Index: clang/include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticLexKinds.td
+++ clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -320,6 +320,8 @@
   "whitespace required after macro name">;
 def warn_missing_whitespace_after_macro_name : Warning<
   "whitespace recommended after macro name">;
+def warn_include_cycle : Warning<"#include cycle">,
+   InGroup<DiagGroup<"include-cycle">>, DefaultIgnore;
 
 class NonportablePath  : Warning<
   "non-portable path to file '%0'; specified path differs in case from file"
Index: clang/include/clang/Basic/FileEntry.h
===================================================================
--- clang/include/clang/Basic/FileEntry.h
+++ clang/include/clang/Basic/FileEntry.h
@@ -359,6 +359,9 @@
   const DirectoryEntry *Dir = nullptr; // Directory file lives in.
   llvm::sys::fs::UniqueID UniqueID;
   unsigned UID = 0; // A unique (small) ID for the file.
+  // Bitfieldiness  vvvvv, mutabley?
+  // Maybe the including machinery should use a map<FileEntry *,unsigned>??
+  mutable unsigned Includedness = 0; // FIXME:DOCUMENT
   bool IsNamedPipe = false;
 
   /// The open file, if it is owned by the \p FileEntry.
@@ -393,6 +396,12 @@
   /// the native FileManager methods).
   bool isNamedPipe() const { return IsNamedPipe; }
 
+  bool isEntered() const { return Includedness != 0; }
+  void EnterOrLeave(bool Enter) const {
+    assert(Enter || Includedness);
+    Includedness += Enter ? +1 : -1;
+  }
+
   void closeFile() const;
 };
 
Index: clang/lib/Headers/emmintrin.h
===================================================================
--- clang/lib/Headers/emmintrin.h
+++ clang/lib/Headers/emmintrin.h
@@ -14,7 +14,11 @@
 #error "This header is only meant to be used on x86 and x64 architecture"
 #endif
 
+#ifndef __XMMINTRIN_H
+/* xmmintrin can #include this file under certain cirumstances, prevent a loop
+   that is problematic for clang modules and C++ header-units.  */
 #include <xmmintrin.h>
+#endif
 
 typedef double __m128d __attribute__((__vector_size__(16), __aligned__(16)));
 typedef long long __m128i __attribute__((__vector_size__(16), __aligned__(16)));
Index: clang/lib/Headers/xmmintrin.h
===================================================================
--- clang/lib/Headers/xmmintrin.h
+++ clang/lib/Headers/xmmintrin.h
@@ -3010,7 +3010,8 @@
 #undef __DEFAULT_FN_ATTRS_MMX
 
 /* Ugly hack for backwards-compatibility (compatible with gcc) */
-#if defined(__SSE2__) && !__building_module(_Builtin_intrinsics)
+#if !defined(__EMMINTRIN_H) && defined(__SSE2__) &&                            \
+    !__building_module(_Builtin_intrinsics)
 #include <emmintrin.h>
 #endif
 
Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -2329,6 +2329,9 @@
 
   bool IsFirstIncludeOfFile = false;
 
+  if (Action == Enter && File && File->getFileEntry().isEntered())
+    Diag(FilenameTok.getLocation(), diag::warn_include_cycle);
+
   // Ask HeaderInfo if we should enter this #include file.  If not, #including
   // this file will have no effect.
   if (Action == Enter && File &&
Index: clang/lib/Lex/PPLexerChange.cpp
===================================================================
--- clang/lib/Lex/PPLexerChange.cpp
+++ clang/lib/Lex/PPLexerChange.cpp
@@ -103,6 +103,10 @@
     }
   }
 
+  if (TheLexer->getPP())
+    if (auto *FE = SourceMgr.getFileEntryForID(FID))
+      FE->EnterOrLeave(true);
+
   EnterSourceFileWithLexer(TheLexer, CurDir);
   return false;
 }
@@ -432,6 +436,10 @@
   // lexing the #includer file.
   if (!IncludeMacroStack.empty()) {
 
+    if (CurPPLexer)
+      if (auto *FE = SourceMgr.getFileEntryForID(CurPPLexer->getFileID()))
+        FE->EnterOrLeave(false);
+
     // If we lexed the code-completion file, act as if we reached EOF.
     if (isCodeCompletionEnabled() && CurPPLexer &&
         SourceMgr.getLocForStartOfFile(CurPPLexer->getFileID()) ==
Index: clang/test/Preprocessor/warn-loop-import-1.h
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/warn-loop-import-1.h
@@ -0,0 +1,2 @@
+// expected-warning@+1 {{#include cycle}}
+#import "warn-loop-import-1.h"
Index: clang/test/Preprocessor/warn-loop-macro-1.h
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/warn-loop-macro-1.h
@@ -0,0 +1,5 @@
+#ifndef LOOP_MACRO_1
+#define LOOP_MACRO_1
+// expected-warning@+1 {{#include cycle}}
+#include "warn-loop-macro-1.h"
+#endif
Index: clang/test/Preprocessor/warn-loop-macro-2.h
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/warn-loop-macro-2.h
@@ -0,0 +1,4 @@
+#ifndef LOOP_MACRO_2
+#define LOOP_MACRO_2
+#include "warn-loop-macro-2a.h"
+#endif
Index: clang/test/Preprocessor/warn-loop-macro-2a.h
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/warn-loop-macro-2a.h
@@ -0,0 +1,5 @@
+#ifndef LOOP_MACRO_2a
+#define LOOP_MACRO_2a
+// expected-warning@+1 {{#include cycle}}
+#include "warn-loop-macro-2.h"
+#endif
Index: clang/test/Preprocessor/warn-loop-main.c
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/warn-loop-main.c
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -Winclude-cycle -fsyntax-only -verify %s
+
+#include "warn-loop-macro-1.h"
+#include "warn-loop-macro-2.h"
+#include "warn-loop-macro-1.h"
+#include "warn-loop-import-1.h"
+#include "warn-loop-pragma-1.h"
Index: clang/test/Preprocessor/warn-loop-pragma-1.h
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/warn-loop-pragma-1.h
@@ -0,0 +1,3 @@
+#pragma once
+// expected-warning@+1 {{#include cycle}}
+#include "warn-loop-pragma-1.h"