diff --git a/clang/include/clang/Serialization/ASTWriter.h b/clang/include/clang/Serialization/ASTWriter.h
index a0ea220b2dcb..940c86ab333c 100644
--- a/clang/include/clang/Serialization/ASTWriter.h
+++ b/clang/include/clang/Serialization/ASTWriter.h
@@ -709,7 +709,7 @@ class PCHGenerator : public SemaConsumer {
   raw_ostream *Out;
   Sema *SemaPtr;
   MemorizeStatCalls *StatCalls; // owned by the FileManager
-  std::vector<unsigned char> Buffer;
+  llvm::SmallVector<char, 128> Buffer;
   llvm::BitstreamWriter Stream;
   ASTWriter Writer;
 
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 3ffb6ecb4b25..c1678d4ef01a 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -2423,7 +2423,7 @@ bool ASTUnit::serialize(raw_ostream &OS) {
   if (getDiagnostics().hasErrorOccurred())
     return true;
 
-  std::vector<unsigned char> Buffer;
+  SmallString<128> Buffer;
   llvm::BitstreamWriter Stream(Buffer);
   ASTWriter Writer(Stream);
   // FIXME: Handle modules
diff --git a/clang/lib/Frontend/SerializedDiagnosticPrinter.cpp b/clang/lib/Frontend/SerializedDiagnosticPrinter.cpp
index 7a8edf7f5c5b..7bf8742a8040 100644
--- a/clang/lib/Frontend/SerializedDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/SerializedDiagnosticPrinter.cpp
@@ -159,7 +159,7 @@ private:
   const DiagnosticOptions &DiagOpts;
   
   /// \brief The byte buffer for the serialized content.
-  std::vector<unsigned char> Buffer;
+  SmallString<1024> Buffer;
 
   /// \brief The BitStreamWriter for the serialized diagnostics.
   llvm::BitstreamWriter Stream;