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

[Sam McCall via Phabricator via cfe-commits]

sammccall created this revision.
sammccall added reviewers: ioeric, ilya-biryukov.
Herald added subscribers: cfe-commits, jfb, mgorny.

This is intended to reduce the number of syscalls when building large
translation units that have long include paths.

This is a WIP for early feedback:

- obviously it needs tests
- it will not be on by default
- the logging and extra instrumentation is temporary to help me tune it

Particularly interested in how to express this without hundreds of
nested switch statements.


Repository:
  rC Clang

https://reviews.llvm.org/D52549

Files:
  include/clang/Basic/VirtualFileSystem.h
  lib/Basic/AvoidStatsVFS.cpp
  lib/Basic/CMakeLists.txt
  lib/Basic/VirtualFileSystem.cpp

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