TwoAddressInstructionPass.cpp revision fb11288109329cb736d9f49769581a0d0c23fe19 
1//===-- TwoAddressInstructionPass.cpp - Two-Address instruction pass ------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the TwoAddress instruction pass which is used
11// by most register allocators. Two-Address instructions are rewritten
12// from:
13//
14//     A = B op C
15//
16// to:
17//
18//     A = B
19//     A op= C
20//
21// Note that if a register allocator chooses to use this pass, that it
22// has to be capable of handling the non-SSA nature of these rewritten
23// virtual registers.
24//
25// It is also worth noting that the duplicate operand of the two
26// address instruction is removed.
27//
28//===----------------------------------------------------------------------===//
29
30#define DEBUG_TYPE "twoaddrinstr"
31#include "llvm/CodeGen/Passes.h"
32#include "llvm/Function.h"
33#include "llvm/CodeGen/LiveVariables.h"
34#include "llvm/CodeGen/MachineFunctionPass.h"
35#include "llvm/CodeGen/MachineInstr.h"
36#include "llvm/CodeGen/MachineRegisterInfo.h"
37#include "llvm/Target/TargetRegisterInfo.h"
38#include "llvm/Target/TargetInstrInfo.h"
39#include "llvm/Target/TargetMachine.h"
40#include "llvm/Target/TargetOptions.h"
41#include "llvm/Support/Compiler.h"
42#include "llvm/Support/Debug.h"
43#include "llvm/ADT/BitVector.h"
44#include "llvm/ADT/DenseMap.h"
45#include "llvm/ADT/SmallSet.h"
46#include "llvm/ADT/Statistic.h"
47#include "llvm/ADT/STLExtras.h"
48using namespace llvm;
49
50STATISTIC(NumTwoAddressInstrs, "Number of two-address instructions");
51STATISTIC(NumCommuted        , "Number of instructions commuted to coalesce");
52STATISTIC(NumAggrCommuted    , "Number of instructions aggressively commuted");
53STATISTIC(NumConvertedTo3Addr, "Number of instructions promoted to 3-address");
54STATISTIC(Num3AddrSunk,        "Number of 3-address instructions sunk");
55STATISTIC(NumReMats,           "Number of instructions re-materialized");
56STATISTIC(NumDeletes,          "Number of dead instructions deleted");
57
58namespace {
59  class VISIBILITY_HIDDEN TwoAddressInstructionPass
60    : public MachineFunctionPass {
61    const TargetInstrInfo *TII;
62    const TargetRegisterInfo *TRI;
63    MachineRegisterInfo *MRI;
64    LiveVariables *LV;
65
66    // DistanceMap - Keep track the distance of a MI from the start of the
67    // current basic block.
68    DenseMap<MachineInstr*, unsigned> DistanceMap;
69
70    // SrcRegMap - A map from virtual registers to physical registers which
71    // are likely targets to be coalesced to due to copies from physical
72    // registers to virtual registers. e.g. v1024 = move r0.
73    DenseMap<unsigned, unsigned> SrcRegMap;
74
75    // DstRegMap - A map from virtual registers to physical registers which
76    // are likely targets to be coalesced to due to copies to physical
77    // registers from virtual registers. e.g. r1 = move v1024.
78    DenseMap<unsigned, unsigned> DstRegMap;
79
80    bool Sink3AddrInstruction(MachineBasicBlock *MBB, MachineInstr *MI,
81                              unsigned Reg,
82                              MachineBasicBlock::iterator OldPos);
83
84    bool isProfitableToReMat(unsigned Reg, const TargetRegisterClass *RC,
85                             MachineInstr *MI, MachineInstr *DefMI,
86                             MachineBasicBlock *MBB, unsigned Loc);
87
88    bool NoUseAfterLastDef(unsigned Reg, MachineBasicBlock *MBB, unsigned Dist,
89                           unsigned &LastDef);
90
91    bool isProfitableToCommute(unsigned regB, unsigned regC,
92                               MachineInstr *MI, MachineBasicBlock *MBB,
93                               unsigned Dist);
94
95    bool CommuteInstruction(MachineBasicBlock::iterator &mi,
96                            MachineFunction::iterator &mbbi,
97                            unsigned RegB, unsigned RegC, unsigned Dist);
98
99    void ProcessCopy(MachineInstr *MI, MachineBasicBlock *MBB,
100                     SmallPtrSet<MachineInstr*, 8> &Processed);
101  public:
102    static char ID; // Pass identification, replacement for typeid
103    TwoAddressInstructionPass() : MachineFunctionPass(&ID) {}
104
105    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
106      AU.addPreserved<LiveVariables>();
107      AU.addPreservedID(MachineLoopInfoID);
108      AU.addPreservedID(MachineDominatorsID);
109      if (StrongPHIElim)
110        AU.addPreservedID(StrongPHIEliminationID);
111      else
112        AU.addPreservedID(PHIEliminationID);
113      MachineFunctionPass::getAnalysisUsage(AU);
114    }
115
116    /// runOnMachineFunction - Pass entry point.
117    bool runOnMachineFunction(MachineFunction&);
118  };
119}
120
121char TwoAddressInstructionPass::ID = 0;
122static RegisterPass<TwoAddressInstructionPass>
123X("twoaddressinstruction", "Two-Address instruction pass");
124
125const PassInfo *const llvm::TwoAddressInstructionPassID = &X;
126
127/// Sink3AddrInstruction - A two-address instruction has been converted to a
128/// three-address instruction to avoid clobbering a register. Try to sink it
129/// past the instruction that would kill the above mentioned register to reduce
130/// register pressure.
131bool TwoAddressInstructionPass::Sink3AddrInstruction(MachineBasicBlock *MBB,
132                                           MachineInstr *MI, unsigned SavedReg,
133                                           MachineBasicBlock::iterator OldPos) {
134  // Check if it's safe to move this instruction.
135  bool SeenStore = true; // Be conservative.
136  if (!MI->isSafeToMove(TII, SeenStore))
137    return false;
138
139  unsigned DefReg = 0;
140  SmallSet<unsigned, 4> UseRegs;
141
142  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
143    const MachineOperand &MO = MI->getOperand(i);
144    if (!MO.isReg())
145      continue;
146    unsigned MOReg = MO.getReg();
147    if (!MOReg)
148      continue;
149    if (MO.isUse() && MOReg != SavedReg)
150      UseRegs.insert(MO.getReg());
151    if (!MO.isDef())
152      continue;
153    if (MO.isImplicit())
154      // Don't try to move it if it implicitly defines a register.
155      return false;
156    if (DefReg)
157      // For now, don't move any instructions that define multiple registers.
158      return false;
159    DefReg = MO.getReg();
160  }
161
162  // Find the instruction that kills SavedReg.
163  MachineInstr *KillMI = NULL;
164  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(SavedReg),
165         UE = MRI->use_end(); UI != UE; ++UI) {
166    MachineOperand &UseMO = UI.getOperand();
167    if (!UseMO.isKill())
168      continue;
169    KillMI = UseMO.getParent();
170    break;
171  }
172
173  if (!KillMI || KillMI->getParent() != MBB)
174    return false;
175
176  // If any of the definitions are used by another instruction between the
177  // position and the kill use, then it's not safe to sink it.
178  //
179  // FIXME: This can be sped up if there is an easy way to query whether an
180  // instruction is before or after another instruction. Then we can use
181  // MachineRegisterInfo def / use instead.
182  MachineOperand *KillMO = NULL;
183  MachineBasicBlock::iterator KillPos = KillMI;
184  ++KillPos;
185
186  unsigned NumVisited = 0;
187  for (MachineBasicBlock::iterator I = next(OldPos); I != KillPos; ++I) {
188    MachineInstr *OtherMI = I;
189    if (NumVisited > 30)  // FIXME: Arbitrary limit to reduce compile time cost.
190      return false;
191    ++NumVisited;
192    for (unsigned i = 0, e = OtherMI->getNumOperands(); i != e; ++i) {
193      MachineOperand &MO = OtherMI->getOperand(i);
194      if (!MO.isReg())
195        continue;
196      unsigned MOReg = MO.getReg();
197      if (!MOReg)
198        continue;
199      if (DefReg == MOReg)
200        return false;
201
202      if (MO.isKill()) {
203        if (OtherMI == KillMI && MOReg == SavedReg)
204          // Save the operand that kills the register. We want to unset the kill
205          // marker if we can sink MI past it.
206          KillMO = &MO;
207        else if (UseRegs.count(MOReg))
208          // One of the uses is killed before the destination.
209          return false;
210      }
211    }
212  }
213
214  // Update kill and LV information.
215  KillMO->setIsKill(false);
216  KillMO = MI->findRegisterUseOperand(SavedReg, false, TRI);
217  KillMO->setIsKill(true);
218
219  if (LV)
220    LV->replaceKillInstruction(SavedReg, KillMI, MI);
221
222  // Move instruction to its destination.
223  MBB->remove(MI);
224  MBB->insert(KillPos, MI);
225
226  ++Num3AddrSunk;
227  return true;
228}
229
230/// isTwoAddrUse - Return true if the specified MI is using the specified
231/// register as a two-address operand.
232static bool isTwoAddrUse(MachineInstr *UseMI, unsigned Reg) {
233  const TargetInstrDesc &TID = UseMI->getDesc();
234  for (unsigned i = 0, e = TID.getNumOperands(); i != e; ++i) {
235    MachineOperand &MO = UseMI->getOperand(i);
236    if (MO.isReg() && MO.getReg() == Reg &&
237        (MO.isDef() || UseMI->isRegTiedToDefOperand(i)))
238      // Earlier use is a two-address one.
239      return true;
240  }
241  return false;
242}
243
244/// isProfitableToReMat - Return true if the heuristics determines it is likely
245/// to be profitable to re-materialize the definition of Reg rather than copy
246/// the register.
247bool
248TwoAddressInstructionPass::isProfitableToReMat(unsigned Reg,
249                                         const TargetRegisterClass *RC,
250                                         MachineInstr *MI, MachineInstr *DefMI,
251                                         MachineBasicBlock *MBB, unsigned Loc) {
252  bool OtherUse = false;
253  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
254         UE = MRI->use_end(); UI != UE; ++UI) {
255    MachineOperand &UseMO = UI.getOperand();
256    MachineInstr *UseMI = UseMO.getParent();
257    MachineBasicBlock *UseMBB = UseMI->getParent();
258    if (UseMBB == MBB) {
259      DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(UseMI);
260      if (DI != DistanceMap.end() && DI->second == Loc)
261        continue;  // Current use.
262      OtherUse = true;
263      // There is at least one other use in the MBB that will clobber the
264      // register.
265      if (isTwoAddrUse(UseMI, Reg))
266        return true;
267    }
268  }
269
270  // If other uses in MBB are not two-address uses, then don't remat.
271  if (OtherUse)
272    return false;
273
274  // No other uses in the same block, remat if it's defined in the same
275  // block so it does not unnecessarily extend the live range.
276  return MBB == DefMI->getParent();
277}
278
279/// NoUseAfterLastDef - Return true if there are no intervening uses between the
280/// last instruction in the MBB that defines the specified register and the
281/// two-address instruction which is being processed. It also returns the last
282/// def location by reference
283bool TwoAddressInstructionPass::NoUseAfterLastDef(unsigned Reg,
284                                           MachineBasicBlock *MBB, unsigned Dist,
285                                           unsigned &LastDef) {
286  LastDef = 0;
287  unsigned LastUse = Dist;
288  for (MachineRegisterInfo::reg_iterator I = MRI->reg_begin(Reg),
289         E = MRI->reg_end(); I != E; ++I) {
290    MachineOperand &MO = I.getOperand();
291    MachineInstr *MI = MO.getParent();
292    if (MI->getParent() != MBB)
293      continue;
294    DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(MI);
295    if (DI == DistanceMap.end())
296      continue;
297    if (MO.isUse() && DI->second < LastUse)
298      LastUse = DI->second;
299    if (MO.isDef() && DI->second > LastDef)
300      LastDef = DI->second;
301  }
302
303  return !(LastUse > LastDef && LastUse < Dist);
304}
305
306/// isCopyToReg - Return true if the specified MI is a copy instruction or
307/// a extract_subreg instruction. It also returns the source and destination
308/// registers and whether they are physical registers by reference.
309static bool isCopyToReg(MachineInstr &MI, const TargetInstrInfo *TII,
310                        unsigned &SrcReg, unsigned &DstReg,
311                        bool &IsSrcPhys, bool &IsDstPhys) {
312  SrcReg = 0;
313  DstReg = 0;
314  unsigned SrcSubIdx, DstSubIdx;
315  if (!TII->isMoveInstr(MI, SrcReg, DstReg, SrcSubIdx, DstSubIdx)) {
316    if (MI.getOpcode() == TargetInstrInfo::EXTRACT_SUBREG) {
317      DstReg = MI.getOperand(0).getReg();
318      SrcReg = MI.getOperand(1).getReg();
319    } else if (MI.getOpcode() == TargetInstrInfo::INSERT_SUBREG) {
320      DstReg = MI.getOperand(0).getReg();
321      SrcReg = MI.getOperand(2).getReg();
322    }
323  }
324
325  if (DstReg) {
326    IsSrcPhys = TargetRegisterInfo::isPhysicalRegister(SrcReg);
327    IsDstPhys = TargetRegisterInfo::isPhysicalRegister(DstReg);
328    return true;
329  }
330  return false;
331}
332
333/// isTwoAddrUse - Return true if the specified MI uses the specified register
334/// as a two-address use. If so, return the destination register by reference.
335static bool isTwoAddrUse(MachineInstr &MI, unsigned Reg, unsigned &DstReg) {
336  const TargetInstrDesc &TID = MI.getDesc();
337  for (unsigned i = 0, e = TID.getNumOperands(); i != e; ++i) {
338    const MachineOperand &MO = MI.getOperand(i);
339    if (!MO.isReg() || !MO.isUse() || MO.getReg() != Reg)
340      continue;
341    unsigned ti;
342    if (MI.isRegTiedToDefOperand(i, &ti)) {
343      DstReg = MI.getOperand(ti).getReg();
344      return true;
345    }
346  }
347  return false;
348}
349
350/// findOnlyInterestingUse - Given a register, if has a single in-basic block
351/// use, return the use instruction if it's a copy or a two-address use.
352static
353MachineInstr *findOnlyInterestingUse(unsigned Reg, MachineBasicBlock *MBB,
354                                     MachineRegisterInfo *MRI,
355                                     const TargetInstrInfo *TII,
356                                     bool &isCopy,
357                                     unsigned &DstReg, bool &IsDstPhys) {
358  MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg);
359  if (UI == MRI->use_end())
360    return 0;
361  MachineInstr &UseMI = *UI;
362  if (++UI != MRI->use_end())
363    // More than one use.
364    return 0;
365  if (UseMI.getParent() != MBB)
366    return 0;
367  unsigned SrcReg;
368  bool IsSrcPhys;
369  if (isCopyToReg(UseMI, TII, SrcReg, DstReg, IsSrcPhys, IsDstPhys))
370    return &UseMI;
371  IsDstPhys = false;
372  if (isTwoAddrUse(UseMI, Reg, DstReg))
373    return &UseMI;
374  return 0;
375}
376
377/// getMappedReg - Return the physical register the specified virtual register
378/// might be mapped to.
379static unsigned
380getMappedReg(unsigned Reg, DenseMap<unsigned, unsigned> &RegMap) {
381  while (TargetRegisterInfo::isVirtualRegister(Reg))  {
382    DenseMap<unsigned, unsigned>::iterator SI = RegMap.find(Reg);
383    if (SI == RegMap.end())
384      return 0;
385    Reg = SI->second;
386  }
387  if (TargetRegisterInfo::isPhysicalRegister(Reg))
388    return Reg;
389  return 0;
390}
391
392/// regsAreCompatible - Return true if the two registers are equal or aliased.
393///
394static bool
395regsAreCompatible(unsigned RegA, unsigned RegB, const TargetRegisterInfo *TRI) {
396  if (RegA == RegB)
397    return true;
398  if (!RegA || !RegB)
399    return false;
400  return TRI->regsOverlap(RegA, RegB);
401}
402
403
404/// isProfitableToReMat - Return true if it's potentially profitable to commute
405/// the two-address instruction that's being processed.
406bool
407TwoAddressInstructionPass::isProfitableToCommute(unsigned regB, unsigned regC,
408                                       MachineInstr *MI, MachineBasicBlock *MBB,
409                                       unsigned Dist) {
410  // Determine if it's profitable to commute this two address instruction. In
411  // general, we want no uses between this instruction and the definition of
412  // the two-address register.
413  // e.g.
414  // %reg1028<def> = EXTRACT_SUBREG %reg1027<kill>, 1
415  // %reg1029<def> = MOV8rr %reg1028
416  // %reg1029<def> = SHR8ri %reg1029, 7, %EFLAGS<imp-def,dead>
417  // insert => %reg1030<def> = MOV8rr %reg1028
418  // %reg1030<def> = ADD8rr %reg1028<kill>, %reg1029<kill>, %EFLAGS<imp-def,dead>
419  // In this case, it might not be possible to coalesce the second MOV8rr
420  // instruction if the first one is coalesced. So it would be profitable to
421  // commute it:
422  // %reg1028<def> = EXTRACT_SUBREG %reg1027<kill>, 1
423  // %reg1029<def> = MOV8rr %reg1028
424  // %reg1029<def> = SHR8ri %reg1029, 7, %EFLAGS<imp-def,dead>
425  // insert => %reg1030<def> = MOV8rr %reg1029
426  // %reg1030<def> = ADD8rr %reg1029<kill>, %reg1028<kill>, %EFLAGS<imp-def,dead>
427
428  if (!MI->killsRegister(regC))
429    return false;
430
431  // Ok, we have something like:
432  // %reg1030<def> = ADD8rr %reg1028<kill>, %reg1029<kill>, %EFLAGS<imp-def,dead>
433  // let's see if it's worth commuting it.
434
435  // Look for situations like this:
436  // %reg1024<def> = MOV r1
437  // %reg1025<def> = MOV r0
438  // %reg1026<def> = ADD %reg1024, %reg1025
439  // r0            = MOV %reg1026
440  // Commute the ADD to hopefully eliminate an otherwise unavoidable copy.
441  unsigned FromRegB = getMappedReg(regB, SrcRegMap);
442  unsigned FromRegC = getMappedReg(regC, SrcRegMap);
443  unsigned ToRegB = getMappedReg(regB, DstRegMap);
444  unsigned ToRegC = getMappedReg(regC, DstRegMap);
445  if (!regsAreCompatible(FromRegB, ToRegB, TRI) &&
446      (regsAreCompatible(FromRegB, ToRegC, TRI) ||
447       regsAreCompatible(FromRegC, ToRegB, TRI)))
448    return true;
449
450  // If there is a use of regC between its last def (could be livein) and this
451  // instruction, then bail.
452  unsigned LastDefC = 0;
453  if (!NoUseAfterLastDef(regC, MBB, Dist, LastDefC))
454    return false;
455
456  // If there is a use of regB between its last def (could be livein) and this
457  // instruction, then go ahead and make this transformation.
458  unsigned LastDefB = 0;
459  if (!NoUseAfterLastDef(regB, MBB, Dist, LastDefB))
460    return true;
461
462  // Since there are no intervening uses for both registers, then commute
463  // if the def of regC is closer. Its live interval is shorter.
464  return LastDefB && LastDefC && LastDefC > LastDefB;
465}
466
467/// CommuteInstruction - Commute a two-address instruction and update the basic
468/// block, distance map, and live variables if needed. Return true if it is
469/// successful.
470bool
471TwoAddressInstructionPass::CommuteInstruction(MachineBasicBlock::iterator &mi,
472                               MachineFunction::iterator &mbbi,
473                               unsigned RegB, unsigned RegC, unsigned Dist) {
474  MachineInstr *MI = mi;
475  DOUT << "2addr: COMMUTING  : " << *MI;
476  MachineInstr *NewMI = TII->commuteInstruction(MI);
477
478  if (NewMI == 0) {
479    DOUT << "2addr: COMMUTING FAILED!\n";
480    return false;
481  }
482
483  DOUT << "2addr: COMMUTED TO: " << *NewMI;
484  // If the instruction changed to commute it, update livevar.
485  if (NewMI != MI) {
486    if (LV)
487      // Update live variables
488      LV->replaceKillInstruction(RegC, MI, NewMI);
489
490    mbbi->insert(mi, NewMI);           // Insert the new inst
491    mbbi->erase(mi);                   // Nuke the old inst.
492    mi = NewMI;
493    DistanceMap.insert(std::make_pair(NewMI, Dist));
494  }
495
496  // Update source register map.
497  unsigned FromRegC = getMappedReg(RegC, SrcRegMap);
498  if (FromRegC) {
499    unsigned RegA = MI->getOperand(0).getReg();
500    SrcRegMap[RegA] = FromRegC;
501  }
502
503  return true;
504}
505
506/// ProcessCopy - If the specified instruction is not yet processed, process it
507/// if it's a copy. For a copy instruction, we find the physical registers the
508/// source and destination registers might be mapped to. These are kept in
509/// point-to maps used to determine future optimizations. e.g.
510/// v1024 = mov r0
511/// v1025 = mov r1
512/// v1026 = add v1024, v1025
513/// r1    = mov r1026
514/// If 'add' is a two-address instruction, v1024, v1026 are both potentially
515/// coalesced to r0 (from the input side). v1025 is mapped to r1. v1026 is
516/// potentially joined with r1 on the output side. It's worthwhile to commute
517/// 'add' to eliminate a copy.
518void TwoAddressInstructionPass::ProcessCopy(MachineInstr *MI,
519                                     MachineBasicBlock *MBB,
520                                     SmallPtrSet<MachineInstr*, 8> &Processed) {
521  if (Processed.count(MI))
522    return;
523
524  bool IsSrcPhys, IsDstPhys;
525  unsigned SrcReg, DstReg;
526  if (!isCopyToReg(*MI, TII, SrcReg, DstReg, IsSrcPhys, IsDstPhys))
527    return;
528
529  if (IsDstPhys && !IsSrcPhys)
530    DstRegMap.insert(std::make_pair(SrcReg, DstReg));
531  else if (!IsDstPhys && IsSrcPhys) {
532    bool isNew =
533      SrcRegMap.insert(std::make_pair(DstReg, SrcReg)).second;
534    isNew = isNew; // Silence compiler warning.
535    assert(isNew && "Can't map to two src physical registers!");
536
537    SmallVector<unsigned, 4> VirtRegPairs;
538    bool isCopy = false;
539    unsigned NewReg = 0;
540    while (MachineInstr *UseMI = findOnlyInterestingUse(DstReg, MBB, MRI,TII,
541                                                   isCopy, NewReg, IsDstPhys)) {
542      if (isCopy) {
543        if (Processed.insert(UseMI))
544          break;
545      }
546
547      DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(UseMI);
548      if (DI != DistanceMap.end())
549        // Earlier in the same MBB.Reached via a back edge.
550        break;
551
552      if (IsDstPhys) {
553        VirtRegPairs.push_back(NewReg);
554        break;
555      }
556      bool isNew = SrcRegMap.insert(std::make_pair(NewReg, DstReg)).second;
557      isNew = isNew; // Silence compiler warning.
558      assert(isNew && "Can't map to two src physical registers!");
559      VirtRegPairs.push_back(NewReg);
560      DstReg = NewReg;
561    }
562
563    if (!VirtRegPairs.empty()) {
564      unsigned ToReg = VirtRegPairs.back();
565      VirtRegPairs.pop_back();
566      while (!VirtRegPairs.empty()) {
567        unsigned FromReg = VirtRegPairs.back();
568        VirtRegPairs.pop_back();
569        bool isNew = DstRegMap.insert(std::make_pair(FromReg, ToReg)).second;
570        isNew = isNew; // Silence compiler warning.
571        assert(isNew && "Can't map to two dst physical registers!");
572        ToReg = FromReg;
573      }
574    }
575  }
576
577  Processed.insert(MI);
578}
579
580/// isSafeToDelete - If the specified instruction does not produce any side
581/// effects and all of its defs are dead, then it's safe to delete.
582static bool isSafeToDelete(MachineInstr *MI, const TargetInstrInfo *TII) {
583  const TargetInstrDesc &TID = MI->getDesc();
584  if (TID.mayStore() || TID.isCall())
585    return false;
586  if (TID.isTerminator() || TID.hasUnmodeledSideEffects())
587    return false;
588
589  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
590    MachineOperand &MO = MI->getOperand(i);
591    if (!MO.isReg() || !MO.isDef())
592      continue;
593    if (!MO.isDead())
594      return false;
595  }
596
597  return true;
598}
599
600/// runOnMachineFunction - Reduce two-address instructions to two operands.
601///
602bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
603  DOUT << "Machine Function\n";
604  const TargetMachine &TM = MF.getTarget();
605  MRI = &MF.getRegInfo();
606  TII = TM.getInstrInfo();
607  TRI = TM.getRegisterInfo();
608  LV = getAnalysisIfAvailable<LiveVariables>();
609
610  bool MadeChange = false;
611
612  DOUT << "********** REWRITING TWO-ADDR INSTRS **********\n";
613  DOUT << "********** Function: " << MF.getFunction()->getName() << '\n';
614
615  // ReMatRegs - Keep track of the registers whose def's are remat'ed.
616  BitVector ReMatRegs;
617  ReMatRegs.resize(MRI->getLastVirtReg()+1);
618
619  SmallPtrSet<MachineInstr*, 8> Processed;
620  for (MachineFunction::iterator mbbi = MF.begin(), mbbe = MF.end();
621       mbbi != mbbe; ++mbbi) {
622    unsigned Dist = 0;
623    DistanceMap.clear();
624    SrcRegMap.clear();
625    DstRegMap.clear();
626    Processed.clear();
627    for (MachineBasicBlock::iterator mi = mbbi->begin(), me = mbbi->end();
628         mi != me; ) {
629      MachineBasicBlock::iterator nmi = next(mi);
630      const TargetInstrDesc &TID = mi->getDesc();
631      bool FirstTied = true;
632
633      DistanceMap.insert(std::make_pair(mi, ++Dist));
634
635      ProcessCopy(&*mi, &*mbbi, Processed);
636
637      unsigned NumOps = (mi->getOpcode() == TargetInstrInfo::INLINEASM)
638        ? mi->getNumOperands() : TID.getNumOperands();
639      for (unsigned si = 0; si < NumOps; ++si) {
640        unsigned ti = 0;
641        if (!mi->isRegTiedToDefOperand(si, &ti))
642          continue;
643
644        if (FirstTied) {
645          ++NumTwoAddressInstrs;
646          DOUT << '\t'; DEBUG(mi->print(*cerr.stream(), &TM));
647        }
648
649        FirstTied = false;
650
651        assert(mi->getOperand(si).isReg() && mi->getOperand(si).getReg() &&
652               mi->getOperand(si).isUse() && "two address instruction invalid");
653
654        // If the two operands are the same we just remove the use
655        // and mark the def as def&use, otherwise we have to insert a copy.
656        if (mi->getOperand(ti).getReg() != mi->getOperand(si).getReg()) {
657          // Rewrite:
658          //     a = b op c
659          // to:
660          //     a = b
661          //     a = a op c
662          unsigned regA = mi->getOperand(ti).getReg();
663          unsigned regB = mi->getOperand(si).getReg();
664
665          assert(TargetRegisterInfo::isVirtualRegister(regB) &&
666                 "cannot update physical register live information");
667
668#ifndef NDEBUG
669          // First, verify that we don't have a use of a in the instruction (a =
670          // b + a for example) because our transformation will not work. This
671          // should never occur because we are in SSA form.
672          for (unsigned i = 0; i != mi->getNumOperands(); ++i)
673            assert(i == ti ||
674                   !mi->getOperand(i).isReg() ||
675                   mi->getOperand(i).getReg() != regA);
676#endif
677
678          // If this instruction is not the killing user of B, see if we can
679          // rearrange the code to make it so.  Making it the killing user will
680          // allow us to coalesce A and B together, eliminating the copy we are
681          // about to insert.
682          if (!mi->killsRegister(regB)) {
683            // If regA is dead and the instruction can be deleted, just delete
684            // it so it doesn't clobber regB.
685            if (mi->getOperand(ti).isDead() && isSafeToDelete(mi, TII)) {
686              mbbi->erase(mi); // Nuke the old inst.
687              mi = nmi;
688              ++NumDeletes;
689              break; // Done with this instruction.
690            }
691
692            // If this instruction is commutative, check to see if C dies.  If
693            // so, swap the B and C operands.  This makes the live ranges of A
694            // and C joinable.
695            // FIXME: This code also works for A := B op C instructions.
696            if (TID.isCommutable() && mi->getNumOperands() >= 3) {
697              assert(mi->getOperand(3-si).isReg() &&
698                     "Not a proper commutative instruction!");
699              unsigned regC = mi->getOperand(3-si).getReg();
700              if (mi->killsRegister(regC)) {
701                if (CommuteInstruction(mi, mbbi, regB, regC, Dist)) {
702                  ++NumCommuted;
703                  regB = regC;
704                  goto InstructionRearranged;
705                }
706              }
707            }
708
709            // If this instruction is potentially convertible to a true
710            // three-address instruction,
711            if (TID.isConvertibleTo3Addr()) {
712              // FIXME: This assumes there are no more operands which are tied
713              // to another register.
714#ifndef NDEBUG
715              for (unsigned i = si + 1, e = TID.getNumOperands(); i < e; ++i)
716                assert(TID.getOperandConstraint(i, TOI::TIED_TO) == -1);
717#endif
718
719              MachineInstr *NewMI = TII->convertToThreeAddress(mbbi, mi, LV);
720              if (NewMI) {
721                DOUT << "2addr: CONVERTING 2-ADDR: " << *mi;
722                DOUT << "2addr:         TO 3-ADDR: " << *NewMI;
723                bool Sunk = false;
724
725                if (NewMI->findRegisterUseOperand(regB, false, TRI))
726                  // FIXME: Temporary workaround. If the new instruction doesn't
727                  // uses regB, convertToThreeAddress must have created more
728                  // then one instruction.
729                  Sunk = Sink3AddrInstruction(mbbi, NewMI, regB, mi);
730
731                mbbi->erase(mi); // Nuke the old inst.
732
733                if (!Sunk) {
734                  DistanceMap.insert(std::make_pair(NewMI, Dist));
735                  mi = NewMI;
736                  nmi = next(mi);
737                }
738
739                ++NumConvertedTo3Addr;
740                break; // Done with this instruction.
741              }
742            }
743          }
744
745          // If it's profitable to commute the instruction, do so.
746          if (TID.isCommutable() && mi->getNumOperands() >= 3) {
747            unsigned regC = mi->getOperand(3-si).getReg();
748            if (isProfitableToCommute(regB, regC, mi, mbbi, Dist))
749              if (CommuteInstruction(mi, mbbi, regB, regC, Dist)) {
750                ++NumAggrCommuted;
751                ++NumCommuted;
752                regB = regC;
753              }
754          }
755
756        InstructionRearranged:
757          const TargetRegisterClass* rc = MRI->getRegClass(regB);
758          MachineInstr *DefMI = MRI->getVRegDef(regB);
759          // If it's safe and profitable, remat the definition instead of
760          // copying it.
761          if (DefMI &&
762              DefMI->getDesc().isAsCheapAsAMove() &&
763              DefMI->isSafeToReMat(TII, regB) &&
764              isProfitableToReMat(regB, rc, mi, DefMI, mbbi, Dist)){
765            DEBUG(cerr << "2addr: REMATTING : " << *DefMI << "\n");
766            TII->reMaterialize(*mbbi, mi, regA, DefMI);
767            ReMatRegs.set(regB);
768            ++NumReMats;
769          } else {
770            TII->copyRegToReg(*mbbi, mi, regA, regB, rc, rc);
771          }
772
773          MachineBasicBlock::iterator prevMI = prior(mi);
774          // Update DistanceMap.
775          DistanceMap.insert(std::make_pair(prevMI, Dist));
776          DistanceMap[mi] = ++Dist;
777
778          // Update live variables for regB.
779          if (LV) {
780            LiveVariables::VarInfo& varInfoB = LV->getVarInfo(regB);
781
782            // regB is used in this BB.
783            varInfoB.UsedBlocks[mbbi->getNumber()] = true;
784
785            if (LV->removeVirtualRegisterKilled(regB,  mi))
786              LV->addVirtualRegisterKilled(regB, prevMI);
787
788            if (LV->removeVirtualRegisterDead(regB, mi))
789              LV->addVirtualRegisterDead(regB, prevMI);
790          }
791
792          DOUT << "\t\tprepend:\t"; DEBUG(prevMI->print(*cerr.stream(), &TM));
793
794          // Replace all occurences of regB with regA.
795          for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
796            if (mi->getOperand(i).isReg() &&
797                mi->getOperand(i).getReg() == regB)
798              mi->getOperand(i).setReg(regA);
799          }
800        }
801
802        assert(mi->getOperand(ti).isDef() && mi->getOperand(si).isUse());
803        mi->getOperand(ti).setReg(mi->getOperand(si).getReg());
804        MadeChange = true;
805
806        DOUT << "\t\trewrite to:\t"; DEBUG(mi->print(*cerr.stream(), &TM));
807      }
808
809      mi = nmi;
810    }
811  }
812
813  // Some remat'ed instructions are dead.
814  int VReg = ReMatRegs.find_first();
815  while (VReg != -1) {
816    if (MRI->use_empty(VReg)) {
817      MachineInstr *DefMI = MRI->getVRegDef(VReg);
818      DefMI->eraseFromParent();
819    }
820    VReg = ReMatRegs.find_next(VReg);
821  }
822
823  return MadeChange;
824}
825