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Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n

To: linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>, David Howells <dhowells@redhat.com>, Markos Chandras <Markos.Chandras@imgtec.com>, linux-mips@linux-mips.org, David Daney <ddaney@caviumnetworks.com>, Andrew Pinski <Andrew.Pinski@caviumnetworks.com>, John Crispin <blogic@openwrt.org>
Subject: Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 30 Sep 2013 16:56:30 +0200
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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@linux-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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