linux-kernel - Re: [PATCH] MIPS: Make BUG() _

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Message-ID: <492748DA.3030805@caviumnetworks.com>
Date:	Fri, 21 Nov 2008 15:48:42 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	gcc@....gnu.org
Subject: Re: [PATCH] MIPS: Make BUG() __noreturn.

Andrew Morton wrote:
>
> 
> Yup, this change will fix some compile warnings which will never be
> fixed in any other way for mips.
> 
>> +static inline void __noreturn BUG(void)
>> +{
>> +	__asm__ __volatile__("break %0" : : "i" (BRK_BUG));
>> +	/* Fool GCC into thinking the function doesn't return. */
>> +	while (1)
>> +		;
>> +}
> 
> This kind of sucks, doesn't it?  It adds instructions into the kernel
> text, very frequently on fast paths.  Those instructions are never
> executed, and we're blowing away i-cache just to quash compiler
> warnings.
> 
> For example, this:
> 
> --- a/arch/x86/include/asm/bug.h~a
> +++ a/arch/x86/include/asm/bug.h
> @@ -22,14 +22,12 @@ do {								\
>  		     ".popsection"				\
>  		     : : "i" (__FILE__), "i" (__LINE__),	\
>  		     "i" (sizeof(struct bug_entry)));		\
> -	for (;;) ;						\
>  } while (0)
>  
>  #else
>  #define BUG()							\
>  do {								\
>  	asm volatile("ud2");					\
> -	for (;;) ;						\
>  } while (0)
>  #endif
>  
> _
> 
> reduces the size of i386 mm/vmalloc.o text by 56 bytes.
> 
> I wonder if there is any clever way in which we can do this without
> introducing additional runtime cost.

As I said in the other part of the thread, We are working on a GCC patch 
that adds a new built-in function '__builtin_noreturn()', that you could 
substitute for 'for(;;);' that emits no instructions in this case.

David Daney
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