Indeed, UB is literally whatever the compiler feels like. A famous one [1] has the compiler deleting code that contains UB and falling through to the next function.

"But it's right there in the name!" Undefined behavior literally places no restrictions on the code generated or the behavior of the program. And the compiler is under no obligation to help you debug your (admittedly buggy) program. It can literally delete your program and replace it with something else that it likes.

[1] https://kristerw.blogspot.com/2017/09/why-undefined-behavior...