[clang-tools-extra] [clangd] [C++20] [Modules] Introduce initial support for C++20 Modules (PR #66462)

Chuanqi Xu via cfe-commits cfe-commits at lists.llvm.org
Fri Sep 15 00:30:52 PDT 2023


https://github.com/ChuanqiXu9 updated https://github.com/llvm/llvm-project/pull/66462

>From a4e9cae646b9a7af5e246075b55c74fc4a152024 Mon Sep 17 00:00:00 2001
From: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
Date: Fri, 15 Sep 2023 11:33:53 +0800
Subject: [PATCH] [clangd] [C++20] [Modules] Introduce initial support for
 C++20 Modules

Alternatives to https://reviews.llvm.org/D153114.

Try to address https://github.com/clangd/clangd/issues/1293.

See the links for design ideas. We want to have some initial support in
clang18.

This is the initial support for C++20 Modules in clangd.
As suggested by sammccall in https://reviews.llvm.org/D153114,
we should minimize the scope of the initial patch to make it easier
to review and understand so that every one are in the same page:

> Don't attempt any cross-file or cross-version coordination: i.e. don't
> try to reuse BMIs between different files, don't try to reuse BMIs
> between (preamble) reparses of the same file, don't try to persist the
> module graph. Instead, when building a preamble, synchronously scan
> for the module graph, build the required PCMs on the single preamble
> thread with filenames private to that preamble, and then proceed to
> build the preamble.

And this patch reflects the above opinions.
---
 clang-tools-extra/clangd/CMakeLists.txt       |   3 +
 clang-tools-extra/clangd/ClangdServer.cpp     |   1 +
 clang-tools-extra/clangd/ClangdServer.h       |   3 +
 .../clangd/GlobalCompilationDatabase.cpp      |  21 ++
 .../clangd/GlobalCompilationDatabase.h        |   6 +
 .../clangd/ModuleDependencyScanner.cpp        |  86 ++++++
 .../clangd/ModuleDependencyScanner.h          |  78 +++++
 clang-tools-extra/clangd/ModuleFilesInfo.cpp  | 282 ++++++++++++++++++
 clang-tools-extra/clangd/ModuleFilesInfo.h    | 118 ++++++++
 clang-tools-extra/clangd/ParsedAST.cpp        |   9 +
 clang-tools-extra/clangd/Preamble.cpp         |  23 +-
 clang-tools-extra/clangd/Preamble.h           |   7 +
 clang-tools-extra/clangd/TUScheduler.cpp      |  13 +
 clang-tools-extra/clangd/TUScheduler.h        |   3 +
 clang-tools-extra/clangd/test/CMakeLists.txt  |   1 +
 clang-tools-extra/clangd/test/modules.test    |  79 +++++
 clang-tools-extra/clangd/tool/Check.cpp       |   8 +-
 clang-tools-extra/clangd/tool/ClangdMain.cpp  |   8 +
 .../clangd/unittests/CMakeLists.txt           |   2 +
 .../clangd/unittests/CodeCompleteTests.cpp    |  18 +-
 .../clangd/unittests/FileIndexTests.cpp       |   5 +-
 .../unittests/ModuleDependencyScannerTest.cpp | 173 +++++++++++
 .../clangd/unittests/ModuleFilesInfoTest.cpp  | 223 ++++++++++++++
 .../clangd/unittests/ModulesTestSetup.h       | 105 +++++++
 .../clangd/unittests/ParsedASTTests.cpp       |   8 +-
 .../clangd/unittests/PreambleTests.cpp        |   6 +-
 clang-tools-extra/clangd/unittests/TestTU.cpp |  14 +-
 clang-tools-extra/docs/ReleaseNotes.rst       |   3 +
 28 files changed, 1285 insertions(+), 21 deletions(-)
 create mode 100644 clang-tools-extra/clangd/ModuleDependencyScanner.cpp
 create mode 100644 clang-tools-extra/clangd/ModuleDependencyScanner.h
 create mode 100644 clang-tools-extra/clangd/ModuleFilesInfo.cpp
 create mode 100644 clang-tools-extra/clangd/ModuleFilesInfo.h
 create mode 100644 clang-tools-extra/clangd/test/modules.test
 create mode 100644 clang-tools-extra/clangd/unittests/ModuleDependencyScannerTest.cpp
 create mode 100644 clang-tools-extra/clangd/unittests/ModuleFilesInfoTest.cpp
 create mode 100644 clang-tools-extra/clangd/unittests/ModulesTestSetup.h

diff --git a/clang-tools-extra/clangd/CMakeLists.txt b/clang-tools-extra/clangd/CMakeLists.txt
index 3911fb6c6c746a8..bcfb49551a02591 100644
--- a/clang-tools-extra/clangd/CMakeLists.txt
+++ b/clang-tools-extra/clangd/CMakeLists.txt
@@ -97,6 +97,8 @@ add_clang_library(clangDaemon
   IncludeFixer.cpp
   InlayHints.cpp
   JSONTransport.cpp
+  ModuleDependencyScanner.cpp
+  ModuleFilesInfo.cpp
   PathMapping.cpp
   Protocol.cpp
   Quality.cpp
@@ -161,6 +163,7 @@ clang_target_link_libraries(clangDaemon
   clangAST
   clangASTMatchers
   clangBasic
+  clangDependencyScanning
   clangDriver
   clangFormat
   clangFrontend
diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
index 13d788162817fb4..e4c85858b6882ae 100644
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -202,6 +202,7 @@ ClangdServer::Options::operator TUScheduler::Options() const {
   Opts.UpdateDebounce = UpdateDebounce;
   Opts.ContextProvider = ContextProvider;
   Opts.PreambleThrottler = PreambleThrottler;
+  Opts.ExperimentalModulesSupport = ExperimentalModulesSupport;
   return Opts;
 }
 
diff --git a/clang-tools-extra/clangd/ClangdServer.h b/clang-tools-extra/clangd/ClangdServer.h
index a416602251428b0..dc546b118cb8f5e 100644
--- a/clang-tools-extra/clangd/ClangdServer.h
+++ b/clang-tools-extra/clangd/ClangdServer.h
@@ -112,6 +112,9 @@ class ClangdServer {
     /// This throttler controls which preambles may be built at a given time.
     clangd::PreambleThrottler *PreambleThrottler = nullptr;
 
+    /// Enable experimental support for modules.
+    bool ExperimentalModulesSupport = false;
+
     /// If true, ClangdServer builds a dynamic in-memory index for symbols in
     /// opened files and uses the index to augment code completion results.
     bool BuildDynamicSymbolIndex = false;
diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
index d1833759917a30f..bcc8f4f0dd9e5ac 100644
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
@@ -729,6 +729,20 @@ DirectoryBasedGlobalCompilationDatabase::getProjectInfo(PathRef File) const {
   return Res->PI;
 }
 
+std::vector<std::string>
+DirectoryBasedGlobalCompilationDatabase::getAllFilesInProjectOf(
+    PathRef File) const {
+  CDBLookupRequest Req;
+  Req.FileName = File;
+  Req.ShouldBroadcast = false;
+  Req.FreshTime = Req.FreshTimeMissing =
+      std::chrono::steady_clock::time_point::min();
+  auto Res = lookupCDB(Req);
+  if (!Res)
+    return {};
+  return Res->CDB->getAllFiles();
+}
+
 OverlayCDB::OverlayCDB(const GlobalCompilationDatabase *Base,
                        std::vector<std::string> FallbackFlags,
                        CommandMangler Mangler)
@@ -805,6 +819,13 @@ std::optional<ProjectInfo> DelegatingCDB::getProjectInfo(PathRef File) const {
   return Base->getProjectInfo(File);
 }
 
+std::vector<std::string>
+DelegatingCDB::getAllFilesInProjectOf(PathRef File) const {
+  if (!Base)
+    return {};
+  return Base->getAllFilesInProjectOf(File);
+}
+
 tooling::CompileCommand DelegatingCDB::getFallbackCommand(PathRef File) const {
   if (!Base)
     return GlobalCompilationDatabase::getFallbackCommand(File);
diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.h b/clang-tools-extra/clangd/GlobalCompilationDatabase.h
index 2bf8c973c534c6f..eaeff8d627a0960 100644
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.h
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.h
@@ -45,6 +45,10 @@ class GlobalCompilationDatabase {
     return std::nullopt;
   }
 
+  virtual std::vector<std::string> getAllFilesInProjectOf(PathRef File) const {
+    return {};
+  }
+
   /// Makes a guess at how to build a file.
   /// The default implementation just runs clang on the file.
   /// Clangd should treat the results as unreliable.
@@ -75,6 +79,7 @@ class DelegatingCDB : public GlobalCompilationDatabase {
   getCompileCommand(PathRef File) const override;
 
   std::optional<ProjectInfo> getProjectInfo(PathRef File) const override;
+  std::vector<std::string> getAllFilesInProjectOf(PathRef File) const override;
 
   tooling::CompileCommand getFallbackCommand(PathRef File) const override;
 
@@ -121,6 +126,7 @@ class DirectoryBasedGlobalCompilationDatabase
   /// Returns the path to first directory containing a compilation database in
   /// \p File's parents.
   std::optional<ProjectInfo> getProjectInfo(PathRef File) const override;
+  std::vector<std::string> getAllFilesInProjectOf(PathRef File) const override;
 
   bool blockUntilIdle(Deadline Timeout) const override;
 
diff --git a/clang-tools-extra/clangd/ModuleDependencyScanner.cpp b/clang-tools-extra/clangd/ModuleDependencyScanner.cpp
new file mode 100644
index 000000000000000..edadfe45d82c368
--- /dev/null
+++ b/clang-tools-extra/clangd/ModuleDependencyScanner.cpp
@@ -0,0 +1,86 @@
+//===---------------- ModuleDependencyScanner.cpp ----------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ModuleDependencyScanner.h"
+
+namespace clang {
+namespace clangd {
+using P1689Rule = tooling::dependencies::P1689Rule;
+
+std::optional<P1689Rule> ModuleDependencyScanner::scan(PathRef FilePath) {
+  if (ScanningCache.count(FilePath))
+    return ScanningCache[FilePath];
+
+  std::optional<tooling::CompileCommand> Cmd = CDB.getCompileCommand(FilePath);
+
+  if (!Cmd)
+    return std::nullopt;
+
+  using namespace clang::tooling::dependencies;
+
+  llvm::SmallString<128> FilePathDir(FilePath);
+  llvm::sys::path::remove_filename(FilePathDir);
+  DependencyScanningTool ScanningTool(
+      Service,
+      TFS ? TFS->view(FilePathDir) : llvm::vfs::createPhysicalFileSystem());
+
+  llvm::Expected<P1689Rule> Result =
+      ScanningTool.getP1689ModuleDependencyFile(*Cmd, Cmd->Directory);
+
+  if (auto E = Result.takeError()) {
+    // Ignore any error.
+    llvm::consumeError(std::move(E));
+    return std::nullopt;
+  }
+
+  if (Result->Provides)
+    ModuleNameToSourceMapper[Result->Provides->ModuleName] = FilePath;
+
+  ScanningCache[FilePath] = *Result;
+  return *Result;
+}
+
+void ModuleDependencyScanner::globalScan(PathRef File) {
+  std::vector<std::string> AllFiles = CDB.getAllFilesInProjectOf(File);
+
+  for (auto &File : AllFiles)
+    scan(File);
+}
+
+PathRef ModuleDependencyScanner::getSourceForModuleName(StringRef ModuleName) const {
+  if (!ModuleNameToSourceMapper.count(ModuleName))
+    return {};
+
+  return ModuleNameToSourceMapper.at(ModuleName);
+}
+
+std::vector<std::string>
+ModuleDependencyScanner::getRequiredModules(PathRef File) const {
+  if (!ScanningCache.count(File))
+    return {};
+
+  const P1689Rule &CachedResult = ScanningCache.at(File);
+  std::vector<std::string> Result;
+
+  for (const auto &Info : CachedResult.Requires)
+    Result.push_back(Info.ModuleName);
+
+  return Result;
+}
+
+StringRef ModuleDependencyScanner::getModuleName(PathRef File) const {
+  if (!ScanningCache.count(File))
+    return {};
+
+  if (!ScanningCache.at(File).Provides)
+    return {};
+
+  return ScanningCache.at(File).Provides->ModuleName;
+}
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/ModuleDependencyScanner.h b/clang-tools-extra/clangd/ModuleDependencyScanner.h
new file mode 100644
index 000000000000000..1d6eb58fda59e20
--- /dev/null
+++ b/clang-tools-extra/clangd/ModuleDependencyScanner.h
@@ -0,0 +1,78 @@
+//===-------------- ModuleDependencyScanner.h --------------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_MODULEDEPENDENCYSCANNER_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_MODULEDEPENDENCYSCANNER_H
+
+#include "GlobalCompilationDatabase.h"
+#include "support/Path.h"
+#include "support/ThreadsafeFS.h"
+
+#include "clang/Tooling/DependencyScanning/DependencyScanningService.h"
+#include "clang/Tooling/DependencyScanning/DependencyScanningTool.h"
+
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringMap.h"
+
+namespace clang {
+namespace clangd {
+
+/// A scanner to produce P1689 format for C++20 Modules.
+///
+/// The scanner can scan a single file with `scan(PathRef)` member function
+/// or scan the whole project with `globalScan(PathRef)` member function. See
+/// the comments of `globalScan` to see the details.
+class ModuleDependencyScanner {
+public:
+  ModuleDependencyScanner(const GlobalCompilationDatabase &CDB,
+                          const ThreadsafeFS *TFS)
+      : CDB(CDB), TFS(TFS),
+        Service(tooling::dependencies::ScanningMode::CanonicalPreprocessing,
+                tooling::dependencies::ScanningOutputFormat::P1689) {}
+
+  /// Scanning the single file specified by \param FilePath.
+  std::optional<clang::tooling::dependencies::P1689Rule> scan(PathRef FilePath);
+
+  /// Scanning every source file in the current project to get the
+  /// <module-name> to <module-unit-source> map.
+  /// It looks unefficiency to scan the whole project especially for
+  /// every version of every file!
+  /// TODO: We should find a efficient method to get the <module-name>
+  /// to <module-unit-source> map. We can make it either by providing
+  /// a global module dependency scanner to monitor every file. Or we
+  /// can simply require the build systems (or even if the end users)
+  /// to provide the map.
+  void globalScan(PathRef File);
+
+  PathRef getSourceForModuleName(StringRef ModuleName) const;
+
+  /// Return the direct required modules. Indirect required modules are not
+  /// included.
+  std::vector<std::string> getRequiredModules(PathRef File) const;
+  StringRef getModuleName(PathRef File) const;
+
+  const ThreadsafeFS *getThreadsafeFS() const { return TFS; }
+
+  const GlobalCompilationDatabase &getCompilationDatabase() const { return CDB; }
+
+private:
+  const GlobalCompilationDatabase &CDB;
+  const ThreadsafeFS *TFS;
+
+  clang::tooling::dependencies::DependencyScanningService Service;
+
+  // Map source file to P1689 Result.
+  llvm::StringMap<clang::tooling::dependencies::P1689Rule> ScanningCache;
+  // Map module name to source file path.
+  llvm::StringMap<std::string> ModuleNameToSourceMapper;
+};
+
+} // namespace clangd
+} // namespace clang
+
+#endif
diff --git a/clang-tools-extra/clangd/ModuleFilesInfo.cpp b/clang-tools-extra/clangd/ModuleFilesInfo.cpp
new file mode 100644
index 000000000000000..845ff01ca09dff3
--- /dev/null
+++ b/clang-tools-extra/clangd/ModuleFilesInfo.cpp
@@ -0,0 +1,282 @@
+//===----------------- ModuleFilesInfo.cpp -----------------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ModuleFilesInfo.h"
+#include "support/Logger.h"
+
+#include "clang/Frontend/FrontendAction.h"
+#include "clang/Frontend/FrontendActions.h"
+#include "clang/Serialization/ASTReader.h"
+
+namespace clang {
+namespace clangd {
+
+namespace {
+llvm::SmallString<128> getAbsolutePath(const tooling::CompileCommand &Cmd) {
+  llvm::SmallString<128> AbsolutePath;
+  if (llvm::sys::path::is_absolute(Cmd.Filename)) {
+    AbsolutePath = Cmd.Filename;
+  } else {
+    AbsolutePath = Cmd.Directory;
+    llvm::sys::path::append(AbsolutePath, Cmd.Filename);
+    llvm::sys::path::remove_dots(AbsolutePath, true);
+  }
+  return AbsolutePath;
+}
+} // namespace
+
+ModuleFilesInfo::ModuleFilesInfo(PathRef MainFile,
+                                 const GlobalCompilationDatabase &CDB) {
+  std::optional<ProjectInfo> PI = CDB.getProjectInfo(MainFile);
+  if (!PI)
+    return;
+
+  llvm::SmallString<128> Result(PI->SourceRoot);
+  llvm::sys::path::append(Result, ".cache");
+  llvm::sys::path::append(Result, "clangd");
+  llvm::sys::path::append(Result, "module_files");
+  llvm::sys::fs::create_directories(Result, /*IgnoreExisting=*/true);
+
+  llvm::sys::path::append(Result, llvm::sys::path::filename(MainFile));
+  llvm::sys::fs::createUniqueDirectory(Result, UniqueModuleFilesPathPrefix);
+
+  log("Initialized module files to {0}", UniqueModuleFilesPathPrefix.str());
+}
+
+ModuleFilesInfo::~ModuleFilesInfo() {
+  DependentModuleNames.clear();
+  Successed = false;
+
+  if (UniqueModuleFilesPathPrefix.empty())
+    return;
+
+  llvm::sys::fs::remove_directories(UniqueModuleFilesPathPrefix);
+  UniqueModuleFilesPathPrefix.clear();
+}
+
+llvm::SmallString<256>
+ModuleFilesInfo::getModuleFilePath(StringRef ModuleName) const {
+  llvm::SmallString<256> ModuleFilePath;
+
+  ModuleFilePath = UniqueModuleFilesPathPrefix;
+  auto [PrimaryModuleName, PartitionName] = ModuleName.split(':');
+  llvm::sys::path::append(ModuleFilePath, PrimaryModuleName);
+  if (!PartitionName.empty()) {
+    ModuleFilePath.append("-");
+    ModuleFilePath.append(PartitionName);
+  }
+  ModuleFilePath.append(".pcm");
+
+  return ModuleFilePath;
+}
+
+bool ModuleFilesInfo::IsModuleUnitBuilt(StringRef ModuleName) const {
+  if (!DependentModuleNames.count(ModuleName))
+    return false;
+
+  auto BMIPath = getModuleFilePath(ModuleName);
+  if (llvm::sys::fs::exists(BMIPath))
+    return true;
+
+  Successed = false;
+
+  DependentModuleNames.erase(ModuleName);
+  return false;
+}
+
+void ModuleFilesInfo::ReplaceHeaderSearchOptions(
+    HeaderSearchOptions &Options) const {
+  if (!IsInited())
+    return;
+
+  Options.PrebuiltModulePaths.insert(Options.PrebuiltModulePaths.begin(),
+                                     UniqueModuleFilesPathPrefix.str().str());
+
+  for (auto Iter = Options.PrebuiltModuleFiles.begin();
+       Iter != Options.PrebuiltModuleFiles.end();) {
+    if (IsModuleUnitBuilt(Iter->first)) {
+      Iter = Options.PrebuiltModuleFiles.erase(Iter);
+      continue;
+    }
+
+    Iter++;
+  }
+}
+
+void ModuleFilesInfo::ReplaceCompileCommands(
+    tooling::CompileCommand &Cmd) const {
+  if (!IsInited())
+    return;
+
+  std::vector<std::string> CommandLine(std::move(Cmd.CommandLine));
+
+  Cmd.CommandLine.emplace_back(CommandLine[0]);
+  Cmd.CommandLine.emplace_back(
+      llvm::Twine("-fprebuilt-module-path=" + UniqueModuleFilesPathPrefix)
+          .str());
+
+  for (std::size_t I = 1; I < CommandLine.size(); I++) {
+    const std::string &Arg = CommandLine[I];
+    const auto &[LHS, RHS] = StringRef(Arg).split("=");
+
+    // Remove original `-fmodule-file=<module-name>=<module-path>` form if it
+    // already built.
+    if (LHS == "-fmodule-file" && RHS.contains("=")) {
+      const auto &[ModuleName, _] = RHS.split("=");
+      if (IsModuleUnitBuilt(ModuleName))
+        continue;
+    }
+
+    Cmd.CommandLine.emplace_back(Arg);
+  }
+}
+
+void ModuleFilesInfo::ReplaceCompileCommands(tooling::CompileCommand &Cmd,
+                                             StringRef OutputModuleName) const {
+  if (!IsInited())
+    return;
+
+  ReplaceCompileCommands(Cmd);
+
+  Cmd.Output = getModuleFilePath(OutputModuleName).str().str();
+}
+
+bool ModuleFilesInfo::buildModuleFile(PathRef ModuleUnitFileName,
+                                      ModuleDependencyScanner &Scanner) {
+  if (ModuleUnitFileName.empty())
+    return false;
+
+  for (auto &ModuleName : Scanner.getRequiredModules(ModuleUnitFileName)) {
+    // Return early if there are errors building the module file.
+    if (!IsModuleUnitBuilt(ModuleName) &&
+        !buildModuleFile(Scanner.getSourceForModuleName(ModuleName), Scanner)) {
+      log("Failed to build module {0}", ModuleName);
+      return false;
+    }
+  }
+
+  auto Cmd =
+      Scanner.getCompilationDatabase().getCompileCommand(ModuleUnitFileName);
+  if (!Cmd)
+    return false;
+
+  ReplaceCompileCommands(*Cmd, Scanner.getModuleName(ModuleUnitFileName));
+
+  ParseInputs Inputs;
+  Inputs.TFS = Scanner.getThreadsafeFS();
+  Inputs.CompileCommand = std::move(*Cmd);
+
+  IgnoreDiagnostics IgnoreDiags;
+  auto CI = buildCompilerInvocation(Inputs, IgnoreDiags);
+  if (!CI)
+    return false;
+
+  auto FS = Inputs.TFS->view(Inputs.CompileCommand.Directory);
+  auto AbsolutePath = getAbsolutePath(Inputs.CompileCommand);
+  auto Buf = FS->getBufferForFile(AbsolutePath);
+  if (!Buf)
+    return false;
+
+  // Hash the contents of input files and store the hash value to the BMI files.
+  // So that we can check if the files are still valid when we want to reuse the
+  // BMI files.
+  CI->getHeaderSearchOpts().ValidateASTInputFilesContent = true;
+
+  CI->getFrontendOpts().OutputFile = Inputs.CompileCommand.Output;
+  auto Clang =
+      prepareCompilerInstance(std::move(CI), /*Preamble=*/nullptr,
+                              std::move(*Buf), std::move(FS), IgnoreDiags);
+  if (!Clang)
+    return false;
+
+  GenerateModuleInterfaceAction Action;
+  Clang->ExecuteAction(Action);
+
+  if (Clang->getDiagnostics().hasErrorOccurred())
+    return false;
+
+  DependentModuleNames.insert(Scanner.getModuleName(ModuleUnitFileName));
+
+  return true;
+}
+
+ModuleFilesInfo
+ModuleFilesInfo::buildModuleFilesInfoFor(PathRef File, const ThreadsafeFS *TFS,
+                                         const GlobalCompilationDatabase &CDB) {
+  ModuleDependencyScanner Scanner(CDB, TFS);
+
+  std::optional<tooling::dependencies::P1689Rule> ScanningResult =
+      Scanner.scan(File);
+  if (!ScanningResult)
+    return {};
+
+  ModuleFilesInfo ModulesInfo(File, CDB);
+
+  Scanner.globalScan(File);
+
+  for (auto &Info : ScanningResult->Requires)
+    // Return early if there is any error.
+    if (!ModulesInfo.buildModuleFile(
+            Scanner.getSourceForModuleName(Info.ModuleName), Scanner)) {
+      log("Failed to build module {0}", Info.ModuleName);
+      return ModulesInfo;
+    }
+
+  ModulesInfo.Successed = true;
+  return ModulesInfo;
+}
+
+bool ModuleFilesInfo::CanReuse(
+    const CompilerInvocation &CI,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS) const {
+  // Try to avoid expensive check as much as possible.
+  if (!IsInited())
+    return true;
+
+  if (!Successed)
+    return false;
+
+  CompilerInstance Clang;
+
+  Clang.setInvocation(std::make_shared<CompilerInvocation>(CI));
+  IntrusiveRefCntPtr<DiagnosticsEngine> Diags =
+      CompilerInstance::createDiagnostics(new DiagnosticOptions());
+  Clang.setDiagnostics(Diags.get());
+
+  FileManager *FM = Clang.createFileManager(VFS);
+  Clang.createSourceManager(*FM);
+
+  if (!Clang.createTarget())
+    return false;
+
+  ReplaceHeaderSearchOptions(Clang.getHeaderSearchOpts());
+  // Since we don't need to compile the source code actually, the TU kind here
+  // doesn't matter.
+  Clang.createPreprocessor(TU_Complete);
+  Clang.getHeaderSearchOpts().ForceCheckCXX20ModulesInputFiles = true;
+  Clang.getHeaderSearchOpts().ValidateASTInputFilesContent = true;
+
+  Clang.createASTReader();
+  for (auto &Iter : DependentModuleNames) {
+    StringRef ModuleName = Iter.first();
+    auto BMIPath = getModuleFilePath(ModuleName);
+    auto ReadResult =
+        Clang.getASTReader()->ReadAST(BMIPath, serialization::MK_MainFile,
+                                      SourceLocation(), ASTReader::ARR_None);
+
+    if (ReadResult != ASTReader::Success) {
+      log("Failed to reuse {0}", BMIPath);
+      return false;
+    }
+  }
+
+  return true;
+}
+
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/ModuleFilesInfo.h b/clang-tools-extra/clangd/ModuleFilesInfo.h
new file mode 100644
index 000000000000000..557233f87076c7a
--- /dev/null
+++ b/clang-tools-extra/clangd/ModuleFilesInfo.h
@@ -0,0 +1,118 @@
+//===----------------- ModuleFilesInfo.h -------------------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Experimental support for C++20 Modules.
+//
+// Currently we simplify the implementations by preventing reusing module files
+// across different versions and different source files. But this is clearly a
+// waste of time and space in the end of the day.
+//
+// FIXME: Supporting reusing module files across different versions and different
+// source files.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_MODULEFILESINFO_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_MODULEFILESINFO_H
+
+#include "Compiler.h"
+#include "GlobalCompilationDatabase.h"
+#include "ModuleDependencyScanner.h"
+#include "support/Path.h"
+
+#include "clang/Lex/HeaderSearchOptions.h"
+
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringMap.h"
+
+namespace clang {
+namespace clangd {
+
+/// Store module files information for a single file.
+///
+/// A ModuleFilesInfo should only be initialized by
+/// `ModuleFilesInfo::buildModuleFilesInfoFor(PathRef, const ThreadsafeFS *, const GlobalCompilationDatabase &)`
+/// static member method. This method will block the current thread and build all the needed
+/// module files. All the built module files won't be shared with other source files.
+///
+/// Users can detect whether the ModuleFilesInfo is still up to date by calling
+/// the `CanReuse()` member function.
+///
+/// The users should call `ReplaceHeaderSearchOptions(...)` or `ReplaceCompileCommands(CompileCommand&)`
+/// member function to update the compilation commands to select the built module files first.
+struct ModuleFilesInfo {
+  ModuleFilesInfo() = default;
+  ~ModuleFilesInfo();
+
+  ModuleFilesInfo(const ModuleFilesInfo &) = delete;
+  ModuleFilesInfo operator=(const ModuleFilesInfo &) = delete;
+
+  ModuleFilesInfo(ModuleFilesInfo &&Other)
+      : Successed(std::exchange(Other.Successed, false)),
+        UniqueModuleFilesPathPrefix(
+            std::move(Other.UniqueModuleFilesPathPrefix)),
+        DependentModuleNames(std::move(Other.DependentModuleNames)) {
+    Other.UniqueModuleFilesPathPrefix.clear();
+  }
+  ModuleFilesInfo &operator=(ModuleFilesInfo &&Other) {
+    if (this == &Other)
+      return *this;
+
+    this->~ModuleFilesInfo();
+    new (this) ModuleFilesInfo(std::move(Other));
+
+    return *this;
+  }
+
+  /// Build all the required module files for \param File.
+  /// Note that only the module files recorded by \param CDB can be built.
+  static ModuleFilesInfo
+  buildModuleFilesInfoFor(PathRef File, const ThreadsafeFS *TFS,
+                          const GlobalCompilationDatabase &CDB);
+
+  /// Return true if the modile file specified by ModuleName is built.
+  /// Note that this interface will only check the existence of the module
+  /// file instead of checking the validness of the module file. 
+  bool IsModuleUnitBuilt(StringRef ModuleName) const;
+
+  /// Change commands to load the module files recorded in this ModuleFilesInfo
+  /// first.
+  void ReplaceHeaderSearchOptions(HeaderSearchOptions &Options) const;
+  void ReplaceCompileCommands(tooling::CompileCommand &Cmd) const;
+  void ReplaceCompileCommands(tooling::CompileCommand &Cmd,
+                              StringRef OutputModuleName) const;
+
+  /// Whether or not the built module files are up to date.
+  /// Note that this can only be used after building the module files.
+  bool CanReuse(const CompilerInvocation &CI,
+                llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>) const;
+
+  /// NOTE: This shouldn't be used by external users except unittests.
+  llvm::SmallString<256> getModuleFilePath(StringRef ModuleName) const;
+
+private:
+  ModuleFilesInfo(PathRef MainFile, const GlobalCompilationDatabase &CDB);
+
+  bool IsInited() const { return !UniqueModuleFilesPathPrefix.empty(); }
+  bool buildModuleFile(PathRef ModuleUnitFileName,
+                       ModuleDependencyScanner &Scanner);
+
+  // Whether the last built is successed.
+  // May change in `IsModuleUnitBuilt`.
+  mutable bool Successed = false;
+
+  llvm::SmallString<256> UniqueModuleFilesPathPrefix;
+  // The language guarantees that the module name is unique in a program.
+  // May change in `IsModuleUnitBuilt`.
+  mutable llvm::StringSet<> DependentModuleNames;
+};
+
+} // namespace clangd
+} // namespace clang
+
+#endif
diff --git a/clang-tools-extra/clangd/ParsedAST.cpp b/clang-tools-extra/clangd/ParsedAST.cpp
index 3f6c0eaebac6c41..3e9480093ebed28 100644
--- a/clang-tools-extra/clangd/ParsedAST.cpp
+++ b/clang-tools-extra/clangd/ParsedAST.cpp
@@ -429,6 +429,12 @@ ParsedAST::build(llvm::StringRef Filename, const ParseInputs &Inputs,
           L->sawDiagnostic(D, Diag);
       });
 
+  // Replace header search options to load the built module files recorded
+  // in DependentModulesInfo.
+  if (Preamble)
+    Preamble->DependentModulesInfo.ReplaceHeaderSearchOptions(
+        CI->getHeaderSearchOpts());
+
   std::optional<PreamblePatch> Patch;
   // We might use an ignoring diagnostic consumer if they are going to be
   // dropped later on to not pay for extra latency by processing them.
@@ -442,6 +448,9 @@ ParsedAST::build(llvm::StringRef Filename, const ParseInputs &Inputs,
       std::move(CI), PreamblePCH,
       llvm::MemoryBuffer::getMemBufferCopy(Inputs.Contents, Filename), VFS,
       *DiagConsumer);
+
+  // Clangd Modules TODO: refactor the command line options of `Clang` here.
+
   if (!Clang) {
     // The last diagnostic contains information about the reason of this
     // failure.
diff --git a/clang-tools-extra/clangd/Preamble.cpp b/clang-tools-extra/clangd/Preamble.cpp
index 60c0f936d6f3810..6e0a39f40e7fa5c 100644
--- a/clang-tools-extra/clangd/Preamble.cpp
+++ b/clang-tools-extra/clangd/Preamble.cpp
@@ -587,11 +587,11 @@ class DiagPatcher {
 };
 } // namespace
 
-std::shared_ptr<const PreambleData>
-buildPreamble(PathRef FileName, CompilerInvocation CI,
-              const ParseInputs &Inputs, bool StoreInMemory,
-              PreambleParsedCallback PreambleCallback,
-              PreambleBuildStats *Stats) {
+std::shared_ptr<const PreambleData> buildPreamble(
+    PathRef FileName, CompilerInvocation CI, const ParseInputs &Inputs,
+    bool StoreInMemory, bool ExperimentalModulesSupport,
+    const GlobalCompilationDatabase &CDB,
+    PreambleParsedCallback PreambleCallback, PreambleBuildStats *Stats) {
   // Note that we don't need to copy the input contents, preamble can live
   // without those.
   auto ContentsBuffer =
@@ -660,10 +660,17 @@ buildPreamble(PathRef FileName, CompilerInvocation CI,
 
   WallTimer PreambleTimer;
   PreambleTimer.startTimer();
+
+  ModuleFilesInfo DependentModulesInfo =
+      !ExperimentalModulesSupport
+          ? ModuleFilesInfo{}
+          : ModuleFilesInfo::buildModuleFilesInfoFor(FileName, Inputs.TFS, CDB);
+
   auto BuiltPreamble = PrecompiledPreamble::Build(
       CI, ContentsBuffer.get(), Bounds, *PreambleDiagsEngine,
       Stats ? TimedFS : StatCacheFS, std::make_shared<PCHContainerOperations>(),
       StoreInMemory, /*StoragePath=*/"", CapturedInfo);
+
   PreambleTimer.stopTimer();
 
   // We have to setup DiagnosticConsumer that will be alife
@@ -696,6 +703,9 @@ buildPreamble(PathRef FileName, CompilerInvocation CI,
     Result->Includes = CapturedInfo.takeIncludes();
     Result->Pragmas = std::make_shared<const include_cleaner::PragmaIncludes>(
         CapturedInfo.takePragmaIncludes());
+
+    // FIXME: If there is no headers?
+    Result->DependentModulesInfo = std::move(DependentModulesInfo);
     Result->Macros = CapturedInfo.takeMacros();
     Result->Marks = CapturedInfo.takeMarks();
     Result->StatCache = StatCache;
@@ -736,7 +746,8 @@ bool isPreambleCompatible(const PreambleData &Preamble,
   auto VFS = Inputs.TFS->view(Inputs.CompileCommand.Directory);
   return compileCommandsAreEqual(Inputs.CompileCommand,
                                  Preamble.CompileCommand) &&
-         Preamble.Preamble.CanReuse(CI, *ContentsBuffer, Bounds, *VFS);
+         Preamble.Preamble.CanReuse(CI, *ContentsBuffer, Bounds, *VFS) &&
+         Preamble.DependentModulesInfo.CanReuse(CI, VFS);
 }
 
 void escapeBackslashAndQuotes(llvm::StringRef Text, llvm::raw_ostream &OS) {
diff --git a/clang-tools-extra/clangd/Preamble.h b/clang-tools-extra/clangd/Preamble.h
index 21a281aac0cce95..fb390c025597452 100644
--- a/clang-tools-extra/clangd/Preamble.h
+++ b/clang-tools-extra/clangd/Preamble.h
@@ -27,6 +27,9 @@
 #include "Diagnostics.h"
 #include "FS.h"
 #include "Headers.h"
+
+#include "ModuleFilesInfo.h"
+
 #include "clang-include-cleaner/Record.h"
 #include "support/Path.h"
 #include "clang/Basic/SourceManager.h"
@@ -104,6 +107,8 @@ struct PreambleData {
   IncludeStructure Includes;
   // Captures #include-mapping information in #included headers.
   std::shared_ptr<const include_cleaner::PragmaIncludes> Pragmas;
+  // Information about module files for this preamble.
+  ModuleFilesInfo DependentModulesInfo;
   // Macros defined in the preamble section of the main file.
   // Users care about headers vs main-file, not preamble vs non-preamble.
   // These should be treated as main-file entities e.g. for code completion.
@@ -147,6 +152,8 @@ struct PreambleBuildStats {
 std::shared_ptr<const PreambleData>
 buildPreamble(PathRef FileName, CompilerInvocation CI,
               const ParseInputs &Inputs, bool StoreInMemory,
+              bool ExperimentalModulesSupport,
+              const GlobalCompilationDatabase &CDB,
               PreambleParsedCallback PreambleCallback,
               PreambleBuildStats *Stats = nullptr);
 
diff --git a/clang-tools-extra/clangd/TUScheduler.cpp b/clang-tools-extra/clangd/TUScheduler.cpp
index 324ba1fc8cb8952..35e0f4205a9c854 100644
--- a/clang-tools-extra/clangd/TUScheduler.cpp
+++ b/clang-tools-extra/clangd/TUScheduler.cpp
@@ -652,6 +652,12 @@ class ASTWorker {
   TUScheduler::FileStats stats() const;
   bool isASTCached() const;
 
+  const GlobalCompilationDatabase &getCompilationDatabase() { return CDB; }
+
+  bool isExperimentalModulesSupportEnabled() const {
+    return ExperimentalModulesSupport;
+  }
+
 private:
   // Details of an update request that are relevant to scheduling.
   struct UpdateType {
@@ -710,6 +716,10 @@ class ASTWorker {
   TUScheduler::ASTCache &IdleASTs;
   TUScheduler::HeaderIncluderCache &HeaderIncluders;
   const bool RunSync;
+
+  /// Enable experimental support for modules.
+  bool ExperimentalModulesSupport = false;
+
   /// Time to wait after an update to see whether another update obsoletes it.
   const DebouncePolicy UpdateDebounce;
   /// File that ASTWorker is responsible for.
@@ -834,6 +844,7 @@ ASTWorker::ASTWorker(PathRef FileName, const GlobalCompilationDatabase &CDB,
                      const TUScheduler::Options &Opts,
                      ParsingCallbacks &Callbacks)
     : IdleASTs(LRUCache), HeaderIncluders(HeaderIncluders), RunSync(RunSync),
+      ExperimentalModulesSupport(Opts.ExperimentalModulesSupport),
       UpdateDebounce(Opts.UpdateDebounce), FileName(FileName),
       ContextProvider(Opts.ContextProvider), CDB(CDB), Callbacks(Callbacks),
       Barrier(Barrier), Done(false), Status(FileName, Callbacks),
@@ -1084,6 +1095,8 @@ void PreambleThread::build(Request Req) {
   bool IsFirstPreamble = !LatestBuild;
   LatestBuild = clang::clangd::buildPreamble(
       FileName, *Req.CI, Inputs, StoreInMemory,
+      ASTPeer.isExperimentalModulesSupportEnabled(),
+      ASTPeer.getCompilationDatabase(),
       [&](CapturedASTCtx ASTCtx,
           std::shared_ptr<const include_cleaner::PragmaIncludes> PI) {
         Callbacks.onPreambleAST(FileName, Inputs.Version, std::move(ASTCtx),
diff --git a/clang-tools-extra/clangd/TUScheduler.h b/clang-tools-extra/clangd/TUScheduler.h
index fb936d46bbcf7e9..706a13c59dd31d0 100644
--- a/clang-tools-extra/clangd/TUScheduler.h
+++ b/clang-tools-extra/clangd/TUScheduler.h
@@ -222,6 +222,9 @@ class TUScheduler {
     /// Cache (large) preamble data in RAM rather than temporary files on disk.
     bool StorePreamblesInMemory = false;
 
+    /// Enable experimental support for modules.
+    bool ExperimentalModulesSupport = false;
+
     /// Time to wait after an update to see if another one comes along.
     /// This tries to ensure we rebuild once the user stops typing.
     DebouncePolicy UpdateDebounce;
diff --git a/clang-tools-extra/clangd/test/CMakeLists.txt b/clang-tools-extra/clangd/test/CMakeLists.txt
index d073267066e0b4c..b51f461a4986659 100644
--- a/clang-tools-extra/clangd/test/CMakeLists.txt
+++ b/clang-tools-extra/clangd/test/CMakeLists.txt
@@ -2,6 +2,7 @@ set(CLANGD_TEST_DEPS
   clangd
   ClangdTests
   clangd-indexer
+  split-file
   # No tests for it, but we should still make sure they build.
   dexp
   )
diff --git a/clang-tools-extra/clangd/test/modules.test b/clang-tools-extra/clangd/test/modules.test
new file mode 100644
index 000000000000000..a5d4fe7ab2dd687
--- /dev/null
+++ b/clang-tools-extra/clangd/test/modules.test
@@ -0,0 +1,79 @@
+# A smoke test to check the modules can work basically.
+#
+# RUN: rm -fr %t
+# RUN: mkdir -p %t
+# RUN: split-file %s %t
+#
+# RUN: sed -e "s|DIR|%/t|g" %t/compile_commands.json.tmpl > %t/compile_commands.json.tmp
+# RUN: sed -e "s|CLANG_CC|%clang|g" %t/compile_commands.json.tmp > %t/compile_commands.json
+# RUN: sed -e "s|DIR|%/t|g" %t/definition.jsonrpc.tmpl > %t/definition.jsonrpc
+#
+# RUN: clangd -experimental-modules-support -lit-test < %t/definition.jsonrpc \
+# RUN:      | FileCheck -strict-whitespace %t/definition.jsonrpc
+
+#--- A.cppm
+export module A;
+export void printA() {}
+
+#--- Use.cpp
+import A;
+void foo() {
+    print
+}
+
+#--- compile_commands.json.tmpl
+[
+    {
+      "directory": "DIR",
+      "command": "CLANG_CC -fprebuilt-module-path=DIR -std=c++20 -o DIR/main.cpp.o -c DIR/Use.cpp",
+      "file": "DIR/Use.cpp"
+    },
+    {
+      "directory": "DIR",
+      "command": "CLANG_CC -std=c++20 DIR/A.cppm --precompile -o DIR/A.pcm",
+      "file": "DIR/A.cppm"
+    }
+]
+
+#--- definition.jsonrpc.tmpl
+{
+  "jsonrpc": "2.0",
+  "id": 0,
+  "method": "initialize",
+  "params": {
+    "processId": 123,
+    "rootPath": "clangd",
+    "capabilities": {
+      "textDocument": {
+        "completion": {
+          "completionItem": {
+            "snippetSupport": true
+          }
+        }
+      }
+    },
+    "trace": "off"
+  }
+}
+---
+{
+  "jsonrpc": "2.0",
+  "method": "textDocument/didOpen",
+  "params": {
+    "textDocument": {
+      "uri": "file://DIR/Use.cpp",
+      "languageId": "cpp",
+      "version": 1,
+      "text": "import A;\nvoid foo() {\n    print\n}\n"
+    }
+  }
+}
+
+# CHECK: "message"{{.*}}printA{{.*}}(fix available)
+
+---
+{"jsonrpc":"2.0","id":1,"method":"textDocument/completion","params":{"textDocument":{"uri":"file://DIR/Use.cpp"},"context":{"triggerKind":1},"position":{"line":2,"character":6}}}
+---
+{"jsonrpc":"2.0","id":2,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}
diff --git a/clang-tools-extra/clangd/tool/Check.cpp b/clang-tools-extra/clangd/tool/Check.cpp
index 46fcab0b69ce005..b3ae59ae7c4f5b1 100644
--- a/clang-tools-extra/clangd/tool/Check.cpp
+++ b/clang-tools-extra/clangd/tool/Check.cpp
@@ -140,6 +140,8 @@ class Checker {
   ClangdLSPServer::Options Opts;
   // from buildCommand
   tooling::CompileCommand Cmd;
+  std::unique_ptr<GlobalCompilationDatabase> BaseCDB;
+  std::unique_ptr<OverlayCDB> CDB;
   // from buildInvocation
   ParseInputs Inputs;
   std::unique_ptr<CompilerInvocation> Invocation;
@@ -162,14 +164,14 @@ class Checker {
     DirectoryBasedGlobalCompilationDatabase::Options CDBOpts(TFS);
     CDBOpts.CompileCommandsDir =
         Config::current().CompileFlags.CDBSearch.FixedCDBPath;
-    std::unique_ptr<GlobalCompilationDatabase> BaseCDB =
+    BaseCDB =
         std::make_unique<DirectoryBasedGlobalCompilationDatabase>(CDBOpts);
     auto Mangler = CommandMangler::detect();
     Mangler.SystemIncludeExtractor =
         getSystemIncludeExtractor(llvm::ArrayRef(Opts.QueryDriverGlobs));
     if (Opts.ResourceDir)
       Mangler.ResourceDir = *Opts.ResourceDir;
-    auto CDB = std::make_unique<OverlayCDB>(
+    CDB = std::make_unique<OverlayCDB>(
         BaseCDB.get(), std::vector<std::string>{}, std::move(Mangler));
 
     if (auto TrueCmd = CDB->getCompileCommand(File)) {
@@ -230,6 +232,8 @@ class Checker {
     log("Building preamble...");
     Preamble = buildPreamble(
         File, *Invocation, Inputs, /*StoreInMemory=*/true,
+        /*ExperimentalModulesSupport*/ Opts.ExperimentalModulesSupport,
+        *CDB.get(),
         [&](CapturedASTCtx Ctx,
             std::shared_ptr<const include_cleaner::PragmaIncludes> PI) {
           if (!Opts.BuildDynamicSymbolIndex)
diff --git a/clang-tools-extra/clangd/tool/ClangdMain.cpp b/clang-tools-extra/clangd/tool/ClangdMain.cpp
index f656a8c587c6533..e38b7c567fb7a97 100644
--- a/clang-tools-extra/clangd/tool/ClangdMain.cpp
+++ b/clang-tools-extra/clangd/tool/ClangdMain.cpp
@@ -550,6 +550,13 @@ opt<std::string> ProjectRoot{
 };
 #endif
 
+opt<bool> ExperimentalModulesSupport{
+    "experimental-modules-support",
+    cat(Features),
+    desc("Experimental support for standard c++ modules"),
+    init(false),
+};
+
 /// Supports a test URI scheme with relaxed constraints for lit tests.
 /// The path in a test URI will be combined with a platform-specific fake
 /// directory to form an absolute path. For example, test:///a.cpp is resolved
@@ -861,6 +868,7 @@ clangd accepts flags on the commandline, and in the CLANGD_FLAGS environment var
 
   ClangdLSPServer::Options Opts;
   Opts.UseDirBasedCDB = (CompileArgsFrom == FilesystemCompileArgs);
+  Opts.ExperimentalModulesSupport = ExperimentalModulesSupport;
 
   switch (PCHStorage) {
   case PCHStorageFlag::Memory:
diff --git a/clang-tools-extra/clangd/unittests/CMakeLists.txt b/clang-tools-extra/clangd/unittests/CMakeLists.txt
index 8d02b91fdd71669..cd79534c6a97923 100644
--- a/clang-tools-extra/clangd/unittests/CMakeLists.txt
+++ b/clang-tools-extra/clangd/unittests/CMakeLists.txt
@@ -72,6 +72,8 @@ add_unittest(ClangdUnitTests ClangdTests
   LoggerTests.cpp
   LSPBinderTests.cpp
   LSPClient.cpp
+  ModuleDependencyScannerTest.cpp
+  ModuleFilesInfoTest.cpp
   ModulesTests.cpp
   ParsedASTTests.cpp
   PathMappingTests.cpp
diff --git a/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp b/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
index 671c0b7da97c6cf..095c05210cafe8f 100644
--- a/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
@@ -132,8 +132,11 @@ CodeCompleteResult completions(const TestTU &TU, Position Point,
     ADD_FAILURE() << "Couldn't build CompilerInvocation";
     return {};
   }
+
+  MockCompilationDatabase CDB;
   auto Preamble = buildPreamble(testPath(TU.Filename), *CI, Inputs,
-                                /*InMemory=*/true, /*Callback=*/nullptr);
+                                /*ExperimentalModulesSupport=*/false,
+                                /*InMemory=*/true, CDB, /*Callback=*/nullptr);
   return codeComplete(testPath(TU.Filename), Point, Preamble.get(), Inputs,
                       Opts);
 }
@@ -1322,8 +1325,11 @@ signatures(llvm::StringRef Text, Position Point,
     ADD_FAILURE() << "Couldn't build CompilerInvocation";
     return {};
   }
+
+  MockCompilationDatabase CDB;
   auto Preamble = buildPreamble(testPath(TU.Filename), *CI, Inputs,
-                                /*InMemory=*/true, /*Callback=*/nullptr);
+                                /*ExperimentalModulesSupport=*/false,
+                                /*InMemory=*/true, CDB, /*Callback=*/nullptr);
   if (!Preamble) {
     ADD_FAILURE() << "Couldn't build Preamble";
     return {};
@@ -1604,8 +1610,12 @@ TEST(SignatureHelpTest, StalePreamble) {
   auto Inputs = TU.inputs(FS);
   auto CI = buildCompilerInvocation(Inputs, Diags);
   ASSERT_TRUE(CI);
-  auto EmptyPreamble = buildPreamble(testPath(TU.Filename), *CI, Inputs,
-                                     /*InMemory=*/true, /*Callback=*/nullptr);
+
+  MockCompilationDatabase CDB;
+  auto EmptyPreamble =
+      buildPreamble(testPath(TU.Filename), *CI, Inputs,
+                    /*ExperimentalModulesSupport=*/false,
+                    /*InMemory=*/true, CDB, /*Callback=*/nullptr);
   ASSERT_TRUE(EmptyPreamble);
 
   TU.AdditionalFiles["a.h"] = "int foo(int x);";
diff --git a/clang-tools-extra/clangd/unittests/FileIndexTests.cpp b/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
index cf30b388d234dfd..d8d1c076a7da0f3 100644
--- a/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
+++ b/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
@@ -336,9 +336,12 @@ TEST(FileIndexTest, RebuildWithPreamble) {
 
   FileIndex Index;
   bool IndexUpdated = false;
+
+  MockCompilationDatabase CDB;
   buildPreamble(
       FooCpp, *CI, PI,
-      /*StoreInMemory=*/true,
+      /*ExperimentalModulesSupport=*/false,
+      /*StoreInMemory=*/true, CDB,
       [&](CapturedASTCtx ASTCtx,
           std::shared_ptr<const include_cleaner::PragmaIncludes> PI) {
         auto &Ctx = ASTCtx.getASTContext();
diff --git a/clang-tools-extra/clangd/unittests/ModuleDependencyScannerTest.cpp b/clang-tools-extra/clangd/unittests/ModuleDependencyScannerTest.cpp
new file mode 100644
index 000000000000000..0a860225b9406ff
--- /dev/null
+++ b/clang-tools-extra/clangd/unittests/ModuleDependencyScannerTest.cpp
@@ -0,0 +1,173 @@
+//===------------ ModuleDependencyScannerTest.cpp ---------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ModuleDependencyScanner.h"
+#include "ModulesTestSetup.h"
+#include "TestFS.h"
+
+using namespace clang;
+using namespace clang::clangd;
+using namespace clang::tooling::dependencies;
+
+namespace {
+class ModuleDependencyScannerTests : public ModuleTestSetup {
+  
+};
+
+TEST_F(ModuleDependencyScannerTests, SingleFile) {
+  addFile("foo.h", R"cpp(
+import foo;
+  )cpp");
+
+  addFile("A.cppm", R"cpp(
+module;
+#include "foo.h"
+export module A;
+export import :partA;
+import :partB;
+import C;
+
+module :private;
+import D;
+  )cpp");
+
+  MockCompilationDatabase CDB(TestDir);
+  CDB.ExtraClangFlags.push_back("-std=c++20");
+
+  ModuleDependencyScanner Scanner(CDB, &TFS);
+  std::optional<P1689Rule> ScanningResult = Scanner.scan(getFullPath("A.cppm"));
+  EXPECT_TRUE(ScanningResult);
+
+  EXPECT_TRUE(ScanningResult->Provides);
+  EXPECT_EQ(ScanningResult->Provides->ModuleName, "A");
+
+  EXPECT_EQ(ScanningResult->Requires.size(), 5u);
+  EXPECT_EQ(ScanningResult->Requires[0].ModuleName, "foo");
+  EXPECT_EQ(ScanningResult->Requires[1].ModuleName, "A:partA");
+  EXPECT_EQ(ScanningResult->Requires[2].ModuleName, "A:partB");
+  EXPECT_EQ(ScanningResult->Requires[3].ModuleName, "C");
+  EXPECT_EQ(ScanningResult->Requires[4].ModuleName, "D");
+}
+
+TEST_F(ModuleDependencyScannerTests, GlobalScanning) {
+  addFile("build/compile_commands.json", R"cpp(
+[
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/foo.cppm -fmodule-output=__DIR__/foo.pcm -c -o __DIR__/foo.o",
+  "file": "__DIR__/foo.cppm",
+  "output": "__DIR__/foo.o"
+},
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/C.cppm -fmodule-output=__DIR__/C.pcm -c -o __DIR__/C.o",
+  "file": "__DIR__/C.cppm",
+  "output": "__DIR__/C.o"
+},
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/D.cppm -fmodule-output=__DIR__/D.pcm -c -o __DIR__/D.o",
+  "file": "__DIR__/D.cppm",
+  "output": "__DIR__/D.o"
+},
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/A-partA.cppm -fmodule-file=foo=__DIR__/foo.pcm -fmodule-output=__DIR__/A-partA.pcm -c -o __DIR__/A-partA.o",
+  "file": "__DIR__/A-partA.cppm",
+  "output": "__DIR__/A-partA.o"
+},
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/A-partB.cppm -fmodule-file=C=__DIR__/C.pcm -fmodule-output=__DIR__/A-partB.pcm -c -o __DIR__/A-partB.o",
+  "file": "__DIR__/A-partB.cppm",
+  "output": "__DIR__/A-partB.o"
+},
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/A.cppm -fmodule-file=A:partB=__DIR__/A-partB.pcm -fmodule-file=A:partA=__DIR__/A-partA.pcm -fmodule-file=foo=__DIR__/foo.pcm -fmodule-file=C=__DIR__/C.pcm -fmodule-file=D=__DIR__/C.pcm -fmodule-output=__DIR__/A.pcm -c -o __DIR__/A.o",
+  "file": "__DIR__/A.cppm",
+  "output": "__DIR__/A.o"
+},
+]
+  )cpp");
+
+  addFile("foo.cppm", R"cpp(
+export module foo;
+  )cpp");
+
+  addFile("foo.h", R"cpp(
+import foo;
+  )cpp");
+
+  addFile("A-partA.cppm", R"cpp(
+export module A:partA;
+import foo;
+  )cpp");
+
+  addFile("A-partB.cppm", R"cpp(
+module A:partB;
+import C;
+  )cpp");
+
+  addFile("C.cppm", R"cpp(
+export module C;
+  )cpp");
+
+  addFile("D.cppm", R"cpp(
+export module D;
+  )cpp");
+
+  addFile("A.cppm", R"cpp(
+module;
+#include "foo.h"
+export module A;
+export import :partA;
+import :partB;
+import C;
+
+module :private;
+import D;
+  )cpp");
+
+  std::unique_ptr<GlobalCompilationDatabase> CDB =
+      getGlobalCompilationDatabase();
+  ModuleDependencyScanner Scanner(*CDB.get(), &TFS);
+  Scanner.globalScan(getFullPath("A.cppm"));
+
+  EXPECT_TRUE(Scanner.getSourceForModuleName("foo").endswith("foo.cppm"));
+  EXPECT_TRUE(Scanner.getSourceForModuleName("A").endswith("A.cppm"));
+  EXPECT_TRUE(Scanner.getSourceForModuleName("A:partA").endswith("A-partA.cppm"));
+  EXPECT_TRUE(Scanner.getSourceForModuleName("A:partB").endswith("A-partB.cppm"));
+  EXPECT_TRUE(Scanner.getSourceForModuleName("C").endswith("C.cppm"));
+  EXPECT_TRUE(Scanner.getSourceForModuleName("D").endswith("D.cppm"));
+
+  EXPECT_TRUE(Scanner.getRequiredModules(getFullPath("foo.cppm")).empty());
+  EXPECT_TRUE(Scanner.getRequiredModules(getFullPath("C.cppm")).empty());
+  EXPECT_TRUE(Scanner.getRequiredModules(getFullPath("D.cppm")).empty());
+
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A-partA.cppm")).size(), 1u);
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A-partA.cppm"))[0], "foo");
+
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A-partB.cppm")).size(), 1u);
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A-partB.cppm"))[0], "C");
+
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A.cppm")).size(), 5u);
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A.cppm"))[0], "foo");
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A.cppm"))[1], "A:partA");
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A.cppm"))[2], "A:partB");
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A.cppm"))[3], "C");
+  EXPECT_EQ(Scanner.getRequiredModules(getFullPath("A.cppm"))[4], "D");
+
+  EXPECT_EQ(Scanner.getModuleName(getFullPath("A.cppm")), "A");
+  EXPECT_EQ(Scanner.getModuleName(getFullPath("A-partA.cppm")), "A:partA");
+  EXPECT_EQ(Scanner.getModuleName(getFullPath("A-partB.cppm")), "A:partB");
+  EXPECT_EQ(Scanner.getModuleName(getFullPath("C.cppm")), "C");
+  EXPECT_EQ(Scanner.getModuleName(getFullPath("D.cppm")), "D");
+}
+
+} // namespace
diff --git a/clang-tools-extra/clangd/unittests/ModuleFilesInfoTest.cpp b/clang-tools-extra/clangd/unittests/ModuleFilesInfoTest.cpp
new file mode 100644
index 000000000000000..b5c34f536898d0d
--- /dev/null
+++ b/clang-tools-extra/clangd/unittests/ModuleFilesInfoTest.cpp
@@ -0,0 +1,223 @@
+//===--------------- ModuleFilesInfoTests.cpp -------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ModuleFilesInfo.h"
+#include "ModulesTestSetup.h"
+
+using namespace clang;
+using namespace clang::clangd;
+
+namespace {
+class ModuleFilesInfoTests : public ModuleTestSetup {
+  
+};
+
+TEST_F(ModuleFilesInfoTests, ModuleFilesInfoTest) {
+  addFile("build/compile_commands.json", R"cpp(
+[
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/M.cppm -fmodule-output=__DIR__/M.pcm -c -o __DIR__/M.o",
+  "file": "__DIR__/M.cppm",
+  "output": "__DIR__/M.o"
+},
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/N.cppm -fmodule-file=M=__DIR__/M.pcm -fmodule-file=N:Part=__DIR__/N-partition.pcm -fprebuilt-module-path=__DIR__ -fmodule-output=__DIR__/N.pcm -c -o __DIR__/N.o",
+  "file": "__DIR__/N.cppm",
+  "output": "__DIR__/N.o"
+},
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/N-part.cppm -fmodule-output=__DIR__/N-partition.pcm -c -o __DIR__/N-part.o",
+  "file": "__DIR__/N-part.cppm",
+  "output": "__DIR__/N-part.o"
+},
+{
+  "directory": "__DIR__",
+  "command": "clang++ -std=c++20 __DIR__/L.cppm -fmodule-output=__DIR__/L.pcm -c -o __DIR__/L.o",
+  "file": "__DIR__/L.cppm",
+  "output": "__DIR__/L.o"
+}
+]
+  )cpp");
+
+  addFile("foo.h", R"cpp(
+inline void foo() {}
+  )cpp");
+
+  addFile("M.cppm", R"cpp(
+module;
+#include "foo.h"
+export module M;
+  )cpp");
+
+  addFile("N.cppm", R"cpp(
+export module N;
+import :Part;
+import M;
+  )cpp");
+
+  addFile("N-part.cppm", R"cpp(
+// Different name with filename intentionally.
+export module N:Part;
+  )cpp");
+
+  addFile("bar.h", R"cpp(
+inline void bar() {}
+  )cpp");
+
+  addFile("L.cppm", R"cpp(
+module;
+#include "bar.h"
+export module L;
+  )cpp");
+
+  std::unique_ptr<GlobalCompilationDatabase> CDB =
+      getGlobalCompilationDatabase();
+  auto MInfo = ModuleFilesInfo::buildModuleFilesInfoFor(getFullPath("M.cppm"),
+                                                        &TFS, *CDB);
+  // buildModuleFilesInfoFor won't built the module itself.
+  EXPECT_FALSE(MInfo.IsModuleUnitBuilt("M"));
+
+  // Module N shouldn't be able to be built.
+  auto NInfo = ModuleFilesInfo::buildModuleFilesInfoFor(getFullPath("N.cppm"),
+                                                        &TFS, *CDB);
+  EXPECT_TRUE(NInfo.IsModuleUnitBuilt("M"));
+  EXPECT_TRUE(NInfo.getModuleFilePath("M").endswith("M.pcm"));
+  EXPECT_TRUE(NInfo.IsModuleUnitBuilt("N:Part"));
+  EXPECT_TRUE(NInfo.getModuleFilePath("N:Part").endswith("N-Part.pcm"));
+
+  ParseInputs NInput = getInputs("N.cppm", *CDB);
+  std::vector<std::string> CC1Args;
+  std::unique_ptr<CompilerInvocation> Invocation =
+      getCompilerInvocation(NInput);
+  // Test that `ModuleFilesInfo::CanReuse` works basically.
+  EXPECT_TRUE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+  // Test that we can still reuse the NInfo after we touch a unrelated file.
+  {
+    addFile("L.cppm", R"cpp(
+module;
+#include "bar.h"
+export module L;
+export int ll = 43;
+  )cpp");
+    EXPECT_TRUE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+    addFile("bar.h", R"cpp(
+inline void bar() {}
+inline void bar(int) {}
+  )cpp");
+    EXPECT_TRUE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+  }
+
+  // Test that we can't reuse the NInfo after we touch a related file.
+  {
+    addFile("M.cppm", R"cpp(
+module;
+#include "foo.h"
+export module M;
+export int mm = 44;
+  )cpp");
+    EXPECT_FALSE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+    NInfo = ModuleFilesInfo::buildModuleFilesInfoFor(getFullPath("N.cppm"),
+                                                     &TFS, *CDB);
+    EXPECT_TRUE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+    addFile("foo.h", R"cpp(
+inline void foo() {}
+inline void foo(int) {}
+  )cpp");
+    EXPECT_FALSE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+    NInfo = ModuleFilesInfo::buildModuleFilesInfoFor(getFullPath("N.cppm"),
+                                                     &TFS, *CDB);
+    EXPECT_TRUE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+  }
+
+  addFile("N-part.cppm", R"cpp(
+export module N:Part;
+// Intentioned to make it uncompilable.
+export int NPart = 4LIdjwldijaw
+  )cpp");
+  EXPECT_FALSE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+  NInfo = ModuleFilesInfo::buildModuleFilesInfoFor(getFullPath("N.cppm"), &TFS,
+                                                   *CDB);
+  // So NInfo should be unreusable even after rebuild.
+  EXPECT_FALSE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+  addFile("N-part.cppm", R"cpp(
+export module N:Part;
+export int NPart = 43;
+  )cpp");
+  EXPECT_FALSE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+  NInfo = ModuleFilesInfo::buildModuleFilesInfoFor(getFullPath("N.cppm"), &TFS,
+                                                   *CDB);
+  // So NInfo should be unreusable even after rebuild.
+  EXPECT_TRUE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+  // Test that if we changed the modification time of the file, the module files
+  // info is still reusable if its content doesn't change.
+  addFile("N-part.cppm", R"cpp(
+export module N:Part;
+export int NPart = 43;
+  )cpp");
+  EXPECT_TRUE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+  addFile("N.cppm", R"cpp(
+export module N;
+import :Part;
+import M;
+
+export int nn = 43;
+  )cpp");
+  // NInfo should be reusable after we change its content.
+  EXPECT_TRUE(NInfo.CanReuse(*Invocation, TFS.view(TestDir)));
+
+  {
+    llvm::StringRef MPath = NInfo.getModuleFilePath("M");
+    llvm::SmallString<256> ModuleFilesPath = MPath;
+    llvm::sys::path::remove_filename(ModuleFilesPath);
+
+    // Check that
+    // `ModuleFilesInfo::ReplaceCompileCommands(tooling::CompileCommand &Cmd)`
+    // can replace compile commands correctly.
+    std::optional<tooling::CompileCommand> NCmd =
+        CDB->getCompileCommand(getFullPath("N.cppm"));
+    EXPECT_TRUE(NCmd);
+    NInfo.ReplaceCompileCommands(*NCmd);
+    EXPECT_EQ(
+        NCmd->CommandLine[1],
+        llvm::Twine("-fprebuilt-module-path=" + ModuleFilesPath.str()).str());
+    for (auto &Argument : NCmd->CommandLine) {
+      EXPECT_FALSE(llvm::StringRef(Argument).startswith("-fmodule-file=M="));
+      EXPECT_FALSE(
+          llvm::StringRef(Argument).startswith("-fmodule-file=N:Part="));
+    }
+
+    // Check that
+    // `ModuleFilesInfo::ReplaceHeaderSearchOptions(HeaderSearchOptions&)` can
+    // replace HeaderSearchOptions correctly.
+    ParseInputs NInput = getInputs("N.cppm", *CDB);
+    std::vector<std::string> CC1Args;
+    std::unique_ptr<CompilerInvocation> NInvocation =
+        getCompilerInvocation(NInput);
+    HeaderSearchOptions &HSOpts = NInvocation->getHeaderSearchOpts();
+    NInfo.ReplaceHeaderSearchOptions(HSOpts);
+
+    EXPECT_EQ(HSOpts.PrebuiltModulePaths.front(), ModuleFilesPath);
+    for (auto &[ModuleName, _] : HSOpts.PrebuiltModuleFiles) {
+      EXPECT_NE(ModuleName, "M");
+      EXPECT_NE(ModuleName, "N:Part");
+    }
+  }
+}
+
+} // namespace
diff --git a/clang-tools-extra/clangd/unittests/ModulesTestSetup.h b/clang-tools-extra/clangd/unittests/ModulesTestSetup.h
new file mode 100644
index 000000000000000..22cbd04353fe999
--- /dev/null
+++ b/clang-tools-extra/clangd/unittests/ModulesTestSetup.h
@@ -0,0 +1,105 @@
+//===-- ModulesTestSetup.h - Setup the module test environment --*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Compiler.h"
+#include "support/ThreadsafeFS.h"
+
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/raw_ostream.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+class ModuleTestSetup : public ::testing::Test {
+protected:
+  void SetUp() override {
+    ASSERT_FALSE(llvm::sys::fs::createUniqueDirectory("modules-test", TestDir));
+  }
+
+  void TearDown() override {
+    llvm::sys::fs::remove_directories(TestDir);
+  }
+
+public:
+  // Add files to the working testing directory and repalce all the
+  // `__DIR__` to TestDir.
+  void addFile(StringRef Path, StringRef Contents) {
+    ASSERT_FALSE(llvm::sys::path::is_absolute(Path));
+
+    SmallString<256> AbsPath(TestDir);
+    llvm::sys::path::append(AbsPath, Path);
+
+    ASSERT_FALSE(llvm::sys::fs::create_directories(
+        llvm::sys::path::parent_path(AbsPath)));
+
+    std::error_code EC;
+    llvm::raw_fd_ostream OS(AbsPath, EC);
+    ASSERT_FALSE(EC);
+
+    std::size_t Pos = Contents.find("__DIR__");
+    while (Pos != llvm::StringRef::npos) {
+      OS << Contents.take_front(Pos);
+      OS << TestDir;
+      Contents = Contents.drop_front(Pos + sizeof("__DIR__") - 1);
+      Pos = Contents.find("__DIR__");
+    }
+
+    OS << Contents;
+  }
+
+  // Get the absolute path for file specified by Path under testing working
+  // directory.
+  std::string getFullPath(StringRef Path) {
+    SmallString<128> Result(TestDir);
+    llvm::sys::path::append(Result, Path);
+    EXPECT_TRUE(llvm::sys::fs::exists(Result.str()));
+    return Result.str().str();
+  }
+
+  std::unique_ptr<GlobalCompilationDatabase> getGlobalCompilationDatabase() {
+    // The compilation flags with modules are much complex so it looks better
+    // to use DirectoryBasedGlobalCompilationDatabase than a mocked compilation
+    // database.
+    DirectoryBasedGlobalCompilationDatabase::Options Opts(TFS);
+    return std::make_unique<DirectoryBasedGlobalCompilationDatabase>(Opts);
+  }
+
+  ParseInputs getInputs(StringRef FileName,
+                        const GlobalCompilationDatabase &CDB) {
+    std::string FullPathName = getFullPath(FileName);
+
+    ParseInputs Inputs;
+    std::optional<tooling::CompileCommand> Cmd =
+        CDB.getCompileCommand(FullPathName);
+    EXPECT_TRUE(Cmd);
+    Inputs.CompileCommand = std::move(*Cmd);
+    Inputs.TFS = &TFS;
+
+    if (auto Contents = TFS.view(TestDir)->getBufferForFile(FullPathName))
+      Inputs.Contents = Contents->get()->getBuffer().str();
+
+    return Inputs;
+  }
+
+  std::unique_ptr<CompilerInvocation>
+  getCompilerInvocation(const ParseInputs &Inputs) {
+    std::vector<std::string> CC1Args;
+    return buildCompilerInvocation(Inputs, DiagConsumer, &CC1Args);
+  }
+
+  SmallString<256> TestDir;
+  // Noticed MockFS but its member variable 'OverlayRealFileSystemForModules'
+  // implies that it will better to use RealThreadsafeFS directly.
+  RealThreadsafeFS TFS;
+
+  DiagnosticConsumer DiagConsumer;
+};
+}
+}
diff --git a/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp b/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
index ec8132645f81fb5..4c7e6c66974dc03 100644
--- a/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
@@ -375,8 +375,10 @@ TEST(ParsedASTTest, PatchesAdditionalIncludes) {
   MockFS FS;
   auto Inputs = TU.inputs(FS);
   auto CI = buildCompilerInvocation(Inputs, Diags);
+  MockCompilationDatabase CDB;
   auto EmptyPreamble =
-      buildPreamble(testPath("foo.cpp"), *CI, Inputs, true, nullptr);
+      buildPreamble(testPath("foo.cpp"), *CI, Inputs,
+                    /*ExperimentalModulesSupport=*/false, true, CDB, nullptr);
   ASSERT_TRUE(EmptyPreamble);
   EXPECT_THAT(EmptyPreamble->Includes.MainFileIncludes, IsEmpty());
 
@@ -417,8 +419,10 @@ TEST(ParsedASTTest, PatchesDeletedIncludes) {
   MockFS FS;
   auto Inputs = TU.inputs(FS);
   auto CI = buildCompilerInvocation(Inputs, Diags);
+  MockCompilationDatabase CDB;
   auto BaselinePreamble =
-      buildPreamble(testPath("foo.cpp"), *CI, Inputs, true, nullptr);
+      buildPreamble(testPath("foo.cpp"), *CI, Inputs,
+                    /*ExperimentalModulesSupport=*/false, true, CDB, nullptr);
   ASSERT_TRUE(BaselinePreamble);
   EXPECT_THAT(BaselinePreamble->Includes.MainFileIncludes,
               ElementsAre(testing::Field(&Inclusion::Written, "<foo.h>")));
diff --git a/clang-tools-extra/clangd/unittests/PreambleTests.cpp b/clang-tools-extra/clangd/unittests/PreambleTests.cpp
index 6da98c55e392706..1d8a014d5d70494 100644
--- a/clang-tools-extra/clangd/unittests/PreambleTests.cpp
+++ b/clang-tools-extra/clangd/unittests/PreambleTests.cpp
@@ -196,8 +196,10 @@ TEST(PreamblePatchTest, PatchesPreambleIncludes) {
   TU.AdditionalFiles["b.h"] = "";
   TU.AdditionalFiles["c.h"] = "";
   auto PI = TU.inputs(FS);
-  auto BaselinePreamble = buildPreamble(
-      TU.Filename, *buildCompilerInvocation(PI, Diags), PI, true, nullptr);
+  MockCompilationDatabase CDB;
+  auto BaselinePreamble =
+      buildPreamble(TU.Filename, *buildCompilerInvocation(PI, Diags), PI,
+                    /*ExperimentalModulesSupport=*/false, true, CDB, nullptr);
   // We drop c.h from modified and add a new header. Since the latter is patched
   // we should only get a.h in preamble includes. d.h shouldn't be part of the
   // preamble, as it's coming from a disabled region.
diff --git a/clang-tools-extra/clangd/unittests/TestTU.cpp b/clang-tools-extra/clangd/unittests/TestTU.cpp
index e65ae825b416773..70c64fa3f8db881 100644
--- a/clang-tools-extra/clangd/unittests/TestTU.cpp
+++ b/clang-tools-extra/clangd/unittests/TestTU.cpp
@@ -107,8 +107,11 @@ TestTU::preamble(PreambleParsedCallback PreambleCallback) const {
     initializeModuleCache(*CI);
   auto ModuleCacheDeleter = llvm::make_scope_exit(
       std::bind(deleteModuleCache, CI->getHeaderSearchOpts().ModuleCachePath));
+  MockCompilationDatabase CDB;
   return clang::clangd::buildPreamble(testPath(Filename), *CI, Inputs,
-                                      /*StoreInMemory=*/true, PreambleCallback);
+                                      /*ExperimentalModulesSupport=*/false,
+                                      /*StoreInMemory=*/true, CDB,
+                                      PreambleCallback);
 }
 
 ParsedAST TestTU::build() const {
@@ -123,9 +126,12 @@ ParsedAST TestTU::build() const {
   auto ModuleCacheDeleter = llvm::make_scope_exit(
       std::bind(deleteModuleCache, CI->getHeaderSearchOpts().ModuleCachePath));
 
-  auto Preamble = clang::clangd::buildPreamble(testPath(Filename), *CI, Inputs,
-                                               /*StoreInMemory=*/true,
-                                               /*PreambleCallback=*/nullptr);
+  MockCompilationDatabase CDB;
+  auto Preamble =
+      clang::clangd::buildPreamble(testPath(Filename), *CI, Inputs,
+                                   /*ExperimentalModulesSupport=*/false,
+                                   /*StoreInMemory=*/true, CDB,
+                                   /*PreambleCallback=*/nullptr);
   auto AST = ParsedAST::build(testPath(Filename), Inputs, std::move(CI),
                               Diags.take(), Preamble);
   if (!AST) {
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 19c977977f9044c..fe56bd1d2db4a11 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -48,6 +48,9 @@ Major New Features
 Improvements to clangd
 ----------------------
 
+- Introduced exmperimental support for C++20 Modules. The experimental support can
+  be enabled by `-experimental-modules-support` option.
+
 Inlay hints
 ^^^^^^^^^^^
 



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