1. Compiler Construction Recap

2. Announcements • PA4: extension until end of exam period • Not a single day more, for any reason! • PA5: bonus exercise • Will be posted immediately after semester ends

3. Exam • 18/02/2013 at 9:00 • Past exams on my website • חומר פתוח • Possible questions • Extend IC • Parsing • Register allocation • …

4. Scanning // An example programclass Hello { boolean state; static void main(string[] args) { Hello h = new Hello(); boolean s = h.rise(); Library.printb(s); h.setState(false); } boolean rise() { boolean oldState = state; state = true; return oldState; } void setState(boolean newState) { state = newState; }} Issues in lexical analysis: • Language changes: • New keywords • New operators • New meta-languagefeatures (e.g., annotations) CLASS,CLASS_ID(Hello),LB,BOOLEAN,ID(state),SEMI …

5. prog class_list class field_method_list field field_method_list method type ID(state) field_method_list … BOOLEAN … Parsing and AST CLASS,CLASS_ID(Hello),LB,BOOLEAN,ID(state),SEMI … Issues in syntax analysis: • Grammars: LL(1), LR(0) • Ambiguity Parser uses token stream, and generates derivation tree

6. Parsing and AST • Should know difference between derivation tree and AST • Know how to build AST from input CLASS,CLASS_ID(Hello),LB,BOOLEAN,ID(state),SEMI … Parser uses token stream, and generates derivation tree prog class_list ProgAST Syntax tree builtduring parsing classList class ClassAST methodList fieldList field_method_list FieldAST[0]type:BoolTypename:state MethodAST[0] … field field_method_list MethodAST[1] … method type ID(state) MethodAST[2] field_method_list … BOOLEAN …

7. Question: Parsing • Is the following grammar is LR(0)? S -> B $ B -> id P | id ( E ] P -> epsilon | ( E ) E -> B | B,E A grammar with epsilon productions is not LR(0)

8. Other possible questions • Is the following grammar in LR(k)? • Build a parser for given grammar • Run an input string using your parser • …

9. Semantic analysis • Representing scopes • Type checking • Semantic checks (Program) ProgAST (Hello) classList ClassAST methodList fieldList FieldAST[0]type:BoolType MethodAST[0] … (setState) MethodAST[1] … … MethodAST[2]

10. Semantic conditions • What is checked at compile time, and what is checked at runtime?

11. Question: IC language • Support Java override annotation inside comments • // @Override • Annotation is written above method to indicate it overrides a method in superclass • Describe the phases in the compiler affected by the change and the changes themselves Legal program Illegal program class A { void rise() {…}}class B extends A { // @Override void ris() {…}} class A { void rise() {…}}class B extends A { // @Override void rise() {…}}

12. Answer • The change affects the lexical analysis, syntax analysis and semantic analysis • Does not affect later phases • User-level semantic condition

13. Changes to scanner • Add pattern for @Override inside comment state patterns • Add Java action code to comments: Instead of not returning any token, we now return a token for the annotation boolean override=false;%%<INITIAL> // { override=false; yybegin(comment); }<comment> @Override { override=true; }<comment> \n { if (override) return new Token(…,override,…) }

14. Changes to parser and AST method  static type name params ‘{‘ mbody ‘}’ | type name params ‘{‘ mbody ‘}’ | OVERRIDE type name params ‘{‘ mbody ‘}’ Add a Boolean flag to the method AST node to indicate that the method is annotated

15. Changes to semantic analysis • Suppose we have an override annotation above a method m in class A • We check the following semantic conditions: • class A extends a superclass (otherwise it does not make sense to override a method) • Traverse the superclasses of A by going up the class hierarchy, until we find the first method m, and check that it has the same signature as A.mIf we fail to find such a method, then we report an error

16. Question: IC language Add constructors to IC (must be called)

17. Answer • Treat the constructor as a function, and call when object allocated • Lexical analysis: nothing • Parsing: AST node for constructor • Semantic analysis: • Check that every class has a constructor • Actual/formal compatibility • IR/code generation: • Call the constructor on allocation

18. Translation to IR • Accept annotated AST and translate functions into lists of instructions • Compute offsets for fields and virtual methods • Issues: dispatch tables, weighted register allocation

19. Question: IR • Give the method tables for Rectangle and Square class Shape {booleanisShape() {return true;}booleanisRectangle() {return false;}booleanisSquare() {return false;} double surfaceArea() {…}}class Rectangle extends Shape { double surfaceArea() {…}booleanisRectangle() {return true;}}class Square extends Rectangle {booleanisSquare() {return true;}}

20. Answer Method table for rectangle Method table for square

21. Question: IR • Suppose we wish to provide type information at runtime • Similar to instanceof in Java • x instanceof A returns true iff x is exactly of type A (in Java it can also be subtype of A) • Describe the changes in runtime organization needed to support this operator and the translation to IR

22. Answer • Use the pointer to the dispatch table as the type indicator • Translate x instanceof A asMove x,R0MoveField R0.0,R0Compare R0,_DV_A • If we want to support the Java operator • Represent the type hierarchy at runtime and generate code to search up the hierarchy • Keep ancestor info for each type to enable constant-time checking

23. Register allocation • Sethi Ullman • can only handle expressions without side effect • Global register allocation • IR registers are treated as local variables • When we have an actual spill we use the stack

24. Weighted register allocation • Can save registers by reordering subtreecomputations • Label each node with its weight • Weight = number of registers needed • Leaf weight known • Internal node weight • w(left) > w(right) then w = left • w(right) > w(left) then w = right • w(right) = w(left) then w = left + 1 • Choose heavier child as first to be translated • Have to check that there are no side effects

25. W=2 W=1 W=2 W=1 W=1 W=0 W=1 Weighted reg. alloc. example R0 :=TR[a+b[5*c]] Phase 1: - check absence of side-effects in expression tree - assign weight to each AST node + a array access base index b * 5 c

26. Reminder R0 := TR[a+(b+(c*d))] left child first right child first + R0 + R0 a a R0 + + R1 R0 b b R1 R2 * R0 * c d c d R2 R0 Translation using all optimizationsshown until now uses 3 registers Managed to save two registers

27. Sethi Ullman • What type of tree is worst case for SU with respect to the tree’s height?

28. Sethi Ullman • What type of tree is best casefor SU with respect to the tree’s height? . . .

29. Sethi Ullman • What type of tree maximizes the ratio between registers allocated by traversing the tree left-to-right and right-to-left? . . .