diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -11173,6 +11173,25 @@
   }
 }
 
+static bool isDefaultStdCall(FunctionDecl *FD, Sema &S) {
+
+  // Default calling convention for main and wmain is __cdecl
+  if (FD->getName() == "main" || FD->getName() == "wmain")
+    return false;
+
+  // Default calling convention for MinGW is __cdecl
+  const llvm::Triple &T = S.Context.getTargetInfo().getTriple();
+  if (T.isWindowsGNUEnvironment())
+    return false;
+
+  // Default calling convention for WinMain, wWinMain and DllMain
+  // is __stdcall on 32 bit Windows
+  if (T.isOSWindows() && T.getArch() == llvm::Triple::x86)
+    return true;
+
+  return false;
+}
+
 void Sema::CheckMSVCRTEntryPoint(FunctionDecl *FD) {
   QualType T = FD->getType();
   assert(T->isFunctionType() && "function decl is not of function type");
@@ -11187,6 +11206,21 @@
     if (FD->getName() != "DllMain")
       FD->setHasImplicitReturnZero(true);
 
+  // Explicity specified calling conventions are applied to MSVC entry points
+  if (!hasExplicitCallingConv(T)) {
+    if (isDefaultStdCall(FD, *this)) {
+      if (FT->getCallConv() != CC_X86StdCall) {
+        FT = Context.adjustFunctionType(
+            FT, FT->getExtInfo().withCallingConv(CC_X86StdCall));
+        FD->setType(QualType(FT, 0));
+      }
+    } else if (FT->getCallConv() != CC_C) {
+      FT = Context.adjustFunctionType(FT,
+                                      FT->getExtInfo().withCallingConv(CC_C));
+      FD->setType(QualType(FT, 0));
+    }
+  }
+
   if (!FD->isInvalidDecl() && FD->getDescribedFunctionTemplate()) {
     Diag(FD->getLocation(), diag::err_mainlike_template_decl) << FD;
     FD->setInvalidDecl();
diff --git a/clang/test/CodeGenCXX/default_calling_conv.cpp b/clang/test/CodeGenCXX/default_calling_conv.cpp
--- a/clang/test/CodeGenCXX/default_calling_conv.cpp
+++ b/clang/test/CodeGenCXX/default_calling_conv.cpp
@@ -4,6 +4,9 @@
 // RUN: %clang_cc1 -triple i486-unknown-linux-gnu -mrtd -emit-llvm -o - %s | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
 // RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=vectorcall -emit-llvm -o - %s | FileCheck %s --check-prefix=VECTORCALL --check-prefix=ALL
 // RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=regcall -emit-llvm -o - %s | FileCheck %s --check-prefix=REGCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i686-pc-win32 -fdefault-calling-conv=vectorcall -emit-llvm -o - %s -DWINDOWS | FileCheck %s --check-prefix=WIN32
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -fdefault-calling-conv=vectorcall -emit-llvm -o - %s -DWINDOWS | FileCheck %s --check-prefix=WIN64
+// RUN: %clang_cc1 -triple i686-pc-win32 -emit-llvm -o - %s -DEXPLICITCC | FileCheck %s --check-prefix=EXPLICITCC
 
 // CDECL: define{{.*}} void @_Z5test1v
 // FASTCALL: define{{.*}} x86_fastcallcc void @_Z5test1v
@@ -50,3 +53,45 @@
 int main() {
   return 1;
 }
+
+#ifdef WINDOWS
+// WIN32: define dso_local i32 @wmain
+// WIN64: define dso_local i32 @wmain
+int wmain() {
+  return 1;
+}
+// WIN32: define dso_local x86_stdcallcc i32 @WinMain
+// WIN64: define dso_local i32 @WinMain
+int WinMain() {
+  return 1;
+}
+// WIN32: define dso_local x86_stdcallcc i32 @wWinMain
+// WIN64: define dso_local i32 @wWinMain
+int wWinMain() {
+  return 1;
+}
+// WIN32: define dso_local x86_stdcallcc i32 @DllMain
+// WIN64: define dso_local i32 @DllMain
+int DllMain() {
+  return 1;
+}
+#endif // Windows
+
+#ifdef EXPLICITCC
+// EXPLICITCC: define dso_local x86_fastcallcc i32 @wmain
+int __fastcall wmain() {
+  return 1;
+}
+// EXPLICITCC: define dso_local x86_fastcallcc i32 @WinMain
+int __fastcall WinMain() {
+  return 1;
+}
+// EXPLICITCC: define dso_local x86_fastcallcc i32 @wWinMain
+int __fastcall wWinMain() {
+  return 1;
+}
+// EXPLICITCC: define dso_local x86_fastcallcc i32 @DllMain
+int __fastcall DllMain() {
+  return 1;
+}
+#endif // ExplicitCC