gcc doesn't issue a warning for very simple cases of uninitialized variable. The options used are '-Wuninitialized -O2' or '-Wmaybe-uninitialized -O2'. It can be reproduced with gcc 5.4, 6.3 and gcc 7.1 (I didn't tried older versions).

One example of code to show this is:

	int g;
	int foo(void)
	{
		int r = 0;
		int u;

		if (g)
			r = u;
		return r;
	}