[PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[behanw]

From: Behan Webster <behanw@converseincode.com>

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@converseincode.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
---
 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..4c41bb8 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);
+		prev = 0;
 	}
 	raw_local_irq_restore(flags);
 	return prev;
-- 
1.8.3.2

Re: [PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[Arnd Bergmann]

On Saturday 22 March 2014, behanw@converseincode.com wrote:
> diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
> index d8d4c89..4c41bb8 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);
> +               prev = 0;
>         }
>         raw_local_irq_restore(flags);
>         return prev;


How about using __unreachable() instead to annotate the fact that you won't
get here?

	Arnd

Re: [PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[Behan Webster]

On 03/22/14 03:01, Arnd Bergmann wrote:
> On Saturday 22 March 2014, behanw@converseincode.com wrote:
>> diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
>> index d8d4c89..4c41bb8 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);
>> +               prev = 0;
>>          }
>>          raw_local_irq_restore(flags);
>>          return prev;
>
> How about using __unreachable() instead to annotate the fact that you won't
> get here?
Yours is a _much_ better idea. Will fix.

Thanks,

Behan

-- 
Behan Webster
behanw@converseincode.com

[PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

[behanw]

From: Behan Webster <behanw@converseincode.com>

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@converseincode.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
---
 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();
 	}
 	raw_local_irq_restore(flags);
 	return prev;
-- 
1.8.3.2

Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

[Sam Ravnborg]

On Sat, Mar 22, 2014 at 08:48:19AM -0700, behanw@converseincode.com wrote:
> From: Behan Webster <behanw@converseincode.com>
> 
> 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@converseincode.com>
> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
> ---
>  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();

It is unreachable() - see compiler.h

	Sam

Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

[James Bottomley]

On Sat, 2014-03-22 at 08:48 -0700, behanw@converseincode.com wrote:
> From: Behan Webster <behanw@converseincode.com>
> 
> 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@converseincode.com>
> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
> ---
>  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).

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.

James

Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

[Behan Webster]

On 03/22/14 09:21, Sam Ravnborg wrote:
> On Sat, Mar 22, 2014 at 08:48:19AM -0700, behanw@converseincode.com wrote:
>> From: Behan Webster <behanw@converseincode.com>
>>
>> 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@converseincode.com>
>> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
>> ---
>>   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();
> It is unreachable() - see compiler.h
Oh I see.

Behan

-- 
Behan Webster
behanw@converseincode.com

[PATCH v3] LLVMLinux: Remove warning about returning an uninitialized variable

[behanw]

From: Behan Webster <behanw@converseincode.com>

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@converseincode.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
---
 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..a4f5882 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);
+		unreachable();
 	}
 	raw_local_irq_restore(flags);
 	return prev;
-- 
1.8.3.2

Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

[Behan Webster]

On 03/22/14 09:29, James Bottomley wrote:
> On Sat, 2014-03-22 at 08:48 -0700, behanw@converseincode.com wrote:
>> From: Behan Webster <behanw@converseincode.com>
>>
>> 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@converseincode.com>
>> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
>> ---
>>   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.

> 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.

Thanks,

Behan

-- 
Behan Webster
behanw@converseincode.com

Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

[James Bottomley]

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@converseincode.com wrote:
> >> From: Behan Webster <behanw@converseincode.com>
> >>
> >> 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@converseincode.com>
> >> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
> >> ---
> >>   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
unreachable problem).

James

---
diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
index d8d4c89..70bef78 100644
--- a/include/asm-generic/cmpxchg-local.h
+++ b/include/asm-generic/cmpxchg-local.h
@@ -4,7 +4,8 @@
 #include <linux/types.h>
 #include <linux/irqflags.h>
 
-extern unsigned long wrong_size_cmpxchg(volatile void *ptr);
+extern unsigned long wrong_size_cmpxchg(volatile void *ptr)
+	__noreturn;
 
 /*
  * Generic version of __cmpxchg_local (disables interrupts). Takes an unsigned

Re: [PATCH v2] LLVMLinux: Remove warning about returning an uninitialized variable

[Behan Webster]

On 03/22/14 09:42, James Bottomley wrote:
> 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@converseincode.com wrote:
>>>> From: Behan Webster <behanw@converseincode.com>
>>>>
>>>> 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@converseincode.com>
>>>> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
>>>> ---
>>>>    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.
Sorry. I wasn't clear. It is not merely a gcc-4 ism; it is also 
supported by clang.

I'm not arguing it should be used. I understand (and agree with) your 
objection to its use since it isn't supported before gcc-4.

>>> 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
> unreachable problem).
>
> James
>
> ---
> diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
> index d8d4c89..70bef78 100644
> --- a/include/asm-generic/cmpxchg-local.h
> +++ b/include/asm-generic/cmpxchg-local.h
> @@ -4,7 +4,8 @@
>   #include <linux/types.h>
>   #include <linux/irqflags.h>
>   
> -extern unsigned long wrong_size_cmpxchg(volatile void *ptr);
> +extern unsigned long wrong_size_cmpxchg(volatile void *ptr)
> +	__noreturn;
>   
>   /*
>    * Generic version of __cmpxchg_local (disables interrupts). Takes an unsigned
Nice. I'll give it a try. It looks like a much better solution.

Behan

-- 
Behan Webster
behanw@converseincode.com

[PATCH v4] LLVMLinux: Remove warning about returning an uninitialized variable

[behanw]

From: Behan Webster <behanw@converseincode.com>

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@converseincode.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
---
 include/asm-generic/cmpxchg-local.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h
index d8d4c89..70bef78 100644
--- a/include/asm-generic/cmpxchg-local.h
+++ b/include/asm-generic/cmpxchg-local.h
@@ -4,7 +4,8 @@
 #include <linux/types.h>
 #include <linux/irqflags.h>
 
-extern unsigned long wrong_size_cmpxchg(volatile void *ptr);
+extern unsigned long wrong_size_cmpxchg(volatile void *ptr)
+	__noreturn;
 
 /*
  * Generic version of __cmpxchg_local (disables interrupts). Takes an unsigned
-- 
1.8.3.2

Re: [PATCH v4] LLVMLinux: Remove warning about returning an uninitialized variable

[David Howells]

behanw@converseincode.com wrote:

> From: Behan Webster <behanw@converseincode.com>
> 
> 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@converseincode.com>
> Signed-off-by: Mark Charlebois <charlebm@gmail.com>

Acked-by: David Howells <dhowells@redhat.com>

Re: [PATCH v4] LLVMLinux: Remove warning about returning an uninitialized variable

[Arnd Bergmann]

On Monday 24 March 2014, behanw@converseincode.com wrote:
> 
> From: Behan Webster <behanw@converseincode.com>
> 
> 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@converseincode.com>
> Signed-off-by: Mark Charlebois <charlebm@gmail.com>

Acked-by: Arnd Bergmann <arnd@arndb.de>

Re: [PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[H. Peter Anvin]

On 03/21/2014 11:38 PM, behanw@converseincode.com wrote:
> From: Behan Webster <behanw@converseincode.com>
> 
> 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@converseincode.com>
> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
> ---
>  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..4c41bb8 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);
> +		prev = 0;
>  	}
>  	raw_local_irq_restore(flags);
>  	return prev;
> 

Shouldn't this be a build time assert (__compiletime_error())?

	-hpa

Re: [PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[Behan Webster]

On 03/31/14 13:52, H. Peter Anvin wrote:
> On 03/21/2014 11:38 PM, behanw@converseincode.com wrote:
>> From: Behan Webster <behanw@converseincode.com>
>>
>> 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@converseincode.com>
>> Signed-off-by: Mark Charlebois <charlebm@gmail.com>
>> ---
>>   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..4c41bb8 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);
>> +		prev = 0;
>>   	}
>>   	raw_local_irq_restore(flags);
>>   	return prev;
>>
> Shouldn't this be a build time assert (__compiletime_error())?
I changed it to a __noreturn on wrong_size_cmpxchg thanks to James 
Bottomley.

Which would be better?

Behan

-- 
Behan Webster
behanw@converseincode.com

Re: [PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[H. Peter Anvin]

On 03/31/2014 03:10 PM, Behan Webster wrote:
>>>
>>> diff --git a/include/asm-generic/cmpxchg-local.h
>>> b/include/asm-generic/cmpxchg-local.h
>>> index d8d4c89..4c41bb8 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);
>>> +        prev = 0;
>>>       }
>>>       raw_local_irq_restore(flags);
>>>       return prev;
>>>
>> Shouldn't this be a build time assert (__compiletime_error())?
> I changed it to a __noreturn on wrong_size_cmpxchg thanks to James
> Bottomley.
> 
> Which would be better?
> 

__compiletime_error traps at compile time rather than link time, which
is what you want.

	-hpa

Re: [PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[Behan Webster]

On 03/31/14 15:11, H. Peter Anvin wrote:
> On 03/31/2014 03:10 PM, Behan Webster wrote:
>>>> diff --git a/include/asm-generic/cmpxchg-local.h
>>>> b/include/asm-generic/cmpxchg-local.h
>>>> index d8d4c89..4c41bb8 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);
>>>> +        prev = 0;
>>>>        }
>>>>        raw_local_irq_restore(flags);
>>>>        return prev;
>>>>
>>> Shouldn't this be a build time assert (__compiletime_error())?
>> I changed it to a __noreturn on wrong_size_cmpxchg thanks to James
>> Bottomley.
>>
>> Which would be better?
>>
> __compiletime_error traps at compile time rather than link time, which
> is what you want.
The idea is to remove the compile time warning that the return code 
"prev" isn't initialized in the default case. Indicating that 
wrong_size_cmpxchg doesn't return fixes that false positive.

Behan

-- 
Behan Webster
behanw@converseincode.com

Re: [PATCH] LLVMLinux: Remove warning about returning an uninitialized variable

[H. Peter Anvin]

On 03/31/2014 03:16 PM, Behan Webster wrote:
>>>
>> __compiletime_error traps at compile time rather than link time, which
>> is what you want.
> The idea is to remove the compile time warning that the return code
> "prev" isn't initialized in the default case. Indicating that
> wrong_size_cmpxchg doesn't return fixes that false positive.
> 

Perhaps __compiletime_error() should have __noreturn built in?

	-hpa