8328480: C2: SubTypeCheckNode in checkcast should use the klass constant of a unique concrete sub class

Reviewed-by: roland, kvn

src/hotspot/share/opto/graphKit.cpp

@@ -3268,8 +3268,9 @@ Node* GraphKit::gen_instanceof(Node* obj, Node* superklass, bool safe_for_replac
 Node* GraphKit::gen_checkcast(Node *obj, Node* superklass,
                               Node* *failure_control) {
   kill_dead_locals();           // Benefit all the uncommon traps
-  const TypeKlassPtr *tk = _gvn.type(superklass)->is_klassptr()->try_improve();
-  const TypeOopPtr *toop = tk->cast_to_exactness(false)->as_instance_type();
+  const TypeKlassPtr* klass_ptr_type = _gvn.type(superklass)->is_klassptr();
+  const TypeKlassPtr* improved_klass_ptr_type = klass_ptr_type->try_improve();
+  const TypeOopPtr* toop = improved_klass_ptr_type->cast_to_exactness(false)->as_instance_type();
 
   // Fast cutout:  Check the case that the cast is vacuously true.
   // This detects the common cases where the test will short-circuit
@@ -3277,10 +3278,10 @@ Node* GraphKit::gen_checkcast(Node *obj, Node* superklass,
   // because if the test is going to turn into zero code, we don't
   // want a residual null check left around.  (Causes a slowdown,
   // for example, in some objArray manipulations, such as a[i]=a[j].)
-  if (tk->singleton()) {
+  if (improved_klass_ptr_type->singleton()) {
     const TypeOopPtr* objtp = _gvn.type(obj)->isa_oopptr();
     if (objtp != nullptr) {
-      switch (C->static_subtype_check(tk, objtp->as_klass_type())) {
+      switch (C->static_subtype_check(improved_klass_ptr_type, objtp->as_klass_type())) {
       case Compile::SSC_always_true:
         // If we know the type check always succeed then we don't use
         // the profiling data at this bytecode. Don't lose it, feed it
@@ -3346,7 +3347,7 @@ Node* GraphKit::gen_checkcast(Node *obj, Node* superklass,
   }
 
   Node* cast_obj = nullptr;
-  if (tk->klass_is_exact()) {
+  if (improved_klass_ptr_type->klass_is_exact()) {
     // The following optimization tries to statically cast the speculative type of the object
     // (for example obtained during profiling) to the type of the superklass and then do a
     // dynamic check that the type of the object is what we expect. To work correctly
@@ -3356,7 +3357,7 @@ Node* GraphKit::gen_checkcast(Node *obj, Node* superklass,
     // a speculative type use it to perform an exact cast.
     ciKlass* spec_obj_type = obj_type->speculative_type();
     if (spec_obj_type != nullptr || data != nullptr) {
-      cast_obj = maybe_cast_profiled_receiver(not_null_obj, tk, spec_obj_type, safe_for_replace);
+      cast_obj = maybe_cast_profiled_receiver(not_null_obj, improved_klass_ptr_type, spec_obj_type, safe_for_replace);
       if (cast_obj != nullptr) {
         if (failure_control != nullptr) // failure is now impossible
           (*failure_control) = top();
@@ -3368,7 +3369,11 @@ Node* GraphKit::gen_checkcast(Node *obj, Node* superklass,
 
   if (cast_obj == nullptr) {
     // Generate the subtype check
-    Node* not_subtype_ctrl = gen_subtype_check(not_null_obj, superklass );
+    Node* improved_superklass = superklass;
+    if (improved_klass_ptr_type != klass_ptr_type && improved_klass_ptr_type->singleton()) {
+      improved_superklass = makecon(improved_klass_ptr_type);
+    }
+    Node* not_subtype_ctrl = gen_subtype_check(not_null_obj, improved_superklass);
 
     // Plug in success path into the merge
     cast_obj = _gvn.transform(new CheckCastPPNode(control(), not_null_obj, toop));

test/hotspot/jtreg/compiler/types/TestSubTypeCheckUniqueSubclass.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2024, Oracle and/or its affiliates. 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.
+ */
+
+/*
+ * @test
+ * @bug 8328480
+ * @summary Test that SubTypeCheckNode takes improved unique concrete klass constant in order to fold consecutive sub
+ *          type checks.
+ * @library /test/lib /
+ * @run driver compiler.types.TestSubTypeCheckUniqueSubclass
+ */
+
+package compiler.types;
+
+import compiler.lib.ir_framework.*;
+
+public class TestSubTypeCheckUniqueSubclass {
+    static Object o = new C(); // Make sure C is loaded.
+    static Object o2 = new C2(); // Make sure C2 is loaded while NeverLoaded is not.
+
+    public static void main(String[] args) {
+        TestFramework.run();
+    }
+
+    @Test
+    @Warmup(0)
+    @IR(counts = {IRNode.SUBTYPE_CHECK, "1"},
+        phase = CompilePhase.ITER_GVN1)
+    static void testAbstractAbstract() {
+         A a = (A)o;
+         A a2 = (B)o;
+    }
+
+    @Test
+    @Warmup(0)
+    @IR(counts = {IRNode.SUBTYPE_CHECK, "1"},
+            phase = CompilePhase.ITER_GVN1)
+    static void testAbstractAbstractWithUnloaded() {
+        A2 a = (A2)o2;
+        A2 a2 = (B2)o2;
+    }
+}
+
+abstract class A {}
+abstract class B extends A {}
+class C extends B {}
+
+abstract class A2 {}
+abstract class B2 extends A2 {}
+class C2 extends B2 {}
+
+// Class never loaded -> C2 looks like unique sub class.
+class NeverLoaded extends B2 {}