Today in CS362: Translating Assignments and Expressions

Admin
* Candy
* CS puzzle: Self-generating programs.
* Daren's talk today!  Go or be a polyomino.
* Upcoming Fridays: the 15th is a "pause for breath"; the 22nd is
  no class, the 29th is cancelled for Thanksgiving.
* How are your projects coming?  
  + Sam reflects on Emily's suggestion

Overview
* Where we are and where we're going (I hope)
* Designing a symbol table interface
* Translating assignments
  + Associated problems
* Translating expressions
  + Simple: variables, constants
  + Compound: addition

Past, Present, and Future
Goal: Translate from Pascal (or a subset) to assembler (or
a simulation thereof)

Stream of characters (source code)
  Lexer
Stream of tokens
  Parser 
Parse tree for syntactically correct programs
  Type checker [BUILDS AND USES SYMBOL TABLE; you build]
Parse tree for syntactically and type correct programs
  Code generator [Talk about; two weeks; you build]
"Intermediate Assembly Code" (infinite registers, simple ops)
  Register analyzer [Talk about; one week; you may build]
"Intermediate Assembly Code" (fixed registers, simple ops)
  Optimizer [Talk about; one week]
"Better Intermediate Assembly Code"
  Naive Translator
Real Assembly Code
  Optimizer
"Better" Real Assembly Code
  Assembler
Machine Code
  Linker
Executable

Side question: Should the type checker change the parse tree
at all?
* It might add information to individual nodes (e.g., type)
* It might be able to simplify the tree
* It might extend the tree
type
  r: real;
  i: integer;
..
  r := i + 2;
Instead of
  Exp
    id(i)
    PLUS
    constant(2)
Use
  Coerce-Integer-to-Real
    Exp
     id(i)
     PLUS
     constant(2)

Observation: Symbol tables are used by more than one phase
of the compiler (type checking, code generation), we'd like
a good interface.

/**
 * A description of what we expect symbol tables to do.
 *
 * @author CSC362.2002F
 */
public interface SymbolTable
{
  /**
   * Given a symbol, get *all* of the information associated
   * with the symbol (type, memory location for variables, 
   * frame size for functions, ...).
   * 
   * @exception NoSuchElementException
   *   if s is not in the table
   */
  public Type lookupType(Symbol s)
    throws NoSuchElementException;

  public MemoryLocation lookupMemloc(Symbol s)
    throws NoSuchElementException,
           AttributeNotPresentException,
           AttributeShouldNotBePresentException

  /**
   * Enter a symbol and attribute in the table.
   *
   * If the symbol is already in the table in the current
   * scope, ... why would we care?  A form of error checking:
   * a symbol shouldn't be defined twice in the same scope.
   */
  public void enterType(Symbol s, Type type)
    throws AlreadySetException;

  public void enterMemLoc(Symbol s, MemoryLocation loc);
    throws AlreadySetException;

  /**
   * Start a new scope.
   */
  public void startScope();

  /**
   * End a scope, forgetting about all declarations in the
   * current scope.
   *
   * @exception Exception
   *   If there has not been a corresponding call to startScope.
   */
  public void endScope()
    throws Exception;
} // interface SymbolTable