# Introduction to Parsing. Comp 412

Size: px
Start display at page:

Transcription

1 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 copies of these materials for their personal use. Faculty from other educational institutions may use these materials for nonprofit educational purposes, provided this copyright notice is preserved.

2 The Front End Source code Scanner tokens Parser IR Errors Parser Checks the stream of words and their parts of speech (produced by the scanner) for grammatical correctness Determines if the input is syntactically well formed Guides checking at deeper levels than syntax Builds an IR representation of the code Think of this chapter as the mathematics of diagramming sentences Comp 412, Fall

3 The Study of Parsing The process of discovering a derivation for some sentence Need a mathematical model of syntax a grammar G Need an algorithm for testing membership in L(G) Need to keep in mind that our goal is building parsers, not studying the mathematics of arbitrary languages Roadmap for our study of parsing 1 Context-free grammars and derivations 2 Top-down parsing Generated LL(1) parsers & hand-coded recursive descent parsers 3 Bottom-up parsing Generated LR(1) parsers Comp 412, Fall

4 Specifying Syntax with a Grammar Context-free syntax is specified with a context-free grammar SheepNoise SheepNoise baa baa This CFG defines the set of noises sheep normally make It is written in a variant of Backus Naur form Formally, a grammar is a four tuple, G = (S,N,T,P) S is the start symbol (set of strings in L(G)) N is a set of nonterminal symbols (syntactic variables) T is a set of terminal symbols (words) P is a set of productions or rewrite rules (P : N (N T) + ) Example due to Dr. Scott K. Warren From Lecture 1 Comp 412, Fall

5 Why Not Use Regular Languages & DFAs? Removed for time Comp 412, Fall

6 Context-free Grammars What makes a grammar context free? The SheepNoise grammar has a specific form: SheepNoise SheepNoise baa baa Productions have a single nonterminal on the left hand side, which makes it impossible to encode left or right context. The grammar is context free. A context-sensitive grammar can have 1 nonterminal on lhs. Notice that L(SheepNoise) is actually a regular language: baa + Comp 412, Fall Classic definition: any language that can be recognized by a push-down automaton is a context-free language.

7 A More Useful Grammar Than Sheep Noise To explore the uses of CFGs,we need a more complex grammar 0 Expr Expr Op Expr 1 number 2 id 3 Op * 6 / Rule Sentential Form Expr 0 Expr Op Expr 2 <id,x> Op Expr 4 <id,x> - Expr 0 <id,x> - Expr Op Expr 1 <id,x> - <num,2> Op Expr 5 <id,x> - <num,2> * Expr 2 <id,x> - <num,2> * <id,y> Such a sequence of rewrites is called a derivation Process of discovering a derivation is called parsing We denote this derivation: Expr * id num * id Comp 412, Fall

8 Derivations The point of parsing is to construct a derivation At each step, we choose a nonterminal to replace Different choices can lead to different derivations Two derivations are of interest Leftmost derivation replace leftmost NT at each step Rightmost derivation replace rightmost NT at each step These are the two systematic derivations (We don t care about randomly-ordered derivations!) The example on the preceding slide was a leftmost derivation Of course, there is also a rightmost derivation Interestingly, it turns out to be different Comp 412, Fall

9 Derivations and Parse Trees Leftmost derivation Rule Sentential Form Expr G 0 Expr Op Expr 2 <id,x> Op Expr E 4 <id,x> - Expr 0 <id,x> - Expr Op Expr 1 <id,x> - <num,2> Op Expr E Op E 5 <id,x> - <num,2> * Expr 2 <id,x> - <num,2> * <id,y> x E Op E This evaluates as x ( 2 * y ) 2 * y Comp 412, Fall

10 Derivations and Parse Trees Rightmost derivation Rule Sentential Form Expr 0 Expr Op Expr 2 Expr Op <id,y> 5 Expr * <id,y> 0 Expr Op Expr * <id,y> 1 Expr Op <num,2> * <id,y> 4 Expr - <num,2> * <id,y> 2 <id,x> - <num,2> * <id,y> E G E Op E E Op E * y This evaluates as ( x 2 ) * y x 2 This ambiguity is NOT good Comp 412, Fall

11 Derivations and Precedence These two derivations point out a problem with the grammar: It has no notion of precedence, or implied order of evaluation To add precedence Create a nonterminal for each level of precedence Isolate the corresponding part of the grammar Force the parser to recognize high precedence subexpressions first For algebraic expressions Parentheses first (level 1 ) Multiplication and division, next (level 2) Subtraction and addition, last (level 3) Comp 412, Fall

12 Derivations and Precedence Adding the standard algebraic precedence produces: level 3 level 2 level 1 0 Goal Expr 1 Expr Expr + Term 2 Expr - Term 3 Term 4 Term Term * Factor 5 Term / Factor 6 Factor 7 Factor ( Expr ) 8 number 9 id This grammar is slightly larger Takes more rewriting to reach some of the terminal symbols Encodes expected precedence Produces same parse tree under leftmost & rightmost derivations Correctness trumps the speed of the parser Let s see how it parses x - 2 * y Cannot handle precedence in an RE for expressions Introduced parentheses, too (beyond power of an RE) One form of the classic expression grammar Comp 412, Fall

13 Derivations and Precedence Rule Sentential Form Goal 0 Expr 2 Expr - Term 4 Expr - Term * Factor 9 Expr - Term * <id,y> 6 Expr - Factor * <id,y> 8 Expr - <num,2> * <id,y> 3 Term - <num,2> * <id,y> 6 Factor - <num,2> * <id,y> 9 <id,x> - <num,2> * <id,y> The rightmost derivation E T F <id,x> G E T T * F F <id,y> <num,2> Its parse tree It derives x ( 2 * y ), along with an appropriate parse tree. Both the leftmost and rightmost derivations give the same expression, because the grammar directly and explicitly encodes the desired precedence. Comp 412, Fall

14 Ambiguity This Removed sentential for time form has two derivations if Expr 1 then if Expr 2 then Stmt 1 else Stmt 2 if if E 1 then else E 1 then if S 2 if E 2 then E 2 then else S 1 S 1 S 2 production 2, then production 1 production 1, then production 2 Comp 412, Fall

15 COMP 412 FALL 2010 Top Down Parsing Copyright 2010, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit permission to make copies of these materials for their personal use. Faculty from other educational institutions may use these materials for nonprofit educational purposes, provided this copyright notice is preserved.

16 Parsing Techniques Top-down parsers (LL(1), recursive descent) Start at the root of the parse tree and grow toward leaves Pick a production & try to match the input Bad pick may need to backtrack Some grammars are backtrack-free (predictive parsing) Bottom-up parsers (LR(1), operator precedence) Start at the leaves and grow toward root As input is consumed, encode possibilities in an internal state Start in a state valid for legal first tokens Bottom-up parsers handle a large class of grammars Comp 412, Fall

17 Top-down Parsing A top-down parser starts with the root of the parse tree The root node is labeled with the goal symbol of the grammar Top-down parsing algorithm: Construct the root node of the parse tree Repeat until lower fringe of the parse tree matches the input string 1 At a node labeled A, select a production with A on its lhs and, for each symbol on its rhs, construct the appropriate child 2 When a terminal symbol is added to the fringe and it doesn t match the fringe, backtrack 3 Find the next node to be expanded (label NT) The key is picking the right production in step 1 That choice should be guided by the input string Comp 412, Fall

18 Remember the expression grammar? We will call this version the classic expression grammar 0 Goal Expr 1 Expr Expr + Term 2 Expr - Term 3 Term 4 Term Term * Factor 5 Term / Factor 6 Factor 7 Factor ( Expr ) 8 number 9 id And the input x 2 * y Comp 412, Fall

19 Example Trying to match the 2 in x 2 * y : Goal Rule Sentential Form Input <id,x> - Term x - 2 * y Expr 6 <id,x> - Factor x - 2 * y 8 <id,x> - <num,2> x - 2 * y Expr - Term <id,x> - <num,2> x - 2 * y Term Fact. Where are we? 2 matches 2 Fact. <id,x> We have more input, but no NTs left to expand The expansion terminated too soon Need to backtrack <num,2> Comp 412, Fall

20 Example Trying again with 2 in x 2 * y : Goal Rule Sentential Form Input <id,x> - Term x - 2 * y Expr 4 <id,x> - Term * Factor x - 2 * y 6 <id,x> - Factor * Factor x - 2 * y Expr Term 8 <id,x> - <num,2> * Factor x - 2 * y <id,x> - <num,2> * Factor x - 2 * y <id,x> - <num,2> * Factor x - 2 * y Term Fact. Term Fact. * Fact. <id,y> 9 <id,x> - <num,2> * <id,y> x - 2 * y <id,x> - <num,2> * <id,y> x - 2 * y <id,x> <num,2> The Point: This time, we matched & consumed all the input The Success! parser must make the right choice when it expands a NT. Wrong choices lead to wasted effort. Comp 412, Fall

21 Left Recursion Top-down parsers cannot handle left-recursive grammars Formally, A grammar is left recursive if A NT such that a derivation A + Aα, for some string α (NT T ) + Our classic expression grammar is left recursive This can lead to non-termination in a top-down parser In a top-down parser, any recursion must be right recursion We would like to convert the left recursion to right recursion Non-termination is always a bad property in a compiler Comp 412, Fall

22 Eliminating Left Recursion To remove left recursion, we can transform the grammar Consider a grammar fragment of the form Fee Fee α β where neither α nor β start with Fee We can rewrite this fragment as Fee β Fie Fie α Fie ε where Fie is a new non-terminal The new grammar defines the same language as the old grammar, using only right recursion. Added a reference to the empty string Comp 412, Fall

23 Eliminating Left Recursion Substituting them back into the grammar yields 0 Goal Expr 1 Expr Term Expr 2 Expr + Term Expr 3 - Term Expr 4 ε 5 Term Factor Term 6 Term * Factor Term 7 / Factor Term 8 ε 9 Factor ( Expr ) 10 number 11 id This grammar is correct, if somewhat non-intuitive. It is left associative, as was the original The naïve transformation yields a right recursive grammar, which changes the implicit associativity A top-down parser will terminate using it. A top-down parser may need to backtrack with it. Comp 412, Fall

24 Picking the Right Production If it picks the wrong production, a top-down parser may backtrack Alternative is to look ahead in input & use context to pick correctly How much lookahead is needed? In general, an arbitrarily large amount Use the Cocke-Younger, Kasami algorithm or Earley s algorithm Fortunately, Large subclasses of CFGs can be parsed with limited lookahead Most programming language constructs fall in those subclasses Among the interesting subclasses are LL(1) and LR(1) grammars We will focus, for now, on LL(1) grammars & predictive parsing Comp 412, Fall

25 Predictive Parsing Basic idea Given A α β, the parser should be able to choose between α & β FIRST sets For some rhs α G, define FIRST(α) as the set of tokens that appear as the first symbol in some string that derives from α That is, x FIRST(α) iff α * x γ, for some γ We will defer the problem of how to compute FIRST sets for the moment. Comp 412, Fall

26 Predictive Parsing What about ε-productions? They complicate the definition of LL(1) If A α and A β and ε FIRST(α), then we need to ensure that FIRST(β) is disjoint from FOLLOW(A), too, where FOLLOW(A) = the set of terminal symbols that can immediately follow A in a sentential form Define FIRST + (A α) as FIRST(α) FOLLOW(A), if ε FIRST(α) FIRST(α), otherwise Then, a grammar is LL(1) iff A α and A β implies FIRST + (A α) FIRST + (A β) = Comp 412, Fall

27 Predictive Parsing Given a grammar that has the LL(1) property Can write a simple routine to recognize each lhs Code is both simple & fast Consider A β 1 β 2 β 3, with FIRST + (A β i ) FIRST + (A β j ) = if i j /* find an A */ if (current_word FIRST(A β 1 )) find a β 1 and return true else if (current_word FIRST(A β 2 )) find a β 2 and return true else if (current_word FIRST(A β 3 )) find a β 3 and return true else report an error and return false Grammars with the LL(1) property are called predictive grammars because the parser can predict the correct expansion at each point in the parse. Parsers that capitalize on the LL(1) property are called predictive parsers. One kind of predictive parser is the recursive descent parser. Of course, there is more detail to find a β i (p. 103 in EAC, 1 st Ed.) Comp 412, Fall

28 Recursive Descent Parsing Recall the expression grammar, after transformation 0 Goal Expr 1 Expr Term Expr 2 Expr + Term Expr 3 - Term Expr 4 ε 5 Term Factor Term 6 Term * Factor Term 7 / Factor Term 8 ε 9 Factor ( Expr ) 10 number 11 id This produces a parser with six mutually recursive routines: Goal Expr EPrime Term TPrime Factor Each recognizes one NT or T The term descent refers to the direction in which the parse tree is built. Comp 412, Fall

29 Recursive Descent Parsing (Procedural) A couple of routines from the expression parser Goal( ) token next_token( ); if (Expr( ) = true & token = EOF) then next compilation step; else report syntax error; return false; Expr( ) if (Term( ) = false) then return false; else return Eprime( ); looking for Number, Identifier, or (, found token instead, or failed to find Expr or ) after ( Factor( ) if (token = Number) then token next_token( ); return true; else if (token = Identifier) then token next_token( ); return true; else if (token = Lparen) token next_token( ); if (Expr( ) = true & token = Rparen) then token next_token( ); return true; // fall out of if statement report syntax error; return false; EPrime, Term, & TPrime follow the same basic lines (Figure 3.7, EAC) Comp 412, Fall

30 Classic Expression Grammar 0 Goal Expr 1 Expr Term Expr 2 Expr + Term Expr 3 - Term Expr 4 ε 5 Term Factor Term 6 Term * Factor Term 7 / Factor Term 8 ε 9 Factor number 10 id 11 ( Expr ) FIRST + (A β ) is identical to FIRST(β ) except for productiond 4 and 8 FIRST + (Expr ε) is {ε,), eof} FIRST + (Term ε) is {ε,+,-, ), eof} Symbol FIRST FOLLOW num num Ø id id Ø + + Ø - - Ø * * Ø / / Ø ( ( Ø ) ) Ø eof eof Ø ε ε Ø Goal (,id,num eof Expr (,id,num ), eof Expr +, -, ε ), eof Term (,id,num +, -, ), eof Term *, /, ε +,-, ), eof Factor (,id,num +,-,*,/,),eof Comp 412, Fall

31 Building Top-down Parsers Building the complete table Need a row for every NT & a column for every T Need an interpreter for the table (skeleton parser) Comp 412, Fall

32 LL(1) Expression Parsing Table Row we built earlier + * / Id Num ( ) EOF Goal Expr Expr Term Term Factor Comp 412, Fall

33 LL(1) Skeleton Parser word NextWord() // Initial conditions, including push EOF onto Stack // a stack to track local goals push the start symbol, S, onto Stack TOS top of Stack loop forever if TOS = EOF and word = EOF then break & report success // exit on success else if TOS is a terminal then if TOS matches word then pop Stack // recognized TOS word NextWord() else report error looking for TOS // error exit else // TOS is a non-terminal if TABLE[TOS,word] is A B 1 B 2 B k then pop Stack // get rid of A push B k, B k-1,, B 1 // in that order else break & report error expanding TOS TOS top of Stack Comp 412, Fall

34 COMP 412 FALL 2010 Bottom-up Parsing Copyright 2010, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit permission to make copies of these materials for their personal use. Faculty from other educational institutions may use these materials for nonprofit educational purposes, provided this copyright notice is preserved.

35 Bottom-up Parsing (definitions) The point of parsing is to construct a derivation A derivation consists of a series of rewrite steps S γ 0 γ 1 γ 2 γ n 1 γ n sentence Each γ i is a sentential form If γ contains only terminal symbols, γ is a sentence in L(G) If γ contains 1 or more non-terminals, γ is a sentential form To get γ i from γ i 1, expand some NT A γ i 1 by using A β Replace the occurrence of A γ i 1 with β to get γ i In a leftmost derivation, it would be the first NT A γ i 1 A left-sentential form occurs in a leftmost derivation A right-sentential form occurs in a rightmost derivation Bottom-up parsers build a rightmost derivation in reverse We saw this definition earlier Comp 412, Fall

36 Bottom-up Parsing (definitions) A bottom-up parser builds a derivation by working from the input sentence back toward the start symbol S S γ 0 γ 1 γ 2 γ n 1 γ n sentence bottom-up To reduce γ i to γ i 1 match some rhs β against γ i then replace β with its corresponding lhs, A. (assuming the production A β) In terms of the parse tree, it works from leaves to root Nodes with no parent in a partial tree form its upper fringe Since each replacement of β with A shrinks the upper fringe, we call it a reduction. Rightmost derivation in reverse processes words left to right The parse tree need not be built, it can be simulated parse tree nodes = terminal symbols + reductions Comp 412, Fall

37 Finding Reductions Consider the grammar 0 Goal a A B e 1 A A b c 2 b 3 B d And the input string abbcde Sentential Next Reduction Form Prod n Pos n abbcde 2 2 a A bcde 1 4 a A de 3 3 a A B e 0 4 Goal The trick is scanning the input and finding the next reduction The mechanism for doing this must be efficient Position specifies where the right end of β occurs in the current sentential form. While the process of finding the next reduction appears to be almost oracular, it can be automated in an efficient way for a large class of grammars Comp 412, Fall

38 Finding Reductions (Handles) The parser must find a substring β of the tree s frontier that matches some production A β that occurs as one step in the rightmost derivation ( β A is in RRD) Informally, we call this substring β a handle Formally, A handle of a right-sentential form γ is a pair <A β,k> where A β P and k is the position in γ of β s rightmost symbol. If <A β,k> is a handle, then replacing β at k with A produces the right sentential form from which γ is derived in the rightmost derivation. Because γ is a right-sentential form, the substring to the right of a handle contains only terminal symbols the parser doesn t need to scan (much) past the handle Most students find handles mystifying; bear with me for a couple more slides. Comp 412, Fall

39 Example derivation 0 Goal Expr 1 Expr Expr + Term 2 Expr - Term 3 Term 4 Term Term * Factor 5 Term / Factor 6 Factor 7 Factor number 8 id 9 ( Expr ) A simple left-recursive form of the classic expression grammar Prod n Sentential Form Handle Goal 0 Expr 0,1 2 Expr - Term 2,3 4 Expr - Term * Factor 4,5 8 Expr - Term * <id,y> 8,5 6 Expr - Factor * <id,y> 6,3 7 Expr - <num,2> * <id,y> 7,3 3 Term - <num,2> * <id,y> 3,1 6 Factor - <num,2> * <id,y> 6,1 8 <id,x> - <num,2> * <id,y> 8,1 parse Handles for rightmost derivation of x 2 * y Comp 412, Fall

40 Bottom-up Parsing (Abstract View) A bottom-up parser repeatedly finds a handle A β in the current right-sentential form and replaces β with A. To construct a rightmost derivation S γ 0 γ 1 γ 2 γ n 1 γ n w Apply the following conceptual algorithm for i n to 1 by 1 Find the handle <A i β i, k i > in γ i Replace β i with A i to generate γ i 1 of course, n is unknown until the derivation is built This takes 2n steps Some authors refer to this algorithm as a handle-pruning parser. The idea is that the parser finds a handle on the upper fringe of the partially complete parse tree and prunes it out of the fringe. The analogy is somewhat strained, so I will try to avoid using it. Comp 412, Fall

41 More on Handles Bottom-up reduce parsers find a rightmost derivation in reverse order Rightmost derivation rightmost NT expanded at each step in the derivation Processed in reverse parser proceeds left to right These statements are somewhat counter-intuitive Comp 412, Fall

42 More on Handles Bottom-up parsers find a reverse rightmost derivation Process input left to right Upper fringe of partially completed parse tree is (NT T)* T* The handle always appears with its right end at the junction between (NT T)* and T* (the hot spot for LR parsing) We can keep the prefix of the upper fringe of the partially completed parse tree on a stack The stack makes the position information irrelevant Handles appear at the top of the stack All the information for the decision is at the hot spot The next word in the input stream The rightmost NT on the fringe & its immediate left neighbors In an LR parser, additional information in the form of a state Comp 412, Fall

43 Shift-reduce Parsing To implement a bottom-up parser, we adopt the shift-reduce paradigm A shift-reduce parser is a stack automaton with four actions Shift next word is shifted onto the stack Reduce right end of handle is at top of stack Locate left end of handle within the stack Pop handle off stack & push appropriate lhs Accept stop parsing & report success Error call an error reporting/recovery routine Accept & Error are simple Shift is just a push and a call to the scanner Reduce takes rhs pops & 1 push But how does the parser know when to shift and when to reduce? It shifts until it has a handle at the top of the stack. Comp 412, Fall

44 Bottom-up Parser A simple shift-reduce parser: push INVALID token next_token( ) repeat until (top of stack = Goal and token = EOF) if the top of the stack is a handle A β then // reduce β to A pop β symbols off the stack push A onto the stack else if (token EOF) then // shift push token token next_token( ) else // need to shift, but out of input report an error What happens on an error? It fails to find a handle Thus, it keeps shifting Eventually, it consumes all input This parser reads all input before reporting an error, not a desirable property. Error localization is an issue in the handle-finding process that affects the practicality of shift-reduce parsers We will fix this issue later. Figure 3.7 in EAC Comp 412, Fall

45 Back to x - 2 * y Stack Input Handle Action \$ id - num * id none shift \$ id - num * id 8,1 reduce 8 \$ Factor - num * id 6,1 reduce 6 \$ Term - num * id 3,1 reduce 3 \$ Expr - num * id none shift \$ Expr - num * id none shift \$ Expr - num * id 7,3 reduce 7 \$ Expr - Factor * id 6,3 reduce 6 \$ Expr - Term * id none shift \$ Expr - Term * id none shift \$ Expr - Term * id 8,5 reduce 8 \$ Expr - Term * Factor 4,5 reduce 4 \$ Expr - Term 2,3 reduce 2 \$ Expr 0,1 reduce 0 \$ Goal none accept 0 Goal Expr 1 Expr Expr + Term 2 Expr - Term 3 Term 4 Term Term * Factor 5 Term / Factor 6 Factor 7 Factor number 8 id 9 ( Expr ) 5 shifts + 9 reduces + 1 accept 1. Shift until the top of the stack is the right end of a handle 2. Find the left end of the handle and reduce Comp 412, Fall

46 Back to x - 2 * y Stack Input Action \$ id - num * id shift \$ id - num * id reduce 8 \$ Factor - num * id reduce 6 \$ Term - num * id reduce 3 \$ Expr - num * id shift \$ Expr - num * id shift \$ Expr - num * id reduce 7 \$ Expr - Factor * id reduce 6 \$ Expr - Term * id shift \$ Expr - Term * id shift \$ Expr - Term * id reduce 8 \$ Expr - Term * Factor reduce 4 \$ Expr - Term reduce 2 \$ Expr reduce 0 \$ Goal accept Goal Expr Expr Term Term Term * Fact. Fact. Fact. <id,y> <id,x> <num,2> Corresponding Parse Tree Comp 412, Fall

47 LR(1) Parsers LR(1) parsers are table-driven, shift-reduce parsers that use a limited right context (1 token) for handle recognition The class of grammars that these parsers recognize is called the set of LR(1) grammars Informal definition: A grammar is LR(1) if, given a rightmost derivation S γ 0 γ 1 γ 2 γ n 1 γ n sentence We can 1. isolate the handle of each right-sentential form γ i, and 2. determine the production by which to reduce, by scanning γ i from left-to-right, going at most 1 symbol beyond the right end of the handle of γ i LR(1) means left-to-right scan of the input, rightmost derivation (in reverse), and 1 word of lookahead. Comp 412, Fall

48 LR(1) Parsers A table-driven LR(1) parser looks like source code Scanner Table-driven Parser IR grammar Parser Generator ACTION & GOTO Tables Tables can be built by hand However, this is a perfect task to automate Comp 412, Fall

49 LR(1) Skeleton Parser stack.push(invalid); stack.push(s 0 ); // initial state token = scanner.next_token(); loop forever { s = stack.top(); if ( ACTION[s,token] == reduce A β ) then { stack.popnum(2* β ); // pop 2* β symbols s = stack.top(); stack.push(a); // push A stack.push(goto[s,a]); // push next state } else if ( ACTION[s,token] == shift s i ) then { stack.push(token); stack.push(s i ); token scanner.next_token(); } else if ( ACTION[s,token] == accept & token == EOF ) then break; else throw a syntax error; } report success; The skeleton parser relies on a stack & a scanner uses two tables, called ACTION & GOTO ACTION: state x word state GOTO: state x NT state shifts words times reduces derivation times accepts at most once detects errors by failure of the other three cases follows basic scheme for shift-reduce parsing from last lecture Comp 412, Fall

50 LR(1) Parsers (parse tables) To make a parser for L(G), need a set of tables The grammar 1 Goal SheepNoise 2 SheepNoise SheepNoise baa 3 baa Remember, this is the left-recursive SheepNoise; EaC shows the rightrecursive version. The tables ACTION Table State EOF baa 0 shift 2 1 accept shift 3 2 reduce 3 reduce 3 3 reduce 2 reduce 2 GOTO Table State SheepNoise Comp 412, Fall

51 Example Parse 1 The string baa Stack Input Action \$ s 0 baa EOF shift 2 \$ s 0 baa s 2 EOF reduce 3 \$ s 0 SN s 1 EOF accept 1 Goal SheepNoise 2 SheepNoise SheepNoise baa 3 baa ACTION Table State EOF baa 0 shift 2 1 accept shift 3 2 reduce 3 reduce 3 3 reduce 2 reduce 2 GOTO Table State SheepNoise Comp 412, Fall

52 Example Parse 2 The string baa baa Stack Input Action \$ s 0 baa baa EOF shift 2 \$ s 0 baa s 2 baa EOF reduce 3 \$ s 0 SN s 1 baa EOF shift 3 \$ s 0 SN s 1 baa s 3 EOF reduce 2 \$ s 0 SN s 1 EOF accept ACTION Table State EOF baa 0 shift 2 1 accept shift 3 2 reduce 3 reduce 3 3 reduce 2 reduce 2 1 Goal SheepNoise 2 SheepNoise SheepNoise baa 3 baa GOTO Table State SheepNoise Comp 412, Fall

53 LR(1) Parsers How does this LR(1) stuff work? Unambiguous grammar unique rightmost derivation Keep upper fringe on a stack All active handles include top of stack (TOS) Shift inputs until TOS is right end of a handle Language of handles is regular (finite) Build a handle-recognizing DFA ACTION & GOTO tables encode the DFA To match subterm, invoke subterm DFA & leave old DFA s state on stack Final state in DFA a reduce action Reduce action New state is GOTO[state at TOS (after pop), lhs] For SN, this takes the DFA to s 1 S 0 SN baa S 1 S 2 baa S 3 Control DFA for SN Comp 412, Fall

54 The Parentheses Language Language of balanced parentheses Beyond power of REs Exhibits role of context in LR(1) parsing 0 Goal List 1 List List Pair 2 Pair 3 Pair ( Pair ) 4 ( ) Comp 412, Fall

55 The Parentheses Language ACTION TABLE State eof ( ) 0 S 3 1 acc S 3 2 R 2 R 2 3 S 6 S 7 4 R 1 R 1 5 S 8 6 S 6 S 10 7 R 4 R 4 8 R 3 R 3 9 S R 4 11 R 3 GOTO TABLE State List Pair Goal List 1 List List Pair 2 Pair 3 Pair ( Pair ) 4 ( ) Comp 412, Fall

56 The Parentheses Language State Lookahead Stack Handle Action ( \$ 0 none 0 ( \$ 0 none shift 3 3 ) \$ 0 ( 3 none shift 7 7 eof \$ 0 ( 3 ) 7 ( ) reduce 4 2 eof \$ 0 Pair 2 Pair reduce 2 1 eof \$ 0 List 1 List accept Parsing ( ) 0 Goal List 1 List List Pair 2 Pair 3 Pair ( Pair ) 4 ( ) Comp 412, Fall

57 The Parentheses Language State L ahead Stack Handle Action ( \$ 0 none 0 ( \$ 0 none shift 3 3 ( \$ 0 ( 3 none shift 6 0 Goal List 1 List List Pair 2 Pair 3 Pair ( Pair ) 4 ( ) Parsing (( )) ( ) 6 ) \$ 0 ( 3 ( 6 none shift ) \$ 0 ( 3 ( 6 ) 10 ( ) reduce 4 5 ) \$ 0 ( 3 Pair 5 none shift 8 8 ( \$ 0 ( 3 Pair 5 ) 8 ( Pair ) reduce 3 2 ( \$ 0 Pair 2 Pair reduce 2 1 ( \$ 0 List 1 none shift 3 3 ) \$ 0 List 1 ( 3 none shift 7 7 eof \$ 0 List 1 ( 3 ) 7 ( ) reduce 4 4 eof \$ 0 List 1 Pair 4 List Pair reduce 1 1 eof \$ 0 List 1 List accept Comp 412, Fall

58 LR(1) Parsers How does this LR(1) stuff work? Unambiguous grammar unique rightmost derivation Keep upper fringe on a stack All active handles include top of stack (TOS) Shift inputs until TOS is right end of a handle Language of handles is regular (finite) Build a handle-recognizing DFA to control the stack-based recognizer ACTION & GOTO tables encode the DFA To match a subterm, invoke the DFA recursively leave old DFA s state on stack and go on Final state in DFA a reduce action Pop rhs off the stack to reveal invoking state It would be legal to recognize an x, and we did New state is GOTO[revealed state, lhs] Take a DFA transition on the new NT the lhs we just pushed Comp 412, Fall

59 LR(1) Parsers The Control DFA for the Parentheses Language List S 0 S 1 ( Pair ( S 4 S 5 S 8 Pair ( ( ) Pair S 3 S 6 S 9 S 11 ) Pair S 2 ) ) S 7 S 10 Control DFA for the Parentheses Language Transitions on terminals represent shift actions Transitions on nonterminals represent reduce actions [ACTION] [GOTO] The table construction derives this DFA from the grammar Comp 412, Fall

60 Building LR(1) Tables Slides removed for time Comp 412, Fall

61 Summary Top-down recursive descent Advantages Fast Good locality Simplicity Good error detection Disadvantages Hand-coded High maintenance Right associativity LR(1) Fast Deterministic langs. Automatable Left associativity Large working sets Poor error messages Large table sizes Comp 412, Fall

### Parsing. 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

### CS 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.

### CSCI312 Principles of Programming Languages

Copyright 2006 The McGraw-Hill 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 McGraw-Hill

### Syntax Analysis, III Comp 412

COMP 412 FALL 2017 Syntax Analysis, III Comp 412 source code IR Front End Optimizer Back End IR target code Copyright 2017, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled in Comp

### Parsing III. CS434 Lecture 8 Spring 2005 Department of Computer Science University of Alabama Joel Jones

Parsing III (Top-down parsing: recursive descent & LL(1) ) (Bottom-up parsing) CS434 Lecture 8 Spring 2005 Department of Computer Science University of Alabama Joel Jones Copyright 2003, Keith D. Cooper,

### Parsing II Top-down parsing. Comp 412

COMP 412 FALL 2018 Parsing II Top-down 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

### Syntax Analysis, III Comp 412

Updated algorithm for removal of indirect left recursion to match EaC3e (3/2018) COMP 412 FALL 2018 Midterm Exam: Thursday October 18, 7PM Herzstein Amphitheater Syntax Analysis, III Comp 412 source code

### Syntax Analysis, V Bottom-up Parsing & The Magic of Handles Comp 412

Midterm Exam: Thursday October 18, 7PM Herzstein Amphitheater Syntax Analysis, V Bottom-up Parsing & The Magic of Handles Comp 412 COMP 412 FALL 2018 source code IR Front End Optimizer Back End IR target

### Syntactic Analysis. Top-Down Parsing

Syntactic Analysis Top-Down 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

### CS415 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

### Parsing III. (Top-down parsing: recursive descent & LL(1) )

Parsing III (Top-down parsing: recursive descent & LL(1) ) Roadmap (Where are we?) Previously We set out to study parsing Specifying syntax Context-free grammars Ambiguity Top-down parsers Algorithm &

### Parsing Part II (Top-down parsing, left-recursion removal)

Parsing Part II (Top-down parsing, left-recursion removal) Copyright 2003, Keith D. Cooper, Ken Kennedy & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit

### Introduction to Parsing

Introduction to Parsing The Front End Source code Scanner tokens Parser IR Errors Parser Checks the stream of words and their parts of speech (produced by the scanner) for grammatical correctness Determines

### Bottom-up Parser. Jungsik Choi

Formal Languages and Compiler (CSE322) Bottom-up Parser Jungsik Choi chjs@khu.ac.kr * Some slides taken from SKKU SWE3010 (Prof. Hwansoo Han) and TAMU CSCE434-500 (Prof. Lawrence Rauchwerger) Bottom-up

### Syntax 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

### Parsing. Roadmap. > Context-free grammars > Derivations and precedence > Top-down parsing > Left-recursion > Look-ahead > Table-driven parsing

Roadmap > Context-free grammars > Derivations and precedence > Top-down parsing > Left-recursion > Look-ahead > Table-driven parsing The role of the parser > performs context-free syntax analysis > guides

### Parsing II Top-down parsing. Comp 412

COMP 412 FALL 2017 Parsing II Top-down parsing Comp 412 source code IR Front End OpMmizer Back End IR target code Copyright 2017, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled

### Compilers. 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

### CS 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.

### Computer Science 160 Translation of Programming Languages

Computer Science 160 Translation of Programming Languages Instructor: Christopher Kruegel Top-Down Parsing Parsing Techniques Top-down parsers (LL(1), recursive descent parsers) Start at the root of the

### 3. 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/

### Parsing Part II. (Ambiguity, Top-down parsing, Left-recursion Removal)

Parsing Part II (Ambiguity, Top-down parsing, Left-recursion Removal) Ambiguous Grammars Definitions If a grammar has more than one leftmost derivation for a single sentential form, the grammar is ambiguous

### Types of parsing. CMSC 430 Lecture 4, Page 1

Types of parsing Top-down 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 backtrack-free (predictive)

### Syntactic Analysis. Top-Down Parsing. Parsing Techniques. Top-Down Parsing. Remember the Expression Grammar? Example. Example

Syntactic Analysis op-down Parsing Parsing echniques op-down Parsers (LL(1), recursive descent) Start at the root of the parse tree and grow toward leaves Pick a production & try to match the input Bad

### Computing Inside The Parser Syntax-Directed Translation. Comp 412 COMP 412 FALL Chapter 4 in EaC2e. source code. IR IR target.

COMP 412 FALL 2017 Computing Inside The Parser Syntax-Directed Translation Comp 412 source code IR IR target Front End Optimizer Back End code Copyright 2017, Keith D. Cooper & Linda Torczon, all rights

### Prelude COMP 181 Tufts University Computer Science Last time Grammar issues Key structure meaning Tufts University Computer Science

Prelude COMP Lecture Topdown Parsing September, 00 What is the Tufts mascot? Jumbo the elephant Why? P. T. Barnum was an original trustee of Tufts : donated \$0,000 for a natural museum on campus Barnum

### Bottom-up parsing. Bottom-Up Parsing. Recall. Goal: For a grammar G, withstartsymbols, any string α such that S α is called a sentential form

Bottom-up parsing Bottom-up 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

### CS 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

### Chapter 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

### Building 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

### CA Compiler Construction

CA4003 - Compiler Construction David Sinclair A top-down 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

### EECS 6083 Intro to Parsing Context Free Grammars

EECS 6083 Intro to Parsing Context Free Grammars Based on slides from text web site: Copyright 2003, Keith D. Cooper, Ken Kennedy & Linda Torczon, all rights reserved. 1 Parsing sequence of tokens parser

### COMP 181. Prelude. Next step. Parsing. Study of parsing. Specifying syntax with a grammar

COMP Lecture Parsing September, 00 Prelude What is the common name of the fruit Synsepalum dulcificum? Miracle fruit from West Africa What is special about miracle fruit? Contains a protein called miraculin

### CS1622. 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

### Syntax 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,

### A left-sentential form is a sentential form that occurs in the leftmost derivation of some sentence.

Bottom-up 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 left-sentential form is a sentential form that occurs

### Computing Inside The Parser Syntax-Directed Translation. Comp 412

COMP 412 FALL 2018 Computing Inside The Parser Syntax-Directed Translation Comp 412 source code IR IR target Front End Optimizer Back End code Copyright 2018, Keith D. Cooper & Linda Torczon, all rights

### Top-Down Parsing and Intro to Bottom-Up Parsing. Lecture 7

Top-Down Parsing and Intro to Bottom-Up Parsing Lecture 7 1 Predictive Parsers Like recursive-descent but parser can predict which production to use Predictive parsers are never wrong Always able to guess

### Top 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

### Chapter 3. Parsing #1

Chapter 3 Parsing #1 Parser source file get next character scanner get token parser AST token A parser recognizes sequences of tokens according to some grammar and generates Abstract Syntax Trees (ASTs)

### Compiler Construction 2016/2017 Syntax Analysis

Compiler Construction 2016/2017 Syntax Analysis Peter Thiemann November 2, 2016 Outline 1 Syntax Analysis Recursive top-down parsing Nonrecursive top-down parsing Bottom-up parsing Syntax Analysis tokens

### Context-sensitive Analysis Part IV Ad-hoc syntax-directed translation, Symbol Tables, andtypes

Context-sensitive Analysis Part IV Ad-hoc syntax-directed translation, Symbol Tables, andtypes Quiz Name two differences between attribute grammars and ad-hoc syntax directed translation techniques? Example:

### Syntax 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

### 4. 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

### Wednesday, 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

### Parsers. 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

### 4. 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

### Top-Down Parsing and Intro to Bottom-Up Parsing. Lecture 7

Top-Down Parsing and Intro to Bottom-Up Parsing Lecture 7 1 Predictive Parsers Like recursive-descent but parser can predict which production to use Predictive parsers are never wrong Always able to guess

### Computing Inside The Parser Syntax-Directed Translation, II. Comp 412 COMP 412 FALL Chapter 4 in EaC2e. source code. IR IR target.

COMP 412 FALL 20167 Computing Inside The Parser Syntax-Directed Translation, II Comp 412 source code IR IR target Front End Optimizer Back End code Copyright 2017, Keith D. Cooper & Linda Torczon, all

### Syntax Analysis. The Big Picture. The Big Picture. COMP 524: Programming Languages Srinivas Krishnan January 25, 2011

Syntax Analysis COMP 524: Programming Languages Srinivas Krishnan January 25, 2011 Based in part on slides and notes by Bjoern Brandenburg, S. Olivier and A. Block. 1 The Big Picture Character Stream Token

### Lexical and Syntax Analysis. Top-Down Parsing

Lexical and Syntax Analysis Top-Down Parsing Easy for humans to write and understand String of characters Lexemes identified String of tokens Easy for programs to transform Data structure Syntax A syntax

### Syntax Analysis. COMP 524: Programming Language Concepts Björn B. Brandenburg. The University of North Carolina at Chapel Hill

Syntax Analysis Björn B. Brandenburg The University of North Carolina at Chapel Hill Based on slides and notes by S. Olivier, A. Block, N. Fisher, F. Hernandez-Campos, and D. Stotts. The Big Picture Character

### Monday, 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 non-terminals S is the start symbol P is the set of productions

### Programming Language Syntax and Analysis

Programming Language Syntax and Analysis 2017 Kwangman Ko (http://compiler.sangji.ac.kr, kkman@sangji.ac.kr) Dept. of Computer Engineering, Sangji University Introduction Syntax the form or structure of

### Parsers. 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

### CSE P 501 Compilers. LR Parsing Hal Perkins Spring UW CSE P 501 Spring 2018 D-1

CSE P 501 Compilers LR Parsing Hal Perkins Spring 2018 UW CSE P 501 Spring 2018 D-1 Agenda LR Parsing Table-driven Parsers Parser States Shift-Reduce and Reduce-Reduce conflicts UW CSE P 501 Spring 2018

### CS 4120 Introduction to Compilers

CS 4120 Introduction to Compilers Andrew Myers Cornell University Lecture 6: Bottom-Up Parsing 9/9/09 Bottom-up parsing A more powerful parsing technology LR grammars -- more expressive than LL can handle

### Syntax Analysis Part I

Syntax Analysis Part I Chapter 4: Context-Free Grammars Slides adapted from : Robert van Engelen, Florida State University Position of a Parser in the Compiler Model Source Program Lexical Analyzer Token,

### 8 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

### Wednesday, 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

### Syntax Analysis Check syntax and construct abstract syntax tree

Syntax Analysis Check syntax and construct abstract syntax tree if == = ; b 0 a b Error reporting and recovery Model using context free grammars Recognize using Push down automata/table Driven Parsers

### Defining syntax using CFGs

Defining syntax using CFGs Roadmap Last time Defined context-free grammar This time CFGs for specifying a language s syntax Language membership List grammars Resolving ambiguity CFG Review G = (N,Σ,P,S)

### CSE 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

### Lexical and Syntax Analysis

Lexical and Syntax Analysis (of Programming Languages) Top-Down Parsing Lexical and Syntax Analysis (of Programming Languages) Top-Down Parsing Easy for humans to write and understand String of characters

SYNTAX 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

### Lexical Analysis. Introduction

Lexical Analysis Introduction Copyright 2015, Pedro C. Diniz, all rights reserved. Students enrolled in the Compilers class at the University of Southern California have explicit permission to make copies

### COP4020 Programming Languages. Syntax Prof. Robert van Engelen

COP4020 Programming Languages Syntax Prof. Robert van Engelen Overview n Tokens and regular expressions n Syntax and context-free grammars n Grammar derivations n More about parse trees n Top-down and

### Front End. Hwansoo Han

Front nd Hwansoo Han Traditional Two-pass Compiler Source code Front nd IR Back nd Machine code rrors High level functions Recognize legal program, generate correct code (OS & linker can accept) Manage

### Parsing #1. Leonidas Fegaras. CSE 5317/4305 L3: Parsing #1 1

Parsing #1 Leonidas Fegaras CSE 5317/4305 L3: Parsing #1 1 Parser source file get next character scanner get token parser AST token A parser recognizes sequences of tokens according to some grammar and

### The Parsing Problem (cont d) Recursive-Descent Parsing. Recursive-Descent Parsing (cont d) ICOM 4036 Programming Languages. The Complexity of Parsing

ICOM 4036 Programming Languages Lexical and Syntax Analysis Lexical Analysis The Parsing Problem Recursive-Descent Parsing Bottom-Up Parsing This lecture covers review questions 14-27 This lecture covers

### MIT 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

### Parsing Wrapup. Roadmap (Where are we?) Last lecture Shift-reduce parser LR(1) parsing. This lecture LR(1) parsing

Parsing Wrapup Roadmap (Where are we?) Last lecture Shift-reduce parser LR(1) parsing LR(1) items Computing closure Computing goto LR(1) canonical collection This lecture LR(1) parsing Building ACTION

### Academic Formalities. CS Modern Compilers: Theory and Practise. Images of the day. What, When and Why of Compilers

Academic Formalities CS6013 - Modern Compilers: Theory and Practise Introduction V. Krishna Nandivada IIT Madras Written assignment = 5 marks. Programming assignments = 40 marks. Midterm = 25 marks, Final

### Compiler Design 1. Bottom-UP Parsing. Goutam Biswas. Lect 6

Compiler Design 1 Bottom-UP 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,

### Syntax 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

### Computing Inside The Parser Syntax-Directed Translation, II. Comp 412

COMP 412 FALL 2018 Computing Inside The Parser Syntax-Directed Translation, II Comp 412 source code IR IR target Front End Optimizer Back End code Copyright 2018, Keith D. Cooper & Linda Torczon, all rights

### Syntax Analysis: Context-free Grammars, Pushdown Automata and Parsing Part - 4. Y.N. Srikant

Syntax Analysis: Context-free 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

### Chapter 4. Lexical and Syntax Analysis. Topics. Compilation. Language Implementation. Issues in Lexical and Syntax Analysis.

Topics Chapter 4 Lexical and Syntax Analysis Introduction Lexical Analysis Syntax Analysis Recursive -Descent Parsing Bottom-Up parsing 2 Language Implementation Compilation There are three possible approaches

### CSE 3302 Programming Languages Lecture 2: Syntax

CSE 3302 Programming Languages Lecture 2: Syntax (based on slides by Chengkai Li) Leonidas Fegaras University of Texas at Arlington CSE 3302 L2 Spring 2011 1 How do we define a PL? Specifying a PL: Syntax:

### EDAN65: Compilers, Lecture 06 A LR parsing. Görel Hedin Revised:

EDAN65: Compilers, Lecture 06 A LR parsing Görel Hedin Revised: 2017-09-11 This lecture Regular expressions Context-free grammar Attribute grammar Lexical analyzer (scanner) Syntactic analyzer (parser)

### Lexical 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.

### Section 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

### Formal Languages and Compilers Lecture VII Part 3: Syntactic A

Formal Languages and Compilers Lecture VII Part 3: Syntactic Analysis Free University of Bozen-Bolzano Faculty of Computer Science POS Building, Room: 2.03 artale@inf.unibz.it http://www.inf.unibz.it/

### Part 5 Program Analysis Principles and Techniques

1 Part 5 Program Analysis Principles and Techniques Front end 2 source code scanner tokens parser il errors Responsibilities: Recognize legal programs Report errors Produce il Preliminary storage map Shape

### Bottom-Up Parsing. Lecture 11-12

Bottom-Up Parsing Lecture 11-12 (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

### Context-free grammars

Context-free grammars Section 4.2 Formal way of specifying rules about the structure/syntax of a program terminals - tokens non-terminals - represent higher-level structures of a program start symbol,

### Outline CS412/413. Administrivia. Review. Grammars. Left vs. Right Recursion. More tips forll(1) grammars Bottom-up parsing LR(0) parser construction

C12/1 Introduction to Compilers and Translators pring 00 Outline More tips forll1) grammars Bottom-up parsing LR0) parser construction Lecture 5: Bottom-up parsing Lecture 5 C 12/1 pring '00 Andrew Myers

### PART 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

### 10/5/17. Lexical and Syntactic Analysis. Lexical and Syntax Analysis. Tokenizing Source. Scanner. Reasons to Separate Lexical and Syntax Analysis

Lexical and Syntactic Analysis Lexical and Syntax Analysis In Text: Chapter 4 Two steps to discover the syntactic structure of a program Lexical analysis (Scanner): to read the input characters and output

### Chapter 3. Describing Syntax and Semantics ISBN

Chapter 3 Describing Syntax and Semantics ISBN 0-321-49362-1 Chapter 3 Topics Introduction The General Problem of Describing Syntax Formal Methods of Describing Syntax Copyright 2009 Addison-Wesley. All

### Administrativia. WA1 due on Thu PA2 in a week. Building a Parser III. Slides on the web site. CS164 3:30-5:00 TT 10 Evans.

Administrativia Building a Parser III CS164 3:30-5:00 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,

### Optimizing Finite Automata

Optimizing Finite Automata We can improve the DFA created by MakeDeterministic. Sometimes a DFA will have more states than necessary. For every DFA there is a unique smallest equivalent DFA (fewest states

### Compiler Construction: Parsing

Compiler Construction: Parsing Mandar Mitra Indian Statistical Institute M. Mitra (ISI) Parsing 1 / 33 Context-free grammars. Reference: Section 4.2 Formal way of specifying rules about the structure/syntax

### CS 406/534 Compiler Construction Putting It All Together

CS 406/534 Compiler Construction Putting It All Together 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

### 3. 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

### LR Parsing Techniques

LR Parsing Techniques Introduction Bottom-Up Parsing LR Parsing as Handle Pruning Shift-Reduce Parser LR(k) Parsing Model Parsing Table Construction: SLR, LR, LALR 1 Bottom-UP Parsing A bottom-up parser

### CS415 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