diff --git a/llvm/include/llvm/DerivedTypes.h b/llvm/include/llvm/DerivedTypes.h
index b9ade512cfb4..da5ad27b1f1c 100644
--- a/llvm/include/llvm/DerivedTypes.h
+++ b/llvm/include/llvm/DerivedTypes.h
@@ -195,9 +195,10 @@ class StructType : public CompositeType {
     // This is the contents of the SubClassData field.
     SCDB_HasBody = 1,
     SCDB_Packed = 2,
-    SCDB_IsLiteral = 4
+    SCDB_IsLiteral = 4,
+    SCDB_IsSized = 8
   };
-  
+
   /// SymbolTableEntry - For a named struct that actually has a name, this is a
   /// pointer to the symbol table entry (maintained by LLVMContext) for the
   /// struct.  This is null if the type is an literal struct or if it is
@@ -248,6 +249,9 @@ public:
   /// isOpaque - Return true if this is a type with an identity that has no body
   /// specified yet.  These prints as 'opaque' in .ll files.
   bool isOpaque() const { return (getSubclassData() & SCDB_HasBody) == 0; }
+
+  /// isSized - Return true if this is a sized type.
+  bool isSized() const;
   
   /// hasName - Return true if this is a named struct that has a non-empty name.
   bool hasName() const { return SymbolTableEntry != 0; }
diff --git a/llvm/lib/VMCore/Type.cpp b/llvm/lib/VMCore/Type.cpp
index 0bc4f74af216..c6f35580e158 100644
--- a/llvm/lib/VMCore/Type.cpp
+++ b/llvm/lib/VMCore/Type.cpp
@@ -185,16 +185,7 @@ bool Type::isSizedDerivedType() const {
   if (!this->isStructTy()) 
     return false;
 
-  // Opaque structs have no size.
-  if (cast<StructType>(this)->isOpaque())
-    return false;
-  
-  // Okay, our struct is sized if all of the elements are.
-  for (subtype_iterator I = subtype_begin(), E = subtype_end(); I != E; ++I)
-    if (!(*I)->isSized()) 
-      return false;
-
-  return true;
+  return cast<StructType>(this)->isSized();
 }
 
 //===----------------------------------------------------------------------===//
@@ -579,6 +570,26 @@ StructType *StructType::create(StringRef Name, Type *type, ...) {
   return llvm::StructType::create(Ctx, StructFields, Name);
 }
 
+bool StructType::isSized() const {
+  if ((getSubclassData() & SCDB_IsSized) != 0)
+    return true;
+  if (isOpaque())
+    return false;
+
+  // Okay, our struct is sized if all of the elements are, but if one of the
+  // elements is opaque, the struct isn't sized *yet*, but may become sized in
+  // the future, so just bail out without caching.
+  for (element_iterator I = element_begin(), E = element_end(); I != E; ++I)
+    if (!(*I)->isSized())
+      return false;
+
+  // Here we cheat a bit and cast away const-ness. The goal is to memoize when
+  // we find a sized type, as types can only move from opaque to sized, not the
+  // other way.
+  const_cast<StructType*>(this)->setSubclassData(
+    getSubclassData() | SCDB_IsSized);
+  return true;
+}
 
 StringRef StructType::getName() const {
   assert(!isLiteral() && "Literal structs never have names");