OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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revealed that none of the constraints were violated in practice. An analysis was
also presented that showed that despecialization did not impact the correctness of
exceptions or synchronization or have a negative impact on the ability to perform
verification.
The initial despecialization study was followed by a second study that was more
comprehensive. While the first study considered seven benchmarks executed using
three virtual machines running on a single platform, the second study considered
20 benchmarks running on a total of eight virtual machine / computing platform
combinations.
The second study also considered grouping the despecializations in a different
manner. In the first study, groups of despecializations were selected based on the
similarity of their general purpose forms. The second study used profiling in order
to determine the frequency of execution of each specialized bytecode. The despecializations
were then performed from least frequently executed to most frequently
executed. This system was used with the expectation that it would minimize the
performance impact of the first despecializations performed. Measuring the performance
of the applications confirmed that this was typically the case with the last few
despecializations showing the majority of the performance loss.
The overall results from the second study confirmed what was found in the first.
When 62 specialized bytecodes were replaced with their equivalent general purpose
forms, an average performance loss of 2.0 percent was observed. Performing approximately
half of the despecializations showed a performance loss of only 0.6 percent.
Evaluating the impact despecialization had on class file size showed that performing
62 despecializations increased average class file size by approximately 3.5 percent
without violating any of the constraints imposed by the Java Virtual Machine Specification.
There are several reasons why specialized bytecodes have little impact on application
performance. These include the fact that many modern virtual machines
make use of a just-in-time compiler and the fact that some specialized bytecodes are
executed extremely infrequently. A just-in-time compiler reduces the performance impact
of specialized bytecodes because it is often possible to generate identical machine
code during the compilation process for both a specialized bytecodes and its equivalent
general purpose form. Thus, despecialization only impacts the performance of
the application up until the point when just-in-time compilation is used.
Infrequently executed specialized bytecodes also have little impact by their very
nature. The difference in performance between the specialized and equivalent general