Index: clang/lib/CodeGen/CGLoopInfo.cpp
===================================================================
--- clang/lib/CodeGen/CGLoopInfo.cpp
+++ clang/lib/CodeGen/CGLoopInfo.cpp
@@ -286,17 +286,18 @@
     Args.push_back(MDNode::get(Ctx, Vals));
   }
 
-  // Setting vectorize.enable
+  // vectorize.enable is set if:
+  // 1) loop hint vectorize.enable is set, or
+  // 2) it is implied when vectorize.predicate is set, or
+  // 3) it is implied when vectorize.width is set.
   if (Attrs.VectorizeEnable != LoopAttributes::Unspecified ||
-      IsVectorPredicateEnabled) {
-    Metadata *Vals[] = {
-        MDString::get(Ctx, "llvm.loop.vectorize.enable"),
-        ConstantAsMetadata::get(ConstantInt::get(
-            llvm::Type::getInt1Ty(Ctx),
-            IsVectorPredicateEnabled
-                ? true
-                : (Attrs.VectorizeEnable == LoopAttributes::Enable)))};
-    Args.push_back(MDNode::get(Ctx, Vals));
+      IsVectorPredicateEnabled ||
+      Attrs.VectorizeWidth > 1 ) {
+    bool AttrVal = Attrs.VectorizeEnable != LoopAttributes::Disable;
+    Args.push_back(
+        MDNode::get(Ctx, {MDString::get(Ctx, "llvm.loop.vectorize.enable"),
+                          ConstantAsMetadata::get(ConstantInt::get(
+                              llvm::Type::getInt1Ty(Ctx), AttrVal))}));
   }
 
   if (FollowupHasTransforms)
Index: clang/test/CodeGenCXX/pragma-loop-pr27643.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/pragma-loop-pr27643.cpp
@@ -0,0 +1,52 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -std=c++11 -emit-llvm -o - %s | FileCheck %s
+
+void loop1(int *List, int Length) {
+// CHECK-LABEL: @{{.*}}loop1{{.*}}(
+// CHECK: br label {{.*}}, !llvm.loop ![[LOOP1:.*]]
+
+  #pragma clang loop vectorize(enable) vectorize_width(1)
+  for (int i = 0; i < Length; i++)
+    List[i] = i * 2;
+}
+
+// Here, vectorize.enable should be set, obviously, but also check that
+// metadata isn't added twice.
+void loop2(int *List, int Length) {
+// CHECK-LABEL: @{{.*}}loop2{{.*}}(
+// CHECK: br label {{.*}}, !llvm.loop ![[LOOP2:.*]]
+
+  #pragma clang loop vectorize(enable) vectorize_width(2)
+  for (int i = 0; i < Length; i++)
+    List[i] = i * 2;
+}
+
+// Test that we do *not* imply vectorize.enable.
+void loop3(int *List, int Length) {
+// CHECK-LABEL: @{{.*}}loop3{{.*}}(
+// CHECK: br label {{.*}}, !llvm.loop ![[LOOP3:.*]]
+
+  #pragma clang loop vectorize_width(1)
+  for (int i = 0; i < Length; i++)
+    List[i] = i * 2;
+}
+
+// Test that we *do* imply vectorize.enable.
+void loop4(int *List, int Length) {
+// CHECK-LABEL: @{{.*}}loop4{{.*}}(
+// CHECK: br label {{.*}}, !llvm.loop ![[LOOP4:.*]]
+
+  #pragma clang loop vectorize_width(2)
+  for (int i = 0; i < Length; i++)
+    List[i] = i * 2;
+}
+
+// CHECK: ![[LOOP1]] = distinct !{![[LOOP1]], ![[VEC_WIDTH_1:.*]], ![[VEC_ENABLE:.*]]}
+// CHECK: ![[VEC_WIDTH_1]] = !{!"llvm.loop.vectorize.width", i32 1}
+// CHECK: ![[VEC_ENABLE]] = !{!"llvm.loop.vectorize.enable", i1 true}
+
+// CHECK: ![[LOOP2]] = distinct !{![[LOOP2]], ![[VEC_WIDTH_2:.*]], ![[VEC_ENABLE]]}
+// CHECK: ![[VEC_WIDTH_2]] = !{!"llvm.loop.vectorize.width", i32 2}
+
+// CHECK: ![[LOOP3]] = distinct !{![[LOOP3]], ![[VEC_WIDTH_1]]}
+
+// CHECK: ![[LOOP4]] = distinct !{![[LOOP4]], ![[VEC_WIDTH_2]], ![[VEC_ENABLE]]}
Index: clang/test/CodeGenCXX/pragma-loop.cpp
===================================================================
--- clang/test/CodeGenCXX/pragma-loop.cpp
+++ clang/test/CodeGenCXX/pragma-loop.cpp
@@ -161,20 +161,20 @@
 // CHECK: ![[LOOP_1]] = distinct !{![[LOOP_1]], ![[UNROLL_FULL:.*]]}
 // CHECK: ![[UNROLL_FULL]] = !{!"llvm.loop.unroll.full"}
 
-// CHECK: ![[LOOP_2]] = distinct !{![[LOOP_2]], ![[UNROLL_DISABLE:.*]], ![[DISTRIBUTE_DISABLE:.*]], ![[WIDTH_8:.*]], ![[INTERLEAVE_4:.*]]}
+// CHECK: ![[LOOP_2]] = distinct !{![[LOOP_2]], ![[UNROLL_DISABLE:.*]], ![[DISTRIBUTE_DISABLE:.*]], ![[WIDTH_8:.*]], ![[INTERLEAVE_4:.*]], ![[VECTORIZE_ENABLE:.*]]}
 // CHECK: ![[UNROLL_DISABLE]] = !{!"llvm.loop.unroll.disable"}
 // CHECK: ![[DISTRIBUTE_DISABLE]] = !{!"llvm.loop.distribute.enable", i1 false}
 // CHECK: ![[WIDTH_8]] = !{!"llvm.loop.vectorize.width", i32 8}
 // CHECK: ![[INTERLEAVE_4]] = !{!"llvm.loop.interleave.count", i32 4}
+// CHECK: ![[VECTORIZE_ENABLE]] = !{!"llvm.loop.vectorize.enable", i1 true}
 
-// CHECK: ![[LOOP_3]] = distinct !{![[LOOP_3]], ![[INTERLEAVE_4:.*]], ![[INTENABLE_1:.*]], ![[FOLLOWUP_VECTOR_3:.*]]}
-// CHECK: ![[INTENABLE_1]] = !{!"llvm.loop.vectorize.enable", i1 true}
+// CHECK: ![[LOOP_3]] = distinct !{![[LOOP_3]], ![[INTERLEAVE_4:.*]], ![[VECTORIZE_ENABLE]], ![[FOLLOWUP_VECTOR_3:.*]]}
 // CHECK: ![[FOLLOWUP_VECTOR_3]] = !{!"llvm.loop.vectorize.followup_all", ![[AFTER_VECTOR_3:.*]]}
 // CHECK: ![[AFTER_VECTOR_3]] = distinct !{![[AFTER_VECTOR_3]], ![[ISVECTORIZED:.*]], ![[UNROLL_8:.*]]}
 // CHECK: ![[ISVECTORIZED]] = !{!"llvm.loop.isvectorized"}
 // CHECK: ![[UNROLL_8]] = !{!"llvm.loop.unroll.count", i32 8}
 
-// CHECK: ![[LOOP_4]] = distinct !{![[LOOP_4]], ![[WIDTH_2:.*]], ![[INTERLEAVE_2:.*]]}
+// CHECK: ![[LOOP_4]] = distinct !{![[LOOP_4]], ![[WIDTH_2:.*]], ![[INTERLEAVE_2:.*]], ![[VECTORIZE_ENABLE]]}
 // CHECK: ![[WIDTH_2]] = !{!"llvm.loop.vectorize.width", i32 2}
 // CHECK: ![[INTERLEAVE_2]] = !{!"llvm.loop.interleave.count", i32 2}
 
@@ -185,7 +185,7 @@
 // CHECK: ![[FOLLOWUP_VECTOR_6]] = !{!"llvm.loop.vectorize.followup_all", ![[AFTER_VECTOR_6:.*]]}
 // CHECK: ![[AFTER_VECTOR_6]] = distinct !{![[AFTER_VECTOR_6]], ![[ISVECTORIZED:.*]], ![[UNROLL_8:.*]]}
 
-// CHECK: ![[LOOP_7]] = distinct !{![[LOOP_7]], ![[WIDTH_5:.*]]}
+// CHECK: ![[LOOP_7]] = distinct !{![[LOOP_7]], ![[WIDTH_5:.*]], ![[VECTORIZE_ENABLE]]}
 // CHECK: ![[WIDTH_5]] = !{!"llvm.loop.vectorize.width", i32 5}
 
 // CHECK: ![[LOOP_8]] = distinct !{![[LOOP_8]], ![[WIDTH_5:.*]]}
@@ -207,11 +207,11 @@
 // CHECK: ![[AFTER_VECTOR_12]] = distinct !{![[AFTER_VECTOR_12]], ![[ISVECTORIZED:.*]], ![[UNROLL_24:.*]]}
 // CHECK: ![[UNROLL_24]] = !{!"llvm.loop.unroll.count", i32 24}
 
-// CHECK: ![[LOOP_13]] = distinct !{![[LOOP_13]], ![[WIDTH_8:.*]], ![[INTERLEAVE_16:.*]], ![[FOLLOWUP_VECTOR_13:.*]]}
+// CHECK: ![[LOOP_13]] = distinct !{![[LOOP_13]], ![[WIDTH_8:.*]], ![[INTERLEAVE_16:.*]], ![[VECTORIZE_ENABLE]], ![[FOLLOWUP_VECTOR_13:.*]]}
 // CHECK: ![[INTERLEAVE_16]] = !{!"llvm.loop.interleave.count", i32 16}
 // CHECK: ![[FOLLOWUP_VECTOR_13]] = !{!"llvm.loop.vectorize.followup_all", ![[AFTER_VECTOR_13:.*]]}
 // CHECK: ![[AFTER_VECTOR_13]] = distinct !{![[AFTER_VECTOR_13]], ![[ISVECTORIZED:.*]], ![[UNROLL_32:.*]]}
 // CHECK: ![[UNROLL_32]] = !{!"llvm.loop.unroll.count", i32 32}
 
-// CHECK: ![[LOOP_14]] = distinct !{![[LOOP_14]], ![[WIDTH_10:.*]]}
+// CHECK: ![[LOOP_14]] = distinct !{![[LOOP_14]], ![[WIDTH_10:.*]], ![[VECTORIZE_ENABLE]]}
 // CHECK: ![[WIDTH_10]] = !{!"llvm.loop.vectorize.width", i32 10}