OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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from one bytecode to the next in a Java interpreter is denoted by the symbol →.
Consequently, this symbol represents the loop, fetch and decode operations, and the
task of advancing the program counter to refer to the next instruction, shown in the
model of a Java interpreter in Figure 2.4. Using this notation, the code that must be
executed for an arbitrary sequence of Java bytecodes can be expressed as:
x 1 → x 2 → x 3 → ... → x n → (7.1)
When a sequence of bytecodes is replaced by a multicode, the transfers of control
between each bytecode are removed. The codelet for the multicode can be constructed
by merging the codelets of its constituent bytecodes, as long as a small number
of minor adjustments are performed during the concatenation. These adjustments
ensure that the correct operands are read from the code stream when the multicode
executes. The multicode that represents the sequence of bytecodes in Equation 7.1
will be denoted as shown in Equation 7.2.
x 1 /x 2 /x 3 /.../x n → (7.2)
It has already been stated that the amount of time required to execute a multicode
is smaller than the amount of time required to execute the equivalent sequence of Java
bytecodes. This is apparent from Equation 7.1 and Equation 7.2 because the number
of transfers of control in Equation 7.1 is n, while the number of transfers of control in
Equation 7.2 is only 1, regardless of the length of the multicode. Equation 7.3 shows
how the difference in performance will be represented. It uses the function Cost(code)
to denote a measure of how much time is needed to execute ‘code’.
∆ x1 /x 2 /x 3 .../x n→ = Cost(x 1 → x 2 → x 3 → ... → x n →) − Cost(x 1 /x 2 /x 3 .../x n →)
(7.3)
If it assumed that the codelet for the multicode is constructed as a concatenation
of the codelets of the multicode’s constituent bytecodes, then Equation 7.3 reduces
to:
∆ x1 /x 2 /x 3 /...x n/→ = (n − 1) × Cost(→) (7.4)
However, in general, the difference in performance is not that simple because
it is possible to perform optimizations on the concatenated codelet representing the
multicode. These optimizations could not be performed before multicode substitution