SmallSet/SmallPtrSet: Refuse huge Small numbers

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.

Differential Revision: http://reviews.llvm.org/D16706

llvm-svn: 259419

llvm/include/llvm/ADT/SmallPtrSet.h

@@ -329,6 +329,11 @@ public:
 /// 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.

llvm/include/llvm/ADT/SmallSet.h

@@ -38,6 +38,11 @@ class SmallSet {
   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() {}

llvm/lib/Target/CppBackend/CPPBackend.cpp

@@ -1651,8 +1651,8 @@ void CppWriter::printFunctionUses(const Function* F) {
 
   // Print type definitions for every type referenced by an instruction and
   // make a note of any global values or constants that are referenced
-  SmallPtrSet<GlobalValue*,64> gvs;
-  SmallPtrSet<Constant*,64> consts;
+  SmallPtrSet<GlobalValue*,32> gvs;
+  SmallPtrSet<Constant*,32> consts;
   for (Function::const_iterator BB = F->begin(), BE = F->end();
        BB != BE; ++BB){
     for (BasicBlock::const_iterator I = BB->begin(), E = BB->end();