1. Symbol Table

2. Role of Symbol Table Essential data structure for compiler
Used for storing information about identifiers appearing in a source program
Lexical Analyzer and Parser fill up symbol table
Code generator and optimizer make use of symbol table
Entities stored in a symbol table
Variables, procedures, functions, defined constants, labels, structures, file identifications, compiler generated temporaries

3. Information in symbol table
Name: May be stored directly in the table or the entry may point to another character string, possibly in an associated string table
Type: type of identifier
Whether variable / function / procedure name
For variables, identify whether integer / real / array …
Location: offset in the program where the identifier is defined
Scope: identifies the region of the program in which the current symbol definition is valid
Other attributes: Array limits, record fields / parameters / return values of functions

4. Usage of symbol table information
Semantic Analysis: check correct semantic usage of language constructs
May need checking types of identifiers
Code generation: All program variables and temporaries need to be allocated some memory locations
Symbol table provides information regarding memory size required for identifiers by their types
Error Detection: Leave variables undefined
Optimization: To reduce the total number of variables used in a program we need to reuse the temporaries generated by the compiler

5. Features of symbol tables
Insert
Delete
Lookup
Modify

6. Symbol Table Design Format of symbol table entries Access mechanism
Linear Lists / arrays / trees….
Access mechanism
Linear search / hashing / binary search ..
Location of storage
Primary Memory (RAM) / Secondary Memory (large symbol table)
Scope issues
Simple symbol table with nested scope
Scoped symbol table with nested scopes

7. Simple symbol table: Operations
Enter a new symbol into the table
Lookup for a symbol
Modify information about a symbol stored earlier

8. Simple Symbol Table Formats
Linear Table
Ordered List
Tree
Hash table

9. Linear Table
int x, y; float z; ….. procedure abc L1:…

10. Linear Table Name Type Location x y z Abc L1 int float procedure label
Offset of x
Offset of y
Offset of abc
Offset of L1
Insert, Lookup and modify operations take O (n) time, n being the number of identifiers
Insertion can be made in O (1) be remembering the pointer to the next free position

11. Ordered List Variation of linear table
List may be sorted and a binary search may be used for access in O (log n)
Insertion needs to be done at proper place to preserve the sorted nature
Self-organizing list: Dynamically re-arrange list based on recency of reference

12. Tree Symbol Table Each entry is used represented as a node in a tree
Based on string comparison of names, entries lesser than a reference node are kept in the left subtree and entries greater than a reference node are kept in the right subtree
Average lookup time is O (log n)
AVL trees may be used

13. Tree symbol table example
abc y L1 z

14. Hash table symbol table
Used in cases where access time is to be minimized
Most common method implementing Symbol Table in compilers
Hash function used to map identifier names to hash table locations, organized as an array
To store a symbol table into the table, hash function is applied which results in the unique location in the table
Symbol along with associated information stored
Accessed using the hash function [O (1) time]
Problem of hash tables
Imperfect hash function – Collision
Collision resolution (Chaining …)

15. Hash function properties
Should depend on name of symbol
Easy and quick to compute
Should be uniform in mapping names to different parts of the table
Computed value must always be within table range

16. Scoped Symbol Table Global Scope File-wide scope
Global Variables
File-wide scope
Modules in more than one file (static variables / functions)
Local scope within a procedure
Function local variables
Local scope within a block

17. Scoping rules Static or lexical scoping Dynamic or runtime scoping
Procedure P1 is callable from P2 and P3
P1 has reference to a non-local variable x
There exist two different definitions of x, one each in P2 and P3
When P1 is called from P2, x will refer to the definition in P2, while when called from P3, it will refer to the definition in P3

18. Nested Lexical Scoping
Procedure P1 ….. Procedure P2 End Procedure Procedure P3 …. Procedure P4 x =

19. Scope resolution rule
If a name is defined in more than one scope, the innermost definition closest to the reference is to be used to interpret the reference to that name
When a scope is exited, all variables declared in that scope are deleted from the symbol table

20. Scope symbol table implementation
One table per scope
One table for all scopes
Operations on symbol tables
Insert: Insert a new symbol in current scope
Delete
Modify
Create: Create a new scope
Lookup

21. One table per scope
List
Tree
Hash tables

22. One symbol table for all scopes
Lists
Trees
Hash tables

23. Reference
Compiler Design: Santanu Chattopadhay