Class 26: Symbol Tables

Held Monday, November 4, 2002

Summary

Today we consider the symbol tables that are core to code generation.

Notes

Overview

• Background: Associating attributes with symbols
• Sample attributes
• The implementation of symbol tables

Some Translation Basics

• Translation is typically done top-down: You translate the subtrees and then tie them together depending on the overall structure of the tree.
• At this stage of our translation, we're somewhat free about using temporary "variables" for partial results. In a pass over the generated code, we'll eliminate duplicates and unused memory.
• When we translate an expression, we don't just generate code; we also identify a memory location (most typically, a temporary location) in which the result of that expression resides.
  • We should also know the type of the result.
• We may want to keep track of what kind of memory we're talking about at every stage:
  • register
  • "temporary" (created by compiler)
  • program variable (local or global)
  • procedure parameter
  • fixed memory location
  • An offset from ...
  • ...

Symbol Tables, Revisited

• At many of the stages of translation, you'll need to keep track of various attributes of the symbols that appear in the program.
  • Their type (for most symbols)
  • Their memory location (for variables)
  • The size of their stack frames (for functions and procedures)
  • Their values (for constants)
  • ...
• The structure used to keep track of this information is called a symbol table.
• You need to think about the design of your symbol tables.