Translating Expressions to Stack Machine
Connection to Postfix
Example: translate expression
(a*b + b*c + a*c) * 2
into posftix form.
* + 2 * + a b * * b c a c
a b * b c * + a c * + 2 *
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