Messages in this thread Patch in this message | | | Date | Mon, 30 Sep 2013 16:56:30 +0200 | From | Ralf Baechle <> | Subject | Issue with BUG() in asm-gemeric/bug.h if CONFIG_BUG=n |
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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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