Index: include/clang/Serialization/ASTReader.h
===================================================================
--- include/clang/Serialization/ASTReader.h
+++ include/clang/Serialization/ASTReader.h
@@ -284,6 +284,21 @@
   void Error(const char *Msg);
 };
 
+/// \brief ASTReaderListenter implementation to set SuggestedPredefines of
+/// ASTReader which is required to use a pch file. This is the replacement
+/// of PCHValidator or SimplePCHValidator when using a pch file without
+/// validating it.
+class SimpleASTReaderListener : public ASTReaderListener {
+  Preprocessor &PP;
+
+public:
+  SimpleASTReaderListener(Preprocessor &PP)
+    : PP(PP) {}
+
+  bool ReadPreprocessorOptions(const PreprocessorOptions &PPOpts, bool Complain,
+                               std::string &SuggestedPredefines) override;
+};
+
 namespace serialization {
 
 class ReadMethodPoolVisitor;
Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -496,12 +496,16 @@
 /// against the preprocessor options in an existing preprocessor.
 ///
 /// \param Diags If non-null, produce diagnostics for any mismatches incurred.
+/// \param Validate If true, validate preprocessor options. If false, allow
+///        macros defined by \p ExistingPPOpts to override those defined by
+///        \p PPOpts in SuggestedPredefines.
 static bool checkPreprocessorOptions(const PreprocessorOptions &PPOpts,
                                      const PreprocessorOptions &ExistingPPOpts,
                                      DiagnosticsEngine *Diags,
                                      FileManager &FileMgr,
                                      std::string &SuggestedPredefines,
-                                     const LangOptions &LangOpts) {
+                                     const LangOptions &LangOpts,
+                                     bool Validate = true) {
   // Check macro definitions.
   MacroDefinitionsMap ASTFileMacros;
   collectMacroDefinitions(PPOpts, ASTFileMacros);
@@ -517,7 +521,7 @@
     // Check whether we know anything about this macro name or not.
     llvm::StringMap<std::pair<StringRef, bool /*IsUndef*/> >::iterator Known
       = ASTFileMacros.find(MacroName);
-    if (Known == ASTFileMacros.end()) {
+    if (!Validate || Known == ASTFileMacros.end()) {
       // FIXME: Check whether this identifier was referenced anywhere in the
       // AST file. If so, we should reject the AST file. Unfortunately, this
       // information isn't in the control block. What shall we do about it?
@@ -560,7 +564,7 @@
   }
 
   // Check whether we're using predefines.
-  if (PPOpts.UsePredefines != ExistingPPOpts.UsePredefines) {
+  if (PPOpts.UsePredefines != ExistingPPOpts.UsePredefines && Validate) {
     if (Diags) {
       Diags->Report(diag::err_pch_undef) << ExistingPPOpts.UsePredefines;
     }
@@ -569,7 +573,7 @@
 
   // Detailed record is important since it is used for the module cache hash.
   if (LangOpts.Modules &&
-      PPOpts.DetailedRecord != ExistingPPOpts.DetailedRecord) {
+      PPOpts.DetailedRecord != ExistingPPOpts.DetailedRecord && Validate) {
     if (Diags) {
       Diags->Report(diag::err_pch_pp_detailed_record) << PPOpts.DetailedRecord;
     }
@@ -618,6 +622,19 @@
                                   PP.getLangOpts());
 }
 
+bool SimpleASTReaderListener::ReadPreprocessorOptions(
+                                  const PreprocessorOptions &PPOpts,
+                                  bool Complain,
+                                  std::string &SuggestedPredefines) {
+  return checkPreprocessorOptions(PPOpts,
+                                  PP.getPreprocessorOpts(),
+                                  nullptr,
+                                  PP.getFileManager(),
+                                  SuggestedPredefines,
+                                  PP.getLangOpts(),
+                                  false);
+}
+
 /// Check the header search options deserialized from the control block
 /// against the header search options in an existing preprocessor.
 ///
@@ -8710,7 +8727,10 @@
   bool AllowConfigurationMismatch, bool ValidateSystemInputs,
   bool UseGlobalIndex,
   std::unique_ptr<llvm::Timer> ReadTimer)
-    : Listener(new PCHValidator(PP, *this)), DeserializationListener(nullptr),
+    : Listener(DisableValidation ?
+        cast<ASTReaderListener>(new SimpleASTReaderListener(PP)) :
+        cast<ASTReaderListener>(new PCHValidator(PP, *this))),
+      DeserializationListener(nullptr),
       OwnsDeserializationListener(false), SourceMgr(PP.getSourceManager()),
       FileMgr(PP.getFileManager()), PCHContainerRdr(PCHContainerRdr),
       Diags(PP.getDiagnostics()), SemaObj(nullptr), PP(PP), Context(Context),
Index: test/PCH/no-validate-pch.cl
===================================================================
--- /dev/null
+++ test/PCH/no-validate-pch.cl
@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -emit-pch -D TWO=2 -D X=4 -o %t %s -triple spir-unknown-unknown
+// RUN: %clang_cc1 -include-pch %t -D THREE=3 -D X=5 -O0 -U__OPTIMIZE__ -fno-validate-pch %s -triple spir-unknown-unknown 2>&1 | FileCheck %s
+// RUN: not %clang_cc1 -include-pch %t -D THREE=3 -D X=5 -D VALIDATE -O0 -fsyntax-only %s -triple spir-unknown-unknown 2>&1 | FileCheck --check-prefix=CHECK-VAL %s
+
+#ifndef HEADER
+#define HEADER
+// Header.
+
+#define ONE 1
+
+#else
+// Using the header.
+
+// CHECK: warning: 'X' macro redefined
+// CHECK: #define X 5
+// CHECK: note: previous definition is here
+// CHECK: #define X 4
+
+// CHECK-VAL: error: __OPTIMIZE__ predefined macro was enabled in PCH file but is currently disabled
+// CHECK-VAL: error: definition of macro 'X' differs between the precompiled header ('4') and the command line ('5')
+
+void test(void) {
+  int a = ONE;
+  int b = TWO;
+  int c = THREE;
+
+#ifndef VALIDATE
+#if X != 5
+#error Definition of X is not overridden!
+#endif
+
+#ifdef __OPTIMIZE__
+#error Optimization is not off!
+#endif
+#endif
+
+}
+
+#endif