JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III PRINCIPLES OF COMPILER CONSTRUCTION Previous Question Paper (August/September - 2021 - R18)

A compiler is a program that translates source code into machine code through several distinct phases. For the statement position = initial + rate * 60, the phases are:

1. Lexical Analysis: Reads the source code and converts it into a stream of tokens. Output: <id, 1> <=> <id, 2> <+> <id, 3> <*> <60>
2. Syntax Analysis: Groups tokens into a hierarchical structure, typically a parse tree. Output: A tree with an assignment node at the root, with 'position' on the left and an expression tree on the right.
3. Semantic Analysis: Checks for semantic consistency, such as type checking. Output: The tree is updated, for example, by inserting a conversion node to change the integer 60 to a float.
4. Intermediate Code Generation: Produces a machine-independent intermediate representation. Output: t1 = inttofloat(60); t2 = id3 * t1; t3 = id2 + t2; id1 = t3
5. Code Optimization: Improves the intermediate code for efficiency. Output: t1 = id3 * 60.0; id1 = id2 + t1
6. Code Generation: Translates the optimized code into target machine code. Output: MOVF id3, R2; MULF #60.0, R2; ADDF id2, R2; MOVF R2, id1

The Symbol Table Manager and Error Handler interact with all phases throughout the process.

The specification of tokens involves defining the patterns that identify valid lexemes in a source language. This is formally achieved using Regular Expressions, which provide a concise notation for describing sets of strings.

Key aspects include:

• Alphabet: A finite set of symbols (e.g., ASCII characters).
• Strings and Languages: Sequences of symbols from the alphabet; a language is a set of such strings.
• Regular Definitions: Used to simplify complex patterns by giving names to regular expressions (e.g., defining 'letter' as [a-zA-Z] and 'digit' as [0-9]).
• Token Patterns: Specific rules such as identifier = letter(letter|digit)* or number = digit+(.digit+)?(E(+|-)?digit+)?.

These specifications are used by the lexical analyzer generator (like Lex or Flex) to construct a deterministic finite automaton (DFA) that recognizes the tokens during the scanning process.