OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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is replaced with a new opcode if and only if the sequence is a complete basic block.
This restricts the number of replacements that can be conducted because bytecode
sequences that occur frequently within many basic blocks (without forming a complete
basic block themselves) are completely disregarded. The performance results
generated for multicodes are also more extensive, considering the determination of 50
multicodes for each of 6 benchmarks.
3.1.3 Selective Inlining
Selective inlining [90] is a runtime optimization which improves the performance of
Java interpreters that make use of direct threading [19] by reducing the number of
bytecode dispatches performed. Direct threading is key to selective inlining because
interpreters that use this bytecode dispatch technique are able to increase the number
of codelets that the interpreter can elect to execute to an arbitrarily large number.
Other interpretation techniques bound the number of codelets at 256 because opcodes
are represented using a single byte.
In many ways, selective inlining can be viewed as a form of just in time compilation
performed on a basic block level instead a method level. As the application
executes, profiling is used in order to determine which basic blocks are executed with
greatest frequency. Once a candidate basic block is identified, a new codelet is added
to the interpreter. This codelet is formed by performing a simple merger of the implementations
of the bytecodes that make up the basic block. The threaded code
is then updated to make use of the new codelet. This update is performed in place
because it is known that the selectively inlined threaded code stream will be shorter
than the original threaded code.
There are many distinctions between selective inlining and multicode substitution.
Multicode substitution is performed ahead of time using arbitrary bytecode
sequences while selective inlining is performed at runtime using only those sequences
that represent a complete basic block. Selective inlining is also dependent on the use
of direct threading because many basic blocks may be inlined. Multicode substitution
does not rely on a specific bytecode dispatch technique because it only converts those
sequences of bytecodes that have been shown to execute with great frequency into
multicodes.