TransformationRuleGuide

Nonterminal Transformation Rules

Let's think about a set of rules for working with integer constants and expressions. We can reduce an INT_LITERAL node to intConstant:

intConstant = INT_LITERAL(void): 1

And we can then use that rule to build up a pattern that can accept an intConstant rewrite of its corresponding child; but suppose we are adding x + 100000 and we can't use a addi instruction? We need to be able to use the INT_LITERAL node as an intExpression. One way to do that would be to define another pattern match rule:

intExpression = INT_LITERAL(void): 2

But this duplication is going to become a maintenance problem very quickly, especially since it'll need to be done for every arithmetic operation that has similar variants.

JBurg provides a facility to continue transforming an already-reduced value, a nonterminal-to-nonterminal transformation. There are two varieties of these transformational rules.

Simple Transformational Rule (Chain Rule)

The simple form of a transformational rule allows one nonterminal state to alias another:

generalExpression = intExpression;

This form of transformational rule is also called a chain rule in the literature. As the example suggests, it's useful to group various other forms of rules.

General Transformational Rule

The second form of transformational rule is more powerful. It defines a generalized conversion from one nonterminal state to another:

intExpression = intConstant: 1
// reducer code to implement conversion, e.g., issue li instruction