# make
gcc -O -DGCC_BUG -c main.c
gcc -O -DGCC_BUG -fPIC -c shared.c
gcc -shared -o libfoo.so  shared.o
gcc -o foo  main.o libfoo.so -Wl,-rpath,.
for f in foo; do echo "Running: $f"; ./$f; \
  if [ $? != 0 ]; then echo Failed; fi; done
Running: foo
Data address: 0x8049924 != Data address from DSO: 0x400187e4
Function address: 0x80484c0 !=  Function address from DSO: 0x4001775f
Data: 100 != Data from DSO: 10
Failed

The testcase will always fail. But without -DGCC_BUG,

Function address: 0x80484c0 !=  Function address from DSO: 0x4001775f

won't be there.