linux-kernel - [PATCH 1/2] objtool: Add a comment for the unreachable annotation macros

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Message-Id: <1570e48d9f87e0fc6f0126c32e7e1de6e109cb67.1509974104.git.jpoimboe@redhat.com>
Date:   Mon,  6 Nov 2017 07:17:37 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 1/2] 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@...ux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 include/linux/compiler.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 3672353a0acd..b863e5ad91a3 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -88,6 +88,11 @@ 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"			\
-- 
2.13.6