Index: lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
+++ lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
@@ -408,6 +408,10 @@
     case ISD::ZERO_EXTEND:
     case ISD::SIGN_EXTEND:
     case ISD::TRUNCATE:
+    case ISD::FP_TO_SINT:
+    case ISD::FP_TO_UINT:
+    case ISD::SINT_TO_FP:
+    case ISD::UINT_TO_FP:
       Res = ScalarizeVecOp_UnaryOp(N);
       break;
     case ISD::CONCAT_VECTORS:
@@ -451,11 +455,11 @@
                      N->getValueType(0), Elt);
 }
 
-/// ScalarizeVecOp_EXTEND - If the value to extend is a vector that needs
-/// to be scalarized, it must be <1 x ty>.  Extend the element instead.
+/// ScalarizeVecOp_UnaryOp - If the input is a vector that needs to be
+/// scalarized, it must be <1 x ty>.  Do the operation on the element instead.
 SDValue DAGTypeLegalizer::ScalarizeVecOp_UnaryOp(SDNode *N) {
   assert(N->getValueType(0).getVectorNumElements() == 1 &&
-         "Unexected vector type!");
+         "Unexpected vector type!");
   SDValue Elt = GetScalarizedVector(N->getOperand(0));
   SDValue Op = DAG.getNode(N->getOpcode(), SDLoc(N),
                            N->getValueType(0).getScalarType(), Elt);
Index: test/CodeGen/AArch64/fpconv-vector-op-scalarize.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/fpconv-vector-op-scalarize.ll
@@ -0,0 +1,30 @@
+; RUN: llc < %s -mtriple=arm64-apple-darwin
+
+; PR20778
+; Check that the legalizer doesn't crash when scalarizing FP conversion
+; instructions' operands.  The operands are all illegal on AArch64,
+; ensuring they are legalized.  The results are all legal.
+
+define <1 x double> @test_sitofp(<1 x i1> %in) {
+entry:
+  %0 = sitofp <1 x i1> %in to <1 x double>
+  ret <1 x double> %0
+}
+
+define <1 x double> @test_uitofp(<1 x i1> %in) {
+entry:
+  %0 = uitofp <1 x i1> %in to <1 x double>
+  ret <1 x double> %0
+}
+
+define <1 x i64> @test_fptosi(<1 x fp128> %in) {
+entry:
+  %0 = fptosi <1 x fp128> %in to <1 x i64>
+  ret <1 x i64> %0
+}
+
+define <1 x i64> @test_fptoui(<1 x fp128> %in) {
+entry:
+  %0 = fptoui <1 x fp128> %in to <1 x i64>
+  ret <1 x i64> %0
+}