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

Flag	Description
-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

Flag	Description
-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

Flag	Description
-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.
-XVerify:none	Disable verification. Verification is not yet supported.
-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:ValueTypePassFieldsAsArgs	Non-product: 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	Non-product: Boolean value, default value is `true` Return value type fields instead of a value type reference
-XX:ValueArrayAtomicAccess	Boolean value, default value is `false` Enable atomic access to values in an array
-XX:-UseTLAB	To see heap allocations which do not use optimized Thread Local Allocation Buffers
-XX:+PrintEliminateAllocations	Non-product builds: Print out when allocations are eliminated
-XX:+PrintEscapeAnalysis	Non-product builds: Print results of escape analysis (e.g. if you believe boxing was not eliminated)