Target/AMDGPU/SIWholeQuadMode.cpp

//===-- SIWholeQuadMode.cpp - enter and suspend whole quad mode -----------===//
//
// 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 pass adds instructions to enable whole quad mode for pixel
/// shaders, and whole wavefront mode for all programs.
///
/// Whole quad mode is required for derivative computations, but it interferes
/// with shader side effects (stores and atomics). This pass is run on the
/// scheduled machine IR but before register coalescing, so that machine SSA is
/// available for analysis. It ensures that WQM is enabled when necessary, but
/// disabled around stores and atomics.
///
/// When necessary, this pass creates a function prolog
///
///   S_MOV_B64 LiveMask, EXEC
///   S_WQM_B64 EXEC, EXEC
///
/// to enter WQM at the top of the function and surrounds blocks of Exact
/// instructions by
///
///   S_AND_SAVEEXEC_B64 Tmp, LiveMask
///   ...
///   S_MOV_B64 EXEC, Tmp
///
/// We also compute when a sequence of instructions requires Whole Wavefront
/// Mode (WWM) and insert instructions to save and restore it:
///
/// S_OR_SAVEEXEC_B64 Tmp, -1
/// ...
/// S_MOV_B64 EXEC, Tmp
///
/// In order to avoid excessive switching during sequences of Exact
/// instructions, the pass first analyzes which instructions must be run in WQM
/// (aka which instructions produce values that lead to derivative
/// computations).
///
/// Basic blocks are always exited in WQM as long as some successor needs WQM.
///
/// There is room for improvement given better control flow analysis:
///
///  (1) at the top level (outside of control flow statements, and as long as
///      kill hasn't been used), one SGPR can be saved by recovering WQM from
///      the LiveMask (this is implemented for the entry block).
///
///  (2) when entire regions (e.g. if-else blocks or entire loops) only
///      consist of exact and don't-care instructions, the switch only has to
///      be done at the entry and exit points rather than potentially in each
///      block of the region.
///
//===----------------------------------------------------------------------===//

#include "AMDGPU.h"
#include "AMDGPUSubtarget.h"
#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
#include "SIInstrInfo.h"
#include "SIMachineFunctionInfo.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/IR/CallingConv.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <vector>

using namespace llvm;

#define DEBUG_TYPE "si-wqm"

namespace {

enum {
  StateWQM = 0x1,
  StateWWM = 0x2,
  StateExact = 0x4,
};

struct PrintState {
public:
  int State;

  explicit PrintState(int State) : State(State) {}
};

#ifndef NDEBUG
static raw_ostream &operator<<(raw_ostream &OS, const PrintState &PS) {
  if (PS.State & StateWQM)
    OS << "WQM";
  if (PS.State & StateWWM) {
    if (PS.State & StateWQM)
      OS << '|';
    OS << "WWM";
  }
  if (PS.State & StateExact) {
    if (PS.State & (StateWQM | StateWWM))
      OS << '|';
    OS << "Exact";
  }

  return OS;
}
#endif

struct InstrInfo {
  char Needs = 0;
  char Disabled = 0;
  char OutNeeds = 0;
};

struct BlockInfo {
  char Needs = 0;
  char InNeeds = 0;
  char OutNeeds = 0;
};

struct WorkItem {
  MachineBasicBlock *MBB = nullptr;
  MachineInstr *MI = nullptr;

  WorkItem() = default;
  WorkItem(MachineBasicBlock *MBB) : MBB(MBB) {}
  WorkItem(MachineInstr *MI) : MI(MI) {}
};

class SIWholeQuadMode : public MachineFunctionPass {
private:
  CallingConv::ID CallingConv;
  const SIInstrInfo *TII;
  const SIRegisterInfo *TRI;
  const GCNSubtarget *ST;
  MachineRegisterInfo *MRI;
  LiveIntervals *LIS;

  unsigned AndOpc;
  unsigned XorTermrOpc;
  unsigned OrSaveExecOpc;
  unsigned Exec;

  DenseMap<const MachineInstr *, InstrInfo> Instructions;
  MapVector<MachineBasicBlock *, BlockInfo> Blocks;
  SmallVector<MachineInstr *, 1> LiveMaskQueries;
  SmallVector<MachineInstr *, 4> LowerToMovInstrs;
  SmallVector<MachineInstr *, 4> LowerToCopyInstrs;

  void printInfo();

  void markInstruction(MachineInstr &MI, char Flag,
                       std::vector<WorkItem> &Worklist);
  void markDefs(const MachineInstr &UseMI, LiveRange &LR, Register Reg,
                unsigned SubReg, char Flag, std::vector<WorkItem> &Worklist);
  void markInstructionUses(const MachineInstr &MI, char Flag,
                           std::vector<WorkItem> &Worklist);
  char scanInstructions(MachineFunction &MF, std::vector<WorkItem> &Worklist);
  void propagateInstruction(MachineInstr &MI, std::vector<WorkItem> &Worklist);
  void propagateBlock(MachineBasicBlock &MBB, std::vector<WorkItem> &Worklist);
  char analyzeFunction(MachineFunction &MF);

  MachineBasicBlock::iterator saveSCC(MachineBasicBlock &MBB,
                                      MachineBasicBlock::iterator Before);
  MachineBasicBlock::iterator
  prepareInsertion(MachineBasicBlock &MBB, MachineBasicBlock::iterator First,
                   MachineBasicBlock::iterator Last, bool PreferLast,
                   bool SaveSCC);
  void toExact(MachineBasicBlock &MBB, MachineBasicBlock::iterator Before,
               unsigned SaveWQM, unsigned LiveMaskReg);
  void toWQM(MachineBasicBlock &MBB, MachineBasicBlock::iterator Before,
             unsigned SavedWQM);
  void toWWM(MachineBasicBlock &MBB, MachineBasicBlock::iterator Before,
             unsigned SaveOrig);
  void fromWWM(MachineBasicBlock &MBB, MachineBasicBlock::iterator Before,
               unsigned SavedOrig);
  void processBlock(MachineBasicBlock &MBB, unsigned LiveMaskReg, bool isEntry);

  void lowerLiveMaskQueries(unsigned LiveMaskReg);
  void lowerCopyInstrs();

public:
  static char ID;

  SIWholeQuadMode() :
    MachineFunctionPass(ID) { }

  bool runOnMachineFunction(MachineFunction &MF) override;

  StringRef getPassName() const override { return "SI Whole Quad Mode"; }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<LiveIntervals>();
    AU.addPreserved<SlotIndexes>();
    AU.addPreserved<LiveIntervals>();
    AU.setPreservesCFG();
    MachineFunctionPass::getAnalysisUsage(AU);
  }
};

} // end anonymous namespace

char SIWholeQuadMode::ID = 0;

INITIALIZE_PASS_BEGIN(SIWholeQuadMode, DEBUG_TYPE, "SI Whole Quad Mode", false,
                      false)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_END(SIWholeQuadMode, DEBUG_TYPE, "SI Whole Quad Mode", false,
                    false)

char &llvm::SIWholeQuadModeID = SIWholeQuadMode::ID;

FunctionPass *llvm::createSIWholeQuadModePass() {
  return new SIWholeQuadMode;
}

#ifndef NDEBUG
LLVM_DUMP_METHOD void SIWholeQuadMode::printInfo() {
  for (const auto &BII : Blocks) {
    dbgs() << "\n"
           << printMBBReference(*BII.first) << ":\n"
           << "  InNeeds = " << PrintState(BII.second.InNeeds)
           << ", Needs = " << PrintState(BII.second.Needs)
           << ", OutNeeds = " << PrintState(BII.second.OutNeeds) << "\n\n";

    for (const MachineInstr &MI : *BII.first) {
      auto III = Instructions.find(&MI);
      if (III == Instructions.end())
        continue;

      dbgs() << "  " << MI << "    Needs = " << PrintState(III->second.Needs)
             << ", OutNeeds = " << PrintState(III->second.OutNeeds) << '\n';
    }
  }
}
#endif

void SIWholeQuadMode::markInstruction(MachineInstr &MI, char Flag,
                                      std::vector<WorkItem> &Worklist) {
  InstrInfo &II = Instructions[&MI];

  assert(!(Flag & StateExact) && Flag != 0);

  LLVM_DEBUG(dbgs() << "markInstruction " << PrintState(Flag) << ": " << MI);

  // Remove any disabled states from the flag. The user that required it gets
  // an undefined value in the helper lanes. For example, this can happen if
  // the result of an atomic is used by instruction that requires WQM, where
  // ignoring the request for WQM is correct as per the relevant specs.
  Flag &= ~II.Disabled;

  // Ignore if the flag is already encompassed by the existing needs, or we
  // just disabled everything.
  if ((II.Needs & Flag) == Flag)
    return;

  II.Needs |= Flag;
  Worklist.push_back(&MI);
}

/// Mark all relevant definitions of register \p Reg in usage \p UseMI.
void SIWholeQuadMode::markDefs(const MachineInstr &UseMI, LiveRange &LR,
                               Register Reg, unsigned SubReg, char Flag,
                               std::vector<WorkItem> &Worklist) {
  assert(!MRI->isSSA());

  LLVM_DEBUG(dbgs() << "markDefs " << PrintState(Flag) << ": " << UseMI);

  LiveQueryResult UseLRQ = LR.Query(LIS->getInstructionIndex(UseMI));
  if (!UseLRQ.valueIn())
    return;

  SmallPtrSet<const VNInfo *, 4> Visited;
  SmallVector<const VNInfo *, 4> ToProcess;
  ToProcess.push_back(UseLRQ.valueIn());
  do {
    const VNInfo *Value = ToProcess.pop_back_val();
    Visited.insert(Value);

    if (Value->isPHIDef()) {
      // Need to mark all defs used in the PHI node
      const MachineBasicBlock *MBB = LIS->getMBBFromIndex(Value->def);
      assert(MBB && "Phi-def has no defining MBB");
      for (MachineBasicBlock::const_pred_iterator PI = MBB->pred_begin(),
                                                  PE = MBB->pred_end();
           PI != PE; ++PI) {
        if (const VNInfo *VN = LR.getVNInfoBefore(LIS->getMBBEndIdx(*PI))) {
          if (!Visited.count(VN))
            ToProcess.push_back(VN);
        }
      }
    } else {
      MachineInstr *MI = LIS->getInstructionFromIndex(Value->def);
      assert(MI && "Def has no defining instruction");
      markInstruction(*MI, Flag, Worklist);

      // Iterate over all operands to find relevant definitions
      for (const MachineOperand &Op : MI->operands()) {
        if (!(Op.isReg() && Op.getReg() == Reg))
          continue;

        // Does this def cover whole register?
        bool DefinesFullReg =
            Op.isUndef() || !Op.getSubReg() || Op.getSubReg() == SubReg;
        if (!DefinesFullReg) {
          // Partial definition; need to follow and mark input value
          LiveQueryResult LRQ = LR.Query(LIS->getInstructionIndex(*MI));
          if (const VNInfo *VN = LRQ.valueIn()) {
            if (!Visited.count(VN))
              ToProcess.push_back(VN);
          }
        }
      }
    }
  } while (!ToProcess.empty());
}

/// Mark all instructions defining the uses in \p MI with \p Flag.
void SIWholeQuadMode::markInstructionUses(const MachineInstr &MI, char Flag,
                                          std::vector<WorkItem> &Worklist) {

  LLVM_DEBUG(dbgs() << "markInstructionUses " << PrintState(Flag) << ": "
                    << MI);

  for (const MachineOperand &Use : MI.uses()) {
    if (!Use.isReg() || !Use.isUse())
      continue;

    Register Reg = Use.getReg();

    // Handle physical registers that we need to track; this is mostly relevant
    // for VCC, which can appear as the (implicit) input of a uniform branch,
    // e.g. when a loop counter is stored in a VGPR.
    if (!Reg.isVirtual()) {
      if (Reg == AMDGPU::EXEC || Reg == AMDGPU::EXEC_LO)
        continue;

      for (MCRegUnitIterator RegUnit(Reg, TRI); RegUnit.isValid(); ++RegUnit) {
        LiveRange &LR = LIS->getRegUnit(*RegUnit);
        const VNInfo *Value = LR.Query(LIS->getInstructionIndex(MI)).valueIn();
        if (!Value)
          continue;

        if (MRI->isSSA()) {
          // Since we're in machine SSA, we do not need to track physical
          // registers across basic blocks.
          if (Value->isPHIDef())
            continue;
          markInstruction(*LIS->getInstructionFromIndex(Value->def), Flag,
                          Worklist);
        } else {
          markDefs(MI, LR, *RegUnit, AMDGPU::NoSubRegister, Flag, Worklist);
        }
      }

      continue;
    }

    if (MRI->isSSA()) {
      for (MachineInstr &DefMI : MRI->def_instructions(Use.getReg()))
        markInstruction(DefMI, Flag, Worklist);
    } else {
      LiveRange &LR = LIS->getInterval(Reg);
      markDefs(MI, LR, Reg, Use.getSubReg(), Flag, Worklist);
    }
  }
}

// Scan instructions to determine which ones require an Exact execmask and
// which ones seed WQM requirements.
char SIWholeQuadMode::scanInstructions(MachineFunction &MF,
                                       std::vector<WorkItem> &Worklist) {
  char GlobalFlags = 0;
  bool WQMOutputs = MF.getFunction().hasFnAttribute("amdgpu-ps-wqm-outputs");
  SmallVector<MachineInstr *, 4> SetInactiveInstrs;
  SmallVector<MachineInstr *, 4> SoftWQMInstrs;

  // We need to visit the basic blocks in reverse post-order so that we visit
  // defs before uses, in particular so that we don't accidentally mark an
  // instruction as needing e.g. WQM before visiting it and realizing it needs
  // WQM disabled.
  ReversePostOrderTraversal<MachineFunction *> RPOT(&MF);
  for (auto BI = RPOT.begin(), BE = RPOT.end(); BI != BE; ++BI) {
    MachineBasicBlock &MBB = **BI;
    BlockInfo &BBI = Blocks[&MBB];

    for (auto II = MBB.begin(), IE = MBB.end(); II != IE; ++II) {
      MachineInstr &MI = *II;
      InstrInfo &III = Instructions[&MI];
      unsigned Opcode = MI.getOpcode();
      char Flags = 0;

      if (TII->isWQM(Opcode)) {
        // Sampling instructions don't need to produce results for all pixels
        // in a quad, they just require all inputs of a quad to have been
        // computed for derivatives.
        markInstructionUses(MI, StateWQM, Worklist);
        GlobalFlags |= StateWQM;
        continue;
      } else if (Opcode == AMDGPU::WQM) {
        // The WQM intrinsic requires its output to have all the helper lanes
        // correct, so we need it to be in WQM.
        Flags = StateWQM;
        LowerToCopyInstrs.push_back(&MI);
      } else if (Opcode == AMDGPU::SOFT_WQM) {
        LowerToCopyInstrs.push_back(&MI);
        SoftWQMInstrs.push_back(&MI);
        continue;
      } else if (Opcode == AMDGPU::WWM) {
        // The WWM intrinsic doesn't make the same guarantee, and plus it needs
        // to be executed in WQM or Exact so that its copy doesn't clobber
        // inactive lanes.
        markInstructionUses(MI, StateWWM, Worklist);
        GlobalFlags |= StateWWM;
        LowerToMovInstrs.push_back(&MI);
        continue;
      } else if (Opcode == AMDGPU::V_SET_INACTIVE_B32 ||
                 Opcode == AMDGPU::V_SET_INACTIVE_B64) {
        III.Disabled = StateWWM;
        MachineOperand &Inactive = MI.getOperand(2);
        if (Inactive.isReg()) {
          if (Inactive.isUndef()) {
            LowerToCopyInstrs.push_back(&MI);
          } else {
            Register Reg = Inactive.getReg();
            if (Reg.isVirtual()) {
              for (MachineInstr &DefMI : MRI->def_instructions(Reg))
                markInstruction(DefMI, StateWWM, Worklist);
            }
          }
        }
        SetInactiveInstrs.push_back(&MI);
        continue;
      } else if (TII->isDisableWQM(MI)) {
        BBI.Needs |= StateExact;
        if (!(BBI.InNeeds & StateExact)) {
          BBI.InNeeds |= StateExact;
          Worklist.push_back(&MBB);
        }
        GlobalFlags |= StateExact;
        III.Disabled = StateWQM | StateWWM;
        continue;
      } else {
        if (Opcode == AMDGPU::SI_PS_LIVE) {
          LiveMaskQueries.push_back(&MI);
        } else if (WQMOutputs) {
          // The function is in machine SSA form, which means that physical
          // VGPRs correspond to shader inputs and outputs. Inputs are
          // only used, outputs are only defined.
          for (const MachineOperand &MO : MI.defs()) {
            if (!MO.isReg())
              continue;

            Register Reg = MO.getReg();

            if (!Reg.isVirtual() &&
                TRI->hasVectorRegisters(TRI->getPhysRegClass(Reg))) {
              Flags = StateWQM;
              break;
            }
          }
        }

        if (!Flags)
          continue;
      }

      markInstruction(MI, Flags, Worklist);
      GlobalFlags |= Flags;
    }
  }

  // Mark sure that any SET_INACTIVE instructions are computed in WQM if WQM is
  // ever used anywhere in the function. This implements the corresponding
  // semantics of @llvm.amdgcn.set.inactive.
  // Similarly for SOFT_WQM instructions, implementing @llvm.amdgcn.softwqm.
  if (GlobalFlags & StateWQM) {
    for (MachineInstr *MI : SetInactiveInstrs)
      markInstruction(*MI, StateWQM, Worklist);
    for (MachineInstr *MI : SoftWQMInstrs)
      markInstruction(*MI, StateWQM, Worklist);
  }

  return GlobalFlags;
}

void SIWholeQuadMode::propagateInstruction(MachineInstr &MI,
                                           std::vector<WorkItem>& Worklist) {
  MachineBasicBlock *MBB = MI.getParent();
  InstrInfo II = Instructions[&MI]; // take a copy to prevent dangling references
  BlockInfo &BI = Blocks[MBB];

  // Control flow-type instructions and stores to temporary memory that are
  // followed by WQM computations must themselves be in WQM.
  if ((II.OutNeeds & StateWQM) && !(II.Disabled & StateWQM) &&
      (MI.isTerminator() || (TII->usesVM_CNT(MI) && MI.mayStore()))) {
    Instructions[&MI].Needs = StateWQM;
    II.Needs = StateWQM;
  }

  // Propagate to block level
  if (II.Needs & StateWQM) {
    BI.Needs |= StateWQM;
    if (!(BI.InNeeds & StateWQM)) {
      BI.InNeeds |= StateWQM;
      Worklist.push_back(MBB);
    }
  }

  // Propagate backwards within block
  if (MachineInstr *PrevMI = MI.getPrevNode()) {
    char InNeeds = (II.Needs & ~StateWWM) | II.OutNeeds;
    if (!PrevMI->isPHI()) {
      InstrInfo &PrevII = Instructions[PrevMI];
      if ((PrevII.OutNeeds | InNeeds) != PrevII.OutNeeds) {
        PrevII.OutNeeds |= InNeeds;
        Worklist.push_back(PrevMI);
      }
    }
  }

  // Propagate WQM flag to instruction inputs
  assert(!(II.Needs & StateExact));

  if (II.Needs != 0)
    markInstructionUses(MI, II.Needs, Worklist);

  // Ensure we process a block containing WWM, even if it does not require any
  // WQM transitions.
  if (II.Needs & StateWWM)
    BI.Needs |= StateWWM;
}

void SIWholeQuadMode::propagateBlock(MachineBasicBlock &MBB,
                                     std::vector<WorkItem>& Worklist) {
  BlockInfo BI = Blocks[&MBB]; // Make a copy to prevent dangling references.

  // Propagate through instructions
  if (!MBB.empty()) {
    MachineInstr *LastMI = &*MBB.rbegin();
    InstrInfo &LastII = Instructions[LastMI];
    if ((LastII.OutNeeds | BI.OutNeeds) != LastII.OutNeeds) {
      LastII.OutNeeds |= BI.OutNeeds;
      Worklist.push_back(LastMI);
    }
  }

  // Predecessor blocks must provide for our WQM/Exact needs.
  for (MachineBasicBlock *Pred : MBB.predecessors()) {
    BlockInfo &PredBI = Blocks[Pred];
    if ((PredBI.OutNeeds | BI.InNeeds) == PredBI.OutNeeds)
      continue;

    PredBI.OutNeeds |= BI.InNeeds;
    PredBI.InNeeds |= BI.InNeeds;
    Worklist.push_back(Pred);
  }

  // All successors must be prepared to accept the same set of WQM/Exact data.
  for (MachineBasicBlock *Succ : MBB.successors()) {
    BlockInfo &SuccBI = Blocks[Succ];
    if ((SuccBI.InNeeds | BI.OutNeeds) == SuccBI.InNeeds)
      continue;

    SuccBI.InNeeds |= BI.OutNeeds;
    Worklist.push_back(Succ);
  }
}

char SIWholeQuadMode::analyzeFunction(MachineFunction &MF) {
  std::vector<WorkItem> Worklist;
  char GlobalFlags = scanInstructions(MF, Worklist);

  while (!Worklist.empty()) {
    WorkItem WI = Worklist.back();
    Worklist.pop_back();

    if (WI.MI)
      propagateInstruction(*WI.MI, Worklist);
    else
      propagateBlock(*WI.MBB, Worklist);
  }

  return GlobalFlags;
}

MachineBasicBlock::iterator
SIWholeQuadMode::saveSCC(MachineBasicBlock &MBB,
                         MachineBasicBlock::iterator Before) {
  Register SaveReg = MRI->createVirtualRegister(&AMDGPU::SReg_32_XM0RegClass);

  MachineInstr *Save =
      BuildMI(MBB, Before, DebugLoc(), TII->get(AMDGPU::COPY), SaveReg)
          .addReg(AMDGPU::SCC);
  MachineInstr *Restore =
      BuildMI(MBB, Before, DebugLoc(), TII->get(AMDGPU::COPY), AMDGPU::SCC)
          .addReg(SaveReg);

  LIS->InsertMachineInstrInMaps(*Save);
  LIS->InsertMachineInstrInMaps(*Restore);
  LIS->createAndComputeVirtRegInterval(SaveReg);

  return Restore;
}

// Return an iterator in the (inclusive) range [First, Last] at which
// instructions can be safely inserted, keeping in mind that some of the
// instructions we want to add necessarily clobber SCC.
MachineBasicBlock::iterator SIWholeQuadMode::prepareInsertion(
    MachineBasicBlock &MBB, MachineBasicBlock::iterator First,
    MachineBasicBlock::iterator Last, bool PreferLast, bool SaveSCC) {
  if (!SaveSCC)
    return PreferLast ? Last : First;

  LiveRange &LR = LIS->getRegUnit(*MCRegUnitIterator(AMDGPU::SCC, TRI));
  auto MBBE = MBB.end();
  SlotIndex FirstIdx = First != MBBE ? LIS->getInstructionIndex(*First)
                                     : LIS->getMBBEndIdx(&MBB);
  SlotIndex LastIdx =
      Last != MBBE ? LIS->getInstructionIndex(*Last) : LIS->getMBBEndIdx(&MBB);
  SlotIndex Idx = PreferLast ? LastIdx : FirstIdx;
  const LiveRange::Segment *S;

  for (;;) {
    S = LR.getSegmentContaining(Idx);
    if (!S)
      break;

    if (PreferLast) {
      SlotIndex Next = S->start.getBaseIndex();
      if (Next < FirstIdx)
        break;
      Idx = Next;
    } else {
      MachineInstr *EndMI = LIS->getInstructionFromIndex(S->end.getBaseIndex());
      assert(EndMI && "Segment does not end on valid instruction");
      auto NextI = std::next(EndMI->getIterator());
      SlotIndex Next = LIS->getInstructionIndex(*NextI);
      if (Next > LastIdx)
        break;
      Idx = Next;
    }
  }

  MachineBasicBlock::iterator MBBI;

  if (MachineInstr *MI = LIS->getInstructionFromIndex(Idx))
    MBBI = MI;
  else {
    assert(Idx == LIS->getMBBEndIdx(&MBB));
    MBBI = MBB.end();
  }

  // Move insertion point past any operations modifying EXEC.
  // This assumes that the value of SCC defined by any of these operations
  // does not need to be preserved.
  while (MBBI != Last) {
    bool IsExecDef = false;
    for (const MachineOperand &MO : MBBI->operands()) {
      if (MO.isReg() && MO.isDef()) {
        IsExecDef |=
            MO.getReg() == AMDGPU::EXEC_LO || MO.getReg() == AMDGPU::EXEC;
      }
    }
    if (!IsExecDef)
      break;
    MBBI++;
    S = nullptr;
  }

  if (S)
    MBBI = saveSCC(MBB, MBBI);

  return MBBI;
}

void SIWholeQuadMode::toExact(MachineBasicBlock &MBB,
                              MachineBasicBlock::iterator Before,
                              unsigned SaveWQM, unsigned LiveMaskReg) {
  MachineInstr *MI;

  if (SaveWQM) {
    MI = BuildMI(MBB, Before, DebugLoc(), TII->get(ST->isWave32() ?
                   AMDGPU::S_AND_SAVEEXEC_B32 : AMDGPU::S_AND_SAVEEXEC_B64),
                 SaveWQM)
             .addReg(LiveMaskReg);
  } else {
    unsigned Exec = ST->isWave32() ? AMDGPU::EXEC_LO : AMDGPU::EXEC;
    MI = BuildMI(MBB, Before, DebugLoc(), TII->get(ST->isWave32() ?
                   AMDGPU::S_AND_B32 : AMDGPU::S_AND_B64),
                 Exec)
             .addReg(Exec)
             .addReg(LiveMaskReg);
  }

  LIS->InsertMachineInstrInMaps(*MI);
}

void SIWholeQuadMode::toWQM(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator Before,
                            unsigned SavedWQM) {
  MachineInstr *MI;

  unsigned Exec = ST->isWave32() ? AMDGPU::EXEC_LO : AMDGPU::EXEC;
  if (SavedWQM) {
    MI = BuildMI(MBB, Before, DebugLoc(), TII->get(AMDGPU::COPY), Exec)
             .addReg(SavedWQM);
  } else {
    MI = BuildMI(MBB, Before, DebugLoc(), TII->get(ST->isWave32() ?
                   AMDGPU::S_WQM_B32 : AMDGPU::S_WQM_B64),
                 Exec)
             .addReg(Exec);
  }

  LIS->InsertMachineInstrInMaps(*MI);
}

void SIWholeQuadMode::toWWM(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator Before,
                            unsigned SaveOrig) {
  MachineInstr *MI;

  assert(SaveOrig);
  MI = BuildMI(MBB, Before, DebugLoc(), TII->get(AMDGPU::ENTER_WWM), SaveOrig)
           .addImm(-1);
  LIS->InsertMachineInstrInMaps(*MI);
}

void SIWholeQuadMode::fromWWM(MachineBasicBlock &MBB,
                              MachineBasicBlock::iterator Before,
                              unsigned SavedOrig) {
  MachineInstr *MI;

  assert(SavedOrig);
  MI = BuildMI(MBB, Before, DebugLoc(), TII->get(AMDGPU::EXIT_WWM),
               ST->isWave32() ? AMDGPU::EXEC_LO : AMDGPU::EXEC)
           .addReg(SavedOrig);
  LIS->InsertMachineInstrInMaps(*MI);
}

void SIWholeQuadMode::processBlock(MachineBasicBlock &MBB, unsigned LiveMaskReg,
                                   bool isEntry) {
  auto BII = Blocks.find(&MBB);
  if (BII == Blocks.end())
    return;

  const BlockInfo &BI = BII->second;

  // This is a non-entry block that is WQM throughout, so no need to do
  // anything.
  if (!isEntry && BI.Needs == StateWQM && BI.OutNeeds != StateExact)
    return;

  LLVM_DEBUG(dbgs() << "\nProcessing block " << printMBBReference(MBB)
                    << ":\n");

  unsigned SavedWQMReg = 0;
  unsigned SavedNonWWMReg = 0;
  bool WQMFromExec = isEntry;
  char State = (isEntry || !(BI.InNeeds & StateWQM)) ? StateExact : StateWQM;
  char NonWWMState = 0;
  const TargetRegisterClass *BoolRC = TRI->getBoolRC();

  auto II = MBB.getFirstNonPHI(), IE = MBB.end();
  if (isEntry) {
    // Skip the instruction that saves LiveMask
    if (II != IE && II->getOpcode() == AMDGPU::COPY)
      ++II;
  }

  // This stores the first instruction where it's safe to switch from WQM to
  // Exact or vice versa.
  MachineBasicBlock::iterator FirstWQM = IE;

  // This stores the first instruction where it's safe to switch from WWM to
  // Exact/WQM or to switch to WWM. It must always be the same as, or after,
  // FirstWQM since if it's safe to switch to/from WWM, it must be safe to
  // switch to/from WQM as well.
  MachineBasicBlock::iterator FirstWWM = IE;

  for (;;) {
    MachineBasicBlock::iterator Next = II;
    char Needs = StateExact | StateWQM; // WWM is disabled by default
    char OutNeeds = 0;

    if (FirstWQM == IE)
      FirstWQM = II;

    if (FirstWWM == IE)
      FirstWWM = II;

    // First, figure out the allowed states (Needs) based on the propagated
    // flags.
    if (II != IE) {
      MachineInstr &MI = *II;

      if (MI.isTerminator() || TII->mayReadEXEC(*MRI, MI)) {
        auto III = Instructions.find(&MI);
        if (III != Instructions.end()) {
          if (III->second.Needs & StateWWM)
            Needs = StateWWM;
          else if (III->second.Needs & StateWQM)
            Needs = StateWQM;
          else
            Needs &= ~III->second.Disabled;
          OutNeeds = III->second.OutNeeds;
        }
      } else {
        // If the instruction doesn't actually need a correct EXEC, then we can
        // safely leave WWM enabled.
        Needs = StateExact | StateWQM | StateWWM;
      }

      if (MI.isTerminator() && OutNeeds == StateExact)
        Needs = StateExact;

      ++Next;
    } else {
      // End of basic block
      if (BI.OutNeeds & StateWQM)
        Needs = StateWQM;
      else if (BI.OutNeeds == StateExact)
        Needs = StateExact;
      else
        Needs = StateWQM | StateExact;
    }

    // Now, transition if necessary.
    if (!(Needs & State)) {
      MachineBasicBlock::iterator First;
      if (State == StateWWM || Needs == StateWWM) {
        // We must switch to or from WWM
        First = FirstWWM;
      } else {
        // We only need to switch to/from WQM, so we can use FirstWQM
        First = FirstWQM;
      }

      MachineBasicBlock::iterator Before =
          prepareInsertion(MBB, First, II, Needs == StateWQM,
                           Needs == StateExact || WQMFromExec);

      if (State == StateWWM) {
        assert(SavedNonWWMReg);
        fromWWM(MBB, Before, SavedNonWWMReg);
        LIS->createAndComputeVirtRegInterval(SavedNonWWMReg);
        SavedNonWWMReg = 0;
        State = NonWWMState;
      }

      if (Needs == StateWWM) {
        NonWWMState = State;
        assert(!SavedNonWWMReg);
        SavedNonWWMReg = MRI->createVirtualRegister(BoolRC);
        toWWM(MBB, Before, SavedNonWWMReg);
        State = StateWWM;
      } else {
        if (State == StateWQM && (Needs & StateExact) && !(Needs & StateWQM)) {
          if (!WQMFromExec && (OutNeeds & StateWQM)) {
            assert(!SavedWQMReg);
            SavedWQMReg = MRI->createVirtualRegister(BoolRC);
          }

          toExact(MBB, Before, SavedWQMReg, LiveMaskReg);
          State = StateExact;
        } else if (State == StateExact && (Needs & StateWQM) &&
                   !(Needs & StateExact)) {
          assert(WQMFromExec == (SavedWQMReg == 0));

          toWQM(MBB, Before, SavedWQMReg);

          if (SavedWQMReg) {
            LIS->createAndComputeVirtRegInterval(SavedWQMReg);
            SavedWQMReg = 0;
          }
          State = StateWQM;
        } else {
          // We can get here if we transitioned from WWM to a non-WWM state that
          // already matches our needs, but we shouldn't need to do anything.
          assert(Needs & State);
        }
      }
    }

    if (Needs != (StateExact | StateWQM | StateWWM)) {
      if (Needs != (StateExact | StateWQM))
        FirstWQM = IE;
      FirstWWM = IE;
    }

    if (II == IE)
      break;

    II = Next;
  }
  assert(!SavedWQMReg);
  assert(!SavedNonWWMReg);
}

void SIWholeQuadMode::lowerLiveMaskQueries(unsigned LiveMaskReg) {
  for (MachineInstr *MI : LiveMaskQueries) {
    const DebugLoc &DL = MI->getDebugLoc();
    Register Dest = MI->getOperand(0).getReg();

    MachineInstr *Copy =
        BuildMI(*MI->getParent(), MI, DL, TII->get(AMDGPU::COPY), Dest)
            .addReg(LiveMaskReg);

    LIS->ReplaceMachineInstrInMaps(*MI, *Copy);
    MI->eraseFromParent();
  }
}

void SIWholeQuadMode::lowerCopyInstrs() {
  for (MachineInstr *MI : LowerToMovInstrs) {
    assert(MI->getNumExplicitOperands() == 2);

    const Register Reg = MI->getOperand(0).getReg();
    const unsigned SubReg = MI->getOperand(0).getSubReg();

    if (TRI->isVGPR(*MRI, Reg)) {
      const TargetRegisterClass *regClass =
          Reg.isVirtual() ? MRI->getRegClass(Reg) : TRI->getPhysRegClass(Reg);
      if (SubReg)
        regClass = TRI->getSubRegClass(regClass, SubReg);

      const unsigned MovOp = TII->getMovOpcode(regClass);
      MI->setDesc(TII->get(MovOp));

      // And make it implicitly depend on exec (like all VALU movs should do).
      MI->addOperand(MachineOperand::CreateReg(AMDGPU::EXEC, false, true));
    } else if (!MRI->isSSA()) {
      // Remove early-clobber and exec dependency from simple SGPR copies.
      // This allows some to be eliminated during/post RA.
      LLVM_DEBUG(dbgs() << "simplify SGPR copy: " << *MI);
      if (MI->getOperand(0).isEarlyClobber()) {
        LIS->removeInterval(Reg);
        MI->getOperand(0).setIsEarlyClobber(false);
        LIS->createAndComputeVirtRegInterval(Reg);
      }
      int Index = MI->findRegisterUseOperandIdx(AMDGPU::EXEC);
      while (Index >= 0) {
        MI->RemoveOperand(Index);
        Index = MI->findRegisterUseOperandIdx(AMDGPU::EXEC);
      }
      MI->setDesc(TII->get(AMDGPU::COPY));
      LLVM_DEBUG(dbgs() << "  -> " << *MI);
    }
  }
  for (MachineInstr *MI : LowerToCopyInstrs) {
    if (MI->getOpcode() == AMDGPU::V_SET_INACTIVE_B32 ||
        MI->getOpcode() == AMDGPU::V_SET_INACTIVE_B64) {
      assert(MI->getNumExplicitOperands() == 3);
      // the only reason we should be here is V_SET_INACTIVE has
      // an undef input so it is being replaced by a simple copy.
      // There should be a second undef source that we should remove.
      assert(MI->getOperand(2).isUndef());
      MI->RemoveOperand(2);
      MI->untieRegOperand(1);
    } else {
      assert(MI->getNumExplicitOperands() == 2);
    }

    MI->setDesc(TII->get(AMDGPU::COPY));
  }
}

bool SIWholeQuadMode::runOnMachineFunction(MachineFunction &MF) {
  Instructions.clear();
  Blocks.clear();
  LiveMaskQueries.clear();
  LowerToCopyInstrs.clear();
  LowerToMovInstrs.clear();
  CallingConv = MF.getFunction().getCallingConv();

  ST = &MF.getSubtarget<GCNSubtarget>();

  TII = ST->getInstrInfo();
  TRI = &TII->getRegisterInfo();
  MRI = &MF.getRegInfo();
  LIS = &getAnalysis<LiveIntervals>();

  if (ST->isWave32()) {
    AndOpc = AMDGPU::S_AND_B32;
    XorTermrOpc = AMDGPU::S_XOR_B32_term;
    OrSaveExecOpc = AMDGPU::S_OR_SAVEEXEC_B32;
    Exec = AMDGPU::EXEC_LO;
  } else {
    AndOpc = AMDGPU::S_AND_B64;
    XorTermrOpc = AMDGPU::S_XOR_B64_term;
    OrSaveExecOpc = AMDGPU::S_OR_SAVEEXEC_B64;
    Exec = AMDGPU::EXEC;
  }

  char GlobalFlags = analyzeFunction(MF);
  unsigned LiveMaskReg = 0;
  if (!(GlobalFlags & StateWQM)) {
    lowerLiveMaskQueries(Exec);
    if (!(GlobalFlags & StateWWM) && LowerToCopyInstrs.empty() && LowerToMovInstrs.empty())
      return !LiveMaskQueries.empty();
  } else {
    // Store a copy of the original live mask when required
    MachineBasicBlock &Entry = MF.front();
    MachineBasicBlock::iterator EntryMI = Entry.getFirstNonPHI();

    if (GlobalFlags & StateExact || !LiveMaskQueries.empty()) {
      LiveMaskReg = MRI->createVirtualRegister(TRI->getBoolRC());
      MachineInstr *MI = BuildMI(Entry, EntryMI, DebugLoc(),
                                 TII->get(AMDGPU::COPY), LiveMaskReg)
                             .addReg(Exec);
      LIS->InsertMachineInstrInMaps(*MI);
    }

    lowerLiveMaskQueries(LiveMaskReg);

    if (GlobalFlags == StateWQM) {
      // For a shader that needs only WQM, we can just set it once.
      auto MI = BuildMI(Entry, EntryMI, DebugLoc(),
                        TII->get(ST->isWave32() ? AMDGPU::S_WQM_B32
                                                : AMDGPU::S_WQM_B64),
                        Exec)
                    .addReg(Exec);
      LIS->InsertMachineInstrInMaps(*MI);

      lowerCopyInstrs();
      // EntryMI may become invalid here
      return true;
    }
  }

  LLVM_DEBUG(printInfo());

  lowerCopyInstrs();

  // Handle the general case
  for (auto BII : Blocks)
    processBlock(*BII.first, LiveMaskReg, BII.first == &*MF.begin());

  if (LiveMaskReg)
    LIS->createAndComputeVirtRegInterval(LiveMaskReg);

  // Physical registers like SCC aren't tracked by default anyway, so just
  // removing the ranges we computed is the simplest option for maintaining
  // the analysis results.
  LIS->removeRegUnit(*MCRegUnitIterator(AMDGPU::SCC, TRI));

  return true;
}