[clang][deps] Print tracing VFS data (#108056)

Clang's `-cc1 -print-stats` shows lots of useful internal data including
basic `FileManager` stats. Since this layer caches some results, it is
unclear how that information translates to actual filesystem accesses.
This PR uses `llvm::vfs::TracingFileSystem` to provide that missing
information.

Similar mechanism is implemented for `clang-scan-deps`'s verbose mode
(`-v`). IO contention proved to be a real bottleneck a couple of times
already and this new feature should make those easier to detect in the
future. The tracing VFS is inserted below the caching FS and above the
real FS.

1 files changed, 11 insertions, 0 deletions

diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
index 4509cee..6097b85 100644
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -692,5 +692,16 @@ void FileManager::PrintStats() const {
   llvm::errs() << NumFileLookups << " file lookups, "
                << NumFileCacheMisses << " file cache misses.\n";
 
+  getVirtualFileSystem().visit([](llvm::vfs::FileSystem &VFS) {
+    if (auto *T = dyn_cast_or_null<llvm::vfs::TracingFileSystem>(&VFS))
+      llvm::errs() << "\n*** Virtual File System Stats:\n"
+                   << T->NumStatusCalls << " status() calls\n"
+                   << T->NumOpenFileForReadCalls << " openFileForRead() calls\n"
+                   << T->NumDirBeginCalls << " dir_begin() calls\n"
+                   << T->NumGetRealPathCalls << " getRealPath() calls\n"
+                   << T->NumExistsCalls << " exists() calls\n"
+                   << T->NumIsLocalCalls << " isLocal() calls\n";
+  });
+
   //llvm::errs() << PagesMapped << BytesOfPagesMapped << FSLookups;
 }