Index: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2685,6 +2685,8 @@
   "mode attribute only supported for integer and floating-point types">;
 def err_mode_wrong_type : Error<
   "type of machine mode does not match type of base type">;
+def err_complex_mode_vector_type : Error<
+  "type of machine mode does not support base vector types">;
 def err_attr_wrong_decl : Error<
   "%0 attribute invalid on this declaration, requires typedef or value">;
 def warn_attribute_nonnull_no_pointers : Warning<
Index: cfe/trunk/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDeclAttr.cpp
+++ cfe/trunk/lib/Sema/SemaDeclAttr.cpp
@@ -3154,16 +3154,22 @@
     return;
   }
 
-  if (!OldTy->getAs<BuiltinType>() && !OldTy->isComplexType())
+  // Base type can also be a vector type (see PR17453).
+  // Distinguish between base type and base element type.
+  QualType OldElemTy = OldTy;
+  if (const VectorType *VT = OldTy->getAs<VectorType>())
+    OldElemTy = VT->getElementType();
+
+  if (!OldElemTy->getAs<BuiltinType>() && !OldElemTy->isComplexType())
     S.Diag(Attr.getLoc(), diag::err_mode_not_primitive);
   else if (IntegerMode) {
-    if (!OldTy->isIntegralOrEnumerationType())
+    if (!OldElemTy->isIntegralOrEnumerationType())
       S.Diag(Attr.getLoc(), diag::err_mode_wrong_type);
   } else if (ComplexMode) {
-    if (!OldTy->isComplexType())
+    if (!OldElemTy->isComplexType())
       S.Diag(Attr.getLoc(), diag::err_mode_wrong_type);
   } else {
-    if (!OldTy->isFloatingType())
+    if (!OldElemTy->isFloatingType())
       S.Diag(Attr.getLoc(), diag::err_mode_wrong_type);
   }
 
@@ -3176,21 +3182,40 @@
     return;
   }
 
-  QualType NewTy;
+  QualType NewElemTy;
 
   if (IntegerMode)
-    NewTy = S.Context.getIntTypeForBitwidth(DestWidth,
-                                            OldTy->isSignedIntegerType());
+    NewElemTy = S.Context.getIntTypeForBitwidth(
+        DestWidth, OldElemTy->isSignedIntegerType());
   else
-    NewTy = S.Context.getRealTypeForBitwidth(DestWidth);
+    NewElemTy = S.Context.getRealTypeForBitwidth(DestWidth);
 
-  if (NewTy.isNull()) {
+  if (NewElemTy.isNull()) {
     S.Diag(Attr.getLoc(), diag::err_machine_mode) << 1 /*Unsupported*/ << Name;
     return;
   }
 
   if (ComplexMode) {
-    NewTy = S.Context.getComplexType(NewTy);
+    NewElemTy = S.Context.getComplexType(NewElemTy);
+  }
+
+  QualType NewTy = NewElemTy;
+  if (const VectorType *OldVT = OldTy->getAs<VectorType>()) {
+    // Complex machine mode does not support base vector types.
+    if (ComplexMode) {
+      S.Diag(Attr.getLoc(), diag::err_complex_mode_vector_type);
+      return;
+    }
+    unsigned NumElements = S.Context.getTypeSize(OldElemTy) *
+                           OldVT->getNumElements() /
+                           S.Context.getTypeSize(NewElemTy);
+    NewTy =
+        S.Context.getVectorType(NewElemTy, NumElements, OldVT->getVectorKind());
+  }
+
+  if (NewTy.isNull()) {
+    S.Diag(Attr.getLoc(), diag::err_mode_wrong_type);
+    return;
   }
 
   // Install the new type.
Index: cfe/trunk/test/CodeGen/attr-mode-vector-types.c
===================================================================
--- cfe/trunk/test/CodeGen/attr-mode-vector-types.c
+++ cfe/trunk/test/CodeGen/attr-mode-vector-types.c
@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -emit-llvm -o - | FileCheck %s
+
+typedef int __attribute__((mode(byte))) __attribute__((vector_size(4))) vec_t1;
+typedef int __attribute__((mode(QI))) __attribute__((vector_size(8))) vec_t2;
+typedef int __attribute__((mode(SI))) __attribute__((vector_size(16))) vec_t3;
+typedef int __attribute__((mode(DI))) __attribute__((vector_size(64)))vec_t4;
+typedef float __attribute__((mode(SF))) __attribute__((vector_size(128))) vec_t5;
+typedef float __attribute__((mode(DF))) __attribute__((vector_size(256))) vec_t6;
+
+void check() {
+  // CHECK: alloca <4 x i8>
+  vec_t1 v1;
+  // CHECK: alloca <8 x i8>
+  vec_t2 v2;
+  // CHECK: alloca <4 x i32>
+  vec_t3 v3;
+  // CHECK: alloca <8 x i64>
+  vec_t4 v4;
+  // CHECK: alloca <32 x float>
+  vec_t5 v5;
+  // CHECK: alloca <32 x double>
+  vec_t6 v6;
+}
+
+// CHECK: ret i32 4
+int check_size1() { return sizeof(vec_t1); }
+
+// CHECK: ret i32 8
+int check_size2() { return sizeof(vec_t2); }
+
+// CHECK: ret i32 16
+int check_size3() { return sizeof(vec_t3); }
+
+// CHECK: ret i32 64
+int check_size4() { return sizeof(vec_t4); }
+
+// CHECK: ret i32 128
+int check_size5() { return sizeof(vec_t5); }
+
+// CHECK: ret i32 256
+int check_size6() { return sizeof(vec_t6); }
Index: cfe/trunk/test/Sema/attr-mode-vector-types.c
===================================================================
--- cfe/trunk/test/Sema/attr-mode-vector-types.c
+++ cfe/trunk/test/Sema/attr-mode-vector-types.c
@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// Correct cases.
+typedef int __attribute__((mode(byte))) __attribute__((vector_size(256))) vec_t1;
+typedef int __attribute__((mode(QI))) __attribute__((vector_size(256))) vec_t2;
+typedef int __attribute__((mode(SI))) __attribute__((vector_size(256))) vec_t3;
+typedef int __attribute__((mode(DI))) __attribute__((vector_size(256)))vec_t4;
+typedef float __attribute__((mode(SF))) __attribute__((vector_size(256))) vec_t5;
+typedef float __attribute__((mode(DF))) __attribute__((vector_size(256))) vec_t6;
+typedef float __attribute__((mode(XF))) __attribute__((vector_size(256))) vec_t7;
+
+// Incorrect cases.
+typedef float __attribute__((mode(QC))) __attribute__((vector_size(256))) vec_t8;
+// expected-error@-1{{unsupported machine mode 'QC'}}
+// expected-error@-2{{type of machine mode does not match type of base type}}
+typedef _Complex float __attribute__((mode(HC))) __attribute__((vector_size(256))) vec_t9;
+// expected-error@-1{{unsupported machine mode 'HC'}}
+// expected-error@-2{{invalid vector element type '_Complex float'}}
+typedef int __attribute__((mode(SC))) __attribute__((vector_size(256))) vec_t10;
+// expected-error@-1{{type of machine mode does not match type of base type}}
+// expected-error@-2{{type of machine mode does not support base vector types}}
+typedef float __attribute__((mode(DC))) __attribute__((vector_size(256))) vec_t11;
+// expected-error@-1{{type of machine mode does not match type of base type}}
+// expected-error@-2{{type of machine mode does not support base vector types}}
+typedef _Complex float __attribute__((mode(XC))) __attribute__((vector_size(256))) vec_t12;
+// expected-error@-1{{invalid vector element type '_Complex float'}}