Index: lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -1239,6 +1239,32 @@
   switch (Opc) {
   default:
     return UnableToLegalize;
+  case TargetOpcode::G_IMPLICIT_DEF: {
+    SmallVector<unsigned, 2> DstRegs;
+
+    unsigned NarrowSize = NarrowTy.getSizeInBits();
+    unsigned DstReg = MI.getOperand(0).getReg();
+    unsigned Size = MRI.getType(DstReg).getSizeInBits();
+    int NumParts = Size / NarrowSize;
+    // FIXME: Don't know how to handle the situation where the small vectors
+    // aren't all the same size yet.
+    if (Size % NarrowSize != 0)
+      return UnableToLegalize;
+
+    for (int i = 0; i < NumParts; ++i) {
+      unsigned TmpReg = MRI.createGenericVirtualRegister(NarrowTy);
+      MIRBuilder.buildUndef(TmpReg);
+      DstRegs.push_back(TmpReg);
+    }
+
+    if (NarrowTy.isVector())
+      MIRBuilder.buildConcatVectors(DstReg, DstRegs);
+    else
+      MIRBuilder.buildBuildVector(DstReg, DstRegs);
+
+    MI.eraseFromParent();
+    return Legalized;
+  }
   case TargetOpcode::G_ADD: {
     unsigned NarrowSize = NarrowTy.getSizeInBits();
     unsigned DstReg = MI.getOperand(0).getReg();
Index: lib/Target/AArch64/AArch64LegalizerInfo.cpp
===================================================================
--- lib/Target/AArch64/AArch64LegalizerInfo.cpp
+++ lib/Target/AArch64/AArch64LegalizerInfo.cpp
@@ -48,9 +48,21 @@
   const LLT v2s64 = LLT::vector(2, 64);
 
   getActionDefinitionsBuilder(G_IMPLICIT_DEF)
-      .legalFor({p0, s1, s8, s16, s32, s64, v2s64})
-      .clampScalar(0, s1, s64)
-      .widenScalarToNextPow2(0, 8);
+    .legalFor({p0, s1, s8, s16, s32, s64, v2s64})
+    .clampScalar(0, s1, s64)
+    .widenScalarToNextPow2(0, 8)
+    .fewerElementsIf(
+      [=](const LegalityQuery &Query) {
+        return Query.Types[0].isVector() &&
+          (Query.Types[0].getElementType() != s64 ||
+           Query.Types[0].getNumElements() != 2);
+      },
+      [=](const LegalityQuery &Query) {
+        LLT EltTy = Query.Types[0].getElementType();
+        if (EltTy == s64)
+          return std::make_pair(0, LLT::vector(2, 64));
+        return std::make_pair(0, EltTy);
+      });
 
   getActionDefinitionsBuilder(G_PHI)
       .legalFor({p0, s16, s32, s64})
Index: test/CodeGen/AArch64/GlobalISel/legalize-undef.mir
===================================================================
--- test/CodeGen/AArch64/GlobalISel/legalize-undef.mir
+++ test/CodeGen/AArch64/GlobalISel/legalize-undef.mir
@@ -35,3 +35,39 @@
 ...
 
 # FIXME: s2 not correctly handled
+
+---
+name: test_implicit_def_v2s32
+body: |
+  bb.0:
+
+    ; CHECK-LABEL: name: test_implicit_def_v2s32
+    ; CHECK: [[DEF:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
+    ; CHECK: [[DEF1:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
+    ; CHECK: [[DEF2:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
+    ; CHECK: [[DEF3:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
+    ; CHECK: [[BUILD_VECTOR:%[0-9]+]]:_(<4 x s32>) = G_BUILD_VECTOR [[DEF]](s32), [[DEF1]](s32), [[DEF2]](s32), [[DEF3]](s32)
+    ; CHECK: [[UV:%[0-9]+]]:_(<2 x s32>), [[UV1:%[0-9]+]]:_(<2 x s32>) = G_UNMERGE_VALUES [[BUILD_VECTOR]](<4 x s32>)
+    ; CHECK: $x0 = COPY [[UV]](<2 x s32>)
+    ; CHECK: $x1 = COPY [[UV1]](<2 x s32>)
+    %0:_(<4 x s32>) = G_IMPLICIT_DEF
+    %1:_(<2 x s32> ), %2:_(<2 x s32>) = G_UNMERGE_VALUES %0
+    $x0 = COPY %1
+    $x1 = COPY %2
+...
+
+---
+name: test_implicit_def_v4s64
+body: |
+  bb.0:
+
+    ; CHECK-LABEL: name: test_implicit_def_v4s64
+    ; CHECK: [[DEF:%[0-9]+]]:_(<2 x s64>) = G_IMPLICIT_DEF
+    ; CHECK: [[DEF1:%[0-9]+]]:_(<2 x s64>) = G_IMPLICIT_DEF
+    ; CHECK: $q0 = COPY [[DEF]](<2 x s64>)
+    ; CHECK: $q1 = COPY [[DEF1]](<2 x s64>)
+    %0:_(<4 x s64>) = G_IMPLICIT_DEF
+    %1:_(<2 x s64> ), %2:_(<2 x s64>) = G_UNMERGE_VALUES %0
+    $q0 = COPY %1
+    $q1 = COPY %2
+...