OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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technique still fundamentally follows the same model of execution, repeatedly fetching,
decoding and executing each bytecode as a discrete item 1 . Multicodes change
this model of execution, making it possible to perform the tasks of several bytecodes
together, reducing the total number of transfers of control that are performed during
the execution of the application. This is accomplished by identifying frequently
executed bytecode sequences and then introducing a new multicode that provides
identical functionality.
The newly created multicode is assigned a unique opcode that is not currently used
by the Java Virtual Machine Specification. There are 52 opcodes that fit this criteria.
This count includes those opcodes numbered 203 to 253, for which no functionality
is defined. They will be represented by the mnemonics o203 through o253. It also
includes opcode 186, which has the mnemonic xxxunusedxxx. The Java Virtual
Machine Specification indicates that this bytecode is unused for historical reasons.
Two additional bytecodes are also potentially available. These opcodes are numbered
254 and 255 and have the mnemonics impdep1 and impdep2 respectively. The purpose
of these bytecodes is to provide traps for accessing implementation specific features
that are present in software and hardware respectively. Sun guarantees that these
implementation specific bytecodes will continue to be available even if the Java Virtual
Machine Specification is revised.
The definition of a multicode also includes a description of its operands. The
number of operand bytes is the total number of operand bytes from all of the bytecodes
represented by the multicode. These operand bytes follow the multicode’s opcode
in the same order as they were present in the original sequence. The creation of
the multicode for the bytecode sequence aload getfield is shown graphically in
Figure 7.1. Recall that aload copies a reference to an object from a local variable to
the operand stack. A single operand byte follows the opcode, which specifies which
local variable is copied. The getfield bytecode pops an object reference from the
operand stack, and pushes the value of a field from the object pointed to by the object
reference. Two operand bytes follow the getfield opcode which specify what field
will be accessed.
Performing a multicode substitution decreases the total size of the code stream.
This occurs because only one opcode is present to represent the functionality of the
sequence when a multicode is employed. In comparison, a sequence of n bytecodes
1 The lone exception to this pattern occurs when selective inlining is used in conjunction with
direct threading.