Short Notes on Compiler Tools & Techniques LEX/YACC, Symbol Table, Predictive Parsing, Peephole Optimization | AI Research

Short Notes on Compiler Tools & Techniques (LEX/YACC, Symbol Table, Predictive Parsing, Peephole Optimization)

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Nov 4, 2026

I can generate a Coursify course section for any two items from: (a) LEX and YACC, (b) Peephole Optimization, (c) Symbol Table, (d) Predictive parser.
However, my web-research tool (tavily_search) is currently failing due to a usage-limit error, so I cannot retrieve external sources to satisfy the requirement that every major claim/statistic/technical detail be backed by web citations.

Web research blocked (citations required)

The required web search tool is returning an error, so I cannot collect sources to provide the mandatory footnote citations.

In the meantime, below is a study-ready course section written as a “short-notes” style package covering (a) LEX and YACC and (c) Symbol Table—but note: it does not include the required external citations because web search is unavailable.

Selected topics

I will cover:

LEX
YACC
Symbol Table
Token
Parse Tree

LEX and YACC (Short Notes)

A typical compiler frontend splits into:

Lexical analysis (turns characters into tokens)
Syntax analysis (validates grammar structure of tokens)

LEX (often implemented as lex/flex) reads rule specifications (typically regular expressions → actions) and generates a scanner program that recognizes tokens.
YACC (often implemented as yacc/bison) reads grammar specifications (productions → semantic actions) and generates a parser.

How LEX and YACC fit together

1
Step 1
Define regular expressions for tokens (e.g., keywords, identifiers, numbers) and attach actions such as returning token types and setting semantic values.
2
Step 2
Run LEX to produce C (or target language) code for scanning input and producing a stream of tokens.
3
Step 3
Provide context-free grammar rules and semantic actions to build AST/parse tree or compute attributes.
4
Step 4
Run YACC to produce a parser that reads tokens from the lexer and checks if the token stream matches the grammar.
5
Step 5
Connect tokenization with parsing; lexer supplies tokens via a function (e.g., yylex), and parser calls it during parsing.
6
Step 6
Semantic actions can perform tasks like expression evaluation, AST construction, and type checking (often involving the symbol table).

Typical LEX specification ideas (what to write)

Rules: patterns for tokens using regular expressions (e.g., identifiers, integers, whitespace).
Actions: code executed when a pattern matches—commonly:
- return a token code (like ID, NUM, IF)
- set yylval (semantic value) for tokens like identifiers and numbers
- ignore whitespace/comments

Tokens are the interface between LEX and YACC.

Typical YACC specification ideas (what to write)

Grammar productions: rules like
- expr : expr PLUS term | term
Terminal symbols: tokens returned by the lexer
Semantic actions: code attached to productions to compute:
- AST nodes
- intermediate values
- types (sometimes deferred)
- error reporting

YACC will generate parsing logic and parsing tables from these rules.

Pro Tip (short-note framing)

In short notes, always mention the pipeline: LEX creates tokens using regex rules; YACC consumes tokens using CFG grammar rules to build/validate parse structure.

Symbol Table (Short Notes)

A Symbol Table stores information about program identifiers (variables, functions, etc.) used by later compiler phases like:

semantic analysis (type checking)
intermediate code generation
optimization and register allocation

Key concepts typically include:

Name → entry mapping: fast lookup (often using hashing or balanced trees)
Scope handling: identifiers can be declared in nested blocks/functions
Attributes stored: type, width, memory location, parameter list, etc.

Why symbol tables matter

When the parser or semantic analyzer encounters an identifier (e.g., x in x = 5;), the compiler needs to know:

Is x declared?
What is its type?
In what scope does it live?
Where should it be stored (address/offset) in memory or registers?

Thus, symbol table operations are often coupled with semantic actions in YACC productions.

Symbol table operations during compilation

1
Step 1
When a declaration is parsed (e.g., int x;), create an entry with attributes like type and scope and store it.
2
Step 2
When an identifier is used (e.g., x + 1), search for the most recent visible declaration in current/outer scopes.
3
Step 3
On entering a new block/function, push a new scope. On exit, pop it so inner declarations don’t leak.
4
Step 4
If lookup fails: undeclared identifier error. If redeclared in the same scope: redeclaration error.
5
Step 5
During semantic analysis, update attributes (e.g., inferred types, function signature details).

Typical symbol table entry (conceptual)

A symbol table entry may include:

Identifier: the lexeme string (e.g., sum)
Scope: current block/function level
Type: e.g., int, float
Address/Offset
For functions: parameter types and return type
For arrays: element type and dimensions

Data structures for implementation

Common structures:

Hash table: expected $O(1)$ average-time lookup/insert
Stack of hash tables: each scope has its own hash table; pushing/popping manages scope
Balanced tree: $O(\log n)$ lookup and ordered traversal

\text{Average lookup time} \approx O(1) \quad\text{(hashing)}

Visual summary

Knowledge Check

Question 1 of 4

Q1Single choice

LEX and YACC are primarily used for…

Generating a lexer and a parser from token rules and grammar rules

Generating machine code directly from source code

Performing register allocation only

Sorting tokens lexicographically

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