This document summarizes feedback from the JVMLS conference on Project Valhalla. Project Valhalla aims to improve how the JVM handles data in memory by introducing value types and specialized generics. It discusses prototypes that have been developed including improvements to memory layout for value types. Further work is still needed on language support features like specialized generics and migration tools. The current status of the project and next steps are outlined.

1. Project Valhalla Update
JVM Language Summit Feedback
#JVMLS_jp

2. Agenda

3. Project Valhalla

4. Project Valhalla - Reboot JVM relationship with data in memory

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

6. Will be discussed in JVMLS 2020...

7. 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

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

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

10. 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

11. Who cares about project Valhalla?

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

13. What the project has done

14. 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...

15. Q World
Q
qload

16. Worked, but...

17. L World
a*

18. L World benefits

19. L World costs

20. Project status

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

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

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

24. Compiler optimizations

26. Considerations

27. Construction
defaultvalue withfield

28. 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."<init>":()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

29. 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
}

30. Arrays

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

32. Default values

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

34. 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;
}
...
}

35. Generics – specialized and erased
Foo<V>
Foo<V?>

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

37. 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<T extents RefObject> { ... }

38. Nullity

39. Nullity
Options

40. Equality

41. Equality
Options

42. 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

43. Other identity-sensitive operations

44. Next action

45. Tons of things to do!

46. From Brian Goetz...

47. Resources

48. 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