University of Technology Department of Computer Sciences. Final Examination st Term. Subject:Compilers Design

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Define the following with simple example (choose five only): 1.)Ambiguity: A grammar that produces more than one parse tree for some sentences.

1 Subject:Compilers Design Division: All Branches Examiner:Dr. Abeer Tariq University of Technology Department of Computer Sciences 2102 Final Examination st Term Year:Third Time: 3 Hours Date: / May /2012 حلول اسئلة الدور األول Q1. Define the following with simple example (choose five only): 1.)Ambiguity: A grammar that produces more than one parse tree for some sentences. An ambiguous grammar is one that produces more than one leftmost or more than one rightmost derivation for the same sentence. (1) three-address code Each statement contains at most 3 operands; in addition to : =, i.e., assignment, at most one operator. An easy and universal format that can be translated into most assembly languages A:=B+C (2) Compiler Is a program (translator) that reads a program written in one language, (the source language) and translates into an equivalent program in another language (the target language). (3) Operator grammar The operator-precedence parser is a shift reduce parser that can be easily constructed by hand. It used for a small class of grammars which is called operator grammar. These grammars have the property:

2 - That no production right side is ε - And no production right side has two adjacent nonterminals. Example: The following grammar for expressions: E EAE (E) E id A + - * / ^ (4) Handle Definition: a handle is a substring that 1- matches a right hand side of a production rule in the grammar and 2- Whose reduction to the nonterminal on the left hand side of that grammar rule is a step along the reverse of a rightmost derivation. (5) Conflicts "Conficts" occur when an ambiguity in the grammar creates a situation where the parser does not know which step to perform at a given point during parsing. There are two kinds of conflicts that occur 1. shift-reduce: a shift reduce conflict occurs when the grammar indicates that different successful parses might occur with either a shift or a reduce at a given point during parsing. The vast majority of situations where this conflict occurs can be correctly resolved by shifting. 2- reduce-reduce : a reduce-reduce conflict occurs when the parser has two or more handles at the same time on the top of the stack. Whatever choice the parser makes is just as likely to be wrong as not. In this case it is usually best to rewrite the grammar to eliminate the conflict, possibly by factoring. Q.2) briefly answer the following (choose three only) A) Draw the diagram showing the position of parser in compiler model. B) What is the difference between static and dynamic semantic? Static semantic checks are performed at compile time

3 Type checking Every variable is declared before used Identifiers are used in appropriate contexts Check subroutine call arguments Dynamic semantic check are performed at run time, and the compiler produces code that performs these checks Array subscript values are within bounds Arithmetic errors, e.g. division by zero A variable is used but hasn't been initialized When a check fails at run time, an exception is raised C) Explain the three techniques of loop optimization with simple examples. Code Motion An important modification that decreases the amount of code in a loop is code motion. This transformation takes an expression that yields the same result independent of the number of times a loop is executed and places the expression before the loop. Ex1: While ( I <= limit-2) Sol: t = limit-2 While (I <= t ) Induction Variable Ex: X = 2 * y + 2 * h + 4 Sol: X = 2 * (y + h) +4 Note: use the algebra method to optimize the mathematical expression. Reduction in strength Reduction in strength, which replaces an expensive operation by a cheaper one, such as a multiplication by an addition. Ex: t4 = 4*j - 4 D) List the four error routines in the bottom-up parsing table, and what is the suitable solution for each type.

4 Q.3) Consider the following grammar G. 1. Eliminate left recursion: 2. Eliminate left factoring. 3. Prove that your grammar is LL(1) by whatever method you like. 3. After finding first and follow we will find that in first nonterminal that contain

5 empty there is not find the same terminal in the follow of that nonterminal so the grammar is LL(1). Q.4) Consider the following grammar: Parse the string " abaaba" by using predictive parsing method. 1. Eliminate the left recursion E TE' E' a b F E' e T FT' T' b a T' e F a b [ E ] 2.Find first and follow First Follow L [, a, b $, ] L' a, e $, ] T [, a, b a, $, ] T' b, e a, $, ] S [, a, b a, b, $, ] 3.parsing table a b [ ] $ L E TE' E TE' E TE' L' E' a b F E' E' e E' e T T FT' T FT' T FT' T' T' e T' b a T' T' e T' e S F a F b F [E] 4.parse the sentence Stack Input Output $E abaaba $ $ E' T abaaba $ E TE' $ E' T'F abaaba $ T FT' $ E' T'a abaaba $ F a $ E' T' $ E' T' a b baaba $ baaba $ T' b a T' $ E' T' a $ E' T' aaba $ aba $ T' e $ E' $ E' F b a aba $ aba $ E' a b F E' $ E' F b $ E' F ba $ a $ F a $ E' a $ E' a $ $ E' e $ $ The sentence is accept

6 Q.5) Given the following grammar: The terminals are = {a,b,0,1}, and the nonterminals are={s,a,b}. Parse the sentence "aa1bbbb" using SLR parsing method.

7 Q.6) A: Write the triple and quadruple form for the following statement: 1. Triple A[j] = B+( c + d ) * X[i] Y/Z op Arg 1 Arg2 (0) + c d (1) =[ ] A i (2) * 0 1 (3) / Y Z (4) + B 2 (5) (6) [ ]= A J = Quadruple op Arg 1 Arg2 result + c d T1 =[ ] A i T2 * T1 T2 T3 / Y Z T4 + B T3 T5 - T5 T4 T6 [ ]= A J T7 = T6 T7

8 B: Show a successful bottom up parse (stack) of input " bcac " using the parsing table below:

1. Explain the input buffer scheme for scanning the source program. How the use of sentinels can improve its performance? Describe in detail.

Code No: R05320502 Set No. 1 1. Explain the input buffer scheme for scanning the source program. How the use of sentinels can improve its performance? Describe in detail. 2. Construct predictive parsing