All JIT:ted VM:s modify their code segments. So not a big deal at all. Code modification is done to implement dynamic dispatch efficiently. For example, suppose

All JIT:ted VM:s modify their code segments. So not a big deal at all. Code modification is done to implement dynamic dispatch efficiently. For example, suppose the compiler generates code for "a + b" If it can't infer the types of a and b, it generates a call to a stub function. When that stub function is called, it looks at the types of a and b and generates efficient dispatch code. For example, if a and b were ints it would generate code like: "if (type(a)==int && type(b)==int) { return fast_add(a, b); } else { slow_but_generic_add(a, b); }" A sophisticated JIT could even keep track on how frequently the fast track is taken and generate a new stub based on that. It could even inline the stub into the calling function and recompile that one.