Index: llvm/include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
+++ llvm/include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
@@ -154,6 +154,10 @@
   /// def by inserting a G_BITCAST from \p CastTy
   void bitcastDst(MachineInstr &MI, LLT CastTy, unsigned OpIdx);
 
+  /// Widen \p OrigReg to \p WideTy by merging to a wider type, padding with
+  /// G_IMPLICIT_DEF, and producing dead results.
+  Register widenWithUnmerge(LLT WideTy, Register OrigReg);
+
 private:
   LegalizeResult
   widenScalarMergeValues(MachineInstr &MI, unsigned TypeIdx, LLT WideTy);
Index: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -1272,10 +1272,8 @@
 void LegalizerHelper::moreElementsVectorDst(MachineInstr &MI, LLT WideTy,
                                             unsigned OpIdx) {
   MachineOperand &MO = MI.getOperand(OpIdx);
-  Register DstExt = MRI.createGenericVirtualRegister(WideTy);
   MIRBuilder.setInsertPt(MIRBuilder.getMBB(), ++MIRBuilder.getInsertPt());
-  MIRBuilder.buildExtract(MO, DstExt, 0);
-  MO.setReg(DstExt);
+  MO.setReg(widenWithUnmerge(WideTy, MO.getReg()));
 }
 
 void LegalizerHelper::moreElementsVectorSrc(MachineInstr &MI, LLT MoreTy,
@@ -1443,6 +1441,40 @@
   return Legalized;
 }
 
+Register LegalizerHelper::widenWithUnmerge(LLT WideTy, Register OrigReg) {
+  Register WideReg = MRI.createGenericVirtualRegister(WideTy);
+  LLT OrigTy = MRI.getType(OrigReg);
+  LLT LCMTy = getLCMType(WideTy, OrigTy);
+
+  const int NumMergeParts = LCMTy.getSizeInBits() / WideTy.getSizeInBits();
+  const int NumUnmergeParts = LCMTy.getSizeInBits() / OrigTy.getSizeInBits();
+
+  Register UnmergeSrc = WideReg;
+
+  // Create a merge to the LCM type, padding with undef
+  // %0:_(<3 x s32>) = G_FOO => <4 x s32>
+  // =>
+  // %1:_(<4 x s32>) = G_FOO
+  // %2:_(<4 x s32>) = G_IMPLICIT_DEF
+  // %3:_(<12 x s32>) = G_CONCAT_VECTORS %1, %2, %2
+  // %0:_(<3 x s32>), %4:_, %5:_, %6:_ = G_UNMERGE_VALUES %3
+  if (NumMergeParts > 1) {
+    Register Undef = MIRBuilder.buildUndef(WideTy).getReg(0);
+    SmallVector<Register, 8> MergeParts(NumMergeParts, Undef);
+    MergeParts[0] = WideReg;
+    UnmergeSrc = MIRBuilder.buildMerge(LCMTy, MergeParts).getReg(0);
+  }
+
+  // Unmerge to the original register and pad with dead defs.
+  SmallVector<Register, 8> UnmergeResults(NumUnmergeParts);
+  UnmergeResults[0] = OrigReg;
+  for (int I = 1; I != NumUnmergeParts; ++I)
+    UnmergeResults[I] = MRI.createGenericVirtualRegister(OrigTy);
+
+  MIRBuilder.buildUnmerge(UnmergeResults, UnmergeSrc);
+  return WideReg;
+}
+
 LegalizerHelper::LegalizeResult
 LegalizerHelper::widenScalarUnmergeValues(MachineInstr &MI, unsigned TypeIdx,
                                           LLT WideTy) {