8256497: Zero: enable G1 and Shenandoah GCs

Reviewed-by: rkennke, erikj, ihse

make/autoconf/jvm-features.m4

@@ -476,7 +476,7 @@ AC_DEFUN([JVM_FEATURES_PREPARE_VARIANT],
     JVM_FEATURES_VARIANT_UNAVAILABLE="cds minimal zero"
   elif test "x$variant" = "xzero"; then
     JVM_FEATURES_VARIANT_UNAVAILABLE="aot cds compiler1 compiler2 \
-        g1gc graal jvmci minimal shenandoahgc zgc"
+        graal jvmci minimal zgc"
   else
     JVM_FEATURES_VARIANT_UNAVAILABLE="minimal zero"
   fi

src/hotspot/cpu/zero/gc/shenandoah/shenandoahBarrierSetAssembler_zero.hpp

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2020, Red Hat, Inc. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef CPU_ZERO_GC_SHENANDOAH_SHENANDOAHBARRIERSETASSEMBLER_ZERO_HPP
+#define CPU_ZERO_GC_SHENANDOAH_SHENANDOAHBARRIERSETASSEMBLER_ZERO_HPP
+
+class ShenandoahBarrierSetAssembler;
+
+#endif // CPU_ZERO_GC_SHENANDOAH_SHENANDOAHBARRIERSETASSEMBLER_ZERO_HPP

src/hotspot/cpu/zero/zeroInterpreter_zero.cpp

@@ -31,6 +31,7 @@
 #include "interpreter/zero/bytecodeInterpreter.hpp"
 #include "interpreter/zero/zeroInterpreter.hpp"
 #include "interpreter/zero/zeroInterpreterGenerator.hpp"
+#include "oops/access.inline.hpp"
 #include "oops/cpCache.inline.hpp"
 #include "oops/methodData.hpp"
 #include "oops/method.hpp"
@@ -120,6 +121,28 @@ int ZeroInterpreter::normal_entry(Method* method, intptr_t UNUSED, TRAPS) {
   return 0;
 }
 
+int ZeroInterpreter::Reference_get_entry(Method* method, intptr_t UNUSED, TRAPS) {
+  JavaThread* thread = THREAD->as_Java_thread();
+  ZeroStack* stack = thread->zero_stack();
+  intptr_t* topOfStack = stack->sp();
+
+  oop ref = STACK_OBJECT(0);
+
+  // Shortcut if reference is known NULL
+  if (ref == NULL) {
+    return normal_entry(method, 0, THREAD);
+  }
+
+  // Read the referent with weaker semantics, and let GCs handle the rest.
+  const int referent_offset = java_lang_ref_Reference::referent_offset();
+  oop obj = HeapAccess<IN_HEAP | ON_WEAK_OOP_REF>::oop_load_at(ref, referent_offset);
+
+  SET_STACK_OBJECT(obj, 0);
+
+  // No deoptimized frames on the stack
+  return 0;
+}
+
 intptr_t narrow(BasicType type, intptr_t result) {
   // mask integer result to narrower return type.
   switch (type) {

src/hotspot/cpu/zero/zeroInterpreter_zero.hpp

@@ -37,6 +37,7 @@
   static int getter_entry(Method* method, intptr_t UNUSED, TRAPS);
   static int setter_entry(Method* method, intptr_t UNUSED, TRAPS);
   static int empty_entry(Method* method, intptr_t UNUSED, TRAPS);
+  static int Reference_get_entry(Method* method, intptr_t UNUSED, TRAPS);
 
  public:
   // Main loop of normal_entry

src/hotspot/share/interpreter/zero/zeroInterpreterGenerator.cpp

@@ -173,23 +173,7 @@ address ZeroInterpreterGenerator::generate_setter_entry() {
 }
 
 address ZeroInterpreterGenerator::generate_Reference_get_entry(void) {
-#if INCLUDE_G1GC
-  if (UseG1GC) {
-    // We need to generate have a routine that generates code to:
-    //   * load the value in the referent field
-    //   * passes that value to the pre-barrier.
-    //
-    // In the case of G1 this will record the value of the
-    // referent in an SATB buffer if marking is active.
-    // This will cause concurrent marking to mark the referent
-    // field as live.
-    Unimplemented();
-  }
-#endif // INCLUDE_G1GC
-
-  // If G1 is not enabled then attempt to go through the normal entry point
-  // Reference.get could be instrumented by jvmti
-  return NULL;
+  return generate_entry((address) ZeroInterpreter::Reference_get_entry);
 }
 
 address ZeroInterpreterGenerator::generate_native_entry(bool synchronized) {