diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index b14250b0662..219fc1877df 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -441,29 +441,22 @@ bool BitcodeReader::ParseAttributeBlock() {
 
   // Read all the records.
   while (1) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of PARAMATTR block");
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return Error("Error at end of PARAMATTR block");
+    case BitstreamEntry::EndBlock:
       return false;
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
     }
-
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      // No known subblocks, always skip them.
-      Stream.ReadSubBlockID();
-      if (Stream.SkipBlock())
-        return Error("Malformed block record");
-      continue;
-    }
-
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
-    }
-
+    
     // Read a record.
     Record.clear();
-    switch (Stream.ReadRecord(Code, Record)) {
+    switch (Stream.readRecord(Entry.ID, Record)) {
     default:  // Default behavior: ignore.
       break;
     case bitc::PARAMATTR_CODE_ENTRY: { // ENTRY: [paramidx0, attr0, ...]
@@ -509,32 +502,26 @@ bool BitcodeReader::ParseTypeTableBody() {
 
   // Read all the records for this type table.
   while (1) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      Error("Error in the type table block");
+      return true;
+    case BitstreamEntry::EndBlock:
       if (NumRecords != TypeList.size())
         return Error("Invalid type forward reference in TYPE_BLOCK");
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of type table block");
       return false;
-    }
-
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      // No known subblocks, always skip them.
-      Stream.ReadSubBlockID();
-      if (Stream.SkipBlock())
-        return Error("Malformed block record");
-      continue;
-    }
-
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
     }
 
     // Read a record.
     Record.clear();
     Type *ResultTy = 0;
-    switch (Stream.ReadRecord(Code, Record)) {
+    switch (Stream.readRecord(Entry.ID, Record)) {
     default: return Error("unknown type in type table");
     case bitc::TYPE_CODE_NUMENTRY: // TYPE_CODE_NUMENTRY: [numentries]
       // TYPE_CODE_NUMENTRY contains a count of the number of types in the
@@ -732,28 +719,22 @@ bool BitcodeReader::ParseValueSymbolTable() {
   // Read all the records for this value table.
   SmallString<128> ValueName;
   while (1) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of value symbol table block");
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return Error("malformed value symbol table block");
+    case BitstreamEntry::EndBlock:
       return false;
-    }
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      // No known subblocks, always skip them.
-      Stream.ReadSubBlockID();
-      if (Stream.SkipBlock())
-        return Error("Malformed block record");
-      continue;
-    }
-
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
     }
 
     // Read a record.
     Record.clear();
-    switch (Stream.ReadRecord(Code, Record)) {
+    switch (Stream.readRecord(Entry.ID, Record)) {
     default:  // Default behavior: unknown type.
       break;
     case bitc::VST_CODE_ENTRY: {  // VST_ENTRY: [valueid, namechar x N]
@@ -793,30 +774,24 @@ bool BitcodeReader::ParseMetadata() {
 
   // Read all the records.
   while (1) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of PARAMATTR block");
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      Error("malformed metadata block");
+      return true;
+    case BitstreamEntry::EndBlock:
       return false;
-    }
-
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      // No known subblocks, always skip them.
-      Stream.ReadSubBlockID();
-      if (Stream.SkipBlock())
-        return Error("Malformed block record");
-      continue;
-    }
-
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
     }
 
     bool IsFunctionLocal = false;
     // Read a record.
     Record.clear();
-    Code = Stream.ReadRecord(Code, Record);
+    unsigned Code = Stream.readRecord(Entry.ID, Record);
     switch (Code) {
     default:  // Default behavior: ignore.
       break;
@@ -827,7 +802,7 @@ bool BitcodeReader::ParseMetadata() {
       Code = Stream.ReadCode();
 
       // METADATA_NAME is always followed by METADATA_NAMED_NODE.
-      unsigned NextBitCode = Stream.ReadRecord(Code, Record);
+      unsigned NextBitCode = Stream.readRecord(Code, Record);
       assert(NextBitCode == bitc::METADATA_NAMED_NODE); (void)NextBitCode;
 
       // Read named metadata elements.
@@ -954,27 +929,29 @@ bool BitcodeReader::ParseConstants() {
   Type *CurTy = Type::getInt32Ty(Context);
   unsigned NextCstNo = ValueList.size();
   while (1) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK)
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return Error("malformed block record in AST file");
+    case BitstreamEntry::EndBlock:
+      if (NextCstNo != ValueList.size())
+        return Error("Invalid constant reference!");
+      
+      // Once all the constants have been read, go through and resolve forward
+      // references.
+      ValueList.ResolveConstantForwardRefs();
+      return false;
+    case BitstreamEntry::Record:
+      // The interesting case.
       break;
-
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      // No known subblocks, always skip them.
-      Stream.ReadSubBlockID();
-      if (Stream.SkipBlock())
-        return Error("Malformed block record");
-      continue;
-    }
-
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
     }
 
     // Read a record.
     Record.clear();
     Value *V = 0;
-    unsigned BitCode = Stream.ReadRecord(Code, Record);
+    unsigned BitCode = Stream.readRecord(Entry.ID, Record);
     switch (BitCode) {
     default:  // Default behavior: unknown constant
     case bitc::CST_CODE_UNDEF:     // UNDEF
@@ -1329,17 +1306,6 @@ bool BitcodeReader::ParseConstants() {
     ValueList.AssignValue(V, NextCstNo);
     ++NextCstNo;
   }
-
-  if (NextCstNo != ValueList.size())
-    return Error("Invalid constant reference!");
-
-  if (Stream.ReadBlockEnd())
-    return Error("Error at end of constants block");
-
-  // Once all the constants have been read, go through and resolve forward
-  // references.
-  ValueList.ResolveConstantForwardRefs();
-  return false;
 }
 
 bool BitcodeReader::ParseUseLists() {
@@ -1350,29 +1316,22 @@ bool BitcodeReader::ParseUseLists() {
 
   // Read all the records.
   while (1) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of use-list table block");
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return Error("malformed use list block");
+    case BitstreamEntry::EndBlock:
       return false;
-    }
-
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      // No known subblocks, always skip them.
-      Stream.ReadSubBlockID();
-      if (Stream.SkipBlock())
-        return Error("Malformed block record");
-      continue;
-    }
-
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
     }
 
     // Read a use list record.
     Record.clear();
-    switch (Stream.ReadRecord(Code, Record)) {
+    switch (Stream.readRecord(Entry.ID, Record)) {
     default:  // Default behavior: unknown type.
       break;
     case bitc::USELIST_CODE_ENTRY: { // USELIST_CODE_ENTRY: TBD.
@@ -1444,17 +1403,18 @@ bool BitcodeReader::ParseModule(bool Resume) {
   std::vector<std::string> GCTable;
 
   // Read all the records for this module.
-  while (!Stream.AtEndOfStream()) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of module block");
-
+  while (1) {
+    BitstreamEntry Entry = Stream.advance();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::Error:
+      Error("malformed module block");
+      return true;
+    case BitstreamEntry::EndBlock:
       return GlobalCleanup();
-    }
-
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      switch (Stream.ReadSubBlockID()) {
+      
+    case BitstreamEntry::SubBlock:
+      switch (Entry.ID) {
       default:  // Skip unknown content.
         if (Stream.SkipBlock())
           return Error("Malformed block record");
@@ -1493,7 +1453,7 @@ bool BitcodeReader::ParseModule(bool Resume) {
             return true;
           SeenFirstFunctionBody = true;
         }
-
+        
         if (RememberAndSkipFunctionBody())
           return true;
         // For streaming bitcode, suspend parsing when we reach the function
@@ -1513,15 +1473,15 @@ bool BitcodeReader::ParseModule(bool Resume) {
         break;
       }
       continue;
+      
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
     }
 
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
-    }
 
     // Read a record.
-    switch (Stream.ReadRecord(Code, Record)) {
+    switch (Stream.readRecord(Entry.ID, Record)) {
     default: break;  // Default behavior, ignore unknown content.
     case bitc::MODULE_CODE_VERSION: {  // VERSION: [version#]
       if (Record.size() < 1)
@@ -1709,8 +1669,6 @@ bool BitcodeReader::ParseModule(bool Resume) {
     }
     Record.clear();
   }
-
-  return Error("Premature end of bitstream");
 }
 
 bool BitcodeReader::ParseBitcodeInto(Module *M) {
@@ -1729,47 +1687,55 @@ bool BitcodeReader::ParseBitcodeInto(Module *M) {
 
   // We expect a number of well-defined blocks, though we don't necessarily
   // need to understand them all.
-  while (!Stream.AtEndOfStream()) {
-    unsigned Code = Stream.ReadCode();
-
-    if (Code != bitc::ENTER_SUBBLOCK) {
-
-      // The ranlib in xcode 4 will align archive members by appending newlines
+  while (1) {
+    if (Stream.AtEndOfStream())
+      return false;
+    
+    BitstreamEntry Entry =
+      Stream.advance(BitstreamCursor::AF_DontAutoprocessAbbrevs);
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::Error:
+      Error("malformed module file");
+      return true;
+    case BitstreamEntry::EndBlock:
+      return false;
+      
+    case BitstreamEntry::SubBlock:
+      switch (Entry.ID) {
+      case bitc::BLOCKINFO_BLOCK_ID:
+        if (Stream.ReadBlockInfoBlock())
+          return Error("Malformed BlockInfoBlock");
+        break;
+      case bitc::MODULE_BLOCK_ID:
+        // Reject multiple MODULE_BLOCK's in a single bitstream.
+        if (TheModule)
+          return Error("Multiple MODULE_BLOCKs in same stream");
+        TheModule = M;
+        if (ParseModule(false))
+          return true;
+        if (LazyStreamer) return false;
+        break;
+      default:
+        if (Stream.SkipBlock())
+          return Error("Malformed block record");
+        break;
+      }
+      continue;
+    case BitstreamEntry::Record:
+      // There should be no records in the top-level of blocks.
+        
+      // The ranlib in Xcode 4 will align archive members by appending newlines
       // to the end of them. If this file size is a multiple of 4 but not 8, we
       // have to read and ignore these final 4 bytes :-(
-      if (Stream.getAbbrevIDWidth() == 2 && Code == 2 &&
+      if (Stream.getAbbrevIDWidth() == 2 && Entry.ID == 2 &&
           Stream.Read(6) == 2 && Stream.Read(24) == 0xa0a0a &&
           Stream.AtEndOfStream())
         return false;
-
+      
       return Error("Invalid record at top-level");
     }
-
-    unsigned BlockID = Stream.ReadSubBlockID();
-
-    // We only know the MODULE subblock ID.
-    switch (BlockID) {
-    case bitc::BLOCKINFO_BLOCK_ID:
-      if (Stream.ReadBlockInfoBlock())
-        return Error("Malformed BlockInfoBlock");
-      break;
-    case bitc::MODULE_BLOCK_ID:
-      // Reject multiple MODULE_BLOCK's in a single bitstream.
-      if (TheModule)
-        return Error("Multiple MODULE_BLOCKs in same stream");
-      TheModule = M;
-      if (ParseModule(false))
-        return true;
-      if (LazyStreamer) return false;
-      break;
-    default:
-      if (Stream.SkipBlock())
-        return Error("Malformed block record");
-      break;
-    }
   }
-
-  return false;
 }
 
 bool BitcodeReader::ParseModuleTriple(std::string &Triple) {
@@ -1779,32 +1745,22 @@ bool BitcodeReader::ParseModuleTriple(std::string &Triple) {
   SmallVector<uint64_t, 64> Record;
 
   // Read all the records for this module.
-  while (!Stream.AtEndOfStream()) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of module block");
-
+  while (1) {
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return Error("malformed module block");
+    case BitstreamEntry::EndBlock:
       return false;
-    }
-
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      switch (Stream.ReadSubBlockID()) {
-      default:  // Skip unknown content.
-        if (Stream.SkipBlock())
-          return Error("Malformed block record");
-        break;
-      }
-      continue;
-    }
-
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
     }
 
     // Read a record.
-    switch (Stream.ReadRecord(Code, Record)) {
+    switch (Stream.readRecord(Entry.ID, Record)) {
     default: break;  // Default behavior, ignore unknown content.
     case bitc::MODULE_CODE_TRIPLE: {  // TRIPLE: [strchr x N]
       std::string S;
@@ -1816,8 +1772,6 @@ bool BitcodeReader::ParseModuleTriple(std::string &Triple) {
     }
     Record.clear();
   }
-
-  return Error("Premature end of bitstream");
 }
 
 bool BitcodeReader::ParseTriple(std::string &Triple) {
@@ -1834,28 +1788,32 @@ bool BitcodeReader::ParseTriple(std::string &Triple) {
 
   // We expect a number of well-defined blocks, though we don't necessarily
   // need to understand them all.
-  while (!Stream.AtEndOfStream()) {
-    unsigned Code = Stream.ReadCode();
-
-    if (Code != bitc::ENTER_SUBBLOCK)
-      return Error("Invalid record at top-level");
-
-    unsigned BlockID = Stream.ReadSubBlockID();
-
-    // We only know the MODULE subblock ID.
-    switch (BlockID) {
-    case bitc::MODULE_BLOCK_ID:
-      if (ParseModuleTriple(Triple))
+  while (1) {
+    BitstreamEntry Entry = Stream.advance();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::Error:
+      Error("malformed module file");
+      return true;
+    case BitstreamEntry::EndBlock:
+      return false;
+      
+    case BitstreamEntry::SubBlock:
+      if (Entry.ID == bitc::MODULE_BLOCK_ID)
+        return ParseModuleTriple(Triple);
+        
+      // Ignore other sub-blocks.
+      if (Stream.SkipBlock()) {
+        Error("malformed block record in AST file");
         return true;
-      break;
-    default:
-      if (Stream.SkipBlock())
-        return Error("Malformed block record");
-      break;
+      }
+      continue;
+      
+    case BitstreamEntry::Record:
+      Stream.skipRecord(Entry.ID);
+      continue;
     }
   }
-
-  return false;
 }
 
 /// ParseMetadataAttachment - Parse metadata attachments.
@@ -1864,20 +1822,23 @@ bool BitcodeReader::ParseMetadataAttachment() {
     return Error("Malformed block record");
 
   SmallVector<uint64_t, 64> Record;
-  while(1) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of PARAMATTR block");
+  while (1) {
+    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::SubBlock: // Handled for us already.
+    case BitstreamEntry::Error:
+      return Error("malformed metadata block");
+    case BitstreamEntry::EndBlock:
+      return false;
+    case BitstreamEntry::Record:
+      // The interesting case.
       break;
     }
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
-    }
+
     // Read a metadata attachment record.
     Record.clear();
-    switch (Stream.ReadRecord(Code, Record)) {
+    switch (Stream.readRecord(Entry.ID, Record)) {
     default:  // Default behavior: ignore.
       break;
     case bitc::METADATA_ATTACHMENT: {
@@ -1898,7 +1859,6 @@ bool BitcodeReader::ParseMetadataAttachment() {
     }
     }
   }
-  return false;
 }
 
 /// ParseFunctionBody - Lazily parse the specified function body block.
@@ -1923,15 +1883,16 @@ bool BitcodeReader::ParseFunctionBody(Function *F) {
   // Read all the records.
   SmallVector<uint64_t, 64> Record;
   while (1) {
-    unsigned Code = Stream.ReadCode();
-    if (Code == bitc::END_BLOCK) {
-      if (Stream.ReadBlockEnd())
-        return Error("Error at end of function block");
-      break;
-    }
-
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      switch (Stream.ReadSubBlockID()) {
+    BitstreamEntry Entry = Stream.advance();
+    
+    switch (Entry.Kind) {
+    case BitstreamEntry::Error:
+      return Error("Bitcode error in function block");
+    case BitstreamEntry::EndBlock:
+      goto OutOfRecordLoop;
+        
+    case BitstreamEntry::SubBlock:
+      switch (Entry.ID) {
       default:  // Skip unknown content.
         if (Stream.SkipBlock())
           return Error("Malformed block record");
@@ -1951,17 +1912,16 @@ bool BitcodeReader::ParseFunctionBody(Function *F) {
         break;
       }
       continue;
+        
+    case BitstreamEntry::Record:
+      // The interesting case.
+      break;
     }
-
-    if (Code == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
-      continue;
-    }
-
+    
     // Read a record.
     Record.clear();
     Instruction *I = 0;
-    unsigned BitCode = Stream.ReadRecord(Code, Record);
+    unsigned BitCode = Stream.readRecord(Entry.ID, Record);
     switch (BitCode) {
     default: // Default behavior: reject
       return Error("Unknown instruction");
@@ -2738,6 +2698,8 @@ bool BitcodeReader::ParseFunctionBody(Function *F) {
       ValueList.AssignValue(I, NextValueNo++);
   }
 
+OutOfRecordLoop:
+  
   // Check the function list for unresolved values.
   if (Argument *A = dyn_cast<Argument>(ValueList.back())) {
     if (A->getParent() == 0) {
diff --git a/lib/Bitcode/Reader/BitstreamReader.cpp b/lib/Bitcode/Reader/BitstreamReader.cpp
index 84d5ca6150c..83df57b8fbe 100644
--- a/lib/Bitcode/Reader/BitstreamReader.cpp
+++ b/lib/Bitcode/Reader/BitstreamReader.cpp
@@ -306,17 +306,21 @@ bool BitstreamCursor::ReadBlockInfoBlock() {
   
   // Read all the records for this module.
   while (1) {
-    unsigned Code = ReadCode();
-    if (Code == bitc::END_BLOCK)
-      return ReadBlockEnd();
-    if (Code == bitc::ENTER_SUBBLOCK) {
-      ReadSubBlockID();
-      if (SkipBlock()) return true;
-      continue;
-    }
+    BitstreamEntry Entry = advanceSkippingSubblocks(AF_DontAutoprocessAbbrevs);
     
+    switch (Entry.Kind) {
+    case llvm::BitstreamEntry::SubBlock: // Handled for us already.
+    case llvm::BitstreamEntry::Error:
+      return true;
+    case llvm::BitstreamEntry::EndBlock:
+      return false;
+    case llvm::BitstreamEntry::Record:
+      // The interesting case.
+      break;
+    }
+
     // Read abbrev records, associate them with CurBID.
-    if (Code == bitc::DEFINE_ABBREV) {
+    if (Entry.ID == bitc::DEFINE_ABBREV) {
       if (!CurBlockInfo) return true;
       ReadAbbrevRecord();
       
@@ -330,7 +334,7 @@ bool BitstreamCursor::ReadBlockInfoBlock() {
     
     // Read a record.
     Record.clear();
-    switch (ReadRecord(Code, Record)) {
+    switch (readRecord(Entry.ID, Record)) {
       default: break;  // Default behavior, ignore unknown content.
       case bitc::BLOCKINFO_CODE_SETBID:
         if (Record.size() < 1) return true;