diff --git a/docs/TableGen/LangRef.rst b/docs/TableGen/LangRef.rst
index 439d646034a..2efee12ec9d 100644
--- a/docs/TableGen/LangRef.rst
+++ b/docs/TableGen/LangRef.rst
@@ -33,7 +33,7 @@ Lexical Analysis
 ================
 
 TableGen supports BCPL (``// ...``) and nestable C-style (``/* ... */``)
-comments.
+comments.  TableGen also provides simple `Preprocessing Support`_.
 
 The following is a listing of the basic punctuation tokens::
 
@@ -448,3 +448,50 @@ applied at the end of parsing the base classes of a record.
    BaseMultiClassList: `MultiClassID` ("," `MultiClassID`)*
    MultiClassID: `TokIdentifier`
    MultiClassObject: `Def` | `Defm` | `Let` | `Foreach`
+
+Preprocessing Support
+=====================
+
+TableGen's embedded preprocessor is only intended for conditional compilation.
+It supports the following directives:
+
+.. productionlist::
+   LineBegin: ^
+   LineEnd: "\n" | "\r" | EOF
+   WhiteSpace: " " | "\t"
+   CStyleComment: "/*" (.* - "*/") "*/"
+   BCPLComment: "//" (.* - `LineEnd`) `LineEnd`
+   WhiteSpaceOrCStyleComment: `WhiteSpace` | `CStyleComment`
+   WhiteSpaceOrAnyComment: `WhiteSpace` | `CStyleComment` | `BCPLComment`
+   MacroName: `ualpha` (`ualpha` | "0"..."9")*
+   PrepDefine: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
+             : "#define" (`WhiteSpace`)+ `MacroName`
+             : (`WhiteSpaceOrAnyComment`)* `LineEnd`
+   PrepIfdef: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
+            : "#ifdef" (`WhiteSpace`)+ `MacroName`
+            : (`WhiteSpaceOrAnyComment`)* `LineEnd`
+   PrepElse: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
+           : "#else" (`WhiteSpaceOrAnyComment`)* `LineEnd`
+   PrepEndif: `LineBegin` (`WhiteSpaceOrCStyleComment`)*
+            : "#endif" (`WhiteSpaceOrAnyComment`)* `LineEnd`
+   PrepRegContentException: `PredIfdef` | `PredElse` | `PredEndif` | EOF
+   PrepRegion: .* - `PrepRegContentException`
+             :| `PrepIfDef`
+             :  (`PrepRegion`)*
+             :  [`PrepElse`]
+             :  (`PrepRegion`)*
+             :  `PrepEndif`
+
+:token:`PrepRegion` may occur anywhere in a TD file, as long as it matches
+the grammar specification.
+
+:token:`PrepDefine` allows defining a :token:`MacroName` so that any following
+:token:`PrepIfdef` - :token:`PrepElse` preprocessing region part and
+:token:`PrepIfdef` - :token:`PrepEndif` preprocessing region
+are enabled for TableGen tokens parsing.
+
+A preprocessing region, starting (i.e. having its :token:`PrepIfdef`) in a file,
+must end (i.e. have its :token:`PrepEndif`) in the same file.
+
+A :token:`MacroName` may be defined externally by using ``{ -D<NAME> }``
+option of TableGen.
diff --git a/lib/TableGen/Main.cpp b/lib/TableGen/Main.cpp
index 3a070162608..df4088ad2f8 100644
--- a/lib/TableGen/Main.cpp
+++ b/lib/TableGen/Main.cpp
@@ -46,6 +46,10 @@ static cl::list<std::string>
 IncludeDirs("I", cl::desc("Directory of include files"),
             cl::value_desc("directory"), cl::Prefix);
 
+static cl::list<std::string>
+MacroNames("D", cl::desc("Name of the macro to be defined"),
+            cl::value_desc("macro name"), cl::Prefix);
+
 static int reportError(const char *ProgName, Twine Msg) {
   errs() << ProgName << ": " << Msg;
   errs().flush();
@@ -91,7 +95,7 @@ int llvm::TableGenMain(char *argv0, TableGenMainFn *MainFn) {
   // it later.
   SrcMgr.setIncludeDirs(IncludeDirs);
 
-  TGParser Parser(SrcMgr, Records);
+  TGParser Parser(SrcMgr, MacroNames, Records);
 
   if (Parser.ParseFile())
     return 1;
diff --git a/lib/TableGen/TGLexer.cpp b/lib/TableGen/TGLexer.cpp
index 652be6e8dbb..16aeee56107 100644
--- a/lib/TableGen/TGLexer.cpp
+++ b/lib/TableGen/TGLexer.cpp
@@ -19,6 +19,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/TableGen/Error.h"
+#include <algorithm>
 #include <cctype>
 #include <cerrno>
 #include <cstdint>
@@ -28,11 +29,35 @@
 
 using namespace llvm;
 
-TGLexer::TGLexer(SourceMgr &SM) : SrcMgr(SM) {
+namespace {
+// A list of supported preprocessing directives with their
+// internal token kinds and names.
+struct {
+  tgtok::TokKind Kind;
+  const char *Word;
+} PreprocessorDirs[] = {
+  { tgtok::Ifdef, "ifdef" },
+  { tgtok::Else, "else" },
+  { tgtok::Endif, "endif" },
+  { tgtok::Define, "define" }
+};
+} // end anonymous namespace
+
+TGLexer::TGLexer(SourceMgr &SM, ArrayRef<std::string> Macros) : SrcMgr(SM) {
   CurBuffer = SrcMgr.getMainFileID();
   CurBuf = SrcMgr.getMemoryBuffer(CurBuffer)->getBuffer();
   CurPtr = CurBuf.begin();
   TokStart = nullptr;
+
+  // Pretend that we enter the "top-level" include file.
+  PrepIncludeStack.push_back(
+      make_unique<std::vector<PreprocessorControlDesc>>());
+
+  // Put all macros defined in the command line into the DefinedMacros set.
+  std::for_each(Macros.begin(), Macros.end(),
+                [this](const std::string &MacroName) {
+                  DefinedMacros.insert(MacroName);
+                });
 }
 
 SMLoc TGLexer::getLoc() const {
@@ -41,11 +66,42 @@ SMLoc TGLexer::getLoc() const {
 
 /// ReturnError - Set the error to the specified string at the specified
 /// location.  This is defined to always return tgtok::Error.
-tgtok::TokKind TGLexer::ReturnError(const char *Loc, const Twine &Msg) {
+tgtok::TokKind TGLexer::ReturnError(SMLoc Loc, const Twine &Msg) {
   PrintError(Loc, Msg);
   return tgtok::Error;
 }
 
+tgtok::TokKind TGLexer::ReturnError(const char *Loc, const Twine &Msg) {
+  return ReturnError(SMLoc::getFromPointer(Loc), Msg);
+}
+
+bool TGLexer::processEOF() {
+  SMLoc ParentIncludeLoc = SrcMgr.getParentIncludeLoc(CurBuffer);
+  if (ParentIncludeLoc != SMLoc()) {
+    // If prepExitInclude() detects a problem with the preprocessing
+    // control stack, it will return false.  Pretend that we reached
+    // the final EOF and stop lexing more tokens by returning false
+    // to LexToken().
+    if (!prepExitInclude(false))
+      return false;
+
+    CurBuffer = SrcMgr.FindBufferContainingLoc(ParentIncludeLoc);
+    CurBuf = SrcMgr.getMemoryBuffer(CurBuffer)->getBuffer();
+    CurPtr = ParentIncludeLoc.getPointer();
+    // Make sure TokStart points into the parent file's buffer.
+    // LexToken() assigns to it before calling getNextChar(),
+    // so it is pointing into the included file now.
+    TokStart = CurPtr;
+    return true;
+  }
+
+  // Pretend that we exit the "top-level" include file.
+  // Note that in case of an error (e.g. control stack imbalance)
+  // the routine will issue a fatal error.
+  prepExitInclude(true);
+  return false;
+}
+
 int TGLexer::getNextChar() {
   char CurChar = *CurPtr++;
   switch (CurChar) {
@@ -57,16 +113,6 @@ int TGLexer::getNextChar() {
     if (CurPtr-1 != CurBuf.end())
       return 0;  // Just whitespace.
 
-    // If this is the end of an included file, pop the parent file off the
-    // include stack.
-    SMLoc ParentIncludeLoc = SrcMgr.getParentIncludeLoc(CurBuffer);
-    if (ParentIncludeLoc != SMLoc()) {
-      CurBuffer = SrcMgr.FindBufferContainingLoc(ParentIncludeLoc);
-      CurBuf = SrcMgr.getMemoryBuffer(CurBuffer)->getBuffer();
-      CurPtr = ParentIncludeLoc.getPointer();
-      return getNextChar();
-    }
-
     // Otherwise, return end of file.
     --CurPtr;  // Another call to lex will return EOF again.
     return EOF;
@@ -83,11 +129,11 @@ int TGLexer::getNextChar() {
   }
 }
 
-int TGLexer::peekNextChar(int Index) {
+int TGLexer::peekNextChar(int Index) const {
   return *(CurPtr + Index);
 }
 
-tgtok::TokKind TGLexer::LexToken() {
+tgtok::TokKind TGLexer::LexToken(bool FileOrLineStart) {
   TokStart = CurPtr;
   // This always consumes at least one character.
   int CurChar = getNextChar();
@@ -100,7 +146,18 @@ tgtok::TokKind TGLexer::LexToken() {
 
     // Unknown character, emit an error.
     return ReturnError(TokStart, "Unexpected character");
-  case EOF: return tgtok::Eof;
+  case EOF:
+    // Lex next token, if we just left an include file.
+    // Note that leaving an include file means that the next
+    // symbol is located at the end of 'include "..."'
+    // construct, so LexToken() is called with default
+    // false parameter.
+    if (processEOF())
+      return LexToken();
+
+    // Return EOF denoting the end of lexing.
+    return tgtok::Eof;
+
   case ':': return tgtok::colon;
   case ';': return tgtok::semi;
   case '.': return tgtok::period;
@@ -114,15 +171,27 @@ tgtok::TokKind TGLexer::LexToken() {
   case ')': return tgtok::r_paren;
   case '=': return tgtok::equal;
   case '?': return tgtok::question;
-  case '#': return tgtok::paste;
+  case '#':
+    if (FileOrLineStart) {
+      tgtok::TokKind Kind = prepIsDirective();
+      if (Kind != tgtok::Error)
+        return lexPreprocessor(Kind);
+    }
+
+    return tgtok::paste;
+
+  case '\r':
+    PrintFatalError("getNextChar() must never return '\r'");
+    return tgtok::Error;
 
   case 0:
   case ' ':
   case '\t':
-  case '\n':
-  case '\r':
     // Ignore whitespace.
-    return LexToken();
+    return LexToken(FileOrLineStart);
+  case '\n':
+    // Ignore whitespace, and identify the new line.
+    return LexToken(true);
   case '/':
     // If this is the start of a // comment, skip until the end of the line or
     // the end of the buffer.
@@ -133,7 +202,7 @@ tgtok::TokKind TGLexer::LexToken() {
         return tgtok::Error;
     } else // Otherwise, this is an error.
       return ReturnError(TokStart, "Unexpected character");
-    return LexToken();
+    return LexToken(FileOrLineStart);
   case '-': case '+':
   case '0': case '1': case '2': case '3': case '4': case '5': case '6':
   case '7': case '8': case '9': {
@@ -249,10 +318,10 @@ tgtok::TokKind TGLexer::LexVarName() {
 }
 
 tgtok::TokKind TGLexer::LexIdentifier() {
-  // The first letter is [a-zA-Z_#].
+  // The first letter is [a-zA-Z_].
   const char *IdentStart = TokStart;
 
-  // Match the rest of the identifier regex: [0-9a-zA-Z_#]*
+  // Match the rest of the identifier regex: [0-9a-zA-Z_]*
   while (isalpha(*CurPtr) || isdigit(*CurPtr) || *CurPtr == '_')
     ++CurPtr;
 
@@ -322,6 +391,9 @@ bool TGLexer::LexInclude() {
   // Save the line number and lex buffer of the includer.
   CurBuf = SrcMgr.getMemoryBuffer(CurBuffer)->getBuffer();
   CurPtr = CurBuf.begin();
+
+  PrepIncludeStack.push_back(
+      make_unique<std::vector<PreprocessorControlDesc>>());
   return false;
 }
 
@@ -496,3 +568,444 @@ tgtok::TokKind TGLexer::LexExclaim() {
 
   return Kind != tgtok::Error ? Kind : ReturnError(Start-1, "Unknown operator");
 }
+
+bool TGLexer::prepExitInclude(bool IncludeStackMustBeEmpty) {
+  // Report an error, if preprocessor control stack for the current
+  // file is not empty.
+  if (!PrepIncludeStack.back()->empty()) {
+    prepReportPreprocessorStackError();
+
+    return false;
+  }
+
+  // Pop the preprocessing controls from the include stack.
+  if (PrepIncludeStack.empty()) {
+    PrintFatalError("Preprocessor include stack is empty");
+  }
+
+  PrepIncludeStack.pop_back();
+
+  if (IncludeStackMustBeEmpty) {
+    if (!PrepIncludeStack.empty())
+      PrintFatalError("Preprocessor include stack is not empty");
+  } else {
+    if (PrepIncludeStack.empty())
+      PrintFatalError("Preprocessor include stack is empty");
+  }
+
+  return true;
+}
+
+tgtok::TokKind TGLexer::prepIsDirective() const {
+  for (unsigned ID = 0; ID < llvm::array_lengthof(PreprocessorDirs); ++ID) {
+    int NextChar = *CurPtr;
+    bool Match = true;
+    unsigned I = 0;
+    for (; I < strlen(PreprocessorDirs[ID].Word); ++I) {
+      if (NextChar != PreprocessorDirs[ID].Word[I]) {
+        Match = false;
+        break;
+      }
+
+      NextChar = peekNextChar(I + 1);
+    }
+
+    // Check for whitespace after the directive.  If there is no whitespace,
+    // then we do not recognize it as a preprocessing directive.
+    if (Match) {
+      tgtok::TokKind Kind = PreprocessorDirs[ID].Kind;
+
+      // New line and EOF may follow only #else/#endif.  It will be reported
+      // as an error for #ifdef/#define after the call to prepLexMacroName().
+      if (NextChar == ' ' || NextChar == '\t' || NextChar == EOF ||
+          NextChar == '\n' ||
+          // It looks like TableGen does not support '\r' as the actual
+          // carriage return, e.g. getNextChar() treats a single '\r'
+          // as '\n'.  So we do the same here.
+          NextChar == '\r')
+        return Kind;
+
+      // Allow comments after some directives, e.g.:
+      //     #else// OR #else/**/
+      //     #endif// OR #endif/**/
+      //
+      // Note that we do allow comments after #ifdef/#define here, e.g.
+      //     #ifdef/**/ AND #ifdef//
+      //     #define/**/ AND #define//
+      //
+      // These cases will be reported as incorrect after calling
+      // prepLexMacroName().  We could have supported C-style comments
+      // after #ifdef/#define, but this would complicate the code
+      // for little benefit.
+      if (NextChar == '/') {
+        NextChar = peekNextChar(I + 1);
+
+        if (NextChar == '*' || NextChar == '/')
+          return Kind;
+
+        // Pretend that we do not recognize the directive.
+      }
+    }
+  }
+
+  return tgtok::Error;
+}
+
+bool TGLexer::prepEatPreprocessorDirective(tgtok::TokKind Kind) {
+  TokStart = CurPtr;
+
+  for (unsigned ID = 0; ID < llvm::array_lengthof(PreprocessorDirs); ++ID)
+    if (PreprocessorDirs[ID].Kind == Kind) {
+      // Advance CurPtr to the end of the preprocessing word.
+      CurPtr += strlen(PreprocessorDirs[ID].Word);
+      return true;
+    }
+
+  PrintFatalError("Unsupported preprocessing token in "
+                  "prepEatPreprocessorDirective()");
+  return false;
+}
+
+tgtok::TokKind TGLexer::lexPreprocessor(
+    tgtok::TokKind Kind, bool ReturnNextLiveToken) {
+
+  // We must be looking at a preprocessing directive.  Eat it!
+  if (!prepEatPreprocessorDirective(Kind))
+    PrintFatalError("lexPreprocessor() called for unknown "
+                    "preprocessor directive");
+
+  if (Kind == tgtok::Ifdef) {
+    StringRef MacroName = prepLexMacroName();
+    if (MacroName.empty())
+      return ReturnError(TokStart, "Expected macro name after #ifdef");
+
+    bool MacroIsDefined = DefinedMacros.count(MacroName) != 0;
+
+    // Regardless of whether we are processing tokens or not,
+    // we put the #ifdef control on stack.
+    PrepIncludeStack.back()->push_back(
+        {Kind, MacroIsDefined, SMLoc::getFromPointer(TokStart)});
+
+    if (!prepSkipDirectiveEnd())
+      return ReturnError(CurPtr,
+                         "Only comments are supported after #ifdef NAME");
+
+    // If we were not processing tokens before this #ifdef,
+    // then just return back to the lines skipping code.
+    if (!ReturnNextLiveToken)
+      return Kind;
+
+    // If we were processing tokens before this #ifdef,
+    // and the macro is defined, then just return the next token.
+    if (MacroIsDefined)
+      return LexToken();
+
+    // We were processing tokens before this #ifdef, and the macro
+    // is not defined, so we have to start skipping the lines.
+    // If the skipping is successful, it will return the token following
+    // either #else or #endif corresponding to this #ifdef.
+    if (prepSkipRegion(ReturnNextLiveToken))
+      return LexToken();
+
+    return tgtok::Error;
+  } else if (Kind == tgtok::Else) {
+    // Check if this #else is correct before calling prepSkipDirectiveEnd(),
+    // which will move CurPtr away from the beginning of #else.
+    if (PrepIncludeStack.back()->empty())
+      return ReturnError(TokStart, "#else without #ifdef");
+
+    PreprocessorControlDesc IfdefEntry = PrepIncludeStack.back()->back();
+
+    if (IfdefEntry.Kind != tgtok::Ifdef) {
+      PrintError(TokStart, "double #else");
+      return ReturnError(IfdefEntry.SrcPos, "Previous #else is here");
+    }
+
+    // Replace the corresponding #ifdef's control with its negation
+    // on the control stack.
+    PrepIncludeStack.back()->pop_back();
+    PrepIncludeStack.back()->push_back(
+        {Kind, !IfdefEntry.IsDefined, SMLoc::getFromPointer(TokStart)});
+
+    if (!prepSkipDirectiveEnd())
+      return ReturnError(CurPtr, "Only comments are supported after #else");
+
+    // If we were processing tokens before this #else,
+    // we have to start skipping lines until the matching #endif.
+    if (ReturnNextLiveToken) {
+      if (prepSkipRegion(ReturnNextLiveToken))
+        return LexToken();
+
+      return tgtok::Error;
+    }
+
+    // Return to the lines skipping code.
+    return Kind;
+  } else if (Kind == tgtok::Endif) {
+    // Check if this #endif is correct before calling prepSkipDirectiveEnd(),
+    // which will move CurPtr away from the beginning of #endif.
+    if (PrepIncludeStack.back()->empty())
+      return ReturnError(TokStart, "#endif without #ifdef");
+
+    auto &IfdefOrElseEntry = PrepIncludeStack.back()->back();
+
+    if (IfdefOrElseEntry.Kind != tgtok::Ifdef &&
+        IfdefOrElseEntry.Kind != tgtok::Else) {
+      PrintFatalError("Invalid preprocessor control on the stack");
+      return tgtok::Error;
+    }
+
+    if (!prepSkipDirectiveEnd())
+      return ReturnError(CurPtr, "Only comments are supported after #endif");
+
+    PrepIncludeStack.back()->pop_back();
+
+    // If we were processing tokens before this #endif, then
+    // we should continue it.
+    if (ReturnNextLiveToken) {
+      return LexToken();
+    }
+
+    // Return to the lines skipping code.
+    return Kind;
+  } else if (Kind == tgtok::Define) {
+    StringRef MacroName = prepLexMacroName();
+    if (MacroName.empty())
+      return ReturnError(TokStart, "Expected macro name after #define");
+
+    if (!DefinedMacros.insert(MacroName).second)
+      PrintWarning(getLoc(),
+                   "Duplicate definition of macro: " + Twine(MacroName));
+
+    if (!prepSkipDirectiveEnd())
+      return ReturnError(CurPtr,
+                         "Only comments are supported after #define NAME");
+
+    if (!ReturnNextLiveToken) {
+      PrintFatalError("#define must be ignored during the lines skipping");
+      return tgtok::Error;
+    }
+
+    return LexToken();
+  }
+
+  PrintFatalError("Preprocessing directive is not supported");
+  return tgtok::Error;
+}
+
+bool TGLexer::prepSkipRegion(bool MustNeverBeFalse) {
+  if (!MustNeverBeFalse)
+    PrintFatalError("Invalid recursion.");
+
+  do {
+    // Skip all symbols to the line end.
+    prepSkipToLineEnd();
+
+    // Find the first non-whitespace symbol in the next line(s).
+    if (!prepSkipLineBegin())
+      return false;
+
+    // If the first non-blank/comment symbol on the line is '#',
+    // it may be a start of preprocessing directive.
+    //
+    // If it is not '#' just go to the next line.
+    if (*CurPtr == '#')
+      ++CurPtr;
+    else
+      continue;
+
+    tgtok::TokKind Kind = prepIsDirective();
+
+    // If we did not find a preprocessing directive or it is #define,
+    // then just skip to the next line.  We do not have to do anything
+    // for #define in the line-skipping mode.
+    if (Kind == tgtok::Error || Kind == tgtok::Define)
+      continue;
+
+    tgtok::TokKind ProcessedKind = lexPreprocessor(Kind, false);
+
+    // If lexPreprocessor() encountered an error during lexing this
+    // preprocessor idiom, then return false to the calling lexPreprocessor().
+    // This will force tgtok::Error to be returned to the tokens processing.
+    if (ProcessedKind == tgtok::Error)
+      return false;
+
+    if (Kind != ProcessedKind)
+      PrintFatalError("prepIsDirective() and lexPreprocessor() "
+                      "returned different token kinds");
+
+    // If this preprocessing directive enables tokens processing,
+    // then return to the lexPreprocessor() and get to the next token.
+    // We can move from line-skipping mode to processing tokens only
+    // due to #else or #endif.
+    if (prepIsProcessingEnabled()) {
+      if (Kind != tgtok::Else && Kind != tgtok::Endif) {
+        PrintFatalError("Tokens processing was enabled by an unexpected "
+                        "preprocessing directive");
+        return false;
+      }
+
+      return true;
+    }
+  } while (CurPtr != CurBuf.end());
+
+  // We have reached the end of the file, but never left the lines-skipping
+  // mode.  This means there is no matching #endif.
+  prepReportPreprocessorStackError();
+  return false;
+}
+
+StringRef TGLexer::prepLexMacroName() {
+  // Skip whitespaces between the preprocessing directive and the macro name.
+  while (*CurPtr == ' ' || *CurPtr == '\t')
+    ++CurPtr;
+
+  TokStart = CurPtr;
+  // Macro names start with [a-zA-Z_].
+  if (*CurPtr != '_' && !isalpha(*CurPtr))
+    return "";
+
+  // Match the rest of the identifier regex: [0-9a-zA-Z_]*
+  while (isalpha(*CurPtr) || isdigit(*CurPtr) || *CurPtr == '_')
+    ++CurPtr;
+
+  return StringRef(TokStart, CurPtr - TokStart);
+}
+
+bool TGLexer::prepSkipLineBegin() {
+  while (CurPtr != CurBuf.end()) {
+    switch (*CurPtr) {
+    case ' ':
+    case '\t':
+    case '\n':
+    case '\r':
+      break;
+
+    case '/': {
+      int NextChar = peekNextChar(1);
+      if (NextChar == '*') {
+        // Skip C-style comment.
+        // Note that we do not care about skipping the C++-style comments.
+        // If the line contains "//", it may not contain any processable
+        // preprocessing directive.  Just return CurPtr pointing to
+        // the first '/' in this case.  We also do not care about
+        // incorrect symbols after the first '/' - we are in lines-skipping
+        // mode, so incorrect code is allowed to some extent.
+
+        // Set TokStart to the beginning of the comment to enable proper
+        // diagnostic printing in case of error in SkipCComment().
+        TokStart = CurPtr;
+
+        // CurPtr must point to '*' before call to SkipCComment().
+        ++CurPtr;
+        if (SkipCComment())
+          return false;
+      } else {
+        // CurPtr points to the non-whitespace '/'.
+        return true;
+      }
+
+      // We must not increment CurPtr after the comment was lexed.
+      continue;
+    }
+
+    default:
+      return true;
+    }
+
+    ++CurPtr;
+  }
+
+  // We have reached the end of the file.  Return to the lines skipping
+  // code, and allow it to handle the EOF as needed.
+  return true;
+}
+
+bool TGLexer::prepSkipDirectiveEnd() {
+  while (CurPtr != CurBuf.end()) {
+    switch (*CurPtr) {
+    case ' ':
+    case '\t':
+      break;
+
+    case '\n':
+    case '\r':
+      return true;
+
+    case '/': {
+      int NextChar = peekNextChar(1);
+      if (NextChar == '/') {
+        // Skip C++-style comment.
+        // We may just return true now, but let's skip to the line/buffer end
+        // to simplify the method specification.
+        ++CurPtr;
+        SkipBCPLComment();
+      } else if (NextChar == '*') {
+        // When we are skipping C-style comment at the end of a preprocessing
+        // directive, we can skip several lines.  If any meaningful TD token
+        // follows the end of the C-style comment on the same line, it will
+        // be considered as an invalid usage of TD token.
+        // For example, we want to forbid usages like this one:
+        //     #define MACRO class Class {}
+        // But with C-style comments we also disallow the following:
+        //     #define MACRO /* This macro is used
+        //                      to ... */ class Class {}
+        // One can argue that this should be allowed, but it does not seem
+        // to be worth of the complication.  Moreover, this matches
+        // the C preprocessor behavior.
+
+        // Set TokStart to the beginning of the comment to enable proper
+        // diagnostic printer in case of error in SkipCComment().
+        TokStart = CurPtr;
+        ++CurPtr;
+        if (SkipCComment())
+          return false;
+      } else {
+        TokStart = CurPtr;
+        PrintError(CurPtr, "Unexpected character");
+        return false;
+      }
+
+      // We must not increment CurPtr after the comment was lexed.
+      continue;
+    }
+
+    default:
+      // Do not allow any non-whitespaces after the directive.
+      TokStart = CurPtr;
+      return false;
+    }
+
+    ++CurPtr;
+  }
+
+  return true;
+}
+
+void TGLexer::prepSkipToLineEnd() {
+  while (*CurPtr != '\n' && *CurPtr != '\r' && CurPtr != CurBuf.end())
+    ++CurPtr;
+}
+
+bool TGLexer::prepIsProcessingEnabled() {
+  for (auto I = PrepIncludeStack.back()->rbegin(),
+            E = PrepIncludeStack.back()->rend();
+       I != E; ++I) {
+    if (!I->IsDefined)
+      return false;
+  }
+
+  return true;
+}
+
+void TGLexer::prepReportPreprocessorStackError() {
+  if (PrepIncludeStack.back()->empty())
+    PrintFatalError("prepReportPreprocessorStackError() called with "
+                    "empty control stack");
+
+  auto &PrepControl = PrepIncludeStack.back()->back();
+  PrintError(CurBuf.end(), "Reached EOF without matching #endif");
+  PrintError(PrepControl.SrcPos, "The latest preprocessor control is here");
+
+  TokStart = CurPtr;
+}
diff --git a/lib/TableGen/TGLexer.h b/lib/TableGen/TGLexer.h
index 2c80743e3a6..e9980b36b97 100644
--- a/lib/TableGen/TGLexer.h
+++ b/lib/TableGen/TGLexer.h
@@ -14,11 +14,14 @@
 #ifndef LLVM_LIB_TABLEGEN_TGLEXER_H
 #define LLVM_LIB_TABLEGEN_TGLEXER_H
 
+#include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/SMLoc.h"
 #include <cassert>
 #include <map>
+#include <memory>
 #include <string>
 
 namespace llvm {
@@ -59,7 +62,11 @@ namespace tgtok {
     BinaryIntVal,
 
     // String valued tokens.
-    Id, StrVal, VarName, CodeFragment
+    Id, StrVal, VarName, CodeFragment,
+
+    // Preprocessing tokens for internal usage by the lexer.
+    // They are never returned as a result of Lex().
+    Ifdef, Else, Endif, Define
   };
 }
 
@@ -87,10 +94,10 @@ private:
   DependenciesMapTy Dependencies;
 
 public:
-  TGLexer(SourceMgr &SrcMgr);
+  TGLexer(SourceMgr &SrcMgr, ArrayRef<std::string> Macros);
 
   tgtok::TokKind Lex() {
-    return CurCode = LexToken();
+    return CurCode = LexToken(CurPtr == CurBuf.begin());
   }
 
   const DependenciesMapTy &getDependencies() const {
@@ -119,12 +126,13 @@ public:
 
 private:
   /// LexToken - Read the next token and return its code.
-  tgtok::TokKind LexToken();
+  tgtok::TokKind LexToken(bool FileOrLineStart = false);
 
+  tgtok::TokKind ReturnError(SMLoc Loc, const Twine &Msg);
   tgtok::TokKind ReturnError(const char *Loc, const Twine &Msg);
 
   int getNextChar();
-  int peekNextChar(int Index);
+  int peekNextChar(int Index) const;
   void SkipBCPLComment();
   bool SkipCComment();
   tgtok::TokKind LexIdentifier();
@@ -134,6 +142,231 @@ private:
   tgtok::TokKind LexNumber();
   tgtok::TokKind LexBracket();
   tgtok::TokKind LexExclaim();
+
+  // Process EOF encountered in LexToken().
+  // If EOF is met in an include file, then the method will update
+  // CurPtr, CurBuf and preprocessing include stack, and return true.
+  // If EOF is met in the top-level file, then the method will
+  // update and check the preprocessing include stack, and return false.
+  bool processEOF();
+
+  // *** Structures and methods for preprocessing support ***
+
+  // A set of macro names that are defined either via command line or
+  // by using:
+  //     #define NAME
+  StringSet<> DefinedMacros;
+
+  // Each of #ifdef and #else directives has a descriptor associated
+  // with it.
+  //
+  // An ordered list of preprocessing controls defined by #ifdef/#else
+  // directives that are in effect currently is called preprocessing
+  // control stack.  It is represented as a vector of PreprocessorControlDesc's.
+  //
+  // The control stack is updated according to the following rules:
+  //
+  // For each #ifdef we add an element to the control stack.
+  // For each #else we replace the top element with a descriptor
+  // with an inverted IsDefined value.
+  // For each #endif we pop the top element from the control stack.
+  //
+  // When CurPtr reaches the current buffer's end, the control stack
+  // must be empty, i.e. #ifdef and the corresponding #endif
+  // must be located in the same file.
+  struct PreprocessorControlDesc {
+    // Either tgtok::Ifdef or tgtok::Else.
+    tgtok::TokKind Kind;
+
+    // True, if the condition for this directive is true, false - otherwise.
+    // Examples:
+    //     #ifdef NAME       : true, if NAME is defined, false - otherwise.
+    //     ...
+    //     #else             : false, if NAME is defined, true - otherwise.
+    bool IsDefined;
+
+    // Pointer into CurBuf to the beginning of the preprocessing directive
+    // word, e.g.:
+    //     #ifdef NAME
+    //      ^ - SrcPos
+    SMLoc SrcPos;
+  };
+
+  // We want to disallow code like this:
+  //     file1.td:
+  //         #define NAME
+  //         #ifdef NAME
+  //         include "file2.td"
+  //     EOF
+  //     file2.td:
+  //         #endif
+  //     EOF
+  //
+  // To do this, we clear the preprocessing control stack on entry
+  // to each of the included file.  PrepIncludeStack is used to store
+  // preprocessing control stacks for the current file and all its
+  // parent files.  The back() element is the preprocessing control
+  // stack for the current file.
+  std::vector<std::unique_ptr<std::vector<PreprocessorControlDesc>>>
+      PrepIncludeStack;
+
+  // Validate that the current preprocessing control stack is empty,
+  // since we are about to exit a file, and pop the include stack.
+  //
+  // If IncludeStackMustBeEmpty is true, the include stack must be empty
+  // after the popping, otherwise, the include stack must not be empty
+  // after the popping.  Basically, the include stack must be empty
+  // only if we exit the "top-level" file (i.e. finish lexing).
+  //
+  // The method returns false, if the current preprocessing control stack
+  // is not empty (e.g. there is an unterminated #ifdef/#else),
+  // true - otherwise.
+  bool prepExitInclude(bool IncludeStackMustBeEmpty);
+
+  // Look ahead for a preprocessing directive starting from CurPtr.  The caller
+  // must only call this method, if *(CurPtr - 1) is '#'.  If the method matches
+  // a preprocessing directive word followed by a whitespace, then it returns
+  // one of the internal token kinds, i.e. Ifdef, Else, Endif, Define.
+  //
+  // CurPtr is not adjusted by this method.
+  tgtok::TokKind prepIsDirective() const;
+
+  // Given a preprocessing token kind, adjusts CurPtr to the end
+  // of the preprocessing directive word.  Returns true, unless
+  // an unsupported token kind is passed in.
+  //
+  // We use look-ahead prepIsDirective() and prepEatPreprocessorDirective()
+  // to avoid adjusting CurPtr before we are sure that '#' is followed
+  // by a preprocessing directive.  If it is not, then we fall back to
+  // tgtok::paste interpretation of '#'.
+  bool prepEatPreprocessorDirective(tgtok::TokKind Kind);
+
+  // The main "exit" point from the token parsing to preprocessor.
+  //
+  // The method is called for CurPtr, when prepIsDirective() returns
+  // true.  The first parameter matches the result of prepIsDirective(),
+  // denoting the actual preprocessor directive to be processed.
+  //
+  // If the preprocessing directive disables the tokens processing, e.g.:
+  //     #ifdef NAME // NAME is undefined
+  // then lexPreprocessor() enters the lines-skipping mode.
+  // In this mode, it does not parse any tokens, because the code under
+  // the #ifdef may not even be a correct tablegen code.  The preprocessor
+  // looks for lines containing other preprocessing directives, which
+  // may be prepended with whitespaces and C-style comments.  If the line
+  // does not contain a preprocessing directive, it is skipped completely.
+  // Otherwise, the preprocessing directive is processed by recursively
+  // calling lexPreprocessor().  The processing of the encountered
+  // preprocessing directives includes updating preprocessing control stack
+  // and adding new macros into DefinedMacros set.
+  //
+  // The second parameter controls whether lexPreprocessor() is called from
+  // LexToken() (true) or recursively from lexPreprocessor() (false).
+  //
+  // If ReturnNextLiveToken is true, the method returns the next
+  // LEX token following the current directive or following the end
+  // of the disabled preprocessing region corresponding to this directive.
+  // If ReturnNextLiveToken is false, the method returns the first parameter,
+  // unless there were errors encountered in the disabled preprocessing
+  // region - in this case, it returns tgtok::Error.
+  tgtok::TokKind lexPreprocessor(tgtok::TokKind Kind,
+                                 bool ReturnNextLiveToken = true);
+
+  // Worker method for lexPreprocessor() to skip lines after some
+  // preprocessing directive up to the buffer end or to the directive
+  // that re-enables token processing.  The method returns true
+  // upon processing the next directive that re-enables tokens
+  // processing.  False is returned if an error was encountered.
+  //
+  // Note that prepSkipRegion() calls lexPreprocessor() to process
+  // encountered preprocessing directives.  In this case, the second
+  // parameter to lexPreprocessor() is set to false.  Being passed
+  // false ReturnNextLiveToken, lexPreprocessor() must never call
+  // prepSkipRegion().  We assert this by passing ReturnNextLiveToken
+  // to prepSkipRegion() and checking that it is never set to false.
+  bool prepSkipRegion(bool MustNeverBeFalse);
+
+  // Lex name of the macro after either #ifdef or #define.  We could have used
+  // LexIdentifier(), but it has special handling of "include" word, which
+  // could result in awkward diagnostic errors.  Consider:
+  // ----
+  // #ifdef include
+  // class ...
+  // ----
+  // LexIdentifier() will engage LexInclude(), which will complain about
+  // missing file with name "class".  Instead, prepLexMacroName() will treat
+  // "include" as a normal macro name.
+  //
+  // On entry, CurPtr points to the end of a preprocessing directive word.
+  // The method allows for whitespaces between the preprocessing directive
+  // and the macro name.  The allowed whitespaces are ' ' and '\t'.
+  //
+  // If the first non-whitespace symbol after the preprocessing directive
+  // is a valid start symbol for an identifier (i.e. [a-zA-Z_]), then
+  // the method updates TokStart to the position of the first non-whitespace
+  // symbol, sets CurPtr to the position of the macro name's last symbol,
+  // and returns a string reference to the macro name.  Otherwise,
+  // TokStart is set to the first non-whitespace symbol after the preprocessing
+  // directive, and the method returns an empty string reference.
+  //
+  // In all cases, TokStart may be used to point to the word following
+  // the preprocessing directive.
+  StringRef prepLexMacroName();
+
+  // Skip any whitespaces starting from CurPtr.  The method is used
+  // only in the lines-skipping mode to find the first non-whitespace
+  // symbol after or at CurPtr.  Allowed whitespaces are ' ', '\t', '\n'
+  // and '\r'.  The method skips C-style comments as well, because
+  // it is used to find the beginning of the preprocessing directive.
+  // If we do not handle C-style comments the following code would
+  // result in incorrect detection of a preprocessing directive:
+  //     /*
+  //     #ifdef NAME
+  //     */
+  // As long as we skip C-style comments, the following code is correctly
+  // recognized as a preprocessing directive:
+  //     /* first line comment
+  //        second line comment */ #ifdef NAME
+  //
+  // The method returns true upon reaching the first non-whitespace symbol
+  // or EOF, CurPtr is set to point to this symbol.  The method returns false,
+  // if an error occured during skipping of a C-style comment.
+  bool prepSkipLineBegin();
+
+  // Skip any whitespaces or comments after a preprocessing directive.
+  // The method returns true upon reaching either end of the line
+  // or end of the file.  If there is a multiline C-style comment
+  // after the preprocessing directive, the method skips
+  // the comment, so the final CurPtr may point to one of the next lines.
+  // The method returns false, if an error occured during skipping
+  // C- or C++-style comment, or a non-whitespace symbol appears
+  // after the preprocessing directive.
+  //
+  // The method maybe called both during lines-skipping and tokens
+  // processing.  It actually verifies that only whitespaces or/and
+  // comments follow a preprocessing directive.
+  //
+  // After the execution of this mehod, CurPtr points either to new line
+  // symbol, buffer end or non-whitespace symbol following the preprocesing
+  // directive.
+  bool prepSkipDirectiveEnd();
+
+  // Skip all symbols to the end of the line/file.
+  // The method adjusts CurPtr, so that it points to either new line
+  // symbol in the current line or the buffer end.
+  void prepSkipToLineEnd();
+
+  // Return true, if the current preprocessor control stack is such that
+  // we should allow lexer to process the next token, false - otherwise.
+  //
+  // In particular, the method returns true, if all the #ifdef/#else
+  // controls on the stack have their IsDefined member set to true.
+  bool prepIsProcessingEnabled();
+
+  // Report an error, if we reach EOF with non-empty preprocessing control
+  // stack.  This means there is no matching #endif for the previous
+  // #ifdef/#else.
+  void prepReportPreprocessorStackError();
 };
 
 } // end namespace llvm
diff --git a/lib/TableGen/TGParser.h b/lib/TableGen/TGParser.h
index 0a28b3a03aa..e3849043513 100644
--- a/lib/TableGen/TGParser.h
+++ b/lib/TableGen/TGParser.h
@@ -115,8 +115,9 @@ class TGParser {
   };
 
 public:
-  TGParser(SourceMgr &SrcMgr, RecordKeeper &records)
-      : Lex(SrcMgr), CurMultiClass(nullptr), Records(records) {}
+  TGParser(SourceMgr &SrcMgr, ArrayRef<std::string> Macros,
+           RecordKeeper &records)
+    : Lex(SrcMgr, Macros), CurMultiClass(nullptr), Records(records) {}
 
   /// ParseFile - Main entrypoint for parsing a tblgen file.  These parser
   /// routines return true on error, or false on success.
diff --git a/test/TableGen/prep-diag1.td b/test/TableGen/prep-diag1.td
new file mode 100644
index 00000000000..41b7d477c69
--- /dev/null
+++ b/test/TableGen/prep-diag1.td
@@ -0,0 +1,26 @@
+// RUN: not llvm-tblgen -DDIAG1 -I %p %s 2>&1 | FileCheck --check-prefixes=DIAG1 %s
+// RUN: not llvm-tblgen -DDIAG4 -I %p %s 2>&1 | FileCheck --check-prefixes=DIAG4 %s
+// RUN: not llvm-tblgen -DDIAG2 -I %p %s 2>&1 | FileCheck --check-prefixes=DIAG2 %s
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck --check-prefixes=DIAG3 %s
+
+#ifdef DIAG1
+// DIAG1: error: Only comments are supported after #define NAME
+#define ENABLED1/*
+*/class C;
+#endif // DIAG1
+
+#ifdef DIAG4
+// DIAG4: warning: Duplicate definition of macro: ENABLED1
+#define ENABLED1
+#define ENABLED1
+#endif // DIAG4
+
+#ifdef DIAG2
+// DIAG2: error: Only comments are supported after #ifdef NAME
+
+// Invalid #ifdef below should be detected even if DIAG2 is not defined.
+// DIAG3: error: Only comments are supported after #ifdef NAME
+#ifdef DIAG2/*
+*/class C;
+#endif
+#endif // DIAG2
diff --git a/test/TableGen/prep-diag10.td b/test/TableGen/prep-diag10.td
new file mode 100644
index 00000000000..eb387a07b06
--- /dev/null
+++ b/test/TableGen/prep-diag10.td
@@ -0,0 +1,6 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Reached EOF without matching #endif
+// CHECK: error: The latest preprocessor control is here
+#ifdef DISABLED
+#else
diff --git a/test/TableGen/prep-diag11-include.inc b/test/TableGen/prep-diag11-include.inc
new file mode 100644
index 00000000000..9f31d7be416
--- /dev/null
+++ b/test/TableGen/prep-diag11-include.inc
@@ -0,0 +1 @@
+#ifdef ENABLED
diff --git a/test/TableGen/prep-diag11.td b/test/TableGen/prep-diag11.td
new file mode 100644
index 00000000000..0042bc04f9e
--- /dev/null
+++ b/test/TableGen/prep-diag11.td
@@ -0,0 +1,8 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Reached EOF without matching #endif
+// CHECK: error: The latest preprocessor control is here
+#ifdef DISABLED
+#else
+#define ENABLED
+include "prep-diag11-include.inc"
diff --git a/test/TableGen/prep-diag12-include.inc b/test/TableGen/prep-diag12-include.inc
new file mode 100644
index 00000000000..995bca60e36
--- /dev/null
+++ b/test/TableGen/prep-diag12-include.inc
@@ -0,0 +1,2 @@
+#ifdef ENABLED
+#else
diff --git a/test/TableGen/prep-diag12.td b/test/TableGen/prep-diag12.td
new file mode 100644
index 00000000000..c26301ee17a
--- /dev/null
+++ b/test/TableGen/prep-diag12.td
@@ -0,0 +1,8 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Reached EOF without matching #endif
+// CHECK: error: The latest preprocessor control is here
+#ifdef DISABLED
+#else
+#define ENABLED
+include "prep-diag12-include.inc"
diff --git a/test/TableGen/prep-diag13.td b/test/TableGen/prep-diag13.td
new file mode 100644
index 00000000000..aa3fdab4802
--- /dev/null
+++ b/test/TableGen/prep-diag13.td
@@ -0,0 +1,9 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Reached EOF without matching #endif
+// CHECK: error: The latest preprocessor control is here
+#ifdef DISABLED
+/*
+#else
+#endif
+*/
diff --git a/test/TableGen/prep-diag14.td b/test/TableGen/prep-diag14.td
new file mode 100644
index 00000000000..cae9bc3b7f5
--- /dev/null
+++ b/test/TableGen/prep-diag14.td
@@ -0,0 +1,6 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Reached EOF without matching #endif
+// CHECK: error: The latest preprocessor control is here
+#ifdef DISABLED
+// #endif
diff --git a/test/TableGen/prep-diag2.td b/test/TableGen/prep-diag2.td
new file mode 100644
index 00000000000..741026b9c8a
--- /dev/null
+++ b/test/TableGen/prep-diag2.td
@@ -0,0 +1,14 @@
+// RUN: not llvm-tblgen -DDIAG1 -I %p %s 2>&1 | FileCheck --check-prefixes=DIAG1 %s
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck --check-prefixes=DIAG2 %s
+
+#ifdef DIAG1
+// DIAG1: error: Only comments are supported after #else
+
+// Invalid #else below should be detected even if DIAG1 is not defined.
+// DIAG2: error: Only comments are supported after #else
+#ifdef DIAG2//DIAG2
+#else/*
+*/class C;
+#endif
+#endif // DIAG1
+
diff --git a/test/TableGen/prep-diag3.td b/test/TableGen/prep-diag3.td
new file mode 100644
index 00000000000..fbedfa290b9
--- /dev/null
+++ b/test/TableGen/prep-diag3.td
@@ -0,0 +1,14 @@
+// RUN: not llvm-tblgen -DDIAG1 -I %p %s 2>&1 | FileCheck --check-prefixes=DIAG1 %s
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck --check-prefixes=DIAG2 %s
+
+#ifdef DIAG1
+// DIAG1: error: Only comments are supported after #endif
+
+// Invalid #else below should be detected even if DIAG1 is not defined.
+// DIAG2: error: Only comments are supported after #endif
+#ifdef DIAG2//DIAG2
+#else/*!DIAG2*/
+#endif/* !DIAG2
+*/class C;
+#endif // DIAG1
+
diff --git a/test/TableGen/prep-diag4.td b/test/TableGen/prep-diag4.td
new file mode 100644
index 00000000000..4661ef8667d
--- /dev/null
+++ b/test/TableGen/prep-diag4.td
@@ -0,0 +1,8 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: double #else
+// CHECK: error: Previous #else is here
+#ifdef DIAG1
+#else
+#else
+#endif
diff --git a/test/TableGen/prep-diag5.td b/test/TableGen/prep-diag5.td
new file mode 100644
index 00000000000..7dc178bef22
--- /dev/null
+++ b/test/TableGen/prep-diag5.td
@@ -0,0 +1,6 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: #else without #ifdef
+#else
+#else
+#endif
diff --git a/test/TableGen/prep-diag6.td b/test/TableGen/prep-diag6.td
new file mode 100644
index 00000000000..f4202d115da
--- /dev/null
+++ b/test/TableGen/prep-diag6.td
@@ -0,0 +1,7 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Expected macro name after #ifdef
+#ifdef
+#else
+#else
+#endif
diff --git a/test/TableGen/prep-diag7.td b/test/TableGen/prep-diag7.td
new file mode 100644
index 00000000000..1efe5260d13
--- /dev/null
+++ b/test/TableGen/prep-diag7.td
@@ -0,0 +1,4 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: #endif without #ifdef
+#endif
diff --git a/test/TableGen/prep-diag8.td b/test/TableGen/prep-diag8.td
new file mode 100644
index 00000000000..7a7bde62c79
--- /dev/null
+++ b/test/TableGen/prep-diag8.td
@@ -0,0 +1,5 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Expected macro name after #define
+#define
+#endif
diff --git a/test/TableGen/prep-diag9.td b/test/TableGen/prep-diag9.td
new file mode 100644
index 00000000000..4ecff575cdc
--- /dev/null
+++ b/test/TableGen/prep-diag9.td
@@ -0,0 +1,5 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Reached EOF without matching #endif
+// CHECK: error: The latest preprocessor control is here
+#ifdef DISABLED
diff --git a/test/TableGen/prep-region-include.inc b/test/TableGen/prep-region-include.inc
new file mode 100644
index 00000000000..7b4e7850b36
--- /dev/null
+++ b/test/TableGen/prep-region-include.inc
@@ -0,0 +1,8 @@
+#ifdef ENABLED4
+def ifdef_enabled4 : C;
+#else
+def ifdef_enabled4_else : C;
+#endif
+
+// EOF immediately after ENABLED5
+#define ENABLED5
diff --git a/test/TableGen/prep-region-processing.td b/test/TableGen/prep-region-processing.td
new file mode 100644
index 00000000000..ce6e19a5ec4
--- /dev/null
+++ b/test/TableGen/prep-region-processing.td
@@ -0,0 +1,150 @@
+// RUN: llvm-tblgen -I %p %s 2>&1 | FileCheck %s --implicit-check-not warning:
+
+class C;
+
+// TableGen prints records in alpabetical order.
+// CHECK-NOT: def ifdef_disabled1
+// CHECK-NOT: def ifdef_disabled2
+// CHECK:     def ifdef_disabled3
+// CHECK-NOT: def ifdef_disabled4
+// CHECK-NOT: def ifdef_disabled5
+// CHECK:     def ifdef_disabled4_else
+// CHECK-NOT: def ifdef_disabled5_else
+// CHECK:     def ifdef_enabled1
+// CHECK-NOT: def ifdef_enabled2
+// CHECK:     def ifdef_enabled3
+// CHECK:     def ifdef_enabled4
+// CHECK-NOT: def ifdef_enabled4_else
+// CHECK:     def ifdef_enabled5
+// CHECK:     def ifdef_enabled6
+// CHECK-NOT: def ifdef_enabled6_else
+// CHECK-NOT: def ifdef_disabled6
+// CHECK-NOT: def ifdef_disabled6_else
+
+#define ENABLED1
+#define ENABLED2
+
+#ifdef DISABLED1
+//
+def ifdef_disabled1 : C;
+
+#define DISABLED2/*This one is disabled,
+                   because DISABLED1 is.
+*/
+#endif
+
+#ifdef ENABLED1
+def ifdef_enabled1 : C;
+#endif
+
+#ifdef DISABLED2/*
+*/
+def ifdef_disabled2 : C;
+#endif
+
+/*
+#ifdef ENABLED2
+def ifdef_enabled2 : C;
+#endif
+*/
+
+//#ifdef DISABLED3
+def ifdef_disabled3 : C;
+
+//#endif
+
+/* _DISABLED4 */  /* padding */ #ifdef _DISABLED4
+def ifdef_disabled4 : C;
+#else//      /*!_DISABLED4
+def ifdef_disabled4_else : C;
+
+#define ENABLED3
+#endif
+
+#ifdef __DISABLED5
+def ifdef_disabled5 : C;
+/*
+
+*/#else
+#ifdef ENABLED3
+def ifdef_enabled3 : C;
+#else /* //!ENABLED3
+*/
+def ifdef_disabled5_else : C;
+#endif
+#endif
+
+#define ENABLED4
+include "prep-region-include.inc"//ENABLED5 is defined inside
+
+#ifdef ENABLED5
+def ifdef_enabled5 : C;
+#endif // ENABLED5
+
+#ifdef DISABLED6__
+// Double inclusion is an error.
+include "prep-region-include.inc"
+#else
+#endif
+
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+#ifdef DIS
+def ifdef_disabled6 : C;
+#endif
+#endif
+#endif
+#endif
+#else
+def ifdef_disabled6_else : C;
+#endif
+#endif
+#endif
+#endif
+#endif
+#endif
+#else
+#define ENAB//ENAB
+#endif
+
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+#ifdef ENAB
+def ifdef_enabled6 : C;
+#endif
+#endif
+#endif
+#endif
+#else
+def ifdef_enabled6_else : C;
+#endif
+#endif
+#endif
+#endif
+#endif
+#endif
+#endif
+
+#ifdef DISABLED_7
+include "non-existent-file.inc"
+#endif
+
+#ifdef DISABLED_8
+\\\\\ invalid TD text /////
+#endif // DISABLED_8
diff --git a/test/TableGen/unterminated-c-comment-include.inc b/test/TableGen/unterminated-c-comment-include.inc
new file mode 100644
index 00000000000..d05d4350c02
--- /dev/null
+++ b/test/TableGen/unterminated-c-comment-include.inc
@@ -0,0 +1,2 @@
+/* comment starts here and finished in the parent file.
+   TableGen used to allow such usage.
diff --git a/test/TableGen/unterminated-c-comment.td b/test/TableGen/unterminated-c-comment.td
new file mode 100644
index 00000000000..0f4cd9d633c
--- /dev/null
+++ b/test/TableGen/unterminated-c-comment.td
@@ -0,0 +1,5 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Unterminated comment!
+
+include "unterminated-c-comment-include.inc" */
diff --git a/test/TableGen/unterminated-code-block-include.inc b/test/TableGen/unterminated-code-block-include.inc
new file mode 100644
index 00000000000..a1363f810fd
--- /dev/null
+++ b/test/TableGen/unterminated-code-block-include.inc
@@ -0,0 +1,8 @@
+class test<code C> {
+  code Code = C;
+}
+
+// TableGen used to allow code blocks starting in one file
+// and finishing in the parent one.  This test checks
+// that this is reported as an error from now on.
+def foo : test<[{ hello world!
diff --git a/test/TableGen/unterminated-code-block.td b/test/TableGen/unterminated-code-block.td
new file mode 100644
index 00000000000..86395abd789
--- /dev/null
+++ b/test/TableGen/unterminated-code-block.td
@@ -0,0 +1,5 @@
+// RUN: not llvm-tblgen -I %p %s 2>&1 | FileCheck %s
+
+// CHECK: error: Unterminated Code Block
+
+include "unterminated-code-block-include.inc" }]>;