linux-kernel - Issue with BUG() in asm-gemeric/bug.h if CONFIG

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Message-ID: <20130930145630.GA14672@linux-mips.org>
Date:	Mon, 30 Sep 2013 16:56:30 +0200
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	David Howells <dhowells@...hat.com>,
	Markos Chandras <Markos.Chandras@...tec.com>,
	linux-mips@...ux-mips.org, David Daney <ddaney@...iumnetworks.com>,
	Andrew Pinski <Andrew.Pinski@...iumnetworks.com>,
	John Crispin <blogic@...nwrt.org>
Subject: Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n

Lately I received several patches for build issues that only strike if
CONFIG_BUG is disabled.  Here's a test case extracted from one of them:

/*
 * Definition of BUG taken from asm-generic/bug.h for the CONFIG_BUG=n case
 */
#define BUG() 	do {} while(0)

int foo(int arg)
{
	int res;

	if (arg == 1)
		res = 23;
	else if (arg == 2)
		res = 42;
	else
		BUG();

	return res;
}

[ralf@h7 ~]$ gcc -O2 -Wall -c bug.c 
bug.c: In function ‘foo’:
bug.c:17:2: warning: ‘res’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  return res;
  ^

It's fairly obvious to see what's happening here - GCC doesn't know that
the else case can not be reached, thus razorsharply concludes that res
may be used uninitialized.

There several locations where MIPS - possibly other architectures as well -
is affected by this.

I think the definition of BUG should be changed to something like

#define BUG()	unreachable()

unreachable() will depending on the compiler being used, expand either
into a call to __builtin_unreachable() or where that function is
unavailable, into do {} while (1).

__builtin_unreachable() was introduce for GCC 4.5.0.

This means there'd be minor bloat for antique compilers - but probably
even better code generation for compilers supporting __builtin_unreachable().

  Ralf

Signed-off-by: Ralf Baechle <ralf@...ux-mips.org>

 include/asm-generic/bug.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 7d10f96..6f78771 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -108,7 +108,7 @@ extern void warn_slowpath_null(const char *file, const int line);
 
 #else /* !CONFIG_BUG */
 #ifndef HAVE_ARCH_BUG
-#define BUG() do {} while(0)
+#define BUG() unreachable()
 #endif
 
 #ifndef HAVE_ARCH_BUG_ON

----- End forwarded message -----

  Ralf
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