Valhalla Project Valhalla Started 2014 JEP 169: Value Objects JEP 218: Generics over Primitive Types Uberto Barbini @ramtop

pe Value Types Inline Classes Inline Types “Codes like a Class works like an int” Reference Immutability Non Nullable Primitive Generics

Which Field Descriptor? B byte signed byte C char Unicode character code point in the Basic Multilingual Plane, encoded with UTF-16 D double double-precision floating-point value F float single-precision floating-point value I int integer J long long integer L ClassName reference an instance of class ClassName Q ClassName inline types S short signed short Z boolean true or false [ reference one array dimension

e Q-Types in L-World For any given named class or interface Foo we say that L-Foo is the L- type of Foo. When the type is obviously not a value type we may omit the “L-” prefix, thus Object and Object[] for L-Object and L-Object[]. As dictated by today’s JVMS, any variable of L-type will accept null. A Q-type is a reference to a value instance. All value instances have a named class, which is incorporated into the spelling of the Q-type. (For regularity we say “reference to a value”, but the reference is not detectable to the user; there are no identity or aliasing relations between values.) For any given named value class Val we say that Q-Val is the Q-type of Val. http://cr.openjdk.java.net/~jrose/values/q-types.html September 2018

LW2 These builds have many limitations: No support for atomic fields containing value types. No support for @Contended value type fields. No JVMTI, AOT, CDS, ZGC, or serviceability agent. Only -Xint and C2 are supported. No C1, no tiered compilation, and no Graal. The interpreter is not optimized, since our focus is on the JIT implementation. Unsafe field and array-accessor APIs are not supported for value types. Low-level unsafe APIs are UNSAFE and will not be changed to support value types. If a value type has been flattened in a container, unsafe does not know the layout, so getObject could return the first flattened element rather than the expected reference. Immutable types can be copied, so updates may not be seen.

pe Constructors final value class Point { public final int x; public final int y; public Point(int x, int y) { this.x = x; this.y = y; } public static Point of(int x, int y) { Point p = Point.default; Point p1 = __WithField(p.x, x); //WithField operator is allowed only with -XDallowWithFieldOperator Point p2 = __WithField(p1.y, y); return p2; }

sync/lock Not possible to lock on InlineTypes No synchronized methods on IT No wait/notify methods in IT

Can Implement Interfaces public value class Dog implements Animal { public final String name; public Dog(String name) { this.name = name; } @Override public String name() { return name; } @Override public String sound() { return "woof"; } }

No Inheritance public class Alsatian implements Animal { //Ok public class Alsatian extends Dog { // Error: cannot inherit from final com.gamasoft.animals.Dog public value class Alsatian extends Dog { // Error: value type may not extend another value or class

toString() public value class Cat implements Animal { public final String name; public Cat(String name) { this.name = name; } @Override public String name() { // return super.toString(); //Error: value types do not support invocation of super.toString return name; }

Equals == var p1 = new Point(3, 4); var p2 = new Point(4, 5); var p3 = p1.displace(1,1); assertEquals(p2, p3); assertTrue(p2 == p3); //Inline Types do support == as byte for byte comparison

Arrays var memBefore = Runtime.getRuntime().freeMemory(); Point[] values = new Point[1_000_000]; var memUsed = memBefore - Runtime.getRuntime().freeMemory(); System.out.println("Memory for 1M Points " + memUsed); //8MB System.out.println("Point default " + values[0]); //default

Nullability var list = new ArrayList(); list.add(Point.of(3, 4)); list.add(null); //compile error: incompatible types: cannot be converted to Point assertEquals(1, list.size()); assertEquals(new Point(3, 4), list.get(0));

Nullable ? Types var list = new ArrayList(); list.add(Point.of(3, 4)); list.add(null); //ok now with ? wrapper assertEquals(2, list.size()); assertEquals(new Point(3, 4), list.get(0)); assertEquals(null, list.get(1));

Nullable Boxes Point QPoint; null-free reference to Point. Point? LPoint; nullable box for Point.

Boxing antbuild/com/ubertob//examples/NullableCats.class void add(Cat? AnotherCat){...} descriptor: (Lcom/ubertob/animals/Cat;)V void addNN(Cat anotherCat) {…} descriptor: (Qcom/ubertob/animals/Cat;)V

Generics var d = new Dog("Lassie"); //VT var c = new Cat("Ginger"); //VT var s = new Shark(); //Object List animalList = new ArrayList<>(); animalList.add(d); animalList.add(c); animalList.add(s); Set catSet = new HashSet<>(); catSet.add(new Cat("Tom")); catSet.add(new Cat("Jerry")); catSet.add(new Cat("Silvester"));

Flattenable public value class Line { public __Flattenable final Point startPoint; public __Flattenable final Point endPoint; public Line(Point p1, Point p2) { this.startPoint = p1; this.endPoint = p2; } } var memBefore = Runtime.getRuntime().freeMemory(); var values = new Line[1_000_000]; var memUsed = memBefore - Runtime.getRuntime().freeMemory(); System.out.println("Memory for 1M Lines " + memUsed); //16MB

Code of Examples https://github.com/uberto/testValueTypes

Source http://hg.openjdk.java.net/valhalla/ valhalla/ Mailing list https://mail.openjdk.java.net/piperm ail/valhalla-dev/

Valhalla Wiki https://wiki.openjdk.java.net/ display/valhalla/Main L-World https://wiki.openjdk.java.net/displa y/valhalla/L-World+Value+Types#L-Wor ldValueTypes-%22LW1%22-MinimalL-Worl d

Browse Source https://hg.openjdk.java.net/valhalla/ valhalla/file/530ea41023b2/test/ benchmarks/src/main/java/oracle/micro/ valhalla/lworld Nightly Builds https://builds.shipilev.net/openjdk-valha lla/