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
@@ -439,8 +439,8 @@
   /// SourceManager keeps an array of these objects, and they are uniquely
   /// identified by the FileID datatype.
   class SLocEntry {
-    unsigned Offset : 31;
-    unsigned IsExpansion : 1;
+    unsigned Offset;
+    bool IsExpansion;
     union {
       FileInfo File;
       ExpansionInfo Expansion;
@@ -465,7 +465,6 @@
     }
 
     static SLocEntry get(unsigned Offset, const FileInfo &FI) {
-      assert(!(Offset & (1u << 31)) && "Offset is too large");
       SLocEntry E;
       E.Offset = Offset;
       E.IsExpansion = false;
@@ -474,7 +473,6 @@
     }
 
     static SLocEntry get(unsigned Offset, const ExpansionInfo &Expansion) {
-      assert(!(Offset & (1u << 31)) && "Offset is too large");
       SLocEntry E;
       E.Offset = Offset;
       E.IsExpansion = true;
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
@@ -885,7 +885,7 @@
     bool Invalid = false;
     unsigned MiddleIndex = (GreaterIndex-LessIndex)/2+LessIndex;
     unsigned MidOffset = getLocalSLocEntry(MiddleIndex, &Invalid).getOffset();
-    if (Invalid)
+    if (LLVM_UNLIKELY(Invalid))
       return FileID::get(0);
 
     ++NumProbes;
@@ -896,13 +896,15 @@
       GreaterIndex = MiddleIndex;
       continue;
     }
+    // Otherwise, move the low-side up to the middle index. Used in next
+    // iteration if !isOffsetInFileID() below.
+    LessIndex = MiddleIndex;
 
     // If the middle index contains the value, succeed and return.
     // FIXME: This could be made faster by using a function that's aware of
     // being in the local area.
-    if (isOffsetInFileID(FileID::get(MiddleIndex), SLocOffset)) {
-      FileID Res = FileID::get(MiddleIndex);
-
+    FileID Res = FileID::get(MiddleIndex);
+    if (LLVM_UNLIKELY(isOffsetInFileID(Res, SLocOffset))) {
       // If this isn't a macro expansion, remember it.  We have good locality
       // across FileID lookups.
       if (!LocalSLocEntryTable[MiddleIndex].isExpansion())
@@ -910,9 +912,6 @@
       NumBinaryProbes += NumProbes;
       return Res;
     }
-
-    // Otherwise, move the low-side up to the middle index.
-    LessIndex = MiddleIndex;
   }
 }