09:29:46 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	behanw@...verseincode.com
Cc:	arnd@...db.de, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, dwmw2@...radead.org,
	pageexec@...email.hu, Mark Charlebois <charlebm@...il.com>
Subject: Re: [PATCH v2] LLVMLinux: Remove warning about returning an
 uninitialized variable

On Sat, 2014-03-22 at 08:48 -0700, behanw@...verseincode.com wrote:
> From: Behan Webster <behanw@...verseincode.com>
> 
> Fix uninitialized return code in default case in cmpxchg-local.h
> 
> This patch fixes the code to prevent an uninitialized return value that is detected
> when compiling with clang. The bug produces numerous warnings when compiling the
> Linux kernel with clang.
> 
> Signed-off-by: Behan Webster <behanw@...verseincode.com>
> Signed-off-by: Mark Charlebois <charlebm@...il.com>
> ---
>  include/asm-generic/cmpxchg-local.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
> index d8d4c89..9112111 100644
> --- a/include/asm-generic/cmpxchg-local.h
> +++ b/include/asm-generic/cmpxchg-local.h
> @@ -41,6 +41,7 @@ static inline unsigned long __cmpxchg_local_generic(volatile void *ptr,
>  		break;
>  	default:
>  		wrong_size_cmpxchg(ptr);
> +		__builtin_unreachable();

No, that's got to be unreachable() so that it works in all compilers,
(__builtin_unreachable is a gcc-4 ism).

Got to say this still looks wrong.  If wrong_size_cmpxchg() cannot
return, the function should be annotated as such with __noreturn (like
panic()) so the unreachable() should be superfluous.

James


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