PHIElimination.cpp

//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass eliminates machine instruction PHI nodes by inserting copy
// instructions.  This destroys SSA information, but is the desired input for
// some register allocators.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/SSARegMap.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include <set>
using namespace llvm;

namespace {
  struct PNE : public MachineFunctionPass {
    bool runOnMachineFunction(MachineFunction &Fn) {
      bool Changed = false;

      // Eliminate PHI instructions by inserting copies into predecessor blocks.
      for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
        Changed |= EliminatePHINodes(Fn, *I);

      return Changed;
    }

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addPreserved<LiveVariables>();
      MachineFunctionPass::getAnalysisUsage(AU);
    }

  private:
    /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
    /// in predecessor basic blocks.
    ///
    bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
    void LowerAtomicPHINode(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator AfterPHIsIt,
                            DenseMap<unsigned, VirtReg2IndexFunctor> &VUC,
                            unsigned BBIsSuccOfPreds);
  };

  RegisterPass<PNE> X("phi-node-elimination",
                      "Eliminate PHI nodes for register allocation");
}


const PassInfo *llvm::PHIEliminationID = X.getPassInfo();

/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
/// predecessor basic blocks.
///
bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
  if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
    return false;   // Quick exit for basic blocks without PHIs.

  // VRegPHIUseCount - Keep track of the number of times each virtual register
  // is used by PHI nodes in successors of this block.
  DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount;
  VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg());

  unsigned BBIsSuccOfPreds = 0;  // Number of times MBB is a succ of preds
  for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(),
         E = MBB.pred_end(); PI != E; ++PI)
    for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(),
           E = (*PI)->succ_end(); SI != E; ++SI) {
    BBIsSuccOfPreds += *SI == &MBB;
    for (MachineBasicBlock::iterator BBI = (*SI)->begin(); BBI !=(*SI)->end() &&
           BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
      for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
        VRegPHIUseCount[BBI->getOperand(i).getReg()]++;
  }

  // Get an iterator to the first instruction after the last PHI node (this may
  // also be the end of the basic block).
  MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
  while (AfterPHIsIt != MBB.end() &&
         AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI)
    ++AfterPHIsIt;    // Skip over all of the PHI nodes...

  while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
    LowerAtomicPHINode(MBB, AfterPHIsIt, VRegPHIUseCount, BBIsSuccOfPreds);
  }
  return true;
}

/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
/// under the assuption that it needs to be lowered in a way that supports
/// atomic execution of PHIs.  This lowering method is always correct all of the
/// time.
void PNE::LowerAtomicPHINode(MachineBasicBlock &MBB,
                             MachineBasicBlock::iterator AfterPHIsIt,
                      DenseMap<unsigned, VirtReg2IndexFunctor> &VRegPHIUseCount,
                             unsigned BBIsSuccOfPreds) {
  // Unlink the PHI node from the basic block, but don't delete the PHI yet.
  MachineInstr *MPhi = MBB.remove(MBB.begin());

  unsigned DestReg = MPhi->getOperand(0).getReg();

  // Create a new register for the incoming PHI arguments/
  MachineFunction &MF = *MBB.getParent();
  const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
  unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);

  // Insert a register to register copy in the top of the current block (but
  // after any remaining phi nodes) which copies the new incoming register
  // into the phi node destination.
  //
  const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
  RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);

  // Update live variable information if there is any...
  LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
  if (LV) {
    MachineInstr *PHICopy = prior(AfterPHIsIt);

    // Add information to LiveVariables to know that the incoming value is
    // killed.  Note that because the value is defined in several places (once
    // each for each incoming block), the "def" block and instruction fields
    // for the VarInfo is not filled in.
    //
    LV->addVirtualRegisterKilled(IncomingReg, PHICopy);

    // Since we are going to be deleting the PHI node, if it is the last use
    // of any registers, or if the value itself is dead, we need to move this
    // information over to the new copy we just inserted.
    //
    LV->removeVirtualRegistersKilled(MPhi);

    std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
      RKs = LV->dead_range(MPhi);
    if (RKs.first != RKs.second) {
      for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I)
        LV->addVirtualRegisterDead(*I, PHICopy);
      LV->removeVirtualRegistersDead(MPhi);
    }
  }

  // Adjust the VRegPHIUseCount map to account for the removal of this PHI
  // node.
  for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
    VRegPHIUseCount[MPhi->getOperand(i).getReg()] -= BBIsSuccOfPreds;

  // Now loop over all of the incoming arguments, changing them to copy into
  // the IncomingReg register in the corresponding predecessor basic block.
  //
  for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) {
    MachineOperand &opVal = MPhi->getOperand(i-1);

    // Get the MachineBasicBlock equivalent of the BasicBlock that is the
    // source path the PHI.
    MachineBasicBlock &opBlock = *MPhi->getOperand(i).getMachineBasicBlock();

    MachineBasicBlock::iterator I = opBlock.getFirstTerminator();

    // Check to make sure we haven't already emitted the copy for this block.
    // This can happen because PHI nodes may have multiple entries for the
    // same basic block.  It doesn't matter which entry we use though, because
    // all incoming values are guaranteed to be the same for a particular bb.
    //
    // If we emitted a copy for this basic block already, it will be right
    // where we want to insert one now.  Just check for a definition of the
    // register we are interested in!
    //
    bool HaveNotEmitted = true;

    if (I != opBlock.begin()) {
      MachineBasicBlock::iterator PrevInst = prior(I);
      for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
        MachineOperand &MO = PrevInst->getOperand(i);
        if (MO.isRegister() && MO.getReg() == IncomingReg)
          if (MO.isDef()) {
            HaveNotEmitted = false;
            break;
          }
      }
    }

    if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
      assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) &&
             "Machine PHI Operands must all be virtual registers!");
      unsigned SrcReg = opVal.getReg();
      RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);

      // Now update live variable information if we have it.
      if (LV) {
        // We want to be able to insert a kill of the register if this PHI
        // (aka, the copy we just inserted) is the last use of the source
        // value.  Live variable analysis conservatively handles this by
        // saying that the value is live until the end of the block the PHI
        // entry lives in.  If the value really is dead at the PHI copy, there
        // will be no successor blocks which have the value live-in.
        //
        // Check to see if the copy is the last use, and if so, update the
        // live variables information so that it knows the copy source
        // instruction kills the incoming value.
        //
        LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);

        // Loop over all of the successors of the basic block, checking to see
        // if the value is either live in the block, or if it is killed in the
        // block.  Also check to see if this register is in use by another PHI
        // node which has not yet been eliminated.  If so, it will be killed
        // at an appropriate point later.
        //
        bool ValueIsLive = false;
        for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
               E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
          MachineBasicBlock *SuccMBB = *SI;

          // Is it alive in this successor?
          unsigned SuccIdx = SuccMBB->getNumber();
          if (SuccIdx < InRegVI.AliveBlocks.size() &&
              InRegVI.AliveBlocks[SuccIdx]) {
            ValueIsLive = true;
            break;
          }

          // Is it killed in this successor?
          for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
            if (InRegVI.Kills[i]->getParent() == SuccMBB) {
              ValueIsLive = true;
              break;
            }

          // Is it used by any PHI instructions in this block?
          if (!ValueIsLive)
            ValueIsLive = VRegPHIUseCount[SrcReg] != 0;
        }

        // Okay, if we now know that the value is not live out of the block,
        // we can add a kill marker to the copy we inserted saying that it
        // kills the incoming value!
        //
        if (!ValueIsLive) {
          MachineBasicBlock::iterator Prev = prior(I);
          LV->addVirtualRegisterKilled(SrcReg, Prev);

          // This vreg no longer lives all of the way through opBlock.
          unsigned opBlockNum = opBlock.getNumber();
          if (opBlockNum < InRegVI.AliveBlocks.size())
            InRegVI.AliveBlocks[opBlockNum] = false;
        }
      }
    }
  }
    
  // Really delete the PHI instruction now!
  delete MPhi;
}