Compilers are the ones who define what happens when UB is encountered. It'd be odd if they couldn't rely on their own behavior!
Some of it is deliberately undefined in the standard so that compilers can use it, e.g. it's UB to use a reserved identifier so that compilers & future standard versions can add new keywords. This is why C's boolean type first got named `_Bool` and C++ defines `__cplusplus`: identifiers starting with an underscore and a capital letter or with two underscores are reserved, and using reserved identifiers is Undefined Behavior.
Some of it is that the compiler authors know how their compiler will generate code, and can rely on changing internal uses of UB when they change the code generation.
