Dave Hanold
CS 536 Notes 
April 26, 1999
===============================================================================

** Final Exam : Tues. May 11
		12:25 - 2:25
		6104 Social Science

** READ 11.1-11.2, 12.1-12.2

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TOPIC: WRITING THE CODE GENERATOR

**DISCLAIMER** The following code was copied off the board during class so
some parts may abridged/wrong.  This code can be found in the CSX CODE
GENERATION ROUTINES handout for Program 5



boolean codeGen(printStream ASMFile)
// avoids need to pass parameters by making the output file global
// contained in the root node

(because the code generation only occurs after a successful type checking,
 NO ERRORS WILL MOST LIKELY BE ENCOUNTERED DURING CODE GENERATION -- just
in case though, return value of false if an error occurs)

void cg() //for all other AST nodes contained in ast.java

----------------

ADR field: int value where you can find a given value
can be: global --> string label
	local --> int varIndex (index into activation record)
	on stack
	literal --> int intVal or String strVal	
	none

----------------

EXAMPLE OF CODE GENERATING SUBROUTINES

void loadI(int val) { //generates ldc val
	ASMFile.println("\tldc " + val);
}

cg() { //intLitNode/charLitNode/bool nodes
	loadI(intVal);
	adr = literal;
}

cg() { //nameNode
	if (subscriptVal.isNull()) {
		if ((varName.idinfo.kind.val == Kinds.Var) ||
            	    (varName.idinfo.kind.val == Kinds.Value)) {
			if (varName.idinfo.adr == Global) {
				label = varName.idinfo.label;
				loadGlobalInt(label);
			} else {
				varIndex = varName.idinfo.varIndex;
				LOADLOCALINDEXVARINDEX);
			}
		}
	adr = stack;
	}
}

cg() { //binaryOpNode
	leftOperand.cg();  //push operands onto stack 
	rightOperand.cg();
	binOp(selectOpCode(operatorCode)); //and perform operation
	adr = stack;
}

EXAMPLE OF BINOPNODE: b + 1

b = Global; label = b$ (add $ to the end of the label to ensure that the
			label doesn't conflict with java commands)

would become: getstatic C/b$ I ==> "getStatic" -> get a static value
 	      ldc 1                 C -> class name
    	      iadd                  b$ -> label name
				    I -> type (I)nteger

--------------------

ASSIGNMENT STATEMENTS (A = B;)

1.  Compute A's address
2.  Compute B onto the stack
3.  Store top of stack at A's address

to Compute an address:

void static computeAdr(nameNode name) {
	if (name.subscriptVal.isNull()) {
		if (name.varName.idinfo.kind.val == Kinds.Var) {
			if (name.varName.idinfo.adr == global) {
				name.adr = global;
				name.label = name.varName.idinfo.label;
			}
		
cg() { //ASG NODE
	computeAdr(target);
	source.cg();
	storeName(target);
}

EXAMPLE: a = b + 1;

b: adr = global
   label = b$
a: adr = local
   index = 0		

resulting code generated:

getStatic c/b$ I
ldc 1
iadd
istore 0