RFR(L): 8185265 [MVT] improve performance of return of value types with new calling convention

[Roland Westrelin]

> http://cr.openjdk.java.net/~roland/8185265/webrev.00/

The following patch on top of the webrev above is needed to not break
the 32 bit build.

Roland.

--- a/src/cpu/x86/vm/sharedRuntime_x86_32.cpp
+++ b/src/cpu/x86/vm/sharedRuntime_x86_32.cpp
@@ -3147,6 +3147,6 @@
 }
 
 BufferedValueTypeBlob* SharedRuntime::generate_buffered_value_type_adapter(const ValueKlass* vk) {
-  ShouldNotReachHere();
+  Unimplemented();
   return NULL;
 }
diff --git a/src/share/vm/oops/valueKlass.cpp b/src/share/vm/oops/valueKlass.cpp
--- a/src/share/vm/oops/valueKlass.cpp
+++ b/src/share/vm/oops/valueKlass.cpp
@@ -344,6 +344,7 @@
 }
 
 void ValueKlass::initialize_calling_convention() {
+  if (ValueTypeReturnedAsFields || ValueTypePassFieldsAsArgs) {
   Thread* THREAD = Thread::current();
   assert(!HAS_PENDING_EXCEPTION, "should have no exception");
   ResourceMark rm;
@@ -355,6 +356,7 @@
     extended_sig->at_put(i, sig_vk.at(i));
   }
 
+    if (ValueTypeReturnedAsFields) {
   BasicType* sig_bt = NEW_RESOURCE_ARRAY(BasicType, nb_fields);
   sig_bt[0] = T_METADATA;
   SigEntry::fill_sig_bt(sig_vk, sig_bt+1, nb_fields-1, true);
@@ -373,6 +375,8 @@
     *((address*)adr_unpack_handler()) = buffered_blob->unpack_fields();
     assert(CodeCache::find_blob(pack_handler()) == buffered_blob, "lost track of blob");
   }
+    }
+  }
   assert(vtable_length() == 0, "a value klass with a vtable?");
 }