AArch64: implement efficient f16 bitcasts

Because i16 is illegal, there's no native DAG method to
represent a bitcast to or from an f16 type. This meant LLVM was
inserting a stack store/load pair which is really not ideal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213378 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -317,6 +317,10 @@ AArch64TargetLowering::AArch64TargetLowering(TargetMachine &TM)
   setTruncStoreAction(MVT::f128, MVT::f64, Expand);
   setTruncStoreAction(MVT::f128, MVT::f32, Expand);
   setTruncStoreAction(MVT::f128, MVT::f16, Expand);
+
+  setOperationAction(ISD::BITCAST, MVT::i16, Custom);
+  setOperationAction(ISD::BITCAST, MVT::f16, Custom);
+
   // Indexed loads and stores are supported.
   for (unsigned im = (unsigned)ISD::PRE_INC;
        im != (unsigned)ISD::LAST_INDEXED_MODE; ++im) {
@@ -1510,12 +1514,30 @@ SDValue AArch64TargetLowering::LowerFSINCOS(SDValue Op,
   return CallResult.first;
 }
 
+static SDValue LowerBITCAST(SDValue Op, SelectionDAG &DAG) {
+  if (Op.getValueType() != MVT::f16)
+    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::BITCAST, DL, MVT::f32, Op);
+  return SDValue(
+      DAG.getMachineNode(TargetOpcode::EXTRACT_SUBREG, DL, MVT::f16, Op,
+                         DAG.getTargetConstant(AArch64::hsub, MVT::i32)),
+      0);
+}
+
+
 SDValue AArch64TargetLowering::LowerOperation(SDValue Op,
                                               SelectionDAG &DAG) const {
   switch (Op.getOpcode()) {
   default:
     llvm_unreachable("unimplemented operand");
     return SDValue();
+  case ISD::BITCAST:
+    return LowerBITCAST(Op, DAG);
   case ISD::GlobalAddress:
     return LowerGlobalAddress(Op, DAG);
   case ISD::GlobalTLSAddress:
@@ -7942,11 +7964,32 @@ bool AArch64TargetLowering::getPostIndexedAddressParts(
   return true;
 }
 
+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");
+  Op = SDValue(
+      DAG.getMachineNode(TargetOpcode::INSERT_SUBREG, DL, MVT::f32,
+                         DAG.getUNDEF(MVT::i32), Op,
+                         DAG.getTargetConstant(AArch64::hsub, MVT::i32)),
+      0);
+  Op = DAG.getNode(ISD::BITCAST, DL, MVT::i32, Op);
+  Results.push_back(DAG.getNode(ISD::TRUNCATE, DL, MVT::i16, Op));
+}
+
 void AArch64TargetLowering::ReplaceNodeResults(
     SDNode *N, SmallVectorImpl<SDValue> &Results, SelectionDAG &DAG) const {
   switch (N->getOpcode()) {
   default:
     llvm_unreachable("Don't know how to custom expand this");
+  case ISD::BITCAST:
+    ReplaceBITCASTResults(N, Results, DAG);
+    return;
   case ISD::FP_TO_UINT:
   case ISD::FP_TO_SINT:
     assert(N->getValueType(0) == MVT::i128 && "unexpected illegal conversion");

test/CodeGen/AArch64/half.ll

@@ -17,6 +17,16 @@ define i16 @test_bitcast_from_half(half* %addr) {
   ret i16 %val_int
 }
 
+define i16 @test_reg_bitcast_from_half(half %in) {
+; CHECK-LABEL: test_reg_bitcast_from_half:
+; CHECK-NOT: str
+; CHECK-NOT: ldr
+; CHECK-DAG: fmov w0, s0
+; CHECK: ret
+  %val = bitcast half %in to i16
+  ret i16 %val
+}
+
 define void @test_bitcast_to_half(half* %addr, i16 %in) {
 ; CHECK-LABEL: test_bitcast_to_half:
 ; CHECK: strh w1, [x0]
@@ -25,6 +35,17 @@ define void @test_bitcast_to_half(half* %addr, i16 %in) {
   ret void
 }
 
+define half @test_reg_bitcast_to_half(i16 %in) {
+; CHECK-LABEL: test_reg_bitcast_to_half:
+; CHECK-NOT: str
+; CHECK-NOT: ldr
+; CHECK-DAG: fmov s0, w0
+; CHECK: ret
+
+  %val = bitcast i16 %in to half
+  ret half %val
+}
+
 define float @test_extend32(half* %addr) {
 ; CHECK-LABEL: test_extend32:
 ; CHECK: fcvt {{s[0-9]+}}, {{h[0-9]+}}