1//===- SSAUpdater.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 SSAUpdater class.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/Transforms/Utils/SSAUpdater.h"
15#include "llvm/ADT/DenseMap.h"
16#include "llvm/ADT/TinyPtrVector.h"
17#include "llvm/Analysis/InstructionSimplify.h"
18#include "llvm/IR/CFG.h"
19#include "llvm/IR/Constants.h"
20#include "llvm/IR/Instructions.h"
21#include "llvm/IR/IntrinsicInst.h"
22#include "llvm/Support/Debug.h"
23#include "llvm/Support/raw_ostream.h"
24#include "llvm/Transforms/Utils/BasicBlockUtils.h"
25#include "llvm/Transforms/Utils/Local.h"
26#include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
27
28using namespace llvm;
29
30#define DEBUG_TYPE "ssaupdater"
31
32typedef DenseMap<BasicBlock*, Value*> AvailableValsTy;
33static AvailableValsTy &getAvailableVals(void *AV) {
34  return *static_cast<AvailableValsTy*>(AV);
35}
36
37SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI)
38  : AV(nullptr), ProtoType(nullptr), ProtoName(), InsertedPHIs(NewPHI) {}
39
40SSAUpdater::~SSAUpdater() {
41  delete static_cast<AvailableValsTy*>(AV);
42}
43
44void SSAUpdater::Initialize(Type *Ty, StringRef Name) {
45  if (!AV)
46    AV = new AvailableValsTy();
47  else
48    getAvailableVals(AV).clear();
49  ProtoType = Ty;
50  ProtoName = Name;
51}
52
53bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
54  return getAvailableVals(AV).count(BB);
55}
56
57void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
58  assert(ProtoType && "Need to initialize SSAUpdater");
59  assert(ProtoType == V->getType() &&
60         "All rewritten values must have the same type");
61  getAvailableVals(AV)[BB] = V;
62}
63
64static bool IsEquivalentPHI(PHINode *PHI,
65                          SmallDenseMap<BasicBlock*, Value*, 8> &ValueMapping) {
66  unsigned PHINumValues = PHI->getNumIncomingValues();
67  if (PHINumValues != ValueMapping.size())
68    return false;
69
70  // Scan the phi to see if it matches.
71  for (unsigned i = 0, e = PHINumValues; i != e; ++i)
72    if (ValueMapping[PHI->getIncomingBlock(i)] !=
73        PHI->getIncomingValue(i)) {
74      return false;
75    }
76
77  return true;
78}
79
80Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
81  Value *Res = GetValueAtEndOfBlockInternal(BB);
82  return Res;
83}
84
85Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
86  // If there is no definition of the renamed variable in this block, just use
87  // GetValueAtEndOfBlock to do our work.
88  if (!HasValueForBlock(BB))
89    return GetValueAtEndOfBlock(BB);
90
91  // Otherwise, we have the hard case.  Get the live-in values for each
92  // predecessor.
93  SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
94  Value *SingularValue = nullptr;
95
96  // We can get our predecessor info by walking the pred_iterator list, but it
97  // is relatively slow.  If we already have PHI nodes in this block, walk one
98  // of them to get the predecessor list instead.
99  if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
100    for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
101      BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
102      Value *PredVal = GetValueAtEndOfBlock(PredBB);
103      PredValues.push_back(std::make_pair(PredBB, PredVal));
104
105      // Compute SingularValue.
106      if (i == 0)
107        SingularValue = PredVal;
108      else if (PredVal != SingularValue)
109        SingularValue = nullptr;
110    }
111  } else {
112    bool isFirstPred = true;
113    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
114      BasicBlock *PredBB = *PI;
115      Value *PredVal = GetValueAtEndOfBlock(PredBB);
116      PredValues.push_back(std::make_pair(PredBB, PredVal));
117
118      // Compute SingularValue.
119      if (isFirstPred) {
120        SingularValue = PredVal;
121        isFirstPred = false;
122      } else if (PredVal != SingularValue)
123        SingularValue = nullptr;
124    }
125  }
126
127  // If there are no predecessors, just return undef.
128  if (PredValues.empty())
129    return UndefValue::get(ProtoType);
130
131  // Otherwise, if all the merged values are the same, just use it.
132  if (SingularValue)
133    return SingularValue;
134
135  // Otherwise, we do need a PHI: check to see if we already have one available
136  // in this block that produces the right value.
137  if (isa<PHINode>(BB->begin())) {
138    SmallDenseMap<BasicBlock*, Value*, 8> ValueMapping(PredValues.begin(),
139                                                       PredValues.end());
140    PHINode *SomePHI;
141    for (BasicBlock::iterator It = BB->begin();
142         (SomePHI = dyn_cast<PHINode>(It)); ++It) {
143      if (IsEquivalentPHI(SomePHI, ValueMapping))
144        return SomePHI;
145    }
146  }
147
148  // Ok, we have no way out, insert a new one now.
149  PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(),
150                                         ProtoName, &BB->front());
151
152  // Fill in all the predecessors of the PHI.
153  for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
154    InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);
155
156  // See if the PHI node can be merged to a single value.  This can happen in
157  // loop cases when we get a PHI of itself and one other value.
158  if (Value *V = SimplifyInstruction(InsertedPHI)) {
159    InsertedPHI->eraseFromParent();
160    return V;
161  }
162
163  // Set the DebugLoc of the inserted PHI, if available.
164  DebugLoc DL;
165  if (const Instruction *I = BB->getFirstNonPHI())
166      DL = I->getDebugLoc();
167  InsertedPHI->setDebugLoc(DL);
168
169  // If the client wants to know about all new instructions, tell it.
170  if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
171
172  DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
173  return InsertedPHI;
174}
175
176void SSAUpdater::RewriteUse(Use &U) {
177  Instruction *User = cast<Instruction>(U.getUser());
178
179  Value *V;
180  if (PHINode *UserPN = dyn_cast<PHINode>(User))
181    V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
182  else
183    V = GetValueInMiddleOfBlock(User->getParent());
184
185  // Notify that users of the existing value that it is being replaced.
186  Value *OldVal = U.get();
187  if (OldVal != V && OldVal->hasValueHandle())
188    ValueHandleBase::ValueIsRAUWd(OldVal, V);
189
190  U.set(V);
191}
192
193void SSAUpdater::RewriteUseAfterInsertions(Use &U) {
194  Instruction *User = cast<Instruction>(U.getUser());
195
196  Value *V;
197  if (PHINode *UserPN = dyn_cast<PHINode>(User))
198    V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
199  else
200    V = GetValueAtEndOfBlock(User->getParent());
201
202  U.set(V);
203}
204
205namespace llvm {
206template<>
207class SSAUpdaterTraits<SSAUpdater> {
208public:
209  typedef BasicBlock BlkT;
210  typedef Value *ValT;
211  typedef PHINode PhiT;
212
213  typedef succ_iterator BlkSucc_iterator;
214  static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
215  static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }
216
217  class PHI_iterator {
218  private:
219    PHINode *PHI;
220    unsigned idx;
221
222  public:
223    explicit PHI_iterator(PHINode *P) // begin iterator
224      : PHI(P), idx(0) {}
225    PHI_iterator(PHINode *P, bool) // end iterator
226      : PHI(P), idx(PHI->getNumIncomingValues()) {}
227
228    PHI_iterator &operator++() { ++idx; return *this; }
229    bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
230    bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
231    Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
232    BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
233  };
234
235  static PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
236  static PHI_iterator PHI_end(PhiT *PHI) {
237    return PHI_iterator(PHI, true);
238  }
239
240  /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
241  /// vector, set Info->NumPreds, and allocate space in Info->Preds.
242  static void FindPredecessorBlocks(BasicBlock *BB,
243                                    SmallVectorImpl<BasicBlock*> *Preds) {
244    // We can get our predecessor info by walking the pred_iterator list,
245    // but it is relatively slow.  If we already have PHI nodes in this
246    // block, walk one of them to get the predecessor list instead.
247    if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
248      for (unsigned PI = 0, E = SomePhi->getNumIncomingValues(); PI != E; ++PI)
249        Preds->push_back(SomePhi->getIncomingBlock(PI));
250    } else {
251      for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
252        Preds->push_back(*PI);
253    }
254  }
255
256  /// GetUndefVal - Get an undefined value of the same type as the value
257  /// being handled.
258  static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
259    return UndefValue::get(Updater->ProtoType);
260  }
261
262  /// CreateEmptyPHI - Create a new PHI instruction in the specified block.
263  /// Reserve space for the operands but do not fill them in yet.
264  static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
265                               SSAUpdater *Updater) {
266    PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds,
267                                   Updater->ProtoName, &BB->front());
268    return PHI;
269  }
270
271  /// AddPHIOperand - Add the specified value as an operand of the PHI for
272  /// the specified predecessor block.
273  static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
274    PHI->addIncoming(Val, Pred);
275  }
276
277  /// InstrIsPHI - Check if an instruction is a PHI.
278  ///
279  static PHINode *InstrIsPHI(Instruction *I) {
280    return dyn_cast<PHINode>(I);
281  }
282
283  /// ValueIsPHI - Check if a value is a PHI.
284  ///
285  static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
286    return dyn_cast<PHINode>(Val);
287  }
288
289  /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
290  /// operands, i.e., it was just added.
291  static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
292    PHINode *PHI = ValueIsPHI(Val, Updater);
293    if (PHI && PHI->getNumIncomingValues() == 0)
294      return PHI;
295    return nullptr;
296  }
297
298  /// GetPHIValue - For the specified PHI instruction, return the value
299  /// that it defines.
300  static Value *GetPHIValue(PHINode *PHI) {
301    return PHI;
302  }
303};
304
305} // End llvm namespace
306
307/// Check to see if AvailableVals has an entry for the specified BB and if so,
308/// return it.  If not, construct SSA form by first calculating the required
309/// placement of PHIs and then inserting new PHIs where needed.
310Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
311  AvailableValsTy &AvailableVals = getAvailableVals(AV);
312  if (Value *V = AvailableVals[BB])
313    return V;
314
315  SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
316  return Impl.GetValue(BB);
317}
318
319//===----------------------------------------------------------------------===//
320// LoadAndStorePromoter Implementation
321//===----------------------------------------------------------------------===//
322
323LoadAndStorePromoter::
324LoadAndStorePromoter(const SmallVectorImpl<Instruction*> &Insts,
325                     SSAUpdater &S, StringRef BaseName) : SSA(S) {
326  if (Insts.empty()) return;
327
328  Value *SomeVal;
329  if (LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
330    SomeVal = LI;
331  else
332    SomeVal = cast<StoreInst>(Insts[0])->getOperand(0);
333
334  if (BaseName.empty())
335    BaseName = SomeVal->getName();
336  SSA.Initialize(SomeVal->getType(), BaseName);
337}
338
339
340void LoadAndStorePromoter::
341run(const SmallVectorImpl<Instruction*> &Insts) const {
342
343  // First step: bucket up uses of the alloca by the block they occur in.
344  // This is important because we have to handle multiple defs/uses in a block
345  // ourselves: SSAUpdater is purely for cross-block references.
346  DenseMap<BasicBlock*, TinyPtrVector<Instruction*> > UsesByBlock;
347
348  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
349    Instruction *User = Insts[i];
350    UsesByBlock[User->getParent()].push_back(User);
351  }
352
353  // Okay, now we can iterate over all the blocks in the function with uses,
354  // processing them.  Keep track of which loads are loading a live-in value.
355  // Walk the uses in the use-list order to be determinstic.
356  SmallVector<LoadInst*, 32> LiveInLoads;
357  DenseMap<Value*, Value*> ReplacedLoads;
358
359  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
360    Instruction *User = Insts[i];
361    BasicBlock *BB = User->getParent();
362    TinyPtrVector<Instruction*> &BlockUses = UsesByBlock[BB];
363
364    // If this block has already been processed, ignore this repeat use.
365    if (BlockUses.empty()) continue;
366
367    // Okay, this is the first use in the block.  If this block just has a
368    // single user in it, we can rewrite it trivially.
369    if (BlockUses.size() == 1) {
370      // If it is a store, it is a trivial def of the value in the block.
371      if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
372        updateDebugInfo(SI);
373        SSA.AddAvailableValue(BB, SI->getOperand(0));
374      } else
375        // Otherwise it is a load, queue it to rewrite as a live-in load.
376        LiveInLoads.push_back(cast<LoadInst>(User));
377      BlockUses.clear();
378      continue;
379    }
380
381    // Otherwise, check to see if this block is all loads.
382    bool HasStore = false;
383    for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
384      if (isa<StoreInst>(BlockUses[i])) {
385        HasStore = true;
386        break;
387      }
388    }
389
390    // If so, we can queue them all as live in loads.  We don't have an
391    // efficient way to tell which on is first in the block and don't want to
392    // scan large blocks, so just add all loads as live ins.
393    if (!HasStore) {
394      for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
395        LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
396      BlockUses.clear();
397      continue;
398    }
399
400    // Otherwise, we have mixed loads and stores (or just a bunch of stores).
401    // Since SSAUpdater is purely for cross-block values, we need to determine
402    // the order of these instructions in the block.  If the first use in the
403    // block is a load, then it uses the live in value.  The last store defines
404    // the live out value.  We handle this by doing a linear scan of the block.
405    Value *StoredValue = nullptr;
406    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
407      if (LoadInst *L = dyn_cast<LoadInst>(II)) {
408        // If this is a load from an unrelated pointer, ignore it.
409        if (!isInstInList(L, Insts)) continue;
410
411        // If we haven't seen a store yet, this is a live in use, otherwise
412        // use the stored value.
413        if (StoredValue) {
414          replaceLoadWithValue(L, StoredValue);
415          L->replaceAllUsesWith(StoredValue);
416          ReplacedLoads[L] = StoredValue;
417        } else {
418          LiveInLoads.push_back(L);
419        }
420        continue;
421      }
422
423      if (StoreInst *SI = dyn_cast<StoreInst>(II)) {
424        // If this is a store to an unrelated pointer, ignore it.
425        if (!isInstInList(SI, Insts)) continue;
426        updateDebugInfo(SI);
427
428        // Remember that this is the active value in the block.
429        StoredValue = SI->getOperand(0);
430      }
431    }
432
433    // The last stored value that happened is the live-out for the block.
434    assert(StoredValue && "Already checked that there is a store in block");
435    SSA.AddAvailableValue(BB, StoredValue);
436    BlockUses.clear();
437  }
438
439  // Okay, now we rewrite all loads that use live-in values in the loop,
440  // inserting PHI nodes as necessary.
441  for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
442    LoadInst *ALoad = LiveInLoads[i];
443    Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
444    replaceLoadWithValue(ALoad, NewVal);
445
446    // Avoid assertions in unreachable code.
447    if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType());
448    ALoad->replaceAllUsesWith(NewVal);
449    ReplacedLoads[ALoad] = NewVal;
450  }
451
452  // Allow the client to do stuff before we start nuking things.
453  doExtraRewritesBeforeFinalDeletion();
454
455  // Now that everything is rewritten, delete the old instructions from the
456  // function.  They should all be dead now.
457  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
458    Instruction *User = Insts[i];
459
460    // If this is a load that still has uses, then the load must have been added
461    // as a live value in the SSAUpdate data structure for a block (e.g. because
462    // the loaded value was stored later).  In this case, we need to recursively
463    // propagate the updates until we get to the real value.
464    if (!User->use_empty()) {
465      Value *NewVal = ReplacedLoads[User];
466      assert(NewVal && "not a replaced load?");
467
468      // Propagate down to the ultimate replacee.  The intermediately loads
469      // could theoretically already have been deleted, so we don't want to
470      // dereference the Value*'s.
471      DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
472      while (RLI != ReplacedLoads.end()) {
473        NewVal = RLI->second;
474        RLI = ReplacedLoads.find(NewVal);
475      }
476
477      replaceLoadWithValue(cast<LoadInst>(User), NewVal);
478      User->replaceAllUsesWith(NewVal);
479    }
480
481    instructionDeleted(User);
482    User->eraseFromParent();
483  }
484}
485
486bool
487LoadAndStorePromoter::isInstInList(Instruction *I,
488                                   const SmallVectorImpl<Instruction*> &Insts)
489                                   const {
490  return std::find(Insts.begin(), Insts.end(), I) != Insts.end();
491}
492