Index: lib/AST/MicrosoftMangle.cpp
===================================================================
--- lib/AST/MicrosoftMangle.cpp
+++ lib/AST/MicrosoftMangle.cpp
@@ -1050,6 +1050,10 @@
     }
     DC = DC->getParent();
   }
+
+  if (const auto *FD = dyn_cast<FunctionDecl>(ND))
+    if (FD->getType()->getAs<FunctionType>()->getCallConv() == CC_Swift)
+      mangleSourceName("__swift_cc");
 }

 void MicrosoftCXXNameMangler::mangleCXXDtorType(CXXDtorType T) {
@@ -1959,6 +1963,10 @@
     mangleQualifiers(Quals, /*IsMember=*/false);
   }

+  // We currently mangle the SwiftCC as `__swift_cc` namespace on the decl.
+  if (CC == CC_Swift)
+    CC = CC_C;
+
   mangleCallingConvention(CC);

   // <return-type> ::= <type>
Index: test/CodeGenCXX/msabi-swiftcall-cc.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCXX/msabi-swiftcall-cc.cpp
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -triple i686-unknown-windows-msvc -emit-llvm %s -o - | FileCheck %s
+
+
+void __attribute__((__swiftcall__)) f(void) {}
+// CHECK-DAG: @"\01?f@__swift_cc@@YAXXZ"
+