# Bottom-Up Parsing. Lecture 11-12

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1 Bottom-Up Parsing Lecture (From slides by G. Necula & R. Bodik) 9/22/06 Prof. Hilfinger CS164 Lecture 11 1

2 Bottom-Up Parsing Bottom-up parsing is more general than topdown parsing And just as efficient Builds on ideas in top-down parsing Most common form is LR parsing L means that tokens are read left to right R means that it constructs a rightmost derivation 9/22/06 Prof. Hilfinger CS164 Lecture 11 2

3 An Introductory xample LR parsers don t need left-factored grammars and can also handle left-recursive grammars Consider the following grammar: + ( ) int Why is this not LL(1)? Consider the string: int + ( int ) + ( int ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 3

4 The Idea LR parsing reduces a string to the start symbol by inverting productions: str input string of terminals while str S: Identify first β in str such that A β is a production and S * α A γ α β γ = str Replace β by A in str (so α A γ becomes new str) Such α β s are called handles 9/22/06 Prof. Hilfinger CS164 Lecture 11 4

5 A Bottom-up Parse in Detail (1) int + (int) + (int) int + ( int ) + ( int ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 5

6 A Bottom-up Parse in Detail (2) int + (int) + (int) + (int) + (int) int + ( int ) + ( int ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 6

7 A Bottom-up Parse in Detail (3) int + (int) + (int) + (int) + (int) + () + (int) int + ( int ) + ( int ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 7

8 A Bottom-up Parse in Detail (4) int + (int) + (int) + (int) + (int) + () + (int) + (int) int + ( int ) + ( int ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 8

9 A Bottom-up Parse in Detail (5) int + (int) + (int) + (int) + (int) + () + (int) + (int) + () int + ( int ) + ( int ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 9

10 A Bottom-up Parse in Detail (6) int + (int) + (int) + (int) + (int) + () + (int) + (int) + () A reverse rightmost derivation int + ( int ) + ( int ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 10

11 Where Do Reductions Happen Because an LR parser produces a reverse rightmost derivation: If αβγ is step of a bottom-up parse with handle αβ And the next reduction is by A β Then γ is a string of terminals! Because αaγ αβγ is a step in a right-most derivation Intuition: We make decisions about what reduction to use after seeing all symbols in handle, rather that before (as for LL(1)) 9/22/06 Prof. Hilfinger CS164 Lecture 11 11

12 Notation Idea: Split the string into two substrings Right substring (a string of terminals) is as yet unexamined by parser Left substring has terminals and non-terminals The dividing point is marked by a I The I is not part of the string Marks end of next potential handle Initially, all input is unexamined: Ix 1 x 2... x n 9/22/06 Prof. Hilfinger CS164 Lecture 11 12

13 Shift-Reduce Parsing Bottom-up parsing uses only two kinds of actions: Shift: Move I one place to the right, shifting a terminal to the left string + (I int ) + (int I ) Reduce: Apply an inverse production at the handle. If + ( ) is a production, then + ( + ( ) I ) +( I ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 13

14 Shift-Reduce xample I int + (int) + (int)\$ shift int + ( int ) + ( int )

15 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int int + ( int ) + ( int )

16 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times int + ( int ) + ( int )

17 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times + (int I ) + (int)\$ red. int int + ( int ) + ( int )

18 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times + (int I ) + (int)\$ red. int + ( I ) + (int)\$ shift int + ( int ) + ( int )

19 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times + (int I ) + (int)\$ red. int + ( I ) + (int)\$ shift + () I + (int)\$ red. + () int + ( int ) + ( int )

20 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times + (int I ) + (int)\$ red. int + ( I ) + (int)\$ shift + () I + (int)\$ red. + () I + (int)\$ shift 3 times int + ( int ) + ( int )

21 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times + (int I ) + (int)\$ red. int + ( I ) + (int)\$ shift + () I + (int)\$ red. + () I + (int)\$ shift 3 times + (int I )\$ red. int int + ( int ) + ( int )

22 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times + (int I ) + (int)\$ red. int + ( I ) + (int)\$ + () I + (int)\$ shift red. + () I + (int)\$ shift 3 times + (int I )\$ red. int + ( I )\$ shift int + ( int ) + ( int )

23 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times + (int I ) + (int)\$ red. int + ( I ) + (int)\$ + () I + (int)\$ shift red. + () I + (int)\$ shift 3 times + (int I )\$ red. int + ( I )\$ + () I \$ shift red. + () int + ( int ) + ( int )

24 Shift-Reduce xample I int + (int) + (int)\$ shift int I + (int) + (int)\$ red. int I + (int) + (int)\$ shift 3 times + (int I ) + (int)\$ red. int + ( I ) + (int)\$ + () I + (int)\$ shift red. + () I + (int)\$ shift 3 times + (int I )\$ red. int + ( I )\$ + () I \$ shift red. + () I \$ accept int + ( int ) + ( int )

25 The Stack Left string can be implemented as a stack Top of the stack is the I Shift pushes a terminal on the stack Reduce pops 0 or more symbols from the stack (production rhs) and pushes a non-terminal on the stack (production lhs) 9/22/06 Prof. Hilfinger CS164 Lecture 11 25

26 Key Issue: When to Shift or Reduce? Decide based on the left string (the stack) Idea: use a finite automaton (DFA) to decide when to shift or reduce The DFA input is the stack up to potential handle DFA alphabet consists of terminals and nonterminals DFA recognizes complete handles We run the DFA on the stack and we examine the resulting state X and the token tok after I If X has a transition labeled tok then shift If X is labeled with A β on tok then reduce 9/22/06 Prof. Hilfinger CS164 Lecture 11 26

27 accept on \$ LR(1) Parsing. An xample + () on \$, + int ( ) ( ) 5 11 int on \$, + int 9 int on ), + int + () on ), + I int + (int) + (int)\$ shift int I + (int) + (int)\$ int I + (int) + (int)\$ + (int I ) + (int)\$ + ( I ) + (int)\$ + () I + (int)\$ I + (int)\$ + (int I )\$ + ( I )\$ + () I \$ I \$ shift(x3) int shift +() shift (x3) int shift +() accept

28 Representing the DFA Parsers represent the DFA as a 2D table As for table-driven lexical analysis Lines correspond to DFA states Columns correspond to terminals and nonterminals In classical treatments, columns are split into: Those for terminals: action table Those for non-terminals: goto table 9/22/06 Prof. Hilfinger CS164 Lecture 11 28

29 Representing the DFA. xample The table for a fragment of our DFA: ( ) 7 int 5 int on ), + + () on \$, int s5 s8 r int r +() r +() 9/22/06 Prof. Hilfinger CS164 Lecture ( s4 s7 ) r int \$ g6

30 The LR Parsing Algorithm After a shift or reduce action we rerun the DFA on the entire stack This is wasteful, since most of the work is repeated So record, for each stack element, state of the DFA after that state LR parser maintains a stack sym 1, state 1... sym n, state n state k is the final state of the DFA on sym 1 sym k 9/22/06 Prof. Hilfinger CS164 Lecture 11 30

31 The LR Parsing Algorithm Let I = w 1 w 2 w n \$ be initial input Let j = 1 Let DFA state 0 be the start state Let stack = dummy, 0 repeat case action[top_state(stack), I[j]] of shift k: push I[j], k ; j += 1 reduce X α: pop α pairs, push X, Goto[top_state(stack), X] accept: halt normally error: halt and report error 9/22/06 Prof. Hilfinger CS164 Lecture 11 31

32 LR Parsing Notes Can be used to parse more grammars than LL Most programming languages grammars are LR Can be described as a simple table There are tools for building the table How is the table constructed? 9/22/06 Prof. Hilfinger CS164 Lecture 11 32

33 To Be Done Review of bottom-up parsing Computing the parsing DFA Using parser generators 9/22/06 Prof. Hilfinger CS164 Lecture 11 33

34 Bottom-up Parsing (Review) A bottom-up parser rewrites the input string to the start symbol The state of the parser is described as α I γ α is a stack of terminals and non-terminals γ is the string of terminals not yet examined Initially: I x 1 x 2... x n 9/22/06 Prof. Hilfinger CS164 Lecture 11 34

35 The Shift and Reduce Actions (Review) Recall the CFG: int + () A bottom-up parser uses two kinds of actions: Shift pushes a terminal from input on the stack + (I int ) + (int I ) Reduce pops 0 or more symbols from the stack (production rhs) and pushes a non-terminal on the stack (production lhs) + ( + ( ) I ) +( I ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 35

36 Key Issue: When to Shift or Reduce? Idea: use a finite automaton (DFA) to decide when to shift or reduce The input is the stack The language consists of terminals and non-terminals We run the DFA on the stack and we examine the resulting state X and the token tok after I If X has a transition labeled tok then shift If X is labeled with A β on tok then reduce 9/22/06 Prof. Hilfinger CS164 Lecture 11 36

37 accept on \$ LR(1) Parsing. An xample + () on \$, + int ( ) ( ) 5 11 int on \$, + int 9 int on ), + int + () on ), + I int + (int) + (int)\$ shift int I + (int) + (int)\$ int I + (int) + (int)\$ + (int I ) + (int)\$ + ( I ) + (int)\$ + () I + (int)\$ I + (int)\$ + (int I )\$ + ( I )\$ + () I \$ I \$ shift(x3) int shift +() shift (x3) int shift +() accept

38 Key Issue: How is the DFA Constructed? The stack describes the context of the parse What non-terminal we are looking for What productions we are looking for What we have seen so far from the rhs 9/22/06 Prof. Hilfinger CS164 Lecture 11 38

39 Parsing Contexts Consider the state: int + ( int ) + ( int ) The stack is + ( I int ) + ( int ) Context: We are looking for an + ( ) Have have seen + ( from the right-hand side We are also looking for int or + ( ) Have seen nothing from the right-hand side One DFA state describes several contexts 9/22/06 Prof. Hilfinger CS164 Lecture 11 39

40 LR(1) Items An LR(1) item is a pair: X α β, a X αβ is a production a is a terminal (the lookahead terminal) LR(1) means 1 lookahead terminal [X α β, a] describes a context of the parser We are trying to find an X followed by an a, and We have α already on top of the stack Thus we need to see next a prefix derived from βa 9/22/06 Prof. Hilfinger CS164 Lecture 11 40

41 Note The symbol I was used before to separate the stack from the rest of input α I γ, where α is the stack and γ is the remaining string of terminals In LR(1) items is used to mark a prefix of a production rhs: X α β, a Here β might contain non-terminals as well In both case the stack is on the left 9/22/06 Prof. Hilfinger CS164 Lecture 11 41

42 Convention We add to our grammar a fresh new start symbol S and a production S Where is the old start symbol No need to do this if had only one production The initial parsing context contains: S, \$ Trying to find an S as a string derived from \$ The stack is empty 9/22/06 Prof. Hilfinger CS164 Lecture 11 42

43 LR(1) Items (Cont.) In context containing + ( ), + If ( follows then we can perform a shift to context containing + ( ), + In context containing + ( ), + We can perform a reduction with + ( ) But only if a + follows 9/22/06 Prof. Hilfinger CS164 Lecture 11 43

44 LR(1) Items (Cont.) Consider a context with the item + ( ), + We expect next a string derived from ) + There are two productions for int and + ( ) We describe this by extending the context with two more items: int, ) + ( ), ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 44

45 The Closure Operation The operation of extending the context with items is called the closure operation Closure(Items) = repeat for each [X α Yβ, a] in Items for each production Y γ for each b First(βa) add [Y γ, b] to Items until Items is unchanged 9/22/06 Prof. Hilfinger CS164 Lecture 11 45

46 Constructing the Parsing DFA (1) Construct the start context: Closure({S, \$}) We abbreviate as: S, \$ +(), \$ int, \$ +(), + int, + S, \$ +(), \$/+ int, \$/+ 9/22/06 Prof. Hilfinger CS164 Lecture 11 46

47 Constructing the Parsing DFA (2) A DFA state is a closed set of LR(1) items This means that we performed Closure The start state is Closure([S, \$]) A state that contains [X α, b] is labeled with reduce with X α on b And now the transitions 9/22/06 Prof. Hilfinger CS164 Lecture 11 47

48 The DFA Transitions A state State that contains [X α yβ, b] has a transition labeled y to a state that contains the items Transition(State, y) y can be a terminal or a non-terminal Transition(State, y) Items for each [X α yβ, b] State add [X αy β, b] to Items return Closure(Items) 9/22/06 Prof. Hilfinger CS164 Lecture 11 48

49 Constructing the Parsing DFA. xample. 2 S, \$ 0 +(), \$/+ int int, \$/+ + S, \$ +(), \$/+ accept on \$ 6 +( ), \$/+ +(), )/+ and so on 1 int, \$/+ + (), \$/+ +( ), \$/+ +(), )/+ int, )/+ int on \$, + int, )/+ 9/22/06 Prof. Hilfinger CS164 Lecture ( int int on ), +

50 LR Parsing Tables. Notes Parsing tables (i.e. the DFA) can be constructed automatically for a CFG But we still need to understand the construction to work with parser generators.g., they report errors in terms of sets of items What kind of errors can we expect? 9/22/06 Prof. Hilfinger CS164 Lecture 11 50

51 Shift/Reduce Conflicts If a DFA state contains both [X α aβ, b] and [Y γ, a] Then on input a we could either Shift into state [X αa β, b], or Reduce with Y γ This is called a shift-reduce conflict 9/22/06 Prof. Hilfinger CS164 Lecture 11 51

52 Shift/Reduce Conflicts Typically due to ambiguities in the grammar Classic example: the dangling else S if then S if then S else S OTHR Will have DFA state containing [S if then S, else] [S if then S else S, \$] If else follows then we can shift or reduce 9/22/06 Prof. Hilfinger CS164 Lecture 11 52

53 More Shift/Reduce Conflicts Consider the ambiguous grammar + * int We will have the states containing [ *, +] [ *, +] [ +, +] [ +, +] Again we have a shift/reduce on input + We need to reduce (* binds more tightly than +) Solution: declare the precedence of * and + 9/22/06 Prof. Hilfinger CS164 Lecture 11 53

54 More Shift/Reduce Conflicts In bison declare precedence and associativity of terminal symbols: %left + %left * Precedence of a rule = that of its last terminal See bison manual for ways to override this default Resolve shift/reduce conflict with a shift if: input terminal has higher precedence than the rule the precedences are the same and right associative 9/22/06 Prof. Hilfinger CS164 Lecture 11 54

55 Using Precedence to Solve S/R Conflicts Back to our example: [ *, +] [ *, +] [ +, +] [ +, +] Will choose reduce because precedence of rule * is higher than of terminal + 9/22/06 Prof. Hilfinger CS164 Lecture 11 55

56 Using Precedence to Solve S/R Conflicts Same grammar as before + * int We will also have the states [ +, +] [ +, +] [ +, +] [ +, +] Now we also have a shift/reduce on input + We choose reduce because + and + have the same precedence and + is left-associative 9/22/06 Prof. Hilfinger CS164 Lecture 11 56

57 Using Precedence to Solve S/R Conflicts Back to our dangling else example [S if then S, else] [S if then S else S, x] Can eliminate conflict by declaring else with higher precedence than then However, best to avoid overuse of precedence declarations or you ll end with unexpected parse trees 9/22/06 Prof. Hilfinger CS164 Lecture 11 57

58 Reduce/Reduce Conflicts If a DFA state contains both [X α, a] and [Y β, a] Then on input a we don t know which production to reduce This is called a reduce/reduce conflict 9/22/06 Prof. Hilfinger CS164 Lecture 11 58

59 Reduce/Reduce Conflicts Usually due to gross ambiguity in the grammar xample: a sequence of identifiers S ε id id S There are two parse trees for the string id S id S id S id How does this confuse the parser? 9/22/06 Prof. Hilfinger CS164 Lecture 11 59

60 More on Reduce/Reduce Conflicts Consider the states [S id, \$] [S S, \$] [S id S, \$] [S, \$] id [S, \$] [S id, \$] [S id, \$] [S id S, \$] [S id S, \$] Reduce/reduce conflict on input \$ S S id S S id S id Better rewrite the grammar: S ε id S 9/22/06 Prof. Hilfinger CS164 Lecture 11 60

61 Using Parser Generators Parser generators construct the parsing DFA given a CFG Use precedence declarations and default conventions to resolve conflicts The parser algorithm is the same for all grammars (and is provided as a library function) But most parser generators do not construct the DFA as described before Because the LR(1) parsing DFA has 1000s of states even for a simple language 9/22/06 Prof. Hilfinger CS164 Lecture 11 61

62 LR(1) Parsing Tables are Big But many states are similar, e.g. 1 int on \$, + int, \$/+ and int, )/+ 5 int on ), + Idea: merge the DFA states whose items differ only in the lookahead tokens We say that such states have the same core We obtain 1 int, \$/+/) int on \$, +, ) 9/22/06 Prof. Hilfinger CS164 Lecture 11 62

63 The Core of a Set of LR Items Definition: The core of a set of LR items is the set of first components Without the lookahead terminals xample: the core of { [X α β, b], [Y γ δ, d]} is {X α β, Y γ δ} 9/22/06 Prof. Hilfinger CS164 Lecture 11 63

64 LALR States Consider for example the LR(1) states {[X α, a], [Y β, c]} {[X α, b], [Y β, d]} They have the same core and can be merged And the merged state contains: {[X α, a/b], [Y β, c/d]} These are called LALR(1) states Stands for LookAhead LR Typically 10 times fewer LALR(1) states than LR(1) 9/22/06 Prof. Hilfinger CS164 Lecture 11 64

65 A LALR(1) DFA Repeat until all states have distinct core Choose two distinct states with same core Merge the states by creating a new one with the union of all the items Point edges from predecessors to new state New state points to all the previous successors A B C A C B D F D F 9/22/06 Prof. Hilfinger CS164 Lecture 11 65

66 Conversion LR(1) to LALR(1). xample. + () on \$, + int ( accept on \$ ) ) ( 5 11 int on \$, + int 9 int on ), + int + () on ), + accept on \$ + () on \$, +, ) int 0 1,5 int ( 2 + 3,8 4,9 7,11 ) + 6,10 int on \$, +, )

67 The LALR Parser Can Have Conflicts Consider for example the LR(1) states {[X α, a], [Y β, b]} {[X α, b], [Y β, a]} And the merged LALR(1) state {[X α, a/b], [Y β, a/b]} Has a new reduce-reduce conflict In practice such cases are rare 9/22/06 Prof. Hilfinger CS164 Lecture 11 67

68 LALR vs. LR Parsing LALR languages are not natural They are an efficiency hack on LR languages But any reasonable programming language has a LALR(1) grammar LALR(1) has become a standard for programming languages and for parser generators 9/22/06 Prof. Hilfinger CS164 Lecture 11 68

69 A Hierarchy of Grammar Classes From Andrew Appel, Modern Compiler Implementation in Java 9/22/06 Prof. Hilfinger CS164 Lecture 11 69

70 Notes on Parsing Parsing A solid foundation: context-free grammars A simple parser: LL(1) A more powerful parser: LR(1) An efficiency hack: LALR(1) We use LALR(1) parser generators 9/22/06 Prof. Hilfinger CS164 Lecture 11 70

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