Index: clangd/index/Index.h
===================================================================
--- clangd/index/Index.h
+++ clangd/index/Index.h
@@ -56,14 +56,19 @@
     uint32_t Column : 12; // 0-based
   };
 
-  // The URI of the source file where a symbol occurs.
-  llvm::StringRef FileURI;
-
   /// The symbol range, using half-open range [Start, End).
   Position Start;
   Position End;
 
-  explicit operator bool() const { return !FileURI.empty(); }
+  explicit operator bool() const { return !StringRef(FileURI).empty(); }
+
+  // The URI of the source file where a symbol occurs.
+  // The string must be null-terminated.
+  //
+  // We avoid using llvm::StringRef here to save memory.
+  // WARNING: unless you know what you are doing, it is recommended to use it
+  // via llvm::StringRef.
+  const char *FileURI = "";
 };
 inline bool operator==(const SymbolLocation::Position &L,
                        const SymbolLocation::Position &R) {
@@ -76,12 +81,16 @@
          std::make_tuple(R.line(), R.column());
 }
 inline bool operator==(const SymbolLocation &L, const SymbolLocation &R) {
-  return std::tie(L.FileURI, L.Start, L.End) ==
-         std::tie(R.FileURI, R.Start, R.End);
+  assert(L.FileURI && R.FileURI);
+  int Cmp = std::strcmp(L.FileURI, R.FileURI);
+  return Cmp == 0 && std::tie(L.Start, L.End) == std::tie(R.Start, R.End);
 }
 inline bool operator<(const SymbolLocation &L, const SymbolLocation &R) {
-  return std::tie(L.FileURI, L.Start, L.End) <
-         std::tie(R.FileURI, R.Start, R.End);
+  assert(L.FileURI && R.FileURI);
+  int Cmp = std::strcmp(L.FileURI, R.FileURI);
+  if (Cmp != 0)
+    return Cmp < 0;
+  return std::tie(L.Start, L.End) < std::tie(R.Start, R.End);
 }
 llvm::raw_ostream &operator<<(llvm::raw_ostream &, const SymbolLocation &);
 
@@ -288,12 +297,18 @@
 template <typename Callback> void visitStrings(Symbol &S, const Callback &CB) {
   CB(S.Name);
   CB(S.Scope);
-  CB(S.CanonicalDeclaration.FileURI);
-  CB(S.Definition.FileURI);
   CB(S.Signature);
   CB(S.CompletionSnippetSuffix);
   CB(S.Documentation);
   CB(S.ReturnType);
+  auto RawCharPointerCB = [&CB](const char *&P) {
+    llvm::StringRef S(P);
+    CB(S);
+    P = S.data();
+  };
+  RawCharPointerCB(S.CanonicalDeclaration.FileURI);
+  RawCharPointerCB(S.Definition.FileURI);
+
   for (auto &Include : S.IncludeHeaders)
     CB(Include.IncludeHeader);
 }
Index: clangd/index/Index.cpp
===================================================================
--- clangd/index/Index.cpp
+++ clangd/index/Index.cpp
@@ -165,7 +165,8 @@
 void RefSlab::Builder::insert(const SymbolID &ID, const Ref &S) {
   auto &M = Refs[ID];
   M.push_back(S);
-  M.back().Location.FileURI = UniqueStrings.save(M.back().Location.FileURI);
+  M.back().Location.FileURI =
+      UniqueStrings.save(M.back().Location.FileURI).data();
 }
 
 RefSlab RefSlab::Builder::build() && {
Index: clangd/index/Merge.cpp
===================================================================
--- clangd/index/Merge.cpp
+++ clangd/index/Merge.cpp
@@ -113,8 +113,9 @@
   bool PreferR = R.Definition && !L.Definition;
   // Merge include headers only if both have definitions or both have no
   // definition; otherwise, only accumulate references of common includes.
+  assert(L.Definition.FileURI && R.Definition.FileURI);
   bool MergeIncludes =
-      L.Definition.FileURI.empty() == R.Definition.FileURI.empty();
+      !std::strlen(L.Definition.FileURI) == !std::strlen(R.Definition.FileURI);
   Symbol S = PreferR ? R : L;        // The target symbol we're merging into.
   const Symbol &O = PreferR ? L : R; // The "other" less-preferred symbol.
 
Index: clangd/index/Serialization.cpp
===================================================================
--- clangd/index/Serialization.cpp
+++ clangd/index/Serialization.cpp
@@ -239,7 +239,7 @@
 
 SymbolLocation readLocation(Reader &Data, ArrayRef<StringRef> Strings) {
   SymbolLocation Loc;
-  Loc.FileURI = Data.consumeString(Strings);
+  Loc.FileURI = Data.consumeString(Strings).data();
   for (auto *Endpoint : {&Loc.Start, &Loc.End}) {
     Endpoint->setLine(Data.consumeVar());
     Endpoint->setColumn(Data.consumeVar());
@@ -409,8 +409,11 @@
   if (Data.Refs) {
     for (const auto &Sym : *Data.Refs) {
       Refs.emplace_back(Sym);
-      for (auto &Ref : Refs.back().second)
-        Strings.intern(Ref.Location.FileURI);
+      for (auto &Ref : Refs.back().second) {
+        StringRef File = Ref.Location.FileURI;
+        Strings.intern(File);
+        Ref.Location.FileURI = File.data();
+      }
     }
   }
 
Index: clangd/index/SymbolCollector.cpp
===================================================================
--- clangd/index/SymbolCollector.cpp
+++ clangd/index/SymbolCollector.cpp
@@ -225,7 +225,7 @@
     return None;
   FileURIStorage = std::move(*U);
   SymbolLocation Result;
-  Result.FileURI = FileURIStorage;
+  Result.FileURI = FileURIStorage.c_str();
   auto Range = getTokenRange(TokLoc, SM, LangOpts);
   Result.Start = Range.first;
   Result.End = Range.second;
@@ -524,7 +524,7 @@
             Ref R;
             R.Location.Start = Range.first;
             R.Location.End = Range.second;
-            R.Location.FileURI = *FileURI;
+            R.Location.FileURI = FileURI->c_str();
             R.Kind = toRefKind(LocAndRole.second);
             Refs.insert(*ID, R);
           }
Index: clangd/index/YAMLSerialization.cpp
===================================================================
--- clangd/index/YAMLSerialization.cpp
+++ clangd/index/YAMLSerialization.cpp
@@ -20,8 +20,10 @@
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Allocator.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/StringSaver.h"
 #include "llvm/Support/YAMLTraits.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cstdint>
@@ -128,9 +130,26 @@
   YPosition P;
 };
 
+struct NormalizedFileURI {
+  NormalizedFileURI(IO &) {}
+  NormalizedFileURI(IO &, const char *FileURI) { URI = FileURI; }
+
+  const char *denormalize(IO &IO) {
+    assert(IO.getContext() &&
+           "Expecting an UniqueStringSaver to allocate data");
+    return static_cast<llvm::UniqueStringSaver *>(IO.getContext())
+        ->save(URI)
+        .data();
+  }
+
+  std::string URI;
+};
+
 template <> struct MappingTraits<SymbolLocation> {
   static void mapping(IO &IO, SymbolLocation &Value) {
-    IO.mapRequired("FileURI", Value.FileURI);
+    MappingNormalization<NormalizedFileURI, const char *> NFile(IO,
+                                                                Value.FileURI);
+    IO.mapRequired("FileURI", NFile->URI);
     MappingNormalization<NormalizedPosition, SymbolLocation::Position> NStart(
         IO, Value.Start);
     IO.mapRequired("Start", NStart->P);
@@ -292,7 +311,9 @@
 Expected<IndexFileIn> readYAML(StringRef Data) {
   SymbolSlab::Builder Symbols;
   RefSlab::Builder Refs;
-  yaml::Input Yin(Data);
+  BumpPtrAllocator Arena; // store the underlying data of Position::FileURI.
+  UniqueStringSaver Strings(Arena);
+  yaml::Input Yin(Data, &Strings);
   do {
     VariantEntry Variant;
     Yin >> Variant;
Index: clangd/index/dex/Dex.cpp
===================================================================
--- clangd/index/dex/Dex.cpp
+++ clangd/index/dex/Dex.cpp
@@ -52,7 +52,7 @@
   std::vector<Token> Result = generateIdentifierTrigrams(Sym.Name);
   Result.emplace_back(Token::Kind::Scope, Sym.Scope);
   // Skip token generation for symbols with unknown declaration location.
-  if (!Sym.CanonicalDeclaration.FileURI.empty())
+  if (!StringRef(Sym.CanonicalDeclaration.FileURI).empty())
     for (const auto &ProximityURI :
          generateProximityURIs(Sym.CanonicalDeclaration.FileURI))
       Result.emplace_back(Token::Kind::ProximityURI, ProximityURI);
Index: unittests/clangd/BackgroundIndexTests.cpp
===================================================================
--- unittests/clangd/BackgroundIndexTests.cpp
+++ unittests/clangd/BackgroundIndexTests.cpp
@@ -9,14 +9,16 @@
 using testing::Not;
 using testing::UnorderedElementsAre;
 
+using namespace llvm;
 namespace clang {
 namespace clangd {
 
 MATCHER_P(Named, N, "") { return arg.Name == N; }
-MATCHER(Declared, "") { return !arg.CanonicalDeclaration.FileURI.empty(); }
-MATCHER(Defined, "") { return !arg.Definition.FileURI.empty(); }
-
-MATCHER_P(FileURI, F, "") { return arg.Location.FileURI == F; }
+MATCHER(Declared, "") {
+  return !StringRef(arg.CanonicalDeclaration.FileURI).empty();
+}
+MATCHER(Defined, "") { return !StringRef(arg.Definition.FileURI).empty(); }
+MATCHER_P(FileURI, F, "") { return StringRef(arg.Location.FileURI) == F; }
 testing::Matcher<const RefSlab &>
 RefsAre(std::vector<testing::Matcher<Ref>> Matchers) {
   return ElementsAre(testing::Pair(_, UnorderedElementsAreArray(Matchers)));
Index: unittests/clangd/CodeCompleteTests.cpp
===================================================================
--- unittests/clangd/CodeCompleteTests.cpp
+++ unittests/clangd/CodeCompleteTests.cpp
@@ -579,7 +579,7 @@
   ClangdServer Server(CDB, FS, DiagConsumer, ClangdServer::optsForTest());
   auto BarURI = URI::createFile(BarHeader).toString();
   Symbol Sym = cls("ns::X");
-  Sym.CanonicalDeclaration.FileURI = BarURI;
+  Sym.CanonicalDeclaration.FileURI = BarURI.c_str();
   Sym.IncludeHeaders.emplace_back(BarURI, 1);
   // Shoten include path based on search dirctory and insert.
   auto Results = completions(Server,
@@ -610,8 +610,8 @@
   Symbol SymY = cls("ns::Y");
   std::string BarHeader = testPath("bar.h");
   auto BarURI = URI::createFile(BarHeader).toString();
-  SymX.CanonicalDeclaration.FileURI = BarURI;
-  SymY.CanonicalDeclaration.FileURI = BarURI;
+  SymX.CanonicalDeclaration.FileURI = BarURI.c_str();
+  SymY.CanonicalDeclaration.FileURI = BarURI.c_str();
   SymX.IncludeHeaders.emplace_back("<bar>", 1);
   SymY.IncludeHeaders.emplace_back("<bar>", 1);
   // Shoten include path based on search dirctory and insert.
@@ -1248,7 +1248,7 @@
 
   // Differences in header-to-insert suppress bundling.
   std::string DeclFile = URI::createFile(testPath("foo")).toString();
-  NoArgsGFunc.CanonicalDeclaration.FileURI = DeclFile;
+  NoArgsGFunc.CanonicalDeclaration.FileURI = DeclFile.c_str();
   NoArgsGFunc.IncludeHeaders.emplace_back("<foo>", 1);
   EXPECT_THAT(
       completions(Context + "int y = GFunc^", {NoArgsGFunc}, Opts).Completions,
@@ -1955,7 +1955,7 @@
 TEST(CompletionTest, InsertTheMostPopularHeader) {
   std::string DeclFile = URI::createFile(testPath("foo")).toString();
   Symbol sym = func("Func");
-  sym.CanonicalDeclaration.FileURI = DeclFile;
+  sym.CanonicalDeclaration.FileURI = DeclFile.c_str();
   sym.IncludeHeaders.emplace_back("\"foo.h\"", 2);
   sym.IncludeHeaders.emplace_back("\"bar.h\"", 1000);
 
@@ -1977,7 +1977,7 @@
 
   std::string DeclFile = URI::createFile(testPath("foo")).toString();
   Symbol sym = func("Func");
-  sym.CanonicalDeclaration.FileURI = DeclFile;
+  sym.CanonicalDeclaration.FileURI = DeclFile.c_str();
   sym.IncludeHeaders.emplace_back("\"foo.h\"", 2);
   sym.IncludeHeaders.emplace_back("\"bar.h\"", 1000);
 
Index: unittests/clangd/DexTests.cpp
===================================================================
--- unittests/clangd/DexTests.cpp
+++ unittests/clangd/DexTests.cpp
@@ -666,7 +666,7 @@
 
 TEST(DexTests, Refs) {
   DenseMap<SymbolID, std::vector<Ref>> Refs;
-  auto AddRef = [&](const Symbol& Sym, StringRef Filename, RefKind Kind) {
+  auto AddRef = [&](const Symbol &Sym, const char *Filename, RefKind Kind) {
     auto& SymbolRefs = Refs[Sym.ID];
     SymbolRefs.emplace_back();
     SymbolRefs.back().Kind = Kind;
Index: unittests/clangd/FileIndexTests.cpp
===================================================================
--- unittests/clangd/FileIndexTests.cpp
+++ unittests/clangd/FileIndexTests.cpp
@@ -31,6 +31,7 @@
 using testing::IsEmpty;
 using testing::Pair;
 using testing::UnorderedElementsAre;
+using namespace llvm;
 
 MATCHER_P(RefRange, Range, "") {
   return std::make_tuple(arg.Location.Start.line(), arg.Location.Start.column(),
@@ -38,12 +39,13 @@
          std::make_tuple(Range.start.line, Range.start.character,
                          Range.end.line, Range.end.character);
 }
-MATCHER_P(FileURI, F, "") { return arg.Location.FileURI == F; }
-MATCHER_P(DeclURI, U, "") { return arg.CanonicalDeclaration.FileURI == U; }
-MATCHER_P(DefURI, U, "") { return arg.Definition.FileURI == U; }
+MATCHER_P(FileURI, F, "") { return StringRef(arg.Location.FileURI) == F; }
+MATCHER_P(DeclURI, U, "") {
+  return StringRef(arg.CanonicalDeclaration.FileURI) == U;
+}
+MATCHER_P(DefURI, U, "") { return StringRef(arg.Definition.FileURI) == U; }
 MATCHER_P(QName, N, "") { return (arg.Scope + arg.Name).str() == N; }
 
-using namespace llvm;
 namespace clang {
 namespace clangd {
 namespace {
@@ -66,7 +68,7 @@
   return llvm::make_unique<SymbolSlab>(std::move(Slab).build());
 }
 
-std::unique_ptr<RefSlab> refSlab(const SymbolID &ID, StringRef Path) {
+std::unique_ptr<RefSlab> refSlab(const SymbolID &ID, const char *Path) {
   RefSlab::Builder Slab;
   Ref R;
   R.Location.FileURI = Path;
Index: unittests/clangd/IndexTests.cpp
===================================================================
--- unittests/clangd/IndexTests.cpp
+++ unittests/clangd/IndexTests.cpp
@@ -36,7 +36,7 @@
          std::make_tuple(Range.start.line, Range.start.character,
                          Range.end.line, Range.end.character);
 }
-MATCHER_P(FileURI, F, "") { return arg.Location.FileURI == F; }
+MATCHER_P(FileURI, F, "") { return StringRef(arg.Location.FileURI) == F; }
 
 TEST(SymbolLocation, Position) {
   using Position = SymbolLocation::Position;
@@ -222,7 +222,7 @@
 
   Symbol M = mergeSymbol(L, R);
   EXPECT_EQ(M.Name, "Foo");
-  EXPECT_EQ(M.CanonicalDeclaration.FileURI, "file:///left.h");
+  EXPECT_EQ(StringRef(M.CanonicalDeclaration.FileURI), "file:///left.h");
   EXPECT_EQ(M.References, 3u);
   EXPECT_EQ(M.Signature, "()");
   EXPECT_EQ(M.CompletionSnippetSuffix, "{$1:0}");
@@ -241,14 +241,14 @@
   R.Name = "right";
 
   Symbol M = mergeSymbol(L, R);
-  EXPECT_EQ(M.CanonicalDeclaration.FileURI, "file:/left.h");
-  EXPECT_EQ(M.Definition.FileURI, "");
+  EXPECT_EQ(StringRef(M.CanonicalDeclaration.FileURI), "file:/left.h");
+  EXPECT_EQ(StringRef(M.Definition.FileURI), "");
   EXPECT_EQ(M.Name, "left");
 
   R.Definition.FileURI = "file:/right.cpp"; // Now right will be favored.
   M = mergeSymbol(L, R);
-  EXPECT_EQ(M.CanonicalDeclaration.FileURI, "file:/right.h");
-  EXPECT_EQ(M.Definition.FileURI, "file:/right.cpp");
+  EXPECT_EQ(StringRef(M.CanonicalDeclaration.FileURI), "file:/right.h");
+  EXPECT_EQ(StringRef(M.Definition.FileURI), "file:/right.cpp");
   EXPECT_EQ(M.Name, "right");
 }
 
Index: unittests/clangd/SerializationTests.cpp
===================================================================
--- unittests/clangd/SerializationTests.cpp
+++ unittests/clangd/SerializationTests.cpp
@@ -111,7 +111,7 @@
   EXPECT_EQ(Sym1.Signature, "");
   EXPECT_EQ(Sym1.Documentation, "Foo doc");
   EXPECT_EQ(Sym1.ReturnType, "int");
-  EXPECT_EQ(Sym1.CanonicalDeclaration.FileURI, "file:///path/foo.h");
+  EXPECT_EQ(StringRef(Sym1.CanonicalDeclaration.FileURI), "file:///path/foo.h");
   EXPECT_EQ(Sym1.Origin, SymbolOrigin::Static);
   EXPECT_TRUE(Sym1.Flags & Symbol::IndexedForCodeCompletion);
   EXPECT_FALSE(Sym1.Flags & Symbol::Deprecated);
@@ -122,7 +122,8 @@
   EXPECT_THAT(Sym2, QName("clang::Foo2"));
   EXPECT_EQ(Sym2.Signature, "-sig");
   EXPECT_EQ(Sym2.ReturnType, "");
-  EXPECT_EQ(Sym2.CanonicalDeclaration.FileURI, "file:///path/bar.h");
+  EXPECT_EQ(llvm::StringRef(Sym2.CanonicalDeclaration.FileURI),
+            "file:///path/bar.h");
   EXPECT_FALSE(Sym2.Flags & Symbol::IndexedForCodeCompletion);
   EXPECT_TRUE(Sym2.Flags & Symbol::Deprecated);
 
@@ -134,7 +135,7 @@
                testing::SizeIs(1))));
   auto Ref1 = ParsedYAML->Refs->begin()->second.front();
   EXPECT_EQ(Ref1.Kind, RefKind::Reference);
-  EXPECT_EQ(Ref1.Location.FileURI, "file:///path/foo.cc");
+  EXPECT_EQ(StringRef(Ref1.Location.FileURI), "file:///path/foo.cc");
 }
 
 std::vector<std::string> YAMLFromSymbols(const SymbolSlab &Slab) {
Index: unittests/clangd/SymbolCollectorTests.cpp
===================================================================
--- unittests/clangd/SymbolCollectorTests.cpp
+++ unittests/clangd/SymbolCollectorTests.cpp
@@ -53,8 +53,10 @@
   return (arg.Name + arg.CompletionSnippetSuffix).str() == S;
 }
 MATCHER_P(QName, Name, "") { return (arg.Scope + arg.Name).str() == Name; }
-MATCHER_P(DeclURI, P, "") { return arg.CanonicalDeclaration.FileURI == P; }
-MATCHER_P(DefURI, P, "") { return arg.Definition.FileURI == P; }
+MATCHER_P(DeclURI, P, "") {
+  return StringRef(arg.CanonicalDeclaration.FileURI) == P;
+}
+MATCHER_P(DefURI, P, "") { return StringRef(arg.Definition.FileURI) == P; }
 MATCHER_P(IncludeHeader, P, "") {
   return (arg.IncludeHeaders.size() == 1) &&
          (arg.IncludeHeaders.begin()->IncludeHeader == P);