OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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4.2.6 Performance Summary
Comparing the results of the three benchmarks tested in this study reveals that the
Sun virtual machine was impacted the least by despecialization. The Kaffe virtual
machine showed the largest performance loss as a result of despecialization, leaving
IBM’s research virtual machine in the middle position. When the average performance
across all of the benchmarks was considered, each of the virtual machines
showed showed a performance loss of less than 2.4 percent. Computing the average
performance across all of the virtual machines and benchmarks revealed an overall
increase in runtime of 2.0 percent. These changes in performance are minor. Using
67 bytecodes (one third of those defined by the Java Virtual Machine Specification)
to achieve this difference is wasteful when those bytecodes can be utilized for other
purposes.
The largest single change in benchmark performance occurred when 227 mtrt
was executed using IBM’s RVM under the complete despecialization condition. This
performance loss of almost 12.7 percent was considerably larger than the largest losses
observed for the other two virtual machines. Note that this change in performance
is the direct result of performing branch despecialization. The introduction of a new
optimization into the IBM virtual machine to handle the branching patterns that occur
in the despecialized bytecode stream would likely result in a considerably smaller
maximum performance loss. Consequently, examining the maximum performance
losses also leads to the previous conclusion.
4.3 Despecialization and Class File Size
The use of specialized bytecodes impacts more than application runtime. Another
result that must be considered is the impact despecialization has on class file size.
Each of the despecializations discussed previously replaces a specialized bytecode with
one or more general purpose bytecodes. In all cases except for the despecialization of
bipush and sipush into ldc or ldc w, the general purpose bytecode or sequence is at
least one byte larger than the original specialized bytecode. This may be the result of
the fact that an operand value is now required in order to specify the position to be
accessed, the fact that the same operand value has been widened to occupy additional
bytes or the fact that additional opcodes reside in the code stream.
It is possible for some occurrences of bipush to be despecialized without increasing
class file size. This can occur when the value being generated already resides in the