OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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Figure 7.22: Overall Performance by Number of Multicode Substitutions Performed
for Transfer Reduction Scoring with Multicodes up to Length 25
is actually executed when the application is run. In this case, some of the multicodes
being introduced will never be executed by some of benchmarks even though they are
of value to others.
The costs associated with new multicodes come in two forms: performance costs
and software engineering costs. Introducing new multicodes has a performance cost
because their introduction into the main execution loop of the interpreter increases
the size of the loop, both with respect to the number of cases that it contains and
the amount of memory it occupies.
Increasing the number of cases in the loop can slow the process of branching to
the codelet that implements the bytecode that is about to execute. Such a slow
down is possible when the compiler generates a switch statement using a binary
search strategy because it may be necessary to perform more comparisons before the
correct target is identified. A slow down can also occur when the switch statement
is implemented using a lookup table. In this case, the slow down can occur because
the size of the lookup table has increased, making it more likely that an entry will be
requested that does not reside within the data cache.
Increasing the total amount of memory required to hold the interpreter’s main loop
can negatively impact performance regardless of how the interpreter is implemented.
The amount of space available in the instruction cache is limited. Increasing the size
of the interpreter’s main loop increases pressure on the instruction cache, because