Index: lib/Target/AArch64/AArch64ISelLowering.cpp
===================================================================
--- lib/Target/AArch64/AArch64ISelLowering.cpp
+++ lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -1652,13 +1652,13 @@
 }
 
 static SDValue LowerBITCAST(SDValue Op, SelectionDAG &DAG) {
-  if (Op.getValueType() != MVT::f16)
+  SDValue Op0 = Op.getOperand(0);
+  if (Op.getValueType() != MVT::f16 || Op0.getValueType() != MVT::i16)
     return SDValue();
 
-  assert(Op.getOperand(0).getValueType() == MVT::i16);
   SDLoc DL(Op);
 
-  Op = DAG.getNode(ISD::ANY_EXTEND, DL, MVT::i32, Op.getOperand(0));
+  Op = DAG.getNode(ISD::ANY_EXTEND, DL, MVT::i32, Op0);
   Op = DAG.getNode(ISD::BITCAST, DL, MVT::f32, Op);
   return SDValue(
       DAG.getMachineNode(TargetOpcode::EXTRACT_SUBREG, DL, MVT::f16, Op,
@@ -8693,13 +8693,12 @@
 
 static void ReplaceBITCASTResults(SDNode *N, SmallVectorImpl<SDValue> &Results,
                                   SelectionDAG &DAG) {
-  if (N->getValueType(0) != MVT::i16)
-    return;
-
   SDLoc DL(N);
   SDValue Op = N->getOperand(0);
-  assert(Op.getValueType() == MVT::f16 &&
-         "Inconsistent bitcast? Only 16-bit types should be i16 or f16");
+
+  if (N->getValueType(0) != MVT::i16 || Op.getValueType() != MVT::f16)
+    return;
+
   Op = SDValue(
       DAG.getMachineNode(TargetOpcode::INSERT_SUBREG, DL, MVT::f32,
                          DAG.getUNDEF(MVT::i32), Op,
Index: test/CodeGen/AArch64/bitcast-v2i8.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/bitcast-v2i8.ll
@@ -0,0 +1,15 @@
+; RUN: llc < %s -mtriple=aarch64-apple-ios | FileCheck %s
+
+; Part of PR21549: going through the stack isn't ideal but is correct.
+
+define i16 @test_bitcast_v2i8_to_i16(<2 x i8> %a) {
+; CHECK-LABEL: test_bitcast_v2i8_to_i16
+; CHECK:      mov.s   [[WREG_HI:w[0-9]+]], v0[1]
+; CHECK-NEXT: fmov    [[WREG_LO:w[0-9]+]], s0
+; CHECK-NEXT: strb    [[WREG_HI]], [sp, #15]
+; CHECK-NEXT: strb    [[WREG_LO]], [sp, #14]
+; CHECK-NEXT: ldrh    w0, [sp, #14]
+
+  %aa = bitcast <2 x i8> %a to i16
+  ret i16 %aa
+}