Index: clang/include/clang/Basic/DiagnosticCommonKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticCommonKinds.td
+++ clang/include/clang/Basic/DiagnosticCommonKinds.td
@@ -251,6 +251,10 @@
 def err_unimplemented_conversion_with_fixed_point_type : Error<
   "conversion between fixed point and %0 is not yet supported">;
 
+def warn_for_global_ctor_for_dllimport : Warning<
+  "global constructor is generated for dllimport global var %0">,
+  InGroup<GlobalConstructors>;
+
 // SEH
 def err_seh_expected_handler : Error<
   "expected '__except' or '__finally' block">;
Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -2228,11 +2228,12 @@
         // FIXME: Diagnostic!
         return false;
 
-      // A dllimport variable never acts like a constant, unless we're
-      // evaluating a value for use only in name mangling.
-      if (!isForManglingOnly(Kind) && Var->hasAttr<DLLImportAttr>())
-        // FIXME: Diagnostic!
-        return false;
+      // In C mode, a dllimport variable never acts like a constant, unless
+      // we're evaluating a value for use only in name mangling.
+      if (!Info.getCtx().getLangOpts().CPlusPlus)
+        if (!isForManglingOnly(Kind) && Var->hasAttr<DLLImportAttr>())
+          // FIXME: Diagnostic!
+          return false;
 
       // In CUDA/HIP device compilation, only device side variables have
       // constant addresses.
Index: clang/lib/CodeGen/CGDeclCXX.cpp
===================================================================
--- clang/lib/CodeGen/CGDeclCXX.cpp
+++ clang/lib/CodeGen/CGDeclCXX.cpp
@@ -504,6 +504,21 @@
     PtrArray->setComdat(C);
 }
 
+void CodeGenModule::EmitCXXCtorInit(const VarDecl *D, llvm::GlobalVariable *Addr,
+                                bool PerformInit, int Priority,
+                                llvm::StringLiteral FuncName) {
+  auto I = DelayedCXXInitPosition.find(D);
+  if (I != DelayedCXXInitPosition.end() && I->second == ~0U)
+    return;
+
+  llvm::FunctionType *FTy = llvm::FunctionType::get(VoidTy, false);
+  llvm::Function *Fn = CreateGlobalInitOrCleanUpFunction(
+      FTy, FuncName.str(), getTypes().arrangeNullaryFunction());
+  CodeGenFunction(*this).GenerateCXXGlobalVarDeclInitFunc(Fn, D, Addr,
+                                                          PerformInit);
+  AddGlobalCtor(Fn, Priority);
+}
+
 void
 CodeGenModule::EmitCXXGlobalVarDeclInitFunc(const VarDecl *D,
                                             llvm::GlobalVariable *Addr,
Index: clang/lib/CodeGen/CGExprConstant.cpp
===================================================================
--- clang/lib/CodeGen/CGExprConstant.cpp
+++ clang/lib/CodeGen/CGExprConstant.cpp
@@ -1920,8 +1920,11 @@
     if (auto VD = dyn_cast<VarDecl>(D)) {
       // We can never refer to a variable with local storage.
       if (!VD->hasLocalStorage()) {
-        if (VD->isFileVarDecl() || VD->hasExternalStorage())
+        if (VD->isFileVarDecl() || VD->hasExternalStorage()) {
+          if (VD->hasAttr<DLLImportAttr>())
+            return nullptr;
           return CGM.GetAddrOfGlobalVar(VD);
+        }
 
         if (VD->isLocalVarDecl()) {
           return CGM.getOrCreateStaticVarDecl(
Index: clang/lib/CodeGen/CodeGenModule.h
===================================================================
--- clang/lib/CodeGen/CodeGenModule.h
+++ clang/lib/CodeGen/CodeGenModule.h
@@ -1623,6 +1623,9 @@
   void EmitCXXGlobalVarDeclInitFunc(const VarDecl *D,
                                     llvm::GlobalVariable *Addr,
                                     bool PerformInit);
+  void EmitCXXCtorInit(const VarDecl *D, llvm::GlobalVariable *Addr,
+                   bool PerformInit, int Priority,
+                   llvm::StringLiteral FnName);
 
   void EmitPointerToInitFunc(const VarDecl *VD, llvm::GlobalVariable *Addr,
                              llvm::Function *InitFunc, InitSegAttr *ISA);
Index: clang/lib/CodeGen/CodeGenModule.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenModule.cpp
+++ clang/lib/CodeGen/CodeGenModule.cpp
@@ -4839,6 +4839,9 @@
       // also don't need to register a destructor.
       if (getLangOpts().CPlusPlus && !NeedsGlobalDtor)
         DelayedCXXInitPosition.erase(D);
+      if (getLangOpts().CPlusPlus && D->hasConstantInitialization() &&
+          NeedsGlobalDtor && D->getType()->isRecordType())
+        NeedsGlobalCtor = true;
 
 #ifndef NDEBUG
       CharUnits VarSize = getContext().getTypeSizeInChars(ASTTy) +
@@ -4992,9 +4995,15 @@
   maybeSetTrivialComdat(*D, *GV);
 
   // Emit the initializer function if necessary.
-  if (NeedsGlobalCtor || NeedsGlobalDtor)
-    EmitCXXGlobalVarDeclInitFunc(D, GV, NeedsGlobalCtor);
 
+  if (D->hasConstantInitialization() && D->getType()->isRecordType()) {
+    EmitCXXCtorInit(D, GV, true, 201, llvm::StringLiteral("ctor"));
+    EmitCXXCtorInit(D, GV, false, 65535, llvm::StringLiteral("dtor"));
+    DelayedCXXInitPosition[D] = ~0U;
+  } else {
+    if (NeedsGlobalCtor || NeedsGlobalDtor)
+      EmitCXXGlobalVarDeclInitFunc(D, GV, NeedsGlobalCtor);
+  }
   SanitizerMD->reportGlobal(GV, *D, NeedsGlobalCtor);
 
   // Emit global variable debug information.
Index: clang/test/CodeGenCXX/dllimport.cpp
===================================================================
--- clang/test/CodeGenCXX/dllimport.cpp
+++ clang/test/CodeGenCXX/dllimport.cpp
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -no-enable-noundef-analysis -triple i686-windows-msvc   -fno-rtti -fno-threadsafe-statics -fms-extensions -emit-llvm -std=c++1y -O0 -o - %s -DMSABI -w | FileCheck --check-prefix=MSC --check-prefix=M32 %s
-// RUN: %clang_cc1 -no-enable-noundef-analysis -triple x86_64-windows-msvc -fno-rtti -fno-threadsafe-statics -fms-extensions -emit-llvm -std=c++1y -O0 -o - %s -DMSABI -w | FileCheck --check-prefix=MSC --check-prefix=M64 %s
+// RUN: %clang_cc1 -no-enable-noundef-analysis -triple i686-windows-msvc   -fno-rtti -fno-threadsafe-statics -fms-extensions -emit-llvm -std=c++1y -O0 -o - %s -DMSABI -w | FileCheck --check-prefix=MSC --check-prefix=M32 --check-prefix=GL32 %s
+// RUN: %clang_cc1 -no-enable-noundef-analysis -triple x86_64-windows-msvc -fno-rtti -fno-threadsafe-statics -fms-extensions -emit-llvm -std=c++1y -O0 -o - %s -DMSABI -w | FileCheck --check-prefix=MSC --check-prefix=M64 --check-prefix=GL64 %s
 // RUN: %clang_cc1 -no-enable-noundef-analysis -triple i686-windows-gnu    -fno-rtti -fno-threadsafe-statics -fms-extensions -emit-llvm -std=c++1y -O0 -o - %s         -w | FileCheck --check-prefix=GNU --check-prefix=G32 %s
 // RUN: %clang_cc1 -no-enable-noundef-analysis -triple x86_64-windows-gnu  -fno-rtti -fno-threadsafe-statics -fms-extensions -emit-llvm -std=c++1y -O0 -o - %s         -w | FileCheck --check-prefix=GNU %s
 // RUN: %clang_cc1 -no-enable-noundef-analysis -triple i686-windows-msvc   -fno-rtti -fno-threadsafe-statics -fms-extensions -fms-compatibility-version=18.00 -emit-llvm -std=c++1y -O1 -disable-llvm-passes -o - %s -DMSABI -w | FileCheck --check-prefix=MO1 --check-prefix=M18 %s
@@ -737,8 +737,6 @@
 
 namespace PR19933 {
 // Don't dynamically initialize dllimport vars.
-// MSC2-NOT: @llvm.global_ctors
-// GNU2-NOT: @llvm.global_ctors
 
   struct NonPOD { NonPOD(); };
   template <typename T> struct A { static NonPOD x; };
@@ -856,6 +854,12 @@
 USEMEMFUNC(PR23770BaseTemplate<int>, f);
 // M32-DAG: declare dllimport x86_thiscallcc void @"?f@?$PR23770BaseTemplate@H@@QAEXXZ"
 
+// MSC-DAG: @"?val@@3HA" = external dllimport global i32
+// GL32-DAG: @"?x@@3PAHA" = dso_local global ptr null
+// GL64-DAG: @"?x@@3PEAHEA" = dso_local global ptr null
+// MSC-DAG: @"?impx@@3HA" = external dllimport global i32
+// MSC-DAG: @llvm.global_ctors = appending global [2 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 201, ptr @_GLOBAL__I_000201, ptr null }, { i32, ptr, ptr } { i32 65535, ptr @_GLOBAL__sub_I_dllimport.cpp, ptr null }]
+
 namespace PR27810 {
   template <class T>
   struct basic_ostream {
@@ -1026,3 +1030,15 @@
 void baz() { U<int> u; u.foo(); } // No diagnostic.
 
 }
+
+void assigndllimporttoconst () {
+  // MSC-DAG: define dso_local void @"?assigndllimporttoconst@@YAXXZ"()
+  // MSC-DAG: %[[VAL_REF:.*]] = alloca ptr
+  // MSC-DAG-NEXT: store ptr @"?val@@3HA", ptr %[[VAL_REF]]
+  extern int _declspec(dllimport) val;
+  constexpr int& val_ref = val;
+}
+
+_declspec(dllimport) int impx;
+constexpr int *y = &impx;
+int *x = &impx;
Index: clang/test/SemaCXX/PR19955.cpp
===================================================================
--- clang/test/SemaCXX/PR19955.cpp
+++ clang/test/SemaCXX/PR19955.cpp
@@ -2,7 +2,7 @@
 // RUN: %clang_cc1 -triple i686-mingw32 -verify -std=c++11 %s
 
 extern int __attribute__((dllimport)) var;
-constexpr int *varp = &var; // expected-error {{must be initialized by a constant expression}}
+constexpr int *varp = &var;
 
 extern __attribute__((dllimport)) void fun();
 constexpr void (*funp)(void) = &fun; // expected-error {{must be initialized by a constant expression}}
Index: clang/test/SemaCXX/constant-expression-cxx11.cpp
===================================================================
--- clang/test/SemaCXX/constant-expression-cxx11.cpp
+++ clang/test/SemaCXX/constant-expression-cxx11.cpp
@@ -1595,7 +1595,7 @@
   void f(int k) { // expected-note {{here}}
     int arr[k]; // expected-warning {{C99}} expected-note {{function parameter 'k'}}
     constexpr int n = 1 +
-        sizeof(arr) // expected-error {{constant expression}}
+        sizeof(arr) // expected-error{{constexpr variable 'n' must be initialized by a constant expression}}
         * 3;
   }
 }
Index: clang/test/SemaCXX/dllimport-constexpr.cpp
===================================================================
--- clang/test/SemaCXX/dllimport-constexpr.cpp
+++ clang/test/SemaCXX/dllimport-constexpr.cpp
@@ -40,7 +40,6 @@
 // constexpr initialization doesn't work for dllimport things.
 // expected-error@+1{{must be initialized by a constant expression}}
 constexpr void (*constexpr_import_func)() = &imported_func;
-// expected-error@+1{{must be initialized by a constant expression}}
 constexpr int *constexpr_import_int = &imported_int;
 // expected-error@+1{{must be initialized by a constant expression}}
 constexpr void (Foo::*constexpr_memptr)() = &Foo::imported_method;
@@ -60,3 +59,11 @@
   // expected-note@+1 {{requested here}}
   StaticConstexpr<imported_func>::g_fp();
 }
+
+extern int __declspec(dllimport) val;
+constexpr int& val_ref = val;
+
+void assigndllimporttoconst () {
+  extern int _declspec(dllimport) val;
+  constexpr int& val_ref = val;
+}