diff --git a/lib/Frontend/PCHReader.cpp b/lib/Frontend/PCHReader.cpp
index f1d7e7a..e7404f8 100644
--- a/lib/Frontend/PCHReader.cpp
+++ b/lib/Frontend/PCHReader.cpp
@@ -507,9 +507,17 @@
     std::string MacroName = Extra.substr(StartOfMacroName,
                                          EndOfMacroName - StartOfMacroName);
 
-    // FIXME: Perform this check!
-    fprintf(stderr, "FIXME: check whether '%s' was used in the PCH file\n",
-            MacroName.c_str());
+    // Check whether this name was used somewhere in the PCH file. If
+    // so, defining it as a macro could change behavior, so we reject
+    // the PCH file.
+    if (IdentifierInfo *II = get(MacroName.c_str(),
+                                 MacroName.c_str() + MacroName.size())) {
+      Diag(diag::warn_macro_name_used_in_pch)
+        << II;
+      Diag(diag::note_ignoring_pch)
+        << FileName;
+      return true;
+    }
 
     // Add this definition to the suggested predefines buffer.
     SuggestedPredefines += Extra;
@@ -818,9 +826,11 @@
                                          Name);
     FileID BufferID = SourceMgr.createFileIDForMemBuffer(Buffer, ID, Offset);
       
-    if (strcmp(Name, "<built-in>") == 0
-        && CheckPredefinesBuffer(BlobStart, BlobLen - 1, BufferID))
-      return IgnorePCH;
+    if (strcmp(Name, "<built-in>") == 0) {
+      PCHPredefinesBufferID = BufferID;
+      PCHPredefines = BlobStart;
+      PCHPredefinesLen = BlobLen - 1;
+    }
 
     break;
   }
@@ -1287,7 +1297,12 @@
   // Load the translation unit declaration
   if (Context)
     ReadDeclRecord(DeclOffsets[0], 0);
-
+  
+  // Check the predefines buffer.
+  if (CheckPredefinesBuffer(PCHPredefines, PCHPredefinesLen, 
+                            PCHPredefinesBufferID))
+    return IgnorePCH;
+  
   // Initialization of builtins and library builtins occurs before the
   // PCH file is read, so there may be some identifiers that were
   // loaded into the IdentifierTable before we intercepted the
diff --git a/lib/Frontend/PCHWriter.cpp b/lib/Frontend/PCHWriter.cpp
index e61cca5..11f6202 100644
--- a/lib/Frontend/PCHWriter.cpp
+++ b/lib/Frontend/PCHWriter.cpp
@@ -1378,11 +1378,21 @@
 
   // Create and write out the blob that contains the identifier
   // strings.
-  IdentifierOffsets.resize(IdentifierIDs.size());
   {
     OnDiskChainedHashTableGenerator<PCHIdentifierTableTrait> Generator;
     
+    // Look for any identifiers that were named while processing the
+    // headers, but are otherwise not needed. We add these to the hash
+    // table to enable checking of the predefines buffer in the case
+    // where the user adds new macro definitions when building the PCH
+    // file.
+    for (IdentifierTable::iterator ID = PP.getIdentifierTable().begin(),
+                                IDEnd = PP.getIdentifierTable().end();
+         ID != IDEnd; ++ID)
+      getIdentifierRef(ID->second);
+
     // Create the on-disk hash table representation.
+    IdentifierOffsets.resize(IdentifierIDs.size());
     for (llvm::DenseMap<const IdentifierInfo *, pch::IdentID>::iterator
            ID = IdentifierIDs.begin(), IDEnd = IdentifierIDs.end();
          ID != IDEnd; ++ID) {