http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58236
--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> --- Note, from the documentation of -Wuninitialized: "Because these warnings depend on optimization, the exact variables or elements for which there are warnings depends on the precise optimization options and version of GCC used." You need to pass -O2 on the command line, and then you'll get warnings.
