objtool: Add a comment for the unreachable annotation macros

Add a comment for the unreachable annotation macros to explain their
purpose and the '__COUNTER__' label hack.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1570e48d9f87e0fc6f0126c32e7e1de6e109cb67.1509974104.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/compiler.h

@@ -187,6 +187,11 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 
 /* Unreachable code */
 #ifdef CONFIG_STACK_VALIDATION
+/*
+ * These macros help objtool understand GCC code flow for unreachable code.
+ * The __COUNTER__ based labels are a hack to make each instance of the macros
+ * unique, to convince GCC not to merge duplicate inline asm statements.
+ */
 #define annotate_reachable() ({						\
 	asm("%c0:\n\t"							\
 	    ".pushsection .discard.reachable\n\t"			\