Minimal Value Types and Valhalla value types command line options

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

javac

• -XDenableValueTypes
  Boolean value, default value is `false`
  Enable direct declaration and compilation of value type classes via the "__ByValue" and other qualifiers.
• -XDmangleClassInfo
  Boolean value, default value is `false`
  Ensure value type names are distinguishable a reference class names by prepending ";Q" to the value type name.
  Should be used in conjunction with java flag "valhalla.mangleClassInfo".
• -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
• -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.
• -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.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
• -Dvalhalla.mangleClassInfo
  Boolean value, default value is `false`
  Ensure value type names are distinguishable a reference class names by prepending ";Q" to the value type name.
  Should be used in conjunction with javac flag "mangleClassInfo".

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: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".
• -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 unwanted allocations (boxing) see the flags:

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