Allow BUG_ON to be optimised away if it is a constant 0.
Useful if we have some BUG_ONs who's conditions become a
constant 0 when debugging functions are turned off.

Signed-off-by: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/include/asm-ppc64/bug.h
===================================================================
--- linux-2.6.orig/include/asm-ppc64/bug.h
+++ linux-2.6/include/asm-ppc64/bug.h
@@ -38,13 +38,17 @@ struct bug_entry *find_bug(unsigned long
 } while (0)
 
 #define BUG_ON(x) do {						\
-	__asm__ __volatile__(					\
-		"1:	tdnei %0,0\n"				\
-		".section __bug_table,\"a\"\n\t"		\
-		"	.llong 1b,%1,%2,%3\n"			\
-		".previous"					\
-		: : "r" ((long long)(x)), "i" (__LINE__),	\
-		    "i" (__FILE__), "i" (__FUNCTION__));	\
+	if (__builtin_constant_p(x) && (x) == 0)		\
+		; /* nothing */					\
+	else {							\
+		__asm__ __volatile__(				\
+			"1:	tdnei %0,0\n"			\
+			".section __bug_table,\"a\"\n\t"	\
+			"	.llong 1b,%1,%2,%3\n"		\
+			".previous"				\
+			: : "r" ((long long)(x)), "i" (__LINE__), \
+			    "i" (__FILE__), "i" (__FUNCTION__)); \
+	}							\
 } while (0)
 
 #define WARN_ON(x) do {						\