SmallSet/SmallPtrSet: Refuse huge SmallSize/N numbers

[MatzeB]

Summary

These sets do linear searching in small mode; It is not a good idea to
use huge numbers as the small value here, save people from themselves by
adding a static_assert.

[echristo]

LGTM.

-eric

include/llvm/ADT/SmallPtrSet.h

 };
 
 /// SmallPtrSet - This class implements a set which is optimized for holding
 /// SmallSize or less elements.  This internally rounds up SmallSize to the next
 /// power of two if it is not already a power of two.  See the comments above
 /// 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.
   enum { SmallSizePowTwo = RoundUpToPowerOfTwo<SmallSize>::Val };
   /// SmallStorage - Fixed size storage used in 'small mode'.
   const void *SmallStorage[SmallSizePowTwo];
 
 public:

include/llvm/ADT/SmallSet.h

class SmallSet {
   /// Use a SmallVector to hold the elements here (even though it will never
   /// reach its 'large' stage) to avoid calling the default ctors of elements
   /// we will never use.
   SmallVector<T, N> Vector;
   std::set<T, C> Set;
   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() {}
 
   bool LLVM_ATTRIBUTE_UNUSED_RESULT empty() const {
     return Vector.empty() && Set.empty();
   }