xref: /freebsd/contrib/llvm-project/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp (revision 0fca6ea1d4eea4c934cfff25ac9ee8ad6fe95583)

//====- X86FlagsCopyLowering.cpp - Lowers COPY nodes of EFLAGS ------------===//
//
// 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
///
/// Lowers COPY nodes of EFLAGS by directly extracting and preserving individual
/// flag bits.
///
/// We have to do this by carefully analyzing and rewriting the usage of the
/// copied EFLAGS register because there is no general way to rematerialize the
/// entire EFLAGS register safely and efficiently. Using `popf` both forces
/// dynamic stack adjustment and can create correctness issues due to IF, TF,
/// and other non-status flags being overwritten. Using sequences involving
/// SAHF don't work on all x86 processors and are often quite slow compared to
/// directly testing a single status preserved in its own GPR.
///
//===----------------------------------------------------------------------===//

#include "X86.h"
#include "X86InstrBuilder.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineSSAUpdater.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSchedule.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/MC/MCSchedule.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <utility>

using namespace llvm;

#define PASS_KEY "x86-flags-copy-lowering"
#define DEBUG_TYPE PASS_KEY

STATISTIC(NumCopiesEliminated, "Number of copies of EFLAGS eliminated");
STATISTIC(NumSetCCsInserted, "Number of setCC instructions inserted");
STATISTIC(NumTestsInserted, "Number of test instructions inserted");
STATISTIC(NumAddsInserted, "Number of adds instructions inserted");
STATISTIC(NumNFsConvertedTo, "Number of NF instructions converted to");

namespace {

// Convenient array type for storing registers associated with each condition.
using CondRegArray = std::array<unsigned, X86::LAST_VALID_COND + 1>;

class X86FlagsCopyLoweringPass : public MachineFunctionPass {
public:
  X86FlagsCopyLoweringPass() : MachineFunctionPass(ID) {}

  StringRef getPassName() const override { return "X86 EFLAGS copy lowering"; }
  bool runOnMachineFunction(MachineFunction &MF) override;
  void getAnalysisUsage(AnalysisUsage &AU) const override;

  /// Pass identification, replacement for typeid.
  static char ID;

private:
  MachineRegisterInfo *MRI = nullptr;
  const X86Subtarget *Subtarget = nullptr;
  const X86InstrInfo *TII = nullptr;
  const TargetRegisterInfo *TRI = nullptr;
  const TargetRegisterClass *PromoteRC = nullptr;
  MachineDominatorTree *MDT = nullptr;

  CondRegArray collectCondsInRegs(MachineBasicBlock &MBB,
                                  MachineBasicBlock::iterator CopyDefI);

  Register promoteCondToReg(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator TestPos,
                            const DebugLoc &TestLoc, X86::CondCode Cond);
  std::pair<unsigned, bool> getCondOrInverseInReg(
      MachineBasicBlock &TestMBB, MachineBasicBlock::iterator TestPos,
      const DebugLoc &TestLoc, X86::CondCode Cond, CondRegArray &CondRegs);
  void insertTest(MachineBasicBlock &MBB, MachineBasicBlock::iterator Pos,
                  const DebugLoc &Loc, unsigned Reg);

  void rewriteSetCC(MachineBasicBlock &MBB, MachineBasicBlock::iterator Pos,
                    const DebugLoc &Loc, MachineInstr &MI,
                    CondRegArray &CondRegs);
  void rewriteArithmetic(MachineBasicBlock &MBB,
                         MachineBasicBlock::iterator Pos, const DebugLoc &Loc,
                         MachineInstr &MI, CondRegArray &CondRegs);
  void rewriteMI(MachineBasicBlock &MBB, MachineBasicBlock::iterator Pos,
                 const DebugLoc &Loc, MachineInstr &MI, CondRegArray &CondRegs);
};

} // end anonymous namespace

INITIALIZE_PASS_BEGIN(X86FlagsCopyLoweringPass, DEBUG_TYPE,
                      "X86 EFLAGS copy lowering", false, false)
INITIALIZE_PASS_END(X86FlagsCopyLoweringPass, DEBUG_TYPE,
                    "X86 EFLAGS copy lowering", false, false)

FunctionPass *llvm::createX86FlagsCopyLoweringPass() {
  return new X86FlagsCopyLoweringPass();
}

char X86FlagsCopyLoweringPass::ID = 0;

void X86FlagsCopyLoweringPass::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.addUsedIfAvailable<MachineDominatorTreeWrapperPass>();
  MachineFunctionPass::getAnalysisUsage(AU);
}

static bool isArithmeticOp(unsigned Opc) {
  return X86::isADC(Opc) || X86::isSBB(Opc) || X86::isRCL(Opc) ||
         X86::isRCR(Opc) || (Opc == X86::SETB_C32r || Opc == X86::SETB_C64r);
}

static MachineBasicBlock &splitBlock(MachineBasicBlock &MBB,
                                     MachineInstr &SplitI,
                                     const X86InstrInfo &TII) {
  MachineFunction &MF = *MBB.getParent();

  assert(SplitI.getParent() == &MBB &&
         "Split instruction must be in the split block!");
  assert(SplitI.isBranch() &&
         "Only designed to split a tail of branch instructions!");
  assert(X86::getCondFromBranch(SplitI) != X86::COND_INVALID &&
         "Must split on an actual jCC instruction!");

  // Dig out the previous instruction to the split point.
  MachineInstr &PrevI = *std::prev(SplitI.getIterator());
  assert(PrevI.isBranch() && "Must split after a branch!");
  assert(X86::getCondFromBranch(PrevI) != X86::COND_INVALID &&
         "Must split after an actual jCC instruction!");
  assert(!std::prev(PrevI.getIterator())->isTerminator() &&
         "Must only have this one terminator prior to the split!");

  // Grab the one successor edge that will stay in `MBB`.
  MachineBasicBlock &UnsplitSucc = *PrevI.getOperand(0).getMBB();

  // Analyze the original block to see if we are actually splitting an edge
  // into two edges. This can happen when we have multiple conditional jumps to
  // the same successor.
  bool IsEdgeSplit =
      std::any_of(SplitI.getIterator(), MBB.instr_end(),
                  [&](MachineInstr &MI) {
                    assert(MI.isTerminator() &&
                           "Should only have spliced terminators!");
                    return llvm::any_of(
                        MI.operands(), [&](MachineOperand &MOp) {
                          return MOp.isMBB() && MOp.getMBB() == &UnsplitSucc;
                        });
                  }) ||
      MBB.getFallThrough() == &UnsplitSucc;

  MachineBasicBlock &NewMBB = *MF.CreateMachineBasicBlock();

  // Insert the new block immediately after the current one. Any existing
  // fallthrough will be sunk into this new block anyways.
  MF.insert(std::next(MachineFunction::iterator(&MBB)), &NewMBB);

  // Splice the tail of instructions into the new block.
  NewMBB.splice(NewMBB.end(), &MBB, SplitI.getIterator(), MBB.end());

  // Copy the necessary succesors (and their probability info) into the new
  // block.
  for (auto SI = MBB.succ_begin(), SE = MBB.succ_end(); SI != SE; ++SI)
    if (IsEdgeSplit || *SI != &UnsplitSucc)
      NewMBB.copySuccessor(&MBB, SI);
  // Normalize the probabilities if we didn't end up splitting the edge.
  if (!IsEdgeSplit)
    NewMBB.normalizeSuccProbs();

  // Now replace all of the moved successors in the original block with the new
  // block. This will merge their probabilities.
  for (MachineBasicBlock *Succ : NewMBB.successors())
    if (Succ != &UnsplitSucc)
      MBB.replaceSuccessor(Succ, &NewMBB);

  // We should always end up replacing at least one successor.
  assert(MBB.isSuccessor(&NewMBB) &&
         "Failed to make the new block a successor!");

  // Now update all the PHIs.
  for (MachineBasicBlock *Succ : NewMBB.successors()) {
    for (MachineInstr &MI : *Succ) {
      if (!MI.isPHI())
        break;

      for (int OpIdx = 1, NumOps = MI.getNumOperands(); OpIdx < NumOps;
           OpIdx += 2) {
        MachineOperand &OpV = MI.getOperand(OpIdx);
        MachineOperand &OpMBB = MI.getOperand(OpIdx + 1);
        assert(OpMBB.isMBB() && "Block operand to a PHI is not a block!");
        if (OpMBB.getMBB() != &MBB)
          continue;

        // Replace the operand for unsplit successors
        if (!IsEdgeSplit || Succ != &UnsplitSucc) {
          OpMBB.setMBB(&NewMBB);

          // We have to continue scanning as there may be multiple entries in
          // the PHI.
          continue;
        }

        // When we have split the edge append a new successor.
        MI.addOperand(MF, OpV);
        MI.addOperand(MF, MachineOperand::CreateMBB(&NewMBB));
        break;
      }
    }
  }

  return NewMBB;
}

enum EFLAGSClobber { NoClobber, EvitableClobber, InevitableClobber };

static EFLAGSClobber getClobberType(const MachineInstr &MI) {
  const MachineOperand *FlagDef =
      MI.findRegisterDefOperand(X86::EFLAGS, /*TRI=*/nullptr);
  if (!FlagDef)
    return NoClobber;
  if (FlagDef->isDead() && X86::getNFVariant(MI.getOpcode()))
    return EvitableClobber;

  return InevitableClobber;
}

bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
  LLVM_DEBUG(dbgs() << "********** " << getPassName() << " : " << MF.getName()
                    << " **********\n");

  Subtarget = &MF.getSubtarget<X86Subtarget>();
  MRI = &MF.getRegInfo();
  TII = Subtarget->getInstrInfo();
  TRI = Subtarget->getRegisterInfo();
  PromoteRC = &X86::GR8RegClass;

  if (MF.empty())
    // Nothing to do for a degenerate empty function...
    return false;

  if (none_of(MRI->def_instructions(X86::EFLAGS), [](const MachineInstr &MI) {
        return MI.getOpcode() == TargetOpcode::COPY;
      }))
    return false;

  // We change the code, so we don't preserve the dominator tree anyway. If we
  // got a valid MDT from the pass manager, use that, otherwise construct one
  // now. This is an optimization that avoids unnecessary MDT construction for
  // functions that have no flag copies.

  auto MDTWrapper = getAnalysisIfAvailable<MachineDominatorTreeWrapperPass>();
  std::unique_ptr<MachineDominatorTree> OwnedMDT;
  if (MDTWrapper) {
    MDT = &MDTWrapper->getDomTree();
  } else {
    OwnedMDT = std::make_unique<MachineDominatorTree>();
    OwnedMDT->getBase().recalculate(MF);
    MDT = OwnedMDT.get();
  }

  // Collect the copies in RPO so that when there are chains where a copy is in
  // turn copied again we visit the first one first. This ensures we can find
  // viable locations for testing the original EFLAGS that dominate all the
  // uses across complex CFGs.
  SmallSetVector<MachineInstr *, 4> Copies;
  ReversePostOrderTraversal<MachineFunction *> RPOT(&MF);
  for (MachineBasicBlock *MBB : RPOT)
    for (MachineInstr &MI : *MBB)
      if (MI.getOpcode() == TargetOpcode::COPY &&
          MI.getOperand(0).getReg() == X86::EFLAGS)
        Copies.insert(&MI);

  // Try to elminate the copys by transform the instructions between copy and
  // copydef to the NF (no flags update) variants, e.g.
  //
  // %1:gr64 = COPY $eflags
  // OP1 implicit-def dead $eflags
  // $eflags = COPY %1
  // OP2 cc, implicit $eflags
  //
  // ->
  //
  // OP1_NF
  // OP2 implicit $eflags
  if (Subtarget->hasNF()) {
    SmallSetVector<MachineInstr *, 4> RemovedCopies;
    // CopyIIt may be invalidated by removing copies.
    auto CopyIIt = Copies.begin(), CopyIEnd = Copies.end();
    while (CopyIIt != CopyIEnd) {
      auto NCopyIIt = std::next(CopyIIt);
      SmallSetVector<MachineInstr *, 4> EvitableClobbers;
      MachineInstr *CopyI = *CopyIIt;
      MachineOperand &VOp = CopyI->getOperand(1);
      MachineInstr *CopyDefI = MRI->getVRegDef(VOp.getReg());
      MachineBasicBlock *CopyIMBB = CopyI->getParent();
      MachineBasicBlock *CopyDefIMBB = CopyDefI->getParent();
      // Walk all basic blocks reachable in depth-first iteration on the inverse
      // CFG from CopyIMBB to CopyDefIMBB. These blocks are all the blocks that
      // may be executed between the execution of CopyDefIMBB and CopyIMBB. On
      // all execution paths, instructions from CopyDefI to CopyI (exclusive)
      // has to be NF-convertible if it clobbers flags.
      for (auto BI = idf_begin(CopyIMBB), BE = idf_end(CopyDefIMBB); BI != BE;
           ++BI) {
        MachineBasicBlock *MBB = *BI;
        for (auto I = (MBB != CopyDefIMBB)
                          ? MBB->begin()
                          : std::next(MachineBasicBlock::iterator(CopyDefI)),
                  E = (MBB != CopyIMBB) ? MBB->end()
                                        : MachineBasicBlock::iterator(CopyI);
             I != E; ++I) {
          MachineInstr &MI = *I;
          EFLAGSClobber ClobberType = getClobberType(MI);
          if (ClobberType == NoClobber)
            continue;

          if (ClobberType == InevitableClobber)
            goto ProcessNextCopyI;

          assert(ClobberType == EvitableClobber && "unexpected workflow");
          EvitableClobbers.insert(&MI);
        }
      }
      // Covert evitable clobbers into NF variants and remove the copyies.
      RemovedCopies.insert(CopyI);
      CopyI->eraseFromParent();
      if (MRI->use_nodbg_empty(CopyDefI->getOperand(0).getReg())) {
        RemovedCopies.insert(CopyDefI);
        CopyDefI->eraseFromParent();
      }
      ++NumCopiesEliminated;
      for (auto *Clobber : EvitableClobbers) {
        unsigned NewOpc = X86::getNFVariant(Clobber->getOpcode());
        assert(NewOpc && "evitable clobber must have a NF variant");
        Clobber->setDesc(TII->get(NewOpc));
        Clobber->removeOperand(
            Clobber->findRegisterDefOperand(X86::EFLAGS, /*TRI=*/nullptr)
                ->getOperandNo());
        ++NumNFsConvertedTo;
      }
      // Update liveins for basic blocks in the path
      for (auto BI = idf_begin(CopyIMBB), BE = idf_end(CopyDefIMBB); BI != BE;
           ++BI)
        if (*BI != CopyDefIMBB)
          BI->addLiveIn(X86::EFLAGS);
    ProcessNextCopyI:
      CopyIIt = NCopyIIt;
    }
    Copies.set_subtract(RemovedCopies);
  }

  // For the rest of copies that cannot be eliminated by NF transform, we use
  // setcc to preserve the flags in GPR32 before OP1, and recheck its value
  // before using the flags, e.g.
  //
  // %1:gr64 = COPY $eflags
  // OP1 implicit-def dead $eflags
  // $eflags = COPY %1
  // OP2 cc, implicit $eflags
  //
  // ->
  //
  // %1:gr8 = SETCCr cc, implicit $eflags
  // OP1 implicit-def dead $eflags
  // TEST8rr %1, %1, implicit-def $eflags
  // OP2 ne, implicit $eflags
  for (MachineInstr *CopyI : Copies) {
    MachineBasicBlock &MBB = *CopyI->getParent();

    MachineOperand &VOp = CopyI->getOperand(1);
    assert(VOp.isReg() &&
           "The input to the copy for EFLAGS should always be a register!");
    MachineInstr &CopyDefI = *MRI->getVRegDef(VOp.getReg());
    if (CopyDefI.getOpcode() != TargetOpcode::COPY) {
      // FIXME: The big likely candidate here are PHI nodes. We could in theory
      // handle PHI nodes, but it gets really, really hard. Insanely hard. Hard
      // enough that it is probably better to change every other part of LLVM
      // to avoid creating them. The issue is that once we have PHIs we won't
      // know which original EFLAGS value we need to capture with our setCCs
      // below. The end result will be computing a complete set of setCCs that
      // we *might* want, computing them in every place where we copy *out* of
      // EFLAGS and then doing SSA formation on all of them to insert necessary
      // PHI nodes and consume those here. Then hoping that somehow we DCE the
      // unnecessary ones. This DCE seems very unlikely to be successful and so
      // we will almost certainly end up with a glut of dead setCC
      // instructions. Until we have a motivating test case and fail to avoid
      // it by changing other parts of LLVM's lowering, we refuse to handle
      // this complex case here.
      LLVM_DEBUG(
          dbgs() << "ERROR: Encountered unexpected def of an eflags copy: ";
          CopyDefI.dump());
      report_fatal_error(
          "Cannot lower EFLAGS copy unless it is defined in turn by a copy!");
    }

    auto Cleanup = make_scope_exit([&] {
      // All uses of the EFLAGS copy are now rewritten, kill the copy into
      // eflags and if dead the copy from.
      CopyI->eraseFromParent();
      if (MRI->use_empty(CopyDefI.getOperand(0).getReg()))
        CopyDefI.eraseFromParent();
      ++NumCopiesEliminated;
    });

    MachineOperand &DOp = CopyI->getOperand(0);
    assert(DOp.isDef() && "Expected register def!");
    assert(DOp.getReg() == X86::EFLAGS && "Unexpected copy def register!");
    if (DOp.isDead())
      continue;

    MachineBasicBlock *TestMBB = CopyDefI.getParent();
    auto TestPos = CopyDefI.getIterator();
    DebugLoc TestLoc = CopyDefI.getDebugLoc();

    LLVM_DEBUG(dbgs() << "Rewriting copy: "; CopyI->dump());

    // Walk up across live-in EFLAGS to find where they were actually def'ed.
    //
    // This copy's def may just be part of a region of blocks covered by
    // a single def of EFLAGS and we want to find the top of that region where
    // possible.
    //
    // This is essentially a search for a *candidate* reaching definition
    // location. We don't need to ever find the actual reaching definition here,
    // but we want to walk up the dominator tree to find the highest point which
    // would be viable for such a definition.
    auto HasEFLAGSClobber = [&](MachineBasicBlock::iterator Begin,
                                MachineBasicBlock::iterator End) {
      // Scan backwards as we expect these to be relatively short and often find
      // a clobber near the end.
      return llvm::any_of(
          llvm::reverse(llvm::make_range(Begin, End)), [&](MachineInstr &MI) {
            // Flag any instruction (other than the copy we are
            // currently rewriting) that defs EFLAGS.
            return &MI != CopyI &&
                   MI.findRegisterDefOperand(X86::EFLAGS, /*TRI=*/nullptr);
          });
    };
    auto HasEFLAGSClobberPath = [&](MachineBasicBlock *BeginMBB,
                                    MachineBasicBlock *EndMBB) {
      assert(MDT->dominates(BeginMBB, EndMBB) &&
             "Only support paths down the dominator tree!");
      SmallPtrSet<MachineBasicBlock *, 4> Visited;
      SmallVector<MachineBasicBlock *, 4> Worklist;
      // We terminate at the beginning. No need to scan it.
      Visited.insert(BeginMBB);
      Worklist.push_back(EndMBB);
      do {
        auto *MBB = Worklist.pop_back_val();
        for (auto *PredMBB : MBB->predecessors()) {
          if (!Visited.insert(PredMBB).second)
            continue;
          if (HasEFLAGSClobber(PredMBB->begin(), PredMBB->end()))
            return true;
          // Enqueue this block to walk its predecessors.
          Worklist.push_back(PredMBB);
        }
      } while (!Worklist.empty());
      // No clobber found along a path from the begin to end.
      return false;
    };
    while (TestMBB->isLiveIn(X86::EFLAGS) && !TestMBB->pred_empty() &&
           !HasEFLAGSClobber(TestMBB->begin(), TestPos)) {
      // Find the nearest common dominator of the predecessors, as
      // that will be the best candidate to hoist into.
      MachineBasicBlock *HoistMBB =
          std::accumulate(std::next(TestMBB->pred_begin()), TestMBB->pred_end(),
                          *TestMBB->pred_begin(),
                          [&](MachineBasicBlock *LHS, MachineBasicBlock *RHS) {
                            return MDT->findNearestCommonDominator(LHS, RHS);
                          });

      // Now we need to scan all predecessors that may be reached along paths to
      // the hoist block. A clobber anywhere in any of these blocks the hoist.
      // Note that this even handles loops because we require *no* clobbers.
      if (HasEFLAGSClobberPath(HoistMBB, TestMBB))
        break;

      // We also need the terminators to not sneakily clobber flags.
      if (HasEFLAGSClobber(HoistMBB->getFirstTerminator()->getIterator(),
                           HoistMBB->instr_end()))
        break;

      // We found a viable location, hoist our test position to it.
      TestMBB = HoistMBB;
      TestPos = TestMBB->getFirstTerminator()->getIterator();
      // Clear the debug location as it would just be confusing after hoisting.
      TestLoc = DebugLoc();
    }
    LLVM_DEBUG({
      auto DefIt = llvm::find_if(
          llvm::reverse(llvm::make_range(TestMBB->instr_begin(), TestPos)),
          [&](MachineInstr &MI) {
            return MI.findRegisterDefOperand(X86::EFLAGS, /*TRI=*/nullptr);
          });
      if (DefIt.base() != TestMBB->instr_begin()) {
        dbgs() << "  Using EFLAGS defined by: ";
        DefIt->dump();
      } else {
        dbgs() << "  Using live-in flags for BB:\n";
        TestMBB->dump();
      }
    });

    // While rewriting uses, we buffer jumps and rewrite them in a second pass
    // because doing so will perturb the CFG that we are walking to find the
    // uses in the first place.
    SmallVector<MachineInstr *, 4> JmpIs;

    // Gather the condition flags that have already been preserved in
    // registers. We do this from scratch each time as we expect there to be
    // very few of them and we expect to not revisit the same copy definition
    // many times. If either of those change sufficiently we could build a map
    // of these up front instead.
    CondRegArray CondRegs = collectCondsInRegs(*TestMBB, TestPos);

    // Collect the basic blocks we need to scan. Typically this will just be
    // a single basic block but we may have to scan multiple blocks if the
    // EFLAGS copy lives into successors.
    SmallVector<MachineBasicBlock *, 2> Blocks;
    SmallPtrSet<MachineBasicBlock *, 2> VisitedBlocks;
    Blocks.push_back(&MBB);

    do {
      MachineBasicBlock &UseMBB = *Blocks.pop_back_val();

      // Track when if/when we find a kill of the flags in this block.
      bool FlagsKilled = false;

      // In most cases, we walk from the beginning to the end of the block. But
      // when the block is the same block as the copy is from, we will visit it
      // twice. The first time we start from the copy and go to the end. The
      // second time we start from the beginning and go to the copy. This lets
      // us handle copies inside of cycles.
      // FIXME: This loop is *super* confusing. This is at least in part
      // a symptom of all of this routine needing to be refactored into
      // documentable components. Once done, there may be a better way to write
      // this loop.
      for (auto MII = (&UseMBB == &MBB && !VisitedBlocks.count(&UseMBB))
                          ? std::next(CopyI->getIterator())
                          : UseMBB.instr_begin(),
                MIE = UseMBB.instr_end();
           MII != MIE;) {
        MachineInstr &MI = *MII++;
        // If we are in the original copy block and encounter either the copy
        // def or the copy itself, break so that we don't re-process any part of
        // the block or process the instructions in the range that was copied
        // over.
        if (&MI == CopyI || &MI == &CopyDefI) {
          assert(&UseMBB == &MBB && VisitedBlocks.count(&MBB) &&
                 "Should only encounter these on the second pass over the "
                 "original block.");
          break;
        }

        MachineOperand *FlagUse =
            MI.findRegisterUseOperand(X86::EFLAGS, /*TRI=*/nullptr);
        FlagsKilled = MI.modifiesRegister(X86::EFLAGS, TRI);

        if (!FlagUse && FlagsKilled)
          break;
        else if (!FlagUse)
          continue;

        LLVM_DEBUG(dbgs() << "  Rewriting use: "; MI.dump());

        // Check the kill flag before we rewrite as that may change it.
        if (FlagUse->isKill())
          FlagsKilled = true;

        // Once we encounter a branch, the rest of the instructions must also be
        // branches. We can't rewrite in place here, so we handle them below.
        //
        // Note that we don't have to handle tail calls here, even conditional
        // tail calls, as those are not introduced into the X86 MI until post-RA
        // branch folding or black placement. As a consequence, we get to deal
        // with the simpler formulation of conditional branches followed by tail
        // calls.
        if (X86::getCondFromBranch(MI) != X86::COND_INVALID) {
          auto JmpIt = MI.getIterator();
          do {
            JmpIs.push_back(&*JmpIt);
            ++JmpIt;
          } while (JmpIt != UseMBB.instr_end() &&
                   X86::getCondFromBranch(*JmpIt) != X86::COND_INVALID);
          break;
        }

        // Otherwise we can just rewrite in-place.
        unsigned Opc = MI.getOpcode();
        if (Opc == TargetOpcode::COPY) {
          // Just replace this copy with the original copy def.
          MRI->replaceRegWith(MI.getOperand(0).getReg(),
                              CopyDefI.getOperand(0).getReg());
          MI.eraseFromParent();
        } else if (X86::isSETCC(Opc)) {
          rewriteSetCC(*TestMBB, TestPos, TestLoc, MI, CondRegs);
        } else if (isArithmeticOp(Opc)) {
          rewriteArithmetic(*TestMBB, TestPos, TestLoc, MI, CondRegs);
        } else {
          rewriteMI(*TestMBB, TestPos, TestLoc, MI, CondRegs);
        }

        // If this was the last use of the flags, we're done.
        if (FlagsKilled)
          break;
      }

      // If the flags were killed, we're done with this block.
      if (FlagsKilled)
        continue;

      // Otherwise we need to scan successors for ones where the flags live-in
      // and queue those up for processing.
      for (MachineBasicBlock *SuccMBB : UseMBB.successors())
        if (SuccMBB->isLiveIn(X86::EFLAGS) &&
            VisitedBlocks.insert(SuccMBB).second) {
          // We currently don't do any PHI insertion and so we require that the
          // test basic block dominates all of the use basic blocks. Further, we
          // can't have a cycle from the test block back to itself as that would
          // create a cycle requiring a PHI to break it.
          //
          // We could in theory do PHI insertion here if it becomes useful by
          // just taking undef values in along every edge that we don't trace
          // this EFLAGS copy along. This isn't as bad as fully general PHI
          // insertion, but still seems like a great deal of complexity.
          //
          // Because it is theoretically possible that some earlier MI pass or
          // other lowering transformation could induce this to happen, we do
          // a hard check even in non-debug builds here.
          if (SuccMBB == TestMBB || !MDT->dominates(TestMBB, SuccMBB)) {
            LLVM_DEBUG({
              dbgs()
                  << "ERROR: Encountered use that is not dominated by our test "
                     "basic block! Rewriting this would require inserting PHI "
                     "nodes to track the flag state across the CFG.\n\nTest "
                     "block:\n";
              TestMBB->dump();
              dbgs() << "Use block:\n";
              SuccMBB->dump();
            });
            report_fatal_error(
                "Cannot lower EFLAGS copy when original copy def "
                "does not dominate all uses.");
          }

          Blocks.push_back(SuccMBB);

          // After this, EFLAGS will be recreated before each use.
          SuccMBB->removeLiveIn(X86::EFLAGS);
        }
    } while (!Blocks.empty());

    // Now rewrite the jumps that use the flags. These we handle specially
    // because if there are multiple jumps in a single basic block we'll have
    // to do surgery on the CFG.
    MachineBasicBlock *LastJmpMBB = nullptr;
    for (MachineInstr *JmpI : JmpIs) {
      // Past the first jump within a basic block we need to split the blocks
      // apart.
      if (JmpI->getParent() == LastJmpMBB)
        splitBlock(*JmpI->getParent(), *JmpI, *TII);
      else
        LastJmpMBB = JmpI->getParent();

      rewriteMI(*TestMBB, TestPos, TestLoc, *JmpI, CondRegs);
    }

    // FIXME: Mark the last use of EFLAGS before the copy's def as a kill if
    // the copy's def operand is itself a kill.
  }

#ifndef NDEBUG
  for (MachineBasicBlock &MBB : MF)
    for (MachineInstr &MI : MBB)
      if (MI.getOpcode() == TargetOpcode::COPY &&
          (MI.getOperand(0).getReg() == X86::EFLAGS ||
           MI.getOperand(1).getReg() == X86::EFLAGS)) {
        LLVM_DEBUG(dbgs() << "ERROR: Found a COPY involving EFLAGS: ";
                   MI.dump());
        llvm_unreachable("Unlowered EFLAGS copy!");
      }
#endif

  return true;
}

/// Collect any conditions that have already been set in registers so that we
/// can re-use them rather than adding duplicates.
CondRegArray X86FlagsCopyLoweringPass::collectCondsInRegs(
    MachineBasicBlock &MBB, MachineBasicBlock::iterator TestPos) {
  CondRegArray CondRegs = {};

  // Scan backwards across the range of instructions with live EFLAGS.
  for (MachineInstr &MI :
       llvm::reverse(llvm::make_range(MBB.begin(), TestPos))) {
    X86::CondCode Cond = X86::getCondFromSETCC(MI);
    if (Cond != X86::COND_INVALID && !MI.mayStore() &&
        MI.getOperand(0).isReg() && MI.getOperand(0).getReg().isVirtual()) {
      assert(MI.getOperand(0).isDef() &&
             "A non-storing SETcc should always define a register!");
      CondRegs[Cond] = MI.getOperand(0).getReg();
    }

    // Stop scanning when we see the first definition of the EFLAGS as prior to
    // this we would potentially capture the wrong flag state.
    if (MI.findRegisterDefOperand(X86::EFLAGS, /*TRI=*/nullptr))
      break;
  }
  return CondRegs;
}

Register X86FlagsCopyLoweringPass::promoteCondToReg(
    MachineBasicBlock &TestMBB, MachineBasicBlock::iterator TestPos,
    const DebugLoc &TestLoc, X86::CondCode Cond) {
  Register Reg = MRI->createVirtualRegister(PromoteRC);
  auto SetI = BuildMI(TestMBB, TestPos, TestLoc, TII->get(X86::SETCCr), Reg)
                  .addImm(Cond);
  (void)SetI;
  LLVM_DEBUG(dbgs() << "    save cond: "; SetI->dump());
  ++NumSetCCsInserted;
  return Reg;
}

std::pair<unsigned, bool> X86FlagsCopyLoweringPass::getCondOrInverseInReg(
    MachineBasicBlock &TestMBB, MachineBasicBlock::iterator TestPos,
    const DebugLoc &TestLoc, X86::CondCode Cond, CondRegArray &CondRegs) {
  unsigned &CondReg = CondRegs[Cond];
  unsigned &InvCondReg = CondRegs[X86::GetOppositeBranchCondition(Cond)];
  if (!CondReg && !InvCondReg)
    CondReg = promoteCondToReg(TestMBB, TestPos, TestLoc, Cond);

  if (CondReg)
    return {CondReg, false};
  else
    return {InvCondReg, true};
}

void X86FlagsCopyLoweringPass::insertTest(MachineBasicBlock &MBB,
                                          MachineBasicBlock::iterator Pos,
                                          const DebugLoc &Loc, unsigned Reg) {
  auto TestI =
      BuildMI(MBB, Pos, Loc, TII->get(X86::TEST8rr)).addReg(Reg).addReg(Reg);
  (void)TestI;
  LLVM_DEBUG(dbgs() << "    test cond: "; TestI->dump());
  ++NumTestsInserted;
}

void X86FlagsCopyLoweringPass::rewriteSetCC(MachineBasicBlock &MBB,
                                            MachineBasicBlock::iterator Pos,
                                            const DebugLoc &Loc,
                                            MachineInstr &MI,
                                            CondRegArray &CondRegs) {
  X86::CondCode Cond = X86::getCondFromSETCC(MI);
  // Note that we can't usefully rewrite this to the inverse without complex
  // analysis of the users of the setCC. Largely we rely on duplicates which
  // could have been avoided already being avoided here.
  unsigned &CondReg = CondRegs[Cond];
  if (!CondReg)
    CondReg = promoteCondToReg(MBB, Pos, Loc, Cond);

  // Rewriting a register def is trivial: we just replace the register and
  // remove the setcc.
  if (!MI.mayStore()) {
    assert(MI.getOperand(0).isReg() &&
           "Cannot have a non-register defined operand to SETcc!");
    Register OldReg = MI.getOperand(0).getReg();
    // Drop Kill flags on the old register before replacing. CondReg may have
    // a longer live range.
    MRI->clearKillFlags(OldReg);
    MRI->replaceRegWith(OldReg, CondReg);
    MI.eraseFromParent();
    return;
  }

  // Otherwise, we need to emit a store.
  auto MIB = BuildMI(*MI.getParent(), MI.getIterator(), MI.getDebugLoc(),
                     TII->get(X86::MOV8mr));
  // Copy the address operands.
  for (int i = 0; i < X86::AddrNumOperands; ++i)
    MIB.add(MI.getOperand(i));

  MIB.addReg(CondReg);
  MIB.setMemRefs(MI.memoperands());
  MI.eraseFromParent();
}

void X86FlagsCopyLoweringPass::rewriteArithmetic(
    MachineBasicBlock &MBB, MachineBasicBlock::iterator Pos,
    const DebugLoc &Loc, MachineInstr &MI, CondRegArray &CondRegs) {
  // Arithmetic is either reading CF or OF.
  X86::CondCode Cond = X86::COND_B; // CF == 1
  // The addend to use to reset CF or OF when added to the flag value.
  // Set up an addend that when one is added will need a carry due to not
  // having a higher bit available.
  int Addend = 255;

  // Now get a register that contains the value of the flag input to the
  // arithmetic. We require exactly this flag to simplify the arithmetic
  // required to materialize it back into the flag.
  unsigned &CondReg = CondRegs[Cond];
  if (!CondReg)
    CondReg = promoteCondToReg(MBB, Pos, Loc, Cond);

  // Insert an instruction that will set the flag back to the desired value.
  Register TmpReg = MRI->createVirtualRegister(PromoteRC);
  auto AddI =
      BuildMI(*MI.getParent(), MI.getIterator(), MI.getDebugLoc(),
              TII->get(Subtarget->hasNDD() ? X86::ADD8ri_ND : X86::ADD8ri))
          .addDef(TmpReg, RegState::Dead)
          .addReg(CondReg)
          .addImm(Addend);
  (void)AddI;
  LLVM_DEBUG(dbgs() << "    add cond: "; AddI->dump());
  ++NumAddsInserted;
  MI.findRegisterUseOperand(X86::EFLAGS, /*TRI=*/nullptr)->setIsKill(true);
}

static X86::CondCode getImplicitCondFromMI(unsigned Opc) {
#define FROM_TO(A, B)                                                          \
  case X86::CMOV##A##_Fp32:                                                    \
  case X86::CMOV##A##_Fp64:                                                    \
  case X86::CMOV##A##_Fp80:                                                    \
    return X86::COND_##B;

  switch (Opc) {
  default:
    return X86::COND_INVALID;
    FROM_TO(B, B)
    FROM_TO(E, E)
    FROM_TO(P, P)
    FROM_TO(BE, BE)
    FROM_TO(NB, AE)
    FROM_TO(NE, NE)
    FROM_TO(NP, NP)
    FROM_TO(NBE, A)
  }
#undef FROM_TO
}

static unsigned getOpcodeWithCC(unsigned Opc, X86::CondCode CC) {
  assert((CC == X86::COND_E || CC == X86::COND_NE) && "Unexpected CC");
#define CASE(A)                                                                \
  case X86::CMOVB_##A:                                                         \
  case X86::CMOVE_##A:                                                         \
  case X86::CMOVP_##A:                                                         \
  case X86::CMOVBE_##A:                                                        \
  case X86::CMOVNB_##A:                                                        \
  case X86::CMOVNE_##A:                                                        \
  case X86::CMOVNP_##A:                                                        \
  case X86::CMOVNBE_##A:                                                       \
    return (CC == X86::COND_E) ? X86::CMOVE_##A : X86::CMOVNE_##A;
  switch (Opc) {
  default:
    llvm_unreachable("Unexpected opcode");
    CASE(Fp32)
    CASE(Fp64)
    CASE(Fp80)
  }
#undef CASE
}

void X86FlagsCopyLoweringPass::rewriteMI(MachineBasicBlock &MBB,
                                         MachineBasicBlock::iterator Pos,
                                         const DebugLoc &Loc, MachineInstr &MI,
                                         CondRegArray &CondRegs) {
  // First get the register containing this specific condition.
  bool IsImplicitCC = false;
  X86::CondCode CC = X86::getCondFromMI(MI);
  if (CC == X86::COND_INVALID) {
    CC = getImplicitCondFromMI(MI.getOpcode());
    IsImplicitCC = true;
  }
  assert(CC != X86::COND_INVALID && "Unknown EFLAG user!");
  unsigned CondReg;
  bool Inverted;
  std::tie(CondReg, Inverted) =
      getCondOrInverseInReg(MBB, Pos, Loc, CC, CondRegs);

  // Insert a direct test of the saved register.
  insertTest(*MI.getParent(), MI.getIterator(), MI.getDebugLoc(), CondReg);

  // Rewrite the instruction to use the !ZF flag from the test, and then kill
  // its use of the flags afterward.
  X86::CondCode NewCC = Inverted ? X86::COND_E : X86::COND_NE;
  if (IsImplicitCC)
    MI.setDesc(TII->get(getOpcodeWithCC(MI.getOpcode(), NewCC)));
  else
    MI.getOperand(MI.getDesc().getNumOperands() - 1).setImm(NewCC);

  MI.findRegisterUseOperand(X86::EFLAGS, /*TRI=*/nullptr)->setIsKill(true);
  LLVM_DEBUG(dbgs() << "    fixed instruction: "; MI.dump());
}