1. CMPE 152: Compiler Design September 4 Class Meeting
Department of Computer Engineering San Jose State University Fall 2018 Instructor: Ron Mak

2. Quick Review of the Framework
Chapter 4
Quick Review of the Framework
TO:
FROM:
Our next topic:
The symbol table

3. The Symbol Table: Conceptual Design
Goal: The symbol table
should be source language
independent.
Each entry in the symbol table has
a name
attributes
At the conceptual level, we don’t worry about implementation.

4. What to Store in Each Symbol Table Entry
Each symbol table entry is designed to store information about an identifier.
The attribute keys indicate what information we will store for each type of identifier.
Store common information in fixed fields (e.g., lineNumbers) and store identifier type-specific information as attributes.
enum class SymTabKeyImpl {
    // Constant.
    CONSTANT_VALUE,
    // Procedure or function.
    ROUTINE_CODE, ROUTINE_SYMTAB, ROUTINE_ICODE,
    ROUTINE_PARMS, ROUTINE_ROUTINES,
    // Variable or record field value.
    DATA_VALUE };
wci/intermediate/symtabimpl/SymTabEntryImpl.h

5. What Needs a Symbol Table?
A Pascal program
Identifiers for constant, type, variable, procedure, and function names.
A Pascal procedure or function
Identifiers for formal parameter (argument) names.
A Pascal record type
Identifiers for field names.

6. The Symbol Table Stack Language constructs can be nested.
Procedures and functions are nested inside a program.
Procedures and functions can be nested inside of each other.
Record types are defined within programs, procedures, and functions.
Record types can be nested inside of each other.
Therefore, symbol tables need to be kept on a symbol table stack.

7. The Symbol Table Stack, cont’d
Whichever symbol table is on top of the stack is the local symbol table.
The first symbol table created (the one at the bottom of the stack) is the global symbol table.
It stores the predefined information, such as entries for the names of the standard types integer, real, char, and boolean.
During the translation process, symbol tables are pushed onto and popped off the stack …
… as the parser enters and exits nested procedures, functions, record types, etc.
Global symbol table

8. The Symbol Table Stack, cont’d
For now, we’ll have only have a single symbol table.
Therefore, the local symbol table is the global symbol table.
We won’t need multiple symbol tables until we start to parse declarations.
Implementing the symbol table stack now will make things easier for us later.
Global symbol table

9. Modifications to Class PascalParserTD
    while ((token = next_token(token)) != nullptr)
    {
        TokenType token_type = token->get_type();
        last_line_number = token->get_line_number();
        Object value = token->get_value();
        string type_str;
        string value_str;
        if (token_type == (TokenType) PT_IDENTIFIER)
        {
            string name = to_lower(token->get_text());
            SymTabEntry *entry = symtab_stack->lookup(name);
            if (entry == nullptr) entry = symtab_stack->enter_local(name);
            entry->append_line_number(token->get_line_number());
        }
        else if (token_type == (TokenType) PT_ERROR)
            PascalErrorCode error_code =
                                    (PascalErrorCode) cast(value, int);
            error_handler.flag(token, error_code, this);
Cross-reference
only identifiers.
If it’s not already in
the symbol table,
create a new entry.
wci/frontend/pascal/PascalParserTD.cpp

10. Cross-Reference Listing
A cross-reference listing verifies the symbol table code: ./Chapter4cpp compile -x newton.pas
Modifications to the main Pascal class:
Pascal.cpp
    parser->parse();
    source->close();
    symtab_stack = parser->get_symtab_stack();
    icode = parser->get_icode();
    if (xref)
    {
        CrossReferencer cross_referencer;
        cross_referencer.print(symtab_stack);
    }
A new utility class CrossReferencer generates the cross-reference listing.
Demo

11. Quick Review of the Framework
Chapter 5
Quick Review of the Framework
FROM:
TO:
Next topic:
Parsing assignment statements and expressions, and
generating parse trees.

12. Pascal Statement Syntax Diagrams

13. Pascal Statement Syntax Diagrams, cont’d
For now,
greatly simplified!

14. Parse Tree: Conceptual Design
More accurately called an
abstract syntax tree (AST).
BEGIN
alpha := -88;
beta := 99;
result := alpha + 3/(beta – gamma) + 5
END

15. Parse Tree: Conceptual Design
At the conceptual design level, we don’t care how we implement the tree.
This should remind you of how we first designed the symbol table.

16. Parse Tree: Basic Tree Operations
Create a new node.
Create a copy of a node.
Set and get the root node of a parse tree.
Set and get an attribute value in a node.
Add a child node to a node.
Get the list of a node’s child nodes.
Get a node’s parent node.

17. Intermediate Code Interfaces
Goal: Keep it source
language-independent.

18. Intermediate Code Implementations

19. An Intermediate Code Factory Class
wci/intermediate/ICodeFactory.cpp
ICode *ICodeFactory::create_icode() {
    return new ICodeImpl(); }
ICodeNode *ICodeFactory::create_icode_node(const ICodeNodeType type)
    return new ICodeNodeImpl(type);

20. Coding to the Interfaces (Again)
    // Create the ASSIGN node.
    ICodeNode *assign_node =
            ICodeFactory::create_icode_node((ICodeNodeType) NT_ASSIGN);
...
    // Create the variable node and set its name attribute.
    ICodeNode *variable_node =
            ICodeFactory::create_icode_node(
                             (ICodeNodeType) NT_VARIABLE);
    // The ASSIGN node adopts the variable node as its first child.
    assign_node->add_child(variable_node);
wci/frontend/pascal/parsers/AssignmentStatementParser.cpp

21. Intermediate Code (ICode) Node Types
enum class ICodeNodeTypeImpl {
    // Program structure
    PROGRAM, PROCEDURE, FUNCTION,
    // Statements
    COMPOUND, ASSIGN, LOOP, TEST, CALL, PARAMETERS,
    IF, SELECT, SELECT_BRANCH, SELECT_CONSTANTS, NO_OP,
    // Relational operators
    EQ, NE, LT, LE, GT, GE, NOT,
    // Additive operators
    ADD, SUBTRACT, OR, NEGATE,
    // Multiplicative operators
    MULTIPLY, INTEGER_DIVIDE, FLOAT_DIVIDE, MOD, AND,
    // Operands
    VARIABLE, SUBSCRIPTS, FIELD,
    INTEGER_CONSTANT, REAL_CONSTANT,
    STRING_CONSTANT, BOOLEAN_CONSTANT,
    // WRITE parameter
    WRITE_PARM, };
Do not confuse
node types (ASSIGN, ADD, etc.)
with data types (integer, real, etc.).
We use the enumerated type
ICodeNodeTypeImpl for node types
which is different from the enumerated
type PascalTokenType to help maintain
source language independence.
wci/intermediate/icodeimpl/ICodeNodeImpl.h

22. Intermediate Code Node Implementation
class ICodeNodeImpl : public ICodeNode {
...
private:
    ICodeNodeType type;            // node type
    ICodeNode *parent;             // node's parent
    vector<ICodeNode *> children;  // node's children
    map<ICodeKey, Object> contents; }
wci/intermediate/icodeimpl/ICodeNodeImpl.h
Each node has an vector<ICodeNode *> of child nodes.
Each node has a map<ICodeKey, Object> for the node contents.

23. A Parent Node Adopts a Child Node
ICodeNode *ICodeNodeImpl::add_child(ICodeNode *node) {
    if (node != nullptr)
    {
        children.push_back(node);
        ((ICodeNodeImpl *) node)->parent = this;
    }
    return node; }
wci/intermediate/icodeimpl/ICodeNodeImpl.cpp
When a parent node adds a child node, we can say that the parent node “adopts” the child node.
Keep the parse tree implementation simple!

24. What Attributes to Store in a Node?
public enum ICodeKeyImpl implements ICodeKey {
LINE, ID, VALUE; }
wci.intermediate.icodeimpl.ICodeNodeImpl.h
Not much! Not every node will have these attributes.
LINE: statement line number
ID: symbol table entry of an identifier
VALUE: data value
Most of the information about what got parsed is encoded in the node type and in the tree structure.

25. Statement Parser Class
Class StatementParser is a subclass of PascalParserTD which is a subclass of Parser.
Its parse() method builds a part of the parse tree and returns the root node of the newly built subtree.

26. Statement Parser Subclasses
StatementParser itself has subclasses:
CompoundStatement-Parser
AssignmentStatement-Parser
ExpressionParser
The parse() method of each subclass returns the root node of the subtree that it builds.
Note the dependency relationships among StatementParser and its subclasses.

27. Building a Parse Tree
Each parse() method builds a subtree and returns the root node of the new subtree.
The caller of the parse() method adopts the subtree’s root node as a child of the subtree that the caller is building.
The caller then returns the root node of its subtree to its caller.
This process continues until the entire source has been parsed and we have the entire parse tree.

28. Building a Parse Tree Example:
BEGIN
alpha := 10;
beta := 20
END
CompoundStatementParser’s parse() method creates a COMPOUND node.
2. AssignmentStatementParser’s parse()method creates an ASSIGN node and a VARIABLE node, which the ASSIGN node adopts as its first child.

29. Building a Parse Tree
3. ExpressionParser’s parse() method creates an INTEGER CONSTANT node which the ASSIGN node adopts as its second child.
BEGIN
alpha := 10;
beta := 20
END
4. The COMPOUND node adopts the ASSIGN node as its first child.

30. Building a Parse Tree
Another set of calls to the parse() methods of AssignmentStatementParser and ExpressionParser builds another assignment statement subtree.
6. The COMPOUND node
adopts the subtree
as its second child.
BEGIN
alpha := 10;
beta := 20
END