OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

13
...
i − 1
iload 0x04 dstore 0x07
i i + 1 i + 2 i + 3
...
i + 5
Figure 2.2: A Code Stream Containing a Load Bytecode and a Store Bytecode
the bytecodes occur at an arbitrary location within the code stream. This notation
will be used to represent code streams graphically throughout the remainder of this
document.
Four additional load and four additional store bytecodes are also provided for each
data type. Each of these bytecodes is only able to access one specific element within
the local variable array. Consequently, these bytecodes do not require an operand
because the slot which is accessed is fully determined by the opcode alone. The
names of these opcodes specify the position that is accessed as the last character
in the opcode name. These bytecodes follow the naming pattern load or
store where is one of the characters listed in Table 2.2 and is
an integer between 0 and 3. For example, the bytecode that would load local variable
0 onto the stack as an object reference is aload 0 while the bytecode istore 3 is
used to pop an integer from the operand stack and store it into local variable 3.
2.3.2 Constant Loading
Constant loading bytecodes are used to place literal values onto the top of the operand
stack. The Java Virtual Machine Specification devotes 20 bytecodes to this purpose.
They are listed in Table 2.3. Like the load and store bytecodes, different bytecodes
are used in order to work with values of different types.
The Java Virtual Machine Specification provides bytecodes that use three different
techniques in order to place a constant value onto the operand stack. The simplest
aconst null dconst 0 iconst m1 iconst 3
fconst 0 dconst 1 iconst 0 iconst 4
fconst 1 lconst 0 iconst 1 iconst 5
fconst 2 lconst 1 iconst 2 bipush
sipush ldc ldc w ldc2 w
Table 2.3: Constant Loading Bytecodes