x86/uaccess: Tell the compiler that uaccess is unlikely to fault

GCC doesn't realize that get_user(), put_user(), and their __
variants are unlikely to fail. Tell it.

I noticed this while playing with the C entry code.

Before:
text data bss dec filename
21828763 5194760 1277952 28301475 vmlinux.baseline

After:
text data bss dec filename
21828379 5194760 1277952 28301091 vmlinux.new

The generated code shrunk by 384 bytes.

Signed-off-by: Andy Lutomirski <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/uaccess.h

···
 		     : "=a" (__ret_gu), "=r" (__val_gu)			\
 		     : "0" (ptr), "i" (sizeof(*(ptr))));		\
 	(x) = (__force __typeof__(*(ptr))) __val_gu;			\
-	__ret_gu;							\
+	__builtin_expect(__ret_gu, 0);					\
 })
 
 #define __put_user_x(size, x, ptr, __ret_pu)			\
···
 		__put_user_x(X, __pu_val, ptr, __ret_pu);	\
 		break;						\
 	}							\
-	__ret_pu;						\
+	__builtin_expect(__ret_pu, 0);				\
 })
 
 #define __put_user_size(x, ptr, size, retval, errret)			\
···
 ({								\
 	int __pu_err;						\
 	__put_user_size((x), (ptr), (size), __pu_err, -EFAULT);	\
-	__pu_err;						\
+	__builtin_expect(__pu_err, 0);				\
 })
 
 #define __get_user_nocheck(x, ptr, size)				\
···
 	unsigned long __gu_val;						\
 	__get_user_size(__gu_val, (ptr), (size), __gu_err, -EFAULT);	\
 	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
-	__gu_err;							\
+	__builtin_expect(__gu_err, 0);					\
 })
 
 /* FIXME: this hack is definitely wrong -AK */