aboutsummaryrefslogtreecommitdiffstats
path: root/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
blob: f3ac44cbd6bcc06075b07fb26682ec2391a0f102 (plain)
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
//===- InstCombineLoadStoreAlloca.cpp -------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the visit functions for load, store and alloca.
//
//===----------------------------------------------------------------------===//

#include "InstCombine.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;

#define DEBUG_TYPE "instcombine"

STATISTIC(NumDeadStore,    "Number of dead stores eliminated");
STATISTIC(NumGlobalCopies, "Number of allocas copied from constant global");

/// pointsToConstantGlobal - Return true if V (possibly indirectly) points to
/// some part of a constant global variable.  This intentionally only accepts
/// constant expressions because we can't rewrite arbitrary instructions.
static bool pointsToConstantGlobal(Value *V) {
  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
    return GV->isConstant();

  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
    if (CE->getOpcode() == Instruction::BitCast ||
        CE->getOpcode() == Instruction::AddrSpaceCast ||
        CE->getOpcode() == Instruction::GetElementPtr)
      return pointsToConstantGlobal(CE->getOperand(0));
  }
  return false;
}

/// isOnlyCopiedFromConstantGlobal - Recursively walk the uses of a (derived)
/// pointer to an alloca.  Ignore any reads of the pointer, return false if we
/// see any stores or other unknown uses.  If we see pointer arithmetic, keep
/// track of whether it moves the pointer (with IsOffset) but otherwise traverse
/// the uses.  If we see a memcpy/memmove that targets an unoffseted pointer to
/// the alloca, and if the source pointer is a pointer to a constant global, we
/// can optimize this.
static bool
isOnlyCopiedFromConstantGlobal(Value *V, MemTransferInst *&TheCopy,
                               SmallVectorImpl<Instruction *> &ToDelete) {
  // We track lifetime intrinsics as we encounter them.  If we decide to go
  // ahead and replace the value with the global, this lets the caller quickly
  // eliminate the markers.

  SmallVector<std::pair<Value *, bool>, 35> ValuesToInspect;
  ValuesToInspect.push_back(std::make_pair(V, false));
  while (!ValuesToInspect.empty()) {
    auto ValuePair = ValuesToInspect.pop_back_val();
    const bool IsOffset = ValuePair.second;
    for (auto &U : ValuePair.first->uses()) {
      Instruction *I = cast<Instruction>(U.getUser());

      if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
        // Ignore non-volatile loads, they are always ok.
        if (!LI->isSimple()) return false;
        continue;
      }

      if (isa<BitCastInst>(I) || isa<AddrSpaceCastInst>(I)) {
        // If uses of the bitcast are ok, we are ok.
        ValuesToInspect.push_back(std::make_pair(I, IsOffset));
        continue;
      }
      if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I)) {
        // If the GEP has all zero indices, it doesn't offset the pointer. If it
        // doesn't, it does.
        ValuesToInspect.push_back(
            std::make_pair(I, IsOffset || !GEP->hasAllZeroIndices()));
        continue;
      }

      if (CallSite CS = I) {
        // If this is the function being called then we treat it like a load and
        // ignore it.
        if (CS.isCallee(&U))
          continue;

        // Inalloca arguments are clobbered by the call.
        unsigned ArgNo = CS.getArgumentNo(&U);
        if (CS.isInAllocaArgument(ArgNo))
          return false;

        // If this is a readonly/readnone call site, then we know it is just a
        // load (but one that potentially returns the value itself), so we can
        // ignore it if we know that the value isn't captured.
        if (CS.onlyReadsMemory() &&
            (CS.getInstruction()->use_empty() || CS.doesNotCapture(ArgNo)))
          continue;

        // If this is being passed as a byval argument, the caller is making a
        // copy, so it is only a read of the alloca.
        if (CS.isByValArgument(ArgNo))
          continue;
      }

      // Lifetime intrinsics can be handled by the caller.
      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
        if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
            II->getIntrinsicID() == Intrinsic::lifetime_end) {
          assert(II->use_empty() && "Lifetime markers have no result to use!");
          ToDelete.push_back(II);
          continue;
        }
      }

      // If this is isn't our memcpy/memmove, reject it as something we can't
      // handle.
      MemTransferInst *MI = dyn_cast<MemTransferInst>(I);
      if (!MI)
        return false;

      // If the transfer is using the alloca as a source of the transfer, then
      // ignore it since it is a load (unless the transfer is volatile).
      if (U.getOperandNo() == 1) {
        if (MI->isVolatile()) return false;
        continue;
      }

      // If we already have seen a copy, reject the second one.
      if (TheCopy) return false;

      // If the pointer has been offset from the start of the alloca, we can't
      // safely handle this.
      if (IsOffset) return false;

      // If the memintrinsic isn't using the alloca as the dest, reject it.
      if (U.getOperandNo() != 0) return false;

      // If the source of the memcpy/move is not a constant global, reject it.
      if (!pointsToConstantGlobal(MI->getSource()))
        return false;

      // Otherwise, the transform is safe.  Remember the copy instruction.
      TheCopy = MI;
    }
  }
  return true;
}

/// isOnlyCopiedFromConstantGlobal - Return true if the specified alloca is only
/// modified by a copy from a constant global.  If we can prove this, we can
/// replace any uses of the alloca with uses of the global directly.
static MemTransferInst *
isOnlyCopiedFromConstantGlobal(AllocaInst *AI,
                               SmallVectorImpl<Instruction *> &ToDelete) {
  MemTransferInst *TheCopy = nullptr;
  if (isOnlyCopiedFromConstantGlobal(AI, TheCopy, ToDelete))
    return TheCopy;
  return nullptr;
}

Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) {
  // Ensure that the alloca array size argument has type intptr_t, so that
  // any casting is exposed early.
  if (DL) {
    Type *IntPtrTy = DL->getIntPtrType(AI.getType());
    if (AI.getArraySize()->getType() != IntPtrTy) {
      Value *V = Builder->CreateIntCast(AI.getArraySize(),
                                        IntPtrTy, false);
      AI.setOperand(0, V);
      return &AI;
    }
  }

  // Convert: alloca Ty, C - where C is a constant != 1 into: alloca [C x Ty], 1
  if (AI.isArrayAllocation()) {  // Check C != 1
    if (const ConstantInt *C = dyn_cast<ConstantInt>(AI.getArraySize())) {
      Type *NewTy =
        ArrayType::get(AI.getAllocatedType(), C->getZExtValue());
      AllocaInst *New = Builder->CreateAlloca(NewTy, nullptr, AI.getName());
      New->setAlignment(AI.getAlignment());

      // Scan to the end of the allocation instructions, to skip over a block of
      // allocas if possible...also skip interleaved debug info
      //
      BasicBlock::iterator It = New;
      while (isa<AllocaInst>(*It) || isa<DbgInfoIntrinsic>(*It)) ++It;

      // Now that I is pointing to the first non-allocation-inst in the block,
      // insert our getelementptr instruction...
      //
      Type *IdxTy = DL
                  ? DL->getIntPtrType(AI.getType())
                  : Type::getInt64Ty(AI.getContext());
      Value *NullIdx = Constant::getNullValue(IdxTy);
      Value *Idx[2] = { NullIdx, NullIdx };
      Instruction *GEP =
        GetElementPtrInst::CreateInBounds(New, Idx, New->getName() + ".sub");
      InsertNewInstBefore(GEP, *It);

      // Now make everything use the getelementptr instead of the original
      // allocation.
      return ReplaceInstUsesWith(AI, GEP);
    } else if (isa<UndefValue>(AI.getArraySize())) {
      return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType()));
    }
  }

  if (DL && AI.getAllocatedType()->isSized()) {
    // If the alignment is 0 (unspecified), assign it the preferred alignment.
    if (AI.getAlignment() == 0)
      AI.setAlignment(DL->getPrefTypeAlignment(AI.getAllocatedType()));

    // Move all alloca's of zero byte objects to the entry block and merge them
    // together.  Note that we only do this for alloca's, because malloc should
    // allocate and return a unique pointer, even for a zero byte allocation.
    if (DL->getTypeAllocSize(AI.getAllocatedType()) == 0) {
      // For a zero sized alloca there is no point in doing an array allocation.
      // This is helpful if the array size is a complicated expression not used
      // elsewhere.
      if (AI.isArrayAllocation()) {
        AI.setOperand(0, ConstantInt::get(AI.getArraySize()->getType(), 1));
        return &AI;
      }

      // Get the first instruction in the entry block.
      BasicBlock &EntryBlock = AI.getParent()->getParent()->getEntryBlock();
      Instruction *FirstInst = EntryBlock.getFirstNonPHIOrDbg();
      if (FirstInst != &AI) {
        // If the entry block doesn't start with a zero-size alloca then move
        // this one to the start of the entry block.  There is no problem with
        // dominance as the array size was forced to a constant earlier already.
        AllocaInst *EntryAI = dyn_cast<AllocaInst>(FirstInst);
        if (!EntryAI || !EntryAI->getAllocatedType()->isSized() ||
            DL->getTypeAllocSize(EntryAI->getAllocatedType()) != 0) {
          AI.moveBefore(FirstInst);
          return &AI;
        }

        // If the alignment of the entry block alloca is 0 (unspecified),
        // assign it the preferred alignment.
        if (EntryAI->getAlignment() == 0)
          EntryAI->setAlignment(
            DL->getPrefTypeAlignment(EntryAI->getAllocatedType()));
        // Replace this zero-sized alloca with the one at the start of the entry
        // block after ensuring that the address will be aligned enough for both
        // types.
        unsigned MaxAlign = std::max(EntryAI->getAlignment(),
                                     AI.getAlignment());
        EntryAI->setAlignment(MaxAlign);
        if (AI.getType() != EntryAI->getType())
          return new BitCastInst(EntryAI, AI.getType());
        return ReplaceInstUsesWith(AI, EntryAI);
      }
    }
  }

  if (AI.getAlignment()) {
    // Check to see if this allocation is only modified by a memcpy/memmove from
    // a constant global whose alignment is equal to or exceeds that of the
    // allocation.  If this is the case, we can change all users to use
    // the constant global instead.  This is commonly produced by the CFE by
    // constructs like "void foo() { int A[] = {1,2,3,4,5,6,7,8,9...}; }" if 'A'
    // is only subsequently read.
    SmallVector<Instruction *, 4> ToDelete;
    if (MemTransferInst *Copy = isOnlyCopiedFromConstantGlobal(&AI, ToDelete)) {
      unsigned SourceAlign = getOrEnforceKnownAlignment(Copy->getSource(),
                                                        AI.getAlignment(),
                                                        DL, AT, &AI, DT);
      if (AI.getAlignment() <= SourceAlign) {
        DEBUG(dbgs() << "Found alloca equal to global: " << AI << '\n');
        DEBUG(dbgs() << "  memcpy = " << *Copy << '\n');
        for (unsigned i = 0, e = ToDelete.size(); i != e; ++i)
          EraseInstFromFunction(*ToDelete[i]);
        Constant *TheSrc = cast<Constant>(Copy->getSource());
        Constant *Cast
          = ConstantExpr::getPointerBitCastOrAddrSpaceCast(TheSrc, AI.getType());
        Instruction *NewI = ReplaceInstUsesWith(AI, Cast);
        EraseInstFromFunction(*Copy);
        ++NumGlobalCopies;
        return NewI;
      }
    }
  }

  // At last, use the generic allocation site handler to aggressively remove
  // unused allocas.
  return visitAllocSite(AI);
}

/// \brief Helper to combine a load to a new type.
///
/// This just does the work of combining a load to a new type. It handles
/// metadata, etc., and returns the new instruction. The \c NewTy should be the
/// loaded *value* type. This will convert it to a pointer, cast the operand to
/// that pointer type, load it, etc.
///
/// Note that this will create all of the instructions with whatever insert
/// point the \c InstCombiner currently is using.
static LoadInst *combineLoadToNewType(InstCombiner &IC, LoadInst &LI, Type *NewTy) {
  Value *Ptr = LI.getPointerOperand();
  unsigned AS = LI.getPointerAddressSpace();
  SmallVector<std::pair<unsigned, MDNode *>, 8> MD;
  LI.getAllMetadata(MD);

  LoadInst *NewLoad = IC.Builder->CreateAlignedLoad(
      IC.Builder->CreateBitCast(Ptr, NewTy->getPointerTo(AS)),
      LI.getAlignment(), LI.getName());
  for (const auto &MDPair : MD) {
    unsigned ID = MDPair.first;
    MDNode *N = MDPair.second;
    // Note, essentially every kind of metadata should be preserved here! This
    // routine is supposed to clone a load instruction changing *only its type*.
    // The only metadata it makes sense to drop is metadata which is invalidated
    // when the pointer type changes. This should essentially never be the case
    // in LLVM, but we explicitly switch over only known metadata to be
    // conservatively correct. If you are adding metadata to LLVM which pertains
    // to loads, you almost certainly want to add it here.
    switch (ID) {
    case LLVMContext::MD_dbg:
    case LLVMContext::MD_tbaa:
    case LLVMContext::MD_prof:
    case LLVMContext::MD_fpmath:
    case LLVMContext::MD_tbaa_struct:
    case LLVMContext::MD_invariant_load:
    case LLVMContext::MD_alias_scope:
    case LLVMContext::MD_noalias:
    case LLVMContext::MD_nontemporal:
    case LLVMContext::MD_mem_parallel_loop_access:
    case LLVMContext::MD_nonnull:
      // All of these directly apply.
      NewLoad->setMetadata(ID, N);
      break;

    case LLVMContext::MD_range:
      // FIXME: It would be nice to propagate this in some way, but the type
      // conversions make it hard.
      break;
    }
  }
  return NewLoad;
}

/// \brief Combine loads to match the type of value their uses after looking
/// through intervening bitcasts.
///
/// The core idea here is that if the result of a load is used in an operation,
/// we should load the type most conducive to that operation. For example, when
/// loading an integer and converting that immediately to a pointer, we should
/// instead directly load a pointer.
///
/// However, this routine must never change the width of a load or the number of
/// loads as that would introduce a semantic change. This combine is expected to
/// be a semantic no-op which just allows loads to more closely model the types
/// of their consuming operations.
///
/// Currently, we also refuse to change the precise type used for an atomic load
/// or a volatile load. This is debatable, and might be reasonable to change
/// later. However, it is risky in case some backend or other part of LLVM is
/// relying on the exact type loaded to select appropriate atomic operations.
static Instruction *combineLoadToOperationType(InstCombiner &IC, LoadInst &LI) {
  // FIXME: We could probably with some care handle both volatile and atomic
  // loads here but it isn't clear that this is important.
  if (!LI.isSimple())
    return nullptr;

  if (LI.use_empty())
    return nullptr;


  // Fold away bit casts of the loaded value by loading the desired type.
  if (LI.hasOneUse())
    if (auto *BC = dyn_cast<BitCastInst>(LI.user_back())) {
      LoadInst *NewLoad = combineLoadToNewType(IC, LI, BC->getDestTy());
      BC->replaceAllUsesWith(NewLoad);
      IC.EraseInstFromFunction(*BC);
      return &LI;
    }

  // FIXME: We should also canonicalize loads of vectors when their elements are
  // cast to other types.
  return nullptr;
}

Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
  Value *Op = LI.getOperand(0);

  // Try to canonicalize the loaded type.
  if (Instruction *Res = combineLoadToOperationType(*this, LI))
    return Res;

  // Attempt to improve the alignment.
  if (DL) {
    unsigned KnownAlign =
      getOrEnforceKnownAlignment(Op, DL->getPrefTypeAlignment(LI.getType()),
                                 DL, AT, &LI, DT);
    unsigned LoadAlign = LI.getAlignment();
    unsigned EffectiveLoadAlign = LoadAlign != 0 ? LoadAlign :
      DL->getABITypeAlignment(LI.getType());

    if (KnownAlign > EffectiveLoadAlign)
      LI.setAlignment(KnownAlign);
    else if (LoadAlign == 0)
      LI.setAlignment(EffectiveLoadAlign);
  }

  // None of the following transforms are legal for volatile/atomic loads.
  // FIXME: Some of it is okay for atomic loads; needs refactoring.
  if (!LI.isSimple()) return nullptr;

  // Do really simple store-to-load forwarding and load CSE, to catch cases
  // where there are several consecutive memory accesses to the same location,
  // separated by a few arithmetic operations.
  BasicBlock::iterator BBI = &LI;
  if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(), BBI,6))
    return ReplaceInstUsesWith(
        LI, Builder->CreateBitCast(AvailableVal, LI.getType()));

  // load(gep null, ...) -> unreachable
  if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
    const Value *GEPI0 = GEPI->getOperand(0);
    // TODO: Consider a target hook for valid address spaces for this xform.
    if (isa<ConstantPointerNull>(GEPI0) && GEPI->getPointerAddressSpace() == 0){
      // Insert a new store to null instruction before the load to indicate
      // that this code is not reachable.  We do this instead of inserting
      // an unreachable instruction directly because we cannot modify the
      // CFG.
      new StoreInst(UndefValue::get(LI.getType()),
                    Constant::getNullValue(Op->getType()), &LI);
      return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
    }
  }

  // load null/undef -> unreachable
  // TODO: Consider a target hook for valid address spaces for this xform.
  if (isa<UndefValue>(Op) ||
      (isa<ConstantPointerNull>(Op) && LI.getPointerAddressSpace() == 0)) {
    // Insert a new store to null instruction before the load to indicate that
    // this code is not reachable.  We do this instead of inserting an
    // unreachable instruction directly because we cannot modify the CFG.
    new StoreInst(UndefValue::get(LI.getType()),
                  Constant::getNullValue(Op->getType()), &LI);
    return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
  }

  if (Op->hasOneUse()) {
    // Change select and PHI nodes to select values instead of addresses: this
    // helps alias analysis out a lot, allows many others simplifications, and
    // exposes redundancy in the code.
    //
    // Note that we cannot do the transformation unless we know that the
    // introduced loads cannot trap!  Something like this is valid as long as
    // the condition is always false: load (select bool %C, int* null, int* %G),
    // but it would not be valid if we transformed it to load from null
    // unconditionally.
    //
    if (SelectInst *SI = dyn_cast<SelectInst>(Op)) {
      // load (select (Cond, &V1, &V2))  --> select(Cond, load &V1, load &V2).
      unsigned Align = LI.getAlignment();
      if (isSafeToLoadUnconditionally(SI->getOperand(1), SI, Align, DL) &&
          isSafeToLoadUnconditionally(SI->getOperand(2), SI, Align, DL)) {
        LoadInst *V1 = Builder->CreateLoad(SI->getOperand(1),
                                           SI->getOperand(1)->getName()+".val");
        LoadInst *V2 = Builder->CreateLoad(SI->getOperand(2),
                                           SI->getOperand(2)->getName()+".val");
        V1->setAlignment(Align);
        V2->setAlignment(Align);
        return SelectInst::Create(SI->getCondition(), V1, V2);
      }

      // load (select (cond, null, P)) -> load P
      if (Constant *C = dyn_cast<Constant>(SI->getOperand(1)))
        if (C->isNullValue()) {
          LI.setOperand(0, SI->getOperand(2));
          return &LI;
        }

      // load (select (cond, P, null)) -> load P
      if (Constant *C = dyn_cast<Constant>(SI->getOperand(2)))
        if (C->isNullValue()) {
          LI.setOperand(0, SI->getOperand(1));
          return &LI;
        }
    }
  }
  return nullptr;
}

/// InstCombineStoreToCast - Fold store V, (cast P) -> store (cast V), P
/// when possible.  This makes it generally easy to do alias analysis and/or
/// SROA/mem2reg of the memory object.
static Instruction *InstCombineStoreToCast(InstCombiner &IC, StoreInst &SI) {
  User *CI = cast<User>(SI.getOperand(1));
  Value *CastOp = CI->getOperand(0);

  Type *DestPTy = CI->getType()->getPointerElementType();
  PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType());
  if (!SrcTy) return nullptr;

  Type *SrcPTy = SrcTy->getElementType();

  if (!DestPTy->isIntegerTy() && !DestPTy->isPointerTy())
    return nullptr;

  /// NewGEPIndices - If SrcPTy is an aggregate type, we can emit a "noop gep"
  /// to its first element.  This allows us to handle things like:
  ///   store i32 xxx, (bitcast {foo*, float}* %P to i32*)
  /// on 32-bit hosts.
  SmallVector<Value*, 4> NewGEPIndices;

  // If the source is an array, the code below will not succeed.  Check to
  // see if a trivial 'gep P, 0, 0' will help matters.  Only do this for
  // constants.
  if (SrcPTy->isArrayTy() || SrcPTy->isStructTy()) {
    // Index through pointer.
    Constant *Zero = Constant::getNullValue(Type::getInt32Ty(SI.getContext()));
    NewGEPIndices.push_back(Zero);

    while (1) {
      if (StructType *STy = dyn_cast<StructType>(SrcPTy)) {
        if (!STy->getNumElements()) /* Struct can be empty {} */
          break;
        NewGEPIndices.push_back(Zero);
        SrcPTy = STy->getElementType(0);
      } else if (ArrayType *ATy = dyn_cast<ArrayType>(SrcPTy)) {
        NewGEPIndices.push_back(Zero);
        SrcPTy = ATy->getElementType();
      } else {
        break;
      }
    }

    SrcTy = PointerType::get(SrcPTy, SrcTy->getAddressSpace());
  }

  if (!SrcPTy->isIntegerTy() && !SrcPTy->isPointerTy())
    return nullptr;

  // If the pointers point into different address spaces don't do the
  // transformation.
  if (SrcTy->getAddressSpace() != CI->getType()->getPointerAddressSpace())
    return nullptr;

  // If the pointers point to values of different sizes don't do the
  // transformation.
  if (!IC.getDataLayout() ||
      IC.getDataLayout()->getTypeSizeInBits(SrcPTy) !=
      IC.getDataLayout()->getTypeSizeInBits(DestPTy))
    return nullptr;

  // If the pointers point to pointers to different address spaces don't do the
  // transformation. It is not safe to introduce an addrspacecast instruction in
  // this case since, depending on the target, addrspacecast may not be a no-op
  // cast.
  if (SrcPTy->isPointerTy() && DestPTy->isPointerTy() &&
      SrcPTy->getPointerAddressSpace() != DestPTy->getPointerAddressSpace())
    return nullptr;

  // Okay, we are casting from one integer or pointer type to another of
  // the same size.  Instead of casting the pointer before
  // the store, cast the value to be stored.
  Value *NewCast;
  Instruction::CastOps opcode = Instruction::BitCast;
  Type* CastSrcTy = DestPTy;
  Type* CastDstTy = SrcPTy;
  if (CastDstTy->isPointerTy()) {
    if (CastSrcTy->isIntegerTy())
      opcode = Instruction::IntToPtr;
  } else if (CastDstTy->isIntegerTy()) {
    if (CastSrcTy->isPointerTy())
      opcode = Instruction::PtrToInt;
  }

  // SIOp0 is a pointer to aggregate and this is a store to the first field,
  // emit a GEP to index into its first field.
  if (!NewGEPIndices.empty())
    CastOp = IC.Builder->CreateInBoundsGEP(CastOp, NewGEPIndices);

  Value *SIOp0 = SI.getOperand(0);
  NewCast = IC.Builder->CreateCast(opcode, SIOp0, CastDstTy,
                                   SIOp0->getName()+".c");
  SI.setOperand(0, NewCast);
  SI.setOperand(1, CastOp);
  return &SI;
}

/// equivalentAddressValues - Test if A and B will obviously have the same
/// value. This includes recognizing that %t0 and %t1 will have the same
/// value in code like this:
///   %t0 = getelementptr \@a, 0, 3
///   store i32 0, i32* %t0
///   %t1 = getelementptr \@a, 0, 3
///   %t2 = load i32* %t1
///
static bool equivalentAddressValues(Value *A, Value *B) {
  // Test if the values are trivially equivalent.
  if (A == B) return true;

  // Test if the values come form identical arithmetic instructions.
  // This uses isIdenticalToWhenDefined instead of isIdenticalTo because
  // its only used to compare two uses within the same basic block, which
  // means that they'll always either have the same value or one of them
  // will have an undefined value.
  if (isa<BinaryOperator>(A) ||
      isa<CastInst>(A) ||
      isa<PHINode>(A) ||
      isa<GetElementPtrInst>(A))
    if (Instruction *BI = dyn_cast<Instruction>(B))
      if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
        return true;

  // Otherwise they may not be equivalent.
  return false;
}

Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
  Value *Val = SI.getOperand(0);
  Value *Ptr = SI.getOperand(1);

  // Attempt to improve the alignment.
  if (DL) {
    unsigned KnownAlign =
      getOrEnforceKnownAlignment(Ptr, DL->getPrefTypeAlignment(Val->getType()),
                                 DL, AT, &SI, DT);
    unsigned StoreAlign = SI.getAlignment();
    unsigned EffectiveStoreAlign = StoreAlign != 0 ? StoreAlign :
      DL->getABITypeAlignment(Val->getType());

    if (KnownAlign > EffectiveStoreAlign)
      SI.setAlignment(KnownAlign);
    else if (StoreAlign == 0)
      SI.setAlignment(EffectiveStoreAlign);
  }

  // Don't hack volatile/atomic stores.
  // FIXME: Some bits are legal for atomic stores; needs refactoring.
  if (!SI.isSimple()) return nullptr;

  // If the RHS is an alloca with a single use, zapify the store, making the
  // alloca dead.
  if (Ptr->hasOneUse()) {
    if (isa<AllocaInst>(Ptr))
      return EraseInstFromFunction(SI);
    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) {
      if (isa<AllocaInst>(GEP->getOperand(0))) {
        if (GEP->getOperand(0)->hasOneUse())
          return EraseInstFromFunction(SI);
      }
    }
  }

  // Do really simple DSE, to catch cases where there are several consecutive
  // stores to the same location, separated by a few arithmetic operations. This
  // situation often occurs with bitfield accesses.
  BasicBlock::iterator BBI = &SI;
  for (unsigned ScanInsts = 6; BBI != SI.getParent()->begin() && ScanInsts;
       --ScanInsts) {
    --BBI;
    // Don't count debug info directives, lest they affect codegen,
    // and we skip pointer-to-pointer bitcasts, which are NOPs.
    if (isa<DbgInfoIntrinsic>(BBI) ||
        (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) {
      ScanInsts++;
      continue;
    }

    if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) {
      // Prev store isn't volatile, and stores to the same location?
      if (PrevSI->isSimple() && equivalentAddressValues(PrevSI->getOperand(1),
                                                        SI.getOperand(1))) {
        ++NumDeadStore;
        ++BBI;
        EraseInstFromFunction(*PrevSI);
        continue;
      }
      break;
    }

    // If this is a load, we have to stop.  However, if the loaded value is from
    // the pointer we're loading and is producing the pointer we're storing,
    // then *this* store is dead (X = load P; store X -> P).
    if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
      if (LI == Val && equivalentAddressValues(LI->getOperand(0), Ptr) &&
          LI->isSimple())
        return EraseInstFromFunction(SI);

      // Otherwise, this is a load from some other location.  Stores before it
      // may not be dead.
      break;
    }

    // Don't skip over loads or things that can modify memory.
    if (BBI->mayWriteToMemory() || BBI->mayReadFromMemory())
      break;
  }

  // store X, null    -> turns into 'unreachable' in SimplifyCFG
  if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) {
    if (!isa<UndefValue>(Val)) {
      SI.setOperand(0, UndefValue::get(Val->getType()));
      if (Instruction *U = dyn_cast<Instruction>(Val))
        Worklist.Add(U);  // Dropped a use.
    }
    return nullptr;  // Do not modify these!
  }

  // store undef, Ptr -> noop
  if (isa<UndefValue>(Val))
    return EraseInstFromFunction(SI);

  // If the pointer destination is a cast, see if we can fold the cast into the
  // source instead.
  if (isa<CastInst>(Ptr))
    if (Instruction *Res = InstCombineStoreToCast(*this, SI))
      return Res;
  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
    if (CE->isCast())
      if (Instruction *Res = InstCombineStoreToCast(*this, SI))
        return Res;


  // If this store is the last instruction in the basic block (possibly
  // excepting debug info instructions), and if the block ends with an
  // unconditional branch, try to move it to the successor block.
  BBI = &SI;
  do {
    ++BBI;
  } while (isa<DbgInfoIntrinsic>(BBI) ||
           (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy()));
  if (BranchInst *BI = dyn_cast<BranchInst>(BBI))
    if (BI->isUnconditional())
      if (SimplifyStoreAtEndOfBlock(SI))
        return nullptr;  // xform done!

  return nullptr;
}

/// SimplifyStoreAtEndOfBlock - Turn things like:
///   if () { *P = v1; } else { *P = v2 }
/// into a phi node with a store in the successor.
///
/// Simplify things like:
///   *P = v1; if () { *P = v2; }
/// into a phi node with a store in the successor.
///
bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) {
  BasicBlock *StoreBB = SI.getParent();

  // Check to see if the successor block has exactly two incoming edges.  If
  // so, see if the other predecessor contains a store to the same location.
  // if so, insert a PHI node (if needed) and move the stores down.
  BasicBlock *DestBB = StoreBB->getTerminator()->getSuccessor(0);

  // Determine whether Dest has exactly two predecessors and, if so, compute
  // the other predecessor.
  pred_iterator PI = pred_begin(DestBB);
  BasicBlock *P = *PI;
  BasicBlock *OtherBB = nullptr;

  if (P != StoreBB)
    OtherBB = P;

  if (++PI == pred_end(DestBB))
    return false;

  P = *PI;
  if (P != StoreBB) {
    if (OtherBB)
      return false;
    OtherBB = P;
  }
  if (++PI != pred_end(DestBB))
    return false;

  // Bail out if all the relevant blocks aren't distinct (this can happen,
  // for example, if SI is in an infinite loop)
  if (StoreBB == DestBB || OtherBB == DestBB)
    return false;

  // Verify that the other block ends in a branch and is not otherwise empty.
  BasicBlock::iterator BBI = OtherBB->getTerminator();
  BranchInst *OtherBr = dyn_cast<BranchInst>(BBI);
  if (!OtherBr || BBI == OtherBB->begin())
    return false;

  // If the other block ends in an unconditional branch, check for the 'if then
  // else' case.  there is an instruction before the branch.
  StoreInst *OtherStore = nullptr;
  if (OtherBr->isUnconditional()) {
    --BBI;
    // Skip over debugging info.
    while (isa<DbgInfoIntrinsic>(BBI) ||
           (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) {
      if (BBI==OtherBB->begin())
        return false;
      --BBI;
    }
    // If this isn't a store, isn't a store to the same location, or is not the
    // right kind of store, bail out.
    OtherStore = dyn_cast<StoreInst>(BBI);
    if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) ||
        !SI.isSameOperationAs(OtherStore))
      return false;
  } else {
    // Otherwise, the other block ended with a conditional branch. If one of the
    // destinations is StoreBB, then we have the if/then case.
    if (OtherBr->getSuccessor(0) != StoreBB &&
        OtherBr->getSuccessor(1) != StoreBB)
      return false;

    // Okay, we know that OtherBr now goes to Dest and StoreBB, so this is an
    // if/then triangle.  See if there is a store to the same ptr as SI that
    // lives in OtherBB.
    for (;; --BBI) {
      // Check to see if we find the matching store.
      if ((OtherStore = dyn_cast<StoreInst>(BBI))) {
        if (OtherStore->getOperand(1) != SI.getOperand(1) ||
            !SI.isSameOperationAs(OtherStore))
          return false;
        break;
      }
      // If we find something that may be using or overwriting the stored
      // value, or if we run out of instructions, we can't do the xform.
      if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() ||
          BBI == OtherBB->begin())
        return false;
    }

    // In order to eliminate the store in OtherBr, we have to
    // make sure nothing reads or overwrites the stored value in
    // StoreBB.
    for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) {
      // FIXME: This should really be AA driven.
      if (I->mayReadFromMemory() || I->mayWriteToMemory())
        return false;
    }
  }

  // Insert a PHI node now if we need it.
  Value *MergedVal = OtherStore->getOperand(0);
  if (MergedVal != SI.getOperand(0)) {
    PHINode *PN = PHINode::Create(MergedVal->getType(), 2, "storemerge");
    PN->addIncoming(SI.getOperand(0), SI.getParent());
    PN->addIncoming(OtherStore->getOperand(0), OtherBB);
    MergedVal = InsertNewInstBefore(PN, DestBB->front());
  }

  // Advance to a place where it is safe to insert the new store and
  // insert it.
  BBI = DestBB->getFirstInsertionPt();
  StoreInst *NewSI = new StoreInst(MergedVal, SI.getOperand(1),
                                   SI.isVolatile(),
                                   SI.getAlignment(),
                                   SI.getOrdering(),
                                   SI.getSynchScope());
  InsertNewInstBefore(NewSI, *BBI);
  NewSI->setDebugLoc(OtherStore->getDebugLoc());

  // If the two stores had AA tags, merge them.
  AAMDNodes AATags;
  SI.getAAMetadata(AATags);
  if (AATags) {
    OtherStore->getAAMetadata(AATags, /* Merge = */ true);
    NewSI->setAAMetadata(AATags);
  }

  // Nuke the old stores.
  EraseInstFromFunction(SI);
  EraseInstFromFunction(*OtherStore);
  return true;
}