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Comment by dbaupp | Hacker News Reader

It is not nearly as simple as you are hoping. The threat of unwinding forces compilers to insert landing pads (increasing code size) and forgo some optimisations because they have to be careful to preserve semantics even during unwinding, restricting things like how much code can be reordered etc.

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It certainly makes things more complex for the compiler. However, I don't know an example where it would prevent an optimization.

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.

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