BottomUp Parsing II (Different types of ShiftReduce Conflicts) Lecture 10. Prof. Aiken (Modified by Professor Vijay Ganesh.


 Chastity Kennedy
 4 years ago
 Views:
Transcription
1 BottomUp Parsing II Different types of ShiftReduce Conflicts) Lecture 10 Ganesh. Lecture 10) 1
2 Review: BottomUp Parsing Bottomup parsing is more general than topdown parsing And just as efficient Doesn t have issues with leftrecursion Doesn t require leftfactoring Many wellknown parser generators Yacc, Bison, ) Can handle many more grammars without backtracking than otherwise LRk) parsers are bottomup 2
3 Review: An Introductory xample Bottomup parsers don t need leftfactored grammars Revert to the natural grammar for our example: + * ) Consider the string: * + 3
4 Review: he Idea Bottomup parsing reduces a string to the start symbol by inverting productions: * + * * + 4
5 Review: Bottomup Parsers and Rightmost derivations in reverse Read the productions in reverse from bottom to top) his is a rightmost derivation! * + * * + 5
6 Review: Notation useful for ShiftReduce Parsers Idea: Split string o two substrings Right substring is as yet unexamined by parsing a string of terminals) Left substring has terminals and nonterminals he dividing po is marked by a he is not part of the string Initially, all input is unexamined x 1 x 2... x n 6
7 Review: ShiftReduce Parsing Bottomup parsing uses only two kinds of actions: Shift Reduce 7
8 Review: Shift Shift: Move one place to the right Shifts a terminal to the left string ABC xyz ABCx yz 8
9 Review: Reduce Apply an inverse production at the right end of the left string If D Cx is a production, then ABCx yz ABD yz 9
10 he Stack Left string can be implemented by a stack op of the stack is the Shift pushes a terminal on the stack Reduce pops 0 or more symbols off of the stack production rhs) and pushes a nonterminal on the stack production lhs) Prof. Aiken CS 143 Lecture 7 10
11 Review: he xample with Reductions Only * + * + reduce reduce * reduce reduce reduce + 11
12 Review: he xample with ShiftReduce Parsing * + * + * + * + * shift shift shift reduce reduce * shift shift reduce reduce reduce + 12
13 Conflicts In a given state, more than one action shift or reduce) may lead to a valid parse If it is legal to shift or reduce, there is a shiftreduce conflict If it is legal to reduce by two different productions, there is a reducereduce conflict Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 13
14 Key Issue: o Shift or Reduce? How do we decide when to shift or reduce? xample grammar: + * ) Consider step * + We could reduce by giving * + A fatal mistake! No way to reduce to the start symbol 14
15 Handles: Symbols replaced by Reduction Intuition: Want to reduce only if the result can still be reduced to the start symbol Handle: Informally, represents the RHS of a production. Let X β be a production in G. hen β in the position after α is a handle of αβω 15
16 Handles Cont.) Handles formalize the uition A handle is a string that can be reduced and also allows further reductions back to the start symbol using a particular production at a specific spot) We only want to reduce at handles Note: We have said what a handle is, not how to find handles 16
17 Important Fact #2 Important Fact #2 about bottomup parsing: In shiftreduce parsing, handles appear only at the top of the stack, never inside Using the symbol already shifted o the stack left context) and the next k lookahead symbols right context), decide to shift or reduce 17
18 A ShiftReduce Parse in Detail 1) * + * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 18
19 A ShiftReduce Parse in Detail 2) * + * + * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 19
20 A ShiftReduce Parse in Detail 3) * + * + * + * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 20
21 A ShiftReduce Parse in Detail 4) * + * + * + * + * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 21
22 A ShiftReduce Parse in Detail 5) * + * + * + * + * + * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 22
23 A ShiftReduce Parse in Detail 6) * + * + * + * + * + + * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 23
24 A ShiftReduce Parse in Detail 7) * + * + * + * + * * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 24
25 A ShiftReduce Parse in Detail 8) * + * + * + * + * * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 25
26 A ShiftReduce Parse in Detail 9) * + * + * + * + * * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 26
27 A ShiftReduce Parse in Detail 10) * + * + * + * + * * + Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) 27
28 A ShiftReduce Parse in Detail 11) * + * + * + * + * * Prof. Alex Aiken Lecture 9 Modified by Professor Vijay Ganesh) + 28
29 heorem: Handle always on top of Stack Proof Method: Informal induction on # of reduce moves Proof: rue initially, stack is empty Immediately after reducing a handle rightmost nonterminal on top of the stack next handle must be to right of rightmost nonterminal, because this is a rightmost derivation Sequence of shift moves reaches next handle 29
30 Summary of Handles In shiftreduce parsing, handles always appear at the top of the stack Handles are never to the left of the rightmost nonterminal herefore, shiftreduce moves are sufficient; the need never move left Bottomup parsing algorithms are based on recognizing handles 30
31 Recognizing Handles here are no known efficient algorithms to recognize handles Solution: use heuristics to guess which stacks are handles On some CFGs, the heuristics always guess correctly For the heuristics we use here, these are the SLR grammars Other heuristics work for other grammars 31
32 Grammars All CFGs Unambiguous CFGs will generate conflicts SLR CFGs 32
33 Viable Prefixes It is not obvious how to detect handles At each step the parser sees only the stack, not the entire input; start with that... α is a viable prefix if there is an ω such that α ω is a state of a shiftreduce parser 33
34 Huh? What does this mean? A few things: A viable prefix does not extend past the right end of the handle It s a viable prefix because it is a prefix of the handle As long as a parser has viable prefixes on the stack no parsing error has been detected 34
35 Important Fact #3 Important Fact #3 about bottomup parsing: For any grammar, the set of viable prefixes is a regular language 35
36 Important Fact #3 Cont.) Important Fact #3 is nonobvious We show how to compute automata that accept viable prefixes 36
37 Items An item is a production with a. somewhere on the rhs he items for ) are.).).) ). 37
38 Items Cont.) he only item for X is X. Items are often called LR0) items 38
39 Intuition he problem in recognizing viable prefixes is that the stack has only bits and pieces of the rhs of productions If it had a complete rhs, we could reduce hese bits and pieces are always prefixes of rhs of productions 39
40 xample Consider the input ) hen ) is a state of a shiftreduce parse is a prefix of the rhs of ) Will be reduced after the next shift Item.) says that so far we have seen of this production and hope to see ) 40
41 Generalization he stack may have many prefixes of rhs s Prefix 1 Prefix 2... Prefix n1 Prefix n Let Prefix i be a prefix of rhs of X i α i Prefix i will eventually reduce to X i he missing part of α i1 starts with X i i.e. there is a X i1 Prefix i1 X i β for some β Recursively, Prefix k+1 Prefix n eventually reduces to the missing part of α k 41
42 An xample Consider the string * ): * ) is a state of a shiftreduce parse is a prefix of the rhs of ) is a prefix of the rhs of * is a prefix of the rhs of * 42
43 An xample Cont.) he stack of items.). *. Says We ve seen of ) We ve seen of We ve seen * of * 43
44 Recognizing Viable Prefixes Idea: o recognize viable prefixes, we must Recognize a sequence of partial rhs s of productions, where ach sequence can eventually reduce to part of the missing suffix of its predecessor 44
45 An NFA Recognizing Viable Prefixes 1. Add a dummy production S S to G 2. he NFA states are the items of G Including the extra production 3. For item α.xβ add transition α.xβ X αx.β 4. For item α.xβ and production X γ add α.xβ X.γ 45
46 An NFA Recognizing Viable Prefixes Cont.) 5. very state is an accepting state 6. Start state is S.S 46
47 NFA for Viable Prefixes of the xample ). ).).) ). S.. + S *..* * *. 47
48 NFA for Viable Prefixes in Detail 1) S. 48
49 NFA for Viable Prefixes in Detail 2) S. S
50 NFA for Viable Prefixes in Detail 3). ) S.. + S
51 NFA for Viable Prefixes in Detail 4). ) S.. + S
52 NFA for Viable Prefixes in Detail 5). ).) S.. + S
53 NFA for Viable Prefixes in Detail 6). ).) S.. + S...)
54 NFA for Viable Prefixes in Detail 7). ).).) ) ). S.. + S
55 NFA for Viable Prefixes in Detail 8). ).).) ) ). S.. + S
56 NFA for Viable Prefixes in Detail 9). ).) S.. + S...).+ + ) )
57 NFA for Viable Prefixes in Detail 10). ).) S.. + S...).+. ) + )
58 NFA for Viable Prefixes in Detail 11). ).) S.. + S...).+. ) + ) *. 58
59 NFA for Viable Prefixes in Detail 12). ).) S.. + S...).+. ) + ) *..* * 59
60 NFA for Viable Prefixes in Detail 13). ).) S.. + S...).+. ) + ) *..* * *. 60
61 +. ranslation to the DFA S * S *.) *..).. +.). *..) +.. ).. +.)... ). ) 61
62 Lingo he states of the DFA are canonical collections of items or canonical collections of LR0) items he Dragon book gives another way of constructing LR0) items 62
63 Valid Items Item X β.γ is valid for a viable prefix αβ if S * αxω αβγω by a rightmost derivation After parsing αβ, the valid items are the possible tops of the stack of items 63
64 Items Valid for a Prefix An item I is valid for a viable prefix α if the DFA recognizing viable prefixes terminates on input α in a state s containing I he items in s describe what the top of the item stack might be after reading input α 64
65 Valid Items xample An item is often valid for many prefixes xample: he item.) is valid for prefixes... 65
66 +. Valid Items for S * S *.) *..) ). *..) +.. ).. +.)... ). ) 66
67 LR0) Parsing Idea: Assume stack contains α next input is t DFA on input α terminates in state s Reduce by X β if s contains item X β. Shift if s contains item X β.tω equivalent to saying s has a transition labeled t 67
68 LR0) Conflicts LR0) has a reduce/reduce conflict if: Any state has two reduce items: X β. and Y ω. LR0) has a shift/reduce conflict if: Any state has a reduce item and a shift item: X β. and Y ω.tδ 68
69 +. LR0) Conflicts S * S *.) *..) ).. ).. + *..).) wo shift/reduce conflicts with LR0) rules... ). ) 69
70 SLR LR = Lefttoright scan SLR = Simple LR SLR improves on LR0) shift/reduce heuristics Fewer states have conflicts 70
71 SLR Parsing Idea: Assume stack contains α next input is t DFA on input α terminates in state s Reduce by X β if s contains item X β. t FollowX) Shift if s contains item X β.tω 71
72 SLR Parsing Cont.) If there are conflicts under these rules, the grammar is not SLR he rules amount to a heuristic for detecting handles he SLR grammars are those where the heuristics detect exactly the handles 72
73 +. SLR Conflicts S... + S... *.. + *.). *..) ). *..) ) ). +.. ).. + Follow) = { ), $ }.) Follow) = { +, ), $ }. No conflicts with SLR rules! 73
74 Precedence Declarations Digression Lots of grammars aren t SLR including all ambiguous grammars We can parse more grammars by using precedence declarations Instructions for resolving conflicts 74
75 Precedence Declarations Cont.) Consider our favorite ambiguous grammar: + * ) he DFA for this grammar contains a state with the following items: *.. + shift/reduce conflict! Declaring * has higher precedence than + resolves this conflict in favor of reducing 75
76 Precedence Declarations Cont.) he term precedence declaration is misleading hese declarations do not define precedence; they define conflict resolutions Not quite the same thing! 76
77 Naïve SLR Parsing Algorithm 1. Let M be DFA for viable prefixes of G 2. Let x 1 x n $ be initial configuration 3. Repeat until configuration is S $ Let α ω be current configuration Run M on current stack α If M rejects α, report parsing error Stack α is not a viable prefix If M accepts α with items I, let a be next input Shift if X β. a γ I Reduce if X β. I and a FollowX) Report parsing error if neither applies 77
78 Notes If there is a conflict in the last step, grammar is not SLRk) k is the amount of lookahead In practice k = 1 78
79 SLR xample Configuration DFA Halt State Action * $ 1 shift 79
80 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
81 SLR xample Configuration DFA Halt State Action * $ 1 shift * $ 3 * not in Follow) shift 81
82 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
83 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
84 SLR xample Configuration DFA Halt State Action * $ 1 shift * $ 3 * not in Follow) shift * $ 11 shift 84
85 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
86 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
87 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
88 SLR xample Configuration DFA Halt State Action * $ 1 shift * $ 3 * not in Follow) shift * $ 11 shift * $ 3 $ Follow) red. 88
89 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
90 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
91 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
92 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
93 SLR xample Configuration DFA Halt State Action * $ 1 shift * $ 3 * not in Follow) shift * $ 11 shift * $ 3 $ Follow) red. * $ 4 $ Follow) red. * 93
94 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
95 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
96 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
97 2 Configuration * $ S.. S... +.) *. * *..) ) ). *..) ) ).. +.)
98 SLR xample Configuration DFA Halt State Action * $ 1 shift * $ 3 * not in Follow) shift * $ 11 shift * $ 3 $ Follow) red. * $ 4 $ Follow) red. * $ 5 $ Follow) red. 98
99 Configuration $ S.. S... +.) *. * *..) ). *..) ) ) ).. +.) 8. 99
100 Configuration $ S.. S... +.) *. * *..) ). *..) ) ) ).. +.)
101 SLR xample Configuration DFA Halt State Action * $ 1 shift * $ 3 * not in Follow) shift * $ 11 shift * $ 3 $ Follow) red. * $ 4 $ Follow) red. * $ 5 $ Follow) red. $ accept 101
102 Notes Skipped using extra start state S in this example to save space on slides Rerunning the automaton at each step is wasteful Most of the work is repeated 102
103 An Improvement Remember the state of the automaton on each prefix of the stack Change stack to contain pairs Symbol, DFA State 103
104 An Improvement Cont.) For a stack sym 1, state 1... sym n, state n state n is the final state of the DFA on sym 1 sym n Detail: he bottom of the stack is any,start where any is any dummy symbol start is the start state of the DFA 104
105 Goto able Define goto[i,a] = j if state i A state j goto is just the transition function of the DFA One of two parsing tables 105
106 Refined Parser Moves Shift x Push a, x on the stack a is current input x is a DFA state Reduce X α As before Accept rror 106
107 Action able For each state s i and terminal a If s i has item X α.aβ and goto[i,a] = j then action[i,a] = shift j If s i has item X α. and a FollowX) and X S then action[i,a] = reduce X α If s i has item S S. then action[i,$] = accept Otherwise, action[i,a] = error 107
108 SLR Parsing Algorithm Let I = w$ be initial input Let j = 0 Let DFA state 1 have item S.S Let stack = dummy, 1 repeat case action[top_statestack),i[j]] of shift k: push I[j++], k reduce X A: pop A pairs, push X, goto[top_statestack),x] accept: halt normally error: halt and report error 108
109 Notes on SLR Parsing Algorithm Note that the algorithm uses only the DFA states and the input he stack symbols are never used! However, we still need the symbols for semantic actions 109
110 More Notes Some common constructs are not SLR1) LR1) is more powerful Build lookahead o the items An LR1) item is a pair: LR0) item x lookahead [. *, $] means After seeing * reduce if lookahead is $ More accurate than just using follow sets ake a look at the LR1) automaton for your parser! 110
Review: ShiftReduce Parsing. Bottomup parsing uses two actions: BottomUp Parsing II. Shift ABC xyz ABCx yz. Lecture 8. Reduce Cbxy ijk CbA ijk
Review: ShiftReduce Parsing Bottomup parsing uses two actions: BottomUp Parsing II Lecture 8 Shift ABC xyz ABCx yz Reduce Cbxy ijk CbA ijk Prof. Aiken CS 13 Lecture 8 1 Prof. Aiken CS 13 Lecture 8 2
More informationBottomUp Parsing II. Lecture 8
BottomUp Parsing II Lecture 8 1 Review: ShiftReduce Parsing Bottomup parsing uses two actions: Shift ABC xyz ABCx yz Reduce Cbxy ijk CbA ijk 2 Recall: he Stack Left string can be implemented by a stack
More informationOutline. The strategy: shiftreduce parsing. Introduction to BottomUp Parsing. A key concept: handles
Outline Introduction to BottomUp Parsing Lecture Notes by Profs. Alex Aiken and George Necula (UCB) he strategy: reduce parsing A key concept: handles Ambiguity and precedence declarations CS780(Prasad)
More informationReview of CFGs and Parsing II Bottomup Parsers. Lecture 5. Review slides 1
Review of CFGs and Parsing II Bottomup Parsers Lecture 5 1 Outline Parser Overview opdown Parsers (Covered largely through labs) Bottomup Parsers 2 he Functionality of the Parser Input: sequence of
More informationIntroduction to BottomUp Parsing
Introduction to BottomUp Parsing Lecture 11 CS 536 Spring 2001 1 Outline he strategy: shiftreduce parsing Ambiguity and precedence declarations Next lecture: bottomup parsing algorithms CS 536 Spring
More informationIntro to Bottomup Parsing. Lecture 9
Intro to Bottomup Parsing Lecture 9 BottomUp Parsing Bottomup parsing is more general than topdown parsing And just as efficient Builds on ideas in topdown parsing Bottomup is the preferred method
More informationBottomUp Parsing. Lecture 1112
BottomUp Parsing Lecture 1112 (From slides by G. Necula & R. Bodik) 2/20/08 Prof. Hilfinger CS164 Lecture 11 1 Administrivia Test I during class on 10 March. 2/20/08 Prof. Hilfinger CS164 Lecture 11
More informationA bottomup parser traces a rightmost derivation in reverse. BottomUp Parsing. Bottomup parsing is more general than topdown.
BottomUp Parsing BottomUp Parsing Bottomup parsing is more general than topdown parsing And just as efficient Builds on ideas in topdown parsing Bottomup is the preferred method Originated from Prof.
More informationBottomUp Parsing. Lecture 1112
BottomUp Parsing Lecture 1112 (From slides by G. Necula & R. Bodik) 9/22/06 Prof. Hilfinger CS164 Lecture 11 1 BottomUp Parsing Bottomup parsing is more general than topdown parsing And just as efficient
More informationLecture 8: Deterministic BottomUp Parsing
Lecture 8: Deterministic BottomUp Parsing (From slides by G. Necula & R. Bodik) Last modified: Fri Feb 12 13:02:57 2010 CS164: Lecture #8 1 Avoiding nondeterministic choice: LR We ve been looking at general
More informationLecture 7: Deterministic BottomUp Parsing
Lecture 7: Deterministic BottomUp Parsing (From slides by G. Necula & R. Bodik) Last modified: Tue Sep 20 12:50:42 2011 CS164: Lecture #7 1 Avoiding nondeterministic choice: LR We ve been looking at general
More informationTopDown Parsing and Intro to BottomUp Parsing. Lecture 7
TopDown Parsing and Intro to BottomUp Parsing Lecture 7 1 Predictive Parsers Like recursivedescent but parser can predict which production to use Predictive parsers are never wrong Always able to guess
More informationTopDown Parsing and Intro to BottomUp Parsing. Lecture 7
TopDown Parsing and Intro to BottomUp Parsing Lecture 7 1 Predictive Parsers Like recursivedescent but parser can predict which production to use Predictive parsers are never wrong Always able to guess
More informationBottomUp Parsing LR Parsing
BottomUp Parsing LR Parsing Maryam Siahbani 2/19/2016 1 What we need for LR parsing LR0) states: Describe all possible states in which parser can be Parsing table ransition between LR0) states Actions
More informationLR Parsing LALR Parser Generators
Outline LR Parsing LALR Parser Generators Review of bottomup parsing Computing the parsing DFA Using parser generators 2 Bottomup Parsing (Review) A bottomup parser rewrites the input string to the
More informationLR Parsing LALR Parser Generators
LR Parsing LALR Parser Generators Outline Review of bottomup parsing Computing the parsing DFA Using parser generators 2 Bottomup Parsing (Review) A bottomup parser rewrites the input string to the
More informationS Y N T A X A N A L Y S I S LR
LR parsing There are three commonly used algorithms to build tables for an LR parser: 1. SLR(1) = LR(0) plus use of FOLLOW set to select between actions smallest class of grammars smallest tables (number
More informationLexical and Syntax Analysis. BottomUp Parsing
Lexical and Syntax Analysis BottomUp Parsing Parsing There are two ways to construct derivation of a grammar. TopDown: begin with start symbol; repeatedly replace an instance of a production s LHS with
More informationMIT Parse Table Construction. Martin Rinard Laboratory for Computer Science Massachusetts Institute of Technology
MIT 6.035 Parse Table Construction Martin Rinard Laboratory for Computer Science Massachusetts Institute of Technology Parse Tables (Review) ACTION Goto State ( ) $ X s0 shift to s2 error error goto s1
More informationBottom up parsing. The sentential forms happen to be a right most derivation in the reverse order. S a A B e a A d e. a A d e a A B e S.
Bottom up parsing Construct a parse tree for an input string beginning at leaves and going towards root OR Reduce a string w of input to start symbol of grammar Consider a grammar S aabe A Abc b B d And
More informationFormal Languages and Compilers Lecture VII Part 3: Syntactic A
Formal Languages and Compilers Lecture VII Part 3: Syntactic Analysis Free University of BozenBolzano Faculty of Computer Science POS Building, Room: 2.03 artale@inf.unibz.it http://www.inf.unibz.it/
More informationCompilers. Bottomup Parsing. (original slides by Sam
Compilers Bottomup Parsing Yannis Smaragdakis U Athens Yannis Smaragdakis, U. Athens (original slides by Sam Guyer@Tufts) BottomUp Parsing More general than topdown parsing And just as efficient Builds
More informationLR Parsing Techniques
LR Parsing Techniques Introduction BottomUp Parsing LR Parsing as Handle Pruning ShiftReduce Parser LR(k) Parsing Model Parsing Table Construction: SLR, LR, LALR 1 BottomUP Parsing A bottomup parser
More informationBottom Up Parsing. Shift and Reduce. Sentential Form. Handle. Parse Tree. Bottom Up Parsing 9/26/2012. Also known as ShiftReduce parsing
Also known as ShiftReduce parsing More powerful than top down Don t need left factored grammars Can handle left recursion Attempt to construct parse tree from an input string eginning at leaves and working
More informationLecture BottomUp Parsing
Lecture 14+15 BottomUp Parsing CS 241: Foundations of Sequential Programs Winter 2018 Troy Vasiga et al University of Waterloo 1 Example CFG 1. S S 2. S AyB 3. A ab 4. A cd 5. B z 6. B wz 2 Stacks in
More informationPART 3  SYNTAX ANALYSIS. F. Wotawa TU Graz) Compiler Construction Summer term / 309
PART 3  SYNTAX ANALYSIS F. Wotawa (IST @ TU Graz) Compiler Construction Summer term 2016 64 / 309 Goals Definition of the syntax of a programming language using context free grammars Methods for parsing
More informationA leftsentential form is a sentential form that occurs in the leftmost derivation of some sentence.
Bottomup parsing Recall For a grammar G, with start symbol S, any string α such that S α is a sentential form If α V t, then α is a sentence in L(G) A leftsentential form is a sentential form that occurs
More informationShift. Reduce. Review: ShiftReduce Parsing. Bottomup parsing uses two actions: BottomUp Parsing II. ABC xyz ABCx yz. Lecture 8.
Rviw: ShiftRduc Parsing Bottomup parsing uss two actions: BottomUp Parsing II Lctur 8 Shift ABC xyz ABCx yz Rduc Cbxy ijk CbA ijk Prof. Aikn CS 13 Lctur 8 1 Prof. Aikn CS 13 Lctur 8 2 Rcall: h Stack
More informationSLR parsers. LR(0) items
SLR parsers LR(0) items As we have seen, in order to make shiftreduce parsing practical, we need a reasonable way to identify viable prefixes (and so, possible handles). Up to now, it has not been clear
More informationIn One Slide. Outline. LR Parsing. Table Construction
LR Parsing Table Construction #1 In One Slide An LR(1) parsing table can be constructed automatically from a CFG. An LR(1) item is a pair made up of a production and a lookahead token; it represents a
More informationshiftreduce parsing
Parsing #2 Bottomup Parsing Rightmost derivations; use of rules from right to left Uses a stack to push symbols the concatenation of the stack symbols with the rest of the input forms a valid bottomup
More informationCompiler Design 1. BottomUP Parsing. Goutam Biswas. Lect 6
Compiler Design 1 BottomUP Parsing Compiler Design 2 The Process The parse tree is built starting from the leaf nodes labeled by the terminals (tokens). The parser tries to discover appropriate reductions,
More informationMODULE 14 SLR PARSER LR(0) ITEMS
MODULE 14 SLR PARSER LR(0) ITEMS In this module we shall discuss one of the LR type parser namely SLR parser. The various steps involved in the SLR parser will be discussed with a focus on the construction
More informationCS 4120 Introduction to Compilers
CS 4120 Introduction to Compilers Andrew Myers Cornell University Lecture 6: BottomUp Parsing 9/9/09 Bottomup parsing A more powerful parsing technology LR grammars  more expressive than LL can handle
More informationCS 2210 Sample Midterm. 1. Determine if each of the following claims is true (T) or false (F).
CS 2210 Sample Midterm 1. Determine if each of the following claims is true (T) or false (F). F A language consists of a set of strings, its grammar structure, and a set of operations. (Note: a language
More informationParsing Wrapup. Roadmap (Where are we?) Last lecture Shiftreduce parser LR(1) parsing. This lecture LR(1) parsing
Parsing Wrapup Roadmap (Where are we?) Last lecture Shiftreduce parser LR(1) parsing LR(1) items Computing closure Computing goto LR(1) canonical collection This lecture LR(1) parsing Building ACTION
More information3. Syntax Analysis. Andrea Polini. Formal Languages and Compilers Master in Computer Science University of Camerino
3. Syntax Analysis Andrea Polini Formal Languages and Compilers Master in Computer Science University of Camerino (Formal Languages and Compilers) 3. Syntax Analysis CS@UNICAM 1 / 54 Syntax Analysis: the
More informationCSE P 501 Compilers. LR Parsing Hal Perkins Spring UW CSE P 501 Spring 2018 D1
CSE P 501 Compilers LR Parsing Hal Perkins Spring 2018 UW CSE P 501 Spring 2018 D1 Agenda LR Parsing Tabledriven Parsers Parser States ShiftReduce and ReduceReduce conflicts UW CSE P 501 Spring 2018
More informationBottomup parsing. BottomUp Parsing. Recall. Goal: For a grammar G, withstartsymbols, any string α such that S α is called a sentential form
Bottomup parsing Bottomup parsing Recall Goal: For a grammar G, withstartsymbols, any string α such that S α is called a sentential form If α V t,thenα is called a sentence in L(G) Otherwise it is just
More informationMonday, September 13, Parsers
Parsers Agenda Terminology LL(1) Parsers Overview of LR Parsing Terminology Grammar G = (Vt, Vn, S, P) Vt is the set of terminals Vn is the set of nonterminals S is the start symbol P is the set of productions
More informationParsing. Handle, viable prefix, items, closures, goto s LR(k): SLR(1), LR(1), LALR(1)
TD parsing  LL(1) Parsing First and Follow sets Parse table construction BU Parsing Handle, viable prefix, items, closures, goto s LR(k): SLR(1), LR(1), LALR(1) Problems with SLR Aho, Sethi, Ullman, Compilers
More informationUNITIII BOTTOMUP PARSING
UNITIII BOTTOMUP PARSING Constructing a parse tree for an input string beginning at the leaves and going towards the root is called bottomup parsing. A general type of bottomup parser is a shiftreduce
More informationConflicts in LR Parsing and More LR Parsing Types
Conflicts in LR Parsing and More LR Parsing Types Lecture 10 Dr. Sean Peisert ECS 142 Spring 2009 1 Status Project 2 Due Friday, Apr. 24, 11:55pm The usual lecture time is being replaced by a discussion
More informationEDAN65: Compilers, Lecture 06 A LR parsing. Görel Hedin Revised:
EDAN65: Compilers, Lecture 06 A LR parsing Görel Hedin Revised: 20170911 This lecture Regular expressions Contextfree grammar Attribute grammar Lexical analyzer (scanner) Syntactic analyzer (parser)
More informationContextfree grammars
Contextfree grammars Section 4.2 Formal way of specifying rules about the structure/syntax of a program terminals  tokens nonterminals  represent higherlevel structures of a program start symbol,
More informationWednesday, August 31, Parsers
Parsers How do we combine tokens? Combine tokens ( words in a language) to form programs ( sentences in a language) Not all combinations of tokens are correct programs (not all sentences are grammatically
More informationPrinciples of Programming Languages
Principles of Programming Languages h"p://www.di.unipi.it/~andrea/dida2ca/plp 14/ Prof. Andrea Corradini Department of Computer Science, Pisa Lesson 8! Bo;om Up Parsing Shi? Reduce LR(0) automata and
More informationParser Generation. BottomUp Parsing. Constructing LR Parser. LR Parsing. Construct parse tree bottomup  from leaves to the root
Parser Generation Main Problem: given a grammar G, how to build a topdown parser or a bottomup parser for it? parser : a program that, given a sentence, reconstructs a derivation for that sentence 
More informationSection A. A grammar that produces more than one parse tree for some sentences is said to be ambiguous.
Section A 1. What do you meant by parser and its types? A parser for grammar G is a program that takes as input a string w and produces as output either a parse tree for w, if w is a sentence of G, or
More informationCompiler Construction: Parsing
Compiler Construction: Parsing Mandar Mitra Indian Statistical Institute M. Mitra (ISI) Parsing 1 / 33 Contextfree grammars. Reference: Section 4.2 Formal way of specifying rules about the structure/syntax
More informationWednesday, September 9, 15. Parsers
Parsers What is a parser A parser has two jobs: 1) Determine whether a string (program) is valid (think: grammatically correct) 2) Determine the structure of a program (think: diagramming a sentence) Agenda
More informationParsers. What is a parser. Languages. Agenda. Terminology. Languages. A parser has two jobs:
What is a parser Parsers A parser has two jobs: 1) Determine whether a string (program) is valid (think: grammatically correct) 2) Determine the structure of a program (think: diagramming a sentence) Agenda
More informationLecture 14: Parser Conflicts, Using Ambiguity, Error Recovery. Last modified: Mon Feb 23 10:05: CS164: Lecture #14 1
Lecture 14: Parser Conflicts, Using Ambiguity, Error Recovery Last modified: Mon Feb 23 10:05:56 2015 CS164: Lecture #14 1 Shift/Reduce Conflicts If a DFA state contains both [X: α aβ, b] and [Y: γ, a],
More informationParsers. Xiaokang Qiu Purdue University. August 31, 2018 ECE 468
Parsers Xiaokang Qiu Purdue University ECE 468 August 31, 2018 What is a parser A parser has two jobs: 1) Determine whether a string (program) is valid (think: grammatically correct) 2) Determine the structure
More informationLR Parsing. Leftmost and Rightmost Derivations. Compiler Design CSE 504. Derivations for id + id: T id = id+id. 1 ShiftReduce Parsing.
LR Parsing Compiler Design CSE 504 1 ShiftReduce Parsing 2 LR Parsers 3 SLR and LR(1) Parsers Last modifled: Fri Mar 06 2015 at 13:50:06 EST Version: 1.7 16:58:46 2016/01/29 Compiled at 12:57 on 2016/02/26
More informationFormal Languages and Compilers Lecture VII Part 4: Syntactic A
Formal Languages and Compilers Lecture VII Part 4: Syntactic Analysis Free University of BozenBolzano Faculty of Computer Science POS Building, Room: 2.03 artale@inf.unibz.it http://www.inf.unibz.it/
More informationSyntax Analysis. Amitabha Sanyal. (www.cse.iitb.ac.in/ as) Department of Computer Science and Engineering, Indian Institute of Technology, Bombay
Syntax Analysis (www.cse.iitb.ac.in/ as) Department of Computer Science and Engineering, Indian Institute of Technology, Bombay September 2007 College of Engineering, Pune Syntax Analysis: 2/124 Syntax
More informationLet us construct the LR(1) items for the grammar given below to construct the LALR parsing table.
MODULE 18 LALR parsing After understanding the most powerful CALR parser, in this module we will learn to construct the LALR parser. The CALR parser has a large set of items and hence the LALR parser is
More informationUNIT III & IV. Bottom up parsing
UNIT III & IV Bottom up parsing 5.0 Introduction Given a grammar and a sentence belonging to that grammar, if we have to show that the given sentence belongs to the given grammar, there are two methods.
More informationCS5371 Theory of Computation. Lecture 8: Automata Theory VI (PDA, PDA = CFG)
CS5371 Theory of Computation Lecture 8: Automata Theory VI (PDA, PDA = CFG) Objectives Introduce Pushdown Automaton (PDA) Show that PDA = CFG In terms of descriptive power Pushdown Automaton (PDA) Roughly
More informationLR Parsing, Part 2. Constructing Parse Tables. An NFA Recognizing Viable Prefixes. Computing the Closure. GOTO Function and DFA States
TDDD16 Compilers and Interpreters TDDB44 Compiler Construction LR Parsing, Part 2 Constructing Parse Tables Parse table construction Grammar conflict handling Categories of LR Grammars and Parsers An NFA
More informationParsing III. CS434 Lecture 8 Spring 2005 Department of Computer Science University of Alabama Joel Jones
Parsing III (Topdown parsing: recursive descent & LL(1) ) (Bottomup parsing) CS434 Lecture 8 Spring 2005 Department of Computer Science University of Alabama Joel Jones Copyright 2003, Keith D. Cooper,
More informationBottom Up Parsing Handout. 1 Introduction. 2 Example illustrating bottomup parsing
Bottom Up Parsing Handout Compiled by: Nomair. Naeem dditional Material by: driel DeanHall and Brad Lushman his handout is intended to accompany material covered during lectures and is not consered a
More informationAmbiguity. Grammar E E + E E * E ( E ) int. The string int * int + int has two parse trees. * int
Administrivia Ambiguity, Precedence, Associativity & opdown Parsing eam assignments this evening for all those not listed as having one. HW#3 is now available, due next uesday morning (Monday is a holiday).
More informationExample CFG. Lectures 16 & 17 BottomUp Parsing. LL(1) Predictor Table Review. Stacks in LR Parsing 1. Sʹ " S. 2. S " AyB. 3. A " ab. 4.
Example CFG Lectures 16 & 17 BottomUp Parsing CS 241: Foundations of Sequential Programs Fall 2016 1. Sʹ " S 2. S " AyB 3. A " ab 4. A " cd Matt Crane University of Waterloo 5. B " z 6. B " wz 2 LL(1)
More informationCSE302: Compiler Design
CSE302: Compiler Design Instructor: Dr. Liang Cheng Department of Computer Science and Engineering P.C. Rossin College of Engineering & Applied Science Lehigh University March 20, 2007 Outline Recap LR(0)
More informationMore BottomUp Parsing
More BottomUp Parsing Lecture 7 Dr. Sean Peisert ECS 142 Spring 2009 1 Status Project 1 Back By Wednesday (ish) savior lexer in ~cs142/s09/bin Project 2 Due Friday, Apr. 24, 11:55pm My office hours 3pm
More informationSyntax Analysis: Contextfree Grammars, Pushdown Automata and Parsing Part  4. Y.N. Srikant
Syntax Analysis: Contextfree Grammars, Pushdown Automata and Part  4 Department of Computer Science and Automation Indian Institute of Science Bangalore 560 012 NPTEL Course on Principles of Compiler
More informationCompiler Construction 2016/2017 Syntax Analysis
Compiler Construction 2016/2017 Syntax Analysis Peter Thiemann November 2, 2016 Outline 1 Syntax Analysis Recursive topdown parsing Nonrecursive topdown parsing Bottomup parsing Syntax Analysis tokens
More informationCSE 401 Compilers. LR Parsing Hal Perkins Autumn /10/ Hal Perkins & UW CSE D1
CSE 401 Compilers LR Parsing Hal Perkins Autumn 2011 10/10/2011 200211 Hal Perkins & UW CSE D1 Agenda LR Parsing Tabledriven Parsers Parser States ShiftReduce and ReduceReduce conflicts 10/10/2011
More informationLR Parsing. Table Construction
#1 LR Parsing Table Construction #2 Outline Review of bottomup parsing Computing the parsing DFA Closures, LR(1) Items, States Transitions Using parser generators Handling Conflicts #3 In One Slide An
More informationCS143 Handout 20 Summer 2011 July 15 th, 2011 CS143 Practice Midterm and Solution
CS143 Handout 20 Summer 2011 July 15 th, 2011 CS143 Practice Midterm and Solution Exam Facts Format Wednesday, July 20 th from 11:00 a.m. 1:00 p.m. in Gates B01 The exam is designed to take roughly 90
More informationLR(0) Parsing Summary. LR(0) Parsing Table. LR(0) Limitations. A NonLR(0) Grammar. LR(0) Parsing Table CS412/CS413
LR(0) Parsing ummary C412/C41 Introduction to Compilers Tim Teitelbaum Lecture 10: LR Parsing February 12, 2007 LR(0) item = a production with a dot in RH LR(0) state = set of LR(0) items valid for viable
More informationLALR Parsing. What Yacc and most compilers employ.
LALR Parsing Canonical sets of LR(1) items Number of states much larger than in the SLR construction LR(1) = Order of thousands for a standard prog. Lang. SLR(1) = order of hundreds for a standard prog.
More informationAmbiguity, Precedence, Associativity & TopDown Parsing. Lecture 910
Ambiguity, Precedence, Associativity & TopDown Parsing Lecture 910 (From slides by G. Necula & R. Bodik) 9/18/06 Prof. Hilfinger CS164 Lecture 9 1 Administrivia Please let me know if there are continued
More informationCS453 : JavaCUP and error recovery. CS453 Shiftreduce Parsing 1
CS453 : JavaCUP and error recovery CS453 Shiftreduce Parsing 1 Shiftreduce parsing in an LR parser LR(k) parser Lefttoright parse Rightmost derivation Ktoken look ahead LR parsing algorithm using
More informationTabledriven using an explicit stack (no recursion!). Stack can be viewed as containing both terminals and nonterminals.
Bottomup Parsing: Tabledriven using an explicit stack (no recursion!). Stack can be viewed as containing both terminals and nonterminals. Basic operation is to shift terminals from the input to the
More informationSYNTAX ANALYSIS 1. Define parser. Hierarchical analysis is one in which the tokens are grouped hierarchically into nested collections with collective meaning. Also termed as Parsing. 2. Mention the basic
More informationPrinciple of Compilers Lecture IV Part 4: Syntactic Analysis. Alessandro Artale
Free University of Bolzano Principles of Compilers Lecture IV Part 4, 2003/2004 AArtale (1) Principle of Compilers Lecture IV Part 4: Syntactic Analysis Alessandro Artale Faculty of Computer Science Free
More informationLR Parsing Techniques
LR Parsing Techniques BottomUp Parsing  LR: a special form of BU Parser LR Parsing as Handle Pruning ShiftReduce Parser (LR Implementation) LR(k) Parsing Model  k lookaheads to determine next action
More informationChapter 4: LR Parsing
Chapter 4: LR Parsing 110 Some definitions Recall For a grammar G, with start symbol S, any string α such that S called a sentential form α is If α Vt, then α is called a sentence in L G Otherwise it is
More informationAdministrativia. WA1 due on Thu PA2 in a week. Building a Parser III. Slides on the web site. CS164 3:305:00 TT 10 Evans.
Administrativia Building a Parser III CS164 3:305:00 10 vans WA1 due on hu PA2 in a week Slides on the web site I do my best to have slides ready and posted by the end of the preceding logical day yesterday,
More informationCS2210: Compiler Construction Syntax Analysis Syntax Analysis
Comparison with Lexical Analysis The second phase of compilation Phase Input Output Lexer string of characters string of tokens Parser string of tokens Parse tree/ast What Parse Tree? CS2210: Compiler
More informationLALR stands for look ahead left right. It is a technique for deciding when reductions have to be made in shift/reduce parsing. Often, it can make the
LALR parsing 1 LALR stands for look ahead left right. It is a technique for deciding when reductions have to be made in shift/reduce parsing. Often, it can make the decisions without using a look ahead.
More informationCS606 compiler instruction Solved MCQS From Midterm Papers
CS606 compiler instruction Solved MCQS From Midterm Papers March 06,2014 MC100401285 Moaaz.pk@gmail.com Mc100401285@gmail.com PSMD01 Final Term MCQ s and Quizzes CS606 compiler instruction If X is a
More informationSyntax Analysis, V Bottomup Parsing & The Magic of Handles Comp 412
Midterm Exam: Thursday October 18, 7PM Herzstein Amphitheater Syntax Analysis, V Bottomup Parsing & The Magic of Handles Comp 412 COMP 412 FALL 2018 source code IR Front End Optimizer Back End IR target
More informationParsing Algorithms. Parsing: continued. Top Down Parsing. Predictive Parser. David Notkin Autumn 2008
Parsing: continued David Notkin Autumn 2008 Parsing Algorithms Earley s algorithm (1970) works for all CFGs O(N 3 ) worst case performance O(N 2 ) for unambiguous grammars Based on dynamic programming,
More informationCompilation 2012 ContextFree Languages Parsers and Scanners. Jan Midtgaard Michael I. Schwartzbach Aarhus University
Compilation 2012 Parsers and Scanners Jan Midtgaard Michael I. Schwartzbach Aarhus University ContextFree Grammars Example: sentence subject verb object subject person person John Joe Zacharias verb asked
More informationCS 164 Programming Languages and Compilers Handout 9. Midterm I Solution
Midterm I Solution Please read all instructions (including these) carefully. There are 5 questions on the exam, some with multiple parts. You have 1 hour and 20 minutes to work on the exam. The exam is
More informationBottomUp Parsing. Parser Generation. LR Parsing. Constructing LR Parser
Parser Generation Main Problem: given a grammar G, how to build a topdown parser or a bottomup parser for it? parser : a program that, given a sentence, reconstructs a derivation for that sentence 
More informationSyntax Analyzer  Parser
Syntax Analyzer  Parser ASU Textbook Chapter 4.24.9 (w/o error handling) Tsansheng Hsu tshsu@iis.sinica.edu.tw http://www.iis.sinica.edu.tw/~tshsu 1 A program represented by a sequence of tokens
More informationTop down vs. bottom up parsing
Parsing A grammar describes the strings that are syntactically legal A recogniser simply accepts or rejects strings A generator produces sentences in the language described by the grammar A parser constructs
More informationConcepts Introduced in Chapter 4
Concepts Introduced in Chapter 4 Grammars ContextFree Grammars Derivations and Parse Trees Ambiguity, Precedence, and Associativity Top Down Parsing Recursive Descent, LL Bottom Up Parsing SLR, LR, LALR
More informationDownloaded from Page 1. LR Parsing
Downloaded from http://himadri.cmsdu.org Page 1 LR Parsing We first understand Context Free Grammars. Consider the input string: x+2*y When scanned by a scanner, it produces the following stream of tokens:
More informationLecture Notes on ShiftReduce Parsing
Lecture Notes on ShiftReduce Parsing 15411: Compiler Design Frank Pfenning, Rob Simmons, André Platzer Lecture 8 September 24, 2015 1 Introduction In this lecture we discuss shiftreduce parsing, which
More informationCS415 Compilers. LR Parsing & Error Recovery
CS415 Compilers LR Parsing & Error Recovery These slides are based on slides copyrighted by Keith Cooper, Ken Kennedy & Linda Torczon at Rice University Review: LR(k) items The LR(1) table construction
More informationReview main idea syntaxdirected evaluation and translation. Recall syntaxdirected interpretation in recursive descent parsers
Plan for Today Review main idea syntaxdirected evaluation and translation Recall syntaxdirected interpretation in recursive descent parsers Syntaxdirected evaluation and translation in shiftreduce
More informationHow do LL(1) Parsers Build Syntax Trees?
How do LL(1) Parsers Build Syntax Trees? So far our LL(1) parser has acted like a recognizer. It verifies that input token are syntactically correct, but it produces no output. Building complete (concrete)
More informationLL(k) Parsing. Predictive Parsers. LL(k) Parser Structure. Sample Parse Table. LL(1) Parsing Algorithm. Push RHS in Reverse Order 10/17/2012
Predictive Parsers LL(k) Parsing Can we avoid backtracking? es, if for a given input symbol and given nonterminal, we can choose the alternative appropriately. his is possible if the first terminal of
More informationParsing. Roadmap. > Contextfree grammars > Derivations and precedence > Topdown parsing > Leftrecursion > Lookahead > Tabledriven parsing
Roadmap > Contextfree grammars > Derivations and precedence > Topdown parsing > Leftrecursion > Lookahead > Tabledriven parsing The role of the parser > performs contextfree syntax analysis > guides
More information