TwoAddressInstructionPass.cpp

//===-- TwoAddressInstructionPass.cpp - Two-Address instruction pass ------===//
//
//                     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 TwoAddress instruction pass which is used
// by most register allocators. Two-Address instructions are rewritten
// from:
//
//     A = B op C
//
// to:
//
//     A = B
//     A op= C
//
// Note that if a register allocator chooses to use this pass, that it
// has to be capable of handling the non-SSA nature of these rewritten
// virtual registers.
//
// It is also worth noting that the duplicate operand of the two
// address instruction is removed.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "twoaddrinstr"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/MachineFunctionPass.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/Target/TargetOptions.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
using namespace llvm;

STATISTIC(NumTwoAddressInstrs, "Number of two-address instructions");
STATISTIC(NumCommuted        , "Number of instructions commuted to coalesce");
STATISTIC(NumAggrCommuted    , "Number of instructions aggressively commuted");
STATISTIC(NumConvertedTo3Addr, "Number of instructions promoted to 3-address");
STATISTIC(Num3AddrSunk,        "Number of 3-address instructions sunk");
STATISTIC(NumReMats,           "Number of instructions re-materialized");
STATISTIC(NumDeletes,          "Number of dead instructions deleted");

namespace {
  class VISIBILITY_HIDDEN TwoAddressInstructionPass
    : public MachineFunctionPass {
    const TargetInstrInfo *TII;
    const TargetRegisterInfo *TRI;
    MachineRegisterInfo *MRI;
    LiveVariables *LV;

    // DistanceMap - Keep track the distance of a MI from the start of the
    // current basic block.
    DenseMap<MachineInstr*, unsigned> DistanceMap;

    // SrcRegMap - A map from virtual registers to physical registers which
    // are likely targets to be coalesced to due to copies from physical
    // registers to virtual registers. e.g. v1024 = move r0.
    DenseMap<unsigned, unsigned> SrcRegMap;

    // DstRegMap - A map from virtual registers to physical registers which
    // are likely targets to be coalesced to due to copies to physical
    // registers from virtual registers. e.g. r1 = move v1024.
    DenseMap<unsigned, unsigned> DstRegMap;

    bool Sink3AddrInstruction(MachineBasicBlock *MBB, MachineInstr *MI,
                              unsigned Reg,
                              MachineBasicBlock::iterator OldPos);

    bool isProfitableToReMat(unsigned Reg, const TargetRegisterClass *RC,
                             MachineInstr *MI, MachineInstr *DefMI,
                             MachineBasicBlock *MBB, unsigned Loc);

    bool NoUseAfterLastDef(unsigned Reg, MachineBasicBlock *MBB, unsigned Dist,
                           unsigned &LastDef);

    MachineInstr *FindLastUseInMBB(unsigned Reg, MachineBasicBlock *MBB,
                                   unsigned Dist);

    bool isProfitableToCommute(unsigned regB, unsigned regC,
                               MachineInstr *MI, MachineBasicBlock *MBB,
                               unsigned Dist);

    bool CommuteInstruction(MachineBasicBlock::iterator &mi,
                            MachineFunction::iterator &mbbi,
                            unsigned RegB, unsigned RegC, unsigned Dist);

    bool isProfitableToConv3Addr(unsigned RegA);

    bool ConvertInstTo3Addr(MachineBasicBlock::iterator &mi,
                            MachineBasicBlock::iterator &nmi,
                            MachineFunction::iterator &mbbi,
                            unsigned RegB, unsigned Dist);

    void ProcessCopy(MachineInstr *MI, MachineBasicBlock *MBB,
                     SmallPtrSet<MachineInstr*, 8> &Processed);
  public:
    static char ID; // Pass identification, replacement for typeid
    TwoAddressInstructionPass() : MachineFunctionPass(&ID) {}

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addPreserved<LiveVariables>();
      AU.addPreservedID(MachineLoopInfoID);
      AU.addPreservedID(MachineDominatorsID);
      if (StrongPHIElim)
        AU.addPreservedID(StrongPHIEliminationID);
      else
        AU.addPreservedID(PHIEliminationID);
      MachineFunctionPass::getAnalysisUsage(AU);
    }

    /// runOnMachineFunction - Pass entry point.
    bool runOnMachineFunction(MachineFunction&);
  };
}

char TwoAddressInstructionPass::ID = 0;
static RegisterPass<TwoAddressInstructionPass>
X("twoaddressinstruction", "Two-Address instruction pass");

const PassInfo *const llvm::TwoAddressInstructionPassID = &X;

/// Sink3AddrInstruction - A two-address instruction has been converted to a
/// three-address instruction to avoid clobbering a register. Try to sink it
/// past the instruction that would kill the above mentioned register to reduce
/// register pressure.
bool TwoAddressInstructionPass::Sink3AddrInstruction(MachineBasicBlock *MBB,
                                           MachineInstr *MI, unsigned SavedReg,
                                           MachineBasicBlock::iterator OldPos) {
  // Check if it's safe to move this instruction.
  bool SeenStore = true; // Be conservative.
  if (!MI->isSafeToMove(TII, SeenStore))
    return false;

  unsigned DefReg = 0;
  SmallSet<unsigned, 4> UseRegs;

  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (!MO.isReg())
      continue;
    unsigned MOReg = MO.getReg();
    if (!MOReg)
      continue;
    if (MO.isUse() && MOReg != SavedReg)
      UseRegs.insert(MO.getReg());
    if (!MO.isDef())
      continue;
    if (MO.isImplicit())
      // Don't try to move it if it implicitly defines a register.
      return false;
    if (DefReg)
      // For now, don't move any instructions that define multiple registers.
      return false;
    DefReg = MO.getReg();
  }

  // Find the instruction that kills SavedReg.
  MachineInstr *KillMI = NULL;
  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(SavedReg),
         UE = MRI->use_end(); UI != UE; ++UI) {
    MachineOperand &UseMO = UI.getOperand();
    if (!UseMO.isKill())
      continue;
    KillMI = UseMO.getParent();
    break;
  }

  if (!KillMI || KillMI->getParent() != MBB || KillMI == MI)
    return false;

  // If any of the definitions are used by another instruction between the
  // position and the kill use, then it's not safe to sink it.
  // 
  // FIXME: This can be sped up if there is an easy way to query whether an
  // instruction is before or after another instruction. Then we can use
  // MachineRegisterInfo def / use instead.
  MachineOperand *KillMO = NULL;
  MachineBasicBlock::iterator KillPos = KillMI;
  ++KillPos;

  unsigned NumVisited = 0;
  for (MachineBasicBlock::iterator I = next(OldPos); I != KillPos; ++I) {
    MachineInstr *OtherMI = I;
    if (NumVisited > 30)  // FIXME: Arbitrary limit to reduce compile time cost.
      return false;