OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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Figure 2.5: A Simple Interpreter Engine
2.4.2 A Better Switch Based Interpreter
When the algorithm presented in Figure 2.4 is executed, two unconditional branches
are performed for each bytecode executed. This can be reduced to one unconditional
branch for each bytecode executed by replacing the while loop with an infinite loop
constructed using a goto statement. One branch is eliminated by replacing the break
statement at the end of each codelet with a goto statement. This goto statement
transfers control to the top of the loop directly, making it unnecessary to branch to
the bottom of the loop. Furthermore, performing this transformation eliminates the
conditional that is tested in order to determine if the while loop should continue to
execute. While an optimizing compiler should be able to eliminate the test if the
conditional is constructed using only constant values, performing this transformation
guarantees that the check will be removed without relying on the optimizer.
Figure 2.6 shows the execution engine once these transformations have been per-