OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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approximately 30 percent of the size of a typical class file is used to represent methods
[14]. However, this includes the bytecodes themselves as well as additional attributes
such as the line number table and local variable table. Since the size of a multicode
attribute is at most the same size as the original code attribute, it is known that
performing this modification will increase class file size by 30 percent at most. In
practice, the increase can be smaller because it is not necessary to duplicate the local
variable table. The line number table can also be omitted for the multicode attribute
if debugging is performed on the original unoptimized bytecode stream instead of the
multicode stream.
The impact on class file size can also be reduced by providing only a single code
attribute for those methods that do not make use of any multicodes. This will require
a small amount of additional work to be performed at class load time. In particular, it
will be necessary for the multicode enabled virtual machine to check for the presence
of a multicode attribute. In the event that the multicode attribute is not present,
it will need to treat the code attribute as if it were the multicode attribute. Since
each class is only normally loaded once during the execution of the application, the
overhead imposed by performing this check should be negligible.
10.2.3 Dynamic Virtual Machine Modification
The multicode substitution process requires the Java Virtual Machine to be modified
in order to support the newly defined multicodes. In the present implementation,
these modifications are performed ahead of time, resulting in a virtual machine that
is only capable of handling one set of multicodes. The virtual machine must be
recompiled in order to change the set of multicodes that are supported. Ideally, the
virtual machine should be capable of handling many sets of multicodes. There are
several options for how this goal could be accomplished.
Standard Multicode Sets
One option would be to develop several standard sets of multicodes. Each set would
attempt to address a specific type of class. For example, one set would contain
multicodes that primarily conduct integer operations while a second set would include
multicodes that handle common array-oriented bytecode patterns. Further sets would
address other data types and program styles.
Multiple execution engines will exist within the virtual machine in order to support
the standard sets of multicodes. The execution engine that will be employed for any