1. Compiler Construction
Chapter 6
Compiler Construction
Dr K. V. N. Sunitha

2. Syntax Directed Translation
Syntax directed definition (SDD) is a high-level language specification for translation.
Syntax directed translation (SDT) describes translation of language constructs guided by context free grammars.
The grammar together with semantic action is called syntax-directed translation
They are used in automatic tools like YACC.
SDT helps the compiler designer to translate the language constructs directly by attaching semantic actions or subroutines.
Aα + {action} = SDT
Compiler Construction
Dr K. V. N. Sunitha

3. Attributes for Grammar Symbols
Synthesized attribute – The value of a synthesized attribute at a node is computed from the values of attributes at the children of that node in the parse tree.
Inherited attribute – The value of an inherited attribute is computed from the values of attributes at the siblings and parent of that node.
Compiler Construction
Dr K. V. N. Sunitha

4. Writing Syntax Directed Translation
Involves three steps as follows:
Define grammar for input string.
Take a simple string and draw parse tree.
Attach semantic actions by looking at expected output.
Compiler Construction
Dr K. V. N. Sunitha

5. SDT for Evaluation of Expressions / Desk Calculator
E → E + T { E•val = E•val + T•val } E → T { E•val = T•val } T → T * F { T•val = T•val * F•val } T → F { T•val = F•val } F → id { F•val = num•lval }
Compiler Construction
Dr K. V. N. Sunitha

6. Compiler Construction
Annotated Parse Tree
Compiler Construction
Dr K. V. N. Sunitha

7. SDT for Converting Infix to Postfix Expression
E → E + T { print(‘+’); } E → T { } T → T * F { print(‘*’); } T → F { } F → id { print(“id•name”); }
Compiler Construction
Dr K. V. N. Sunitha

8. Creation of Syntax Tree
E → E + T { E•nptr = mknode ( E•nptr , ‘+’ , T•nptr ) ; } E → T { E•nptr = T•nptr; } T → T * F { T•nptr = mknode ( T•nptr , ‘*’ , F•nptr ) ; } T → F { T•nptr = F•nptr; } F → id { F•nptr = mknode ( NULL, id•lvalue , NULL ) ; }
Compiler Construction
Dr K. V. N. Sunitha

9. S-Attributed Definition
It uses only synthesized attributes only.
Semantic actions can be placed only at the end of right hand side of a production.
Attributes are generally evaluated during bottom up parsing.
Compiler Construction
Dr K. V. N. Sunitha

10. L-Attributed Definition
It allows both types. But if an inherited attribute is present there is a restriction. The restriction is – each inherited attribute is restricted to inherit either from parent or left sibling only.
Translation rules can be placed anywhere on right hand side of the production.
Attributes are generally evaluated by traversing the parse tree depth first and left to right.
Compiler Construction
Dr K. V. N. Sunitha

11. SDT for Storing Type Information in Symbol Table
D → TL {L•type = T•type ; } T → int { T•type = int } T → char { T•type = char } L → L1 , id { L1•in = L•in ; add_type(L1•in , id) ; } L → id { add_type(L•in , id•name) ; }
Compiler Construction
Dr K. V. N. Sunitha

12. Converting L-Attributed to S-Attributed Definition
D → D1 , id {D•type = D1•type ; add_type(D1•type , id•name) ; } D → T , id {L•type = T•type ; add_type(T•type , id•name) } T → int { T•type = int } T → char { T•type = char }
Compiler Construction
Dr K. V. N. Sunitha

13. Compiler Construction
YACC
Compiler Construction
Dr K. V. N. Sunitha

14. Compiler Construction
What Is YACC ?
Automatic tool to build a parser for a given grammar.
YACC (Yet Another Compiler Compiler) implements LALR(1) technique.
Input is a CFG and actions to take upon recognizing a rule, i.e., SDT.
Output is a parser in C.
Compiler Construction
Dr K. V. N. Sunitha

15. Compiler Construction
LEX and YACC: A Team
call yylex()
[0-9]+
next token is NUM
NUM ‘+’ NUM
Compiler Construction
Dr K. V. N. Sunitha

16. Compiler Construction
Availability
lex, yacc on most UNIX systems
bison: a yacc replacement from GNU
flex: fast lexical analyzer
BSD yacc
Windows/MS-DOS versions exist
Compiler Construction
Dr K. V. N. Sunitha

17. YACC Basic Operational Sequence
gram.y
File containing desired grammar in YACC format
YACC program
yacc
y.tab.c
C source program created by YACC cc
or gcc
C compiler
a.out
Executable program that will parse grammar given
in gram.y
Compiler Construction
Dr K. V. N. Sunitha

18. YACC – A Parser Generator
A language for specifying parsers and semantic analyzers
lang.y
Yacc compiler
y.tab.c
y.tab.c
C compiler
a.out
tokens
a.out
syntax tree
Compiler Construction
Dr K. V. N. Sunitha

19. Compiler Construction
YACC Programs %{
C declarations %}
Yacc declarations %%
Grammar rules
Additional C code
Compiler Construction
Dr K. V. N. Sunitha

20. YACC Program for Evaluation of Expressions
Input: 2+3*4
Output: 14
Compiler Construction
Dr K. V. N. Sunitha

21. YACC Program for Evaluation of Expressions%{
#include<stdio.h> %}
%token NUM %%
L:L E '\n' {printf("value of expr=%d\n",$2);} |;
E:E'+'T {$$=$1+$3;}
| T ;
T:T'*' F {$$=$1*$3;} |F
F:NUM {$$=$1;}
Compiler Construction
Dr K. V. N. Sunitha

22. Compiler Construction
main() {
yyparse(); }
void yyerror(char *s)
{printf("error\n");
yylex()
char c;
while((c=getchar())!='\n')
if(isdigit(c))
yylval=c-'0';
return NUM;
else return c; }
return c;
Compiler Construction
Dr K. V. N. Sunitha

23. Compiler Construction
Input: 2+3*4
Output: 14
$yacc –v x.y gives y.output FSM
Compiler Construction
Dr K. V. N. Sunitha

24. Compiler Construction
Lex Program %%
[ \t] { }
[0-9]+ {sscanf(yytext,"%d",&yylval);
return num;}
[\n] {return(yytext[0]);}
[+*-/] {return(yytext[0]);}
yywrap() {
return 1; }
Compiler Construction
Dr K. V. N. Sunitha

25. Compiler Construction
Parsing
Parsers are already being used extensively
for compiler construction,
in database interfaces,
in self-describing databases,
in linguistics for text analysis, corpora analysis, machine
translation, text analysis of b,
in document preparation and conversion
Compiler Construction
Dr K. V. N. Sunitha