Yeah a decent chunk of UB is about reducing the burden on the compiler. Null derefs being an obvious such example. If it was defined behavior, the compiler would be endlessly adding & later attempting to optimize-away null checks. Which isn't something anyone actually wants when reaching for C/C++.
Similarly with C/C++ it's not actually possible for the compiler to ensure you don't access a pointer past the end of the array - the array size often isn't "known" in a way the compiler can understand.
> Which isn't something anyone actually wants when reaching for C/C++.
Disagree. I think a lot of people want some kind of "cross-platform assembler" (i.e. they want e.g. null deref to trap on architectures where it traps, and silently succeed on architectures where it succeeds), and get told C is this, which it very much isn't.
Except every other sane systems programming language does indeed do null checks, even those older than C, but they didn't come with UNIX, so here we are.
Similarly with C/C++ it's not actually possible for the compiler to ensure you don't access a pointer past the end of the array - the array size often isn't "known" in a way the compiler can understand.