# Syntax Analysis, VII One more LR(1) example, plus some more stuff. Comp 412 COMP 412 FALL Chapter 3 in EaC2e. target code.

Size: px
Start display at page:

Download "Syntax Analysis, VII One more LR(1) example, plus some more stuff. Comp 412 COMP 412 FALL Chapter 3 in EaC2e. target code."

Transcription

1 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, 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. Chapter 3 in EaC2e

2 Computing Closures Review Closure(s) adds all the possibilities for the items already in s Any item [A b Bd,a] where B Î NT implies [B t,x] for each production that has B on the lhs, and each x Î FIRST(da) Since bbd is valid, any way to derive bbd is valid, too The Algorithm Closure( s ) while ( s is still changing ) " items [A b Bd,a] Î s lookahead FIRST(da) // d might be e " productions B t Î P " b Î lookahead if [B t,b] Ï s then s s È { [B t,b] } Classic fixed-point method Halts because s Ì I, the set of all items (finite) Worklist version is faster Closure fills out a state s Generate new lookaheads. See note on p. 128 COMP 412, Fall

3 Computing Gotos Review Goto(s,x) computes the state that the parser would reach if it recognized an x while in state s Goto( { [A b Xd,a] }, X ) produces {[A bx d,a]} (obviously) It finds all such items & uses Closure() to fill out the state The Algorithm Goto( s, X ) new Ø " items [A b Xd,a] Î s new new È {[A bx d,a]} return Closure( new ) Goto( ) models a transition in the automaton Straightforward computation Goto() is not a fixed-point method (but it calls Closure()) COMP Goto412, in this Fall construction 2017 is analogous to Move in the subset construction. 2

4 Building the Canonical Collection Review Start from s 0 = Closure( [S S,EOF] ) Repeatedly construct new states, until all are found The Algorithm s 0 Closure( { [S S,EOF] } ) S { s 0 } k 1 while (S is still changing) " s j Î S and " x Î (T È NT ) s k Goto(s j,x) record s j s k on x if s k Ï S then S S È { s k } k k + 1 Fixed-point computation Loop adds to S (monotone) S Í 2 ITEMS, so S is finite Worklist version is faster because it avoids duplicated effort This membership / equality test requires careful and/or clever implementation. COMP 412, Fall

5 Filling in the ACTION and GOTO Tables Review The Table Construction Algorithm x is the state number " set S x Î S " item i Î S x if i is [A b ad,b] and goto(s x,a) = S k, a Î T then ACTION[x,a] shift k else if i is [S S,EOF] then ACTION[x,EOF] accept else if i is [A b,a] then ACTION[x,a] reduce A b " n Î NT if goto(s x,n) = S k then GOTO[x,n] k before T Þ shift have Goal Þ accept at end Þ reduce Many items generate no table entry Placeholder before a NT does not generate an ACTION table entry Closure( ) instantiates FIRST(X) directly for [A b Xd, a] COMP 412, Fall

6 Another Example (grammar & sets) Simplified, right recursive expression grammar 0 Goal Expr 1 Expr Term - Expr 2 Term 3 Term Factor * Term 4 Factor 5 Factor id SYMBOL FIRST Goal { id } Expr { id } Term { id } Factor { id } { } * { * } id { id } COMP 412, Fall

7 Simplified Expression Grammar Building the Collection Initialization Step 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 id, EOF], [Factor id, ], [Factor id, *] } S {s 0 } Item in black is the initial item. COMP Items 412, in gray Fall 2017 are added by closure(). 6

8 Simplified Expression Grammar Building the Collection 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, id ) Iteration 2 s 5 goto(s 2, ) s 6 goto(s 3, * ) Iteration 3 s 7 goto(s 5, Expr ) s 8 goto(s 6, Term ) Goal, *, & - generate empty sets Goal, Expr, Term, Factor, & id generate empty sets Goal, *, & - generate empty sets. Term, Factor, & id start to re-create existing sets. COMP 412, Fall

9 Simplified Expression Grammar The Details 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 id, EOF], [Factor id, ], [Factor id, *] } 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, ] } Items in black are core items, generated by moving the placeholder. COMP Items 412, in gray Fall 2017 are added by closure(). 8

10 Simplified Expression Grammar The Details s 4 goto(s 0, id) { [Factor id, EOF],[Factor id, ], [Factor id, *] } 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 id, *], [Factor id, ], [Factor id, 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 id, EOF], [Factor id, ], [Factor id, *] } Items in black are core items, generated by moving the placeholder. COMP Items 412, in gray Fall 2017 are added by closure(). 9

11 Simplified Expression Grammar The Details s 7 goto(s 5, Expr ) { [Expr Term Expr, EOF] } goto(s 5, Term) recreates s 2 goto(s 5, Factor) recreates s 3 goto(s 5, id) recreates s 4 s 8 goto(s 6, Term ) { [Term Factor * Term, EOF], [Term Factor * Term, ] } goto(s 6, Term ) recreates s 3 goto(s 6, id) recreates s 4 The next iteration creates no new sets. Items in black are core items, generated by moving the placeholder. COMP Items 412, in gray Fall 2017 are added by closure(). 10

12 Simplified Expression Grammar Recorded Transitions The Goto Relationship (recorded during the construction) State Expr Term Factor - * id s s 1 s 2 5 s 3 6 s 4 s s s 7 s 8 COMP 412, Fall

13 Simplified Expression Grammar Filling in the Tables The algorithm produces the following tables ACTION GOTO State id - * EOF Expr Term Factor s 0 s s 1 acc s 2 s 5 r 3 s 3 r 5 s 6 r 5 s 4 r 6 r 6 r 6 s 5 s s 6 s s 7 r 2 s 8 r 4 r 4 COMP 412, Fall

14 Brief Commercial: Why Are We Doing This? <word, category> pairs Skeleton LR Parser Table IR Knowledge encoded in tables to drive skeleton specifications (as a CFG) LR(1) Parser Generator The goal of this exercise is to automate construction of parsers Compiler writer provides a CFG written in modified BNF Tools provide an efficient and correct parser One that works well with an automatically generated scanner LR parser generators accept the largest class of grammars that are deterministically parsable, and they are highly efficient Generated parsers are preferable to hand-coded ones for large grammars COMP 412, Fall

15 3.6.2 in EaC2e Shrinking the ACTION and GOTO Tables Three classic 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 mappings for ACTION & GOTO Use another construction algorithm Both LALR(1) and SLR(1) produce smaller tables LALR(1) represents each state with its core items SLR(1) uses LR(0) items and the FOLLOW set Implementations are readily available COMP 412, Fall

16 Pages in EaC2e Shrinking the Grammar 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 Canonical construction produces 32 states 32 x (9 + 3) = 384 ACTION/GOTO entries Large table, but still just 1.5kb Action Table State eof + ( ) num name 0 s 4 s5 s 6 1 acc s 7 s 8 2 r 4 r 4 r 4 s 9 s 10 3 r 7 r 7 r 7 r 7 r 7 4 s 14 s 15 s 16 5 r 9 r 9 r 9 r 9 r 9 6 r 10 r 10 r 10 r 10 r 10 7 s 4 s 5 s 6 8 s 4 s 5 s 6 9 s 4 s 5 s 6 10 s 4 s 5 s 6 11 s 21 s 22 s r 4 r 4 s 24 s 25 r 4 13 r 7 r 7 r 7 r 7 r 7 14 s 14 s 15 s r 9 r 9 r 9 r 9 r 9 16 r 10 r 10 r 10 r 10 r r 2 r 2 r 2 s 9 s r 3 r 3 r 3 s 9 s r 5 r 5 r 5 r 5 r 5 20 r 6 r 6 r 6 r 6 r 6 21 s 14 s 15 s s 14 s 15 s r 8 r 8 r 8 r 8 r 8 24 s 14 s 15 s s 14 s 15 s s 21 s 22 s r 2 r 2 s 24 s 25 r 2 28 r 3 r 3 s 24 s 25 r 3 29 r 5 r 5 r 5 r 5 r 5 30 r 6 r 6 r 6 r 6 r 6 31 r 8 r 8 r 8 r 8 r 8 COMP 412, Fall 2017 n FIGURE 3.31 Action Table for the Classic Expression Grammar. 15

17 Pages in EaC2e Shrinking the Grammar We can combine some of the syntactically equivalent symbols Combine + and into AddSub Combine * and / into MulDiv Combine identifier and number into Val 0 Goal Expr 1 Expr Expr AddSub Term 2 Term 3 Term Term MulDiv Factor 4 Factor 5 Factor ( Expr ) 6 Val The Reduced Expression Grammar This grammar has Fewer terminals Fewer productions Which leads to Fewer columns in ACTION Fewer states, which leads to fewer rows in both tables COMP 412, Fall

18 Pages in EaC2e Shrinking the Grammar The Resulting Tables 0 Goal Expr 1 Expr Expr AddSub Term 2 Term 3 Term Term MulDiv Factor 4 Factor 5 Factor ( Expr ) 6 Val 22 states 22 * (6 + 3) = 198 ACTION/GOTO entries 48.4% reduction ( ) / 384 Builds (essentially) the same parse tree Action Table Goto Table eof addsub muldiv ( ) val Expr Term Factor 0 s 4 s acc s 6 2 r 3 r 3 s 7 3 r 5 r 5 r 5 4 s 11 s r 7 r 7 r 7 6 s 4 s s 4 s s 15 s 16 9 r 3 s 17 r 3 10 r 5 r 5 r 5 11 s 11 s r 7 r 7 r 7 13 r 2 r 2 s 7 14 r 4 r 4 r 4 15 s 11 s r 6 r 6 r 6 17 s 11 s s 15 s r 2 s 17 r 2 20 r 4 r 4 r 4 21 r 6 r 6 r 6 (b) Action and Goto Tables for the Reduced Expression Grammar n FIGURE 3.33 The Reduced Expression Grammar and its Tables. COMP 412, Fall

19 3.6.2 in EaC2e Shrinking the ACTION and GOTO Tables Three classic 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 mappings for ACTION & GOTO Use another construction algorithm Both LALR(1) and SLR(1) produce smaller tables LALR(1) represents each state with its core items SLR(1) uses LR(0) items and the FOLLOW set Implementations are readily available left-recursive expression grammar with precedence, see in EAC classic space-time tradeoff Fewer grammars, same languages COMP 412, Fall

20 LR(k) versus LL(k) Finding the next step in a derivation LR(k) Þ Each reduction in the parse is detectable with the complete left context, the reducible phrase, itself, and the k terminal symbols to its right generalizations of LR(1) and LL(1) to longer lookaheads LL(k) Þ Parser must select the expansion based on The complete left context The next k terminals Thus, LR(k) examines more context The question is, do languages fall in the gap between LR(k) and LL(k)? COMP 412, Fall

21 Example due to Bill Waite, UC Boulder LR(1) versus LL(1) The following LR(1) grammar has no LL(1) counterpart The Canonical Collection has 18 sets of LR(1) Items It is not a simple grammar It is, however, LR(1) 0 Goal S 1 S A 2 B 3 A ( A ) 4 a 5 B ( B > 6 b It requires an arbitrary lookahead to choose between A & B An LR(1) parser can carry the left context (the ( s) until it sees a or b The table construction will handle it In contrast, an LL(1) parser cannot decide whether to expand Goal by A or B No amount of massaging the grammar and no amount of lookahead will resolve this problem COMP 412, Fall More precisely, the language described by this LR(1) grammar cannot be described with an LL(1) grammar. In fact, the language has no LL(k) grammar, for finite k.

22 ACTION & GOTO Tables for Waite s Example EOF ( ) a } S A B s 0 s 4 s 5 s s 1 acc s 1 s 2 r 2 s 2 s 3 r 3 s 3 s 4 s 8 s 9 s s 5 r 5 s 5 0 Start A 1 B 2 A ( A ) 3 a 4 B ( B } 5 a s 6 s 10 s 6 s 7 s 11 s 7 s 8 s 8 s 9 s s 9 r 5 r 7 s 9 s 10 r 4 s 10 s 11 r 6 s 11 s 12 s 14 s 12 s 13 s 15 s 13 s 14 r 4 s 14 COMP S , Fall 2017 r 6 S 15 21

23 LR(k) versus LL(k) Other Non-LL Grammars 0 B R 1 ( B ) 2 R E = E 3 E a 4 b 5 ( E + E ) 0 S a A b 1 c 2 A b S 3 B b 4 B a A 5 c Example from D.E Knuth, Top-Down Syntactic Analysis, Acta Informatica, 1:2 (1971), pages Example from Lewis, Rosenkrantz, & Stearns book, Compiler Design Theory, (1976), Figure 13.1 This grammar is actually LR(0) COMP 412, Fall

24 LR(k) versus LL(k) Finding the next step in a derivation LR(k) Þ Each reduction in the parse is detectable with the complete left context, the reducible phrase, itself, and the k terminal symbols to its right LL(k) Þ Parser must select the expansion based on The complete left context The next k terminals Thus, LR(k) examines more context in practice, programming languages do not actually seem to fall in the gap between LL(1) languages and deterministic languages J.J. Horning, LR Grammars and Analysers, in Compiler Construction, An Advanced Course, Springer-Verlag, 1976 COMP 412, Fall

25 Left Recursion versus Right Recursion Right recursion Required for termination in top-down parsers Uses (on average) more stack space Naïve right recursion produces right-associativity Left recursion Works fine in bottom-up parsers Limits required stack space Naïve left recursion produces left-associativity Rule of thumb Left recursion for bottom-up parsers Right recursion for top-down parsers * * w * x z y w * ( x * ( y * z ) ) * * * z y w x ( (w * x ) * y ) * z COMP 412, Fall

26 Left Recursion versus Right Recursion A real example, from the lab 1 ILOC simulator s front end The simulator was built by two of my successful Ph.D.s It is actually a more complex piece of software than you might guess The front end is an LR(1) parser, generated by Bison The grammar contained the following productions: instruction_list : instruction When my colleague first ran the timing blocks through the simulator, it exploded with the error message memory exhausted. What happened? label_def instruction instruction instruction_list label_def instruction instruction_list COMP 412, Fall

27 Left Recursion versus Right Recursion A real example, from the lab 1 simulator s front end instruction_list : instruction label_def instruction instruction instruction_list label_def instruction instruction_list right recursion The parse stack overflowed as it tried to instantiate the instruction_list COMP 412, Fall

28 Left Recursion versus Right Recursion A real example, from the lab 1 simulator s front end instruction_list : instruction label_def instruction instruction instruction_list label_def instruction instruction_list right recursion The parse stack overflowed as it tried to instantiate the instruction_list The fix was easy instruction_list : instruction label_def instruction instruction_list instruction instruction_list label_def instruction left recursion This grammar has (small) bounded stack space & (thus) scales well COMP 412, Fall 2017 See pp in EaC2e 27

29 Error Detection and Recovery Error Detection Recursive descent Parser takes the last else clause in a routine Compiler writer can code almost any arbitrary action Table-driven LL(1) In state s i facing word x, entry is an error Report the error, valid entries in row for s i encode possibilities Table-driven LR(1) In state s i facing word x, entry is an error Report the error, shift states in row encode possibilities Can precompute better messages from LR(1) items COMP 412, Fall

30 Error Detection and Recovery Error Recovery Table-driven LL(1) Treat as missing token, e.g. ), Þ expand by desired symbol Treat as extra token, e.g., x-+y, Þ pop stack and move ahead Table-driven LR(1) Treat as missing token, e.g. ), Þ shift the token Treat as extra token, e.g., x-+y, Þ don t shift the token Can pre-compute sets of states with appropriate entries COMP 412, Fall

31 Error Detection and Recovery One common strategy is hard token recovery Skip ahead in input until we find some hard token, e.g. ; ; separates statements, makes a logical break in the parse Resynchronize state, stack, and input to point after hard token LL(1): pop stack until we find a row with entry for ; LR(1): pop stack until we find a state with a reduction on ; Does not correct the input, rather it allows parse to proceed NT pop() repeat until Table[NT, ; ] ¹ error NT pop() token NextToken() repeat until token = ; token NextToken() Resynchronizing an LL(1) parser repeat until token = ; shift token shift s e token NextToken() reduce by error production // pops all that state off stack Resynchronizing an LR(1) parser COMP 412, Fall

32 Hierarchy of Context-Free Languages Context-free languages LR(k) º LR(1) Deterministic languages (LR(k)) LL(k) languages Simple precedence languages LL(1) languages Operator precedence languages The inclusion hierarchy for context-free languages COMP 412, Fall

33 Hierarchy of Context-Free Grammars Context-free grammars Floyd-Evans Parsable Unambiguous CFGs Operator Precedence LR(k) LR(1) LALR(1) LL(k) Operator precedence includes some ambiguous grammars Note sub-categories of LR(1) LL(1) is a subset of SLR(1) SLR(1) LL(1) LR(0) The inclusion hierarchy for context-free grammars COMP 412, Fall

### 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 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

### CS415 Compilers. LR Parsing & Error Recovery

CS415 Compilers LR Parsing & Error Recovery These slides are based on slides copyrighted by Keith Cooper, Ken Kennedy & Linda Torczon at Rice University Review: LR(k) items The LR(1) table construction

### 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

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

### 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

### 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

### 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

### 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

### Introduction 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

### 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,

### 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

### 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

### 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)

### 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

### 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

### 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

### 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.

### 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

### 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

### shift-reduce parsing

Parsing #2 Bottom-up 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 bottom-up

### 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

### Grammars. CS434 Lecture 15 Spring 2005 Department of Computer Science University of Alabama Joel Jones

Grammars CS434 Lecture 5 Spring 2005 Department of Computer Science University of Alabama Joel Jones Copyright 2003, Keith D. Cooper, Ken Kennedy & Linda Torczon, all rights reserved. Students enrolled

### 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

### 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

### Lexical Analysis. Note by Baris Aktemur: Our slides are adapted from Cooper and Torczon s slides that they prepared for COMP 412 at Rice.

Lexical Analysis 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.

### 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

### 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 &

### 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

### 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.

### Part III : Parsing. From Regular to Context-Free Grammars. Deriving a Parser from a Context-Free Grammar. Scanners and Parsers.

Part III : Parsing From Regular to Context-Free Grammars Deriving a Parser from a Context-Free Grammar Scanners and Parsers A Parser for EBNF Left-Parsable Grammars Martin Odersky, LAMP/DI 1 From Regular

### Let 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

### S Y N T A X A N A L Y S I S LR

LR parsing There are three commonly used algorithms to build tables for an LR parser: 1. SLR(1) = LR(0) plus use of FOLLOW set to select between actions smallest class of grammars smallest tables (number

### 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

### CS 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

### 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.

### 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

### 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

### 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

### 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

### 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

### 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

### 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,

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:

### 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

### 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

### CS 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

### Lecture 7: Deterministic Bottom-Up Parsing

Lecture 7: Deterministic Bottom-Up 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

### 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

### 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

### 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

### Lecture 8: Deterministic Bottom-Up Parsing

Lecture 8: Deterministic Bottom-Up 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

### 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

### 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,

### CSE P 501 Compilers. Parsing & Context-Free Grammars Hal Perkins Spring UW CSE P 501 Spring 2018 C-1

CSE P 501 Compilers Parsing & Context-Free Grammars Hal Perkins Spring 2018 UW CSE P 501 Spring 2018 C-1 Administrivia Project partner signup: please find a partner and fill out the signup form by noon

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

Bottom-Up Parsing Lecture 11-12 (From slides by G. Necula & R. Bodik) 9/22/06 Prof. Hilfinger CS164 Lecture 11 1 Bottom-Up Parsing Bottom-up parsing is more general than topdown parsing And just as efficient

### 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

### Principle 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

### 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

WWW.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.

### 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

### 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

### Parser. Larissa von Witte. 11. Januar Institut für Softwaretechnik und Programmiersprachen. L. v. Witte 11. Januar /23

Parser Larissa von Witte Institut für oftwaretechnik und Programmiersprachen 11. Januar 2016 L. v. Witte 11. Januar 2016 1/23 Contents Introduction Taxonomy Recursive Descent Parser hift Reduce Parser

### Lecture 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],

### Lexical and Syntax Analysis. Bottom-Up Parsing

Lexical and Syntax Analysis Bottom-Up Parsing Parsing There are two ways to construct derivation of a grammar. Top-Down: begin with start symbol; repeatedly replace an instance of a production s LHS with

### 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

### Code Shape Comp 412 COMP 412 FALL Chapters 4, 5, 6 & 7 in EaC2e. source code. IR IR target. code. Front End Optimizer Back End

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

### EXAM. CS331 Compiler Design Spring Please read all instructions, including these, carefully

EXAM Please read all instructions, including these, carefully There are 7 questions on the exam, with multiple parts. You have 3 hours to work on the exam. The exam is open book, open notes. Please write

### 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

### 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/

### 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

### Conflicts 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

### Generating Code for Assignment Statements back to work. Comp 412 COMP 412 FALL Chapters 4, 6 & 7 in EaC2e. source code. IR IR target.

COMP 412 FALL 2017 Generating Code for Assignment Statements back to work Comp 412 source code IR IR target Front End Optimizer Back End code Copyright 2017, Keith D. Cooper & Linda Torczon, all rights

### Bottom Up Parsing. Shift and Reduce. Sentential Form. Handle. Parse Tree. Bottom Up Parsing 9/26/2012. Also known as Shift-Reduce parsing

Also known as Shift-Reduce parsing More powerful than top down Don t need left factored grammars Can handle left recursion Attempt to construct parse tree from an input string eginning at leaves and working

### Compiler Optimisation

Compiler Optimisation 1 Introductory Lecture Hugh Leather IF 1.18a hleather@inf.ed.ac.uk Institute for Computing Systems Architecture School of Informatics University of Edinburgh 2018 Textbooks Engineering

### Algorithms for NLP. LR Parsing. Reading: Hopcroft and Ullman, Intro. to Automata Theory, Lang. and Comp. Section , pp.

11-711 Algorithms for NL L arsing eading: Hopcroft and Ullman, Intro. to Automata Theory, Lang. and Comp. Section 10.6-10.7, pp. 248 256 Shift-educe arsing A class of parsers with the following principles:

### 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

### LR Parsing LALR Parser Generators

LR Parsing LALR Parser Generators Outline Review of bottom-up parsing Computing the parsing DFA Using parser generators 2 Bottom-up Parsing (Review) A bottom-up parser rewrites the input string to the

### Parsing Algorithms. Parsing: continued. Top Down Parsing. Predictive Parser. David Notkin Autumn 2008

Parsing: continued David Notkin Autumn 2008 Parsing Algorithms Earley s algorithm (1970) works for all CFGs O(N 3 ) worst case performance O(N 2 ) for unambiguous grammars Based on dynamic programming,

### 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

### 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

### Table-driven using an explicit stack (no recursion!). Stack can be viewed as containing both terminals and non-terminals.

Bottom-up Parsing: Table-driven using an explicit stack (no recursion!). Stack can be viewed as containing both terminals and non-terminals. Basic operation is to shift terminals from the input to the

### 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

### 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

### 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

### Review main idea syntax-directed evaluation and translation. Recall syntax-directed interpretation in recursive descent parsers

Plan for Today Review main idea syntax-directed evaluation and translation Recall syntax-directed interpretation in recursive descent parsers Syntax-directed evaluation and translation in shift-reduce

### Action Table for CSX-Lite. LALR Parser Driver. Example of LALR(1) Parsing. GoTo Table for CSX-Lite

LALR r Driver Action Table for CSX-Lite 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){

### EDAN65: Compilers, Lecture 04 Grammar transformations: Eliminating ambiguities, adapting to LL parsing. Görel Hedin Revised:

EDAN65: Compilers, Lecture 04 Grammar transformations: Eliminating ambiguities, adapting to LL parsing Görel Hedin Revised: 2017-09-04 This lecture Regular expressions Context-free grammar Attribute grammar

### Chapter 3: Lexing and Parsing

Chapter 3: Lexing and Parsing Aarne Ranta Slides for the book Implementing Programming Languages. An Introduction to Compilers and Interpreters, College Publications, 2012. Lexing and Parsing* Deeper understanding

### LR Parsing LALR Parser Generators

Outline LR Parsing LALR Parser Generators Review of bottom-up parsing Computing the parsing DFA Using parser generators 2 Bottom-up Parsing (Review) A bottom-up parser rewrites the input string to the

### Bottom-Up Parsing. Parser Generation. LR Parsing. Constructing LR Parser

Parser Generation Main Problem: given a grammar G, how to build a top-down parser or a bottom-up parser for it? parser : a program that, given a sentence, reconstructs a derivation for that sentence ----

### 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

### CS502: Compilers & Programming Systems

CS502: Compilers & Programming Systems Top-down Parsing Zhiyuan Li Department of Computer Science Purdue University, USA There exist two well-known schemes to construct deterministic top-down parsers:

### CSE 401 Compilers. LR Parsing Hal Perkins Autumn /10/ Hal Perkins & UW CSE D-1

CSE 401 Compilers LR Parsing Hal Perkins Autumn 2011 10/10/2011 2002-11 Hal Perkins & UW CSE D-1 Agenda LR Parsing Table-driven Parsers Parser States Shift-Reduce and Reduce-Reduce conflicts 10/10/2011

### Intermediate Representations

COMP 506 Rice University Spring 2018 Intermediate Representations source code IR Front End Optimizer Back End IR target code Copyright 2018, Keith D. Cooper & Linda Torczon, all rights reserved. Students

### 3.5 Practical Issues PRACTICAL ISSUES Error Recovery

3.5 Practical Issues 141 3.5 PRACTICAL ISSUES Even with automatic parser generators, the compiler writer must manage several issues to produce a robust, efficient parser for a real programming language.