diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -2059,6 +2059,15 @@
     { ISD::BITCAST, MVT::nxv2i16, MVT::nxv2f16, 0 },
     { ISD::BITCAST, MVT::nxv4i16, MVT::nxv4f16, 0 },
     { ISD::BITCAST, MVT::nxv2i32, MVT::nxv2f32, 0 },
+
+    // Add high cost for extending to illegal -too widen- scalable vectors.
+    { ISD::ZERO_EXTEND, MVT::nxv16i16, MVT::nxv16i8, 16},
+    { ISD::ZERO_EXTEND, MVT::nxv16i32, MVT::nxv16i8, 32},
+    { ISD::ZERO_EXTEND, MVT::nxv16i64, MVT::nxv16i8, 64},
+
+    { ISD::SIGN_EXTEND, MVT::nxv16i16, MVT::nxv16i8, 16},
+    { ISD::SIGN_EXTEND, MVT::nxv16i32, MVT::nxv16i8, 32},
+    { ISD::SIGN_EXTEND, MVT::nxv16i64, MVT::nxv16i8, 64},
   };
 
   if (const auto *Entry = ConvertCostTableLookup(ConversionTbl, ISD,