llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

//===-- llvm/CodeGen/GlobalISel/LegalizerHelper.cpp -----------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
/// \file This file implements the LegalizerHelper class to legalize
/// individual instructions and the LegalizeMachineIR wrapper pass for the
/// primary legalization.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/GlobalISel/LegalizerHelper.h"
#include "llvm/CodeGen/GlobalISel/CallLowering.h"
#include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetSubtargetInfo.h"

#include <sstream>

#define DEBUG_TYPE "legalize-mir"

using namespace llvm;

LegalizerHelper::LegalizerHelper(MachineFunction &MF)
    : MRI(MF.getRegInfo()), LI(*MF.getSubtarget().getLegalizerInfo()) {
  MIRBuilder.setMF(MF);
}

LegalizerHelper::LegalizeResult
LegalizerHelper::legalizeInstrStep(MachineInstr &MI) {
  auto Action = LI.getAction(MI, MRI);
  switch (std::get<0>(Action)) {
  case LegalizerInfo::Legal:
    return AlreadyLegal;
  case LegalizerInfo::Libcall:
    return libcall(MI);
  case LegalizerInfo::NarrowScalar:
    return narrowScalar(MI, std::get<1>(Action), std::get<2>(Action));
  case LegalizerInfo::WidenScalar:
    return widenScalar(MI, std::get<1>(Action), std::get<2>(Action));
  case LegalizerInfo::Lower:
    return lower(MI, std::get<1>(Action), std::get<2>(Action));
  case LegalizerInfo::FewerElements:
    return fewerElementsVector(MI, std::get<1>(Action), std::get<2>(Action));
  case LegalizerInfo::Custom:
    return LI.legalizeCustom(MI, MRI, MIRBuilder) ? Legalized
                                                  : UnableToLegalize;
  default:
    return UnableToLegalize;
  }
}

LegalizerHelper::LegalizeResult
LegalizerHelper::legalizeInstr(MachineInstr &MI) {
  SmallVector<MachineInstr *, 4> WorkList;
  MIRBuilder.recordInsertions(
      [&](MachineInstr *MI) { WorkList.push_back(MI); });
  WorkList.push_back(&MI);

  bool Changed = false;
  LegalizeResult Res;
  unsigned Idx = 0;
  do {
    Res = legalizeInstrStep(*WorkList[Idx]);
    if (Res == UnableToLegalize) {
      MIRBuilder.stopRecordingInsertions();
      return UnableToLegalize;
    }
    Changed |= Res == Legalized;
    ++Idx;
  } while (Idx < WorkList.size());

  MIRBuilder.stopRecordingInsertions();

  return Changed ? Legalized : AlreadyLegal;
}

void LegalizerHelper::extractParts(unsigned Reg, LLT Ty, int NumParts,
                                   SmallVectorImpl<unsigned> &VRegs) {
  for (int i = 0; i < NumParts; ++i)
    VRegs.push_back(MRI.createGenericVirtualRegister(Ty));
  MIRBuilder.buildUnmerge(VRegs, Reg);
}

static RTLIB::Libcall getRTLibDesc(unsigned Opcode, unsigned Size) {
  switch (Opcode) {
  case TargetOpcode::G_FREM:
    return Size == 64 ? RTLIB::REM_F64 : RTLIB::REM_F32;
  case TargetOpcode::G_FPOW:
    return Size == 64 ? RTLIB::POW_F64 : RTLIB::POW_F32;
  }
  llvm_unreachable("Unknown libcall function");
}

LegalizerHelper::LegalizeResult
LegalizerHelper::libcall(MachineInstr &MI) {
  LLT Ty = MRI.getType(MI.getOperand(0).getReg());
  unsigned Size = Ty.getSizeInBits();
  MIRBuilder.setInstr(MI);

  switch (MI.getOpcode()) {
  default:
    return UnableToLegalize;
  case TargetOpcode::G_FPOW:
  case TargetOpcode::G_FREM: {
    auto &Ctx = MIRBuilder.getMF().getFunction()->getContext();
    Type *Ty = Size == 64 ? Type::getDoubleTy(Ctx) : Type::getFloatTy(Ctx);
    auto &CLI = *MIRBuilder.getMF().getSubtarget().getCallLowering();
    auto &TLI = *MIRBuilder.getMF().getSubtarget().getTargetLowering();
    const char *Name = TLI.getLibcallName(getRTLibDesc(MI.getOpcode(), Size));
    MIRBuilder.getMF().getFrameInfo().setHasCalls(true);
    CLI.lowerCall(
        MIRBuilder, MachineOperand::CreateES(Name),
        {MI.getOperand(0).getReg(), Ty},
        {{MI.getOperand(1).getReg(), Ty}, {MI.getOperand(2).getReg(), Ty}});
    MI.eraseFromParent();
    return Legalized;
  }
  }
}

LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
                                                              unsigned TypeIdx,
                                                              LLT NarrowTy) {
  // FIXME: Don't know how to handle secondary types yet.
  if (TypeIdx != 0)
    return UnableToLegalize;

  MIRBuilder.setInstr(MI);

  switch (MI.getOpcode()) {
  default:
    return UnableToLegalize;
  case TargetOpcode::G_ADD: {
    // Expand in terms of carry-setting/consuming G_ADDE instructions.
    int NumParts = MRI.getType(MI.getOperand(0).getReg()).getSizeInBits() /
                   NarrowTy.getSizeInBits();

    SmallVector<unsigned, 2> Src1Regs, Src2Regs, DstRegs;
    extractParts(MI.getOperand(1).getReg(), NarrowTy, NumParts, Src1Regs);
    extractParts(MI.getOperand(2).getReg(), NarrowTy, NumParts, Src2Regs);

    unsigned CarryIn = MRI.createGenericVirtualRegister(LLT::scalar(1));
    MIRBuilder.buildConstant(CarryIn, 0);

    for (int i = 0; i < NumParts; ++i) {
      unsigned DstReg = MRI.createGenericVirtualRegister(NarrowTy);
      unsigned CarryOut = MRI.createGenericVirtualRegister(LLT::scalar(1));

      MIRBuilder.buildUAdde(DstReg, CarryOut, Src1Regs[i],
                            Src2Regs[i], CarryIn);

      DstRegs.push_back(DstReg);
      CarryIn = CarryOut;
    }
    unsigned DstReg = MI.getOperand(0).getReg();
    MIRBuilder.buildMerge(DstReg, DstRegs);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_INSERT: {
    if (TypeIdx != 0)
      return UnableToLegalize;

    int64_t NarrowSize = NarrowTy.getSizeInBits();
    int NumParts =
        MRI.getType(MI.getOperand(0).getReg()).getSizeInBits() / NarrowSize;

    SmallVector<unsigned, 2> SrcRegs, DstRegs;
    SmallVector<uint64_t, 2> Indexes;
    extractParts(MI.getOperand(1).getReg(), NarrowTy, NumParts, SrcRegs);

    unsigned OpReg = MI.getOperand(2).getReg();
    int64_t OpStart = MI.getOperand(3).getImm();
    int64_t OpSize = MRI.getType(OpReg).getSizeInBits();
    for (int i = 0; i < NumParts; ++i) {
      unsigned DstStart = i * NarrowSize;

      if (DstStart + NarrowSize <= OpStart || DstStart >= OpStart + OpSize) {
        // No part of the insert affects this subregister, forward the original.
        DstRegs.push_back(SrcRegs[i]);
        continue;
      } else if (DstStart == OpStart && NarrowTy == MRI.getType(OpReg)) {
        // The entire subregister is defined by this insert, forward the new
        // value.
        DstRegs.push_back(OpReg);
        continue;
      }

      // OpSegStart is where this destination segment would start in OpReg if it
      // extended infinitely in both directions.
      int64_t ExtractOffset, InsertOffset, SegSize;
      if (OpStart < DstStart) {
        InsertOffset = 0;
        ExtractOffset = DstStart - OpStart;
        SegSize = std::min(NarrowSize, OpStart + OpSize - DstStart);
      } else {
        InsertOffset = OpStart - DstStart;
        ExtractOffset = 0;
        SegSize =
            std::min(NarrowSize - InsertOffset, OpStart + OpSize - DstStart);
      }

      unsigned SegReg = OpReg;
      if (ExtractOffset != 0 || SegSize != OpSize) {
        // A genuine extract is needed.
        SegReg = MRI.createGenericVirtualRegister(LLT::scalar(SegSize));
        MIRBuilder.buildExtract(SegReg, OpReg, ExtractOffset);
      }

      unsigned DstReg = MRI.createGenericVirtualRegister(NarrowTy);
      MIRBuilder.buildInsert(DstReg, SrcRegs[i], SegReg, InsertOffset);
      DstRegs.push_back(DstReg);
    }

    assert(DstRegs.size() == (unsigned)NumParts && "not all parts covered");
    MIRBuilder.buildMerge(MI.getOperand(0).getReg(), DstRegs);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_LOAD: {
    unsigned NarrowSize = NarrowTy.getSizeInBits();
    int NumParts =
        MRI.getType(MI.getOperand(0).getReg()).getSizeInBits() / NarrowSize;
    LLT NarrowPtrTy = LLT::pointer(
        MRI.getType(MI.getOperand(1).getReg()).getAddressSpace(), NarrowSize);

    SmallVector<unsigned, 2> DstRegs;
    for (int i = 0; i < NumParts; ++i) {
      unsigned DstReg = MRI.createGenericVirtualRegister(NarrowTy);
      unsigned SrcReg = MRI.createGenericVirtualRegister(NarrowPtrTy);
      unsigned Offset = MRI.createGenericVirtualRegister(LLT::scalar(64));

      MIRBuilder.buildConstant(Offset, i * NarrowSize / 8);
      MIRBuilder.buildGEP(SrcReg, MI.getOperand(1).getReg(), Offset);
      // TODO: This is conservatively correct, but we probably want to split the
      // memory operands in the future.
      MIRBuilder.buildLoad(DstReg, SrcReg, **MI.memoperands_begin());

      DstRegs.push_back(DstReg);
    }
    unsigned DstReg = MI.getOperand(0).getReg();
    MIRBuilder.buildMerge(DstReg, DstRegs);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_STORE: {
    unsigned NarrowSize = NarrowTy.getSizeInBits();
    int NumParts =
        MRI.getType(MI.getOperand(0).getReg()).getSizeInBits() / NarrowSize;
    LLT NarrowPtrTy = LLT::pointer(
        MRI.getType(MI.getOperand(1).getReg()).getAddressSpace(), NarrowSize);

    SmallVector<unsigned, 2> SrcRegs;
    extractParts(MI.getOperand(0).getReg(), NarrowTy, NumParts, SrcRegs);

    for (int i = 0; i < NumParts; ++i) {
      unsigned DstReg = MRI.createGenericVirtualRegister(NarrowPtrTy);
      unsigned Offset = MRI.createGenericVirtualRegister(LLT::scalar(64));
      MIRBuilder.buildConstant(Offset, i * NarrowSize / 8);
      MIRBuilder.buildGEP(DstReg, MI.getOperand(1).getReg(), Offset);
      // TODO: This is conservatively correct, but we probably want to split the
      // memory operands in the future.
      MIRBuilder.buildStore(SrcRegs[i], DstReg, **MI.memoperands_begin());
    }
    MI.eraseFromParent();
    return Legalized;
  }
  }
}

LegalizerHelper::LegalizeResult
LegalizerHelper::widenScalar(MachineInstr &MI, unsigned TypeIdx, LLT WideTy) {
  MIRBuilder.setInstr(MI);

  switch (MI.getOpcode()) {
  default:
    return UnableToLegalize;
  case TargetOpcode::G_ADD:
  case TargetOpcode::G_AND:
  case TargetOpcode::G_MUL:
  case TargetOpcode::G_OR:
  case TargetOpcode::G_XOR:
  case TargetOpcode::G_SUB:
  case TargetOpcode::G_SHL: {
    // Perform operation at larger width (any extension is fine here, high bits
    // don't affect the result) and then truncate the result back to the
    // original type.
    unsigned Src1Ext = MRI.createGenericVirtualRegister(WideTy);
    unsigned Src2Ext = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildAnyExt(Src1Ext, MI.getOperand(1).getReg());
    MIRBuilder.buildAnyExt(Src2Ext, MI.getOperand(2).getReg());

    unsigned DstExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildInstr(MI.getOpcode())
        .addDef(DstExt)
        .addUse(Src1Ext)
        .addUse(Src2Ext);

    MIRBuilder.buildTrunc(MI.getOperand(0).getReg(), DstExt);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_SDIV:
  case TargetOpcode::G_UDIV:
  case TargetOpcode::G_ASHR:
  case TargetOpcode::G_LSHR: {
    unsigned ExtOp = MI.getOpcode() == TargetOpcode::G_SDIV ||
                             MI.getOpcode() == TargetOpcode::G_ASHR
                         ? TargetOpcode::G_SEXT
                         : TargetOpcode::G_ZEXT;

    unsigned LHSExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildInstr(ExtOp).addDef(LHSExt).addUse(
        MI.getOperand(1).getReg());

    unsigned RHSExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildInstr(ExtOp).addDef(RHSExt).addUse(
        MI.getOperand(2).getReg());

    unsigned ResExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildInstr(MI.getOpcode())
        .addDef(ResExt)
        .addUse(LHSExt)
        .addUse(RHSExt);

    MIRBuilder.buildTrunc(MI.getOperand(0).getReg(), ResExt);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_SELECT: {
    if (TypeIdx != 0)
      return UnableToLegalize;

    // Perform operation at larger width (any extension is fine here, high bits
    // don't affect the result) and then truncate the result back to the
    // original type.
    unsigned Src1Ext = MRI.createGenericVirtualRegister(WideTy);
    unsigned Src2Ext = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildAnyExt(Src1Ext, MI.getOperand(2).getReg());
    MIRBuilder.buildAnyExt(Src2Ext, MI.getOperand(3).getReg());

    unsigned DstExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildInstr(TargetOpcode::G_SELECT)
        .addDef(DstExt)
        .addReg(MI.getOperand(1).getReg())
        .addUse(Src1Ext)
        .addUse(Src2Ext);

    MIRBuilder.buildTrunc(MI.getOperand(0).getReg(), DstExt);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_FPTOSI:
  case TargetOpcode::G_FPTOUI: {
    if (TypeIdx != 0)
      return UnableToLegalize;

    unsigned DstExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildInstr(MI.getOpcode())
        .addDef(DstExt)
        .addUse(MI.getOperand(1).getReg());

    MIRBuilder.buildTrunc(MI.getOperand(0).getReg(), DstExt);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_SITOFP:
  case TargetOpcode::G_UITOFP: {
    if (TypeIdx != 1)
      return UnableToLegalize;

    unsigned Src = MI.getOperand(1).getReg();
    unsigned SrcExt = MRI.createGenericVirtualRegister(WideTy);

    if (MI.getOpcode() == TargetOpcode::G_SITOFP) {
      MIRBuilder.buildSExt(SrcExt, Src);
    } else {
      assert(MI.getOpcode() == TargetOpcode::G_UITOFP && "Unexpected conv op");
      MIRBuilder.buildZExt(SrcExt, Src);
    }

    MIRBuilder.buildInstr(MI.getOpcode())
        .addDef(MI.getOperand(0).getReg())
        .addUse(SrcExt);

    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_INSERT: {
    if (TypeIdx != 0)
      return UnableToLegalize;

    unsigned Src = MI.getOperand(1).getReg();
    unsigned SrcExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildAnyExt(SrcExt, Src);

    unsigned DstExt = MRI.createGenericVirtualRegister(WideTy);
    auto MIB = MIRBuilder.buildInsert(DstExt, SrcExt, MI.getOperand(2).getReg(),
                                      MI.getOperand(3).getImm());
    for (unsigned OpNum = 4; OpNum < MI.getNumOperands(); OpNum += 2) {
      MIB.addReg(MI.getOperand(OpNum).getReg());
      MIB.addImm(MI.getOperand(OpNum + 1).getImm());
    }

    MIRBuilder.buildTrunc(MI.getOperand(0).getReg(), DstExt);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_LOAD: {
    assert(alignTo(MRI.getType(MI.getOperand(0).getReg()).getSizeInBits(), 8) ==
               WideTy.getSizeInBits() &&
           "illegal to increase number of bytes loaded");

    unsigned DstExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildLoad(DstExt, MI.getOperand(1).getReg(),
                         **MI.memoperands_begin());
    MIRBuilder.buildTrunc(MI.getOperand(0).getReg(), DstExt);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_STORE: {
    assert(alignTo(MRI.getType(MI.getOperand(0).getReg()).getSizeInBits(), 8) ==
               WideTy.getSizeInBits() &&
           "illegal to increase number of bytes modified by a store");

    unsigned SrcExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildAnyExt(SrcExt, MI.getOperand(0).getReg());
    MIRBuilder.buildStore(SrcExt, MI.getOperand(1).getReg(),
                          **MI.memoperands_begin());
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_CONSTANT: {
    unsigned DstExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildConstant(DstExt, *MI.getOperand(1).getCImm());
    MIRBuilder.buildTrunc(MI.getOperand(0).getReg(), DstExt);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_FCONSTANT: {
    unsigned DstExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildFConstant(DstExt, *MI.getOperand(1).getFPImm());
    MIRBuilder.buildFPTrunc(MI.getOperand(0).getReg(), DstExt);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_BRCOND: {
    unsigned TstExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildAnyExt(TstExt, MI.getOperand(0).getReg());
    MIRBuilder.buildBrCond(TstExt, *MI.getOperand(1).getMBB());
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_ICMP: {
    assert(TypeIdx == 1 && "unable to legalize predicate");
    bool IsSigned = CmpInst::isSigned(
        static_cast<CmpInst::Predicate>(MI.getOperand(1).getPredicate()));
    unsigned Op0Ext = MRI.createGenericVirtualRegister(WideTy);
    unsigned Op1Ext = MRI.createGenericVirtualRegister(WideTy);
    if (IsSigned) {
      MIRBuilder.buildSExt(Op0Ext, MI.getOperand(2).getReg());
      MIRBuilder.buildSExt(Op1Ext, MI.getOperand(3).getReg());
    } else {
      MIRBuilder.buildZExt(Op0Ext, MI.getOperand(2).getReg());
      MIRBuilder.buildZExt(Op1Ext, MI.getOperand(3).getReg());
    }
    MIRBuilder.buildICmp(
        static_cast<CmpInst::Predicate>(MI.getOperand(1).getPredicate()),
        MI.getOperand(0).getReg(), Op0Ext, Op1Ext);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_GEP: {
    assert(TypeIdx == 1 && "unable to legalize pointer of GEP");
    unsigned OffsetExt = MRI.createGenericVirtualRegister(WideTy);
    MIRBuilder.buildSExt(OffsetExt, MI.getOperand(2).getReg());
    MI.getOperand(2).setReg(OffsetExt);
    return Legalized;
  }
  }
}

LegalizerHelper::LegalizeResult
LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT Ty) {
  using namespace TargetOpcode;
  MIRBuilder.setInstr(MI);

  switch(MI.getOpcode()) {
  default:
    return UnableToLegalize;
  case TargetOpcode::G_SREM:
  case TargetOpcode::G_UREM: {
    unsigned QuotReg = MRI.createGenericVirtualRegister(Ty);
    MIRBuilder.buildInstr(MI.getOpcode() == G_SREM ? G_SDIV : G_UDIV)
        .addDef(QuotReg)
        .addUse(MI.getOperand(1).getReg())
        .addUse(MI.getOperand(2).getReg());

    unsigned ProdReg = MRI.createGenericVirtualRegister(Ty);
    MIRBuilder.buildMul(ProdReg, QuotReg, MI.getOperand(2).getReg());
    MIRBuilder.buildSub(MI.getOperand(0).getReg(), MI.getOperand(1).getReg(),
                        ProdReg);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_SMULO:
  case TargetOpcode::G_UMULO: {
    // Generate G_UMULH/G_SMULH to check for overflow and a normal G_MUL for the
    // result.
    unsigned Res = MI.getOperand(0).getReg();
    unsigned Overflow = MI.getOperand(1).getReg();
    unsigned LHS = MI.getOperand(2).getReg();
    unsigned RHS = MI.getOperand(3).getReg();

    MIRBuilder.buildMul(Res, LHS, RHS);

    unsigned Opcode = MI.getOpcode() == TargetOpcode::G_SMULO
                          ? TargetOpcode::G_SMULH
                          : TargetOpcode::G_UMULH;

    unsigned HiPart = MRI.createGenericVirtualRegister(Ty);
    MIRBuilder.buildInstr(Opcode)
      .addDef(HiPart)
      .addUse(LHS)
      .addUse(RHS);

    unsigned Zero = MRI.createGenericVirtualRegister(Ty);
    MIRBuilder.buildConstant(Zero, 0);
    MIRBuilder.buildICmp(CmpInst::ICMP_NE, Overflow, HiPart, Zero);
    MI.eraseFromParent();
    return Legalized;
  }
  case TargetOpcode::G_FNEG: {
    // TODO: Handle vector types once we are able to
    // represent them.
    if (Ty.isVector())
      return UnableToLegalize;
    unsigned Res = MI.getOperand(0).getReg();
    Type *ZeroTy;
    LLVMContext &Ctx = MIRBuilder.getMF().getFunction()->getContext();
    switch (Ty.getSizeInBits()) {
    case 16:
      ZeroTy = Type::getHalfTy(Ctx);
      break;
    case 32:
      ZeroTy = Type::getFloatTy(Ctx);
      break;
    case 64:
      ZeroTy = Type::getDoubleTy(Ctx);
      break;
    default:
      llvm_unreachable("unexpected floating-point type");
    }
    ConstantFP &ZeroForNegation =
        *cast<ConstantFP>(ConstantFP::getZeroValueForNegation(ZeroTy));
    unsigned Zero = MRI.createGenericVirtualRegister(Ty);
    MIRBuilder.buildFConstant(Zero, ZeroForNegation);
    MIRBuilder.buildInstr(TargetOpcode::G_FSUB)
        .addDef(Res)
        .addUse(Zero)
        .addUse(MI.getOperand(1).getReg());
    MI.eraseFromParent();
    return Legalized;
  }
  }
}

LegalizerHelper::LegalizeResult
LegalizerHelper::fewerElementsVector(MachineInstr &MI, unsigned TypeIdx,
                                     LLT NarrowTy) {
  // FIXME: Don't know how to handle secondary types yet.
  if (TypeIdx != 0)
    return UnableToLegalize;
  switch (MI.getOpcode()) {
  default:
    return UnableToLegalize;
  case TargetOpcode::G_ADD: {
    unsigned NarrowSize = NarrowTy.getSizeInBits();
    unsigned DstReg = MI.getOperand(0).getReg();
    int NumParts = MRI.getType(DstReg).getSizeInBits() / NarrowSize;

    MIRBuilder.setInstr(MI);

    SmallVector<unsigned, 2> Src1Regs, Src2Regs, DstRegs;
    extractParts(MI.getOperand(1).getReg(), NarrowTy, NumParts, Src1Regs);
    extractParts(MI.getOperand(2).getReg(), NarrowTy, NumParts, Src2Regs);

    for (int i = 0; i < NumParts; ++i) {
      unsigned DstReg = MRI.createGenericVirtualRegister(NarrowTy);
      MIRBuilder.buildAdd(DstReg, Src1Regs[i], Src2Regs[i]);
      DstRegs.push_back(DstReg);
    }

    MIRBuilder.buildMerge(DstReg, DstRegs);
    MI.eraseFromParent();
    return Legalized;
  }
  }
}