Index: lib/CodeGen/TargetInfo.cpp
===================================================================
--- lib/CodeGen/TargetInfo.cpp
+++ lib/CodeGen/TargetInfo.cpp
@@ -3536,7 +3536,17 @@
 
 void X86_64ABIInfo::computeInfo(CGFunctionInfo &FI) const {
 
-  bool IsRegCall = FI.getCallingConvention() == llvm::CallingConv::X86_RegCall;
+  const unsigned CallingConv = FI.getCallingConvention();
+  // It is possible to force Win64 calling convention on any x86_64 target by
+  // using __attribute__((ms_abi)). In such case to correctly emit Win64
+  // compatible code delegate this call to WinX86_64ABIInfo::computeInfo.
+  if (CallingConv == llvm::CallingConv::Win64) {
+    WinX86_64ABIInfo Win64ABIInfo(CGT);
+    Win64ABIInfo.computeInfo(FI);
+    return;
+  }
+
+  bool IsRegCall = CallingConv == llvm::CallingConv::X86_RegCall;
 
   // Keep track of the number of assigned registers.
   unsigned FreeIntRegs = IsRegCall ? 11 : 6;
Index: test/CodeGen/ms_abi.c
===================================================================
--- test/CodeGen/ms_abi.c
+++ test/CodeGen/ms_abi.c
@@ -146,3 +146,16 @@
   // WIN64: %[[AP_VAL:.*]] = load i8*, i8** %[[AP]]
   // WIN64-NEXT: store i8* %[[AP_VAL]], i8** %[[AP2:.*]]
 }
+
+// This test checks if structs are passed according to Win64 calling convention
+// when it's enforced by __attribute((ms_abi)).
+struct i128 {
+  unsigned long long a;
+  unsigned long long b;
+};
+
+__attribute__((ms_abi)) struct i128 f7(struct i128 a) {
+  // WIN64: define void @f7(%struct.i128* noalias sret %agg.result, %struct.i128* %a)
+  // FREEBSD: define win64cc void @f7(%struct.i128* noalias sret %agg.result, %struct.i128* %a)
+  return a;
+}