Introduction to Syntax Analysis in Compiler Design

Syntax Analysis (also known as parsing) is the step after Lexical Analysis. The Lexical analysis breaks source code into tokens.

• Tokens are inputs for Syntax Analysis.
• The goal of Syntax Analysis is to interpret the meaning of these tokens.
• It checks whether the tokens produced by the lexical analyzer are arranged according to the language's grammar.
• The syntax analyzer attempts to build a Parse Tree or Abstract Syntax Tree (AST), which represents the program's structure.

Concepts for Syntax Analysis in Compiler Design

In syntax analysis, the following key concepts help in understanding and verifying the structure of the source code.

1. Context-Free Grammars (CFG)

Context-Free Grammars define the syntax rules of a programming language. They consist of production rules that describe how valid strings (sequences of tokens) are formed. CFGs are used to specify the grammar of the language, ensuring that the source code adheres to the language's syntax.

2. Derivations

A derivation is the process of applying the rules of a Context-Free Grammar to generate a sequence of tokens, ultimately forming a valid structure. It helps in constructing a parse tree, which represents the syntactic structure of the source code.

3. Concrete and Abstract Syntax Trees

• Concrete Syntax Tree (CST): Represents the full syntactic structure of the source code, including every detail of the grammar.
• Abstract Syntax Tree (AST): A simplified version of the CST, focusing on the essential elements and removing redundant syntax to make it easier for further processing.

4. Ambiguity

Ambiguity occurs when a grammar allows multiple interpretations for the same string of tokens. This can lead to errors or inconsistencies in parsing, making it essential to avoid ambiguous grammar in programming languages.

These formalisms are crucial for performing accurate syntax analysis and ensuring that the source code follows the correct grammatical structure.

Features of Syntax Analysis

Syntax Trees: Syntax analysis creates a syntax tree, which is a hierarchical representation of the code's structure. The tree shows the relationship between the various parts of the code, including statements, expressions, and operators.

Context-Free Grammar: Syntax analysis uses context-free grammar to define the syntax of the programming language. Context-free grammar is a formal language used to describe the structure of programming languages.

Top-Down and Bottom-Up Parsing: Syntax analysis can be performed using two main approaches: top-down parsing and bottom-up parsing. Top-down parsing starts from the highest level of the syntax tree and works its way down, while bottom-up parsing starts from the lowest level and works its way up.

Error Detection: Syntax analysis is responsible for detecting syntax errors in the code. If the code does not conform to the rules of the programming language, the parser will report an error and halt the compilation process.

Intermediate Code Generation: Syntax analysis generates an intermediate representation of the code, which is used by the subsequent phases of the compiler. The intermediate representation is usually a more abstract form of the code, which is easier to work with than the original source code.

Optimization: Syntax analysis can perform basic optimizations on the code, such as removing redundant code and simplifying expressions.

Context-Free Grammar (CFG)

A Context-Free Grammar (CFG) offers a powerful way to define languages, overcoming the limitations of regular expressions. Unlike regular expressions, CFGs can handle complex structures, such as:

• Properly balanced parentheses.
• Functions with nested block structures.

CFGs define context-free languages, which are a strict superset of regular languages. They use production rules to describe how symbols in a language can be replaced, allowing for more flexibility in defining programming language syntax. This makes CFGs ideal for representing the grammar of programming languages.

Parse Tree

A parse tree, also known as a syntax tree, is a tree structure that represents the syntactic structure of a string according to a given Context-Free Grammar (CFG). It shows how a particular string can be derived from the start symbol of the grammar using its production rules.

• The root of the tree represents the start symbol of the grammar.
• Internal nodes represent non-terminal symbols, which are expanded according to the production rules.
• Leaf nodes represent terminal symbols, which are the actual tokens from the input string.

Example: Suppose Production rules for the Grammar of a language are:

  S -> cAd
  A -> bc|a
  And the input string is “cad”.

Now the parser attempts to construct a syntax tree from this grammar for the given input string. It uses the given production rules and applies those as needed to generate the string. To generate string “cad” it uses the rules as shown in the given diagram:

In step (iii) above, the production rule A->bc was not a suitable one to apply (because the string produced is “cbcd” not “cad”), here the parser needs to backtrack, and apply the next production rule available with A which is shown in step (iv), and the string “cad” is produced.

Thus, the given input can be produced by the given grammar, therefore the input is correct in syntax. But backtrack was needed to get the correct syntax tree, which is really a complex process to implement.

Steps in Syntax Analysis Phase

The Syntax Analysis phase, also known as parsing, is a crucial step in the compilation process where the structure of the source code is verified according to the grammar of the programming language.

• Parsing: The tokens are analyzed according to the grammar rules of the programming language, and a parse tree or AST is constructed that represents the hierarchical structure of the program.
• Error handling: If the input program contains syntax errors, the syntax analyzer detects and reports them to the user, along with an indication of where the error occurred.
• Symbol table creation: The syntax analyzer creates a symbol table, which is a data structure that stores information about the identifiers used in the program, such as their type, scope, and location.