Index: include/llvm/ADT/SmallPtrSet.h
===================================================================
--- include/llvm/ADT/SmallPtrSet.h
+++ include/llvm/ADT/SmallPtrSet.h
@@ -351,6 +351,11 @@
 /// SmallPtrSetImplBase for details of the algorithm.
 template<class PtrType, unsigned SmallSize>
 class SmallPtrSet : public SmallPtrSetImpl<PtrType> {
+  // In small mode SmallPtrSet uses linear search for the elements, so it is
+  // not a good idea to choose this value too high. You may consider using a
+  // DenseSet<> instead if you expect many elements in the set.
+  static_assert(SmallSize <= 32, "SmallSize should be small");
+
   typedef SmallPtrSetImpl<PtrType> BaseT;
 
   // Make sure that SmallSize is a power of two, round up if not.
Index: include/llvm/ADT/SmallSet.h
===================================================================
--- include/llvm/ADT/SmallSet.h
+++ include/llvm/ADT/SmallSet.h
@@ -38,6 +38,11 @@
   typedef typename SmallVector<T, N>::const_iterator VIterator;
   typedef typename SmallVector<T, N>::iterator mutable_iterator;
 
+  // In small mode SmallPtrSet uses linear search for the elements, so it is
+  // not a good idea to choose this value too high. You may consider using a
+  // DenseSet<> instead if you expect many elements in the set.
+  static_assert(N <= 32, "N should be small");
+
 public:
   typedef size_t size_type;
   SmallSet() {}