On 09/30/2013 10:15 AM, Pinski, Andrew wrote:
On Sep 30, 2013, at 9:20 AM, "David Daney" <ddaney.cavm@xxxxxxxxx> 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@xxxxxxxxxxxxxx>
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
