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Re: [PATCH] MIPS: Make BUG() __noreturn.

To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] MIPS: Make BUG() __noreturn.
From: David Daney <ddaney@caviumnetworks.com>
Date: Fri, 21 Nov 2008 15:48:42 -0800
Cc: linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, gcc@gcc.gnu.org
In-reply-to: <20081121150023.032f7b5b.akpm@linux-foundation.org>
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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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