[clangd] Use Builder for symbol slabs, and use sorted-vector for storage

Summary:
This improves a few things:
 - the insert -> freeze -> read sequence is now enforced/communicated by the
   type system
 - SymbolSlab::const_iterator iterates over symbols, not over id-symbol pairs
 - we avoid permanently storing a second copy of the IDs, and the
   string map's hashtable

The slab size is now down to 21.8MB for the LLVM project.
Of this only 2.7MB is strings, the rest is #symbols * `sizeof(Symbol)`.
`sizeof(Symbol)` is currently 96, which seems too big - I think
SymbolInfo isn't efficiently packed. That's a topic for another patch!

Also added simple API to see the memory usage/#symbols of a slab, since
it seems likely we will continue to care about this.

Reviewers: ilya-biryukov

Subscribers: klimek, mgrang, cfe-commits

Differential Revision: https://reviews.llvm.org/D41506

llvm-svn: 321412

11 files changed, 177 insertions, 119 deletions

diff --git a/clang-tools-extra/clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp b/clang-tools-extra/clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp
index 5715c9dce25..30a92fea5ae 100644
--- a/clang-tools-extra/clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp
+++ b/clang-tools-extra/clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp
@@ -80,16 +80,16 @@ int main(int argc, const char **argv) {
 
   // Found compilation database, we iterate all TUs from database to get all
   // symbols, and then merge them into a single SymbolSlab.
-  SymbolSlab GlobalSymbols;
+  SymbolSlab::Builder GlobalSymbols;
   std::mutex SymbolMutex;
   auto AddSymbols = [&](const SymbolSlab& NewSymbols) {
     // Synchronize set accesses.
     std::unique_lock<std::mutex> LockGuard(SymbolMutex);
-    for (auto It : NewSymbols) {
+    for (auto Sym : NewSymbols) {
       // FIXME: Better handling the overlap symbols, currently we overwrite it
       // with the latest one, but we always want to good declarations (class
       // definitions, instead of forward declarations).
-      GlobalSymbols.insert(It.second);
+      GlobalSymbols.insert(Sym);
     }
   };
 
@@ -105,6 +105,6 @@ int main(int argc, const char **argv) {
     }
   }
 
-  llvm::outs() << SymbolToYAML(GlobalSymbols);
+  llvm::outs() << SymbolToYAML(std::move(GlobalSymbols).build());
   return 0;
 }
diff --git a/clang-tools-extra/clangd/index/FileIndex.cpp b/clang-tools-extra/clangd/index/FileIndex.cpp
index 5f4e157bd35..ed2500eed9f 100644
--- a/clang-tools-extra/clangd/index/FileIndex.cpp
+++ b/clang-tools-extra/clangd/index/FileIndex.cpp
@@ -54,7 +54,7 @@ std::shared_ptr<std::vector<const Symbol *>> FileSymbols::allSymbols() {
     for (const auto &FileAndSlab : FileToSlabs) {
       Snap->KeepAlive.push_back(FileAndSlab.second);
       for (const auto &Iter : *FileAndSlab.second)
-        Snap->Pointers.push_back(&Iter.second);
+        Snap->Pointers.push_back(&Iter);
     }
   }
   auto *Pointers = &Snap->Pointers;
diff --git a/clang-tools-extra/clangd/index/Index.cpp b/clang-tools-extra/clangd/index/Index.cpp
index cac3ceab6f1..95348582739 100644
--- a/clang-tools-extra/clangd/index/Index.cpp
+++ b/clang-tools-extra/clangd/index/Index.cpp
@@ -13,16 +13,17 @@
 
 namespace clang {
 namespace clangd {
+using namespace llvm;
 
-SymbolID::SymbolID(llvm::StringRef USR)
-    : HashValue(llvm::SHA1::hash(arrayRefFromStringRef(USR))) {}
+SymbolID::SymbolID(StringRef USR)
+    : HashValue(SHA1::hash(arrayRefFromStringRef(USR))) {}
 
-llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const SymbolID &ID) {
-  OS << toHex(llvm::toStringRef(ID.HashValue));
+raw_ostream &operator<<(raw_ostream &OS, const SymbolID &ID) {
+  OS << toHex(toStringRef(ID.HashValue));
   return OS;
 }
 
-void operator>>(llvm::StringRef Str, SymbolID &ID) {
+void operator>>(StringRef Str, SymbolID &ID) {
   std::string HexString = fromHex(Str);
   assert(HexString.size() == ID.HashValue.size());
   std::copy(HexString.begin(), HexString.end(), ID.HashValue.begin());
@@ -32,23 +33,58 @@ SymbolSlab::const_iterator SymbolSlab::begin() const { return Symbols.begin(); }
 
 SymbolSlab::const_iterator SymbolSlab::end() const { return Symbols.end(); }
 
-SymbolSlab::const_iterator SymbolSlab::find(const SymbolID &SymID) const {
-  return Symbols.find(SymID);
+SymbolSlab::const_iterator SymbolSlab::find(const SymbolID &ID) const {
+  auto It = std::lower_bound(Symbols.begin(), Symbols.end(), ID,
+                             [](const Symbol &S, const SymbolID &I) {
+                               return S.ID < I;
+                             });
+  if (It != Symbols.end() && It->ID == ID)
+    return It;
+  return Symbols.end();
 }
 
-void SymbolSlab::freeze() { Frozen = true; }
+// Copy the underlying data of the symbol into the owned arena.
+static void own(Symbol &S, DenseSet<StringRef> &Strings,
+                BumpPtrAllocator &Arena) {
+  // Intern replaces V with a reference to the same string owned by the arena.
+  auto Intern = [&](StringRef &V) {
+    auto R = Strings.insert(V);
+    if (R.second) { // New entry added to the table, copy the string.
+      char *Data = Arena.Allocate<char>(V.size());
+      memcpy(Data, V.data(), V.size());
+      *R.first = StringRef(Data, V.size());
+    }
+    V = *R.first;
+  };
 
-void SymbolSlab::insert(const Symbol &S) {
-  assert(!Frozen && "Can't insert a symbol after the slab has been frozen!");
-  auto ItInserted = Symbols.try_emplace(S.ID, S);
-  if (!ItInserted.second)
-    return;
-  auto &Sym = ItInserted.first->second;
+  // We need to copy every StringRef field onto the arena.
+  Intern(S.Name);
+  Intern(S.Scope);
+  Intern(S.CanonicalDeclaration.FilePath);
+}
+
+void SymbolSlab::Builder::insert(const Symbol &S) {
+  auto R = SymbolIndex.try_emplace(S.ID, Symbols.size());
+  if (R.second) {
+    Symbols.push_back(S);
+    own(Symbols.back(), Strings, Arena);
+  } else {
+    auto &Copy = Symbols[R.first->second] = S;
+    own(Copy, Strings, Arena);
+  }
+}
 
-  // We inserted a new symbol, so copy the underlying data.
-  intern(Sym.Name);
-  intern(Sym.Scope);
-  intern(Sym.CanonicalDeclaration.FilePath);
+SymbolSlab SymbolSlab::Builder::build() && {
+  Symbols = {Symbols.begin(), Symbols.end()}; // Force shrink-to-fit.
+  // Sort symbols so the slab can binary search over them.
+  std::sort(Symbols.begin(), Symbols.end(),
+            [](const Symbol &L, const Symbol &R) { return L.ID < R.ID; });
+  // We may have unused strings from overwritten symbols. Build a new arena.
+  BumpPtrAllocator NewArena;
+  DenseSet<StringRef> Strings;
+  for (auto &S : Symbols)
+    own(S, Strings, NewArena);
+  return SymbolSlab(std::move(NewArena), std::move(Symbols));
 }
 
 } // namespace clangd
diff --git a/clang-tools-extra/clangd/index/Index.h b/clang-tools-extra/clangd/index/Index.h
index b5044634d3c..c69a6e34f6c 100644
--- a/clang-tools-extra/clangd/index/Index.h
+++ b/clang-tools-extra/clangd/index/Index.h
@@ -13,9 +13,9 @@
 #include "../Context.h"
 #include "clang/Index/IndexSymbol.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/StringExtras.h"
-#include "llvm/ADT/StringSet.h"
 #include <array>
 #include <string>
 
@@ -49,6 +49,9 @@ public:
   bool operator==(const SymbolID &Sym) const {
     return HashValue == Sym.HashValue;
   }
+  bool operator<(const SymbolID &Sym) const {
+    return HashValue < Sym.HashValue;
+  }
 
 private:
   friend llvm::hash_code hash_value(const SymbolID &ID) {
@@ -72,13 +75,38 @@ llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const SymbolID &ID);
 // "<<" operator.
 void operator>>(llvm::StringRef HexStr, SymbolID &ID);
 
+} // namespace clangd
+} // namespace clang
+namespace llvm {
+// Support SymbolIDs as DenseMap keys.
+template <> struct DenseMapInfo<clang::clangd::SymbolID> {
+  static inline clang::clangd::SymbolID getEmptyKey() {
+    static clang::clangd::SymbolID EmptyKey("EMPTYKEY");
+    return EmptyKey;
+  }
+  static inline clang::clangd::SymbolID getTombstoneKey() {
+    static clang::clangd::SymbolID TombstoneKey("TOMBSTONEKEY");
+    return TombstoneKey;
+  }
+  static unsigned getHashValue(const clang::clangd::SymbolID &Sym) {
+    return hash_value(Sym);
+  }
+  static bool isEqual(const clang::clangd::SymbolID &LHS,
+                      const clang::clangd::SymbolID &RHS) {
+    return LHS == RHS;
+  }
+};
+} // namespace llvm
+namespace clang {
+namespace clangd {
+
 // The class presents a C++ symbol, e.g. class, function.
 //
 // WARNING: Symbols do not own much of their underlying data - typically strings
 // are owned by a SymbolSlab. They should be treated as non-owning references.
 // Copies are shallow.
 // When adding new unowned data fields to Symbol, remember to update
-// SymbolSlab::insert to copy them to the slab's storage.
+// SymbolSlab::Builder in Index.cpp to copy them to the slab's storage.
 struct Symbol {
   // The ID of the symbol.
   SymbolID ID;
@@ -104,11 +132,11 @@ struct Symbol {
   // FIXME: add code completion information.
 };
 
-// A symbol container that stores a set of symbols. The container will maintain
-// the lifetime of the symbols.
+// An immutable symbol container that stores a set of symbols.
+// The container will maintain the lifetime of the symbols.
 class SymbolSlab {
 public:
-  using const_iterator = llvm::DenseMap<SymbolID, Symbol>::const_iterator;
+  using const_iterator = std::vector<Symbol>::const_iterator;
 
   SymbolSlab() = default;
 
@@ -116,26 +144,45 @@ public:
   const_iterator end() const;
   const_iterator find(const SymbolID &SymID) const;
 
-  // Once called, no more symbols would be added to the SymbolSlab. This
-  // operation is irreversible.
-  void freeze();
+  size_t size() const { return Symbols.size(); }
+  // Estimates the total memory usage.
+  size_t bytes() const {
+    return sizeof(*this) + Arena.getTotalMemory() +
+           Symbols.capacity() * sizeof(Symbol);
+  }
 
-  // Adds the symbol to this slab.
-  // This is a deep copy: underlying strings will be owned by the slab.
-  void insert(const Symbol& S);
+  // SymbolSlab::Builder is a mutable container that can 'freeze' to SymbolSlab.
+  // The frozen SymbolSlab will use less memory.
+  class Builder {
+   public:
+     // Adds a symbol, overwriting any existing one with the same ID.
+     // This is a deep copy: underlying strings will be owned by the slab.
+     void insert(const Symbol& S);
+
+     // Returns the symbol with an ID, if it exists. Valid until next insert().
+     const Symbol* find(const SymbolID &ID) {
+       auto I = SymbolIndex.find(ID);
+       return I == SymbolIndex.end() ? nullptr : &Symbols[I->second];
+     }
+
+     // Consumes the builder to finalize the slab.
+     SymbolSlab build() &&;
+
+   private:
+     llvm::BumpPtrAllocator Arena;
+     // Intern table for strings. Contents are on the arena.
+     llvm::DenseSet<llvm::StringRef> Strings;
+     std::vector<Symbol> Symbols;
+     // Values are indices into Symbols vector.
+     llvm::DenseMap<SymbolID, size_t> SymbolIndex;
+  };
 
 private:
-  // Replaces S with a reference to the same string, owned by this slab.
-  void intern(llvm::StringRef &S) {
-    S = S.empty() ? llvm::StringRef() : Strings.insert(S).first->getKey();
-  }
-
-  bool Frozen = false;
+  SymbolSlab(llvm::BumpPtrAllocator Arena, std::vector<Symbol> Symbols)
+      : Arena(std::move(Arena)), Symbols(std::move(Symbols)) {}
 
-  // Intern table for strings. Not StringPool as we don't refcount, just insert.
-  // We use BumpPtrAllocator to avoid lots of tiny allocations for nodes.
-  llvm::StringSet<llvm::BumpPtrAllocator> Strings;
-  llvm::DenseMap<SymbolID, Symbol> Symbols;
+  llvm::BumpPtrAllocator Arena; // Owns Symbol data that the Symbols do not.
+  std::vector<Symbol> Symbols;  // Sorted by SymbolID to allow lookup.
 };
 
 struct FuzzyFindRequest {
@@ -176,27 +223,4 @@ public:
 
 } // namespace clangd
 } // namespace clang
-
-namespace llvm {
-
-template <> struct DenseMapInfo<clang::clangd::SymbolID> {
-  static inline clang::clangd::SymbolID getEmptyKey() {
-    static clang::clangd::SymbolID EmptyKey("EMPTYKEY");
-    return EmptyKey;
-  }
-  static inline clang::clangd::SymbolID getTombstoneKey() {
-    static clang::clangd::SymbolID TombstoneKey("TOMBSTONEKEY");
-    return TombstoneKey;
-  }
-  static unsigned getHashValue(const clang::clangd::SymbolID &Sym) {
-    return hash_value(Sym);
-  }
-  static bool isEqual(const clang::clangd::SymbolID &LHS,
-                      const clang::clangd::SymbolID &RHS) {
-    return LHS == RHS;
-  }
-};
-
-} // namespace llvm
-
 #endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_INDEX_INDEX_H
diff --git a/clang-tools-extra/clangd/index/SymbolCollector.cpp b/clang-tools-extra/clangd/index/SymbolCollector.cpp
index c53522a40f2..1442f9364a6 100644
--- a/clang-tools-extra/clangd/index/SymbolCollector.cpp
+++ b/clang-tools-extra/clangd/index/SymbolCollector.cpp
@@ -91,7 +91,7 @@ bool SymbolCollector::handleDeclOccurence(
 
     std::string USR(Buff.data(), Buff.size());
     auto ID = SymbolID(USR);
-    if (Symbols.find(ID) != Symbols.end())
+    if (Symbols.find(ID) != nullptr)
       return true;
 
     auto &SM = ND->getASTContext().getSourceManager();
@@ -114,7 +114,5 @@ bool SymbolCollector::handleDeclOccurence(
   return true;
 }
 
-void SymbolCollector::finish() { Symbols.freeze(); }
-
 } // namespace clangd
 } // namespace clang
diff --git a/clang-tools-extra/clangd/index/SymbolCollector.h b/clang-tools-extra/clangd/index/SymbolCollector.h
index dfd66b94afb..55840b59fd2 100644
--- a/clang-tools-extra/clangd/index/SymbolCollector.h
+++ b/clang-tools-extra/clangd/index/SymbolCollector.h
@@ -29,13 +29,11 @@ public:
                       unsigned Offset,
                       index::IndexDataConsumer::ASTNodeInfo ASTNode) override;
 
-  void finish() override;
-
-  SymbolSlab takeSymbols() { return std::move(Symbols); }
+  SymbolSlab takeSymbols() { return std::move(Symbols).build(); }
 
 private:
   // All Symbols collected from the AST.
-  SymbolSlab Symbols;
+  SymbolSlab::Builder Symbols;
 };
 
 } // namespace clangd
diff --git a/clang-tools-extra/clangd/index/SymbolYAML.cpp b/clang-tools-extra/clangd/index/SymbolYAML.cpp
index beb0ab55100..dc6753f7116 100644
--- a/clang-tools-extra/clangd/index/SymbolYAML.cpp
+++ b/clang-tools-extra/clangd/index/SymbolYAML.cpp
@@ -127,20 +127,18 @@ SymbolSlab SymbolFromYAML(llvm::StringRef YAMLContent) {
   std::vector<Symbol> S;
   llvm::yaml::Input Yin(YAMLContent);
   Yin >> S;
-  SymbolSlab Syms;
+  SymbolSlab::Builder Syms;
   for (auto& Sym : S)
-    Syms.insert(std::move(Sym));
-  return Syms;
+    Syms.insert(Sym);
+  return std::move(Syms).build();
 }
 
 std::string SymbolToYAML(const SymbolSlab& Symbols) {
   std::string Str;
   llvm::raw_string_ostream OS(Str);
   llvm::yaml::Output Yout(OS);
-  for (auto &Pair : Symbols) {
-    Symbol MutableSymbol = Pair.second;
-    Yout<< MutableSymbol;
-  }
+  for (Symbol S : Symbols) // copy: Yout<< requires mutability.
+    Yout<< S;
   return OS.str();
 }
 
diff --git a/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp b/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
index db998b0805a..e3042c3d827 100644
--- a/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
+++ b/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
@@ -449,6 +449,7 @@ std::unique_ptr<SymbolIndex> simpleIndexFromSymbols(
     std::vector<const Symbol *> Pointers;
   };
   auto Snap = std::make_shared<Snapshot>();
+  SymbolSlab::Builder Slab;
   for (const auto &Pair : Symbols) {
     Symbol Sym;
     Sym.ID = SymbolID(Pair.first);
@@ -462,10 +463,11 @@ std::unique_ptr<SymbolIndex> simpleIndexFromSymbols(
       Sym.Scope = QName.substr(0, Pos);
     }
     Sym.SymInfo.Kind = Pair.second;
-    Snap->Slab.insert(std::move(Sym));
+    Slab.insert(Sym);
   }
+  Snap->Slab = std::move(Slab).build();
   for (auto &Iter : Snap->Slab)
-    Snap->Pointers.push_back(&Iter.second);
+    Snap->Pointers.push_back(&Iter);
   auto S = std::shared_ptr<std::vector<const Symbol *>>(std::move(Snap),
                                                         &Snap->Pointers);
   I->build(std::move(S));
diff --git a/clang-tools-extra/unittests/clangd/FileIndexTests.cpp b/clang-tools-extra/unittests/clangd/FileIndexTests.cpp
index c7bce7b5e79..d0c32d17234 100644
--- a/clang-tools-extra/unittests/clangd/FileIndexTests.cpp
+++ b/clang-tools-extra/unittests/clangd/FileIndexTests.cpp
@@ -28,9 +28,11 @@ Symbol symbol(llvm::StringRef ID) {
   return Sym;
 }
 
-void addNumSymbolsToSlab(int Begin, int End, SymbolSlab *Slab) {
+std::unique_ptr<SymbolSlab> numSlab(int Begin, int End) {
+  SymbolSlab::Builder Slab;
   for (int i = Begin; i <= End; i++)
-    Slab->insert(symbol(std::to_string(i)));
+    Slab.insert(symbol(std::to_string(i)));
+  return llvm::make_unique<SymbolSlab>(std::move(Slab).build());
 }
 
 std::vector<std::string>
@@ -45,28 +47,15 @@ TEST(FileSymbolsTest, UpdateAndGet) {
   FileSymbols FS;
   EXPECT_THAT(getSymbolNames(*FS.allSymbols()), UnorderedElementsAre());
 
-  auto Slab = llvm::make_unique<SymbolSlab>();
-  addNumSymbolsToSlab(1, 3, Slab.get());
-
-  FS.update("f1", std::move(Slab));
-
+  FS.update("f1", numSlab(1, 3));
   EXPECT_THAT(getSymbolNames(*FS.allSymbols()),
               UnorderedElementsAre("1", "2", "3"));
 }
 
 TEST(FileSymbolsTest, Overlap) {
   FileSymbols FS;
-
-  auto Slab = llvm::make_unique<SymbolSlab>();
-  addNumSymbolsToSlab(1, 3, Slab.get());
-
-  FS.update("f1", std::move(Slab));
-
-  Slab = llvm::make_unique<SymbolSlab>();
-  addNumSymbolsToSlab(3, 5, Slab.get());
-
-  FS.update("f2", std::move(Slab));
-
+  FS.update("f1", numSlab(1, 3));
+  FS.update("f2", numSlab(3, 5));
   EXPECT_THAT(getSymbolNames(*FS.allSymbols()),
               UnorderedElementsAre("1", "2", "3", "3", "4", "5"));
 }
@@ -74,17 +63,13 @@ TEST(FileSymbolsTest, Overlap) {
 TEST(FileSymbolsTest, SnapshotAliveAfterRemove) {
   FileSymbols FS;
 
-  auto Slab = llvm::make_unique<SymbolSlab>();
-  addNumSymbolsToSlab(1, 3, Slab.get());
-
-  FS.update("f1", std::move(Slab));
+  FS.update("f1", numSlab(1, 3));
 
   auto Symbols = FS.allSymbols();
   EXPECT_THAT(getSymbolNames(*Symbols), UnorderedElementsAre("1", "2", "3"));
 
   FS.update("f1", nullptr);
   EXPECT_THAT(getSymbolNames(*FS.allSymbols()), UnorderedElementsAre());
-
   EXPECT_THAT(getSymbolNames(*Symbols), UnorderedElementsAre("1", "2", "3"));
 }
 
diff --git a/clang-tools-extra/unittests/clangd/IndexTests.cpp b/clang-tools-extra/unittests/clangd/IndexTests.cpp
index 0a4f31a7f1b..fac2895ec63 100644
--- a/clang-tools-extra/unittests/clangd/IndexTests.cpp
+++ b/clang-tools-extra/unittests/clangd/IndexTests.cpp
@@ -13,10 +13,10 @@
 #include "gtest/gtest.h"
 
 using testing::UnorderedElementsAre;
+using testing::Pointee;
 
 namespace clang {
 namespace clangd {
-
 namespace {
 
 Symbol symbol(llvm::StringRef QName) {
@@ -33,6 +33,24 @@ Symbol symbol(llvm::StringRef QName) {
   return Sym;
 }
 
+MATCHER_P(Named, N, "") { return arg.Name == N; }
+
+TEST(SymbolSlab, FindAndIterate) {
+  SymbolSlab::Builder B;
+  B.insert(symbol("Z"));
+  B.insert(symbol("Y"));
+  B.insert(symbol("X"));
+  EXPECT_EQ(nullptr, B.find(SymbolID("W")));
+  for (const char *Sym : {"X", "Y", "Z"})
+    EXPECT_THAT(B.find(SymbolID(Sym)), Pointee(Named(Sym)));
+
+  SymbolSlab S = std::move(B).build();
+  EXPECT_THAT(S, UnorderedElementsAre(Named("X"), Named("Y"), Named("Z")));
+  EXPECT_EQ(S.end(), S.find(SymbolID("W")));
+  for (const char *Sym : {"X", "Y", "Z"})
+    EXPECT_THAT(*S.find(SymbolID(Sym)), Named(Sym));
+}
+
 struct SlabAndPointers {
   SymbolSlab Slab;
   std::vector<const Symbol *> Pointers;
@@ -45,18 +63,18 @@ struct SlabAndPointers {
 std::shared_ptr<std::vector<const Symbol *>>
 generateSymbols(std::vector<std::string> QualifiedNames,
                 std::weak_ptr<SlabAndPointers> *WeakSymbols = nullptr) {
-  auto Slab = std::make_shared<SlabAndPointers>();
-  if (WeakSymbols)
-    *WeakSymbols = Slab;
-
+  SymbolSlab::Builder Slab;
   for (llvm::StringRef QName : QualifiedNames)
-    Slab->Slab.insert(symbol(QName));
-
-  for (const auto &Sym : Slab->Slab)
-    Slab->Pointers.push_back(&Sym.second);
+    Slab.insert(symbol(QName));
 
-  auto *Pointers = &Slab->Pointers;
-  return {std::move(Slab), Pointers};
+  auto Storage = std::make_shared<SlabAndPointers>();
+  Storage->Slab = std::move(Slab).build();
+  for (const auto &Sym : Storage->Slab)
+    Storage->Pointers.push_back(&Sym);
+  if (WeakSymbols)
+    *WeakSymbols = Storage;
+  auto *Pointers = &Storage->Pointers;
+  return {std::move(Storage), Pointers};
 }
 
 // Create a slab of symbols with IDs and names [Begin, End], otherwise identical
diff --git a/clang-tools-extra/unittests/clangd/SymbolCollectorTests.cpp b/clang-tools-extra/unittests/clangd/SymbolCollectorTests.cpp
index acbecbae7db..1a6dc970242 100644
--- a/clang-tools-extra/unittests/clangd/SymbolCollectorTests.cpp
+++ b/clang-tools-extra/unittests/clangd/SymbolCollectorTests.cpp
@@ -32,8 +32,7 @@ using testing::UnorderedElementsAre;
 
 // GMock helpers for matching Symbol.
 MATCHER_P(QName, Name, "") {
-  return (arg.second.Scope + (arg.second.Scope.empty() ? "" : "::") +
-          arg.second.Name).str() == Name;
+  return (arg.Scope + (arg.Scope.empty() ? "" : "::") + arg.Name).str() == Name;
 }
 
 namespace clang {