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#define DEBUG_TYPE "ssaupdater"
15#include "llvm/Constants.h"
16#include "llvm/Instructions.h"
17#include "llvm/IntrinsicInst.h"
18#include "llvm/ADT/DenseMap.h"
19#include "llvm/ADT/TinyPtrVector.h"
20#include "llvm/Analysis/InstructionSimplify.h"
21#include "llvm/Support/AlignOf.h"
22#include "llvm/Support/Allocator.h"
23#include "llvm/Support/CFG.h"
24#include "llvm/Support/Debug.h"
25#include "llvm/Support/raw_ostream.h"
26#include "llvm/Transforms/Utils/BasicBlockUtils.h"
27#include "llvm/Transforms/Utils/Local.h"
28#include "llvm/Transforms/Utils/SSAUpdater.h"
29#include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
30
31using namespace llvm;
32
33typedef DenseMap<BasicBlock*, Value*> AvailableValsTy;
34static AvailableValsTy &getAvailableVals(void *AV) {
35  return *static_cast<AvailableValsTy*>(AV);
36}
37
38SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI)
39  : AV(0), ProtoType(0), ProtoName(), InsertedPHIs(NewPHI) {}
40
41SSAUpdater::~SSAUpdater() {
42  delete &getAvailableVals(AV);
43}
44
45/// Initialize - Reset this object to get ready for a new set of SSA
46/// updates with type 'Ty'.  PHI nodes get a name based on 'Name'.
47void SSAUpdater::Initialize(Type *Ty, StringRef Name) {
48  if (AV == 0)
49    AV = new AvailableValsTy();
50  else
51    getAvailableVals(AV).clear();
52  ProtoType = Ty;
53  ProtoName = Name;
54}
55
56/// HasValueForBlock - Return true if the SSAUpdater already has a value for
57/// the specified block.
58bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
59  return getAvailableVals(AV).count(BB);
60}
61
62/// AddAvailableValue - Indicate that a rewritten value is available in the
63/// specified block with the specified value.
64void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
65  assert(ProtoType != 0 && "Need to initialize SSAUpdater");
66  assert(ProtoType == V->getType() &&
67         "All rewritten values must have the same type");
68  getAvailableVals(AV)[BB] = V;
69}
70
71/// IsEquivalentPHI - Check if PHI has the same incoming value as specified
72/// in ValueMapping for each predecessor block.
73static bool IsEquivalentPHI(PHINode *PHI,
74                            DenseMap<BasicBlock*, Value*> &ValueMapping) {
75  unsigned PHINumValues = PHI->getNumIncomingValues();
76  if (PHINumValues != ValueMapping.size())
77    return false;
78
79  // Scan the phi to see if it matches.
80  for (unsigned i = 0, e = PHINumValues; i != e; ++i)
81    if (ValueMapping[PHI->getIncomingBlock(i)] !=
82        PHI->getIncomingValue(i)) {
83      return false;
84    }
85
86  return true;
87}
88
89/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
90/// live at the end of the specified block.
91Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
92  Value *Res = GetValueAtEndOfBlockInternal(BB);
93  return Res;
94}
95
96/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
97/// is live in the middle of the specified block.
98///
99/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
100/// important case: if there is a definition of the rewritten value after the
101/// 'use' in BB.  Consider code like this:
102///
103///      X1 = ...
104///   SomeBB:
105///      use(X)
106///      X2 = ...
107///      br Cond, SomeBB, OutBB
108///
109/// In this case, there are two values (X1 and X2) added to the AvailableVals
110/// set by the client of the rewriter, and those values are both live out of
111/// their respective blocks.  However, the use of X happens in the *middle* of
112/// a block.  Because of this, we need to insert a new PHI node in SomeBB to
113/// merge the appropriate values, and this value isn't live out of the block.
114///
115Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
116  // If there is no definition of the renamed variable in this block, just use
117  // GetValueAtEndOfBlock to do our work.
118  if (!HasValueForBlock(BB))
119    return GetValueAtEndOfBlock(BB);
120
121  // Otherwise, we have the hard case.  Get the live-in values for each
122  // predecessor.
123  SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
124  Value *SingularValue = 0;
125
126  // We can get our predecessor info by walking the pred_iterator list, but it
127  // is relatively slow.  If we already have PHI nodes in this block, walk one
128  // of them to get the predecessor list instead.
129  if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
130    for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
131      BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
132      Value *PredVal = GetValueAtEndOfBlock(PredBB);
133      PredValues.push_back(std::make_pair(PredBB, PredVal));
134
135      // Compute SingularValue.
136      if (i == 0)
137        SingularValue = PredVal;
138      else if (PredVal != SingularValue)
139        SingularValue = 0;
140    }
141  } else {
142    bool isFirstPred = true;
143    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
144      BasicBlock *PredBB = *PI;
145      Value *PredVal = GetValueAtEndOfBlock(PredBB);
146      PredValues.push_back(std::make_pair(PredBB, PredVal));
147
148      // Compute SingularValue.
149      if (isFirstPred) {
150        SingularValue = PredVal;
151        isFirstPred = false;
152      } else if (PredVal != SingularValue)
153        SingularValue = 0;
154    }
155  }
156
157  // If there are no predecessors, just return undef.
158  if (PredValues.empty())
159    return UndefValue::get(ProtoType);
160
161  // Otherwise, if all the merged values are the same, just use it.
162  if (SingularValue != 0)
163    return SingularValue;
164
165  // Otherwise, we do need a PHI: check to see if we already have one available
166  // in this block that produces the right value.
167  if (isa<PHINode>(BB->begin())) {
168    DenseMap<BasicBlock*, Value*> ValueMapping(PredValues.begin(),
169                                               PredValues.end());
170    PHINode *SomePHI;
171    for (BasicBlock::iterator It = BB->begin();
172         (SomePHI = dyn_cast<PHINode>(It)); ++It) {
173      if (IsEquivalentPHI(SomePHI, ValueMapping))
174        return SomePHI;
175    }
176  }
177
178  // Ok, we have no way out, insert a new one now.
179  PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(),
180                                         ProtoName, &BB->front());
181
182  // Fill in all the predecessors of the PHI.
183  for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
184    InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);
185
186  // See if the PHI node can be merged to a single value.  This can happen in
187  // loop cases when we get a PHI of itself and one other value.
188  if (Value *V = SimplifyInstruction(InsertedPHI)) {
189    InsertedPHI->eraseFromParent();
190    return V;
191  }
192
193  // Set DebugLoc.
194  InsertedPHI->setDebugLoc(GetFirstDebugLocInBasicBlock(BB));
195
196  // If the client wants to know about all new instructions, tell it.
197  if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
198
199  DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
200  return InsertedPHI;
201}
202
203/// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
204/// which use their value in the corresponding predecessor.
205void SSAUpdater::RewriteUse(Use &U) {
206  Instruction *User = cast<Instruction>(U.getUser());
207
208  Value *V;
209  if (PHINode *UserPN = dyn_cast<PHINode>(User))
210    V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
211  else
212    V = GetValueInMiddleOfBlock(User->getParent());
213
214  U.set(V);
215}
216
217/// RewriteUseAfterInsertions - Rewrite a use, just like RewriteUse.  However,
218/// this version of the method can rewrite uses in the same block as a
219/// definition, because it assumes that all uses of a value are below any
220/// inserted values.
221void SSAUpdater::RewriteUseAfterInsertions(Use &U) {
222  Instruction *User = cast<Instruction>(U.getUser());
223
224  Value *V;
225  if (PHINode *UserPN = dyn_cast<PHINode>(User))
226    V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
227  else
228    V = GetValueAtEndOfBlock(User->getParent());
229
230  U.set(V);
231}
232
233/// PHIiter - Iterator for PHI operands.  This is used for the PHI_iterator
234/// in the SSAUpdaterImpl template.
235namespace {
236  class PHIiter {
237  private:
238    PHINode *PHI;
239    unsigned idx;
240
241  public:
242    explicit PHIiter(PHINode *P) // begin iterator
243      : PHI(P), idx(0) {}
244    PHIiter(PHINode *P, bool) // end iterator
245      : PHI(P), idx(PHI->getNumIncomingValues()) {}
246
247    PHIiter &operator++() { ++idx; return *this; }
248    bool operator==(const PHIiter& x) const { return idx == x.idx; }
249    bool operator!=(const PHIiter& x) const { return !operator==(x); }
250    Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
251    BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
252  };
253}
254
255/// SSAUpdaterTraits<SSAUpdater> - Traits for the SSAUpdaterImpl template,
256/// specialized for SSAUpdater.
257namespace llvm {
258template<>
259class SSAUpdaterTraits<SSAUpdater> {
260public:
261  typedef BasicBlock BlkT;
262  typedef Value *ValT;
263  typedef PHINode PhiT;
264
265  typedef succ_iterator BlkSucc_iterator;
266  static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
267  static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }
268
269  typedef PHIiter PHI_iterator;
270  static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
271  static inline PHI_iterator PHI_end(PhiT *PHI) {
272    return PHI_iterator(PHI, true);
273  }
274
275  /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
276  /// vector, set Info->NumPreds, and allocate space in Info->Preds.
277  static void FindPredecessorBlocks(BasicBlock *BB,
278                                    SmallVectorImpl<BasicBlock*> *Preds) {
279    // We can get our predecessor info by walking the pred_iterator list,
280    // but it is relatively slow.  If we already have PHI nodes in this
281    // block, walk one of them to get the predecessor list instead.
282    if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
283      for (unsigned PI = 0, E = SomePhi->getNumIncomingValues(); PI != E; ++PI)
284        Preds->push_back(SomePhi->getIncomingBlock(PI));
285    } else {
286      for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
287        Preds->push_back(*PI);
288    }
289  }
290
291  /// GetUndefVal - Get an undefined value of the same type as the value
292  /// being handled.
293  static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
294    return UndefValue::get(Updater->ProtoType);
295  }
296
297  /// CreateEmptyPHI - Create a new PHI instruction in the specified block.
298  /// Reserve space for the operands but do not fill them in yet.
299  static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
300                               SSAUpdater *Updater) {
301    PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds,
302                                   Updater->ProtoName, &BB->front());
303    return PHI;
304  }
305
306  /// AddPHIOperand - Add the specified value as an operand of the PHI for
307  /// the specified predecessor block.
308  static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
309    PHI->addIncoming(Val, Pred);
310  }
311
312  /// InstrIsPHI - Check if an instruction is a PHI.
313  ///
314  static PHINode *InstrIsPHI(Instruction *I) {
315    return dyn_cast<PHINode>(I);
316  }
317
318  /// ValueIsPHI - Check if a value is a PHI.
319  ///
320  static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
321    return dyn_cast<PHINode>(Val);
322  }
323
324  /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
325  /// operands, i.e., it was just added.
326  static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
327    PHINode *PHI = ValueIsPHI(Val, Updater);
328    if (PHI && PHI->getNumIncomingValues() == 0)
329      return PHI;
330    return 0;
331  }
332
333  /// GetPHIValue - For the specified PHI instruction, return the value
334  /// that it defines.
335  static Value *GetPHIValue(PHINode *PHI) {
336    return PHI;
337  }
338};
339
340} // End llvm namespace
341
342/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
343/// for the specified BB and if so, return it.  If not, construct SSA form by
344/// first calculating the required placement of PHIs and then inserting new
345/// PHIs where needed.
346Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
347  AvailableValsTy &AvailableVals = getAvailableVals(AV);
348  if (Value *V = AvailableVals[BB])
349    return V;
350
351  SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
352  return Impl.GetValue(BB);
353}
354
355//===----------------------------------------------------------------------===//
356// LoadAndStorePromoter Implementation
357//===----------------------------------------------------------------------===//
358
359LoadAndStorePromoter::
360LoadAndStorePromoter(const SmallVectorImpl<Instruction*> &Insts,
361                     SSAUpdater &S, StringRef BaseName) : SSA(S) {
362  if (Insts.empty()) return;
363
364  Value *SomeVal;
365  if (LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
366    SomeVal = LI;
367  else
368    SomeVal = cast<StoreInst>(Insts[0])->getOperand(0);
369
370  if (BaseName.empty())
371    BaseName = SomeVal->getName();
372  SSA.Initialize(SomeVal->getType(), BaseName);
373}
374
375
376void LoadAndStorePromoter::
377run(const SmallVectorImpl<Instruction*> &Insts) const {
378
379  // First step: bucket up uses of the alloca by the block they occur in.
380  // This is important because we have to handle multiple defs/uses in a block
381  // ourselves: SSAUpdater is purely for cross-block references.
382  DenseMap<BasicBlock*, TinyPtrVector<Instruction*> > UsesByBlock;
383
384  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
385    Instruction *User = Insts[i];
386    UsesByBlock[User->getParent()].push_back(User);
387  }
388
389  // Okay, now we can iterate over all the blocks in the function with uses,
390  // processing them.  Keep track of which loads are loading a live-in value.
391  // Walk the uses in the use-list order to be determinstic.
392  SmallVector<LoadInst*, 32> LiveInLoads;
393  DenseMap<Value*, Value*> ReplacedLoads;
394
395  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
396    Instruction *User = Insts[i];
397    BasicBlock *BB = User->getParent();
398    TinyPtrVector<Instruction*> &BlockUses = UsesByBlock[BB];
399
400    // If this block has already been processed, ignore this repeat use.
401    if (BlockUses.empty()) continue;
402
403    // Okay, this is the first use in the block.  If this block just has a
404    // single user in it, we can rewrite it trivially.
405    if (BlockUses.size() == 1) {
406      // If it is a store, it is a trivial def of the value in the block.
407      if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
408        updateDebugInfo(SI);
409        SSA.AddAvailableValue(BB, SI->getOperand(0));
410      } else
411        // Otherwise it is a load, queue it to rewrite as a live-in load.
412        LiveInLoads.push_back(cast<LoadInst>(User));
413      BlockUses.clear();
414      continue;
415    }
416
417    // Otherwise, check to see if this block is all loads.
418    bool HasStore = false;
419    for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
420      if (isa<StoreInst>(BlockUses[i])) {
421        HasStore = true;
422        break;
423      }
424    }
425
426    // If so, we can queue them all as live in loads.  We don't have an
427    // efficient way to tell which on is first in the block and don't want to
428    // scan large blocks, so just add all loads as live ins.
429    if (!HasStore) {
430      for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
431        LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
432      BlockUses.clear();
433      continue;
434    }
435
436    // Otherwise, we have mixed loads and stores (or just a bunch of stores).
437    // Since SSAUpdater is purely for cross-block values, we need to determine
438    // the order of these instructions in the block.  If the first use in the
439    // block is a load, then it uses the live in value.  The last store defines
440    // the live out value.  We handle this by doing a linear scan of the block.
441    Value *StoredValue = 0;
442    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
443      if (LoadInst *L = dyn_cast<LoadInst>(II)) {
444        // If this is a load from an unrelated pointer, ignore it.
445        if (!isInstInList(L, Insts)) continue;
446
447        // If we haven't seen a store yet, this is a live in use, otherwise
448        // use the stored value.
449        if (StoredValue) {
450          replaceLoadWithValue(L, StoredValue);
451          L->replaceAllUsesWith(StoredValue);
452          ReplacedLoads[L] = StoredValue;
453        } else {
454          LiveInLoads.push_back(L);
455        }
456        continue;
457      }
458
459      if (StoreInst *SI = dyn_cast<StoreInst>(II)) {
460        // If this is a store to an unrelated pointer, ignore it.
461        if (!isInstInList(SI, Insts)) continue;
462        updateDebugInfo(SI);
463
464        // Remember that this is the active value in the block.
465        StoredValue = SI->getOperand(0);
466      }
467    }
468
469    // The last stored value that happened is the live-out for the block.
470    assert(StoredValue && "Already checked that there is a store in block");
471    SSA.AddAvailableValue(BB, StoredValue);
472    BlockUses.clear();
473  }
474
475  // Okay, now we rewrite all loads that use live-in values in the loop,
476  // inserting PHI nodes as necessary.
477  for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
478    LoadInst *ALoad = LiveInLoads[i];
479    Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
480    replaceLoadWithValue(ALoad, NewVal);
481
482    // Avoid assertions in unreachable code.
483    if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType());
484    ALoad->replaceAllUsesWith(NewVal);
485    ReplacedLoads[ALoad] = NewVal;
486  }
487
488  // Allow the client to do stuff before we start nuking things.
489  doExtraRewritesBeforeFinalDeletion();
490
491  // Now that everything is rewritten, delete the old instructions from the
492  // function.  They should all be dead now.
493  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
494    Instruction *User = Insts[i];
495
496    // If this is a load that still has uses, then the load must have been added
497    // as a live value in the SSAUpdate data structure for a block (e.g. because
498    // the loaded value was stored later).  In this case, we need to recursively
499    // propagate the updates until we get to the real value.
500    if (!User->use_empty()) {
501      Value *NewVal = ReplacedLoads[User];
502      assert(NewVal && "not a replaced load?");
503
504      // Propagate down to the ultimate replacee.  The intermediately loads
505      // could theoretically already have been deleted, so we don't want to
506      // dereference the Value*'s.
507      DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
508      while (RLI != ReplacedLoads.end()) {
509        NewVal = RLI->second;
510        RLI = ReplacedLoads.find(NewVal);
511      }
512
513      replaceLoadWithValue(cast<LoadInst>(User), NewVal);
514      User->replaceAllUsesWith(NewVal);
515    }
516
517    instructionDeleted(User);
518    User->eraseFromParent();
519  }
520}
521