SEMANTIC ANALYSISSYMBOL TABLE

Shimmi Asokan

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Contents

Introduction

Symbol Table Implementation

Global Symbol Table Structure

Global Symbol Table

Local Symbol Table

Functions on Symbol Table

Conclusion

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INTRODUCTION

Last chance for the compiler to weed out incorrect programs

It consists of

• Identifiers are defined

• no multiple definition of same identifier

• local variables are defined before used

Major Tasks Involved

• Symbol Table Construction

• Type Checking

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SYMBOL TABLE

Used to keep track of scope and binding information about names

Declarations can be

• Global

• Local

Each symbol has a set of attributes associated with it

• Name

• Type

• Size

• Binding information

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Implementation

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Implementation using hash tables

Entry 1.1

Entry 2.3

Entry n.2

Entry 2.2

Entry n.1

….

Entry 2.1

Entry n.3

Key 1

Key 2

Key n

Hash Keys

Symbol table entries

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GLOBAL SYMBOL TABLE STRUCTURE

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Program e;var a, b, c: integer;procedure f;

var a, b, c: integer;end;procedure g;

var a, b: integer;procedure h;

var c, d: integer;end;procedure i;

var b, d: integer;end;

end;procedure j;

var b, d: integer;end;

end

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Program e

var a, b, c, d

procedure f()

var b

procedure g()

var d

import a, b from P

end

end

package P

export a, b

end

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Structure – Global Symbol Table

struct GSymbol_table

{

char name[MAXLEN];

int type;

int binding;

int size;

Argstruct* arglist;

int FuncRetType;

int isFuncDefined;

int argcount;

struct node *fbody;

struct node *freturn;

struct GSymbol_table* next;

struct list *head;

int count;

}*Ghead;

struct argstruct

{

char name[MAXLEN];

int type;

struct argstruct* next;

} * list_head;

#define NODPTR struct node*

struct node

{

GSymbolTable* Gstptr;

LSymbolTable* Lstptr;

char name[MAXLEN];

int RetType;

int NodeType;int count;

struct list1 *head;

int value;

NODPTR list;

NODPTR next;

NODPTR Lptr;

NODPTR Mptr;

NODPTR Rptr;

};

Fdef : func_ret_type func_name '(' FargList ')' '{' Ldecl_sec BEG stmt_list ret_stmt END '}'{ $$->Gstptr->fbody=$9;

$$->Gstptr->freturn=$10; }

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Structure – Local Symbol Table

struct LSymbol_table

{

char name[MAXLEN];

char fname[MAXLEN];

int type; // INT / BOOL

int binding;

struct LSymbol_table* next;

}* Lhead;

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Functions

Lookup()

• Searches the given symbol table for a given symbol

GSymbolTable* Glookup(char* sym_name)

• $1->Gstptr=Glookup($1->name);

LSymbolTable* Llookup(char* sym_name,char *fun)

Insert()

• Inserts an entry for the given symbol in the given symbol table

GSymbolTable* Ginsert(char* name, int type, int rtype, int size, Argstruct* list)

• Ginsert($1->name, type_flag, -1, -1, NULL);

• Ginsert($1->name, FUNC, type_flag, -1, list_head);

LSymbolTable* Linsert(char* sym_name, int sym_type, int arg_var, char *fun)

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GSymbolTable* Glookup(char* sym_name)

{

GSymbolTable* ptr;

for(ptr = Ghead; ptr != NULL; ptr = ptr->next)

{

if(strcmp(ptr->name, sym_name) == 0)

return ptr;

}

return NULL;

}

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GSymbolTable* Ginsert(char* name, int type, int rtype, int size, Argstruct* list)

{

int argc = 0;

Argstruct* tmp = list;

while(tmp) {

argc++;

tmp = tmp->next; }

if(Ghead == NULL) {

Ghead = malloc(sizeof(GSymbolTable));

strcpy(Ghead->name, name);

Ghead->type = type;

Ghead->FuncRetType = rtype;

Ghead->size = size;

Ghead->argcount = argc;

Ghead->arglist = list;

Ghead->isFuncDefined = 0;

Ghead->next = NULL;

return Ghead; }

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else

{

GSymbolTable *ptr;

ptr = malloc(sizeof(GSymbolTable));

strcpy(ptr->name, name);

ptr->type = type;

ptr->FuncRetType = rtype;

ptr->size = size;

ptr->argcount = argc;

ptr->arglist = list;

ptr->isFuncDefined = 0;

ptr->next = Ghead;

Ghead = ptr;

return ptr;

}

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CONCLUSION

Semantic Analysis ensures the combination of tokens forms a sensible set of instructions in the programming language

Symbol table collects together the attributes of a particular symbol in a way that allows them to be easily set and retrieved

Global symbol table can be build as a stack of local symbol tables

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References

[1] A. Aho, R. Sethi, J. Ullman, Compilers: Principles, Techniques, and Tools, Pearson Education.

[2] Steven S. Muchnick, Advanced Compiler Design and Implementation, Morgan Kaufmann Publishers

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THANK YOU