Example: translate expression

(a*b + b*c + a*c) * 2

into posftix form.

Tree:

        * 
     +     2
  *     +
 a b  *   *
     b c a c

Postfix expression:

a b * b c * + a c * + 2 *

Translated expression:

  iload slot_a
  iload slot_b
  imul
  iload slot_b
  iload slot_c
  imul
  iadd
  iload slot_a
  iload slot_c
  imul
  iadd
  iconst_2 
  imul

Compare to cubeArea in Compiled Expression Examples

In compiled code what corresponds to:

• '+'
• '*'
• variables, like a,b,c
• constants, like 2

Conclusion: Translating expressions to stack machine is like printing expression in postfix form.

For constant ,

The iconst is just special case of bipush

Translation for operations:

and similarly for other operations.

• we will sometimes omit List(…) when it is clear from the context

Intuition: to evaluate e1 * e2, an interpreter would

• evaluate e1
• evaluate e2
• perform operation on these two values

Compiled code should do the same

• generate code that will evaluate e1 and store it somewhere
• generate code that will evaluate e2 and store it somewhere else
• retrieve these two values and perform the operation on them

In stack machine, “somewhere” is an appropriate part of the stack

• we need to know that “somewhere” and “somewhere else” are different
• what would happen if compile code “ate” stack before and removed previous value?

Properties of generated code for expression:

• do not erase anything that was on the stack
• increase stack exactly by one, and leave the result of the expression on the stack