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10.2.9 Expanding the Class File Mutator’s Capabilities
The class file mutator currently supports two primary rule types. Despecialization
rules replace all occurrences of a specialized bytecode with its equivalent general purpose
representation. Substitution rules replace occurrences of a bytecode sequence
with the target bytecode indicated within the rule. In each case, the mutator reconstructs
all of the branch targets and exception ranges in order to handle the fact that
each of these rule types may change the size of the code attribute.
At the present time there are three bytecodes which are not supported in substitution
rules. These are lookupswitch, tableswitch and wide. Each of these
bytecodes is variable in size, making them more difficult to work with than the other
bytecodes. However, there is nothing that makes it impossible to include them within
a substitution rule. As a result, future work should extend the implementation of the
class file mutator so that applications that make heavy use of these bytecodes can be
handled optimally.
A related extension should also be considered. At the present time, the target
of each substitution rule must be an opcode from o203 through o253. However,
when substitution is used in conjunction with despecialization, opcodes that are despecialized
are potentially available as the targets of additional substitution rules.
If 67 despecializations were performed, making this modification would allow a total
of 119 multicodes to be implemented without making extensive changes to the
implementation of the Java Virtual Machine.
10.3 Conclusion
This chapter has identified several areas of future work. Some of the areas presented
are implementation issues that need to be resolved in order for applications to take
full advantage of multicodes. Other ideas presented in this section represent areas
of interesting future research that will lead to additional valuable results. The author
believes that exploring issues related to dynamic virtual machine or hardware
modification is particularly likely to reveal interesting research problems.