Compiler Design Yacc Example " Yet Another Compiler Compiler"

1. Kanat Bolazar Compiler DesignYacc Example"Yet Another Compiler Compiler"

2. 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 Lex Yacc Input yylex() yyparse() Parsed Input

3. Lex and Yacc • Lex and Yacc generate C code for your analyzer & parser. Grammar Rules Lexical Rules C code C code Yacc Lex Parsed Input yyparse() Input yylex() token stream char stream C code C code Lexical Analyzer (Tokenizer) Parser

4. 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 Yacc (so it is Yacc). • Resources: • 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/

5. 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

6. 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.

7. Yacc Calculator Example: Declarations %{ # include <stdio.h> # include <ctype.h> int regs[26]; int base; %} %start list %token DIGIT LETTER %left '+' '-' %left '*' '/' '%' %left UMINUS /* precedence for unary minus */ • Directly included C code • list is our start symbol; a list of one-line statements / expressions. • DIGIT & LETTER are tokens; • (other tokens use ASCII codes, as in '+', '=', etc) • Precedence and associativity (left) of operators: • +, - have lowest precedence • *, / have higher precedence • Unary minus has highest precedence

8. Yacc Calculator Example: Rules %% /* begin rules section */ list : /* empty */ | list stat '\n' | list error '\n' { yyerrok; } ; stat : expr { printf( "%d\n", $1 ); } | LETTER '=' expr { regs[$1] = $3; } ; number: DIGIT { $$ = $1; base = ($1==0) ? 8 : 10; } | number DIGIT { $$ = base * $1 + $2; } ; 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 number: made up of digits (tokenizer should handle this, but this is a simple example).

9. 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

10. 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 ); }