Index: llvm/trunk/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
+++ llvm/trunk/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
@@ -411,6 +411,10 @@
     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:
Index: llvm/trunk/test/CodeGen/AArch64/fpconv-vector-op-scalarize.ll
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/fpconv-vector-op-scalarize.ll
+++ llvm/trunk/test/CodeGen/AArch64/fpconv-vector-op-scalarize.ll
@@ -0,0 +1,44 @@
+; RUN: llc < %s -mtriple=arm64-apple-darwin | FileCheck %s
+
+; 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) {
+; CHECK-LABEL: test_sitofp:
+; CHECK:       sbfx  w8, w0, #0, #1
+; CHECK-NEXT:  scvtf d0, w8
+; CHECK-NEXT:  ret
+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) {
+; CHECK-LABEL: test_uitofp:
+; CHECK:       and   w8, w0, #0x1
+; CHECK-NEXT:  ucvtf d0, w8
+; CHECK-NEXT:  ret
+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) {
+; CHECK-LABEL: test_fptosi:
+; CHECK:       bl    ___fixtfdi
+; CHECK-NEXT:  fmov  d0, x0
+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) {
+; CHECK-LABEL: test_fptoui:
+; CHECK:       bl    ___fixunstfdi
+; CHECK-NEXT:  fmov  d0, x0
+entry:
+  %0 = fptoui <1 x fp128> %in to <1 x i64>
+  ret <1 x i64> %0
+}