llvm.org GIT mirror llvm / d94715e lib / Transforms / Scalar / DeadStoreElimination.cpp
d94715e

Tree @d94715e (Download .tar.gz)

DeadStoreElimination.cpp @d94715eraw · history · blame

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a trivial dead store elimination that only considers
// basic-block local redundant stores.
//
// FIXME: This should eventually be extended to be a post-dominator tree
// traversal.  Doing so would be pretty trivial.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;

#define DEBUG_TYPE "dse"

STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther , "Number of other instrs removed");

namespace {
  struct DSE : public FunctionPass {
    AliasAnalysis *AA;
    MemoryDependenceAnalysis *MD;
    DominatorTree *DT;
    const TargetLibraryInfo *TLI;

    static char ID; // Pass identification, replacement for typeid
    DSE() : FunctionPass(ID), AA(nullptr), MD(nullptr), DT(nullptr) {
      initializeDSEPass(*PassRegistry::getPassRegistry());
    }

    bool runOnFunction(Function &F) override {
      if (skipOptnoneFunction(F))
        return false;

      AA = &getAnalysis<AliasAnalysis>();
      MD = &getAnalysis<MemoryDependenceAnalysis>();
      DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
      TLI = AA->getTargetLibraryInfo();

      bool Changed = false;
      for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
        // Only check non-dead blocks.  Dead blocks may have strange pointer
        // cycles that will confuse alias analysis.
        if (DT->isReachableFromEntry(I))
          Changed |= runOnBasicBlock(*I);

      AA = nullptr; MD = nullptr; DT = nullptr;
      return Changed;
    }

    bool runOnBasicBlock(BasicBlock &BB);
    bool HandleFree(CallInst *F);
    bool handleEndBlock(BasicBlock &BB);
    void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
                               SmallSetVector<Value*, 16> &DeadStackObjects);

    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.setPreservesCFG();
      AU.addRequired<DominatorTreeWrapperPass>();
      AU.addRequired<AliasAnalysis>();
      AU.addRequired<MemoryDependenceAnalysis>();
      AU.addPreserved<AliasAnalysis>();
      AU.addPreserved<DominatorTreeWrapperPass>();
      AU.addPreserved<MemoryDependenceAnalysis>();
    }
  };
}

char DSE::ID = 0;
INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)

FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }

//===----------------------------------------------------------------------===//
// Helper functions
//===----------------------------------------------------------------------===//

/// DeleteDeadInstruction - Delete this instruction.  Before we do, go through
/// and zero out all the operands of this instruction.  If any of them become
/// dead, delete them and the computation tree that feeds them.
///
/// If ValueSet is non-null, remove any deleted instructions from it as well.
///
static void DeleteDeadInstruction(Instruction *I,
                               MemoryDependenceAnalysis &MD,
                               const TargetLibraryInfo *TLI,
                               SmallSetVector<Value*, 16> *ValueSet = nullptr) {
  SmallVector<Instruction*, 32> NowDeadInsts;

  NowDeadInsts.push_back(I);
  --NumFastOther;

  // Before we touch this instruction, remove it from memdep!
  do {
    Instruction *DeadInst = NowDeadInsts.pop_back_val();
    ++NumFastOther;

    // This instruction is dead, zap it, in stages.  Start by removing it from
    // MemDep, which needs to know the operands and needs it to be in the
    // function.
    MD.removeInstruction(DeadInst);

    for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
      Value *Op = DeadInst->getOperand(op);
      DeadInst->setOperand(op, nullptr);

      // If this operand just became dead, add it to the NowDeadInsts list.
      if (!Op->use_empty()) continue;

      if (Instruction *OpI = dyn_cast<Instruction>(Op))
        if (isInstructionTriviallyDead(OpI, TLI))
          NowDeadInsts.push_back(OpI);
    }

    DeadInst->eraseFromParent();

    if (ValueSet) ValueSet->remove(DeadInst);
  } while (!NowDeadInsts.empty());
}


/// hasMemoryWrite - Does this instruction write some memory?  This only returns
/// true for things that we can analyze with other helpers below.
static bool hasMemoryWrite(Instruction *I, const TargetLibraryInfo *TLI) {
  if (isa<StoreInst>(I))
    return true;
  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
    switch (II->getIntrinsicID()) {
    default:
      return false;
    case Intrinsic::memset:
    case Intrinsic::memmove:
    case Intrinsic::memcpy:
    case Intrinsic::init_trampoline:
    case Intrinsic::lifetime_end:
      return true;
    }
  }
  if (CallSite CS = I) {
    if (Function *F = CS.getCalledFunction()) {
      if (TLI && TLI->has(LibFunc::strcpy) &&
          F->getName() == TLI->getName(LibFunc::strcpy)) {
        return true;
      }
      if (TLI && TLI->has(LibFunc::strncpy) &&
          F->getName() == TLI->getName(LibFunc::strncpy)) {
        return true;
      }
      if (TLI && TLI->has(LibFunc::strcat) &&
          F->getName() == TLI->getName(LibFunc::strcat)) {
        return true;
      }
      if (TLI && TLI->has(LibFunc::strncat) &&
          F->getName() == TLI->getName(LibFunc::strncat)) {
        return true;
      }
    }
  }
  return false;
}

/// getLocForWrite - Return a Location stored to by the specified instruction.
/// If isRemovable returns true, this function and getLocForRead completely
/// describe the memory operations for this instruction.
static AliasAnalysis::Location
getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
  const DataLayout *DL = AA.getDataLayout();
  if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
    return AA.getLocation(SI);

  if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
    // memcpy/memmove/memset.
    AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
    // If we don't have target data around, an unknown size in Location means
    // that we should use the size of the pointee type.  This isn't valid for
    // memset/memcpy, which writes more than an i8.
    if (Loc.Size == AliasAnalysis::UnknownSize && DL == nullptr)
      return AliasAnalysis::Location();
    return Loc;
  }

  IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
  if (!II) return AliasAnalysis::Location();

  switch (II->getIntrinsicID()) {
  default: return AliasAnalysis::Location(); // Unhandled intrinsic.
  case Intrinsic::init_trampoline:
    // If we don't have target data around, an unknown size in Location means
    // that we should use the size of the pointee type.  This isn't valid for
    // init.trampoline, which writes more than an i8.
    if (!DL) return AliasAnalysis::Location();

    // FIXME: We don't know the size of the trampoline, so we can't really
    // handle it here.
    return AliasAnalysis::Location(II->getArgOperand(0));
  case Intrinsic::lifetime_end: {
    uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
    return AliasAnalysis::Location(II->getArgOperand(1), Len);
  }
  }
}

/// getLocForRead - Return the location read by the specified "hasMemoryWrite"
/// instruction if any.
static AliasAnalysis::Location
getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
  assert(hasMemoryWrite(Inst, AA.getTargetLibraryInfo()) &&
         "Unknown instruction case");

  // The only instructions that both read and write are the mem transfer
  // instructions (memcpy/memmove).
  if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
    return AA.getLocationForSource(MTI);
  return AliasAnalysis::Location();
}


/// isRemovable - If the value of this instruction and the memory it writes to
/// is unused, may we delete this instruction?
static bool isRemovable(Instruction *I) {
  // Don't remove volatile/atomic stores.
  if (StoreInst *SI = dyn_cast<StoreInst>(I))
    return SI->isUnordered();

  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
    switch (II->getIntrinsicID()) {
    default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate");
    case Intrinsic::lifetime_end:
      // Never remove dead lifetime_end's, e.g. because it is followed by a
      // free.
      return false;
    case Intrinsic::init_trampoline:
      // Always safe to remove init_trampoline.
      return true;

    case Intrinsic::memset:
    case Intrinsic::memmove:
    case Intrinsic::memcpy:
      // Don't remove volatile memory intrinsics.
      return !cast<MemIntrinsic>(II)->isVolatile();
    }
  }

  if (CallSite CS = I)
    return CS.getInstruction()->use_empty();

  return false;
}


/// isShortenable - Returns true if this instruction can be safely shortened in
/// length.
static bool isShortenable(Instruction *I) {
  // Don't shorten stores for now
  if (isa<StoreInst>(I))
    return false;

  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
    switch (II->getIntrinsicID()) {
      default: return false;
      case Intrinsic::memset:
      case Intrinsic::memcpy:
        // Do shorten memory intrinsics.
        return true;
    }
  }

  // Don't shorten libcalls calls for now.

  return false;
}

/// getStoredPointerOperand - Return the pointer that is being written to.
static Value *getStoredPointerOperand(Instruction *I) {
  if (StoreInst *SI = dyn_cast<StoreInst>(I))
    return SI->getPointerOperand();
  if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
    return MI->getDest();

  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
    switch (II->getIntrinsicID()) {
    default: llvm_unreachable("Unexpected intrinsic!");
    case Intrinsic::init_trampoline:
      return II->getArgOperand(0);
    }
  }

  CallSite CS = I;
  // All the supported functions so far happen to have dest as their first
  // argument.
  return CS.getArgument(0);
}

static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) {
  uint64_t Size;
  if (getObjectSize(V, Size, AA.getDataLayout(), AA.getTargetLibraryInfo()))
    return Size;
  return AliasAnalysis::UnknownSize;
}

namespace {
  enum OverwriteResult
  {
    OverwriteComplete,
    OverwriteEnd,
    OverwriteUnknown
  };
}

/// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
/// completely overwrites a store to the 'Earlier' location.
/// 'OverwriteEnd' if the end of the 'Earlier' location is completely
/// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
                                   const AliasAnalysis::Location &Earlier,
                                   AliasAnalysis &AA,
                                   int64_t &EarlierOff,
                                   int64_t &LaterOff) {
  const DataLayout *DL = AA.getDataLayout();
  const Value *P1 = Earlier.Ptr->stripPointerCasts();
  const Value *P2 = Later.Ptr->stripPointerCasts();

  // If the start pointers are the same, we just have to compare sizes to see if
  // the later store was larger than the earlier store.
  if (P1 == P2) {
    // If we don't know the sizes of either access, then we can't do a
    // comparison.
    if (Later.Size == AliasAnalysis::UnknownSize ||
        Earlier.Size == AliasAnalysis::UnknownSize) {
      // If we have no DataLayout information around, then the size of the store
      // is inferrable from the pointee type.  If they are the same type, then
      // we know that the store is safe.
      if (DL == nullptr && Later.Ptr->getType() == Earlier.Ptr->getType())
        return OverwriteComplete;

      return OverwriteUnknown;
    }

    // Make sure that the Later size is >= the Earlier size.
    if (Later.Size >= Earlier.Size)
      return OverwriteComplete;
  }

  // Otherwise, we have to have size information, and the later store has to be
  // larger than the earlier one.
  if (Later.Size == AliasAnalysis::UnknownSize ||
      Earlier.Size == AliasAnalysis::UnknownSize || DL == nullptr)
    return OverwriteUnknown;

  // Check to see if the later store is to the entire object (either a global,
  // an alloca, or a byval/inalloca argument).  If so, then it clearly
  // overwrites any other store to the same object.
  const Value *UO1 = GetUnderlyingObject(P1, DL),
              *UO2 = GetUnderlyingObject(P2, DL);

  // If we can't resolve the same pointers to the same object, then we can't
  // analyze them at all.
  if (UO1 != UO2)
    return OverwriteUnknown;

  // If the "Later" store is to a recognizable object, get its size.
  uint64_t ObjectSize = getPointerSize(UO2, AA);
  if (ObjectSize != AliasAnalysis::UnknownSize)
    if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
      return OverwriteComplete;

  // Okay, we have stores to two completely different pointers.  Try to
  // decompose the pointer into a "base + constant_offset" form.  If the base
  // pointers are equal, then we can reason about the two stores.
  EarlierOff = 0;
  LaterOff = 0;
  const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL);
  const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL);

  // If the base pointers still differ, we have two completely different stores.
  if (BP1 != BP2)
    return OverwriteUnknown;

  // The later store completely overlaps the earlier store if:
  //
  // 1. Both start at the same offset and the later one's size is greater than
  //    or equal to the earlier one's, or
  //
  //      |--earlier--|
  //      |--   later   --|
  //
  // 2. The earlier store has an offset greater than the later offset, but which
  //    still lies completely within the later store.
  //
  //        |--earlier--|
  //    |-----  later  ------|
  //
  // We have to be careful here as *Off is signed while *.Size is unsigned.
  if (EarlierOff >= LaterOff &&
      Later.Size >= Earlier.Size &&
      uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
    return OverwriteComplete;

  // The other interesting case is if the later store overwrites the end of
  // the earlier store
  //
  //      |--earlier--|
  //                |--   later   --|
  //
  // In this case we may want to trim the size of earlier to avoid generating
  // writes to addresses which will definitely be overwritten later
  if (LaterOff > EarlierOff &&
      LaterOff < int64_t(EarlierOff + Earlier.Size) &&
      int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
    return OverwriteEnd;

  // Otherwise, they don't completely overlap.
  return OverwriteUnknown;
}

/// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
/// memory region into an identical pointer) then it doesn't actually make its
/// input dead in the traditional sense.  Consider this case:
///
///   memcpy(A <- B)
///   memcpy(A <- A)
///
/// In this case, the second store to A does not make the first store to A dead.
/// The usual situation isn't an explicit A<-A store like this (which can be
/// trivially removed) but a case where two pointers may alias.
///
/// This function detects when it is unsafe to remove a dependent instruction
/// because the DSE inducing instruction may be a self-read.
static bool isPossibleSelfRead(Instruction *Inst,
                               const AliasAnalysis::Location &InstStoreLoc,
                               Instruction *DepWrite, AliasAnalysis &AA) {
  // Self reads can only happen for instructions that read memory.  Get the
  // location read.
  AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
  if (!InstReadLoc.Ptr) return false;  // Not a reading instruction.

  // If the read and written loc obviously don't alias, it isn't a read.
  if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;

  // Okay, 'Inst' may copy over itself.  However, we can still remove a the
  // DepWrite instruction if we can prove that it reads from the same location
  // as Inst.  This handles useful cases like:
  //   memcpy(A <- B)
  //   memcpy(A <- B)
  // Here we don't know if A/B may alias, but we do know that B/B are must
  // aliases, so removing the first memcpy is safe (assuming it writes <= #
  // bytes as the second one.
  AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);

  if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
    return false;

  // If DepWrite doesn't read memory or if we can't prove it is a must alias,
  // then it can't be considered dead.
  return true;
}


//===----------------------------------------------------------------------===//
// DSE Pass
//===----------------------------------------------------------------------===//

bool DSE::runOnBasicBlock(BasicBlock &BB) {
  bool MadeChange = false;

  // Do a top-down walk on the BB.
  for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
    Instruction *Inst = BBI++;

    // Handle 'free' calls specially.
    if (CallInst *F = isFreeCall(Inst, TLI)) {
      MadeChange |= HandleFree(F);
      continue;
    }

    // If we find something that writes memory, get its memory dependence.
    if (!hasMemoryWrite(Inst, TLI))
      continue;

    MemDepResult InstDep = MD->getDependency(Inst);

    // Ignore any store where we can't find a local dependence.
    // FIXME: cross-block DSE would be fun. :)
    if (!InstDep.isDef() && !InstDep.isClobber())
      continue;

    // If we're storing the same value back to a pointer that we just
    // loaded from, then the store can be removed.
    if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
      if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
        if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
            SI->getOperand(0) == DepLoad && isRemovable(SI)) {
          DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n  "
                       << "LOAD: " << *DepLoad << "\n  STORE: " << *SI << '\n');

          // DeleteDeadInstruction can delete the current instruction.  Save BBI
          // in case we need it.
          WeakVH NextInst(BBI);

          DeleteDeadInstruction(SI, *MD, TLI);

          if (!NextInst)  // Next instruction deleted.
            BBI = BB.begin();
          else if (BBI != BB.begin())  // Revisit this instruction if possible.
            --BBI;
          ++NumFastStores;
          MadeChange = true;
          continue;
        }
      }
    }

    // Figure out what location is being stored to.
    AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);

    // If we didn't get a useful location, fail.
    if (!Loc.Ptr)
      continue;

    while (InstDep.isDef() || InstDep.isClobber()) {
      // Get the memory clobbered by the instruction we depend on.  MemDep will
      // skip any instructions that 'Loc' clearly doesn't interact with.  If we
      // end up depending on a may- or must-aliased load, then we can't optimize
      // away the store and we bail out.  However, if we depend on on something
      // that overwrites the memory location we *can* potentially optimize it.
      //
      // Find out what memory location the dependent instruction stores.
      Instruction *DepWrite = InstDep.getInst();
      AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
      // If we didn't get a useful location, or if it isn't a size, bail out.
      if (!DepLoc.Ptr)
        break;

      // If we find a write that is a) removable (i.e., non-volatile), b) is
      // completely obliterated by the store to 'Loc', and c) which we know that
      // 'Inst' doesn't load from, then we can remove it.
      if (isRemovable(DepWrite) &&
          !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
        int64_t InstWriteOffset, DepWriteOffset;
        OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
                                         DepWriteOffset, InstWriteOffset);
        if (OR == OverwriteComplete) {
          DEBUG(dbgs() << "DSE: Remove Dead Store:\n  DEAD: "
                << *DepWrite << "\n  KILLER: " << *Inst << '\n');

          // Delete the store and now-dead instructions that feed it.
          DeleteDeadInstruction(DepWrite, *MD, TLI);
          ++NumFastStores;
          MadeChange = true;

          // DeleteDeadInstruction can delete the current instruction in loop
          // cases, reset BBI.
          BBI = Inst;
          if (BBI != BB.begin())
            --BBI;
          break;
        } else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
          // TODO: base this on the target vector size so that if the earlier
          // store was too small to get vector writes anyway then its likely
          // a good idea to shorten it
          // Power of 2 vector writes are probably always a bad idea to optimize
          // as any store/memset/memcpy is likely using vector instructions so
          // shortening it to not vector size is likely to be slower
          MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
          unsigned DepWriteAlign = DepIntrinsic->getAlignment();
          if (llvm::isPowerOf2_64(InstWriteOffset) ||
              ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {

            DEBUG(dbgs() << "DSE: Remove Dead Store:\n  OW END: "
                  << *DepWrite << "\n  KILLER (offset "
                  << InstWriteOffset << ", "
                  << DepLoc.Size << ")"
                  << *Inst << '\n');

            Value* DepWriteLength = DepIntrinsic->getLength();
            Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
                                                    InstWriteOffset -
                                                    DepWriteOffset);
            DepIntrinsic->setLength(TrimmedLength);
            MadeChange = true;
          }
        }
      }

      // If this is a may-aliased store that is clobbering the store value, we
      // can keep searching past it for another must-aliased pointer that stores
      // to the same location.  For example, in:
      //   store -> P
      //   store -> Q
      //   store -> P
      // we can remove the first store to P even though we don't know if P and Q
      // alias.
      if (DepWrite == &BB.front()) break;

      // Can't look past this instruction if it might read 'Loc'.
      if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
        break;

      InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
    }
  }

  // If this block ends in a return, unwind, or unreachable, all allocas are
  // dead at its end, which means stores to them are also dead.
  if (BB.getTerminator()->getNumSuccessors() == 0)
    MadeChange |= handleEndBlock(BB);

  return MadeChange;
}

/// Find all blocks that will unconditionally lead to the block BB and append
/// them to F.
static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
                                   BasicBlock *BB, DominatorTree *DT) {
  for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
    BasicBlock *Pred = *I;
    if (Pred == BB) continue;
    TerminatorInst *PredTI = Pred->getTerminator();
    if (PredTI->getNumSuccessors() != 1)
      continue;

    if (DT->isReachableFromEntry(Pred))
      Blocks.push_back(Pred);
  }
}

/// HandleFree - Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
bool DSE::HandleFree(CallInst *F) {
  bool MadeChange = false;

  AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
  SmallVector<BasicBlock *, 16> Blocks;
  Blocks.push_back(F->getParent());

  while (!Blocks.empty()) {
    BasicBlock *BB = Blocks.pop_back_val();
    Instruction *InstPt = BB->getTerminator();
    if (BB == F->getParent()) InstPt = F;

    MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
    while (Dep.isDef() || Dep.isClobber()) {
      Instruction *Dependency = Dep.getInst();
      if (!hasMemoryWrite(Dependency, TLI) || !isRemovable(Dependency))
        break;

      Value *DepPointer =
        GetUnderlyingObject(getStoredPointerOperand(Dependency));

      // Check for aliasing.
      if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
        break;

      Instruction *Next = std::next(BasicBlock::iterator(Dependency));

      // DCE instructions only used to calculate that store
      DeleteDeadInstruction(Dependency, *MD, TLI);
      ++NumFastStores;
      MadeChange = true;

      // Inst's old Dependency is now deleted. Compute the next dependency,
      // which may also be dead, as in
      //    s[0] = 0;
      //    s[1] = 0; // This has just been deleted.
      //    free(s);
      Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB);
    }

    if (Dep.isNonLocal())
      FindUnconditionalPreds(Blocks, BB, DT);
  }

  return MadeChange;
}

/// handleEndBlock - Remove dead stores to stack-allocated locations in the
/// function end block.  Ex:
/// %A = alloca i32
/// ...
/// store i32 1, i32* %A
/// ret void
bool DSE::handleEndBlock(BasicBlock &BB) {
  bool MadeChange = false;

  // Keep track of all of the stack objects that are dead at the end of the
  // function.
  SmallSetVector<Value*, 16> DeadStackObjects;

  // Find all of the alloca'd pointers in the entry block.
  BasicBlock *Entry = BB.getParent()->begin();
  for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
    if (isa<AllocaInst>(I))
      DeadStackObjects.insert(I);

    // Okay, so these are dead heap objects, but if the pointer never escapes
    // then it's leaked by this function anyways.
    else if (isAllocLikeFn(I, TLI) && !PointerMayBeCaptured(I, true, true))
      DeadStackObjects.insert(I);
  }

  // Treat byval or inalloca arguments the same, stores to them are dead at the
  // end of the function.
  for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
       AE = BB.getParent()->arg_end(); AI != AE; ++AI)
    if (AI->hasByValOrInAllocaAttr())
      DeadStackObjects.insert(AI);

  // Scan the basic block backwards
  for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
    --BBI;

    // If we find a store, check to see if it points into a dead stack value.
    if (hasMemoryWrite(BBI, TLI) && isRemovable(BBI)) {
      // See through pointer-to-pointer bitcasts
      SmallVector<Value *, 4> Pointers;
      GetUnderlyingObjects(getStoredPointerOperand(BBI), Pointers);

      // Stores to stack values are valid candidates for removal.
      bool AllDead = true;
      for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
           E = Pointers.end(); I != E; ++I)
        if (!DeadStackObjects.count(*I)) {
          AllDead = false;
          break;
        }

      if (AllDead) {
        Instruction *Dead = BBI++;

        DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n  DEAD: "
                     << *Dead << "\n  Objects: ";
              for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
                   E = Pointers.end(); I != E; ++I) {
                dbgs() << **I;
                if (std::next(I) != E)
                  dbgs() << ", ";
              }
              dbgs() << '\n');

        // DCE instructions only used to calculate that store.
        DeleteDeadInstruction(Dead, *MD, TLI, &DeadStackObjects);
        ++NumFastStores;
        MadeChange = true;
        continue;
      }
    }

    // Remove any dead non-memory-mutating instructions.
    if (isInstructionTriviallyDead(BBI, TLI)) {
      Instruction *Inst = BBI++;
      DeleteDeadInstruction(Inst, *MD, TLI, &DeadStackObjects);
      ++NumFastOther;
      MadeChange = true;
      continue;
    }

    if (isa<AllocaInst>(BBI)) {
      // Remove allocas from the list of dead stack objects; there can't be
      // any references before the definition.
      DeadStackObjects.remove(BBI);
      continue;
    }

    if (CallSite CS = cast<Value>(BBI)) {
      // Remove allocation function calls from the list of dead stack objects; 
      // there can't be any references before the definition.
      if (isAllocLikeFn(BBI, TLI))
        DeadStackObjects.remove(BBI);

      // If this call does not access memory, it can't be loading any of our
      // pointers.
      if (AA->doesNotAccessMemory(CS))
        continue;

      // If the call might load from any of our allocas, then any store above
      // the call is live.
      DeadStackObjects.remove_if([&](Value *I) {
        // See if the call site touches the value.
        AliasAnalysis::ModRefResult A =
            AA->getModRefInfo(CS, I, getPointerSize(I, *AA));

        return A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref;
      });

      // If all of the allocas were clobbered by the call then we're not going
      // to find anything else to process.
      if (DeadStackObjects.empty())
        break;

      continue;
    }

    AliasAnalysis::Location LoadedLoc;

    // If we encounter a use of the pointer, it is no longer considered dead
    if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
      if (!L->isUnordered()) // Be conservative with atomic/volatile load
        break;
      LoadedLoc = AA->getLocation(L);
    } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
      LoadedLoc = AA->getLocation(V);
    } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
      LoadedLoc = AA->getLocationForSource(MTI);
    } else if (!BBI->mayReadFromMemory()) {
      // Instruction doesn't read memory.  Note that stores that weren't removed
      // above will hit this case.
      continue;
    } else {
      // Unknown inst; assume it clobbers everything.
      break;
    }

    // Remove any allocas from the DeadPointer set that are loaded, as this
    // makes any stores above the access live.
    RemoveAccessedObjects(LoadedLoc, DeadStackObjects);

    // If all of the allocas were clobbered by the access then we're not going
    // to find anything else to process.
    if (DeadStackObjects.empty())
      break;
  }

  return MadeChange;
}

/// RemoveAccessedObjects - Check to see if the specified location may alias any
/// of the stack objects in the DeadStackObjects set.  If so, they become live
/// because the location is being loaded.
void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
                                SmallSetVector<Value*, 16> &DeadStackObjects) {
  const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);

  // A constant can't be in the dead pointer set.
  if (isa<Constant>(UnderlyingPointer))
    return;

  // If the kill pointer can be easily reduced to an alloca, don't bother doing
  // extraneous AA queries.
  if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
    DeadStackObjects.remove(const_cast<Value*>(UnderlyingPointer));
    return;
  }

  // Remove objects that could alias LoadedLoc.
  DeadStackObjects.remove_if([&](Value *I) {
    // See if the loaded location could alias the stack location.
    AliasAnalysis::Location StackLoc(I, getPointerSize(I, *AA));
    return !AA->isNoAlias(StackLoc, LoadedLoc);
  });
}