lib/CodeGen/LiveVariables.cpp

//===-- LiveVariables.cpp - Live Variable Analysis for Machine Code -------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the LiveVariable analysis pass.  For each machine
// instruction in the function, this pass calculates the set of registers that
// are immediately dead after the instruction (i.e., the instruction calculates
// the value, but it is never used) and the set of registers that are used by
// the instruction, but are never used after the instruction (i.e., they are
// killed).
//
// This class computes live variables using are sparse implementation based on
// the machine code SSA form.  This class computes live variable information for
// each virtual and _register allocatable_ physical register in a function.  It
// uses the dominance properties of SSA form to efficiently compute live
// variables for virtual registers, and assumes that physical registers are only
// live within a single basic block (allowing it to do a single local analysis
// to resolve physical register lifetimes in each basic block).  If a physical
// register is not register allocatable, it is not tracked.  This is useful for
// things like the stack pointer and condition codes.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Config/alloca.h"
#include <algorithm>
using namespace llvm;

char LiveVariables::ID = 0;
static RegisterPass<LiveVariables> X("livevars", "Live Variable Analysis");

void LiveVariables::VarInfo::dump() const {
  cerr << "  Alive in blocks: ";
  for (unsigned i = 0, e = AliveBlocks.size(); i != e; ++i)
    if (AliveBlocks[i]) cerr << i << ", ";
  cerr << "  Used in blocks: ";
  for (unsigned i = 0, e = UsedBlocks.size(); i != e; ++i)
    if (UsedBlocks[i]) cerr << i << ", ";
  cerr << "\n  Killed by:";
  if (Kills.empty())
    cerr << " No instructions.\n";
  else {
    for (unsigned i = 0, e = Kills.size(); i != e; ++i)
      cerr << "\n    #" << i << ": " << *Kills[i];
    cerr << "\n";
  }
}

/// getVarInfo - Get (possibly creating) a VarInfo object for the given vreg.
LiveVariables::VarInfo &LiveVariables::getVarInfo(unsigned RegIdx) {
  assert(TargetRegisterInfo::isVirtualRegister(RegIdx) &&
         "getVarInfo: not a virtual register!");
  RegIdx -= TargetRegisterInfo::FirstVirtualRegister;
  if (RegIdx >= VirtRegInfo.size()) {
    if (RegIdx >= 2*VirtRegInfo.size())
      VirtRegInfo.resize(RegIdx*2);
    else
      VirtRegInfo.resize(2*VirtRegInfo.size());
  }
  VarInfo &VI = VirtRegInfo[RegIdx];
  VI.AliveBlocks.resize(MF->getNumBlockIDs());
  VI.UsedBlocks.resize(MF->getNumBlockIDs());
  return VI;
}

void LiveVariables::MarkVirtRegAliveInBlock(VarInfo& VRInfo,
                                            MachineBasicBlock *DefBlock,
                                            MachineBasicBlock *MBB,
                                    std::vector<MachineBasicBlock*> &WorkList) {
  unsigned BBNum = MBB->getNumber();
  
  // Check to see if this basic block is one of the killing blocks.  If so,
  // remove it.
  for (unsigned i = 0, e = VRInfo.Kills.size(); i != e; ++i)
    if (VRInfo.Kills[i]->getParent() == MBB) {
      VRInfo.Kills.erase(VRInfo.Kills.begin()+i);  // Erase entry
      break;
    }
  
  if (MBB == DefBlock) return;  // Terminate recursion

  if (VRInfo.AliveBlocks[BBNum])
    return;  // We already know the block is live

  // Mark the variable known alive in this bb
  VRInfo.AliveBlocks[BBNum] = true;

  for (MachineBasicBlock::const_pred_reverse_iterator PI = MBB->pred_rbegin(),
         E = MBB->pred_rend(); PI != E; ++PI)
    WorkList.push_back(*PI);
}

void LiveVariables::MarkVirtRegAliveInBlock(VarInfo &VRInfo,
                                            MachineBasicBlock *DefBlock,
                                            MachineBasicBlock *MBB) {
  std::vector<MachineBasicBlock*> WorkList;
  MarkVirtRegAliveInBlock(VRInfo, DefBlock, MBB, WorkList);

  while (!WorkList.empty()) {
    MachineBasicBlock *Pred = WorkList.back();
    WorkList.pop_back();
    MarkVirtRegAliveInBlock(VRInfo, DefBlock, Pred, WorkList);
  }
}

void LiveVariables::HandleVirtRegUse(unsigned reg, MachineBasicBlock *MBB,
                                     MachineInstr *MI) {
  const MachineRegisterInfo &MRI = MBB->getParent()->getRegInfo();
  assert(MRI.getVRegDef(reg) && "Register use before def!");

  unsigned BBNum = MBB->getNumber();

  VarInfo& VRInfo = getVarInfo(reg);
  VRInfo.UsedBlocks[BBNum] = true;
  VRInfo.NumUses++;

  // Check to see if this basic block is already a kill block.
  if (!VRInfo.Kills.empty() && VRInfo.Kills.back()->getParent() == MBB) {
    // Yes, this register is killed in this basic block already. Increase the
    // live range by updating the kill instruction.
    VRInfo.Kills.back() = MI;
    return;
  }

#ifndef NDEBUG
  for (unsigned i = 0, e = VRInfo.Kills.size(); i != e; ++i)
    assert(VRInfo.Kills[i]->getParent() != MBB && "entry should be at end!");
#endif

  assert(MBB != MRI.getVRegDef(reg)->getParent() &&
         "Should have kill for defblock!");

  // Add a new kill entry for this basic block. If this virtual register is
  // already marked as alive in this basic block, that means it is alive in at
  // least one of the successor blocks, it's not a kill.
  if (!VRInfo.AliveBlocks[BBNum])
    VRInfo.Kills.push_back(MI);

  // Update all dominating blocks to mark them as "known live".
  for (MachineBasicBlock::const_pred_iterator PI = MBB->pred_begin(),
         E = MBB->pred_end(); PI != E; ++PI)
    MarkVirtRegAliveInBlock(VRInfo, MRI.getVRegDef(reg)->getParent(), *PI);
}

/// HandlePhysRegUse - Turn previous partial def's into read/mod/writes. Add
/// implicit defs to a machine instruction if there was an earlier def of its
/// super-register.
void LiveVariables::HandlePhysRegUse(unsigned Reg, MachineInstr *MI) {
  // Turn previous partial def's into read/mod/write.
  for (unsigned i = 0, e = PhysRegPartDef[Reg].size(); i != e; ++i) {
    MachineInstr *Def = PhysRegPartDef[Reg][i];

    // First one is just a def. This means the use is reading some undef bits.
    if (i != 0)
      Def->addOperand(MachineOperand::CreateReg(Reg,
                                                false /*IsDef*/,
                                                true  /*IsImp*/,
                                                true  /*IsKill*/));

    Def->addOperand(MachineOperand::CreateReg(Reg,
                                              true  /*IsDef*/,
                                              true  /*IsImp*/));
  }

  PhysRegPartDef[Reg].clear();

  // There was an earlier def of a super-register. Add implicit def to that MI.
  //
  //   A: EAX = ...
  //   B: ... = AX
  //
  // Add implicit def to A.
  if (PhysRegInfo[Reg] && PhysRegInfo[Reg] != PhysRegPartUse[Reg] &&
      !PhysRegUsed[Reg]) {
    MachineInstr *Def = PhysRegInfo[Reg];

    if (!Def->modifiesRegister(Reg))
      Def->addOperand(MachineOperand::CreateReg(Reg,
                                                true  /*IsDef*/,
                                                true  /*IsImp*/));
  }

  // There is a now a proper use, forget about the last partial use.
  PhysRegPartUse[Reg] = NULL;
  PhysRegInfo[Reg] = MI;
  PhysRegUsed[Reg] = true;

  // Now reset the use information for the sub-registers.
  for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
       unsigned SubReg = *SubRegs; ++SubRegs) {
    PhysRegPartUse[SubReg] = NULL;
    PhysRegInfo[SubReg] = MI;
    PhysRegUsed[SubReg] = true;
  }

  for (const unsigned *SuperRegs = TRI->getSuperRegisters(Reg);
       unsigned SuperReg = *SuperRegs; ++SuperRegs) {
    // Remember the partial use of this super-register if it was previously
    // defined.
    bool HasPrevDef = PhysRegInfo[SuperReg] != NULL;

    if (!HasPrevDef)
      // No need to go up more levels. A def of a register also sets its sub-
      // registers. So if PhysRegInfo[SuperReg] is NULL, it means SuperReg's
      // super-registers are not previously defined.
      for (const unsigned *SSRegs = TRI->getSuperRegisters(SuperReg);
           unsigned SSReg = *SSRegs; ++SSRegs)
        if (PhysRegInfo[SSReg] != NULL) {
          HasPrevDef = true;
          break;
        }

    if (HasPrevDef) {
      PhysRegInfo[SuperReg] = MI;
      PhysRegPartUse[SuperReg] = MI;
    }
  }
}

/// addRegisterKills - For all of a register's sub-registers that are killed in
/// at this machine instruction, mark them as "killed". (If the machine operand
/// isn't found, add it first.)
void LiveVariables::addRegisterKills(unsigned Reg, MachineInstr *MI,
                                     SmallSet<unsigned, 4> &SubKills) {
  if (SubKills.count(Reg) == 0) {
    MI->addRegisterKilled(Reg, TRI, true);
    return;
  }

  for (const unsigned *SubRegs = TRI->getImmediateSubRegisters(Reg);
       unsigned SubReg = *SubRegs; ++SubRegs)
    addRegisterKills(SubReg, MI, SubKills);
}

/// HandlePhysRegKill - The recursive version of HandlePhysRegKill. Returns true
/// if:
///
///   - The register has no sub-registers and the machine instruction is the
///     last def/use of the register, or
///   - The register has sub-registers and none of them are killed elsewhere.
///
/// SubKills is filled with the set of sub-registers that are killed elsewhere.
bool LiveVariables::HandlePhysRegKill(unsigned Reg, const MachineInstr *RefMI,
                                      SmallSet<unsigned, 4> &SubKills) {
  const unsigned *SubRegs = TRI->getImmediateSubRegisters(Reg);

  for (; unsigned SubReg = *SubRegs; ++SubRegs) {
    const MachineInstr *LastRef = PhysRegInfo[SubReg];

    if (LastRef != RefMI ||
        !HandlePhysRegKill(SubReg, RefMI, SubKills))
      SubKills.insert(SubReg);
  }

  if (*SubRegs == 0) {
    // No sub-registers, just check if reg is killed by RefMI.
    if (PhysRegInfo[Reg] == RefMI)
      return true;
  } else if (SubKills.empty()) {
    // None of the sub-registers are killed elsewhere.
    return true;
  }

  return false;
}

/// HandlePhysRegKill - Returns true if the whole register is killed in the
/// machine instruction. If only some of its sub-registers are killed in this
/// machine instruction, then mark those as killed and return false.
bool LiveVariables::HandlePhysRegKill(unsigned Reg, MachineInstr *RefMI) {
  SmallSet<unsigned, 4> SubKills;

  if (HandlePhysRegKill(Reg, RefMI, SubKills)) {
    // This machine instruction kills this register.
    RefMI->addRegisterKilled(Reg, TRI, true);
    return true;
  }

  // Some sub-registers are killed by another machine instruction.
  for (const unsigned *SubRegs = TRI->getImmediateSubRegisters(Reg);
       unsigned SubReg = *SubRegs; ++SubRegs)
    addRegisterKills(SubReg, RefMI, SubKills);

  return false;
}

void LiveVariables::HandlePhysRegDef(unsigned Reg, MachineInstr *MI) {
  // Does this kill a previous version of this register?
  if (MachineInstr *LastRef = PhysRegInfo[Reg]) {
    if (PhysRegUsed[Reg]) {
      if (!HandlePhysRegKill(Reg, LastRef)) {
        if (PhysRegPartUse[Reg])
          PhysRegPartUse[Reg]->addRegisterKilled(Reg, TRI, true);
      }
    } else if (PhysRegPartUse[Reg]) {
      // Add implicit use / kill to last partial use.
      PhysRegPartUse[Reg]->addRegisterKilled(Reg, TRI, true);
    } else if (LastRef != MI) {
      // Defined, but not used. However, watch out for cases where a super-reg
      // is also defined on the same MI.
      LastRef->addRegisterDead(Reg, TRI);
    }
  }

  for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
       unsigned SubReg = *SubRegs; ++SubRegs) {
    if (MachineInstr *LastRef = PhysRegInfo[SubReg]) {
      if (PhysRegUsed[SubReg]) {
        if (!HandlePhysRegKill(SubReg, LastRef)) {
          if (PhysRegPartUse[SubReg])
            PhysRegPartUse[SubReg]->addRegisterKilled(SubReg, TRI, true);
        }
      } else if (PhysRegPartUse[SubReg]) {
        // Add implicit use / kill to last use of a sub-register.
        PhysRegPartUse[SubReg]->addRegisterKilled(SubReg, TRI, true);
      } else if (LastRef != MI) {
        // This must be a def of the subreg on the same MI.
        LastRef->addRegisterDead(SubReg, TRI);
      }
    }
  }

  if (MI) {
    for (const unsigned *SuperRegs = TRI->getSuperRegisters(Reg);
         unsigned SuperReg = *SuperRegs; ++SuperRegs) {
      if (PhysRegInfo[SuperReg] && PhysRegInfo[SuperReg] != MI) {
        // The larger register is previously defined. Now a smaller part is
        // being re-defined. Treat it as read/mod/write.
        // EAX =
        // AX  =        EAX<imp-use,kill>, EAX<imp-def>
        MI->addOperand(MachineOperand::CreateReg(SuperReg, false/*IsDef*/,
                                                 true/*IsImp*/,true/*IsKill*/));
        MI->addOperand(MachineOperand::CreateReg(SuperReg, true/*IsDef*/,
                                                 true/*IsImp*/));
        PhysRegInfo[SuperReg] = MI;
        PhysRegUsed[SuperReg] = false;
        PhysRegPartUse[SuperReg] = NULL;
      } else {
        // Remember this partial def.
        PhysRegPartDef[SuperReg].push_back(MI);
      }
    }

    PhysRegInfo[Reg] = MI;
    PhysRegUsed[Reg] = false;
    PhysRegPartDef[Reg].clear();
    PhysRegPartUse[Reg] = NULL;

    for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
         unsigned SubReg = *SubRegs; ++SubRegs) {
      PhysRegInfo[SubReg] = MI;
      PhysRegUsed[SubReg] = false;
      PhysRegPartDef[SubReg].clear();
      PhysRegPartUse[SubReg] = NULL;
    }
  }
}

bool LiveVariables::runOnMachineFunction(MachineFunction &mf) {
  MF = &mf;
  TRI = MF->getTarget().getRegisterInfo();
  MachineRegisterInfo& MRI = mf.getRegInfo();

  ReservedRegisters = TRI->getReservedRegs(mf);

  unsigned NumRegs = TRI->getNumRegs();
  PhysRegInfo = new MachineInstr*[NumRegs];
  PhysRegUsed = new bool[NumRegs];
  PhysRegPartUse = new MachineInstr*[NumRegs];
  PhysRegPartDef = new SmallVector<MachineInstr*,4>[NumRegs];
  PHIVarInfo = new SmallVector<unsigned, 4>[MF->getNumBlockIDs()];
  std::fill(PhysRegInfo, PhysRegInfo + NumRegs, (MachineInstr*)0);
  std::fill(PhysRegUsed, PhysRegUsed + NumRegs, false);
  std::fill(PhysRegPartUse, PhysRegPartUse + NumRegs, (MachineInstr*)0);

  /// Get some space for a respectable number of registers.
  VirtRegInfo.resize(64);

  analyzePHINodes(mf);

  // Calculate live variable information in depth first order on the CFG of the
  // function.  This guarantees that we will see the definition of a virtual
  // register before its uses due to dominance properties of SSA (except for PHI
  // nodes, which are treated as a special case).
  MachineBasicBlock *Entry = MF->begin();
  SmallPtrSet<MachineBasicBlock*,16> Visited;

  for (df_ext_iterator<MachineBasicBlock*, SmallPtrSet<MachineBasicBlock*,16> >
         DFI = df_ext_begin(Entry, Visited), E = df_ext_end(Entry, Visited);
       DFI != E; ++DFI) {
    MachineBasicBlock *MBB = *DFI;

    // Mark live-in registers as live-in.
    for (MachineBasicBlock::const_livein_iterator II = MBB->livein_begin(),
           EE = MBB->livein_end(); II != EE; ++II) {
      assert(TargetRegisterInfo::isPhysicalRegister(*II) &&
             "Cannot have a live-in virtual register!");
      HandlePhysRegDef(*II, 0);
    }

    // Loop over all of the instructions, processing them.
    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
         I != E; ++I) {
      MachineInstr *MI = I;

      // Process all of the operands of the instruction...
      unsigned NumOperandsToProcess = MI->getNumOperands();

      // Unless it is a PHI node.  In this case, ONLY process the DEF, not any
      // of the uses.  They will be handled in other basic blocks.
      if (MI->getOpcode() == TargetInstrInfo::PHI)
        NumOperandsToProcess = 1;

      // Process all uses.
      for (unsigned i = 0; i != NumOperandsToProcess; ++i) {
        const MachineOperand &MO = MI->getOperand(i);

        if (MO.isRegister() && MO.isUse() && MO.getReg()) {
          unsigned MOReg = MO.getReg();

          if (TargetRegisterInfo::isVirtualRegister(MOReg))
            HandleVirtRegUse(MOReg, MBB, MI);
          else if (TargetRegisterInfo::isPhysicalRegister(MOReg) &&
                   !ReservedRegisters[MOReg])
            HandlePhysRegUse(MOReg, MI);
        }
      }

      // Process all defs.
      for (unsigned i = 0; i != NumOperandsToProcess; ++i) {
        const MachineOperand &MO = MI->getOperand(i);

        if (MO.isRegister() && MO.isDef() && MO.getReg()) {
          unsigned MOReg = MO.getReg();

          if (TargetRegisterInfo::isVirtualRegister(MOReg)) {
            VarInfo &VRInfo = getVarInfo(MOReg);

            if (VRInfo.AliveBlocks.none())
              // If vr is not alive in any block, then defaults to dead.
              VRInfo.Kills.push_back(MI);
          } else if (TargetRegisterInfo::isPhysicalRegister(MOReg) &&
                     !ReservedRegisters[MOReg]) {
            HandlePhysRegDef(MOReg, MI);
          }
        }
      }
    }

    // Handle any virtual assignments from PHI nodes which might be at the
    // bottom of this basic block.  We check all of our successor blocks to see
    // if they have PHI nodes, and if so, we simulate an assignment at the end
    // of the current block.
    if (!PHIVarInfo[MBB->getNumber()].empty()) {
      SmallVector<unsigned, 4>& VarInfoVec = PHIVarInfo[MBB->getNumber()];

      for (SmallVector<unsigned, 4>::iterator I = VarInfoVec.begin(),
             E = VarInfoVec.end(); I != E; ++I)
        // Mark it alive only in the block we are representing.
        MarkVirtRegAliveInBlock(getVarInfo(*I), MRI.getVRegDef(*I)->getParent(),
                                MBB);
    }

    // Finally, if the last instruction in the block is a return, make sure to
    // mark it as using all of the live-out values in the function.
    if (!MBB->empty() && MBB->back().getDesc().isReturn()) {
      MachineInstr *Ret = &MBB->back();

      for (MachineRegisterInfo::liveout_iterator
           I = MF->getRegInfo().liveout_begin(),
           E = MF->getRegInfo().liveout_end(); I != E; ++I) {
        assert(TargetRegisterInfo::isPhysicalRegister(*I) &&
               "Cannot have a live-in virtual register!");
        HandlePhysRegUse(*I, Ret);

        // Add live-out registers as implicit uses.
        if (!Ret->readsRegister(*I))
          Ret->addOperand(MachineOperand::CreateReg(*I, false, true));
      }
    }

    // Loop over PhysRegInfo, killing any registers that are available at the
    // end of the basic block. This also resets the PhysRegInfo map.
    for (unsigned i = 0; i != NumRegs; ++i)
      if (PhysRegInfo[i])
        HandlePhysRegDef(i, 0);

    // Clear some states between BB's. These are purely local information.
    for (unsigned i = 0; i != NumRegs; ++i)
      PhysRegPartDef[i].clear();

    std::fill(PhysRegInfo, PhysRegInfo + NumRegs, (MachineInstr*)0);
    std::fill(PhysRegUsed, PhysRegUsed + NumRegs, false);
    std::fill(PhysRegPartUse, PhysRegPartUse + NumRegs, (MachineInstr*)0);
  }

  // Convert and transfer the dead / killed information we have gathered into
  // VirtRegInfo onto MI's.
  for (unsigned i = 0, e1 = VirtRegInfo.size(); i != e1; ++i)
    for (unsigned j = 0, e2 = VirtRegInfo[i].Kills.size(); j != e2; ++j)
      if (VirtRegInfo[i].Kills[j] ==
          MRI.getVRegDef(i + TargetRegisterInfo::FirstVirtualRegister))
        VirtRegInfo[i]
          .Kills[j]->addRegisterDead(i +
                                     TargetRegisterInfo::FirstVirtualRegister,
                                     TRI);
      else
        VirtRegInfo[i]
          .Kills[j]->addRegisterKilled(i +
                                       TargetRegisterInfo::FirstVirtualRegister,
                                       TRI);

  // Check to make sure there are no unreachable blocks in the MC CFG for the
  // function.  If so, it is due to a bug in the instruction selector or some
  // other part of the code generator if this happens.
#ifndef NDEBUG
  for(MachineFunction::iterator i = MF->begin(), e = MF->end(); i != e; ++i)
    assert(Visited.count(&*i) != 0 && "unreachable basic block found");
#endif

  delete[] PhysRegInfo;
  delete[] PhysRegUsed;
  delete[] PhysRegPartUse;
  delete[] PhysRegPartDef;
  delete[] PHIVarInfo;

  return false;
}

/// instructionChanged - When the address of an instruction changes, this method
/// should be called so that live variables can update its internal data
/// structures.  This removes the records for OldMI, transfering them to the
/// records for NewMI.
void LiveVariables::instructionChanged(MachineInstr *OldMI,
                                       MachineInstr *NewMI) {
  // If the instruction defines any virtual registers, update the VarInfo,
  // kill and dead information for the instruction.
  for (unsigned i = 0, e = OldMI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = OldMI->getOperand(i);
    if (MO.isRegister() && MO.getReg() &&
        TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
      unsigned Reg = MO.getReg();
      VarInfo &VI = getVarInfo(Reg);
      if (MO.isDef()) {
        if (MO.isDead()) {
          MO.setIsDead(false);
          addVirtualRegisterDead(Reg, NewMI);
        }
      }
      if (MO.isKill()) {
        MO.setIsKill(false);
        addVirtualRegisterKilled(Reg, NewMI);
      }
      // If this is a kill of the value, update the VI kills list.
      if (VI.removeKill(OldMI))
        VI.Kills.push_back(NewMI);   // Yes, there was a kill of it
    }
  }
}

/// removeVirtualRegistersKilled - Remove all killed info for the specified
/// instruction.
void LiveVariables::removeVirtualRegistersKilled(MachineInstr *MI) {
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI->getOperand(i);
    if (MO.isRegister() && MO.isKill()) {
      MO.setIsKill(false);
      unsigned Reg = MO.getReg();
      if (TargetRegisterInfo::isVirtualRegister(Reg)) {
        bool removed = getVarInfo(Reg).removeKill(MI);
        assert(removed && "kill not in register's VarInfo?");
      }
    }
  }
}

/// removeVirtualRegistersDead - Remove all of the dead registers for the
/// specified instruction from the live variable information.
void LiveVariables::removeVirtualRegistersDead(MachineInstr *MI) {
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI->getOperand(i);
    if (MO.isRegister() && MO.isDead()) {
      MO.setIsDead(false);
      unsigned Reg = MO.getReg();
      if (TargetRegisterInfo::isVirtualRegister(Reg)) {
        bool removed = getVarInfo(Reg).removeKill(MI);
        assert(removed && "kill not in register's VarInfo?");
      }
    }
  }
}

/// analyzePHINodes - Gather information about the PHI nodes in here. In
/// particular, we want to map the variable information of a virtual register
/// which is used in a PHI node. We map that to the BB the vreg is coming from.
///
void LiveVariables::analyzePHINodes(const MachineFunction& Fn) {
  for (MachineFunction::const_iterator I = Fn.begin(), E = Fn.end();
       I != E; ++I)
    for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end();
         BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
      for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
        PHIVarInfo[BBI->getOperand(i + 1).getMBB()->getNumber()]
          .push_back(BBI->getOperand(i).getReg());
}