[clang-scan-deps] Fix contention when updating TrackingStatistics in hot code paths in FileManager. (#88427)

`FileManager::getDirectoryRef()` and `FileManager::getFileRef()` are hot code paths in `clang-scan-deps`. These functions are updating on every call a few atomics related to printing statistics, which causes contention on high core count machines.

![Screenshot 2024-04-10
214123](https://github.com/llvm/llvm-project/assets/37383324/5756b1bc-cab5-4612-8769-ee7e03a66479)

![Screenshot 2024-04-10
214246](https://github.com/llvm/llvm-project/assets/37383324/3d560e89-61c7-4fb9-9330-f9e660e8fc8b)

![Screenshot 2024-04-10
214315](https://github.com/llvm/llvm-project/assets/37383324/006341fc-49d4-4720-a348-7af435c21b17)

After this patch we make the variables local to the `FileManager`.

In our test case, this saves about 49 sec over 1 min 47 sec of `clang-scan-deps` run time (1 min 47 sec before, 58 sec after). These figures are after applying my suggestion in #88152 (comment), that is:
```
static bool shouldCacheStatFailures(StringRef Filename) {
  return true;
}
```
Without the above, there's just too much OS noise from the high volume of `status()` calls with regular non-modules C++ code. Tested on Windows with clang-cl.

Author: aganea

clang/include/clang/Basic/FileManager.h

@@ -114,6 +114,12 @@ class FileManager : public RefCountedBase<FileManager> {
   ///
   unsigned NextFileUID;
 
+  /// Statistics gathered during the lifetime of the FileManager.
+  unsigned NumDirLookups = 0;
+  unsigned NumFileLookups = 0;
+  unsigned NumDirCacheMisses = 0;
+  unsigned NumFileCacheMisses = 0;
+
   // Caching.
   std::unique_ptr<FileSystemStatCache> StatCache;
 
@@ -341,6 +347,10 @@ class FileManager : public RefCountedBase<FileManager> {
 
 public:
   void PrintStats() const;
+
+  /// Import statistics from a child FileManager and add them to this current
+  /// FileManager.
+  void AddStats(const FileManager &Other);
 };
 
 } // end namespace clang

clang/lib/Basic/FileManager.cpp

@@ -39,12 +39,6 @@ using namespace clang;
 
 #define DEBUG_TYPE "file-search"
 
-ALWAYS_ENABLED_STATISTIC(NumDirLookups, "Number of directory lookups.");
-ALWAYS_ENABLED_STATISTIC(NumFileLookups, "Number of file lookups.");
-ALWAYS_ENABLED_STATISTIC(NumDirCacheMisses,
-                         "Number of directory cache misses.");
-ALWAYS_ENABLED_STATISTIC(NumFileCacheMisses, "Number of file cache misses.");
-
 //===----------------------------------------------------------------------===//
 // Common logic.
 //===----------------------------------------------------------------------===//
@@ -656,6 +650,14 @@ StringRef FileManager::getCanonicalName(const void *Entry, StringRef Name) {
   return CanonicalName;
 }
 
+void FileManager::AddStats(const FileManager &Other) {
+  assert(&Other != this && "Collecting stats into the same FileManager");
+  NumDirLookups += Other.NumDirLookups;
+  NumFileLookups += Other.NumFileLookups;
+  NumDirCacheMisses += Other.NumDirCacheMisses;
+  NumFileCacheMisses += Other.NumFileCacheMisses;
+}
+
 void FileManager::PrintStats() const {
   llvm::errs() << "\n*** File Manager Stats:\n";
   llvm::errs() << UniqueRealFiles.size() << " real files found, "

clang/lib/Frontend/CompilerInstance.cpp

@@ -1293,6 +1293,10 @@ compileModuleImpl(CompilerInstance &ImportingInstance, SourceLocation ImportLoc,
                                             diag::remark_module_build_done)
     << ModuleName;
 
+  // Propagate the statistics to the parent FileManager.
+  if (!FrontendOpts.ModulesShareFileManager)
+    ImportingInstance.getFileManager().AddStats(Instance.getFileManager());
+
   if (Crashed) {
     // Clear the ASTConsumer if it hasn't been already, in case it owns streams
     // that must be closed before clearing output files.

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

@@ -439,6 +439,9 @@ class DependencyScanningAction : public tooling::ToolAction {
     if (Result)
       setLastCC1Arguments(std::move(OriginalInvocation));
 
+    // Propagate the statistics to the parent FileManager.
+    DriverFileMgr->AddStats(ScanInstance.getFileManager());
+
     return Result;
   }