[Bug c/59801] GCC does not warn on unused global variable

["su at cs dot ucdavis.edu" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59801

Zhendong Su <su at cs dot ucdavis.edu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |su at cs dot ucdavis.edu

--- Comment #3 from Zhendong Su <su at cs dot ucdavis.edu> ---
For the given code, it does seem make sense to issue a warning (whether or not
"volatile" is used) because this is likely a bug in the code. 

$ gcc-trunk -Wunused-variable small.c
$ clang-trunk -Wunused-variable small.c
small.c:1:21: warning: unused variable 'a' [-Wunused-variable]
static volatile int a;
                    ^
1 warning generated.
$ cat small.c
static volatile int a;

int 
main () 
{
  return 0;
}

----------------------

This also seems to be a missed optimization opportunity: 

$ gcc-trunk -O3 -S small.c
$ cat small.s
    .file    "small.c"
    .section    .text.unlikely,"ax",@progbits
.LCOLDB0:
    .section    .text.startup,"ax",@progbits
.LHOTB0:
    .p2align 4,,15
    .globl    main
    .type    main, @function
main:
.LFB0:
    .cfi_startproc
    xorl    %eax, %eax
    ret
    .cfi_endproc
.LFE0:
    .size    main, .-main
    .section    .text.unlikely
.LCOLDE0:
    .section    .text.startup
.LHOTE0:
    .local    a
    .comm    a,4,4
    .ident    "GCC: (GNU) 4.9.0 20140115 (experimental) [trunk revision
206638]"
    .section    .note.GNU-stack,"",@progbits