diff --git a/clang/lib/Basic/Targets/X86.h b/clang/lib/Basic/Targets/X86.h
index c1e1da22978bf..f5efd290e63b7 100644
--- a/clang/lib/Basic/Targets/X86.h
+++ b/clang/lib/Basic/Targets/X86.h
@@ -819,11 +819,10 @@ class LLVM_LIBRARY_VISIBILITY WindowsX86_64TargetInfo
     case CC_PreserveAll:
     case CC_X86_64SysV:
     case CC_Swift:
+    case CC_SwiftAsync:
     case CC_X86RegCall:
     case CC_OpenCLKernel:
       return CCCR_OK;
-    case CC_SwiftAsync:
-      return CCCR_Error;
     default:
       return CCCR_Warning;
     }
diff --git a/llvm/lib/Target/X86/X86FrameLowering.cpp b/llvm/lib/Target/X86/X86FrameLowering.cpp
index b6fa3d5d4e543..11987d265efd0 100644
--- a/llvm/lib/Target/X86/X86FrameLowering.cpp
+++ b/llvm/lib/Target/X86/X86FrameLowering.cpp
@@ -1510,7 +1510,8 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF,
   // Space reserved for stack-based arguments when making a (ABI-guaranteed)
   // tail call.
   unsigned TailCallArgReserveSize = -X86FI->getTCReturnAddrDelta();
-  if (TailCallArgReserveSize  && IsWin64Prologue)
+  if (TailCallArgReserveSize && IsWin64Prologue &&
+      !MF.getFunction().getAttributes().hasAttrSomewhere(Attribute::SwiftAsync))
     report_fatal_error("Can't handle guaranteed tail call under win64 yet");
 
   const bool EmitStackProbeCall =