[Karliss]

The UB comes from how C++ standard defines expression sequencing which is not relevant for Java. Languages other than C++ typically define such details more strictly so there is no UB or even concept of UB. JIT compilers don't change it as any non toy JIT will generate native instructions directly or through intermediate representation (instead of generating C++ text and passing that through regular C++ compiler) both of which should have much stricter semantics compared to what C++ guarantees.

[chasd00]

late to this but what i meant was the JIT Compiler had to be compiled by a compiler at some point so its behavior is defined by the compiler used to compile it (this goes all the way down to the poor soul who hand compiled the first compiler). I'm assuming things like JIT are platform dependent and therefore the compiler used to build it would also be platform dependent. So the JIT on different platforms could have been built from different compilers choosing their own way of handling undefined behavior. ...unless i'm confusing myself which is likely.

[dmoy]

The compiler is compiled via java nowadays. It's bootstrapped.

Yes the first one was probably written in C, however long ago. Not super relevant now though.

JVM/JRE, probably a different story