Abstract Syntax Tree Design and Construction in Compilers, Lecture notes of Computer Science

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Lecture 7 Abstract Syntax Tree

Xiaoyin Wang

CS 5363 Programming

Languages and Compilers

Abstract Syntax Tree (AST)

• The Most Important Intermediate

Presentation in Compilers

• AST Design

• AST Decoration

• (^) Type Inference
• (^) Scope Analysis
• (^) Symbol Table

AST Design

• (^) Do not use single class AST_node
• (^) E.g., need information for if, while, +, *, ID,

NUM

class AST_node {

int node_type;

AST_node[ ] children;

String name; int value; …etc…

• (^) Problem: must have fields for every

different kind of node with attributes

• (^) Not extensible, Java type checking no help

Use Class Hierarchy

• (^) Use subclassing to solve problem
  • (^) Use abstract class for each “interesting” set

of nonterminals (e.g., expressions)

E  E+E | E*E | -E | (E)

abstract class Expr { … }

class Add extends Expr { Expr left, right; … }

class Mult extends Expr { Expr left, right; … }

// or: class BinExpr extends Expr { Oper o; Expr l,

r; }

class Minus extends Expr { Expr e; …}

a - b - c Valid

a > b > c invalid

AST Construction

• (^) Separate AST construction from semantic

checking phase

• (^) Traverse AST (visit) and perform semantic

checks (or other actions) only after tree has

been built and its structure is stable

• (^) Approach is more flexible and less error-prone
  • (^) It is better when efficiency is not a critical issue

7

Illustration of the Parsing Phase

• (^) Parsing: LL(1)
• (^) AST Data Structure Design
• (^) AST Generation
  • (^) Tree reduction and transformation (parser tree to

abstract syntax tree)

• (^) AST Generation on the fly
• (^) AST Visitor Design
• (^) Symbol Table Generation
• (^) Type Annotation

Left-factoring

• The only production requiring left-factoring

• (^) → + |
• (^) →
• (^) → + | ε

Left-factored Grammar for

Illustration

1. → begin
2. → | ε
3. → int ident | bool ident
4. → if then
5. →
6. → + | ε
7. → ident

LL(1) Parsing: First/Follow Sets

• (^) Follow Set
  • (^) Follow() = {$}
  • (^) Follow() = {begin}
  • (^) Follow() = {int, bool, begin}
  • (^) Follow() = {$}
  • (^) Follow() = Follow() = {$, then}
  • (^) Follow() = {+, then, $}

Left-factored Grammar for

Illustration

1. → begin

2. → | ε

3. → int ident | bool ident

4. → if then

5. →

6. → + | ε

7. → ε

8. → ident

Parser Tree Generation

• (^) Input:

int x

bool y

begin

if y then x + x

Parser Tree Generation

• (^) Derivation

-> begin

-> begin

-> int x begin

-> int x begin

=> int x bool y begin

-> int x bool y begin if then

-> int x bool y begin if then

begin

int

bool

ε

if

then

x

x ε

y

ε

y

x

AST Data Structure

• (^) What are important nonterminals and terminals
  • (^) prog: decl + code
  • (^) decl: a list of defs
  • (^) def: type + identifier
  • (^) code: condition + executing expression
  • (^) expression: left, right
  • (^) identifier