Index: include/clang/Frontend/CompilerInstance.h
===================================================================
--- include/clang/Frontend/CompilerInstance.h
+++ include/clang/Frontend/CompilerInstance.h
@@ -183,6 +183,9 @@
   /// The list of active output files.
   std::list<OutputFile> OutputFiles;
 
+  /// Force an output buffer.
+  std::unique_ptr<llvm::raw_pwrite_stream> OutputStream;
+
   CompilerInstance(const CompilerInstance &) = delete;
   void operator=(const CompilerInstance &) = delete;
 public:
@@ -773,6 +776,14 @@
 
   /// }
 
+  void setOutputStream(std::unique_ptr<llvm::raw_pwrite_stream> OutStream) {
+    OutputStream = std::move(OutStream);
+  }
+
+  std::unique_ptr<llvm::raw_pwrite_stream> takeOutputStream() {
+    return std::move(OutputStream);
+  }
+
   // Create module manager.
   void createModuleManager();
 
Index: lib/CodeGen/CodeGenAction.cpp
===================================================================
--- lib/CodeGen/CodeGenAction.cpp
+++ lib/CodeGen/CodeGenAction.cpp
@@ -846,7 +846,10 @@
 std::unique_ptr<ASTConsumer>
 CodeGenAction::CreateASTConsumer(CompilerInstance &CI, StringRef InFile) {
   BackendAction BA = static_cast<BackendAction>(Act);
-  std::unique_ptr<raw_pwrite_stream> OS = GetOutputStream(CI, InFile, BA);
+  std::unique_ptr<raw_pwrite_stream> OS = CI.takeOutputStream();
+  if (!OS)
+    OS = GetOutputStream(CI, InFile, BA);
+
   if (BA != Backend_EmitNothing && !OS)
     return nullptr;
 
Index: unittests/Frontend/CMakeLists.txt
===================================================================
--- unittests/Frontend/CMakeLists.txt
+++ unittests/Frontend/CMakeLists.txt
@@ -9,6 +9,7 @@
   CodeGenActionTest.cpp
   ParsedSourceLocationTest.cpp
   PCHPreambleTest.cpp
+  OutputStreamTest.cpp
   )
 target_link_libraries(FrontendTests
   PRIVATE
@@ -18,4 +19,5 @@
   clangLex
   clangSema
   clangCodeGen
+  clangFrontendTool
   )
Index: unittests/Frontend/OutputStreamTest.cpp
===================================================================
--- unittests/Frontend/OutputStreamTest.cpp
+++ unittests/Frontend/OutputStreamTest.cpp
@@ -0,0 +1,46 @@
+//===- unittests/Frontend/OutputStreamTest.cpp --- FrontendAction tests --===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/CodeGen/BackendUtil.h"
+#include "clang/CodeGen/CodeGenAction.h"
+#include "clang/Frontend/CompilerInstance.h"
+#include "clang/FrontendTool/Utils.h"
+#include "clang/Lex/PreprocessorOptions.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+using namespace clang;
+using namespace clang::frontend;
+
+namespace {
+
+TEST(FrontendOutputTests, TestOutputStream) {
+  auto Invocation = std::make_shared<CompilerInvocation>();
+  Invocation->getPreprocessorOpts().addRemappedFile(
+      "test.cc", MemoryBuffer::getMemBuffer("").release());
+  Invocation->getFrontendOpts().Inputs.push_back(
+      FrontendInputFile("test.cc", InputKind::CXX));
+  Invocation->getFrontendOpts().ProgramAction = EmitBC;
+  Invocation->getTargetOpts().Triple = "i386-unknown-linux-gnu";
+  CompilerInstance Compiler;
+
+  SmallVector<char, 256> IRBuffer;
+  std::unique_ptr<raw_pwrite_stream> IRStream(
+      new raw_svector_ostream(IRBuffer));
+
+  Compiler.setOutputStream(std::move(IRStream));
+  Compiler.setInvocation(std::move(Invocation));
+  Compiler.createDiagnostics();
+
+  bool Success = ExecuteCompilerInvocation(&Compiler);
+  EXPECT_TRUE(Success);
+  EXPECT_TRUE(!IRBuffer.empty());
+  EXPECT_TRUE(StringRef(IRBuffer.data()).startswith("BC"));
+}
+}