Syntax-Directed Translation (SDT): Deriving Program Meaning from Syntax
Explore Syntax-Directed Translation (SDT), a compiler technique that extracts program meaning directly from its grammatical structure. This guide explains how SDT uses parse trees and semantic actions to compute values and perform operations, illustrating the process with examples.
Syntax Directed Translation (SDT)
What is Syntax Directed Translation?
Syntax-Directed Translation (SDT) is a compiler technique where the program's meaning is derived directly from its grammatical structure (syntax). It works by building a parse tree (a tree representation of the program's syntax) and performing semantic actions at each node of the tree. These actions calculate values or perform other operations based on the program's structure. The actions are typically performed in a depth-first, left-to-right manner.
How SDT Works
The SDT process involves these steps:
- Parsing: The input program is parsed to create a parse tree.
- Semantic Actions: Semantic rules (actions) are associated with each production rule in the grammar. These rules specify what calculations or operations should be performed at each node in the parse tree.
- Tree Traversal: The parse tree is traversed (usually depth-first and left-to-right). As each node is visited, the associated semantic action is executed.
- Result: The final result of the translation is obtained by combining the results of the semantic actions.
Example: SDT with Arithmetic Expressions
Consider this grammar and its semantic rules:
| Production Rule | Semantic Rule |
|---|---|
S → E $ |
{ print E.VAL } |
E → E + T |
{ E.VAL := E.VAL + T.VAL } |
E → T |
{ E.VAL := T.VAL } |
T → T * F |
{ T.VAL := T.VAL * F.VAL } |
T → F |
{ T.VAL := F.VAL } |
F → (E) |
{ F.VAL := E.VAL } |
F → digit |
{ F.VAL := LEXVAL } (LEXVAL is the value of the digit) |
Let's evaluate the expression "3 + 4 * 2". A parse tree would visually represent this expression's structure. Each node in the parse tree would have a semantic action associated with it. By traversing the parse tree and applying the semantic rules, the compiler would compute the value of the expression step by step.