1//===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===//
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 promotes memory references to be register references.  It promotes
11// alloca instructions which only have loads and stores as uses.  An alloca is
12// transformed by using iterated dominator frontiers to place PHI nodes, then
13// traversing the function in depth-first order to rewrite loads and stores as
14// appropriate.
15//
16//===----------------------------------------------------------------------===//
17
18#include "llvm/Transforms/Utils/PromoteMemToReg.h"
19#include "llvm/ADT/ArrayRef.h"
20#include "llvm/ADT/DenseMap.h"
21#include "llvm/ADT/STLExtras.h"
22#include "llvm/ADT/SmallPtrSet.h"
23#include "llvm/ADT/SmallVector.h"
24#include "llvm/ADT/Statistic.h"
25#include "llvm/Analysis/AliasSetTracker.h"
26#include "llvm/Analysis/InstructionSimplify.h"
27#include "llvm/Analysis/IteratedDominanceFrontier.h"
28#include "llvm/Analysis/ValueTracking.h"
29#include "llvm/IR/CFG.h"
30#include "llvm/IR/Constants.h"
31#include "llvm/IR/DIBuilder.h"
32#include "llvm/IR/DebugInfo.h"
33#include "llvm/IR/DerivedTypes.h"
34#include "llvm/IR/Dominators.h"
35#include "llvm/IR/Function.h"
36#include "llvm/IR/Instructions.h"
37#include "llvm/IR/IntrinsicInst.h"
38#include "llvm/IR/Metadata.h"
39#include "llvm/IR/Module.h"
40#include "llvm/Transforms/Utils/Local.h"
41#include <algorithm>
42using namespace llvm;
43
44#define DEBUG_TYPE "mem2reg"
45
46STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
47STATISTIC(NumSingleStore,   "Number of alloca's promoted with a single store");
48STATISTIC(NumDeadAlloca,    "Number of dead alloca's removed");
49STATISTIC(NumPHIInsert,     "Number of PHI nodes inserted");
50
51bool llvm::isAllocaPromotable(const AllocaInst *AI) {
52  // FIXME: If the memory unit is of pointer or integer type, we can permit
53  // assignments to subsections of the memory unit.
54  unsigned AS = AI->getType()->getAddressSpace();
55
56  // Only allow direct and non-volatile loads and stores...
57  for (const User *U : AI->users()) {
58    if (const LoadInst *LI = dyn_cast<LoadInst>(U)) {
59      // Note that atomic loads can be transformed; atomic semantics do
60      // not have any meaning for a local alloca.
61      if (LI->isVolatile())
62        return false;
63    } else if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
64      if (SI->getOperand(0) == AI)
65        return false; // Don't allow a store OF the AI, only INTO the AI.
66      // Note that atomic stores can be transformed; atomic semantics do
67      // not have any meaning for a local alloca.
68      if (SI->isVolatile())
69        return false;
70    } else if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
71      if (II->getIntrinsicID() != Intrinsic::lifetime_start &&
72          II->getIntrinsicID() != Intrinsic::lifetime_end)
73        return false;
74    } else if (const BitCastInst *BCI = dyn_cast<BitCastInst>(U)) {
75      if (BCI->getType() != Type::getInt8PtrTy(U->getContext(), AS))
76        return false;
77      if (!onlyUsedByLifetimeMarkers(BCI))
78        return false;
79    } else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(U)) {
80      if (GEPI->getType() != Type::getInt8PtrTy(U->getContext(), AS))
81        return false;
82      if (!GEPI->hasAllZeroIndices())
83        return false;
84      if (!onlyUsedByLifetimeMarkers(GEPI))
85        return false;
86    } else {
87      return false;
88    }
89  }
90
91  return true;
92}
93
94namespace {
95
96struct AllocaInfo {
97  SmallVector<BasicBlock *, 32> DefiningBlocks;
98  SmallVector<BasicBlock *, 32> UsingBlocks;
99
100  StoreInst *OnlyStore;
101  BasicBlock *OnlyBlock;
102  bool OnlyUsedInOneBlock;
103
104  Value *AllocaPointerVal;
105  DbgDeclareInst *DbgDeclare;
106
107  void clear() {
108    DefiningBlocks.clear();
109    UsingBlocks.clear();
110    OnlyStore = nullptr;
111    OnlyBlock = nullptr;
112    OnlyUsedInOneBlock = true;
113    AllocaPointerVal = nullptr;
114    DbgDeclare = nullptr;
115  }
116
117  /// Scan the uses of the specified alloca, filling in the AllocaInfo used
118  /// by the rest of the pass to reason about the uses of this alloca.
119  void AnalyzeAlloca(AllocaInst *AI) {
120    clear();
121
122    // As we scan the uses of the alloca instruction, keep track of stores,
123    // and decide whether all of the loads and stores to the alloca are within
124    // the same basic block.
125    for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
126      Instruction *User = cast<Instruction>(*UI++);
127
128      if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
129        // Remember the basic blocks which define new values for the alloca
130        DefiningBlocks.push_back(SI->getParent());
131        AllocaPointerVal = SI->getOperand(0);
132        OnlyStore = SI;
133      } else {
134        LoadInst *LI = cast<LoadInst>(User);
135        // Otherwise it must be a load instruction, keep track of variable
136        // reads.
137        UsingBlocks.push_back(LI->getParent());
138        AllocaPointerVal = LI;
139      }
140
141      if (OnlyUsedInOneBlock) {
142        if (!OnlyBlock)
143          OnlyBlock = User->getParent();
144        else if (OnlyBlock != User->getParent())
145          OnlyUsedInOneBlock = false;
146      }
147    }
148
149    DbgDeclare = FindAllocaDbgDeclare(AI);
150  }
151};
152
153// Data package used by RenamePass()
154class RenamePassData {
155public:
156  typedef std::vector<Value *> ValVector;
157
158  RenamePassData() : BB(nullptr), Pred(nullptr), Values() {}
159  RenamePassData(BasicBlock *B, BasicBlock *P, const ValVector &V)
160      : BB(B), Pred(P), Values(V) {}
161  BasicBlock *BB;
162  BasicBlock *Pred;
163  ValVector Values;
164
165  void swap(RenamePassData &RHS) {
166    std::swap(BB, RHS.BB);
167    std::swap(Pred, RHS.Pred);
168    Values.swap(RHS.Values);
169  }
170};
171
172/// \brief This assigns and keeps a per-bb relative ordering of load/store
173/// instructions in the block that directly load or store an alloca.
174///
175/// This functionality is important because it avoids scanning large basic
176/// blocks multiple times when promoting many allocas in the same block.
177class LargeBlockInfo {
178  /// \brief For each instruction that we track, keep the index of the
179  /// instruction.
180  ///
181  /// The index starts out as the number of the instruction from the start of
182  /// the block.
183  DenseMap<const Instruction *, unsigned> InstNumbers;
184
185public:
186
187  /// This code only looks at accesses to allocas.
188  static bool isInterestingInstruction(const Instruction *I) {
189    return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) ||
190           (isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1)));
191  }
192
193  /// Get or calculate the index of the specified instruction.
194  unsigned getInstructionIndex(const Instruction *I) {
195    assert(isInterestingInstruction(I) &&
196           "Not a load/store to/from an alloca?");
197
198    // If we already have this instruction number, return it.
199    DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I);
200    if (It != InstNumbers.end())
201      return It->second;
202
203    // Scan the whole block to get the instruction.  This accumulates
204    // information for every interesting instruction in the block, in order to
205    // avoid gratuitus rescans.
206    const BasicBlock *BB = I->getParent();
207    unsigned InstNo = 0;
208    for (const Instruction &BBI : *BB)
209      if (isInterestingInstruction(&BBI))
210        InstNumbers[&BBI] = InstNo++;
211    It = InstNumbers.find(I);
212
213    assert(It != InstNumbers.end() && "Didn't insert instruction?");
214    return It->second;
215  }
216
217  void deleteValue(const Instruction *I) { InstNumbers.erase(I); }
218
219  void clear() { InstNumbers.clear(); }
220};
221
222struct PromoteMem2Reg {
223  /// The alloca instructions being promoted.
224  std::vector<AllocaInst *> Allocas;
225  DominatorTree &DT;
226  DIBuilder DIB;
227
228  /// An AliasSetTracker object to update.  If null, don't update it.
229  AliasSetTracker *AST;
230
231  /// A cache of @llvm.assume intrinsics used by SimplifyInstruction.
232  AssumptionCache *AC;
233
234  /// Reverse mapping of Allocas.
235  DenseMap<AllocaInst *, unsigned> AllocaLookup;
236
237  /// \brief The PhiNodes we're adding.
238  ///
239  /// That map is used to simplify some Phi nodes as we iterate over it, so
240  /// it should have deterministic iterators.  We could use a MapVector, but
241  /// since we already maintain a map from BasicBlock* to a stable numbering
242  /// (BBNumbers), the DenseMap is more efficient (also supports removal).
243  DenseMap<std::pair<unsigned, unsigned>, PHINode *> NewPhiNodes;
244
245  /// For each PHI node, keep track of which entry in Allocas it corresponds
246  /// to.
247  DenseMap<PHINode *, unsigned> PhiToAllocaMap;
248
249  /// If we are updating an AliasSetTracker, then for each alloca that is of
250  /// pointer type, we keep track of what to copyValue to the inserted PHI
251  /// nodes here.
252  std::vector<Value *> PointerAllocaValues;
253
254  /// For each alloca, we keep track of the dbg.declare intrinsic that
255  /// describes it, if any, so that we can convert it to a dbg.value
256  /// intrinsic if the alloca gets promoted.
257  SmallVector<DbgDeclareInst *, 8> AllocaDbgDeclares;
258
259  /// The set of basic blocks the renamer has already visited.
260  ///
261  SmallPtrSet<BasicBlock *, 16> Visited;
262
263  /// Contains a stable numbering of basic blocks to avoid non-determinstic
264  /// behavior.
265  DenseMap<BasicBlock *, unsigned> BBNumbers;
266
267  /// Lazily compute the number of predecessors a block has.
268  DenseMap<const BasicBlock *, unsigned> BBNumPreds;
269
270public:
271  PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
272                 AliasSetTracker *AST, AssumptionCache *AC)
273      : Allocas(Allocas.begin(), Allocas.end()), DT(DT),
274        DIB(*DT.getRoot()->getParent()->getParent(), /*AllowUnresolved*/ false),
275        AST(AST), AC(AC) {}
276
277  void run();
278
279private:
280  void RemoveFromAllocasList(unsigned &AllocaIdx) {
281    Allocas[AllocaIdx] = Allocas.back();
282    Allocas.pop_back();
283    --AllocaIdx;
284  }
285
286  unsigned getNumPreds(const BasicBlock *BB) {
287    unsigned &NP = BBNumPreds[BB];
288    if (NP == 0)
289      NP = std::distance(pred_begin(BB), pred_end(BB)) + 1;
290    return NP - 1;
291  }
292
293  void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
294                           const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
295                           SmallPtrSetImpl<BasicBlock *> &LiveInBlocks);
296  void RenamePass(BasicBlock *BB, BasicBlock *Pred,
297                  RenamePassData::ValVector &IncVals,
298                  std::vector<RenamePassData> &Worklist);
299  bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version);
300};
301
302} // end of anonymous namespace
303
304static void removeLifetimeIntrinsicUsers(AllocaInst *AI) {
305  // Knowing that this alloca is promotable, we know that it's safe to kill all
306  // instructions except for load and store.
307
308  for (auto UI = AI->user_begin(), UE = AI->user_end(); UI != UE;) {
309    Instruction *I = cast<Instruction>(*UI);
310    ++UI;
311    if (isa<LoadInst>(I) || isa<StoreInst>(I))
312      continue;
313
314    if (!I->getType()->isVoidTy()) {
315      // The only users of this bitcast/GEP instruction are lifetime intrinsics.
316      // Follow the use/def chain to erase them now instead of leaving it for
317      // dead code elimination later.
318      for (auto UUI = I->user_begin(), UUE = I->user_end(); UUI != UUE;) {
319        Instruction *Inst = cast<Instruction>(*UUI);
320        ++UUI;
321        Inst->eraseFromParent();
322      }
323    }
324    I->eraseFromParent();
325  }
326}
327
328/// \brief Rewrite as many loads as possible given a single store.
329///
330/// When there is only a single store, we can use the domtree to trivially
331/// replace all of the dominated loads with the stored value. Do so, and return
332/// true if this has successfully promoted the alloca entirely. If this returns
333/// false there were some loads which were not dominated by the single store
334/// and thus must be phi-ed with undef. We fall back to the standard alloca
335/// promotion algorithm in that case.
336static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
337                                     LargeBlockInfo &LBI,
338                                     DominatorTree &DT,
339                                     AliasSetTracker *AST) {
340  StoreInst *OnlyStore = Info.OnlyStore;
341  bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
342  BasicBlock *StoreBB = OnlyStore->getParent();
343  int StoreIndex = -1;
344
345  // Clear out UsingBlocks.  We will reconstruct it here if needed.
346  Info.UsingBlocks.clear();
347
348  for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
349    Instruction *UserInst = cast<Instruction>(*UI++);
350    if (!isa<LoadInst>(UserInst)) {
351      assert(UserInst == OnlyStore && "Should only have load/stores");
352      continue;
353    }
354    LoadInst *LI = cast<LoadInst>(UserInst);
355
356    // Okay, if we have a load from the alloca, we want to replace it with the
357    // only value stored to the alloca.  We can do this if the value is
358    // dominated by the store.  If not, we use the rest of the mem2reg machinery
359    // to insert the phi nodes as needed.
360    if (!StoringGlobalVal) { // Non-instructions are always dominated.
361      if (LI->getParent() == StoreBB) {
362        // If we have a use that is in the same block as the store, compare the
363        // indices of the two instructions to see which one came first.  If the
364        // load came before the store, we can't handle it.
365        if (StoreIndex == -1)
366          StoreIndex = LBI.getInstructionIndex(OnlyStore);
367
368        if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) {
369          // Can't handle this load, bail out.
370          Info.UsingBlocks.push_back(StoreBB);
371          continue;
372        }
373
374      } else if (LI->getParent() != StoreBB &&
375                 !DT.dominates(StoreBB, LI->getParent())) {
376        // If the load and store are in different blocks, use BB dominance to
377        // check their relationships.  If the store doesn't dom the use, bail
378        // out.
379        Info.UsingBlocks.push_back(LI->getParent());
380        continue;
381      }
382    }
383
384    // Otherwise, we *can* safely rewrite this load.
385    Value *ReplVal = OnlyStore->getOperand(0);
386    // If the replacement value is the load, this must occur in unreachable
387    // code.
388    if (ReplVal == LI)
389      ReplVal = UndefValue::get(LI->getType());
390    LI->replaceAllUsesWith(ReplVal);
391    if (AST && LI->getType()->isPointerTy())
392      AST->deleteValue(LI);
393    LI->eraseFromParent();
394    LBI.deleteValue(LI);
395  }
396
397  // Finally, after the scan, check to see if the store is all that is left.
398  if (!Info.UsingBlocks.empty())
399    return false; // If not, we'll have to fall back for the remainder.
400
401  // Record debuginfo for the store and remove the declaration's
402  // debuginfo.
403  if (DbgDeclareInst *DDI = Info.DbgDeclare) {
404    DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false);
405    ConvertDebugDeclareToDebugValue(DDI, Info.OnlyStore, DIB);
406    DDI->eraseFromParent();
407    LBI.deleteValue(DDI);
408  }
409  // Remove the (now dead) store and alloca.
410  Info.OnlyStore->eraseFromParent();
411  LBI.deleteValue(Info.OnlyStore);
412
413  if (AST)
414    AST->deleteValue(AI);
415  AI->eraseFromParent();
416  LBI.deleteValue(AI);
417  return true;
418}
419
420/// Many allocas are only used within a single basic block.  If this is the
421/// case, avoid traversing the CFG and inserting a lot of potentially useless
422/// PHI nodes by just performing a single linear pass over the basic block
423/// using the Alloca.
424///
425/// If we cannot promote this alloca (because it is read before it is written),
426/// return false.  This is necessary in cases where, due to control flow, the
427/// alloca is undefined only on some control flow paths.  e.g. code like
428/// this is correct in LLVM IR:
429///  // A is an alloca with no stores so far
430///  for (...) {
431///    int t = *A;
432///    if (!first_iteration)
433///      use(t);
434///    *A = 42;
435///  }
436static bool promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
437                                     LargeBlockInfo &LBI,
438                                     AliasSetTracker *AST) {
439  // The trickiest case to handle is when we have large blocks. Because of this,
440  // this code is optimized assuming that large blocks happen.  This does not
441  // significantly pessimize the small block case.  This uses LargeBlockInfo to
442  // make it efficient to get the index of various operations in the block.
443
444  // Walk the use-def list of the alloca, getting the locations of all stores.
445  typedef SmallVector<std::pair<unsigned, StoreInst *>, 64> StoresByIndexTy;
446  StoresByIndexTy StoresByIndex;
447
448  for (User *U : AI->users())
449    if (StoreInst *SI = dyn_cast<StoreInst>(U))
450      StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI));
451
452  // Sort the stores by their index, making it efficient to do a lookup with a
453  // binary search.
454  std::sort(StoresByIndex.begin(), StoresByIndex.end(), less_first());
455
456  // Walk all of the loads from this alloca, replacing them with the nearest
457  // store above them, if any.
458  for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
459    LoadInst *LI = dyn_cast<LoadInst>(*UI++);
460    if (!LI)
461      continue;
462
463    unsigned LoadIdx = LBI.getInstructionIndex(LI);
464
465    // Find the nearest store that has a lower index than this load.
466    StoresByIndexTy::iterator I =
467        std::lower_bound(StoresByIndex.begin(), StoresByIndex.end(),
468                         std::make_pair(LoadIdx,
469                                        static_cast<StoreInst *>(nullptr)),
470                         less_first());
471    if (I == StoresByIndex.begin()) {
472      if (StoresByIndex.empty())
473        // If there are no stores, the load takes the undef value.
474        LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
475      else
476        // There is no store before this load, bail out (load may be affected
477        // by the following stores - see main comment).
478        return false;
479    }
480    else
481      // Otherwise, there was a store before this load, the load takes its value.
482      LI->replaceAllUsesWith(std::prev(I)->second->getOperand(0));
483
484    if (AST && LI->getType()->isPointerTy())
485      AST->deleteValue(LI);
486    LI->eraseFromParent();
487    LBI.deleteValue(LI);
488  }
489
490  // Remove the (now dead) stores and alloca.
491  while (!AI->use_empty()) {
492    StoreInst *SI = cast<StoreInst>(AI->user_back());
493    // Record debuginfo for the store before removing it.
494    if (DbgDeclareInst *DDI = Info.DbgDeclare) {
495      DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false);
496      ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
497    }
498    SI->eraseFromParent();
499    LBI.deleteValue(SI);
500  }
501
502  if (AST)
503    AST->deleteValue(AI);
504  AI->eraseFromParent();
505  LBI.deleteValue(AI);
506
507  // The alloca's debuginfo can be removed as well.
508  if (DbgDeclareInst *DDI = Info.DbgDeclare) {
509    DDI->eraseFromParent();
510    LBI.deleteValue(DDI);
511  }
512
513  ++NumLocalPromoted;
514  return true;
515}
516
517void PromoteMem2Reg::run() {
518  Function &F = *DT.getRoot()->getParent();
519
520  if (AST)
521    PointerAllocaValues.resize(Allocas.size());
522  AllocaDbgDeclares.resize(Allocas.size());
523
524  AllocaInfo Info;
525  LargeBlockInfo LBI;
526  ForwardIDFCalculator IDF(DT);
527
528  for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
529    AllocaInst *AI = Allocas[AllocaNum];
530
531    assert(isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!");
532    assert(AI->getParent()->getParent() == &F &&
533           "All allocas should be in the same function, which is same as DF!");
534
535    removeLifetimeIntrinsicUsers(AI);
536
537    if (AI->use_empty()) {
538      // If there are no uses of the alloca, just delete it now.
539      if (AST)
540        AST->deleteValue(AI);
541      AI->eraseFromParent();
542
543      // Remove the alloca from the Allocas list, since it has been processed
544      RemoveFromAllocasList(AllocaNum);
545      ++NumDeadAlloca;
546      continue;
547    }
548
549    // Calculate the set of read and write-locations for each alloca.  This is
550    // analogous to finding the 'uses' and 'definitions' of each variable.
551    Info.AnalyzeAlloca(AI);
552
553    // If there is only a single store to this value, replace any loads of
554    // it that are directly dominated by the definition with the value stored.
555    if (Info.DefiningBlocks.size() == 1) {
556      if (rewriteSingleStoreAlloca(AI, Info, LBI, DT, AST)) {
557        // The alloca has been processed, move on.
558        RemoveFromAllocasList(AllocaNum);
559        ++NumSingleStore;
560        continue;
561      }
562    }
563
564    // If the alloca is only read and written in one basic block, just perform a
565    // linear sweep over the block to eliminate it.
566    if (Info.OnlyUsedInOneBlock &&
567        promoteSingleBlockAlloca(AI, Info, LBI, AST)) {
568      // The alloca has been processed, move on.
569      RemoveFromAllocasList(AllocaNum);
570      continue;
571    }
572
573    // If we haven't computed a numbering for the BB's in the function, do so
574    // now.
575    if (BBNumbers.empty()) {
576      unsigned ID = 0;
577      for (auto &BB : F)
578        BBNumbers[&BB] = ID++;
579    }
580
581    // If we have an AST to keep updated, remember some pointer value that is
582    // stored into the alloca.
583    if (AST)
584      PointerAllocaValues[AllocaNum] = Info.AllocaPointerVal;
585
586    // Remember the dbg.declare intrinsic describing this alloca, if any.
587    if (Info.DbgDeclare)
588      AllocaDbgDeclares[AllocaNum] = Info.DbgDeclare;
589
590    // Keep the reverse mapping of the 'Allocas' array for the rename pass.
591    AllocaLookup[Allocas[AllocaNum]] = AllocaNum;
592
593    // At this point, we're committed to promoting the alloca using IDF's, and
594    // the standard SSA construction algorithm.  Determine which blocks need PHI
595    // nodes and see if we can optimize out some work by avoiding insertion of
596    // dead phi nodes.
597
598
599    // Unique the set of defining blocks for efficient lookup.
600    SmallPtrSet<BasicBlock *, 32> DefBlocks;
601    DefBlocks.insert(Info.DefiningBlocks.begin(), Info.DefiningBlocks.end());
602
603    // Determine which blocks the value is live in.  These are blocks which lead
604    // to uses.
605    SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
606    ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks);
607
608    // At this point, we're committed to promoting the alloca using IDF's, and
609    // the standard SSA construction algorithm.  Determine which blocks need phi
610    // nodes and see if we can optimize out some work by avoiding insertion of
611    // dead phi nodes.
612    IDF.setLiveInBlocks(LiveInBlocks);
613    IDF.setDefiningBlocks(DefBlocks);
614    SmallVector<BasicBlock *, 32> PHIBlocks;
615    IDF.calculate(PHIBlocks);
616    if (PHIBlocks.size() > 1)
617      std::sort(PHIBlocks.begin(), PHIBlocks.end(),
618                [this](BasicBlock *A, BasicBlock *B) {
619                  return BBNumbers.lookup(A) < BBNumbers.lookup(B);
620                });
621
622    unsigned CurrentVersion = 0;
623    for (unsigned i = 0, e = PHIBlocks.size(); i != e; ++i)
624      QueuePhiNode(PHIBlocks[i], AllocaNum, CurrentVersion);
625  }
626
627  if (Allocas.empty())
628    return; // All of the allocas must have been trivial!
629
630  LBI.clear();
631
632  // Set the incoming values for the basic block to be null values for all of
633  // the alloca's.  We do this in case there is a load of a value that has not
634  // been stored yet.  In this case, it will get this null value.
635  //
636  RenamePassData::ValVector Values(Allocas.size());
637  for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
638    Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());
639
640  // Walks all basic blocks in the function performing the SSA rename algorithm
641  // and inserting the phi nodes we marked as necessary
642  //
643  std::vector<RenamePassData> RenamePassWorkList;
644  RenamePassWorkList.emplace_back(&F.front(), nullptr, std::move(Values));
645  do {
646    RenamePassData RPD;
647    RPD.swap(RenamePassWorkList.back());
648    RenamePassWorkList.pop_back();
649    // RenamePass may add new worklist entries.
650    RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList);
651  } while (!RenamePassWorkList.empty());
652
653  // The renamer uses the Visited set to avoid infinite loops.  Clear it now.
654  Visited.clear();
655
656  // Remove the allocas themselves from the function.
657  for (unsigned i = 0, e = Allocas.size(); i != e; ++i) {
658    Instruction *A = Allocas[i];
659
660    // If there are any uses of the alloca instructions left, they must be in
661    // unreachable basic blocks that were not processed by walking the dominator
662    // tree. Just delete the users now.
663    if (!A->use_empty())
664      A->replaceAllUsesWith(UndefValue::get(A->getType()));
665    if (AST)
666      AST->deleteValue(A);
667    A->eraseFromParent();
668  }
669
670  const DataLayout &DL = F.getParent()->getDataLayout();
671
672  // Remove alloca's dbg.declare instrinsics from the function.
673  for (unsigned i = 0, e = AllocaDbgDeclares.size(); i != e; ++i)
674    if (DbgDeclareInst *DDI = AllocaDbgDeclares[i])
675      DDI->eraseFromParent();
676
677  // Loop over all of the PHI nodes and see if there are any that we can get
678  // rid of because they merge all of the same incoming values.  This can
679  // happen due to undef values coming into the PHI nodes.  This process is
680  // iterative, because eliminating one PHI node can cause others to be removed.
681  bool EliminatedAPHI = true;
682  while (EliminatedAPHI) {
683    EliminatedAPHI = false;
684
685    // Iterating over NewPhiNodes is deterministic, so it is safe to try to
686    // simplify and RAUW them as we go.  If it was not, we could add uses to
687    // the values we replace with in a non-deterministic order, thus creating
688    // non-deterministic def->use chains.
689    for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
690             I = NewPhiNodes.begin(),
691             E = NewPhiNodes.end();
692         I != E;) {
693      PHINode *PN = I->second;
694
695      // If this PHI node merges one value and/or undefs, get the value.
696      if (Value *V = SimplifyInstruction(PN, DL, nullptr, &DT, AC)) {
697        if (AST && PN->getType()->isPointerTy())
698          AST->deleteValue(PN);
699        PN->replaceAllUsesWith(V);
700        PN->eraseFromParent();
701        NewPhiNodes.erase(I++);
702        EliminatedAPHI = true;
703        continue;
704      }
705      ++I;
706    }
707  }
708
709  // At this point, the renamer has added entries to PHI nodes for all reachable
710  // code.  Unfortunately, there may be unreachable blocks which the renamer
711  // hasn't traversed.  If this is the case, the PHI nodes may not
712  // have incoming values for all predecessors.  Loop over all PHI nodes we have
713  // created, inserting undef values if they are missing any incoming values.
714  //
715  for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
716           I = NewPhiNodes.begin(),
717           E = NewPhiNodes.end();
718       I != E; ++I) {
719    // We want to do this once per basic block.  As such, only process a block
720    // when we find the PHI that is the first entry in the block.
721    PHINode *SomePHI = I->second;
722    BasicBlock *BB = SomePHI->getParent();
723    if (&BB->front() != SomePHI)
724      continue;
725
726    // Only do work here if there the PHI nodes are missing incoming values.  We
727    // know that all PHI nodes that were inserted in a block will have the same
728    // number of incoming values, so we can just check any of them.
729    if (SomePHI->getNumIncomingValues() == getNumPreds(BB))
730      continue;
731
732    // Get the preds for BB.
733    SmallVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
734
735    // Ok, now we know that all of the PHI nodes are missing entries for some
736    // basic blocks.  Start by sorting the incoming predecessors for efficient
737    // access.
738    std::sort(Preds.begin(), Preds.end());
739
740    // Now we loop through all BB's which have entries in SomePHI and remove
741    // them from the Preds list.
742    for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
743      // Do a log(n) search of the Preds list for the entry we want.
744      SmallVectorImpl<BasicBlock *>::iterator EntIt = std::lower_bound(
745          Preds.begin(), Preds.end(), SomePHI->getIncomingBlock(i));
746      assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) &&
747             "PHI node has entry for a block which is not a predecessor!");
748
749      // Remove the entry
750      Preds.erase(EntIt);
751    }
752
753    // At this point, the blocks left in the preds list must have dummy
754    // entries inserted into every PHI nodes for the block.  Update all the phi
755    // nodes in this block that we are inserting (there could be phis before
756    // mem2reg runs).
757    unsigned NumBadPreds = SomePHI->getNumIncomingValues();
758    BasicBlock::iterator BBI = BB->begin();
759    while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
760           SomePHI->getNumIncomingValues() == NumBadPreds) {
761      Value *UndefVal = UndefValue::get(SomePHI->getType());
762      for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred)
763        SomePHI->addIncoming(UndefVal, Preds[pred]);
764    }
765  }
766
767  NewPhiNodes.clear();
768}
769
770/// \brief Determine which blocks the value is live in.
771///
772/// These are blocks which lead to uses.  Knowing this allows us to avoid
773/// inserting PHI nodes into blocks which don't lead to uses (thus, the
774/// inserted phi nodes would be dead).
775void PromoteMem2Reg::ComputeLiveInBlocks(
776    AllocaInst *AI, AllocaInfo &Info,
777    const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
778    SmallPtrSetImpl<BasicBlock *> &LiveInBlocks) {
779
780  // To determine liveness, we must iterate through the predecessors of blocks
781  // where the def is live.  Blocks are added to the worklist if we need to
782  // check their predecessors.  Start with all the using blocks.
783  SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(),
784                                                    Info.UsingBlocks.end());
785
786  // If any of the using blocks is also a definition block, check to see if the
787  // definition occurs before or after the use.  If it happens before the use,
788  // the value isn't really live-in.
789  for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) {
790    BasicBlock *BB = LiveInBlockWorklist[i];
791    if (!DefBlocks.count(BB))
792      continue;
793
794    // Okay, this is a block that both uses and defines the value.  If the first
795    // reference to the alloca is a def (store), then we know it isn't live-in.
796    for (BasicBlock::iterator I = BB->begin();; ++I) {
797      if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
798        if (SI->getOperand(1) != AI)
799          continue;
800
801        // We found a store to the alloca before a load.  The alloca is not
802        // actually live-in here.
803        LiveInBlockWorklist[i] = LiveInBlockWorklist.back();
804        LiveInBlockWorklist.pop_back();
805        --i;
806        --e;
807        break;
808      }
809
810      if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
811        if (LI->getOperand(0) != AI)
812          continue;
813
814        // Okay, we found a load before a store to the alloca.  It is actually
815        // live into this block.
816        break;
817      }
818    }
819  }
820
821  // Now that we have a set of blocks where the phi is live-in, recursively add
822  // their predecessors until we find the full region the value is live.
823  while (!LiveInBlockWorklist.empty()) {
824    BasicBlock *BB = LiveInBlockWorklist.pop_back_val();
825
826    // The block really is live in here, insert it into the set.  If already in
827    // the set, then it has already been processed.
828    if (!LiveInBlocks.insert(BB).second)
829      continue;
830
831    // Since the value is live into BB, it is either defined in a predecessor or
832    // live into it to.  Add the preds to the worklist unless they are a
833    // defining block.
834    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
835      BasicBlock *P = *PI;
836
837      // The value is not live into a predecessor if it defines the value.
838      if (DefBlocks.count(P))
839        continue;
840
841      // Otherwise it is, add to the worklist.
842      LiveInBlockWorklist.push_back(P);
843    }
844  }
845}
846
847/// \brief Queue a phi-node to be added to a basic-block for a specific Alloca.
848///
849/// Returns true if there wasn't already a phi-node for that variable
850bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
851                                  unsigned &Version) {
852  // Look up the basic-block in question.
853  PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)];
854
855  // If the BB already has a phi node added for the i'th alloca then we're done!
856  if (PN)
857    return false;
858
859  // Create a PhiNode using the dereferenced type... and add the phi-node to the
860  // BasicBlock.
861  PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB),
862                       Allocas[AllocaNo]->getName() + "." + Twine(Version++),
863                       &BB->front());
864  ++NumPHIInsert;
865  PhiToAllocaMap[PN] = AllocaNo;
866
867  if (AST && PN->getType()->isPointerTy())
868    AST->copyValue(PointerAllocaValues[AllocaNo], PN);
869
870  return true;
871}
872
873/// \brief Recursively traverse the CFG of the function, renaming loads and
874/// stores to the allocas which we are promoting.
875///
876/// IncomingVals indicates what value each Alloca contains on exit from the
877/// predecessor block Pred.
878void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
879                                RenamePassData::ValVector &IncomingVals,
880                                std::vector<RenamePassData> &Worklist) {
881NextIteration:
882  // If we are inserting any phi nodes into this BB, they will already be in the
883  // block.
884  if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
885    // If we have PHI nodes to update, compute the number of edges from Pred to
886    // BB.
887    if (PhiToAllocaMap.count(APN)) {
888      // We want to be able to distinguish between PHI nodes being inserted by
889      // this invocation of mem2reg from those phi nodes that already existed in
890      // the IR before mem2reg was run.  We determine that APN is being inserted
891      // because it is missing incoming edges.  All other PHI nodes being
892      // inserted by this pass of mem2reg will have the same number of incoming
893      // operands so far.  Remember this count.
894      unsigned NewPHINumOperands = APN->getNumOperands();
895
896      unsigned NumEdges = std::count(succ_begin(Pred), succ_end(Pred), BB);
897      assert(NumEdges && "Must be at least one edge from Pred to BB!");
898
899      // Add entries for all the phis.
900      BasicBlock::iterator PNI = BB->begin();
901      do {
902        unsigned AllocaNo = PhiToAllocaMap[APN];
903
904        // Add N incoming values to the PHI node.
905        for (unsigned i = 0; i != NumEdges; ++i)
906          APN->addIncoming(IncomingVals[AllocaNo], Pred);
907
908        // The currently active variable for this block is now the PHI.
909        IncomingVals[AllocaNo] = APN;
910
911        // Get the next phi node.
912        ++PNI;
913        APN = dyn_cast<PHINode>(PNI);
914        if (!APN)
915          break;
916
917        // Verify that it is missing entries.  If not, it is not being inserted
918        // by this mem2reg invocation so we want to ignore it.
919      } while (APN->getNumOperands() == NewPHINumOperands);
920    }
921  }
922
923  // Don't revisit blocks.
924  if (!Visited.insert(BB).second)
925    return;
926
927  for (BasicBlock::iterator II = BB->begin(); !isa<TerminatorInst>(II);) {
928    Instruction *I = &*II++; // get the instruction, increment iterator
929
930    if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
931      AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand());
932      if (!Src)
933        continue;
934
935      DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src);
936      if (AI == AllocaLookup.end())
937        continue;
938
939      Value *V = IncomingVals[AI->second];
940
941      // Anything using the load now uses the current value.
942      LI->replaceAllUsesWith(V);
943      if (AST && LI->getType()->isPointerTy())
944        AST->deleteValue(LI);
945      BB->getInstList().erase(LI);
946    } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
947      // Delete this instruction and mark the name as the current holder of the
948      // value
949      AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
950      if (!Dest)
951        continue;
952
953      DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
954      if (ai == AllocaLookup.end())
955        continue;
956
957      // what value were we writing?
958      IncomingVals[ai->second] = SI->getOperand(0);
959      // Record debuginfo for the store before removing it.
960      if (DbgDeclareInst *DDI = AllocaDbgDeclares[ai->second])
961        ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
962      BB->getInstList().erase(SI);
963    }
964  }
965
966  // 'Recurse' to our successors.
967  succ_iterator I = succ_begin(BB), E = succ_end(BB);
968  if (I == E)
969    return;
970
971  // Keep track of the successors so we don't visit the same successor twice
972  SmallPtrSet<BasicBlock *, 8> VisitedSuccs;
973
974  // Handle the first successor without using the worklist.
975  VisitedSuccs.insert(*I);
976  Pred = BB;
977  BB = *I;
978  ++I;
979
980  for (; I != E; ++I)
981    if (VisitedSuccs.insert(*I).second)
982      Worklist.emplace_back(*I, Pred, IncomingVals);
983
984  goto NextIteration;
985}
986
987void llvm::PromoteMemToReg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
988                           AliasSetTracker *AST, AssumptionCache *AC) {
989  // If there is nothing to do, bail out...
990  if (Allocas.empty())
991    return;
992
993  PromoteMem2Reg(Allocas, DT, AST, AC).run();
994}
995