1//===- MachineSSAUpdater.cpp - Unstructured SSA Update Tool ---------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements the MachineSSAUpdater class. It's based on SSAUpdater
10// class in lib/Transforms/Utils.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/CodeGen/MachineSSAUpdater.h"
15#include "llvm/ADT/DenseMap.h"
16#include "llvm/ADT/SmallVector.h"
17#include "llvm/CodeGen/MachineBasicBlock.h"
18#include "llvm/CodeGen/MachineFunction.h"
19#include "llvm/CodeGen/MachineInstr.h"
20#include "llvm/CodeGen/MachineInstrBuilder.h"
21#include "llvm/CodeGen/MachineOperand.h"
22#include "llvm/CodeGen/MachineRegisterInfo.h"
23#include "llvm/CodeGen/TargetInstrInfo.h"
24#include "llvm/CodeGen/TargetOpcodes.h"
25#include "llvm/CodeGen/TargetSubtargetInfo.h"
26#include "llvm/IR/DebugLoc.h"
27#include "llvm/Support/Debug.h"
28#include "llvm/Support/ErrorHandling.h"
29#include "llvm/Support/raw_ostream.h"
30#include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
31#include <utility>
32
33using namespace llvm;
34
35#define DEBUG_TYPE "machine-ssaupdater"
36
37using AvailableValsTy = DenseMap<MachineBasicBlock *, Register>;
38
39static AvailableValsTy &getAvailableVals(void *AV) {
40  return *static_cast<AvailableValsTy*>(AV);
41}
42
43MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
44                                     SmallVectorImpl<MachineInstr*> *NewPHI)
45  : InsertedPHIs(NewPHI), TII(MF.getSubtarget().getInstrInfo()),
46    MRI(&MF.getRegInfo()) {}
47
48MachineSSAUpdater::~MachineSSAUpdater() {
49  delete static_cast<AvailableValsTy*>(AV);
50}
51
52/// Initialize - Reset this object to get ready for a new set of SSA
53/// updates.
54void MachineSSAUpdater::Initialize(const TargetRegisterClass *RC) {
55  if (!AV)
56    AV = new AvailableValsTy();
57  else
58    getAvailableVals(AV).clear();
59
60  VRC = RC;
61}
62
63void MachineSSAUpdater::Initialize(Register V) {
64  Initialize(MRI->getRegClass(V));
65}
66
67/// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for
68/// the specified block.
69bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const {
70  return getAvailableVals(AV).count(BB);
71}
72
73/// AddAvailableValue - Indicate that a rewritten value is available in the
74/// specified block with the specified value.
75void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, Register V) {
76  getAvailableVals(AV)[BB] = V;
77}
78
79/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
80/// live at the end of the specified block.
81Register MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) {
82  return GetValueAtEndOfBlockInternal(BB);
83}
84
85static
86Register LookForIdenticalPHI(MachineBasicBlock *BB,
87        SmallVectorImpl<std::pair<MachineBasicBlock *, Register>> &PredValues) {
88  if (BB->empty())
89    return Register();
90
91  MachineBasicBlock::iterator I = BB->begin();
92  if (!I->isPHI())
93    return Register();
94
95  AvailableValsTy AVals;
96  for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
97    AVals[PredValues[i].first] = PredValues[i].second;
98  while (I != BB->end() && I->isPHI()) {
99    bool Same = true;
100    for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) {
101      Register SrcReg = I->getOperand(i).getReg();
102      MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB();
103      if (AVals[SrcBB] != SrcReg) {
104        Same = false;
105        break;
106      }
107    }
108    if (Same)
109      return I->getOperand(0).getReg();
110    ++I;
111  }
112  return Register();
113}
114
115/// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define
116/// a value of the given register class at the start of the specified basic
117/// block. It returns the virtual register defined by the instruction.
118static
119MachineInstrBuilder InsertNewDef(unsigned Opcode,
120                           MachineBasicBlock *BB, MachineBasicBlock::iterator I,
121                           const TargetRegisterClass *RC,
122                           MachineRegisterInfo *MRI,
123                           const TargetInstrInfo *TII) {
124  Register NewVR = MRI->createVirtualRegister(RC);
125  return BuildMI(*BB, I, DebugLoc(), TII->get(Opcode), NewVR);
126}
127
128/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
129/// is live in the middle of the specified block. If ExistingValueOnly is
130/// true then this will only return an existing value or $noreg; otherwise new
131/// instructions may be inserted to materialize a value.
132///
133/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
134/// important case: if there is a definition of the rewritten value after the
135/// 'use' in BB.  Consider code like this:
136///
137///      X1 = ...
138///   SomeBB:
139///      use(X)
140///      X2 = ...
141///      br Cond, SomeBB, OutBB
142///
143/// In this case, there are two values (X1 and X2) added to the AvailableVals
144/// set by the client of the rewriter, and those values are both live out of
145/// their respective blocks.  However, the use of X happens in the *middle* of
146/// a block.  Because of this, we need to insert a new PHI node in SomeBB to
147/// merge the appropriate values, and this value isn't live out of the block.
148Register MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB,
149                                                    bool ExistingValueOnly) {
150  // If there is no definition of the renamed variable in this block, just use
151  // GetValueAtEndOfBlock to do our work.
152  if (!HasValueForBlock(BB))
153    return GetValueAtEndOfBlockInternal(BB, ExistingValueOnly);
154
155  // If there are no predecessors, just return undef.
156  if (BB->pred_empty()) {
157    // If we cannot insert new instructions, just return $noreg.
158    if (ExistingValueOnly)
159      return Register();
160    // Insert an implicit_def to represent an undef value.
161    MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
162                                        BB, BB->getFirstTerminator(),
163                                        VRC, MRI, TII);
164    return NewDef->getOperand(0).getReg();
165  }
166
167  // Otherwise, we have the hard case.  Get the live-in values for each
168  // predecessor.
169  SmallVector<std::pair<MachineBasicBlock*, Register>, 8> PredValues;
170  Register SingularValue;
171
172  bool isFirstPred = true;
173  for (MachineBasicBlock *PredBB : BB->predecessors()) {
174    Register PredVal = GetValueAtEndOfBlockInternal(PredBB, ExistingValueOnly);
175    PredValues.push_back(std::make_pair(PredBB, PredVal));
176
177    // Compute SingularValue.
178    if (isFirstPred) {
179      SingularValue = PredVal;
180      isFirstPred = false;
181    } else if (PredVal != SingularValue)
182      SingularValue = Register();
183  }
184
185  // Otherwise, if all the merged values are the same, just use it.
186  if (SingularValue)
187    return SingularValue;
188
189  // If an identical PHI is already in BB, just reuse it.
190  Register DupPHI = LookForIdenticalPHI(BB, PredValues);
191  if (DupPHI)
192    return DupPHI;
193
194  // If we cannot create new instructions, return $noreg now.
195  if (ExistingValueOnly)
196    return Register();
197
198  // Otherwise, we do need a PHI: insert one now.
199  MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
200  MachineInstrBuilder InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB,
201                                                 Loc, VRC, MRI, TII);
202
203  // Fill in all the predecessors of the PHI.
204  for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
205    InsertedPHI.addReg(PredValues[i].second).addMBB(PredValues[i].first);
206
207  // See if the PHI node can be merged to a single value.  This can happen in
208  // loop cases when we get a PHI of itself and one other value.
209  if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
210    InsertedPHI->eraseFromParent();
211    return ConstVal;
212  }
213
214  // If the client wants to know about all new instructions, tell it.
215  if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
216
217  LLVM_DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
218  return InsertedPHI.getReg(0);
219}
220
221static
222MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI,
223                                         MachineOperand *U) {
224  for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
225    if (&MI->getOperand(i) == U)
226      return MI->getOperand(i+1).getMBB();
227  }
228
229  llvm_unreachable("MachineOperand::getParent() failure?");
230}
231
232/// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
233/// which use their value in the corresponding predecessor.
234void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
235  MachineInstr *UseMI = U.getParent();
236  Register NewVR;
237  if (UseMI->isPHI()) {
238    MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U);
239    NewVR = GetValueAtEndOfBlockInternal(SourceBB);
240  } else {
241    NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
242  }
243
244  U.setReg(NewVR);
245}
246
247namespace llvm {
248
249/// SSAUpdaterTraits<MachineSSAUpdater> - Traits for the SSAUpdaterImpl
250/// template, specialized for MachineSSAUpdater.
251template<>
252class SSAUpdaterTraits<MachineSSAUpdater> {
253public:
254  using BlkT = MachineBasicBlock;
255  using ValT = Register;
256  using PhiT = MachineInstr;
257  using BlkSucc_iterator = MachineBasicBlock::succ_iterator;
258
259  static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return BB->succ_begin(); }
260  static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return BB->succ_end(); }
261
262  /// Iterator for PHI operands.
263  class PHI_iterator {
264  private:
265    MachineInstr *PHI;
266    unsigned idx;
267
268  public:
269    explicit PHI_iterator(MachineInstr *P) // begin iterator
270      : PHI(P), idx(1) {}
271    PHI_iterator(MachineInstr *P, bool) // end iterator
272      : PHI(P), idx(PHI->getNumOperands()) {}
273
274    PHI_iterator &operator++() { idx += 2; return *this; }
275    bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
276    bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
277
278    unsigned getIncomingValue() { return PHI->getOperand(idx).getReg(); }
279
280    MachineBasicBlock *getIncomingBlock() {
281      return PHI->getOperand(idx+1).getMBB();
282    }
283  };
284
285  static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
286
287  static inline PHI_iterator PHI_end(PhiT *PHI) {
288    return PHI_iterator(PHI, true);
289  }
290
291  /// FindPredecessorBlocks - Put the predecessors of BB into the Preds
292  /// vector.
293  static void FindPredecessorBlocks(MachineBasicBlock *BB,
294                                    SmallVectorImpl<MachineBasicBlock*> *Preds){
295    append_range(*Preds, BB->predecessors());
296  }
297
298  /// GetUndefVal - Create an IMPLICIT_DEF instruction with a new register.
299  /// Add it into the specified block and return the register.
300  static Register GetUndefVal(MachineBasicBlock *BB,
301                              MachineSSAUpdater *Updater) {
302    // Insert an implicit_def to represent an undef value.
303    MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
304                                        BB, BB->getFirstNonPHI(),
305                                        Updater->VRC, Updater->MRI,
306                                        Updater->TII);
307    return NewDef->getOperand(0).getReg();
308  }
309
310  /// CreateEmptyPHI - Create a PHI instruction that defines a new register.
311  /// Add it into the specified block and return the register.
312  static Register CreateEmptyPHI(MachineBasicBlock *BB, unsigned NumPreds,
313                                 MachineSSAUpdater *Updater) {
314    MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
315    MachineInstr *PHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
316                                     Updater->VRC, Updater->MRI,
317                                     Updater->TII);
318    return PHI->getOperand(0).getReg();
319  }
320
321  /// AddPHIOperand - Add the specified value as an operand of the PHI for
322  /// the specified predecessor block.
323  static void AddPHIOperand(MachineInstr *PHI, Register Val,
324                            MachineBasicBlock *Pred) {
325    MachineInstrBuilder(*Pred->getParent(), PHI).addReg(Val).addMBB(Pred);
326  }
327
328  /// InstrIsPHI - Check if an instruction is a PHI.
329  static MachineInstr *InstrIsPHI(MachineInstr *I) {
330    if (I && I->isPHI())
331      return I;
332    return nullptr;
333  }
334
335  /// ValueIsPHI - Check if the instruction that defines the specified register
336  /// is a PHI instruction.
337  static MachineInstr *ValueIsPHI(Register Val, MachineSSAUpdater *Updater) {
338    return InstrIsPHI(Updater->MRI->getVRegDef(Val));
339  }
340
341  /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
342  /// operands, i.e., it was just added.
343  static MachineInstr *ValueIsNewPHI(Register Val, MachineSSAUpdater *Updater) {
344    MachineInstr *PHI = ValueIsPHI(Val, Updater);
345    if (PHI && PHI->getNumOperands() <= 1)
346      return PHI;
347    return nullptr;
348  }
349
350  /// GetPHIValue - For the specified PHI instruction, return the register
351  /// that it defines.
352  static Register GetPHIValue(MachineInstr *PHI) {
353    return PHI->getOperand(0).getReg();
354  }
355};
356
357} // end namespace llvm
358
359/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
360/// for the specified BB and if so, return it.  If not, construct SSA form by
361/// first calculating the required placement of PHIs and then inserting new
362/// PHIs where needed.
363Register
364MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB,
365                                                bool ExistingValueOnly) {
366  AvailableValsTy &AvailableVals = getAvailableVals(AV);
367  Register ExistingVal = AvailableVals.lookup(BB);
368  if (ExistingVal || ExistingValueOnly)
369    return ExistingVal;
370
371  SSAUpdaterImpl<MachineSSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
372  return Impl.GetValue(BB);
373}
374