Download presentation

Presentation is loading. Please wait.

1
**Parsing 4 Dr William Harrison Fall 2008 HarrisonWL@missouri.edu**

CS4430 – Compilers I

2
**Today Continuing the second phase “Predictive” parsing**

“parsing” or grammatical analysis discovers the real structure of a program and represents it in a computationally useful way “Predictive” parsing also called “recursive descent” parsing Last time: basics of LL(1) parsing follow sets, table-driven predictive parsing, etc. Today: finish predictive parsing Reading: You should be reading Chapter 2 of “Modern Compiler Design”

3
**Parsing concepts Language Grammar Parser**

* all inter-related, but different notions

4
**Review: Parse Trees from Derivations**

S if E then S else S S begin S L S print E L end L ; S L E num = num Parse Tree Associated with Derivation S begin S L print E end 1 = 1 S begin S L begin print E L begin print 1=1 L begin print 1=1 end

5
**The Process of constructing Parsers**

Language Design Construct CFG Recognize its Form Parser Language Parser start again if not in appropriate form (e.g., LL(1), LR(1),…)

6
**Predictive Parsing S if E then S else S S begin S L S print E**

The first token on the RHS of each rule is unique. S if E then S else S S begin S L S print E L end L ; S L E num = num a.k.a. “recursive descent” If the first symbol on the r.h.s. of the productions is unique, then this grammar is LL(1).

7
**“Rolling your own” Predictive Parsers**

Has one function for each non-terminal, and one clause for each production int tok = getToken(); void advance() { tok = getToken(); } void eat(int t) { if (tok = t) { advance() } else { error(); } } void S(){switch(tok) { case IF: eat(IF); E(); eat(THEN); S(); eat(ELSE); S(); break; case BEGIN: … void E() { eat(NUM); eat(EQ); eat(NUM); … S if E then S else S S begin S L S print E L end L ; S L E num = num

8
**Road map for today Determining if a grammar is LL(1)**

First sets “Follow” sets “Massaging” a grammar into LL Using the technique of “left factoring” …and eliminating “left recursion”

9
**Review: Table-driven Predictive Parsing**

S E $ E T E’ E’ + T E’ E’ - T E’ E’ T F T’ T’ * F T’ T’ / F T’ T’ F id F num F ( E ) front of token stream + id $ * S E E’ T T’ F 1 2 3 5 6 9 7 9 10 Actions are: predict(production) match, accept, and error top of the “parse stack” empty=error

10
**Remaining Questions about Predictive Parsing**

We were “given” that grammar and prediction table Can all grammars be given a similar table Alas, no – only LL(1) grammars How did we come up with that table? “First” and “Follow” sets Techniques for converting some grammars into LL(1) Eliminating left-recursion Left factoring We’ll discuss these now

11
**FIRST(g) g is a sequence of symbols (terminal or non-terminal)**

For example, the right hand side of a production FIRST returns the set of all possible terminal symbols that begin any string derivable from g. Consider two productions X g1 and X g2 If FIRST (g1 ) and FIRST (g2 ) have symbols in common, then the prediction mechanism will not know which way to choose. The Grammar is not LL(1)! If FIRST (g1 ) and FIRST (g2 ) have no symbols in common, then perhaps LL(1) can be used. We need some more formalisms.

12
**FOLLOW sets and nullable productions**

FOLLOW(X) X is a non-terminal The set of terminals that can immediately follow X. nullable(X) True if, and only if, X can derive to the empty string λ. FIRST, FOLLOW & nullable can be used to construct predictive parsing tables for LL(1) parsers. Can build tables that support LL(2), LL(3), etc. X A X B C X d B a b FOLLOW(X) = ? X a B X B B d B λ nullable(X) = ?

13
**FOLLOW sets and nullable productions**

FOLLOW(X) X is a non-terminal The set of terminals that can immediately follow X. nullable(X) True if, and only if, X can derive to the empty string λ. FIRST, FOLLOW & nullable can be used to construct predictive parsing tables for LL(1) parsers. Can build tables that support LL(2), LL(3), etc. X A X B C X d B a b FOLLOW(X) = { d, a } X a B X B B d B λ nullable(X) = true

14
**Constructing the parse table**

For every non-terminal X and token t: t X X g Enter the production (X g) if t FIRST(g), If X is nullable, enter the production (X g) if t FOLLOW(g)

15
**Constructing the parse table**

What if there are more than one production? t X X g1,X g

16
**Constructing the parse table**

What if there are more than one production? t X g1,X g X Then the grammar cannot be parsed with “predictive parsing”, and it is (by definition) not LL(1).

17
**Shortcomings of LL Parsers**

Recursive descent renders a readable parser. depends on the first terminal symbol of each sub-expression providing enough information to choose which production to use. But consider a predictive parser for this grammar E E + T E T void E(){switch(tok) { case ?: E(); eat(TIMES); T(); no way of choosing production case ?: T(); … } void T(){eat(ID);}

18
**Eliminating left recursion**

Consider this grammar it’s “left-recursive” Can not use LL(1) – why? Consider this alternative different grammar This derives the same language Now there is no left recursion. There is a generalization for more complex grammars. E E + T E T E T E’ E’ + T E’ E’ λ

19
**Removing Common Prefixes**

Consider It’s not LL(1) – why? We can transform this grammar, and make it LL(1). S if E then S else S S if E then S S if E then S X X λ X else S * Remark: The resulting grammar is not as readable.

20
**In Class Discussion (1) S S ; S S id := E E id E num E E + E**

How can we transform this grammar So that it accepts the same language, but Has no left recursion We may have to use left factoring

21
In Class Discussion (2) S S1 ; S was S S ; S S S1 S1 id := E E E + E1 was E E+E E E1 E1 id E1 num Prefix elimination doesn’t work here Write a grammar that accepts the same language, but Is not ambiguous Has no left recursion

22
**Class Discussion (3) S S1 S2 S1 id := E S2 ; S1 S2 S2 **

E E1 E2 E1 id E1 num E2 + E1 E2 E2 Write a grammar that accepts the same language, but … Has no left recursion Before: (# is a terminal) X X # Y X Y After: X Y X2 X2 # Y X2 X2 This final grammar is LL(1).

23
Next time LR Parsing LR(k) grammars are more expressive than LL(k) grammars, …but it’s not as obvious how to parse with them.

Similar presentations

Presentation is loading. Please wait....

OK

Parsing V: Bottom-up Parsing

Parsing V: Bottom-up Parsing

© 2017 SlidePlayer.com Inc.

All rights reserved.

Ads by Google

Ppt on wind power generation Ppt on foundation of buildings Ppt on collection of secondary data Ppt on standing order medical Ppt on france in french language Ppt on project tiger Ppt on molecular biology of head neck cancer Ppt on electricity for class 10th roll Ppt on total internal reflection critical angle Download ppt on basic concepts of chemistry