OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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The aload 0 bytecode was the most frequently executed instruction used for loading
a local variable, representing almost 9.2 percent of the total bytecodes executed.
The large execution frequency for this bytecode is a direct result of the design of
the Java Virtual Machine. When any virtual method is invoked, the Java Virtual
Machine Specification requires that a pointer to the receiver object be passed to the
invoked method. This object reference is always passed as the first parameter. The
Java Virtual Machine Specification also dictates that the values passed as parameters
to methods will reside in the first local variable slots in the called scope. These
conditions guarantee that the receiver object will always reside in slot 0 of the local
variable array.
A reference to the receiver object must be placed on the operand stack before any
field in the object can be accessed or another method can be invoked on it. Both of
these operations are performed frequently in object oriented languages. As a result,
the aload 0 bytecode is executed frequently in order to place the required object
references on the stack.
The next most frequently executed load bytecodes are iload 1 and aload 1 respectively.
In instance methods that take one or more parameters in addition to
the receiver object, these bytecodes reflect the process of loading a copy of the first
parameter onto the operand stack when its type is integer or object reference. If
the method being invoked is static, these bytecodes are used to access the second
parameter when it exists and has the appropriate type.
An additional analysis was performed in order to determine the distribution of
method parameter counts. It is described in more detail in Section 6.2.4. This
analysis includes the receiver object as the first parameter to the method when the
invoked method is in an instance method. It found that approximately 38.4 percent
of the methods invoked have either 0 or 1 parameters. As a consequence, it can be
concluded that aload 1 and iload 1 refer to a programmer declared local variable
rather than a method parameter in over one third of the methods invoked. In the
remaining methods, these bytecodes are being used to load the value of a method
parameter onto the stack.
The remaining 3 specialized bytecodes that appear in the 10 most frequently
executed load bytecodes are iload 3 at position 5, iload 2 at position 7 and aload 2
at position 9. Initially it may appear that these bytecodes are being employed for the
purpose of accessing method parameters. However,the analysis of method parameters
indicates that only approximately 19 percent of the methods invoked have more than 2
parameters. As a result, it is concluded that these bytecodes are generally being used