MachineSSAUpdater.cpp revision fdc2d0faf321224393f1a5dbf05c3e3f88bb6e3e 
1//===- MachineSSAUpdater.cpp - Unstructured SSA Update Tool ---------------===//
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 MachineSSAUpdater class. It's based on SSAUpdater
11// class in lib/Transforms/Utils.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/CodeGen/MachineSSAUpdater.h"
16#include "llvm/CodeGen/MachineInstr.h"
17#include "llvm/CodeGen/MachineInstrBuilder.h"
18#include "llvm/CodeGen/MachineRegisterInfo.h"
19#include "llvm/Target/TargetInstrInfo.h"
20#include "llvm/Target/TargetMachine.h"
21#include "llvm/Target/TargetRegisterInfo.h"
22#include "llvm/ADT/DenseMap.h"
23#include "llvm/ADT/SmallVector.h"
24#include "llvm/Support/AlignOf.h"
25#include "llvm/Support/Allocator.h"
26#include "llvm/Support/Debug.h"
27#include "llvm/Support/ErrorHandling.h"
28#include "llvm/Support/raw_ostream.h"
29#include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
30using namespace llvm;
31
32typedef DenseMap<MachineBasicBlock*, unsigned> AvailableValsTy;
33static AvailableValsTy &getAvailableVals(void *AV) {
34  return *static_cast<AvailableValsTy*>(AV);
35}
36
37MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
38                                     SmallVectorImpl<MachineInstr*> *NewPHI)
39  : AV(0), InsertedPHIs(NewPHI) {
40  TII = MF.getTarget().getInstrInfo();
41  MRI = &MF.getRegInfo();
42}
43
44MachineSSAUpdater::~MachineSSAUpdater() {
45  delete &getAvailableVals(AV);
46}
47
48/// Initialize - Reset this object to get ready for a new set of SSA
49/// updates.  ProtoValue is the value used to name PHI nodes.
50void MachineSSAUpdater::Initialize(unsigned V) {
51  if (AV == 0)
52    AV = new AvailableValsTy();
53  else
54    getAvailableVals(AV).clear();
55
56  VR = V;
57  VRC = MRI->getRegClass(VR);
58}
59
60/// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for
61/// the specified block.
62bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const {
63  return getAvailableVals(AV).count(BB);
64}
65
66/// AddAvailableValue - Indicate that a rewritten value is available in the
67/// specified block with the specified value.
68void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, unsigned V) {
69  getAvailableVals(AV)[BB] = V;
70}
71
72/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
73/// live at the end of the specified block.
74unsigned MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) {
75  return GetValueAtEndOfBlockInternal(BB);
76}
77
78static
79unsigned LookForIdenticalPHI(MachineBasicBlock *BB,
80          SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> &PredValues) {
81  if (BB->empty())
82    return 0;
83
84  MachineBasicBlock::iterator I = BB->begin();
85  if (!I->isPHI())
86    return 0;
87
88  AvailableValsTy AVals;
89  for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
90    AVals[PredValues[i].first] = PredValues[i].second;
91  while (I != BB->end() && I->isPHI()) {
92    bool Same = true;
93    for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) {
94      unsigned SrcReg = I->getOperand(i).getReg();
95      MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB();
96      if (AVals[SrcBB] != SrcReg) {
97        Same = false;
98        break;
99      }
100    }
101    if (Same)
102      return I->getOperand(0).getReg();
103    ++I;
104  }
105  return 0;
106}
107
108/// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define
109/// a value of the given register class at the start of the specified basic
110/// block. It returns the virtual register defined by the instruction.
111static
112MachineInstr *InsertNewDef(unsigned Opcode,
113                           MachineBasicBlock *BB, MachineBasicBlock::iterator I,
114                           const TargetRegisterClass *RC,
115                           MachineRegisterInfo *MRI,
116                           const TargetInstrInfo *TII) {
117  unsigned NewVR = MRI->createVirtualRegister(RC);
118  return BuildMI(*BB, I, DebugLoc(), TII->get(Opcode), NewVR);
119}
120
121/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
122/// is live in the middle of the specified block.
123///
124/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
125/// important case: if there is a definition of the rewritten value after the
126/// 'use' in BB.  Consider code like this:
127///
128///      X1 = ...
129///   SomeBB:
130///      use(X)
131///      X2 = ...
132///      br Cond, SomeBB, OutBB
133///
134/// In this case, there are two values (X1 and X2) added to the AvailableVals
135/// set by the client of the rewriter, and those values are both live out of
136/// their respective blocks.  However, the use of X happens in the *middle* of
137/// a block.  Because of this, we need to insert a new PHI node in SomeBB to
138/// merge the appropriate values, and this value isn't live out of the block.
139///
140unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) {
141  // If there is no definition of the renamed variable in this block, just use
142  // GetValueAtEndOfBlock to do our work.
143  if (!HasValueForBlock(BB))
144    return GetValueAtEndOfBlockInternal(BB);
145
146  // If there are no predecessors, just return undef.
147  if (BB->pred_empty()) {
148    // Insert an implicit_def to represent an undef value.
149    MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
150                                        BB, BB->getFirstTerminator(),
151                                        VRC, MRI, TII);
152    return NewDef->getOperand(0).getReg();
153  }
154
155  // Otherwise, we have the hard case.  Get the live-in values for each
156  // predecessor.
157  SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> PredValues;
158  unsigned SingularValue = 0;
159
160  bool isFirstPred = true;
161  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
162         E = BB->pred_end(); PI != E; ++PI) {
163    MachineBasicBlock *PredBB = *PI;
164    unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
165    PredValues.push_back(std::make_pair(PredBB, PredVal));
166
167    // Compute SingularValue.
168    if (isFirstPred) {
169      SingularValue = PredVal;
170      isFirstPred = false;
171    } else if (PredVal != SingularValue)
172      SingularValue = 0;
173  }
174
175  // Otherwise, if all the merged values are the same, just use it.
176  if (SingularValue != 0)
177    return SingularValue;
178
179  // If an identical PHI is already in BB, just reuse it.
180  unsigned DupPHI = LookForIdenticalPHI(BB, PredValues);
181  if (DupPHI)
182    return DupPHI;
183
184  // Otherwise, we do need a PHI: insert one now.
185  MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
186  MachineInstr *InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB,
187                                           Loc, VRC, MRI, TII);
188
189  // Fill in all the predecessors of the PHI.
190  MachineInstrBuilder MIB(InsertedPHI);
191  for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
192    MIB.addReg(PredValues[i].second).addMBB(PredValues[i].first);
193
194  // See if the PHI node can be merged to a single value.  This can happen in
195  // loop cases when we get a PHI of itself and one other value.
196  if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
197    InsertedPHI->eraseFromParent();
198    return ConstVal;
199  }
200
201  // If the client wants to know about all new instructions, tell it.
202  if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
203
204  DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
205  return InsertedPHI->getOperand(0).getReg();
206}
207
208static
209MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI,
210                                         MachineOperand *U) {
211  for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
212    if (&MI->getOperand(i) == U)
213      return MI->getOperand(i+1).getMBB();
214  }
215
216  llvm_unreachable("MachineOperand::getParent() failure?");
217}
218
219/// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
220/// which use their value in the corresponding predecessor.
221void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
222  MachineInstr *UseMI = U.getParent();
223  unsigned NewVR = 0;
224  if (UseMI->isPHI()) {
225    MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U);
226    NewVR = GetValueAtEndOfBlockInternal(SourceBB);
227  } else {
228    NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
229  }
230
231  U.setReg(NewVR);
232}
233
234void MachineSSAUpdater::ReplaceRegWith(unsigned OldReg, unsigned NewReg) {
235  MRI->replaceRegWith(OldReg, NewReg);
236
237  AvailableValsTy &AvailableVals = getAvailableVals(AV);
238  for (DenseMap<MachineBasicBlock*, unsigned>::iterator
239         I = AvailableVals.begin(), E = AvailableVals.end(); I != E; ++I)
240    if (I->second == OldReg)
241      I->second = NewReg;
242}
243
244/// SSAUpdaterTraits<MachineSSAUpdater> - Traits for the SSAUpdaterImpl
245/// template, specialized for MachineSSAUpdater.
246namespace llvm {
247template<>
248class SSAUpdaterTraits<MachineSSAUpdater> {
249public:
250  typedef MachineBasicBlock BlkT;
251  typedef unsigned ValT;
252  typedef MachineInstr PhiT;
253
254  typedef MachineBasicBlock::succ_iterator BlkSucc_iterator;
255  static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return BB->succ_begin(); }
256  static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return BB->succ_end(); }
257
258  /// Iterator for PHI operands.
259  class PHI_iterator {
260  private:
261    MachineInstr *PHI;
262    unsigned idx;
263
264  public:
265    explicit PHI_iterator(MachineInstr *P) // begin iterator
266      : PHI(P), idx(1) {}
267    PHI_iterator(MachineInstr *P, bool) // end iterator
268      : PHI(P), idx(PHI->getNumOperands()) {}
269
270    PHI_iterator &operator++() { idx += 2; return *this; }
271    bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
272    bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
273    unsigned getIncomingValue() { return PHI->getOperand(idx).getReg(); }
274    MachineBasicBlock *getIncomingBlock() {
275      return PHI->getOperand(idx+1).getMBB();
276    }
277  };
278  static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
279  static inline PHI_iterator PHI_end(PhiT *PHI) {
280    return PHI_iterator(PHI, true);
281  }
282
283  /// FindPredecessorBlocks - Put the predecessors of BB into the Preds
284  /// vector.
285  static void FindPredecessorBlocks(MachineBasicBlock *BB,
286                                    SmallVectorImpl<MachineBasicBlock*> *Preds){
287    for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
288           E = BB->pred_end(); PI != E; ++PI)
289      Preds->push_back(*PI);
290  }
291
292  /// GetUndefVal - Create an IMPLICIT_DEF instruction with a new register.
293  /// Add it into the specified block and return the register.
294  static unsigned GetUndefVal(MachineBasicBlock *BB,
295                              MachineSSAUpdater *Updater) {
296    // Insert an implicit_def to represent an undef value.
297    MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
298                                        BB, BB->getFirstTerminator(),
299                                        Updater->VRC, Updater->MRI,
300                                        Updater->TII);
301    return NewDef->getOperand(0).getReg();
302  }
303
304  /// CreateEmptyPHI - Create a PHI instruction that defines a new register.
305  /// Add it into the specified block and return the register.
306  static unsigned CreateEmptyPHI(MachineBasicBlock *BB, unsigned NumPreds,
307                                 MachineSSAUpdater *Updater) {
308    MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
309    MachineInstr *PHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
310                                     Updater->VRC, Updater->MRI,
311                                     Updater->TII);
312    return PHI->getOperand(0).getReg();
313  }
314
315  /// AddPHIOperand - Add the specified value as an operand of the PHI for
316  /// the specified predecessor block.
317  static void AddPHIOperand(MachineInstr *PHI, unsigned Val,
318                            MachineBasicBlock *Pred) {
319    PHI->addOperand(MachineOperand::CreateReg(Val, false));
320    PHI->addOperand(MachineOperand::CreateMBB(Pred));
321  }
322
323  /// InstrIsPHI - Check if an instruction is a PHI.
324  ///
325  static MachineInstr *InstrIsPHI(MachineInstr *I) {
326    if (I && I->isPHI())
327      return I;
328    return 0;
329  }
330
331  /// ValueIsPHI - Check if the instruction that defines the specified register
332  /// is a PHI instruction.
333  static MachineInstr *ValueIsPHI(unsigned Val, MachineSSAUpdater *Updater) {
334    return InstrIsPHI(Updater->MRI->getVRegDef(Val));
335  }
336
337  /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
338  /// operands, i.e., it was just added.
339  static MachineInstr *ValueIsNewPHI(unsigned Val, MachineSSAUpdater *Updater) {
340    MachineInstr *PHI = ValueIsPHI(Val, Updater);
341    if (PHI && PHI->getNumOperands() <= 1)
342      return PHI;
343    return 0;
344  }
345
346  /// GetPHIValue - For the specified PHI instruction, return the register
347  /// that it defines.
348  static unsigned GetPHIValue(MachineInstr *PHI) {
349    return PHI->getOperand(0).getReg();
350  }
351};
352
353} // End llvm namespace
354
355/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
356/// for the specified BB and if so, return it.  If not, construct SSA form by
357/// first calculating the required placement of PHIs and then inserting new
358/// PHIs where needed.
359unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
360  AvailableValsTy &AvailableVals = getAvailableVals(AV);
361  if (unsigned V = AvailableVals[BB])
362    return V;
363
364  SSAUpdaterImpl<MachineSSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
365  return Impl.GetValue(BB);
366}
367