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

To: "Pinski, Andrew" <Andrew.Pinski@caviumnetworks.com>
Subject: Re: Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n
From: David Daney <ddaney.cavm@gmail.com>
Date: Mon, 30 Sep 2013 10:45:08 -0700
Cc: Ralf Baechle <ralf@linux-mips.org>, "linux-kernel@vger.kernel.org" <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" <linux-mips@linux-mips.org>, John Crispin <blogic@openwrt.org>
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On 09/30/2013 10:15 AM, Pinski, Andrew wrote:
On Sep 30, 2013, at 9:20 AM, "David Daney" <ddaney.cavm@gmail.com> wrote:


On 09/30/2013 07:56 AM, Ralf Baechle wrote:
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()
16304
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).

The *only* reason we have CONFIG_BUG=n is to reduce code size.

Sticking in that empty loop, negates the entire point.

IMHO: We should do one of:
o Make CONFIG_BUG=y mandatory
o Ignore the warnings.
o Fix the warning sites so they quit Warning.

So I don't think the patch is really an improvement over the status quo.

What about using __builtin_unreachable when we can but turn off warnings and 
use do{}while(0) when __builtin_unreachable does not exist?  This seems the 
both worlds.  Newer compilers produce better code with unreachable anyways.


Simply not true.

do{}while(0) is a NOP it is no more useful than an ';' statement. It doesn't serve as a magic uninitialized variable hiding mechanism.

David Daney


Thanks,
Andrew



David Daney

__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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