Add frontend method getVFTEntry

The optimizer should ideally not fetch class fields directly.
Factor it out into a ClassEnv method instead.

Signed-off-by: Noah Weninger <Noah.Weninger@ibm.com>

compiler/env/OMRClassEnv.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2000, 2016 IBM Corp. and others
+ * Copyright (c) 2000, 2018 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -42,3 +42,9 @@ OMR::ClassEnv::getArrayElementWidthInBytes(TR::Compilation *comp, TR_OpaqueClass
    notImplemented("getArrayElementWidthInBytes");
    return 0;
    }
+
+intptrj_t
+OMR::ClassEnv::getVFTEntry(TR::Compilation *comp, TR_OpaqueClassBlock* clazz, int32_t offset)
+   {
+   return *(intptrj_t*) (((uint8_t *)clazz) + offset);
+   }

compiler/env/OMRClassEnv.hpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2000, 2016 IBM Corp. and others
+ * Copyright (c) 2000, 2018 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -111,6 +111,15 @@ class OMR_EXTENSIBLE ClassEnv
    char *classSignature_DEPRECATED(TR::Compilation *comp, TR_OpaqueClassBlock * clazz, int32_t & length, TR_Memory *) { return NULL; }
    char *classSignature(TR::Compilation *comp, TR_OpaqueClassBlock * clazz, TR_Memory *) { return NULL; }
 
+   /**
+    * Get the virtual function table entry at a specific offset from the class
+    *
+    * @param clazz The RAM class pointer to read from
+    * @param offset An offset into the virtual function table (VFT) of clazz
+    * @return The entry point of the method at the given offset
+    */
+   intptrj_t getVFTEntry(TR::Compilation *comp, TR_OpaqueClassBlock* clazz, int32_t offset);
+
    };
 
 }

compiler/optimizer/VPHandlers.cpp

@@ -9678,9 +9678,9 @@ static TR::Node *constrainIfcmpeqne(OMR::ValuePropagation *vp, TR::Node *node, b
                      TR::VPConstraint *classConstraint = vp->getConstraint(classNode, isGlobal);
                      if (ignoreVirtualGuard && classConstraint && classConstraint->isFixedClass())
                         {
-                        uint8_t   *clazz            = (uint8_t*)classConstraint->getClass();
+                        TR_OpaqueClassBlock *clazz  = classConstraint->getClass();
                         int32_t    vftOffset        = vtableEntryNode->getSymbolReference()->getOffset();
-                        intptrj_t  vftEntry         = *(intptrj_t*)(clazz + vftOffset);
+                        intptrj_t  vftEntry         = TR::Compiler->cls.getVFTEntry(vp->comp(), clazz, vftOffset);
                         bool       childrenAreEqual = (vftEntry == methodPtrNode->getAddress());
                         bool       testForEquality  = (node->getOpCodeValue() == TR::ifacmpeq);
                         traceMsg(vp->comp(), "TR_MethodTest: node=%p, vtableEntryNode=%p, clazz=%p, vftOffset=%d, vftEntry=%p, childrenAreEqual=%d, testForEquality=%d\n",