LLVM  3.7.0
DeadStoreElimination.cpp
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1 //===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
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 a trivial dead store elimination that only considers
11 // basic-block local redundant stores.
12 //
13 // FIXME: This should eventually be extended to be a post-dominator tree
14 // traversal. Doing so would be pretty trivial.
15 //
16 //===----------------------------------------------------------------------===//
17 
18 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/ADT/STLExtras.h"
20 #include "llvm/ADT/SetVector.h"
21 #include "llvm/ADT/Statistic.h"
28 #include "llvm/IR/Constants.h"
29 #include "llvm/IR/DataLayout.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/IR/Function.h"
32 #include "llvm/IR/GlobalVariable.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/Pass.h"
36 #include "llvm/Support/Debug.h"
39 using namespace llvm;
40 
41 #define DEBUG_TYPE "dse"
42 
43 STATISTIC(NumFastStores, "Number of stores deleted");
44 STATISTIC(NumFastOther , "Number of other instrs removed");
45 
46 namespace {
47  struct DSE : public FunctionPass {
48  AliasAnalysis *AA;
50  DominatorTree *DT;
51  const TargetLibraryInfo *TLI;
52 
53  static char ID; // Pass identification, replacement for typeid
54  DSE() : FunctionPass(ID), AA(nullptr), MD(nullptr), DT(nullptr) {
56  }
57 
58  bool runOnFunction(Function &F) override {
59  if (skipOptnoneFunction(F))
60  return false;
61 
62  AA = &getAnalysis<AliasAnalysis>();
63  MD = &getAnalysis<MemoryDependenceAnalysis>();
64  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
65  TLI = AA->getTargetLibraryInfo();
66 
67  bool Changed = false;
68  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
69  // Only check non-dead blocks. Dead blocks may have strange pointer
70  // cycles that will confuse alias analysis.
71  if (DT->isReachableFromEntry(I))
72  Changed |= runOnBasicBlock(*I);
73 
74  AA = nullptr; MD = nullptr; DT = nullptr;
75  return Changed;
76  }
77 
78  bool runOnBasicBlock(BasicBlock &BB);
79  bool HandleFree(CallInst *F);
80  bool handleEndBlock(BasicBlock &BB);
81  void RemoveAccessedObjects(const MemoryLocation &LoadedLoc,
82  SmallSetVector<Value *, 16> &DeadStackObjects,
83  const DataLayout &DL);
84 
85  void getAnalysisUsage(AnalysisUsage &AU) const override {
86  AU.setPreservesCFG();
93  }
94  };
95 }
96 
97 char DSE::ID = 0;
98 INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
102 INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
103 
104 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
105 
106 //===----------------------------------------------------------------------===//
107 // Helper functions
108 //===----------------------------------------------------------------------===//
109 
110 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
111 /// and zero out all the operands of this instruction. If any of them become
112 /// dead, delete them and the computation tree that feeds them.
113 ///
114 /// If ValueSet is non-null, remove any deleted instructions from it as well.
115 ///
118  const TargetLibraryInfo *TLI,
119  SmallSetVector<Value*, 16> *ValueSet = nullptr) {
120  SmallVector<Instruction*, 32> NowDeadInsts;
121 
122  NowDeadInsts.push_back(I);
123  --NumFastOther;
124 
125  // Before we touch this instruction, remove it from memdep!
126  do {
127  Instruction *DeadInst = NowDeadInsts.pop_back_val();
128  ++NumFastOther;
129 
130  // This instruction is dead, zap it, in stages. Start by removing it from
131  // MemDep, which needs to know the operands and needs it to be in the
132  // function.
133  MD.removeInstruction(DeadInst);
134 
135  for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
136  Value *Op = DeadInst->getOperand(op);
137  DeadInst->setOperand(op, nullptr);
138 
139  // If this operand just became dead, add it to the NowDeadInsts list.
140  if (!Op->use_empty()) continue;
141 
142  if (Instruction *OpI = dyn_cast<Instruction>(Op))
143  if (isInstructionTriviallyDead(OpI, TLI))
144  NowDeadInsts.push_back(OpI);
145  }
146 
147  DeadInst->eraseFromParent();
148 
149  if (ValueSet) ValueSet->remove(DeadInst);
150  } while (!NowDeadInsts.empty());
151 }
152 
153 
154 /// hasMemoryWrite - Does this instruction write some memory? This only returns
155 /// true for things that we can analyze with other helpers below.
156 static bool hasMemoryWrite(Instruction *I, const TargetLibraryInfo *TLI) {
157  if (isa<StoreInst>(I))
158  return true;
159  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
160  switch (II->getIntrinsicID()) {
161  default:
162  return false;
163  case Intrinsic::memset:
164  case Intrinsic::memmove:
165  case Intrinsic::memcpy:
166  case Intrinsic::init_trampoline:
167  case Intrinsic::lifetime_end:
168  return true;
169  }
170  }
171  if (auto CS = CallSite(I)) {
172  if (Function *F = CS.getCalledFunction()) {
173  if (TLI && TLI->has(LibFunc::strcpy) &&
174  F->getName() == TLI->getName(LibFunc::strcpy)) {
175  return true;
176  }
177  if (TLI && TLI->has(LibFunc::strncpy) &&
178  F->getName() == TLI->getName(LibFunc::strncpy)) {
179  return true;
180  }
181  if (TLI && TLI->has(LibFunc::strcat) &&
182  F->getName() == TLI->getName(LibFunc::strcat)) {
183  return true;
184  }
185  if (TLI && TLI->has(LibFunc::strncat) &&
186  F->getName() == TLI->getName(LibFunc::strncat)) {
187  return true;
188  }
189  }
190  }
191  return false;
192 }
193 
194 /// getLocForWrite - Return a Location stored to by the specified instruction.
195 /// If isRemovable returns true, this function and getLocForRead completely
196 /// describe the memory operations for this instruction.
198  if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
199  return MemoryLocation::get(SI);
200 
201  if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
202  // memcpy/memmove/memset.
204  return Loc;
205  }
206 
207  IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
208  if (!II)
209  return MemoryLocation();
210 
211  switch (II->getIntrinsicID()) {
212  default:
213  return MemoryLocation(); // Unhandled intrinsic.
214  case Intrinsic::init_trampoline:
215  // FIXME: We don't know the size of the trampoline, so we can't really
216  // handle it here.
217  return MemoryLocation(II->getArgOperand(0));
218  case Intrinsic::lifetime_end: {
219  uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
220  return MemoryLocation(II->getArgOperand(1), Len);
221  }
222  }
223 }
224 
225 /// getLocForRead - Return the location read by the specified "hasMemoryWrite"
226 /// instruction if any.
228  assert(hasMemoryWrite(Inst, AA.getTargetLibraryInfo()) &&
229  "Unknown instruction case");
230 
231  // The only instructions that both read and write are the mem transfer
232  // instructions (memcpy/memmove).
233  if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
234  return MemoryLocation::getForSource(MTI);
235  return MemoryLocation();
236 }
237 
238 
239 /// isRemovable - If the value of this instruction and the memory it writes to
240 /// is unused, may we delete this instruction?
241 static bool isRemovable(Instruction *I) {
242  // Don't remove volatile/atomic stores.
243  if (StoreInst *SI = dyn_cast<StoreInst>(I))
244  return SI->isUnordered();
245 
246  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
247  switch (II->getIntrinsicID()) {
248  default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate");
249  case Intrinsic::lifetime_end:
250  // Never remove dead lifetime_end's, e.g. because it is followed by a
251  // free.
252  return false;
253  case Intrinsic::init_trampoline:
254  // Always safe to remove init_trampoline.
255  return true;
256 
257  case Intrinsic::memset:
258  case Intrinsic::memmove:
259  case Intrinsic::memcpy:
260  // Don't remove volatile memory intrinsics.
261  return !cast<MemIntrinsic>(II)->isVolatile();
262  }
263  }
264 
265  if (auto CS = CallSite(I))
266  return CS.getInstruction()->use_empty();
267 
268  return false;
269 }
270 
271 
272 /// isShortenable - Returns true if this instruction can be safely shortened in
273 /// length.
274 static bool isShortenable(Instruction *I) {
275  // Don't shorten stores for now
276  if (isa<StoreInst>(I))
277  return false;
278 
279  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
280  switch (II->getIntrinsicID()) {
281  default: return false;
282  case Intrinsic::memset:
283  case Intrinsic::memcpy:
284  // Do shorten memory intrinsics.
285  return true;
286  }
287  }
288 
289  // Don't shorten libcalls calls for now.
290 
291  return false;
292 }
293 
294 /// getStoredPointerOperand - Return the pointer that is being written to.
296  if (StoreInst *SI = dyn_cast<StoreInst>(I))
297  return SI->getPointerOperand();
298  if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
299  return MI->getDest();
300 
301  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
302  switch (II->getIntrinsicID()) {
303  default: llvm_unreachable("Unexpected intrinsic!");
304  case Intrinsic::init_trampoline:
305  return II->getArgOperand(0);
306  }
307  }
308 
309  CallSite CS(I);
310  // All the supported functions so far happen to have dest as their first
311  // argument.
312  return CS.getArgument(0);
313 }
314 
315 static uint64_t getPointerSize(const Value *V, const DataLayout &DL,
316  const TargetLibraryInfo *TLI) {
317  uint64_t Size;
318  if (getObjectSize(V, Size, DL, TLI))
319  return Size;
321 }
322 
323 namespace {
325  {
326  OverwriteComplete,
327  OverwriteEnd,
328  OverwriteUnknown
329  };
330 }
331 
332 /// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
333 /// completely overwrites a store to the 'Earlier' location.
334 /// 'OverwriteEnd' if the end of the 'Earlier' location is completely
335 /// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
337  const MemoryLocation &Earlier,
338  const DataLayout &DL,
339  const TargetLibraryInfo *TLI,
340  int64_t &EarlierOff, int64_t &LaterOff) {
341  const Value *P1 = Earlier.Ptr->stripPointerCasts();
342  const Value *P2 = Later.Ptr->stripPointerCasts();
343 
344  // If the start pointers are the same, we just have to compare sizes to see if
345  // the later store was larger than the earlier store.
346  if (P1 == P2) {
347  // If we don't know the sizes of either access, then we can't do a
348  // comparison.
349  if (Later.Size == MemoryLocation::UnknownSize ||
351  return OverwriteUnknown;
352 
353  // Make sure that the Later size is >= the Earlier size.
354  if (Later.Size >= Earlier.Size)
355  return OverwriteComplete;
356  }
357 
358  // Otherwise, we have to have size information, and the later store has to be
359  // larger than the earlier one.
360  if (Later.Size == MemoryLocation::UnknownSize ||
362  return OverwriteUnknown;
363 
364  // Check to see if the later store is to the entire object (either a global,
365  // an alloca, or a byval/inalloca argument). If so, then it clearly
366  // overwrites any other store to the same object.
367  const Value *UO1 = GetUnderlyingObject(P1, DL),
368  *UO2 = GetUnderlyingObject(P2, DL);
369 
370  // If we can't resolve the same pointers to the same object, then we can't
371  // analyze them at all.
372  if (UO1 != UO2)
373  return OverwriteUnknown;
374 
375  // If the "Later" store is to a recognizable object, get its size.
376  uint64_t ObjectSize = getPointerSize(UO2, DL, TLI);
377  if (ObjectSize != MemoryLocation::UnknownSize)
378  if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
379  return OverwriteComplete;
380 
381  // Okay, we have stores to two completely different pointers. Try to
382  // decompose the pointer into a "base + constant_offset" form. If the base
383  // pointers are equal, then we can reason about the two stores.
384  EarlierOff = 0;
385  LaterOff = 0;
386  const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL);
387  const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL);
388 
389  // If the base pointers still differ, we have two completely different stores.
390  if (BP1 != BP2)
391  return OverwriteUnknown;
392 
393  // The later store completely overlaps the earlier store if:
394  //
395  // 1. Both start at the same offset and the later one's size is greater than
396  // or equal to the earlier one's, or
397  //
398  // |--earlier--|
399  // |-- later --|
400  //
401  // 2. The earlier store has an offset greater than the later offset, but which
402  // still lies completely within the later store.
403  //
404  // |--earlier--|
405  // |----- later ------|
406  //
407  // We have to be careful here as *Off is signed while *.Size is unsigned.
408  if (EarlierOff >= LaterOff &&
409  Later.Size >= Earlier.Size &&
410  uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
411  return OverwriteComplete;
412 
413  // The other interesting case is if the later store overwrites the end of
414  // the earlier store
415  //
416  // |--earlier--|
417  // |-- later --|
418  //
419  // In this case we may want to trim the size of earlier to avoid generating
420  // writes to addresses which will definitely be overwritten later
421  if (LaterOff > EarlierOff &&
422  LaterOff < int64_t(EarlierOff + Earlier.Size) &&
423  int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
424  return OverwriteEnd;
425 
426  // Otherwise, they don't completely overlap.
427  return OverwriteUnknown;
428 }
429 
430 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
431 /// memory region into an identical pointer) then it doesn't actually make its
432 /// input dead in the traditional sense. Consider this case:
433 ///
434 /// memcpy(A <- B)
435 /// memcpy(A <- A)
436 ///
437 /// In this case, the second store to A does not make the first store to A dead.
438 /// The usual situation isn't an explicit A<-A store like this (which can be
439 /// trivially removed) but a case where two pointers may alias.
440 ///
441 /// This function detects when it is unsafe to remove a dependent instruction
442 /// because the DSE inducing instruction may be a self-read.
443 static bool isPossibleSelfRead(Instruction *Inst,
444  const MemoryLocation &InstStoreLoc,
445  Instruction *DepWrite, AliasAnalysis &AA) {
446  // Self reads can only happen for instructions that read memory. Get the
447  // location read.
448  MemoryLocation InstReadLoc = getLocForRead(Inst, AA);
449  if (!InstReadLoc.Ptr) return false; // Not a reading instruction.
450 
451  // If the read and written loc obviously don't alias, it isn't a read.
452  if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
453 
454  // Okay, 'Inst' may copy over itself. However, we can still remove a the
455  // DepWrite instruction if we can prove that it reads from the same location
456  // as Inst. This handles useful cases like:
457  // memcpy(A <- B)
458  // memcpy(A <- B)
459  // Here we don't know if A/B may alias, but we do know that B/B are must
460  // aliases, so removing the first memcpy is safe (assuming it writes <= #
461  // bytes as the second one.
462  MemoryLocation DepReadLoc = getLocForRead(DepWrite, AA);
463 
464  if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
465  return false;
466 
467  // If DepWrite doesn't read memory or if we can't prove it is a must alias,
468  // then it can't be considered dead.
469  return true;
470 }
471 
472 
473 //===----------------------------------------------------------------------===//
474 // DSE Pass
475 //===----------------------------------------------------------------------===//
476 
477 bool DSE::runOnBasicBlock(BasicBlock &BB) {
478  bool MadeChange = false;
479 
480  // Do a top-down walk on the BB.
481  for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
482  Instruction *Inst = BBI++;
483 
484  // Handle 'free' calls specially.
485  if (CallInst *F = isFreeCall(Inst, TLI)) {
486  MadeChange |= HandleFree(F);
487  continue;
488  }
489 
490  // If we find something that writes memory, get its memory dependence.
491  if (!hasMemoryWrite(Inst, TLI))
492  continue;
493 
494  MemDepResult InstDep = MD->getDependency(Inst);
495 
496  // Ignore any store where we can't find a local dependence.
497  // FIXME: cross-block DSE would be fun. :)
498  if (!InstDep.isDef() && !InstDep.isClobber())
499  continue;
500 
501  // If we're storing the same value back to a pointer that we just
502  // loaded from, then the store can be removed.
503  if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
504  if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
505  if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
506  SI->getOperand(0) == DepLoad && isRemovable(SI)) {
507  DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
508  << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
509 
510  // DeleteDeadInstruction can delete the current instruction. Save BBI
511  // in case we need it.
512  WeakVH NextInst(BBI);
513 
514  DeleteDeadInstruction(SI, *MD, TLI);
515 
516  if (!NextInst) // Next instruction deleted.
517  BBI = BB.begin();
518  else if (BBI != BB.begin()) // Revisit this instruction if possible.
519  --BBI;
520  ++NumFastStores;
521  MadeChange = true;
522  continue;
523  }
524  }
525  }
526 
527  // Figure out what location is being stored to.
528  MemoryLocation Loc = getLocForWrite(Inst, *AA);
529 
530  // If we didn't get a useful location, fail.
531  if (!Loc.Ptr)
532  continue;
533 
534  while (InstDep.isDef() || InstDep.isClobber()) {
535  // Get the memory clobbered by the instruction we depend on. MemDep will
536  // skip any instructions that 'Loc' clearly doesn't interact with. If we
537  // end up depending on a may- or must-aliased load, then we can't optimize
538  // away the store and we bail out. However, if we depend on on something
539  // that overwrites the memory location we *can* potentially optimize it.
540  //
541  // Find out what memory location the dependent instruction stores.
542  Instruction *DepWrite = InstDep.getInst();
543  MemoryLocation DepLoc = getLocForWrite(DepWrite, *AA);
544  // If we didn't get a useful location, or if it isn't a size, bail out.
545  if (!DepLoc.Ptr)
546  break;
547 
548  // If we find a write that is a) removable (i.e., non-volatile), b) is
549  // completely obliterated by the store to 'Loc', and c) which we know that
550  // 'Inst' doesn't load from, then we can remove it.
551  if (isRemovable(DepWrite) &&
552  !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
553  int64_t InstWriteOffset, DepWriteOffset;
554  const DataLayout &DL = BB.getModule()->getDataLayout();
556  isOverwrite(Loc, DepLoc, DL, AA->getTargetLibraryInfo(),
557  DepWriteOffset, InstWriteOffset);
558  if (OR == OverwriteComplete) {
559  DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
560  << *DepWrite << "\n KILLER: " << *Inst << '\n');
561 
562  // Delete the store and now-dead instructions that feed it.
563  DeleteDeadInstruction(DepWrite, *MD, TLI);
564  ++NumFastStores;
565  MadeChange = true;
566 
567  // DeleteDeadInstruction can delete the current instruction in loop
568  // cases, reset BBI.
569  BBI = Inst;
570  if (BBI != BB.begin())
571  --BBI;
572  break;
573  } else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
574  // TODO: base this on the target vector size so that if the earlier
575  // store was too small to get vector writes anyway then its likely
576  // a good idea to shorten it
577  // Power of 2 vector writes are probably always a bad idea to optimize
578  // as any store/memset/memcpy is likely using vector instructions so
579  // shortening it to not vector size is likely to be slower
580  MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
581  unsigned DepWriteAlign = DepIntrinsic->getAlignment();
582  if (llvm::isPowerOf2_64(InstWriteOffset) ||
583  ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
584 
585  DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
586  << *DepWrite << "\n KILLER (offset "
587  << InstWriteOffset << ", "
588  << DepLoc.Size << ")"
589  << *Inst << '\n');
590 
591  Value* DepWriteLength = DepIntrinsic->getLength();
592  Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
593  InstWriteOffset -
594  DepWriteOffset);
595  DepIntrinsic->setLength(TrimmedLength);
596  MadeChange = true;
597  }
598  }
599  }
600 
601  // If this is a may-aliased store that is clobbering the store value, we
602  // can keep searching past it for another must-aliased pointer that stores
603  // to the same location. For example, in:
604  // store -> P
605  // store -> Q
606  // store -> P
607  // we can remove the first store to P even though we don't know if P and Q
608  // alias.
609  if (DepWrite == &BB.front()) break;
610 
611  // Can't look past this instruction if it might read 'Loc'.
612  if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
613  break;
614 
615  InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
616  }
617  }
618 
619  // If this block ends in a return, unwind, or unreachable, all allocas are
620  // dead at its end, which means stores to them are also dead.
621  if (BB.getTerminator()->getNumSuccessors() == 0)
622  MadeChange |= handleEndBlock(BB);
623 
624  return MadeChange;
625 }
626 
627 /// Find all blocks that will unconditionally lead to the block BB and append
628 /// them to F.
630  BasicBlock *BB, DominatorTree *DT) {
631  for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
632  BasicBlock *Pred = *I;
633  if (Pred == BB) continue;
634  TerminatorInst *PredTI = Pred->getTerminator();
635  if (PredTI->getNumSuccessors() != 1)
636  continue;
637 
638  if (DT->isReachableFromEntry(Pred))
639  Blocks.push_back(Pred);
640  }
641 }
642 
643 /// HandleFree - Handle frees of entire structures whose dependency is a store
644 /// to a field of that structure.
645 bool DSE::HandleFree(CallInst *F) {
646  bool MadeChange = false;
647 
650  Blocks.push_back(F->getParent());
651  const DataLayout &DL = F->getModule()->getDataLayout();
652 
653  while (!Blocks.empty()) {
654  BasicBlock *BB = Blocks.pop_back_val();
655  Instruction *InstPt = BB->getTerminator();
656  if (BB == F->getParent()) InstPt = F;
657 
658  MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
659  while (Dep.isDef() || Dep.isClobber()) {
660  Instruction *Dependency = Dep.getInst();
661  if (!hasMemoryWrite(Dependency, TLI) || !isRemovable(Dependency))
662  break;
663 
664  Value *DepPointer =
666 
667  // Check for aliasing.
668  if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
669  break;
670 
671  Instruction *Next = std::next(BasicBlock::iterator(Dependency));
672 
673  // DCE instructions only used to calculate that store
674  DeleteDeadInstruction(Dependency, *MD, TLI);
675  ++NumFastStores;
676  MadeChange = true;
677 
678  // Inst's old Dependency is now deleted. Compute the next dependency,
679  // which may also be dead, as in
680  // s[0] = 0;
681  // s[1] = 0; // This has just been deleted.
682  // free(s);
683  Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB);
684  }
685 
686  if (Dep.isNonLocal())
687  FindUnconditionalPreds(Blocks, BB, DT);
688  }
689 
690  return MadeChange;
691 }
692 
693 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
694 /// function end block. Ex:
695 /// %A = alloca i32
696 /// ...
697 /// store i32 1, i32* %A
698 /// ret void
699 bool DSE::handleEndBlock(BasicBlock &BB) {
700  bool MadeChange = false;
701 
702  // Keep track of all of the stack objects that are dead at the end of the
703  // function.
704  SmallSetVector<Value*, 16> DeadStackObjects;
705 
706  // Find all of the alloca'd pointers in the entry block.
707  BasicBlock *Entry = BB.getParent()->begin();
708  for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
709  if (isa<AllocaInst>(I))
710  DeadStackObjects.insert(I);
711 
712  // Okay, so these are dead heap objects, but if the pointer never escapes
713  // then it's leaked by this function anyways.
714  else if (isAllocLikeFn(I, TLI) && !PointerMayBeCaptured(I, true, true))
715  DeadStackObjects.insert(I);
716  }
717 
718  // Treat byval or inalloca arguments the same, stores to them are dead at the
719  // end of the function.
720  for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
721  AE = BB.getParent()->arg_end(); AI != AE; ++AI)
722  if (AI->hasByValOrInAllocaAttr())
723  DeadStackObjects.insert(AI);
724 
725  const DataLayout &DL = BB.getModule()->getDataLayout();
726 
727  // Scan the basic block backwards
728  for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
729  --BBI;
730 
731  // If we find a store, check to see if it points into a dead stack value.
732  if (hasMemoryWrite(BBI, TLI) && isRemovable(BBI)) {
733  // See through pointer-to-pointer bitcasts
734  SmallVector<Value *, 4> Pointers;
735  GetUnderlyingObjects(getStoredPointerOperand(BBI), Pointers, DL);
736 
737  // Stores to stack values are valid candidates for removal.
738  bool AllDead = true;
739  for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
740  E = Pointers.end(); I != E; ++I)
741  if (!DeadStackObjects.count(*I)) {
742  AllDead = false;
743  break;
744  }
745 
746  if (AllDead) {
747  Instruction *Dead = BBI++;
748 
749  DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
750  << *Dead << "\n Objects: ";
751  for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
752  E = Pointers.end(); I != E; ++I) {
753  dbgs() << **I;
754  if (std::next(I) != E)
755  dbgs() << ", ";
756  }
757  dbgs() << '\n');
758 
759  // DCE instructions only used to calculate that store.
760  DeleteDeadInstruction(Dead, *MD, TLI, &DeadStackObjects);
761  ++NumFastStores;
762  MadeChange = true;
763  continue;
764  }
765  }
766 
767  // Remove any dead non-memory-mutating instructions.
768  if (isInstructionTriviallyDead(BBI, TLI)) {
769  Instruction *Inst = BBI++;
770  DeleteDeadInstruction(Inst, *MD, TLI, &DeadStackObjects);
771  ++NumFastOther;
772  MadeChange = true;
773  continue;
774  }
775 
776  if (isa<AllocaInst>(BBI)) {
777  // Remove allocas from the list of dead stack objects; there can't be
778  // any references before the definition.
779  DeadStackObjects.remove(BBI);
780  continue;
781  }
782 
783  if (auto CS = CallSite(BBI)) {
784  // Remove allocation function calls from the list of dead stack objects;
785  // there can't be any references before the definition.
786  if (isAllocLikeFn(BBI, TLI))
787  DeadStackObjects.remove(BBI);
788 
789  // If this call does not access memory, it can't be loading any of our
790  // pointers.
791  if (AA->doesNotAccessMemory(CS))
792  continue;
793 
794  // If the call might load from any of our allocas, then any store above
795  // the call is live.
796  DeadStackObjects.remove_if([&](Value *I) {
797  // See if the call site touches the value.
798  AliasAnalysis::ModRefResult A = AA->getModRefInfo(
799  CS, I, getPointerSize(I, DL, AA->getTargetLibraryInfo()));
800 
801  return A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref;
802  });
803 
804  // If all of the allocas were clobbered by the call then we're not going
805  // to find anything else to process.
806  if (DeadStackObjects.empty())
807  break;
808 
809  continue;
810  }
811 
812  MemoryLocation LoadedLoc;
813 
814  // If we encounter a use of the pointer, it is no longer considered dead
815  if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
816  if (!L->isUnordered()) // Be conservative with atomic/volatile load
817  break;
818  LoadedLoc = MemoryLocation::get(L);
819  } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
820  LoadedLoc = MemoryLocation::get(V);
821  } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
822  LoadedLoc = MemoryLocation::getForSource(MTI);
823  } else if (!BBI->mayReadFromMemory()) {
824  // Instruction doesn't read memory. Note that stores that weren't removed
825  // above will hit this case.
826  continue;
827  } else {
828  // Unknown inst; assume it clobbers everything.
829  break;
830  }
831 
832  // Remove any allocas from the DeadPointer set that are loaded, as this
833  // makes any stores above the access live.
834  RemoveAccessedObjects(LoadedLoc, DeadStackObjects, DL);
835 
836  // If all of the allocas were clobbered by the access then we're not going
837  // to find anything else to process.
838  if (DeadStackObjects.empty())
839  break;
840  }
841 
842  return MadeChange;
843 }
844 
845 /// RemoveAccessedObjects - Check to see if the specified location may alias any
846 /// of the stack objects in the DeadStackObjects set. If so, they become live
847 /// because the location is being loaded.
848 void DSE::RemoveAccessedObjects(const MemoryLocation &LoadedLoc,
849  SmallSetVector<Value *, 16> &DeadStackObjects,
850  const DataLayout &DL) {
851  const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr, DL);
852 
853  // A constant can't be in the dead pointer set.
854  if (isa<Constant>(UnderlyingPointer))
855  return;
856 
857  // If the kill pointer can be easily reduced to an alloca, don't bother doing
858  // extraneous AA queries.
859  if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
860  DeadStackObjects.remove(const_cast<Value*>(UnderlyingPointer));
861  return;
862  }
863 
864  // Remove objects that could alias LoadedLoc.
865  DeadStackObjects.remove_if([&](Value *I) {
866  // See if the loaded location could alias the stack location.
867  MemoryLocation StackLoc(I,
868  getPointerSize(I, DL, AA->getTargetLibraryInfo()));
869  return !AA->isNoAlias(StackLoc, LoadedLoc);
870  });
871 }
unsigned getAlignment() const
iplist< Instruction >::iterator eraseFromParent()
eraseFromParent - This method unlinks 'this' from the containing basic block and deletes it...
Definition: Instruction.cpp:70
A parsed version of the target data layout string in and methods for querying it. ...
Definition: DataLayout.h:104
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
static MemoryLocation getLocForWrite(Instruction *Inst, AliasAnalysis &AA)
getLocForWrite - Return a Location stored to by the specified instruction.
bool isDef() const
isDef - Return true if this MemDepResult represents a query that is an instruction definition depende...
STATISTIC(NumFunctions,"Total number of functions")
ValTy * getArgument(unsigned ArgNo) const
Definition: CallSite.h:119
iterator end()
Definition: Function.h:459
Intrinsic::ID getIntrinsicID() const
getIntrinsicID - Return the intrinsic ID of this intrinsic.
Definition: IntrinsicInst.h:44
unsigned getNumOperands() const
Definition: User.h:138
bool getObjectSize(const Value *Ptr, uint64_t &Size, const DataLayout &DL, const TargetLibraryInfo *TLI, bool RoundToAlign=false)
Compute the size of the object pointed by Ptr.
CallInst - This class represents a function call, abstracting a target machine's calling convention...
static OverwriteResult isOverwrite(const MemoryLocation &Later, const MemoryLocation &Earlier, const DataLayout &DL, const TargetLibraryInfo *TLI, int64_t &EarlierOff, int64_t &LaterOff)
isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location completely overwrites a s...
static bool isPossibleSelfRead(Instruction *Inst, const MemoryLocation &InstStoreLoc, Instruction *DepWrite, AliasAnalysis &AA)
isPossibleSelfRead - If 'Inst' might be a self read (i.e.
const Function * getParent() const
Return the enclosing method, or null if none.
Definition: BasicBlock.h:111
arg_iterator arg_end()
Definition: Function.h:480
const Instruction & front() const
Definition: BasicBlock.h:243
F(f)
LoadInst - an instruction for reading from memory.
Definition: Instructions.h:177
void initializeDSEPass(PassRegistry &)
static bool isShortenable(Instruction *I)
isShortenable - Returns true if this instruction can be safely shortened in length.
void GetUnderlyingObjects(Value *V, SmallVectorImpl< Value * > &Objects, const DataLayout &DL, LoopInfo *LI=nullptr, unsigned MaxLookup=6)
This method is similar to GetUnderlyingObject except that it can look through phi and select instruct...
#define op(i)
bool isClobber() const
isClobber - Return true if this MemDepResult represents a query that is an instruction clobber depend...
const CallInst * isFreeCall(const Value *I, const TargetLibraryInfo *TLI)
isFreeCall - Returns non-null if the value is a call to the builtin free()
StringRef getName() const
Return a constant reference to the value's name.
Definition: Value.cpp:188
bool isMustAlias(const MemoryLocation &LocA, const MemoryLocation &LocB)
isMustAlias - A convenience wrapper.
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:231
AnalysisUsage & addRequired()
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:70
bool has(LibFunc::Func F) const
Tests whether a library function is available.
const Module * getModule() const
Return the module owning the function this basic block belongs to, or nullptr it the function does no...
Definition: BasicBlock.cpp:116
T LLVM_ATTRIBUTE_UNUSED_RESULT pop_back_val()
Definition: SmallVector.h:406
#define llvm_unreachable(msg)
Marks that the current location is not supposed to be reachable.
Definition: ErrorHandling.h:98
This class consists of common code factored out of the SmallVector class to reduce code duplication b...
Definition: APInt.h:33
#define INITIALIZE_PASS_END(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:75
MemoryDependenceAnalysis - This is an analysis that determines, for a given memory operation...
bool remove(const value_type &X)
Remove an item from the set vector.
Definition: SetVector.h:118
#define false
Definition: ConvertUTF.c:65
bool insert(const value_type &X)
Insert a new element into the SetVector.
Definition: SetVector.h:102
static Value * getStoredPointerOperand(Instruction *I)
getStoredPointerOperand - Return the pointer that is being written to.
bool LLVM_ATTRIBUTE_UNUSED_RESULT empty() const
Definition: SmallVector.h:57
static MemoryLocation getForDest(const MemIntrinsic *MI)
Return a location representing the destination of a memory set or transfer.
Value handle that is nullable, but tries to track the Value.
Definition: ValueHandle.h:141
Value * GetPointerBaseWithConstantOffset(Value *Ptr, int64_t &Offset, const DataLayout &DL)
GetPointerBaseWithConstantOffset - Analyze the specified pointer to see if it can be expressed as a b...
bool isReachableFromEntry(const Use &U) const
Provide an overload for a Use.
Definition: Dominators.cpp:265
static uint64_t getPointerSize(const Value *V, const DataLayout &DL, const TargetLibraryInfo *TLI)
bool empty() const
Determine if the SetVector is empty or not.
Definition: SetVector.h:59
StoreInst - an instruction for storing to memory.
Definition: Instructions.h:316
static bool hasMemoryWrite(Instruction *I, const TargetLibraryInfo *TLI)
hasMemoryWrite - Does this instruction write some memory? This only returns true for things that we c...
iterator begin()
Definition: Function.h:457
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree...
Definition: Dominators.h:67
unsigned getNumSuccessors() const
Return the number of successors that this terminator has.
Definition: InstrTypes.h:57
static MemoryLocation get(const LoadInst *LI)
Return a location with information about the memory reference by the given instruction.
Subclasses of this class are all able to terminate a basic block.
Definition: InstrTypes.h:35
LLVM Basic Block Representation.
Definition: BasicBlock.h:65
static MemoryLocation getLocForRead(Instruction *Inst, AliasAnalysis &AA)
getLocForRead - Return the location read by the specified "hasMemoryWrite" instruction if any...
machine Machine Common Subexpression Elimination
Definition: MachineCSE.cpp:115
This file contains the declarations for the subclasses of Constant, which represent the different fla...
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
static void DeleteDeadInstruction(Instruction *I, MemoryDependenceAnalysis &MD, const TargetLibraryInfo *TLI, SmallSetVector< Value *, 16 > *ValueSet=nullptr)
DeleteDeadInstruction - Delete this instruction.
Represent the analysis usage information of a pass.
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:294
Value * getOperand(unsigned i) const
Definition: User.h:118
Interval::pred_iterator pred_end(Interval *I)
Definition: Interval.h:117
arg_iterator arg_begin()
Definition: Function.h:472
#define INITIALIZE_AG_DEPENDENCY(depName)
Definition: PassSupport.h:72
VAArgInst - This class represents the va_arg llvm instruction, which returns an argument of the speci...
bool PointerMayBeCaptured(const Value *V, bool ReturnCaptures, bool StoreCaptures)
PointerMayBeCaptured - Return true if this pointer value may be captured by the enclosing function (w...
Value * GetUnderlyingObject(Value *V, const DataLayout &DL, unsigned MaxLookup=6)
GetUnderlyingObject - This method strips off any GEP address adjustments and pointer casts from the s...
MemDepResult - A memory dependence query can return one of three different answers, described below.
const Value * Ptr
The address of the start of the location.
Representation for a specific memory location.
A SetVector that performs no allocations if smaller than a certain size.
Definition: SetVector.h:217
MemIntrinsic - This is the common base class for memset/memcpy/memmove.
iterator end()
Definition: BasicBlock.h:233
const Module * getModule() const
Return the module owning the function this instruction belongs to or nullptr it the function does not...
Definition: Instruction.cpp:57
FunctionPass * createDeadStoreEliminationPass()
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small...
Definition: SmallVector.h:861
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:222
Provides information about what library functions are available for the current target.
#define INITIALIZE_PASS_BEGIN(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:67
Value * getLength() const
bool isNonLocal() const
isNonLocal - Return true if this MemDepResult represents a query that is transparent to the start of ...
Value * stripPointerCasts()
Strip off pointer casts, all-zero GEPs, and aliases.
Definition: Value.cpp:458
static Constant * get(Type *Ty, uint64_t V, bool isSigned=false)
If Ty is a vector type, return a Constant with a splat of the given value.
Definition: Constants.cpp:582
void setPreservesCFG()
This function should be called by the pass, iff they do not:
Definition: Pass.cpp:263
void setOperand(unsigned i, Value *Val)
Definition: User.h:122
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:123
static void FindUnconditionalPreds(SmallVectorImpl< BasicBlock * > &Blocks, BasicBlock *BB, DominatorTree *DT)
Find all blocks that will unconditionally lead to the block BB and append them to F...
Value * getArgOperand(unsigned i) const
getArgOperand/setArgOperand - Return/set the i-th call argument.
bool isPowerOf2_64(uint64_t Value)
isPowerOf2_64 - This function returns true if the argument is a power of two 0 (64 bit edition...
Definition: MathExtras.h:360
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
void setLength(Value *L)
MemTransferInst - This class wraps the llvm.memcpy/memmove intrinsics.
const DataLayout & getDataLayout() const
Get the data layout for the module's target platform.
Definition: Module.cpp:372
Instruction * getInst() const
getInst() - If this is a normal dependency, return the instruction that is depended on...
size_type count(const key_type &key) const
Count the number of elements of a given key in the SetVector.
Definition: SetVector.h:156
StringRef getName(LibFunc::Func F) const
#define I(x, y, z)
Definition: MD5.cpp:54
TerminatorInst * getTerminator()
Returns the terminator instruction if the block is well formed or null if the block is not well forme...
Definition: BasicBlock.cpp:124
bool isAllocLikeFn(const Value *V, const TargetLibraryInfo *TLI, bool LookThroughBitCast=false)
Tests if a value is a call or invoke to a library function that allocates memory (either malloc...
bool use_empty() const
Definition: Value.h:275
bool isInstructionTriviallyDead(Instruction *I, const TargetLibraryInfo *TLI=nullptr)
isInstructionTriviallyDead - Return true if the result produced by the instruction is not used...
Definition: Local.cpp:282
LLVM Value Representation.
Definition: Value.h:69
ModRefResult
Simple mod/ref information...
bool remove_if(UnaryPredicate P)
Remove items from the set vector based on a predicate function.
Definition: SetVector.h:143
static bool isRemovable(Instruction *I)
isRemovable - If the value of this instruction and the memory it writes to is unused, may we delete this instruction?
#define DEBUG(X)
Definition: Debug.h:92
const TargetLibraryInfo * getTargetLibraryInfo() const
getTargetLibraryInfo - Return a pointer to the current TargetLibraryInfo object, or null if no Target...
bool isNoAlias(const MemoryLocation &LocA, const MemoryLocation &LocB)
isNoAlias - A trivial helper function to check to see if the specified pointers are no-alias...
Legacy analysis pass which computes a DominatorTree.
Definition: Dominators.h:203
static bool isVolatile(Instruction *Inst)
NodeTy * remove(iterator &IT)
Definition: ilist.h:435
const BasicBlock * getParent() const
Definition: Instruction.h:72
void removeInstruction(Instruction *InstToRemove)
removeInstruction - Remove an instruction from the dependence analysis, updating the dependence of in...
IntrinsicInst - A useful wrapper class for inspecting calls to intrinsic functions.
Definition: IntrinsicInst.h:37
uint64_t Size
The maximum size of the location, in address-units, or UnknownSize if the size is not known...
static MemoryLocation getForSource(const MemTransferInst *MTI)
Return a location representing the source of a memory transfer.