Project Valhalla Update - JVMLS Feedback

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Project Valhalla Update JVM Language Summit Feedback #JVMLS_jp

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Agenda

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Project Valhalla

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Project Valhalla - Reboot JVM relationship with data in memory

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Project Valhalla - Reboot JVM relationship with data in memory  Value Types  Generic Specialization

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Will be discussed in JVMLS 2020...

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Why are flatter and denser memory layouts required? e.g. Intel Broadwell-EP L1 L2 Main Memory Intel® 64 and IA-32 Architectures Optimization Reference Manual https://software.intel.com/sites/default/files/managed/9e/bc/64-ia-32-architectures-optimization-manual.pdf L3 Cache

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For example... class Point { int x; int y; } public class App { ... Point[] points = new Point[5]; points[0] = new Point(); ... }

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As is... header x y header x y header x y header x y header x y header

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To be... header x y header x y header x y header x y header x y header header x y x y x y x y x y

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Who cares about project Valhalla?

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Value Type is available in CLR System.ValueType https://docs.microsoft.com/ja-jp/dotnet/csharp/language-reference/keywords/value-types

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What the project has done

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Drilling from JVM side and programming model side L World Early prototypes (M1-M3) Compiler-only Exploring language side • Specialized generics, wildcards • Informed VM requirements • Many of these problems still require considerable work “MVT” prototype VM-only Exploring the “Q-World” model For more detail, check the JVMLS 2018 video! Current prototype VM and compiler support No support yet for specialized generics or migration Still drilling...

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Q World Q qload

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Worked, but...

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L World a*

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L World benefits

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L World costs

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Project status

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Milestone L2 (LW2) https://wiki.openjdk.java.net/display/valhalla/LW2

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LW2 prototype binary http://jdk.java.net/valhalla/

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Inline class value class Point { int x; int y; } inline class Point { int x; int y; }

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Compiler optimizations

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Considerations

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Construction defaultvalue withfield

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class Point { int x; int y; Point() { this.x = 0; this.y = 0; } Point(int x, int y) { this.x = x; this.y = y; } } Point(int, int); descriptor: (II)V flags: (0x0000) Code: stack=2, locals=3, args_size=3 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: aload_0 5: iload_1 6: putfield #7 // Field x:I 9: aload_0 10: iload_2 11: putfield #13 // Field y:I 14: return LineNumberTable: line 10: 0 line 11: 4 line 12: 9 line 13: 14

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inline class Point { final int x; final int y; Point() { this.x = 0; this.y = 0; } Point(int x, int y) { this.x = x; this.y = y; } } static Point Point(int, int); descriptor: (II)QPoint; flags: (0x0008) ACC_STATIC Code: stack=2, locals=3, args_size=2 0: defaultvalue #1 // class Point 3: astore_2 4: iload_0 5: aload_2 6: swap 7: withfield #3 // Field x:I 10: astore_2 11: iload_1 12: aload_2 13: swap 14: withfield #7 // Field y:I 17: astore_2 18: aload_2 19: areturn LineNumberTable: line 10: 0 line 11: 4 line 12: 11 line 13: 18 }

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Arrays

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Default values Foo.default x = T.default x = null

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Default values

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Flattening Value types are flattenable, but flattening is not mandatory in every scene.

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Flattening V? ( http://hg.openjdk.java.net/valhalla/valhalla/file/a32457198d 3e/test/jdk/valhalla/valuetypes/NonFlattenValue.java public inline class NonFlattenValue { Point? nfp; NonFlattenValue() { this.nfp = Point.makePoint(0,0); } NonFlattenValue(Point p) { this.nfp = p; } public Point? point() { return nfp; } public Point pointValue() { return (Point) nfp; } ... }

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Generics – specialized and erased Foo Foo

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New top types (not yet in LW2, under discussion)

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New top types (not yet in LW2, under discussion) By bringing ref-ness and val-ness into the type system x instance of RefObject RefObject m() { ... } void m(RefObject o) { … } class Foo { ... }

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Nullity

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Nullity Options

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Equality

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Equality Options

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Migration https://docs.oracle.com/en/java/javase/12/docs/api/java.base/java/lang/doc- files/ValueBased.html https://docs.oracle.com/javase/jp/12/docs/api/java.base/java/lang/doc-files/ValueBased.html

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Other identity-sensitive operations

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Next action

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Tons of things to do!

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From Brian Goetz...

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Resources

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Resources https://youtu.be/1H4vmT-Va4o https://youtu.be/_26KZAegYRM https://youtu.be/49GUljUmGHg https://www.slideshare.net/DavidBuck7/valhalla-update-jjug-ccc-spring-2019

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