Syntax-Directed Definitions and Attributes

In the field of compiler design, syntax-directed definitions and attributes play a crucial role in building efficient and reliable compilers. These definitions and attributes allow us to associate semantic actions and attributes with specific parts of the source code during the compilation process. This article explores the concept of syntax-directed definitions and attributes, their significance, and their application in building a compiler.

Understanding Syntax-Directed Definitions

Syntax-directed definitions (SDDs) serve as a formal way to specify the translation of syntactic constructs into executable code. They define the translation of each element in the programming language's grammar rules and associate semantic actions with the production rules of a context-free grammar. These semantic actions can be instructions to perform calculations, store values in memory, or any other operations required during compilation.

The main idea behind SDDs is to define a correspondence between the production rules in the grammar and the high-level language statements that carry out the desired translation. This correspondence is established by associating attributes with the symbols in the grammar, which hold information about the computation or translation performed by the production rule.

Attributes in Syntax-Directed Definitions

Attributes in SDDs are used to carry information from one production rule to another. They provide a way to propagate values throughout the syntax tree, enabling the sharing of data between different parts of the program. In other words, attributes allow us to evaluate and store information corresponding to the non-terminal symbols or terminals in the grammar.

There are two types of attributes commonly used:

  1. Synthesized Attributes: These attributes are synthesized by each non-terminal symbol in the production rule based on the attributes of its child symbols. The synthesized attribute is computed by applying a semantic rule and is typically associated with the non-terminal symbol on the left-hand side of the production rule.

  2. Inherited Attributes: Inherited attributes are passed to non-terminal symbols from their parent symbols. They allow the parent symbol to communicate relevant information to its child symbols. Inherited attributes are usually associated with the non-terminal symbol on the right-hand side of the production rule.

By combining synthesized and inherited attributes, we can establish a bidirectional flow of information within the syntax tree, enabling complex computations and transformations during the compilation process.

Applications in Compiler Design

Syntax-directed definitions and attributes are extensively used in various stages of the compiler, such as lexical analysis, parsing, and semantic analysis.

During lexical analysis, attributes can be used to store information about token values, line numbers, or other metadata associated with the source code. This information can later be utilized in error reporting or generating meaningful compiler messages.

In parsing, attributes play a crucial role in constructing the parse tree and ensuring the correct order of operations. By associating attributes with grammar symbols, we can propagate information and compute values needed for further analysis or code generation.

In semantic analysis, syntax-directed attributes aid in type-checking, symbol-table management, and generating intermediate representations such as abstract syntax trees (ASTs) or three-address code. These attributes help enforce language constraints, perform static checks, and annotate the AST with semantic information for subsequent code generation steps.

Conclusion

Syntax-directed definitions and attributes provide a powerful mechanism for associating semantic actions and attributes with different parts of a programming language's grammar. By utilizing synthesized and inherited attributes, compilers can perform various computations, propagate information, and transform the source code during compilation. Understanding and effectively utilizing syntax-directed definitions and attributes are key components in designing efficient and reliable compilers.

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