My guess at AIX and OS/2 bustage was wrong -- backing it out. Someone with access to a machine with one of those compilers will have to try various things here.

content/base/src/nsStyleContext.cpp

@@ -206,9 +206,8 @@ nsStyleContext::QueryInterface(const nsIID& aIID, void** aInstancePtr)
 
 nsIStyleContext* nsStyleContext::GetParent(void) const
 {
-  nsIStyleContext *rv = mParent;
-  NS_IF_ADDREF(rv);
-  return rv;
+  NS_IF_ADDREF(mParent);
+  return mParent;
 }
 
 NS_IMETHODIMP

layout/style/nsStyleContext.cpp

@@ -206,9 +206,8 @@ nsStyleContext::QueryInterface(const nsIID& aIID, void** aInstancePtr)
 
 nsIStyleContext* nsStyleContext::GetParent(void) const
 {
-  nsIStyleContext *rv = mParent;
-  NS_IF_ADDREF(rv);
-  return rv;
+  NS_IF_ADDREF(mParent);
+  return mParent;
 }
 
 NS_IMETHODIMP