JITs do indeed modify already-compiled code. For example, JavaScriptCore has a notion of a "patchable jump." These are jump instructions intended to be modified

JITs do indeed modify already-compiled code. For example, JavaScriptCore has a notion of a "patchable jump." These are jump instructions intended to be modified later. They may initially point to slow paths, and are later patched to point to faster paths as type information accumulates. https://webkit.org/blog/10298/inline-caching-delete/ https://webkit.org/blog/10298/inline-caching-delete/