OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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Chapter 8
Multicodes and Despecialization
In many respects, despecialization and multicode substitution are competitors. Despecialization
attempts to minimize the number of bytecodes in the Java instruction
set while multicode substitution introduces new instructions. Despecialization increases
the size of the bytecode stream due to the need for additional opcodes and
operands to represent the program. In contrast, multicodes reduce the size of the
bytecode stream by reducing the number of opcodes used to represent the program.
Performing despecialization typically results in a performance loss, while performing
multicode substitution improves the performance of the applications it is applied to.
This chapters takes the notions presented in Chapters 4, 5 and 7 and brings them
together, testing benchmarks when both despecialization and multicode substitution
have been performed. The impact that bringing these techniques together has on
multicode identification is considered and performance results are presented for the
benchmarks in the SPEC JVM98 Benchmark Suite.
8.1 Multicode Identification in the Presence of Despecialization
The previous chapter presented an algorithm for identifying a set of multicodes that
would have a beneficial impact on execution performance. The algorithm relies on
profiling an application, or collection of applications, to obtain a dynamic stream of
bytecodes, divided into multicode blocks. Each contiguous sub-sequence of bytecodes
within a multicode block is a candidate sequence, and the number of times each
candidate sequence occurs is accumulated.
When an application is despecialized, the dynamic sequence of bytecodes gener-