LLVM  3.7.0
PromoteMemoryToRegister.cpp
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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 
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"
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"
41 #include <algorithm>
42 using namespace llvm;
43 
44 #define DEBUG_TYPE "mem2reg"
45 
46 STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
47 STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store");
48 STATISTIC(NumDeadAlloca, "Number of dead alloca's removed");
49 STATISTIC(NumPHIInsert, "Number of PHI nodes inserted");
50 
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 
94 namespace {
95 
96 struct AllocaInfo {
97  SmallVector<BasicBlock *, 32> DefiningBlocks;
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()
154 class RenamePassData {
155 public:
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.
177 class 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.
184 
185 public:
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.
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 (BasicBlock::const_iterator BBI = BB->begin(), E = BB->end(); BBI != E;
209  ++BBI)
210  if (isInterestingInstruction(BBI))
211  InstNumbers[BBI] = InstNo++;
212  It = InstNumbers.find(I);
213 
214  assert(It != InstNumbers.end() && "Didn't insert instruction?");
215  return It->second;
216  }
217 
218  void deleteValue(const Instruction *I) { InstNumbers.erase(I); }
219 
220  void clear() { InstNumbers.clear(); }
221 };
222 
223 struct PromoteMem2Reg {
224  /// The alloca instructions being promoted.
225  std::vector<AllocaInst *> Allocas;
226  DominatorTree &DT;
227  DIBuilder DIB;
228 
229  /// An AliasSetTracker object to update. If null, don't update it.
230  AliasSetTracker *AST;
231 
232  /// A cache of @llvm.assume intrinsics used by SimplifyInstruction.
233  AssumptionCache *AC;
234 
235  /// Reverse mapping of Allocas.
237 
238  /// \brief The PhiNodes we're adding.
239  ///
240  /// That map is used to simplify some Phi nodes as we iterate over it, so
241  /// it should have deterministic iterators. We could use a MapVector, but
242  /// since we already maintain a map from BasicBlock* to a stable numbering
243  /// (BBNumbers), the DenseMap is more efficient (also supports removal).
245 
246  /// For each PHI node, keep track of which entry in Allocas it corresponds
247  /// to.
248  DenseMap<PHINode *, unsigned> PhiToAllocaMap;
249 
250  /// If we are updating an AliasSetTracker, then for each alloca that is of
251  /// pointer type, we keep track of what to copyValue to the inserted PHI
252  /// nodes here.
253  std::vector<Value *> PointerAllocaValues;
254 
255  /// For each alloca, we keep track of the dbg.declare intrinsic that
256  /// describes it, if any, so that we can convert it to a dbg.value
257  /// intrinsic if the alloca gets promoted.
258  SmallVector<DbgDeclareInst *, 8> AllocaDbgDeclares;
259 
260  /// The set of basic blocks the renamer has already visited.
261  ///
263 
264  /// Contains a stable numbering of basic blocks to avoid non-determinstic
265  /// behavior.
267 
268  /// Lazily compute the number of predecessors a block has.
270 
271 public:
272  PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
274  : Allocas(Allocas.begin(), Allocas.end()), DT(DT),
275  DIB(*DT.getRoot()->getParent()->getParent(), /*AllowUnresolved*/ false),
276  AST(AST), AC(AC) {}
277 
278  void run();
279 
280 private:
281  void RemoveFromAllocasList(unsigned &AllocaIdx) {
282  Allocas[AllocaIdx] = Allocas.back();
283  Allocas.pop_back();
284  --AllocaIdx;
285  }
286 
287  unsigned getNumPreds(const BasicBlock *BB) {
288  unsigned &NP = BBNumPreds[BB];
289  if (NP == 0)
290  NP = std::distance(pred_begin(BB), pred_end(BB)) + 1;
291  return NP - 1;
292  }
293 
294  void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
295  const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
296  SmallPtrSetImpl<BasicBlock *> &LiveInBlocks);
297  void RenamePass(BasicBlock *BB, BasicBlock *Pred,
298  RenamePassData::ValVector &IncVals,
299  std::vector<RenamePassData> &Worklist);
300  bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version);
301 };
302 
303 } // end of anonymous namespace
304 
306  // Knowing that this alloca is promotable, we know that it's safe to kill all
307  // instructions except for load and store.
308 
309  for (auto UI = AI->user_begin(), UE = AI->user_end(); UI != UE;) {
310  Instruction *I = cast<Instruction>(*UI);
311  ++UI;
312  if (isa<LoadInst>(I) || isa<StoreInst>(I))
313  continue;
314 
315  if (!I->getType()->isVoidTy()) {
316  // The only users of this bitcast/GEP instruction are lifetime intrinsics.
317  // Follow the use/def chain to erase them now instead of leaving it for
318  // dead code elimination later.
319  for (auto UUI = I->user_begin(), UUE = I->user_end(); UUI != UUE;) {
320  Instruction *Inst = cast<Instruction>(*UUI);
321  ++UUI;
322  Inst->eraseFromParent();
323  }
324  }
325  I->eraseFromParent();
326  }
327 }
328 
329 /// \brief Rewrite as many loads as possible given a single store.
330 ///
331 /// When there is only a single store, we can use the domtree to trivially
332 /// replace all of the dominated loads with the stored value. Do so, and return
333 /// true if this has successfully promoted the alloca entirely. If this returns
334 /// false there were some loads which were not dominated by the single store
335 /// and thus must be phi-ed with undef. We fall back to the standard alloca
336 /// promotion algorithm in that case.
337 static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
338  LargeBlockInfo &LBI,
339  DominatorTree &DT,
340  AliasSetTracker *AST) {
341  StoreInst *OnlyStore = Info.OnlyStore;
342  bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
343  BasicBlock *StoreBB = OnlyStore->getParent();
344  int StoreIndex = -1;
345 
346  // Clear out UsingBlocks. We will reconstruct it here if needed.
347  Info.UsingBlocks.clear();
348 
349  for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
350  Instruction *UserInst = cast<Instruction>(*UI++);
351  if (!isa<LoadInst>(UserInst)) {
352  assert(UserInst == OnlyStore && "Should only have load/stores");
353  continue;
354  }
355  LoadInst *LI = cast<LoadInst>(UserInst);
356 
357  // Okay, if we have a load from the alloca, we want to replace it with the
358  // only value stored to the alloca. We can do this if the value is
359  // dominated by the store. If not, we use the rest of the mem2reg machinery
360  // to insert the phi nodes as needed.
361  if (!StoringGlobalVal) { // Non-instructions are always dominated.
362  if (LI->getParent() == StoreBB) {
363  // If we have a use that is in the same block as the store, compare the
364  // indices of the two instructions to see which one came first. If the
365  // load came before the store, we can't handle it.
366  if (StoreIndex == -1)
367  StoreIndex = LBI.getInstructionIndex(OnlyStore);
368 
369  if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) {
370  // Can't handle this load, bail out.
371  Info.UsingBlocks.push_back(StoreBB);
372  continue;
373  }
374 
375  } else if (LI->getParent() != StoreBB &&
376  !DT.dominates(StoreBB, LI->getParent())) {
377  // If the load and store are in different blocks, use BB dominance to
378  // check their relationships. If the store doesn't dom the use, bail
379  // out.
380  Info.UsingBlocks.push_back(LI->getParent());
381  continue;
382  }
383  }
384 
385  // Otherwise, we *can* safely rewrite this load.
386  Value *ReplVal = OnlyStore->getOperand(0);
387  // If the replacement value is the load, this must occur in unreachable
388  // code.
389  if (ReplVal == LI)
390  ReplVal = UndefValue::get(LI->getType());
391  LI->replaceAllUsesWith(ReplVal);
392  if (AST && LI->getType()->isPointerTy())
393  AST->deleteValue(LI);
394  LI->eraseFromParent();
395  LBI.deleteValue(LI);
396  }
397 
398  // Finally, after the scan, check to see if the store is all that is left.
399  if (!Info.UsingBlocks.empty())
400  return false; // If not, we'll have to fall back for the remainder.
401 
402  // Record debuginfo for the store and remove the declaration's
403  // debuginfo.
404  if (DbgDeclareInst *DDI = Info.DbgDeclare) {
405  DIBuilder DIB(*AI->getParent()->getParent()->getParent(),
406  /*AllowUnresolved*/ false);
407  ConvertDebugDeclareToDebugValue(DDI, Info.OnlyStore, DIB);
408  DDI->eraseFromParent();
409  LBI.deleteValue(DDI);
410  }
411  // Remove the (now dead) store and alloca.
412  Info.OnlyStore->eraseFromParent();
413  LBI.deleteValue(Info.OnlyStore);
414 
415  if (AST)
416  AST->deleteValue(AI);
417  AI->eraseFromParent();
418  LBI.deleteValue(AI);
419  return true;
420 }
421 
422 /// Many allocas are only used within a single basic block. If this is the
423 /// case, avoid traversing the CFG and inserting a lot of potentially useless
424 /// PHI nodes by just performing a single linear pass over the basic block
425 /// using the Alloca.
426 ///
427 /// If we cannot promote this alloca (because it is read before it is written),
428 /// return true. This is necessary in cases where, due to control flow, the
429 /// alloca is potentially undefined on some control flow paths. e.g. code like
430 /// this is potentially correct:
431 ///
432 /// for (...) { if (c) { A = undef; undef = B; } }
433 ///
434 /// ... so long as A is not used before undef is set.
435 static void promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
436  LargeBlockInfo &LBI,
437  AliasSetTracker *AST) {
438  // The trickiest case to handle is when we have large blocks. Because of this,
439  // this code is optimized assuming that large blocks happen. This does not
440  // significantly pessimize the small block case. This uses LargeBlockInfo to
441  // make it efficient to get the index of various operations in the block.
442 
443  // Walk the use-def list of the alloca, getting the locations of all stores.
444  typedef SmallVector<std::pair<unsigned, StoreInst *>, 64> StoresByIndexTy;
445  StoresByIndexTy StoresByIndex;
446 
447  for (User *U : AI->users())
448  if (StoreInst *SI = dyn_cast<StoreInst>(U))
449  StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI));
450 
451  // Sort the stores by their index, making it efficient to do a lookup with a
452  // binary search.
453  std::sort(StoresByIndex.begin(), StoresByIndex.end(), less_first());
454 
455  // Walk all of the loads from this alloca, replacing them with the nearest
456  // store above them, if any.
457  for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
458  LoadInst *LI = dyn_cast<LoadInst>(*UI++);
459  if (!LI)
460  continue;
461 
462  unsigned LoadIdx = LBI.getInstructionIndex(LI);
463 
464  // Find the nearest store that has a lower index than this load.
465  StoresByIndexTy::iterator I =
466  std::lower_bound(StoresByIndex.begin(), StoresByIndex.end(),
467  std::make_pair(LoadIdx,
468  static_cast<StoreInst *>(nullptr)),
469  less_first());
470 
471  if (I == StoresByIndex.begin())
472  // If there is no store before this load, the load takes the undef value.
474  else
475  // Otherwise, there was a store before this load, the load takes its value.
476  LI->replaceAllUsesWith(std::prev(I)->second->getOperand(0));
477 
478  if (AST && LI->getType()->isPointerTy())
479  AST->deleteValue(LI);
480  LI->eraseFromParent();
481  LBI.deleteValue(LI);
482  }
483 
484  // Remove the (now dead) stores and alloca.
485  while (!AI->use_empty()) {
486  StoreInst *SI = cast<StoreInst>(AI->user_back());
487  // Record debuginfo for the store before removing it.
488  if (DbgDeclareInst *DDI = Info.DbgDeclare) {
489  DIBuilder DIB(*AI->getParent()->getParent()->getParent(),
490  /*AllowUnresolved*/ false);
491  ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
492  }
493  SI->eraseFromParent();
494  LBI.deleteValue(SI);
495  }
496 
497  if (AST)
498  AST->deleteValue(AI);
499  AI->eraseFromParent();
500  LBI.deleteValue(AI);
501 
502  // The alloca's debuginfo can be removed as well.
503  if (DbgDeclareInst *DDI = Info.DbgDeclare) {
504  DDI->eraseFromParent();
505  LBI.deleteValue(DDI);
506  }
507 
508  ++NumLocalPromoted;
509 }
510 
511 void PromoteMem2Reg::run() {
512  Function &F = *DT.getRoot()->getParent();
513 
514  if (AST)
515  PointerAllocaValues.resize(Allocas.size());
516  AllocaDbgDeclares.resize(Allocas.size());
517 
518  AllocaInfo Info;
519  LargeBlockInfo LBI;
520  IDFCalculator IDF(DT);
521 
522  for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
523  AllocaInst *AI = Allocas[AllocaNum];
524 
525  assert(isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!");
526  assert(AI->getParent()->getParent() == &F &&
527  "All allocas should be in the same function, which is same as DF!");
528 
530 
531  if (AI->use_empty()) {
532  // If there are no uses of the alloca, just delete it now.
533  if (AST)
534  AST->deleteValue(AI);
535  AI->eraseFromParent();
536 
537  // Remove the alloca from the Allocas list, since it has been processed
538  RemoveFromAllocasList(AllocaNum);
539  ++NumDeadAlloca;
540  continue;
541  }
542 
543  // Calculate the set of read and write-locations for each alloca. This is
544  // analogous to finding the 'uses' and 'definitions' of each variable.
545  Info.AnalyzeAlloca(AI);
546 
547  // If there is only a single store to this value, replace any loads of
548  // it that are directly dominated by the definition with the value stored.
549  if (Info.DefiningBlocks.size() == 1) {
550  if (rewriteSingleStoreAlloca(AI, Info, LBI, DT, AST)) {
551  // The alloca has been processed, move on.
552  RemoveFromAllocasList(AllocaNum);
553  ++NumSingleStore;
554  continue;
555  }
556  }
557 
558  // If the alloca is only read and written in one basic block, just perform a
559  // linear sweep over the block to eliminate it.
560  if (Info.OnlyUsedInOneBlock) {
561  promoteSingleBlockAlloca(AI, Info, LBI, AST);
562 
563  // The alloca has been processed, move on.
564  RemoveFromAllocasList(AllocaNum);
565  continue;
566  }
567 
568  // If we haven't computed a numbering for the BB's in the function, do so
569  // now.
570  if (BBNumbers.empty()) {
571  unsigned ID = 0;
572  for (auto &BB : F)
573  BBNumbers[&BB] = ID++;
574  }
575 
576  // If we have an AST to keep updated, remember some pointer value that is
577  // stored into the alloca.
578  if (AST)
579  PointerAllocaValues[AllocaNum] = Info.AllocaPointerVal;
580 
581  // Remember the dbg.declare intrinsic describing this alloca, if any.
582  if (Info.DbgDeclare)
583  AllocaDbgDeclares[AllocaNum] = Info.DbgDeclare;
584 
585  // Keep the reverse mapping of the 'Allocas' array for the rename pass.
586  AllocaLookup[Allocas[AllocaNum]] = AllocaNum;
587 
588  // At this point, we're committed to promoting the alloca using IDF's, and
589  // the standard SSA construction algorithm. Determine which blocks need PHI
590  // nodes and see if we can optimize out some work by avoiding insertion of
591  // dead phi nodes.
592 
593 
594  // Unique the set of defining blocks for efficient lookup.
596  DefBlocks.insert(Info.DefiningBlocks.begin(), Info.DefiningBlocks.end());
597 
598  // Determine which blocks the value is live in. These are blocks which lead
599  // to uses.
600  SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
601  ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks);
602 
603  // At this point, we're committed to promoting the alloca using IDF's, and
604  // the standard SSA construction algorithm. Determine which blocks need phi
605  // nodes and see if we can optimize out some work by avoiding insertion of
606  // dead phi nodes.
607  IDF.setLiveInBlocks(LiveInBlocks);
608  IDF.setDefiningBlocks(DefBlocks);
610  IDF.calculate(PHIBlocks);
611  if (PHIBlocks.size() > 1)
612  std::sort(PHIBlocks.begin(), PHIBlocks.end(),
613  [this](BasicBlock *A, BasicBlock *B) {
614  return BBNumbers.lookup(A) < BBNumbers.lookup(B);
615  });
616 
617  unsigned CurrentVersion = 0;
618  for (unsigned i = 0, e = PHIBlocks.size(); i != e; ++i)
619  QueuePhiNode(PHIBlocks[i], AllocaNum, CurrentVersion);
620  }
621 
622  if (Allocas.empty())
623  return; // All of the allocas must have been trivial!
624 
625  LBI.clear();
626 
627  // Set the incoming values for the basic block to be null values for all of
628  // the alloca's. We do this in case there is a load of a value that has not
629  // been stored yet. In this case, it will get this null value.
630  //
631  RenamePassData::ValVector Values(Allocas.size());
632  for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
633  Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());
634 
635  // Walks all basic blocks in the function performing the SSA rename algorithm
636  // and inserting the phi nodes we marked as necessary
637  //
638  std::vector<RenamePassData> RenamePassWorkList;
639  RenamePassWorkList.emplace_back(F.begin(), nullptr, std::move(Values));
640  do {
641  RenamePassData RPD;
642  RPD.swap(RenamePassWorkList.back());
643  RenamePassWorkList.pop_back();
644  // RenamePass may add new worklist entries.
645  RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList);
646  } while (!RenamePassWorkList.empty());
647 
648  // The renamer uses the Visited set to avoid infinite loops. Clear it now.
649  Visited.clear();
650 
651  // Remove the allocas themselves from the function.
652  for (unsigned i = 0, e = Allocas.size(); i != e; ++i) {
653  Instruction *A = Allocas[i];
654 
655  // If there are any uses of the alloca instructions left, they must be in
656  // unreachable basic blocks that were not processed by walking the dominator
657  // tree. Just delete the users now.
658  if (!A->use_empty())
660  if (AST)
661  AST->deleteValue(A);
662  A->eraseFromParent();
663  }
664 
665  const DataLayout &DL = F.getParent()->getDataLayout();
666 
667  // Remove alloca's dbg.declare instrinsics from the function.
668  for (unsigned i = 0, e = AllocaDbgDeclares.size(); i != e; ++i)
669  if (DbgDeclareInst *DDI = AllocaDbgDeclares[i])
670  DDI->eraseFromParent();
671 
672  // Loop over all of the PHI nodes and see if there are any that we can get
673  // rid of because they merge all of the same incoming values. This can
674  // happen due to undef values coming into the PHI nodes. This process is
675  // iterative, because eliminating one PHI node can cause others to be removed.
676  bool EliminatedAPHI = true;
677  while (EliminatedAPHI) {
678  EliminatedAPHI = false;
679 
680  // Iterating over NewPhiNodes is deterministic, so it is safe to try to
681  // simplify and RAUW them as we go. If it was not, we could add uses to
682  // the values we replace with in a non-deterministic order, thus creating
683  // non-deterministic def->use chains.
684  for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
685  I = NewPhiNodes.begin(),
686  E = NewPhiNodes.end();
687  I != E;) {
688  PHINode *PN = I->second;
689 
690  // If this PHI node merges one value and/or undefs, get the value.
691  if (Value *V = SimplifyInstruction(PN, DL, nullptr, &DT, AC)) {
692  if (AST && PN->getType()->isPointerTy())
693  AST->deleteValue(PN);
694  PN->replaceAllUsesWith(V);
695  PN->eraseFromParent();
696  NewPhiNodes.erase(I++);
697  EliminatedAPHI = true;
698  continue;
699  }
700  ++I;
701  }
702  }
703 
704  // At this point, the renamer has added entries to PHI nodes for all reachable
705  // code. Unfortunately, there may be unreachable blocks which the renamer
706  // hasn't traversed. If this is the case, the PHI nodes may not
707  // have incoming values for all predecessors. Loop over all PHI nodes we have
708  // created, inserting undef values if they are missing any incoming values.
709  //
710  for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
711  I = NewPhiNodes.begin(),
712  E = NewPhiNodes.end();
713  I != E; ++I) {
714  // We want to do this once per basic block. As such, only process a block
715  // when we find the PHI that is the first entry in the block.
716  PHINode *SomePHI = I->second;
717  BasicBlock *BB = SomePHI->getParent();
718  if (&BB->front() != SomePHI)
719  continue;
720 
721  // Only do work here if there the PHI nodes are missing incoming values. We
722  // know that all PHI nodes that were inserted in a block will have the same
723  // number of incoming values, so we can just check any of them.
724  if (SomePHI->getNumIncomingValues() == getNumPreds(BB))
725  continue;
726 
727  // Get the preds for BB.
729 
730  // Ok, now we know that all of the PHI nodes are missing entries for some
731  // basic blocks. Start by sorting the incoming predecessors for efficient
732  // access.
733  std::sort(Preds.begin(), Preds.end());
734 
735  // Now we loop through all BB's which have entries in SomePHI and remove
736  // them from the Preds list.
737  for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
738  // Do a log(n) search of the Preds list for the entry we want.
739  SmallVectorImpl<BasicBlock *>::iterator EntIt = std::lower_bound(
740  Preds.begin(), Preds.end(), SomePHI->getIncomingBlock(i));
741  assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) &&
742  "PHI node has entry for a block which is not a predecessor!");
743 
744  // Remove the entry
745  Preds.erase(EntIt);
746  }
747 
748  // At this point, the blocks left in the preds list must have dummy
749  // entries inserted into every PHI nodes for the block. Update all the phi
750  // nodes in this block that we are inserting (there could be phis before
751  // mem2reg runs).
752  unsigned NumBadPreds = SomePHI->getNumIncomingValues();
753  BasicBlock::iterator BBI = BB->begin();
754  while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
755  SomePHI->getNumIncomingValues() == NumBadPreds) {
756  Value *UndefVal = UndefValue::get(SomePHI->getType());
757  for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred)
758  SomePHI->addIncoming(UndefVal, Preds[pred]);
759  }
760  }
761 
762  NewPhiNodes.clear();
763 }
764 
765 /// \brief Determine which blocks the value is live in.
766 ///
767 /// These are blocks which lead to uses. Knowing this allows us to avoid
768 /// inserting PHI nodes into blocks which don't lead to uses (thus, the
769 /// inserted phi nodes would be dead).
770 void PromoteMem2Reg::ComputeLiveInBlocks(
771  AllocaInst *AI, AllocaInfo &Info,
772  const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
773  SmallPtrSetImpl<BasicBlock *> &LiveInBlocks) {
774 
775  // To determine liveness, we must iterate through the predecessors of blocks
776  // where the def is live. Blocks are added to the worklist if we need to
777  // check their predecessors. Start with all the using blocks.
778  SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(),
779  Info.UsingBlocks.end());
780 
781  // If any of the using blocks is also a definition block, check to see if the
782  // definition occurs before or after the use. If it happens before the use,
783  // the value isn't really live-in.
784  for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) {
785  BasicBlock *BB = LiveInBlockWorklist[i];
786  if (!DefBlocks.count(BB))
787  continue;
788 
789  // Okay, this is a block that both uses and defines the value. If the first
790  // reference to the alloca is a def (store), then we know it isn't live-in.
791  for (BasicBlock::iterator I = BB->begin();; ++I) {
792  if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
793  if (SI->getOperand(1) != AI)
794  continue;
795 
796  // We found a store to the alloca before a load. The alloca is not
797  // actually live-in here.
798  LiveInBlockWorklist[i] = LiveInBlockWorklist.back();
799  LiveInBlockWorklist.pop_back();
800  --i, --e;
801  break;
802  }
803 
804  if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
805  if (LI->getOperand(0) != AI)
806  continue;
807 
808  // Okay, we found a load before a store to the alloca. It is actually
809  // live into this block.
810  break;
811  }
812  }
813  }
814 
815  // Now that we have a set of blocks where the phi is live-in, recursively add
816  // their predecessors until we find the full region the value is live.
817  while (!LiveInBlockWorklist.empty()) {
818  BasicBlock *BB = LiveInBlockWorklist.pop_back_val();
819 
820  // The block really is live in here, insert it into the set. If already in
821  // the set, then it has already been processed.
822  if (!LiveInBlocks.insert(BB).second)
823  continue;
824 
825  // Since the value is live into BB, it is either defined in a predecessor or
826  // live into it to. Add the preds to the worklist unless they are a
827  // defining block.
828  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
829  BasicBlock *P = *PI;
830 
831  // The value is not live into a predecessor if it defines the value.
832  if (DefBlocks.count(P))
833  continue;
834 
835  // Otherwise it is, add to the worklist.
836  LiveInBlockWorklist.push_back(P);
837  }
838  }
839 }
840 
841 /// \brief Queue a phi-node to be added to a basic-block for a specific Alloca.
842 ///
843 /// Returns true if there wasn't already a phi-node for that variable
844 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
845  unsigned &Version) {
846  // Look up the basic-block in question.
847  PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)];
848 
849  // If the BB already has a phi node added for the i'th alloca then we're done!
850  if (PN)
851  return false;
852 
853  // Create a PhiNode using the dereferenced type... and add the phi-node to the
854  // BasicBlock.
855  PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB),
856  Allocas[AllocaNo]->getName() + "." + Twine(Version++),
857  BB->begin());
858  ++NumPHIInsert;
859  PhiToAllocaMap[PN] = AllocaNo;
860 
861  if (AST && PN->getType()->isPointerTy())
862  AST->copyValue(PointerAllocaValues[AllocaNo], PN);
863 
864  return true;
865 }
866 
867 /// \brief Recursively traverse the CFG of the function, renaming loads and
868 /// stores to the allocas which we are promoting.
869 ///
870 /// IncomingVals indicates what value each Alloca contains on exit from the
871 /// predecessor block Pred.
872 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
873  RenamePassData::ValVector &IncomingVals,
874  std::vector<RenamePassData> &Worklist) {
875 NextIteration:
876  // If we are inserting any phi nodes into this BB, they will already be in the
877  // block.
878  if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
879  // If we have PHI nodes to update, compute the number of edges from Pred to
880  // BB.
881  if (PhiToAllocaMap.count(APN)) {
882  // We want to be able to distinguish between PHI nodes being inserted by
883  // this invocation of mem2reg from those phi nodes that already existed in
884  // the IR before mem2reg was run. We determine that APN is being inserted
885  // because it is missing incoming edges. All other PHI nodes being
886  // inserted by this pass of mem2reg will have the same number of incoming
887  // operands so far. Remember this count.
888  unsigned NewPHINumOperands = APN->getNumOperands();
889 
890  unsigned NumEdges = std::count(succ_begin(Pred), succ_end(Pred), BB);
891  assert(NumEdges && "Must be at least one edge from Pred to BB!");
892 
893  // Add entries for all the phis.
894  BasicBlock::iterator PNI = BB->begin();
895  do {
896  unsigned AllocaNo = PhiToAllocaMap[APN];
897 
898  // Add N incoming values to the PHI node.
899  for (unsigned i = 0; i != NumEdges; ++i)
900  APN->addIncoming(IncomingVals[AllocaNo], Pred);
901 
902  // The currently active variable for this block is now the PHI.
903  IncomingVals[AllocaNo] = APN;
904 
905  // Get the next phi node.
906  ++PNI;
907  APN = dyn_cast<PHINode>(PNI);
908  if (!APN)
909  break;
910 
911  // Verify that it is missing entries. If not, it is not being inserted
912  // by this mem2reg invocation so we want to ignore it.
913  } while (APN->getNumOperands() == NewPHINumOperands);
914  }
915  }
916 
917  // Don't revisit blocks.
918  if (!Visited.insert(BB).second)
919  return;
920 
921  for (BasicBlock::iterator II = BB->begin(); !isa<TerminatorInst>(II);) {
922  Instruction *I = II++; // get the instruction, increment iterator
923 
924  if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
926  if (!Src)
927  continue;
928 
929  DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src);
930  if (AI == AllocaLookup.end())
931  continue;
932 
933  Value *V = IncomingVals[AI->second];
934 
935  // Anything using the load now uses the current value.
936  LI->replaceAllUsesWith(V);
937  if (AST && LI->getType()->isPointerTy())
938  AST->deleteValue(LI);
939  BB->getInstList().erase(LI);
940  } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
941  // Delete this instruction and mark the name as the current holder of the
942  // value
943  AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
944  if (!Dest)
945  continue;
946 
947  DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
948  if (ai == AllocaLookup.end())
949  continue;
950 
951  // what value were we writing?
952  IncomingVals[ai->second] = SI->getOperand(0);
953  // Record debuginfo for the store before removing it.
954  if (DbgDeclareInst *DDI = AllocaDbgDeclares[ai->second])
955  ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
956  BB->getInstList().erase(SI);
957  }
958  }
959 
960  // 'Recurse' to our successors.
961  succ_iterator I = succ_begin(BB), E = succ_end(BB);
962  if (I == E)
963  return;
964 
965  // Keep track of the successors so we don't visit the same successor twice
966  SmallPtrSet<BasicBlock *, 8> VisitedSuccs;
967 
968  // Handle the first successor without using the worklist.
969  VisitedSuccs.insert(*I);
970  Pred = BB;
971  BB = *I;
972  ++I;
973 
974  for (; I != E; ++I)
975  if (VisitedSuccs.insert(*I).second)
976  Worklist.emplace_back(*I, Pred, IncomingVals);
977 
978  goto NextIteration;
979 }
980 
982  AliasSetTracker *AST, AssumptionCache *AC) {
983  // If there is nothing to do, bail out...
984  if (Allocas.empty())
985  return;
986 
987  PromoteMem2Reg(Allocas, DT, AST, AC).run();
988 }
iplist< Instruction >::iterator eraseFromParent()
eraseFromParent - This method unlinks 'this' from the containing basic block and deletes it...
Definition: Instruction.cpp:70
void push_back(const T &Elt)
Definition: SmallVector.h:222
A parsed version of the target data layout string in and methods for querying it. ...
Definition: DataLayout.h:104
const_iterator end(StringRef path)
Get end iterator over path.
Definition: Path.cpp:240
void addIncoming(Value *V, BasicBlock *BB)
addIncoming - Add an incoming value to the end of the PHI list
const Instruction & back() const
Definition: BasicBlock.h:245
STATISTIC(NumFunctions,"Total number of functions")
static void promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info, LargeBlockInfo &LBI, AliasSetTracker *AST)
Many allocas are only used within a single basic block.
DbgDeclareInst * FindAllocaDbgDeclare(Value *V)
FindAllocaDbgDeclare - Finds the llvm.dbg.declare intrinsic corresponding to an alloca, if any.
Definition: Local.cpp:1070
static void removeLifetimeIntrinsicUsers(AllocaInst *AI)
This file contains the declarations for metadata subclasses.
size_type count(PtrType Ptr) const
count - Return 1 if the specified pointer is in the set, 0 otherwise.
Definition: SmallPtrSet.h:276
A cache of .assume calls within a function.
const_iterator begin(StringRef path)
Get begin iterator over path.
Definition: Path.cpp:232
const Function * getParent() const
Return the enclosing method, or null if none.
Definition: BasicBlock.h:111
const Instruction & front() const
Definition: BasicBlock.h:243
F(f)
unsigned getAddressSpace() const
Return the address space of the Pointer type.
Definition: DerivedTypes.h:472
LoadInst - an instruction for reading from memory.
Definition: Instructions.h:177
bool isAllocaPromotable(const AllocaInst *AI)
Return true if this alloca is legal for promotion.
A templated base class for SmallPtrSet which provides the typesafe interface that is common across al...
Definition: SmallPtrSet.h:242
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:231
Twine - A lightweight data structure for efficiently representing the concatenation of temporary valu...
Definition: Twine.h:79
static StringRef getName(Value *V)
#define false
Definition: ConvertUTF.c:65
Interval::succ_iterator succ_begin(Interval *I)
succ_begin/succ_end - define methods so that Intervals may be used just like BasicBlocks can with the...
Definition: Interval.h:104
void PromoteMemToReg(ArrayRef< AllocaInst * > Allocas, DominatorTree &DT, AliasSetTracker *AST=nullptr, AssumptionCache *AC=nullptr)
Promote the specified list of alloca instructions into scalar registers, inserting PHI nodes as appro...
This class represents a no-op cast from one type to another.
ArrayRef - Represent a constant reference to an array (0 or more elements consecutively in memory)...
Definition: ArrayRef.h:31
StoreInst - an instruction for storing to memory.
Definition: Instructions.h:316
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:351
iterator begin()
Definition: Function.h:457
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree...
Definition: Dominators.h:67
Interval::succ_iterator succ_end(Interval *I)
Definition: Interval.h:107
unsigned getNumIncomingValues() const
getNumIncomingValues - Return the number of incoming edges
GetElementPtrInst - an instruction for type-safe pointer arithmetic to access elements of arrays and ...
Definition: Instructions.h:830
#define P(N)
LLVM Basic Block Representation.
Definition: BasicBlock.h:65
PointerType * getType() const
getType - Overload to return most specific pointer type
Definition: Instructions.h:115
This file contains the declarations for the subclasses of Constant, which represent the different fla...
bool onlyUsedByLifetimeMarkers(const Value *V)
onlyUsedByLifetimeMarkers - Return true if the only users of this pointer are lifetime markers...
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:264
Interval::pred_iterator pred_begin(Interval *I)
pred_begin/pred_end - define methods so that Intervals may be used just like BasicBlocks can with the...
Definition: Interval.h:114
BasicBlock * getIncomingBlock(unsigned i) const
getIncomingBlock - Return incoming basic block number i.
const InstListType & getInstList() const
Return the underlying instruction list container.
Definition: BasicBlock.h:252
Value * getOperand(unsigned i) const
Definition: User.h:118
Interval::pred_iterator pred_end(Interval *I)
Definition: Interval.h:117
Value * getPointerOperand()
Definition: Instructions.h:284
bool empty() const
empty - Check if the array is empty.
Definition: ArrayRef.h:129
bool isPointerTy() const
isPointerTy - True if this is an instance of PointerType.
Definition: Type.h:217
static UndefValue * get(Type *T)
get() - Static factory methods - Return an 'undef' object of the specified type.
Definition: Constants.cpp:1473
static PointerType * getInt8PtrTy(LLVMContext &C, unsigned AS=0)
Definition: Type.cpp:283
iterator erase(iterator where)
Definition: ilist.h:465
hexagon gen pred
bool dominates(const Instruction *Def, const Use &U) const
Return true if Def dominates a use in User.
Definition: Dominators.cpp:214
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:299
bool ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI, StoreInst *SI, DIBuilder &Builder)
===---------------------------------------------------------------——===// Dbg Intrinsic utilities ...
Definition: Local.cpp:979
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:861
Module.h This file contains the declarations for the Module class.
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:222
Instruction * user_back()
user_back - Specialize the methods defined in Value, as we know that an instruction can only be used ...
Definition: Instruction.h:69
static PHINode * Create(Type *Ty, unsigned NumReservedValues, const Twine &NameStr="", Instruction *InsertBefore=nullptr)
Constructors - NumReservedValues is a hint for the number of incoming edges that this phi node will h...
void swap(llvm::BitVector &LHS, llvm::BitVector &RHS)
Implement std::swap in terms of BitVector swap.
Definition: BitVector.h:576
iterator_range< user_iterator > users()
Definition: Value.h:300
LLVM_ATTRIBUTE_UNUSED_RESULT std::enable_if< !is_simple_type< Y >::value, typename cast_retty< X, const Y >::ret_type >::type dyn_cast(const Y &Val)
Definition: Casting.h:285
const DataLayout & getDataLayout() const
Get the data layout for the module's target platform.
Definition: Module.cpp:372
#define I(x, y, z)
Definition: MD5.cpp:54
iterator find(const KeyT &Val)
Definition: DenseMap.h:124
static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info, LargeBlockInfo &LBI, DominatorTree &DT, AliasSetTracker *AST)
Rewrite as many loads as possible given a single store.
Determine the iterated dominance frontier, given a set of defining blocks, and optionally, a set of live-in blocks.
bool use_empty() const
Definition: Value.h:275
user_iterator user_begin()
Definition: Value.h:294
Module * getParent()
Get the module that this global value is contained inside of...
Definition: GlobalValue.h:365
LLVM Value Representation.
Definition: Value.h:69
static const Function * getParent(const Value *V)
Value * SimplifyInstruction(Instruction *I, const DataLayout &DL, const TargetLibraryInfo *TLI=nullptr, const DominatorTree *DT=nullptr, AssumptionCache *AC=nullptr)
SimplifyInstruction - See if we can compute a simplified version of this instruction.
DbgDeclareInst - This represents the llvm.dbg.declare instruction.
Definition: IntrinsicInst.h:82
Function object to check whether the first component of a std::pair compares less than the first comp...
Definition: STLExtras.h:205
const BasicBlock * getParent() const
Definition: Instruction.h:72
void deleteValue(Value *PtrVal)
deleteValue method - This method is used to remove a pointer value from the AliasSetTracker entirely...
IntrinsicInst - A useful wrapper class for inspecting calls to intrinsic functions.
Definition: IntrinsicInst.h:37
bool isVoidTy() const
isVoidTy - Return true if this is 'void'.
Definition: Type.h:137
AllocaInst - an instruction to allocate memory on the stack.
Definition: Instructions.h:76
user_iterator user_end()
Definition: Value.h:296