This page describes the command line for the experimental java compiler, the java launcher and HotSpot for the support of value types in the valhalla project.

Minimal Value Types flags:

javac

• -Xlint:values
  Lint category for warnings associated to bad usage of value capable classes - examples:
  • VCC not final
  • VCC cannot 'extend' another class
  • overrides of 'bad' Object methods (such as wait/notify)
  • illegal modifiers on VCC such as 'synchronized'
  • non-final instance fields in VCC
  • cannot assign 'null' to variable of VCC type

java

• -Djava.lang.invoke.MethodHandle.DUMP_CLASS_FILES
  Boolean value, default value is `false`
  If true dump class files generated for lambda forms and derived value types into a directory named DUMP_CLASS_FILES under the current directory
• -Dvalhalla.enableValueLambdaForms
  Boolean value, default value is `true`
  If true then lambda forms with value types in their signature will use the Q type __Value and value-type specific byte code will be generated.
• -Dvalhalla.dumpProxyClasses
  Boolean value, default value is `false`
  If true, dumps MVT lambda forms

• -Dvalhalla.enablePoolPatches
  Boolean value, default value is `false`
  If true and if "valhalla.enableValueLambdaForms" is true then lambda forms with value types in their signature will generate byte code with constant pool patching where appropriate

HotSpot

• -XX:EnableMVT
  Boolean value, default value is `false`
  If true enable support for minimal value types with a value capable class and an associated derived value type.
• -XX:ValueTypePassFieldsAsArgs
  Boolean value, default value is `true`
  Pass each value type field as an argument to a method call instead of a value type reference
• -XX:ValueTypeReturnedAsFields
  Boolean value, default value is `true`
  Return value type fields instead of a value type reference
• -XX:ValueArrayFlatten
  Boolean value, default value is `true`
  Flatten arrays of values, if possible
• -XX:ValueArrayElemMaxFlatSize
  Integer value, default is -1
  Max size for flattening an array of values, < 0 means no limit
• -XX:ValueArrayElemMaxFlatOops
  Integer value, default is 4
  Max number of embedded object references in value type to flatten in an array, < 0 means no limit 
• -XX:ValueArrayAtomicAccess
  Boolean value, default value is `false`
  Enable atomic access to values in an array

To determine if there are unwanted allocations (boxing) see the flags:

  -XX:-UseTLAB
  -XX:+PrintEliminateAllocations
  -XX:+PrintEscapeAnalysis

Post-MVT, Valhalla Value Class Flags 

• Prototypes under these additional flags are even more experimental, may change at any time, and are not regularly tested

javac

• -XDenableValueTypes
  Boolean value, default value is `false`
  Enable direct declaration and compilation of value type classes via the "__ByValue" and other qualifiers.

java

• -Djdk.lang.reflect.DVT
  Boolean value, default value is `false`
  If true, allow reflection on derived value types.  This is a temporary workaround until existing hotspot tests convert to VCC/DVT and use reflection on VCC only.

Hotspot

• -XX:EnableValhalla
  Boolean value, default value is `false`
  If true enable support for "valhalla" value types, which in contrast to MVT supports direct value types in conjunction with byte code generation or with the javac flag "enableValueTypes".