OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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Because despecialization increases the size of the code attribute, it was necessary to
update the start pc and end pc values so as to guarantee that any of the general
purpose bytecodes generated for a specialized bytecode are within the range of each
exception handler if and only if the original specialized bytecode was within the range
of the exception handler. Furthermore, it was also necessary to update handler pc
in order to ensure that control is transferred to the correct location in the event that
an exception is caught.
Performing despecialization does not inhibit verification. It has already been
noted previously that, in practice, despecialization does not violate any of the size
constraints imposed by the Java Virtual Machine Specification. Consequently, verification
is performed on the larger class files. Since the despecialized code attributes
are semantically equivalent to their original specialized forms, a despecialized code
attribute will pass verification if and only if the original specialized form passed verification.
Similarly, a code attribute that failed verification before despecialization
will still fail verification after despecialization is performed.
4.5 Summary
This chapter has described the difference between general purpose bytecodes and
specialized bytecodes. A total of 76 specialized bytecodes were identified along with
their corresponding general purpose forms. Of these, 67 were implemented and tested
using three Java virtual machines. Performance results were presented for the SPEC
JVM98 Benchmark Suite. These results showed that an average increase of 2.0 percent
was observed across all three virtual machines when complete despecialization
was performed on all of the benchmarks. An analysis was also performed on despecialization’s
impact on class file size. It was found that despecialization had a
similarly small impact on class file size, increasing average class file size across all of
the library and application classes by between 5.3 and 6.3 percent depending on the
set of classes under consideration.
While there are minor performance and class file size penalties associated with
performing despecialization, a large benefit is achieved: the number of Java bytecodes
in use is reduced from 201 to 134. This opens up many opportunities for expanding
the bytecode language. New instructions could include:
• Supporting frequently occurring operations that require several bytecodes to
perform. The task of incrementing a field by a small integer is a particularly
good example, requiring six bytecodes to perform.