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

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Message-Id: <20081121150023.032f7b5b.akpm@linux-foundation.org>
Date:	Fri, 21 Nov 2008 15:00:23 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MIPS: Make BUG() __noreturn.

On Thu, 20 Nov 2008 17:26:36 -0800
David Daney <ddaney@...iumnetworks.com> wrote:

> MIPS: Make BUG() __noreturn.
> 
> Often we do things like put BUG() in the default clause of a case
> statement.  Since it was not declared __noreturn, this could sometimes
> lead to bogus compiler warnings that variables were used
> uninitialized.
> 
> There is a small problem in that we have to put a magic while(1); loop to
> fool GCC into really thinking it is noreturn.  This makes the new
> BUG() function 3 instructions long instead of just 1, but I think it
> is worth it as it is now unnecessary to do extra work to silence the
> 'used uninitialized' warnings.
> 
> I also re-wrote BUG_ON so that if it is given a constant condition, it
> just does BUG() instead of loading a constant value in to a register
> and testing it.
> 

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.
--
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