Index: lib/CodeGen/CGExprScalar.cpp
===================================================================
--- lib/CodeGen/CGExprScalar.cpp
+++ lib/CodeGen/CGExprScalar.cpp
@@ -1166,6 +1166,16 @@
   return llvm::ConstantInt::get(I32Ty, Off+MV);
 }
 
+static llvm::Constant *getAsInt32(llvm::ConstantInt *C, llvm::Type *I32Ty) {
+  if (C->getBitWidth() != 32) {
+      assert(llvm::ConstantInt::isValueValidForType(I32Ty,
+                                                    C->getZExtValue()) &&
+             "Index operand too large for shufflevector mask!");
+      return llvm::ConstantInt::get(I32Ty, C->getZExtValue());
+  }
+  return C;
+}
+
 Value *ScalarExprEmitter::VisitInitListExpr(InitListExpr *E) {
   bool Ignore = TestAndClearIgnoreResultAssign();
   (void)Ignore;
@@ -1216,7 +1226,8 @@
           Value *LHS = nullptr, *RHS = nullptr;
           if (CurIdx == 0) {
             // insert into undef -> shuffle (src, undef)
-            Args.push_back(C);
+            // shufflemask must use an i32
+            Args.push_back(getAsInt32(C, CGF.Int32Ty));
             Args.resize(ResElts, llvm::UndefValue::get(CGF.Int32Ty));
 
             LHS = EI->getVectorOperand();
Index: test/CodeGenOpenCL/vector_shufflevector_valid.cl
===================================================================
--- /dev/null
+++ test/CodeGenOpenCL/vector_shufflevector_valid.cl
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -emit-llvm -O0 %s -o - | FileCheck %s
+
+// The shuffle vector mask must always be of i32 vector type
+// See http://reviews.llvm.org/D10838 and https://llvm.org/bugs/show_bug.cgi?id=23800#c2
+// for more information about a bug where a 64 bit index operand causes the generation
+// of an invalid mask
+
+typedef unsigned int uint2 __attribute((ext_vector_type(2)));
+
+void vector_shufflevector_valid(void) {
+    //CHECK: {{%.*}} = shufflevector <2 x i32> {{%.*}}, <2 x i32> undef, <2 x i32> <i32 0, i32 undef>
+    (uint2)(((uint2)(0)).s0, 0);
+}