Given that practically anything under the sun is undefined behavior for C and C++, that isn’t saying much.
Often, what constitutes undefined behavior is non-obvious (and not well justified). For example, when adding two signed integers results in an overflow, it is undefined behavior even if your program never uses the result.
Due to C's definition of "undefined" behavior, it means that all of the guarantees we rely on to ensure security go out the window whenever the programmer steps on one of these land mines.
If something is "undefined" according to C, you can't rely on what the hardware does, because the hardware might not even get a chance to do anything. The compiler may completely elide sections of your program -- and they do in practice (for example, bounds checks).
Actually, hardware always does something reasonable for add instructions (throw exception or overflow or saturate). It's additions in C that can have unreasonable results.
That problem is you can no longer depend on that because the compiler writers have decided they can do anything during AST optimization when there is undefined behavior.
Who knew he was talking about C compilers??