On May 18, 2017, at 12:55 PM, John Rose <john.r.r...@oracle.com> wrote: > > I'll post some info about that in a moment.
Here's what we are thinking about doing with condy in the short term. Comments are welcome. This work is landing in branches of the Amber project. — John Just as the linkage of an invokedynamic call site involves an upcall from the JVM to Java-based linkage logic, we can apply this same trick to the resolution of a constant pool entry. A CONSTANT_Dynamic constant pool entry encodes the bootstrap method to perform the resolution (a MethodHandle), the type of the constant (a Class), and any static bootstrap arguments (an arbitrary sequence of constants, barring cycles in the CP between dynamic constants.) We add a new constant pool form, CONSTANT_Dynamic (new constant tag 17), which has two components following its tag byte: the index of a bootstrap method, in the same format as the index found in a CONSTANT_InvokeDynamic, and a CONSTANT_NameAndType) which encodes the expected type. Behaviorally, a CONSTANT_Dynamic constant is resolved by executing its bootstrap method on the following parameters: 1. a local Lookup object, 2. the String representing the name component of the constant, 3. the Classrepresenting the expected constant type, and 4. any remaining bootstrap arguments. As with invokedynamic, multiple threads can race to resolve, but a unique winner will be chosen and any other contending answers discarded. Instead of returning a CallSite object (as the invokedynamicinstruction requires) the bootstrap method would return a value which would be immediately converted to the required type. As with invokedynamic, the name component is an additional channel, besides the type, for passing expression information to the bootstrap method. It is expected that just as invokedynamic instructions find uses for the name component (e.g., a method name or some ad hoc descriptor) dynamic constants will also find uses for the name (e.g., the name of a enum constant or the spelling of a symbolic constant). Putting the CONSTANT_NameAndTypein both places makes for a more regular design. In effect, a CONSTANT_Methodref and CONSTANT_Fieldref constants are used to refer to named members of classes, while the analogous CONSTANT_InvokeDynamic and CONSTANT_Dynamic constants are used to refer to named entities with user-programmed bootstraps. The type component of the constant, with both invokedynamic andCONSTANT_Dynamic, determines the effective type of the call site or constant (respectively). The bootstrap method does not contribute or constrain this type information, so that bootstrap methods may be (and often are) weakly typed whereas the bytecodes themselves are always strongly typed. To relax length restrictions on bootstrap specifiers, the language which defines the invocation of bootstrap methods will be adjusted (with complete backward compatibility) to allow variable arity (ACC_VARARGS) bootstrap methods to absorb, into their trailing arguments, all remaining static arguments, even if there are 2^16-1 of them. (The classfile format already allows this, although there is no way to read over-long bootstrap argument lists.) For consistency, the invokeWithArguments methods of MethodHandle will also be expanded in this way, if the target method has variable arity. In this way the invocation of bootstrap methods can be specified in terms of the weakly typed methods invokeWithArguments and invoke, just as today it is specified in terms of invoke alone.
_______________________________________________ mlvm-dev mailing list mlvm-dev@openjdk.java.net http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev
