OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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// istore_1:
local_vars[1] = stack[sp];
// iload_1:
stack[sp] = local_vars[1];
With the stack adjustments removed, it becomes apparent that the second assignment
is redundant. Generalizing, the first assignment is of the form A = B while
the second is of the form B = A. Since B is a stack location which is only accessible
within the current thread of execution, there is no possibility that the value of B has
changed since it was written by the previous assignment. Consequently, the second
assignment can be removed.
A similar argument holds for the case where B is a local variable. Because local
variables are part of the call stack entry for a method, they are also inaccessible to
other threads of execution. As a result, there is no possibility that the value of B has
been updated in this case either. The codelet representing the multicode istore 1
iload 1 after the redundant assignment has been removed is shown below.
// istore_1:
local_vars[1] = stack[sp];
// iload_1:
Examining the assembly language code generated before and after the redundant
assignment has been removed from the codelet reveals that removing the assignment
results in a shorter assembly language code segment. When the second assignment
is present, the optimizer in g++ is unable to determine that the second assignment is
redundant. Consequently, an extra statement is generated in order to store a value in
a register into the memory location for stack[sp]. The author speculates that this
is the case because g++ is unaware that access to the local array is restricted to the
current thread of execution. Thus the compiler is unable to determine conclusively
that the value of local vars[1] has not changed between the statements.
Unfortunately there are many sequences of bytecodes that cannot be optimized as
much as the examples presented here. Sometimes such sequences do not optimize well
because the adjacent bytecodes have minimal interaction with each other such as the
process of loading several values onto the stack in preparation for invoking a method.
In other cases, the optimization process becomes difficult because the sequence of
bytecodes includes complex operations such as method invocations or field accesses.