Managing Scope Information in Symbol Tables: Compiler Design

Learn how compilers manage scope information using symbol tables. This guide explains scope rules in block-structured languages, the use of hierarchical symbol table organization (often implemented as a stack), and how compilers track variable visibility and accessibility.

Managing Scope Information in Symbol Tables

Scope Rules in Programming Languages

In programming languages, the scope of a name (variable, function, etc.) determines where that name is visible and can be accessed within the program’s code. Understanding scope is crucial for writing correct and maintainable programs, especially for larger and more complex programs. This section explains how scope is handled in block-structured languages (languages that support nested blocks of code).

Scope Rules for Block-Structured Languages

Block-structured languages (like C, C++, Java) follow these scope rules:

1. A name declared within a block is only accessible within that block (local scope).
2. If block B1 is nested within block B2, names declared in B2 are also accessible in B1, unless the name is redeclared within B1 (inner blocks override outer blocks).

Organizing Symbol Tables for Scope

To manage scope effectively, compilers use a hierarchical symbol table organization. This is often implemented using a stack data structure:

1. Entering a Block: A new symbol table is pushed onto the stack to store names declared within the block.
2. Declaration: When a new name is declared, the compiler searches the current symbol table. If the name is not found, a new entry is created.
3. Name Lookup: When a name is referenced, the compiler searches the stack of symbol tables, starting from the most recently added table and moving downwards. The first occurrence of the name determines the correct symbol.

Example: Symbol Table Organization

Consider this C-like code:

int x;
void f(int m) {
  float x, y;
  {
    int i, j;
  }
}
int g(int n) {
  bool t;
}

The symbol table would be organized as follows:

• Global Symbol Table: Contains `x`, `f`, `g`.
• `f`'s Symbol Table: Contains `x`, `y`, `i`, `j`.
• `g`'s Symbol Table: Contains `t`.