Yacc (yet another compiler compiler)

PhD Program in Computer Science

Seminar Title:

YACC (Yet Another Compiler Compiler)

Prepared By:

Omer Salih Dawood Omer

Email:[email protected]

Supervised By:

Dr. Mohamed Elhafiz Mustafa Musa

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Agenda: Overview and History

What Do Compilers Do?

The Structure of a Compiler

Review of Parser

How Does YACC Work?

References.

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Overview and History

The first real compiler

FORTRAN compilers of the late 1950s

18 person-years to build

Today, we can build a simple compiler in a few month.

Crafting and efficient and reliable compiler is still challenging.

Another kind of language processor, called an interpreter, differs from a compiler in that it executes programs without explicitly performing a translation

Advantages and Disadvantages of an interpreter

What Do Compilers Do?

Source Program Encoding

OutputInterpreter

Data

What Do Compilers Do? (Cont’d.) Advantage

Modification to program during execution Interactive debugging

Not for every language, e.g., Basic, Pascal

Dynamic-typed languages Variable types may change at run time, e.g., LISP.

Difficult to compile

Better diagnostics Source code is available.

Machine independence However, the interpreter itself must be portable.

What Do Compilers Do? (Cont’d.) Disadvantage

Slower execution due to repeated examination

Substantial space overhead

The Structure of a Compiler

Modern compilers are syntax-directed

Compilation is driven the syntactic structure of programs; i.e., actions are associated with the structures.

Any compiler must perform two major tasks

Analysis of the source program

Synthesis of a machine-language program

The Structure of a Compiler

Scanner ParserSemanticRoutines

CodeGenerator

Optimizer

SourceProgram

(CharacterStream)

Tokens Syntactic

Structure

IntermediateRepresentation

Target MachineCode

Symbol andAttribute

Tables

(Used by all Phases of The Compiler)

The structure of a Syntax-Directed Compiler

The Structure of a Compiler (Cont’d.)

Scanner The scanner begins the analysis of the source program

by reading the input, character by character, and grouping characters into individual words and symbols (tokens)

The tokens are encoded and then are fetched to the parser for syntactic analysis

For details.

Scanner generators

regular

exp for

tokens

finite

automata

as programs

lex or scangen

The Structure of a Compiler (Cont’d.) Parser

Given a formal syntax specification (typically as a context-free grammar [CFG]), the parse reads tokens and groups them into units as specified by the productions of the CFG being used.

While parsing, the parser verifies correct syntax, and if a syntax error is found, it issues a suitable diagnostic.

As syntactic structure is recognized, the parser either calls corresponding semantic routines directly or builds a syntax tree.

grammaryacc or llgen

parser

The Structure of a Compiler (Cont’d.) Semantic Routines

Perform two functions Check the static semantics of each construct

Do the actual translation for generating IR

The heart of a compiler

Optimizer The IR code generated by the semantic routines is

analyzed and transformed into functionally equivalent but improved IR code.

This phase can be very complex and slow.

The Structure of a Compiler (Cont’d.) One-pass compiler

No optimization is required

Retargetable compiler

Many machine description files, e.g., gcc

Compiler writing tools

Compiler generators or compiler-compilers

Lex and Yacc

E.g., scanner and parser generators[1]

Review of Parser Yacc provides a general tool for imposing structure on

the input to a computer program. The Yacc userprepares a specification of the input process; thisincludes rules describing the input structure, code tobe invoked when these rules are recognized, and a low-level routine to do the basic input. Yacc then generatesa function to control the input process[2].

Parser invokes scanner for tokens.

Parser analyze the syntactic structure according togrammars.

Finally, parser executes the semantic routines[3].

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yacc

Generate a new parser code from grammar

YACC source (*.y)

y.tab.h

y.tab.c

C compiler/linker

Compile a new parser

y.tab.c a.out

a.out

Parse source code

Token stream

Abstract

Syntax

Tree

y.output

How Does YACC Work?

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YACC Format

%{

C declarations

%}

yacc declarations

%%

Grammar rules

%%

Additional C code

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YACC Example

%{

#include <stdio.h>

%}

%token NAME NUMBER

%%

statement: NAME '=' expression

| expression { printf("= %d\n", $1); }

;

expression: expression '+' NUMBER { $$ = $1 + $3; }

| expression '-' NUMBER { $$ = $1 - $3; }

| NUMBER { $$ = $1; }

;

%%

int yyerror(char *s)

{

fprintf(stderr, "%s\n", s);

return 0;

}

int main(void)

{

yyparse();

return 0;

}

C declarations

yacc declarations

Grammar rules

Additional C code

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YACC Declaration Summary

`%start'

Specify the grammar's start symbol

`%union'

Declare the collection of data types that semantic values may have

`%token'

Declare a terminal symbol (token type name) with no precedence or associativity specified

`%type'

Declare the type of semantic values for a nonterminal symbol

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YACC Declaration Summary

`%right'

Declare a terminal symbol (token type name) that is

right-associative

`%left'

Declare a terminal symbol (token type name) that is left-associative

`%nonassoc'

Declare a terminal symbol (token type name) that is nonassociative

(using it in a way that would be associative is a syntax error)

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YACC and Bison Command

Yacc (AT&T)

yacc –d xxx.y

Bison (GNU)

bison –d –y xxx.y

Produce y.tab.c, the

same as above yacc

command

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Work between LEX and YACC%{

#include <stdio.h>

#include "y.tab.h"

%}

id [_a-zA-Z][_a-zA-Z0-9]*

%%

int { return INT; }

char { return CHAR; }

float { return FLOAT; }

{id} { return ID;}

%{

#include <stdio.h>

#include <stdlib.h>

%}

%token CHAR, FLOAT, ID, INT

%%

yacc -d xxx.y

Produced

y.tab.h

# define CHAR 258

# define FLOAT 259

# define ID 260

# define INT 261

parser.y

scanner.l

References: 1- Book: Crafting a Compiler with C

Author: Charles N. Fischer and Richard J. LeBlanc, Jr.The

Benjamin/Cumming Publishing Company, Inc

2- dinosaur.compilertools.net/yacc/

3- Ying-Hung Jiang “Introduction to Yacc “

4- “YACC Parser Generator” ,www.csie.ntu.edu.tw/~b92006/YACC-

present-2005.ppt

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