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

[Peter Maydell]

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?)

thanks
-- PMM