[Developers] g++ warning: operation on *bla* may be undefined

[Tom Goodale]

Interesting.  What happens when you do use an __attribute__((unused)) 
variable ?

Tom

On Thu, 26 Oct 2006, Erik Schnetter wrote:

> gcc has __attribute__((unused)) to mark variables that are intentionally 
> unused.  This suppresses this warning.  Other compilers may have similar ways 
> for annotating variables.
>
> I have implemented a bit of generic infrastructure to declare variables, 
> which makes it easier to experiment with these things.  I have also 
> implemented autodetecting whether __attribute__((unused)) is supported, so 
> that it is used when present, and an old-style mechanism is used otherwise. 
> A similar thing is done for Fortran code.
>
> The changes are straightforward, but touch quite a number of files.  I would 
> need to isolate them from other changes, and then they need to be tested 
> thoroughly.  I can do the first part if you are interested.
>
> We discussed this before, and then the consensus was that using 
> __attribute__((unused)) was not worth the effort because the existing 
> mechanisms were sufficient.  Maybe this has changed with gcc 4 having become 
> too clever.
>
> -erik
>
> On Oct 26, 2006, at 15:50:38, Steve White wrote:
>
>> Hi again.
>> 
>> Unfortunately, Portland Group's C++ compiler (which I hadn't tried
>> in my tests) now warns:
>>
>>      variable "parameter_name" was set but never used
>> 
>> for every CCTK parameter.  I don't think it was warning about the
>> old code.
>> 
>> It seems that one can't please everybody.
>> 
>> 
>> On 20.10.06, Steve White wrote:
>>> Hi,
>>> 
>>> This is concerned with g++ warnings like:
>>>
>>> 	Preprocessing arrangements/Carpet/Carpet/src/Timing.cc
>>> 	Compiling arrangements/Carpet/Carpet/src/Timing.cc
>>> 	configs/chain_gcc_carpet/build/Carpet/Timing.cc:
>>> 		In function ‘void Carpet::InitTimingVariables(const cGH*)’:
>>> 	configs/chain_gcc_carpet/build/Carpet/Timing.cc:120:
>>> 		warning: operation on ‘cctki_use’ may be undefined
>>> 
>>> The culprit was in generated bindings code meant to get rid of warnings
>>> about unused variables corresponding to Cactus parameters included in
>>> code by the DECLARE_CCTK_PARAMETERS macro.
>>> 
>>> Typically, a struct was defined to hold copies of the macros, then
>>> such very interesting constructs would appear.
>>>
>>>  const void *PRIVATE_CACTUS_STRUCT_use = ( \
>>>    PRIVATE_CACTUS_STRUCT_use = &cctk_run_title, \
>>>    PRIVATE_CACTUS_STRUCT_use = &*PRIVATE_CACTUS_STRUCT_use \
>>>  )
>>> 
>>> This code replaces that with a do-nothing enum declaration
>>>  enum {
>>>    dummy_cctk_run_title = sizeof( cctk_run_title )
>>>    //,etc.
>>>  };
>>> The nature of all the entities involved is here much clearer, both to
>>> compilers and to me.
>>> 
>>> Like the existing solution, this permits the user to proceed to make more
>>> declarations after the DECLARE_CCTK_PARAMETERS macro (simply putting a 
>>> list
>>> of variable assignments after the declaration would not have that 
>>> property).
>>> 
>>> See also the patches list posting of the same name, and PR 2070
>>> http://www.cactuscode.org/cactus_cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=cactus&pr=2070
>>> for a patch.
>>> 
>>> It has been tested on Gnu 4x, Intel 9x, IBM XL
>>> 
>>> Cheers!
>>> 
>> 
>> -- 
>> Steve White : Programmer
>> Max-Planck-Institut für Gravitationsphysik      Albert-Einstein-Institut
>> Am Mühlenberg 1, D-14476 Golm, Germany                  +49-331-567-7625
>> 
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>
>
>