Target/ARM/ARMBaseRegisterInfo.cpp

//===-- ARMBaseRegisterInfo.cpp - ARM Register Information ----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains the base ARM implementation of TargetRegisterInfo class.
//
//===----------------------------------------------------------------------===//

#include "ARMBaseRegisterInfo.h"
#include "ARM.h"
#include "ARMBaseInstrInfo.h"
#include "ARMFrameLowering.h"
#include "ARMMachineFunctionInfo.h"
#include "ARMSubtarget.h"
#include "MCTargetDesc/ARMAddressingModes.h"
#include "MCTargetDesc/ARMBaseInfo.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/VirtRegMap.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Type.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include <cassert>
#include <utility>

#define DEBUG_TYPE "arm-register-info"

#define GET_REGINFO_TARGET_DESC
#include "ARMGenRegisterInfo.inc"

using namespace llvm;

ARMBaseRegisterInfo::ARMBaseRegisterInfo()
    : ARMGenRegisterInfo(ARM::LR, 0, 0, ARM::PC) {}

static unsigned getFramePointerReg(const ARMSubtarget &STI) {
  return STI.useR7AsFramePointer() ? ARM::R7 : ARM::R11;
}

const MCPhysReg*
ARMBaseRegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
  const ARMSubtarget &STI = MF->getSubtarget<ARMSubtarget>();
  bool UseSplitPush = STI.splitFramePushPop(*MF);
  const MCPhysReg *RegList =
      STI.isTargetDarwin()
          ? CSR_iOS_SaveList
          : (UseSplitPush ? CSR_AAPCS_SplitPush_SaveList : CSR_AAPCS_SaveList);

  const Function &F = MF->getFunction();
  if (F.getCallingConv() == CallingConv::GHC) {
    // GHC set of callee saved regs is empty as all those regs are
    // used for passing STG regs around
    return CSR_NoRegs_SaveList;
  } else if (F.hasFnAttribute("interrupt")) {
    if (STI.isMClass()) {
      // M-class CPUs have hardware which saves the registers needed to allow a
      // function conforming to the AAPCS to function as a handler.
      return UseSplitPush ? CSR_AAPCS_SplitPush_SaveList : CSR_AAPCS_SaveList;
    } else if (F.getFnAttribute("interrupt").getValueAsString() == "FIQ") {
      // Fast interrupt mode gives the handler a private copy of R8-R14, so less
      // need to be saved to restore user-mode state.
      return CSR_FIQ_SaveList;
    } else {
      // Generally only R13-R14 (i.e. SP, LR) are automatically preserved by
      // exception handling.
      return CSR_GenericInt_SaveList;
    }
  }

  if (STI.getTargetLowering()->supportSwiftError() &&
      F.getAttributes().hasAttrSomewhere(Attribute::SwiftError)) {
    if (STI.isTargetDarwin())
      return CSR_iOS_SwiftError_SaveList;

    return UseSplitPush ? CSR_AAPCS_SplitPush_SwiftError_SaveList :
      CSR_AAPCS_SwiftError_SaveList;
  }

  if (STI.isTargetDarwin() && F.getCallingConv() == CallingConv::CXX_FAST_TLS)
    return MF->getInfo<ARMFunctionInfo>()->isSplitCSR()
               ? CSR_iOS_CXX_TLS_PE_SaveList
               : CSR_iOS_CXX_TLS_SaveList;
  return RegList;
}

const MCPhysReg *ARMBaseRegisterInfo::getCalleeSavedRegsViaCopy(
    const MachineFunction *MF) const {
  assert(MF && "Invalid MachineFunction pointer.");
  if (MF->getFunction().getCallingConv() == CallingConv::CXX_FAST_TLS &&
      MF->getInfo<ARMFunctionInfo>()->isSplitCSR())
    return CSR_iOS_CXX_TLS_ViaCopy_SaveList;
  return nullptr;
}

const uint32_t *
ARMBaseRegisterInfo::getCallPreservedMask(const MachineFunction &MF,
                                          CallingConv::ID CC) const {
  const ARMSubtarget &STI = MF.getSubtarget<ARMSubtarget>();
  if (CC == CallingConv::GHC)
    // This is academic because all GHC calls are (supposed to be) tail calls
    return CSR_NoRegs_RegMask;

  if (STI.getTargetLowering()->supportSwiftError() &&
      MF.getFunction().getAttributes().hasAttrSomewhere(Attribute::SwiftError))
    return STI.isTargetDarwin() ? CSR_iOS_SwiftError_RegMask
                                : CSR_AAPCS_SwiftError_RegMask;

  if (STI.isTargetDarwin() && CC == CallingConv::CXX_FAST_TLS)
    return CSR_iOS_CXX_TLS_RegMask;
  return STI.isTargetDarwin() ? CSR_iOS_RegMask : CSR_AAPCS_RegMask;
}

const uint32_t*
ARMBaseRegisterInfo::getNoPreservedMask() const {
  return CSR_NoRegs_RegMask;
}

const uint32_t *
ARMBaseRegisterInfo::getTLSCallPreservedMask(const MachineFunction &MF) const {
  assert(MF.getSubtarget<ARMSubtarget>().isTargetDarwin() &&
         "only know about special TLS call on Darwin");
  return CSR_iOS_TLSCall_RegMask;
}

const uint32_t *
ARMBaseRegisterInfo::getSjLjDispatchPreservedMask(const MachineFunction &MF) const {
  const ARMSubtarget &STI = MF.getSubtarget<ARMSubtarget>();
  if (!STI.useSoftFloat() && STI.hasVFP2Base() && !STI.isThumb1Only())
    return CSR_NoRegs_RegMask;
  else
    return CSR_FPRegs_RegMask;
}

const uint32_t *
ARMBaseRegisterInfo::getThisReturnPreservedMask(const MachineFunction &MF,
                                                CallingConv::ID CC) const {
  const ARMSubtarget &STI = MF.getSubtarget<ARMSubtarget>();
  // This should return a register mask that is the same as that returned by
  // getCallPreservedMask but that additionally preserves the register used for
  // the first i32 argument (which must also be the register used to return a
  // single i32 return value)
  //
  // In case that the calling convention does not use the same register for
  // both or otherwise does not want to enable this optimization, the function
  // should return NULL
  if (CC == CallingConv::GHC)
    // This is academic because all GHC calls are (supposed to be) tail calls
    return nullptr;
  return STI.isTargetDarwin() ? CSR_iOS_ThisReturn_RegMask
                              : CSR_AAPCS_ThisReturn_RegMask;
}

BitVector ARMBaseRegisterInfo::
getReservedRegs(const MachineFunction &MF) const {
  const ARMSubtarget &STI = MF.getSubtarget<ARMSubtarget>();
  const ARMFrameLowering *TFI = getFrameLowering(MF);

  // FIXME: avoid re-calculating this every time.
  BitVector Reserved(getNumRegs());
  markSuperRegs(Reserved, ARM::SP);
  markSuperRegs(Reserved, ARM::PC);
  markSuperRegs(Reserved, ARM::FPSCR);
  markSuperRegs(Reserved, ARM::APSR_NZCV);
  if (TFI->hasFP(MF))
    markSuperRegs(Reserved, getFramePointerReg(STI));
  if (hasBasePointer(MF))
    markSuperRegs(Reserved, BasePtr);
  // Some targets reserve R9.
  if (STI.isR9Reserved())
    markSuperRegs(Reserved, ARM::R9);
  // Reserve D16-D31 if the subtarget doesn't support them.
  if (!STI.hasD32()) {
    static_assert(ARM::D31 == ARM::D16 + 15, "Register list not consecutive!");
    for (unsigned R = 0; R < 16; ++R)
      markSuperRegs(Reserved, ARM::D16 + R);
  }
  const TargetRegisterClass &RC = ARM::GPRPairRegClass;
  for (unsigned Reg : RC)
    for (MCSubRegIterator SI(Reg, this); SI.isValid(); ++SI)
      if (Reserved.test(*SI))
        markSuperRegs(Reserved, Reg);
  // For v8.1m architecture
  markSuperRegs(Reserved, ARM::ZR);

  assert(checkAllSuperRegsMarked(Reserved));
  return Reserved;
}

bool ARMBaseRegisterInfo::
isAsmClobberable(const MachineFunction &MF, unsigned PhysReg) const {
  return !getReservedRegs(MF).test(PhysReg);
}

const TargetRegisterClass *
ARMBaseRegisterInfo::getLargestLegalSuperClass(const TargetRegisterClass *RC,
                                               const MachineFunction &) const {
  const TargetRegisterClass *Super = RC;
  TargetRegisterClass::sc_iterator I = RC->getSuperClasses();
  do {
    switch (Super->getID()) {
    case ARM::GPRRegClassID:
    case ARM::SPRRegClassID:
    case ARM::DPRRegClassID:
    case ARM::QPRRegClassID:
    case ARM::QQPRRegClassID:
    case ARM::QQQQPRRegClassID:
    case ARM::GPRPairRegClassID:
      return Super;
    }
    Super = *I++;
  } while (Super);
  return RC;
}

const TargetRegisterClass *
ARMBaseRegisterInfo::getPointerRegClass(const MachineFunction &MF, unsigned Kind)
                                                                         const {
  return &ARM::GPRRegClass;
}

const TargetRegisterClass *
ARMBaseRegisterInfo::getCrossCopyRegClass(const TargetRegisterClass *RC) const {
  if (RC == &ARM::CCRRegClass)
    return &ARM::rGPRRegClass;  // Can't copy CCR registers.
  return RC;
}

unsigned
ARMBaseRegisterInfo::getRegPressureLimit(const TargetRegisterClass *RC,
                                         MachineFunction &MF) const {
  const ARMSubtarget &STI = MF.getSubtarget<ARMSubtarget>();
  const ARMFrameLowering *TFI = getFrameLowering(MF);

  switch (RC->getID()) {
  default:
    return 0;
  case ARM::tGPRRegClassID: {
    // hasFP ends up calling getMaxCallFrameComputed() which may not be
    // available when getPressureLimit() is called as part of
    // ScheduleDAGRRList.
    bool HasFP = MF.getFrameInfo().isMaxCallFrameSizeComputed()
                 ? TFI->hasFP(MF) : true;
    return 5 - HasFP;
  }
  case ARM::GPRRegClassID: {
    bool HasFP = MF.getFrameInfo().isMaxCallFrameSizeComputed()
                 ? TFI->hasFP(MF) : true;
    return 10 - HasFP - (STI.isR9Reserved() ? 1 : 0);
  }
  case ARM::SPRRegClassID:  // Currently not used as 'rep' register class.
  case ARM::DPRRegClassID:
    return 32 - 10;
  }
}

// Get the other register in a GPRPair.
static unsigned getPairedGPR(unsigned Reg, bool Odd, const MCRegisterInfo *RI) {
  for (MCSuperRegIterator Supers(Reg, RI); Supers.isValid(); ++Supers)
    if (ARM::GPRPairRegClass.contains(*Supers))
      return RI->getSubReg(*Supers, Odd ? ARM::gsub_1 : ARM::gsub_0);
  return 0;
}

// Resolve the RegPairEven / RegPairOdd register allocator hints.
bool
ARMBaseRegisterInfo::getRegAllocationHints(unsigned VirtReg,
                                           ArrayRef<MCPhysReg> Order,
                                           SmallVectorImpl<MCPhysReg> &Hints,
                                           const MachineFunction &MF,
                                           const VirtRegMap *VRM,
                                           const LiveRegMatrix *Matrix) const {
  const MachineRegisterInfo &MRI = MF.getRegInfo();
  std::pair<unsigned, unsigned> Hint = MRI.getRegAllocationHint(VirtReg);

  unsigned Odd;
  switch (Hint.first) {
  case ARMRI::RegPairEven:
    Odd = 0;
    break;
  case ARMRI::RegPairOdd:
    Odd = 1;
    break;
  default:
    TargetRegisterInfo::getRegAllocationHints(VirtReg, Order, Hints, MF, VRM);
    return false;
  }

  // This register should preferably be even (Odd == 0) or odd (Odd == 1).
  // Check if the other part of the pair has already been assigned, and provide
  // the paired register as the first hint.
  unsigned Paired = Hint.second;
  if (Paired == 0)
    return false;

  unsigned PairedPhys = 0;
  if (TargetRegisterInfo::isPhysicalRegister(Paired)) {
    PairedPhys = Paired;
  } else if (VRM && VRM->hasPhys(Paired)) {
    PairedPhys = getPairedGPR(VRM->getPhys(Paired), Odd, this);
  }

  // First prefer the paired physreg.
  if (PairedPhys && is_contained(Order, PairedPhys))
    Hints.push_back(PairedPhys);

  // Then prefer even or odd registers.
  for (unsigned Reg : Order) {
    if (Reg == PairedPhys || (getEncodingValue(Reg) & 1) != Odd)
      continue;
    // Don't provide hints that are paired to a reserved register.
    unsigned Paired = getPairedGPR(Reg, !Odd, this);
    if (!Paired || MRI.isReserved(Paired))
      continue;
    Hints.push_back(Reg);
  }
  return false;
}

void
ARMBaseRegisterInfo::updateRegAllocHint(unsigned Reg, unsigned NewReg,
                                        MachineFunction &MF) const {
  MachineRegisterInfo *MRI = &MF.getRegInfo();
  std::pair<unsigned, unsigned> Hint = MRI->getRegAllocationHint(Reg);
  if ((Hint.first == (unsigned)ARMRI::RegPairOdd ||
       Hint.first == (unsigned)ARMRI::RegPairEven) &&
      TargetRegisterInfo::isVirtualRegister(Hint.second)) {
    // If 'Reg' is one of the even / odd register pair and it's now changed
    // (e.g. coalesced) into a different register. The other register of the
    // pair allocation hint must be updated to reflect the relationship
    // change.
    unsigned OtherReg = Hint.second;
    Hint = MRI->getRegAllocationHint(OtherReg);
    // Make sure the pair has not already divorced.
    if (Hint.second == Reg) {
      MRI->setRegAllocationHint(OtherReg, Hint.first, NewReg);
      if (TargetRegisterInfo::isVirtualRegister(NewReg))
        MRI->setRegAllocationHint(NewReg,
            Hint.first == (unsigned)ARMRI::RegPairOdd ? ARMRI::RegPairEven
            : ARMRI::RegPairOdd, OtherReg);
    }
  }
}

bool ARMBaseRegisterInfo::hasBasePointer(const MachineFunction &MF) const {
  const MachineFrameInfo &MFI = MF.getFrameInfo();
  const ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
  const ARMFrameLowering *TFI = getFrameLowering(MF);

  // If we have stack realignment and VLAs, we have no pointer to use to
  // access the stack. If we have stack realignment, and a large call frame,
  // we have no place to allocate the emergency spill slot.
  if (needsStackRealignment(MF) && !TFI->hasReservedCallFrame(MF))
    return true;

  // Thumb has trouble with negative offsets from the FP. Thumb2 has a limited
  // negative range for ldr/str (255), and thumb1 is positive offsets only.
  //
  // It's going to be better to use the SP or Base Pointer instead. When there
  // are variable sized objects, we can't reference off of the SP, so we
  // reserve a Base Pointer.
  //
  // For Thumb2, estimate whether a negative offset from the frame pointer
  // will be sufficient to reach the whole stack frame. If a function has a
  // smallish frame, it's less likely to have lots of spills and callee saved
  // space, so it's all more likely to be within range of the frame pointer.
  // If it's wrong, the scavenger will still enable access to work, it just
  // won't be optimal.  (We should always be able to reach the emergency
  // spill slot from the frame pointer.)
  if (AFI->isThumb2Function() && MFI.hasVarSizedObjects() &&
      MFI.getLocalFrameSize() >= 128)
    return true;
  // For Thumb1, if sp moves, nothing is in range, so force a base pointer.
  // This is necessary for correctness in cases where we need an emergency
  // spill slot. (In Thumb1, we can't use a negative offset from the frame
  // pointer.)
  if (AFI->isThumb1OnlyFunction() && !TFI->hasReservedCallFrame(MF))
    return true;
  return false;
}

bool ARMBaseRegisterInfo::canRealignStack(const MachineFunction &MF) const {
  const MachineRegisterInfo *MRI = &MF.getRegInfo();
  const ARMFrameLowering *TFI = getFrameLowering(MF);
  // We can't realign the stack if:
  // 1. Dynamic stack realignment is explicitly disabled,
  // 2. There are VLAs in the function and the base pointer is disabled.
  if (!TargetRegisterInfo::canRealignStack(MF))
    return false;
  // Stack realignment requires a frame pointer.  If we already started
  // register allocation with frame pointer elimination, it is too late now.
  if (!MRI->canReserveReg(getFramePointerReg(MF.getSubtarget<ARMSubtarget>())))
    return false;
  // We may also need a base pointer if there are dynamic allocas or stack
  // pointer adjustments around calls.
  if (TFI->hasReservedCallFrame(MF))
    return true;
  // A base pointer is required and allowed.  Check that it isn't too late to
  // reserve it.
  return MRI->canReserveReg(BasePtr);
}

bool ARMBaseRegisterInfo::
cannotEliminateFrame(const MachineFunction &MF) const {
  const MachineFrameInfo &MFI = MF.getFrameInfo();
  if (MF.getTarget().Options.DisableFramePointerElim(MF) && MFI.adjustsStack())
    return true;
  return MFI.hasVarSizedObjects() || MFI.isFrameAddressTaken()
    || needsStackRealignment(MF);
}

Register
ARMBaseRegisterInfo::getFrameRegister(const MachineFunction &MF) const {
  const ARMSubtarget &STI = MF.getSubtarget<ARMSubtarget>();
  const ARMFrameLowering *TFI = getFrameLowering(MF);

  if (TFI->hasFP(MF))
    return getFramePointerReg(STI);
  return ARM::SP;
}

/// emitLoadConstPool - Emits a load from constpool to materialize the
/// specified immediate.
void ARMBaseRegisterInfo::emitLoadConstPool(
    MachineBasicBlock &MBB, MachineBasicBlock::iterator &MBBI,
    const DebugLoc &dl, unsigned DestReg, unsigned SubIdx, int Val,
    ARMCC::CondCodes Pred, unsigned PredReg, unsigned MIFlags) const {
  MachineFunction &MF = *MBB.getParent();
  const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
  MachineConstantPool *ConstantPool = MF.getConstantPool();
  const Constant *C =
        ConstantInt::get(Type::getInt32Ty(MF.getFunction().getContext()), Val);
  unsigned Idx = ConstantPool->getConstantPoolIndex(C, 4);

  BuildMI(MBB, MBBI, dl, TII.get(ARM::LDRcp))
      .addReg(DestReg, getDefRegState(true), SubIdx)
      .addConstantPoolIndex(Idx)
      .addImm(0)
      .add(predOps(Pred, PredReg))
      .setMIFlags(MIFlags);
}

bool ARMBaseRegisterInfo::
requiresRegisterScavenging(const MachineFunction &MF) const {
  return true;
}

bool ARMBaseRegisterInfo::
trackLivenessAfterRegAlloc(const MachineFunction &MF) const {
  return true;
}

bool ARMBaseRegisterInfo::
requiresFrameIndexScavenging(const MachineFunction &MF) const {
  return true;
}

bool ARMBaseRegisterInfo::
requiresVirtualBaseRegisters(const MachineFunction &MF) const {
  return true;
}

int64_t ARMBaseRegisterInfo::
getFrameIndexInstrOffset(const MachineInstr *MI, int Idx) const {
  const MCInstrDesc &Desc = MI->getDesc();
  unsigned AddrMode = (Desc.TSFlags & ARMII::AddrModeMask);
  int64_t InstrOffs = 0;
  int Scale = 1;
  unsigned ImmIdx = 0;
  switch (AddrMode) {
  case ARMII::AddrModeT2_i8:
  case ARMII::AddrModeT2_i12:
  case ARMII::AddrMode_i12:
    InstrOffs = MI->getOperand(Idx+1).getImm();
    Scale = 1;
    break;
  case ARMII::AddrMode5: {
    // VFP address mode.
    const MachineOperand &OffOp = MI->getOperand(Idx+1);
    InstrOffs = ARM_AM::getAM5Offset(OffOp.getImm());
    if (ARM_AM::getAM5Op(OffOp.getImm()) == ARM_AM::sub)
      InstrOffs = -InstrOffs;
    Scale = 4;
    break;
  }
  case ARMII::AddrMode2:
    ImmIdx = Idx+2;
    InstrOffs = ARM_AM::getAM2Offset(MI->getOperand(ImmIdx).getImm());
    if (ARM_AM::getAM2Op(MI->getOperand(ImmIdx).getImm()) == ARM_AM::sub)
      InstrOffs = -InstrOffs;
    break;
  case ARMII::AddrMode3:
    ImmIdx = Idx+2;
    InstrOffs = ARM_AM::getAM3Offset(MI->getOperand(ImmIdx).getImm());
    if (ARM_AM::getAM3Op(MI->getOperand(ImmIdx).getImm()) == ARM_AM::sub)
      InstrOffs = -InstrOffs;
    break;
  case ARMII::AddrModeT1_s:
    ImmIdx = Idx+1;
    InstrOffs = MI->getOperand(ImmIdx).getImm();
    Scale = 4;
    break;
  default:
    llvm_unreachable("Unsupported addressing mode!");
  }

  return InstrOffs * Scale;
}

/// needsFrameBaseReg - Returns true if the instruction's frame index
/// reference would be better served by a base register other than FP
/// or SP. Used by LocalStackFrameAllocation to determine which frame index
/// references it should create new base registers for.
bool ARMBaseRegisterInfo::
needsFrameBaseReg(MachineInstr *MI, int64_t Offset) const {
  for (unsigned i = 0; !MI->getOperand(i).isFI(); ++i) {
    assert(i < MI->getNumOperands() &&"Instr doesn't have FrameIndex operand!");
  }

  // It's the load/store FI references that cause issues, as it can be difficult
  // to materialize the offset if it won't fit in the literal field. Estimate
  // based on the size of the local frame and some conservative assumptions
  // about the rest of the stack frame (note, this is pre-regalloc, so
  // we don't know everything for certain yet) whether this offset is likely
  // to be out of range of the immediate. Return true if so.

  // We only generate virtual base registers for loads and stores, so
  // return false for everything else.
  unsigned Opc = MI->getOpcode();
  switch (Opc) {
  case ARM::LDRi12: case ARM::LDRH: case ARM::LDRBi12:
  case ARM::STRi12: case ARM::STRH: case ARM::STRBi12:
  case ARM::t2LDRi12: case ARM::t2LDRi8:
  case ARM::t2STRi12: case ARM::t2STRi8:
  case ARM::VLDRS: case ARM::VLDRD:
  case ARM::VSTRS: case ARM::VSTRD:
  case ARM::tSTRspi: case ARM::tLDRspi:
    break;
  default:
    return false;
  }

  // Without a virtual base register, if the function has variable sized
  // objects, all fixed-size local references will be via the frame pointer,
  // Approximate the offset and see if it's legal for the instruction.
  // Note that the incoming offset is based on the SP value at function entry,
  // so it'll be negative.
  MachineFunction &MF = *MI->getParent()->getParent();
  const ARMFrameLowering *TFI = getFrameLowering(MF);
  MachineFrameInfo &MFI = MF.getFrameInfo();
  ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();

  // Estimate an offset from the frame pointer.
  // Conservatively assume all callee-saved registers get pushed. R4-R6
  // will be earlier than the FP, so we ignore those.
  // R7, LR
  int64_t FPOffset = Offset - 8;
  // ARM and Thumb2 functions also need to consider R8-R11 and D8-D15
  if (!AFI->isThumbFunction() || !AFI->isThumb1OnlyFunction())
    FPOffset -= 80;
  // Estimate an offset from the stack pointer.
  // The incoming offset is relating to the SP at the start of the function,
  // but when we access the local it'll be relative to the SP after local
  // allocation, so adjust our SP-relative offset by that allocation size.
  Offset += MFI.getLocalFrameSize();
  // Assume that we'll have at least some spill slots allocated.
  // FIXME: This is a total SWAG number. We should run some statistics
  //        and pick a real one.
  Offset += 128; // 128 bytes of spill slots

  // If there's a frame pointer and the addressing mode allows it, try using it.
  // The FP is only available if there is no dynamic realignment. We
  // don't know for sure yet whether we'll need that, so we guess based
  // on whether there are any local variables that would trigger it.
  unsigned StackAlign = TFI->getStackAlignment();
  if (TFI->hasFP(MF) &&
      !((MFI.getLocalFrameMaxAlign() > StackAlign) && canRealignStack(MF))) {
    if (isFrameOffsetLegal(MI, getFrameRegister(MF), FPOffset))
      return false;
  }
  // If we can reference via the stack pointer, try that.
  // FIXME: This (and the code that resolves the references) can be improved
  //        to only disallow SP relative references in the live range of
  //        the VLA(s). In practice, it's unclear how much difference that
  //        would make, but it may be worth doing.
  if (!MFI.hasVarSizedObjects() && isFrameOffsetLegal(MI, ARM::SP, Offset))
    return false;

  // The offset likely isn't legal, we want to allocate a virtual base register.
  return true;
}

/// materializeFrameBaseRegister - Insert defining instruction(s) for BaseReg to
/// be a pointer to FrameIdx at the beginning of the basic block.
void ARMBaseRegisterInfo::
materializeFrameBaseRegister(MachineBasicBlock *MBB,
                             unsigned BaseReg, int FrameIdx,
                             int64_t Offset) const {
  ARMFunctionInfo *AFI = MBB->getParent()->getInfo<ARMFunctionInfo>();
  unsigned ADDriOpc = !AFI->isThumbFunction() ? ARM::ADDri :
    (AFI->isThumb1OnlyFunction() ? ARM::tADDframe : ARM::t2ADDri);

  MachineBasicBlock::iterator Ins = MBB->begin();
  DebugLoc DL;                  // Defaults to "unknown"
  if (Ins != MBB->end())
    DL = Ins->getDebugLoc();

  const MachineFunction &MF = *MBB->getParent();
  MachineRegisterInfo &MRI = MBB->getParent()->getRegInfo();
  const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
  const MCInstrDesc &MCID = TII.get(ADDriOpc);
  MRI.constrainRegClass(BaseReg, TII.getRegClass(MCID, 0, this, MF));

  MachineInstrBuilder MIB = BuildMI(*MBB, Ins, DL, MCID, BaseReg)
    .addFrameIndex(FrameIdx).addImm(Offset);

  if (!AFI->isThumb1OnlyFunction())
    MIB.add(predOps(ARMCC::AL)).add(condCodeOp());
}

void ARMBaseRegisterInfo::resolveFrameIndex(MachineInstr &MI, unsigned BaseReg,
                                            int64_t Offset) const {
  MachineBasicBlock &MBB = *MI.getParent();
  MachineFunction &MF = *MBB.getParent();
  const ARMBaseInstrInfo &TII =
      *static_cast<const ARMBaseInstrInfo *>(MF.getSubtarget().getInstrInfo());
  ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
  int Off = Offset; // ARM doesn't need the general 64-bit offsets
  unsigned i = 0;

  assert(!AFI->isThumb1OnlyFunction() &&
         "This resolveFrameIndex does not support Thumb1!");

  while (!MI.getOperand(i).isFI()) {
    ++i;
    assert(i < MI.getNumOperands() && "Instr doesn't have FrameIndex operand!");
  }
  bool Done = false;
  if (!AFI->isThumbFunction())
    Done = rewriteARMFrameIndex(MI, i, BaseReg, Off, TII);
  else {
    assert(AFI->isThumb2Function());
    Done = rewriteT2FrameIndex(MI, i, BaseReg, Off, TII);
  }
  assert(Done && "Unable to resolve frame index!");
  (void)Done;
}

bool ARMBaseRegisterInfo::isFrameOffsetLegal(const MachineInstr *MI, unsigned BaseReg,
                                             int64_t Offset) const {
  const MCInstrDesc &Desc = MI->getDesc();
  unsigned AddrMode = (Desc.TSFlags & ARMII::AddrModeMask);
  unsigned i = 0;
  for (; !MI->getOperand(i).isFI(); ++i)
    assert(i+1 < MI->getNumOperands() && "Instr doesn't have FrameIndex operand!");

  // AddrMode4 and AddrMode6 cannot handle any offset.
  if (AddrMode == ARMII::AddrMode4 || AddrMode == ARMII::AddrMode6)
    return Offset == 0;

  unsigned NumBits = 0;
  unsigned Scale = 1;
  bool isSigned = true;
  switch (AddrMode) {
  case ARMII::AddrModeT2_i8:
  case ARMII::AddrModeT2_i12:
    // i8 supports only negative, and i12 supports only positive, so
    // based on Offset sign, consider the appropriate instruction
    Scale = 1;
    if (Offset < 0) {
      NumBits = 8;
      Offset = -Offset;
    } else {
      NumBits = 12;
    }
    break;
  case ARMII::AddrMode5:
    // VFP address mode.
    NumBits = 8;
    Scale = 4;
    break;
  case ARMII::AddrMode_i12:
  case ARMII::AddrMode2:
    NumBits = 12;
    break;
  case ARMII::AddrMode3:
    NumBits = 8;
    break;
  case ARMII::AddrModeT1_s:
    NumBits = (BaseReg == ARM::SP ? 8 : 5);
    Scale = 4;
    isSigned = false;
    break;
  default:
    llvm_unreachable("Unsupported addressing mode!");
  }

  Offset += getFrameIndexInstrOffset(MI, i);
  // Make sure the offset is encodable for instructions that scale the
  // immediate.
  if ((Offset & (Scale-1)) != 0)
    return false;

  if (isSigned && Offset < 0)
    Offset = -Offset;

  unsigned Mask = (1 << NumBits) - 1;
  if ((unsigned)Offset <= Mask * Scale)
    return true;

  return false;
}

void
ARMBaseRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
                                         int SPAdj, unsigned FIOperandNum,
                                         RegScavenger *RS) const {
  MachineInstr &MI = *II;
  MachineBasicBlock &MBB = *MI.getParent();
  MachineFunction &MF = *MBB.getParent();
  const ARMBaseInstrInfo &TII =
      *static_cast<const ARMBaseInstrInfo *>(MF.getSubtarget().getInstrInfo());
  const ARMFrameLowering *TFI = getFrameLowering(MF);
  ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
  assert(!AFI->isThumb1OnlyFunction() &&
         "This eliminateFrameIndex does not support Thumb1!");
  int FrameIndex = MI.getOperand(FIOperandNum).getIndex();
  unsigned FrameReg;

  int Offset = TFI->ResolveFrameIndexReference(MF, FrameIndex, FrameReg, SPAdj);

  // PEI::scavengeFrameVirtualRegs() cannot accurately track SPAdj because the
  // call frame setup/destroy instructions have already been eliminated.  That
  // means the stack pointer cannot be used to access the emergency spill slot
  // when !hasReservedCallFrame().
#ifndef NDEBUG
  if (RS && FrameReg == ARM::SP && RS->isScavengingFrameIndex(FrameIndex)){
    assert(TFI->hasReservedCallFrame(MF) &&
           "Cannot use SP to access the emergency spill slot in "
           "functions without a reserved call frame");
    assert(!MF.getFrameInfo().hasVarSizedObjects() &&
           "Cannot use SP to access the emergency spill slot in "
           "functions with variable sized frame objects");
  }
#endif // NDEBUG

  assert(!MI.isDebugValue() && "DBG_VALUEs should be handled in target-independent code");

  // Modify MI as necessary to handle as much of 'Offset' as possible
  bool Done = false;
  if (!AFI->isThumbFunction())
    Done = rewriteARMFrameIndex(MI, FIOperandNum, FrameReg, Offset, TII);
  else {
    assert(AFI->isThumb2Function());
    Done = rewriteT2FrameIndex(MI, FIOperandNum, FrameReg, Offset, TII);
  }
  if (Done)
    return;

  // If we get here, the immediate doesn't fit into the instruction.  We folded
  // as much as possible above, handle the rest, providing a register that is
  // SP+LargeImm.
  assert((Offset ||
          (MI.getDesc().TSFlags & ARMII::AddrModeMask) == ARMII::AddrMode4 ||
          (MI.getDesc().TSFlags & ARMII::AddrModeMask) == ARMII::AddrMode6) &&
         "This code isn't needed if offset already handled!");

  unsigned ScratchReg = 0;
  int PIdx = MI.findFirstPredOperandIdx();
  ARMCC::CondCodes Pred = (PIdx == -1)
    ? ARMCC::AL : (ARMCC::CondCodes)MI.getOperand(PIdx).getImm();
  Register PredReg = (PIdx == -1) ? Register() : MI.getOperand(PIdx+1).getReg();
  if (Offset == 0)
    // Must be addrmode4/6.
    MI.getOperand(FIOperandNum).ChangeToRegister(FrameReg, false, false, false);
  else {
    ScratchReg = MF.getRegInfo().createVirtualRegister(&ARM::GPRRegClass);
    if (!AFI->isThumbFunction())
      emitARMRegPlusImmediate(MBB, II, MI.getDebugLoc(), ScratchReg, FrameReg,
                              Offset, Pred, PredReg, TII);
    else {
      assert(AFI->isThumb2Function());
      emitT2RegPlusImmediate(MBB, II, MI.getDebugLoc(), ScratchReg, FrameReg,
                             Offset, Pred, PredReg, TII);
    }
    // Update the original instruction to use the scratch register.
    MI.getOperand(FIOperandNum).ChangeToRegister(ScratchReg, false, false,true);
  }
}

bool ARMBaseRegisterInfo::shouldCoalesce(MachineInstr *MI,
                                  const TargetRegisterClass *SrcRC,
                                  unsigned SubReg,
                                  const TargetRegisterClass *DstRC,
                                  unsigned DstSubReg,
                                  const TargetRegisterClass *NewRC,
                                  LiveIntervals &LIS) const {
  auto MBB = MI->getParent();
  auto MF = MBB->getParent();
  const MachineRegisterInfo &MRI = MF->getRegInfo();
  // If not copying into a sub-register this should be ok because we shouldn't
  // need to split the reg.
  if (!DstSubReg)
    return true;
  // Small registers don't frequently cause a problem, so we can coalesce them.
  if (getRegSizeInBits(*NewRC) < 256 && getRegSizeInBits(*DstRC) < 256 &&
      getRegSizeInBits(*SrcRC) < 256)
    return true;

  auto NewRCWeight =
              MRI.getTargetRegisterInfo()->getRegClassWeight(NewRC);
  auto SrcRCWeight =
              MRI.getTargetRegisterInfo()->getRegClassWeight(SrcRC);
  auto DstRCWeight =
              MRI.getTargetRegisterInfo()->getRegClassWeight(DstRC);
  // If the source register class is more expensive than the destination, the
  // coalescing is probably profitable.
  if (SrcRCWeight.RegWeight > NewRCWeight.RegWeight)
    return true;
  if (DstRCWeight.RegWeight > NewRCWeight.RegWeight)
    return true;

  // If the register allocator isn't constrained, we can always allow coalescing
  // unfortunately we don't know yet if we will be constrained.
  // The goal of this heuristic is to restrict how many expensive registers
  // we allow to coalesce in a given basic block.
  auto AFI = MF->getInfo<ARMFunctionInfo>();
  auto It = AFI->getCoalescedWeight(MBB);

  LLVM_DEBUG(dbgs() << "\tARM::shouldCoalesce - Coalesced Weight: "
                    << It->second << "\n");
  LLVM_DEBUG(dbgs() << "\tARM::shouldCoalesce - Reg Weight: "
                    << NewRCWeight.RegWeight << "\n");

  // This number is the largest round number that which meets the criteria:
  //  (1) addresses PR18825
  //  (2) generates better code in some test cases (like vldm-shed-a9.ll)
  //  (3) Doesn't regress any test cases (in-tree, test-suite, and SPEC)
  // In practice the SizeMultiplier will only factor in for straight line code
  // that uses a lot of NEON vectors, which isn't terribly common.
  unsigned SizeMultiplier = MBB->size()/100;
  SizeMultiplier = SizeMultiplier ? SizeMultiplier : 1;
  if (It->second < NewRCWeight.WeightLimit * SizeMultiplier) {
    It->second += NewRCWeight.RegWeight;
    return true;
  }
  return false;
}