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Last Updated: Jun 23, 2024
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Symbol Tables in Compiler Design

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Introduction

Compilers consist of the fundamental building blocks which help in checking the correctness of the code, execution, allocation of resources, and many more such tasks. The symbol table is a data structure used in compiler design. 

symbol table in compiler design

 

Compilers keep track of the occurrence of various entities, including variable names, function names, objects, classes, and interfaces in the symbol table. A compiler's symbol table is used for the analysis and synthesis processes. 

What is a symbol table in compiler design?

A symbol table is an important Data Structure used by compilers to manage identifiers in a program. An identifier is a name given to a variable, function, class or other programming construct that is used to represent some data or functionality in a program. Identifiers must be declared before they are used in a program, and their properties (such as data type, scope, and memory location) must be known by the compiler to generate the correct code. A symbol table provides a way for the compiler to store this information and access it as needed during the compilation process.
 

A symbol table typically consists of a set of entries, each of which represents an identifier used in the program. Each entry contains information such as the identifier's name, data type, scope, memory location, and any other attributes that may be needed by the compiler. The symbol table is created and populated during the compilation phase, as the compiler scans the program for identifiers and their declarations. Once the symbol table is populated, it can be used by the compiler in several ways. For example, the compiler can use the symbol table to check for errors in the program, such as undeclared variables or type mismatches.

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Implementation of Symbol table

If a compiler only needs to process a small quantity of data, the symbol table can be implemented as an unordered list, which is simple to code but only works for small tables. The following methods can be used to create a symbol table:

List

A List is a collection of elements in which each element has a position or an index. This is one of the way to create a symbol table is to use a List to store the entries for each identifier. This method is simple and easy to implement, but it may not be efficient for large symbol tables because searching for a specific identifier requires iterating through the entire List.

Linked List

A Linked List is a data structure in which each element contains a reference to the next element in the list. One way to create a symbol table is to use a Linked List to store the entries for each identifier. This method can be more efficient than a List for searching because it allows for faster traversal of the entries. However, it can be slower than other methods for inserting or deleting entries because it requires modifying the references between elements.

Binary Search Tree

A Binary Search Tree is a data structure in which each node has at most two children, and the values in the left subtree are less than the value in the node, and the values in the right subtree are greater than the value in the node. One way to create a symbol table is to use a Binary Search Tree to store the entries for each identifier. This method is efficient for searching because it allows for a binary search to quickly find a specific identifier. However, it can be slower than other methods for inserting or deleting entries because it may require rebalancing the tree.

Hash Table

A Hash Table is a data structure that uses a hash function to map keys to values. One way to create a symbol table is to use a Hash Table to store the entries for each identifier. This method is efficient for searching, inserting, and deleting entries because it allows for constant-time access to entries. However, it may require more memory than other methods because it needs to store the hash table and handle collisions between entries.

Features Of Symbol Table 

This symbol table is used for the following purposes:

  • It is used to keep all of the names of all entities in one place in a structured format.
  • It is used to check whether or not a variable has been declared.
  • It is used to identify the scope of a name.
  • It is used to implement type checking in source code by ensuring if assignments and expressions are semantically accurate or not.
     
  • It is utilized in the compiler's different phases, as shown below:
    Lexical Analysis: New table entries are created in the table, For example, entries about tokens.
    Syntax Analysis: Adds the information about attribute type,, dimension, scope line of reference, use, etc in the table.
    Semantic Analysis: Checks for semantics in the table, i.e., verifies that expressions and assignments are semantically accurate (type checking) and updates the table appropriately.
    Intermediate Code generation: The symbol table is used to determine how much and what type of run-time is allocated, as well as to add temporary variable data.
    Code Optimization: Uses information from the symbol table for machine-dependent optimization.
    Target Code generation: Uses the address information of the identifier in the table to generate code.

Symbol Table entries in Compiler Design

Each item in the symbol table is linked to a set of properties that assist the compiler at various stages.

Items stored in Symbol table: 

  • Labels in source languages
  • Procedure and function names
  • Variable names and constants
  • Compiler generated temporaries
  • Literal constants and strings

Information used by the compiler from the Symbol table:  

  • Data type and name
  • Offset in storage
  • Declaring procedures
  • For parameters, whether parameter passing by value or by reference
  • Number and type of arguments passed to function
  • Base Address

Basic Operations of Symbol Table in Compiler Design

The core operations of a symbol table are Allocate, free, insert, lookup, set attribute, and get attribute. The allocation operation creates n empty symbol table. The free operation is used to remove all records and free the storage of a symbol table. As the name implies, the insert operation puts a name into a symbol table and returns a pointer to its entry. The lookup function looks up a name and returns a reference to the corresponding entry. The set and get attributes associate an attribute with a given entry and get an attribute associated with a provided. Other steps may be introduced depending upon the requirements. For example, a delete action deletes a previously entered name.

Operations Of Symbol Table

OperationsFunctions
allocateTo allocate a new empty symbol table
freeTo remove all entries and free storage of symbol table
lookupTo search for a name and return a pointer to its entry
insertTo insert a name in a symbol table and return a pointer to its entry
set_attributeTo associate an attribute with a given array
get_attributeTo get an attribute associated with a given array

Information used by the compiler from Symbol table: 

Symbol Identification

The symbol table enables the compiler to apprehend and keep track of variables, functions, and different symbols in your application.

Data Type Information

It stores data about the data types of variables, like whether they're numbers, text, or something else.

Variable Scope

It is information wherein variables are declared and where they may be used within your code.

Function Details

For functions, it stores their names, parameters, and what they do.

Error Detection

The symbol desk aids in detecting mistakes for your code by way of making sure that variables and functions are used correctly and consistently.

Code Generation

It assists in generating machine code or executable programs primarily based on the statistics saved, making your code run effectively.

Advantages of Symbol Table

Here are the advantages of the symbol table:

  • Efficient identifier management: Symbol tables provide an efficient mechanism for managing identifiers such as variables, functions, and data types used in a program.
  • Error detection: Symbol tables can help detect errors such as duplicate declarations, undeclared variables, and type mismatches in a program. 
  • Code optimization: Symbol tables can be used to track the usage of variables and functions in a program. This information can be used during code optimization to eliminate unused variables and optimize function calls.
  • Language extensions: Symbol tables can be extended to support new language features or extensions. 

Disadvantages of Symbol Table

Symbol table can have certain disadvantages:

  • Symbol tables can take up a lot of memory because they store a lot of information about the names used in a program.
  • Creating and maintaining a symbol table can be slow and can make the compilation process take longer, especially for large programs.
  • Symbol tables can be difficult to implement correctly because they have to handle different kinds of names used in different parts of the program.

Applications of Symbol Table

There are many applications of the Symbol table. Here are some:

  • Symbol tables are used to store information about the names used in a program, including variables, functions, and other identifiers. This information is used by the compiler to generate code and ensure that the program works correctly.
  • Symbol tables are used to keep track of where variables are defined and used within a program, and to ensure that they are used correctly.
  • Symbol tables are used to handle scoping and namespaces within a program, so that variables and functions can be defined with unique names in different parts of the program.
  • Symbol tables are used to handle data types and ensure that variables are assigned the correct type of data. This helps prevent errors and ensures that the program works as intended.
  • Symbol tables are used to provide error messages and debugging information, so that programmers can understand what went wrong in the program and how to fix it. 

Frequently Asked Questions

What are the types of symbol table?

There are two types of symbol tables: Global symbol table and Scope symbol table. Both are created and maintained by the compiler. The Global Symbol table can be utilised by all the procedures, but the scope symbol table is just for the scope of a program.

What is symbol table in data structures?

A symbol table is a data structure used to store and retrieve information about symbols, such as variable and function names, in a programming language. It is commonly used by compilers and interpreters to keep track of identifiers and their associated attributes.

What are the functions of symbol table?

The functions of a symbol table are to store and manage information about symbols in a programming language, including their names, data types, scope, and memory locations. It is used by compilers and interpreters to perform tasks such as type checking, name resolution, and code generation.

What is a symbol table in compiler?

A symbol table in a compiler is a data structure used to store information about identifiers (variables, functions, objects) in a program, including their types, scopes, and memory locations.

What is '-' symbol called?

The '-' symbol is called a hyphen or minus sign. In mathematics and programming, it denotes subtraction or negation, while in writing, it connects words or parts of words.

What is the use of symbol table in assembler?

In an assembler, a symbol table is used to store the addresses and types of all symbols (variables, labels) in the source code, facilitating efficient address resolution and error checking during assembly.

Conclusion

In this article, we discussed the symbol tables in compiler design and their implementation.Later in the blog, we also discussed on the advantages, disadvantages and applications of compiler design. 

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Topics covered
1.
Introduction
2.
What is a symbol table in compiler design?
3.
Implementation of Symbol table
3.1.
List
3.2.
Linked List
3.3.
Binary Search Tree
3.4.
Hash Table
4.
Features Of Symbol Table 
5.
Symbol Table entries in Compiler Design
5.1.
Information used by the compiler from the Symbol table:  
6.
Basic Operations of Symbol Table in Compiler Design
7.
Operations Of Symbol Table
8.
Information used by the compiler from Symbol table: 
8.1.
Symbol Identification
8.2.
Data Type Information
8.3.
Variable Scope
8.4.
Function Details
8.5.
Error Detection
8.6.
Code Generation
9.
Advantages of Symbol Table
10.
Disadvantages of Symbol Table
11.
Applications of Symbol Table
12.
Frequently Asked Questions
12.1.
What are the types of symbol table?
12.2.
What is symbol table in data structures?
12.3.
What are the functions of symbol table?
12.4.
What is a symbol table in compiler?
12.5.
What is '-' symbol called?
12.6.
What is the use of symbol table in assembler?
13.
Conclusion