1 October 14, 2015 1 October 14, 2015October 14, 2015October 14, 2015 Azusa, CA Sheldon X. Liang Ph....

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April 19, 20231

April 19, 2023April 19, 2023 Azusa, CAAzusa, CA

Sheldon X. Liang Ph. D.

Computer Science at Computer Science at Azusa Pacific UniversityAzusa Pacific University

Azusa Pacific University, Azusa, CA 91702, Tel: (800) 825-5278 Department of Computer Science, http://www.apu.edu/clas/computerscience/

CS400 Compiler ConstructionCS400 Compiler Construction

http://www.apu.edu/clas/computerscience/

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CS@APU: CS400 Compiler ConstructionCS@APU: CS400 Compiler Construction

Got it with following questionsGot it with following questions• Yacc Specification

– Yacc declaration– C Declaration– User-defined procedures

• Deal with ambiguity– L/R operators’ associativity– Precedence level definition– Removing ambiguity

• Error recovery in Yacc– Set e-mode & skip input until NL– Reset parser– Diagnosis as much as possible

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ANTLR, Yacc, and Bison

• ANTLR tool– Generates LL(k) parsers

• Yacc (Yet Another Compiler Compiler)– Generates LALR(1) parsers

• Bison– Improved version of Yacc

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Creating an LALR(1) Parser with Yacc/Bison

Yacc or Bisoncompiler

yaccspecificationyacc.y

y.tab.c

inputstream

Ccompiler

a.out outputstream

y.tab.c

a.out

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Yacc Specification

• A yacc specification consists of three parts:yacc declarations, and C declarations within %{ %}%% translation rules%%user-defined auxiliary procedures

• The translation rules are productions with actions:production1 { semantic action1 }production2 { semantic action2 }…productionn { semantic actionn }

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Writing a Grammar in Yacc

• Productions in Yacc are of the formNonterminal : tokens/nonterminals { action }

| tokens/nonterminals { action }…;

• Tokens that are single characters can be used directly within productions, e.g. ‘+’

• Named tokens must be declared first in the declaration part using

%token TokenName

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Synthesized Attributes

• Semantic actions may refer to values of the synthesized attributes of terminals and nonterminals in a production:

X : Y1 Y2 Y3 … Yn { action }– $$ refers to the value of the attribute of X– $i refers to the value of the attribute of Yi

• For examplefactor : ‘(’ expr ‘)’ { $$=$2; }

factor.val=x

expr.val=x )($$=$2

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Example 1

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Dealing With Ambiguous Grammars

• By defining operator precedence levels and left/right associativity of the operators, we can specify ambiguous grammars in Yacc, such asE E+E | E-E | E*E | E/E | (E) | -E | num

• To define precedence levels and associativity in Yacc’s declaration part:

%left ‘+’ ‘-’%left ‘*’ ‘/’%right UMINUS

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Example 2

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Example 2 (cont’d)

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Combining Lex/Flex with Yacc/Bison

Yacc or Bisoncompiler

Yacc Specificationyacc.y

lex.yy.cy.tab.c

inputstream

Ccompiler

a.out outputstream

y.tab.cy.tab.h

a.out

Lex or Flexcompiler

Lex specification lex.land token definitions

y.tab.hlex.yy.c

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Lex Specification for Example 2%option noyywrap%{#include “y.tab.h”

extern double yylval;%}number [0-9]+\.?|[0-9]*\.[0-9]+%%[ ] { /* skip blanks */ }{number} { sscanf(yytext, “%lf”, &yylval);

return NUMBER;}

\n|. { return yytext[0]; }

Generated by Yacc, contains#define NUMBER xxx

yacc -d example2.ylex example2.lgcc y.tab.c lex.yy.c./a.out

bison -d -y example2.yflex example2.lgcc y.tab.c lex.yy.c./a.out

Defined in y.tab.c

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Error Recovery in Yacc%{…%}…%%lines : lines expr ‘\n’ { printf(“%g\n”, $2; }

| lines ‘\n’| /* empty */| error ‘\n’ { yyerror(“reenter last line: ”);

yyerrok;}

;…

Reset parser to normal modeError production:set error mode and

skip input until newlineApril 19, 2023

14Azusa Pacific University, Azusa, CA 91702, Tel: (800) 825-5278 Department of Computer Science, http://www.apu.edu/clas/computerscience/


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Organization

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Got it with following questionsGot it with following questions• Yacc Specification

– Yacc declaration– C Declaration– User-defined procedures

• Deal with ambiguity– L/R operators’ associativity– Precedence level definition– Removing ambiguity

• Error recovery in Yacc– Set e-mode & skip input until NL– Reset parser– Diagnosis as much as possible


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Thank you very much!

Questions?

April 19, 202317




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