Index: llvm/include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h
===================================================================
--- llvm/include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h
+++ llvm/include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h
@@ -208,7 +208,7 @@
 
   /// ReductionList contains the reduction descriptors for all
   /// of the reductions that were found in the loop.
-  using ReductionList = DenseMap<PHINode *, RecurrenceDescriptor>;
+  using ReductionList = MapVector<PHINode *, RecurrenceDescriptor>;
 
   /// InductionList saves induction variables and maps them to the
   /// induction descriptor.
Index: llvm/test/Transforms/LoopVectorize/reduction-order.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopVectorize/reduction-order.ll
@@ -0,0 +1,48 @@
+; RUN: opt -loop-vectorize -S < %s 2>&1 | FileCheck %s
+; RUN: opt -passes='loop-vectorize' -S < %s 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-grtev4-linux-gnu"
+
+; Make sure the selects generated from reduction are always emitted
+; in deterministic order.
+; CHECK-LABEL: @foo(
+; CHECK: vector.body:
+; CHECK: %[[VAR1:.*]] = add <4 x i32>
+; CHECK-NEXT:  = add <4 x i32>
+; CHECK-NEXT:  = add <4 x i32>
+; CHECK-NEXT:  = add <4 x i32>
+; CHECK-NEXT: %[[VAR2:.*]] = add <4 x i32>
+; CHECK: select <4 x i1> {{.*}}, <4 x i32> %[[VAR2]], <4 x i32>
+; CHECK-NEXT: select <4 x i1> {{.*}}, <4 x i32> %[[VAR1]], <4 x i32>
+; CHECK: br i1 {{.*}}, label %middle.block, label %vector.body
+;
+define internal i64 @foo(i32* %t0) !prof !1 {
+t16:
+  br label %t20
+
+t17:                                               ; preds = %t20
+  %t18 = phi i32 [ %t24, %t20 ]
+  %t19 = phi i32 [ %t28, %t20 ]
+  br label %t31
+
+t20:                                               ; preds = %t20, %t16
+  %t21 = phi i64 [ 0, %t16 ], [ %t29, %t20 ]
+  %t22 = phi i32 [ undef, %t16 ], [ %t28, %t20 ]
+  %t23 = phi i32 [ 0, %t16 ], [ %t24, %t20 ]
+  %t24 = add i32 undef, %t23
+  %t25 = add i32 %t22, undef
+  %t26 = add i32 %t25, undef
+  %t27 = add i32 %t26, undef
+  %t28 = add i32 %t27, undef
+  %t29 = add nuw nsw i64 %t21, 1
+  %t30 = icmp eq i64 %t29, undef
+  br i1 %t30, label %t17, label %t20, !prof !2
+
+t31:
+  ret i64 undef
+}
+
+!0 = !{!"clang version google3-trunk (fe5f233a938f5bc31c458c39cca54d7dcc2667ef)"}
+!1 = !{!"function_entry_count", i64 801}
+!2 = !{!"branch_weights", i32 746, i32 1}