Compiler Design

Yacc Example

"

Yet Another Compiler Compiler"

Kanat Bolazar

Lex and Yacc

• • • Two classical tools for compilers: – –

Lex

: A Lexical Analyzer Generator

Yacc

: “Yet Another Compiler Compiler” (Parser Generator)

Lex

creates programs that scan your tokens one by one.

Yacc

takes a grammar (sentence structure) and generates a parser.

Lexical Rules Grammar Rules Input

Lex

yylex()

Yacc

yyparse() Parsed Input 2

Lex and Yacc

•

Lex

and

Yacc

generate C code for your analyzer & parser.

Lexical Rules Grammar Rules C code

Lex

C code

Yacc

Input char stream yylex() C code Lexical Analyzer (Tokenizer) token stream yyparse() C code Parser Parsed Input 3

Flex, Yacc, Bison, Byacc

• Often, instead of the standard

Lex

and

Yacc

,

Flex

and

Bison

are used: –

Flex

: A fast lexical analyzer – (GNU)

Bison

: A drop-in replacement for (backwards compatible with) Yacc •

Byacc

is Berkeley implementation of • Resources:

Yacc

(so it is

Yacc

).

http://en.wikipedia.org/wiki/Flex_lexical_analyser http://en.wikipedia.org/wiki/GNU_Bison • The Lex & Yacc Page (manuals, links): http://dinosaur.compilertools.net/ 4

Yacc: A Standard Parser Generator

• Yacc is not a new tool, and yet, it is still used in many projects.

• Yacc syntax is similar to Lex/Flex at the top level.

• Lex/Flex rules were regular expression – action pairs.

• Yacc rules are grammar rule – action pairs.

declarations %% rules %% programs 5

Yacc Examples: Calculator

• • • • • A standard Yacc example is the int-valued calculator.

Appendix A of Yacc manual at Lex and Yacc Page shows such a calculator.

We'll examine this example in parts.

Let's start with four operations: E -> E + E | E – E | E * E | E / E Note that this grammar is ambiguous because 2 + 5 * 7 could be parsed 2 + 5 first or 5 * 7 first.

6

Yacc Calculator Example: Declarations

%{ # include # include Directly included C code int regs[26]; int base; list is our start symbol; a list of one-line statements / expressions.

%} %start list %token DIGIT LETTER DIGIT & LETTER are tokens; (other tokens use ASCII codes, as in '+', '=', etc) %left '+' '-' %left '*' '/' '%' Precedence and associativity (left) of %left UMINUS /* precedence for unary minus */ operators: +, - have lowest precedence *, / have higher precedence Unary minus has highest precedence 7

Yacc Calculator Example: Rules

%% /* begin rules section */ list : /* empty */ | list stat '\n' ; | list error '\n' { yyerrok; } list: a list of one-line statements / expressions.

Error handling allows a statement to be corrupt, but list continues with next statement.

statement: expression to calculate, or assignment stat : expr { printf( "%d\n", $1 ); } ; | LETTER '=' expr { regs[$1] = $3; } number: made up of digits (tokenizer should handle this, but this is a simple example).

number: DIGIT { $$ = $1; base = ($1==0) ? 8 : 10; } ; | number DIGIT { $$ = base * $1 + $2; } 8

Yacc Calculator Example: Rules, cont'd

expr : '(' expr ')' { $$ = $2; } | expr '+' expr { $$ = $1 + $3; } | expr '-' expr { $$ = $1 - $3; } | expr '*' expr { $$ = $1 * $3; } | expr '/' expr { $$ = $1 / $3; } | '-' expr %prec UMINUS { $$ = - $2; } | LETTER { $$ = regs[$1]; } ; | number Unary minus Letter: Register/var 9

Yacc Calculator Example: Programs (C Code)

%% /* start of programs */ yylex() { /* lexical analysis routine */ /* returns LETTER for a lower case letter, yylval = 0 through 25 */ /* return DIGIT for a digit, yylval = 0 through 9 */ /* all other characters are returned immediately */ int c; while( (c=getchar()) == ' ' ) {/* skip blanks */ } /* c is now nonblank */ if( islower( c ) ) { yylval = c - 'a'; } return ( LETTER ); if( isdigit( c ) ) { yylval = c - '0'; } return( DIGIT ); } return( c ); 10