The example in the slide uses `-Wall -O`, and `-Wall` includes `-Wuninitialized`. Recent versions of GCC have a revamped syntax tree analyzer, which was needed

The example in the slide uses `-Wall -O`, and `-Wall` includes `-Wuninitialized`. Recent versions of GCC have a revamped syntax tree analyzer, which was needed for the new tree-vectorize features. It does more work from the outset these days, so I'm not surprised if it throws the warning earlier.