What does Oop Maps means in Hotspot VM exactly

Question

I read from some documents that Hotspot VM utilizes a data structure called Oop Maps to manage all OOPs in VM. My question is that when does this Oop Map data structure generated? At compile time or runtime? Any further detailed documents regarding to this will be more than welcomed. Thank you guys.

Answer 1

OopMap is a structure that records where object references (OOPs) are located on the Java stack. Its primary purpose is to find GC roots on Java stacks and to update the references whenever objects are moved within the Heap.

There are three kinds of OopMaps:

1. OopMaps for interpreted methods. They are computed lazily, i.e. when GC happens, by analyzing bytecode flow. The best reference is the source code (with lots of comments), see generateOopMap.cpp. InterpreterOopMaps are stored in OopMapCache.
2. OopMaps for JIT-compiled methods. They are generated during JIT-compilation and kept along with the compiled code so that VM can quickly find by instruction address the stack locations and the registers where the object references are held.
3. OopMaps for generated shared runtime stubs. These maps are constructed manually by the developers - authors of these runtime stubs.

During GC JVM walks through all thread stacks. Each stack is parsed as a stream of stack frames. The frames are either interpreted or compiled or stubs. Interpreted frames contain information about Java method and bci (bytecode index). OopMapCache helps to find an OopMap corresponding to the given method and bci. The method of a compiled frame is discovered by instruction address lookup.