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Undefined behaviour enables only a fairly small set of optimisations. There's a large set of optimisations that can be implemented completely safely without having to make such dangerous assumptions. Other programming languages do this all the time, it's not just C/C++ that have optimisers!

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This is simply not true. Almost all optimisations rely on undefined behavior, see https://news.ycombinator.com/item?id=38760475

Note that I am not talking about UB like signed integer overflow. Removing that would slow down programs by a couple of percent. The important type of UB is pointer provenance. This ensures that e.g., writing to a random memory address is UB.

Almost all optimisations in C/C++ compilers depend on undefined behaviour, because practically no behaviour is defined!

The trick is to define behaviour, which is what other programming languages do.

E.g.: in both C# and Rust, integers have fixed sizes. A C# Int32 is equivalent to a Rust i32. Only God knows what a C/C++ "int" is. It could have 17 bits and use ternary.

As I said, this isn't about ints, it's about pointers. What behavior will you define in C for writing to a random memory address?

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