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

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Date:	Sun, 23 Nov 2008 10:58:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Daney <ddaney@...iumnetworks.com>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MIPS: Make BUG() __noreturn.


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> > +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.

yes - the total image effect is significantly - recently looked at how 
much larger !CONFIG_BUG builds would get if we inserted an infinite 
loop into them - it was in the 50K text range (!).

but in the x86 ud2 case we could guarantee that we wont ever return 
from that exception. Mind sending a patch with a signoff, a 
description and an infinite loop in the u2d handler?

	Ingo
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