Understanding Syntax Analysis in Compiler Design

Ordered, rooted tree that represents the syntactic structure of a string according to some context-free grammar.

Syntax analysis, also known as parsing, is the second phase of the compilation process. It plays a crucial role in transforming the linear sequence of tokens produced by the lexical analyzer into a hierarchical structure, known as a parse tree, which is used by the subsequent phases of the compiler.

Role of the Syntax Analyzer

The syntax analyzer checks the token sequence for syntactic correctness. It uses a formal grammar of the programming language to verify if the sequence of tokens can be generated by that grammar. If the sequence of tokens is syntactically correct, the syntax analyzer generates a parse tree, which represents the syntactic structure of the token sequence. If the sequence of tokens is not syntactically correct, the syntax analyzer generates an error message.

Context-Free Grammars and Syntax Analysis

A context-free grammar (CFG) is a set of production rules that describe all possible strings in a given language. In the context of syntax analysis, the CFG is the formal grammar of the programming language. Each production rule in a CFG has a single non-terminal symbol on the left-hand side and a sequence of terminal and/or non-terminal symbols on the right-hand side.

The syntax analyzer uses the CFG to guide the construction of the parse tree. Each node in the parse tree corresponds to a symbol (terminal or non-terminal) in the CFG, and each edge in the parse tree corresponds to a production rule in the CFG.

Parse Trees and Derivations

A parse tree is a hierarchical representation of the syntactic structure of a token sequence. The root of the parse tree corresponds to the start symbol of the CFG, the leaves of the parse tree correspond to the tokens in the token sequence, and the internal nodes of the parse tree correspond to the non-terminal symbols in the CFG.

A derivation is a sequence of production rule applications that generate a string from the start symbol of the CFG. There are two types of derivations: leftmost and rightmost. A leftmost derivation always applies a production rule to the leftmost non-terminal symbol, and a rightmost derivation always applies a production rule to the rightmost non-terminal symbol.

Designing a Syntax Analyzer

Designing a syntax analyzer involves the following steps:

1. Define the CFG of the programming language.
2. Choose a parsing technique. There are two main types of parsing techniques: top-down and bottom-up. Top-down parsing starts from the root of the parse tree and works its way down to the leaves, while bottom-up parsing starts from the leaves of the parse tree and works its way up to the root.
3. Implement the parser. This can be done manually or using a parser generator, which is a software tool that generates a parser from a CFG.

In conclusion, syntax analysis is a critical phase in the compilation process. It ensures that the token sequence generated by the lexical analyzer is syntactically correct according to the formal grammar of the programming language, and it generates a parse tree that represents the syntactic structure of the token sequence.