Index: clangd/CMakeLists.txt
===================================================================
--- clangd/CMakeLists.txt
+++ clangd/CMakeLists.txt
@@ -47,3 +47,4 @@
   add_subdirectory(fuzzer)
 endif()
 add_subdirectory(tool)
+add_subdirectory(global-symbol-builder)
Index: clangd/global-symbol-builder/CMakeLists.txt
===================================================================
--- /dev/null
+++ clangd/global-symbol-builder/CMakeLists.txt
@@ -0,0 +1,19 @@
+include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../)
+
+set(LLVM_LINK_COMPONENTS
+    Support
+    )
+
+add_clang_executable(global-symbol-builder
+  GlobalSymbolBuilderMain.cpp
+  )
+
+target_link_libraries(global-symbol-builder
+  PRIVATE
+  clangAST
+  clangIndex
+  clangDaemon
+  clangBasic
+  clangFrontend
+  clangTooling
+)
Index: clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp
===================================================================
--- /dev/null
+++ clangd/global-symbol-builder/GlobalSymbolBuilderMain.cpp
@@ -0,0 +1,127 @@
+//===--- GlobalSymbolBuilderMain.cpp -----------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// GlobalSymbolBuilder is a tool to generate YAML-format symbols across the
+// whole project. This tools is for **experimental** only. Don't use it in
+// production code.
+//
+//===---------------------------------------------------------------------===//
+
+#include "index/Index.h"
+#include "index/SymbolCollector.h"
+#include "index/SymbolYAML.h"
+#include "clang/Frontend/FrontendActions.h"
+#include "clang/Index/IndexingAction.h"
+#include "clang/Index/IndexDataConsumer.h"
+#include "clang/Tooling/CommonOptionsParser.h"
+#include "clang/Tooling/Tooling.h"
+#include "clang/Tooling/CommonOptionsParser.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Signals.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/ThreadPool.h"
+
+using namespace llvm;
+using namespace clang::tooling;
+using clang::clangd::SymbolSlab;
+
+namespace clang {
+namespace clangd {
+
+class SymbolIndexActionFactory : public tooling::FrontendActionFactory {
+public:
+  SymbolIndexActionFactory() = default;
+
+  clang::FrontendAction *create() override {
+    index::IndexingOptions IndexOpts;
+    IndexOpts.SystemSymbolFilter =
+        index::IndexingOptions::SystemSymbolFilterKind::All;
+    IndexOpts.IndexFunctionLocals = false;
+    Collector = std::make_shared<SymbolCollector>();
+    return index::createIndexingAction(Collector, IndexOpts, nullptr).release();
+  }
+
+  std::shared_ptr<SymbolCollector> Collector;
+};
+
+} // namespace clangd
+} // namespace clang
+
+static cl::OptionCategory GlobalSymbolCategory("global-symbol-builder options");
+
+static cl::opt<std::string> ResultPath("result-path", cl::desc(R"(
+File path for storing global symbols.)"),
+                                       cl::init("./clangd-global-symbols.yaml"),
+                                       cl::cat(GlobalSymbolCategory));
+
+bool WriteFile(llvm::StringRef OutputFile, const SymbolSlab& Symbols) {
+  std::error_code EC;
+  llvm::raw_fd_ostream OS(OutputFile, EC, llvm::sys::fs::F_None);
+  if (EC) {
+    llvm::errs() << "Can't open '" << OutputFile << "': " << EC.message()
+                 << '\n';
+    return false;
+  }
+  OS << clang::clangd::SymbolToYAML(Symbols);
+  return true;
+}
+
+int main(int argc, const char **argv) {
+  llvm::sys::PrintStackTraceOnErrorSignal(argv[0]);
+
+  const char* Overview =
+      "This is an **experimental** tool to generate YAML-format "
+      "project-wide symbols for clangd (global code completion). It would be "
+      "changed and deprecated eventually. Don't use it in production code!";
+  CommonOptionsParser OptionsParser(argc, argv, GlobalSymbolCategory,
+                                    /*Overview=*/Overview);
+
+  // No compilation database found, fallback to single TU analysis, this is
+  // mainly for debugging purpose:
+  //   global-symbol-buidler /tmp/t.cc -- -std=c++11.
+  if (OptionsParser.getCompilations().getAllFiles().empty()) {
+    ClangTool Tool(OptionsParser.getCompilations(),
+                   OptionsParser.getSourcePathList());
+    clang::clangd::SymbolIndexActionFactory IndexAction;
+    Tool.run(&IndexAction);
+    llvm::outs() << SymbolToYAML(IndexAction.Collector->takeSymbols());
+    return 0;
+  }
+
+  // Found compilation database, we iterate all TUs from database to get all
+  // symbols, and then merge them into a single SymbolSlab.
+  SymbolSlab GlobalSymbols;
+  std::mutex SymbolMutex;
+  auto AddSymbols = [&](const SymbolSlab& NewSymbols) {
+    // Synchronize set accesses.
+    std::unique_lock<std::mutex> LockGuard(SymbolMutex);
+    for (auto It : 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);
+    }
+  };
+
+  {
+    llvm::ThreadPool Pool;
+    for (auto& file : OptionsParser.getCompilations().getAllFiles()) {
+      Pool.async([&OptionsParser, &AddSymbols](llvm::StringRef Path) {
+        ClangTool Tool(OptionsParser.getCompilations(), {Path});
+        clang::clangd::SymbolIndexActionFactory IndexAction;
+        Tool.run(&IndexAction);
+        AddSymbols(IndexAction.Collector->takeSymbols());
+      }, file);
+    }
+  }
+
+  WriteFile(ResultPath, GlobalSymbols);
+  return 0;
+}