Types of SDT

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This lecture segment provides a detailed comparison between S-Attributed and L-Attributes Syntax Directed Translation (SDT) schemes. The instructor utilizes a structured table to outline key differences regarding attribute types, semantic action placement, and evaluation mechanisms. S-Attributed SDT is characterized by the exclusive use of synthesized attributes, whereas L-Attributes SDT incorporates both inherited and synthesized attributes. The lecture clarifies that while S-Attributed SDT is evaluated during Bottom-Up Parsing, L-Attributes SDT requires a depth-first left-to-right traversal of the parse tree.

Chapters

1. 0:00 – 2:00 00:00-02:00

   The instructor presents a side-by-side comparison table for S-Attributed and L-Attributes SDT. He underlines the first row, noting S-Attributed uses only synthesized attributes, while L-Attributes uses both inherited and synthesized attributes. He specifically underlines the restriction for L-Attributes: inherited attributes must inherit from either the parent or the left sibling only. Moving to the third row, he explains semantic action placement. For S-Attributed, actions are at the extreme right end of the production. For L-Attributes, they can be placed anywhere on the right-hand side. He writes alpha -> a b [S] c on the screen to visualize this placement. Finally, he notes that S-Attributed attributes are evaluated during Bottom-Up Parsing (BUP).

2. 2:00 – 2:45 02:00-02:45

   The discussion moves to the evaluation strategy for L-Attributes SDT. The instructor underlines the text: Attributes are evaluated by traversing parse tree depth first left to right. To clarify this, he draws a triangular diagram representing a parse tree. He sketches lines across the nodes of the triangle to demonstrate the traversal order. He emphasizes that unlike S-Attributed SDT which uses BUP, L-Attributes requires a specific depth-first traversal. He crosses out parts of his drawing to indicate the sequence of evaluation, reinforcing the concept that inherited attributes must be available before the node is processed.

The core distinction lies in attribute dependency and evaluation order. S-Attributed SDT is restricted to synthesized attributes evaluated bottom-up, making it suitable for simpler translations. In contrast, L-Attributes SDT allows inherited attributes but restricts their source to parents or left siblings, necessitating a depth-first traversal. This flexibility allows for more complex semantic analysis while maintaining a predictable evaluation order.