OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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Chapter 11
Summary and Conclusions
This thesis has conducted a thorough evaluation of the Java Virtual Machine instruction
set. It began by providing a brief review of the events that lead to the
development and success of Java. This was followed by a description of the Java Virtual
Machine and a summary of the optimization strategies that have been employed
in order to improve its performance.
Chapter 4 introduced the notion of despecialization. It identified a total of 76 specialized
bytecodes which provide functionality that is duplicated by other bytecodes
within the Java Virtual Machine instruction set. Many of these specialized bytecodes
can be replaced with exactly one other general purpose bytecode. Examples
of bytecodes within this category include 40 bytecodes of the form load
and store and 14 constant loading bytecodes of the form const .
Other specialized bytecodes require two or more general purpose bytecodes in order
to achieve identical functionality. These include the if bytecodes that can be
replaced with exactly two general purpose bytecodes and several instructions which
require a longer sequence.
Performance results were collected for 7 benchmarks executing on three virtual
machines. These results showed that the change in runtime associated with despecialization
was minor. While the worst performance change for any one benchmark was a
loss of approximately 12.7 percent, this result was not typical. When all benchmarks
were considered across all virtual machines, the overall performance loss averaged 2.0
percent when 67 despecializations were performed.
The Java Virtual Machine Specification includes numerous constraints on the content
of a Java class file. Performing despecialization has the potential to violate some
of those constraints. An analysis was performed in order to determine which of these
limits could be violated. Examining the class files before and after despecialization