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

Tree @d94715e (Download .tar.gz)

LoopRerollPass.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
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
//===-- LoopReroll.cpp - Loop rerolling pass ------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a simple loop reroller.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"

using namespace llvm;

#define DEBUG_TYPE "loop-reroll"

STATISTIC(NumRerolledLoops, "Number of rerolled loops");

static cl::opt<unsigned>
MaxInc("max-reroll-increment", cl::init(2048), cl::Hidden,
  cl::desc("The maximum increment for loop rerolling"));

// This loop re-rolling transformation aims to transform loops like this:
//
// int foo(int a);
// void bar(int *x) {
//   for (int i = 0; i < 500; i += 3) {
//     foo(i);
//     foo(i+1);
//     foo(i+2);
//   }
// }
//
// into a loop like this:
//
// void bar(int *x) {
//   for (int i = 0; i < 500; ++i)
//     foo(i);
// }
//
// It does this by looking for loops that, besides the latch code, are composed
// of isomorphic DAGs of instructions, with each DAG rooted at some increment
// to the induction variable, and where each DAG is isomorphic to the DAG
// rooted at the induction variable (excepting the sub-DAGs which root the
// other induction-variable increments). In other words, we're looking for loop
// bodies of the form:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// f(%iv)
// %iv.1 = add %iv, 1                <-- a root increment
// f(%iv.1)
// %iv.2 = add %iv, 2                <-- a root increment
// f(%iv.2)
// %iv.scale_m_1 = add %iv, scale-1  <-- a root increment
// f(%iv.scale_m_1)
// ...
// %iv.next = add %iv, scale
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit
//
// where each f(i) is a set of instructions that, collectively, are a function
// only of i (and other loop-invariant values).
//
// As a special case, we can also reroll loops like this:
//
// int foo(int);
// void bar(int *x) {
//   for (int i = 0; i < 500; ++i) {
//     x[3*i] = foo(0);
//     x[3*i+1] = foo(0);
//     x[3*i+2] = foo(0);
//   }
// }
//
// into this:
//
// void bar(int *x) {
//   for (int i = 0; i < 1500; ++i)
//     x[i] = foo(0);
// }
//
// in which case, we're looking for inputs like this:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// %scaled.iv = mul %iv, scale
// f(%scaled.iv)
// %scaled.iv.1 = add %scaled.iv, 1
// f(%scaled.iv.1)
// %scaled.iv.2 = add %scaled.iv, 2
// f(%scaled.iv.2)
// %scaled.iv.scale_m_1 = add %scaled.iv, scale-1
// f(%scaled.iv.scale_m_1)
// ...
// %iv.next = add %iv, 1
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit

namespace {
  class LoopReroll : public LoopPass {
  public:
    static char ID; // Pass ID, replacement for typeid
    LoopReroll() : LoopPass(ID) {
      initializeLoopRerollPass(*PassRegistry::getPassRegistry());
    }

    bool runOnLoop(Loop *L, LPPassManager &LPM) override;

    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.addRequired<AliasAnalysis>();
      AU.addRequired<LoopInfo>();
      AU.addPreserved<LoopInfo>();
      AU.addRequired<DominatorTreeWrapperPass>();
      AU.addPreserved<DominatorTreeWrapperPass>();
      AU.addRequired<ScalarEvolution>();
      AU.addRequired<TargetLibraryInfo>();
    }

protected:
    AliasAnalysis *AA;
    LoopInfo *LI;
    ScalarEvolution *SE;
    const DataLayout *DL;
    TargetLibraryInfo *TLI;
    DominatorTree *DT;

    typedef SmallVector<Instruction *, 16> SmallInstructionVector;
    typedef SmallSet<Instruction *, 16>   SmallInstructionSet;

    // A chain of isomorphic instructions, indentified by a single-use PHI,
    // representing a reduction. Only the last value may be used outside the
    // loop.
    struct SimpleLoopReduction {
      SimpleLoopReduction(Instruction *P, Loop *L)
        : Valid(false), Instructions(1, P) {
        assert(isa<PHINode>(P) && "First reduction instruction must be a PHI");
        add(L);
      }

      bool valid() const {
        return Valid;
      }

      Instruction *getPHI() const {
        assert(Valid && "Using invalid reduction");
        return Instructions.front();
      }

      Instruction *getReducedValue() const {
        assert(Valid && "Using invalid reduction");
        return Instructions.back();
      }

      Instruction *get(size_t i) const {
        assert(Valid && "Using invalid reduction");
        return Instructions[i+1];
      }

      Instruction *operator [] (size_t i) const { return get(i); }

      // The size, ignoring the initial PHI.
      size_t size() const {
        assert(Valid && "Using invalid reduction");
        return Instructions.size()-1;
      }

      typedef SmallInstructionVector::iterator iterator;
      typedef SmallInstructionVector::const_iterator const_iterator;

      iterator begin() {
        assert(Valid && "Using invalid reduction");
        return std::next(Instructions.begin());
      }

      const_iterator begin() const {
        assert(Valid && "Using invalid reduction");
        return std::next(Instructions.begin());
      }

      iterator end() { return Instructions.end(); }
      const_iterator end() const { return Instructions.end(); }

    protected:
      bool Valid;
      SmallInstructionVector Instructions;

      void add(Loop *L);
    };

    // The set of all reductions, and state tracking of possible reductions
    // during loop instruction processing.
    struct ReductionTracker {
      typedef SmallVector<SimpleLoopReduction, 16> SmallReductionVector;

      // Add a new possible reduction.
      void addSLR(SimpleLoopReduction &SLR) {
        PossibleReds.push_back(SLR);
      }

      // Setup to track possible reductions corresponding to the provided
      // rerolling scale. Only reductions with a number of non-PHI instructions
      // that is divisible by the scale are considered. Three instructions sets
      // are filled in:
      //   - A set of all possible instructions in eligible reductions.
      //   - A set of all PHIs in eligible reductions
      //   - A set of all reduced values (last instructions) in eligible reductions.
      void restrictToScale(uint64_t Scale,
                           SmallInstructionSet &PossibleRedSet,
                           SmallInstructionSet &PossibleRedPHISet,
                           SmallInstructionSet &PossibleRedLastSet) {
        PossibleRedIdx.clear();
        PossibleRedIter.clear();
        Reds.clear();

        for (unsigned i = 0, e = PossibleReds.size(); i != e; ++i)
          if (PossibleReds[i].size() % Scale == 0) {
            PossibleRedLastSet.insert(PossibleReds[i].getReducedValue());
            PossibleRedPHISet.insert(PossibleReds[i].getPHI());
      
            PossibleRedSet.insert(PossibleReds[i].getPHI());
            PossibleRedIdx[PossibleReds[i].getPHI()] = i;
            for (SimpleLoopReduction::iterator J = PossibleReds[i].begin(),
                 JE = PossibleReds[i].end(); J != JE; ++J) {
              PossibleRedSet.insert(*J);
              PossibleRedIdx[*J] = i;
            }
          }
      }

      // The functions below are used while processing the loop instructions.

      // Are the two instructions both from reductions, and furthermore, from
      // the same reduction?
      bool isPairInSame(Instruction *J1, Instruction *J2) {
        DenseMap<Instruction *, int>::iterator J1I = PossibleRedIdx.find(J1);
        if (J1I != PossibleRedIdx.end()) {
          DenseMap<Instruction *, int>::iterator J2I = PossibleRedIdx.find(J2);
          if (J2I != PossibleRedIdx.end() && J1I->second == J2I->second)
            return true;
        }

        return false;
      }

      // The two provided instructions, the first from the base iteration, and
      // the second from iteration i, form a matched pair. If these are part of
      // a reduction, record that fact.
      void recordPair(Instruction *J1, Instruction *J2, unsigned i) {
        if (PossibleRedIdx.count(J1)) {
          assert(PossibleRedIdx.count(J2) &&
                 "Recording reduction vs. non-reduction instruction?");

          PossibleRedIter[J1] = 0;
          PossibleRedIter[J2] = i;

          int Idx = PossibleRedIdx[J1];
          assert(Idx == PossibleRedIdx[J2] &&
                 "Recording pair from different reductions?");
          Reds.insert(Idx);
        }
      }

      // The functions below can be called after we've finished processing all
      // instructions in the loop, and we know which reductions were selected.

      // Is the provided instruction the PHI of a reduction selected for
      // rerolling?
      bool isSelectedPHI(Instruction *J) {
        if (!isa<PHINode>(J))
          return false;

        for (DenseSet<int>::iterator RI = Reds.begin(), RIE = Reds.end();
             RI != RIE; ++RI) {
          int i = *RI;
          if (cast<Instruction>(J) == PossibleReds[i].getPHI())
            return true;
        }

        return false;
      }

      bool validateSelected();
      void replaceSelected();

    protected:
      // The vector of all possible reductions (for any scale).
      SmallReductionVector PossibleReds;

      DenseMap<Instruction *, int> PossibleRedIdx;
      DenseMap<Instruction *, int> PossibleRedIter;
      DenseSet<int> Reds;
    };

    void collectPossibleIVs(Loop *L, SmallInstructionVector &PossibleIVs);
    void collectPossibleReductions(Loop *L,
           ReductionTracker &Reductions);
    void collectInLoopUserSet(Loop *L,
           const SmallInstructionVector &Roots,
           const SmallInstructionSet &Exclude,
           const SmallInstructionSet &Final,
           DenseSet<Instruction *> &Users);
    void collectInLoopUserSet(Loop *L,
           Instruction * Root,
           const SmallInstructionSet &Exclude,
           const SmallInstructionSet &Final,
           DenseSet<Instruction *> &Users);
    bool findScaleFromMul(Instruction *RealIV, uint64_t &Scale,
                          Instruction *&IV,
                          SmallInstructionVector &LoopIncs);
    bool collectAllRoots(Loop *L, uint64_t Inc, uint64_t Scale, Instruction *IV,
                         SmallVector<SmallInstructionVector, 32> &Roots,
                         SmallInstructionSet &AllRoots,
                         SmallInstructionVector &LoopIncs);
    bool reroll(Instruction *IV, Loop *L, BasicBlock *Header, const SCEV *IterCount,
                ReductionTracker &Reductions);
  };
}

char LoopReroll::ID = 0;
INITIALIZE_PASS_BEGIN(LoopReroll, "loop-reroll", "Reroll loops", false, false)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
INITIALIZE_PASS_END(LoopReroll, "loop-reroll", "Reroll loops", false, false)

Pass *llvm::createLoopRerollPass() {
  return new LoopReroll;
}

// Returns true if the provided instruction is used outside the given loop.
// This operates like Instruction::isUsedOutsideOfBlock, but considers PHIs in
// non-loop blocks to be outside the loop.
static bool hasUsesOutsideLoop(Instruction *I, Loop *L) {
  for (User *U : I->users())
    if (!L->contains(cast<Instruction>(U)))
      return true;

  return false;
}

// Collect the list of loop induction variables with respect to which it might
// be possible to reroll the loop.
void LoopReroll::collectPossibleIVs(Loop *L,
                                    SmallInstructionVector &PossibleIVs) {
  BasicBlock *Header = L->getHeader();
  for (BasicBlock::iterator I = Header->begin(),
       IE = Header->getFirstInsertionPt(); I != IE; ++I) {
    if (!isa<PHINode>(I))
      continue;
    if (!I->getType()->isIntegerTy())
      continue;

    if (const SCEVAddRecExpr *PHISCEV =
        dyn_cast<SCEVAddRecExpr>(SE->getSCEV(I))) {
      if (PHISCEV->getLoop() != L)
        continue;
      if (!PHISCEV->isAffine())
        continue;
      if (const SCEVConstant *IncSCEV =
          dyn_cast<SCEVConstant>(PHISCEV->getStepRecurrence(*SE))) {
        if (!IncSCEV->getValue()->getValue().isStrictlyPositive())
          continue;
        if (IncSCEV->getValue()->uge(MaxInc))
          continue;

        DEBUG(dbgs() << "LRR: Possible IV: " << *I << " = " <<
              *PHISCEV << "\n");
        PossibleIVs.push_back(I);
      }
    }
  }
}

// Add the remainder of the reduction-variable chain to the instruction vector
// (the initial PHINode has already been added). If successful, the object is
// marked as valid.
void LoopReroll::SimpleLoopReduction::add(Loop *L) {
  assert(!Valid && "Cannot add to an already-valid chain");

  // The reduction variable must be a chain of single-use instructions
  // (including the PHI), except for the last value (which is used by the PHI
  // and also outside the loop).
  Instruction *C = Instructions.front();

  do {
    C = cast<Instruction>(*C->user_begin());
    if (C->hasOneUse()) {
      if (!C->isBinaryOp())
        return;

      if (!(isa<PHINode>(Instructions.back()) ||
            C->isSameOperationAs(Instructions.back())))
        return;

      Instructions.push_back(C);
    }
  } while (C->hasOneUse());

  if (Instructions.size() < 2 ||
      !C->isSameOperationAs(Instructions.back()) ||
      C->use_empty())
    return;

  // C is now the (potential) last instruction in the reduction chain.
  for (User *U : C->users())
    // The only in-loop user can be the initial PHI.
    if (L->contains(cast<Instruction>(U)))
      if (cast<Instruction>(U) != Instructions.front())
        return;

  Instructions.push_back(C);
  Valid = true;
}

// Collect the vector of possible reduction variables.
void LoopReroll::collectPossibleReductions(Loop *L,
  ReductionTracker &Reductions) {
  BasicBlock *Header = L->getHeader();
  for (BasicBlock::iterator I = Header->begin(),
       IE = Header->getFirstInsertionPt(); I != IE; ++I) {
    if (!isa<PHINode>(I))
      continue;
    if (!I->getType()->isSingleValueType())
      continue;

    SimpleLoopReduction SLR(I, L);
    if (!SLR.valid())
      continue;

    DEBUG(dbgs() << "LRR: Possible reduction: " << *I << " (with " <<
          SLR.size() << " chained instructions)\n");
    Reductions.addSLR(SLR);
  }
}

// Collect the set of all users of the provided root instruction. This set of
// users contains not only the direct users of the root instruction, but also
// all users of those users, and so on. There are two exceptions:
//
//   1. Instructions in the set of excluded instructions are never added to the
//   use set (even if they are users). This is used, for example, to exclude
//   including root increments in the use set of the primary IV.
//
//   2. Instructions in the set of final instructions are added to the use set
//   if they are users, but their users are not added. This is used, for
//   example, to prevent a reduction update from forcing all later reduction
//   updates into the use set.
void LoopReroll::collectInLoopUserSet(Loop *L,
  Instruction *Root, const SmallInstructionSet &Exclude,
  const SmallInstructionSet &Final,
  DenseSet<Instruction *> &Users) {
  SmallInstructionVector Queue(1, Root);
  while (!Queue.empty()) {
    Instruction *I = Queue.pop_back_val();
    if (!Users.insert(I).second)
      continue;

    if (!Final.count(I))
      for (Use &U : I->uses()) {
        Instruction *User = cast<Instruction>(U.getUser());
        if (PHINode *PN = dyn_cast<PHINode>(User)) {
          // Ignore "wrap-around" uses to PHIs of this loop's header.
          if (PN->getIncomingBlock(U) == L->getHeader())
            continue;
        }
  
        if (L->contains(User) && !Exclude.count(User)) {
          Queue.push_back(User);
        }
      }

    // We also want to collect single-user "feeder" values.
    for (User::op_iterator OI = I->op_begin(),
         OIE = I->op_end(); OI != OIE; ++OI) {
      if (Instruction *Op = dyn_cast<Instruction>(*OI))
        if (Op->hasOneUse() && L->contains(Op) && !Exclude.count(Op) &&
            !Final.count(Op))
          Queue.push_back(Op);
    }
  }
}

// Collect all of the users of all of the provided root instructions (combined
// into a single set).
void LoopReroll::collectInLoopUserSet(Loop *L,
  const SmallInstructionVector &Roots,
  const SmallInstructionSet &Exclude,
  const SmallInstructionSet &Final,
  DenseSet<Instruction *> &Users) {
  for (SmallInstructionVector::const_iterator I = Roots.begin(),
       IE = Roots.end(); I != IE; ++I)
    collectInLoopUserSet(L, *I, Exclude, Final, Users);
}

static bool isSimpleLoadStore(Instruction *I) {
  if (LoadInst *LI = dyn_cast<LoadInst>(I))
    return LI->isSimple();
  if (StoreInst *SI = dyn_cast<StoreInst>(I))
    return SI->isSimple();
  if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
    return !MI->isVolatile();
  return false;
}

// Recognize loops that are setup like this:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// %scaled.iv = mul %iv, scale
// f(%scaled.iv)
// %scaled.iv.1 = add %scaled.iv, 1
// f(%scaled.iv.1)
// %scaled.iv.2 = add %scaled.iv, 2
// f(%scaled.iv.2)
// %scaled.iv.scale_m_1 = add %scaled.iv, scale-1
// f(%scaled.iv.scale_m_1)
// ...
// %iv.next = add %iv, 1
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit
//
// and, if found, set IV = %scaled.iv, and add %iv.next to LoopIncs.
bool LoopReroll::findScaleFromMul(Instruction *RealIV, uint64_t &Scale,
                                  Instruction *&IV,
                                  SmallInstructionVector &LoopIncs) {
  // This is a special case: here we're looking for all uses (except for
  // the increment) to be multiplied by a common factor. The increment must
  // be by one. This is to capture loops like:
  //   for (int i = 0; i < 500; ++i) {
  //     foo(3*i); foo(3*i+1); foo(3*i+2);
  //   }
  if (RealIV->getNumUses() != 2)
    return false;
  const SCEVAddRecExpr *RealIVSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(RealIV));
  Instruction *User1 = cast<Instruction>(*RealIV->user_begin()),
              *User2 = cast<Instruction>(*std::next(RealIV->user_begin()));
  if (!SE->isSCEVable(User1->getType()) || !SE->isSCEVable(User2->getType()))
    return false;
  const SCEVAddRecExpr *User1SCEV =
                         dyn_cast<SCEVAddRecExpr>(SE->getSCEV(User1)),
                       *User2SCEV =
                         dyn_cast<SCEVAddRecExpr>(SE->getSCEV(User2));
  if (!User1SCEV || !User1SCEV->isAffine() ||
      !User2SCEV || !User2SCEV->isAffine())
    return false;

  // We assume below that User1 is the scale multiply and User2 is the
  // increment. If this can't be true, then swap them.
  if (User1SCEV == RealIVSCEV->getPostIncExpr(*SE)) {
    std::swap(User1, User2);
    std::swap(User1SCEV, User2SCEV);
  }

  if (User2SCEV != RealIVSCEV->getPostIncExpr(*SE))
    return false;
  assert(User2SCEV->getStepRecurrence(*SE)->isOne() &&
         "Invalid non-unit step for multiplicative scaling");
  LoopIncs.push_back(User2);

  if (const SCEVConstant *MulScale =
      dyn_cast<SCEVConstant>(User1SCEV->getStepRecurrence(*SE))) {
    // Make sure that both the start and step have the same multiplier.
    if (RealIVSCEV->getStart()->getType() != MulScale->getType())
      return false;
    if (SE->getMulExpr(RealIVSCEV->getStart(), MulScale) !=
        User1SCEV->getStart())
      return false;

    ConstantInt *MulScaleCI = MulScale->getValue();
    if (!MulScaleCI->uge(2) || MulScaleCI->uge(MaxInc))
      return false;
    Scale = MulScaleCI->getZExtValue();
    IV = User1;
  } else
    return false;

  DEBUG(dbgs() << "LRR: Found possible scaling " << *User1 << "\n");
  return true;
}

// Collect all root increments with respect to the provided induction variable
// (normally the PHI, but sometimes a multiply). A root increment is an
// instruction, normally an add, with a positive constant less than Scale. In a
// rerollable loop, each of these increments is the root of an instruction
// graph isomorphic to the others. Also, we collect the final induction
// increment (the increment equal to the Scale), and its users in LoopIncs.
bool LoopReroll::collectAllRoots(Loop *L, uint64_t Inc, uint64_t Scale,
                                 Instruction *IV,
                                 SmallVector<SmallInstructionVector, 32> &Roots,
                                 SmallInstructionSet &AllRoots,
                                 SmallInstructionVector &LoopIncs) {
  for (User *U : IV->users()) {
    Instruction *UI = cast<Instruction>(U);
    if (!SE->isSCEVable(UI->getType()))
      continue;
    if (UI->getType() != IV->getType())
      continue;
    if (!L->contains(UI))
      continue;
    if (hasUsesOutsideLoop(UI, L))
      continue;

    if (const SCEVConstant *Diff = dyn_cast<SCEVConstant>(SE->getMinusSCEV(
          SE->getSCEV(UI), SE->getSCEV(IV)))) {
      uint64_t Idx = Diff->getValue()->getValue().getZExtValue();
      if (Idx > 0 && Idx < Scale) {
        Roots[Idx-1].push_back(UI);
        AllRoots.insert(UI);
      } else if (Idx == Scale && Inc > 1) {
        LoopIncs.push_back(UI);
      }
    }
  }

  if (Roots[0].empty())
    return false;
  bool AllSame = true;
  for (unsigned i = 1; i < Scale-1; ++i)
    if (Roots[i].size() != Roots[0].size()) {
      AllSame = false;
      break;
    }

  if (!AllSame)
    return false;

  return true;
}

// Validate the selected reductions. All iterations must have an isomorphic
// part of the reduction chain and, for non-associative reductions, the chain
// entries must appear in order.
bool LoopReroll::ReductionTracker::validateSelected() {
  // For a non-associative reduction, the chain entries must appear in order.
  for (DenseSet<int>::iterator RI = Reds.begin(), RIE = Reds.end();
       RI != RIE; ++RI) {
    int i = *RI;
    int PrevIter = 0, BaseCount = 0, Count = 0;
    for (SimpleLoopReduction::iterator J = PossibleReds[i].begin(),
         JE = PossibleReds[i].end(); J != JE; ++J) {
	// Note that all instructions in the chain must have been found because
	// all instructions in the function must have been assigned to some
	// iteration.
      int Iter = PossibleRedIter[*J];
      if (Iter != PrevIter && Iter != PrevIter + 1 &&
          !PossibleReds[i].getReducedValue()->isAssociative()) {
        DEBUG(dbgs() << "LRR: Out-of-order non-associative reduction: " <<
                        *J << "\n");
        return false;
      }

      if (Iter != PrevIter) {
        if (Count != BaseCount) {
          DEBUG(dbgs() << "LRR: Iteration " << PrevIter <<
                " reduction use count " << Count <<
                " is not equal to the base use count " <<
                BaseCount << "\n");
          return false;
        }

        Count = 0;
      }

      ++Count;
      if (Iter == 0)
        ++BaseCount;

      PrevIter = Iter;
    }
  }

  return true;
}

// For all selected reductions, remove all parts except those in the first
// iteration (and the PHI). Replace outside uses of the reduced value with uses
// of the first-iteration reduced value (in other words, reroll the selected
// reductions).
void LoopReroll::ReductionTracker::replaceSelected() {
  // Fixup reductions to refer to the last instruction associated with the
  // first iteration (not the last).
  for (DenseSet<int>::iterator RI = Reds.begin(), RIE = Reds.end();
       RI != RIE; ++RI) {
    int i = *RI;
    int j = 0;
    for (int e = PossibleReds[i].size(); j != e; ++j)
      if (PossibleRedIter[PossibleReds[i][j]] != 0) {
        --j;
        break;
      }

    // Replace users with the new end-of-chain value.
    SmallInstructionVector Users;
    for (User *U : PossibleReds[i].getReducedValue()->users())
      Users.push_back(cast<Instruction>(U));

    for (SmallInstructionVector::iterator J = Users.begin(),
         JE = Users.end(); J != JE; ++J)
      (*J)->replaceUsesOfWith(PossibleReds[i].getReducedValue(),
                              PossibleReds[i][j]);
  }
}

// Reroll the provided loop with respect to the provided induction variable.
// Generally, we're looking for a loop like this:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// f(%iv)
// %iv.1 = add %iv, 1                <-- a root increment
// f(%iv.1)
// %iv.2 = add %iv, 2                <-- a root increment
// f(%iv.2)
// %iv.scale_m_1 = add %iv, scale-1  <-- a root increment
// f(%iv.scale_m_1)
// ...
// %iv.next = add %iv, scale
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit
//
// Notably, we do not require that f(%iv), f(%iv.1), etc. be isolated groups of
// instructions. In other words, the instructions in f(%iv), f(%iv.1), etc. can
// be intermixed with eachother. The restriction imposed by this algorithm is
// that the relative order of the isomorphic instructions in f(%iv), f(%iv.1),
// etc. be the same.
//
// First, we collect the use set of %iv, excluding the other increment roots.
// This gives us f(%iv). Then we iterate over the loop instructions (scale-1)
// times, having collected the use set of f(%iv.(i+1)), during which we:
//   - Ensure that the next unmatched instruction in f(%iv) is isomorphic to
//     the next unmatched instruction in f(%iv.(i+1)).
//   - Ensure that both matched instructions don't have any external users
//     (with the exception of last-in-chain reduction instructions).
//   - Track the (aliasing) write set, and other side effects, of all
//     instructions that belong to future iterations that come before the matched
//     instructions. If the matched instructions read from that write set, then
//     f(%iv) or f(%iv.(i+1)) has some dependency on instructions in
//     f(%iv.(j+1)) for some j > i, and we cannot reroll the loop. Similarly,
//     if any of these future instructions had side effects (could not be
//     speculatively executed), and so do the matched instructions, when we
//     cannot reorder those side-effect-producing instructions, and rerolling
//     fails.
//
// Finally, we make sure that all loop instructions are either loop increment
// roots, belong to simple latch code, parts of validated reductions, part of
// f(%iv) or part of some f(%iv.i). If all of that is true (and all reductions
// have been validated), then we reroll the loop.
bool LoopReroll::reroll(Instruction *IV, Loop *L, BasicBlock *Header,
                        const SCEV *IterCount,
                        ReductionTracker &Reductions) {
  const SCEVAddRecExpr *RealIVSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(IV));
  uint64_t Inc = cast<SCEVConstant>(RealIVSCEV->getOperand(1))->
                   getValue()->getZExtValue();
  // The collection of loop increment instructions.
  SmallInstructionVector LoopIncs;
  uint64_t Scale = Inc;

  // The effective induction variable, IV, is normally also the real induction
  // variable. When we're dealing with a loop like:
  //   for (int i = 0; i < 500; ++i)
  //     x[3*i] = ...;
  //     x[3*i+1] = ...;
  //     x[3*i+2] = ...;
  // then the real IV is still i, but the effective IV is (3*i).
  Instruction *RealIV = IV;
  if (Inc == 1 && !findScaleFromMul(RealIV, Scale, IV, LoopIncs))
    return false;

  assert(Scale <= MaxInc && "Scale is too large");
  assert(Scale > 1 && "Scale must be at least 2");

  // The set of increment instructions for each increment value.
  SmallVector<SmallInstructionVector, 32> Roots(Scale-1);
  SmallInstructionSet AllRoots;
  if (!collectAllRoots(L, Inc, Scale, IV, Roots, AllRoots, LoopIncs))
    return false;

  DEBUG(dbgs() << "LRR: Found all root induction increments for: " <<
                  *RealIV << "\n");

  // An array of just the possible reductions for this scale factor. When we
  // collect the set of all users of some root instructions, these reduction
  // instructions are treated as 'final' (their uses are not considered).
  // This is important because we don't want the root use set to search down
  // the reduction chain.
  SmallInstructionSet PossibleRedSet;
  SmallInstructionSet PossibleRedLastSet, PossibleRedPHISet;
  Reductions.restrictToScale(Scale, PossibleRedSet, PossibleRedPHISet,
                             PossibleRedLastSet);

  // We now need to check for equivalence of the use graph of each root with
  // that of the primary induction variable (excluding the roots). Our goal
  // here is not to solve the full graph isomorphism problem, but rather to
  // catch common cases without a lot of work. As a result, we will assume
  // that the relative order of the instructions in each unrolled iteration
  // is the same (although we will not make an assumption about how the
  // different iterations are intermixed). Note that while the order must be
  // the same, the instructions may not be in the same basic block.
  SmallInstructionSet Exclude(AllRoots);
  Exclude.insert(LoopIncs.begin(), LoopIncs.end());

  DenseSet<Instruction *> BaseUseSet;
  collectInLoopUserSet(L, IV, Exclude, PossibleRedSet, BaseUseSet);

  DenseSet<Instruction *> AllRootUses;
  std::vector<DenseSet<Instruction *> > RootUseSets(Scale-1);

  bool MatchFailed = false;
  for (unsigned i = 0; i < Scale-1 && !MatchFailed; ++i) {
    DenseSet<Instruction *> &RootUseSet = RootUseSets[i];
    collectInLoopUserSet(L, Roots[i], SmallInstructionSet(),
                         PossibleRedSet, RootUseSet);

    DEBUG(dbgs() << "LRR: base use set size: " << BaseUseSet.size() <<
                    " vs. iteration increment " << (i+1) <<
                    " use set size: " << RootUseSet.size() << "\n");

    if (BaseUseSet.size() != RootUseSet.size()) {
      MatchFailed = true;
      break;
    }

    // In addition to regular aliasing information, we need to look for
    // instructions from later (future) iterations that have side effects
    // preventing us from reordering them past other instructions with side
    // effects.
    bool FutureSideEffects = false;
    AliasSetTracker AST(*AA);

    // The map between instructions in f(%iv.(i+1)) and f(%iv).
    DenseMap<Value *, Value *> BaseMap;

    assert(L->getNumBlocks() == 1 && "Cannot handle multi-block loops");
    for (BasicBlock::iterator J1 = Header->begin(), J2 = Header->begin(),
         JE = Header->end(); J1 != JE && !MatchFailed; ++J1) {
      if (cast<Instruction>(J1) == RealIV)
        continue;
      if (cast<Instruction>(J1) == IV)
        continue;
      if (!BaseUseSet.count(J1))
        continue;
      if (PossibleRedPHISet.count(J1)) // Skip reduction PHIs.
        continue;

      while (J2 != JE && (!RootUseSet.count(J2) ||
             std::find(Roots[i].begin(), Roots[i].end(), J2) !=
               Roots[i].end())) {
        // As we iterate through the instructions, instructions that don't
        // belong to previous iterations (or the base case), must belong to
        // future iterations. We want to track the alias set of writes from
        // previous iterations.
        if (!isa<PHINode>(J2) && !BaseUseSet.count(J2) &&
            !AllRootUses.count(J2)) {
          if (J2->mayWriteToMemory())
            AST.add(J2);

          // Note: This is specifically guarded by a check on isa<PHINode>,
          // which while a valid (somewhat arbitrary) micro-optimization, is
          // needed because otherwise isSafeToSpeculativelyExecute returns
          // false on PHI nodes.
          if (!isSimpleLoadStore(J2) && !isSafeToSpeculativelyExecute(J2, DL))
            FutureSideEffects = true; 
        }

        ++J2;
      }

      if (!J1->isSameOperationAs(J2)) {
        DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                        " vs. " << *J2 << "\n");
        MatchFailed = true;
        break;
      }

      // Make sure that this instruction, which is in the use set of this
      // root instruction, does not also belong to the base set or the set of
      // some previous root instruction.
      if (BaseUseSet.count(J2) || AllRootUses.count(J2)) {
        DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                        " vs. " << *J2 << " (prev. case overlap)\n");
        MatchFailed = true;
        break;
      }

      // Make sure that we don't alias with any instruction in the alias set
      // tracker. If we do, then we depend on a future iteration, and we
      // can't reroll.
      if (J2->mayReadFromMemory()) {
        for (AliasSetTracker::iterator K = AST.begin(), KE = AST.end();
             K != KE && !MatchFailed; ++K) {
          if (K->aliasesUnknownInst(J2, *AA)) {
            DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                            " vs. " << *J2 << " (depends on future store)\n");
            MatchFailed = true;
            break;
          }
        }
      }

      // If we've past an instruction from a future iteration that may have
      // side effects, and this instruction might also, then we can't reorder
      // them, and this matching fails. As an exception, we allow the alias
      // set tracker to handle regular (simple) load/store dependencies.
      if (FutureSideEffects &&
            ((!isSimpleLoadStore(J1) &&
              !isSafeToSpeculativelyExecute(J1, DL)) ||
             (!isSimpleLoadStore(J2) &&
              !isSafeToSpeculativelyExecute(J2, DL)))) {
        DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                        " vs. " << *J2 <<
                        " (side effects prevent reordering)\n");
        MatchFailed = true;
        break;
      }

      // For instructions that are part of a reduction, if the operation is
      // associative, then don't bother matching the operands (because we
      // already know that the instructions are isomorphic, and the order
      // within the iteration does not matter). For non-associative reductions,
      // we do need to match the operands, because we need to reject
      // out-of-order instructions within an iteration!
      // For example (assume floating-point addition), we need to reject this:
      //   x += a[i]; x += b[i];
      //   x += a[i+1]; x += b[i+1];
      //   x += b[i+2]; x += a[i+2];
      bool InReduction = Reductions.isPairInSame(J1, J2);

      if (!(InReduction && J1->isAssociative())) {
        bool Swapped = false, SomeOpMatched = false;
        for (unsigned j = 0; j < J1->getNumOperands() && !MatchFailed; ++j) {
          Value *Op2 = J2->getOperand(j);

	  // If this is part of a reduction (and the operation is not
	  // associatve), then we match all operands, but not those that are
	  // part of the reduction.
          if (InReduction)
            if (Instruction *Op2I = dyn_cast<Instruction>(Op2))
              if (Reductions.isPairInSame(J2, Op2I))
                continue;

          DenseMap<Value *, Value *>::iterator BMI = BaseMap.find(Op2);
          if (BMI != BaseMap.end())
            Op2 = BMI->second;
          else if (std::find(Roots[i].begin(), Roots[i].end(),
                             (Instruction*) Op2) != Roots[i].end())
            Op2 = IV;

          if (J1->getOperand(Swapped ? unsigned(!j) : j) != Op2) {
	    // If we've not already decided to swap the matched operands, and
	    // we've not already matched our first operand (note that we could
	    // have skipped matching the first operand because it is part of a
	    // reduction above), and the instruction is commutative, then try
	    // the swapped match.
            if (!Swapped && J1->isCommutative() && !SomeOpMatched &&
                J1->getOperand(!j) == Op2) {
              Swapped = true;
            } else {
              DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                              " vs. " << *J2 << " (operand " << j << ")\n");
              MatchFailed = true;
              break;
            }
          }

          SomeOpMatched = true;
        }
      }

      if ((!PossibleRedLastSet.count(J1) && hasUsesOutsideLoop(J1, L)) ||
          (!PossibleRedLastSet.count(J2) && hasUsesOutsideLoop(J2, L))) {
        DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                        " vs. " << *J2 << " (uses outside loop)\n");
        MatchFailed = true;
        break;
      }

      if (!MatchFailed)
        BaseMap.insert(std::pair<Value *, Value *>(J2, J1));

      AllRootUses.insert(J2);
      Reductions.recordPair(J1, J2, i+1);

      ++J2;
    }
  }

  if (MatchFailed)
    return false;

  DEBUG(dbgs() << "LRR: Matched all iteration increments for " <<
                  *RealIV << "\n");

  DenseSet<Instruction *> LoopIncUseSet;
  collectInLoopUserSet(L, LoopIncs, SmallInstructionSet(),
                       SmallInstructionSet(), LoopIncUseSet);
  DEBUG(dbgs() << "LRR: Loop increment set size: " <<
                  LoopIncUseSet.size() << "\n");

  // Make sure that all instructions in the loop have been included in some
  // use set.
  for (BasicBlock::iterator J = Header->begin(), JE = Header->end();
       J != JE; ++J) {
    if (isa<DbgInfoIntrinsic>(J))
      continue;
    if (cast<Instruction>(J) == RealIV)
      continue;
    if (cast<Instruction>(J) == IV)
      continue;
    if (BaseUseSet.count(J) || AllRootUses.count(J) ||
        (LoopIncUseSet.count(J) && (J->isTerminator() ||
                                    isSafeToSpeculativelyExecute(J, DL))))
      continue;

    if (AllRoots.count(J))
      continue;

    if (Reductions.isSelectedPHI(J))
      continue;

    DEBUG(dbgs() << "LRR: aborting reroll based on " << *RealIV <<
                    " unprocessed instruction found: " << *J << "\n");
    MatchFailed = true;
    break;
  }

  if (MatchFailed)
    return false;

  DEBUG(dbgs() << "LRR: all instructions processed from " <<
                  *RealIV << "\n");

  if (!Reductions.validateSelected())
    return false;

  // At this point, we've validated the rerolling, and we're committed to
  // making changes!

  Reductions.replaceSelected();

  // Remove instructions associated with non-base iterations.
  for (BasicBlock::reverse_iterator J = Header->rbegin();
       J != Header->rend();) {
    if (AllRootUses.count(&*J)) {
      Instruction *D = &*J;
      DEBUG(dbgs() << "LRR: removing: " << *D << "\n");
      D->eraseFromParent();
      continue;
    }

    ++J; 
  }

  // Insert the new induction variable.
  const SCEV *Start = RealIVSCEV->getStart();
  if (Inc == 1)
    Start = SE->getMulExpr(Start,
                           SE->getConstant(Start->getType(), Scale));
  const SCEVAddRecExpr *H =
    cast<SCEVAddRecExpr>(SE->getAddRecExpr(Start,
                           SE->getConstant(RealIVSCEV->getType(), 1),
                           L, SCEV::FlagAnyWrap));
  { // Limit the lifetime of SCEVExpander.
    SCEVExpander Expander(*SE, "reroll");
    Value *NewIV = Expander.expandCodeFor(H, IV->getType(), Header->begin());

    for (DenseSet<Instruction *>::iterator J = BaseUseSet.begin(),
         JE = BaseUseSet.end(); J != JE; ++J)
      (*J)->replaceUsesOfWith(IV, NewIV);

    if (BranchInst *BI = dyn_cast<BranchInst>(Header->getTerminator())) {
      if (LoopIncUseSet.count(BI)) {
        const SCEV *ICSCEV = RealIVSCEV->evaluateAtIteration(IterCount, *SE);
        if (Inc == 1)
          ICSCEV =
            SE->getMulExpr(ICSCEV, SE->getConstant(ICSCEV->getType(), Scale));
        // Iteration count SCEV minus 1
        const SCEV *ICMinus1SCEV =
          SE->getMinusSCEV(ICSCEV, SE->getConstant(ICSCEV->getType(), 1));

        Value *ICMinus1; // Iteration count minus 1
        if (isa<SCEVConstant>(ICMinus1SCEV)) {
          ICMinus1 = Expander.expandCodeFor(ICMinus1SCEV, NewIV->getType(), BI);
        } else {
          BasicBlock *Preheader = L->getLoopPreheader();
          if (!Preheader)
            Preheader = InsertPreheaderForLoop(L, this);

          ICMinus1 = Expander.expandCodeFor(ICMinus1SCEV, NewIV->getType(),
                                            Preheader->getTerminator());
        }
 
        Value *Cond = new ICmpInst(BI, CmpInst::ICMP_EQ, NewIV, ICMinus1,
                                   "exitcond");
        BI->setCondition(Cond);

        if (BI->getSuccessor(1) != Header)
          BI->swapSuccessors();
      }
    }
  }

  SimplifyInstructionsInBlock(Header, DL, TLI);
  DeleteDeadPHIs(Header, TLI);
  ++NumRerolledLoops;
  return true;
}

bool LoopReroll::runOnLoop(Loop *L, LPPassManager &LPM) {
  if (skipOptnoneFunction(L))
    return false;

  AA = &getAnalysis<AliasAnalysis>();
  LI = &getAnalysis<LoopInfo>();
  SE = &getAnalysis<ScalarEvolution>();
  TLI = &getAnalysis<TargetLibraryInfo>();
  DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
  DL = DLP ? &DLP->getDataLayout() : nullptr;
  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();

  BasicBlock *Header = L->getHeader();
  DEBUG(dbgs() << "LRR: F[" << Header->getParent()->getName() <<
        "] Loop %" << Header->getName() << " (" <<
        L->getNumBlocks() << " block(s))\n");

  bool Changed = false;

  // For now, we'll handle only single BB loops.
  if (L->getNumBlocks() > 1)
    return Changed;

  if (!SE->hasLoopInvariantBackedgeTakenCount(L))
    return Changed;

  const SCEV *LIBETC = SE->getBackedgeTakenCount(L);
  const SCEV *IterCount =
    SE->getAddExpr(LIBETC, SE->getConstant(LIBETC->getType(), 1));
  DEBUG(dbgs() << "LRR: iteration count = " << *IterCount << "\n");

  // First, we need to find the induction variable with respect to which we can
  // reroll (there may be several possible options).
  SmallInstructionVector PossibleIVs;
  collectPossibleIVs(L, PossibleIVs);

  if (PossibleIVs.empty()) {
    DEBUG(dbgs() << "LRR: No possible IVs found\n");
    return Changed;
  }

  ReductionTracker Reductions;
  collectPossibleReductions(L, Reductions);

  // For each possible IV, collect the associated possible set of 'root' nodes
  // (i+1, i+2, etc.).
  for (SmallInstructionVector::iterator I = PossibleIVs.begin(),
       IE = PossibleIVs.end(); I != IE; ++I)
    if (reroll(*I, L, Header, IterCount, Reductions)) {
      Changed = true;
      break;
    }

  return Changed;
}