OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

105
these despecializations in a method that contains a lookupswitch or a tableswitch
bytecode can result in a class file size change of anywhere from zero to three bytes.
Examining Figure 5.12 further revealed that 52 despecializations must be performed
before average class file size increases by one percent. The remaining change
in average class file size, approximately 2.5 percent, is the result of performing the
final 10 despecializations listed in Table 5.1.
The overall impact on average class file size when 62 despecializations were performed
was an increase of approximately 3.5 percent. An increase of this magnitude
is a minor concern at most. Class file compression is an area of active research.
Techniques have been developed which easily overcome an increase of this magnitude
[61, 68, 94]. Consequently, it is concluded that despecialization has only a minor
impact on class file size which is not sufficient to justify the use of 62 Java bytecodes.
5.4 Generality of Despecialization
The impact performing despecialization has on the correctness of class files was discussed
in detail in Section 4.4. That discussion is not duplicated here. It is simply
pointed out that none of the concerns addressed previously, including the limits imposed
by the Java Virtual Machine Specification, either implicitly or explicitly, were
violated during the despecialization process. Consequently, it is concluded that specialized
bytecodes are not required in order to maintain the correctness of the Java
bytecode language or the Java Virtual Machine.
5.5 Conclusion
This chapter has presented an extension to the initial despecialization study discussed
in Chapter 4. The results presented here cover 20 benchmarks executed on a total
of 8 distinct virtual machine / hardware architectures. The results were consistent
with those presented previously, showing minor changes in application performance
and class file size. When approximately half of the despecializations under consideration
were performed, the observed change in application performance was a loss of
approximately 0.6 percent. This was accompanied by an increase in class file size of
approximately 0.2 percent.
Performing additional despecializations had a larger impact on application performance.
When all 62 despecializations were utilized, average application performance
slowed by approximately 2.0 percent. The largest average increase observed for any