Compiler Design: Semantic Analysis

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Questions and Answers

What is the crucial difference between synthesized and inherited attributes?

Synthesized attributes are evaluated independently from their children.

Synthesized attributes flow up from child nodes, while inherited attributes flow down from parent nodes. (correct)

Attributes can be both synthesized and inherited simultaneously.

Inherited attributes can only be used in leaf nodes.

Which statement best describes attribute computation rules in the context of productions?

Rules for attributes are local to the production and cannot have side effects. (correct)

Attribute rules can affect symbols across different productions globally.

Rules can introduce synthesized attributes in the leaf nodes of a parse tree.

Inherited attributes must always be initialized before use.

In the provided example of attribute grammar, what value does AS(S) contain?

count ↑: integer

final count of non-terminals

equal ↑: {T, F} (correct)

no attributes defined

How are synthesized attributes initialized in the context of a parse tree?

They are initialized by the lexical analyzer and modified during tree traversal. Signup and view all the answers

What characterizes attribute grammars in relation to the parse tree and production rules?

Attribute dependencies can be represented as a graph. Signup and view all the answers

Which of the following statements is true regarding synthesized attributes within attribute grammars?

Synthesized attributes are always computed in a bottom-up manner. Signup and view all the answers

In the attribute grammar example provided, how many levels of non-terminals contain attributes?

All non-terminals have synthesized attributes. Signup and view all the answers

What is a restriction on the rules for attribute computation in the given context?

They can only use attributes from the production p involved. Signup and view all the answers

What is the purpose of the attribute dependence graph in compiler design?

To represent the relationships between attribute instances Signup and view all the answers

Which statement best describes an acyclic dependence graph?

It is a prerequisite for non-circular attribute grammars Signup and view all the answers

What does attribute evaluation involve?

Assigning consistent values to attribute instances Signup and view all the answers

What is the main challenge in determining non-circularity of an attribute grammar?

It is exponential in the size of the grammar Signup and view all the answers

Which method is used to obtain an evaluation order in the attribute evaluation strategy?

Topological sort of the dependence graph Signup and view all the answers

In attribute grammars, what distinguishes synthesized attributes from inherited attributes?

Synthesized attributes propagate values upwards in the tree. Signup and view all the answers

What might result from having multiple attributes at a node in the parse tree?

The node may need to be visited multiple times. Signup and view all the answers

Which attribute evaluation strategy step usually follows constructing the dependence graph?

Performing a topological sort Signup and view all the answers

What is the primary purpose of type checking in semantic analysis?

To verify that variables and functions adhere to their declared types during program execution Signup and view all the answers

Which of the following describes static type checking?

Types are assigned and verified at compile-time, before execution Signup and view all the answers

What are synthesized attributes in attribute grammars?

Attributes that are computed from child nodes and used to define parent nodes Signup and view all the answers

What is the distinction between static and dynamic type coercion?

Static coercion occurs at compile-time, while dynamic occurs at runtime Signup and view all the answers

What role do inherited attributes play in attribute grammars?

They carry information from parent nodes down to child nodes Signup and view all the answers

Which best describes attribute evaluation in the context of attribute grammars?

The method through which values of attributes are computed and assigned Signup and view all the answers

What is a key characteristic of high-level intermediate representations (HLIR)?

They maintain a close relation to the original source code for better readability Signup and view all the answers

In the context of semantic analysis, which aspect does 'scope resolution' primarily address?

How names (variables and functions) are resolved and accessed in programs Signup and view all the answers

Study Notes

GRP 1: Semantics Analysis

Semantic analysis is a phase in a compiler.
It follows lexical and syntax analysis.
It checks if the program is meaningful. The toothbrush sang a lullaby, is syntactically correct but semantically incorrect, as a toothbrush cannot sing.
It involves analyzing the meaning of the source code rather than just its structure.
A semantic analyzer uses a symbol table to verify elements in the program. This includes the identifier's type, internal structure, and scope.

Stages in Compilation

Source code is input into the compiler.
Lexical analysis breaks source code into tokens.
Syntax analysis uses grammar rules to identify the grammatical structure. A parse tree emerges.
A parse tree is a hierarchical representation of the syntactic structure of the source code.
Semantic analysis checks the meaning of the program.
The program is then optimized.
The code is translated into assembly language.

High-Level Representations

High-level program representations are simplified versions of code, used in compiler optimizations.
It’s an intermediate representation nearly similar to the source language.
It represents all facts a compiler derives from a program.
The intermediate representation is a critical component of how compilers work, being responsible for multiple tasks including, but not limited to optimization.
Two forms:
- Graphical IR: Nodes and edges, including syntax trees and graphs.
- Linear IR: Instructions in order of appearance.
- Hybrid IR: A blend of graphical and linear forms.

Intermediate Representations (IRs) Overview

A high-level, intermediate representation (HLIR) describes the program in a structured way that is similar but simpler than the original.
Includes:
- Intermediate representations are different from syntax trees.
- A taxonomy of intermediate representations on several axes:
  - Structural axes: The way data is organized.
  - Abstraction level axes: High or low level of code abstraction.
  - Naming discipline axes: How variables are named and referenced..
- Graphical IRS, and taxonomy of IRS.

Parse Tree

Example of program derivation: ax2+ax2xb
Syntax tree and Abstract Syntax Tree (AST)
- AST is more concise than syntax tree

Abstract Syntax Tree (AST)

An AST is essentially a parse tree, but simplified.
Extraneous nodes are condensed.
Example: A graphical representation of operations (+, ×) and operands (variables, numbers) in an expression.

Directed Acyclic Graph (DAG)

Another form of representation.
It compactly represents repeated subtrees in an AST.
Example: Represents code with repeated computations by sharing nodes instead of redundant copies.

Scoping and Binding Resolutions

Determines how names (variables, functions) resolve.
It checks how identifiers are accessible.
Identifiers are bound to memory locations based on their scope.
Types of scope:
- Global: Accessible anywhere after declaration.
- Local: Within a function or block.
- Block/Nested: Within nested blocks, e.g., loops.
Scope checking: The compiler checks accessibility of identifiers.
Binding: Association of identifiers (names) with entities (values or functions).

Symbol Table

Maintains information mapping each identifier to its type, internal structure, and scope.
Enables semantic analysis.
Example entry for x: Identifier = 'x', Type = 'int', Scope = 'local', Value = 10.
The role in scope and binding resolution: The symbol table helps the compiler determine where each variable or function is visible and accessible.

Type Checking

Verifies each operation respects the type system.
Ensures operands are of the correct types and numbers.
Types in code can be checked in one of two ways: statically or dynamically.
Dynamic type-checking is performed during run time. It checks type information later. Example: Python.
Static type checking is often done during compilation. Example: C++.

Type Coercion

Implicit type conversion; the compiler changes a variable's type to an appropriate one.

Static vs. Dynamic Scoping

Static: Variables' scopes are determined at compile time.
Dynamic: Scopes depend on the runtime call stack.

Declarative Specifications

Attributes (such as synthesized, inherited) are described and used.

Attribute Grammars

Specify semantic rules of a programming language.
Extends context-free grammars.
Attributes contain:
- Synthesized attributes.
  - Computed bottom-up, starting from leaves.
  - Used for computing the values of attributes higher up in the tree.
- Inherited attributes.
  - Computed top-down, from parent or siblings.
  - Used to provide information to children in a parse tree.

Attribute Dependence Graph

Represents dependencies between attributes in a parse tree.
Example: A graph that shows how attribute values calculate each other.

Attribute Evaluation Strategy

Systematically executes attribute evaluation to obtain valid attribute values.

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Description

This quiz covers the concept of semantic analysis in compiler design, a crucial phase that evaluates the meaning and correctness of programs following lexical and syntax analysis. It involves the verification of program elements using a symbol table and emphasizes the importance of meaningful code. Test your understanding of the stages in compilation and high-level representations.