OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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Figure 10.1: Multicode Blocks under the Original and Proposed Future Definition
• The bytecodes within the sequence are adjacent to each other within the code
stream.
• A bytecode within the sequence is the target of a branch if and only if it is the
first bytecode in the sequence.
• Each bytecode within the sequence will be executed zero or one times. If bytecode
i will not be executed then bytecode i + j will not execute for j = 1..n − i.
• An exception handler covers a bytecode within the sequence if and only if that
exception handler covers all bytecodes within the sequence.
Under the revised definition, it is possible that only some initial bytecodes within
the multicode will execute. The advantage of such a definition is that it would allow
conditional branch bytecodes to exist within a multicode at a point other than the
last bytecode. Consequently, the bytecode sequence would have a single point of entry
but many potential points of exit. This is illustrated if Figure 10.1 which shows the
multicodes blocks that will be identified under the original multicode block definition
and the revised multicode block definition.
Care must be used when performing optimizations using the revised multicode
block definition. Because there is no guarantee that every bytecode within the sequence
will execute, it is necessary to ensure that the optimizer does not move any
code across branch points which changes the semantics of the multicode when the
branch is taken. Otherwise the multicode will provide different functionality than the
original bytecode sequence, resulting in incorrect application behaviour.