lib/CodeGen/MachineVerifier.cpp

//===-- MachineVerifier.cpp - Machine Code Verifier -------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Pass to verify generated machine code. The following is checked:
//
// Operand counts: All explicit operands must be present.
//
// Register classes: All physical and virtual register operands must be
// compatible with the register class required by the instruction descriptor.
//
// Register live intervals: Registers must be defined only once, and must be
// defined before use.
//
// The machine code verifier is enabled from LLVMTargetMachine.cpp with the
// command-line option -verify-machineinstrs, or by defining the environment
// variable LLVM_VERIFY_MACHINEINSTRS to the name of a file that will receive
// the verifier errors.
//===----------------------------------------------------------------------===//

#include "llvm/Function.h"
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

namespace {
  struct MachineVerifier {

    MachineVerifier(Pass *pass) :
      PASS(pass),
      OutFileName(getenv("LLVM_VERIFY_MACHINEINSTRS"))
      {}

    bool runOnMachineFunction(MachineFunction &MF);

    Pass *const PASS;
    const char *const OutFileName;
    raw_ostream *OS;
    const MachineFunction *MF;
    const TargetMachine *TM;
    const TargetRegisterInfo *TRI;
    const MachineRegisterInfo *MRI;

    unsigned foundErrors;

    typedef SmallVector<unsigned, 16> RegVector;
    typedef DenseSet<unsigned> RegSet;
    typedef DenseMap<unsigned, const MachineInstr*> RegMap;

    BitVector regsReserved;
    RegSet regsLive;
    RegVector regsDefined, regsDead, regsKilled;
    RegSet regsLiveInButUnused;

    // Add Reg and any sub-registers to RV
    void addRegWithSubRegs(RegVector &RV, unsigned Reg) {
      RV.push_back(Reg);
      if (TargetRegisterInfo::isPhysicalRegister(Reg))
        for (const unsigned *R = TRI->getSubRegisters(Reg); *R; R++)
          RV.push_back(*R);
    }

    struct BBInfo {
      // Is this MBB reachable from the MF entry point?
      bool reachable;

      // Vregs that must be live in because they are used without being
      // defined. Map value is the user.
      RegMap vregsLiveIn;

      // Regs killed in MBB. They may be defined again, and will then be in both
      // regsKilled and regsLiveOut.
      RegSet regsKilled;

      // Regs defined in MBB and live out. Note that vregs passing through may
      // be live out without being mentioned here.
      RegSet regsLiveOut;

      // Vregs that pass through MBB untouched. This set is disjoint from
      // regsKilled and regsLiveOut.
      RegSet vregsPassed;

      // Vregs that must pass through MBB because they are needed by a successor
      // block. This set is disjoint from regsLiveOut.
      RegSet vregsRequired;

      BBInfo() : reachable(false) {}

      // Add register to vregsPassed if it belongs there. Return true if
      // anything changed.
      bool addPassed(unsigned Reg) {
        if (!TargetRegisterInfo::isVirtualRegister(Reg))
          return false;
        if (regsKilled.count(Reg) || regsLiveOut.count(Reg))
          return false;
        return vregsPassed.insert(Reg).second;
      }

      // Same for a full set.
      bool addPassed(const RegSet &RS) {
        bool changed = false;
        for (RegSet::const_iterator I = RS.begin(), E = RS.end(); I != E; ++I)
          if (addPassed(*I))
            changed = true;
        return changed;
      }

      // Add register to vregsRequired if it belongs there. Return true if
      // anything changed.
      bool addRequired(unsigned Reg) {
        if (!TargetRegisterInfo::isVirtualRegister(Reg))
          return false;
        if (regsLiveOut.count(Reg))
          return false;
        return vregsRequired.insert(Reg).second;
      }

      // Same for a full set.
      bool addRequired(const RegSet &RS) {
        bool changed = false;
        for (RegSet::const_iterator I = RS.begin(), E = RS.end(); I != E; ++I)
          if (addRequired(*I))
            changed = true;
        return changed;
      }

      // Same for a full map.
      bool addRequired(const RegMap &RM) {
        bool changed = false;
        for (RegMap::const_iterator I = RM.begin(), E = RM.end(); I != E; ++I)
          if (addRequired(I->first))
            changed = true;
        return changed;
      }

      // Live-out registers are either in regsLiveOut or vregsPassed.
      bool isLiveOut(unsigned Reg) const {
        return regsLiveOut.count(Reg) || vregsPassed.count(Reg);
      }
    };

    // Extra register info per MBB.
    DenseMap<const MachineBasicBlock*, BBInfo> MBBInfoMap;

    bool isReserved(unsigned Reg) {
      return Reg < regsReserved.size() && regsReserved.test(Reg);
    }

    // Analysis information if available
    LiveVariables *LiveVars;

    void visitMachineFunctionBefore();
    void visitMachineBasicBlockBefore(const MachineBasicBlock *MBB);
    void visitMachineInstrBefore(const MachineInstr *MI);
    void visitMachineOperand(const MachineOperand *MO, unsigned MONum);
    void visitMachineInstrAfter(const MachineInstr *MI);
    void visitMachineBasicBlockAfter(const MachineBasicBlock *MBB);
    void visitMachineFunctionAfter();

    void report(const char *msg, const MachineFunction *MF);
    void report(const char *msg, const MachineBasicBlock *MBB);
    void report(const char *msg, const MachineInstr *MI);
    void report(const char *msg, const MachineOperand *MO, unsigned MONum);

    void markReachable(const MachineBasicBlock *MBB);
    void calcRegsPassed();
    void checkPHIOps(const MachineBasicBlock *MBB);

    void calcRegsRequired();
    void verifyLiveVariables();
  };

  struct MachineVerifierPass : public MachineFunctionPass {
    static char ID; // Pass ID, replacement for typeid

    MachineVerifierPass()
      : MachineFunctionPass(&ID) {}

    void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesAll();
      MachineFunctionPass::getAnalysisUsage(AU);
    }

    bool runOnMachineFunction(MachineFunction &MF) {
      MF.verify(this);
      return false;
    }
  };

}

char MachineVerifierPass::ID = 0;
static RegisterPass<MachineVerifierPass>
MachineVer("machineverifier", "Verify generated machine code");
static const PassInfo *const MachineVerifyID = &MachineVer;

FunctionPass *llvm::createMachineVerifierPass() {
  return new MachineVerifierPass();
}

void MachineFunction::verify(Pass *p) const {
  MachineVerifier(p).runOnMachineFunction(const_cast<MachineFunction&>(*this));
}

bool MachineVerifier::runOnMachineFunction(MachineFunction &MF) {
  raw_ostream *OutFile = 0;
  if (OutFileName) {
    std::string ErrorInfo;
    OutFile = new raw_fd_ostream(OutFileName, ErrorInfo,
                                 raw_fd_ostream::F_Append);
    if (!ErrorInfo.empty()) {
      errs() << "Error opening '" << OutFileName << "': " << ErrorInfo << '\n';
      exit(1);
    }

    OS = OutFile;
  } else {
    OS = &errs();
  }

  foundErrors = 0;

  this->MF = &MF;
  TM = &MF.getTarget();
  TRI = TM->getRegisterInfo();
  MRI = &MF.getRegInfo();

  if (PASS) {
    LiveVars = PASS->getAnalysisIfAvailable<LiveVariables>();
  } else {
    LiveVars = NULL;
  }

  visitMachineFunctionBefore();
  for (MachineFunction::const_iterator MFI = MF.begin(), MFE = MF.end();
       MFI!=MFE; ++MFI) {
    visitMachineBasicBlockBefore(MFI);
    for (MachineBasicBlock::const_iterator MBBI = MFI->begin(),
           MBBE = MFI->end(); MBBI != MBBE; ++MBBI) {
      visitMachineInstrBefore(MBBI);
      for (unsigned I = 0, E = MBBI->getNumOperands(); I != E; ++I)
        visitMachineOperand(&MBBI->getOperand(I), I);
      visitMachineInstrAfter(MBBI);
    }
    visitMachineBasicBlockAfter(MFI);
  }
  visitMachineFunctionAfter();

  if (OutFile)
    delete OutFile;
  else if (foundErrors)
    report_fatal_error("Found "+Twine(foundErrors)+" machine code errors.");

  // Clean up.
  regsLive.clear();
  regsDefined.clear();
  regsDead.clear();
  regsKilled.clear();
  regsLiveInButUnused.clear();
  MBBInfoMap.clear();

  return false;                 // no changes
}

void MachineVerifier::report(const char *msg, const MachineFunction *MF) {
  assert(MF);
  *OS << '\n';
  if (!foundErrors++)
    MF->print(*OS);
  *OS << "*** Bad machine code: " << msg << " ***\n"
      << "- function:    " << MF->getFunction()->getNameStr() << "\n";
}

void MachineVerifier::report(const char *msg, const MachineBasicBlock *MBB) {
  assert(MBB);
  report(msg, MBB->getParent());
  *OS << "- basic block: " << MBB->getName()
      << " " << (void*)MBB
      << " (BB#" << MBB->getNumber() << ")\n";
}

void MachineVerifier::report(const char *msg, const MachineInstr *MI) {
  assert(MI);
  report(msg, MI->getParent());
  *OS << "- instruction: ";
  MI->print(*OS, TM);
}

void MachineVerifier::report(const char *msg,
                             const MachineOperand *MO, unsigned MONum) {
  assert(MO);
  report(msg, MO->getParent());
  *OS << "- operand " << MONum << ":   ";
  MO->print(*OS, TM);
  *OS << "\n";
}

void MachineVerifier::markReachable(const MachineBasicBlock *MBB) {
  BBInfo &MInfo = MBBInfoMap[MBB];
  if (!MInfo.reachable) {
    MInfo.reachable = true;
    for (MachineBasicBlock::const_succ_iterator SuI = MBB->succ_begin(),
           SuE = MBB->succ_end(); SuI != SuE; ++SuI)
      markReachable(*SuI);
  }
}

void MachineVerifier::visitMachineFunctionBefore() {
  regsReserved = TRI->getReservedRegs(*MF);

  // A sub-register of a reserved register is also reserved
  for (int Reg = regsReserved.find_first(); Reg>=0;
       Reg = regsReserved.find_next(Reg)) {
    for (const unsigned *Sub = TRI->getSubRegisters(Reg); *Sub; ++Sub) {
      // FIXME: This should probably be:
      // assert(regsReserved.test(*Sub) && "Non-reserved sub-register");
      regsReserved.set(*Sub);
    }
  }
  markReachable(&MF->front());
}

// Does iterator point to a and b as the first two elements?
static bool matchPair(MachineBasicBlock::const_succ_iterator i,
                      const MachineBasicBlock *a, const MachineBasicBlock *b) {
  if (*i == a)
    return *++i == b;
  if (*i == b)
    return *++i == a;
  return false;
}

void
MachineVerifier::visitMachineBasicBlockBefore(const MachineBasicBlock *MBB) {
  const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();

  // Call AnalyzeBranch. If it succeeds, there several more conditions to check.
  MachineBasicBlock *TBB = 0, *FBB = 0;
  SmallVector<MachineOperand, 4> Cond;
  if (!TII->AnalyzeBranch(*const_cast<MachineBasicBlock *>(MBB),
                          TBB, FBB, Cond)) {
    // Ok, AnalyzeBranch thinks it knows what's going on with this block. Let's
    // check whether its answers match up with reality.
    if (!TBB && !FBB) {
      // Block falls through to its successor.
      MachineFunction::const_iterator MBBI = MBB;
      ++MBBI;
      if (MBBI == MF->end()) {
        // It's possible that the block legitimately ends with a noreturn
        // call or an unreachable, in which case it won't actually fall
        // out the bottom of the function.
      } else if (MBB->succ_empty()) {
        // It's possible that the block legitimately ends with a noreturn
        // call or an unreachable, in which case it won't actuall fall
        // out of the block.
      } else if (MBB->succ_size() != 1) {
        report("MBB exits via unconditional fall-through but doesn't have "
               "exactly one CFG successor!", MBB);
      } else if (MBB->succ_begin()[0] != MBBI) {
        report("MBB exits via unconditional fall-through but its successor "
               "differs from its CFG successor!", MBB);
      }
      if (!MBB->empty() && MBB->back().getDesc().isBarrier() &&
          !TII->isPredicated(&MBB->back())) {
        report("MBB exits via unconditional fall-through but ends with a "
               "barrier instruction!", MBB);
      }
      if (!Cond.empty()) {
        report("MBB exits via unconditional fall-through but has a condition!",
               MBB);
      }
    } else if (TBB && !FBB && Cond.empty()) {
      // Block unconditionally branches somewhere.
      if (MBB->succ_size() != 1) {
        report("MBB exits via unconditional branch but doesn't have "
               "exactly one CFG successor!", MBB);
      } else if (MBB->succ_begin()[0] != TBB) {
        report("MBB exits via unconditional branch but the CFG "
               "successor doesn't match the actual successor!", MBB);
      }
      if (MBB->empty()) {
        report("MBB exits via unconditional branch but doesn't contain "
               "any instructions!", MBB);
      } else if (!MBB->back().getDesc().isBarrier()) {
        report("MBB exits via unconditional branch but doesn't end with a "
               "barrier instruction!", MBB);
      } else if (!MBB->back().getDesc().isTerminator()) {
        report("MBB exits via unconditional branch but the branch isn't a "
               "terminator instruction!", MBB);
      }
    } else if (TBB && !FBB && !Cond.empty()) {
      // Block conditionally branches somewhere, otherwise falls through.
      MachineFunction::const_iterator MBBI = MBB;
      ++MBBI;
      if (MBBI == MF->end()) {
        report("MBB conditionally falls through out of function!", MBB);
      } if (MBB->succ_size() != 2) {
        report("MBB exits via conditional branch/fall-through but doesn't have "
               "exactly two CFG successors!", MBB);
      } else if (!matchPair(MBB->succ_begin(), TBB, MBBI)) {
        report("MBB exits via conditional branch/fall-through but the CFG "
               "successors don't match the actual successors!", MBB);
      }
      if (MBB->empty()) {
        report("MBB exits via conditional branch/fall-through but doesn't "
               "contain any instructions!", MBB);
      } else if (MBB->back().getDesc().isBarrier()) {
        report("MBB exits via conditional branch/fall-through but ends with a "
               "barrier instruction!", MBB);
      } else if (!MBB->back().getDesc().isTerminator()) {
        report("MBB exits via conditional branch/fall-through but the branch "
               "isn't a terminator instruction!", MBB);
      }
    } else if (TBB && FBB) {
      // Block conditionally branches somewhere, otherwise branches
      // somewhere else.
      if (MBB->succ_size() != 2) {
        report("MBB exits via conditional branch/branch but doesn't have "
               "exactly two CFG successors!", MBB);
      } else if (!matchPair(MBB->succ_begin(), TBB, FBB)) {
        report("MBB exits via conditional branch/branch but the CFG "
               "successors don't match the actual successors!", MBB);
      }
      if (MBB->empty()) {
        report("MBB exits via conditional branch/branch but doesn't "
               "contain any instructions!", MBB);
      } else if (!MBB->back().getDesc().isBarrier()) {
        report("MBB exits via conditional branch/branch but doesn't end with a "
               "barrier instruction!", MBB);
      } else if (!MBB->back().getDesc().isTerminator()) {
        report("MBB exits via conditional branch/branch but the branch "
               "isn't a terminator instruction!", MBB);
      }
      if (Cond.empty()) {
        report("MBB exits via conditinal branch/branch but there's no "
               "condition!", MBB);
      }
    } else {
      report("AnalyzeBranch returned invalid data!", MBB);
    }
  }

  regsLive.clear();
  for (MachineBasicBlock::livein_iterator I = MBB->livein_begin(),
         E = MBB->livein_end(); I != E; ++I) {
    if (!TargetRegisterInfo::isPhysicalRegister(*I)) {
      report("MBB live-in list contains non-physical register", MBB);
      continue;
    }
    regsLive.insert(*I);
    for (const unsigned *R = TRI->getSubRegisters(*I); *R; R++)
      regsLive.insert(*R);
  }
  regsLiveInButUnused = regsLive;

  const MachineFrameInfo *MFI = MF->getFrameInfo();
  assert(MFI && "Function has no frame info");
  BitVector PR = MFI->getPristineRegs(MBB);
  for (int I = PR.find_first(); I>0; I = PR.find_next(I)) {
    regsLive.insert(I);
    for (const unsigned *R = TRI->getSubRegisters(I); *R; R++)
      regsLive.insert(*R);
  }

  regsKilled.clear();
  regsDefined.clear();
}

void MachineVerifier::visitMachineInstrBefore(const MachineInstr *MI) {
  const TargetInstrDesc &TI = MI->getDesc();
  if (MI->getNumOperands() < TI.getNumOperands()) {
    report("Too few operands", MI);
    *OS << TI.getNumOperands() << " operands expected, but "
        << MI->getNumExplicitOperands() << " given.\n";
  }

  // Check the MachineMemOperands for basic consistency.
  for (MachineInstr::mmo_iterator I = MI->memoperands_begin(),
       E = MI->memoperands_end(); I != E; ++I) {
    if ((*I)->isLoad() && !TI.mayLoad())
      report("Missing mayLoad flag", MI);
    if ((*I)->isStore() && !TI.mayStore())
      report("Missing mayStore flag", MI);
  }
}

void
MachineVerifier::visitMachineOperand(const MachineOperand *MO, unsigned MONum) {
  const MachineInstr *MI = MO->getParent();
  const TargetInstrDesc &TI = MI->getDesc();

  // The first TI.NumDefs operands must be explicit register defines
  if (MONum < TI.getNumDefs()) {
    if (!MO->isReg())
      report("Explicit definition must be a register", MO, MONum);
    else if (!MO->isDef())
      report("Explicit definition marked as use", MO, MONum);
    else if (MO->isImplicit())
      report("Explicit definition marked as implicit", MO, MONum);
  } else if (MONum < TI.getNumOperands()) {
    if (MO->isReg()) {
      if (MO->isDef())
        report("Explicit operand marked as def", MO, MONum);
      if (MO->isImplicit())
        report("Explicit operand marked as implicit", MO, MONum);
    }
  } else {
    // ARM adds %reg0 operands to indicate predicates. We'll allow that.
    if (MO->isReg() && !MO->isImplicit() && !TI.isVariadic() && MO->getReg())
      report("Extra explicit operand on non-variadic instruction", MO, MONum);
  }

  switch (MO->getType()) {
  case MachineOperand::MO_Register: {
    const unsigned Reg = MO->getReg();
    if (!Reg)
      return;

    // Check Live Variables.
    if (MO->isUndef()) {
      // An <undef> doesn't refer to any register, so just skip it.
    } else if (MO->isUse()) {
      regsLiveInButUnused.erase(Reg);

      bool isKill = false;
      unsigned defIdx;
      if (MI->isRegTiedToDefOperand(MONum, &defIdx)) {
        // A two-addr use counts as a kill if use and def are the same.
        unsigned DefReg = MI->getOperand(defIdx).getReg();
        if (Reg == DefReg) {
          isKill = true;
          // ANd in that case an explicit kill flag is not allowed.
          if (MO->isKill())
            report("Illegal kill flag on two-address instruction operand",
                   MO, MONum);
        } else if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
          report("Two-address instruction operands must be identical",
                 MO, MONum);
        }
      } else
        isKill = MO->isKill();

      if (isKill) {
        addRegWithSubRegs(regsKilled, Reg);

        // Check that LiveVars knows this kill
        if (LiveVars && TargetRegisterInfo::isVirtualRegister(Reg)) {
          LiveVariables::VarInfo &VI = LiveVars->getVarInfo(Reg);
          if (std::find(VI.Kills.begin(),
                        VI.Kills.end(), MI) == VI.Kills.end())
            report("Kill missing from LiveVariables", MO, MONum);
        }
      }

      // Use of a dead register.
      if (!regsLive.count(Reg)) {
        if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
          // Reserved registers may be used even when 'dead'.
          if (!isReserved(Reg))
            report("Using an undefined physical register", MO, MONum);
        } else {
          BBInfo &MInfo = MBBInfoMap[MI->getParent()];
          // We don't know which virtual registers are live in, so only complain
          // if vreg was killed in this MBB. Otherwise keep track of vregs that
          // must be live in. PHI instructions are handled separately.
          if (MInfo.regsKilled.count(Reg))
            report("Using a killed virtual register", MO, MONum);
          else if (!MI->isPHI())
            MInfo.vregsLiveIn.insert(std::make_pair(Reg, MI));
        }
      }
    } else {
      assert(MO->isDef());
      // Register defined.
      // TODO: verify that earlyclobber ops are not used.
      if (MO->isDead())
        addRegWithSubRegs(regsDead, Reg);
      else
        addRegWithSubRegs(regsDefined, Reg);
    }

    // Check register classes.
    if (MONum < TI.getNumOperands() && !MO->isImplicit()) {
      const TargetOperandInfo &TOI = TI.OpInfo[MONum];
      unsigned SubIdx = MO->getSubReg();

      if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
        unsigned sr = Reg;
        if (SubIdx) {
          unsigned s = TRI->getSubReg(Reg, SubIdx);
          if (!s) {
            report("Invalid subregister index for physical register",
                   MO, MONum);
            return;
          }
          sr = s;
        }
        if (const TargetRegisterClass *DRC = TOI.getRegClass(TRI)) {
          if (!DRC->contains(sr)) {
            report("Illegal physical register for instruction", MO, MONum);
            *OS << TRI->getName(sr) << " is not a "
                << DRC->getName() << " register.\n";
          }
        }
      } else {
        // Virtual register.
        const TargetRegisterClass *RC = MRI->getRegClass(Reg);
        if (SubIdx) {
          const TargetRegisterClass *SRC = RC->getSubRegisterRegClass(SubIdx);
          if (!SRC) {
            report("Invalid subregister index for virtual register", MO, MONum);
            *OS << "Register class " << RC->getName()
                << " does not support subreg index " << SubIdx << "\n";
            return;
          }
          RC = SRC;
        }
        if (const TargetRegisterClass *DRC = TOI.getRegClass(TRI)) {
          if (RC != DRC && !RC->hasSuperClass(DRC)) {
            report("Illegal virtual register for instruction", MO, MONum);
            *OS << "Expected a " << DRC->getName() << " register, but got a "
                << RC->getName() << " register\n";
          }
        }
      }
    }
    break;
  }

  case MachineOperand::MO_MachineBasicBlock:
    if (MI->isPHI() && !MO->getMBB()->isSuccessor(MI->getParent()))
      report("PHI operand is not in the CFG", MO, MONum);
    break;

  default:
    break;
  }
}

void MachineVerifier::visitMachineInstrAfter(const MachineInstr *MI) {
  BBInfo &MInfo = MBBInfoMap[MI->getParent()];
  set_union(MInfo.regsKilled, regsKilled);
  set_subtract(regsLive, regsKilled); regsKilled.clear();
  set_subtract(regsLive, regsDead);   regsDead.clear();
  set_union(regsLive, regsDefined);   regsDefined.clear();
}

void
MachineVerifier::visitMachineBasicBlockAfter(const MachineBasicBlock *MBB) {
  MBBInfoMap[MBB].regsLiveOut = regsLive;
  regsLive.clear();
}

// Calculate the largest possible vregsPassed sets. These are the registers that
// can pass through an MBB live, but may not be live every time. It is assumed
// that all vregsPassed sets are empty before the call.
void MachineVerifier::calcRegsPassed() {
  // First push live-out regs to successors' vregsPassed. Remember the MBBs that
  // have any vregsPassed.
  DenseSet<const MachineBasicBlock*> todo;
  for (MachineFunction::const_iterator MFI = MF->begin(), MFE = MF->end();
       MFI != MFE; ++MFI) {
    const MachineBasicBlock &MBB(*MFI);
    BBInfo &MInfo = MBBInfoMap[&MBB];
    if (!MInfo.reachable)
      continue;
    for (MachineBasicBlock::const_succ_iterator SuI = MBB.succ_begin(),
           SuE = MBB.succ_end(); SuI != SuE; ++SuI) {
      BBInfo &SInfo = MBBInfoMap[*SuI];
      if (SInfo.addPassed(MInfo.regsLiveOut))
        todo.insert(*SuI);
    }
  }

  // Iteratively push vregsPassed to successors. This will converge to the same
  // final state regardless of DenseSet iteration order.
  while (!todo.empty()) {
    const MachineBasicBlock *MBB = *todo.begin();
    todo.erase(MBB);
    BBInfo &MInfo = MBBInfoMap[MBB];
    for (MachineBasicBlock::const_succ_iterator SuI = MBB->succ_begin(),
           SuE = MBB->succ_end(); SuI != SuE; ++SuI) {
      if (*SuI == MBB)
        continue;
      BBInfo &SInfo = MBBInfoMap[*SuI];
      if (SInfo.addPassed(MInfo.vregsPassed))
        todo.insert(*SuI);
    }
  }
}

// Calculate the set of virtual registers that must be passed through each basic
// block in order to satisfy the requirements of successor blocks. This is very
// similar to calcRegsPassed, only backwards.
void MachineVerifier::calcRegsRequired() {
  // First push live-in regs to predecessors' vregsRequired.
  DenseSet<const MachineBasicBlock*> todo;
  for (MachineFunction::const_iterator MFI = MF->begin(), MFE = MF->end();
       MFI != MFE; ++MFI) {
    const MachineBasicBlock &MBB(*MFI);
    BBInfo &MInfo = MBBInfoMap[&MBB];
    for (MachineBasicBlock::const_pred_iterator PrI = MBB.pred_begin(),
           PrE = MBB.pred_end(); PrI != PrE; ++PrI) {
      BBInfo &PInfo = MBBInfoMap[*PrI];
      if (PInfo.addRequired(MInfo.vregsLiveIn))
        todo.insert(*PrI);
    }
  }

  // Iteratively push vregsRequired to predecessors. This will converge to the
  // same final state regardless of DenseSet iteration order.
  while (!todo.empty()) {
    const MachineBasicBlock *MBB = *todo.begin();
    todo.erase(MBB);
    BBInfo &MInfo = MBBInfoMap[MBB];
    for (MachineBasicBlock::const_pred_iterator PrI = MBB->pred_begin(),
           PrE = MBB->pred_end(); PrI != PrE; ++PrI) {
      if (*PrI == MBB)
        continue;
      BBInfo &SInfo = MBBInfoMap[*PrI];
      if (SInfo.addRequired(MInfo.vregsRequired))
        todo.insert(*PrI);
    }
  }
}

// Check PHI instructions at the beginning of MBB. It is assumed that
// calcRegsPassed has been run so BBInfo::isLiveOut is valid.
void MachineVerifier::checkPHIOps(const MachineBasicBlock *MBB) {
  for (MachineBasicBlock::const_iterator BBI = MBB->begin(), BBE = MBB->end();
       BBI != BBE && BBI->isPHI(); ++BBI) {
    DenseSet<const MachineBasicBlock*> seen;

    for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
      unsigned Reg = BBI->getOperand(i).getReg();
      const MachineBasicBlock *Pre = BBI->getOperand(i + 1).getMBB();
      if (!Pre->isSuccessor(MBB))
        continue;
      seen.insert(Pre);
      BBInfo &PrInfo = MBBInfoMap[Pre];
      if (PrInfo.reachable && !PrInfo.isLiveOut(Reg))
        report("PHI operand is not live-out from predecessor",
               &BBI->getOperand(i), i);
    }

    // Did we see all predecessors?
    for (MachineBasicBlock::const_pred_iterator PrI = MBB->pred_begin(),
           PrE = MBB->pred_end(); PrI != PrE; ++PrI) {
      if (!seen.count(*PrI)) {
        report("Missing PHI operand", BBI);
        *OS << "BB#" << (*PrI)->getNumber()
            << " is a predecessor according to the CFG.\n";
      }
    }
  }
}

void MachineVerifier::visitMachineFunctionAfter() {
  calcRegsPassed();

  for (MachineFunction::const_iterator MFI = MF->begin(), MFE = MF->end();
       MFI != MFE; ++MFI) {
    BBInfo &MInfo = MBBInfoMap[MFI];

    // Skip unreachable MBBs.
    if (!MInfo.reachable)
      continue;

    checkPHIOps(MFI);
  }

  // Now check LiveVariables info if available
  if (LiveVars) {
    calcRegsRequired();
    verifyLiveVariables();
  }
}

void MachineVerifier::verifyLiveVariables() {
  assert(LiveVars && "Don't call verifyLiveVariables without LiveVars");
  for (unsigned Reg = TargetRegisterInfo::FirstVirtualRegister,
         RegE = MRI->getLastVirtReg()-1; Reg != RegE; ++Reg) {
    LiveVariables::VarInfo &VI = LiveVars->getVarInfo(Reg);
    for (MachineFunction::const_iterator MFI = MF->begin(), MFE = MF->end();
         MFI != MFE; ++MFI) {
      BBInfo &MInfo = MBBInfoMap[MFI];

      // Our vregsRequired should be identical to LiveVariables' AliveBlocks
      if (MInfo.vregsRequired.count(Reg)) {
        if (!VI.AliveBlocks.test(MFI->getNumber())) {
          report("LiveVariables: Block missing from AliveBlocks", MFI);
          *OS << "Virtual register %reg" << Reg
              << " must be live through the block.\n";
        }
      } else {
        if (VI.AliveBlocks.test(MFI->getNumber())) {
          report("LiveVariables: Block should not be in AliveBlocks", MFI);
          *OS << "Virtual register %reg" << Reg
              << " is not needed live through the block.\n";
        }
      }
    }
  }
}