diff --git a/llvm/include/llvm/Support/raw_ostream.h b/llvm/include/llvm/Support/raw_ostream.h
--- a/llvm/include/llvm/Support/raw_ostream.h
+++ b/llvm/include/llvm/Support/raw_ostream.h
@@ -137,6 +137,10 @@
   // Configuration Interface
   //===--------------------------------------------------------------------===//
 
+  /// If possible, pre-allocate \p Capacity bytes for stream data.
+  /// reserve() does not change size/data of the stream.
+  virtual void reserve(size_t Capacity) {}
+
   /// Set the stream to be buffered, with an automatically determined buffer
   /// size.
   void SetBuffered();
@@ -626,6 +630,8 @@
     flush();
     return OS;
   }
+
+  void reserve(size_t Capacity) override { OS.reserve(Capacity); }
 };
 
 /// A raw_ostream that writes to an SmallVector or SmallString.  This is a
@@ -659,6 +665,8 @@
 
   /// Return a StringRef for the vector contents.
   StringRef str() const { return StringRef(OS.data(), OS.size()); }
+
+  void reserve(size_t Capacity) override { OS.reserve(Capacity); }
 };
 
 /// A raw_ostream that discards all output.
diff --git a/llvm/unittests/Support/raw_ostream_test.cpp b/llvm/unittests/Support/raw_ostream_test.cpp
--- a/llvm/unittests/Support/raw_ostream_test.cpp
+++ b/llvm/unittests/Support/raw_ostream_test.cpp
@@ -455,4 +455,18 @@
   TiedStream << "0";
   EXPECT_EQ("acegostuv", TiedToBuffer);
 }
+
+TEST(raw_ostreamTest, reserve_stream) {
+  std::string Str;
+  size_t OrigCapacity = Str.capacity();
+  raw_string_ostream OS(Str);
+  size_t NewCapacity = OrigCapacity + 1000;
+  OS.reserve(NewCapacity);
+  EXPECT_EQ(Str.capacity(), NewCapacity);
+  OS << "hello";
+  OS << 1;
+  OS << 'w' << 'o' << 'r' << 'l' << 'd';
+  OS.flush();
+  EXPECT_EQ("hello1world", Str);
+}
 }