CSC362 2004S, Class 34: Translating Conditionals

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Overview:
* Perspectives on Boolean expression
* Translating strict ifs
* Translating short-circuit ifs
* Translating case statements

* On Booleans
  * Suppose code: test1 and test2 (test && test2)
  * Suppose test1 evaluates to false
  * Do you evalute test2?
* No (Short-circuit evaluation): Stop evaluating as soon as you know result
  * Sometimes the successful evaluation of the second depends on the first holding
    * if (x >= 0) and (sqrt(x) < 100) ...
    * if ((p != null) && (p->x > 100))
  * "Easier" for the programmer to write a series of nested if statements
  * Wastes computing resources to do the extra test
  * Easier for programmers in funky languages
* Yes (Strict evaluation): Evaluate the whole expression no matter what
  * Permits parallel evaluation
  * Might rely on side effects
  * Easier to analyze/think about
* The choice is an issue of language design
  * Short-circuit: C, Java, Scheme
  * Strict: Java (special notation), Pascal
* The choice affects implementation

Traditional internal representation of Booleans
* 1: true;0: false

To translate 
  IF <exp> THEN <statement>0 ELSE <statement>1
1. Translate the expression: code_e, location_e
2. Translate the first statement: code_0
3. Translate the second statement: code_1
4. Generate three new labels.  For convenience, I'll call them TRUEPART, FALSEPART, and ENDPART
5. Shove everything together

  code_e
  JumpIfZero location_e FALSEPART
  Jump TRUEPART
TRUEPART:
  code_0
  Jump ENDPART
FALSEPART:
  code_1
  Jump ENDPART
ENDPART:

Optimiziation will remove those jumps

How do we evaluate the expression part?

* Idea: Reserve memory locations for true (1) and false (0)

true
  code: none
  location: the reserved location for true
false
  code: none
  location: the reserved location for false
Boolean variable
  code: none
  location: the location of the variable
<exp> AND <exp>
  Get location for expression 0 and expression 1: loc_0, loc_1
  Get code for expression 0 and expression 1: code_0, code_1
  Create a new temp location, temp
  code: code_0 + code_1 +
	MULT loc_0 loc_1 -> temp
  location: 
<exp>0 OR <exp>1
  Get location for expression 0 and expression 1: loc_0, loc_1
  Get code for expression 0 and expression 1: code_0, code_1
  Create a new temp location, temp
  code: 
	code_0
	code_1
	BITWISEOR loc_0 loc_1 -> temp
  alternative code:
	code_0
	code_1
        JUMPNOTZERO loc_0 STORE_ONE
        JUMPNOTZERO loc_1 STORE_ONE
        MOVE 0 into temp
        JUMP END
      STORE_ONE:
        MOVE 1 into temp
      END:
  location:  return temp
NOT <exp>
  code:
    code_for_exp
    SUB 1 loc_of_exp loc_of_exp
  location: loc_of_exp
<exp> <relop> <exp>

x := 5;
y := -5;
if (x) println("x is true.");
if (y) println("y is true.");
if (x or y) println("both x and y are true");