[PATCH] D52549: [VFS] Add a VFS wrapper which avoids opening nonexistent files by reading dirs.

[Eric Liu via Phabricator via cfe-commits]

ioeric added a comment.

This looks like a great improvement. The design looks reasonable to me. The cache states were a bit confusing at first, but I think more documentation when polishing the prototype could help.

I'd be really interested in seeing some profiles before/after this optimization e.g. for clang or clangd.



================
Comment at: lib/Basic/AvoidStatsVFS.cpp:44
+
+class StatLessFS : public ProxyFileSystem {
+public:
----------------
`LessStatFS` sounds more accurate lol


================
Comment at: lib/Basic/AvoidStatsVFS.cpp:46
+public:
+  // In fact we only read a directory once we've wanted its contents several
+  // times. This avoids a common pattern:
----------------
The comment is very clear, but it's hard to tell how it relates to `ReadDirThreshold = 3`.


================
Comment at: lib/Basic/AvoidStatsVFS.cpp:186
+  //   - NormPath is a directory whose children can't be listed
+  bool populateCacheForDir(StringRef NormPath) {
+    // First, just see if we have any work to do.
----------------
Is there any overhead for reading all parent directories, e.g. when directories are large? Or would they be read anyway somewhere else?


Repository:
  rC Clang

https://reviews.llvm.org/D52549