diff --git a/clang/include/clang/Basic/SourceManager.h b/clang/include/clang/Basic/SourceManager.h
--- a/clang/include/clang/Basic/SourceManager.h
+++ b/clang/include/clang/Basic/SourceManager.h
@@ -692,6 +692,9 @@
   /// Positive FileIDs are indexes into this table. Entry 0 indicates an invalid
   /// expansion.
   SmallVector<SrcMgr::SLocEntry, 0> LocalSLocEntryTable;
+  /// An in-parallel SlocEntry offset table, merely used for speeding up the
+  /// FileID lookup.
+  SmallVector<SourceLocation::UIntTy> LocalLocOffsetTable;
 
   /// The table of SLocEntries that are loaded from other modules.
   ///
diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -336,6 +336,7 @@
 void SourceManager::clearIDTables() {
   MainFileID = FileID();
   LocalSLocEntryTable.clear();
+  LocalLocOffsetTable.clear();
   LoadedSLocEntryTable.clear();
   SLocEntryLoaded.clear();
   LastLineNoFileIDQuery = FileID();
@@ -618,6 +619,7 @@
   LocalSLocEntryTable.push_back(
       SLocEntry::get(NextLocalOffset,
                      FileInfo::get(IncludePos, File, FileCharacter, Filename)));
+  LocalLocOffsetTable.push_back(NextLocalOffset);
   // We do a +1 here because we want a SourceLocation that means "the end of the
   // file", e.g. for the "no newline at the end of the file" diagnostic.
   NextLocalOffset += FileSize + 1;
@@ -669,6 +671,7 @@
     return SourceLocation::getMacroLoc(LoadedOffset);
   }
   LocalSLocEntryTable.push_back(SLocEntry::get(NextLocalOffset, Info));
+  LocalLocOffsetTable.push_back(NextLocalOffset);
   assert(NextLocalOffset + Length + 1 > NextLocalOffset &&
          NextLocalOffset + Length + 1 <= CurrentLoadedOffset &&
          "Ran out of source locations!");
@@ -799,10 +802,10 @@
   // SLocOffset.
   unsigned LessIndex = 0;
   // upper bound of the search range.
-  unsigned GreaterIndex = LocalSLocEntryTable.size();
+  unsigned GreaterIndex = LocalLocOffsetTable.size();
   if (LastFileIDLookup.ID >= 0) {
     // Use the LastFileIDLookup to prune the search space.
-    if (LocalSLocEntryTable[LastFileIDLookup.ID].getOffset() < SLocOffset)
+    if (LocalLocOffsetTable[LastFileIDLookup.ID] < SLocOffset)
       LessIndex = LastFileIDLookup.ID;
     else
       GreaterIndex = LastFileIDLookup.ID;
@@ -812,8 +815,8 @@
   unsigned NumProbes = 0;
   while (true) {
     --GreaterIndex;
-    assert(GreaterIndex < LocalSLocEntryTable.size());
-    if (LocalSLocEntryTable[GreaterIndex].getOffset() <= SLocOffset) {
+    assert(GreaterIndex < LocalLocOffsetTable.size());
+    if (LocalLocOffsetTable[GreaterIndex] <= SLocOffset) {
       FileID Res = FileID::get(int(GreaterIndex));
       // Remember it.  We have good locality across FileID lookups.
       LastFileIDLookup = Res;
@@ -827,8 +830,7 @@
   NumProbes = 0;
   while (true) {
     unsigned MiddleIndex = (GreaterIndex-LessIndex)/2+LessIndex;
-    SourceLocation::UIntTy MidOffset =
-        getLocalSLocEntry(MiddleIndex).getOffset();
+    SourceLocation::UIntTy MidOffset = LocalLocOffsetTable[MiddleIndex];
 
     ++NumProbes;
 
@@ -840,8 +842,8 @@
     }
 
     // If the middle index contains the value, succeed and return.
-    if (MiddleIndex + 1 == LocalSLocEntryTable.size() ||
-        SLocOffset < getLocalSLocEntry(MiddleIndex + 1).getOffset()) {
+    if (MiddleIndex + 1 == LocalLocOffsetTable.size() ||
+        SLocOffset < LocalLocOffsetTable[MiddleIndex + 1]) {
       FileID Res = FileID::get(MiddleIndex);
 
       // Remember it.  We have good locality across FileID lookups.
@@ -2254,6 +2256,7 @@
 size_t SourceManager::getDataStructureSizes() const {
   size_t size = llvm::capacity_in_bytes(MemBufferInfos)
     + llvm::capacity_in_bytes(LocalSLocEntryTable)
+    + llvm::capacity_in_bytes(LocalLocOffsetTable)
     + llvm::capacity_in_bytes(LoadedSLocEntryTable)
     + llvm::capacity_in_bytes(SLocEntryLoaded)
     + llvm::capacity_in_bytes(FileInfos);