Compiler Construction Sohail Aslam Lecture 9 DFA Minimization

Compiler Construction Sohail Aslam Lecture 9

DFA Minimization § The generated DFA may have a large number of states. § Hopcroft’s algorithm: minimizes DFA states 2

DFA Minimization § The generated DFA may have a large number of states. § Hopcroft’s algorithm: minimizes DFA states 3

DFA Minimization § Idea: find groups of equivalent states. § All transitions from states in one group G 1 go to states in the same group G 2 4

DFA Minimization § Idea: find groups of equivalent states. § All transitions from states in one group G 1 go to states in the same group G 2 5

DFA Minimization § Construct the minimized DFA such that there is one state for each group of states from the initial DFA. 6

DFA Minimization a a A b a B a b D b E a b C b DFA for (a | b )*abb 7

DFA Minimization b A, C a a B a b D b E a b Minimized DFA for (a | b )*abb 8

Optimized Acceptor RE R RE=>NFA NFA=>DFA Min. DFA input w string Simulate DFA yes, if w e L(R) no, if w e L(R) 9

Lexical Analyzers § Lexical analyzers (scanners) use the same mechanism § but they: • Have multiple RE descriptions for multiple tokens • Have a character stream at the input 10

Lexical Analyzers § Lexical analyzers (scanners) use the same mechanism § but they: • Have multiple RE descriptions for multiple tokens • Have a character stream at the input 11

Lexical Analyzers § Lexical analyzers (scanners) use the same mechanism § but they: • Have multiple RE descriptions for multiple tokens • Have a character stream at the input 12

Lexical Analyzers • Return a sequence of matching tokens at the output (or an error) • Always return the longest matching token 13

Lexical Analyzers • Return a sequence of matching tokens at the output (or an error) • Always return the longest matching token 14

Lexical Analyzers R 1…R 2 RE=>NFA NFA=>DFA Min. DFA character stream Simulate Token stream DFA 15

Lexical Analyzer Generators § The lexical analysis process can automated § We only need to specify • Regular expressions for tokens • Rule priorities for multiple longest match cases 16

Lexical Analyzer Generators § The lexical analysis process can automated § We only need to specify • Regular expressions for tokens • Rule priorities for multiple longest match cases 17

Lexical Analyzer Generators § Flex generates lexical analyzer in C or C++ § Jlex written in Java. Generates lexical analyzer in Java 18

Lexical Analyzer Generators § Flex generates lexical analyzer in C or C++ § Jlex written in Java. Generates lexical analyzer in Java 19

Using Flex § Provide a specification file § Flex reads this file and produces C or C++ output file contains the scanner. § The file consists of three sections 20

Using Flex § Provide a specification file § Flex reads this file and produces C or C++ output file contains the scanner. § The file consists of three sections 21

Using Flex § Provide a specification file § Flex reads this file and produces C or C++ output file contains the scanner. § The file consists of three sections 22

Flex Specification File 1 C or C++ and flex definitions 23

Flex Specification File 1 C or C++ and flex definitions 2 %% token definitions and actions 24

Flex Specification File 1 C or C++ and flex definitions 2 %% token definitions and actions %% 3 user code 25

Specification File lex. l %{ #include “tokdefs. h” %} D [0 -9] L [a-z. A-Z_] id {L}({L}|{D})* %% "void" {return(TOK_VOID); } "int" {return(TOK_INT); } "if" {return(TOK_IF); } 26

Specification File lex. l "else" {return(TOK_ELSE); } "while"{return(TOK_WHILE)}; "<=" {return(TOK_LE); } ">=" {return(TOK_GE); } "==" {return(TOK_EQ); } "!=" {return(TOK_NE); } {D}+ {return(TOK_INT); } {id} {return(TOK_ID); } [n]|[t]|[ ] ; %% 27

File tokdefs. h #define #define #define TOK_VOID 1 TOK_INT 2 TOK_IF 3 TOK_ELSE 4 TOK_WHILE 5 TOK_LE 6 TOK_GE 7 TOK_EQ 8 TOK_NE 9 TOK_INT 10 TOK_ID 111 28

Invoking Flex lex. l flex lex. cpp 29

Using Generated Scanner void main() { Flex. Lexer lex; int tc = lex. yylex(); while(tc != 0) cout << tc << “, ” <<lex. YYText() << endl; tc = lex. yylex(); } 30

Creating Scanner EXE flex lex. l g++ –c lex. cpp g++ –c main. cpp g++ –o lex. exe lex. o main. o lex <main. cpp 31