diff --git a/clang/include/clang/Basic/Diagnostic.h b/clang/include/clang/Basic/Diagnostic.h
--- a/clang/include/clang/Basic/Diagnostic.h
+++ b/clang/include/clang/Basic/Diagnostic.h
@@ -25,6 +25,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/Error.h"
 #include <cassert>
 #include <cstdint>
 #include <limits>
@@ -1303,6 +1304,11 @@
   return DiagnosticBuilder(this);
 }
 
+inline const DiagnosticBuilder &operator<<(const DiagnosticBuilder &DB, llvm::Error &&E) {
+  DB.AddString(toString(std::move(E)));
+  return DB;
+}
+
 inline DiagnosticBuilder DiagnosticsEngine::Report(unsigned DiagID) {
   return Report(SourceLocation(), DiagID);
 }
diff --git a/clang/include/clang/Frontend/FrontendAction.h b/clang/include/clang/Frontend/FrontendAction.h
--- a/clang/include/clang/Frontend/FrontendAction.h
+++ b/clang/include/clang/Frontend/FrontendAction.h
@@ -23,6 +23,7 @@
 #include "clang/Frontend/ASTUnit.h"
 #include "clang/Frontend/FrontendOptions.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Error.h"
 #include <memory>
 #include <string>
 #include <vector>
@@ -229,7 +230,8 @@
   bool BeginSourceFile(CompilerInstance &CI, const FrontendInputFile &Input);
 
   /// Set the source manager's main input file, and run the action.
-  bool Execute();
+  __attribute__((deprecated("The semantics of this API have changed from bool to Error, changing the polarity of the return value"))) // FIXME remove this, it's here to help me catch all uses of this API.
+  llvm::Error Execute();
 
   /// Perform any per-file post processing, deallocate per-file
   /// objects, and run statistics and output file cleanup code.
diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
--- a/clang/include/clang/Serialization/ASTReader.h
+++ b/clang/include/clang/Serialization/ASTReader.h
@@ -1437,6 +1437,7 @@
   void Error(StringRef Msg) const;
   void Error(unsigned DiagID, StringRef Arg1 = StringRef(),
              StringRef Arg2 = StringRef()) const;
+  void Error(llvm::Error &&Err) const;
 
 public:
   /// Load the AST file and validate its contents against the given
@@ -2379,7 +2380,7 @@
 
   /// Reads a record with id AbbrevID from Cursor, resetting the
   /// internal state.
-  unsigned readRecord(llvm::BitstreamCursor &Cursor, unsigned AbbrevID);
+  Expected<unsigned> readRecord(llvm::BitstreamCursor &Cursor, unsigned AbbrevID);
 
   /// Is this a module file for a module (rather than a PCH or similar).
   bool isModule() const { return F->isModule(); }
@@ -2679,7 +2680,8 @@
       : Cursor(Cursor), Offset(Cursor.GetCurrentBitNo()) {}
 
   ~SavedStreamPosition() {
-    Cursor.JumpToBit(Offset);
+    if (llvm::Error Err = Cursor.JumpToBit(Offset))
+      llvm::report_fatal_error("Cursor should always be able to go back, failed: " + toString(std::move(Err)));
   }
 
 private:
diff --git a/clang/include/clang/Serialization/GlobalModuleIndex.h b/clang/include/clang/Serialization/GlobalModuleIndex.h
--- a/clang/include/clang/Serialization/GlobalModuleIndex.h
+++ b/clang/include/clang/Serialization/GlobalModuleIndex.h
@@ -20,6 +20,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Error.h"
 #include <memory>
 #include <utility>
 
@@ -122,28 +123,14 @@
 public:
   ~GlobalModuleIndex();
 
-  /// An error code returned when trying to read an index.
-  enum ErrorCode {
-    /// No error occurred.
-    EC_None,
-    /// No index was found.
-    EC_NotFound,
-    /// Some other process is currently building the index; it is not
-    /// available yet.
-    EC_Building,
-    /// There was an unspecified I/O error reading or writing the index.
-    EC_IOError
-  };
-
   /// Read a global index file for the given directory.
   ///
   /// \param Path The path to the specific module cache where the module files
   /// for the intended configuration reside.
   ///
   /// \returns A pair containing the global module index (if it exists) and
-  /// the error code.
-  static std::pair<GlobalModuleIndex *, ErrorCode>
-  readIndex(llvm::StringRef Path);
+  /// the error.
+  static std::pair<GlobalModuleIndex *, llvm::Error> readIndex(llvm::StringRef Path);
 
   /// Returns an iterator for identifiers stored in the index table.
   ///
@@ -194,9 +181,7 @@
   /// creating modules.
   /// \param Path The path to the directory containing module files, into
   /// which the global index will be written.
-  static ErrorCode writeIndex(FileManager &FileMgr,
-                              const PCHContainerReader &PCHContainerRdr,
-                              llvm::StringRef Path);
+  static llvm::Error writeIndex(FileManager &FileMgr, const PCHContainerReader &PCHContainerRdr, llvm::StringRef Path);
 };
 }
 
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -1206,8 +1206,8 @@
   else
     PreambleSrcLocCache.clear();
 
-  if (!Act->Execute())
-    goto error;
+  if (llvm::Error Err = Act->Execute())
+    goto error; // FIXME this drops the error on the floor.
 
   transferASTDataFromCompilerInstance(*Clang);
 
@@ -1632,7 +1632,8 @@
     Clang->setASTConsumer(
         llvm::make_unique<MultiplexConsumer>(std::move(Consumers)));
   }
-  if (!Act->Execute()) {
+  if (llvm::Error Err = Act->Execute()) {
+    // FIXME this drops the error on the floor.
     AST->transferASTDataFromCompilerInstance(*Clang);
     if (OwnAST && ErrAST)
       ErrAST->swap(OwnAST);
@@ -2280,7 +2281,9 @@
   std::unique_ptr<SyntaxOnlyAction> Act;
   Act.reset(new SyntaxOnlyAction);
   if (Act->BeginSourceFile(*Clang.get(), Clang->getFrontendOpts().Inputs[0])) {
-    Act->Execute();
+    if (llvm::Error Err = Act->Execute()) {
+      // FIXME this drops the error on the floor.
+    }
     Act->EndSourceFile();
   }
 }
diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -945,7 +945,9 @@
       getSourceManager().clearIDTables();
 
     if (Act.BeginSourceFile(*this, FIF)) {
-      Act.Execute();
+      if (llvm::Error Err = Act.Execute()) {
+        // FIXME this drops the error on the floor.
+      }
       Act.EndSourceFile();
     }
   }
@@ -2049,9 +2051,11 @@
       hasPreprocessor()) {
     llvm::sys::fs::create_directories(
       getPreprocessor().getHeaderSearchInfo().getModuleCachePath());
-    GlobalModuleIndex::writeIndex(
-        getFileManager(), getPCHContainerReader(),
-        getPreprocessor().getHeaderSearchInfo().getModuleCachePath());
+    if (llvm::Error Err = GlobalModuleIndex::writeIndex(getFileManager(), getPCHContainerReader(), getPreprocessor().getHeaderSearchInfo().getModuleCachePath())) {
+      // FIXME this drops the error on the floor. This code is only used for
+      // typo correction and drops more than just this one source of errors
+      // (such as the directory creation failure above).
+    }
     ModuleManager->resetForReload();
     ModuleManager->loadGlobalIndex();
     GlobalIndex = ModuleManager->getGlobalIndex();
@@ -2076,9 +2080,9 @@
       }
     }
     if (RecreateIndex) {
-      GlobalModuleIndex::writeIndex(
-          getFileManager(), getPCHContainerReader(),
-          getPreprocessor().getHeaderSearchInfo().getModuleCachePath());
+      if (llvm::Error Err = GlobalModuleIndex::writeIndex(getFileManager(), getPCHContainerReader(), getPreprocessor().getHeaderSearchInfo().getModuleCachePath())) {
+        // FIXME As above, this drops the error on the floor.
+      }
       ModuleManager->resetForReload();
       ModuleManager->loadGlobalIndex();
       GlobalIndex = ModuleManager->getGlobalIndex();
diff --git a/clang/lib/Frontend/FrontendAction.cpp b/clang/lib/Frontend/FrontendAction.cpp
--- a/clang/lib/Frontend/FrontendAction.cpp
+++ b/clang/lib/Frontend/FrontendAction.cpp
@@ -924,7 +924,7 @@
   return false;
 }
 
-bool FrontendAction::Execute() {
+llvm::Error FrontendAction::Execute() {
   CompilerInstance &CI = getCompilerInstance();
 
   if (CI.hasFrontendTimer()) {
@@ -939,12 +939,13 @@
       CI.hasPreprocessor()) {
     StringRef Cache =
         CI.getPreprocessor().getHeaderSearchInfo().getModuleCachePath();
-    if (!Cache.empty())
-      GlobalModuleIndex::writeIndex(CI.getFileManager(),
-                                    CI.getPCHContainerReader(), Cache);
+    if (!Cache.empty()) {
+      if (llvm::Error Err = GlobalModuleIndex::writeIndex(CI.getFileManager(), CI.getPCHContainerReader(), Cache))
+        return Err;
+    }
   }
 
-  return true;
+  return llvm::Error::success();
 }
 
 void FrontendAction::EndSourceFile() {
diff --git a/clang/lib/Frontend/PrecompiledPreamble.cpp b/clang/lib/Frontend/PrecompiledPreamble.cpp
--- a/clang/lib/Frontend/PrecompiledPreamble.cpp
+++ b/clang/lib/Frontend/PrecompiledPreamble.cpp
@@ -352,7 +352,8 @@
   if (auto CommentHandler = Callbacks.getCommentHandler())
     Clang->getPreprocessor().addCommentHandler(CommentHandler);
 
-  Act->Execute();
+  if (llvm::Error Err = Act->Execute())
+    return errorToErrorCode(std::move(Err));
 
   // Run the callbacks.
   Callbacks.AfterExecute(*Clang);
diff --git a/clang/lib/Frontend/Rewrite/FrontendActions.cpp b/clang/lib/Frontend/Rewrite/FrontendActions.cpp
--- a/clang/lib/Frontend/Rewrite/FrontendActions.cpp
+++ b/clang/lib/Frontend/Rewrite/FrontendActions.cpp
@@ -129,7 +129,8 @@
       FixItOpts->FixOnlyWarnings = FEOpts.FixOnlyWarnings;
       FixItRewriter Rewriter(CI.getDiagnostics(), CI.getSourceManager(),
                              CI.getLangOpts(), FixItOpts.get());
-      FixAction->Execute();
+      if (llvm::Error Err = FixAction->Execute())
+        err = true; // FIXME this drops the error on the floor.
 
       err = Rewriter.WriteFixedFiles(&RewrittenFiles);
 
diff --git a/clang/lib/Frontend/SerializedDiagnosticReader.cpp b/clang/lib/Frontend/SerializedDiagnosticReader.cpp
--- a/clang/lib/Frontend/SerializedDiagnosticReader.cpp
+++ b/clang/lib/Frontend/SerializedDiagnosticReader.cpp
@@ -41,21 +41,38 @@
     return SDError::InvalidSignature;
 
   // Sniff for the signature.
-  if (Stream.Read(8) != 'D' ||
-      Stream.Read(8) != 'I' ||
-      Stream.Read(8) != 'A' ||
-      Stream.Read(8) != 'G')
+  for (unsigned char C : {'D', 'I', 'A', 'G'}) {
+    if (Expected<llvm::SimpleBitstreamCursor::word_t> Res = Stream.Read(8)) {
+      if (Res.get() == C)
+        continue;
+    } else
+      (void)Res.takeError(); // FIXME propagate the error details.
     return SDError::InvalidSignature;
+  }
 
   // Read the top level blocks.
   while (!Stream.AtEndOfStream()) {
-    if (Stream.ReadCode() != llvm::bitc::ENTER_SUBBLOCK)
-      return SDError::InvalidDiagnostics;
+    if (Expected<unsigned> Res = Stream.ReadCode()) {
+      if (Res.get() != llvm::bitc::ENTER_SUBBLOCK)
+        return SDError::InvalidDiagnostics;
+    } else
+        return SDError::InvalidDiagnostics; // FIXME propagate the error details.
 
     std::error_code EC;
-    switch (Stream.ReadSubBlockID()) {
+    Expected<unsigned> SubBlockID = Stream.ReadSubBlockID();
+    if (!SubBlockID)
+        return SDError::InvalidDiagnostics; // FIXME propagate the error details.
+
+    switch (SubBlockID.get()) {
     case llvm::bitc::BLOCKINFO_BLOCK_ID:
-      BlockInfo = Stream.ReadBlockInfoBlock();
+      {
+        Expected<Optional<llvm::BitstreamBlockInfo>> MaybeBlockInfo = Stream.ReadBlockInfoBlock();
+        if (!MaybeBlockInfo) {
+          (void)MaybeBlockInfo.takeError(); // FIXME propagate the error details.
+          return SDError::InvalidDiagnostics;
+        }
+        BlockInfo = std::move(MaybeBlockInfo.get());
+      }
       if (!BlockInfo)
         return SDError::MalformedBlockInfoBlock;
       Stream.setBlockInfo(&*BlockInfo);
@@ -89,11 +106,18 @@
   BlockOrRecordID = 0;
 
   while (!Stream.AtEndOfStream()) {
-    unsigned Code = Stream.ReadCode();
+    unsigned Code;
+    if (Expected<unsigned> Res = Stream.ReadCode())
+      Code = Res.get();
+    else
+      return llvm::errorToErrorCode(Res.takeError());
 
     switch ((llvm::bitc::FixedAbbrevIDs)Code) {
     case llvm::bitc::ENTER_SUBBLOCK:
-      BlockOrRecordID = Stream.ReadSubBlockID();
+      if (Expected<unsigned> Res = Stream.ReadSubBlockID())
+        BlockOrRecordID = Res.get();
+      else
+        return llvm::errorToErrorCode(Res.takeError());
       return Cursor::BlockBegin;
 
     case llvm::bitc::END_BLOCK:
@@ -102,7 +126,8 @@
       return Cursor::BlockEnd;
 
     case llvm::bitc::DEFINE_ABBREV:
-      Stream.ReadAbbrevRecord();
+      if (llvm::Error Err = Stream.ReadAbbrevRecord())
+        return llvm::errorToErrorCode(std::move(Err));
       continue;
 
     case llvm::bitc::UNABBREV_RECORD:
@@ -145,7 +170,10 @@
     }
 
     SmallVector<uint64_t, 1> Record;
-    unsigned RecordID = Stream.readRecord(BlockOrCode, Record);
+    Expected<unsigned> MaybeRecordID = Stream.readRecord(BlockOrCode, Record);
+    if (!MaybeRecordID)
+      return errorToErrorCode(MaybeRecordID.takeError());
+    unsigned RecordID = MaybeRecordID.get();
 
     if (RecordID == RECORD_VERSION) {
       if (Record.size() < 1)
@@ -193,7 +221,10 @@
     // Read the record.
     Record.clear();
     StringRef Blob;
-    unsigned RecID = Stream.readRecord(BlockOrCode, Record, &Blob);
+    Expected<unsigned> MaybeRecID = Stream.readRecord(BlockOrCode, Record, &Blob);
+    if (!MaybeRecID)
+      return errorToErrorCode(MaybeRecID.takeError());
+    unsigned RecID = MaybeRecID.get();
 
     if (RecID < serialized_diags::RECORD_FIRST ||
         RecID > serialized_diags::RECORD_LAST)
diff --git a/clang/lib/Frontend/TestModuleFileExtension.cpp b/clang/lib/Frontend/TestModuleFileExtension.cpp
--- a/clang/lib/Frontend/TestModuleFileExtension.cpp
+++ b/clang/lib/Frontend/TestModuleFileExtension.cpp
@@ -48,7 +48,11 @@
   // Read the extension block.
   SmallVector<uint64_t, 4> Record;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    llvm::Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      (void)MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::SubBlock:
     case llvm::BitstreamEntry::EndBlock:
@@ -61,8 +65,10 @@
 
     Record.clear();
     StringRef Blob;
-    unsigned RecCode = Stream.readRecord(Entry.ID, Record, &Blob);
-    switch (RecCode) {
+    Expected<unsigned> MaybeRecCode = Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecCode)
+      fprintf(stderr, "Failed reading rec code: %s\n", toString(MaybeRecCode.takeError()).c_str());
+    switch (MaybeRecCode.get()) {
     case FIRST_EXTENSION_RECORD_ID: {
       StringRef Message = Blob.substr(0, Record[0]);
       fprintf(stderr, "Read extension block message: %s\n",
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -1148,12 +1148,26 @@
   assert(Offset != 0);
 
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(Offset);
+  if (llvm::Error Err = Cursor.JumpToBit(Offset)) {
+    Error(std::move(Err));
+    return true;
+  }
 
   RecordData Record;
   StringRef Blob;
-  unsigned Code = Cursor.ReadCode();
-  unsigned RecCode = Cursor.readRecord(Code, Record, &Blob);
+  Expected<unsigned> MaybeCode = Cursor.ReadCode();
+  if (!MaybeCode) {
+    Error(MaybeCode.takeError());
+    return true;
+  }
+  unsigned Code = MaybeCode.get();
+
+  Expected<unsigned> MaybeRecCode = Cursor.readRecord(Code, Record, &Blob);
+  if (!MaybeRecCode) {
+    Error(MaybeRecCode.takeError());
+    return true;
+  }
+  unsigned RecCode = MaybeRecCode.get();
   if (RecCode != DECL_CONTEXT_LEXICAL) {
     Error("Expected lexical block");
     return true;
@@ -1184,12 +1198,26 @@
   assert(Offset != 0);
 
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(Offset);
+  if (llvm::Error Err = Cursor.JumpToBit(Offset)) {
+    Error(std::move(Err));
+    return true;
+  }
 
   RecordData Record;
   StringRef Blob;
-  unsigned Code = Cursor.ReadCode();
-  unsigned RecCode = Cursor.readRecord(Code, Record, &Blob);
+  Expected<unsigned> MaybeCode = Cursor.ReadCode();
+  if (!MaybeCode) {
+    Error(MaybeCode.takeError());
+    return true;
+  }
+  unsigned Code = MaybeCode.get();
+
+  Expected<unsigned> MaybeRecCode = Cursor.readRecord(Code, Record, &Blob);
+  if (!MaybeRecCode) {
+    Error(MaybeRecCode.takeError());
+    return true;
+  }
+  unsigned RecCode = MaybeRecCode.get();
   if (RecCode != DECL_CONTEXT_VISIBLE) {
     Error("Expected visible lookup table block");
     return true;
@@ -1219,6 +1247,10 @@
     Diag(DiagID) << Arg1 << Arg2;
 }
 
+void ASTReader::Error(llvm::Error &&Err) const {
+  Error(toString(std::move(Err)));
+}
+
 //===----------------------------------------------------------------------===//
 // Source Manager Deserialization
 //===----------------------------------------------------------------------===//
@@ -1295,7 +1327,12 @@
 
   RecordData Record;
   while (true) {
-    llvm::BitstreamEntry E = SLocEntryCursor.advanceSkippingSubblocks();
+    Expected<llvm::BitstreamEntry> MaybeE = SLocEntryCursor.advanceSkippingSubblocks();
+    if (!MaybeE) {
+      Error(MaybeE.takeError());
+      return true;
+    }
+    llvm::BitstreamEntry E = MaybeE.get();
 
     switch (E.Kind) {
     case llvm::BitstreamEntry::SubBlock: // Handled for us already.
@@ -1312,7 +1349,12 @@
     // Read a record.
     Record.clear();
     StringRef Blob;
-    switch (SLocEntryCursor.readRecord(E.ID, Record, &Blob)) {
+    Expected<unsigned> MaybeRecord = SLocEntryCursor.readRecord(E.ID, Record, &Blob);
+    if (!MaybeRecord) {
+      Error(MaybeRecord.takeError());
+      return true;
+    }
+    switch (MaybeRecord.get()) {
     default:  // Default behavior: ignore.
       break;
 
@@ -1376,8 +1418,19 @@
       StringRef Name) -> std::unique_ptr<llvm::MemoryBuffer> {
     RecordData Record;
     StringRef Blob;
-    unsigned Code = SLocEntryCursor.ReadCode();
-    unsigned RecCode = SLocEntryCursor.readRecord(Code, Record, &Blob);
+    Expected<unsigned> MaybeCode = SLocEntryCursor.ReadCode();
+    if (!MaybeCode) {
+      Error(MaybeCode.takeError());
+      return nullptr;
+    }
+    unsigned Code = MaybeCode.get();
+
+    Expected<unsigned> MaybeRecCode = SLocEntryCursor.readRecord(Code, Record, &Blob);
+    if (!MaybeRecCode) {
+      Error(MaybeRecCode.takeError());
+      return nullptr;
+    }
+    unsigned RecCode = MaybeRecCode.get();
 
     if (RecCode == SM_SLOC_BUFFER_BLOB_COMPRESSED) {
       if (!llvm::zlib::isAvailable()) {
@@ -1401,12 +1454,22 @@
   };
 
   ModuleFile *F = GlobalSLocEntryMap.find(-ID)->second;
-  F->SLocEntryCursor.JumpToBit(F->SLocEntryOffsets[ID - F->SLocEntryBaseID]);
+  if (llvm::Error Err = F->SLocEntryCursor.JumpToBit(F->SLocEntryOffsets[ID - F->SLocEntryBaseID])) {
+    Error(std::move(Err));
+    return true;
+  }
+
   BitstreamCursor &SLocEntryCursor = F->SLocEntryCursor;
   unsigned BaseOffset = F->SLocEntryBaseOffset;
 
   ++NumSLocEntriesRead;
-  llvm::BitstreamEntry Entry = SLocEntryCursor.advance();
+  Expected<llvm::BitstreamEntry> MaybeEntry = SLocEntryCursor.advance();
+  if (!MaybeEntry) {
+    Error(MaybeEntry.takeError());
+    return true;
+  }
+  llvm::BitstreamEntry Entry = MaybeEntry.get();
+
   if (Entry.Kind != llvm::BitstreamEntry::Record) {
     Error("incorrectly-formatted source location entry in AST file");
     return true;
@@ -1414,7 +1477,12 @@
 
   RecordData Record;
   StringRef Blob;
-  switch (SLocEntryCursor.readRecord(Entry.ID, Record, &Blob)) {
+  Expected<unsigned> MaybeSLOC = SLocEntryCursor.readRecord(Entry.ID, Record, &Blob);
+  if (!MaybeSLOC) {
+    Error(MaybeSLOC.takeError());
+    return true;
+  }
+  switch (MaybeSLOC.get()) {
   default:
     Error("incorrectly-formatted source location entry in AST file");
     return true;
@@ -1538,23 +1606,34 @@
   return F->ImportedBy[0]->FirstLoc;
 }
 
-/// ReadBlockAbbrevs - Enter a subblock of the specified BlockID with the
-/// specified cursor.  Read the abbreviations that are at the top of the block
-/// and then leave the cursor pointing into the block.
+/// Enter a subblock of the specified BlockID with the specified cursor. Read
+/// the abbreviations that are at the top of the block and then leave the cursor
+/// pointing into the block.
 bool ASTReader::ReadBlockAbbrevs(BitstreamCursor &Cursor, unsigned BlockID) {
   if (Cursor.EnterSubBlock(BlockID))
     return true;
 
   while (true) {
     uint64_t Offset = Cursor.GetCurrentBitNo();
-    unsigned Code = Cursor.ReadCode();
+    Expected<unsigned> MaybeCode = Cursor.ReadCode();
+    if (!MaybeCode) {
+      // FIXME this drops errors on the floor.
+      return true;
+    }
+    unsigned Code = MaybeCode.get();
 
     // We expect all abbrevs to be at the start of the block.
     if (Code != llvm::bitc::DEFINE_ABBREV) {
-      Cursor.JumpToBit(Offset);
+      if (llvm::Error Err = Cursor.JumpToBit(Offset)) {
+        // FIXME this drops errors on the floor.
+        return true;
+      }
       return false;
     }
-    Cursor.ReadAbbrevRecord();
+    if (llvm::Error Err = Cursor.ReadAbbrevRecord()) {
+      // FIXME this drops errors on the floor.
+      return true;
+    }
   }
 }
 
@@ -1578,7 +1657,10 @@
   // after reading this macro.
   SavedStreamPosition SavedPosition(Stream);
 
-  Stream.JumpToBit(Offset);
+  if (llvm::Error Err = Stream.JumpToBit(Offset)) {
+    // FIXME this drops errors on the floor.
+    return nullptr;
+  }
   RecordData Record;
   SmallVector<IdentifierInfo*, 16> MacroParams;
   MacroInfo *Macro = nullptr;
@@ -1588,7 +1670,12 @@
     // pop it (removing all the abbreviations from the cursor) since we want to
     // be able to reseek within the block and read entries.
     unsigned Flags = BitstreamCursor::AF_DontPopBlockAtEnd;
-    llvm::BitstreamEntry Entry = Stream.advanceSkippingSubblocks(Flags);
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks(Flags);
+    if (!MaybeEntry) {
+      Error(MaybeEntry.takeError());
+      return Macro;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::SubBlock: // Handled for us already.
@@ -1604,8 +1691,13 @@
 
     // Read a record.
     Record.clear();
-    PreprocessorRecordTypes RecType =
-      (PreprocessorRecordTypes)Stream.readRecord(Entry.ID, Record);
+    PreprocessorRecordTypes RecType;
+    if (Expected<unsigned> MaybeRecType = Stream.readRecord(Entry.ID, Record))
+      RecType = (PreprocessorRecordTypes)MaybeRecType.get();
+    else {
+      Error(MaybeRecType.takeError());
+      return Macro;
+    }
     switch (RecType) {
     case PP_MODULE_MACRO:
     case PP_MACRO_DIRECTIVE_HISTORY:
@@ -1828,11 +1920,19 @@
       continue;
 
     BitstreamCursor Cursor = MacroCursor;
-    Cursor.JumpToBit(I.MacroStartOffset);
+    if (llvm::Error Err = Cursor.JumpToBit(I.MacroStartOffset)) {
+      Error(std::move(Err));
+      return;
+    }
 
     RecordData Record;
     while (true) {
-      llvm::BitstreamEntry E = Cursor.advanceSkippingSubblocks();
+      Expected<llvm::BitstreamEntry> MaybeE = Cursor.advanceSkippingSubblocks();
+      if (!MaybeE) {
+        Error(MaybeE.takeError());
+        return;
+      }
+      llvm::BitstreamEntry E = MaybeE.get();
 
       switch (E.Kind) {
       case llvm::BitstreamEntry::SubBlock: // Handled for us already.
@@ -1842,9 +1942,14 @@
       case llvm::BitstreamEntry::EndBlock:
         goto NextCursor;
 
-      case llvm::BitstreamEntry::Record:
+      case llvm::BitstreamEntry::Record: {
         Record.clear();
-        switch (Cursor.readRecord(E.ID, Record)) {
+        Expected<unsigned> MaybeRecord = Cursor.readRecord(E.ID, Record);
+        if (!MaybeRecord) {
+          Error(MaybeRecord.takeError());
+          return;
+        }
+        switch (MaybeRecord.get()) {
         default:  // Default behavior: ignore.
           break;
 
@@ -1862,6 +1967,7 @@
         }
         break;
       }
+      }
     }
     NextCursor:  ;
   }
@@ -1962,7 +2068,10 @@
 
   BitstreamCursor &Cursor = M.MacroCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(PMInfo.MacroDirectivesOffset);
+  if (llvm::Error Err = Cursor.JumpToBit(PMInfo.MacroDirectivesOffset)) {
+    Error(std::move(Err));
+    return;
+  }
 
   struct ModuleMacroRecord {
     SubmoduleID SubModID;
@@ -1976,15 +2085,26 @@
   // macro histroy.
   RecordData Record;
   while (true) {
-    llvm::BitstreamEntry Entry =
+    Expected<llvm::BitstreamEntry> MaybeEntry =
         Cursor.advance(BitstreamCursor::AF_DontPopBlockAtEnd);
+    if (!MaybeEntry) {
+      Error(MaybeEntry.takeError());
+      return;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     if (Entry.Kind != llvm::BitstreamEntry::Record) {
       Error("malformed block record in AST file");
       return;
     }
 
     Record.clear();
-    switch ((PreprocessorRecordTypes)Cursor.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybePP = Cursor.readRecord(Entry.ID, Record);
+    if (!MaybePP) {
+      Error(MaybePP.takeError());
+      return;
+    }
+    switch ((PreprocessorRecordTypes)MaybePP.get()) {
     case PP_MACRO_DIRECTIVE_HISTORY:
       break;
 
@@ -2070,16 +2190,23 @@
   // Go find this input file.
   BitstreamCursor &Cursor = F.InputFilesCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(F.InputFileOffsets[ID-1]);
+  if (llvm::Error Err = Cursor.JumpToBit(F.InputFileOffsets[ID-1])) {
+    // FIXME this drops errors on the floor.
+  }
 
-  unsigned Code = Cursor.ReadCode();
+  Expected<unsigned> MaybeCode = Cursor.ReadCode();
+  if (!MaybeCode) {
+    // FIXME this drops errors on the floor.
+  }
+  unsigned Code = MaybeCode.get();
   RecordData Record;
   StringRef Blob;
 
-  unsigned Result = Cursor.readRecord(Code, Record, &Blob);
-  assert(static_cast<InputFileRecordTypes>(Result) == INPUT_FILE &&
-         "invalid record type for input file");
-  (void)Result;
+  if (Expected<unsigned> Maybe = Cursor.readRecord(Code, Record, &Blob))
+    assert(static_cast<InputFileRecordTypes>(Maybe.get()) == INPUT_FILE && "invalid record type for input file");
+  else {
+    // FIXME this drops errors on the floor.
+  }
 
   assert(Record[0] == ID && "Bogus stored ID or offset");
   InputFileInfo R;
@@ -2109,7 +2236,9 @@
   // Go find this input file.
   BitstreamCursor &Cursor = F.InputFilesCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(F.InputFileOffsets[ID-1]);
+  if (llvm::Error Err = Cursor.JumpToBit(F.InputFileOffsets[ID-1])) {
+    // FIXME this drops errors on the floor.
+  }
 
   InputFileInfo FI = readInputFileInfo(F, ID);
   off_t StoredSize = FI.StoredSize;
@@ -2265,7 +2394,12 @@
   RecordData Record;
   ASTReadResult Result = Success;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry) {
+      // FIXME this drops errors on the floor.
+      return Failure;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::Error:
@@ -2282,7 +2416,12 @@
 
     // Read and process a record.
     Record.clear();
-    switch ((OptionsRecordTypes)Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecordType = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecordType) {
+      // FIXME this drops errors on the floor.
+      return Failure;
+    }
+    switch ((OptionsRecordTypes)MaybeRecordType.get()) {
     case LANGUAGE_OPTIONS: {
       bool Complain = (ClientLoadCapabilities & ARR_ConfigurationMismatch) == 0;
       if (ParseLanguageOptions(Record, Complain, Listener,
@@ -2362,7 +2501,12 @@
   unsigned NumUserInputs = 0;
   StringRef BaseDirectoryAsWritten;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry) {
+      Error(MaybeEntry.takeError());
+      return Failure;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::Error:
@@ -2485,7 +2629,12 @@
     // Read and process a record.
     Record.clear();
     StringRef Blob;
-    switch ((ControlRecordTypes)Stream.readRecord(Entry.ID, Record, &Blob)) {
+    Expected<unsigned> MaybeRecordType = Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecordType) {
+      Error(MaybeRecordType.takeError());
+      return Failure;
+    }
+    switch ((ControlRecordTypes)MaybeRecordType.get()) {
     case METADATA: {
       if (Record[0] != VERSION_MAJOR && !DisableValidation) {
         if ((ClientLoadCapabilities & ARR_VersionMismatch) == 0)
@@ -2690,7 +2839,12 @@
   // Read all of the records and blocks for the AST file.
   RecordData Record;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry) {
+      Error(MaybeEntry.takeError());
+      return Failure;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::Error:
@@ -2794,8 +2948,12 @@
     // Read and process a record.
     Record.clear();
     StringRef Blob;
-    auto RecordType =
-        (ASTRecordTypes)Stream.readRecord(Entry.ID, Record, &Blob);
+    Expected<unsigned> MaybeRecordType = Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecordType) {
+      Error(MaybeRecordType.takeError());
+      return Failure;
+    }
+    ASTRecordTypes RecordType = (ASTRecordTypes)MaybeRecordType.get();
 
     // If we're not loading an AST context, we don't care about most records.
     if (!ContextObj) {
@@ -3814,8 +3972,8 @@
   TriedLoadingGlobalIndex = true;
   StringRef ModuleCachePath
     = getPreprocessor().getHeaderSearchInfo().getModuleCachePath();
-  std::pair<GlobalModuleIndex *, GlobalModuleIndex::ErrorCode> Result
-    = GlobalModuleIndex::readIndex(ModuleCachePath);
+  std::pair<GlobalModuleIndex *, llvm::Error> Result = GlobalModuleIndex::readIndex(ModuleCachePath);
+  // FIXME this drops errors on the floor.
   if (!Result.first)
     return true;
 
@@ -3846,7 +4004,13 @@
 /// true on failure.
 static bool SkipCursorToBlock(BitstreamCursor &Cursor, unsigned BlockID) {
   while (true) {
-    llvm::BitstreamEntry Entry = Cursor.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Cursor.advance();
+    if (!MaybeEntry) {
+      // FIXME this drops errors on the floor.
+      return true;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::Error:
     case llvm::BitstreamEntry::EndBlock:
@@ -3854,8 +4018,12 @@
 
     case llvm::BitstreamEntry::Record:
       // Ignore top-level records.
-      Cursor.skipRecord(Entry.ID);
-      break;
+      if (Expected<unsigned> Skipped = Cursor.skipRecord(Entry.ID))
+        break;
+      else {
+        // FIXME this drops errors on the floor.
+        return true;
+      }
 
     case llvm::BitstreamEntry::SubBlock:
       if (Entry.ID == BlockID) {
@@ -4108,13 +4276,18 @@
 
 static ASTFileSignature readASTFileSignature(StringRef PCH);
 
-/// Whether \p Stream starts with the AST/PCH file magic number 'CPCH'.
-static bool startsWithASTFileMagic(BitstreamCursor &Stream) {
-  return Stream.canSkipToPos(4) &&
-         Stream.Read(8) == 'C' &&
-         Stream.Read(8) == 'P' &&
-         Stream.Read(8) == 'C' &&
-         Stream.Read(8) == 'H';
+/// Whether \p Stream doesn't start with the AST/PCH file magic number 'CPCH'.
+static llvm::Error doesntStartWithASTFileMagic(BitstreamCursor &Stream) {
+  // FIXME checking magic headers is done in other places such as SerializedDiagnosticReader and GlobalModuleIndex, but error handling isn't always done the same. Unify it all with a helper.
+  if (!Stream.canSkipToPos(4))
+    return llvm::createStringError(std::errc::illegal_byte_sequence, "file too small to contain AST file magic");
+  for (unsigned C : {'C','P','C','H'})
+    if (Expected<llvm::SimpleBitstreamCursor::word_t> Res = Stream.Read(8)) {
+      if (Res.get() != C)
+        return llvm::createStringError(std::errc::illegal_byte_sequence, "file doesn't start with AST file magic");
+    } else
+      return Res.takeError();
+  return llvm::Error::success();
 }
 
 static unsigned moduleKindForDiagnostic(ModuleKind Kind) {
@@ -4201,16 +4374,20 @@
   F.SizeInBits = F.Buffer->getBufferSize() * 8;
 
   // Sniff for the signature.
-  if (!startsWithASTFileMagic(Stream)) {
-    Diag(diag::err_module_file_invalid) << moduleKindForDiagnostic(Type)
-                                        << FileName;
+  if (llvm::Error Err = doesntStartWithASTFileMagic(Stream)) {
+    Diag(diag::err_module_file_invalid) << moduleKindForDiagnostic(Type) << FileName << std::move(Err);
     return Failure;
   }
 
   // This is used for compatibility with older PCH formats.
   bool HaveReadControlBlock = false;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry) {
+      Error(MaybeEntry.takeError());
+      return Failure;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::Error:
@@ -4342,8 +4519,10 @@
   BitstreamCursor Stream(StreamData);
 
   // Sniff for the signature.
-  if (!startsWithASTFileMagic(Stream))
+  if (llvm::Error Err = doesntStartWithASTFileMagic(Stream)) {
+    // FIXME this drops the error on the floor.
     return Failure;
+  }
 
   // Scan for the UNHASHED_CONTROL_BLOCK_ID block.
   if (SkipCursorToBlock(Stream, UNHASHED_CONTROL_BLOCK_ID))
@@ -4353,7 +4532,12 @@
   RecordData Record;
   ASTReadResult Result = Success;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry) {
+      // FIXME this drops the error on the floor.
+      return Failure;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::Error:
@@ -4370,8 +4554,12 @@
 
     // Read and process a record.
     Record.clear();
-    switch (
-        (UnhashedControlBlockRecordTypes)Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecordType = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecordType) {
+      // FIXME this drops the error.
+      return Failure;
+    }
+    switch ((UnhashedControlBlockRecordTypes)MaybeRecordType.get()) {
     case SIGNATURE:
       if (F)
         std::copy(Record.begin(), Record.end(), F->Signature.data());
@@ -4423,7 +4611,13 @@
 
   RecordData Record;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry) {
+      Error(MaybeEntry.takeError());
+      return Failure;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::SubBlock:
       if (Stream.SkipBlock())
@@ -4443,8 +4637,12 @@
 
     Record.clear();
     StringRef Blob;
-    unsigned RecCode = Stream.readRecord(Entry.ID, Record, &Blob);
-    switch (RecCode) {
+    Expected<unsigned> MaybeRecCode = Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecCode) {
+      Error(MaybeRecCode.takeError());
+      return Failure;
+    }
+    switch (MaybeRecCode.get()) {
     case EXTENSION_METADATA: {
       ModuleFileExtensionMetadata Metadata;
       if (parseModuleFileExtensionMetadata(Record, Blob, Metadata))
@@ -4615,8 +4813,10 @@
 /// else returns 0.
 static ASTFileSignature readASTFileSignature(StringRef PCH) {
   BitstreamCursor Stream(PCH);
-  if (!startsWithASTFileMagic(Stream))
+  if (llvm::Error Err = doesntStartWithASTFileMagic(Stream)) {
+    // FIXME this drops the error on the floor.
     return ASTFileSignature();
+  }
 
   // Scan for the UNHASHED_CONTROL_BLOCK_ID block.
   if (SkipCursorToBlock(Stream, UNHASHED_CONTROL_BLOCK_ID))
@@ -4625,13 +4825,24 @@
   // Scan for SIGNATURE inside the diagnostic options block.
   ASTReader::RecordData Record;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry) {
+      // FIXME this drops the error on the floor.
+      return ASTFileSignature();
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     if (Entry.Kind != llvm::BitstreamEntry::Record)
       return ASTFileSignature();
 
     Record.clear();
     StringRef Blob;
-    if (SIGNATURE == Stream.readRecord(Entry.ID, Record, &Blob))
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecord) {
+      // FIXME this drops the error on the floor.
+      return ASTFileSignature();
+    }
+    if (SIGNATURE == MaybeRecord.get())
       return {{{(uint32_t)Record[0], (uint32_t)Record[1], (uint32_t)Record[2],
                 (uint32_t)Record[3], (uint32_t)Record[4]}}};
   }
@@ -4655,8 +4866,8 @@
   BitstreamCursor Stream(PCHContainerRdr.ExtractPCH(**Buffer));
 
   // Sniff for the signature.
-  if (!startsWithASTFileMagic(Stream)) {
-    Diags.Report(diag::err_fe_not_a_pch_file) << ASTFileName;
+  if (llvm::Error Err = doesntStartWithASTFileMagic(Stream)) {
+    Diags.Report(diag::err_fe_not_a_pch_file) << ASTFileName << std::move(Err);
     return std::string();
   }
 
@@ -4669,7 +4880,13 @@
   // Scan for ORIGINAL_FILE inside the control block.
   RecordData Record;
   while (true) {
-    llvm::BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry) {
+      // FIXME this drops errors on the floor.
+      return std::string();
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     if (Entry.Kind == llvm::BitstreamEntry::EndBlock)
       return std::string();
 
@@ -4680,7 +4897,12 @@
 
     Record.clear();
     StringRef Blob;
-    if (Stream.readRecord(Entry.ID, Record, &Blob) == ORIGINAL_FILE)
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecord) {
+      // FIXME this drops the errors on the floor.
+      return std::string();
+    }
+    if (ORIGINAL_FILE == MaybeRecord.get())
       return Blob.str();
   }
 }
@@ -4754,8 +4976,10 @@
   BitstreamCursor Stream(Bytes);
 
   // Sniff for the signature.
-  if (!startsWithASTFileMagic(Stream))
+  if (llvm::Error Err = doesntStartWithASTFileMagic(Stream)) {
+    // FIXME this drops the error on the floor.
     return true;
+  }
 
   // Scan for the CONTROL_BLOCK_ID block.
   if (SkipCursorToBlock(Stream, CONTROL_BLOCK_ID))
@@ -4770,7 +4994,12 @@
   std::string ModuleDir;
   bool DoneWithControlBlock = false;
   while (!DoneWithControlBlock) {
-    llvm::BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry) {
+      // FIXME this drops the error on the floor.
+      return true;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::SubBlock: {
@@ -4816,8 +5045,12 @@
 
     Record.clear();
     StringRef Blob;
-    unsigned RecCode = Stream.readRecord(Entry.ID, Record, &Blob);
-    switch ((ControlRecordTypes)RecCode) {
+    Expected<unsigned> MaybeRecCode = Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecCode) {
+      // FIXME this drops the error.
+      return Failure;
+    }
+    switch ((ControlRecordTypes)MaybeRecCode.get()) {
     case METADATA:
       if (Record[0] != VERSION_MAJOR)
         return true;
@@ -4854,13 +5087,24 @@
 
         BitstreamCursor &Cursor = InputFilesCursor;
         SavedStreamPosition SavedPosition(Cursor);
-        Cursor.JumpToBit(InputFileOffs[I]);
+        if (llvm::Error Err = Cursor.JumpToBit(InputFileOffs[I])) {
+          // FIXME this drops errors on the floor.
+        }
+
+        Expected<unsigned> MaybeCode = Cursor.ReadCode();
+        if (!MaybeCode) {
+          // FIXME this drops errors on the floor.
+        }
+        unsigned Code = MaybeCode.get();
 
-        unsigned Code = Cursor.ReadCode();
         RecordData Record;
         StringRef Blob;
         bool shouldContinue = false;
-        switch ((InputFileRecordTypes)Cursor.readRecord(Code, Record, &Blob)) {
+        Expected<unsigned> MaybeRecordType = Cursor.readRecord(Code, Record, &Blob);
+        if (!MaybeRecordType) {
+          // FIXME this drops errors on the floor.
+        }
+        switch ((InputFileRecordTypes)MaybeRecordType.get()) {
         case INPUT_FILE:
           bool Overridden = static_cast<bool>(Record[3]);
           std::string Filename = Blob;
@@ -4903,7 +5147,12 @@
     while (!SkipCursorToBlock(Stream, EXTENSION_BLOCK_ID)) {
       bool DoneWithExtensionBlock = false;
       while (!DoneWithExtensionBlock) {
-       llvm::BitstreamEntry Entry = Stream.advance();
+        Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+        if (!MaybeEntry) {
+          // FIXME this drops the error.
+          return true;
+        }
+        llvm::BitstreamEntry Entry = MaybeEntry.get();
 
        switch (Entry.Kind) {
        case llvm::BitstreamEntry::SubBlock:
@@ -4925,8 +5174,12 @@
 
        Record.clear();
        StringRef Blob;
-       unsigned RecCode = Stream.readRecord(Entry.ID, Record, &Blob);
-       switch (RecCode) {
+       Expected<unsigned> MaybeRecCode = Stream.readRecord(Entry.ID, Record, &Blob);
+       if (!MaybeRecCode) {
+          // FIXME this drops the error.
+         return true;
+       }
+       switch (MaybeRecCode.get()) {
        case EXTENSION_METADATA: {
          ModuleFileExtensionMetadata Metadata;
          if (parseModuleFileExtensionMetadata(Record, Blob, Metadata))
@@ -4978,7 +5231,12 @@
   Module *CurrentModule = nullptr;
   RecordData Record;
   while (true) {
-    llvm::BitstreamEntry Entry = F.Stream.advanceSkippingSubblocks();
+    Expected<llvm::BitstreamEntry> MaybeEntry = F.Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry) {
+      Error(MaybeEntry.takeError());
+      return Failure;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::SubBlock: // Handled for us already.
@@ -4995,7 +5253,12 @@
     // Read a record.
     StringRef Blob;
     Record.clear();
-    auto Kind = F.Stream.readRecord(Entry.ID, Record, &Blob);
+    Expected<unsigned> MaybeKind = F.Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeKind) {
+      Error(MaybeKind.takeError());
+      return Failure;
+    }
+    unsigned Kind = MaybeKind.get();
 
     if ((Kind == SUBMODULE_METADATA) != First) {
       Error("submodule metadata record should be at beginning of block");
@@ -5472,10 +5735,18 @@
   }
 
   SavedStreamPosition SavedPosition(M.PreprocessorDetailCursor);
-  M.PreprocessorDetailCursor.JumpToBit(PPOffs.BitOffset);
+  if (llvm::Error Err = M.PreprocessorDetailCursor.JumpToBit(PPOffs.BitOffset)) {
+    Error(std::move(Err));
+    return nullptr;
+  }
 
-  llvm::BitstreamEntry Entry =
-    M.PreprocessorDetailCursor.advance(BitstreamCursor::AF_DontPopBlockAtEnd);
+  Expected<llvm::BitstreamEntry> MaybeEntry = M.PreprocessorDetailCursor.advance(BitstreamCursor::AF_DontPopBlockAtEnd);
+  if (!MaybeEntry) {
+    Error(MaybeEntry.takeError());
+    return nullptr;
+  }
+  llvm::BitstreamEntry Entry = MaybeEntry.get();
+    
   if (Entry.Kind != llvm::BitstreamEntry::Record)
     return nullptr;
 
@@ -5485,10 +5756,12 @@
   PreprocessingRecord &PPRec = *PP.getPreprocessingRecord();
   StringRef Blob;
   RecordData Record;
-  PreprocessorDetailRecordTypes RecType =
-    (PreprocessorDetailRecordTypes)M.PreprocessorDetailCursor.readRecord(
-                                          Entry.ID, Record, &Blob);
-  switch (RecType) {
+  Expected<unsigned> MaybeRecType = M.PreprocessorDetailCursor.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeRecType) {
+      Error(MaybeRecType.takeError());
+      return nullptr;
+    }
+  switch ((PreprocessorDetailRecordTypes)MaybeRecType.get()) {
   case PPD_MACRO_EXPANSION: {
     bool isBuiltin = Record[0];
     IdentifierInfo *Name = nullptr;
@@ -5897,10 +6170,24 @@
   Deserializing AType(this);
 
   unsigned Idx = 0;
-  DeclsCursor.JumpToBit(Loc.Offset);
+  if (llvm::Error Err = DeclsCursor.JumpToBit(Loc.Offset)) {
+    Error(std::move(Err));
+    return QualType();
+  }
   RecordData Record;
-  unsigned Code = DeclsCursor.ReadCode();
-  switch ((TypeCode)DeclsCursor.readRecord(Code, Record)) {
+  Expected<unsigned> MaybeCode = DeclsCursor.ReadCode();
+  if (!MaybeCode) {
+    Error(MaybeCode.takeError());
+    return QualType();
+  }
+  unsigned Code = MaybeCode.get();
+
+  Expected<unsigned> MaybeTypeCode = DeclsCursor.readRecord(Code, Record);
+  if (!MaybeTypeCode) {
+    Error(MaybeTypeCode.takeError());
+    return QualType();
+  }
+  switch ((TypeCode)MaybeTypeCode.get()) {
   case TYPE_EXT_QUAL: {
     if (Record.size() != 2) {
       Error("Incorrect encoding of extended qualifier type");
@@ -7205,13 +7492,26 @@
   RecordLocation Loc = getLocalBitOffset(Offset);
   BitstreamCursor &Cursor = Loc.F->DeclsCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(Loc.Offset);
+  if (llvm::Error Err = Cursor.JumpToBit(Loc.Offset)) {
+    Error(std::move(Err));
+    return nullptr;
+  }
   ReadingKindTracker ReadingKind(Read_Decl, *this);
 
   RecordData Record;
-  unsigned Code = Cursor.ReadCode();
-  unsigned RecCode = Cursor.readRecord(Code, Record);
-  if (RecCode != DECL_CXX_CTOR_INITIALIZERS) {
+  Expected<unsigned> MaybeCode = Cursor.ReadCode();
+  if (!MaybeCode) {
+    Error(MaybeCode.takeError());
+    return nullptr;
+  }
+  unsigned Code = MaybeCode.get();
+
+  Expected<unsigned> MaybeRecCode = Cursor.readRecord(Code, Record);
+  if (!MaybeRecCode) {
+    Error(MaybeRecCode.takeError());
+    return nullptr;
+  }
+  if (MaybeRecCode.get() != DECL_CXX_CTOR_INITIALIZERS) {
     Error("malformed AST file: missing C++ ctor initializers");
     return nullptr;
   }
@@ -7227,11 +7527,27 @@
   RecordLocation Loc = getLocalBitOffset(Offset);
   BitstreamCursor &Cursor = Loc.F->DeclsCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(Loc.Offset);
+  if (llvm::Error Err = Cursor.JumpToBit(Loc.Offset)) {
+    Error(std::move(Err));
+    return nullptr;
+  }
   ReadingKindTracker ReadingKind(Read_Decl, *this);
   RecordData Record;
-  unsigned Code = Cursor.ReadCode();
-  unsigned RecCode = Cursor.readRecord(Code, Record);
+
+  Expected<unsigned> MaybeCode = Cursor.ReadCode();
+  if (!MaybeCode) {
+    Error(MaybeCode.takeError());
+    return nullptr;
+  }
+  unsigned Code = MaybeCode.get();
+
+  Expected<unsigned> MaybeRecCode = Cursor.readRecord(Code, Record);
+  if (!MaybeRecCode) {
+    Error(MaybeCode.takeError());
+    return nullptr;
+  }
+  unsigned RecCode = MaybeRecCode.get();
+
   if (RecCode != DECL_CXX_BASE_SPECIFIERS) {
     Error("malformed AST file: missing C++ base specifiers");
     return nullptr;
@@ -7439,7 +7755,10 @@
 
   // Offset here is a global offset across the entire chain.
   RecordLocation Loc = getLocalBitOffset(Offset);
-  Loc.F->DeclsCursor.JumpToBit(Loc.Offset);
+  if (llvm::Error Err = Loc.F->DeclsCursor.JumpToBit(Loc.Offset)) {
+    Error(std::move(Err));
+    return nullptr;
+  }
   assert(NumCurrentElementsDeserializing == 0 &&
          "should not be called while already deserializing");
   Deserializing D(this);
@@ -9269,8 +9588,12 @@
 
     RecordData Record;
     while (true) {
-      llvm::BitstreamEntry Entry =
-        Cursor.advanceSkippingSubblocks(BitstreamCursor::AF_DontPopBlockAtEnd);
+      Expected<llvm::BitstreamEntry> MaybeEntry = Cursor.advanceSkippingSubblocks(BitstreamCursor::AF_DontPopBlockAtEnd);
+      if (!MaybeEntry) {
+        Error(MaybeEntry.takeError());
+        return;
+      }
+      llvm::BitstreamEntry Entry = MaybeEntry.get();
 
       switch (Entry.Kind) {
       case llvm::BitstreamEntry::SubBlock: // Handled for us already.
@@ -9286,7 +9609,12 @@
 
       // Read a record.
       Record.clear();
-      switch ((CommentRecordTypes)Cursor.readRecord(Entry.ID, Record)) {
+      Expected<unsigned> MaybeComment = Cursor.readRecord(Entry.ID, Record);
+      if (!MaybeComment) {
+        Error(MaybeComment.takeError());
+        return;
+      }
+      switch ((CommentRecordTypes)MaybeComment.get()) {
       case COMMENTS_RAW_COMMENT: {
         unsigned Idx = 0;
         SourceRange SR = ReadSourceRange(F, Record, Idx);
@@ -11756,8 +12084,7 @@
   return SemaObj ? SemaObj->IdResolver : DummyIdResolver;
 }
 
-unsigned ASTRecordReader::readRecord(llvm::BitstreamCursor &Cursor,
-                                     unsigned AbbrevID) {
+Expected<unsigned> ASTRecordReader::readRecord(llvm::BitstreamCursor &Cursor, unsigned AbbrevID) {
   Idx = 0;
   Record.clear();
   return Cursor.readRecord(AbbrevID, Record);
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -3679,14 +3679,25 @@
   // Note that we are loading a declaration record.
   Deserializing ADecl(this);
 
-  DeclsCursor.JumpToBit(Loc.Offset);
+  auto Fail = [] (const char *what, llvm::Error &&Err) {
+    llvm::report_fatal_error(Twine("ASTReader::ReadDeclRecord failed ") + what + ": " + toString(std::move(Err)));
+  };
+
+  if (llvm::Error JumpFailed = DeclsCursor.JumpToBit(Loc.Offset))
+    Fail("jumping", std::move(JumpFailed));
   ASTRecordReader Record(*this, *Loc.F);
   ASTDeclReader Reader(*this, Record, Loc, ID, DeclLoc);
-  unsigned Code = DeclsCursor.ReadCode();
+  Expected<unsigned> MaybeCode = DeclsCursor.ReadCode();
+  if (!MaybeCode)
+    Fail("reading code", MaybeCode.takeError());
+  unsigned Code = MaybeCode.get();
 
   ASTContext &Context = getContext();
   Decl *D = nullptr;
-  switch ((DeclCode)Record.readRecord(DeclsCursor, Code)) {
+  Expected<unsigned> MaybeDeclCode = Record.readRecord(DeclsCursor, Code);
+  if (!MaybeDeclCode)
+    llvm::report_fatal_error("ASTReader::ReadDeclRecord failed reading decl code: " + toString(MaybeDeclCode.takeError()));
+  switch ((DeclCode)MaybeDeclCode.get()) {
   case DECL_CONTEXT_LEXICAL:
   case DECL_CONTEXT_VISIBLE:
     llvm_unreachable("Record cannot be de-serialized with ReadDeclRecord");
@@ -4025,12 +4036,18 @@
       uint64_t Offset = FileAndOffset.second;
       llvm::BitstreamCursor &Cursor = F->DeclsCursor;
       SavedStreamPosition SavedPosition(Cursor);
-      Cursor.JumpToBit(Offset);
-      unsigned Code = Cursor.ReadCode();
+      if (llvm::Error JumpFailed = Cursor.JumpToBit(Offset))
+        // FIXME don't do a fatal error.
+        llvm::report_fatal_error("ASTReader::loadDeclUpdateRecords failed jumping: " + toString(std::move(JumpFailed)));
+      Expected<unsigned> MaybeCode = Cursor.ReadCode();
+      if (!MaybeCode)
+        llvm::report_fatal_error("ASTReader::loadDeclUpdateRecords failed reading code: " + toString(MaybeCode.takeError()));
+      unsigned Code = MaybeCode.get();
       ASTRecordReader Record(*this, *F);
-      unsigned RecCode = Record.readRecord(Cursor, Code);
-      (void)RecCode;
-      assert(RecCode == DECL_UPDATES && "Expected DECL_UPDATES record!");
+      if (Expected<unsigned> MaybeRecCode = Record.readRecord(Cursor, Code))
+        assert(MaybeRecCode.get() == DECL_UPDATES && "Expected DECL_UPDATES record!");
+      else
+        llvm::report_fatal_error("ASTReader::loadDeclUpdateRecords failed reading rec code: " + toString(MaybeCode.takeError()));
 
       ASTDeclReader Reader(*this, Record, RecordLocation(F, Offset), ID,
                            SourceLocation());
@@ -4094,13 +4111,18 @@
 
   llvm::BitstreamCursor &Cursor = M->DeclsCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(LocalOffset);
+  if (llvm::Error JumpFailed = Cursor.JumpToBit(LocalOffset))
+    llvm::report_fatal_error("ASTReader::loadPendingDeclChain failed jumping: " + toString(std::move(JumpFailed)));
 
   RecordData Record;
-  unsigned Code = Cursor.ReadCode();
-  unsigned RecCode = Cursor.readRecord(Code, Record);
-  (void)RecCode;
-  assert(RecCode == LOCAL_REDECLARATIONS && "expected LOCAL_REDECLARATIONS record!");
+  Expected<unsigned> MaybeCode = Cursor.ReadCode();
+  if (!MaybeCode)
+    llvm::report_fatal_error("ASTReader::loadPendingDeclChain failed reading code: " + toString(MaybeCode.takeError()));
+  unsigned Code = MaybeCode.get();
+  if (Expected<unsigned> MaybeRecCode = Cursor.readRecord(Code, Record))
+    assert(MaybeRecCode.get() == LOCAL_REDECLARATIONS && "expected LOCAL_REDECLARATIONS record!");
+  else
+    llvm::report_fatal_error("ASTReader::loadPendingDeclChain failed reading rec code: " + toString(MaybeCode.takeError()));
 
   // FIXME: We have several different dispatches on decl kind here; maybe
   // we should instead generate one loop per kind and dispatch up-front?
diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -2392,7 +2392,12 @@
   Stmt::EmptyShell Empty;
 
   while (true) {
-    llvm::BitstreamEntry Entry = Cursor.advanceSkippingSubblocks();
+    llvm::Expected<llvm::BitstreamEntry> MaybeEntry = Cursor.advanceSkippingSubblocks();
+    if (!MaybeEntry) {
+      Error(toString(MaybeEntry.takeError()));
+      return nullptr;
+    }
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::SubBlock: // Handled for us already.
@@ -2410,7 +2415,12 @@
     Stmt *S = nullptr;
     bool Finished = false;
     bool IsStmtReference = false;
-    switch ((StmtCode)Record.readRecord(Cursor, Entry.ID)) {
+    Expected<unsigned> MaybeStmtCode = Record.readRecord(Cursor, Entry.ID);
+    if (!MaybeStmtCode) {
+      Error(toString(MaybeStmtCode.takeError()));
+      return nullptr;
+    }
+    switch ((StmtCode)MaybeStmtCode.get()) {
     case STMT_STOP:
       Finished = true;
       break;
diff --git a/clang/lib/Serialization/GlobalModuleIndex.cpp b/clang/lib/Serialization/GlobalModuleIndex.cpp
--- a/clang/lib/Serialization/GlobalModuleIndex.cpp
+++ b/clang/lib/Serialization/GlobalModuleIndex.cpp
@@ -128,12 +128,20 @@
                                      llvm::BitstreamCursor Cursor)
     : Buffer(std::move(Buffer)), IdentifierIndex(), NumIdentifierLookups(),
       NumIdentifierLookupHits() {
+  auto Fail = [&Buffer] (llvm::Error &&Err) {
+    report_fatal_error("Module index '" + Buffer->getBufferIdentifier() + "' failed: " + toString(std::move(Err)));
+  };
+
   llvm::TimeTraceScope TimeScope("Module LoadIndex", StringRef(""));
   // Read the global index.
   bool InGlobalIndexBlock = false;
   bool Done = false;
   while (!Done) {
-    llvm::BitstreamEntry Entry = Cursor.advance();
+    llvm::BitstreamEntry Entry;
+    if (Expected<llvm::BitstreamEntry> Res = Cursor.advance())
+      Entry = Res.get();
+    else
+      Fail(Res.takeError());
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::Error:
@@ -169,7 +177,11 @@
 
     SmallVector<uint64_t, 64> Record;
     StringRef Blob;
-    switch ((IndexRecordTypes)Cursor.readRecord(Entry.ID, Record, &Blob)) {
+    Expected<unsigned> MaybeIndexRecord = Cursor.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeIndexRecord)
+      Fail(MaybeIndexRecord.takeError());
+    IndexRecordTypes IndexRecord = static_cast<IndexRecordTypes>(MaybeIndexRecord.get());
+    switch (IndexRecord) {
     case INDEX_METADATA:
       // Make sure that the version matches.
       if (Record.size() < 1 || Record[0] != CurrentVersion)
@@ -234,7 +246,7 @@
   delete static_cast<IdentifierIndexTable *>(IdentifierIndex);
 }
 
-std::pair<GlobalModuleIndex *, GlobalModuleIndex::ErrorCode>
+std::pair<GlobalModuleIndex *, llvm::Error>
 GlobalModuleIndex::readIndex(StringRef Path) {
   // Load the index file, if it's there.
   llvm::SmallString<128> IndexPath;
@@ -244,22 +256,22 @@
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> BufferOrErr =
       llvm::MemoryBuffer::getFile(IndexPath.c_str());
   if (!BufferOrErr)
-    return std::make_pair(nullptr, EC_NotFound);
+    return std::make_pair(nullptr, llvm::errorCodeToError(BufferOrErr.getError()));
   std::unique_ptr<llvm::MemoryBuffer> Buffer = std::move(BufferOrErr.get());
 
   /// The main bitstream cursor for the main block.
   llvm::BitstreamCursor Cursor(*Buffer);
 
   // Sniff for the signature.
-  if (Cursor.Read(8) != 'B' ||
-      Cursor.Read(8) != 'C' ||
-      Cursor.Read(8) != 'G' ||
-      Cursor.Read(8) != 'I') {
-    return std::make_pair(nullptr, EC_IOError);
+  for (unsigned char C : {'B', 'C', 'G', 'I'}) {
+    if (Expected<llvm::SimpleBitstreamCursor::word_t> Res = Cursor.Read(8)) {
+      if (Res.get() != C)
+        return std::make_pair(nullptr, llvm::createStringError(std::errc::illegal_byte_sequence, "expected signature BCGI"));
+    } else
+      return std::make_pair(nullptr, Res.takeError());
   }
 
-  return std::make_pair(new GlobalModuleIndex(std::move(Buffer), Cursor),
-                        EC_None);
+  return std::make_pair(new GlobalModuleIndex(std::move(Buffer), Cursor), llvm::Error::success());
 }
 
 void
@@ -438,9 +450,7 @@
         : FileMgr(FileMgr), PCHContainerRdr(PCHContainerRdr) {}
 
     /// Load the contents of the given module file into the builder.
-    ///
-    /// \returns true if an error occurred, false otherwise.
-    bool loadModuleFile(const FileEntry *File);
+    llvm::Error loadModuleFile(const FileEntry *File);
 
     /// Write the index to the given bitstream.
     /// \returns true if an error occurred, false otherwise.
@@ -511,24 +521,23 @@
   };
 }
 
-bool GlobalModuleIndexBuilder::loadModuleFile(const FileEntry *File) {
+llvm::Error GlobalModuleIndexBuilder::loadModuleFile(const FileEntry *File) {
   // Open the module file.
 
   auto Buffer = FileMgr.getBufferForFile(File, /*isVolatile=*/true);
-  if (!Buffer) {
-    return true;
-  }
+  if (!Buffer)
+    return llvm::createStringError(Buffer.getError(), "failed getting buffer for module file");
 
   // Initialize the input stream
   llvm::BitstreamCursor InStream(PCHContainerRdr.ExtractPCH(**Buffer));
 
   // Sniff for the signature.
-  if (InStream.Read(8) != 'C' ||
-      InStream.Read(8) != 'P' ||
-      InStream.Read(8) != 'C' ||
-      InStream.Read(8) != 'H') {
-    return true;
-  }
+  for (unsigned char C : {'C','P','C','H'})
+    if (Expected<llvm::SimpleBitstreamCursor::word_t> Res = InStream.Read(8)) {
+      if (Res.get() != C)
+        return llvm::createStringError(std::errc::illegal_byte_sequence, "expected signature CPCH");
+    } else
+      return Res.takeError();
 
   // Record this module file and assign it a unique ID (if it doesn't have
   // one already).
@@ -538,7 +547,11 @@
   enum { Other, ControlBlock, ASTBlock, DiagnosticOptionsBlock } State = Other;
   bool Done = false;
   while (!Done) {
-    llvm::BitstreamEntry Entry = InStream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = InStream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::Error:
       Done = true;
@@ -547,8 +560,10 @@
     case llvm::BitstreamEntry::Record:
       // In the 'other' state, just skip the record. We don't care.
       if (State == Other) {
-        InStream.skipRecord(Entry.ID);
-        continue;
+        if (llvm::Expected<unsigned> Skipped = InStream.skipRecord(Entry.ID))
+          continue;
+        else
+          return Skipped.takeError();
       }
 
       // Handle potentially-interesting records below.
@@ -557,7 +572,7 @@
     case llvm::BitstreamEntry::SubBlock:
       if (Entry.ID == CONTROL_BLOCK_ID) {
         if (InStream.EnterSubBlock(CONTROL_BLOCK_ID))
-          return true;
+          return llvm::createStringError(std::errc::illegal_byte_sequence, "invalid CONTROL_BLOCK_ID sub block");
 
         // Found the control block.
         State = ControlBlock;
@@ -566,7 +581,7 @@
 
       if (Entry.ID == AST_BLOCK_ID) {
         if (InStream.EnterSubBlock(AST_BLOCK_ID))
-          return true;
+          return llvm::createStringError(std::errc::illegal_byte_sequence, "invalid UNHASHED_CONTROL_BLOCK_ID sub block");
 
         // Found the AST block.
         State = ASTBlock;
@@ -575,7 +590,7 @@
 
       if (Entry.ID == UNHASHED_CONTROL_BLOCK_ID) {
         if (InStream.EnterSubBlock(UNHASHED_CONTROL_BLOCK_ID))
-          return true;
+          return llvm::createStringError(std::errc::illegal_byte_sequence, "invalid UNHASHED_CONTROL_BLOCK_ID sub block");
 
         // Found the Diagnostic Options block.
         State = DiagnosticOptionsBlock;
@@ -583,7 +598,7 @@
       }
 
       if (InStream.SkipBlock())
-        return true;
+          return llvm::createStringError(std::errc::illegal_byte_sequence, "failed skipping block");
 
       continue;
 
@@ -595,7 +610,10 @@
     // Read the given record.
     SmallVector<uint64_t, 64> Record;
     StringRef Blob;
-    unsigned Code = InStream.readRecord(Entry.ID, Record, &Blob);
+    Expected<unsigned> MaybeCode = InStream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeCode)
+      return MaybeCode.takeError();
+    unsigned Code = MaybeCode.get();
 
     // Handle module dependencies.
     if (State == ControlBlock && Code == IMPORTS) {
@@ -637,7 +655,7 @@
                             /*cacheFailure=*/false);
 
         if (!DependsOnFile)
-          return true;
+          return llvm::createStringError(std::errc::bad_file_descriptor, "imported file \"%s\" not found", ImportedFile.c_str());
 
         // Save the information in ImportedModuleFileInfo so we can verify after
         // loading all pcms.
@@ -682,7 +700,7 @@
     // We don't care about this record.
   }
 
-  return false;
+  return llvm::Error::success();
 }
 
 namespace {
@@ -820,7 +838,7 @@
   return false;
 }
 
-GlobalModuleIndex::ErrorCode
+llvm::Error
 GlobalModuleIndex::writeIndex(FileManager &FileMgr,
                               const PCHContainerReader &PCHContainerRdr,
                               StringRef Path) {
@@ -833,7 +851,7 @@
   llvm::LockFileManager Locked(IndexPath);
   switch (Locked) {
   case llvm::LockFileManager::LFS_Error:
-    return EC_IOError;
+    return llvm::createStringError(std::errc::io_error, "LFS error");
 
   case llvm::LockFileManager::LFS_Owned:
     // We're responsible for building the index ourselves. Do so below.
@@ -842,7 +860,7 @@
   case llvm::LockFileManager::LFS_Shared:
     // Someone else is responsible for building the index. We don't care
     // when they finish, so we're done.
-    return EC_Building;
+    return llvm::createStringError(std::errc::device_or_resource_busy, "someone else is building the index");
   }
 
   // The module index builder.
@@ -859,7 +877,7 @@
       // in the process of rebuilding a module. They'll rebuild the index
       // at the end of that translation unit, so we don't have to.
       if (llvm::sys::path::extension(D->path()) == ".pcm.lock")
-        return EC_Building;
+        return llvm::createStringError(std::errc::device_or_resource_busy, "someone else is building the index");
 
       continue;
     }
@@ -870,8 +888,8 @@
       continue;
 
     // Load this module file.
-    if (Builder.loadModuleFile(ModuleFile))
-      return EC_IOError;
+    if (llvm::Error Err = Builder.loadModuleFile(ModuleFile))
+      return Err;
   }
 
   // The output buffer, into which the global index will be written.
@@ -879,7 +897,7 @@
   {
     llvm::BitstreamWriter OutputStream(OutputBuffer);
     if (Builder.writeIndex(OutputStream))
-      return EC_IOError;
+      return llvm::createStringError(std::errc::io_error, "failed writing index");
   }
 
   // Write the global index file to a temporary file.
@@ -887,31 +905,30 @@
   int TmpFD;
   if (llvm::sys::fs::createUniqueFile(IndexPath + "-%%%%%%%%", TmpFD,
                                       IndexTmpPath))
-    return EC_IOError;
+    return llvm::createStringError(std::errc::io_error, "failed creating unique file");
 
   // Open the temporary global index file for output.
   llvm::raw_fd_ostream Out(TmpFD, true);
   if (Out.has_error())
-    return EC_IOError;
+    return llvm::createStringError(Out.error(), "failed outputting to stream");
 
   // Write the index.
   Out.write(OutputBuffer.data(), OutputBuffer.size());
   Out.close();
   if (Out.has_error())
-    return EC_IOError;
+    return llvm::createStringError(Out.error(), "failed writing to stream");
 
   // Remove the old index file. It isn't relevant any more.
   llvm::sys::fs::remove(IndexPath);
 
   // Rename the newly-written index file to the proper name.
-  if (llvm::sys::fs::rename(IndexTmpPath, IndexPath)) {
-    // Rename failed; just remove the
+  if (std::error_code Err = llvm::sys::fs::rename(IndexTmpPath, IndexPath)) {
+    // Remove the file on failure, don't check whether removal succeeded.
     llvm::sys::fs::remove(IndexTmpPath);
-    return EC_IOError;
+    return llvm::createStringError(Err, "failed renaming file \"%s\" to \"%s\"", IndexTmpPath.c_str(), IndexPath.c_str());
   }
 
-  // We're done.
-  return EC_None;
+  return llvm::Error::success();
 }
 
 namespace {
diff --git a/llvm/include/llvm/Bitcode/BitstreamReader.h b/llvm/include/llvm/Bitcode/BitstreamReader.h
--- a/llvm/include/llvm/Bitcode/BitstreamReader.h
+++ b/llvm/include/llvm/Bitcode/BitstreamReader.h
@@ -127,7 +127,7 @@
   ArrayRef<uint8_t> getBitcodeBytes() const { return BitcodeBytes; }
 
   /// Reset the stream to the specified bit number.
-  void JumpToBit(uint64_t BitNo) {
+  Error JumpToBit(uint64_t BitNo) {
     size_t ByteNo = size_t(BitNo/8) & ~(sizeof(word_t)-1);
     unsigned WordBitNo = unsigned(BitNo & (sizeof(word_t)*8-1));
     assert(canSkipToPos(ByteNo) && "Invalid location");
@@ -137,8 +137,14 @@
     BitsInCurWord = 0;
 
     // Skip over any bits that are already consumed.
-    if (WordBitNo)
-      Read(WordBitNo);
+    if (WordBitNo) {
+      if (Expected<word_t> Res = Read(WordBitNo))
+        return Error::success();
+      else
+        return Res.takeError();
+    }
+
+    return Error::success();
   }
 
   /// Get a pointer into the bitstream at the specified byte offset.
@@ -154,9 +160,9 @@
     return getPointerToByte(BitNo / 8, NumBytes);
   }
 
-  void fillCurWord() {
+  Error fillCurWord() {
     if (NextChar >= BitcodeBytes.size())
-      report_fatal_error("Unexpected end of file");
+      return createStringError(std::errc::io_error, "Unexpected end of file reading %u of %u bytes", NextChar, BitcodeBytes.size());
 
     // Read the next word from the stream.
     const uint8_t *NextCharPtr = BitcodeBytes.data() + NextChar;
@@ -175,9 +181,10 @@
     }
     NextChar += BytesRead;
     BitsInCurWord = BytesRead * 8;
+    return Error::success();
   }
 
-  word_t Read(unsigned NumBits) {
+  Expected<word_t> Read(unsigned NumBits) {
     static const unsigned BitsInWord = MaxChunkSize;
 
     assert(NumBits && NumBits <= BitsInWord &&
@@ -199,11 +206,12 @@
     word_t R = BitsInCurWord ? CurWord : 0;
     unsigned BitsLeft = NumBits - BitsInCurWord;
 
-    fillCurWord();
+    if (Error fillResult = fillCurWord())
+      return std::move(fillResult);
 
     // If we run out of data, abort.
     if (BitsLeft > BitsInCurWord)
-      report_fatal_error("Unexpected end of file");
+      return createStringError(std::errc::io_error, "Unexpected end of file reading %u of %u bits", BitsInCurWord, BitsLeft);
 
     word_t R2 = CurWord & (~word_t(0) >> (BitsInWord - BitsLeft));
 
@@ -217,8 +225,12 @@
     return R;
   }
 
-  uint32_t ReadVBR(unsigned NumBits) {
-    uint32_t Piece = Read(NumBits);
+  Expected<uint32_t> ReadVBR(unsigned NumBits) {
+    Expected<unsigned> MaybeRead = Read(NumBits);
+    if (!MaybeRead)
+      return MaybeRead;
+    uint32_t Piece = MaybeRead.get();
+
     if ((Piece & (1U << (NumBits-1))) == 0)
       return Piece;
 
@@ -231,14 +243,21 @@
         return Result;
 
       NextBit += NumBits-1;
-      Piece = Read(NumBits);
+      MaybeRead = Read(NumBits);
+      if (!MaybeRead)
+        return MaybeRead;
+      Piece = MaybeRead.get();
     }
   }
 
   // Read a VBR that may have a value up to 64-bits in size. The chunk size of
   // the VBR must still be <= 32 bits though.
-  uint64_t ReadVBR64(unsigned NumBits) {
-    uint32_t Piece = Read(NumBits);
+  Expected<uint64_t> ReadVBR64(unsigned NumBits) {
+    Expected<unsigned> MaybeRead = Read(NumBits);
+    if (!MaybeRead)
+      return MaybeRead;
+    uint32_t Piece = MaybeRead.get();
+
     if ((Piece & (1U << (NumBits-1))) == 0)
       return uint64_t(Piece);
 
@@ -251,7 +270,10 @@
         return Result;
 
       NextBit += NumBits-1;
-      Piece = Read(NumBits);
+      MaybeRead = Read(NumBits);
+      if (!MaybeRead)
+        return MaybeRead;
+      Piece = MaybeRead.get();
     }
   }
 
@@ -365,12 +387,16 @@
   };
 
   /// Advance the current bitstream, returning the next entry in the stream.
-  BitstreamEntry advance(unsigned Flags = 0) {
+  Expected<BitstreamEntry> advance(unsigned Flags = 0) {
     while (true) {
       if (AtEndOfStream())
         return BitstreamEntry::getError();
 
-      unsigned Code = ReadCode();
+      Expected<unsigned> MaybeCode = ReadCode();
+      if (!MaybeCode)
+        return MaybeCode.takeError();
+      unsigned Code = MaybeCode.get();
+
       if (Code == bitc::END_BLOCK) {
         // Pop the end of the block unless Flags tells us not to.
         if (!(Flags & AF_DontPopBlockAtEnd) && ReadBlockEnd())
@@ -378,14 +404,19 @@
         return BitstreamEntry::getEndBlock();
       }
 
-      if (Code == bitc::ENTER_SUBBLOCK)
-        return BitstreamEntry::getSubBlock(ReadSubBlockID());
+      if (Code == bitc::ENTER_SUBBLOCK) {
+        if (Expected<unsigned> MaybeSubBlock = ReadSubBlockID())
+          return BitstreamEntry::getSubBlock(MaybeSubBlock.get());
+        else
+          return MaybeSubBlock.takeError();
+      }
 
       if (Code == bitc::DEFINE_ABBREV &&
           !(Flags & AF_DontAutoprocessAbbrevs)) {
         // We read and accumulate abbrev's, the client can't do anything with
         // them anyway.
-        ReadAbbrevRecord();
+        if (Error Err = ReadAbbrevRecord())
+          return std::move(Err);
         continue;
       }
 
@@ -395,10 +426,14 @@
 
   /// This is a convenience function for clients that don't expect any
   /// subblocks. This just skips over them automatically.
-  BitstreamEntry advanceSkippingSubblocks(unsigned Flags = 0) {
+  Expected<BitstreamEntry> advanceSkippingSubblocks(unsigned Flags = 0) {
     while (true) {
       // If we found a normal entry, return it.
-      BitstreamEntry Entry = advance(Flags);
+      Expected<BitstreamEntry> MaybeEntry = advance(Flags);
+      if (!MaybeEntry)
+        return MaybeEntry;
+      BitstreamEntry Entry = MaybeEntry.get();
+
       if (Entry.Kind != BitstreamEntry::SubBlock)
         return Entry;
 
@@ -408,7 +443,7 @@
     }
   }
 
-  unsigned ReadCode() {
+  Expected<unsigned> ReadCode() {
     return Read(CurCodeSize);
   }
 
@@ -416,18 +451,24 @@
   //    [ENTER_SUBBLOCK, blockid, newcodelen, <align4bytes>, blocklen]
 
   /// Having read the ENTER_SUBBLOCK code, read the BlockID for the block.
-  unsigned ReadSubBlockID() {
+  Expected<unsigned> ReadSubBlockID() {
     return ReadVBR(bitc::BlockIDWidth);
   }
 
   /// Having read the ENTER_SUBBLOCK abbrevid and a BlockID, skip over the body
   /// of this block. If the block record is malformed, return true.
-  bool SkipBlock() {
-    // Read and ignore the codelen value.  Since we are skipping this block, we
-    // don't care what code widths are used inside of it.
-    ReadVBR(bitc::CodeLenWidth);
+  Expected<bool> SkipBlock() {
+    // Read and ignore the codelen value.
+    if (Expected<uint32_t> Res = ReadVBR(bitc::CodeLenWidth))
+      ; // Since we are skipping this block, we don't care what code widths are used inside of it.
+    else
+      return Res.takeError();
+
     SkipToFourByteBoundary();
-    size_t NumFourBytes = Read(bitc::BlockSizeWidth);
+    Expected<unsigned> MaybeNum = Read(bitc::BlockSizeWidth);
+    if (!MaybeNum)
+      return MaybeNum;
+    size_t NumFourBytes = MaybeNum.get();
 
     // Check that the block wasn't partially defined, and that the offset isn't
     // bogus.
@@ -435,13 +476,15 @@
     if (AtEndOfStream() || !canSkipToPos(SkipTo/8))
       return true;
 
-    JumpToBit(SkipTo);
+    if (Error Res = JumpToBit(SkipTo))
+      return std::move(Res);
+
     return false;
   }
 
   /// Having read the ENTER_SUBBLOCK abbrevid, enter the block, and return true
   /// if the block has an error.
-  bool EnterSubBlock(unsigned BlockID, unsigned *NumWordsP = nullptr);
+  Expected<bool> EnterSubBlock(unsigned BlockID, unsigned *NumWordsP = nullptr);
 
   bool ReadBlockEnd() {
     if (BlockScope.empty()) return true;
@@ -476,22 +519,21 @@
   }
 
   /// Read the current record and discard it, returning the code for the record.
-  unsigned skipRecord(unsigned AbbrevID);
+  Expected<unsigned> skipRecord(unsigned AbbrevID);
 
-  unsigned readRecord(unsigned AbbrevID, SmallVectorImpl<uint64_t> &Vals,
-                      StringRef *Blob = nullptr);
+  Expected<unsigned> readRecord(unsigned AbbrevID, SmallVectorImpl<uint64_t> &Vals, StringRef *Blob = nullptr);
 
   //===--------------------------------------------------------------------===//
   // Abbrev Processing
   //===--------------------------------------------------------------------===//
-  void ReadAbbrevRecord();
+  Error ReadAbbrevRecord();
 
   /// Read and return a block info block from the bitstream. If an error was
   /// encountered, return None.
   ///
   /// \param ReadBlockInfoNames Whether to read block/record name information in
   /// the BlockInfo block. Only llvm-bcanalyzer uses this.
-  Optional<BitstreamBlockInfo>
+  Expected<Optional<BitstreamBlockInfo>>
   ReadBlockInfoBlock(bool ReadBlockInfoNames = false);
 
   /// Set the block info to be used by this BitstreamCursor to interpret
diff --git a/llvm/include/llvm/Support/Error.h b/llvm/include/llvm/Support/Error.h
--- a/llvm/include/llvm/Support/Error.h
+++ b/llvm/include/llvm/Support/Error.h
@@ -1160,7 +1160,7 @@
 
 /// Create formatted StringError object.
 template <typename... Ts>
-Error createStringError(std::error_code EC, char const *Fmt,
+inline Error createStringError(std::error_code EC, char const *Fmt,
                         const Ts &... Vals) {
   std::string Buffer;
   raw_string_ostream Stream(Buffer);
@@ -1170,6 +1170,11 @@
 
 Error createStringError(std::error_code EC, char const *Msg);
 
+template <typename... Ts>
+inline Error createStringError(std::errc EC, char const *Fmt, const Ts &... Vals) {
+  return createStringError(std::make_error_code(EC), Fmt, Vals...);
+}
+
 /// This class wraps a filename and another Error.
 ///
 /// In some cases, an error needs to live along a 'source' name, in order to
diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -105,18 +105,22 @@
       Message, make_error_code(BitcodeError::CorruptedBitcode));
 }
 
-/// Helper to read the header common to all bitcode files.
-static bool hasValidBitcodeHeader(BitstreamCursor &Stream) {
-  // Sniff for the signature.
-  if (!Stream.canSkipToPos(4) ||
-      Stream.Read(8) != 'B' ||
-      Stream.Read(8) != 'C' ||
-      Stream.Read(4) != 0x0 ||
-      Stream.Read(4) != 0xC ||
-      Stream.Read(4) != 0xE ||
-      Stream.Read(4) != 0xD)
-    return false;
-  return true;
+static Error hasInvalidBitcodeHeader(BitstreamCursor &Stream) {
+  if (!Stream.canSkipToPos(4))
+    return createStringError(std::errc::illegal_byte_sequence, "file too small to contain bitcode header");
+  for (unsigned C : {'B', 'C'})
+    if (Expected<SimpleBitstreamCursor::word_t> Res = Stream.Read(8)) {
+      if (Res.get() != C)
+        return createStringError(std::errc::illegal_byte_sequence, "file doesn't start with bitcode header");
+    } else
+      return Res.takeError();
+  for (unsigned C : {0x0, 0xC, 0xE, 0xD})
+    if (Expected<SimpleBitstreamCursor::word_t> Res = Stream.Read(4)) {
+      if (Res.get() != C)
+        return createStringError(std::errc::illegal_byte_sequence, "file doesn't start with bitcode header");
+    } else
+      return Res.takeError();
+  return Error::success();
 }
 
 static Expected<BitstreamCursor> initStream(MemoryBufferRef Buffer) {
@@ -133,8 +137,8 @@
       return error("Invalid bitcode wrapper header");
 
   BitstreamCursor Stream(ArrayRef<uint8_t>(BufPtr, BufEnd));
-  if (!hasValidBitcodeHeader(Stream))
-    return error("Invalid bitcode signature");
+  if (Error Err = hasInvalidBitcodeHeader(Stream))
+    return std::move(Err);
 
   return std::move(Stream);
 }
@@ -173,7 +177,11 @@
   std::string ProducerIdentification;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    BitstreamEntry Entry;
+    if (Expected<BitstreamEntry> Res = Stream.advance())
+      Entry = Res.get();
+    else
+      return Res.takeError();
 
     switch (Entry.Kind) {
     default:
@@ -188,8 +196,10 @@
 
     // Read a record.
     Record.clear();
-    unsigned BitCode = Stream.readRecord(Entry.ID, Record);
-    switch (BitCode) {
+    Expected<unsigned> MaybeBitCode = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeBitCode)
+      return MaybeBitCode.takeError();
+    switch (unsigned BitCode = MaybeBitCode.get()) {
     default: // Default behavior: reject
       return error("Invalid value");
     case bitc::IDENTIFICATION_CODE_STRING: // IDENTIFICATION: [strchr x N]
@@ -214,7 +224,12 @@
     if (Stream.AtEndOfStream())
       return "";
 
-    BitstreamEntry Entry = Stream.advance();
+    BitstreamEntry Entry;
+    if (Expected<BitstreamEntry> Res = Stream.advance())
+      Entry = std::move(Res.get());
+    else
+      return Res.takeError();
+
     switch (Entry.Kind) {
     case BitstreamEntry::EndBlock:
     case BitstreamEntry::Error:
@@ -229,8 +244,10 @@
         return error("Malformed block");
       continue;
     case BitstreamEntry::Record:
-      Stream.skipRecord(Entry.ID);
-      continue;
+      if (auto Skipped = Stream.skipRecord(Entry.ID))
+        continue;
+      else
+        return Skipped.takeError();
     }
   }
 }
@@ -243,7 +260,10 @@
   // Read all the records for this module.
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -257,7 +277,10 @@
     }
 
     // Read a record.
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default:
       break; // Default behavior, ignore unknown content.
     case bitc::MODULE_CODE_SECTIONNAME: { // SECTIONNAME: [strchr x N]
@@ -280,7 +303,11 @@
   // We expect a number of well-defined blocks, though we don't necessarily
   // need to understand them all.
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    BitstreamEntry Entry;
+    if (Expected<BitstreamEntry> Res = Stream.advance())
+      Entry = std::move(Res.get());
+    else
+      return Res.takeError();
 
     switch (Entry.Kind) {
     case BitstreamEntry::Error:
@@ -298,8 +325,10 @@
       continue;
 
     case BitstreamEntry::Record:
-      Stream.skipRecord(Entry.ID);
-      continue;
+      if (auto Skipped = Stream.skipRecord(Entry.ID))
+        continue;
+      else
+        return Skipped.takeError();
     }
   }
 }
@@ -314,7 +343,10 @@
 
   // Read all the records for this module.
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -328,7 +360,10 @@
     }
 
     // Read a record.
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default: break;  // Default behavior, ignore unknown content.
     case bitc::MODULE_CODE_TRIPLE: {  // TRIPLE: [strchr x N]
       std::string S;
@@ -347,7 +382,10 @@
   // We expect a number of well-defined blocks, though we don't necessarily
   // need to understand them all.
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::Error:
@@ -365,8 +403,10 @@
       continue;
 
     case BitstreamEntry::Record:
-      Stream.skipRecord(Entry.ID);
-      continue;
+      if (llvm::Expected<unsigned> Skipped = Stream.skipRecord(Entry.ID))
+        continue;
+      else
+        return Skipped.takeError();
     }
   }
 }
@@ -1265,7 +1305,10 @@
 
   // Read all the records.
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -1280,7 +1323,10 @@
 
     // Read a record.
     Record.clear();
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default:  // Default behavior: ignore.
       break;
     case bitc::PARAMATTR_CODE_ENTRY_OLD: // ENTRY: [paramidx0, attr0, ...]
@@ -1464,7 +1510,10 @@
 
   // Read all the records.
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -1479,7 +1528,10 @@
 
     // Read a record.
     Record.clear();
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default:  // Default behavior: ignore.
       break;
     case bitc::PARAMATTR_GRP_CODE_ENTRY: { // ENTRY: [grpid, idx, a0, a1, ...]
@@ -1572,7 +1624,10 @@
 
   // Read all the records for this type table.
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -1590,7 +1645,10 @@
     // Read a record.
     Record.clear();
     Type *ResultTy = nullptr;
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default:
       return error("Invalid value");
     case bitc::TYPE_CODE_NUMENTRY: // TYPE_CODE_NUMENTRY: [numentries]
@@ -1809,7 +1867,10 @@
   SmallVector<uint64_t, 64> Record;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -1824,7 +1885,10 @@
 
     // Tags are implicitly mapped to integers by their order.
 
-    if (Stream.readRecord(Entry.ID, Record) != bitc::OPERAND_BUNDLE_TAG)
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    if (MaybeRecord.get() != bitc::OPERAND_BUNDLE_TAG)
       return error("Invalid record");
 
     // OPERAND_BUNDLE_TAG: [strchr x N]
@@ -1844,7 +1908,11 @@
 
   SmallVector<uint64_t, 64> Record;
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
     case BitstreamEntry::Error:
@@ -1861,7 +1929,10 @@
     // Synchronization scope names are implicitly mapped to synchronization
     // scope IDs by their order.
 
-    if (Stream.readRecord(Entry.ID, Record) != bitc::SYNC_SCOPE_NAME)
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    if (MaybeRecord.get() != bitc::SYNC_SCOPE_NAME)
       return error("Invalid record");
 
     SmallString<16> SSN;
@@ -1902,22 +1973,17 @@
 
 /// Helper to note and return the current location, and jump to the given
 /// offset.
-static uint64_t jumpToValueSymbolTable(uint64_t Offset,
-                                       BitstreamCursor &Stream) {
+static Expected<uint64_t> jumpToValueSymbolTable(uint64_t Offset, BitstreamCursor &Stream) {
   // Save the current parsing location so we can jump back at the end
   // of the VST read.
   uint64_t CurrentBit = Stream.GetCurrentBitNo();
-  Stream.JumpToBit(Offset * 32);
-#ifndef NDEBUG
-  // Do some checking if we are in debug mode.
-  BitstreamEntry Entry = Stream.advance();
-  assert(Entry.Kind == BitstreamEntry::SubBlock);
-  assert(Entry.ID == bitc::VALUE_SYMTAB_BLOCK_ID);
-#else
-  // In NDEBUG mode ignore the output so we don't get an unused variable
-  // warning.
-  Stream.advance();
-#endif
+  if (Error JumpFailed = Stream.JumpToBit(Offset * 32))
+    return std::move(JumpFailed);
+  Expected<BitstreamEntry> MaybeEntry = Stream.advance();
+  if (!MaybeEntry)
+    return MaybeEntry.takeError();
+  assert(MaybeEntry.get().Kind == BitstreamEntry::SubBlock);
+  assert(MaybeEntry.get().ID == bitc::VALUE_SYMTAB_BLOCK_ID);
   return CurrentBit;
 }
 
@@ -1947,7 +2013,10 @@
 
   SmallVector<uint64_t, 64> Record;
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock:
@@ -1960,7 +2029,10 @@
     }
 
     Record.clear();
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     case bitc::VST_CODE_FNENTRY: // [valueid, offset]
       setDeferredFunctionInfo(FuncBitcodeOffsetDelta,
                               cast<Function>(ValueList[Record[0]]), Record);
@@ -1977,12 +2049,16 @@
   // VST (where we want to jump to the VST offset) and the function-level
   // VST (where we don't).
   if (Offset > 0) {
-    CurrentBit = jumpToValueSymbolTable(Offset, Stream);
+    Expected<uint64_t> MaybeCurrentBit = jumpToValueSymbolTable(Offset, Stream);
+    if (!MaybeCurrentBit)
+      return MaybeCurrentBit.takeError();
+    CurrentBit = MaybeCurrentBit.get();
     // If this module uses a string table, read this as a module-level VST.
     if (UseStrtab) {
       if (Error Err = parseGlobalValueSymbolTable())
         return Err;
-      Stream.JumpToBit(CurrentBit);
+      if (Error JumpFailed = Stream.JumpToBit(CurrentBit))
+        return JumpFailed;
       return Error::success();
     }
     // Otherwise, the VST will be in a similar format to a function-level VST,
@@ -2014,7 +2090,10 @@
   SmallString<128> ValueName;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -2022,7 +2101,8 @@
       return error("Malformed block");
     case BitstreamEntry::EndBlock:
       if (Offset > 0)
-        Stream.JumpToBit(CurrentBit);
+        if (Error JumpFailed = Stream.JumpToBit(CurrentBit))
+          return JumpFailed;
       return Error::success();
     case BitstreamEntry::Record:
       // The interesting case.
@@ -2031,7 +2111,10 @@
 
     // Read a record.
     Record.clear();
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default:  // Default behavior: unknown type.
       break;
     case bitc::VST_CODE_ENTRY: {  // VST_CODE_ENTRY: [valueid, namechar x N]
@@ -2186,7 +2269,10 @@
   unsigned NextCstNo = ValueList.size();
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -2209,8 +2295,10 @@
     Record.clear();
     Type *VoidType = Type::getVoidTy(Context);
     Value *V = nullptr;
-    unsigned BitCode = Stream.readRecord(Entry.ID, Record);
-    switch (BitCode) {
+    Expected<unsigned> MaybeBitCode = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeBitCode)
+      return MaybeBitCode.takeError();
+    switch (unsigned BitCode = MaybeBitCode.get()) {
     default:  // Default behavior: unknown constant
     case bitc::CST_CODE_UNDEF:     // UNDEF
       V = UndefValue::get(CurTy);
@@ -2676,7 +2764,10 @@
   SmallVector<uint64_t, 64> Record;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -2692,7 +2783,10 @@
     // Read a use list record.
     Record.clear();
     bool IsBB = false;
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default:  // Default behavior: unknown type.
       break;
     case bitc::USELIST_CODE_BB:
@@ -2749,7 +2843,8 @@
 Error BitcodeReader::materializeMetadata() {
   for (uint64_t BitPos : DeferredMetadataInfo) {
     // Move the bit stream to the saved position.
-    Stream.JumpToBit(BitPos);
+    if (Error JumpFailed = Stream.JumpToBit(BitPos))
+      return JumpFailed;
     if (Error Err = MDLoader->parseModuleMetadata())
       return Err;
   }
@@ -2835,7 +2930,8 @@
 /// or if we have an anonymous function being materialized, since anonymous
 /// functions do not have a name and are therefore not in the VST.
 Error BitcodeReader::rememberAndSkipFunctionBodies() {
-  Stream.JumpToBit(NextUnreadBit);
+  if (Error JumpFailed = Stream.JumpToBit(NextUnreadBit))
+    return JumpFailed;
 
   if (Stream.AtEndOfStream())
     return error("Could not find function in stream");
@@ -2850,7 +2946,11 @@
   SmallVector<uint64_t, 64> Record;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     default:
       return error("Expect SubBlock");
@@ -2869,7 +2969,10 @@
 }
 
 bool BitcodeReaderBase::readBlockInfo() {
-  Optional<BitstreamBlockInfo> NewBlockInfo = Stream.ReadBlockInfoBlock();
+  Expected<Optional<BitstreamBlockInfo>> MaybeNewBlockInfo = Stream.ReadBlockInfoBlock();
+  if (!MaybeNewBlockInfo)
+    return true; // FIXME Handle the error.
+  Optional<BitstreamBlockInfo> NewBlockInfo = std::move(MaybeNewBlockInfo.get());
   if (!NewBlockInfo)
     return true;
   BlockInfo = std::move(*NewBlockInfo);
@@ -3205,16 +3308,20 @@
 
 Error BitcodeReader::parseModule(uint64_t ResumeBit,
                                  bool ShouldLazyLoadMetadata) {
-  if (ResumeBit)
-    Stream.JumpToBit(ResumeBit);
-  else if (Stream.EnterSubBlock(bitc::MODULE_BLOCK_ID))
+  if (ResumeBit) {
+    if (Error JumpFailed = Stream.JumpToBit(ResumeBit))
+      return JumpFailed;
+  } else if (Stream.EnterSubBlock(bitc::MODULE_BLOCK_ID))
     return error("Invalid record");
 
   SmallVector<uint64_t, 64> Record;
 
   // Read all the records for this module.
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::Error:
@@ -3357,8 +3464,10 @@
     }
 
     // Read a record.
-    auto BitCode = Stream.readRecord(Entry.ID, Record);
-    switch (BitCode) {
+    Expected<unsigned> MaybeBitCode = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeBitCode)
+      return MaybeBitCode.takeError();
+    switch (unsigned BitCode = MaybeBitCode.get()) {
     default: break;  // Default behavior, ignore unknown content.
     case bitc::MODULE_CODE_VERSION: {
       Expected<unsigned> VersionOrErr = parseVersionRecord(Record);
@@ -3520,7 +3629,10 @@
   SmallVector<uint64_t, 64> Record;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::Error:
@@ -3568,8 +3680,10 @@
     // Read a record.
     Record.clear();
     Instruction *I = nullptr;
-    unsigned BitCode = Stream.readRecord(Entry.ID, Record);
-    switch (BitCode) {
+    Expected<unsigned> MaybeBitCode = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeBitCode)
+      return MaybeBitCode.takeError();
+    switch (unsigned BitCode = MaybeBitCode.get()) {
     default: // Default behavior: reject
       return error("Invalid value");
     case bitc::FUNC_CODE_DECLAREBLOCKS: {   // DECLAREBLOCKS: [nblocks]
@@ -4922,8 +5036,8 @@
     return Err;
 
   // Move the bit stream to the saved position of the deferred function body.
-  Stream.JumpToBit(DFII->second);
-
+  if (Error JumpFailed = Stream.JumpToBit(DFII->second))
+    return JumpFailed;
   if (Error Err = parseFunctionBody(F))
     return Err;
   F->setIsMaterializable(false);
@@ -5086,7 +5200,10 @@
     return Error::success();
 
   assert(Offset > 0 && "Expected non-zero VST offset");
-  uint64_t CurrentBit = jumpToValueSymbolTable(Offset, Stream);
+  Expected<uint64_t> MaybeCurrentBit = jumpToValueSymbolTable(Offset, Stream);
+  if (!MaybeCurrentBit)
+    return MaybeCurrentBit.takeError();
+  uint64_t CurrentBit = MaybeCurrentBit.get();
 
   if (Stream.EnterSubBlock(bitc::VALUE_SYMTAB_BLOCK_ID))
     return error("Invalid record");
@@ -5097,7 +5214,10 @@
   SmallString<128> ValueName;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -5105,7 +5225,8 @@
       return error("Malformed block");
     case BitstreamEntry::EndBlock:
       // Done parsing VST, jump back to wherever we came from.
-      Stream.JumpToBit(CurrentBit);
+      if (Error JumpFailed = Stream.JumpToBit(CurrentBit))
+        return JumpFailed;
       return Error::success();
     case BitstreamEntry::Record:
       // The interesting case.
@@ -5114,7 +5235,10 @@
 
     // Read a record.
     Record.clear();
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default: // Default behavior: ignore (e.g. VST_CODE_BBENTRY records).
       break;
     case bitc::VST_CODE_ENTRY: { // VST_CODE_ENTRY: [valueid, namechar x N]
@@ -5171,7 +5295,10 @@
 
   // Read the index for this module.
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::Error:
@@ -5228,8 +5355,10 @@
 
     case BitstreamEntry::Record: {
         Record.clear();
-        auto BitCode = Stream.readRecord(Entry.ID, Record);
-        switch (BitCode) {
+        Expected<unsigned> MaybeBitCode = Stream.readRecord(Entry.ID, Record);
+        if (!MaybeBitCode)
+          return MaybeBitCode.takeError();
+        switch (unsigned BitCode = MaybeBitCode.get()) {
         default:
           break; // Default behavior, ignore unknown content.
         case bitc::MODULE_CODE_VERSION: {
@@ -5392,10 +5521,17 @@
 
   // Parse version
   {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
+
     if (Entry.Kind != BitstreamEntry::Record)
       return error("Invalid Summary Block: record for version expected");
-    if (Stream.readRecord(Entry.ID, Record) != bitc::FS_VERSION)
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    if (MaybeRecord.get() != bitc::FS_VERSION)
       return error("Invalid Summary Block: version expected");
   }
   const uint64_t Version = Record[0];
@@ -5420,7 +5556,10 @@
       PendingTypeCheckedLoadConstVCalls;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -5441,8 +5580,10 @@
     // in the combined index VST entries). The records also contain
     // information used for ThinLTO renaming and importing.
     Record.clear();
-    auto BitCode = Stream.readRecord(Entry.ID, Record);
-    switch (BitCode) {
+    Expected<unsigned> MaybeBitCode = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeBitCode)
+      return MaybeBitCode.takeError();
+    switch (unsigned BitCode = MaybeBitCode.get()) {
     default: // Default behavior: ignore.
       break;
     case bitc::FS_FLAGS: {  // [flags]
@@ -5774,7 +5915,10 @@
   ModuleSummaryIndex::ModuleInfo *LastSeenModule = nullptr;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -5788,7 +5932,10 @@
     }
 
     Record.clear();
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default: // Default behavior: ignore.
       break;
     case bitc::MST_CODE_ENTRY: {
@@ -5859,7 +6006,11 @@
 
   StringRef Strtab;
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case BitstreamEntry::EndBlock:
       return Strtab;
@@ -5875,7 +6026,10 @@
     case BitstreamEntry::Record:
       StringRef Blob;
       SmallVector<uint64_t, 1> Record;
-      if (Stream.readRecord(Entry.ID, Record, &Blob) == RecordID)
+      Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record, &Blob);
+      if (!MaybeRecord)
+        return MaybeRecord.takeError();
+      if (MaybeRecord.get() == RecordID)
         Strtab = Blob;
       break;
     }
@@ -5911,7 +6065,11 @@
     if (BCBegin + 8 >= Stream.getBitcodeBytes().size())
       return F;
 
-    BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case BitstreamEntry::EndBlock:
     case BitstreamEntry::Error:
@@ -5924,7 +6082,13 @@
         if (Stream.SkipBlock())
           return error("Malformed block");
 
-        Entry = Stream.advance();
+        {
+          Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+          if (!MaybeEntry)
+            return MaybeEntry.takeError();
+          Entry = MaybeEntry.get();
+        }
+
         if (Entry.Kind != BitstreamEntry::SubBlock ||
             Entry.ID != bitc::MODULE_BLOCK_ID)
           return error("Malformed block");
@@ -5986,8 +6150,10 @@
       continue;
     }
     case BitstreamEntry::Record:
-      Stream.skipRecord(Entry.ID);
-      continue;
+      if (Expected<unsigned> StreamFailed = Stream.skipRecord(Entry.ID))
+        continue;
+      else
+        return StreamFailed.takeError();
     }
   }
 }
@@ -6007,7 +6173,8 @@
 
   std::string ProducerIdentification;
   if (IdentificationBit != -1ull) {
-    Stream.JumpToBit(IdentificationBit);
+    if (Error JumpFailed = Stream.JumpToBit(IdentificationBit))
+      return std::move(JumpFailed);
     Expected<std::string> ProducerIdentificationOrErr =
         readIdentificationBlock(Stream);
     if (!ProducerIdentificationOrErr)
@@ -6016,7 +6183,8 @@
     ProducerIdentification = *ProducerIdentificationOrErr;
   }
 
-  Stream.JumpToBit(ModuleBit);
+  if (Error JumpFailed = Stream.JumpToBit(ModuleBit))
+    return std::move(JumpFailed);
   auto *R = new BitcodeReader(std::move(Stream), Strtab, ProducerIdentification,
                               Context);
 
@@ -6054,7 +6222,8 @@
 Error BitcodeModule::readSummary(ModuleSummaryIndex &CombinedIndex,
                                  StringRef ModulePath, uint64_t ModuleId) {
   BitstreamCursor Stream(Buffer);
-  Stream.JumpToBit(ModuleBit);
+  if (Error JumpFailed = Stream.JumpToBit(ModuleBit))
+    return JumpFailed;
 
   ModuleSummaryIndexBitcodeReader R(std::move(Stream), Strtab, CombinedIndex,
                                     ModulePath, ModuleId);
@@ -6064,7 +6233,8 @@
 // Parse the specified bitcode buffer, returning the function info index.
 Expected<std::unique_ptr<ModuleSummaryIndex>> BitcodeModule::getSummary() {
   BitstreamCursor Stream(Buffer);
-  Stream.JumpToBit(ModuleBit);
+  if (Error JumpFailed = Stream.JumpToBit(ModuleBit))
+    return std::move(JumpFailed);
 
   auto Index = llvm::make_unique<ModuleSummaryIndex>(/*HaveGVs=*/false);
   ModuleSummaryIndexBitcodeReader R(std::move(Stream), Strtab, *Index,
@@ -6083,7 +6253,10 @@
   SmallVector<uint64_t, 64> Record;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -6100,8 +6273,10 @@
 
     // Look for the FS_FLAGS record.
     Record.clear();
-    auto BitCode = Stream.readRecord(Entry.ID, Record);
-    switch (BitCode) {
+    Expected<unsigned> MaybeBitCode = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeBitCode)
+      return MaybeBitCode.takeError();
+    switch (MaybeBitCode.get()) {
     default: // Default behavior: ignore.
       break;
     case bitc::FS_FLAGS: { // [flags]
@@ -6119,13 +6294,17 @@
 // Check if the given bitcode buffer contains a global value summary block.
 Expected<BitcodeLTOInfo> BitcodeModule::getLTOInfo() {
   BitstreamCursor Stream(Buffer);
-  Stream.JumpToBit(ModuleBit);
+  if (Error JumpFailed = Stream.JumpToBit(ModuleBit))
+    return std::move(JumpFailed);
 
   if (Stream.EnterSubBlock(bitc::MODULE_BLOCK_ID))
     return error("Invalid record");
 
   while (true) {
-    BitstreamEntry Entry = Stream.advance();
+    Expected<llvm::BitstreamEntry> MaybeEntry = Stream.advance();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    llvm::BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::Error:
@@ -6159,8 +6338,10 @@
       continue;
 
     case BitstreamEntry::Record:
-      Stream.skipRecord(Entry.ID);
-      continue;
+      if (Expected<unsigned> StreamFailed = Stream.skipRecord(Entry.ID))
+        continue;
+      else
+        return StreamFailed.takeError();
     }
   }
 }
diff --git a/llvm/lib/Bitcode/Reader/BitstreamReader.cpp b/llvm/lib/Bitcode/Reader/BitstreamReader.cpp
--- a/llvm/lib/Bitcode/Reader/BitstreamReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitstreamReader.cpp
@@ -19,7 +19,7 @@
 
 /// EnterSubBlock - Having read the ENTER_SUBBLOCK abbrevid, enter
 /// the block, and return true if the block has an error.
-bool BitstreamCursor::EnterSubBlock(unsigned BlockID, unsigned *NumWordsP) {
+Expected<bool> BitstreamCursor::EnterSubBlock(unsigned BlockID, unsigned *NumWordsP) {
   // Save the current block's state on BlockScope.
   BlockScope.push_back(Block(CurCodeSize));
   BlockScope.back().PrevAbbrevs.swap(CurAbbrevs);
@@ -34,21 +34,28 @@
   }
 
   // Get the codesize of this block.
-  CurCodeSize = ReadVBR(bitc::CodeLenWidth);
+  Expected<uint32_t> MaybeVBR = ReadVBR(bitc::CodeLenWidth);
+  if (!MaybeVBR)
+    return MaybeVBR.takeError();
+  CurCodeSize = MaybeVBR.get();
+
   // We can't read more than MaxChunkSize at a time
   if (CurCodeSize > MaxChunkSize)
     return true;
 
   SkipToFourByteBoundary();
-  unsigned NumWords = Read(bitc::BlockSizeWidth);
-  if (NumWordsP) *NumWordsP = NumWords;
+  Expected<word_t> MaybeNum = Read(bitc::BlockSizeWidth);
+  if (!MaybeNum)
+    return MaybeNum.takeError();
+  word_t NumWords = MaybeNum.get();
+  if (NumWordsP)
+    *NumWordsP = NumWords;
 
   // Validate that this block is sane.
   return CurCodeSize == 0 || AtEndOfStream();
 }
 
-static uint64_t readAbbreviatedField(BitstreamCursor &Cursor,
-                                     const BitCodeAbbrevOp &Op) {
+static Expected<uint64_t> readAbbreviatedField(BitstreamCursor &Cursor, const BitCodeAbbrevOp &Op) {
   assert(!Op.isLiteral() && "Not to be used with literals!");
 
   // Decode the value as we are commanded.
@@ -63,13 +70,15 @@
     assert((unsigned)Op.getEncodingData() <= Cursor.MaxChunkSize);
     return Cursor.ReadVBR64((unsigned)Op.getEncodingData());
   case BitCodeAbbrevOp::Char6:
-    return BitCodeAbbrevOp::DecodeChar6(Cursor.Read(6));
+    if (Expected<unsigned> Res = Cursor.Read(6))
+      return BitCodeAbbrevOp::DecodeChar6(Res.get());
+    else
+      return Res.takeError();
   }
   llvm_unreachable("invalid abbreviation encoding");
 }
 
-static void skipAbbreviatedField(BitstreamCursor &Cursor,
-                                 const BitCodeAbbrevOp &Op) {
+static Error skipAbbreviatedField(BitstreamCursor &Cursor, const BitCodeAbbrevOp &Op) {
   assert(!Op.isLiteral() && "Not to be used with literals!");
 
   // Decode the value as we are commanded.
@@ -79,26 +88,42 @@
     llvm_unreachable("Should not reach here");
   case BitCodeAbbrevOp::Fixed:
     assert((unsigned)Op.getEncodingData() <= Cursor.MaxChunkSize);
-    Cursor.Read((unsigned)Op.getEncodingData());
-    break;
+    if (Expected<unsigned> Res = Cursor.Read((unsigned)Op.getEncodingData()))
+      break;
+    else
+      return Res.takeError();
   case BitCodeAbbrevOp::VBR:
     assert((unsigned)Op.getEncodingData() <= Cursor.MaxChunkSize);
-    Cursor.ReadVBR64((unsigned)Op.getEncodingData());
-    break;
+    if (Expected<uint64_t> Res = Cursor.ReadVBR64((unsigned)Op.getEncodingData()))
+      break;
+    else
+      return Res.takeError();
   case BitCodeAbbrevOp::Char6:
-    Cursor.Read(6);
-    break;
+    if (Expected<unsigned> Res = Cursor.Read(6))
+      break;
+    else
+      return Res.takeError();
   }
+  return ErrorSuccess();
 }
 
 /// skipRecord - Read the current record and discard it.
-unsigned BitstreamCursor::skipRecord(unsigned AbbrevID) {
+Expected<unsigned> BitstreamCursor::skipRecord(unsigned AbbrevID) {
   // Skip unabbreviated records by reading past their entries.
   if (AbbrevID == bitc::UNABBREV_RECORD) {
-    unsigned Code = ReadVBR(6);
-    unsigned NumElts = ReadVBR(6);
+    Expected<uint32_t> MaybeCode = ReadVBR(6);
+    if (!MaybeCode)
+      return MaybeCode.takeError();
+    unsigned Code = MaybeCode.get();
+    Expected<uint32_t> MaybeVBR = ReadVBR(6);
+    if (!MaybeVBR)
+      return MaybeVBR.get();
+    unsigned NumElts = MaybeVBR.get();
     for (unsigned i = 0; i != NumElts; ++i)
-      (void)ReadVBR64(6);
+      if (Expected<uint64_t> Res = ReadVBR64(6))
+        ; // Skip!
+      else
+        return Res.takeError();
     return Code;
   }
 
@@ -111,7 +136,10 @@
     if (CodeOp.getEncoding() == BitCodeAbbrevOp::Array ||
         CodeOp.getEncoding() == BitCodeAbbrevOp::Blob)
       report_fatal_error("Abbreviation starts with an Array or a Blob");
-    Code = readAbbreviatedField(*this, CodeOp);
+    Expected<uint64_t> MaybeCode = readAbbreviatedField(*this, CodeOp);
+    if (!MaybeCode)
+      return MaybeCode.takeError();
+    Code = MaybeCode.get();
   }
 
   for (unsigned i = 1, e = Abbv->getNumOperandInfos(); i < e; ++i) {
@@ -121,13 +149,17 @@
 
     if (Op.getEncoding() != BitCodeAbbrevOp::Array &&
         Op.getEncoding() != BitCodeAbbrevOp::Blob) {
-      skipAbbreviatedField(*this, Op);
+      if (Error Err = skipAbbreviatedField(*this, Op))
+        return std::move(Err);
       continue;
     }
 
     if (Op.getEncoding() == BitCodeAbbrevOp::Array) {
       // Array case.  Read the number of elements as a vbr6.
-      unsigned NumElts = ReadVBR(6);
+      Expected<uint32_t> MaybeNum = ReadVBR(6);
+      if (!MaybeNum)
+        return MaybeNum.takeError();
+      unsigned NumElts = MaybeNum.get();
 
       // Get the element encoding.
       assert(i+2 == e && "array op not second to last?");
@@ -140,15 +172,20 @@
         report_fatal_error("Array element type can't be an Array or a Blob");
       case BitCodeAbbrevOp::Fixed:
         assert((unsigned)EltEnc.getEncodingData() <= MaxChunkSize);
-        JumpToBit(GetCurrentBitNo() + NumElts * EltEnc.getEncodingData());
+        if (Error Err = JumpToBit(GetCurrentBitNo() + NumElts * EltEnc.getEncodingData()))
+          return std::move(Err);
         break;
       case BitCodeAbbrevOp::VBR:
         assert((unsigned)EltEnc.getEncodingData() <= MaxChunkSize);
         for (; NumElts; --NumElts)
-          ReadVBR64((unsigned)EltEnc.getEncodingData());
+          if (Expected<uint64_t> Res = ReadVBR64((unsigned)EltEnc.getEncodingData()))
+            ; // Skip!
+          else
+            return Res.takeError();
         break;
       case BitCodeAbbrevOp::Char6:
-        JumpToBit(GetCurrentBitNo() + NumElts * 6);
+        if (Error Err = JumpToBit(GetCurrentBitNo() + NumElts * 6))
+          return std::move(Err);
         break;
       }
       continue;
@@ -156,7 +193,10 @@
 
     assert(Op.getEncoding() == BitCodeAbbrevOp::Blob);
     // Blob case.  Read the number of bytes as a vbr6.
-    unsigned NumElts = ReadVBR(6);
+    Expected<uint32_t> MaybeNum = ReadVBR(6);
+    if (!MaybeNum)
+      return MaybeNum.takeError();
+    unsigned NumElts = MaybeNum.get();
     SkipToFourByteBoundary();  // 32-bit alignment
 
     // Figure out where the end of this blob will be including tail padding.
@@ -170,19 +210,28 @@
     }
 
     // Skip over the blob.
-    JumpToBit(NewEnd);
+    if (Error Err = JumpToBit(NewEnd))
+      return std::move(Err);
   }
   return Code;
 }
 
-unsigned BitstreamCursor::readRecord(unsigned AbbrevID,
-                                     SmallVectorImpl<uint64_t> &Vals,
-                                     StringRef *Blob) {
+Expected<unsigned> BitstreamCursor::readRecord(unsigned AbbrevID, SmallVectorImpl<uint64_t> &Vals, StringRef *Blob) {
   if (AbbrevID == bitc::UNABBREV_RECORD) {
-    unsigned Code = ReadVBR(6);
-    unsigned NumElts = ReadVBR(6);
+    Expected<uint32_t> MaybeCode = ReadVBR(6);
+    if (!MaybeCode)
+      return MaybeCode.takeError();
+    uint32_t Code = MaybeCode.get();
+    Expected<uint32_t> MaybeNumElts = ReadVBR(6);
+    if (!MaybeNumElts)
+      return MaybeNumElts.takeError();
+    uint32_t NumElts = MaybeNumElts.get();
+
     for (unsigned i = 0; i != NumElts; ++i)
-      Vals.push_back(ReadVBR64(6));
+      if (Expected<uint64_t> MaybeVal = ReadVBR64(6))
+        Vals.push_back(MaybeVal.get());
+      else
+        return MaybeVal.takeError();
     return Code;
   }
 
@@ -198,7 +247,10 @@
     if (CodeOp.getEncoding() == BitCodeAbbrevOp::Array ||
         CodeOp.getEncoding() == BitCodeAbbrevOp::Blob)
       report_fatal_error("Abbreviation starts with an Array or a Blob");
-    Code = readAbbreviatedField(*this, CodeOp);
+    if (Expected<uint64_t> MaybeCode = readAbbreviatedField(*this, CodeOp))
+      Code = MaybeCode.get();
+    else
+      return MaybeCode.takeError();
   }
 
   for (unsigned i = 1, e = Abbv->getNumOperandInfos(); i != e; ++i) {
@@ -210,13 +262,19 @@
 
     if (Op.getEncoding() != BitCodeAbbrevOp::Array &&
         Op.getEncoding() != BitCodeAbbrevOp::Blob) {
-      Vals.push_back(readAbbreviatedField(*this, Op));
+      if (Expected<uint64_t> MaybeVal = readAbbreviatedField(*this, Op))
+        Vals.push_back(MaybeVal.get());
+      else
+        return MaybeVal.takeError();
       continue;
     }
 
     if (Op.getEncoding() == BitCodeAbbrevOp::Array) {
       // Array case.  Read the number of elements as a vbr6.
-      unsigned NumElts = ReadVBR(6);
+      Expected<uint32_t> MaybeNumElts = ReadVBR(6);
+      if (!MaybeNumElts)
+        return MaybeNumElts.takeError();
+      uint32_t NumElts = MaybeNumElts.get();
 
       // Get the element encoding.
       if (i + 2 != e)
@@ -232,22 +290,34 @@
         report_fatal_error("Array element type can't be an Array or a Blob");
       case BitCodeAbbrevOp::Fixed:
         for (; NumElts; --NumElts)
-          Vals.push_back(Read((unsigned)EltEnc.getEncodingData()));
+          if (Expected<SimpleBitstreamCursor::word_t> MaybeVal = Read((unsigned)EltEnc.getEncodingData()))
+            Vals.push_back(MaybeVal.get());
+          else
+            return MaybeVal.takeError();
         break;
       case BitCodeAbbrevOp::VBR:
         for (; NumElts; --NumElts)
-          Vals.push_back(ReadVBR64((unsigned)EltEnc.getEncodingData()));
+          if (Expected<uint64_t> MaybeVal = ReadVBR64((unsigned)EltEnc.getEncodingData()))
+            Vals.push_back(MaybeVal.get());
+          else
+            return MaybeVal.takeError();
         break;
       case BitCodeAbbrevOp::Char6:
         for (; NumElts; --NumElts)
-          Vals.push_back(BitCodeAbbrevOp::DecodeChar6(Read(6)));
+          if (Expected<SimpleBitstreamCursor::word_t> MaybeVal = Read(6))
+            Vals.push_back(BitCodeAbbrevOp::DecodeChar6(MaybeVal.get()));
+          else
+            return MaybeVal.takeError();
       }
       continue;
     }
 
     assert(Op.getEncoding() == BitCodeAbbrevOp::Blob);
     // Blob case.  Read the number of bytes as a vbr6.
-    unsigned NumElts = ReadVBR(6);
+    Expected<uint32_t> MaybeNumElts = ReadVBR(6);
+    if (!MaybeNumElts)
+      return MaybeNumElts.takeError();
+    uint32_t NumElts = MaybeNumElts.get();
     SkipToFourByteBoundary();  // 32-bit alignment
 
     // Figure out where the end of this blob will be including tail padding.
@@ -265,7 +335,8 @@
     // Otherwise, inform the streamer that we need these bytes in memory.  Skip
     // over tail padding first, in case jumping to NewEnd invalidates the Blob
     // pointer.
-    JumpToBit(NewEnd);
+    if (Error Err = JumpToBit(NewEnd))
+      return std::move(Err);
     const char *Ptr = (const char *)getPointerToBit(CurBitPos, NumElts);
 
     // If we can return a reference to the data, do so to avoid copying it.
@@ -281,19 +352,34 @@
   return Code;
 }
 
-void BitstreamCursor::ReadAbbrevRecord() {
+Error BitstreamCursor::ReadAbbrevRecord() {
   auto Abbv = std::make_shared<BitCodeAbbrev>();
-  unsigned NumOpInfo = ReadVBR(5);
+  Expected<uint32_t> MaybeNumOpInfo = ReadVBR(5);
+  if (!MaybeNumOpInfo)
+    return MaybeNumOpInfo.takeError();
+  unsigned NumOpInfo = MaybeNumOpInfo.get();
   for (unsigned i = 0; i != NumOpInfo; ++i) {
-    bool IsLiteral = Read(1);
+    Expected<word_t> MaybeIsLiteral = Read(1);
+    if (!MaybeIsLiteral)
+      return MaybeIsLiteral.takeError();
+    bool IsLiteral = MaybeIsLiteral.get();
     if (IsLiteral) {
-      Abbv->Add(BitCodeAbbrevOp(ReadVBR64(8)));
+      Expected<uint64_t> MaybeOp = ReadVBR64(8);
+      if (!MaybeOp)
+        return MaybeOp.takeError();
+      Abbv->Add(BitCodeAbbrevOp(MaybeOp.get()));
       continue;
     }
 
-    BitCodeAbbrevOp::Encoding E = (BitCodeAbbrevOp::Encoding)Read(3);
+    Expected<word_t> MaybeEncoding = Read(3);
+    if (!MaybeEncoding)
+      return MaybeEncoding.takeError();
+    BitCodeAbbrevOp::Encoding E = (BitCodeAbbrevOp::Encoding)MaybeEncoding.get();
     if (BitCodeAbbrevOp::hasEncodingData(E)) {
-      uint64_t Data = ReadVBR64(5);
+      Expected<uint64_t> MaybeData = ReadVBR64(5);
+      if (!MaybeData)
+        return MaybeData.takeError();
+      uint64_t Data = MaybeData.get();
 
       // As a special case, handle fixed(0) (i.e., a fixed field with zero bits)
       // and vbr(0) as a literal zero.  This is decoded the same way, and avoids
@@ -317,9 +403,11 @@
   if (Abbv->getNumOperandInfos() == 0)
     report_fatal_error("Abbrev record with no operands");
   CurAbbrevs.push_back(std::move(Abbv));
+
+  return Error::success();
 }
 
-Optional<BitstreamBlockInfo>
+Expected<Optional<BitstreamBlockInfo>>
 BitstreamCursor::ReadBlockInfoBlock(bool ReadBlockInfoNames) {
   if (EnterSubBlock(bitc::BLOCKINFO_BLOCK_ID)) return None;
 
@@ -330,7 +418,10 @@
 
   // Read all the records for this module.
   while (true) {
-    BitstreamEntry Entry = advanceSkippingSubblocks(AF_DontAutoprocessAbbrevs);
+    Expected<BitstreamEntry> MaybeEntry = advanceSkippingSubblocks(AF_DontAutoprocessAbbrevs);
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case llvm::BitstreamEntry::SubBlock: // Handled for us already.
@@ -346,7 +437,8 @@
     // Read abbrev records, associate them with CurBID.
     if (Entry.ID == bitc::DEFINE_ABBREV) {
       if (!CurBlockInfo) return None;
-      ReadAbbrevRecord();
+      if (Error Err = ReadAbbrevRecord())
+        return std::move(Err);
 
       // ReadAbbrevRecord installs the abbrev in CurAbbrevs.  Move it to the
       // appropriate BlockInfo.
@@ -357,7 +449,10 @@
 
     // Read a record.
     Record.clear();
-    switch (readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeBlockInfo = readRecord(Entry.ID, Record);
+    if (!MaybeBlockInfo)
+      return MaybeBlockInfo.takeError();
+    switch (MaybeBlockInfo.get()) {
       default: break;  // Default behavior, ignore unknown content.
       case bitc::BLOCKINFO_CODE_SETBID:
         if (Record.size() < 1) return None;
diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -674,8 +674,11 @@
   SmallVector<uint64_t, 64> Record;
   // Get the abbrevs, and preload record positions to make them lazy-loadable.
   while (true) {
-    BitstreamEntry Entry = IndexCursor.advanceSkippingSubblocks(
-        BitstreamCursor::AF_DontPopBlockAtEnd);
+    Expected<BitstreamEntry> MaybeEntry = IndexCursor.advanceSkippingSubblocks(BitstreamCursor::AF_DontPopBlockAtEnd);
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
     case BitstreamEntry::Error:
@@ -687,14 +690,21 @@
       // The interesting case.
       ++NumMDRecordLoaded;
       uint64_t CurrentPos = IndexCursor.GetCurrentBitNo();
-      auto Code = IndexCursor.skipRecord(Entry.ID);
+      Expected<unsigned> MaybeCode = IndexCursor.skipRecord(Entry.ID);
+      if (!MaybeCode)
+        return MaybeCode.takeError();
+      unsigned Code = MaybeCode.get();
       switch (Code) {
       case bitc::METADATA_STRINGS: {
         // Rewind and parse the strings.
-        IndexCursor.JumpToBit(CurrentPos);
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
         StringRef Blob;
         Record.clear();
-        IndexCursor.readRecord(Entry.ID, Record, &Blob);
+        if (Expected<unsigned> MaybeRecord = IndexCursor.readRecord(Entry.ID, Record, &Blob))
+          ;
+        else
+          return MaybeRecord.takeError();
         unsigned NumStrings = Record[0];
         MDStringRef.reserve(NumStrings);
         auto IndexNextMDString = [&](StringRef Str) {
@@ -707,26 +717,31 @@
       case bitc::METADATA_INDEX_OFFSET: {
         // This is the offset to the index, when we see this we skip all the
         // records and load only an index to these.
-        IndexCursor.JumpToBit(CurrentPos);
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
         Record.clear();
-        IndexCursor.readRecord(Entry.ID, Record);
+        if (Expected<unsigned> MaybeRecord = IndexCursor.readRecord(Entry.ID, Record))
+          ;
+        else
+          return MaybeRecord.takeError();
         if (Record.size() != 2)
           return error("Invalid record");
         auto Offset = Record[0] + (Record[1] << 32);
         auto BeginPos = IndexCursor.GetCurrentBitNo();
-        IndexCursor.JumpToBit(BeginPos + Offset);
-        Entry = IndexCursor.advanceSkippingSubblocks(
-            BitstreamCursor::AF_DontPopBlockAtEnd);
+        if (Error Err = IndexCursor.JumpToBit(BeginPos + Offset))
+          return std::move(Err);
+        Expected<BitstreamEntry> MaybeEntry = IndexCursor.advanceSkippingSubblocks(BitstreamCursor::AF_DontPopBlockAtEnd);
+        if (!MaybeEntry)
+          return MaybeEntry.takeError();
+        Entry = MaybeEntry.get();
         assert(Entry.Kind == BitstreamEntry::Record &&
                "Corrupted bitcode: Expected `Record` when trying to find the "
                "Metadata index");
         Record.clear();
-        auto Code = IndexCursor.readRecord(Entry.ID, Record);
-        (void)Code;
-        assert(Code == bitc::METADATA_INDEX && "Corrupted bitcode: Expected "
-                                               "`METADATA_INDEX` when trying "
-                                               "to find the Metadata index");
-
+        if (Expected<unsigned> MaybeCode = IndexCursor.readRecord(Entry.ID, Record))
+          assert(MaybeCode.get() == bitc::METADATA_INDEX && "Corrupted bitcode: Expected `METADATA_INDEX` when trying to find the Metadata index");
+        else
+          return MaybeCode.takeError();
         // Delta unpack
         auto CurrentValue = BeginPos;
         GlobalMetadataBitPosIndex.reserve(Record.size());
@@ -742,21 +757,31 @@
         return error("Corrupted Metadata block");
       case bitc::METADATA_NAME: {
         // Named metadata need to be materialized now and aren't deferred.
-        IndexCursor.JumpToBit(CurrentPos);
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
         Record.clear();
-        unsigned Code = IndexCursor.readRecord(Entry.ID, Record);
-        assert(Code == bitc::METADATA_NAME);
+
+        unsigned Code;
+        if (Expected<unsigned> MaybeCode = IndexCursor.readRecord(Entry.ID, Record)) {
+          Code = MaybeCode.get();
+          assert(Code == bitc::METADATA_NAME);
+        } else
+          return MaybeCode.takeError();
 
         // Read name of the named metadata.
         SmallString<8> Name(Record.begin(), Record.end());
-        Code = IndexCursor.ReadCode();
+        if (Expected<unsigned> MaybeCode = IndexCursor.ReadCode())
+          Code = MaybeCode.get();
+        else
+          return MaybeCode.takeError();
 
         // Named Metadata comes in two parts, we expect the name to be followed
         // by the node
         Record.clear();
-        unsigned NextBitCode = IndexCursor.readRecord(Code, Record);
-        assert(NextBitCode == bitc::METADATA_NAMED_NODE);
-        (void)NextBitCode;
+        if (Expected<unsigned> MaybeNextBitCode = IndexCursor.readRecord(Code, Record))
+          assert(MaybeNextBitCode.get() == bitc::METADATA_NAMED_NODE);
+        else
+          return MaybeNextBitCode.takeError();
 
         // Read named metadata elements.
         unsigned Size = Record.size();
@@ -775,9 +800,13 @@
       case bitc::METADATA_GLOBAL_DECL_ATTACHMENT: {
         // FIXME: we need to do this early because we don't materialize global
         // value explicitly.
-        IndexCursor.JumpToBit(CurrentPos);
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
         Record.clear();
-        IndexCursor.readRecord(Entry.ID, Record);
+        if (Expected<unsigned> MaybeRecord = IndexCursor.readRecord(Entry.ID, Record))
+          ;
+        else
+          return MaybeRecord.takeError();
         if (Record.size() % 2 == 0)
           return error("Invalid record");
         unsigned ValueID = Record[0];
@@ -871,7 +900,8 @@
       // Return at the beginning of the block, since it is easy to skip it
       // entirely from there.
       Stream.ReadBlockEnd(); // Pop the abbrev block context.
-      Stream.JumpToBit(EntryPos);
+        if (Error Err = IndexCursor.JumpToBit(EntryPos))
+          return Err;
       if (Stream.SkipBlock())
         return error("Invalid record");
       return Error::success();
@@ -883,7 +913,10 @@
 
   // Read all the records.
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -902,10 +935,11 @@
     Record.clear();
     StringRef Blob;
     ++NumMDRecordLoaded;
-    unsigned Code = Stream.readRecord(Entry.ID, Record, &Blob);
-    if (Error Err =
-            parseOneMetadata(Record, Code, Placeholders, Blob, NextMetadataNo))
-      return Err;
+    if (Expected<unsigned> MaybeCode = Stream.readRecord(Entry.ID, Record, &Blob)) {
+      if (Error Err = parseOneMetadata(Record, MaybeCode.get(), Placeholders, Blob, NextMetadataNo))
+        return Err;
+    } else
+      return MaybeCode.takeError();
   }
 }
 
@@ -930,12 +964,19 @@
   }
   SmallVector<uint64_t, 64> Record;
   StringRef Blob;
-  IndexCursor.JumpToBit(GlobalMetadataBitPosIndex[ID - MDStringRef.size()]);
-  auto Entry = IndexCursor.advanceSkippingSubblocks();
+  if (Error Err = IndexCursor.JumpToBit(GlobalMetadataBitPosIndex[ID - MDStringRef.size()]))
+    report_fatal_error("lazyLoadOneMetadata failed jumping: " + toString(std::move(Err)));
+  Expected<BitstreamEntry> MaybeEntry = IndexCursor.advanceSkippingSubblocks();
+  if (!MaybeEntry)
+    // FIXME this drops the error on the floor.
+    report_fatal_error("lazyLoadOneMetadata failed advanceSkippingSubblocks");
+  BitstreamEntry Entry = MaybeEntry.get();
   ++NumMDRecordLoaded;
-  unsigned Code = IndexCursor.readRecord(Entry.ID, Record, &Blob);
-  if (Error Err = parseOneMetadata(Record, Code, Placeholders, Blob, ID))
-    report_fatal_error("Can't lazyload MD");
+  if (Expected<unsigned> MaybeCode = IndexCursor.readRecord(Entry.ID, Record, &Blob)) {
+    if (Error Err = parseOneMetadata(Record, MaybeCode.get(), Placeholders, Blob, ID))
+      report_fatal_error("Can't lazyload MD, parseOneMetadata: " + toString(std::move(Err)));
+  } else
+    report_fatal_error("Can't lazyload MD: " + toString(MaybeCode.takeError()));
 }
 
 /// Ensure that all forward-references and placeholders are resolved.
@@ -1032,12 +1073,17 @@
     // Read name of the named metadata.
     SmallString<8> Name(Record.begin(), Record.end());
     Record.clear();
-    Code = Stream.ReadCode();
+    Expected<unsigned> MaybeCode = Stream.ReadCode();
+    if (!MaybeCode)
+      return MaybeCode.takeError();
+    Code = MaybeCode.get();
 
     ++NumMDRecordLoaded;
-    unsigned NextBitCode = Stream.readRecord(Code, Record);
-    if (NextBitCode != bitc::METADATA_NAMED_NODE)
-      return error("METADATA_NAME not followed by METADATA_NAMED_NODE");
+    if (Expected<unsigned> MaybeNextBitCode = Stream.readRecord(Code, Record)) {
+      if (MaybeNextBitCode.get() != bitc::METADATA_NAMED_NODE)
+        return error("METADATA_NAME not followed by METADATA_NAMED_NODE");
+    } else
+      return MaybeNextBitCode.takeError();
 
     // Read named metadata elements.
     unsigned Size = Record.size();
@@ -1863,7 +1909,10 @@
     if (R.AtEndOfStream())
       return error("Invalid record: metadata strings bad length");
 
-    unsigned Size = R.ReadVBR(6);
+    Expected<uint32_t> MaybeSize = R.ReadVBR(6);
+    if (!MaybeSize)
+      return MaybeSize.takeError();
+    uint32_t Size = MaybeSize.get();
     if (Strings.size() < Size)
       return error("Invalid record: metadata strings truncated chars");
 
@@ -1899,7 +1948,10 @@
   PlaceholderQueue Placeholders;
 
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -1916,7 +1968,10 @@
     // Read a metadata attachment record.
     Record.clear();
     ++NumMDRecordLoaded;
-    switch (Stream.readRecord(Entry.ID, Record)) {
+    Expected<unsigned> MaybeRecord = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeRecord)
+      return MaybeRecord.takeError();
+    switch (MaybeRecord.get()) {
     default: // Default behavior: ignore.
       break;
     case bitc::METADATA_ATTACHMENT: {
@@ -1997,7 +2052,10 @@
 
   // Read all the records.
   while (true) {
-    BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
+    Expected<BitstreamEntry> MaybeEntry = Stream.advanceSkippingSubblocks();
+    if (!MaybeEntry)
+      return MaybeEntry.takeError();
+    BitstreamEntry Entry = MaybeEntry.get();
 
     switch (Entry.Kind) {
     case BitstreamEntry::SubBlock: // Handled for us already.
@@ -2013,8 +2071,10 @@
     // Read a record.
     Record.clear();
     ++NumMDRecordLoaded;
-    unsigned Code = Stream.readRecord(Entry.ID, Record);
-    switch (Code) {
+    Expected<unsigned> MaybeCode = Stream.readRecord(Entry.ID, Record);
+    if (!MaybeCode)
+      return MaybeCode.takeError();
+    switch (MaybeCode.get()) {
     default: // Default behavior: ignore.
       break;
     case bitc::METADATA_KIND: {
diff --git a/llvm/tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp b/llvm/tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
--- a/llvm/tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
+++ b/llvm/tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
@@ -451,13 +451,17 @@
 
 
 
-/// ReportError - All bitcode analysis errors go through this function, making this a
-/// good place to breakpoint if debugging.
+/// All bitcode analysis errors go through this function, making this a good
+/// place to breakpoint if debugging.
 static bool ReportError(const Twine &Err) {
   WithColor::error() << Err << "\n";
   return true;
 }
 
+static bool ReportError(Error &&Err) {
+  return ReportError(toString(std::move(Err)));
+}
+
 static bool decodeMetadataStringsBlob(StringRef Indent,
                                       ArrayRef<uint64_t> Record,
                                       StringRef Blob) {
@@ -478,7 +482,10 @@
     if (R.AtEndOfStream())
       return ReportError("bad length");
 
-    unsigned Size = R.ReadVBR(6);
+    Expected<uint32_t> MaybeSize = R.ReadVBR(6);
+    if (!MaybeSize)
+      return ReportError(MaybeSize.takeError());
+    uint32_t Size = MaybeSize.get();
     if (Strings.size() < Size)
       return ReportError("truncated chars");
 
@@ -518,12 +525,16 @@
   bool DumpRecords = Dump;
   if (BlockID == bitc::BLOCKINFO_BLOCK_ID) {
     if (Dump) outs() << Indent << "<BLOCKINFO_BLOCK/>\n";
-    Optional<BitstreamBlockInfo> NewBlockInfo =
+    Expected<Optional<BitstreamBlockInfo>> MaybeNewBlockInfo =
         Stream.ReadBlockInfoBlock(/*ReadBlockInfoNames=*/true);
+    if (!MaybeNewBlockInfo)
+      return ReportError(MaybeNewBlockInfo.takeError());
+    Optional<BitstreamBlockInfo> NewBlockInfo = std::move(MaybeNewBlockInfo.get());
     if (!NewBlockInfo)
       return ReportError("Malformed BlockInfoBlock");
     BlockInfo = std::move(*NewBlockInfo);
-    Stream.JumpToBit(BlockBitStart);
+    if (Error Err = Stream.JumpToBit(BlockBitStart))
+      return ReportError(std::move(Err));
     // It's not really interesting to dump the contents of the blockinfo block.
     DumpRecords = false;
   }
@@ -562,9 +573,11 @@
 
     uint64_t RecordStartBit = Stream.GetCurrentBitNo();
 
-    BitstreamEntry Entry =
-      Stream.advance(BitstreamCursor::AF_DontAutoprocessAbbrevs);
-    
+    Expected<BitstreamEntry> MaybeEntry = Stream.advance(BitstreamCursor::AF_DontAutoprocessAbbrevs);
+    if (!MaybeEntry)
+      return ReportError(MaybeEntry.takeError());
+    BitstreamEntry Entry = MaybeEntry.get();
+
     switch (Entry.Kind) {
     case BitstreamEntry::Error:
       return ReportError("malformed bitcode file");
@@ -599,7 +612,8 @@
     }
 
     if (Entry.ID == bitc::DEFINE_ABBREV) {
-      Stream.ReadAbbrevRecord();
+      if (Error Err = Stream.ReadAbbrevRecord())
+        return ReportError(std::move(Err));
       ++BlockStats.NumAbbrevs;
       continue;
     }
@@ -610,7 +624,10 @@
 
     StringRef Blob;
     uint64_t CurrentRecordPos = Stream.GetCurrentBitNo();
-    unsigned Code = Stream.readRecord(Entry.ID, Record, &Blob);
+    Expected<unsigned> MaybeCode = Stream.readRecord(Entry.ID, Record, &Blob);
+    if (!MaybeCode)
+      return ReportError(MaybeCode.takeError());
+    unsigned Code = MaybeCode.get();
 
     // Increment the # occurrences of this code.
     if (BlockStats.CodeFreq.size() <= Code)
@@ -742,8 +759,12 @@
     }
 
     // Make sure that we can skip the current record.
-    Stream.JumpToBit(CurrentRecordPos);
-    Stream.skipRecord(Entry.ID);
+    if (Error Err = Stream.JumpToBit(CurrentRecordPos))
+      return ReportError(std::move(Err));
+    if (Expected<unsigned> Skipped = Stream.skipRecord(Entry.ID))
+      ; // Do nothing.
+    else
+      return ReportError(Skipped.takeError());
   }
 }
 
@@ -755,27 +776,35 @@
                    (double)Bits/8, (unsigned long)(Bits/32));
 }
 
-static CurStreamTypeType ReadSignature(BitstreamCursor &Stream) {
+static Expected<CurStreamTypeType> ReadSignature(BitstreamCursor &Stream) {
+  auto tryRead = [&Stream] (char &Dest, size_t size) -> Error {
+    if (Expected<SimpleBitstreamCursor::word_t> MaybeWord = Stream.Read(size))
+      Dest = MaybeWord.get();
+    else
+      return MaybeWord.takeError();
+    return Error::success();
+  };
+
   char Signature[6];
-  Signature[0] = Stream.Read(8);
-  Signature[1] = Stream.Read(8);
+  if (Error Err = tryRead(Signature[0], 8)) return std::move(Err);
+  if (Error Err = tryRead(Signature[1], 8)) return std::move(Err);
 
   // Autodetect the file contents, if it is one we know.
   if (Signature[0] == 'C' && Signature[1] == 'P') {
-    Signature[2] = Stream.Read(8);
-    Signature[3] = Stream.Read(8);
+    if (Error Err = tryRead(Signature[2], 8)) return std::move(Err);
+    if (Error Err = tryRead(Signature[3], 8)) return std::move(Err);
     if (Signature[2] == 'C' && Signature[3] == 'H')
       return ClangSerializedASTBitstream;
   } else if (Signature[0] == 'D' && Signature[1] == 'I') {
-    Signature[2] = Stream.Read(8);
-    Signature[3] = Stream.Read(8);
+    if (Error Err = tryRead(Signature[2], 8)) return std::move(Err);
+    if (Error Err = tryRead(Signature[3], 8)) return std::move(Err);
     if (Signature[2] == 'A' && Signature[3] == 'G')
       return ClangSerializedDiagnosticsBitstream;
   } else {
-    Signature[2] = Stream.Read(4);
-    Signature[3] = Stream.Read(4);
-    Signature[4] = Stream.Read(4);
-    Signature[5] = Stream.Read(4);
+    if (Error Err = tryRead(Signature[2], 4)) return std::move(Err);
+    if (Error Err = tryRead(Signature[3], 4)) return std::move(Err);
+    if (Error Err = tryRead(Signature[4], 4)) return std::move(Err);
+    if (Error Err = tryRead(Signature[5], 4)) return std::move(Err);
     if (Signature[0] == 'B' && Signature[1] == 'C' &&
         Signature[2] == 0x0 && Signature[3] == 0xC &&
         Signature[4] == 0xE && Signature[5] == 0xD)
@@ -827,7 +856,10 @@
   }
 
   Stream = BitstreamCursor(ArrayRef<uint8_t>(BufPtr, EndBufPtr));
-  CurStreamType = ReadSignature(Stream);
+  Expected<CurStreamTypeType> MaybeSignature = ReadSignature(Stream);
+  if (!MaybeSignature)
+    return ReportError(MaybeSignature.takeError());
+  CurStreamType = std::move(MaybeSignature.get());
 
   return false;
 }
@@ -853,21 +885,31 @@
       return true;
 
     while (!BlockInfoCursor.AtEndOfStream()) {
-      unsigned Code = BlockInfoCursor.ReadCode();
-      if (Code != bitc::ENTER_SUBBLOCK)
+      Expected<unsigned> MaybeCode = BlockInfoCursor.ReadCode();
+      if (!MaybeCode)
+        return ReportError(MaybeCode.takeError());
+      if (MaybeCode.get() != bitc::ENTER_SUBBLOCK)
         return ReportError("Invalid record at top-level in block info file");
 
-      unsigned BlockID = BlockInfoCursor.ReadSubBlockID();
-      if (BlockID == bitc::BLOCKINFO_BLOCK_ID) {
-        Optional<BitstreamBlockInfo> NewBlockInfo =
-            BlockInfoCursor.ReadBlockInfoBlock(/*ReadBlockInfoNames=*/true);
+      Expected<unsigned> MaybeBlockID = BlockInfoCursor.ReadSubBlockID();
+      if (!MaybeBlockID)
+        return ReportError(MaybeBlockID.takeError());
+      if (MaybeBlockID.get() == bitc::BLOCKINFO_BLOCK_ID) {
+        Expected<Optional<BitstreamBlockInfo>> MaybeNewBlockInfo =
+          BlockInfoCursor.ReadBlockInfoBlock(/*ReadBlockInfoNames=*/true);
+        if (!MaybeNewBlockInfo)
+          return ReportError(MaybeNewBlockInfo.takeError());
+        Optional<BitstreamBlockInfo> NewBlockInfo = std::move(MaybeNewBlockInfo.get());
         if (!NewBlockInfo)
           return ReportError("Malformed BlockInfoBlock in block info file");
         BlockInfo = std::move(*NewBlockInfo);
         break;
       }
 
-      BlockInfoCursor.SkipBlock();
+      if (Expected<bool> SkipBlock = BlockInfoCursor.SkipBlock())
+        ; // Ignore result.
+      else
+        return ReportError(SkipBlock.takeError());
     }
   }
 
@@ -875,13 +917,17 @@
 
   // Parse the top-level structure.  We only allow blocks at the top-level.
   while (!Stream.AtEndOfStream()) {
-    unsigned Code = Stream.ReadCode();
-    if (Code != bitc::ENTER_SUBBLOCK)
+    Expected<unsigned> MaybeCode = Stream.ReadCode();
+    if (!MaybeCode)
+      return ReportError(MaybeCode.takeError());
+    if (MaybeCode.get() != bitc::ENTER_SUBBLOCK)
       return ReportError("Invalid record at top-level");
 
-    unsigned BlockID = Stream.ReadSubBlockID();
+    Expected<unsigned> MaybeBlockID = Stream.ReadSubBlockID();
+    if (!MaybeBlockID)
+      return ReportError(MaybeBlockID.takeError());
 
-    if (ParseBlock(Stream, BlockInfo, BlockID, 0, CurStreamType))
+    if (ParseBlock(Stream, BlockInfo, MaybeBlockID.get(), 0, CurStreamType))
       return true;
     ++NumTopBlocks;
   }