Index: cfe/trunk/lib/Basic/Targets.cpp
===================================================================
--- cfe/trunk/lib/Basic/Targets.cpp
+++ cfe/trunk/lib/Basic/Targets.cpp
@@ -3997,6 +3997,7 @@
     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.
+    case 'k': // AVX512 arch mask registers: k1-k7.
       Info.setAllowsRegister();
       return true;
     }
@@ -4018,6 +4019,8 @@
   case 'q': // Any register accessible as [r]l: a, b, c, and d.
   case 'y': // Any MMX register.
   case 'x': // Any SSE register.
+  case 'k': // Any AVX512 mask register (same as Yk, additionaly allows k0
+            // for intermideate k reg operations).
   case 'Q': // Any register accessible as [r]h: a, b, c, and d.
   case 'R': // "Legacy" registers: ax, bx, cx, dx, di, si, sp, bp.
   case 'l': // "Index" registers: any general register that can be used as an
@@ -4051,6 +4054,8 @@
                                         unsigned Size) const {
   switch (Constraint[0]) {
   default: break;
+  case 'k':
+  // Registers k0-k7 (AVX512) size limit is 64 bit.
   case 'y':
     return Size <= 64;
   case 'f':
@@ -4071,6 +4076,7 @@
     default: break;
     case 'm':
       // 'Ym' is synonymous with 'y'.
+    case 'k':
       return Size <= 64;
     case 'i':
     case 't':
@@ -4102,6 +4108,20 @@
     return std::string("{st}");
   case 'u': // second from top of floating point stack.
     return std::string("{st(1)}"); // second from top of floating point stack.
+  case 'Y':
+    switch (Constraint[1]) {
+    default:
+      // Break from inner switch and fall through (copy single char),
+      // continue parsing after copying the current constraint into 
+      // the return string.
+      break;
+    case 'k':
+      // "^" hints llvm that this is a 2 letter constraint.
+      // "Constraint++" is used to promote the string iterator 
+      // to the next constraint.
+      return std::string("^") + std::string(Constraint++, 2);
+    } 
+    LLVM_FALLTHROUGH;
   default:
     return std::string(1, *Constraint);
   }
Index: cfe/trunk/test/CodeGen/avx512-kconstraints-att_inline_asm.c
===================================================================
--- cfe/trunk/test/CodeGen/avx512-kconstraints-att_inline_asm.c
+++ cfe/trunk/test/CodeGen/avx512-kconstraints-att_inline_asm.c
@@ -0,0 +1,59 @@
+// RUN: %clang_cc1 %s -triple=x86_64-apple-darwin -target-cpu skylake-avx512 -O0  -emit-llvm -S -o - -Wall -Werror | FileCheck %s
+// This test checks validity of att\gcc style inline assmebly for avx512 k and Yk constraints.
+// Also checks mask register allows flexible type (size <= 64 bit)
+
+void mask_Yk_i8(char msk){ 
+//CHECK: vpaddb %xmm1, %xmm0, %xmm1 {%k1}
+ asm ("vpaddb\t %%xmm1, %%xmm0, %%xmm1 %{%0%}\t"
+       :                //output
+       : "Yk" (msk));   //inputs
+}
+
+void mask_Yk_i16(short msk){
+//CHECK: vpaddb %xmm1, %xmm0, %xmm1 {%k1}
+ asm ("vpaddb\t %%xmm1, %%xmm0, %%xmm1 %{%0%}\t"
+       :                //output
+       :  "Yk" (msk));  //inputs
+}
+
+void mask_Yk_i32(int msk){
+//CHECK: vpaddb %xmm1, %xmm0, %xmm1 {%k1}
+    asm ("vpaddb\t %%xmm1, %%xmm0, %%xmm1 %{%0%}\t"
+       :                //output
+       :  "Yk" (msk));   //inputs
+}
+
+void mask_Yk_i64(long long msk){
+//CHECK: vpaddb %xmm1, %xmm0, %xmm1 {%k1}
+ asm ("vpaddb\t %%xmm1, %%xmm0, %%xmm1 %{%0%}\t"
+       :                //output
+       :  "Yk" (msk));   //inputs
+}
+
+void k_wise_op_i8(char msk_dst,char msk_src1,char msk_src2){
+//CHECK: kandw %k1, %k0, %k0
+ asm ("kandw\t%2, %1, %0"
+       : "=k" (msk_dst)
+       : "k" (msk_src1), "k" (msk_src2));
+}
+
+void k_wise_op_i16(short msk_dst, short msk_src1, short msk_src2){
+//CHECK: kandw %k1, %k0, %k0
+  asm ("kandw\t%2, %1, %0"
+       : "=k" (msk_dst)
+       : "k" (msk_src1), "k" (msk_src2));
+}
+
+void k_wise_op_i32(int msk_dst, int msk_src1, int msk_src2){
+//CHECK: kandw %k1, %k0, %k0
+  asm ("kandw\t%2, %1, %0"
+       : "=k" (msk_dst)
+       : "k" (msk_src1), "k" (msk_src2));
+}
+
+void k_wise_op_i64(long long msk_dst, long long msk_src1, long long msk_src2){
+//CHECK: kandw %k1, %k0, %k0
+  asm ("kandw\t%2, %1, %0"
+       : "=k" (msk_dst)
+       : "k" (msk_src1), "k" (msk_src2));
+}