8373248: C2: CastPP should not change the type of the oop

Reviewed-by: bmaillard, dfenacci, rcastanedalo, mchevalier

Author: merykitty

src/hotspot/share/opto/castnode.cpp

@@ -413,6 +413,43 @@ Node* CastLLNode::Ideal(PhaseGVN* phase, bool can_reshape) {
   return nullptr;
 }
 
+// CastPPNodes are removed before matching, while alias classes are needed in global code motion.
+// As a result, it is not valid for a CastPPNode to change the oop such that the derived pointers
+// lie in different alias classes with and without the node. For example, a CastPPNode c may not
+// cast an Object to a Bottom[], because later removal of c would affect the alias class of c's
+// array length field (c + arrayOopDesc::length_offset_in_bytes()).
+//
+// This function verifies that a CastPPNode on an oop does not violate the aforementioned property.
+//
+// TODO 8382147: Currently, this verification only applies during the construction of a CastPPNode,
+// we may want to apply the same verification during IGVN transformations, as well as final graph
+// reshaping.
+void CastPPNode::verify_type(const Type* in_type, const Type* out_type) {
+#ifdef ASSERT
+  out_type = out_type->join(in_type);
+  if (in_type->empty() || out_type->empty()) {
+    return;
+  }
+  if (in_type == TypePtr::NULL_PTR || out_type == TypePtr::NULL_PTR) {
+    return;
+  }
+  if (!in_type->isa_oopptr() && !out_type->isa_oopptr()) {
+    return;
+  }
+
+  assert(in_type->isa_oopptr() && out_type->isa_oopptr(), "must be both oops or both non-oops");
+  if (in_type->isa_aryptr() && out_type->isa_aryptr()) {
+    const Type* e1 = in_type->is_aryptr()->elem();
+    const Type* e2 = out_type->is_aryptr()->elem();
+    assert(e1->basic_type() == e2->basic_type(), "must both be arrays of the same primitive type or both be oops arrays");
+    return;
+  }
+
+  assert(in_type->isa_instptr() && out_type->isa_instptr(), "must be both array oops or both non-array oops");
+  assert(in_type->is_instptr()->instance_klass() == out_type->is_instptr()->instance_klass(), "must not cast to a different type");
+#endif // ASSERT
+}
+
 //------------------------------Value------------------------------------------
 // Take 'join' of input and cast-up type, unless working with an Interface
 const Type* CheckCastPPNode::Value(PhaseGVN* phase) const {
@@ -440,6 +477,11 @@ const Type* CheckCastPPNode::Value(PhaseGVN* phase) const {
   return result;
 }
 
+Node* CheckCastPPNode::pin_node_under_control_impl() const {
+  assert(_dependency.is_floating(), "already pinned");
+  return new CheckCastPPNode(in(0), in(1), bottom_type(), _dependency.with_pinned_dependency(), _extra_types);
+}
+
 //=============================================================================
 //------------------------------Value------------------------------------------
 const Type* CastX2PNode::Value(PhaseGVN* phase) const {

src/hotspot/share/opto/castnode.hpp

@@ -303,14 +303,18 @@ class CastVVNode: public ConstraintCastNode {
 
 //------------------------------CastPPNode-------------------------------------
 // cast pointer to pointer (different type)
-class CastPPNode: public ConstraintCastNode {
-  public:
-  CastPPNode (Node* ctrl, Node* n, const Type* t, const DependencyType& dependency = DependencyType::FloatingNarrowing, const TypeTuple* types = nullptr)
+class CastPPNode : public ConstraintCastNode {
+public:
+  CastPPNode(Node* ctrl, Node* n, const Type* t, const DependencyType& dependency = DependencyType::FloatingNarrowing, const TypeTuple* types = nullptr)
     : ConstraintCastNode(ctrl, n, t, dependency, types) {
     init_class_id(Class_CastPP);
+    verify_type(n->bottom_type(), t);
   }
   virtual int Opcode() const;
   virtual uint ideal_reg() const { return Op_RegP; }
+
+private:
+  static void verify_type(const Type* in_type, const Type* out_type);
 };
 
 //------------------------------CheckCastPPNode--------------------------------
@@ -329,6 +333,7 @@ class CheckCastPPNode: public ConstraintCastNode {
 
 private:
   virtual bool depends_only_on_test_impl() const { return !type()->isa_rawptr() && ConstraintCastNode::depends_only_on_test_impl(); }
+  virtual Node* pin_node_under_control_impl() const;
 };
 
 

src/hotspot/share/opto/library_call.cpp

@@ -4311,7 +4311,7 @@ Node* LibraryCallKit::generate_array_guard_common(Node* kls, RegionNode* region,
   if (obj != nullptr && is_array_ctrl != nullptr && is_array_ctrl != top()) {
     // Keep track of the fact that 'obj' is an array to prevent
     // array specific accesses from floating above the guard.
-    *obj = _gvn.transform(new CastPPNode(is_array_ctrl, *obj, TypeAryPtr::BOTTOM));
+    *obj = _gvn.transform(new CheckCastPPNode(is_array_ctrl, *obj, TypeAryPtr::BOTTOM));
   }
   return ctrl;
 }

src/hotspot/share/opto/node.hpp

@@ -1186,6 +1186,7 @@ class Node {
       return nullptr;
     }
     assert(!res->depends_only_on_test(), "the result must not depends_only_on_test");
+    assert(Opcode() == res->Opcode(), "pinning must result in the same kind of node %s - %s", Name(), res->Name());
     return res;
   }
 

src/hotspot/share/opto/vector.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2020, 2026, 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
@@ -455,14 +455,12 @@ void PhaseVector::expand_vunbox_node(VectorUnboxNode* vec_unbox) {
       gvn.record_for_igvn(local_mem);
       BarrierSetC2* bs = BarrierSet::barrier_set()->barrier_set_c2();
       C2OptAccess access(gvn, ctrl, local_mem, decorators, T_OBJECT, obj, addr);
-      const Type* type = TypeOopPtr::make_from_klass(field->type()->as_klass());
-      vec_field_ld = bs->load_at(access, type);
-    }
 
-    // For proper aliasing, attach concrete payload type.
-    ciKlass* payload_klass = ciTypeArrayKlass::make(bt);
-    const Type* payload_type = TypeAryPtr::make_from_klass(payload_klass)->cast_to_ptr_type(TypePtr::NotNull);
-    vec_field_ld = gvn.transform(new CastPPNode(nullptr, vec_field_ld, payload_type));
+      // For proper aliasing, attach concrete payload type.
+      ciKlass* payload_klass = ciTypeArrayKlass::make(bt);
+      const Type* payload_type = TypeAryPtr::make_from_klass(payload_klass)->cast_to_ptr_type(TypePtr::NotNull);
+      vec_field_ld = bs->load_at(access, payload_type);
+    }
 
     Node* adr = kit.array_element_address(vec_field_ld, gvn.intcon(0), bt);
     const TypePtr* adr_type = adr->bottom_type()->is_ptr();