Compiling Objects in Class-Based Languages CS 153: Compilers

Object-Oriented Language? It should be clear that there are several families of OO languages: Prototype-based (e.g. Javascript, Lua) Class-based (e.g. C++, Java, C#) We will focus on the compilation of class-based OO languages.

A brief incomplete history of OO (Early 60’s) Key concepts emerge in various languages/programs: sketchpad (Sutherland), SIMSCRIPT (Hoare), and probably many others. (1967) Simula 67 (Dahl, Nygaard) crystalizes many ideas (class, object, subclass, dispatch) into a coherent OO language (1972) Smalltalk (Kay) introduces the concept of object- oriented programming (you should try Squeak!) (1978) Modula-2 (Wirth) (1985) Eiffel (Meyer) (1990’s) OO programming becomes mainstream: C++, Java, C#, …

Classes What’s the difference between a class and an object?

Classes What’s the difference between a class and an object? Class (a blueprint for objects) declared fields / instance variables values may differ from object to object usually mutable methods shared by all objects of a class inherited from superclasses usually immutable implicit receiver object ( this, self ) all have visibility: public / private/ protected

Code generation for objects Methods Generate method body code Generate method calls (dispatching) Fields Memory layout Alignment

Jish Abstract Syntax type tipe = Int_t|Bool_t |Class_t of class_name type exp = Var of var | Int of int | Nil | Var of var | Assign of var * exp | New of class_name | Invoke of exp * var * (exp list) |... type stmt = Exp of exp | Seq of stmt*stmt |... type method = Method of {mname:var, mret_tipe:tipe option, margs:var*tipe list, mbody:stmt} type class = Class of {cname:class_name, csuper:class_name, cinstance_vars:var*tipe list, cmethods:method list}

The need for dynamic dispatching Methods look like functions, type-checked like functions… what’s different? Problem: compiler can’t tell what code to run when a method is called, in general. interface Point { int getx(); float norm(); } class ColoredPoint implements Point { … float norm() { return sqrt(x*x+y*y); } class 3DPoint implements Point { … float norm() return sqrt(x*x+y*y+z*z); } Point p = new 3DPoint(...); p.norm(); Solution: dispatch table (vtable, virtual method table)

Method dispatch Idea: every method has its own offset Offset is used to look up method in vtable class A { void foo() {.. } } class B extends A { void bar() {.. } void baz() {.. } } class C extends B { void foo() {…} void bar() {…} void baz() {…} void quux() {…} }

vtable A foo | B bar, baz | C quux

Fields Work similarly, calculate offset into object. class Pt2d extends Object { int x; /* offset 4 */ int y; /* offset 8 */ void movex(int i) { x = x + i; } void movey(int i) { y = y + i; } } class Pt3d extends Pt2d { int z; /* offset 12 */ void movez(int i) { z = z + i; } }

Fields Values of fields placed in object Accesses to fields are indexed loads Need to know size of superclasses – can be a problem e.g., Java – field offsets resolved at dynamic link/load time class Pt3d extends Pt2d { int z; /* offset = size(Pt2d) + 4 = 12 */ void movez(int i) { z = z + i; } }

Object layout

Field layout in Java (actually) public abstract class AbstractMap implements Map { Set keySet; Collection values; } public class HashMap extends AbstractMap { Entry[] table; int size; int threshold; float loadFactor; int modCount; boolean useAltHashing; int hashSeed; } Will keySet be the first field in HashMap? Will table be the 3rd field in HashMap?

Field Layout in Java (actually) $ java -jar target/java-object-layout.jar java.util.HashMap java.util.HashMap offset size type description 0 12 (object header + first field alignment) 12 4 Set AbstractMap.keySet 16 4 Collection AbstractMap.values 20 4 int HashMap.size 24 4 int HashMap.threshold 28 4 float HashMap.loadFactor 32 4 int HashMap.modCount 36 4 int HashMap.hashSeed 40 1 boolean HashMap.useAltHashing 41 3 (alignment/padding gap) 44 4 Entry[] HashMap.table 48 4 Set HashMap.entrySet 52 4 (loss due to the next object alignment) 56 (object boundary, size estimate) VM reports 56 bytes per instance

Field Layout in Java Instead, the attributes are organized in memory in the following order: 1.doubles and longs 2.ints and floats 3.shorts and chars 4.booleans and bytes 5.references Why was there a reference as the first field in HashMap ?

Compiling to Cish For every method m(x 1,…,x n ), generate a Cish function m(self,vtables,x 1,…,x n ). At startup, for every class C, create a record of C's methods (the vtable.) Collect all of the vtables into a big record. we will pass this data structure to each method as the vtables argument. wouldn't need this if we had a global variable in Cish for storing the vtables. Create a Main object and invoke its main() method.

Operations new C create a record big enough to hold a C object initialize the object's vtable pointer. initialize instance variables with default values 0 is default for int, false for bool, null for objects. return pointer to object as result e.m(e1,…,en) evaluate e to an object. extract a pointer to the m method from e's vtable invoke m, passing to it e,e1,…,en e is passed as self, or this. in a real system, must check that e isn't null!

Operations (continued) x, x := e read or write a variable. the variable could be a local or an instance variable. if it's an instance variable, we must use the "self" pointer to access the value. *(this+offset(x)) = [| e |] (C)e -- type casts if e has type D and D ≤ C, succeeds. (upcast) if e has type D and C ≤ D, performs a run-time check to make sure the object is actually (at least) a C. (downcast) if e has type D, and C is unrelated to D, then generates a compile-time error.

Subtleties in Type-Checking: Every object has a run-time type. Essentially its vtable. The type-checker tracks a static type. some super-type of the object. NB: Java confuses super-types and super-classes. In reality, if e is of type C, then e could be nil or a C object. Java "C" = ML "C option"

Subtyping vs. Inheritance Inheritance is a way to assemble classes Simple inheritance: D extends C implies D  C a read of instance variable x defined in C? okay because D has it too. an invocation of method m defined in C? okay because D has it too. m : (C self,T 1,…,T n )  T Read C instance variables, invoke C methods.

Overriding: class List { int hd; List tl; void append(List y) { if (tl == Nil) tl := y; else tl.append(y); } class DList extends List { DList prev; void append(DList y) { if (tl == Nil) { tl := y; if (y != Nil) y.prev := self; } else { tl.append(y); } Java won't let you say this…

Best you can do: class List { int hd; List tl; void append(List y) { if (tl == Nil) tl := y; else tl.append(y); } class DList extends List { DList prev; void append(List y) { if (tl == Nil) { tl := y; if (y != Nil) ((DList)y).prev := self; } else { tl.append(y); } Run-time type-check

What We Wish we Had… Don't just "copy" when inheriting: Also replace super-class name with sub-class name. That is, we need a "self" type as much as a self value. But this will not, in general, give you that the sub-class is a sub-type of the super-class. why?

Overloading In Java you can override a method, and you can also overload a method With overloading, you don’t update the behavior of a method, instead, within a class hierarchy, you have different methods with the same name However, for a call to such an overloaded method, we must be able to figure out which one to call at compile time

Danger of Overriding + Overloading Class C { Boolean equals(C other) { … } // 1 } Class SC extends C { Boolean equals(C other) { … } // 2 Boolean equals(SC other) { … } // 3 } C c = new C(); SC sc = new SC(); C c’ = new SC(); c.equals(c) ; c.equals(sc) ; c.equals(c’); c’.equals(c) ; c’.equals(sc) ; c’.equals(c’) ; sc.equals(c) ; sc.equals(sc) ; sc.equals(c’);