Native call framework prototype

[Vladimir Ivanov]

Hi,

As you, probably, already noticed, I've just pushed raw prototype of 
java.lang.invoke-based native function invocation framework, inspired by 
[1].

The first version is a raw prototype which is able to call getpid (see 
jdk/test/java/lang/invoke/NativeCallTest.java):

   MethodHandle mh =
     LOOKUP.findNative(null, "getpid", MethodType.methodType(int.class));
   int pid = (int) mh.invokeExact();

NativeMethodHandle and NativeEntryPoint (MemberName counterpart) are 
used to represent native function.

The implementation relies on 2 VM primitives:
   MethodHandle.linkToNative() & Unsafe.findNativeAddress()

Native part is mostly a skeleton for future work:
   * linkToNative adapter generation is present only on x64;
   * supported adapter signature is trivial, just enough to call getpid()
   * generated adapter doesn't do thread state transition before calling 
native function (it corresponds to privileged "raw" invocation mode in [2]);

On the other hand, Java part is in much better shape. No access & 
security checks yet, but once VM can generate more adapters, everything 
should "just work" :-)

The next steps are:

   - extend linkToNative adapter to work on all Java signatures in raw 
invocation mode

   - introduce "state transitioning" calling mode (like JNI does) and 
different calling conventions support; the idea is to encapsulate such 
info in NativeEntryPoint

   - JIT-compiler support for "raw" mode: issue direct native call 
instead of using linkToNative adapter when possible

   - try some simple Java<->C type conversions (like String <-> char*); 
they can be done on LambdaForm level

   - try commoning thread state transitions in JIT-compiled code in 
order to reduce native call overhead

Best regards,
Vladimir Ivanov

[1] http://cr.openjdk.java.net/~jrose/panama/native-call-primitive.html
[2] https://blogs.oracle.com/jrose/entry/the_isthmus_in_the_vm