Symbol Table

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

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

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

Features of symbol tables

• Insert• Delete• Lookup • Modify

Symbol Table Design

• Format of symbol table entries– 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

Simple symbol table: Operations

• Enter a new symbol into the table• Lookup for a symbol• Modify information about a symbol stored

earlier

Simple Symbol Table Formats

• Linear Table• Ordered List• Tree• Hash table

Linear Table

int x, y;float z;…..

procedure abc…..L1:……..

Linear TableName Type Location

xyzAbcL1

intintfloatprocedure label

Offset of xOffset of yOffset of yOffset of abcOffset 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

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

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

Tree symbol table example

x

abc

L1

y

z

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 …)

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

Scoped Symbol Table

• Global 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

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

Nested Lexical ScopingProcedure P1

…..Procedure P2

…..End ProcedureProcedure P3

…. Procedure P4 ….

x = ….

End ProcedureEnd Procedure

End Procedure

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

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

One table per scope

• List• Tree• Hash tables

One symbol table for all scopes

• Lists• Trees • Hash tables

Reference

• Compiler Design: Santanu Chattopadhay