OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

15
...
aload 3 iconst 2
faload
...
i − 1
i i + 1 i + 2
i + 3
Figure 2.3: A Code Stream Showing An Array Element Access
dup dup x1 dup x2
dup2 dup2 x1 dup2 x2
pop pop2 swap
Table 2.5: Stack Manipulation Bytecodes
copy a pointer to the array onto the operand stack. The second bytecode utilized will
be iconst 2. It will put the constant integer 2 onto the operand stack, specifying
what index will be accessed. Finally, a faload bytecode will be used in order to pop
the array pointer and index from the operand stack and replace them with a copy of
the value stored at index 2 of the array.
2.3.4 Stack Manipulation
Most Java bytecodes are only permitted to access elements that are at the top of
the operand stack. Occasionally, this restriction presents difficulties. Consequently,
there are several bytecodes that are able to reorder stack values or duplicate values
“trapped” under other operands on the stack. One such bytecode is swap, which
reverses the order of the top two elements on the operand stack. Other bytecodes are
capable of duplicating one or two elements that are located somewhere on the stack.
The dup bytecode is the simplest of these, replicating the top element on the stack.
Other variants of dup replicate elements from deeper within the stack. Each of the 9
stack manipulation bytecodes are listed in Table 2.5.
2.3.5 Arithmetic Operations
Of the 201 bytecodes defined by the Java Virtual Machine Specification, 36 are devoted
to arithmetic operations. These bytecodes are listed in Table 2.6. All bytecodes
that perform arithmetic operations are typed. Their names begin with one of the
characters listed in Table 2.2, which indicates the type of data that the operation
is performed on. The values used as input to these arithmetic operations are read