Comment by kibwen

> In fact, take any C program with UB, compile it, and get a dangerous executable. Now disassemble the executable, and you get an equally dangerous program, yet it doesn't have any UB.

I'd put it like this:

Undefined behavior is a property of an abstract machine. When you write any high-level language with an optimizing compiler, you're writing code against that abstract machine.

The goal of an optimizing compiler for a high-level language is to be "semantics-preserving", such that whatever eventual assembly code that gets spit out at the end of the process guarantees certain behaviors about the runtime behavior of the program.

When you write high-level code that exhibits UB for a given abstract machine, what happens is that the compiler can no longer guarantee that the resulting assembly code is semantics-preserving.