CS 321 Programming Languages and Compilers Bottom Up Parsing.

CS 321Programming Languages and

Compilers

CS 321Programming Languages and

Compilers

Bottom Up Parsing

Bottom-Up ParsingBottom-Up ParsingBottom-Up ParsingBottom-Up Parsing22

Bottom-up Parsing: Shift-reduce parsingBottom-up Parsing: Shift-reduce parsing

Grammar H: L E;L | E

E a | b

Input: a;a;b

has parse tree LL;E

L;E

baa


Data for Shift-reduce ParserData for Shift-reduce Parser

• Input string: sequence of tokens being checked for grammatical correctness

• Stack: sentential form representing the input seen so far

• Trees Constructed: the parse trees that have been constructed so far at some point in the parsing


Operations of shift-reduce parserOperations of shift-reduce parser

• Shift: move input token to the set of trees as a singleton tree.

• Reduce: coalesce one or more trees into single tree (according to some production).

• Accept: terminate and accept the token stream as grammatically correct.

• Reject: Terminate and reject the token stream.


A parse for grammar HA parse for grammar H

Input Stack Action Treesa;a;b Shift

;a;b a Reduce E a

;a;b E Shift

a;b E; Shift

a

E

a

E

a

;



Input Stack Action Trees;b E;a Reduce E a

;b E;E Shift

b E;E; Shift

E

a

; a

E

a

; E

a

E

a

; E

a

;



Input Stack Action Trees$ E;E;b Reduce Eb

$ E;E;E Reduce LE

$ E;E;L Reduce LE;L

E

a

; E

a

; b

E

a

; E

a

; E

b

E

a

; E

a

;

E

b

L


A parse for grammar HA parse for grammar HInput Stack Action Trees

$ E;L Reduce LE;L

$ L Acc

E

a

; E

a

;

E

b

L

L

E

a

; E

a

;

E

b

L

LL


Bottom-up ParsingBottom-up Parsing

• Characteristic automata for an “LR” grammar: tells when to shift/reduce/accept or reject

• handles

• viable prefixes



Input Stack Action Stack+Inputa;a;b Shift a;a;b ;a;b a Reduce E a a;a;b ;a;b E Shift E;a;b a;b E; Shift E;a;b ;b E;a Reduce E a E;a;b ;b E;E Shift E;E;b b E;E; Shift E;E;b $ E;E;b Reduce Eb E;E;b $ E;E;E Reduce LE E;E;E$ E;E;L Reduce LE;L E;E;L$ E;L Reduce LE;L E;L$ L Accept L


“Bottom-up” parsing?“Bottom-up” parsing?Stack+Input

LE;LE;E;LE;E;EE;E;b E;E;b E;E;b E;a;b E;a;b E;a;b a;a;b a;a;b

E

a

; E

a

;

E

b

L

LL

E

a

; E

a

;

E

b

L

LL


Handles and viable prefixesHandles and viable prefixes

• Stack+remaining input = sentential form

• Handle = the part of the sentential form that is reduced in each step

• Viable prefix = the prefix of the sentential form in a right-most derivation that do not extend beyond the end of the handle

• E.g. viable prefixes for H: (E;)*(E | L | a | b)

• Viable prefixes form a regular set.


Characteristic Finite State Machine (CFSM)Characteristic Finite State Machine (CFSM)

Viable prefixes of H are recognized by this CFSM:

0

1

2

3

4

E

L

a

b

5 6E

a

b

L;


How a Bottom-Up Parser WorksHow a Bottom-Up Parser Works

• Run the CFSM on symbols in the stack

• If a transition possible on the incoming input symbol, then shift, else reduce.

– Still need to decide which rule to use for the reduction.


Characteristic automatonCharacteristic automaton

0

1

2

3

4

E

L

a

b

5 6E

a

b

L;

a;a;b leads to state 3 after aE;a;b leads to state 3 after E;aE;E;b leads to state 4 after E;E;bE;E;E leads to state 2 after E;E;EE;E;L leads to state 6 after E;E;LE;L leads to state 6 after E;L

start

ViablePrefixes


Characteristic automatonCharacteristic automaton

0

1

2

3

4

E

L

a

b

5 6E

a

b

L;start

State Action0,5 shift (if possible)1 accept2 reduce LE, if EOF

shift otherwise3 reduce Ea4 reduce Eb6 reduce LE;L


Example: expression grammarExample: expression grammar

E E+T | T

T T*P | P

P id

id+id+id+idhas parse tree:

ET+E

T+ET+E

T

P

id

P

id

P

id

P

id


A parse in this grammarA parse in this grammar

id+id+id+id Shift+id+id+id id Reduce+id+id+id P Reduce+id+id+id T Reduce+id+id+id E Shiftid+id+id E + Shift+id+id E+id Reduce+id+id E+P Reduce+id+id E+T Reduce+id+id E Shiftid+id E+ Shift


A parse in this grammar (cont.)A parse in this grammar (cont.)

+id E+ id Reduce+id E+P Reduce+id E+T Reduce+id E Shiftid E+ Shift$ E+id Reduce$ E+P Reduce$ E+T Reduce$ E Reduce$ E Accept


Characteristic Finite State MachineCharacteristic Finite State Machine

The CFSM recognizes viable prefixes (strings of grammar symbols that can appear on the stack):

0

1

4

2

3

id

E

T

P

5 8

id

*

T

id

+

7 9P

*P


DefinitionsDefinitions

• Rightmost derivation

• Right-sentential form

• Handle

• Viable prefix

• Characteristic automaton


Rightmost DerivationRightmost Derivation

Definition: A rightmost derivation in G is a derivation:

1ii1 wwwS

such that for each step i, the rightmost non-terminal in iw

1iw is replaced to obtain

ba;a;a;a;a EE;E;E;E;EE;E;LE;LL

ba;a;ba;babb ;E;;L;E;L;LE;LL

1.

2.

is right-most

is not right-most


Right-sentential FormsRight-sentential Forms

Definition: A right-sentential form is any sentential form that occurs in a right-most derivation.

ba;a;ba;babb ;E;;L;E;L;LE;LL

E.g., any of these


HandlesHandles

Definition: Assume the i-th step of a rightmost derivation is:

wi=uiAvi uivi=wi+1

Then, (, |ui|) is the handle of wi+1

In an unambiguous grammar, any sentence has a unique rightmostderivation, and so we can talk about “the” handle rather than “a”handle.


The PlanThe Plan

• Construct a parser by first constructing the CFSM, then constructing GOTO and ACTION tables from it.

• Construction has two parts:– “LR(0)” construction of the CFSM

– “SLR(1)” construction of tables


Constructing the CFSM: StatesConstructing the CFSM: States

The states in the CFSM are created by taking the “closure” of “LR(0) items”

L • E ; LL E • ; LL E ; • LL E ; L •

Given a production “L E ; L”, these are all induced “LR(0) items”


What is “•”? What is “•”?

The “•” in “L E • ; L” represents the state of the parse.

L

E ; LOnly this part of thetree is fully developed


A State in the CFSM: Closure of LR(0) ItemA State in the CFSM: Closure of LR(0) Item

For set I of LR(0) items, calculate closure(I):1. if “A • B ” is in closure(I), then for every production “B ”,

“B • ” is in closure(I)2. closure(I) is the smallest set with property (1)


Closure of LR(0) Item: ExampleClosure of LR(0) Item: Example

H: L E;L | E E a | b

closure({L E ; • L}) = {L E ; • L , L • E ; L , L • E, E • a , E • b}


LR(0) MachineLR(0) Machine

• Given grammar G with goal symbol S, augment the grammar by adding new goal symbol S’ and production S’ S.

• States = sets of LR(0) items

• Start state = closure({S’ •S})

• All states are considered to be final (set of viable prefixes closed under prefix)

• transition(I, X) = closure({A X• | A •XI}).


Example: LR(0) CFSM Construction Example: LR(0) CFSM Construction

H: L’ L L E;L | E E a | b

Augment the grammar:

Initial State is closure of this “augmenting rule”:

closure({L’ • L}) =

L’ • LL • E;LL • EE • aE •b

:I0


Example: Transitions from I0 Example: Transitions from I0

transition( , L) = {L’ L •} = 0I 1I

transition( , E) = {L E •;L , L E •} = 0I 2I

transition( , a ) = {E a •} = 0I

transition( , b) = {E b •} = 0I

3I

4I

There are no other transitions from I0. There are no transitions possible from I1.Now consider the transitions from I2.


Transitions from I2 Transitions from I2

transition( , ;) = 2I

L E;• LL • E;LL • EE • aE •b

:I5

transition( , L) = { L E;L• }= 5I

transition( , E) =5I 2Itransition( , a) =5I 3I transition( , b) =5I 4I

New state: 6I


The CFSM Transition Diagram for HThe CFSM Transition Diagram for H

L E;• LL • E;LL • EE • aE •b

:I5

L’ L •

L E •;LL E •

E a •

E b •

L’ • LL • E;LL • EE • aE •b

:I0

L E;L•

:I6:I2

:I4

:I3

:I1

L

E

a

b

E

a

b

;

L


Characteristic Finite State Machine for HCharacteristic Finite State Machine for H

0

1

2

3

4

E

L

a

b

5 6E

a

b

L;


How LR(1) parsers workHow LR(1) parsers work

• GOTO table: transition function of characteristic automaton in tabular form

• ACTION table: State Action

• Procedure:– Use the GOTO table to run the CFSM over the stack. Suppose

state reached at top of stack is .

– Take action given by ACTION(,a), where a is incoming input symbol.


Action Table for HAction Table for H

a b ; $

0 Shift Shift1 Accept2 Shift Reduce L E3 Reduce E a Reduce E a4 Reduce E b Reduce E b5 Shift Shift6 Reduce L E;L


SLR(1) Parser ConstructionSLR(1) Parser Construction

• GOTO table is the move function from the LR(0) CFSM.

• ACTION table is constructed from the CFSM as follows:

– If state i contains A •a, then ACTION(i,a) = Shift.

– If state i contains A •, then ACTION(i,a) = Reduce A(But, if A is L’, action is Accept.)

– Otherwise, reject.

• But,...


SLR(1) Parser ConstructionSLR(1) Parser Construction

• Rules for the ACTION table can involve shift/reduce conflicts.

• So the actual rule for reduce actions is: If state i contains A •, then ACTION(i,a) = Reduce A , for

all a FOLLOW(A).

• E.g. state 2 for grammar H yields a shift/reduce conflict. Namely, should you shift the “;” or reduce by “LE”. This is resolved by looking at the “follow set” for L.

• Follow(L) = {$}


FIRST and FOLLOW setsFIRST and FOLLOW sets

• FIRST() = {a | a} { | a }

• FOLLOW(A) = {a | S Aa}


Calculating FIRST setsCalculating FIRST sets

• Create table Fi mapping N to {}; initially, Fi(A) = for all A.

• Repeat until Fi does not change:– For each A P,

Fi(A) := Fi(A) FIRST(, Fi)

where FIRST(, Fi) is defined as follows:– FIRST(, Fi) = {}

– FIRST(a, Fi) = {a}

– FIRST(B, Fi) = Fi(B) FIRST(b,Fi), if Fi(B) Fi(B), o.w.


Calculating FOLLOW setsCalculating FOLLOW sets

• Calculate FOLLOW sets

• Create table Fo : N {$}; initially, Fo(A) = for all A, except Fo(S) = {$}

• Repeat until Fo does not change:– For each production A B,

Fo(B) := Fo(B) FIRST() - {}

– For each production A BFo(B) := Fo(B) Fo(A)

– For each production A Bif FIRST(), Fo(B) := Fo(B) Fo(A)

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CS 321 Programming Languages and Compilers Bottom Up Parsing.

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