It's not just a matter of the compiler missing optimizations. Some (unsafe, low-level) algorithms have to be written differently (and less efficiently) because of the possibility of unwinding. Disallowed patterns typically look something like

  * Put an object in an invalid state
  * Perform some operation that might unwind
  * Fix the object again

If you unwind while the object is in an invalid state, and its destructor is called during unwinding, you can end up with undefined behavior.