Add Cache Pruning support

Incremental LTO will usea cache to store object files.
This patch handles the pruning part of the cache, exposing
a few knobs:

- Pruning interval: the implementation keeps a "timestamp" file in the
  directory and will scan it only after a given interval since the
  last modification of the timestamp file. This is for performance
  purpose, we don't want to scan continuously the folder.
- Entry expiration: this is the time after which a file that hasn't
  been used is remove from the cache.
- Maximum size: expressed in percentage of the available disk space,
  it helps to avoid that we blow up the disk space.

http://reviews.llvm.org/D18422

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 265209

1 files changed, 69 insertions, 0 deletions

diff --git a/llvm/include/llvm/Support/CachePruning.h b/llvm/include/llvm/Support/CachePruning.h
new file mode 100644
index 00000000000..38341411913
--- /dev/null
+++ b/llvm/include/llvm/Support/CachePruning.h
@@ -0,0 +1,69 @@
+//=- CachePruning.h - Helper to manage the pruning of a cache dir -*- C++ -*-=//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements pruning of a directory intended for cache storage, using
+// various policies.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_CACHE_PRUNING_H
+#define LLVM_SUPPORT_CACHE_PRUNING_H
+
+#include "llvm/ADT/StringRef.h"
+
+namespace llvm {
+
+/// Handle pruning a directory provided a path and some options to control what
+/// to prune.
+class CachePruning {
+public:
+  /// Prepare to prune \p Path.
+  CachePruning(StringRef Path) : Path(Path) {}
+
+  /// Define the pruning interval. This is intended to be used to avoid scanning
+  /// the directory too often. It does not impact the decision of which file to
+  /// prune. A value of 0 forces the scan to occurs.
+  CachePruning &setPruningInterval(int PruningInterval) {
+    Interval = PruningInterval;
+    return *this;
+  }
+
+  /// Define the expiration for a file. When a file hasn't been accessed for
+  /// \p ExpireAfter seconds, it is removed from the cache. A value of 0 disable
+  /// the expiration-based pruning.
+  CachePruning &setEntryExpiration(unsigned ExpireAfter) {
+    Expiration = ExpireAfter;
+    return *this;
+  }
+
+  /// Define the maximum size for the cache directory, in terms of percentage of
+  /// the available space on the the disk. Set to 100 to indicate no limit, 50
+  /// to indicate that the cache size will not be left over half the
+  /// available disk space. A value over 100 will be reduced to 100. A value of
+  /// 0 disable the size-based pruning.
+  CachePruning &setMaxSize(unsigned Percentage) {
+    PercentageOfAvailableSpace = std::min(100u, Percentage);
+    return *this;
+  }
+
+  /// Peform pruning using the supplied options, returns true if pruning
+  /// occured, i.e. if PruningInterval was expired.
+  bool prune();
+
+private:
+  // Options that matches the setters above.
+  std::string Path;
+  unsigned Expiration = 0;
+  unsigned Interval = 0;
+  unsigned PercentageOfAvailableSpace = 0;
+};
+
+} // namespace llvm
+
+#endif
\ No newline at end of file