diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 9c65eec..ae1f98f 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -107,7 +107,12 @@
   bool AnyFwdRefs;
   unsigned MinFwdRef;
   unsigned MaxFwdRef;
-  std::vector<TrackingMDRef> MetadataPtrs;
+
+  /// Array of metadata references.
+  ///
+  /// Don't use std::vector here.  Some versions of libc++ copy (instead of
+  /// move) on resize, and TrackingMDRef is very expensive to copy.
+  SmallVector<TrackingMDRef, 1> MetadataPtrs;
 
   LLVMContext &Context;
 public: