objtool: Prevent GCC from merging annotate_unreachable(), take 2

This fixes the following warning with GCC 4.6:

  mm/migrate.o: warning: objtool: migrate_misplaced_transhuge_page()+0x71: unreachable instruction

The problem is that the compiler merged identical annotate_unreachable()
inline asm blocks, resulting in a missing 'unreachable' annotation.

This problem happened before, and was partially fixed with:

  3d1e236 ("objtool: Prevent GCC from merging annotate_unreachable()")

That commit tried to ensure that each instance of the
annotate_unreachable() inline asm statement has a unique label.  It used
the __LINE__ macro to generate the label number.  However, even the line
number isn't necessarily unique when used in an inline function with
multiple callers (in this case, __alloc_pages_node()'s use of
VM_BUG_ON).

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kbuild-all@01.org
Cc: tipbuild@zytor.com
Fixes: 3d1e236 ("objtool: Prevent GCC from merging annotate_unreachable()")
Link: http://lkml.kernel.org/r/20171103221941.cajpwszir7ujxyc4@treble
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Author: jpoimboe

include/linux/compiler.h

@@ -191,13 +191,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	asm("%c0:\n\t"							\
 	    ".pushsection .discard.reachable\n\t"			\
 	    ".long %c0b - .\n\t"					\
-	    ".popsection\n\t" : : "i" (__LINE__));			\
+	    ".popsection\n\t" : : "i" (__COUNTER__));			\
 })
 #define annotate_unreachable() ({					\
 	asm("%c0:\n\t"							\
 	    ".pushsection .discard.unreachable\n\t"			\
 	    ".long %c0b - .\n\t"					\
-	    ".popsection\n\t" : : "i" (__LINE__));			\
+	    ".popsection\n\t" : : "i" (__COUNTER__));			\
 })
 #define ASM_UNREACHABLE							\
 	"999:\n\t"							\