diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index bf15460..f660429 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -13,6 +13,7 @@
 #include "clang/AST/Decl.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/FileManager.h"
+#include "clang/Basic/MemoryBufferCache.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Basic/Version.h"
@@ -55,9 +56,15 @@
 
 CompilerInstance::CompilerInstance(
     std::shared_ptr<PCHContainerOperations> PCHContainerOps,
-    bool BuildingModule)
-    : ModuleLoader(BuildingModule), Invocation(new CompilerInvocation()),
-      ThePCHContainerOperations(std::move(PCHContainerOps)) {}
+    MemoryBufferCache *SharedPCMCache)
+    : ModuleLoader(/* BuildingModule = */ SharedPCMCache),
+      Invocation(new CompilerInvocation()),
+      PCMCache(SharedPCMCache ? SharedPCMCache : new MemoryBufferCache),
+      ThePCHContainerOperations(std::move(PCHContainerOps)) {
+  // Don't allow this to invalidate buffers in use by others.
+  if (SharedPCMCache)
+    getPCMCache().finalizeCurrentBuffers();
+}
 
 CompilerInstance::~CompilerInstance() {
   assert(OutputFiles.empty() && "Still output files in flight?");
@@ -128,6 +135,8 @@
   return ModuleManager;
 }
 void CompilerInstance::setModuleManager(IntrusiveRefCntPtr<ASTReader> Reader) {
+  assert(PCMCache.get() == &Reader->getModuleManager().getPCMCache() &&
+         "Expected ASTReader to use the same PCM cache");
   ModuleManager = std::move(Reader);
 }
 
@@ -370,7 +379,7 @@
                        getDiagnostics(), getLangOpts(), &getTarget());
   PP = std::make_shared<Preprocessor>(
       Invocation->getPreprocessorOptsPtr(), getDiagnostics(), getLangOpts(),
-      getSourceManager(), *HeaderInfo, *this, PTHMgr,
+      getSourceManager(), getPCMCache(), *HeaderInfo, *this, PTHMgr,
       /*OwnsHeaderSearch=*/true, TUKind);
   PP->Initialize(getTarget(), getAuxTarget());
 
@@ -1073,9 +1082,11 @@
          Invocation->getModuleHash() && "Module hash mismatch!");
   
   // Construct a compiler instance that will be used to actually create the
-  // module.
+  // module.  Since we're sharing a PCMCache,
+  // CompilerInstance::CompilerInstance is responsible for finalizing the
+  // buffers to prevent use-after-frees.
   CompilerInstance Instance(ImportingInstance.getPCHContainerOperations(),
-                            /*BuildingModule=*/true);
+                            &ImportingInstance.getPreprocessor().getPCMCache());
   auto &Inv = *Invocation;
   Instance.setInvocation(std::move(Invocation));