Syntax Directed Translation in Compiler Design

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Questions and Answers

What is the primary advantage of using postfix notation in expressions?

  • It allows operators to precede operands.
  • It eliminates the need for parentheses. (correct)
  • It increases computational complexity.
  • It requires explicit operator precedence.

What is the standard form of a three-address statement?

  • result = operator operand1 operand2
  • operator = operand1 operand2
  • operand1 = operand2 operator result
  • result = operand1 operator operand2 (correct)

How many references are typically involved in a standard three-address statement?

  • One reference for the result only.
  • Three references for the result only.
  • No references at all.
  • Two references for operands and one for the result. (correct)

What is the role of internal nodes in a syntax tree?

<p>They represent operators and keywords. (A)</p> Signup and view all the answers

What does a syntax tree represent in relation to a parse tree?

<p>A condensed representation of the parse tree. (D)</p> Signup and view all the answers

What are temporary variables in three-address code typically referred to as?

<p>Temps (A)</p> Signup and view all the answers

Which of the following is NOT a method for representing three-address code?

<p>Direct Calls (D)</p> Signup and view all the answers

How does creating a syntax tree improve expression representation?

<p>By strategically placing parentheses. (C)</p> Signup and view all the answers

What happens to control when the expression E is false within a while loop?

<p>Control jumps to the next statement after S. (D)</p> Signup and view all the answers

What is the characteristic of a postfix syntax-directed translation (SDT)?

<p>Semantic actions occur at the end of the production. (A)</p> Signup and view all the answers

Which of the following correctly defines the function of S1.NEXT in a while loop implementation?

<p>S1.NEXT points to the execution of the next statement after S. (D)</p> Signup and view all the answers

In parser stack implementation of postfix SDTs, what is primarily stored in the parser stack?

<p>Records of non-terminals and their attributes. (D)</p> Signup and view all the answers

How is control managed in the translation syntax for a while loop?

<p>By establishing jump labels for both true and false conditions of E. (C)</p> Signup and view all the answers

What does the semantic rule 'E.FALSE = S.NEXT' indicate in the context of a while loop?

<p>The next statement after the while loop will be executed. (B)</p> Signup and view all the answers

In the context of postfix SDTs, which semantic rule corresponds to 'S.CODE = GEN(S.BEGIN '− ') || E.CODE || GEN(E.TRUE '− ') || S1.CODE'?

<p>It creates jump instructions for both conditions and calls the next loop. (B)</p> Signup and view all the answers

What is the significance of the label S.BEGIN in the control flow of a while loop?

<p>It marks the beginning point for evaluating the loop's condition. (D)</p> Signup and view all the answers

What happens when reduction occurs at the top of the stack during syntax-directed translation?

<p>The corresponding LHS non-terminal and its attributes replace existing attributes. (C)</p> Signup and view all the answers

In an SDT with actions inside the production, when are the actions performed if using a bottom-up parser?

<p>Immediately after the occurrence of a non-terminal at the top of the parser stack. (D)</p> Signup and view all the answers

What should be done to a grammar that contains left recursion to enable parsing with a top-down parser?

<p>Eliminate the left recursion. (D)</p> Signup and view all the answers

In L-attributed definitions used in SDT, where should the actions for inherited attributes be placed?

<p>Before the non-terminal in the production. (C)</p> Signup and view all the answers

In the provided production P ⇢ Pr | q, how is it transformed after eliminating left recursion?

<p>P ⇢ qA. (C)</p> Signup and view all the answers

What is an example of a synthesized attribute in the provided productions?

<p>A.str = B.str + C.str (C)</p> Signup and view all the answers

How are arithmetic expression references, like A[i] + B[j], converted into intermediate representations in syntax-directed translation?

<p>Calculating indices first, then dereferencing the values. (B)</p> Signup and view all the answers

What is an outcome of using synthesized attributes in syntax-directed translation?

<p>They facilitate semantic actions during parsing. (C)</p> Signup and view all the answers

Flashcards

While loop control

A program construct that executes a block of code repeatedly as long as a condition (expression) is true.

Postfix SDT

Syntax-directed translation scheme where semantic actions are placed at the right end of the production.

Semantic Rule

A rule that defines how to compute the values, actions, and other attributes associated with a production during a syntax analysis.

Production

A rule in a grammar that defines a non-terminal in terms of terminals and/or other non-terminals.

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Parser Stack

A data structure used by a bottom-up parser (like LR parser) to keep track of grammar symbols and their attributes.

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Syntax-Directed Translation

A method of translation that incorporates actions during parsing to build the translated code or data.

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Synthesized Attribute

Attribute of a non-terminal that is computed from the attributes of its children.

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Bottom-up parser

Parsing method that works by combining or reducing the input tokens to higher-level grammar symbols.

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SDT with action inside production

Semantic actions are placed anywhere on the right-hand side of a production rule. The actions are evaluated and performed immediately after the non-terminal is processed in both top-down and bottom-up parsing.

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Eliminating Left Recursion from SDT

A technique used to modify grammars with left recursion so that they can be parsed by top-down parsing algorithms. This transformation alters the grammar's structure to eliminate left-recursive productions.

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Inherited Attribute

An attribute of a non-terminal computed from attributes of non-terminals on the left of it in the production.

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L-attributed Definition

SDT using both synthesized and inherited attributes to define how attributes are passed.

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Production Rule

A rule defining how a non-terminal can be derived from terminals and non-terminals.

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Array References in Arithmetic Expressions

Converts array references (e.g., A[i] + B[j]) to intermediate representation (IR) for execution.

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Procedure Calls

Translation of function calls, parameter passing, and return statements.

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Postfix Notation

A way to write expressions where the operator follows the operands. Also known as Reverse Polish Notation.

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Infix Notation

A way to write expressions where the operator is placed between the operands. (e.g., a + b).

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Three-Address Code

A representation of code using maximum three references in each statement (two operands, one result).

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Intermediate Code

A form of code that is between the source code and target code in a compiler. It can be language-specific (like Bytecode) or language-independent (like three-address code).

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Syntax Tree

A tree-like representation of a program. Internal nodes are operators and child nodes are operands. Parentheses help structure it.

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Example of Postfix

(a + b) * c is equivalent to ab + c * in Postfix notation.

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Three-Address-Statement

A statement with a maximum of 3 references (2 operands, 1 result) and expressed as x = y + z, where x, y, z are memory addresses.

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Quadruples

One way -- a specific representation style -- to express a three-address code.

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Study Notes

Syntax Directed Translation in Compiler Design

  • Parsers use Context-Free Grammars (CFGs) to validate input strings and produce output for the next compiler phase.
  • Outputs can be parse trees or abstract syntax trees.
  • Syntax Directed Translation (SDT) interleaves semantic analysis with syntax analysis.
  • Conceptually, SDT parses the input, builds a parse tree, then traverses the tree to evaluate semantic rules at every node.
  • Semantic rule evaluation can generate code, manage the symbol table, produce error messages, or perform other actions.

Definition

  • Syntax Directed Translation augments the grammar.
  • SDT involves passing information (bottom-up and/or top-down) to the parse tree, attaching attributes to nodes.
  • Attributes can be lexical values, constants, and attributes associated with non-terminals.
  • SDT generally constructs a parse tree, evaluating attribute values at the nodes in a specific order.

Example

  • A grammar example for expressions with addition and multiplication is given.
  • Semantic analysis can be performed using SDT rules.
  • An example of how semantic analysis happens is presented with a sample input and its corresponding parse tree; showing how attribute values are calculated at the parse tree's different nodes in a left-to-right bottom-up evaluation.

Intermediate Code Generation in Compiler Design

  • A compiler uses a frontend to translate a source program into intermediate code.
  • The backend uses this intermediate code to generate target code.
  • The role of intermediate code is platform independence.
  • Portability is enhanced.
  • Efficient code generation and code optimization can occur through intermediate code.

Advantages of Syntax Directed Translation

  • Implementation is easier.
  • Separates concerns of parsing and translation.
  • Modularity and extensibility are facilitated when designing compilers.

Disadvantages of Syntax Directed Translation

  • Limited expressiveness compared to other methods (like attribute grammars).
  • Can be inflexible for complex translation rules.
  • Limited error recovery capabilities, which could impact error messages and diagnostics.

Intermediate Code Representations

  • Postfix notation (reverse Polish notation) places operators after operands (e.g., a + b becomes ab+).
  • Three-address code has a maximum of three references per statement (two for operands, one for result).
    • Example, the expression a+bc will compile to: T1 = bc ; T2 = a+T1;

Syntax Tree

  • Condensed representation of a parse tree.
  • Simplifies visual representation of the program's syntactic structure.
  • Relocates operators and keywords and places them within parent nodes.

Intermediate code generation advantages

  • Implementation is easier.
  • Facilitates code optimization.
  • Platform-independent: code can run on multiple platforms without modification.
  • Code reuse: Intermediate code can be reused for other platforms or languages.
  • Easier debugging: Closer to source code, thus allowing for easier debugging.

Intermediate code generation disadvantages

  • Increased compilation time.
  • Additional memory usage.
  • Increased complexity.
  • Reduced performance (than generating machine code directly)

Abstract Translation Scheme

  • An abstract translation scheme describes how to translate programming constructions into intermediate code.
  • Includes examples for if-then, if-then-else, and while-loop constructs.

Control Statements

  • Statements like if–then,if–then–else, and while–do, which change the flow of the program's execution.
  • These are translated by generating jump instructions, based on the Boolean expression's result.

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