slike bài giảng môn chương trình dịch chương 4 syntax analysis

Syntax Analysis II Quan Thanh Tho qttho@dit.hcmut.edu.vn LR Parsing • The most general parsing technique • Usually known as LR(k) parsing technique –Left-to-right scanning –Rightmost derivation detected in reverse –k lookahead tokens, (pratically k ≤ 1) Faculty of CSE Syntax Analysis II 2 LR Parsing Techniques • Simple LR (SLR): easy to implement but not practically powerful • Canonical LR: powerful but expensive in terms of computation • Lookahead LR (LALR): reasonably powerful and expensive Faculty of CSE Syntax Analysis II 3 LR Parsing Algorithm LR Parsing Program Input a+b$ Stack Output Grammatically correct? Which productions involved? s m X m s m-1 action goto Faculty of CSE Syntax Analysis II 4 Stack • Stack-type data structure • Store a string of the form s 0 X 1 s 1 X 2 …X m s m where s m is on the top •X i : a grammar symbol •s i : a state • When implemented, grammar symbols are not necessarily stored in the stack •X 1 X 2 …X m î a part of a sentential form corresponding to the scanned part of input Faculty of CSE Syntax Analysis II 5 Action – Goto Table • Rows: States • Columns: grammar symbols, consisting of two parts: action (for terminals) and goto (for nonterminal) • action[s m ,a i ] – shift s, where s is a state – reduce by a rule A å β – accept – error • goto[s m ,A i ]ås n Faculty of CSE Syntax Analysis II 6 Input • Similar to that of LL(1) table-driven predictive parser • When parsed, the contents of input imply the unexpended part of the initial input string Faculty of CSE Syntax Analysis II 7 Example 1 0 1 2 3 4 5 s3 1 2 + id $ E T s4 a 5 r2 r2 r3 r3 s3 5 r1 r1 (1) EåE+T (2) EåT (3) Tåid Faculty of CSE Syntax Analysis II 8 Configuration • Configuration = stack contents + unexpended input • A configuration represents a sentential form (s 0 X 1 s 1 X 2 …X m s m ,a i a i+1 …a n $) úX 1 X 2 …X m a i a i+1 …a n Faculty of CSE Syntax Analysis II 9 LR Parsing Operational Mechanism • Initial configuration: (0, a 1 …a n $)) • Moving from the current configuration to a new one until reaching accept or error status • action[s m ,a i ] = shift s (s 0 X 1 s 1 X 2 …X m s m ,a i a i+1 …a n $) å (s 0 X 1 s 1 X 2 …X m s m a i s,a i+1 …a n $) • action[s m ,a i ] = reduce Aåβ (s 0 X 1 s 1 X 2 …X m s m ,a i a i+1 …a n $) å (s 0 X 1 s 1 X 2 … X m-r s m-r As, a i a i+1 …a n $) where r = |β|, goto[s m-r ,A] = s Faculty of CSE Syntax Analysis II 10 [...]... T 1 1 s4 a 2 r2 s3 5 r1 Faculty of CSE E T 0E1 +id$ T id 0E1 +4 id$ 5 0 E 1 + 4 id 3 $ $ 0E1 r3 4 +id$ 0E1+4T5 r2 3 r3 2 T id 0T2 + +id$ $ E E+T accept 5 r1 id + id Syntax Analysis II T + id E+ id E+T E 11 Example 3 (1)EåE+T (2) EåT (3) Tåid Stack 0 Input Rule id+$ 0 id 3 0 id $ s3 E T 1 1 s4 a 2 r2 +$ E 0E1 +$ 0E1 +4 $ T error r2 3 r3 2 T id 0T2 + +$ r3 4 s3 5 r1 Faculty of CSE 5 5 r1 Syntax Analysis. .. Tåid Faculty of CSE Syntax Analysis II 27 Example 7 (cont’d) I0 = closure({[E’å E]}) E’å E Eå E+T Eå T Tå id I1 = goto(I0,E) E’åE EåE +T Faculty of CSE Syntax Analysis II 28 Example 7 (cont’d) I2 = goto(I0,T) EåT I3 = goto(I0,id) Tåid I4 = goto(I1,+) EåE+ T Tå id Faculty of CSE Syntax Analysis II 29 Example 7 (cont’d) I5 = goto(I4,T) EåE+T goto(I4,id) = I3 Faculty of CSE Syntax Analysis II 30 Shift... CSE Syntax Analysis II 14 In-depth Operational Mechanism Analysis (cont’d) • How can we keep track of stack and on the – Re-check the grammar whenever the stack contents are changed å high computational cost • How can we manage many instances of and that may occur concurrently Faculty of CSE Syntax Analysis II 15 Example 4 • Given configuration (E+T,+id$) – 1 Faculty of CSE = T, 2 = E+T Syntax Analysis. .. Faculty of CSE Syntax Analysis II 24 Kernel Items • Kernel Items: S’å S and all items whose dots are not at the left end • Nonkernel items: other else (generate by applying closure on kernel items) Faculty of CSE Syntax Analysis II 25 Construction of Item Sets • C = {closure({[S’å S]})} • For each item set I and each grammar symbol X – if I’ = goto(I,X) and I C • add I’ to C Faculty of CSE Syntax Analysis. .. current input symbol: id Tå id is moveable, meaning so is EåE+ T Faculty of CSE Syntax Analysis II 21 Goto Operation (cont’d) • Goto operation: goto(I,X)= closure([Aå X is an item in I Faculty of CSE Syntax Analysis II ]) where Aå X 22 Example 6 • I: EåE+ T EåT Tå id • goto(I,T) EåE+ T • goto(I,id) Tåid EåE+T Faculty of CSE Syntax Analysis II 23 Augmented Grammar • When would we know that we have reached... grammar given in Example 1, compare your result which that given in Example 1 Faculty of CSE Syntax Analysis II 33 Conventional Notations • SLR ú SLR(1) (1 is usually omitted by default in conventional notation) • SLR ú Canonical LR(0), or LR(0) for short • Why 1? Why 0? Faculty of CSE Syntax Analysis II 34 Conventional Notations • SLR ú SLR(1) (1 is usually omitted by default in conventional notation)... = set of items – A å XYZ – A å XYZ – A å X YZ – A å XY Z – A å XYZ Faculty of CSE Syntax Analysis II 17 Closure Operation • An item corresponds to a track of a production currently applied • Problem: there may be multiple tracks coexisting E å E+ T å id T • Solution: Closure operation introduced Faculty of CSE Syntax Analysis II 18 Closure Operation (cont’d) • Closure(I): find all of tracks (items)... action-goto table Faculty of CSE Syntax Analysis II 31 Simple LR Parsing Table Construction • Construct C = {I0,I1,I3,…,In} • Ii ústate i • if Aå a in Ii and goto(Ii,a) = Ij, set action(i,a) = shift j • if Aå X in Ii and goto(Ii,X) = Ij, set goto(i,X) = shift j • If Aå in Ii , set action(i,a) = reduce Aå for each a in Follow(A) • [S’åS ,$] = accept Faculty of CSE Syntax Analysis II 32 Hands-on Exercise... possibly inferred from a set of items I – Add all items in I to closure(I) – If Aå B in I and B å is a production, add Bå to closure(I) Faculty of CSE Syntax Analysis II 19 Example 5 • I: EåE+ T EåT • closure(I) EåE+ T EåT Tå id Faculty of CSE Syntax Analysis II 20 Goto Operation • How to follow tracks in an item set, given a new input symbol a? – Aå – Aå a : possible to move on b : impossible to move... – 1: 1 lookahead token for whole item set Faculty of CSE Syntax Analysis II 35 Conventional Notations • SLR ú SLR(1) (1 is usually omitted by default in conventional notation) • SLR ú Canonical LR(0), or LR(0) for short • Why 1? Why 0? – 1: 1 lookahead token for whole item set – 0: 0 lookahead token for individual item Faculty of CSE Syntax Analysis II 36 . input string Faculty of CSE Syntax Analysis II 7 Example 1 0 1 2 3 4 5 s3 1 2 + id $ E T s4 a 5 r2 r2 r3 r3 s3 5 r1 r1 (1) EåE+T (2) EåT (3) Tåid Faculty of CSE Syntax Analysis II 8 Configuration •. CSE Syntax Analysis II 10 Example 2 (1)Eå E+T (2) EåT (3) Tå id Stack 0 0 id 3 0 T 2 0 E 1 0 E 1 + 4 0 E 1 + 4 id 3 0 E 1 + 4 T 5 0 E 1 Input id+id$ +id$ +id$ +id$ id$ $ $ $ Faculty of CSE Syntax. 3 (1)Eå E+T (2) EåT (3) Tå id Faculty of CSE Syntax Analysis II 12 0 1 2 3 4 5 s3 1 2 + id $ E T s4 a 5 r2 r2 r3 r3 s3 5 r1 r1 Stack 0 0 id 3 0 T 2 0 E 1 0 E 1 + 4 Input id+$ +$ +$ +$ $ Rule T→id E → T error Hands-on