Correct the handling of hidden class field comparisions

When using jitFieldsAreSame to compare the fields of two distinct hidden classes, the result can be a false positive if the field names and data types match. This can result in disastrous symref sharing for hidden class stores/loads. Hidden classes generated within the same host class do not have distinct class names, but share the same field names with different field data types and offsets. Therefore, name-based check for whether fields are same can result in false positives when it comes to hidden classes unless the fields are from the same j9class objects. Signed-off-by: Nazim Bhuiyan <[email protected]>
eclipse-openj9 · Nov 7, 2023 · 9d68a73 · 9d68a73
1 parent 3ba8567
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diff --git a/runtime/compiler/env/VMJ9.cpp b/runtime/compiler/env/VMJ9.cpp
@@ -6988,6 +6988,17 @@ TR_J9VMBase::jitFieldsAreSame(TR_ResolvedMethod * method1, I_32 cpIndex1, TR_Res
    TR::VMAccessCriticalSection jitFieldsAreSame(this);
    bool result = false;
 
+   // Hidden classes generated within the same host class do not have distinct class names,
+   // but share the same field names with different field data types and offsets. Therefore,
+   // name-based check for whether fields are same can result in false positives when it comes
+   // to hidden classes unless the fields are from the same j9class objects.
+   if (method1->classOfMethod()
+       && method2->classOfMethod()
+       && (isHiddenClass(method1->classOfMethod())
+          || isHiddenClass(method2->classOfMethod()))
+       && method1->classOfMethod() != method2->classOfMethod())
+      return false;
+
    bool sigSame = true;
    if (method1->fieldsAreSame(cpIndex1, method2, cpIndex2, sigSame))
       result = true;

diff --git a/runtime/compiler/env/VMJ9.h b/runtime/compiler/env/VMJ9.h
@@ -1295,6 +1295,15 @@ class TR_J9VMBase : public TR_FrontEnd
 
    virtual bool isAnonymousClass(TR_OpaqueClassBlock *j9clazz) { return (J9_ARE_ALL_BITS_SET(((J9Class*)j9clazz)->romClass->extraModifiers, J9AccClassAnonClass)); }
    virtual bool isAnonymousClass(J9ROMClass *romClass) { return (J9_ARE_ALL_BITS_SET(romClass->extraModifiers, J9AccClassAnonClass)); }
+
+   /**
+    * \brief Check whether or not the class object is a hidden class
+    *
+    * \param j9clazz the class location
+    * \return true if class is hidden class, false otherwise
+    */
+   virtual bool isHiddenClass(TR_OpaqueClassBlock *j9clazz) { return (J9_ARE_ALL_BITS_SET(((J9Class*)j9clazz)->romClass->extraModifiers, J9AccClassHidden)); }
+
    virtual int64_t getCpuTimeSpentInCompThread(TR::Compilation * comp); // resolution is 0.5 sec or worse. Returns -1 if unavailable
 
    virtual void * getClassLoader(TR_OpaqueClassBlock * classPointer);