Index: lib/Basic/Targets/X86.h
===================================================================
--- lib/Basic/Targets/X86.h
+++ lib/Basic/Targets/X86.h
@@ -437,9 +437,12 @@
     // In case the constraint is 'r' we need to return Expression
     case 'r':
       return Expression;
+    // Double letters Y<x> constraints
+    case 'Y':
+      if ((++I != E) && ((*I == '0') || (*I == 'z')))
+        return "xmm0";
     default:
-      // Default value if there is no constraint for the register
-      return "";
+      break;
     }
     return "";
   }
Index: lib/Basic/Targets/X86.cpp
===================================================================
--- lib/Basic/Targets/X86.cpp
+++ lib/Basic/Targets/X86.cpp
@@ -1386,7 +1386,9 @@
     switch (*Name) {
     default:
       return false;
+    case 'z':
     case '0': // First SSE register.
+    case '2':
     case 't': // Any SSE register, when SSE2 is enabled.
     case 'i': // Any SSE register, when SSE2 and inter-unit moves enabled.
     case 'm': // Any MMX register, when inter-unit moves enabled.
@@ -1455,33 +1457,39 @@
   case 't':
   case 'u':
     return Size <= 128;
-  case 'v':
-  case 'x':
-    if (SSELevel >= AVX512F)
-      // 512-bit zmm registers can be used if target supports AVX512F.
-      return Size <= 512U;
-    else if (SSELevel >= AVX)
-      // 256-bit ymm registers can be used if target supports AVX.
-      return Size <= 256U;
-    return Size <= 128U;
   case 'Y':
     // 'Y' is the first character for several 2-character constraints.
     switch (Constraint[1]) {
     default:
-      break;
+      return false;
     case 'm':
       // 'Ym' is synonymous with 'y'.
     case 'k':
       return Size <= 64;
+    case 'z':
+    case '0':
+      // XMM0
+      if (SSELevel >= SSE1)
+        return Size <= 128U;
+      return false;
     case 'i':
     case 't':
-      // 'Yi' and 'Yt' are synonymous with 'x' when SSE2 is enabled.
-      if (SSELevel >= AVX512F)
-        return Size <= 512U;
-      else if (SSELevel >= AVX)
-        return Size <= 256U;
-      return SSELevel >= SSE2 && Size <= 128U;
+    case '2':
+      // 'Yi','Yt','Y2' are synonymous with 'x' when SSE2 is enabled.
+      if (SSELevel < SSE2)
+        return false;
+      break;
     }
+  case 'v':
+  case 'x':
+    if (SSELevel >= AVX512F)
+      // 512-bit zmm registers can be used if target supports AVX512F.
+      return Size <= 512U;
+    else if (SSELevel >= AVX)
+      // 256-bit ymm registers can be used if target supports AVX.
+      return Size <= 256U;
+    return Size <= 128U;
+
   }
 
   return true;
@@ -1515,6 +1523,12 @@
       // the return string.
       break;
     case 'k':
+    case 'm':
+    case 'i':
+    case 't':
+    case 'z':
+    case '0':
+    case '2':
       // "^" hints llvm that this is a 2 letter constraint.
       // "Constraint++" is used to promote the string iterator
       // to the next constraint.
Index: lib/CodeGen/TargetInfo.cpp
===================================================================
--- lib/CodeGen/TargetInfo.cpp
+++ lib/CodeGen/TargetInfo.cpp
@@ -22,6 +22,7 @@
 #include "clang/CodeGen/SwiftCallingConv.h"
 #include "clang/Frontend/CodeGenOptions.h"
 #include "llvm/ADT/StringExtras.h"
+#include "llvm/ADT/StringSwitch.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Type.h"
@@ -870,7 +871,10 @@
 static llvm::Type* X86AdjustInlineAsmType(CodeGen::CodeGenFunction &CGF,
                                           StringRef Constraint,
                                           llvm::Type* Ty) {
-  if ((Constraint == "y" || Constraint == "&y") && Ty->isVectorTy()) {
+  bool IsMMXCons = llvm::StringSwitch<bool>(Constraint)
+                     .Cases("y", "&y", "^Ym", true)
+                     .Default(false);
+  if (IsMMXCons && Ty->isVectorTy()) {
     if (cast<llvm::VectorType>(Ty)->getBitWidth() != 64) {
       // Invalid MMX constraint
       return nullptr;
Index: test/CodeGen/x86-GCC-inline-asm-Y-constraints.c
===================================================================
--- test/CodeGen/x86-GCC-inline-asm-Y-constraints.c
+++ test/CodeGen/x86-GCC-inline-asm-Y-constraints.c
@@ -0,0 +1,68 @@
+// RUN: %clang_cc1 -ffreestanding  -triple=x86_64-apple-darwin -target-cpu skx %s -emit-llvm -o - | FileCheck %s
+#include <xmmintrin.h>
+// This test is complimented by the .ll test under llvm/test/MC/X86/. 
+// At this level we can only check if the constarints are passed correctly
+// from inline asm to llvm IR.
+
+// CHECK-LABEL: @f_Ym
+void f_Ym(__m64 m)
+  {
+  // CHECK: movq  $0, %mm1
+  // CHECK-SAME: "=^Ym,~{dirflag},~{fpsr},~{flags}"
+  __asm__ volatile ("movq %0, %%mm1\n\t"
+          :"=Ym" (m));
+}
+
+// CHECK-LABEL: f_Yi
+void f_Yi(__m128 x, __m128 y, __m128 z)
+  {
+  // CHECK: vpaddq
+  // CHECK-SAME: "=^Yi,^Yi,^Yi,~{dirflag},~{fpsr},~{flags}"
+  __asm__ volatile ("vpaddq %0, %1, %2\n\t"
+          :"=Yi" (x)
+          :"Yi" (y),"Yi"(z));
+}
+
+// CHECK-LABEL: f_Yt
+void f_Yt(__m128 x, __m128 y, __m128 z)
+  {
+  // CHECK: vpaddq
+  // CHECK-SAME: "=^Yt,^Yt,^Yt,~{dirflag},~{fpsr},~{flags}"
+  __asm__ volatile ("vpaddq %0, %1, %2\n\t"
+          :"=Yt" (x)
+          :"Yt" (y),"Yt"(z));
+}
+
+// CHECK-LABEL: f_Y2
+void f_Y2(__m128 x, __m128 y, __m128 z)
+    {
+  // CHECK: vpaddq
+  // CHECK-SAME: "=^Y2,^Y2,^Y2,~{dirflag},~{fpsr},~{flags}"
+  __asm__ volatile ("vpaddq %0, %1, %2\n\t"
+            :"=Y2" (x)
+            :"Y2" (y),"Y2"(z));
+}
+
+// CHECK-LABEL: f_Yz
+void f_Yz(__m128 x, __m128 y, __m128 z)
+  {
+  // CHECK: vpaddq
+  // CHECK-SAME: vpaddq
+  // CHECK-SAME: "=^Yi,=^Yz,^Yi,0,~{dirflag},~{fpsr},~{flags}"
+  __asm__ volatile ("vpaddq %0,%2,%1\n\t"
+       "vpaddq %1,%0,%2\n\t"
+          :"+Yi"(z),"=Yz" (x)
+          :"Yi" (y) );
+}
+
+// CHECK-LABEL: f_Y0
+void f_Y0(__m128 x, __m128 y, __m128 z)
+  {
+  // CHECK: vpaddq
+  // CHECK-SAME: "=^Yi,=^Y0,^Yi,0,~{dirflag},~{fpsr},~{flags}"
+  __asm__ volatile ("vpaddq %0,%2,%1\n\t"
+          "vpaddq %1,%0,%2\n\t"
+          :"+Yi"(z),"=Y0" (x)
+          :"Yi" (y) );
+}
+