Tabledriven using an explicit stack (no recursion!). Stack can be viewed as containing both terminals and nonterminals.


 Corey Hamilton
 3 years ago
 Views:
Transcription
1 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 stack until the righthand side of an appropriate grammar rule is seen, and then to reduce the stuff on the stack that matches the righthand side to the single nonterminal of the rule. Hence, bottomup parsers are often called shiftreduce parsers. 1 Example Grammar: E E + n n Input: 2 + 3, or n + n Parse: ($ is EOF input, also bottom of stack) Parsing stack Input Action 1 $ n + n $ shift 2 $ n + n $ reduce E n 3 $ E + n $ shift 4 $ E + n $ shift 5 $ E + n $ reduce E E + n 6 $ E $ accept 2 Compiler Construction F6S 1
2 Notes: Left recursion is not a problem in bottomup parsing. Indeed, as we shall see, lookahead is not as serious an issue. Keeping track of what is on the stack, however, is an issue (note the difference in the grammar rule reductions at lines 2 and 5 of the previous example). See later discussion on stack state. Right recursion is actually a bit of a problem, because it makes the stack grow large (see next example). 3 Example Grammar: E n + E n Input: 2 + 3, or n + n Parse: Parsing stack Input Action 1 $ n + n $ shift 2 $ n + n $ shift 3 $ n + n $ shift 4 $ n + n $ reduce E n 5 $ n + E $ reduce E n + E 6 $ E $ accept 4 Compiler Construction F6S 2
3 Decision Problems in Bottomup Parsing (parsing conflicts): Shiftreduce conflicts: Almost always come from ambiguities, and almost always the right disambiguating rule is to shift (danglingelse). Reducereduce conflicts: More difficult, bottomup parsers try to resolve them using Follow contexts. There are no shiftshift conflicts. 5 Danglingelse Example: Grammar: S I o I i S i S e S Input: i i o e o Parse: Parsing stack Input Action 1 $ i i o e o $ shift 2 $ i i o e o $ shift 3 $ i i o e o $ shift 4 $ i i o e o $ reduce S o 5 $ i i S e o $ shift/reduce (shift) 6 $ i i S e o $ shift 7 $ i i S e o $ reduce S o 8 $ i i S e S $ reduce I i S e S 9 $ i I $ reduce S I 10 $ i S $ reduce I i S 11 $ I $ reduce S I 12 $ S $ accept 6 Compiler Construction F6S 3
4 Reducereduce Example Grammar: S AB A x B x Input: x x Parse: (Follow(A) = {x}, Follow(B) = {$}) Parsing stack Input Action 1 $ x x $ shift 2 $ x x $ reduce A x (reduce B x) 3 $ A x $ shift 4 $ A x $ reduce B x 5 $ A B $ reduce S A B 6 $ S $ accept 7 Shiftreduce parsers differ in their use of Follow information: LR(0) parsers never consult the lookahead at all. SLR(1) parsers use the Follow sets as previously constructed. LR(1) parsers use context to split the Follow sets into subsets for different parsing paths (huge, inefficient parsers). LALR(1) parsers: like LR(1) but coarser subsets are used (achieves most of the benefit, but much smaller and faster). 8 Compiler Construction F6S 4
5 Technical Addendum Shiftreduce parsers have trouble figuring out when to accept, so acceptance is turned into a reduction by a new rule S S with a new start symbol S. Adding this rule is called augmenting the grammar: Parsing stack Input Action <previous example> $ A B $ reduce S A B 6 $ S $ reduce S' S 7 $ S' $ accept 9 Format of a Yacc/Bison definition file {definitions} %% {rules} %% {auxiliary C functions} Typical file extensions:.y.yacc.bison 10 Compiler Construction F6S 5
6 Stack States: the DFA of sets of LR(0) items An item is a grammar rule option with a distinguished position (indicated by a period or other symbol): A α. β The position in an item indicates that a parse has reached that position in recognizing that rule. α is then on the stack, and a β may be coming in the input (the α is called a viable prefix). A stack state consists of the set of items which have compatible viable prefixes. 11 Computing the DFA of sets of LR(0) items Add the augmentation item S. S to the start state of the DFA. Then add all the initial items for S to the state: S. α. Continue by adding all the initial items for those nonterminals which appear right after the dot in any previous item (this is called the closure of the set of items). Every symbol that comes immediately after the dot gives rise to a transition to a state generated by adding closure items to the item with the dot moved past that symbol. 12 Compiler Construction F6S 6
7 Example: S A B, A x, B x The DFA of sets of items: S' S A x. S. A B. x 0 S A S' S. 5 A x. 1 B S A. B S A B. B. x 4 2 x B x Parsing Table from the DFA (1) S AB, (2) A x, (3) B x State Input Goto x other $ S A B 0 s r2 r2 r2 2 s3 4 3 r3 r3 r3 4 r1 r1 r1 5 accept Note that this table is LR(0), since each state is either a shift state or a reduce state by a single rule. Thus, no lookahead needs to be consulted. 14 Compiler Construction F6S 7
8 Example of a parse by the table Parsing stack Input Action 1 $0 x x $ s1 2 $ 0 x 1 x $ r2 (A x) 3 $ 0 A 2 x $ s3 4 $ 0 A 2 x 3 $ r3 (B x) 5 $ 0 A 2 B 4 $ r1 (S A B) 6 $ 0 S 5 $ accept Note that, with the state numbers on the stack, the symbols themselves need not be put on the stack, and we will delete them occasionally from future examples. 15 Bison table is essentially the same: state 0 'x' shift, and go to state 1 S go to state 5 A go to state 2 state 1 A > 'x'.(rule 2) $default reduce using rule 2 (A) state 2 S > A. B (rule 1) 'x' shift, and go to state 3 B go to state 4 state 3 B > 'x'. (rule 3) $default reduce using rule 3 (B) state 4 S > A B. (rule 1) $default reduce using rule 1 (S) state 5 $ go to state 6 state 6 $ go to state 7 state 7 $default accept 16 Compiler Construction F6S 8
9 Example: E E + n n The DFA of sets of items: E'. E E. E + n E. n 0 n E E' E. E E. + n 1 + E n. E E +. n n E E + n Parsing Table (not( LR(0)!): ): State Input Goto n + $ E 0 s2 1 1 s3 accept 2 r (E n) r (E n) 3 s4 4 r (E E + n) r (E E + n) This table uses the SLR(1) rule: consult the Follow sets to decide between a shift and a reduce, or between two reduces: Follow(E) = { + $ }, Follow(E ) = {$} (Note the clear need for augmentation in this case) 18 Compiler Construction F6S 9
10 Sample Parse: Parsing stack Input Action 1 $ 0 n + n + n $ shift 2 2 $ 0 n 2 + n + n $ reduce E n 3 $ 0 E 1 + n + n $ shift 3 4 $ 0 E n + n $ shift 4 5 $ 0 E n 4 + n $ reduce E E + n 6 $ 0 E 1 + n $ shift 3 7 $ 0 E n $ shift 4 8 $ 0 E n 4 $ reduce E E + n 9 $ 0 E 1 $ accept 19 Example with ε (Bison table): S A B, A x, A ε, B x state 0 'x' shift, and go to state 1 'x' [reduce using rule 3 (A)] $default reduce using rule 3 (A) S go to state 5 A go to state 2 state 1 A > 'x'. (rule 2) $default reduce using rule 2 (A) state 2 etc Compiler Construction F6S 10
11 Remarks on Example with ε This grammar is unambiguous, but cannot be parsed with one symbol of lookahead The default disambiguation rule in Bison means it will never accept the string x, only the string x x Thus, even bottomup parsers have their power limited by lookahead 21 A Contrasting Example with ε: S A B, A x, B x, B ε state 0 'x' shift, and go to state 1 S go to state 5 A go to state 2 state 1 A > 'x'. (rule 2) $default reduce using rule 2 (A) state 2 S > A. B (rule 1) 'x' shift, and go to state 3 $default reduce using rule 4 (B) B go to state 4 22 Compiler Construction F6S 11
12 Contrasting Example with ε (continued) : state 3 B > 'x'. (rule 3) $default reduce using rule 3 (B) state 4 S > A B. (rule 1) $default reduce using rule 1 (S) state 5 $ go to state 6 state 6 $ go to state 7 state 7 $default accept 23 Example: E n + E n state 0 'n' shift, and go to state 1 E go to state 4 state 1 E > 'n'. '+' E (rule 1) E > 'n'. (rule 2) '+' shift, and go to state 2 $default reduce using rule 2 (E) state 2 E > 'n' '+'. E (rule 1) 'n' shift, and go to state 1 E go to state 3 state 3 E > 'n' '+' E. (rule 1) $default reduce using rule 1 (E) state 4 $... accept 24 Compiler Construction F6S 12
13 Danglingelse: Grammar: (1) S I (2) S other (3) I if S (4) I if S else S DFA of LR(0) items on next slide 25 S'. S S. I S S' S. 1 S. other I. if S I S I. I. if S else S 2 0 other if I I S other. 3 other if I if. S I if. S else S S. I S. other I. if S I. if S else S 4 if I if S else. S S. I S. other I. if S I. if S else S 6 else S I if S. I if S. else S 5 other S I if S else S Compiler Construction F6S 13
14 The parsing table (with conflict): State Input Goto if else other $ S I 0 s4 s accept 2 r1 r1 3 r2 r2 4 s4 s s6/r3 r3 6 s4 s r4 r4 27 When Follow Sets Fail: S id V = E V id E V n But Yacc/Bison has no trouble with this grammar! state 0 id shift, and go to state 1 S go to state 8 V go to state 2 state 1 S > id. (rule 1) V > id. (rule 3) '=' reduce using rule 3 (V) $default reduce using rule 1 (S) Compiler Construction F6S 14
15 Notes on this example Legal strings are: { id, id=id, id=n } First(S) = First(V) = {id}, First(E) = {id, n} Follow(S) = Follow(E) = {$} Follow(V) = {$, id, =} There is a reducereduce SLR(1) conflict on $ in state 1 (Follow(S) and Follow(V) both contain $) 29 LALR(1) parsers add lookahead An LR(1) item has a lookahead component: [ S.S, $] Begin with the above item, propagate lookahead to closure items as follows: given item [ A α.bγ,a], add closure item [B.β, b] for all b in First(γa). 30 Compiler Construction F6S 15
16 The DFA for the example is then: [ S'. S, $ ] [ S.id, $ ] [ S. V = E, $ ] [ V.id, = ] V 0 S id [ S' S., $ ] 8 [ S id., $ ] [ V id., = ] 1 [ S V. = E, $ ] [ S V = E.,$ ] 2 := E [ S V =. E, $ ] [ E. V, $ ] [ E.n, $ ] [ V.id, $ ] 3 7 V n id [ E V., $ ] [ E n., $ ] [ V id., $ ] Definition of First Sets for Symbols (and ε): If X is a terminal or ε, then First(X) = {X}. If X is a nonterminal, then for each production choice X X 1 X 2...X n, First(X) contains First(X 1 ) {ε}. If also for some i < n, all of the sets First(X 1 ),..., First (X i ) contain ε, then First(X) contains First(X i+1 ) {ε}. If all of the sets First(X 1 ),..., First (X n ) contain ε then First(X) also contains ε. 32 Compiler Construction F6S 16
17 Definition of First Sets for Strings of Symbols: First (α) for any string α = X 1 X 2...X n (a string of terminals and nonterminals) is as follows. First(α) contains First(X 1 ) {ε}. For each i = 2,..., n, if First (X k ) contains ε for all k = 1,..., i 1, then First(α) contains First (X i ) {ε}. Finally, if for all i = 1,..., n, First (X i ) contains ε, then First(α) contains ε. 33 Example S A B z, A x ε, B y ε First(A) = { x, ε } First(B) = { y, ε } First(S) = { x, y, z, ε } First(ASw) = { x, y, z, w } 34 Compiler Construction F6S 17
18 Algorithm for Computing First Sets for Nonterminals: For all nonterminals A do First(A) := {}; while there are changes to any First(A) do for each production choice A X 1 X 2...X n do k := 1 ; Continue := true ; while Continue = true and k <= n do add First (X k ) {ε} to First(A); if ε is not in First (X k ) then Continue := false ; k := k + 1 ; if Continue = true then add ε to First(A) ; 35 Example of Algorithm Grammar Rule Pass 1 Pass 2 statement ifstmt First(statement) = {if, other} statement other First(statement) = {other} ifstmt if ( exp) statement elsepart First(ifstmt) = {if} elsepart else statement First(elsepart) = {else} elsepart ε First(elsepart) = {else, ε} exp 0 First(exp) = {0} exp 1 First(exp) = {0, 1} 36 Compiler Construction F6S 18
19 Definition of Follow Sets for Nonterminals: Given a nonterminal A, the set Follow(A), consisting of terminals, and possibly $, is defined as follows. 1. If A is the start symbol, then $ is in Follow(A). 2. If there is a production B αa γ, then First(γ) {ε} is in Follow(A). 3. If there is a production B αa γ such that ε is in First(γ), then Follow(A) contains Follow(B). 37 Algorithm for Computing Follow Sets for Nonterminals: Follow (startsymbol) := {$} ; for all nonterminals A startsymbol do Follow (A) := {} ; while there are changes to any Follow sets do for each production A X 1 X 2... X n do for each X i that is a nonterminal do add First(X i+1 X i+2... X n ) {ε} to Follow (X i ) (* Note: if i=n, then X i+1 X i+2... X n = ε *) if ε is in First(X i+1 X i+2... X n ) then add Follow(A) to Follow (X i ) 38 Compiler Construction F6S 19
20 Example S A B z, A x ε, B y ε First(A) = { x, ε } First(B) = { y, ε } First(S) = { x, y, z, ε } First(ASw) = { x, y, z, w } Follow(S) = { $ } Follow(A) = { y, $ } Follow(B) = { $ } 39 A Second Example statement ifstmt other ifstmt if ( exp )statement elsepart elsepart else statement ε exp 0 First(statement) = {if, other} First(ifstmt) = {if} First(elsepart) = {else, ε} First(exp) = {0, 1} Follow(statement) = {$,else} Follow(ifstmt) = {$,else} Follow(elsepart) = {$,else} Follow(exp) = {)} 40 Compiler Construction F6S 20
S 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 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 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 informationNote that for recursive descent to work, if A ::= B1 B2 is a grammar rule we need First k (B1) disjoint from First k (B2).
LL(k) Grammars We need a bunch of terminology. For any terminal string a we write First k (a) is the prefix of a of length k (or all of a if its length is less than k) For any string g of terminal and
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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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(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 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 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 informationParsing. Lecture 11: Parsing. Recursive Descent Parser. Arithmetic grammar.  drops irrelevant details from parse tree
Parsing Lecture 11: Parsing CSC 131 Fall, 2014 Kim Bruce Build parse tree from an expression Interested in abstract syntax tree  drops irrelevant details from parse tree Arithmetic grammar ::=
More informationCS1622. Today. A Recursive Descent Parser. Preliminaries. Lecture 9 Parsing (4)
CS1622 Lecture 9 Parsing (4) CS 1622 Lecture 9 1 Today Example of a recursive descent parser Predictive & LL(1) parsers Building parse tables CS 1622 Lecture 9 2 A Recursive Descent Parser. Preliminaries
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 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 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 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 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 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 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 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 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 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 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 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 information3. Parsing. Oscar Nierstrasz
3. Parsing Oscar Nierstrasz Thanks to Jens Palsberg and Tony Hosking for their kind permission to reuse and adapt the CS132 and CS502 lecture notes. http://www.cs.ucla.edu/~palsberg/ http://www.cs.purdue.edu/homes/hosking/
More informationCS502: Compilers & Programming Systems
CS502: Compilers & Programming Systems Topdown Parsing Zhiyuan Li Department of Computer Science Purdue University, USA There exist two wellknown schemes to construct deterministic topdown parsers:
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 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 informationSyntactic Analysis. Chapter 4. Compiler Construction Syntactic Analysis 1
Syntactic Analysis Chapter 4 Compiler Construction Syntactic Analysis 1 Contextfree Grammars The syntax of programming language constructs can be described by contextfree grammars (CFGs) Relatively simple
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 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 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 informationCSC 4181 Compiler Construction. Parsing. Outline. Introduction
CC 4181 Compiler Construction Parsing 1 Outline Topdown v.s. Bottomup Topdown parsing Recursivedescent parsing LL1) parsing LL1) parsing algorithm First and follow sets Constructing LL1) parsing table
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 informationTypes of parsing. CMSC 430 Lecture 4, Page 1
Types of parsing Topdown parsers start at the root of derivation tree and fill in picks a production and tries to match the input may require backtracking some grammars are backtrackfree (predictive)
More informationTableDriven Parsing
TableDriven Parsing It is possible to build a nonrecursive predictive parser by maintaining a stack explicitly, rather than implicitly via recursive calls [1] The nonrecursive parser looks up the production
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 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 informationECE 468/573 Midterm 1 October 1, 2014
ECE 468/573 Midterm 1 October 1, 2014 Name: Purdue email: Please sign the following: I affirm that the answers given on this test are mine and mine alone. I did not receive help from any person or material
More informationParser. Larissa von Witte. 11. Januar Institut für Softwaretechnik und Programmiersprachen. L. v. Witte 11. Januar /23
Parser Larissa von Witte Institut für oftwaretechnik und Programmiersprachen 11. Januar 2016 L. v. Witte 11. Januar 2016 1/23 Contents Introduction Taxonomy Recursive Descent Parser hift Reduce Parser
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 informationSyntactic Analysis. TopDown Parsing
Syntactic Analysis TopDown Parsing Copyright 2017, Pedro C. Diniz, all rights reserved. Students enrolled in Compilers class at University of Southern California (USC) have explicit permission to make
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 informationAcknowledgements. The slides for this lecture are a modified versions of the offering by Prof. Sanjeev K Aggarwal
Acknowledgements The slides for this lecture are a modified versions of the offering by Prof. Sanjeev K Aggarwal Syntax Analysis Check syntax and construct abstract syntax tree if == = ; b 0 a b Error
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 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 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 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 informationCA Compiler Construction
CA4003  Compiler Construction David Sinclair A topdown parser starts with the root of the parse tree, labelled with the goal symbol of the grammar, and repeats the following steps until the fringe of
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 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 Parsing  The Items
LR Parsing  The Items Lecture 10 Sections 4.5, 4.7 Robb T. Koether HampdenSydney College Fri, Feb 13, 2015 Robb T. Koether (HampdenSydney College) LR Parsing  The Items Fri, Feb 13, 2015 1 / 31 1 LR
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 informationCompiler Construction Class Notes
Compiler Construction Class Notes Reg Dodds Department of Computer Science University of the Western Cape rdodds@uwc.ac.za c 2006,2017 Reg Dodds March 22, 2017 Introduction What is a Compiler? What is
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 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 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 informationSyn S t yn a t x a Ana x lysi y s si 1
Syntax Analysis 1 Position of a Parser in the Compiler Model Source Program Lexical Analyzer Token, tokenval Get next token Parser and rest of frontend Intermediate representation Lexical error Syntax
More informationWWW.STUDENTSFOCUS.COM UNIT 3 SYNTAX ANALYSIS 3.1 ROLE OF THE PARSER Parser obtains a string of tokens from the lexical analyzer and verifies that it can be generated by the language for the source program.
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 informationCS 314 Principles of Programming Languages
CS 314 Principles of Programming Languages Lecture 5: Syntax Analysis (Parsing) Zheng (Eddy) Zhang Rutgers University January 31, 2018 Class Information Homework 1 is being graded now. The sample solution
More informationCSCI312 Principles of Programming Languages
Copyright 2006 The McGrawHill Companies, Inc. CSCI312 Principles of Programming Languages! LL Parsing!! Xu Liu Derived from Keith Cooper s COMP 412 at Rice University Recap Copyright 2006 The McGrawHill
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 information8 Parsing. Parsing. Top Down Parsing Methods. Parsing complexity. Top down vs. bottom up parsing. Top down vs. bottom up parsing
8 Parsing Parsing A grammar describes syntactically legal strings in a language A recogniser simply accepts or rejects strings A generator produces strings A parser constructs a parse tree for a string
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 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 information4. Lexical and Syntax Analysis
4. Lexical and Syntax Analysis 4.1 Introduction Language implementation systems must analyze source code, regardless of the specific implementation approach Nearly all syntax analysis is based on a formal
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 informationSyntax Analysis. Martin Sulzmann. Martin Sulzmann Syntax Analysis 1 / 38
Syntax Analysis Martin Sulzmann Martin Sulzmann Syntax Analysis 1 / 38 Syntax Analysis Objective Recognize individual tokens as sentences of a language (beyond regular languages). Example 1 (OK) Program
More informationVIVA QUESTIONS WITH ANSWERS
VIVA QUESTIONS WITH ANSWERS 1. What is a compiler? A compiler is a program that reads a program written in one language the source language and translates it into an equivalent program in another languagethe
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 informationConfiguration Sets for CSX Lite. Parser Action Table
Configuration Sets for CSX Lite State s 6 s 7 Cofiguration Set Prog { Stmts } Eof Stmts Stmt Stmts State s s Cofiguration Set Prog { Stmts } Eof Prog { Stmts } Eof Stmts Stmt Stmts Stmts λ Stmt if ( Expr
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 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 information4. Lexical and Syntax Analysis
4. Lexical and Syntax Analysis 4.1 Introduction Language implementation systems must analyze source code, regardless of the specific implementation approach Nearly all syntax analysis is based on a formal
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 informationCSE 130 Programming Language Principles & Paradigms Lecture # 5. Chapter 4 Lexical and Syntax Analysis
Chapter 4 Lexical and Syntax Analysis Introduction  Language implementation systems must analyze source code, regardless of the specific implementation approach  Nearly all syntax analysis is based on
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 information