ioeric created this revision.
ioeric added reviewers: sammccall, hokein.
Herald added subscribers: cfe-commits, kadircet, arphaman, jkorous, MaskRay, 
ilya-biryukov.

This allows us to deduplicate header symbols across TUs. File digests
are collects when collecting symbols/refs. And the index store deduplicates
file symbols based on the file digest.

WIP: This is still a prototype and apparently needs better names and more tests.
Looking for early feedback to make sure this is heading the right direction.


Repository:
  rCTE Clang Tools Extra

https://reviews.llvm.org/D53433

Files:
  clangd/index/Background.cpp
  clangd/index/Background.h
  clangd/index/FileIndex.cpp
  clangd/index/FileIndex.h
  clangd/index/IndexAction.cpp
  clangd/index/IndexAction.h
  clangd/index/SymbolCollector.cpp
  clangd/index/SymbolCollector.h
  clangd/indexer/IndexerMain.cpp
  unittests/clangd/BackgroundIndexTests.cpp
  unittests/clangd/FileIndexTests.cpp
  unittests/clangd/SyncAPI.cpp
  unittests/clangd/SyncAPI.h

Index: unittests/clangd/SyncAPI.h
===================================================================
--- unittests/clangd/SyncAPI.h
+++ unittests/clangd/SyncAPI.h
@@ -52,6 +52,7 @@
 
 SymbolSlab runFuzzyFind(const SymbolIndex &Index, StringRef Query);
 SymbolSlab runFuzzyFind(const SymbolIndex &Index, const FuzzyFindRequest &Req);
+RefSlab getRefs(const SymbolIndex &Index, SymbolID ID);
 
 } // namespace clangd
 } // namespace clang
Index: unittests/clangd/SyncAPI.cpp
===================================================================
--- unittests/clangd/SyncAPI.cpp
+++ unittests/clangd/SyncAPI.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "SyncAPI.h"
+#include "index/Index.h"
 
 namespace clang {
 namespace clangd {
@@ -137,5 +138,14 @@
   return std::move(Builder).build();
 }
 
+RefSlab getRefs(const SymbolIndex &Index, SymbolID ID) {
+  RefsRequest Req;
+  Req.IDs = {ID};
+  RefSlab::Builder Slab;
+  Index.refs(Req, [&](const Ref &S) { Slab.insert(ID, S); });
+  return std::move(Slab).build();
+}
+
+
 } // namespace clangd
 } // namespace clang
Index: unittests/clangd/FileIndexTests.cpp
===================================================================
--- unittests/clangd/FileIndexTests.cpp
+++ unittests/clangd/FileIndexTests.cpp
@@ -72,14 +72,6 @@
   return llvm::make_unique<RefSlab>(std::move(Slab).build());
 }
 
-RefSlab getRefs(const SymbolIndex &I, SymbolID ID) {
-  RefsRequest Req;
-  Req.IDs = {ID};
-  RefSlab::Builder Slab;
-  I.refs(Req, [&](const Ref &S) { Slab.insert(ID, S); });
-  return std::move(Slab).build();
-}
-
 TEST(FileSymbolsTest, UpdateAndGet) {
   FileSymbols FS;
   EXPECT_THAT(runFuzzyFind(*FS.buildIndex(IndexType::Light), ""), IsEmpty());
Index: unittests/clangd/BackgroundIndexTests.cpp
===================================================================
--- unittests/clangd/BackgroundIndexTests.cpp
+++ unittests/clangd/BackgroundIndexTests.cpp
@@ -1,36 +1,78 @@
 #include "SyncAPI.h"
 #include "TestFS.h"
 #include "index/Background.h"
+#include "gmock/gmock-generated-matchers.h"
+#include "gmock/gmock-matchers.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
+using testing::_;
+using testing::AllOf;
+using testing::Not;
 using testing::UnorderedElementsAre;
 
 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; }
+testing::Matcher<const RefSlab &>
+RefsAre(std::vector<testing::Matcher<Ref>> Matchers) {
+  return ElementsAre(testing::Pair(_, UnorderedElementsAreArray(Matchers)));
+}
 
 TEST(BackgroundIndexTest, IndexTwoFiles) {
   MockFSProvider FS;
   // a.h yields different symbols when included by A.cc vs B.cc.
   // Currently we store symbols for each TU, so we get both.
-  FS.Files[testPath("root/A.h")] = "void a_h(); void NAME(){}";
-  FS.Files[testPath("root/A.cc")] = "#include \"A.h\"";
-  FS.Files[testPath("root/B.cc")] = "#define NAME bar\n#include \"A.h\"";
+  FS.Files[testPath("root/A.h")] = R"(
+      void common();
+      void f_b();
+      #if CC == A
+        class A_H {};
+      #elif CC == B
+        class B_H {};
+      #else
+        class _H {};
+      #endif
+      )";
+  FS.Files[testPath("root/A.cc")] =
+      "#include \"A.h\"\nvoid g() { (void)common; }";
+  FS.Files[testPath("root/B.cc")] =
+      "#define CC B\n#include \"A.h\"\nvoid f_b() { (void)common; }";
   BackgroundIndex Idx(Context::empty(), "", FS);
 
   tooling::CompileCommand Cmd;
   Cmd.Filename = testPath("root/A.cc");
   Cmd.Directory = testPath("root");
-  Cmd.CommandLine = {"clang++", "-DNAME=foo", testPath("root/A.cc")};
+  Cmd.CommandLine = {"clang++", "-DCC=A", testPath("root/A.cc")};
   Idx.enqueue(testPath("root"), Cmd);
-  Cmd.CommandLine.back() = Cmd.Filename = testPath("root/B.cc");
+
+  Idx.blockUntilIdleForTest();
+  EXPECT_THAT(
+      runFuzzyFind(Idx, ""),
+      UnorderedElementsAre(Named("common"), Named("A_H"),
+                           AllOf(Named("f_b"), Declared(), Not(Defined()))));
+
+  Cmd.Filename = testPath("root/B.cc");
+  Cmd.CommandLine = {"clang++", Cmd.Filename};
   Idx.enqueue(testPath("root"), Cmd);
 
   Idx.blockUntilIdleForTest();
+  // B_H is dropped as we don't collect symbols from A.h in this compilation.
   EXPECT_THAT(runFuzzyFind(Idx, ""),
-              UnorderedElementsAre(Named("a_h"), Named("foo"), Named("bar")));
+              UnorderedElementsAre(Named("common"), Named("A_H"),
+                                   AllOf(Named("f_b"), Declared(), Defined())));
+  auto Syms = runFuzzyFind(Idx, "common");
+  EXPECT_THAT(Syms, UnorderedElementsAre(Named("common")));
+  auto Common = *Syms.begin();
+  EXPECT_THAT(getRefs(Idx, Common.ID),
+              RefsAre({FileURI("file:///clangd-test/root/A.h"),
+                       FileURI("file:///clangd-test/root/A.cc"),
+                       FileURI("file:///clangd-test/root/B.cc")}));
 }
 
 } // namespace clangd
Index: clangd/indexer/IndexerMain.cpp
===================================================================
--- clangd/indexer/IndexerMain.cpp
+++ clangd/indexer/IndexerMain.cpp
@@ -63,7 +63,8 @@
                    for (const auto &Ref : Sym.second)
                      Refs.insert(Sym.first, Ref);
                  }
-               })
+               },
+               nullptr)
         .release();
   }
 
Index: clangd/index/SymbolCollector.h
===================================================================
--- clangd/index/SymbolCollector.h
+++ clangd/index/SymbolCollector.h
@@ -10,12 +10,14 @@
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_INDEX_SYMBOL_COLLECTOR_H
 
 #include "CanonicalIncludes.h"
+#include "FileIndex.h"
 #include "Index.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Decl.h"
 #include "clang/Index/IndexDataConsumer.h"
 #include "clang/Index/IndexSymbol.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
+#include "llvm/ADT/DenseSet.h"
 
 namespace clang {
 namespace clangd {
@@ -96,6 +98,7 @@
 
   SymbolSlab takeSymbols() { return std::move(Symbols).build(); }
   RefSlab takeRefs() { return std::move(Refs).build(); }
+  FileDigests takeFileDigests() { return std::move(IndexedFileDigests); }
 
   void finish() override;
 
@@ -109,6 +112,7 @@
   // Only symbols declared in preamble (from #include) and referenced from the
   // main file will be included.
   RefSlab::Builder Refs;
+  FileDigests IndexedFileDigests;
   ASTContext *ASTCtx;
   std::shared_ptr<Preprocessor> PP;
   std::shared_ptr<GlobalCodeCompletionAllocator> CompletionAllocator;
Index: clangd/index/SymbolCollector.cpp
===================================================================
--- clangd/index/SymbolCollector.cpp
+++ clangd/index/SymbolCollector.cpp
@@ -24,10 +24,12 @@
 #include "clang/Basic/Specifiers.h"
 #include "clang/Index/IndexSymbol.h"
 #include "clang/Index/USRGeneration.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/SHA1.h"
 
 namespace clang {
 namespace clangd {
@@ -202,18 +204,32 @@
           CreatePosition(TokLoc.getLocWithOffset(TokenLength))};
 }
 
+void digestFile(const SourceManager &SM, SourceLocation Loc,
+                llvm::StringRef FileURI, FileDigests *Digest) {
+  if (Digest->count(FileURI) != 0)
+    return;
+  FileID FID = SM.getFileID(Loc);
+  if (FID.isInvalid())
+    return;
+  bool Invalid = false;
+  StringRef Content = SM.getBufferData(FID, &Invalid);
+  if (!Invalid)
+    (*Digest)[FileURI] = digest(Content);
+}
+
 // Return the symbol location of the token at \p TokLoc.
 llvm::Optional<SymbolLocation>
 getTokenLocation(SourceLocation TokLoc, const SourceManager &SM,
                  const SymbolCollector::Options &Opts,
-                 const clang::LangOptions &LangOpts,
+                 const clang::LangOptions &LangOpts, FileDigests &Digest,
                  std::string &FileURIStorage) {
   auto U = toURI(SM, SM.getFilename(TokLoc), Opts);
   if (!U)
     return llvm::None;
   FileURIStorage = std::move(*U);
   SymbolLocation Result;
   Result.FileURI = FileURIStorage;
+  digestFile(SM, TokLoc, Result.FileURI, &Digest);
   auto Range = getTokenRange(TokLoc, SM, LangOpts);
   Result.Start = Range.first;
   Result.End = Range.second;
@@ -425,8 +441,9 @@
   S.Flags |= Symbol::IndexedForCodeCompletion;
   S.SymInfo = index::getSymbolInfoForMacro(*MI);
   std::string FileURI;
-  if (auto DeclLoc = getTokenLocation(MI->getDefinitionLoc(), SM, Opts,
-                                      PP->getLangOpts(), FileURI))
+  if (auto DeclLoc =
+          getTokenLocation(MI->getDefinitionLoc(), SM, Opts, PP->getLangOpts(),
+                           IndexedFileDigests, FileURI))
     S.CanonicalDeclaration = *DeclLoc;
 
   CodeCompletionResult SymbolCompletion(Name);
@@ -503,6 +520,7 @@
         for (const auto &LocAndRole : It.second) {
           auto FileID = SM.getFileID(LocAndRole.first);
           if (auto FileURI = GetURI(FileID)) {
+            digestFile(SM, LocAndRole.first, *FileURI, &IndexedFileDigests);
             auto Range =
                 getTokenRange(LocAndRole.first, SM, ASTCtx->getLangOpts());
             Ref R;
@@ -540,8 +558,9 @@
     S.Flags |= Symbol::ImplementationDetail;
   S.SymInfo = index::getSymbolInfo(&ND);
   std::string FileURI;
-  if (auto DeclLoc = getTokenLocation(findNameLoc(&ND), SM, Opts,
-                                      ASTCtx->getLangOpts(), FileURI))
+  if (auto DeclLoc =
+          getTokenLocation(findNameLoc(&ND), SM, Opts, ASTCtx->getLangOpts(),
+                           IndexedFileDigests, FileURI))
     S.CanonicalDeclaration = *DeclLoc;
 
   // Add completion info.
@@ -592,9 +611,9 @@
   // in clang::index. We should only see one definition.
   Symbol S = DeclSym;
   std::string FileURI;
-  if (auto DefLoc = getTokenLocation(findNameLoc(&ND),
-                                     ND.getASTContext().getSourceManager(),
-                                     Opts, ASTCtx->getLangOpts(), FileURI))
+  if (auto DefLoc = getTokenLocation(
+          findNameLoc(&ND), ND.getASTContext().getSourceManager(), Opts,
+          ASTCtx->getLangOpts(), IndexedFileDigests, FileURI))
     S.Definition = *DefLoc;
   Symbols.insert(S);
 }
Index: clangd/index/IndexAction.h
===================================================================
--- clangd/index/IndexAction.h
+++ clangd/index/IndexAction.h
@@ -9,6 +9,7 @@
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_INDEX_INDEX_ACTION_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_INDEX_INDEX_ACTION_H
+#include "FileIndex.h"
 #include "SymbolCollector.h"
 #include "clang/Frontend/FrontendActions.h"
 
@@ -23,10 +24,11 @@
 //   - references are always counted
 //   - all references are collected (if RefsCallback is non-null)
 //   - the symbol origin is always Static
-std::unique_ptr<FrontendAction>
-createStaticIndexingAction(SymbolCollector::Options Opts,
-                           std::function<void(SymbolSlab)> SymbolsCallback,
-                           std::function<void(RefSlab)> RefsCallback);
+std::unique_ptr<FrontendAction> createStaticIndexingAction(
+    SymbolCollector::Options Opts,
+    std::function<void(SymbolSlab)> SymbolsCallback,
+    std::function<void(RefSlab)> RefsCallback,
+    std::function<void(FileDigests)> FileDigestsCallback);
 
 } // namespace clangd
 } // namespace clang
Index: clangd/index/IndexAction.cpp
===================================================================
--- clangd/index/IndexAction.cpp
+++ clangd/index/IndexAction.cpp
@@ -1,8 +1,10 @@
 #include "IndexAction.h"
+#include "FileIndex.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Index/IndexDataConsumer.h"
 #include "clang/Index/IndexingAction.h"
 #include "clang/Tooling/Tooling.h"
+#include <functional>
 namespace clang {
 namespace clangd {
 namespace {
@@ -14,10 +16,12 @@
               std::unique_ptr<CanonicalIncludes> Includes,
               const index::IndexingOptions &Opts,
               std::function<void(SymbolSlab)> SymbolsCallback,
-              std::function<void(RefSlab)> RefsCallback)
+              std::function<void(RefSlab)> RefsCallback,
+              std::function<void(FileDigests)> FileDigestsCallback)
       : WrapperFrontendAction(index::createIndexingAction(C, Opts, nullptr)),
         SymbolsCallback(SymbolsCallback), RefsCallback(RefsCallback),
-        Collector(C), Includes(std::move(Includes)),
+        FileDigestsCallback(FileDigestsCallback), Collector(C),
+        Includes(std::move(Includes)),
         PragmaHandler(collectIWYUHeaderMaps(this->Includes.get())) {}
 
   std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &CI,
@@ -44,22 +48,26 @@
     SymbolsCallback(Collector->takeSymbols());
     if (RefsCallback != nullptr)
       RefsCallback(Collector->takeRefs());
+    if (FileDigestsCallback)
+      FileDigestsCallback(Collector->takeFileDigests());
   }
 
 private:
   std::function<void(SymbolSlab)> SymbolsCallback;
   std::function<void(RefSlab)> RefsCallback;
+  std::function<void(FileDigests)> FileDigestsCallback;
   std::shared_ptr<SymbolCollector> Collector;
   std::unique_ptr<CanonicalIncludes> Includes;
   std::unique_ptr<CommentHandler> PragmaHandler;
 };
 
 } // namespace
 
-std::unique_ptr<FrontendAction>
-createStaticIndexingAction(SymbolCollector::Options Opts,
-                           std::function<void(SymbolSlab)> SymbolsCallback,
-                           std::function<void(RefSlab)> RefsCallback) {
+std::unique_ptr<FrontendAction> createStaticIndexingAction(
+    SymbolCollector::Options Opts,
+    std::function<void(SymbolSlab)> SymbolsCallback,
+    std::function<void(RefSlab)> RefsCallback,
+    std::function<void(FileDigests)> FileDigestsCallback) {
   index::IndexingOptions IndexOpts;
   IndexOpts.SystemSymbolFilter =
       index::IndexingOptions::SystemSymbolFilterKind::All;
@@ -75,7 +83,7 @@
   Opts.Includes = Includes.get();
   return llvm::make_unique<IndexAction>(
       std::make_shared<SymbolCollector>(std::move(Opts)), std::move(Includes),
-      IndexOpts, SymbolsCallback, RefsCallback);
+      IndexOpts, SymbolsCallback, RefsCallback, FileDigestsCallback);
 }
 
 } // namespace clangd
Index: clangd/index/FileIndex.h
===================================================================
--- clangd/index/FileIndex.h
+++ clangd/index/FileIndex.h
@@ -21,6 +21,9 @@
 #include "MemIndex.h"
 #include "Merge.h"
 #include "clang/Lex/Preprocessor.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/SHA1.h"
 #include <memory>
 
 namespace clang {
@@ -34,6 +37,44 @@
   Heavy,
 };
 
+using FileDigest = decltype(llvm::SHA1::hash({}));
+using FileDigests = llvm::StringMap<FileDigest>;
+
+inline FileDigest digest(llvm::StringRef Content) {
+  return llvm::SHA1::hash({(const uint8_t *)Content.data(), Content.size()});
+}
+
+/// FIXME: rename to something else. FileSymbols is a good fit but taken. Maybe
+/// rename the existing FileSymbols to something else e.g. TUSymbols?
+class SymbolsGroupedByFiles {
+public:
+  /// Update with index results from indexing a TU. Index data is partitioned
+  /// into results and stores based on file. This only stores symbols/references
+  /// from files (in FileDigests) with new contents (i.e. new digest).
+  /// FIXME: make sure indexing a header file as main file and as included
+  /// header generate the same index results.
+  void update(SymbolSlab Symbols, RefSlab Refs, FileDigests Digests);
+
+  // Build an index on all symbols from all files.
+  std::unique_ptr<SymbolIndex>
+  buildIndex(IndexType, ArrayRef<std::string> URISchemes = {});
+
+private:
+  struct OneFileSymbols {
+    OneFileSymbols(llvm::StringRef FileURI, FileDigest Digest,
+                      SymbolSlab Syms, RefSlab Refs)
+        : FileURI(FileURI), Digest(Digest), Symbols(std::move(Syms)),
+          Refs(std::move(Refs)) {}
+
+    llvm::StringRef FileURI;
+    const FileDigest Digest;
+    const SymbolSlab Symbols;
+    const RefSlab Refs;
+  };
+
+  llvm::StringMap<std::unique_ptr<OneFileSymbols>> AllSymbols;
+};
+
 /// A container of Symbols from several source files. It can be updated
 /// at source-file granularity, replacing all symbols from one file with a new
 /// set.
@@ -52,7 +93,7 @@
   /// Updates all symbols and refs in a file.
   /// If either is nullptr, corresponding data for \p Path will be removed.
   void update(PathRef Path, std::unique_ptr<SymbolSlab> Slab,
-              std::unique_ptr<RefSlab> Refs);
+              std::unique_ptr<RefSlab> Ref);
 
   // The index keeps the symbols alive.
   std::unique_ptr<SymbolIndex>
@@ -65,6 +106,7 @@
   llvm::StringMap<std::shared_ptr<SymbolSlab>> FileToSymbols;
   /// Stores the latest ref snapshots for all active files.
   llvm::StringMap<std::shared_ptr<RefSlab>> FileToRefs;
+  llvm::StringMap<FileDigests> Digests;
 };
 
 /// This manages symbols from files and an in-memory index on all symbols.
Index: clangd/index/FileIndex.cpp
===================================================================
--- clangd/index/FileIndex.cpp
+++ clangd/index/FileIndex.cpp
@@ -12,11 +12,16 @@
 #include "Logger.h"
 #include "SymbolCollector.h"
 #include "index/Index.h"
+#include "index/MemIndex.h"
 #include "index/Merge.h"
 #include "index/dex/Dex.h"
 #include "clang/Index/IndexingAction.h"
 #include "clang/Lex/MacroInfo.h"
 #include "clang/Lex/Preprocessor.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/StringRef.h"
 #include <memory>
 
 namespace clang {
@@ -86,6 +91,96 @@
       .first;
 }
 
+void SymbolsGroupedByFiles::update(SymbolSlab Symbols, RefSlab Refs,
+                                   FileDigests Digests) {
+  // Find files with new digests or not seen before.
+  llvm::DenseSet<llvm::StringRef> FilesToUpdate;
+  for (const auto &D : Digests) {
+    auto I = AllSymbols.find(D.first());
+    if (I == AllSymbols.end() || I->getValue()->Digest != D.getValue())
+      FilesToUpdate.insert(D.first());
+  }
+
+  // Partition symbols/references into files.
+  struct File {
+    llvm::DenseSet<const Symbol *> Symbols;
+    llvm::DenseSet<const Ref *> Refs;
+  };
+  llvm::StringMap<File> Files;
+  for (const auto &Sym : Symbols) {
+    if (FilesToUpdate.count(Sym.CanonicalDeclaration.FileURI) != 0) {
+      auto &F = Files[Sym.CanonicalDeclaration.FileURI];
+      F.Symbols.insert(&Sym);
+    }
+    if (Sym.Definition.FileURI != Sym.CanonicalDeclaration.FileURI &&
+        FilesToUpdate.count(Sym.Definition.FileURI) != 0) {
+      auto &F = Files[Sym.Definition.FileURI];
+      F.Symbols.insert(&Sym);
+    }
+  }
+  llvm::DenseMap<const Ref *, SymbolID> RefToIDs;
+  for (const auto &SymRefs : Refs) {
+    for (const auto &R : SymRefs.second) {
+      if (FilesToUpdate.count(R.Location.FileURI) != 0) {
+        auto &F = Files[R.Location.FileURI];
+        RefToIDs[&R] = SymRefs.first;
+        F.Refs.insert(&R);
+      }
+    }
+  }
+
+  // Build and store new slabs for each updated file.
+  for (const auto &F : Files) {
+    log("> update symbols in {0}", F.first());
+    StringRef FileURI = F.getKey();
+    SymbolSlab::Builder Syms;
+    RefSlab::Builder Refs;
+    for (const auto *S : F.getValue().Symbols) {
+      Syms.insert(*S);
+      log("   add sym: {0}{1}, {2}", S->Scope, S->Name,
+          S->CanonicalDeclaration.FileURI);
+    }
+    for (const auto *R : F.getValue().Refs) {
+      Refs.insert(RefToIDs[R], *R);
+      log("   add ref: {0} at {1}", RefToIDs[R], R->Location);
+    }
+    AllSymbols[FileURI] = llvm::make_unique<OneFileSymbols>(
+        FileURI, Digests[FileURI], std::move(Syms).build(),
+        std::move(Refs).build());
+  }
+}
+
+std::unique_ptr<SymbolIndex>
+SymbolsGroupedByFiles::buildIndex(IndexType Type,
+                                  ArrayRef<std::string> URISchemes) {
+  SymbolSlab::Builder Syms;
+  RefSlab::Builder Refs;
+  for (const auto &FileSyms : AllSymbols) {
+    for (const auto &Sym : FileSyms.getValue()->Symbols) {
+      // FIXME: add comment explaining why this works for fwd decls vs actual
+      // decls.
+      if (const auto *Existing = Syms.find(Sym.ID))
+        Syms.insert(mergeSymbol(*Existing, Sym));
+      else
+        Syms.insert(Sym);
+    }
+    // FIXME: aggregate symbol reference count based on references.
+    for (const auto &Sym : FileSyms.getValue()->Refs) {
+      for (const auto &Ref : Sym.second)
+        Refs.insert(Sym.first, Ref);
+    }
+  }
+  auto Symbols = std::move(Syms).build();
+  auto References = std::move(Refs).build();
+  switch (Type) {
+  case IndexType::Light:
+    return MemIndex::build(std::move(Symbols), std::move(References));
+  case IndexType::Heavy:
+    return dex::Dex::build(std::move(Symbols), std::move(References),
+                           URISchemes);
+  }
+}
+
 void FileSymbols::update(PathRef Path, std::unique_ptr<SymbolSlab> Symbols,
                          std::unique_ptr<RefSlab> Refs) {
   std::lock_guard<std::mutex> Lock(Mutex);
Index: clangd/index/Background.h
===================================================================
--- clangd/index/Background.h
+++ clangd/index/Background.h
@@ -15,6 +15,7 @@
 #include "index/FileIndex.h"
 #include "index/Index.h"
 #include "clang/Tooling/CompilationDatabase.h"
+#include "llvm/ADT/StringMap.h"
 #include "llvm/Support/SHA1.h"
 #include <condition_variable>
 #include <deque>
@@ -27,6 +28,7 @@
 // all commands in a compilation database. Indexing happens in the background.
 // FIXME: it should also persist its state on disk for fast start.
 // FIXME: it should watch for changes to files on disk.
+// FIXME: support different URI schemes.
 class BackgroundIndex : public SwapIndex {
 public:
   // FIXME: resource-dir injection should be hoisted somewhere common.
@@ -57,9 +59,9 @@
 
   // index state
   llvm::Error index(tooling::CompileCommand);
-  FileSymbols IndexedSymbols; // Index contents.
-  using Hash = decltype(llvm::SHA1::hash({}));
-  llvm::StringMap<Hash> FileHash; // Digest of indexed file.
+
+  SymbolsGroupedByFiles IndexedSymbols;
+  FileDigests MainFileDigests;
 
   // queue management
   using Task = std::function<void()>; // FIXME: use multiple worker threads.
Index: clangd/index/Background.cpp
===================================================================
--- clangd/index/Background.cpp
+++ clangd/index/Background.cpp
@@ -126,10 +126,9 @@
   auto Buf = FS->getBufferForFile(AbsolutePath);
   if (!Buf)
     return errorCodeToError(Buf.getError());
-  StringRef Contents = Buf->get()->getBuffer();
-  auto Hash = SHA1::hash({(const uint8_t *)Contents.data(), Contents.size()});
+  auto Hash = digest(Buf->get()->getBuffer());
 
-  if (FileHash.lookup(AbsolutePath) == Hash) {
+  if (MainFileDigests.lookup(AbsolutePath) == Hash) {
     vlog("No need to index {0}, already up to date", AbsolutePath);
     return Error::success();
   }
@@ -154,10 +153,12 @@
   SymbolCollector::Options IndexOpts;
   SymbolSlab Symbols;
   RefSlab Refs;
+  FileDigests Digests;
   IndexFileIn IndexData;
   auto Action = createStaticIndexingAction(
       IndexOpts, [&](SymbolSlab S) { Symbols = std::move(S); },
-      [&](RefSlab R) { Refs = std::move(R); });
+      [&](RefSlab R) { Refs = std::move(R); },
+      [&](FileDigests D) { Digests = std::move(D); });
 
   // We're going to run clang here, and it could potentially crash.
   // We could use CrashRecoveryContext to try to make indexing crashes nonfatal,
@@ -177,11 +178,9 @@
       Symbols.size(), Refs.size());
   SPAN_ATTACH(Tracer, "symbols", int(Symbols.size()));
   SPAN_ATTACH(Tracer, "refs", int(Refs.size()));
-  // FIXME: partition the symbols by file rather than TU, to avoid duplication.
-  IndexedSymbols.update(AbsolutePath,
-                        llvm::make_unique<SymbolSlab>(std::move(Symbols)),
-                        llvm::make_unique<RefSlab>(std::move(Refs)));
-  FileHash[AbsolutePath] = Hash;
+  IndexedSymbols.update(std::move(Symbols), std::move(Refs),
+                        std::move(Digests));
+  MainFileDigests[AbsolutePath] = Hash;
 
   // FIXME: this should rebuild once-in-a-while, not after every file.
   //       At that point we should use Dex, too.
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