diff --git a/lib/Basic/SourceManager.cpp b/lib/Basic/SourceManager.cpp
index c5d275a..ea95650 100644
--- a/lib/Basic/SourceManager.cpp
+++ b/lib/Basic/SourceManager.cpp
@@ -1855,7 +1855,32 @@
   Loc = SM.getDecomposedLoc(UpperLoc);
   return false;
 }
-  
+
+/// Return the cache entry for comparing the given file IDs
+/// for isBeforeInTranslationUnit.
+InBeforeInTUCacheEntry &SourceManager::getInBeforeInTUCache(FileID LFID,
+                                                            FileID RFID) const {
+  // This is a magic number for limiting the cache size.  It was experimentally
+  // derived from a small Objective-C project (where the cache filled
+  // out to ~250 items).  We can make it larger if necessary.
+  enum { MagicCacheSize = 300 };
+  IsBeforeInTUCacheKey Key(LFID, RFID);
+
+  // If the cache size isn't too large, do a lookup and if necessary default
+  // construct an entry.  We can then return it to the caller for direct
+  // use.  When they update the value, the cache will get automatically
+  // updated as well.
+  if (IBTUCache.size() < MagicCacheSize)
+    return IBTUCache[Key];
+
+  // Otherwise, do a lookup that will not construct a new value.
+  InBeforeInTUCache::iterator I = IBTUCache.find(Key);
+  if (I != IBTUCache.end())
+    return I->second;
+
+  // Fall back to the overflow value.
+  return IBTUCacheOverflow;
+}
 
 /// \brief Determines the order of 2 source locations in the translation unit.
 ///
@@ -1875,6 +1900,11 @@
 
   // If we are comparing a source location with multiple locations in the same
   // file, we get a big win by caching the result.
+  InBeforeInTUCacheEntry &IsBeforeInTUCache =
+    getInBeforeInTUCache(LOffs.first, ROffs.first);
+
+  // If we are comparing a source location with multiple locations in the same
+  // file, we get a big win by caching the result.
   if (IsBeforeInTUCache.isCacheValid(LOffs.first, ROffs.first))
     return IsBeforeInTUCache.getCachedResult(LOffs.second, ROffs.second);