diff --git a/clang/include/clang/Sema/HLSLExternalSemaSource.h b/clang/include/clang/Sema/HLSLExternalSemaSource.h
--- a/clang/include/clang/Sema/HLSLExternalSemaSource.h
+++ b/clang/include/clang/Sema/HLSLExternalSemaSource.h
@@ -15,6 +15,7 @@
 #include "llvm/ADT/DenseMap.h"
 
 #include "clang/Sema/ExternalSemaSource.h"
+#include "clang/Sema/MultiplexExternalSemaSource.h"
 
 namespace clang {
 class NamespaceDecl;
@@ -24,6 +25,7 @@
   Sema *SemaPtr = nullptr;
   NamespaceDecl *HLSLNamespace;
   CXXRecordDecl *ResourceDecl;
+  ExternalSemaSource *ExternalSema = nullptr;
 
   using CompletionFunction = std::function<void(CXXRecordDecl *)>;
   llvm::DenseMap<CXXRecordDecl *, CompletionFunction> Completions;
@@ -48,6 +50,31 @@
   using ExternalASTSource::CompleteType;
   /// Complete an incomplete HLSL builtin type
   void CompleteType(TagDecl *Tag) override;
+  void SetExternalSema(ExternalSemaSource *ExtSema) { ExternalSema = ExtSema; }
+};
+
+/// Members of ChainedHLSLExternalSemaSource, factored out so we can initialize
+/// them before we initialize the ExternalSemaSource base class.
+struct ChainedHLSLExternalSemaSourceMembers {
+  ChainedHLSLExternalSemaSourceMembers(ExternalSemaSource *ExtSema)
+      : ExternalSema(ExtSema) {
+    HLSLSema.SetExternalSema(ExtSema);
+  }
+  HLSLExternalSemaSource HLSLSema;
+  IntrusiveRefCntPtr<ExternalSemaSource> ExternalSema;
+};
+
+class ChainedHLSLExternalSemaSource
+    : private ChainedHLSLExternalSemaSourceMembers,
+      public MultiplexExternalSemaSource {
+public:
+  ChainedHLSLExternalSemaSource(ExternalSemaSource *ExtSema)
+      : ChainedHLSLExternalSemaSourceMembers(ExtSema),
+        // NOTE: HLSLSema after ExternalSema so HLSLSema initialize after
+        // ExternalSema.
+        // When initialize HLSLSema, reuse hlsl decls if already in
+        //  ExternalSema.
+        MultiplexExternalSemaSource(*ExternalSema.get(), HLSLSema) {}
 };
 
 } // namespace clang
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
@@ -1018,9 +1018,16 @@
 
   // Setup HLSL External Sema Source
   if (CI.getLangOpts().HLSL && CI.hasASTContext()) {
-    IntrusiveRefCntPtr<ExternalASTSource> HLSLSema(
-        new HLSLExternalSemaSource());
-    CI.getASTContext().setExternalSource(HLSLSema);
+    if (auto *SemaSource = dyn_cast_if_present<ExternalSemaSource>(
+            CI.getASTContext().getExternalSource())) {
+      IntrusiveRefCntPtr<ExternalSemaSource> HLSLSema(
+          new ChainedHLSLExternalSemaSource(SemaSource));
+      CI.getASTContext().setExternalSource(HLSLSema);
+    } else {
+      IntrusiveRefCntPtr<ExternalSemaSource> HLSLSema(
+          new HLSLExternalSemaSource());
+      CI.getASTContext().setExternalSource(HLSLSema);
+    }
   }
 
   FailureCleanup.release();
diff --git a/clang/lib/Sema/HLSLExternalSemaSource.cpp b/clang/lib/Sema/HLSLExternalSemaSource.cpp
--- a/clang/lib/Sema/HLSLExternalSemaSource.cpp
+++ b/clang/lib/Sema/HLSLExternalSemaSource.cpp
@@ -250,9 +250,51 @@
 
 HLSLExternalSemaSource::~HLSLExternalSemaSource() {}
 
+static NamedDecl *findDecl(ASTContext &AST, Sema &S, StringRef Name,
+                           Sema::LookupNameKind Kind) {
+  IdentifierInfo &II = AST.Idents.get(Name, tok::TokenKind::identifier);
+  DeclarationNameInfo NameInfo{DeclarationName{&II}, SourceLocation()};
+  LookupResult R(S, NameInfo, Kind);
+  S.LookupName(R, S.getCurScope());
+  NamedDecl *D = nullptr;
+  if (!R.isAmbiguous() && !R.empty())
+    D = R.getRepresentativeDecl();
+  return D;
+}
+
 void HLSLExternalSemaSource::InitializeSema(Sema &S) {
   SemaPtr = &S;
   ASTContext &AST = SemaPtr->getASTContext();
+
+  if (ExternalSema) {
+    NamespaceDecl *ExternlHLSL = llvm::dyn_cast_if_present<NamespaceDecl>(
+        findDecl(AST, S, "hlsl", Sema::LookupNameKind::LookupNamespaceName));
+    // Try to initailize from ExternalSema.
+    if (ExternlHLSL) {
+      auto *Resource = llvm::dyn_cast_if_present<CXXRecordDecl>(findDecl(
+          AST, S, "Resource", Sema::LookupNameKind::LookupOrdinaryName));
+
+      auto *RWBuffer = llvm::dyn_cast_if_present<ClassTemplateDecl>(
+          findDecl(AST, S, "RWBuffer", Sema::LookupNameKind::LookupAnyName));
+
+      // Find all things from ExternalSema, use them and return.
+      if (Resource && RWBuffer) {
+        HLSLNamespace = ExternlHLSL;
+        ResourceDecl = Resource;
+        CXXRecordDecl *Decl = RWBuffer->getTemplatedDecl();
+        if (!Decl->hasDefinition()) {
+          // Mark ExternalLexicalStorage so complete type will be called for
+          // ExternalAST path.
+          Decl->setHasExternalLexicalStorage();
+          Completions.insert(std::make_pair(
+              Decl, std::bind(&HLSLExternalSemaSource::completeBufferType, this,
+                              std::placeholders::_1)));
+        }
+        return;
+      }
+    }
+  }
+
   IdentifierInfo &HLSL = AST.Idents.get("hlsl", tok::TokenKind::identifier);
   HLSLNamespace =
       NamespaceDecl::Create(AST, AST.getTranslationUnitDecl(), false,
diff --git a/clang/test/AST/HLSL/Inputs/pch.hlsl b/clang/test/AST/HLSL/Inputs/pch.hlsl
new file mode 100644
--- /dev/null
+++ b/clang/test/AST/HLSL/Inputs/pch.hlsl
@@ -0,0 +1,4 @@
+
+float2 foo(float2 a, float2 b) {
+  return a + b;
+}
diff --git a/clang/test/AST/HLSL/pch.hlsl b/clang/test/AST/HLSL/pch.hlsl
new file mode 100644
--- /dev/null
+++ b/clang/test/AST/HLSL/pch.hlsl
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.3-library -x hlsl \
+// RUN:  -finclude-default-header -emit-pch -o %t %S/Inputs/pch.hlsl
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.3-library -x hlsl \
+// RUN:  -finclude-default-header -include-pch %t -fsyntax-only -ast-dump-all %s \
+// RUN: | FileCheck  %s
+
+// Make sure PCH works by using function declared in PCH header and declare a RWBuffer in current file.
+// CHECK:FunctionDecl 0x[[FOO:[0-9a-f]+]] <{{.*}}:2:1, line:4:1> line:2:8 imported used foo 'float2 (float2, float2)'
+// CHECK:VarDecl 0x{{[0-9a-f]+}} <{{.*}}:10:1, col:23> col:23 Buffer 'hlsl::RWBuffer<float>':'hlsl::RWBuffer<>'
+hlsl::RWBuffer<float> Buffer;
+
+float2 bar(float2 a, float2 b) {
+// CHECK:CallExpr 0x{{[0-9a-f]+}} <col:10, col:18> 'float2':'float __attribute__((ext_vector_type(2)))'
+// CHECK-NEXT:ImplicitCastExpr 0x{{[0-9a-f]+}} <col:10> 'float2 (*)(float2, float2)' <FunctionToPointerDecay>
+// CHECK-NEXT:`-DeclRefExpr 0x{{[0-9a-f]+}} <col:10> 'float2 (float2, float2)' lvalue Function 0x[[FOO]] 'foo' 'float2 (float2, float2)'
+  return foo(a, b);
+}