Bottom-up syntactic parsing. LR(k) grammars. LR(0) grammars. Bottom-up parser. Definition Properties
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1 Course
2 Bottom-up syntactic parsing Bottom-up parser LR(k) grammars Definition Properties LR(0) grammars Characterization theorem for LR(0) LR(0) automaton 2
3 a 1... a i... a n # X 1 X 1 Control Parsing table... # p 1 p
4 S S, S asa bsb c 4
5 LR(0) Syntactic analysis Syntactic analyzer generators FIRST, FOLLOW SLR(1) Grammars SLR(1) parsing table SLR(1) syntactic analysis LR(1) grammars 5
6 The parsing table is the LR(0) automaton, M. Configuration: (σ, u#, π) where σεt 0 T*, uεt*, πεp*. Initial configuration (t 0, w#, ε), Transitions: Shift: (σt, au#, π) (σtt, u#, π) if g(t, a) = t. Reduce: (σtσ t, u#, π) ( σtt, u#, πr) if A β ε t, r = A β, σ t = β şi t = g (t, A). Accept: (t 0 t 1, #, π) is the acceptance configuration if S S ε t1, π is the parsing of the word. Error: a configuration from which no transitions are possible 6
7 char ps[]= w# ; //ps is the input string w i = 0; // position in the input string STACK.push(t0); // the stack is initialized with t0 while(true) { // repeat until accept or error t = STACK.top(); a = ps[i] // a is the current symbol at the input if( g(t, a) { //shift STACK.push(g(t, a)); i++; //move forward in the input string } else { if(a X 1 X 2 X m t){ if(a == S ) if(a == # )exit( accept ); else exit( error ); else // reduce for( i = 1; i <= m; i++) STACK.pop(); STACK.push(g(top(STACK), A)); } //endif else exit( error ); }//endelse }//endwhile 7
8 S S S E$ E E+T T (E) E T T a 8
9 S S S E$ E E+T T (E) E T T a Stack Input Action Output 0 a+(a+a)$# shift 05 +(a+a)$# reduce T a 03 +(a+a)$# reduce E T 02 +(a+a)$# shift 027 (a+a)$# shift 0274 a+a)$# shift a)$# reduce T a a)$# reduce E T a)$# shift a)$# shift )$# reduce T a )$# reduce E E+T )$# shift 02748'10' $# reduce T (E) 0279 $# reduce E E+T 02 $# shift 026 # reduce S E$ 01 # accept 9
10 Lemma 1, 2 Let G = (N, T, S, P) be an LR(0), t 0 σ, t 0 τ paths in the LR(0) automaton, labeled with φ and γ respectively and u, v ε T*. It follows that, if in the LR(0) parser the calculation (t 0 σ, uv#, ε) + (t 0 τ, v#, π) holds, then the derivation φ dr π u holds for the grammar G, and reciprocally. Theorem If G is an LR(0) grammar, then for any word w ε T*, the LR(0) parser will reach an acceptance configuration for w, i.e. (t 0 σ, uv#, ε) + (t 0 τ, v#, π) iff φ dr π u. 10
11 YACC ( Yet Another Compiler Compiler ) designed and built in 1975 by S. C. Johnson at the Bell Laboratories Bison, compatible with YACC, written by Robert Corbett and Richard Stallmann 11
12 yacc <file_name> The result is a C source file Structure of a yacc file C and yacc declarations %% Rules (context free grammar) %% C code 12
13 %{C declarations%} Declaring terminals: %token <terminal> Declaring the starting symbol of the grammar : %start <non-terminal> Other declarations: type defines types; left operators associative to the left; right - operators associative to the right; prec operator precedence; nonassoc non-associative. 13
14 non-terminal: <right hand side> {C code} expr:nr{$$=$1;} expr "+"expr {$$=$1+$3;} expr "-" expr{$$=$1-$3;} expr "*" expr{$$=$1*$3;} expr "/" expr{$$=$1/$3;} "("expr")" {$$=$2;} ; 14
15 Definition Let G be a grammar for which the LR(0) automaton contains inconsistent states (G is not LR(0)). The grammar G is SLR(1) if for any state t of its LR(0) automaton, the following are true: If A α, B β t then FOLLOW(A) FOLLOW(B) = ; If A α, B β aγ t then a FOLLOW(A). The SLR(1) syntactic analysis is similar to the LR(0) one; the syntactic analysis table has two main components: The first, ACTION, determines whether the parser will shift or reduce, depending on the state at the top of the stack and the next input symbol The second, GOTO, determines the state which will be added to the stack following a reduction. 15
16 FIRST(α) = {a a ε T, α st * au } if (α st * ε) then {ε} else. FOLLOW(A) = {a a ε T {ε}, S st * uaγ, a ε FIRST (γ) } 16
17 1.for(X ε Σ) 2.if(X ε T)FIRST(X)={X} else FIRST(X)= ; 3.for(A aβ ε P) 4.FIRST(A)=FIRST(A) {a}; 5.FLAG=true; 6.while(FLAG){ 7.FLAG=false; 8.for(A X 1 X 2 X n ε P){ 9.i=1; 10.if((FIRST(X1) FIRST(A)){ 11.FIRST(A)=FIRST(A) (FIRST(X1); 12.FLAG=true; 13.}//endif 14.while(i<n&&X i st * ε) 15.if((FIRST(X i+1 ) FIRST(A)){ 16.FIRST(A)=FIRST(A) FIRST(X i+1 ); 17.FLAG=true;i++; }//endif }//endwhile }//endfor }//endwhile for(a ε N) if(a st * ε)first(a)=first(a) {ε}; 17
18 Input: the grammar G=(N,T,S,P). the sets FIRST(X),X ε Σ. α =X 1 X 2 X n, X i ε Σ,1 i n. Output: FIRST(α). 1.FIRST(α)=FIRST(X 1 )-{ε};i=1; 2.while(i<n && X i + ε){ 3.FIRST(α)=FIRST(α) (FIRST(X i+1 )-{ε}); 4.i=i+1; }//endwhile 5.if(i==n && X n + ε) 6.FIRST(α)=FIRST(α) {ε}; 18
19 Let the grammar G be: S E B, E ε, B a beginsc end, C ε ; SC FIRST(S) = {a, begin, ε} FIRST(E) = {ε} FIRST(B) = {a, begin} FIRST(C) = {;, ε}. FIRST(SEC) = {a, begin, ;, ε}, FIRST(SB)= {a, begin}, FIRST(;SC)= {;}. 19
20 ε ε FOLLOW(S). If A αbβxγ ε P and β + ε, then FIRST(X) -{ε} FOLLOW (B). S * α 1 A β 1 α 1 αbβxγβ 1 * α 1 αbxγβ 1 it then follows that FIRST(X)-{ε} FOLLOW (B). If A αbβ ε P then FIRST(β)-{ε} FOLLOW (B). If A αbβ ε P and β + ε, then FOLLOW(A) FOLLOW(B). 20
21 1. for(a Σ)FOLLOW(A)= ; 2.FOLLOW(S)={ε}; 3.for(A X 1 X 2 X n ){ 4.i=1; 5.while(i<n){ 6.while(X i N)++i; 7.if(i<n){ 8.FOLLOW(Xi)= FOLLOW(X i ) (FIRST(X i+1 X i+2 X n )-{ε}); 9.++i; }//endif }//endwhile }//endfor 21
22 10.FLAG=true; 11.while(FLAG){ 12.FLAG=false; 13.for(A X 1 X 2 X n ){ 14.i=n; 15.while(i>0 && X i N){ 16.if(FOLLOW(A) FOLLOW(X i )){ 17.FOLLOW(Xi)=FOLLOW(X i ) FOLLOW(A); 18.FLAG=true; 19.}//endif 20.if(X i + ε)--i; 21.else continue; 22.}//endwhile 23.}//endfor 24.}//endwhile 22
23 Let the grammar G be: S E B, E ε, B a begin SC end, C ε ; SC FOLLOW(S)=FOLLOW(E)=FOLLOW(B) ={ε, ;, end} FOLLOW(C) = {end}. 23
24 Definition Let G be a grammar for which the LR(0) automaton contains inconsistent states (G is not LR(0)). The grammar G is SLR(1) if for any state t of its LR(0) automaton, the following are true: If A α, B β t then FOLLOW(A) FOLLOW(B) = ; If A α, B β aγ t then a FOLLOW(A). The SLR(1) syntactic analysis is similar to the LR(0) one; the syntactic analysis table has two main components: The first, ACTION, determines whether the parser will shift or reduce, depending on the state at the top of the stack and the next input symbol The second, GOTO, determines the state which will be added to the stack following a reduction. 24
25 Input: The grammar G = (N, T, S, P) augmented with S S; The automaton M = (Q, Σ, g, t 0, Q ); The sets FOLLOW(A), A V Output: The SLR(1) parsing table, made up of two parts: ACTION(t, a), t Q, a T { # }, GOTO(t, A), t Q, A N. 25
26 for(t Q) for (a T) ACTION(t, a) = error ; for (A V) GOTO(t, A) = error ; for(t Q}{ for(a α aβ t) ACTION(t,a)= D g(t, a) ;//shift to g(t, a) for(b γ t ){// accept or reduce if(b == S ) ACTION(t, a) = accept ; else for(a FOLLOW(B)) ACTION(t,a)= R B γ ; }// endfor for (A N) GOTO(t, A) = g(t, A); }//endfor 26
27 shift: (σt, au#, π) (σtt, u#, π) if ACTION(t, a)=st ; reduce: (σtσ t, u#, π) ( σtt, u#, πr) ACTION(t, a) = Rp where p= A β, σ t = β and t = GOTO(t, A); accept: (t 0 t, #, π) if ACTION(t,a) = accept ; The analyzer stops and accepts the analyzed word and π is the parse for the word (the sequence of productions, in reverse, for the rightmost derivation of w). error: (σ t, au#, π) error if ACTION(t,a) = error ; The analyzer stops and rejects the analyzed word. 27
28 Input: The grammar G = (N, T, S, P) which is SLR(1) ; The SLR(1) parsing table ( ACTION, GOTO); The input word w T*. Output: The bottom-up syntactic parse of w, if w L(G); error, otherwise. The analyzer uses the stack St for performing the shift/reduce transitions 28
29 char ps[] = w# ; //ps is the input word w int i = 0; // the current position of the input letter St.push(t0); // initialize the stack with t0 while(true) { // repeat until success or error t = St.top(); a = ps[i] // a is the current input symbol if(action(t,a) == accept ) exit( accept ); if(action(t,a) == Dt ){ St.push(t ); i++; // move forward in w }//endif else { if(action(t,a) == R A X 1 X 2 X m ){ for( i = 1; i m; i++) St.pop(); St.push(GOTO(St.top, A)); } //endif else exit( error ); }//endelse }//endwhile 29
30 0.S E, 1.E E+T, 2.E T, 3.T T*F, 4.T F, 5.F (E), 6.F a 30
31 a + * ( ) E T F
32 G is not LR(0), as the states 1, 2, 9 contain shift/reduce conflicts FOLLOW(S)={#}, FOLLOW(E)={#,+,)} The grammar is SLR(1) because: In state 1: + FOLLOW(S); In state 2: * FOLLOW(E); In state 9: * FOLLOW(E). 32
33 State ACTION GOTO a + * ( ) # E T F 0 S5 S S6 accept 2 R2 S7 R2 R2 3 R4 R4 R4 R4 4 S5 S R6 R6 R6 R6 6 S5 S S5 S S6 S11 9 R1 S7 R1 R1 10 R2 R3 R2 R2 11 R5 R5 R5 R5 33
34 Stack Input Action Output 0 a*(a+a)# shift 05 *(a+a)# reduce 6.F a 03 *(a+a)# reduce 4.T F 02 *(a+a)# shift 027 (a+a)# shift 0274 a+a)# shift a)# reduce 6.F a a)# reduce 4.T F a)# reduce 2.E T a)# shift 34
35 Stack Input Action Output a)# shift )# reduce 6.F a )# reduce 4.T F )# reduce 1.E E+T )# shift 02748(11) # reduce 5.F (E) 027(10) # reduce 3.T T*F 02 # reduce 2.E T 01 # accept 35
36 Definition Let G = (V, T, S, P) be a reduced grammar. An LR(1) article for the grammar G is a pair (A α β, a), where A α β is an LR(0) article, and a FOLLOW(A) (# instead of ε). Definition The article (A β1 β2, a) is valid for the viable prefix αβ1 if the following are true S r *αau αβ1β2u and a = 1:u (a = # if u = ε). Theorem A grammar G = (N, T, S, P) is LR(1) iff for any viable prefix φ, there are no two distinct articles, valid for φ, with (A α, a), (B β γ, b) where a FIRST(γb). 36
37 There are no shift/reduce conflicts. Such a conflict would mean that two articles (A α, a) and (B β aβ, b) are both valid for the same prefix. There are no reduce/reduce conflicts. Such a conflict would mean that two complete articles (A α, a) and (B β, a) are both valid for the same prefix. To check if a grammar is LR(1) we build the LR(1) automaton in a similar manner to the LR(0) automaton: States contain sets of LR(1) articles Transitions are made by reading symbols to the right of the point Closure is based on the fact that, if the article (B β Aβ, b) is valid for the viable prefix φ then all articles of the form (A α, a) where a FIRTST(αa) are valid for the same prefix. 37
38 flag= true; while( flag) { flag= false; for ( (A α Bβ, a) I) { for B γ P) for( b FIRST(βa)){ if( (B γ, b) I) { I = I {(B γ, b)}; flag = true; }//endif }//endforb }//endforb }//endfora }//endwhile return I; 38
39 t0 = closure((s S,#));T={t 0 };marked(t 0 )=false; while( t T&&!marked(t)){ // marked(t) = false for( X Σ) { t = Φ; for( (A α Xβ,a) t ) t = t {(B αx β,a) (B B α Xβ,a) t}; if( t Φ){ t = closure( t ); if( t T) { T= T { t }; marked( t ) = false; }//endif g(t, X) = t ; } //endif } //endfor marked( t ) = true; } // endwhile 39
40 Theorem The automaton M built in algorithm 2 is deterministic and L(M) is the set of viable prefixes for G. Moreover, for any viable prefix γ, g(t 0,γ) is the set of LR(1) articles valid for γ. The LR(1) automaton can be used to check if G is LR(1) Reduce/reduce conflict: If a state t contains articles of the form (A α, a), (B β, a) then G is not LR(1); Shift/reduce conflict : If a state t contains articles of the form (A α, a) and (B β 1 aβ 2, b), then G is not LR(1). G is LR(1) if all the states t T are free of conflicts 40
41 S L=R R, L *R a, R L 41
42 42
43 Grigoraş Gh., Construcţia compilatoarelor. Algoritmi fundamentali, Editura Universităţii Alexandru Ioan Cuza, Iaşi,
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