[NFC] [C++20] [Modules] [P1689] [Scanner] Don't use thread pool in P1689 per file mode (#84285)

I suddenly found that the clang scan deps may use all concurrent threads
to scan the files. It makes sense in the batch mode. But in P1689 per
file mode, it simply wastes times and resources.

This patch itself should be a NFC patch. It simply moves codes.

1 files changed, 100 insertions, 95 deletions

diff --git a/clang/tools/clang-scan-deps/ClangScanDeps.cpp b/clang/tools/clang-scan-deps/ClangScanDeps.cpp
index d042fec..eaa76dd 100644
--- a/clang/tools/clang-scan-deps/ClangScanDeps.cpp
+++ b/clang/tools/clang-scan-deps/ClangScanDeps.cpp
@@ -867,13 +867,6 @@ int clang_scan_deps_main(int argc, char **argv, const llvm::ToolContext &) {
   // Print out the dependency results to STDOUT by default.
   SharedStream DependencyOS(llvm::outs());
 
-  DependencyScanningService Service(ScanMode, Format, OptimizeArgs,
-                                    EagerLoadModules);
-  llvm::DefaultThreadPool Pool(llvm::hardware_concurrency(NumThreads));
-  std::vector<std::unique_ptr<DependencyScanningTool>> WorkerTools;
-  for (unsigned I = 0; I < Pool.getMaxConcurrency(); ++I)
-    WorkerTools.push_back(std::make_unique<DependencyScanningTool>(Service));
-
   std::vector<tooling::CompileCommand> Inputs =
       AdjustingCompilations->getAllCompileCommands();
 
@@ -893,102 +886,114 @@ int clang_scan_deps_main(int argc, char **argv, const llvm::ToolContext &) {
   if (Format == ScanningOutputFormat::Full)
     FD.emplace(ModuleName.empty() ? Inputs.size() : 0);
 
-  if (Verbose) {
-    llvm::outs() << "Running clang-scan-deps on " << Inputs.size()
-                 << " files using " << Pool.getMaxConcurrency() << " workers\n";
-  }
+  auto ScanningTask = [&](DependencyScanningService &Service) {
+    DependencyScanningTool WorkerTool(Service);
+
+    llvm::DenseSet<ModuleID> AlreadySeenModules;
+    while (auto MaybeInputIndex = GetNextInputIndex()) {
+      size_t LocalIndex = *MaybeInputIndex;
+      const tooling::CompileCommand *Input = &Inputs[LocalIndex];
+      std::string Filename = std::move(Input->Filename);
+      std::string CWD = std::move(Input->Directory);
+
+      std::optional<StringRef> MaybeModuleName;
+      if (!ModuleName.empty())
+        MaybeModuleName = ModuleName;
+
+      std::string OutputDir(ModuleFilesDir);
+      if (OutputDir.empty())
+        OutputDir = getModuleCachePath(Input->CommandLine);
+      auto LookupOutput = [&](const ModuleID &MID, ModuleOutputKind MOK) {
+        return ::lookupModuleOutput(MID, MOK, OutputDir);
+      };
+
+      // Run the tool on it.
+      if (Format == ScanningOutputFormat::Make) {
+        auto MaybeFile = WorkerTool.getDependencyFile(Input->CommandLine, CWD);
+        if (handleMakeDependencyToolResult(Filename, MaybeFile, DependencyOS,
+                                           Errs))
+          HadErrors = true;
+      } else if (Format == ScanningOutputFormat::P1689) {
+        // It is useful to generate the make-format dependency output during
+        // the scanning for P1689. Otherwise the users need to scan again for
+        // it. We will generate the make-format dependency output if we find
+        // `-MF` in the command lines.
+        std::string MakeformatOutputPath;
+        std::string MakeformatOutput;
+
+        auto MaybeRule = WorkerTool.getP1689ModuleDependencyFile(
+            *Input, CWD, MakeformatOutput, MakeformatOutputPath);
+
+        if (handleP1689DependencyToolResult(Filename, MaybeRule, PD, Errs))
+          HadErrors = true;
+
+        if (!MakeformatOutputPath.empty() && !MakeformatOutput.empty() &&
+            !HadErrors) {
+          static std::mutex Lock;
+          // With compilation database, we may open different files
+          // concurrently or we may write the same file concurrently. So we
+          // use a map here to allow multiple compile commands to write to the
+          // same file. Also we need a lock here to avoid data race.
+          static llvm::StringMap<llvm::raw_fd_ostream> OSs;
+          std::unique_lock<std::mutex> LockGuard(Lock);
+
+          auto OSIter = OSs.find(MakeformatOutputPath);
+          if (OSIter == OSs.end()) {
+            std::error_code EC;
+            OSIter =
+                OSs.try_emplace(MakeformatOutputPath, MakeformatOutputPath, EC)
+                    .first;
+            if (EC)
+              llvm::errs() << "Failed to open P1689 make format output file \""
+                           << MakeformatOutputPath << "\" for " << EC.message()
+                           << "\n";
+          }
+
+          SharedStream MakeformatOS(OSIter->second);
+          llvm::Expected<std::string> MaybeOutput(MakeformatOutput);
+          if (handleMakeDependencyToolResult(Filename, MaybeOutput,
+                                             MakeformatOS, Errs))
+            HadErrors = true;
+        }
+      } else if (MaybeModuleName) {
+        auto MaybeModuleDepsGraph = WorkerTool.getModuleDependencies(
+            *MaybeModuleName, Input->CommandLine, CWD, AlreadySeenModules,
+            LookupOutput);
+        if (handleModuleResult(*MaybeModuleName, MaybeModuleDepsGraph, *FD,
+                               LocalIndex, DependencyOS, Errs))
+          HadErrors = true;
+      } else {
+        auto MaybeTUDeps = WorkerTool.getTranslationUnitDependencies(
+            Input->CommandLine, CWD, AlreadySeenModules, LookupOutput);
+        if (handleTranslationUnitResult(Filename, MaybeTUDeps, *FD, LocalIndex,
+                                        DependencyOS, Errs))
+          HadErrors = true;
+      }
+    }
+  };
+
+  DependencyScanningService Service(ScanMode, Format, OptimizeArgs,
+                                    EagerLoadModules);
 
   llvm::Timer T;
   T.startTimer();
 
-  for (unsigned I = 0; I < Pool.getMaxConcurrency(); ++I) {
-    Pool.async([&, I]() {
-      llvm::DenseSet<ModuleID> AlreadySeenModules;
-      while (auto MaybeInputIndex = GetNextInputIndex()) {
-        size_t LocalIndex = *MaybeInputIndex;
-        const tooling::CompileCommand *Input = &Inputs[LocalIndex];
-        std::string Filename = std::move(Input->Filename);
-        std::string CWD = std::move(Input->Directory);
-
-        std::optional<StringRef> MaybeModuleName;
-        if (!ModuleName.empty())
-          MaybeModuleName = ModuleName;
-
-        std::string OutputDir(ModuleFilesDir);
-        if (OutputDir.empty())
-          OutputDir = getModuleCachePath(Input->CommandLine);
-        auto LookupOutput = [&](const ModuleID &MID, ModuleOutputKind MOK) {
-          return ::lookupModuleOutput(MID, MOK, OutputDir);
-        };
+  if (Inputs.size() == 1) {
+    ScanningTask(Service);
+  } else {
+    llvm::DefaultThreadPool Pool(llvm::hardware_concurrency(NumThreads));
 
-        // Run the tool on it.
-        if (Format == ScanningOutputFormat::Make) {
-          auto MaybeFile =
-              WorkerTools[I]->getDependencyFile(Input->CommandLine, CWD);
-          if (handleMakeDependencyToolResult(Filename, MaybeFile, DependencyOS,
-                                             Errs))
-            HadErrors = true;
-        } else if (Format == ScanningOutputFormat::P1689) {
-          // It is useful to generate the make-format dependency output during
-          // the scanning for P1689. Otherwise the users need to scan again for
-          // it. We will generate the make-format dependency output if we find
-          // `-MF` in the command lines.
-          std::string MakeformatOutputPath;
-          std::string MakeformatOutput;
-
-          auto MaybeRule = WorkerTools[I]->getP1689ModuleDependencyFile(
-              *Input, CWD, MakeformatOutput, MakeformatOutputPath);
-
-          if (handleP1689DependencyToolResult(Filename, MaybeRule, PD, Errs))
-            HadErrors = true;
+    if (Verbose) {
+      llvm::outs() << "Running clang-scan-deps on " << Inputs.size()
+                   << " files using " << Pool.getMaxConcurrency()
+                   << " workers\n";
+    }
 
-          if (!MakeformatOutputPath.empty() && !MakeformatOutput.empty() &&
-              !HadErrors) {
-            static std::mutex Lock;
-            // With compilation database, we may open different files
-            // concurrently or we may write the same file concurrently. So we
-            // use a map here to allow multiple compile commands to write to the
-            // same file. Also we need a lock here to avoid data race.
-            static llvm::StringMap<llvm::raw_fd_ostream> OSs;
-            std::unique_lock<std::mutex> LockGuard(Lock);
-
-            auto OSIter = OSs.find(MakeformatOutputPath);
-            if (OSIter == OSs.end()) {
-              std::error_code EC;
-              OSIter = OSs.try_emplace(MakeformatOutputPath,
-                                       MakeformatOutputPath, EC)
-                           .first;
-              if (EC)
-                llvm::errs()
-                    << "Failed to open P1689 make format output file \""
-                    << MakeformatOutputPath << "\" for " << EC.message()
-                    << "\n";
-            }
+    for (unsigned I = 0; I < Pool.getMaxConcurrency(); ++I)
+      Pool.async([ScanningTask, &Service]() { ScanningTask(Service); });
 
-            SharedStream MakeformatOS(OSIter->second);
-            llvm::Expected<std::string> MaybeOutput(MakeformatOutput);
-            if (handleMakeDependencyToolResult(Filename, MaybeOutput,
-                                               MakeformatOS, Errs))
-              HadErrors = true;
-          }
-        } else if (MaybeModuleName) {
-          auto MaybeModuleDepsGraph = WorkerTools[I]->getModuleDependencies(
-              *MaybeModuleName, Input->CommandLine, CWD, AlreadySeenModules,
-              LookupOutput);
-          if (handleModuleResult(*MaybeModuleName, MaybeModuleDepsGraph, *FD,
-                                 LocalIndex, DependencyOS, Errs))
-            HadErrors = true;
-        } else {
-          auto MaybeTUDeps = WorkerTools[I]->getTranslationUnitDependencies(
-              Input->CommandLine, CWD, AlreadySeenModules, LookupOutput);
-          if (handleTranslationUnitResult(Filename, MaybeTUDeps, *FD,
-                                          LocalIndex, DependencyOS, Errs))
-            HadErrors = true;
-        }
-      }
-    });
+    Pool.wait();
   }
-  Pool.wait();
 
   T.stopTimer();
   if (PrintTiming)