Chapter 2. Design of a Simple Compiler

J. H. WangSep. 21, 2015

Outline

• An Informal Definition of the ac Language

• Formal Definition of ac• Phases of a Simple compiler• Scanning• Parsing• Abstract Syntax Trees• Semantic Analysis• Code Generation

Introduction

• An overview of compilation process by considering a simple language

• A quick overview of a compiler’s phases and their associated data structures

An Informal Definition of the ac Language

• ac: adding calculator• Types

– integer– float: allows 5 fractional digits after the decimal point– Automatic type conversion from integer to float

• Keywords– f: float– i: integer– p: print

• Variables– 23 names from lowercase Roman alphabet except the three

reserved keywords f, i, and p• Target of translation: dc (desk calculator)

– Reverse Polish notation (RPN)

An Example ac Program

• Example ac program:– f b

i aa = 5b = a + 3.2p b$

• Corresponding dc code– 5

sala3.2+sblbp

Formal Definition of ac

• Syntax specification: context-free grammar (CFG)– (Chap. 4)

• Token specification: regular expressions– (Sec. 3.2)

Syntax Specification

• CFG:– A set of productions or rewriting rules– E.g.: Stmt id assign Val Expr

| print id– Two kinds of symbols

• Terminals: cannot be rewritten– E.g.: id, assign, print– Empty or null string: λ– End of input stream or file: $

• Nonterminals:– Start symbol: Prog– E.g.: Val, Expr

– Left-hand side (LHS)– Right-hand side (RHS)

• Starting from the start symbol• Choosing some nonterminal symbol

and finding a production for it• Replacing it with the string of

symbols on the RHS• Any string of terminals that can be

produced: syntactically valid• Otherwise: syntax error

Token Specification

Phases of a Simple Compiler

• Scanner: source ac program -> tokens– Chap. 3

• Parser: tokens -> abstract syntax tree (AST)– Chap. 5 & 6

• Symbol table: created from AST– Chap. 8

• Semantic analysis: AST decoration• Translation: by traversing AST

Scanning

• To translate a stream of characters into a stream of tokens– Automatic construction of scanners: Chap.3– Token:

• Type: membership in the terminal alphabet• Semantic value: additional information

– For most programming languages, the scanner’s job is not so easy

• +, ++• //, “, \”• Variable-length tokens

CANNER

PEEK

PEEK

ADVANCE

ADVANCE

CAN IGITS

EXICAL RROR

CAN IGITS

PEEK

PEEK

PEEK

ADVANCE

ADVANCE

ADVANCE

Parsing

• To determine if the stream of tokens conforms to the language’s grammar (Chap. 4, 5, 6)– e.g.: Are these valid statements?

• b = a + 3.2 • p b

– For ac, a simple parsing technique called recursive descent is used

• “Mutually recursive parsing routines that descend through a derivation tree”

• Each nonterminal has an associated parsing procedure for determining if the token stream contains a sequence of tokens derivable from that nonterminal

Predicting a Parsing Procedure

• Examine the next input token to predict which production should be applied– E.g.:

• Stmt id assign Val Expr• Stmt print id

– Predict set• {id} [1]• {print} [6]

TMT

PEEK

PEEK

MATCH

MATCH

MATCH

MATCH

ERROR

AL

XPR

TMT

PEEK

PEEK

MATCH

MATCH

MATCH

MATCH

ERROR

AL

XPR

• Consider the productions for Stmts– Stmts Stmt Stmts– Stmts λ

• The predict sets– {id, print} [8]– {$} [11]

TMTS

PEEK PEEK

PEEK

TMT

TMTS

ERROR

Implementing the Production

• When a terminal is encountered, a call to MATCH() is placed

• For each nonterminal, the corresponding procedure will be called

• For the symbol λ, no code is executed

Abstract Syntax Trees

• Some aspects of compilation that can be difficult to perform during syntax analysis– Some aspects of language cannot be specified

in a CFG• E.g.: symbol usage consistency with type declaration

– Context sensitive

• In Java: x.y.z– Package x, class y, static field z– Variable x, field y, another field z

• Operator overloading– +: numerical addition or appending of strings

– Separation into phases makes the compiler much easier to write and maintain

• Parse trees are large and unnecessarily detailed (Fig. 2.4)– Abstract syntax tree (AST) (Fig. 2.9)

• Inessential punctuation and delimiters are not included

– A common intermediate representation for all phases after syntax analysis

• Declarations need not be in source form• Order of executable statements explicitly represented• Assignment statement must retain identifier and

expression• Nodes representing computation: operation and operands• Print statement must retain name of identifier

Semantic Analysis

• Example processing include:– Declarations and name scopes are

processed to construct a symbol table– Type consistency– Make type-dependent behavior explicit

Symbol Tables

• To record all identifiers and their types – 23 entries for 23 distinct identifiers in ac

(Fig. 2.11)• Type info.: integer, float, unused (null)• Attributes: scope, storage class, protection

properties

– Symbol table construction (Fig. 2.10)• Symbol declaration nodes call

VISIT(SymDeclaring n)• ENTERSYMBOL checks the given symbol has

not been previously declared

VISIT

GET YPE

NTER YMBOL

NTER YMBOL

NTER YMBOL

OOKUP YMBOL

ERROR

GET D

GET D

Type Checking

• Only two types in ac– Integer– Float

• Type hierarchy– Float wider than integer– Automatic widening (or casting)

• integer -> float

Type Analysis

VISIT

VISIT

VISIT

VISIT

VISIT

ONSISTENT

ONVERT

OOKUP YMBOL

ONSISTENT

ENERALIZE

ONVERT

ONVERT

ONVERT

ENERALIZE

ERROR

• Type checking– Constants and symbol reference: simply set

the node’s type based on the node’s contents– Computation nodes: CONSISTENT(n.c1, n.c2)– Assignment operation: CONVERT(n.c2,

n.c1.type)

• CONSISTENT()– GENERALIZE(): determines the least general

type– CONVERT(): checks whether conversion is

necessary

Code Generation

• The formulation of target-machine instructions that faithfully represent the semantics of the source program– Chap. 11 & 13– dc: stack machine model– Code generation proceeds by traversing the

AST, starting at its root• VISIT (Computing n): +, -• VISIT (Assigning n): =• VISIT (SymReferencing n)• VISIT (Printing n)• VISIT (Converting n)

VISIT

VISIT

VISIT

VISIT

VISIT

VISIT

ODE EN

ODE EN

ODE EN

ODE EN

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

VISIT

VISIT

VISIT

VISIT

VISIT

VISIT

ODE EN

ODE EN

ODE EN

ODE EN

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

MIT

End of Chapter 2