llvm.org GIT mirror llvm / 7716d65 lib / Transforms / Scalar / MergeICmps.cpp
7716d65

Tree @7716d65 (Download .tar.gz)

MergeICmps.cpp @7716d65raw · 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
//===- MergeICmps.cpp - Optimize chains of integer comparisons ------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass turns chains of integer comparisons into memcmp (the memcmp is
// later typically inlined as a chain of efficient hardware comparisons). This
// typically benefits c++ member or nonmember operator==().
//
// The basic idea is to replace a longer chain of integer comparisons loaded
// from contiguous memory locations into a shorter chain of larger integer
// comparisons. Benefits are double:
//  - There are less jumps, and therefore less opportunities for mispredictions
//    and I-cache misses.
//  - Code size is smaller, both because jumps are removed and because the
//    encoding of a 2*n byte compare is smaller than that of two n-byte
//    compares.
//
// Example:
//
//  struct S {
//    int a;
//    char b;
//    char c;
//    uint16_t d;
//    bool operator==(const S& o) const {
//      return a == o.a && b == o.b && c == o.c && d == o.d;
//    }
//  };
//
//  Is optimized as :
//
//    bool S::operator==(const S& o) const {
//      return memcmp(this, &o, 8) == 0;
//    }
//
//  Which will later be expanded (ExpandMemCmp) as a single 8-bytes icmp.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BuildLibCalls.h"
#include <algorithm>
#include <numeric>
#include <utility>
#include <vector>

using namespace llvm;

namespace {

#define DEBUG_TYPE "mergeicmps"

// Returns true if the instruction is a simple load or a simple store
static bool isSimpleLoadOrStore(const Instruction *I) {
  if (const LoadInst *LI = dyn_cast<LoadInst>(I))
    return LI->isSimple();
  if (const StoreInst *SI = dyn_cast<StoreInst>(I))
    return SI->isSimple();
  return false;
}

// A BCE atom "Binary Compare Expression Atom" represents an integer load
// that is a constant offset from a base value, e.g. `a` or `o.c` in the example
// at the top.
struct BCEAtom {
  BCEAtom() = default;
  BCEAtom(GetElementPtrInst *GEP, LoadInst *LoadI, int BaseId, APInt Offset)
      : GEP(GEP), LoadI(LoadI), BaseId(BaseId), Offset(Offset) {}

  // We want to order BCEAtoms by (Base, Offset). However we cannot use
  // the pointer values for Base because these are non-deterministic.
  // To make sure that the sort order is stable, we first assign to each atom
  // base value an index based on its order of appearance in the chain of
  // comparisons. We call this index `BaseOrdering`. For example, for:
  //    b[3] == c[2] && a[1] == d[1] && b[4] == c[3]
  //    |  block 1 |    |  block 2 |    |  block 3 |
  // b gets assigned index 0 and a index 1, because b appears as LHS in block 1,
  // which is before block 2.
  // We then sort by (BaseOrdering[LHS.Base()], LHS.Offset), which is stable.
  bool operator<(const BCEAtom &O) const {
    return BaseId != O.BaseId ? BaseId < O.BaseId : Offset.slt(O.Offset);
  }

  GetElementPtrInst *GEP = nullptr;
  LoadInst *LoadI = nullptr;
  unsigned BaseId = 0;
  APInt Offset;
};

// A class that assigns increasing ids to values in the order in which they are
// seen. See comment in `BCEAtom::operator<()``.
class BaseIdentifier {
public:
  // Returns the id for value `Base`, after assigning one if `Base` has not been
  // seen before.
  int getBaseId(const Value *Base) {
    assert(Base && "invalid base");
    const auto Insertion = BaseToIndex.try_emplace(Base, Order);
    if (Insertion.second)
      ++Order;
    return Insertion.first->second;
  }

private:
  unsigned Order = 1;
  DenseMap<const Value*, int> BaseToIndex;
};

// If this value is a load from a constant offset w.r.t. a base address, and
// there are no other users of the load or address, returns the base address and
// the offset.
BCEAtom visitICmpLoadOperand(Value *const Val, BaseIdentifier &BaseId) {
  auto *const LoadI = dyn_cast<LoadInst>(Val);
  if (!LoadI)
    return {};
  LLVM_DEBUG(dbgs() << "load\n");
  if (LoadI->isUsedOutsideOfBlock(LoadI->getParent())) {
    LLVM_DEBUG(dbgs() << "used outside of block\n");
    return {};
  }
  // Do not optimize atomic loads to non-atomic memcmp
  if (!LoadI->isSimple()) {
    LLVM_DEBUG(dbgs() << "volatile or atomic\n");
    return {};
  }
  Value *const Addr = LoadI->getOperand(0);
  auto *const GEP = dyn_cast<GetElementPtrInst>(Addr);
  if (!GEP)
    return {};
  LLVM_DEBUG(dbgs() << "GEP\n");
  if (GEP->isUsedOutsideOfBlock(LoadI->getParent())) {
    LLVM_DEBUG(dbgs() << "used outside of block\n");
    return {};
  }
  const auto &DL = GEP->getModule()->getDataLayout();
  if (!isDereferenceablePointer(GEP, DL)) {
    LLVM_DEBUG(dbgs() << "not dereferenceable\n");
    // We need to make sure that we can do comparison in any order, so we
    // require memory to be unconditionnally dereferencable.
    return {};
  }
  APInt Offset = APInt(DL.getPointerTypeSizeInBits(GEP->getType()), 0);
  if (!GEP->accumulateConstantOffset(DL, Offset))
    return {};
  return BCEAtom(GEP, LoadI, BaseId.getBaseId(GEP->getPointerOperand()),
                 Offset);
}

// A basic block with a comparison between two BCE atoms, e.g. `a == o.a` in the
// example at the top.
// The block might do extra work besides the atom comparison, in which case
// doesOtherWork() returns true. Under some conditions, the block can be
// split into the atom comparison part and the "other work" part
// (see canSplit()).
// Note: the terminology is misleading: the comparison is symmetric, so there
// is no real {l/r}hs. What we want though is to have the same base on the
// left (resp. right), so that we can detect consecutive loads. To ensure this
// we put the smallest atom on the left.
class BCECmpBlock {
 public:
  BCECmpBlock() {}

  BCECmpBlock(BCEAtom L, BCEAtom R, int SizeBits)
      : Lhs_(L), Rhs_(R), SizeBits_(SizeBits) {
    if (Rhs_ < Lhs_) std::swap(Rhs_, Lhs_);
  }

  bool IsValid() const { return Lhs_.BaseId != 0 && Rhs_.BaseId != 0; }

  // Assert the block is consistent: If valid, it should also have
  // non-null members besides Lhs_ and Rhs_.
  void AssertConsistent() const {
    if (IsValid()) {
      assert(BB);
      assert(CmpI);
      assert(BranchI);
    }
  }

  const BCEAtom &Lhs() const { return Lhs_; }
  const BCEAtom &Rhs() const { return Rhs_; }
  int SizeBits() const { return SizeBits_; }

  // Returns true if the block does other works besides comparison.
  bool doesOtherWork() const;

  // Returns true if the non-BCE-cmp instructions can be separated from BCE-cmp
  // instructions in the block.
  bool canSplit(AliasAnalysis *AA) const;

  // Return true if this all the relevant instructions in the BCE-cmp-block can
  // be sunk below this instruction. By doing this, we know we can separate the
  // BCE-cmp-block instructions from the non-BCE-cmp-block instructions in the
  // block.
  bool canSinkBCECmpInst(const Instruction *, DenseSet<Instruction *> &,
                         AliasAnalysis *AA) const;

  // We can separate the BCE-cmp-block instructions and the non-BCE-cmp-block
  // instructions. Split the old block and move all non-BCE-cmp-insts into the
  // new parent block.
  void split(BasicBlock *NewParent, AliasAnalysis *AA) const;

  // The basic block where this comparison happens.
  BasicBlock *BB = nullptr;
  // The ICMP for this comparison.
  ICmpInst *CmpI = nullptr;
  // The terminating branch.
  BranchInst *BranchI = nullptr;
  // The block requires splitting.
  bool RequireSplit = false;

private:
  BCEAtom Lhs_;
  BCEAtom Rhs_;
  int SizeBits_ = 0;
};

bool BCECmpBlock::canSinkBCECmpInst(const Instruction *Inst,
                                    DenseSet<Instruction *> &BlockInsts,
                                    AliasAnalysis *AA) const {
  // If this instruction has side effects and its in middle of the BCE cmp block
  // instructions, then bail for now.
  if (Inst->mayHaveSideEffects()) {
    // Bail if this is not a simple load or store
    if (!isSimpleLoadOrStore(Inst))
      return false;
    // Disallow stores that might alias the BCE operands
    MemoryLocation LLoc = MemoryLocation::get(Lhs_.LoadI);
    MemoryLocation RLoc = MemoryLocation::get(Rhs_.LoadI);
    if (isModSet(AA->getModRefInfo(Inst, LLoc)) ||
        isModSet(AA->getModRefInfo(Inst, RLoc)))
        return false;
  }
  // Make sure this instruction does not use any of the BCE cmp block
  // instructions as operand.
  for (auto BI : BlockInsts) {
    if (is_contained(Inst->operands(), BI))
      return false;
  }
  return true;
}

void BCECmpBlock::split(BasicBlock *NewParent, AliasAnalysis *AA) const {
  DenseSet<Instruction *> BlockInsts(
      {Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
  llvm::SmallVector<Instruction *, 4> OtherInsts;
  for (Instruction &Inst : *BB) {
    if (BlockInsts.count(&Inst))
      continue;
      assert(canSinkBCECmpInst(&Inst, BlockInsts, AA) &&
             "Split unsplittable block");
    // This is a non-BCE-cmp-block instruction. And it can be separated
    // from the BCE-cmp-block instruction.
    OtherInsts.push_back(&Inst);
  }

  // Do the actual spliting.
  for (Instruction *Inst : reverse(OtherInsts)) {
    Inst->moveBefore(&*NewParent->begin());
  }
}

bool BCECmpBlock::canSplit(AliasAnalysis *AA) const {
  DenseSet<Instruction *> BlockInsts(
      {Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
  for (Instruction &Inst : *BB) {
    if (!BlockInsts.count(&Inst)) {
      if (!canSinkBCECmpInst(&Inst, BlockInsts, AA))
        return false;
    }
  }
  return true;
}

bool BCECmpBlock::doesOtherWork() const {
  AssertConsistent();
  // All the instructions we care about in the BCE cmp block.
  DenseSet<Instruction *> BlockInsts(
      {Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
  // TODO(courbet): Can we allow some other things ? This is very conservative.
  // We might be able to get away with anything does not have any side
  // effects outside of the basic block.
  // Note: The GEPs and/or loads are not necessarily in the same block.
  for (const Instruction &Inst : *BB) {
    if (!BlockInsts.count(&Inst))
      return true;
  }
  return false;
}

// Visit the given comparison. If this is a comparison between two valid
// BCE atoms, returns the comparison.
BCECmpBlock visitICmp(const ICmpInst *const CmpI,
                      const ICmpInst::Predicate ExpectedPredicate,
                      BaseIdentifier &BaseId) {
  // The comparison can only be used once:
  //  - For intermediate blocks, as a branch condition.
  //  - For the final block, as an incoming value for the Phi.
  // If there are any other uses of the comparison, we cannot merge it with
  // other comparisons as we would create an orphan use of the value.
  if (!CmpI->hasOneUse()) {
    LLVM_DEBUG(dbgs() << "cmp has several uses\n");
    return {};
  }
  if (CmpI->getPredicate() != ExpectedPredicate)
    return {};
  LLVM_DEBUG(dbgs() << "cmp "
                    << (ExpectedPredicate == ICmpInst::ICMP_EQ ? "eq" : "ne")
                    << "\n");
  auto Lhs = visitICmpLoadOperand(CmpI->getOperand(0), BaseId);
  if (!Lhs.BaseId)
    return {};
  auto Rhs = visitICmpLoadOperand(CmpI->getOperand(1), BaseId);
  if (!Rhs.BaseId)
    return {};
  const auto &DL = CmpI->getModule()->getDataLayout();
  return BCECmpBlock(std::move(Lhs), std::move(Rhs),
                     DL.getTypeSizeInBits(CmpI->getOperand(0)->getType()));
}

// Visit the given comparison block. If this is a comparison between two valid
// BCE atoms, returns the comparison.
BCECmpBlock visitCmpBlock(Value *const Val, BasicBlock *const Block,
                          const BasicBlock *const PhiBlock,
                          BaseIdentifier &BaseId) {
  if (Block->empty()) return {};
  auto *const BranchI = dyn_cast<BranchInst>(Block->getTerminator());
  if (!BranchI) return {};
  LLVM_DEBUG(dbgs() << "branch\n");
  if (BranchI->isUnconditional()) {
    // In this case, we expect an incoming value which is the result of the
    // comparison. This is the last link in the chain of comparisons (note
    // that this does not mean that this is the last incoming value, blocks
    // can be reordered).
    auto *const CmpI = dyn_cast<ICmpInst>(Val);
    if (!CmpI) return {};
    LLVM_DEBUG(dbgs() << "icmp\n");
    auto Result = visitICmp(CmpI, ICmpInst::ICMP_EQ, BaseId);
    Result.CmpI = CmpI;
    Result.BranchI = BranchI;
    return Result;
  } else {
    // In this case, we expect a constant incoming value (the comparison is
    // chained).
    const auto *const Const = dyn_cast<ConstantInt>(Val);
    LLVM_DEBUG(dbgs() << "const\n");
    if (!Const->isZero()) return {};
    LLVM_DEBUG(dbgs() << "false\n");
    auto *const CmpI = dyn_cast<ICmpInst>(BranchI->getCondition());
    if (!CmpI) return {};
    LLVM_DEBUG(dbgs() << "icmp\n");
    assert(BranchI->getNumSuccessors() == 2 && "expecting a cond branch");
    BasicBlock *const FalseBlock = BranchI->getSuccessor(1);
    auto Result = visitICmp(
        CmpI, FalseBlock == PhiBlock ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE,
        BaseId);
    Result.CmpI = CmpI;
    Result.BranchI = BranchI;
    return Result;
  }
  return {};
}

static inline void enqueueBlock(std::vector<BCECmpBlock> &Comparisons,
                                BCECmpBlock &Comparison) {
  LLVM_DEBUG(dbgs() << "Block '" << Comparison.BB->getName()
                    << "': Found cmp of " << Comparison.SizeBits()
                    << " bits between " << Comparison.Lhs().BaseId << " + "
                    << Comparison.Lhs().Offset << " and "
                    << Comparison.Rhs().BaseId << " + "
                    << Comparison.Rhs().Offset << "\n");
  LLVM_DEBUG(dbgs() << "\n");
  Comparisons.push_back(Comparison);
}

// A chain of comparisons.
class BCECmpChain {
 public:
  BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi,
              AliasAnalysis *AA);

  int size() const { return Comparisons_.size(); }

#ifdef MERGEICMPS_DOT_ON
  void dump() const;
#endif  // MERGEICMPS_DOT_ON

  bool simplify(const TargetLibraryInfo *const TLI, AliasAnalysis *AA);

 private:
  static bool IsContiguous(const BCECmpBlock &First,
                           const BCECmpBlock &Second) {
    return First.Lhs().BaseId == Second.Lhs().BaseId &&
           First.Rhs().BaseId == Second.Rhs().BaseId &&
           First.Lhs().Offset + First.SizeBits() / 8 == Second.Lhs().Offset &&
           First.Rhs().Offset + First.SizeBits() / 8 == Second.Rhs().Offset;
  }

  // Merges the given comparison blocks into one memcmp block and update
  // branches. Comparisons are assumed to be continguous. If NextBBInChain is
  // null, the merged block will link to the phi block.
  void mergeComparisons(ArrayRef<BCECmpBlock> Comparisons,
                        BasicBlock *const NextBBInChain, PHINode &Phi,
                        const TargetLibraryInfo *const TLI, AliasAnalysis *AA);

  PHINode &Phi_;
  std::vector<BCECmpBlock> Comparisons_;
  // The original entry block (before sorting);
  BasicBlock *EntryBlock_;
};

BCECmpChain::BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi,
                         AliasAnalysis *AA)
    : Phi_(Phi) {
  assert(!Blocks.empty() && "a chain should have at least one block");
  // Now look inside blocks to check for BCE comparisons.
  std::vector<BCECmpBlock> Comparisons;
  BaseIdentifier BaseId;
  for (size_t BlockIdx = 0; BlockIdx < Blocks.size(); ++BlockIdx) {
    BasicBlock *const Block = Blocks[BlockIdx];
    assert(Block && "invalid block");
    BCECmpBlock Comparison = visitCmpBlock(Phi.getIncomingValueForBlock(Block),
                                           Block, Phi.getParent(), BaseId);
    Comparison.BB = Block;
    if (!Comparison.IsValid()) {
      LLVM_DEBUG(dbgs() << "chain with invalid BCECmpBlock, no merge.\n");
      return;
    }
    if (Comparison.doesOtherWork()) {
      LLVM_DEBUG(dbgs() << "block '" << Comparison.BB->getName()
                        << "' does extra work besides compare\n");
      if (Comparisons.empty()) {
        // This is the initial block in the chain, in case this block does other
        // work, we can try to split the block and move the irrelevant
        // instructions to the predecessor.
        //
        // If this is not the initial block in the chain, splitting it wont
        // work.
        //
        // As once split, there will still be instructions before the BCE cmp
        // instructions that do other work in program order, i.e. within the
        // chain before sorting. Unless we can abort the chain at this point
        // and start anew.
        //
        // NOTE: we only handle block with single predecessor for now.
        if (Comparison.canSplit(AA)) {
          LLVM_DEBUG(dbgs()
                     << "Split initial block '" << Comparison.BB->getName()
                     << "' that does extra work besides compare\n");
          Comparison.RequireSplit = true;
          enqueueBlock(Comparisons, Comparison);
        } else {
          LLVM_DEBUG(dbgs()
                     << "ignoring initial block '" << Comparison.BB->getName()
                     << "' that does extra work besides compare\n");
        }
        continue;
      }
      // TODO(courbet): Right now we abort the whole chain. We could be
      // merging only the blocks that don't do other work and resume the
      // chain from there. For example:
      //  if (a[0] == b[0]) {  // bb1
      //    if (a[1] == b[1]) {  // bb2
      //      some_value = 3; //bb3
      //      if (a[2] == b[2]) { //bb3
      //        do a ton of stuff  //bb4
      //      }
      //    }
      //  }
      //
      // This is:
      //
      // bb1 --eq--> bb2 --eq--> bb3* -eq--> bb4 --+
      //  \            \           \               \
      //   ne           ne          ne              \
      //    \            \           \               v
      //     +------------+-----------+----------> bb_phi
      //
      // We can only merge the first two comparisons, because bb3* does
      // "other work" (setting some_value to 3).
      // We could still merge bb1 and bb2 though.
      return;
    }
    enqueueBlock(Comparisons, Comparison);
  }

  // It is possible we have no suitable comparison to merge.
  if (Comparisons.empty()) {
    LLVM_DEBUG(dbgs() << "chain with no BCE basic blocks, no merge\n");
    return;
  }
  EntryBlock_ = Comparisons[0].BB;
  Comparisons_ = std::move(Comparisons);
#ifdef MERGEICMPS_DOT_ON
  errs() << "BEFORE REORDERING:\n\n";
  dump();
#endif  // MERGEICMPS_DOT_ON
  // Reorder blocks by LHS. We can do that without changing the
  // semantics because we are only accessing dereferencable memory.
  llvm::sort(Comparisons_,
             [](const BCECmpBlock &LhsBlock, const BCECmpBlock &RhsBlock) {
               return LhsBlock.Lhs() < RhsBlock.Lhs();
             });
#ifdef MERGEICMPS_DOT_ON
  errs() << "AFTER REORDERING:\n\n";
  dump();
#endif  // MERGEICMPS_DOT_ON
}

#ifdef MERGEICMPS_DOT_ON
void BCECmpChain::dump() const {
  errs() << "digraph dag {\n";
  errs() << " graph [bgcolor=transparent];\n";
  errs() << " node [color=black,style=filled,fillcolor=lightyellow];\n";
  errs() << " edge [color=black];\n";
  for (size_t I = 0; I < Comparisons_.size(); ++I) {
    const auto &Comparison = Comparisons_[I];
    errs() << " \"" << I << "\" [label=\"%"
           << Comparison.Lhs().Base()->getName() << " + "
           << Comparison.Lhs().Offset << " == %"
           << Comparison.Rhs().Base()->getName() << " + "
           << Comparison.Rhs().Offset << " (" << (Comparison.SizeBits() / 8)
           << " bytes)\"];\n";
    const Value *const Val = Phi_.getIncomingValueForBlock(Comparison.BB);
    if (I > 0) errs() << " \"" << (I - 1) << "\" -> \"" << I << "\";\n";
    errs() << " \"" << I << "\" -> \"Phi\" [label=\"" << *Val << "\"];\n";
  }
  errs() << " \"Phi\" [label=\"Phi\"];\n";
  errs() << "}\n\n";
}
#endif  // MERGEICMPS_DOT_ON

bool BCECmpChain::simplify(const TargetLibraryInfo *const TLI,
                           AliasAnalysis *AA) {
  // First pass to check if there is at least one merge. If not, we don't do
  // anything and we keep analysis passes intact.
  {
    bool AtLeastOneMerged = false;
    for (size_t I = 1; I < Comparisons_.size(); ++I) {
      if (IsContiguous(Comparisons_[I - 1], Comparisons_[I])) {
        AtLeastOneMerged = true;
        break;
      }
    }
    if (!AtLeastOneMerged) return false;
  }

  // Remove phi references to comparison blocks, they will be rebuilt as we
  // merge the blocks.
  for (const auto &Comparison : Comparisons_) {
    Phi_.removeIncomingValue(Comparison.BB, false);
  }

  // If entry block is part of the chain, we need to make the first block
  // of the chain the new entry block of the function.
  BasicBlock *Entry = &Comparisons_[0].BB->getParent()->getEntryBlock();
  for (size_t I = 1; I < Comparisons_.size(); ++I) {
    if (Entry == Comparisons_[I].BB) {
      BasicBlock *NEntryBB = BasicBlock::Create(Entry->getContext(), "",
                                                Entry->getParent(), Entry);
      BranchInst::Create(Entry, NEntryBB);
      break;
    }
  }

  // Point the predecessors of the chain to the first comparison block (which is
  // the new entry point) and update the entry block of the chain.
  if (EntryBlock_ != Comparisons_[0].BB) {
    EntryBlock_->replaceAllUsesWith(Comparisons_[0].BB);
    EntryBlock_ = Comparisons_[0].BB;
  }

  // Effectively merge blocks.
  int NumMerged = 1;
  for (size_t I = 1; I < Comparisons_.size(); ++I) {
    if (IsContiguous(Comparisons_[I - 1], Comparisons_[I])) {
      ++NumMerged;
    } else {
      // Merge all previous comparisons and start a new merge block.
      mergeComparisons(
          makeArrayRef(Comparisons_).slice(I - NumMerged, NumMerged),
          Comparisons_[I].BB, Phi_, TLI, AA);
      NumMerged = 1;
    }
  }
  mergeComparisons(makeArrayRef(Comparisons_)
                       .slice(Comparisons_.size() - NumMerged, NumMerged),
                   nullptr, Phi_, TLI, AA);

  return true;
}

void BCECmpChain::mergeComparisons(ArrayRef<BCECmpBlock> Comparisons,
                                   BasicBlock *const NextBBInChain,
                                   PHINode &Phi,
                                   const TargetLibraryInfo *const TLI,
                                   AliasAnalysis *AA) {
  assert(!Comparisons.empty());
  const auto &FirstComparison = *Comparisons.begin();
  BasicBlock *const BB = FirstComparison.BB;
  LLVMContext &Context = BB->getContext();

  if (Comparisons.size() >= 2) {
    // If there is one block that requires splitting, we do it now, i.e.
    // just before we know we will collapse the chain. The instructions
    // can be executed before any of the instructions in the chain.
    auto C = std::find_if(Comparisons.begin(), Comparisons.end(),
                          [](const BCECmpBlock &B) { return B.RequireSplit; });
    if (C != Comparisons.end())
      C->split(EntryBlock_, AA);

    LLVM_DEBUG(dbgs() << "Merging " << Comparisons.size() << " comparisons\n");
    const auto TotalSize =
        std::accumulate(Comparisons.begin(), Comparisons.end(), 0,
                        [](int Size, const BCECmpBlock &C) {
                          return Size + C.SizeBits();
                        }) /
        8;

    // Incoming edges do not need to be updated, and both GEPs are already
    // computing the right address, we just need to:
    //   - replace the two loads and the icmp with the memcmp
    //   - update the branch
    //   - update the incoming values in the phi.
    FirstComparison.BranchI->eraseFromParent();
    FirstComparison.CmpI->eraseFromParent();
    FirstComparison.Lhs().LoadI->eraseFromParent();
    FirstComparison.Rhs().LoadI->eraseFromParent();

    IRBuilder<> Builder(BB);
    const auto &DL = Phi.getModule()->getDataLayout();
    Value *const MemCmpCall = emitMemCmp(
        FirstComparison.Lhs().GEP, FirstComparison.Rhs().GEP,
        ConstantInt::get(DL.getIntPtrType(Context), TotalSize),
        Builder, DL, TLI);
    Value *const MemCmpIsZero = Builder.CreateICmpEQ(
        MemCmpCall, ConstantInt::get(Type::getInt32Ty(Context), 0));

    // Add a branch to the next basic block in the chain.
    if (NextBBInChain) {
      Builder.CreateCondBr(MemCmpIsZero, NextBBInChain, Phi.getParent());
      Phi.addIncoming(ConstantInt::getFalse(Context), BB);
    } else {
      Builder.CreateBr(Phi.getParent());
      Phi.addIncoming(MemCmpIsZero, BB);
    }

    // Delete merged blocks.
    for (size_t I = 1; I < Comparisons.size(); ++I) {
      BasicBlock *CBB = Comparisons[I].BB;
      CBB->replaceAllUsesWith(BB);
      CBB->eraseFromParent();
    }
  } else {
    assert(Comparisons.size() == 1);
    // There are no blocks to merge, but we still need to update the branches.
    LLVM_DEBUG(dbgs() << "Only one comparison, updating branches\n");
    if (NextBBInChain) {
      if (FirstComparison.BranchI->isConditional()) {
        LLVM_DEBUG(dbgs() << "conditional -> conditional\n");
        // Just update the "true" target, the "false" target should already be
        // the phi block.
        assert(FirstComparison.BranchI->getSuccessor(1) == Phi.getParent());
        FirstComparison.BranchI->setSuccessor(0, NextBBInChain);
        Phi.addIncoming(ConstantInt::getFalse(Context), BB);
      } else {
        LLVM_DEBUG(dbgs() << "unconditional -> conditional\n");
        // Replace the unconditional branch by a conditional one.
        FirstComparison.BranchI->eraseFromParent();
        IRBuilder<> Builder(BB);
        Builder.CreateCondBr(FirstComparison.CmpI, NextBBInChain,
                             Phi.getParent());
        Phi.addIncoming(FirstComparison.CmpI, BB);
      }
    } else {
      if (FirstComparison.BranchI->isConditional()) {
        LLVM_DEBUG(dbgs() << "conditional -> unconditional\n");
        // Replace the conditional branch by an unconditional one.
        FirstComparison.BranchI->eraseFromParent();
        IRBuilder<> Builder(BB);
        Builder.CreateBr(Phi.getParent());
        Phi.addIncoming(FirstComparison.CmpI, BB);
      } else {
        LLVM_DEBUG(dbgs() << "unconditional -> unconditional\n");
        Phi.addIncoming(FirstComparison.CmpI, BB);
      }
    }
  }
}

std::vector<BasicBlock *> getOrderedBlocks(PHINode &Phi,
                                           BasicBlock *const LastBlock,
                                           int NumBlocks) {
  // Walk up from the last block to find other blocks.
  std::vector<BasicBlock *> Blocks(NumBlocks);
  assert(LastBlock && "invalid last block");
  BasicBlock *CurBlock = LastBlock;
  for (int BlockIndex = NumBlocks - 1; BlockIndex > 0; --BlockIndex) {
    if (CurBlock->hasAddressTaken()) {
      // Somebody is jumping to the block through an address, all bets are
      // off.
      LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
                        << " has its address taken\n");
      return {};
    }
    Blocks[BlockIndex] = CurBlock;
    auto *SinglePredecessor = CurBlock->getSinglePredecessor();
    if (!SinglePredecessor) {
      // The block has two or more predecessors.
      LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
                        << " has two or more predecessors\n");
      return {};
    }
    if (Phi.getBasicBlockIndex(SinglePredecessor) < 0) {
      // The block does not link back to the phi.
      LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
                        << " does not link back to the phi\n");
      return {};
    }
    CurBlock = SinglePredecessor;
  }
  Blocks[0] = CurBlock;
  return Blocks;
}

bool processPhi(PHINode &Phi, const TargetLibraryInfo *const TLI,
                AliasAnalysis *AA) {
  LLVM_DEBUG(dbgs() << "processPhi()\n");
  if (Phi.getNumIncomingValues() <= 1) {
    LLVM_DEBUG(dbgs() << "skip: only one incoming value in phi\n");
    return false;
  }
  // We are looking for something that has the following structure:
  //   bb1 --eq--> bb2 --eq--> bb3 --eq--> bb4 --+
  //     \            \           \               \
  //      ne           ne          ne              \
  //       \            \           \               v
  //        +------------+-----------+----------> bb_phi
  //
  //  - The last basic block (bb4 here) must branch unconditionally to bb_phi.
  //    It's the only block that contributes a non-constant value to the Phi.
  //  - All other blocks (b1, b2, b3) must have exactly two successors, one of
  //    them being the phi block.
  //  - All intermediate blocks (bb2, bb3) must have only one predecessor.
  //  - Blocks cannot do other work besides the comparison, see doesOtherWork()

  // The blocks are not necessarily ordered in the phi, so we start from the
  // last block and reconstruct the order.
  BasicBlock *LastBlock = nullptr;
  for (unsigned I = 0; I < Phi.getNumIncomingValues(); ++I) {
    if (isa<ConstantInt>(Phi.getIncomingValue(I))) continue;
    if (LastBlock) {
      // There are several non-constant values.
      LLVM_DEBUG(dbgs() << "skip: several non-constant values\n");
      return false;
    }
    if (!isa<ICmpInst>(Phi.getIncomingValue(I)) ||
        cast<ICmpInst>(Phi.getIncomingValue(I))->getParent() !=
            Phi.getIncomingBlock(I)) {
      // Non-constant incoming value is not from a cmp instruction or not
      // produced by the last block. We could end up processing the value
      // producing block more than once.
      //
      // This is an uncommon case, so we bail.
      LLVM_DEBUG(
          dbgs()
          << "skip: non-constant value not from cmp or not from last block.\n");
      return false;
    }
    LastBlock = Phi.getIncomingBlock(I);
  }
  if (!LastBlock) {
    // There is no non-constant block.
    LLVM_DEBUG(dbgs() << "skip: no non-constant block\n");
    return false;
  }
  if (LastBlock->getSingleSuccessor() != Phi.getParent()) {
    LLVM_DEBUG(dbgs() << "skip: last block non-phi successor\n");
    return false;
  }

  const auto Blocks =
      getOrderedBlocks(Phi, LastBlock, Phi.getNumIncomingValues());
  if (Blocks.empty()) return false;
  BCECmpChain CmpChain(Blocks, Phi, AA);

  if (CmpChain.size() < 2) {
    LLVM_DEBUG(dbgs() << "skip: only one compare block\n");
    return false;
  }

  return CmpChain.simplify(TLI, AA);
}

class MergeICmps : public FunctionPass {
 public:
  static char ID;

  MergeICmps() : FunctionPass(ID) {
    initializeMergeICmpsPass(*PassRegistry::getPassRegistry());
  }

  bool runOnFunction(Function &F) override {
    if (skipFunction(F)) return false;
    const auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
    const auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
    AliasAnalysis *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
    auto PA = runImpl(F, &TLI, &TTI, AA);
    return !PA.areAllPreserved();
  }

 private:
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<TargetLibraryInfoWrapperPass>();
    AU.addRequired<TargetTransformInfoWrapperPass>();
    AU.addRequired<AAResultsWrapperPass>();
  }

  PreservedAnalyses runImpl(Function &F, const TargetLibraryInfo *TLI,
                            const TargetTransformInfo *TTI, AliasAnalysis *AA);
};

PreservedAnalyses MergeICmps::runImpl(Function &F, const TargetLibraryInfo *TLI,
                                      const TargetTransformInfo *TTI,
                                      AliasAnalysis *AA) {
  LLVM_DEBUG(dbgs() << "MergeICmpsPass: " << F.getName() << "\n");

  // We only try merging comparisons if the target wants to expand memcmp later.
  // The rationale is to avoid turning small chains into memcmp calls.
  if (!TTI->enableMemCmpExpansion(true)) return PreservedAnalyses::all();

  // If we don't have memcmp avaiable we can't emit calls to it.
  if (!TLI->has(LibFunc_memcmp))
    return PreservedAnalyses::all();

  bool MadeChange = false;

  for (auto BBIt = ++F.begin(); BBIt != F.end(); ++BBIt) {
    // A Phi operation is always first in a basic block.
    if (auto *const Phi = dyn_cast<PHINode>(&*BBIt->begin()))
      MadeChange |= processPhi(*Phi, TLI, AA);
  }

  if (MadeChange) return PreservedAnalyses::none();
  return PreservedAnalyses::all();
}

}  // namespace

char MergeICmps::ID = 0;
INITIALIZE_PASS_BEGIN(MergeICmps, "mergeicmps",
                      "Merge contiguous icmps into a memcmp", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(MergeICmps, "mergeicmps",
                    "Merge contiguous icmps into a memcmp", false, false)

Pass *llvm::createMergeICmpsPass() { return new MergeICmps(); }