diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -32,6 +32,7 @@
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/ErrorOr.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/FileSystem/UniqueID.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Process.h"
@@ -655,6 +656,9 @@
   Status getStatus(const Twine &RequestedName) const {
     return Status::copyWithNewName(Stat, RequestedName);
   }
+
+  UniqueID getUniqueID() const { return Stat.getUniqueID(); }
+
   InMemoryNode *getChild(StringRef Name) {
     auto I = Entries.find(Name);
     if (I != Entries.end())
@@ -698,10 +702,28 @@
 } // namespace
 } // namespace detail
 
+// The UniqueID of in-memory files is derived from path and content.
+// This avoids difficulties in creating exactly equivalent in-memory FSes,
+// as often needed in multithreaded programs.
+static sys::fs::UniqueID getUniqueID(hash_code Hash) {
+  return sys::fs::UniqueID(std::numeric_limits<uint64_t>::max(),
+                           uint64_t(size_t(Hash)));
+}
+static sys::fs::UniqueID getFileID(sys::fs::UniqueID Parent,
+                                   llvm::StringRef Name,
+                                   llvm::StringRef Contents) {
+  return getUniqueID(llvm::hash_combine(Parent.getFile(), Name, Contents));
+}
+static sys::fs::UniqueID getDirectoryID(sys::fs::UniqueID Parent,
+                                        llvm::StringRef Name) {
+  return getUniqueID(llvm::hash_combine(Parent.getFile(), Name));
+}
+
 InMemoryFileSystem::InMemoryFileSystem(bool UseNormalizedPaths)
     : Root(new detail::InMemoryDirectory(
-          Status("", getNextVirtualUniqueID(), llvm::sys::TimePoint<>(), 0, 0,
-                 0, llvm::sys::fs::file_type::directory_file,
+          Status("", getDirectoryID(llvm::sys::fs::UniqueID(), ""),
+                 llvm::sys::TimePoint<>(), 0, 0, 0,
+                 llvm::sys::fs::file_type::directory_file,
                  llvm::sys::fs::perms::all_all))),
       UseNormalizedPaths(UseNormalizedPaths) {}
 
@@ -754,10 +776,14 @@
           Child.reset(new detail::InMemoryHardLink(P.str(), *HardLinkTarget));
         else {
           // Create a new file or directory.
-          Status Stat(P.str(), getNextVirtualUniqueID(),
-                      llvm::sys::toTimePoint(ModificationTime), ResolvedUser,
-                      ResolvedGroup, Buffer->getBufferSize(), ResolvedType,
-                      ResolvedPerms);
+          Status Stat(
+              P.str(),
+              (ResolvedType == sys::fs::file_type::directory_file)
+                  ? getDirectoryID(Dir->getUniqueID(), Name)
+                  : getFileID(Dir->getUniqueID(), Name, Buffer->getBuffer()),
+              llvm::sys::toTimePoint(ModificationTime), ResolvedUser,
+              ResolvedGroup, Buffer->getBufferSize(), ResolvedType,
+              ResolvedPerms);
           if (ResolvedType == sys::fs::file_type::directory_file) {
             Child.reset(new detail::InMemoryDirectory(std::move(Stat)));
           } else {
@@ -772,9 +798,9 @@
       // Create a new directory. Use the path up to here.
       Status Stat(
           StringRef(Path.str().begin(), Name.end() - Path.str().begin()),
-          getNextVirtualUniqueID(), llvm::sys::toTimePoint(ModificationTime),
-          ResolvedUser, ResolvedGroup, 0, sys::fs::file_type::directory_file,
-          NewDirectoryPerms);
+          getDirectoryID(Dir->getUniqueID(), Name),
+          llvm::sys::toTimePoint(ModificationTime), ResolvedUser, ResolvedGroup,
+          0, sys::fs::file_type::directory_file, NewDirectoryPerms);
       Dir = cast<detail::InMemoryDirectory>(Dir->addChild(
           Name, std::make_unique<detail::InMemoryDirectory>(std::move(Stat))));
       continue;
diff --git a/llvm/unittests/Support/VirtualFileSystemTest.cpp b/llvm/unittests/Support/VirtualFileSystemTest.cpp
--- a/llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ b/llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -1279,6 +1279,26 @@
   EXPECT_THAT(Nodes, testing::UnorderedElementsAre("/a", "/a/b", "/c", "/c/d"));
 }
 
+TEST_F(InMemoryFileSystemTest, UniqueID) {
+  ASSERT_TRUE(FS.addFile("/a/b", 0, MemoryBuffer::getMemBuffer("text")));
+  ASSERT_TRUE(FS.addFile("/c/d", 0, MemoryBuffer::getMemBuffer("text")));
+  ASSERT_TRUE(FS.addHardLink("/e/f", "/a/b"));
+
+  EXPECT_EQ(FS.status("/a/b")->getUniqueID(), FS.status("/a/b")->getUniqueID());
+  EXPECT_NE(FS.status("/a/b")->getUniqueID(), FS.status("/c/d")->getUniqueID());
+  EXPECT_EQ(FS.status("/a/b")->getUniqueID(), FS.status("/e/f")->getUniqueID());
+  EXPECT_EQ(FS.status("/a")->getUniqueID(), FS.status("/a")->getUniqueID());
+  EXPECT_NE(FS.status("/a")->getUniqueID(), FS.status("/c")->getUniqueID());
+  EXPECT_NE(FS.status("/a")->getUniqueID(), FS.status("/e")->getUniqueID());
+
+  // Recreating the "same" FS yields the same UniqueIDs.
+  vfs::InMemoryFileSystem FS2;
+  ASSERT_TRUE(FS2.addFile("/a/b", 0, MemoryBuffer::getMemBuffer("text")));
+  EXPECT_EQ(FS.status("/a/b")->getUniqueID(),
+            FS2.status("/a/b")->getUniqueID());
+  EXPECT_EQ(FS.status("/a")->getUniqueID(), FS2.status("/a")->getUniqueID());
+}
+
 // NOTE: in the tests below, we use '//root/' as our root directory, since it is
 // a legal *absolute* path on Windows as well as *nix.
 class VFSFromYAMLTest : public ::testing::Test {