Why is null-terminated C string considered a "billion dollar mistake", but UB isn't?
Anyway, one argument is that UB is fundamentally useful in languages that are insufficiently type-safe, like C and C++. The "holes" in the specification allow for regions where the compiler can optimize the code in ways you may not expect.
As we have developed more advanced type systems, the utility of undefined behavior has lessened considerably.
> As far as I can tell, C89 did not use performance as a justification for any of its undefined behaviors. They were non-portabilities, like signed overflow and null pointer dereferences, or they were outright bugs, like use-after-free. But now experts like Chris Lattner and Hans Boehm point to optimization potential, not portability, as justification for undefined behaviors. I conclude that the rationales really have shifted from the mid-1980s to today: an idea that meant to capture non-portability has been preserved for performance, trumping concerns like correctness and debuggability.
Null being an "allowed" value for pointers is the mistake e.g. what became nullptr. "Allowed" because garbage values are garbage.
Think of this piece of code - `y * x / y`.
Would you like to simplify it to just `x` ?
You need to either lean on UB to do so or have some magical way to prove that y can not be 0.
Otherwise this transformation changes behavior, and is illegal.
> Maybe I'm doing some rounding?
Compilers won't do this optimization when it is illegal to. If you disagree with the compiler's idea of what is legal, you can either write inline assembly, or put this code into an always inlined, but never optimized function.