I'm pretty sure this is an upstream GCC bug because it happens on Ubuntu as well, but GCC requests that bugs be filed against distributions, so I report it here...

$ gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.7.0/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --disable-build-with-cxx --disable-build-poststage1-with-cxx --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin --enable-initfini-array --enable-java-awt=gtk --disable-dssi --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre --enable-libgcj-multifile --enable-java-maintainer-mode --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib --with-ppl --with-cloog --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.7.0 20120507 (Red Hat 4.7.0-5) (GCC) 


Consider the following three files:

==> a.c <==
__attribute__((visibility("hidden"))) const int x;

void b (int);

void a (void) { b (x); }

==> main.c <==
#include <stdio.h>

void b (int x) { printf ("x = %d\n", x); }

void a (void);

int main (void) { a (); return 0; }

==> x.c <==
__attribute__((visibility("hidden"))) const int x = 5;



This is more or less the classic case of failing to mark the declaration of a global variable in a header file as 'extern' leading to the classical unix "common symbol" treatment (which is questionable, but quite well-defined).

If you build this program, you get the expected output:

$ gcc -o x {a,x,main}.c
$ ./x
x = 5

Of course, this is depending on the common-symbol-combining functionality of the linker, as can be seen by using --warn-common:

$ gcc -o x {a,x,main}.c -Wl,--warn-common
/tmp/ccQm2lYI.o: warning: definition of `x' overriding common
/tmp/ccogwEdV.o: warning: common is here

We can explicitly request that this feature is disabled in GCC:

$ gcc -o x {a,x,main}.c -fno-common
/tmp/ccKhuZE1.o:(.rodata+0x0): multiple definition of `x'
/tmp/cc1cz0t6.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status

The problem comes when we compile with -O2 -fcommon (note: -fcommon is the default in F17 but I write it here for sake of being explicit):

$ gcc -o x {a,x,main}.c -O2 -fcommon
$ ./x
x = 0

The assembly code for a() looks like so:

a:
.LFB0:
        .cfi_startproc
        xorl    %edi, %edi
        jmp     b
        .cfi_endproc

We now have an incorrect output.  The reason is obvious.  GCC considers that it sees a constant global variable defined in a file with no value written to it.  Interposition is not possible (due to the symbol being marked as hidden).  Of course the value is zero, so we may as well just replace it with a constant zero ("xorl %edi, %edi").

This reasoning fails to consider the common symbol merging that the linker may very well do.  -fcommon was specified, after all, so this possibility should be expected.