Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable
From: James Bottomley
Date: Sat Mar 22 2014 - 12:42:58 EST
On Sat, 2014-03-22 at 09:37 -0700, Behan Webster wrote:
> On 03/22/14 09:29, James Bottomley wrote:
> > On Sat, 2014-03-22 at 08:48 -0700, behanw@xxxxxxxxxxxxxxxxxx wrote:
> >> From: Behan Webster <behanw@xxxxxxxxxxxxxxxxxx>
> >> Fix uninitialized return code in default case in cmpxchg-local.h
> >> This patch fixes the code to prevent an uninitialized return value that is detected
> >> when compiling with clang. The bug produces numerous warnings when compiling the
> >> Linux kernel with clang.
> >> Signed-off-by: Behan Webster <behanw@xxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Mark Charlebois <charlebm@xxxxxxxxx>
> >> ---
> >> include/asm-generic/cmpxchg-local.h | 1 +
> >> 1 file changed, 1 insertion(+)
> >> diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
> >> index d8d4c89..9112111 100644
> >> --- a/include/asm-generic/cmpxchg-local.h
> >> +++ b/include/asm-generic/cmpxchg-local.h
> >> @@ -41,6 +41,7 @@ static inline unsigned long __cmpxchg_local_generic(volatile void *ptr,
> >> break;
> >> default:
> >> wrong_size_cmpxchg(ptr);
> >> + __builtin_unreachable();
> > No, that's got to be unreachable() so that it works in all compilers,
> > (__builtin_unreachable is a gcc-4 ism).
> It is also supported by clang.
OK, but it's not supported by gcc-3. So gcc-3 would crap out seeing the
statement, which is why we have to wrapper it.
> > Got to say this still looks wrong. If wrong_size_cmpxchg() cannot
> > return, the function should be annotated as such with __noreturn (like
> > panic()) so the unreachable() should be superfluous.
> Okay. I can try that instead.
Great; This seems to work for me (but then my compiler doesn't see the
diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
index d8d4c89..70bef78 100644
@@ -4,7 +4,8 @@
-extern unsigned long wrong_size_cmpxchg(volatile void *ptr);
+extern unsigned long wrong_size_cmpxchg(volatile void *ptr)
* Generic version of __cmpxchg_local (disables interrupts). Takes an unsigned
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