Re: [Consult] g++: About "-Wunused-variable" for constant variable initialized by function

[Chen Gang <gang dot chen dot 5i5j at gmail dot com>]

On 10/13/14 18:58, Peter Maydell wrote:
> On 12 October 2014 16:32, Chen Gang <gang.chen.5i5j@gmail.com> wrote:
>> Hello All:
>>
>> I found an issue about g++, it is OK for "-Wall -O0", but will report
>> -Wunused-variable for "-Wall -O1|2|3|s". The original version (e.g.
>> gcc 4.8.3 redhat version) does not report warning for "-Wall -O?".
>>
>> The related operation:
>>
>>   [root@localhost qemu_cc]# cat test.cc
>>   const char n() { return 1; }
>>   const char c =  n();
>>   [root@localhost qemu_cc]# /usr/local/bin/g++ -Wall -O0 -c -o test.o test.cc
>>   [root@localhost qemu_cc]# /usr/local/bin/g++ -Wall -O2 -c -o test.o test.cc
>>   test.cc:2:12: warning: 'c' defined but not used [-Wunused-variable]
>>    const char c =  n();
>>               ^
> 
> This cut down test case seems to have discarded two interesting
> things about the original issue
> (context: http://lists.gnu.org/archive/html/qemu-devel/2014-10/msg01253.html)
>  (1) the definitions of the const variables are in a header
>  (2) gcc doesn't warn about const declarations in the same
>      header which use 'int' rather than 'float' or 'double'
> 
> That said, how does g++ know that the variable isn't defined
> for the benefit of another translation unit? (Conversely, how
> should a library define constants in a header for the benefit
> of users of the library in a way that doesn't make g++ complain
> if the library using code happens not to use the constant?)
> 

Oh, yes. Originally I got this warning by compiling Qemu. And sorry for
my sample (test.cc) may be not quite precise.

For me, I guess:

 - If the constant number is defined in the header file, and never be
   used, our g++ need not report warning about [-Wunused-variable].

 - But if the constant number is defined in the source file, and never
   be used, our g++ need report warning about [-Wunused-variable] (just
   like what Jason Merrill said).

Is my guess correct?

Thanks.
-- 
Chen Gang

Open, share, and attitude like air, water, and life which God blessed