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

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

Index: linux-2.6/include/asm-powerpc/bug.h
===================================================================
--- linux-2.6.orig/include/asm-powerpc/bug.h
+++ linux-2.6/include/asm-powerpc/bug.h
@@ -40,24 +40,26 @@ struct bug_entry *find_bug(unsigned long
 } while (0)
 
 #define BUG_ON(x) do {						\
-	__asm__ __volatile__(					\
-		"1:	"PPC_TLNEI"	%0,0\n"			\
-		".section __bug_table,\"a\"\n"			\
-		"\t"PPC_LONG"	1b,%1,%2,%3\n"		\
-		".previous"					\
-		: : "r" ((long)(x)), "i" (__LINE__),		\
-		    "i" (__FILE__), "i" (__FUNCTION__));	\
+	if (!__builtin_constant_p(x) || (x))			\
+		__asm__ __volatile__(				\
+			"1:	"PPC_TLNEI"	%0,0\n"		\
+			".section __bug_table,\"a\"\n"		\
+			"\t"PPC_LONG"	1b,%1,%2,%3\n"		\
+			".previous"				\
+			: : "r" ((long)(x)), "i" (__LINE__),	\
+			    "i" (__FILE__), "i" (__FUNCTION__));\
 } while (0)
 
 #define WARN_ON(x) do {						\
-	__asm__ __volatile__(					\
-		"1:	"PPC_TLNEI"	%0,0\n"			\
-		".section __bug_table,\"a\"\n"			\
-		"\t"PPC_LONG"	1b,%1,%2,%3\n"		\
-		".previous"					\
-		: : "r" ((long)(x)),				\
-		    "i" (__LINE__ + BUG_WARNING_TRAP),		\
-		    "i" (__FILE__), "i" (__FUNCTION__));	\
+	if (!__builtin_constant_p(x) || (x))			\
+		__asm__ __volatile__(				\
+			"1:	"PPC_TLNEI"	%0,0\n"		\
+			".section __bug_table,\"a\"\n"		\
+			"\t"PPC_LONG"	1b,%1,%2,%3\n"		\
+			".previous"				\
+			: : "r" ((long)(x)),			\
+			    "i" (__LINE__ + BUG_WARNING_TRAP),	\
+			    "i" (__FILE__), "i" (__FUNCTION__));\
 } while (0)
 
 #define HAVE_ARCH_BUG