Compiler Design: Syntax and Parsing for Exams, Exams of Compiler Design

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Syntax and Parsing

COMS W

Prof. Stephen A. Edwards Spring 2002 Columbia University Department of Computer Science

Last Time

Administrivia

Class Project

Types of Programming Languages:

Imperative, Object-Oriented, Functional, Logic, Dataflow

The Compilation Process

Interpreters

Source Program ↓ Input → Interpreter → Output

Structure of a Compiler

↓ Program Text Lexer ↓ Token Stream Parser ↓ Abstract Syntax Tree Static semantics (type checking) ↓ Annotated AST Translation to intermediate form ↓ Three-address code Code generation ↓ Assembly Code

Compiling a Simple Program

int gcd(int a, int b) { while (a != b) { if (a > b) a -= b; else b -= a; } return a; }

After Lexical Analysis

int gcd(int a, int b) { while (a != b) { if (a > b) a -= b; else b -= a; } return a; }

int gcd ( int a , int b ) – while ( a

!= b ) – if ( a > b ) a -= b ;

else b -= a ; ˝ return a ; ˝

A stream of tokens. Whitespace, comments removed.

After Parsing

func

int gcd args

arg int a

arg int b

seq while != a b

if

a b

a b

b a

return a

int gcd(int a, int b) { while (a != b) { if (a > b) a -= b; else b -= a; } return a; }

Abstract syntax tree built from parsing rules.

After Translation into 3-Address Code

L0: sne $1, a, b seq $0, $1, 0 btrue $0, L1 % while (a != b) sl $3, b, a seq $2, $3, 0 btrue $2, L4 % if (a < b) sub a, a, b % a -= b jmp L L4: sub b, b, a % b -= a L5: jmp L L1: ret a

int gcd(int a, int b) { while (a != b) { if (a > b) a -= b; else b -= a; } return a; }

Idealized assembly language w/ infinite registers

After Translation to 80386 Assembly

gcd: pushl %ebp % Save frame pointer movl %esp,%ebp movl 8(%ebp),%eax % Load a from stack movl 12(%ebp),%edx % Load b from stack .L8: cmpl %edx,%eax je .L3 % while (a != b) jle .L5 % if (a < b) subl %edx,%eax % a -= b jmp .L .L5: subl %eax,%edx % b -= a jmp .L .L3: leave % Restore SP, BP ret

Lexical Analysis (Scanning)

Goal is to translate a stream of characters

i n t sp g c d ( i n t sp a , sp i n t sp b

into a stream of tokens

ID int

ID

gcd

LPAREN

ID

int

ID

a

COMMA

ID

int

ID

b

Each token consists of a token type and its text.

Whitespace and comments are discarded.

Lexical Analysis

Goal: simplify the job of the parser.

Scanners are usually much faster than parsers.

Discard as many irrelevant details as possible (e.g., whitespace, comments).

Parser does not care that the the identifer is “supercalifragilisticexpialidocious.”

Parser rules are only concerned with token types.

An ANTLR File for a Simple Scanner

class CalcLexer extends Lexer;

LPAREN : ’(’ ; // Rules for puctuation RPAREN : ’)’ ; STAR : ’*’ ; PLUS : ’+’ ; SEMI : ’;’ ; protected // Can only be used as a sub-rule DIGIT : ’0’..’9’ ; // Any character between 0 and 9 INT : (DIGIT)+ ; // One or more digits

WS : (’ ’ | ’\t’ | ’\n’| ’\r’) // Whitespace { $setType(Token.SKIP); } ; // Action: ignore

ANTLR Specifications for Scanners

Rules are names starting with a capital letter.

A character in single quotes matches that character.

LPAREN : ’(’ ;

A string in double quotes matches the string

IF : "if" ;

A vertical bar indicates a choice:

OP : ’+’ | ’-’ | ’*’ | ’/’ ;