CS415 Compilers. LR Parsing & Error Recovery


 Ross Cannon
 4 years ago
 Views:
Transcription
1 CS415 Compilers LR Parsing & Error Recovery These slides are based on slides copyrighted by Keith Cooper, Ken Kennedy & Linda Torczon at Rice University
2 Review: LR(k) items The LR(1) table construction algorithm uses LR(1) items to represent valid configurations of an LR(1) parser An LR(k) item is a pair [P, δ], where P is a production A β with a at some position in the rhs δ is a lookahead string of length k (words or EOF) The in an item indicates the position of the top of the stack LR(1): [A βγ,a] means that the input seen so far is consistent with the use of A βγ immediately after the symbol on top of the stack [A β γ,a] means that the input seen so far is consistent with the use of A βγ at this point in the parse, and that the parser has already recognized β. [A βγ,a] means that the parser has seen βγ, and that a lookahead symbol of a is consistent with reducing to A. Lecture 13 2
3 Review  Computing Closures Closure(s) adds all the items implied by items already in s Any item [A β Bδ,a] implies [B τ,x] for each production with B on the lhs, and each x FIRST(δa) for LR(1) item The algorithm Closure( s ) while ( s is still changing ) items [A β Bδ,a] s productions B τ P b FIRST(δa) // δ might be ε if [B τ,b] s then add [B τ,b] to s Ø Classic fixedpoint method Ø Halts because s ITEMS Closure fills out a state 3
4 Review  Computing Gotos Goto(s,x) computes the state that the parser would reach if it recognized an x while in state s Goto( { [A β Xδ,a] }, X ) produces [A βx δ,a] (easy part) Should also includes closure( [A βx δ,a] ) (fill out the state) The algorithm Goto( s, X ) new Ø items [A β Xδ,a] s new new [A βx δ,a] return closure(new) Ø Not a fixedpoint method! Ø Straightforward computation Ø Uses closure ( ) Goto() moves forward 4
5 Review  Building the Canonical Collection Start from s 0 = closure( [S S,EOF ] ) Repeatedly construct new states, until all are found The algorithm cc 0 closure ( [S S, EOF] ) CC { cc 0 } while ( new sets are still being added to CC) for each unmarked set cc j CC mark cc j as processed for each x following a in an item in cc j temp goto(cc j, x) if temp CC then CC CC { temp } record transitions from cc j to temp on x Ø Fixedpoint computation (worklist version) Ø Loop adds to CC Ø CC 2 ITEMS, so CC is finite 5
6 Review LR(1) Table Construction Highlevel overview 1 Build the canonical collection of sets of LR(1) Items, I a Begin in an appropriate state, s 0 Assume: S S, and S is unique start symbol that does not occur on any RHS of a production (extended CFG  ECFG) [S S,EOF], along with any equivalent items Derive equivalent items as closure( s 0 ) b Repeatedly compute, for each s k, and each X, goto(s k,x) If the set is not already in the collection, add it Record all the transitions created by goto( ) This eventually reaches a fixed point 2 Fill in the table from the collection of sets of LR(1) items The canonical collection completely encodes the transition diagram for the handlefinding DFA 6
7 Review: Example (building the collection) 1: Goal Expr 2: Expr Term Expr 3: Expr Term 4: Term Factor * Term 5: Term Factor 6: Factor ident Initialization Step Symbol FIRST Goal { ident } Expr { ident } Term { ident } Factor { ident } { } * { * } ident { ident } s 0 closure( { [Goal Expr, EOF] } ) = {[Goal Expr, EOF], [Expr à Term Expr, EOF], [Expr à Term, EOF], [Term à Factor * Term, ], [Term à Factor, ], [Term à Factor * Term, EOF], [Term à Factor, EOF], [Factor à ident, *], [Factor à ident, ], [Factor à ident, EOF]} S { S 0 } 7
8 Example (building the collection) s 0 closure( { [Goal Expr, EOF] } ) { [Goal Expr, EOF], [Expr Term Expr, EOF], [Expr Term, EOF], [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ], [Factor ident, EOF], [Factor ident, ], [Factor ident, *] } Iteration 1 s 1 goto(s 0, Expr) s 2 goto(s 0, Term) s 3 goto(s 0, Factor) s 4 goto(s 0, ident ) 8
9 Example (building the collection) s 0 closure( { [Goal Expr, EOF] } ) { [Goal Expr, EOF], [Expr Term Expr, EOF], [Expr Term, EOF], [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ], [Factor ident, EOF], [Factor ident, ], [Factor ident, *] } Iteration 1 s 1 goto(s 0, Expr) = { [Goal Expr, EOF] } s 2 goto(s 0, Term) = { [Expr Term Expr, EOF], [Expr Term, EOF] } s 3 goto(s 0, Factor) = { [Term Factor * Term, EOF],[Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ] } s 4 goto(s 0, ident ) = { [Factor ident, EOF],[Factor ident, ], [Factor ident, *] } 9
10 Example (building the collection) Iteration 1 s 1 goto(s 0, Expr) = { [Goal Expr, EOF] } s 2 goto(s 0, Term) = { [Expr Term Expr, EOF], [Expr Term, EOF] } s 3 goto(s 0, Factor) = { [Term Factor * Term, EOF],[Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ] } s 4 goto(s 0, ident ) = { [Factor ident, EOF],[Factor ident, ], [Factor ident, *] } Iteration 2 s 5 goto(s 2, ) s 6 goto(s 3, * ) 10
11 Example (building the collection) Iteration 1 s 1 goto(s 0, Expr) = { [Goal Expr, EOF] } s 2 goto(s 0, Term) = { [Expr Term Expr, EOF], [Expr Term, EOF] } s 3 goto(s 0, Factor) = { [Term Factor * Term, EOF],[Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ] } s 4 goto(s 0, ident ) = { [Factor ident, EOF],[Factor ident, ], [Factor ident, *] } Iteration 2 s 5 goto(s 2, ) = { [Expr Term Expr, EOF], [Expr Term Expr, EOF], [Expr Term, EOF], [Term Factor * Term, ], [Term Factor, ], [Term Factor * Term, EOF], [Term Factor, EOF], [Factor ident, *], [Factor ident, ], [Factor ident, EOF] } s 6 goto(s 3, * ) = see next page 11
12 Example (building the collection) Iteration 2 s 5 goto(s 2, ) = { [Expr Term Expr, EOF], [Expr Term Expr, EOF], [Expr Term, EOF], [Term Factor * Term, ], [Term Factor * Term, EOF], [Term Factor, ], [Term Factor, EOF], [Factor ident, *], [Factor ident, ], [Factor ident, EOF] } s 6 goto(s 3, * ) = { [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ], [Factor ident, EOF], [Factor ident, ], [Factor ident, *] } Iteration 3 s 7 goto(s 5, Expr ) = {? } s 8 goto(s 6, Term ) = {? } s 2 goto(s 5, Term), s 3 goto(s 5, factor), s 4 goto(s 5, ident), s 3 goto(s 6, Factor), s 4 goto(s 6, ident) 12
13 Example (building the collection) Iteration 2 s 5 goto(s 2, ) = { [Expr Term Expr, EOF], [Expr Term Expr, EOF], [Expr Term, EOF], [Term Factor * Term, ], [Term Factor * Term, EOF], [Term Factor, ], [Term Factor, EOF], [Factor ident, *], [Factor ident, ], [Factor ident, EOF] } s 6 goto(s 3, * ) = { [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ], [Factor ident, EOF], [Factor ident, ], [Factor ident, *] } Iteration 3 s 7 goto(s 5, Expr ) = { [Expr Term Expr, EOF] } s 8 goto(s 6, Term ) = { [Term Factor * Term, EOF], [Term Factor * Term, ] } 13
14 Example (Summary) S 0 : { [Goal Expr, EOF], [Expr Term Expr, EOF], [Expr Term, EOF], [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ], [Factor ident, EOF], [Factor ident, ], [Factor ident, *] } S 1 : { [Goal Expr, EOF] } S 2 : { [Expr Term Expr, EOF], [Expr Term, EOF] } S 3 : { [Term Factor * Term, EOF],[Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ] } S 4 : { [Factor ident, EOF],[Factor ident, ], [Factor ident, *] } S 5 : { [Expr Term Expr, EOF], [Expr Term Expr, EOF], [Expr Term, EOF], [Term Factor * Term, ], [Term Factor, ], [Term Factor * Term, EOF], [Term Factor, EOF], [Factor ident, *], [Factor ident, ], [Factor ident, EOF] } 14
15 Example (Summary) S 6 : { [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor * Term, EOF], [Term Factor * Term, ], [Term Factor, EOF], [Term Factor, ], [Factor ident, EOF], [Factor ident, ], [Factor ident, *] } S 7 : { [Expr Term Expr, EOF] } S 8 : { [Term Factor * Term, EOF], [Term Factor * Term, ] } 15
16 Example (DFA) term s 1 s 2 s 7 term  expr expr ident ident s 0 s 4 s 5 term ident factor s 6 * s 3 s 8 factor factor The State Transition Table State Ident  * Expr Term Factor
17 Example (DFA) term s 1 s 2 s 7 term  expr expr ident ident s 0 s 4 s 5 term ident factor s 6 * s 3 s 8 factor factor The State Transition Table State Ident  * Expr Term Factor
18 Filling in the ACTION and GOTO Tables The algorithm set s x S item i s x if i is [A β ad,b] and goto(s x,a) = s k, a T then ACTION[x,a] shift k else if i is [S S,EOF] then ACTION[x, EOF] accept else if i is [A β,a] then ACTION[x,a] reduce A β n NT if goto(s x,n) = s k then GOTO[x,n] k Many items generate no table entry 18
19 Example (Filling in the tables) The algorithm produces LR(1) parse table ACTION GOTO Ident  * EOF Expr Term Factor 0 s acc 2 s 5 r 3 3 r 5 s 6 r 5 4 r 6 r 6 r 6 5 s s r 2 8 r 4 r 4 Plugs into the skeleton LR(1) parser Remember the state transition table? State Ident  * Expr Term Factor
20 An Example for Table Filling Practice A Parse Table Filling Example For pdf lecture notes readers, see attached LR(1) parse table example file Lecture 15 20
21 What can go wrong? What if set s contains [A β aγ,b] and [B β,a]? First item generates shift, second generates reduce Both define ACTION[s,a] cannot do both actions This is a fundamental ambiguity, called a shift/reduce error Modify the grammar to eliminate it (ifthenelse) What if set s contains [A γ, a] and [B γ, a]? Each generates reduce, but with a different production Both define ACTION[s,a] cannot do both reductions This fundamental ambiguity is called a reduce/reduce error Modify the grammar to eliminate it In either case, the grammar is not LR(1) EaC includes a worked example 21
22 Shrinking the Tables Three options: Combine terminals such as number & identifier, + & , * & / Directly removes a column, may remove a row For expression grammar, 198 (vs. 384) table entries Combine rows or columns Implement identical rows once & remap states Requires extra indirection on each lookup Use separate mapping for ACTION & for GOTO Use another construction algorithm Both LALR(1) and SLR(1) produce smaller tables Implementations are readily available (table compression) 22
23 LR(0) versus SLR(1) versus LR(1) LR(0)?  set of LR(0) items as states LR(1)?  set of LR(1) items as states, different states compared to LR(0) SLR(1)?  LR(0) items and canonical sets, same as LR(0) SLR(1): add FOLLOW(A) to each LR(0) item [A γ ] as its second component: [A γ, a], a FOLLOW(A) 23
24 LR(0) versus SLR(1) versus LR(1) Example: S S S S ; a a LR(0)? LR(1)? SLR(1)? Lecture 15 24
25 LR(0) versus LR(1) versus SLR(1) LR(0) States s0 = Closure({[S.S]}) = {[S >.S], [S >.S; a], [S >.a] } s1 = Closure( GoTo (s0, S)) = {[S S. ], [S S.; a] } s2 = Closure( GoTo (s0, a)) = {[S a.]} s3 = Closure( GoTo (s1, ;)) = {[S S;. a]} LR(1) States s4 = Closure( GoTo (s3, a)) = {[S S;a.] } s0 = Closure({[S.S,eof]}) = {[S >.S,eof], [S >.S; a,eof], [S >.a,;] } s1 = Closure( GoTo (s0, S)) = {[S S. eof], [S S.; a,eof] } s2 = Closure( GoTo (s0, a)) = {[S a.,;]} s3 = Closure( GoTo (s1, ;)) = {[S S;. a,eof]} s4 = Closure( GoTo (s3, a)) = {[S S;a., eof] } Grammar is not LR(0), but LR(1) and SLR(1) Lecture 15 25
26 LALR(1) versus LR(1) LALR(1)? LR(1) items, State > Grouped LR(1) states LALR(1): Merge two sets of LR(1) items (states), if they have the same core. core of set of LR(1) items: set of LR(0) items derived by ignoring the lookahead symbols FACT: collapsing LR(1) states into LALR(1) states cannot introduce shift/reduce conflicts 26
27 LALR(1) versus LR(1) s0 = Closure({[S.S, eof]}) s1 = Closure( GoTo (s0, a)) = {[S a. Ad, eof], [S a. Be, eof], [A.c, d], [ B.c, e]} s2 = Closure( GoTo (s0, b)) = {[S b. Ae, eof], [S b. Bd, eof], [A.c, e], [B.c, d]} s3 = Closure( GoTo (s1, c)) = {[A c., d], [B c., e]} s4 = Closure( GoTo (s2, c)) = {[A c., e], [B c., d]} There are other states that are not listed here! Grammar is LR(1), but not LALR(1), since collapsing s3 and s4 (same core) will introduce reducereduce conflict. Lecture 15 27
28 Hierarchy of ContextFree Grammars FloydEvans Parsable Contextfree grammars Unambiguous CFGs Operator Precedence Operator precedence includes some ambiguous grammars LL(1) is a subset of SLR(1) LR(k) LR(1) LALR(1) LL(k) The inclusion hierarchy for contextfree grammars SLR(1) LR(0) LL(1) Ref Book: Michael Sipser, Introduction to the Theory of Computation, 3 rd Edition Lecture 16 28
29 Error Recovery in ShiftReduce Parsers The problem: parser encounters an invalid token Goal: Want to parse the rest of the file Basic idea (panic mode): Assume something went wrong while trying to find handle for nonterminal A Pretend handle for A has been found; pop handle, skip over input to find terminal that can follow A Restarting the parser (panic mode): find a restartable state on the stack (has transition for nonterminal A) move to a consistent place in the input (token that can follow A) perform (error) reduction (for nonterminal A) print an informative message Lecture 15 29
30 Error Recovery in YACC Yacc s (bison s) error mechanism (note: version dependent!) designated token error used in error productions of the form A error α // basic case α specifies synchronization points When error is discovered pops stack until it finds state where it can shift the error token resumes parsing to match α special cases: α = w, where w is string of terminals: skip input until w has been read α = ε : skip input until state transition on input token is defined error productions can have actions Lecture 15 30
31 Error Recovery in YACC cmpdstmt: BEG stmt_list END stmt_list : stmt stmt_list ; stmt error { yyerror( \n***error: illegal statement\n );} This should throw out the erroneous statement synchronize at ; or end (implicit: α = ε) writes message ***Error: illegal statement to stderr Example: begin a & 5 hello ; a := 3 end resume parsing ***Error: illegal statement Lecture 15 31
32 Project #2 (see lex & yacc, Levine et al., O Reilly) You do have to (slightly) change the scanner (scan.l) How to specify and use attributes in YACC? Define attributes as types in attr.h typedef struct info_node {int a; int b} infonode; Include type attribute name in %union in parse.y %union {tokentype token; infonode myinfo; } Assign attributes in parse.y to Terminals: %token <token> ID ICONST Nonterminals: %type <myinfo> block variables procdecls cmpdstmt Accessing attribute values in parse.y use $$, $1, $2 etc. notation: block : variables procdecls {$2.b = $1.b + 1;} cmpdstmt { $$.a = $1.a + $2.a + $3.b;} Lecture 16 32
33 YACC : parse.y parse.y : %{ #include <stdio.h> #include "attr.h" int yylex(); void yyerror(char * s); #include "symtab.h" %} %union {tokentype token; } Will be included verbatim in parse.tab.c List and assign attributes %token PROG PERIOD PROC VAR ARRAY RANGE OF %token INT REAL DOUBLE WRITELN THEN ELSE IF %token BEG END ASG NOT %token EQ NEQ LT LEQ GEQ GT OR EXOR AND DIV NOT %token <token> ID CCONST ICONST RCONST %start program %% program : PROG ID ';' block PERIOD { } ; block : BEG ID ASG ICONST END { } ; %% void yyerror(char* s) { fprintf(stderr,"%s\n",s); } int main() { printf("1\t"); yyparse(); return 1; } Rules with semantic actions Main program and helper functions; may contain initialization code of global structures. Will be included verbatim in parse.tab.c Lecture 16 33
34 Project #2 : Things to do Learn/Review the C programming language Add error productions (syntax errors) Define and assign attributes to nonterminals Implement singlelevel symbol table Perform type checking and produce required error messages; note: actions may occur at any location on righthand side (implicit use of marker productions) Lecture 16 34
35 Next two classes Adhoc, syntax directed translation schemes,type checking Read EaC: Chapters Lecture 15 35
Parsing 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 informationSyntax Analysis, VII One more LR(1) example, plus some more stuff. Comp 412 COMP 412 FALL Chapter 3 in EaC2e. target code.
COMP 412 FALL 2017 Syntax Analysis, VII One more LR(1) example, plus some more stuff Comp 412 source code IR Front End Optimizer Back End IR target code Copyright 2017, Keith D. Cooper & Linda Torczon,
More informationCS415 Compilers ContextSensitive Analysis Type checking Symbol tables
CS415 Compilers ContextSensitive Analysis Type checking Symbol tables These slides are based on slides copyrighted by Keith Cooper, Ken Kennedy & Linda Torczon at Rice University Lecture 18 1 Announcements
More informationBottomup Parser. Jungsik Choi
Formal Languages and Compiler (CSE322) Bottomup Parser Jungsik Choi chjs@khu.ac.kr * Some slides taken from SKKU SWE3010 (Prof. Hwansoo Han) and TAMU CSCE434500 (Prof. Lawrence Rauchwerger) Bottomup
More informationCS 406/534 Compiler Construction Parsing Part II LL(1) and LR(1) Parsing
CS 406/534 Compiler Construction Parsing Part II LL(1) and LR(1) Parsing Prof. Li Xu Dept. of Computer Science UMass Lowell Fall 2004 Part of the course lecture notes are based on Prof. Keith Cooper, Prof.
More informationSyntax Analysis, VI Examples from LR Parsing. Comp 412
Midterm Exam: Thursday October 18, 7PM Herzstein Amphitheater Syntax Analysis, VI Examples from LR Parsing Comp 412 COMP 412 FALL 2018 source code IR IR target Front End Optimizer Back End code Copyright
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 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 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 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 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 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 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 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 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 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 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 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 informationCMSC 430 Introduction to Compilers. Fall Lexing and Parsing
CMSC 430 Introduction to Compilers Fall 2014 Lexing and Parsing Overview Compilers are roughly divided into two parts Frontend deals with surface syntax of the language Backend analysis and code generation
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 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 informationCS 406/534 Compiler Construction LR(1) Parsing and CSA
CS 406/534 Compiler Construction LR(1) Parsing and CSA Prof. Li Xu Dept. of Computer Science UMass Lowell Fall 2004 Part of the course lecture notes are based on Prof. Keith Cooper, Prof. Ken Kennedy and
More informationBuilding a Parser Part III
COMP 506 Rice University Spring 2018 Building a Parser Part III With Practical Application To Lab One source code IR Front End Optimizer Back End IR target code Copyright 2018, Keith D. Cooper & Linda
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 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 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 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 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 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 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 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 informationAction Table for CSXLite. LALR Parser Driver. Example of LALR(1) Parsing. GoTo Table for CSXLite
LALR r Driver Action Table for CSXLite Given the GoTo and parser action tables, a Shift/Reduce (LALR) parser is fairly simple: { S 5 9 5 9 void LALRDriver(){ Push(S ); } R S R R R R5 if S S R S R5 while(true){
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 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 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 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 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 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 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 informationSimple LR (SLR) LR(0) Drawbacks LR(1) SLR Parse. LR(1) Start State and Reduce. LR(1) Items 10/3/2012
LR(0) Drawbacks Consider the unambiguous augmented grammar: 0.) S E $ 1.) E T + E 2.) E T 3.) T x If we build the LR(0) DFA table, we find that there is a shiftreduce conflict. This arises because 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 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 informationUsing an LALR(1) Parser Generator
Using an LALR(1) Parser Generator Yacc is an LALR(1) parser generator Developed by S.C. Johnson and others at AT&T Bell Labs Yacc is an acronym for Yet another compiler compiler Yacc generates an integrated
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 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 informationCOMPILER (CSE 4120) (Lecture 6: Parsing 4 Bottomup Parsing )
COMPILR (CS 4120) (Lecture 6: Parsing 4 Bottomup Parsing ) Sungwon Jung Mobile Computing & Data ngineering Lab Dept. of Computer Science and ngineering Sogang University Seoul, Korea Tel: +8227058930
More informationCS415 Compilers. Syntax Analysis. These slides are based on slides copyrighted by Keith Cooper, Ken Kennedy & Linda Torczon at Rice University
CS415 Compilers Syntax Analysis These slides are based on slides copyrighted by Keith Cooper, Ken Kennedy & Linda Torczon at Rice University Limits of Regular Languages Advantages of Regular Expressions
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. 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 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 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 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 informationIntroduction to Parsing. Comp 412
COMP 412 FALL 2010 Introduction to Parsing Comp 412 Copyright 2010, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit permission to make
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 informationBottomUp Parsing II (Different types of ShiftReduce Conflicts) Lecture 10. Prof. Aiken (Modified by Professor Vijay Ganesh.
BottomUp Parsing II Different types of ShiftReduce Conflicts) Lecture 10 Ganesh. Lecture 10) 1 Review: BottomUp Parsing Bottomup parsing is more general than topdown parsing And just as efficient Doesn
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 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 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 informationPart III : Parsing. From Regular to ContextFree Grammars. Deriving a Parser from a ContextFree Grammar. Scanners and Parsers.
Part III : Parsing From Regular to ContextFree Grammars Deriving a Parser from a ContextFree Grammar Scanners and Parsers A Parser for EBNF LeftParsable Grammars Martin Odersky, LAMP/DI 1 From Regular
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 informationReview: 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 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 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 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 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 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 informationSyntaxDirected Translation
SyntaxDirected Translation ALSU Textbook Chapter 5.1 5.4, 4.8, 4.9 Tsansheng Hsu tshsu@iis.sinica.edu.tw http://www.iis.sinica.edu.tw/~tshsu 1 What is syntaxdirected translation? Definition: The compilation
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 informationSyntax Analysis Part IV
Syntax Analysis Part IV Chapter 4: Bison Slides adapted from : Robert van Engelen, Florida State University Yacc and Bison Yacc (Yet Another Compiler Compiler) Generates LALR(1) parsers Bison Improved
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 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 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 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 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 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 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 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 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 informationSyntax Analysis Part I
Syntax Analysis Part I Chapter 4: ContextFree Grammars Slides adapted from : Robert van Engelen, Florida State University Position of a Parser in the Compiler Model Source Program Lexical Analyzer Token,
More informationCS415 Compilers. Lexical Analysis
CS415 Compilers Lexical Analysis These slides are based on slides copyrighted by Keith Cooper, Ken Kennedy & Linda Torczon at Rice University Lecture 7 1 Announcements First project and second homework
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 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 informationParsing II Topdown parsing. Comp 412
COMP 412 FALL 2018 Parsing II Topdown parsing Comp 412 source code IR Front End Optimizer Back End IR target code Copyright 2018, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled
More informationError Detection in LALR Parsers. LALR is More Powerful. { b + c = a; } Eof. Expr Expr + id Expr id we can first match an id:
Error Detection in LALR Parsers In bottomup, LALR parsers syntax errors are discovered when a blank (error) entry is fetched from the parser action table. Let s again trace how the following illegal CSXlite
More informationCS 406/534 Compiler Construction Parsing Part I
CS 406/534 Compiler Construction Parsing Part I Prof. Li Xu Dept. of Computer Science UMass Lowell Fall 2004 Part of the course lecture notes are based on Prof. Keith Cooper, Prof. Ken Kennedy and Dr.
More informationLexical Analysis  An Introduction. Lecture 4 Spring 2005 Department of Computer Science University of Alabama Joel Jones
Lexical Analysis  An Introduction Lecture 4 Spring 2005 Department of Computer Science University of Alabama Joel Jones Copyright 2003, Keith D. Cooper, Ken Kennedy & Linda Torczon, all rights reserved.
More informationChapter 4. Lexical and Syntax Analysis
Chapter 4 Lexical and Syntax Analysis Chapter 4 Topics Introduction Lexical Analysis The Parsing Problem RecursiveDescent Parsing BottomUp Parsing Copyright 2012 AddisonWesley. All rights reserved.
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 informationParsing. Note by Baris Aktemur: Our slides are adapted from Cooper and Torczon s slides that they prepared for COMP 412 at Rice.
Parsing Note by Baris Aktemur: Our slides are adapted from Cooper and Torczon s slides that they prepared for COMP 412 at Rice. Copyright 2010, Keith D. Cooper & Linda Torczon, all rights reserved. Students
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 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 informationThe Parsing Problem (cont d) RecursiveDescent Parsing. RecursiveDescent Parsing (cont d) ICOM 4036 Programming Languages. The Complexity of Parsing
ICOM 4036 Programming Languages Lexical and Syntax Analysis Lexical Analysis The Parsing Problem RecursiveDescent Parsing BottomUp Parsing This lecture covers review questions 1427 This lecture covers
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 informationParsing Part II (Topdown parsing, leftrecursion removal)
Parsing Part II (Topdown parsing, leftrecursion removal) Copyright 2003, Keith D. Cooper, Ken Kennedy & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit
More informationCompilers. Yannis Smaragdakis, U. Athens (original slides by Sam
Compilers Parsing Yannis Smaragdakis, U. Athens (original slides by Sam Guyer@Tufts) Next step text chars Lexical analyzer tokens Parser IR Errors Parsing: Organize tokens into sentences Do tokens conform
More informationProgramming Languages (CS 550) Lecture 4 Summary Scanner and Parser Generators. Jeremy R. Johnson
Programming Languages (CS 550) Lecture 4 Summary Scanner and Parser Generators Jeremy R. Johnson 1 Theme We have now seen how to describe syntax using regular expressions and grammars and how to create
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 information