diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -22581,6 +22581,11 @@
     }
   }
 
+  // Simplify scalar inserts into an undef vector:
+  // insert_subvector undef, (splat X), N2 -> splat X
+  if (N0.isUndef() && N1.getOpcode() == ISD::SPLAT_VECTOR)
+    return DAG.getNode(ISD::SPLAT_VECTOR, SDLoc(N), VT, N1.getOperand(0));
+
   // Canonicalize insert_subvector dag nodes.
   // Example:
   // (insert_subvector (insert_subvector A, Idx0), Idx1)
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -10092,10 +10092,9 @@
     // lowering code.
     if (auto *ConstVal = dyn_cast<ConstantSDNode>(SplatVal)) {
       // We can hande the zero case during isel.
-      if (ConstVal->isZero())
-        return Op;
-      if (ConstVal->isOne())
+      if (ConstVal->getZExtValue() & 0x1)
         return getPTrue(DAG, dl, VT, AArch64SVEPredPattern::all);
+      return Op;
     }
     // The general case of i1.  There isn't any natural way to do this,
     // so we use some trickery with whilelo.
diff --git a/llvm/test/CodeGen/AArch64/sve-insert-vector.ll b/llvm/test/CodeGen/AArch64/sve-insert-vector.ll
--- a/llvm/test/CodeGen/AArch64/sve-insert-vector.ll
+++ b/llvm/test/CodeGen/AArch64/sve-insert-vector.ll
@@ -1,5 +1,7 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -mattr=+bf16 < %s | FileCheck %s --check-prefixes=CHECK
+; RUN: llc -mattr=+sve -mattr=+bf16 < %s | FileCheck %s --check-prefixes=CHECK
+
+target triple = "aarch64-unknown-linux-gnu"
 
 define <vscale x 2 x i64> @insert_v2i64_nxv2i64(<vscale x 2 x i64> %vec, <2 x i64> %subvec) nounwind {
 ; CHECK-LABEL: insert_v2i64_nxv2i64:
@@ -571,6 +573,42 @@
   ret <vscale x 16 x i1> %v0
 }
 
+; Test constant preicate insert into undef
+define <vscale x 2 x i1> @insert_nxv2i1_v8i1_const_true_into_undef() vscale_range(4,4) {
+; CHECK-LABEL: insert_nxv2i1_v8i1_const_true_into_undef:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ptrue p0.d
+; CHECK-NEXT:    ret
+  %v0 = call <vscale x 2 x i1> @llvm.experimental.vector.insert.nxv2i1.v8i1 (<vscale x 2 x i1> undef, <8 x i1> <i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1>, i64 0)
+  ret <vscale x 2 x i1> %v0
+}
+
+define <vscale x 4 x i1> @insert_nxv4i1_v16i1_const_true_into_undef() vscale_range(4,4) {
+; CHECK-LABEL: insert_nxv4i1_v16i1_const_true_into_undef:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ptrue p0.s
+; CHECK-NEXT:    ret
+  %v0 = call <vscale x 4 x i1> @llvm.experimental.vector.insert.nxv4i1.v16i1 (<vscale x 4 x i1> undef, <16 x i1> <i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1>, i64 0)
+  ret <vscale x 4 x i1> %v0
+}
+
+define <vscale x 8 x i1> @insert_nxv8i1_v32i1_const_true_into_undef() vscale_range(4,4) {
+; CHECK-LABEL: insert_nxv8i1_v32i1_const_true_into_undef:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ptrue p0.h
+; CHECK-NEXT:    ret
+  %v0 = call <vscale x 8 x i1> @llvm.experimental.vector.insert.nxv8i1.v32i1 (<vscale x 8 x i1> undef, <32 x i1> <i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1>, i64 0)
+  ret <vscale x 8 x i1> %v0
+}
+
+define <vscale x 16 x i1> @insert_nxv16i1_v64i1_const_true_into_undef() vscale_range(4,4) {
+; CHECK-LABEL: insert_nxv16i1_v64i1_const_true_into_undef:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ptrue p0.b
+; CHECK-NEXT:    ret
+  %v0 = call <vscale x 16 x i1> @llvm.experimental.vector.insert.nxv16i1.v64i1 (<vscale x 16 x i1> undef, <64 x i1> <i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1>, i64 0)
+  ret <vscale x 16 x i1> %v0
+}
 
 declare <vscale x 3 x i32> @llvm.experimental.vector.insert.nxv3i32.nxv2i32(<vscale x 3 x i32>, <vscale x 2 x i32>, i64)
 declare <vscale x 3 x float> @llvm.experimental.vector.insert.nxv3f32.nxv2f32(<vscale x 3 x float>, <vscale x 2 x float>, i64)
@@ -583,5 +621,9 @@
 declare <vscale x 4 x bfloat> @llvm.experimental.vector.insert.nxv4bf16.v4bf16(<vscale x 4 x bfloat>, <4 x bfloat>, i64)
 declare <vscale x 2 x bfloat> @llvm.experimental.vector.insert.nxv2bf16.nxv2bf16(<vscale x 2 x bfloat>, <vscale x 2 x bfloat>, i64)
 
+declare <vscale x 2 x i1> @llvm.experimental.vector.insert.nxv2i1.v8i1(<vscale x 2 x i1>, <8 x i1>, i64)
+declare <vscale x 4 x i1> @llvm.experimental.vector.insert.nxv4i1.v16i1(<vscale x 4 x i1>, <16 x i1>, i64)
+declare <vscale x 8 x i1> @llvm.experimental.vector.insert.nxv8i1.v32i1(<vscale x 8 x i1>, <32 x i1>, i64)
 declare <vscale x 16 x i1> @llvm.experimental.vector.insert.nx16i1.nxv4i1(<vscale x 16 x i1>, <vscale x 4 x i1>, i64)
 declare <vscale x 16 x i1> @llvm.experimental.vector.insert.nx16i1.nxv8i1(<vscale x 16 x i1>, <vscale x 8 x i1>, i64)
+declare <vscale x 16 x i1> @llvm.experimental.vector.insert.nxv16i1.v64i1(<vscale x 16 x i1>, <64 x i1>, i64)