In Phase 2: Lexical Analysis of compiler construction, the focus is on breaking the source code into a
sequence of tokens that are easier to analyze and process in later phases. Here's an explanation of the
concepts you've mentioned:
1. Difference between Token, Pattern, and Lexeme:
Token:
A token is a category or class of lexemes that share a common structure. It represents a
meaningful unit in the source code. For example, int , + , while , or numeric literals like 10
are tokens. The purpose of tokens is to categorize strings of characters from the source code into
a meaningful group.
Example:
In the code int x = 10; , the tokens would be:
int (keyword)
x (identifier)
= (operator)
10 (literal)
; (punctuation)
Pattern:
A pattern is a rule or regular expression that describes the syntactic structure of a token. It
defines the set of strings that can be considered as instances of a token. In other words, patterns
describe how lexemes (actual sequences of characters) match specific types of tokens.
Example:
A pattern for an identifier token could be: [a-zA-Z_][a-zA-Z0-9_]* , which describes
the structure of variable names (starting with a letter or underscore, followed by any
combination of letters, digits, or underscores).
Lexeme:
A lexeme is an actual sequence of characters in the source code that matches a pattern and is
classified as a token. Lexemes are the concrete instances that the lexical analyzer identifies in the
input source code. They are the actual "words" of the program.
Example:
In the statement int x = 10; , x is a lexeme that corresponds to the identifier token.
2. Compiler Construction Tools (e.g., Lex):
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Lex (or Flex, the faster version) is a tool used for generating lexical analyzers. A lexical analyzer (or
lexer) is responsible for scanning the input source code and breaking it down into tokens.
How Lex Works:
Lex uses regular expressions to define patterns for different types of tokens and then
generates code (usually in C or C++) to perform the actual tokenization process.
Input: A specification file containing regular expressions and corresponding token
names.
Output: A C program that performs lexical analysis.
Example:
If you define a regular expression in Lex for integer literals ( [0-9]+ ), the Lex tool will
generate a lexer that recognizes integer literals and classifies them as the token INTEGER .
3. Directed Acyclic Graph (DAG) – for optimization and value numbering:
A Directed Acyclic Graph (DAG) is a graph used for representing expressions in an optimized form,
particularly in the context of compiler optimizations. It is used to minimize redundant calculations,
making expressions more efficient.
DAG in Compiler Optimization:
After generating intermediate code for a program, a DAG can be constructed for each expression
in the code. In this graph, nodes represent operations (like addition or multiplication), and edges
represent operands. The key property of a DAG is that there are no cycles, meaning no operand is
dependent on itself (directly or indirectly). This allows for optimization, such as common
subexpression elimination (reusing the results of previously computed expressions) and value
numbering (assigning unique identifiers to expressions with the same value).
Value Numbering:
In value numbering, expressions that yield the same result are assigned the same "number" or
identifier. This helps eliminate redundant computations by reusing the previously computed
results. For instance, if you have an expression a + b and later b + a , the compiler can
recognize that they are equivalent and compute the result only once, reusing the same value.
DAG Example:
Consider the expression (a + b) + (a + b) . In a simple linear evaluation, this expression
would be computed twice, but in a DAG, it can be optimized by recognizing that the two instances
of (a + b) are the same and only computing it once.
Summary:
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Token: Abstract classification of characters from source code (e.g., int , + , identifier ).
Pattern: Regular expression or rule defining the structure of a token.
Lexeme: The actual string in the source code that corresponds to a token.
Lex: A tool for generating lexical analyzers that identify tokens from source code.
DAG: A graph used for optimization, such as common subexpression elimination and value
numbering, by eliminating redundant calculations.
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