On Stack Replacement - OSR

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Normally the way Java code ends up in compiled code is that when invoking a method the interpreter detects that there's compiled code for it, and it dispatches to that instead of staying in the interpreter.  This does not help long-running loops that started in the interpreter since they are not being invoked again.

When a long-running loop is detected at runtime, HotSpot VM requests a compile that starts its execution at the first bytecode of loop instead of starting at the first bytecode in the method.  The resulting generated code takes an interpreter frame as its input and uses that state to begin its execution.  In this way, long-running loops are able to take advantage of compiled code.  The act of the generated code taking an interpreter frame as input to be execution is called On Stack Replace.

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OSR is useful in situations where you identify a function as "hot" while it is running. This might not necessarily be because the function gets called frequently; it might be called only once, but it spends a lot of time in a big loop which could benefit from optimization. When OSR occurs, the VM is paused, and the stack frame for the target function is replaced by an equivalent frame which may have variables in different locations.