Index: llvm/trunk/include/llvm/IR/Instructions.h
===================================================================
--- llvm/trunk/include/llvm/IR/Instructions.h
+++ llvm/trunk/include/llvm/IR/Instructions.h
@@ -2261,6 +2261,19 @@
     return Mask;
   }
 
+  /// Change values in a shuffle permute mask assuming the two vector operands
+  /// of length InVecNumElts have swapped position.
+  static void commuteShuffleMask(MutableArrayRef<int> Mask,
+                                 unsigned InVecNumElts) {
+    for (int &Idx : Mask) {
+      if (Idx == -1)
+        continue;
+      Idx = Idx < (int)InVecNumElts ? Idx + InVecNumElts : Idx - InVecNumElts;
+      assert(Idx >= 0 && Idx < (int)InVecNumElts * 2 &&
+             "shufflevector mask index out of range");
+    }
+  }
+
   // Methods for support type inquiry through isa, cast, and dyn_cast:
   static inline bool classof(const Instruction *I) {
     return I->getOpcode() == Instruction::ShuffleVector;
Index: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp
@@ -4126,13 +4126,7 @@
   // second one.
   if (Op0Const && !Op1Const) {
     std::swap(Op0, Op1);
-    for (int &Idx : Indices) {
-      if (Idx == -1)
-        continue;
-      Idx = Idx < (int)InVecNumElts ? Idx + InVecNumElts : Idx - InVecNumElts;
-      assert(Idx >= 0 && Idx < (int)InVecNumElts * 2 &&
-             "shufflevector mask index out of range");
-    }
+    ShuffleVectorInst::commuteShuffleMask(Indices, InVecNumElts);
     Mask = ConstantDataVector::get(
         Mask->getContext(),
         makeArrayRef(reinterpret_cast<uint32_t *>(Indices.data()),
Index: llvm/trunk/unittests/IR/InstructionsTest.cpp
===================================================================
--- llvm/trunk/unittests/IR/InstructionsTest.cpp
+++ llvm/trunk/unittests/IR/InstructionsTest.cpp
@@ -21,6 +21,7 @@
 #include "llvm/IR/Module.h"
 #include "llvm/IR/NoFolder.h"
 #include "llvm/IR/Operator.h"
+#include "gmock/gmock-matchers.h"
 #include "gtest/gtest.h"
 #include <memory>
 
@@ -740,5 +741,11 @@
   EXPECT_EQ(BB1.get(), Handle.getCaseSuccessor());
 }
 
+TEST(InstructionsTest, CommuteShuffleMask) {
+  SmallVector<int, 16> Indices({-1, 0, 7});
+  ShuffleVectorInst::commuteShuffleMask(Indices, 4);
+  EXPECT_THAT(Indices, testing::ContainerEq(ArrayRef<int>({-1, 4, 3})));
+}
+
 } // end anonymous namespace
 } // end namespace llvm