xref: /llvm-project-15.0.7/llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp (revision 7df225df)

//===-- llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp - Call lowering -----===//
//
// 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
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file implements the lowering of LLVM calls to machine code calls for
/// GlobalISel.
///
//===----------------------------------------------------------------------===//

#include "AMDGPUCallLowering.h"
#include "AMDGPU.h"
#include "AMDGPUISelLowering.h"
#include "AMDGPUSubtarget.h"
#include "SIISelLowering.h"
#include "SIMachineFunctionInfo.h"
#include "SIRegisterInfo.h"
#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/CallingConvLower.h"
#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/Support/LowLevelTypeImpl.h"

using namespace llvm;

namespace {

struct OutgoingArgHandler : public CallLowering::ValueHandler {
  OutgoingArgHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
                     MachineInstrBuilder MIB, CCAssignFn *AssignFn)
      : ValueHandler(MIRBuilder, MRI, AssignFn), MIB(MIB) {}

  MachineInstrBuilder MIB;

  Register getStackAddress(uint64_t Size, int64_t Offset,
                           MachinePointerInfo &MPO) override {
    llvm_unreachable("not implemented");
  }

  void assignValueToAddress(Register ValVReg, Register Addr, uint64_t Size,
                            MachinePointerInfo &MPO, CCValAssign &VA) override {
    llvm_unreachable("not implemented");
  }

  void assignValueToReg(Register ValVReg, Register PhysReg,
                        CCValAssign &VA) override {
    MIB.addUse(PhysReg);
    MIRBuilder.buildCopy(PhysReg, ValVReg);
  }

  bool assignArg(unsigned ValNo, MVT ValVT, MVT LocVT,
                 CCValAssign::LocInfo LocInfo,
                 const CallLowering::ArgInfo &Info,
                 CCState &State) override {
    return AssignFn(ValNo, ValVT, LocVT, LocInfo, Info.Flags, State);
  }
};

struct IncomingArgHandler : public CallLowering::ValueHandler {
  uint64_t StackUsed = 0;

  IncomingArgHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
                     CCAssignFn *AssignFn)
    : ValueHandler(MIRBuilder, MRI, AssignFn) {}

  Register getStackAddress(uint64_t Size, int64_t Offset,
                           MachinePointerInfo &MPO) override {
    auto &MFI = MIRBuilder.getMF().getFrameInfo();
    int FI = MFI.CreateFixedObject(Size, Offset, true);
    MPO = MachinePointerInfo::getFixedStack(MIRBuilder.getMF(), FI);
    Register AddrReg = MRI.createGenericVirtualRegister(
      LLT::pointer(AMDGPUAS::PRIVATE_ADDRESS, 32));
    MIRBuilder.buildFrameIndex(AddrReg, FI);
    StackUsed = std::max(StackUsed, Size + Offset);
    return AddrReg;
  }

  void assignValueToReg(Register ValVReg, Register PhysReg,
                        CCValAssign &VA) override {
    markPhysRegUsed(PhysReg);

    if (VA.getLocVT().getSizeInBits() < 32) {
      // 16-bit types are reported as legal for 32-bit registers. We need to do
      // a 32-bit copy, and truncate to avoid the verifier complaining about it.
      auto Copy = MIRBuilder.buildCopy(LLT::scalar(32), PhysReg);
      MIRBuilder.buildTrunc(ValVReg, Copy);
      return;
    }

    switch (VA.getLocInfo()) {
    case CCValAssign::LocInfo::SExt:
    case CCValAssign::LocInfo::ZExt:
    case CCValAssign::LocInfo::AExt: {
      auto Copy = MIRBuilder.buildCopy(LLT{VA.getLocVT()}, PhysReg);
      MIRBuilder.buildTrunc(ValVReg, Copy);
      break;
    }
    default:
      MIRBuilder.buildCopy(ValVReg, PhysReg);
      break;
    }
  }

  void assignValueToAddress(Register ValVReg, Register Addr, uint64_t Size,
                            MachinePointerInfo &MPO, CCValAssign &VA) override {
    // FIXME: Get alignment
    auto MMO = MIRBuilder.getMF().getMachineMemOperand(
      MPO, MachineMemOperand::MOLoad | MachineMemOperand::MOInvariant, Size, 1);
    MIRBuilder.buildLoad(ValVReg, Addr, *MMO);
  }

  /// How the physical register gets marked varies between formal
  /// parameters (it's a basic-block live-in), and a call instruction
  /// (it's an implicit-def of the BL).
  virtual void markPhysRegUsed(unsigned PhysReg) = 0;

  // FIXME: What is the point of this being a callback?
  bool isArgumentHandler() const override { return true; }
};

struct FormalArgHandler : public IncomingArgHandler {
  FormalArgHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
                   CCAssignFn *AssignFn)
    : IncomingArgHandler(MIRBuilder, MRI, AssignFn) {}

  void markPhysRegUsed(unsigned PhysReg) override {
    MIRBuilder.getMBB().addLiveIn(PhysReg);
  }
};

}

AMDGPUCallLowering::AMDGPUCallLowering(const AMDGPUTargetLowering &TLI)
  : CallLowering(&TLI) {
}

void AMDGPUCallLowering::splitToValueTypes(
    const ArgInfo &OrigArg, SmallVectorImpl<ArgInfo> &SplitArgs,
    const DataLayout &DL, MachineRegisterInfo &MRI, CallingConv::ID CallConv,
    SplitArgTy PerformArgSplit) const {
  const SITargetLowering &TLI = *getTLI<SITargetLowering>();
  LLVMContext &Ctx = OrigArg.Ty->getContext();

  if (OrigArg.Ty->isVoidTy())
    return;

  SmallVector<EVT, 4> SplitVTs;
  ComputeValueVTs(TLI, DL, OrigArg.Ty, SplitVTs);

  assert(OrigArg.Regs.size() == SplitVTs.size());

  int SplitIdx = 0;
  for (EVT VT : SplitVTs) {
    unsigned NumParts = TLI.getNumRegistersForCallingConv(Ctx, CallConv, VT);
    Type *Ty = VT.getTypeForEVT(Ctx);



    if (NumParts == 1) {
      // No splitting to do, but we want to replace the original type (e.g. [1 x
      // double] -> double).
      SplitArgs.emplace_back(OrigArg.Regs[SplitIdx], Ty,
                             OrigArg.Flags, OrigArg.IsFixed);

      ++SplitIdx;
      continue;
    }

    LLT LLTy = getLLTForType(*Ty, DL);

    SmallVector<Register, 8> SplitRegs;

    EVT PartVT = TLI.getRegisterTypeForCallingConv(Ctx, CallConv, VT);
    Type *PartTy = PartVT.getTypeForEVT(Ctx);
    LLT PartLLT = getLLTForType(*PartTy, DL);

    // FIXME: Should we be reporting all of the part registers for a single
    // argument, and let handleAssignments take care of the repacking?
    for (unsigned i = 0; i < NumParts; ++i) {
      Register PartReg = MRI.createGenericVirtualRegister(PartLLT);
      SplitRegs.push_back(PartReg);
      SplitArgs.emplace_back(ArrayRef<Register>(PartReg), PartTy, OrigArg.Flags);
    }

    PerformArgSplit(SplitRegs, LLTy, PartLLT, SplitIdx);

    ++SplitIdx;
  }
}

bool AMDGPUCallLowering::lowerReturn(MachineIRBuilder &MIRBuilder,
                                     const Value *Val,
                                     ArrayRef<Register> VRegs) const {

  MachineFunction &MF = MIRBuilder.getMF();
  MachineRegisterInfo &MRI = MF.getRegInfo();
  SIMachineFunctionInfo *MFI = MF.getInfo<SIMachineFunctionInfo>();
  MFI->setIfReturnsVoid(!Val);

  if (!Val) {
    MIRBuilder.buildInstr(AMDGPU::S_ENDPGM).addImm(0);
    return true;
  }

  Register VReg = VRegs[0];

  const Function &F = MF.getFunction();
  auto &DL = F.getParent()->getDataLayout();
  if (!AMDGPU::isShader(F.getCallingConv()))
    return false;


  const AMDGPUTargetLowering &TLI = *getTLI<AMDGPUTargetLowering>();
  SmallVector<EVT, 4> SplitVTs;
  SmallVector<uint64_t, 4> Offsets;
  ArgInfo OrigArg{VReg, Val->getType()};
  setArgFlags(OrigArg, AttributeList::ReturnIndex, DL, F);
  ComputeValueVTs(TLI, DL, OrigArg.Ty, SplitVTs, &Offsets, 0);

  SmallVector<ArgInfo, 8> SplitArgs;
  CCAssignFn *AssignFn = CCAssignFnForReturn(F.getCallingConv(), false);
  for (unsigned i = 0, e = Offsets.size(); i != e; ++i) {
    Type *SplitTy = SplitVTs[i].getTypeForEVT(F.getContext());
    SplitArgs.push_back({VRegs[i], SplitTy, OrigArg.Flags, OrigArg.IsFixed});
  }
  auto RetInstr = MIRBuilder.buildInstrNoInsert(AMDGPU::SI_RETURN_TO_EPILOG);
  OutgoingArgHandler Handler(MIRBuilder, MRI, RetInstr, AssignFn);
  if (!handleAssignments(MIRBuilder, SplitArgs, Handler))
    return false;
  MIRBuilder.insertInstr(RetInstr);

  return true;
}

Register AMDGPUCallLowering::lowerParameterPtr(MachineIRBuilder &MIRBuilder,
                                               Type *ParamTy,
                                               uint64_t Offset) const {

  MachineFunction &MF = MIRBuilder.getMF();
  const SIMachineFunctionInfo *MFI = MF.getInfo<SIMachineFunctionInfo>();
  MachineRegisterInfo &MRI = MF.getRegInfo();
  const Function &F = MF.getFunction();
  const DataLayout &DL = F.getParent()->getDataLayout();
  PointerType *PtrTy = PointerType::get(ParamTy, AMDGPUAS::CONSTANT_ADDRESS);
  LLT PtrType = getLLTForType(*PtrTy, DL);
  Register DstReg = MRI.createGenericVirtualRegister(PtrType);
  Register KernArgSegmentPtr =
    MFI->getPreloadedReg(AMDGPUFunctionArgInfo::KERNARG_SEGMENT_PTR);
  Register KernArgSegmentVReg = MRI.getLiveInVirtReg(KernArgSegmentPtr);

  Register OffsetReg = MRI.createGenericVirtualRegister(LLT::scalar(64));
  MIRBuilder.buildConstant(OffsetReg, Offset);

  MIRBuilder.buildGEP(DstReg, KernArgSegmentVReg, OffsetReg);

  return DstReg;
}

void AMDGPUCallLowering::lowerParameter(MachineIRBuilder &MIRBuilder,
                                        Type *ParamTy, uint64_t Offset,
                                        unsigned Align,
                                        Register DstReg) const {
  MachineFunction &MF = MIRBuilder.getMF();
  const Function &F = MF.getFunction();
  const DataLayout &DL = F.getParent()->getDataLayout();
  PointerType *PtrTy = PointerType::get(ParamTy, AMDGPUAS::CONSTANT_ADDRESS);
  MachinePointerInfo PtrInfo(UndefValue::get(PtrTy));
  unsigned TypeSize = DL.getTypeStoreSize(ParamTy);
  Register PtrReg = lowerParameterPtr(MIRBuilder, ParamTy, Offset);

  MachineMemOperand *MMO =
      MF.getMachineMemOperand(PtrInfo, MachineMemOperand::MOLoad |
                                       MachineMemOperand::MODereferenceable |
                                       MachineMemOperand::MOInvariant,
                                       TypeSize, Align);

  MIRBuilder.buildLoad(DstReg, PtrReg, *MMO);
}

// Allocate special inputs passed in user SGPRs.
static void allocateHSAUserSGPRs(CCState &CCInfo,
                                 MachineIRBuilder &MIRBuilder,
                                 MachineFunction &MF,
                                 const SIRegisterInfo &TRI,
                                 SIMachineFunctionInfo &Info) {
  // FIXME: How should these inputs interact with inreg / custom SGPR inputs?
  if (Info.hasPrivateSegmentBuffer()) {
    unsigned PrivateSegmentBufferReg = Info.addPrivateSegmentBuffer(TRI);
    MF.addLiveIn(PrivateSegmentBufferReg, &AMDGPU::SGPR_128RegClass);
    CCInfo.AllocateReg(PrivateSegmentBufferReg);
  }

  if (Info.hasDispatchPtr()) {
    unsigned DispatchPtrReg = Info.addDispatchPtr(TRI);
    MF.addLiveIn(DispatchPtrReg, &AMDGPU::SGPR_64RegClass);
    CCInfo.AllocateReg(DispatchPtrReg);
  }

  if (Info.hasQueuePtr()) {
    unsigned QueuePtrReg = Info.addQueuePtr(TRI);
    MF.addLiveIn(QueuePtrReg, &AMDGPU::SGPR_64RegClass);
    CCInfo.AllocateReg(QueuePtrReg);
  }

  if (Info.hasKernargSegmentPtr()) {
    MachineRegisterInfo &MRI = MF.getRegInfo();
    Register InputPtrReg = Info.addKernargSegmentPtr(TRI);
    const LLT P4 = LLT::pointer(AMDGPUAS::CONSTANT_ADDRESS, 64);
    Register VReg = MRI.createGenericVirtualRegister(P4);
    MRI.addLiveIn(InputPtrReg, VReg);
    MIRBuilder.getMBB().addLiveIn(InputPtrReg);
    MIRBuilder.buildCopy(VReg, InputPtrReg);
    CCInfo.AllocateReg(InputPtrReg);
  }

  if (Info.hasDispatchID()) {
    unsigned DispatchIDReg = Info.addDispatchID(TRI);
    MF.addLiveIn(DispatchIDReg, &AMDGPU::SGPR_64RegClass);
    CCInfo.AllocateReg(DispatchIDReg);
  }

  if (Info.hasFlatScratchInit()) {
    unsigned FlatScratchInitReg = Info.addFlatScratchInit(TRI);
    MF.addLiveIn(FlatScratchInitReg, &AMDGPU::SGPR_64RegClass);
    CCInfo.AllocateReg(FlatScratchInitReg);
  }

  // TODO: Add GridWorkGroupCount user SGPRs when used. For now with HSA we read
  // these from the dispatch pointer.
}

bool AMDGPUCallLowering::lowerFormalArgumentsKernel(
    MachineIRBuilder &MIRBuilder, const Function &F,
    ArrayRef<ArrayRef<Register>> VRegs) const {
  MachineFunction &MF = MIRBuilder.getMF();
  const GCNSubtarget *Subtarget = &MF.getSubtarget<GCNSubtarget>();
  MachineRegisterInfo &MRI = MF.getRegInfo();
  SIMachineFunctionInfo *Info = MF.getInfo<SIMachineFunctionInfo>();
  const SIRegisterInfo *TRI = Subtarget->getRegisterInfo();
  const SITargetLowering &TLI = *getTLI<SITargetLowering>();

  const DataLayout &DL = F.getParent()->getDataLayout();

  SmallVector<CCValAssign, 16> ArgLocs;
  CCState CCInfo(F.getCallingConv(), F.isVarArg(), MF, ArgLocs, F.getContext());

  allocateHSAUserSGPRs(CCInfo, MIRBuilder, MF, *TRI, *Info);

  unsigned i = 0;
  const unsigned KernArgBaseAlign = 16;
  const unsigned BaseOffset = Subtarget->getExplicitKernelArgOffset(F);
  uint64_t ExplicitArgOffset = 0;

  // TODO: Align down to dword alignment and extract bits for extending loads.
  for (auto &Arg : F.args()) {
    Type *ArgTy = Arg.getType();
    unsigned AllocSize = DL.getTypeAllocSize(ArgTy);
    if (AllocSize == 0)
      continue;

    unsigned ABIAlign = DL.getABITypeAlignment(ArgTy);

    uint64_t ArgOffset = alignTo(ExplicitArgOffset, ABIAlign) + BaseOffset;
    ExplicitArgOffset = alignTo(ExplicitArgOffset, ABIAlign) + AllocSize;

    ArrayRef<Register> OrigArgRegs = VRegs[i];
    Register ArgReg =
      OrigArgRegs.size() == 1
      ? OrigArgRegs[0]
      : MRI.createGenericVirtualRegister(getLLTForType(*ArgTy, DL));
    unsigned Align = MinAlign(KernArgBaseAlign, ArgOffset);
    ArgOffset = alignTo(ArgOffset, DL.getABITypeAlignment(ArgTy));
    lowerParameter(MIRBuilder, ArgTy, ArgOffset, Align, ArgReg);
    if (OrigArgRegs.size() > 1)
      unpackRegs(OrigArgRegs, ArgReg, ArgTy, MIRBuilder);
    ++i;
  }

  TLI.allocateSpecialEntryInputVGPRs(CCInfo, MF, *TRI, *Info);
  TLI.allocateSystemSGPRs(CCInfo, MF, *Info, F.getCallingConv(), false);
  return true;
}

static void packSplitRegsToOrigType(MachineIRBuilder &MIRBuilder,
                                    ArrayRef<Register> OrigRegs,
                                    ArrayRef<Register> Regs,
                                    LLT LLTy,
                                    LLT PartLLT) {
  if (!LLTy.isVector() && !PartLLT.isVector()) {
    MIRBuilder.buildMerge(OrigRegs[0], Regs);
    return;
  }

  if (LLTy.isVector() && PartLLT.isVector()) {
    assert(LLTy.getElementType() == PartLLT.getElementType());

    int DstElts = LLTy.getNumElements();
    int PartElts = PartLLT.getNumElements();
    if (DstElts % PartElts == 0)
      MIRBuilder.buildConcatVectors(OrigRegs[0], Regs);
    else {
      // Deal with v3s16 split into v2s16
      assert(PartElts == 2 && DstElts % 2 != 0);
      int RoundedElts = PartElts * ((DstElts + PartElts - 1) / PartElts);

      LLT RoundedDestTy = LLT::vector(RoundedElts, PartLLT.getElementType());
      auto RoundedConcat = MIRBuilder.buildConcatVectors(RoundedDestTy, Regs);
      MIRBuilder.buildExtract(OrigRegs[0], RoundedConcat, 0);
    }

    return;
  }

  assert(LLTy.isVector() && !PartLLT.isVector());

  LLT DstEltTy = LLTy.getElementType();
  if (DstEltTy == PartLLT) {
    // Vector was trivially scalarized.
    MIRBuilder.buildBuildVector(OrigRegs[0], Regs);
  } else if (DstEltTy.getSizeInBits() > PartLLT.getSizeInBits()) {
    // Deal with vector with 64-bit elements decomposed to 32-bit
    // registers. Need to create intermediate 64-bit elements.
    SmallVector<Register, 8> EltMerges;
    int PartsPerElt = DstEltTy.getSizeInBits() / PartLLT.getSizeInBits();

    assert(DstEltTy.getSizeInBits() % PartLLT.getSizeInBits() == 0);

    for (int I = 0, NumElts = LLTy.getNumElements(); I != NumElts; ++I)  {
      auto Merge = MIRBuilder.buildMerge(DstEltTy,
                                         Regs.take_front(PartsPerElt));
      EltMerges.push_back(Merge.getReg(0));
      Regs = Regs.drop_front(PartsPerElt);
    }

    MIRBuilder.buildBuildVector(OrigRegs[0], EltMerges);
  } else {
    // Vector was split, and elements promoted to a wider type.
    LLT BVType = LLT::vector(LLTy.getNumElements(), PartLLT);
    auto BV = MIRBuilder.buildBuildVector(BVType, Regs);
    MIRBuilder.buildTrunc(OrigRegs[0], BV);
  }
}

bool AMDGPUCallLowering::lowerFormalArguments(
    MachineIRBuilder &MIRBuilder, const Function &F,
    ArrayRef<ArrayRef<Register>> VRegs) const {
  CallingConv::ID CC = F.getCallingConv();

  // The infrastructure for normal calling convention lowering is essentially
  // useless for kernels. We want to avoid any kind of legalization or argument
  // splitting.
  if (CC == CallingConv::AMDGPU_KERNEL)
    return lowerFormalArgumentsKernel(MIRBuilder, F, VRegs);

  // AMDGPU_GS and AMDGP_HS are not supported yet.
  if (CC == CallingConv::AMDGPU_GS || CC == CallingConv::AMDGPU_HS)
    return false;

  const bool IsShader = AMDGPU::isShader(CC);
  const bool IsEntryFunc = AMDGPU::isEntryFunctionCC(CC);

  MachineFunction &MF = MIRBuilder.getMF();
  MachineBasicBlock &MBB = MIRBuilder.getMBB();
  MachineRegisterInfo &MRI = MF.getRegInfo();
  SIMachineFunctionInfo *Info = MF.getInfo<SIMachineFunctionInfo>();
  const GCNSubtarget &Subtarget = MF.getSubtarget<GCNSubtarget>();
  const SIRegisterInfo *TRI = Subtarget.getRegisterInfo();
  const DataLayout &DL = F.getParent()->getDataLayout();


  SmallVector<CCValAssign, 16> ArgLocs;
  CCState CCInfo(CC, F.isVarArg(), MF, ArgLocs, F.getContext());

  if (Info->hasImplicitBufferPtr()) {
    Register ImplicitBufferPtrReg = Info->addImplicitBufferPtr(*TRI);
    MF.addLiveIn(ImplicitBufferPtrReg, &AMDGPU::SGPR_64RegClass);
    CCInfo.AllocateReg(ImplicitBufferPtrReg);
  }


  SmallVector<ArgInfo, 32> SplitArgs;
  unsigned Idx = 0;
  unsigned PSInputNum = 0;

  for (auto &Arg : F.args()) {
    if (DL.getTypeStoreSize(Arg.getType()) == 0)
      continue;

    const bool InReg = Arg.hasAttribute(Attribute::InReg);

    // SGPR arguments to functions not implemented.
    if (!IsShader && InReg)
      return false;

    // TODO: Handle sret.
    if (Arg.hasAttribute(Attribute::StructRet) ||
        Arg.hasAttribute(Attribute::SwiftSelf) ||
        Arg.hasAttribute(Attribute::SwiftError) ||
        Arg.hasAttribute(Attribute::Nest))
      return false;

    if (CC == CallingConv::AMDGPU_PS && !InReg && PSInputNum <= 15) {
      const bool ArgUsed = !Arg.use_empty();
      bool SkipArg = !ArgUsed && !Info->isPSInputAllocated(PSInputNum);

      if (!SkipArg) {
        Info->markPSInputAllocated(PSInputNum);
        if (ArgUsed)
          Info->markPSInputEnabled(PSInputNum);
      }

      ++PSInputNum;

      if (SkipArg) {
        for (int I = 0, E = VRegs[Idx].size(); I != E; ++I)
          MIRBuilder.buildUndef(VRegs[Idx][I]);

        ++Idx;
        continue;
      }
    }

    ArgInfo OrigArg(VRegs[Idx], Arg.getType());
    setArgFlags(OrigArg, Idx + AttributeList::FirstArgIndex, DL, F);

    splitToValueTypes(
      OrigArg, SplitArgs, DL, MRI, CC,
      // FIXME: We should probably be passing multiple registers to
      // handleAssignments to do this
      [&](ArrayRef<Register> Regs, LLT LLTy, LLT PartLLT, int VTSplitIdx) {
        packSplitRegsToOrigType(MIRBuilder, VRegs[Idx][VTSplitIdx], Regs,
                                LLTy, PartLLT);
      });

    ++Idx;
  }

  // At least one interpolation mode must be enabled or else the GPU will
  // hang.
  //
  // Check PSInputAddr instead of PSInputEnable. The idea is that if the user
  // set PSInputAddr, the user wants to enable some bits after the compilation
  // based on run-time states. Since we can't know what the final PSInputEna
  // will look like, so we shouldn't do anything here and the user should take
  // responsibility for the correct programming.
  //
  // Otherwise, the following restrictions apply:
  // - At least one of PERSP_* (0xF) or LINEAR_* (0x70) must be enabled.
  // - If POS_W_FLOAT (11) is enabled, at least one of PERSP_* must be
  //   enabled too.
  if (CC == CallingConv::AMDGPU_PS) {
    if ((Info->getPSInputAddr() & 0x7F) == 0 ||
        ((Info->getPSInputAddr() & 0xF) == 0 &&
         Info->isPSInputAllocated(11))) {
      CCInfo.AllocateReg(AMDGPU::VGPR0);
      CCInfo.AllocateReg(AMDGPU::VGPR1);
      Info->markPSInputAllocated(0);
      Info->markPSInputEnabled(0);
    }

    if (Subtarget.isAmdPalOS()) {
      // For isAmdPalOS, the user does not enable some bits after compilation
      // based on run-time states; the register values being generated here are
      // the final ones set in hardware. Therefore we need to apply the
      // workaround to PSInputAddr and PSInputEnable together.  (The case where
      // a bit is set in PSInputAddr but not PSInputEnable is where the frontend
      // set up an input arg for a particular interpolation mode, but nothing
      // uses that input arg. Really we should have an earlier pass that removes
      // such an arg.)
      unsigned PsInputBits = Info->getPSInputAddr() & Info->getPSInputEnable();
      if ((PsInputBits & 0x7F) == 0 ||
          ((PsInputBits & 0xF) == 0 &&
           (PsInputBits >> 11 & 1)))
        Info->markPSInputEnabled(
          countTrailingZeros(Info->getPSInputAddr(), ZB_Undefined));
    }
  }

  const SITargetLowering &TLI = *getTLI<SITargetLowering>();
  CCAssignFn *AssignFn = TLI.CCAssignFnForCall(CC, F.isVarArg());

  if (!MBB.empty())
    MIRBuilder.setInstr(*MBB.begin());

  FormalArgHandler Handler(MIRBuilder, MRI, AssignFn);
  if (!handleAssignments(CCInfo, ArgLocs, MIRBuilder, SplitArgs, Handler))
    return false;

  if (!IsEntryFunc) {
    // Special inputs come after user arguments.
    TLI.allocateSpecialInputVGPRs(CCInfo, MF, *TRI, *Info);
  }

  // Start adding system SGPRs.
  if (IsEntryFunc) {
    TLI.allocateSystemSGPRs(CCInfo, MF, *Info, CC, IsShader);
  } else {
    CCInfo.AllocateReg(Info->getScratchRSrcReg());
    CCInfo.AllocateReg(Info->getScratchWaveOffsetReg());
    CCInfo.AllocateReg(Info->getFrameOffsetReg());
    TLI.allocateSpecialInputSGPRs(CCInfo, MF, *TRI, *Info);
  }

  // Move back to the end of the basic block.
  MIRBuilder.setMBB(MBB);

  return true;
}