CS453 Intro and PA1 1CS453 Lecture More tranlation to Tree IR 1Plan for todayTranslating program features to the Tree IR This time:– call expressions– less than operator– if statement– while statements Lvalues versus rvaluesCS453 Lecture More tranlation to Tree IR 2Call expression... this . otherFunc(y,f)...CS453 Lecture More tranlation to Tree IR 3Call expression example with recursive call expressions class MethodCalls { public static void main(String[] a){ System.out.println( new Bar().getBaz().getFoo().testing()); } } class Foo { public int testing() { return 42; } } class Bar { public Baz getBaz() { return new Baz(); } } class Baz { public Foo getFoo() { return new Foo();} }CS453 Lecture More tranlation to Tree IR 4outACallExp Steps– Figure out the class type for the receiver– if (node.getExp() instanceof AThisExp)– else if (node.getExp() instanceof ANewExp)– else if (node.getExp() instanceof AIdExp)– else if (node.getExp() instanceof ACallExp)– look up class in current symbol table– look up method in class’s symbol table– create an ExpNAME for the method, get name from the methods frame– collect Tree.Exp from argument children– create a StmMOVE that stores the result of the call– collect statements from children and place before StmMOVE in stmtlist– map stm list and Tree.Exp for temp to ACallExp nodeCS453 Intro and PA1 2CS453 Lecture More tranlation to Tree IR 5operator < Low level pseudocode for result of translationif (lhs < rhs) goto truebody else goto falsebodyfalsebody:temp = 0goto endiftruebody:temp = 1goto endifendif: Tree.Exp for ALTExp node– ExpTEMP( Temp instance for temp )CS453 Lecture More tranlation to Tree IR 6If statement Low level pseudocode for result of translationif (test == true) goto truebody else goto falsebodyfalsebody:...goto endiftruebody:...goto endifendif: Tree.Exp for AIfStatement node– none, AIfStatement is a statementCS453 Lecture More tranlation to Tree IR 7while statement Low level pseudocode for result of translationloop:if (test != true) goto endloop else goto bodybody:...goto loop Tree.Exp for AWhileStatement node– none, AWhileStatement is a statementCS453 Lecture More tranlation to Tree IR 8StmCJUMP public class StmCJUMP extends Stm { public int relop; public Exp left, right; public Label iftrue, iffalse; public StmCJUMP(int rel, Exp l, Exp r, Label t,Label f) { ... } public final static int EQ=0, NE=1, LT=2, GT=3, ...CS453 Intro and PA1 3CS453 Lecture More tranlation to Tree IR 9Tree intermediate representation ExpCONST(int i) - The integer constant i ExpNAME(Label n) - Constant address. Corresponds to label in assembly. ExpTEMP(Temp t) - Temporary t. “Infinite” Temps in Tree IR. ExpBINOP(int binop, Exp left, Exp right) - left binop right ExpMEM(Exp exp) - If left child of move, then store into address calculated by exp.Otherwise, fetch value at address calculated by exp. ExpCALL(Exp func, List<Exp> args) - evaluate func to find func address, thenevaluate args left to right. StmMOVE(Temp t, e) - Eval e and put result in t. StmMOVE(ExpMEM(e1), e2) - Eval e2 and store into address e1. StmEXP(Exp e) - Eval e and ignore result StmJUMP(Label targ) - Transfer control to given label. StmCJUMP(int rel, Exp l, Exp r, Label t, Label f) - if l rel r then goto t else goto f StmLABEL(Label l) - Label in assembly.CS453 Lecture More tranlation to Tree IR 10Lvalues versus RvaluesLvalue– “result of an expression that can occur on the left of an assignment statement”– examples: *(&a), b.membervar, p->membervar Rvalue– an expression whose result can only appear as a subexpression or on the rhs of astatement– examples: &a, 3*4, new Foo() Why?– explains compiler errors you might see in the future, e.g. non-lvalue assignment– x+3 = 5– 4 = 5– emphasizes the difference between equality in math and assignment inprogramming languages– think about where values are stored during the progression of a program– this is why the lhs of a StmMOVE must be an ExpMEM or ExpTEMP– what if we could pass around user defined types (not just references to
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