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

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

2 Agenda LR Parsing Table-driven Parsers Parser States Shift-Reduce and Reduce-Reduce conflicts 10/10/ Hal Perkins & UW CSE D-2

3 LR(1) Parsing We ll look at LR(1) parsers Left to right scan, Rightmost derivation, 1 symbol lookahead Almost all practical programming languages have an LR(1) grammar LALR(1), SLR(1), etc. subsets of LR(1) LALR(1) can parse most real languages, tables are more compact, and is used by YACC/Bison/ CUP/etc. 10/10/ Hal Perkins & UW CSE D-3

4 Bottom-Up Parsing Idea: Read the input left to right Whenever we ve matched the right hand side of a production, reduce it to the appropriate non-terminal and add that non-terminal to the parse tree The upper edge of this partial parse tree is known as the frontier 10/10/ Hal Perkins & UW CSE D-4

5 Example Grammar Bottom-up Parse S ::= aab e A ::= Abc b B ::= d a b b c d e 10/10/ Hal Perkins & UW CSE D-5

6 Details The bottom-up parser reconstructs a reverse rightmost derivation Given the rightmost derivation S => 1 => 2 => => n-2 => n-1 => n = w the parser will first discover n-1 => n, then n-2 => n-1, etc. Parsing terminates when 1 reduced to S (start symbol, success), or No match can be found (syntax error) 10/10/ Hal Perkins & UW CSE D-6

7 How Do We Parse with This? Key: given what we ve already seen and the next input symbol, decide what to do. Choices: Perform a reduction Look ahead further Can reduce A=> if both of these hold: A=> is a valid production A=> is a step in this rightmost derivation This is known as a shift-reduce parser 10/10/ Hal Perkins & UW CSE D-7

8 Sentential Forms If S =>*, the string is called a sentential form of the of the grammar In the derivation S => 1 => 2 => => n-2 => n-1 => n = w each of the i are sentential forms A sentential form in a rightmost derivation is called a right-sentential form (similarly for leftmost and left-sentential) 10/10/ Hal Perkins & UW CSE D-8

9 Handles Informally, a substring of the tree frontier that matches the right side of a production Even if A::= is a production, is a handle only if it matches the frontier at a point where A::= was used in that derivation may appear in many other places in the frontier without being a handle for that particular production 10/10/ Hal Perkins & UW CSE D-9

10 Handles (cont.) Formally, a handle of a right-sentential form is a production A ::= and a position in where may be replaced by A to produce the previous rightsentential form in the rightmost derivation of 10/10/ Hal Perkins & UW CSE D-10

11 Handle Examples In the derivation S => aabe => aade => aabcde => abbcde abbcde is a right sentential form whose handle is A::=b at position 2 aabcde is a right sentential form whose handle is A::=Abc at position 4 Note: some books take the left of the match as the position 10/10/ Hal Perkins & UW CSE D-11

12 Implementing Shift-Reduce Parsers Key Data structures A stack holding the frontier of the tree A string with the remaining input 10/10/ Hal Perkins & UW CSE D-12

13 Shift-Reduce Parser Operations Reduce if the top of the stack is the right side of a handle A::=, pop the right side and push the left side A Shift push the next input symbol onto the stack Accept announce success Error syntax error discovered 10/10/ Hal Perkins & UW CSE D-13

14 Shift-Reduce Example S ::= aabe A ::= Abc b B ::= d Stack Input Action \$ abbcde\$ shift 10/10/ Hal Perkins & UW CSE D-14

15 How Do We Automate This? Def. Viable prefix a prefix of a rightsentential form that can appear on the stack of the shift-reduce parser Equivalent: a prefix of a right-sentential form that does not continue past the rightmost handle of that sentential form Idea: Construct a DFA to recognize viable prefixes given the stack and remaining input Perform reductions when we recognize them 10/10/ Hal Perkins & UW CSE D-15

16 DFA for prefixes of S ::= aabe A ::= Abc b B ::= d e accept 8 9 \$ B start 1 a 2 A 3 b 6 c 7 S ::= aabe A ::= Abc b d 4 5 A ::= b B ::= d 10/10/ Hal Perkins & UW CSE D-16

17 Trace S ::= aabe A ::= Abc b B ::= d Stack Input \$ abbcde\$ accept e 8 9 B \$ start a A b c S ::= aabe A ::= Abc b d 4 5 A ::= b B ::= d 10/10/ Hal Perkins & UW CSE D-17

18 Observations Way too much backtracking We want the parser to run in time proportional to the length of the input Where the heck did this DFA come from anyway? From the underlying grammar We ll defer construction details for now 10/10/ Hal Perkins & UW CSE D-18

19 Avoiding DFA Rescanning Observation: after a reduction, the contents of the stack are the same as before except for the new non-terminal on top Scanning the stack will take us through the same transitions as before until the last one If we record state numbers on the stack, we can go directly to the appropriate state when we pop the right hand side of a production from the stack 10/10/ Hal Perkins & UW CSE D-19

20 Stack Change the stack to contain pairs of states and symbols from the grammar \$s 0 X 1 s 1 X 2 s 2 X n s n State s 0 represents the accept state (Not always added depends on particular presentation) Observation: in an actual parser, only the state numbers need to be pushed, since they implicitly contain the symbol information, but for explanations it s clearer to use both. 10/10/ Hal Perkins & UW CSE D-20

21 Encoding the DFA in a Table A shift-reduce parser s DFA can be encoded in two tables One row for each state action table encodes what to do given the current state and the next input symbol goto table encodes the transitions to take after a reduction 10/10/ Hal Perkins & UW CSE D-21

22 Actions (1) Given the current state and input symbol, the main possible actions are si shift the input symbol and state i onto the stack (i.e., shift and move to state i ) rj reduce using grammar production j The production number tells us how many <symbol, state> pairs to pop off the stack 10/10/ Hal Perkins & UW CSE D-22

23 Actions (2) Other possible action table entries accept blank no transition syntax error A LR parser will detect an error as soon as possible on a left-to-right scan A real compiler needs to produce an error message, recover, and continue parsing when this happens 10/10/ Hal Perkins & UW CSE D-23

24 Goto When a reduction is performed, <symbol, state> pairs are popped from the stack revealing a state uncovered_s on the top of the stack goto[uncovered_s, A] is the new state to push on the stack when reducing production A ::= (after popping and revealing state uncovered_s on top) 10/10/ Hal Perkins & UW CSE D-24

25 Reminder: DFA for S ::= aabe A ::= Abc b B ::= d accept 8 e 9 \$ B start 1 a 2 A 3 b 6 c 7 S ::= aabe A ::= Abc b d 4 5 A ::= b B ::= d 10/10/ Hal Perkins & UW CSE D-25

26 LR Parse Table for 1. S ::= aabe 2. A ::= Abc 3. A ::= b 4. B ::= d State action goto a b c d e \$ A B S 1 s2 acc g1 2 s4 g3 3 s6 s5 g8 4 r3 r3 r3 r3 r3 r3 5 r4 r4 r4 r4 r4 r4 6 s7 7 r2 r2 r2 r2 r2 r2 8 s9 9 r1 r1 r1 r1 r1 r1 10/10/ Hal Perkins & UW CSE D-26

27 LR Parsing Algorithm (1) word = scanner.gettoken(); while (true) { s = top of stack; if (action[s, word] = si ) { push word; push i (state); word = scanner.gettoken(); } else if (action[s, word] = rj ) { pop 2 * length of right side of production j (2* ); uncovered_s = top of stack; push left side A of production j ; push state goto[uncovered_s, A]; } } else if (action[s, word] = accept ) { return; } else { // no entry in action table report syntax error; halt or attempt recovery; } 10/10/ Hal Perkins & UW CSE D-27

28 Example 1. S ::= aabe 2. A ::= Abc 3. A ::= b 4. B ::= d Stack Input \$ abbcde\$ action goto S a b c d e \$ A B S 1 s2 ac g1 2 s4 g3 3 s6 s5 g8 4 r3 r3 r3 r3 r3 r3 5 r4 r4 r4 r4 r4 r4 6 s7 7 r2 r2 r2 r2 r2 r2 8 s9 9 r1 r1 r1 r1 r1 r1 10/10/ Hal Perkins & UW CSE D-28

29 LR States Idea is that each state encodes The set of all possible productions that we could be looking at, given the current state of the parse, and Where we are in the right hand side of each of those productions 10/10/ Hal Perkins & UW CSE D-29

30 Items An item is a production with a dot in the right hand side Example: Items for production A ::= XY A ::=.XY A ::= X.Y A ::= XY. Idea: The dot represents a position in the production 10/10/ Hal Perkins & UW CSE D-30

31 DFA for S ::= aabe A ::= Abc b B ::= d S ::=.aabe a S ::= a.abe A ::=.Abc A ::=.b b A ::= b. \$ A 3 5 accept S ::= aa.be A ::= A.bc B ::=.d d B ::= d. 8 9 B 6 b 7 S ::= aab.e e S ::= aabe. A ::= Ab.c c A ::= Abc. 10/10/ Hal Perkins & UW CSE D-31

32 Problems with Grammars Grammars can cause problems when constructing a LR parser Shift-reduce conflicts Reduce-reduce conflicts 10/10/ Hal Perkins & UW CSE D-32

33 Shift-Reduce Conflicts Situation: both a shift and a reduce are possible at a given point in the parse (equivalently: in a particular state of the DFA) Classic example: if-else statement S ::= ifthen S ifthen S else S 10/10/ Hal Perkins & UW CSE D-33

34 Parser States for 1. S ::= ifthen S 2. S ::= ifthen S else S S ::=. ifthen S S ::=. ifthen S else S ifthen S ::= ifthen. S S ::= ifthen. S else S S S ::= ifthen S. S ::= ifthen S. else S else S ::= ifthen S else. S State 3 has a shiftreduce conflict Can shift past else into state 4 (s4) Can reduce (r1) S ::= ifthen S (Note: other S ::=. ifthen items not included in states 2-4 to save space) 10/10/ Hal Perkins & UW CSE D-34

35 Solving Shift-Reduce Conflicts Fix the grammar Done in Java reference grammar, others Use a parse tool with a longest match rule i.e., if there is a conflict, choose to shift instead of reduce Does exactly what we want for if-else case Guideline: a few shift-reduce conflicts are fine, but be sure they do what you want 10/10/ Hal Perkins & UW CSE D-35

36 Reduce-Reduce Conflicts Situation: two different reductions are possible in a given state Contrived example S ::= A S ::= B A ::= x B ::= x 10/10/ Hal Perkins & UW CSE D-36

37 Parser States for 1. S ::= A 2. S ::= B 3. A ::= x 4. B ::= x 1 2 S ::=.A S ::=.B A ::=.x B ::=.x x A ::= x. B ::= x. State 2 has a reduce-reduce conflict (r3, r4) 10/10/ Hal Perkins & UW CSE D-37

38 Handling Reduce-Reduce Conflicts These normally indicate a serious problem with the grammar. Fixes Use a different kind of parser generator that takes lookahead information into account when constructing the states Most practical tools use this information Fix the grammar 10/10/ Hal Perkins & UW CSE D-38

39 Another Reduce-Reduce Conflict Suppose the grammar separates arithmetic and boolean expressions expr ::= aexp bexp aexp ::= aexp * aident aident bexp ::= bexp && bident bident aident ::= id bident ::= id This will create a reduce-reduce conflict 10/10/ Hal Perkins & UW CSE D-39

40 Covering Grammars A solution is to merge aident and bident into a single non-terminal (or use id in place of aident and bident everywhere they appear) This is a covering grammar Includes some programs that are not generated by the original grammar Use the type checker or other static semantic analysis to weed out illegal programs later 10/10/ Hal Perkins & UW CSE D-40

41 Coming Attractions Constructing LR tables We ll present a simple version (SLR(0)) in lecture, then talk about extending it to LR(1) LL parsers and recursive descent Continue reading ch. 3 10/10/ Hal Perkins & UW CSE D-41

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