Which evaluation order is suitable for evaluating L-attributed Syntax Directed Definitions during parsing?
L-attributed definitions are designed so attributes can be evaluated in a single depth-first, left-to-right traversal (matching LL parsing or bottom-up LR with embedded actions).
Consider the SDD:
E -> E1 + T { E.val = E1.val + T.val }
E -> T { E.val = T.val }
T -> num { T.val = num.lexval }
What type of attribute is 'val'?The value of E.val is computed from its children E1.val and T.val, making 'val' a synthesized attribute.
What is a Dependency Graph in the context of Syntax Directed Translation?
A dependency graph depicts dependencies among attribute instances in a parse tree. A topological sort of the dependency graph gives a valid evaluation order.
If a Syntax Directed Definition (SDD) contains a cycle in its Dependency Graph for a given parse tree, what does it imply?
A cycle in a dependency graph implies a circular dependency, meaning no topological sort exists and the attributes cannot be evaluated.
Consider the grammar production A -> B C with semantic action { B.$i = A.i + 1$; C.$i = B.s + 1$; A.$s = C.s$; }. Assume 'i' denotes inherited and 's' denotes synthesized. Is this SDD L-attributed?
In L-attributed definitions, inherited attributes can depend on parent attributes (A.i) and attributes of left siblings (B.s). Here B is to the left of C, so C.i depending on B.s is perfectly valid for L-attributed definitions.