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Section 5.5

Section 5.5. Aggelos Kiayias Computer Science & Engineering Department The University of Connecticut 371 Fairfield Road, Unit 1155 Storrs, CT 06269. aggelos@cse.uconn.edu http://www.cse.uconn.edu/~akiayias. Top-Down Translation. F  ( E ) | id.

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Section 5.5

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  1. Section 5.5 Aggelos Kiayias Computer Science & Engineering Department The University of Connecticut 371 Fairfield Road, Unit 1155 Storrs, CT 06269 aggelos@cse.uconn.edu http://www.cse.uconn.edu/~akiayias

  2. Top-Down Translation F  ( E ) | id E  TE’ E’  + TE’ |  T  FT’ T’  * FT’ |  • Implementing L-attributed Definitions during “predictive parsing.” • Recall predictive parsing (chapter 4): • Grammar has no left-recursion and it is left-factored. • For each non-terminal construct a procedure thatimplements all of its productions. • Example: recall the grammar

  3. Fragment of a Predictive Parser main() { E(); } E’() { if lookahead = ‘+’ then { match(‘+’); T(); E’(); } } F() { if lookahead = ‘(’ then { match(‘(‘); E(); match(‘)’); } else ... } TE() { T(); E’(); } T() { F(); T’(); } E’() { if lookahead = ‘*’ then { match(‘*’); F(); T’(); } }

  4. Eliminating Left-Recursion From Translation Schemes TRANSLATION SCHEME E E1+ T {E.val = E1.val + T.val} E E1- T {E.val = E1.val - T.val} E T {E.val = T.val} T (E) {T.val = E.val} T  num {T.val = text.val } Grammar with left recursion removed: E T R {E.val = ?} R + T R {?} R - T R {?} R   {?} T (E) {T.val = E.val} T  num {T.val = text.val }

  5. Eliminating Left-Recursion From Translation Schemes, II TRANSLATION SCHEME E E1+ T {E.val = E1.val + T.val} E E1- T {E.val = E1.val - T.val} E T {E.val = T.val} T (E) {T.val = E.val} T  num {T.val = text.val } Grammar with left recursion removed: E T {R.i = T.val} R {E.val = R.s } R + T {R1.i = R.i + T.val} R1 {R.s = R1.s} R - T {R1.i = R.i - T.val} R1 {R.s = R1.s} R   {R.s = R.i} T (E) {T.val = E.val} T  num {T.val = text.val } Employ for R an inherited and a synthesized attribute i, s. Example: 9-5+2

  6. Eliminating Left-Recursion From Translation Schemes, III In GENERAL: TRANSLATION SCHEME with synthesized attributes A A1 Y {A.a = g(A1.a , Y.y) } A X {A.a = f(X.x) } Translation scheme with left recursion removed: A X {R.i = f(X.x)} R {A.a = R.s) } R Y {R1.i = g(R.i, Y.y)} R1 {A.a = R.s) } R  {R.s = R.i} Example: XYY

  7. Eliminating Left-Recursion from Translation Schemes, IV E E1+ T {E.nptr = mknode(“+”, E1.nptr , T.nptr )} E E1- T {E.nptr = mknode(“-”, E1.nptr , T.nptr )} E T {E.nptr = T.nptr } T (E) {T.nptr = E.nptr} T id {T.nptr = mkleaf(id, id.entry)} T num {T.nptr = mkleaf(num, num.entry)} Translation Scheme with left recursion removed: E T { ??? } R { ??? } R + T { ??? } R1 { ??? } R - T { ??? } R1 { ??? } R   {R.s = R.i} T (E) {T.nptr = E.nptr} T id {T.nptr = mkleaf(id, id.entry)} T num {T.nptr = mkleaf(num, num.entry)}

  8. Eliminating Left-Recursion from Translation Schemes, V E E1+ T {E.nptr = mknode(“+”, E1.nptr , T.nptr )} E E1- T {E.nptr = mknode(“-”, E1.nptr , T.nptr )} E T {E.nptr = T.nptr } T (E) {T.nptr = E.nptr} T id {T.nptr = mkleaf(id, id.entry)} T num {T.nptr = mkleaf(num, num.entry)} Translation Scheme with left recursion removed: E T { R.i = T.nptr } R { E.nptr =R.s} R + T { R1.i = mknode(“+”, R.i , T.nptr )} R1 { R.s = R1.s } R - T { R1.i = mknode(“-”, R.i , T.nptr )} R1 { R.s = R1.s } R   {R.s = R.i} T (E) {T.nptr = E.nptr} T id {T.nptr = mkleaf(id, id.entry)} T num {T.nptr = mkleaf(num, num.entry)}

  9. Translation over a Predictive Parser (Simplified) Translation Scheme E T { R.i = T.nptr } R { E.nptr =R.s} R addop T { R1.i = mknode(addop,lex , R.i , T.nptr )} R1 { R.s = R1.s } R   {R.s = R.i} T (E) {T.nptr = E.nptr} T id {T.nptr = mkleaf(id, id.entry)} T num {T.nptr = mkleaf(num, num.entry)} main() { E(); } Plain Predictive Parser: T() {if lookahead = ‘(’ then { match(‘(’); E(); match(‘)’); } else if lookahead = ‘id’ then {match(‘id’);} else if lookahead = ‘num’ then {match(‘num’);} else error} R() {if lookahead=‘addop’ then { match(‘addop’); T(); R(); } else { } } E() { T(); R(); }

  10. Translation over a Predictive Parser, II node T() {if lookahead = ‘(’ then { match(‘(’); nptr=E(); match(‘)’); } else if lookahead = ‘id’ then { match(‘id’); label = lexval; nptr=mkleaf(label);} else if lookahead = ‘num’ then { match(‘num’); label = lexval; nptr=mkleaf(label);} else error; return nptr} node main() { return E(); } node TD_E() { nptr = T(); return R(nptr); } node R(i:node) {if lookahead=‘addop’ then { match(‘addop’); addoplex=lexval; nptr = T(); i = mknode(addoplex,i,nptr); s = R(i); } else { s=i } return s }

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