OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

23
Figure 2.6: Interpreter Engine Constructed Using Goto Statements
formed. The arcs are used to illustrate the branches performed in order to execute
the aload 0 bytecode.
2.4.3 Indirect Threading
It is still possible to reduce the number of branches performed. This is accomplished
by using a technique referred to as threading. The general idea behind threading
techniques is to remove the need to branch back to the top of the loop between each
bytecode. Instead, the the flow of control transfers directly from the codelet for one
bytecode to the codelet for the next bytecode, eliminating the need to branch back
to the decode operation operation at the top of the loop.
Indirect threading [41] accomplishes this task using a special array and goto statements.
Each element in the array is initialized to the address of a codelet. One element
is present in the array for each bytecode defined by the Java Virtual Machine
Specification. Once the array is initialized, a bytecode can be decoded by loading the
address of its codelet from the array and branching to it using a goto statement.
Indirect threading completely eliminates the need for the switch statement. Instead,
a small amount of extra code is added at the end of each of the codelets.