GCC should have prepared a preprocessed testcase for you, please provide it.  If it has not, please prepare it yourself.
In any case, don't use -combine, it is known to be broken.

Gcc did not provide a test case.  I have come up with a test case:

============================================================
cat > file1.c << EOF
struct s1_s {
    int v;
};
struct s1_s g1;
void __attribute__((externally_visible)) func1() {
    struct s1_s *l1 = &g1;
}
EOF

cat > file2.c << EOF
extern struct s1_s g1;
void func2() {
    &g1;
}
EOF

cc -O -g -fwhole-program -combine -c file1.c file2.c
============================================================

It happens on GCC mainline as well, filed there as http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41893

Should be fixed in gcc-4.4.2-8 and later.

This came back in FC12:

$ rpm -q gcc
gcc-4.4.2-20.fc12.x86_64

The test case above no longer causes the problem - but simply changing the test case to also assign a value to the variable shows the problem.

New test case:

============================================================
cat > file1.c << EOF
struct s1_s {
    int v;
};
struct s1_s g1;
void __attribute__((externally_visible)) func1() {
    struct s1_s *l1 = &g1;
    l1->v = 0;
}
EOF

cat > file2.c << EOF
extern struct s1_s g1;
void func2() {
    &g1;
}
EOF

cc -O -g -fwhole-program -combine -c file1.c file2.c
============================================================

Should be fixed in gcc-4.4.3-14 and above.