OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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Removing these unnecessary writes is accomplished by performing copy propagation
to eliminate uses of memory locations above the top of the stack on the right
hand side of assignments. Performing replacements where the value being propagated
consists of only a single value results in the following codelet:
// iload_1:
stack[sp+1] = local_vars[1];
// iconst_1:
stack[sp+2] = 1;
// iadd:
stack[sp+1] = local_vars[1] + 1;
// istore_2:
local_vars[2] = stack[sp+1];
Further uses of stack locations on the right side of assignment operations can be
eliminated when complete expressions are propagated rather than only single values.
Care needs to be used to ensure that performing such a propagation does not result
in recomputing the same expression on several occasions. However, in this instance,
propagating local vars[1] + 1 as the value of stack[sp+1] will still only require
its value to be computed once when all of the optimizations have been performed.
// iload_1:
stack[sp+1] = local_vars[1];
// iconst_1:
stack[sp+2] = 1;
// iadd:
stack[sp+1] = local_vars[1] + 1;
// istore_2:
local_vars[2] = local_vars[1] + 1;
Once these copy propagations have been performed, unnecessary assignments can
be removed. In general, an assignment is unnecessary if the memory location on the
left side of the assignment is known conclusively to be written to before it is read
again. This is the case for the first 3 assignments in the preceding codelet because they
each write to a memory location that is above the top of the stack when the codelet
finishes executing. Consequently, the codelet required to implement the multicode
iload 1 iconst 1 iadd istore 2 is reduced to a single assignment.