The Java HotSpot Performance Engine: On-Stack Replacement Example

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public class Benchmark {
  public static void main(
             String[] arg) {
 long before = 
       System.currentTimeMillis();
 int sum = 0;
 
 
 for (int index = 0; index <
        10*1000*1000; index += 1) {
 sum += index;
     }
 long after = 
         System.currentTimeMillis();
 System.out.println("Elapsed time: " +
         Long.toString(after - before) +
           " milliseconds");
     }
  }
  

The inner loop could be anything. "sum += index" is just one operation that someone might try to measure. The method begins interpreting, but on every backward branch (corresponding to the closing brace of the loop), a counter associated with the method is incremented. When it goes past a threshold (currently 10,000 in Java HotSpot version 1.0) then the method is compiled.

The timeline is:

• Interpreter starts interpreting main() method.
• Counter hits 10,000, and compilation begins, but still interpreting main().
• Compilation finishes, still interpreting main().
• main() finishes.

Because the main method is never reentered, the compiled version will never be executed. That's where on-stack replacement comes in. A special version of the method is compiled that allows execution to continue from the middle of the loop.

With on-stack replacement in place, our timeline is now:

• Interpreter starts interpreting main() method
• Counter hits 10,000, and compilation begins, but still interpreting main()
• Compilation finishes, still interpreting main()
• Counter hits 14,000 (in HotSpot 1.0), and interpreting stops—time for OSR!
• main() is compiled a second time via OSR to allow entry in the middle of the loop
• main() resumes in the compiled code
• main() finishes

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