OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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Changing the definition of a multicode block in this manner also presents a challenge
for profiling. At the present time, the profiler is able to end the current block as
soon as a branch bytecode is encountered. Under the new system, the profiler would
have to determine if the branch was taken before the block was ended. In the event
that the branch was taken, a new block would begin. Otherwise, bytecodes would
continue to be added to the current block. Consequently, changing the definition
of a multicode block in this manner would only impact the selection of multicodes
for branches that normally “fall through” to the following instruction rather than
branching to another location. If bytecodes continued to be added to the same block
after a branch was taken, the sequence would represent execution from two distinct
points within the file. Attempting to represent code from two distinct locations within
the file with a single opcode would present several large challenges. Whether these
challenges could be overcome, and whether overcoming the challenges would offer any
gains compared to current multicode selection techniques is unknown at the present
time.
10.2.2 Compatibility with Virtual Machines that do not Support
Multicodes
In its present implementation, performing multicode substitution generates a class
file that does not strictly follow the Java Virtual Machine Specification. While the
general structure of the file is identical, the specification dictates that bytecodes that
are not defined by the Java Virtual Machine Specification will not occur in the code
attribute of a legal class files. Consequently, performing multicode substitution results
in illegal class files because they contain such bytecodes.
It is possible to perform multicode substitution while retaining a legal Java class
file. This can be accomplished by introducing a new attribute for each method in
the class which contains a second copy of the code where multicode substitution has
been performed. Because the Java Virtual Machine Specification requires unknown
attributes to be silently ignored, introducing the new attribute has no impact on the
functionality of virtual machines that do not include multicode functionality. Such
virtual machines will ignore the multicode attribute and execute the original code.
However, a virtual machine that supports multicodes will recognize the presence of
the additional attribute and execute it, gaining the benefits multicodes provide.
The cost associated with achieving compatibility with virtual machines that do
not support multicodes relates to class file size. Previous studies have shown that