diff --git a/lib/Lex/HeaderMap.cpp b/lib/Lex/HeaderMap.cpp
new file mode 100644
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--- /dev/null
+++ b/lib/Lex/HeaderMap.cpp
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+//===--- HeaderMap.cpp - A file that acts like dir of symlinks ------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the HeaderMap interface.
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Lex/HeaderMap.h"
+#include "clang/Basic/FileManager.h"
+#include "llvm/ADT/OwningPtr.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/System/DataTypes.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include <cstdio>
+using namespace clang;
+
+//===----------------------------------------------------------------------===//
+// Data Structures and Manifest Constants
+//===----------------------------------------------------------------------===//
+
+enum {
+  HMAP_HeaderMagicNumber = ('h' << 24) | ('m' << 16) | ('a' << 8) | 'p',
+  HMAP_HeaderVersion = 1,
+
+  HMAP_EmptyBucketKey = 0
+};
+
+namespace clang {
+struct HMapBucket {
+  uint32_t Key;          // Offset (into strings) of key.
+
+  uint32_t Prefix;     // Offset (into strings) of value prefix.
+  uint32_t Suffix;     // Offset (into strings) of value suffix.
+};
+
+struct HMapHeader {
+  uint32_t Magic;           // Magic word, also indicates byte order.
+  uint16_t Version;         // Version number -- currently 1.
+  uint16_t Reserved;        // Reserved for future use - zero for now.
+  uint32_t StringsOffset;   // Offset to start of string pool.
+  uint32_t NumEntries;      // Number of entries in the string table.
+  uint32_t NumBuckets;      // Number of buckets (always a power of 2).
+  uint32_t MaxValueLength;  // Length of longest result path (excluding nul).
+  // An array of 'NumBuckets' HMapBucket objects follows this header.
+  // Strings follow the buckets, at StringsOffset.
+};
+} // end namespace clang.
+
+/// HashHMapKey - This is the 'well known' hash function required by the file
+/// format, used to look up keys in the hash table.  The hash table uses simple
+/// linear probing based on this function.
+static inline unsigned HashHMapKey(llvm::StringRef Str) {
+  unsigned Result = 0;
+  const char *S = Str.begin(), *End = Str.end();
+
+  for (; S != End; S++)
+    Result += tolower(*S) * 13;
+  return Result;
+}
+
+
+
+//===----------------------------------------------------------------------===//
+// Verification and Construction
+//===----------------------------------------------------------------------===//
+
+/// HeaderMap::Create - This attempts to load the specified file as a header
+/// map.  If it doesn't look like a HeaderMap, it gives up and returns null.
+/// If it looks like a HeaderMap but is obviously corrupted, it puts a reason
+/// into the string error argument and returns null.
+const HeaderMap *HeaderMap::Create(const FileEntry *FE) {
+  // If the file is too small to be a header map, ignore it.
+  unsigned FileSize = FE->getSize();
+  if (FileSize <= sizeof(HMapHeader)) return 0;
+
+  llvm::OwningPtr<const llvm::MemoryBuffer> FileBuffer(
+    llvm::MemoryBuffer::getFile(FE->getName(), 0, FE->getSize()));
+  if (FileBuffer == 0) return 0;  // Unreadable file?
+  const char *FileStart = FileBuffer->getBufferStart();
+
+  // We know the file is at least as big as the header, check it now.
+  const HMapHeader *Header = reinterpret_cast<const HMapHeader*>(FileStart);
+
+  // Sniff it to see if it's a headermap by checking the magic number and
+  // version.
+  bool NeedsByteSwap;
+  if (Header->Magic == HMAP_HeaderMagicNumber &&
+      Header->Version == HMAP_HeaderVersion)
+    NeedsByteSwap = false;
+  else if (Header->Magic == llvm::ByteSwap_32(HMAP_HeaderMagicNumber) &&
+           Header->Version == llvm::ByteSwap_16(HMAP_HeaderVersion))
+    NeedsByteSwap = true;  // Mixed endianness headermap.
+  else
+    return 0;  // Not a header map.
+
+  if (Header->Reserved != 0) return 0;
+
+  // Okay, everything looks good, create the header map.
+  return new HeaderMap(FileBuffer.take(), NeedsByteSwap);
+}
+
+HeaderMap::~HeaderMap() {
+  delete FileBuffer;
+}
+
+//===----------------------------------------------------------------------===//
+//  Utility Methods
+//===----------------------------------------------------------------------===//
+
+
+/// getFileName - Return the filename of the headermap.
+const char *HeaderMap::getFileName() const {
+  return FileBuffer->getBufferIdentifier();
+}
+
+unsigned HeaderMap::getEndianAdjustedWord(unsigned X) const {
+  if (!NeedsBSwap) return X;
+  return llvm::ByteSwap_32(X);
+}
+
+/// getHeader - Return a reference to the file header, in unbyte-swapped form.
+/// This method cannot fail.
+const HMapHeader &HeaderMap::getHeader() const {
+  // We know the file is at least as big as the header.  Return it.
+  return *reinterpret_cast<const HMapHeader*>(FileBuffer->getBufferStart());
+}
+
+/// getBucket - Return the specified hash table bucket from the header map,
+/// bswap'ing its fields as appropriate.  If the bucket number is not valid,
+/// this return a bucket with an empty key (0).
+HMapBucket HeaderMap::getBucket(unsigned BucketNo) const {
+  HMapBucket Result;
+  Result.Key = HMAP_EmptyBucketKey;
+
+  const HMapBucket *BucketArray =
+    reinterpret_cast<const HMapBucket*>(FileBuffer->getBufferStart() +
+                                        sizeof(HMapHeader));
+
+  const HMapBucket *BucketPtr = BucketArray+BucketNo;
+  if ((char*)(BucketPtr+1) > FileBuffer->getBufferEnd()) {
+    Result.Prefix = 0;
+    Result.Suffix = 0;
+    return Result;  // Invalid buffer, corrupt hmap.
+  }
+
+  // Otherwise, the bucket is valid.  Load the values, bswapping as needed.
+  Result.Key    = getEndianAdjustedWord(BucketPtr->Key);
+  Result.Prefix = getEndianAdjustedWord(BucketPtr->Prefix);
+  Result.Suffix = getEndianAdjustedWord(BucketPtr->Suffix);
+  return Result;
+}
+
+/// getString - Look up the specified string in the string table.  If the string
+/// index is not valid, it returns an empty string.
+const char *HeaderMap::getString(unsigned StrTabIdx) const {
+  // Add the start of the string table to the idx.
+  StrTabIdx += getEndianAdjustedWord(getHeader().StringsOffset);
+
+  // Check for invalid index.
+  if (StrTabIdx >= FileBuffer->getBufferSize())
+    return 0;
+
+  // Otherwise, we have a valid pointer into the file.  Just return it.  We know
+  // that the "string" can not overrun the end of the file, because the buffer
+  // is nul terminated by virtue of being a MemoryBuffer.
+  return FileBuffer->getBufferStart()+StrTabIdx;
+}
+
+//===----------------------------------------------------------------------===//
+// The Main Drivers
+//===----------------------------------------------------------------------===//
+
+/// dump - Print the contents of this headermap to stderr.
+void HeaderMap::dump() const {
+  const HMapHeader &Hdr = getHeader();
+  unsigned NumBuckets = getEndianAdjustedWord(Hdr.NumBuckets);
+
+  fprintf(stderr, "Header Map %s:\n  %d buckets, %d entries\n",
+          getFileName(), NumBuckets,
+          getEndianAdjustedWord(Hdr.NumEntries));
+
+  for (unsigned i = 0; i != NumBuckets; ++i) {
+    HMapBucket B = getBucket(i);
+    if (B.Key == HMAP_EmptyBucketKey) continue;
+
+    const char *Key    = getString(B.Key);
+    const char *Prefix = getString(B.Prefix);
+    const char *Suffix = getString(B.Suffix);
+    fprintf(stderr, "  %d. %s -> '%s' '%s'\n", i, Key, Prefix, Suffix);
+  }
+}
+
+/// LookupFile - Check to see if the specified relative filename is located in
+/// this HeaderMap.  If so, open it and return its FileEntry.
+const FileEntry *HeaderMap::LookupFile(llvm::StringRef Filename,
+                                       FileManager &FM) const {
+  const HMapHeader &Hdr = getHeader();
+  unsigned NumBuckets = getEndianAdjustedWord(Hdr.NumBuckets);
+
+  // If the number of buckets is not a power of two, the headermap is corrupt.
+  // Don't probe infinitely.
+  if (NumBuckets & (NumBuckets-1))
+    return 0;
+
+  // Linearly probe the hash table.
+  for (unsigned Bucket = HashHMapKey(Filename);; ++Bucket) {
+    HMapBucket B = getBucket(Bucket & (NumBuckets-1));
+    if (B.Key == HMAP_EmptyBucketKey) return 0; // Hash miss.
+
+    // See if the key matches.  If not, probe on.
+    if (!Filename.equals_lower(getString(B.Key)))
+      continue;
+
+    // If so, we have a match in the hash table.  Construct the destination
+    // path.
+    llvm::SmallString<1024> DestPath;
+    DestPath += getString(B.Prefix);
+    DestPath += getString(B.Suffix);
+    return FM.getFile(DestPath.begin(), DestPath.end());
+  }
+}