Csc 302: Compiler Construction 1 Lecture 5 Department of Computer Science

CSC 302: COMPILER CONSTRUCTION 1 LECTURE 5 DEPARTMENT OF COMPUTER SCIENCE

ON MONDAY @ 9AM; TUESDAY @ 10AM

INSTRUCTOR: OBIENU, A. C. ([email protected]) CSC 302 SECOND SEMESTER

© 2018 Obienu, A. C. All rights reserved. Students enrolled in Csc 302 at MCIU and other educational institutions have explicit permission to make copies of these materials for their personal use, provided this copyright notice is preserved. Parser Generator

PARSERS

Top Down Parser (TDP) Bottom Up Parser (BUP)

TDP with full TDP without Backtracking Backtracking

Bruteforce Recursive Non-Recursive Method descent descent (LL(1))

Operator precedence LL Parsers Parser

LR(0) SLR(0) LALR(1) CLR(1)

Top-down parsers ❑ start at the root of derivation tree and fill in ❑ picks a production and tries to match the input ❑ may require backtracking ❑ some grammars are backtrack-free (predictive) Bottom-up parsers ❑ start at the leaves and fill in ❑ start in a state valid for legal first tokens ❑ as input is consumed, change state to encode possibilities (recognize valid prefixes) ❑ use a stack to store both state and sentential forms

S → aABe w = abbcde A → Abc | b B → D

S Top Down a A B e A b c d b

NB: TDP uses leftmost derivation

S → aABe w = abbcde A → Abc | b B → D

S Top Down a A B e A b c d b

(Issue : What to use)

S → aABe w = abbcde A → Abc | b B → D

S Bottom Up B A A a b b c d e NB: BUP follow rightmost derivation

S → aABe w = abbcde A → Abc | b B → D

S Bottom Up B A A a b b c d e

Top Down Parsing (TDP)

Once grammar is on its optimal form: I. Goal is to construct the parse tree for some input (token) string a) starting with the root, b) recursively descending through the tree, c) while building the parse tree on preorder form (depth-first). 2. Find the left-most derivation to guide the preorder construction. 3. At each step of the parse tree construction: a) determine which (unique) production to be applied, b) match terminal symbols in the production body with the input (token) string symbols.

First, bring grammar on optimal form E → E + T | T T → T * F | F F → (E) |id Can be rewritten (left-recursion elimination): E → T E’ E’ → + T E’ / 휀 T → F T ’ T → *F T ‘ / 휀 F → ( E ) / id

Then, Top down parsing

Recursive-Descent Parser: general form of top-down parsers. ❑ May require backtracking. Predictive Parser special case of recursive-descent parsers. ❑ No backtracking necessary. ❑ Constructed from LL(k) grammars, k >= 1.

An LL(k) grammar implies scanning input from Left, retrieving Leftmost derivation, using k lookahead symbols to reach a resolution.

A predictive parser looks like:

A parser generator system often looks like:

This is true for both top-down (LL) and bottom-up (LR) parsers

LL(k) implies scanning input from Left, retrieving

Leftmost derivation, using k lookahead symbols to reach a resolution.

Stack: A data structure used for the procedure of parsing

Input Buffer holds the input string NB: Both Stack and Input buffer has a “$”, which is used as a stopping criterium.

Key parsing problem: determining which production to apply whilst constructing the parse tree.

FIRST and FOLLOW sets of a grammar helps choose a production based on the next input symbol.

For the next week, we will look at:

THE FIRST AND FOLLOW SETS

QUESTIONS ?

E-mail: [email protected] Office hours: Mon, Wed & Fri, 1.30pm to 3.30pm Office: Power of Faith Building, Floor 2, Rm 59