IndexedMap: Document why SmallVector<T, 0> is preferable here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231028 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/ADT/IndexedMap.h b/include/llvm/ADT/IndexedMap.h
index a830a41f56db7691845f652cdefe8f3ed4f943f8..5ba85c02792095ff75ef49e21ee1d172481234eb 100644 (file)
--- a/include/llvm/ADT/IndexedMap.h
+++ b/include/llvm/ADT/IndexedMap.h
@@ -30,6 +30,9 @@ namespace llvm {
 template <typename T, typename ToIndexT = llvm::identity<unsigned> >
   class IndexedMap {
     typedef typename ToIndexT::argument_type IndexT;
+    // Prefer SmallVector with zero inline storage over std::vector. IndexedMaps
+    // can grow very large and SmallVector grows more efficiently as long as T
+    // is trivially copyable.
     typedef SmallVector<T, 0> StorageT;
     StorageT storage_;
     T nullVal_;