[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

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;
 }

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;

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);
+}
 
-  // We inserted a new symbol, so copy the underlying data.
-  intern(Sym.Name);
-  intern(Sym.Scope);
-  intern(Sym.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);
+  }
+}
+
+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

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();
-
-  // Adds the symbol to this slab.
-  // This is a deep copy: underlying strings will be owned by the slab.
-  void insert(const Symbol& S);
-
-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();
+  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);
   }
 
-  bool Frozen = false;
+  // 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);
 
-  // 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;
+     // 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:
+  SymbolSlab(llvm::BumpPtrAllocator Arena, std::vector<Symbol> Symbols)
+      : Arena(std::move(Arena)), Symbols(std::move(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

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

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

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();
 }
 

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));

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"));
 }
 

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));
+    Slab.insert(symbol(QName));
 
-  for (const auto &Sym : Slab->Slab)
-    Slab->Pointers.push_back(&Sym.second);
-
-  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

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 {