aboutsummaryrefslogtreecommitdiffstats
path: root/contrib/libs/llvm12/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
blob: 6e09dec198c2c6b7288160e8e80d629de0a3a5a4 (plain) (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
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
//===- InductiveRangeCheckElimination.cpp - -------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// The InductiveRangeCheckElimination pass splits a loop's iteration space into
// three disjoint ranges.  It does that in a way such that the loop running in
// the middle loop provably does not need range checks. As an example, it will
// convert
//
//   len = < known positive >
//   for (i = 0; i < n; i++) {
//     if (0 <= i && i < len) {
//       do_something();
//     } else {
//       throw_out_of_bounds();
//     }
//   }
//
// to
//
//   len = < known positive >
//   limit = smin(n, len)
//   // no first segment
//   for (i = 0; i < limit; i++) {
//     if (0 <= i && i < len) { // this check is fully redundant
//       do_something();
//     } else {
//       throw_out_of_bounds();
//     }
//   }
//   for (i = limit; i < n; i++) {
//     if (0 <= i && i < len) {
//       do_something();
//     } else {
//       throw_out_of_bounds();
//     }
//   }
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar/InductiveRangeCheckElimination.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/PriorityWorklist.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <limits>
#include <utility>
#include <vector>

using namespace llvm;
using namespace llvm::PatternMatch;

static cl::opt<unsigned> LoopSizeCutoff("irce-loop-size-cutoff", cl::Hidden,
                                        cl::init(64));

static cl::opt<bool> PrintChangedLoops("irce-print-changed-loops", cl::Hidden,
                                       cl::init(false));

static cl::opt<bool> PrintRangeChecks("irce-print-range-checks", cl::Hidden,
                                      cl::init(false));

static cl::opt<bool> SkipProfitabilityChecks("irce-skip-profitability-checks",
                                             cl::Hidden, cl::init(false));

static cl::opt<unsigned> MinRuntimeIterations("irce-min-runtime-iterations",
                                              cl::Hidden, cl::init(10));

static cl::opt<bool> AllowUnsignedLatchCondition("irce-allow-unsigned-latch",
                                                 cl::Hidden, cl::init(true));

static cl::opt<bool> AllowNarrowLatchCondition(
    "irce-allow-narrow-latch", cl::Hidden, cl::init(true),
    cl::desc("If set to true, IRCE may eliminate wide range checks in loops "
             "with narrow latch condition."));

static const char *ClonedLoopTag = "irce.loop.clone";

#define DEBUG_TYPE "irce"

namespace {

/// An inductive range check is conditional branch in a loop with
///
///  1. a very cold successor (i.e. the branch jumps to that successor very
///     rarely)
///
///  and
///
///  2. a condition that is provably true for some contiguous range of values
///     taken by the containing loop's induction variable.
///
class InductiveRangeCheck {

  const SCEV *Begin = nullptr;
  const SCEV *Step = nullptr;
  const SCEV *End = nullptr;
  Use *CheckUse = nullptr;

  static bool parseRangeCheckICmp(Loop *L, ICmpInst *ICI, ScalarEvolution &SE,
                                  Value *&Index, Value *&Length,
                                  bool &IsSigned);

  static void
  extractRangeChecksFromCond(Loop *L, ScalarEvolution &SE, Use &ConditionUse,
                             SmallVectorImpl<InductiveRangeCheck> &Checks,
                             SmallPtrSetImpl<Value *> &Visited);

public:
  const SCEV *getBegin() const { return Begin; }
  const SCEV *getStep() const { return Step; }
  const SCEV *getEnd() const { return End; }

  void print(raw_ostream &OS) const {
    OS << "InductiveRangeCheck:\n";
    OS << "  Begin: ";
    Begin->print(OS);
    OS << "  Step: ";
    Step->print(OS);
    OS << "  End: ";
    End->print(OS);
    OS << "\n  CheckUse: ";
    getCheckUse()->getUser()->print(OS);
    OS << " Operand: " << getCheckUse()->getOperandNo() << "\n";
  }

  LLVM_DUMP_METHOD
  void dump() {
    print(dbgs());
  }

  Use *getCheckUse() const { return CheckUse; }

  /// Represents an signed integer range [Range.getBegin(), Range.getEnd()).  If
  /// R.getEnd() le R.getBegin(), then R denotes the empty range.

  class Range {
    const SCEV *Begin;
    const SCEV *End;

  public:
    Range(const SCEV *Begin, const SCEV *End) : Begin(Begin), End(End) {
      assert(Begin->getType() == End->getType() && "ill-typed range!");
    }

    Type *getType() const { return Begin->getType(); }
    const SCEV *getBegin() const { return Begin; }
    const SCEV *getEnd() const { return End; }
    bool isEmpty(ScalarEvolution &SE, bool IsSigned) const {
      if (Begin == End)
        return true;
      if (IsSigned)
        return SE.isKnownPredicate(ICmpInst::ICMP_SGE, Begin, End);
      else
        return SE.isKnownPredicate(ICmpInst::ICMP_UGE, Begin, End);
    }
  };

  /// This is the value the condition of the branch needs to evaluate to for the
  /// branch to take the hot successor (see (1) above).
  bool getPassingDirection() { return true; }

  /// Computes a range for the induction variable (IndVar) in which the range
  /// check is redundant and can be constant-folded away.  The induction
  /// variable is not required to be the canonical {0,+,1} induction variable.
  Optional<Range> computeSafeIterationSpace(ScalarEvolution &SE,
                                            const SCEVAddRecExpr *IndVar,
                                            bool IsLatchSigned) const;

  /// Parse out a set of inductive range checks from \p BI and append them to \p
  /// Checks.
  ///
  /// NB! There may be conditions feeding into \p BI that aren't inductive range
  /// checks, and hence don't end up in \p Checks.
  static void
  extractRangeChecksFromBranch(BranchInst *BI, Loop *L, ScalarEvolution &SE,
                               BranchProbabilityInfo *BPI,
                               SmallVectorImpl<InductiveRangeCheck> &Checks);
};

struct LoopStructure;

class InductiveRangeCheckElimination {
  ScalarEvolution &SE;
  BranchProbabilityInfo *BPI;
  DominatorTree &DT;
  LoopInfo &LI;

  using GetBFIFunc =
      llvm::Optional<llvm::function_ref<llvm::BlockFrequencyInfo &()> >;
  GetBFIFunc GetBFI;

  // Returns true if it is profitable to do a transform basing on estimation of
  // number of iterations.
  bool isProfitableToTransform(const Loop &L, LoopStructure &LS);

public:
  InductiveRangeCheckElimination(ScalarEvolution &SE,
                                 BranchProbabilityInfo *BPI, DominatorTree &DT,
                                 LoopInfo &LI, GetBFIFunc GetBFI = None)
      : SE(SE), BPI(BPI), DT(DT), LI(LI), GetBFI(GetBFI) {}

  bool run(Loop *L, function_ref<void(Loop *, bool)> LPMAddNewLoop);
};

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

  IRCELegacyPass() : FunctionPass(ID) {
    initializeIRCELegacyPassPass(*PassRegistry::getPassRegistry());
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<BranchProbabilityInfoWrapperPass>();
    AU.addRequired<DominatorTreeWrapperPass>();
    AU.addPreserved<DominatorTreeWrapperPass>();
    AU.addRequired<LoopInfoWrapperPass>();
    AU.addPreserved<LoopInfoWrapperPass>();
    AU.addRequired<ScalarEvolutionWrapperPass>();
    AU.addPreserved<ScalarEvolutionWrapperPass>();
  }

  bool runOnFunction(Function &F) override;
};

} // end anonymous namespace

char IRCELegacyPass::ID = 0;

INITIALIZE_PASS_BEGIN(IRCELegacyPass, "irce",
                      "Inductive range check elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(BranchProbabilityInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_END(IRCELegacyPass, "irce", "Inductive range check elimination",
                    false, false)

/// Parse a single ICmp instruction, `ICI`, into a range check.  If `ICI` cannot
/// be interpreted as a range check, return false and set `Index` and `Length`
/// to `nullptr`.  Otherwise set `Index` to the value being range checked, and
/// set `Length` to the upper limit `Index` is being range checked.
bool
InductiveRangeCheck::parseRangeCheckICmp(Loop *L, ICmpInst *ICI,
                                         ScalarEvolution &SE, Value *&Index,
                                         Value *&Length, bool &IsSigned) {
  auto IsLoopInvariant = [&SE, L](Value *V) {
    return SE.isLoopInvariant(SE.getSCEV(V), L);
  };

  ICmpInst::Predicate Pred = ICI->getPredicate();
  Value *LHS = ICI->getOperand(0);
  Value *RHS = ICI->getOperand(1);

  switch (Pred) {
  default:
    return false;

  case ICmpInst::ICMP_SLE:
    std::swap(LHS, RHS);
    LLVM_FALLTHROUGH;
  case ICmpInst::ICMP_SGE:
    IsSigned = true;
    if (match(RHS, m_ConstantInt<0>())) {
      Index = LHS;
      return true; // Lower.
    }
    return false;

  case ICmpInst::ICMP_SLT:
    std::swap(LHS, RHS);
    LLVM_FALLTHROUGH;
  case ICmpInst::ICMP_SGT:
    IsSigned = true;
    if (match(RHS, m_ConstantInt<-1>())) {
      Index = LHS;
      return true; // Lower.
    }

    if (IsLoopInvariant(LHS)) {
      Index = RHS;
      Length = LHS;
      return true; // Upper.
    }
    return false;

  case ICmpInst::ICMP_ULT:
    std::swap(LHS, RHS);
    LLVM_FALLTHROUGH;
  case ICmpInst::ICMP_UGT:
    IsSigned = false;
    if (IsLoopInvariant(LHS)) {
      Index = RHS;
      Length = LHS;
      return true; // Both lower and upper.
    }
    return false;
  }

  llvm_unreachable("default clause returns!");
}

void InductiveRangeCheck::extractRangeChecksFromCond(
    Loop *L, ScalarEvolution &SE, Use &ConditionUse,
    SmallVectorImpl<InductiveRangeCheck> &Checks,
    SmallPtrSetImpl<Value *> &Visited) {
  Value *Condition = ConditionUse.get();
  if (!Visited.insert(Condition).second)
    return;

  // TODO: Do the same for OR, XOR, NOT etc?
  if (match(Condition, m_And(m_Value(), m_Value()))) {
    extractRangeChecksFromCond(L, SE, cast<User>(Condition)->getOperandUse(0),
                               Checks, Visited);
    extractRangeChecksFromCond(L, SE, cast<User>(Condition)->getOperandUse(1),
                               Checks, Visited);
    return;
  }

  ICmpInst *ICI = dyn_cast<ICmpInst>(Condition);
  if (!ICI)
    return;

  Value *Length = nullptr, *Index;
  bool IsSigned;
  if (!parseRangeCheckICmp(L, ICI, SE, Index, Length, IsSigned))
    return;

  const auto *IndexAddRec = dyn_cast<SCEVAddRecExpr>(SE.getSCEV(Index));
  bool IsAffineIndex =
      IndexAddRec && (IndexAddRec->getLoop() == L) && IndexAddRec->isAffine();

  if (!IsAffineIndex)
    return;

  const SCEV *End = nullptr;
  // We strengthen "0 <= I" to "0 <= I < INT_SMAX" and "I < L" to "0 <= I < L".
  // We can potentially do much better here.
  if (Length)
    End = SE.getSCEV(Length);
  else {
    // So far we can only reach this point for Signed range check. This may
    // change in future. In this case we will need to pick Unsigned max for the
    // unsigned range check.
    unsigned BitWidth = cast<IntegerType>(IndexAddRec->getType())->getBitWidth();
    const SCEV *SIntMax = SE.getConstant(APInt::getSignedMaxValue(BitWidth));
    End = SIntMax;
  }

  InductiveRangeCheck IRC;
  IRC.End = End;
  IRC.Begin = IndexAddRec->getStart();
  IRC.Step = IndexAddRec->getStepRecurrence(SE);
  IRC.CheckUse = &ConditionUse;
  Checks.push_back(IRC);
}

void InductiveRangeCheck::extractRangeChecksFromBranch(
    BranchInst *BI, Loop *L, ScalarEvolution &SE, BranchProbabilityInfo *BPI,
    SmallVectorImpl<InductiveRangeCheck> &Checks) {
  if (BI->isUnconditional() || BI->getParent() == L->getLoopLatch())
    return;

  BranchProbability LikelyTaken(15, 16);

  if (!SkipProfitabilityChecks && BPI &&
      BPI->getEdgeProbability(BI->getParent(), (unsigned)0) < LikelyTaken)
    return;

  SmallPtrSet<Value *, 8> Visited;
  InductiveRangeCheck::extractRangeChecksFromCond(L, SE, BI->getOperandUse(0),
                                                  Checks, Visited);
}

// Add metadata to the loop L to disable loop optimizations. Callers need to
// confirm that optimizing loop L is not beneficial.
static void DisableAllLoopOptsOnLoop(Loop &L) {
  // We do not care about any existing loopID related metadata for L, since we
  // are setting all loop metadata to false.
  LLVMContext &Context = L.getHeader()->getContext();
  // Reserve first location for self reference to the LoopID metadata node.
  MDNode *Dummy = MDNode::get(Context, {});
  MDNode *DisableUnroll = MDNode::get(
      Context, {MDString::get(Context, "llvm.loop.unroll.disable")});
  Metadata *FalseVal =
      ConstantAsMetadata::get(ConstantInt::get(Type::getInt1Ty(Context), 0));
  MDNode *DisableVectorize = MDNode::get(
      Context,
      {MDString::get(Context, "llvm.loop.vectorize.enable"), FalseVal});
  MDNode *DisableLICMVersioning = MDNode::get(
      Context, {MDString::get(Context, "llvm.loop.licm_versioning.disable")});
  MDNode *DisableDistribution= MDNode::get(
      Context,
      {MDString::get(Context, "llvm.loop.distribute.enable"), FalseVal});
  MDNode *NewLoopID =
      MDNode::get(Context, {Dummy, DisableUnroll, DisableVectorize,
                            DisableLICMVersioning, DisableDistribution});
  // Set operand 0 to refer to the loop id itself.
  NewLoopID->replaceOperandWith(0, NewLoopID);
  L.setLoopID(NewLoopID);
}

namespace {

// Keeps track of the structure of a loop.  This is similar to llvm::Loop,
// except that it is more lightweight and can track the state of a loop through
// changing and potentially invalid IR.  This structure also formalizes the
// kinds of loops we can deal with -- ones that have a single latch that is also
// an exiting block *and* have a canonical induction variable.
struct LoopStructure {
  const char *Tag = "";

  BasicBlock *Header = nullptr;
  BasicBlock *Latch = nullptr;

  // `Latch's terminator instruction is `LatchBr', and it's `LatchBrExitIdx'th
  // successor is `LatchExit', the exit block of the loop.
  BranchInst *LatchBr = nullptr;
  BasicBlock *LatchExit = nullptr;
  unsigned LatchBrExitIdx = std::numeric_limits<unsigned>::max();

  // The loop represented by this instance of LoopStructure is semantically
  // equivalent to:
  //
  // intN_ty inc = IndVarIncreasing ? 1 : -1;
  // pred_ty predicate = IndVarIncreasing ? ICMP_SLT : ICMP_SGT;
  //
  // for (intN_ty iv = IndVarStart; predicate(iv, LoopExitAt); iv = IndVarBase)
  //   ... body ...

  Value *IndVarBase = nullptr;
  Value *IndVarStart = nullptr;
  Value *IndVarStep = nullptr;
  Value *LoopExitAt = nullptr;
  bool IndVarIncreasing = false;
  bool IsSignedPredicate = true;

  LoopStructure() = default;

  template <typename M> LoopStructure map(M Map) const {
    LoopStructure Result;
    Result.Tag = Tag;
    Result.Header = cast<BasicBlock>(Map(Header));
    Result.Latch = cast<BasicBlock>(Map(Latch));
    Result.LatchBr = cast<BranchInst>(Map(LatchBr));
    Result.LatchExit = cast<BasicBlock>(Map(LatchExit));
    Result.LatchBrExitIdx = LatchBrExitIdx;
    Result.IndVarBase = Map(IndVarBase);
    Result.IndVarStart = Map(IndVarStart);
    Result.IndVarStep = Map(IndVarStep);
    Result.LoopExitAt = Map(LoopExitAt);
    Result.IndVarIncreasing = IndVarIncreasing;
    Result.IsSignedPredicate = IsSignedPredicate;
    return Result;
  }

  static Optional<LoopStructure> parseLoopStructure(ScalarEvolution &, Loop &,
                                                    const char *&);
};

/// This class is used to constrain loops to run within a given iteration space.
/// The algorithm this class implements is given a Loop and a range [Begin,
/// End).  The algorithm then tries to break out a "main loop" out of the loop
/// it is given in a way that the "main loop" runs with the induction variable
/// in a subset of [Begin, End).  The algorithm emits appropriate pre and post
/// loops to run any remaining iterations.  The pre loop runs any iterations in
/// which the induction variable is < Begin, and the post loop runs any
/// iterations in which the induction variable is >= End.
class LoopConstrainer {
  // The representation of a clone of the original loop we started out with.
  struct ClonedLoop {
    // The cloned blocks
    std::vector<BasicBlock *> Blocks;

    // `Map` maps values in the clonee into values in the cloned version
    ValueToValueMapTy Map;

    // An instance of `LoopStructure` for the cloned loop
    LoopStructure Structure;
  };

  // Result of rewriting the range of a loop.  See changeIterationSpaceEnd for
  // more details on what these fields mean.
  struct RewrittenRangeInfo {
    BasicBlock *PseudoExit = nullptr;
    BasicBlock *ExitSelector = nullptr;
    std::vector<PHINode *> PHIValuesAtPseudoExit;
    PHINode *IndVarEnd = nullptr;

    RewrittenRangeInfo() = default;
  };

  // Calculated subranges we restrict the iteration space of the main loop to.
  // See the implementation of `calculateSubRanges' for more details on how
  // these fields are computed.  `LowLimit` is None if there is no restriction
  // on low end of the restricted iteration space of the main loop.  `HighLimit`
  // is None if there is no restriction on high end of the restricted iteration
  // space of the main loop.

  struct SubRanges {
    Optional<const SCEV *> LowLimit;
    Optional<const SCEV *> HighLimit;
  };

  // Compute a safe set of limits for the main loop to run in -- effectively the
  // intersection of `Range' and the iteration space of the original loop.
  // Return None if unable to compute the set of subranges.
  Optional<SubRanges> calculateSubRanges(bool IsSignedPredicate) const;

  // Clone `OriginalLoop' and return the result in CLResult.  The IR after
  // running `cloneLoop' is well formed except for the PHI nodes in CLResult --
  // the PHI nodes say that there is an incoming edge from `OriginalPreheader`
  // but there is no such edge.
  void cloneLoop(ClonedLoop &CLResult, const char *Tag) const;

  // Create the appropriate loop structure needed to describe a cloned copy of
  // `Original`.  The clone is described by `VM`.
  Loop *createClonedLoopStructure(Loop *Original, Loop *Parent,
                                  ValueToValueMapTy &VM, bool IsSubloop);

  // Rewrite the iteration space of the loop denoted by (LS, Preheader). The
  // iteration space of the rewritten loop ends at ExitLoopAt.  The start of the
  // iteration space is not changed.  `ExitLoopAt' is assumed to be slt
  // `OriginalHeaderCount'.
  //
  // If there are iterations left to execute, control is made to jump to
  // `ContinuationBlock', otherwise they take the normal loop exit.  The
  // returned `RewrittenRangeInfo' object is populated as follows:
  //
  //  .PseudoExit is a basic block that unconditionally branches to
  //      `ContinuationBlock'.
  //
  //  .ExitSelector is a basic block that decides, on exit from the loop,
  //      whether to branch to the "true" exit or to `PseudoExit'.
  //
  //  .PHIValuesAtPseudoExit are PHINodes in `PseudoExit' that compute the value
  //      for each PHINode in the loop header on taking the pseudo exit.
  //
  // After changeIterationSpaceEnd, `Preheader' is no longer a legitimate
  // preheader because it is made to branch to the loop header only
  // conditionally.
  RewrittenRangeInfo
  changeIterationSpaceEnd(const LoopStructure &LS, BasicBlock *Preheader,
                          Value *ExitLoopAt,
                          BasicBlock *ContinuationBlock) const;

  // The loop denoted by `LS' has `OldPreheader' as its preheader.  This
  // function creates a new preheader for `LS' and returns it.
  BasicBlock *createPreheader(const LoopStructure &LS, BasicBlock *OldPreheader,
                              const char *Tag) const;

  // `ContinuationBlockAndPreheader' was the continuation block for some call to
  // `changeIterationSpaceEnd' and is the preheader to the loop denoted by `LS'.
  // This function rewrites the PHI nodes in `LS.Header' to start with the
  // correct value.
  void rewriteIncomingValuesForPHIs(
      LoopStructure &LS, BasicBlock *ContinuationBlockAndPreheader,
      const LoopConstrainer::RewrittenRangeInfo &RRI) const;

  // Even though we do not preserve any passes at this time, we at least need to
  // keep the parent loop structure consistent.  The `LPPassManager' seems to
  // verify this after running a loop pass.  This function adds the list of
  // blocks denoted by BBs to this loops parent loop if required.
  void addToParentLoopIfNeeded(ArrayRef<BasicBlock *> BBs);

  // Some global state.
  Function &F;
  LLVMContext &Ctx;
  ScalarEvolution &SE;
  DominatorTree &DT;
  LoopInfo &LI;
  function_ref<void(Loop *, bool)> LPMAddNewLoop;

  // Information about the original loop we started out with.
  Loop &OriginalLoop;

  const SCEV *LatchTakenCount = nullptr;
  BasicBlock *OriginalPreheader = nullptr;

  // The preheader of the main loop.  This may or may not be different from
  // `OriginalPreheader'.
  BasicBlock *MainLoopPreheader = nullptr;

  // The range we need to run the main loop in.
  InductiveRangeCheck::Range Range;

  // The structure of the main loop (see comment at the beginning of this class
  // for a definition)
  LoopStructure MainLoopStructure;

public:
  LoopConstrainer(Loop &L, LoopInfo &LI,
                  function_ref<void(Loop *, bool)> LPMAddNewLoop,
                  const LoopStructure &LS, ScalarEvolution &SE,
                  DominatorTree &DT, InductiveRangeCheck::Range R)
      : F(*L.getHeader()->getParent()), Ctx(L.getHeader()->getContext()),
        SE(SE), DT(DT), LI(LI), LPMAddNewLoop(LPMAddNewLoop), OriginalLoop(L),
        Range(R), MainLoopStructure(LS) {}

  // Entry point for the algorithm.  Returns true on success.
  bool run();
};

} // end anonymous namespace

/// Given a loop with an deccreasing induction variable, is it possible to
/// safely calculate the bounds of a new loop using the given Predicate.
static bool isSafeDecreasingBound(const SCEV *Start,
                                  const SCEV *BoundSCEV, const SCEV *Step,
                                  ICmpInst::Predicate Pred,
                                  unsigned LatchBrExitIdx,
                                  Loop *L, ScalarEvolution &SE) {
  if (Pred != ICmpInst::ICMP_SLT && Pred != ICmpInst::ICMP_SGT &&
      Pred != ICmpInst::ICMP_ULT && Pred != ICmpInst::ICMP_UGT)
    return false;

  if (!SE.isAvailableAtLoopEntry(BoundSCEV, L))
    return false;

  assert(SE.isKnownNegative(Step) && "expecting negative step");

  LLVM_DEBUG(dbgs() << "irce: isSafeDecreasingBound with:\n");
  LLVM_DEBUG(dbgs() << "irce: Start: " << *Start << "\n");
  LLVM_DEBUG(dbgs() << "irce: Step: " << *Step << "\n");
  LLVM_DEBUG(dbgs() << "irce: BoundSCEV: " << *BoundSCEV << "\n");
  LLVM_DEBUG(dbgs() << "irce: Pred: " << ICmpInst::getPredicateName(Pred)
                    << "\n");
  LLVM_DEBUG(dbgs() << "irce: LatchExitBrIdx: " << LatchBrExitIdx << "\n");

  bool IsSigned = ICmpInst::isSigned(Pred);
  // The predicate that we need to check that the induction variable lies
  // within bounds.
  ICmpInst::Predicate BoundPred =
    IsSigned ? CmpInst::ICMP_SGT : CmpInst::ICMP_UGT;

  if (LatchBrExitIdx == 1)
    return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, BoundSCEV);

  assert(LatchBrExitIdx == 0 &&
         "LatchBrExitIdx should be either 0 or 1");

  const SCEV *StepPlusOne = SE.getAddExpr(Step, SE.getOne(Step->getType()));
  unsigned BitWidth = cast<IntegerType>(BoundSCEV->getType())->getBitWidth();
  APInt Min = IsSigned ? APInt::getSignedMinValue(BitWidth) :
    APInt::getMinValue(BitWidth);
  const SCEV *Limit = SE.getMinusSCEV(SE.getConstant(Min), StepPlusOne);

  const SCEV *MinusOne =
    SE.getMinusSCEV(BoundSCEV, SE.getOne(BoundSCEV->getType()));

  return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, MinusOne) &&
         SE.isLoopEntryGuardedByCond(L, BoundPred, BoundSCEV, Limit);

}

/// Given a loop with an increasing induction variable, is it possible to
/// safely calculate the bounds of a new loop using the given Predicate.
static bool isSafeIncreasingBound(const SCEV *Start,
                                  const SCEV *BoundSCEV, const SCEV *Step,
                                  ICmpInst::Predicate Pred,
                                  unsigned LatchBrExitIdx,
                                  Loop *L, ScalarEvolution &SE) {
  if (Pred != ICmpInst::ICMP_SLT && Pred != ICmpInst::ICMP_SGT &&
      Pred != ICmpInst::ICMP_ULT && Pred != ICmpInst::ICMP_UGT)
    return false;

  if (!SE.isAvailableAtLoopEntry(BoundSCEV, L))
    return false;

  LLVM_DEBUG(dbgs() << "irce: isSafeIncreasingBound with:\n");
  LLVM_DEBUG(dbgs() << "irce: Start: " << *Start << "\n");
  LLVM_DEBUG(dbgs() << "irce: Step: " << *Step << "\n");
  LLVM_DEBUG(dbgs() << "irce: BoundSCEV: " << *BoundSCEV << "\n");
  LLVM_DEBUG(dbgs() << "irce: Pred: " << ICmpInst::getPredicateName(Pred)
                    << "\n");
  LLVM_DEBUG(dbgs() << "irce: LatchExitBrIdx: " << LatchBrExitIdx << "\n");

  bool IsSigned = ICmpInst::isSigned(Pred);
  // The predicate that we need to check that the induction variable lies
  // within bounds.
  ICmpInst::Predicate BoundPred =
      IsSigned ? CmpInst::ICMP_SLT : CmpInst::ICMP_ULT;

  if (LatchBrExitIdx == 1)
    return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, BoundSCEV);

  assert(LatchBrExitIdx == 0 && "LatchBrExitIdx should be 0 or 1");

  const SCEV *StepMinusOne =
    SE.getMinusSCEV(Step, SE.getOne(Step->getType()));
  unsigned BitWidth = cast<IntegerType>(BoundSCEV->getType())->getBitWidth();
  APInt Max = IsSigned ? APInt::getSignedMaxValue(BitWidth) :
    APInt::getMaxValue(BitWidth);
  const SCEV *Limit = SE.getMinusSCEV(SE.getConstant(Max), StepMinusOne);

  return (SE.isLoopEntryGuardedByCond(L, BoundPred, Start,
                                      SE.getAddExpr(BoundSCEV, Step)) &&
          SE.isLoopEntryGuardedByCond(L, BoundPred, BoundSCEV, Limit));
}

Optional<LoopStructure>
LoopStructure::parseLoopStructure(ScalarEvolution &SE, Loop &L,
                                  const char *&FailureReason) {
  if (!L.isLoopSimplifyForm()) {
    FailureReason = "loop not in LoopSimplify form";
    return None;
  }

  BasicBlock *Latch = L.getLoopLatch();
  assert(Latch && "Simplified loops only have one latch!");

  if (Latch->getTerminator()->getMetadata(ClonedLoopTag)) {
    FailureReason = "loop has already been cloned";
    return None;
  }

  if (!L.isLoopExiting(Latch)) {
    FailureReason = "no loop latch";
    return None;
  }

  BasicBlock *Header = L.getHeader();
  BasicBlock *Preheader = L.getLoopPreheader();
  if (!Preheader) {
    FailureReason = "no preheader";
    return None;
  }

  BranchInst *LatchBr = dyn_cast<BranchInst>(Latch->getTerminator());
  if (!LatchBr || LatchBr->isUnconditional()) {
    FailureReason = "latch terminator not conditional branch";
    return None;
  }

  unsigned LatchBrExitIdx = LatchBr->getSuccessor(0) == Header ? 1 : 0;

  ICmpInst *ICI = dyn_cast<ICmpInst>(LatchBr->getCondition());
  if (!ICI || !isa<IntegerType>(ICI->getOperand(0)->getType())) {
    FailureReason = "latch terminator branch not conditional on integral icmp";
    return None;
  }

  const SCEV *LatchCount = SE.getExitCount(&L, Latch);
  if (isa<SCEVCouldNotCompute>(LatchCount)) {
    FailureReason = "could not compute latch count";
    return None;
  }

  ICmpInst::Predicate Pred = ICI->getPredicate();
  Value *LeftValue = ICI->getOperand(0);
  const SCEV *LeftSCEV = SE.getSCEV(LeftValue);
  IntegerType *IndVarTy = cast<IntegerType>(LeftValue->getType());

  Value *RightValue = ICI->getOperand(1);
  const SCEV *RightSCEV = SE.getSCEV(RightValue);

  // We canonicalize `ICI` such that `LeftSCEV` is an add recurrence.
  if (!isa<SCEVAddRecExpr>(LeftSCEV)) {
    if (isa<SCEVAddRecExpr>(RightSCEV)) {
      std::swap(LeftSCEV, RightSCEV);
      std::swap(LeftValue, RightValue);
      Pred = ICmpInst::getSwappedPredicate(Pred);
    } else {
      FailureReason = "no add recurrences in the icmp";
      return None;
    }
  }

  auto HasNoSignedWrap = [&](const SCEVAddRecExpr *AR) {
    if (AR->getNoWrapFlags(SCEV::FlagNSW))
      return true;

    IntegerType *Ty = cast<IntegerType>(AR->getType());
    IntegerType *WideTy =
        IntegerType::get(Ty->getContext(), Ty->getBitWidth() * 2);

    const SCEVAddRecExpr *ExtendAfterOp =
        dyn_cast<SCEVAddRecExpr>(SE.getSignExtendExpr(AR, WideTy));
    if (ExtendAfterOp) {
      const SCEV *ExtendedStart = SE.getSignExtendExpr(AR->getStart(), WideTy);
      const SCEV *ExtendedStep =
          SE.getSignExtendExpr(AR->getStepRecurrence(SE), WideTy);

      bool NoSignedWrap = ExtendAfterOp->getStart() == ExtendedStart &&
                          ExtendAfterOp->getStepRecurrence(SE) == ExtendedStep;

      if (NoSignedWrap)
        return true;
    }

    // We may have proved this when computing the sign extension above.
    return AR->getNoWrapFlags(SCEV::FlagNSW) != SCEV::FlagAnyWrap;
  };

  // `ICI` is interpreted as taking the backedge if the *next* value of the
  // induction variable satisfies some constraint.

  const SCEVAddRecExpr *IndVarBase = cast<SCEVAddRecExpr>(LeftSCEV);
  if (!IndVarBase->isAffine()) {
    FailureReason = "LHS in icmp not induction variable";
    return None;
  }
  const SCEV* StepRec = IndVarBase->getStepRecurrence(SE);
  if (!isa<SCEVConstant>(StepRec)) {
    FailureReason = "LHS in icmp not induction variable";
    return None;
  }
  ConstantInt *StepCI = cast<SCEVConstant>(StepRec)->getValue();

  if (ICI->isEquality() && !HasNoSignedWrap(IndVarBase)) {
    FailureReason = "LHS in icmp needs nsw for equality predicates";
    return None;
  }

  assert(!StepCI->isZero() && "Zero step?");
  bool IsIncreasing = !StepCI->isNegative();
  bool IsSignedPredicate;
  const SCEV *StartNext = IndVarBase->getStart();
  const SCEV *Addend = SE.getNegativeSCEV(IndVarBase->getStepRecurrence(SE));
  const SCEV *IndVarStart = SE.getAddExpr(StartNext, Addend);
  const SCEV *Step = SE.getSCEV(StepCI);

  const SCEV *FixedRightSCEV = nullptr;

  // If RightValue resides within loop (but still being loop invariant),
  // regenerate it as preheader.
  if (auto *I = dyn_cast<Instruction>(RightValue))
    if (L.contains(I->getParent()))
      FixedRightSCEV = RightSCEV;

  if (IsIncreasing) {
    bool DecreasedRightValueByOne = false;
    if (StepCI->isOne()) {
      // Try to turn eq/ne predicates to those we can work with.
      if (Pred == ICmpInst::ICMP_NE && LatchBrExitIdx == 1)
        // while (++i != len) {         while (++i < len) {
        //   ...                 --->     ...
        // }                            }
        // If both parts are known non-negative, it is profitable to use
        // unsigned comparison in increasing loop. This allows us to make the
        // comparison check against "RightSCEV + 1" more optimistic.
        if (isKnownNonNegativeInLoop(IndVarStart, &L, SE) &&
            isKnownNonNegativeInLoop(RightSCEV, &L, SE))
          Pred = ICmpInst::ICMP_ULT;
        else
          Pred = ICmpInst::ICMP_SLT;
      else if (Pred == ICmpInst::ICMP_EQ && LatchBrExitIdx == 0) {
        // while (true) {               while (true) {
        //   if (++i == len)     --->     if (++i > len - 1)
        //     break;                       break;
        //   ...                          ...
        // }                            }
        if (IndVarBase->getNoWrapFlags(SCEV::FlagNUW) &&
            cannotBeMinInLoop(RightSCEV, &L, SE, /*Signed*/false)) {
          Pred = ICmpInst::ICMP_UGT;
          RightSCEV = SE.getMinusSCEV(RightSCEV,
                                      SE.getOne(RightSCEV->getType()));
          DecreasedRightValueByOne = true;
        } else if (cannotBeMinInLoop(RightSCEV, &L, SE, /*Signed*/true)) {
          Pred = ICmpInst::ICMP_SGT;
          RightSCEV = SE.getMinusSCEV(RightSCEV,
                                      SE.getOne(RightSCEV->getType()));
          DecreasedRightValueByOne = true;
        }
      }
    }

    bool LTPred = (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_ULT);
    bool GTPred = (Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_UGT);
    bool FoundExpectedPred =
        (LTPred && LatchBrExitIdx == 1) || (GTPred && LatchBrExitIdx == 0);

    if (!FoundExpectedPred) {
      FailureReason = "expected icmp slt semantically, found something else";
      return None;
    }

    IsSignedPredicate = ICmpInst::isSigned(Pred);
    if (!IsSignedPredicate && !AllowUnsignedLatchCondition) {
      FailureReason = "unsigned latch conditions are explicitly prohibited";
      return None;
    }

    if (!isSafeIncreasingBound(IndVarStart, RightSCEV, Step, Pred,
                               LatchBrExitIdx, &L, SE)) {
      FailureReason = "Unsafe loop bounds";
      return None;
    }
    if (LatchBrExitIdx == 0) {
      // We need to increase the right value unless we have already decreased
      // it virtually when we replaced EQ with SGT.
      if (!DecreasedRightValueByOne)
        FixedRightSCEV =
            SE.getAddExpr(RightSCEV, SE.getOne(RightSCEV->getType()));
    } else {
      assert(!DecreasedRightValueByOne &&
             "Right value can be decreased only for LatchBrExitIdx == 0!");
    }
  } else {
    bool IncreasedRightValueByOne = false;
    if (StepCI->isMinusOne()) {
      // Try to turn eq/ne predicates to those we can work with.
      if (Pred == ICmpInst::ICMP_NE && LatchBrExitIdx == 1)
        // while (--i != len) {         while (--i > len) {
        //   ...                 --->     ...
        // }                            }
        // We intentionally don't turn the predicate into UGT even if we know
        // that both operands are non-negative, because it will only pessimize
        // our check against "RightSCEV - 1".
        Pred = ICmpInst::ICMP_SGT;
      else if (Pred == ICmpInst::ICMP_EQ && LatchBrExitIdx == 0) {
        // while (true) {               while (true) {
        //   if (--i == len)     --->     if (--i < len + 1)
        //     break;                       break;
        //   ...                          ...
        // }                            }
        if (IndVarBase->getNoWrapFlags(SCEV::FlagNUW) &&
            cannotBeMaxInLoop(RightSCEV, &L, SE, /* Signed */ false)) {
          Pred = ICmpInst::ICMP_ULT;
          RightSCEV = SE.getAddExpr(RightSCEV, SE.getOne(RightSCEV->getType()));
          IncreasedRightValueByOne = true;
        } else if (cannotBeMaxInLoop(RightSCEV, &L, SE, /* Signed */ true)) {
          Pred = ICmpInst::ICMP_SLT;
          RightSCEV = SE.getAddExpr(RightSCEV, SE.getOne(RightSCEV->getType()));
          IncreasedRightValueByOne = true;
        }
      }
    }

    bool LTPred = (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_ULT);
    bool GTPred = (Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_UGT);

    bool FoundExpectedPred =
        (GTPred && LatchBrExitIdx == 1) || (LTPred && LatchBrExitIdx == 0);

    if (!FoundExpectedPred) {
      FailureReason = "expected icmp sgt semantically, found something else";
      return None;
    }

    IsSignedPredicate =
        Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SGT;

    if (!IsSignedPredicate && !AllowUnsignedLatchCondition) {
      FailureReason = "unsigned latch conditions are explicitly prohibited";
      return None;
    }

    if (!isSafeDecreasingBound(IndVarStart, RightSCEV, Step, Pred,
                               LatchBrExitIdx, &L, SE)) {
      FailureReason = "Unsafe bounds";
      return None;
    }

    if (LatchBrExitIdx == 0) {
      // We need to decrease the right value unless we have already increased
      // it virtually when we replaced EQ with SLT.
      if (!IncreasedRightValueByOne)
        FixedRightSCEV =
            SE.getMinusSCEV(RightSCEV, SE.getOne(RightSCEV->getType()));
    } else {
      assert(!IncreasedRightValueByOne &&
             "Right value can be increased only for LatchBrExitIdx == 0!");
    }
  }
  BasicBlock *LatchExit = LatchBr->getSuccessor(LatchBrExitIdx);

  assert(SE.getLoopDisposition(LatchCount, &L) ==
             ScalarEvolution::LoopInvariant &&
         "loop variant exit count doesn't make sense!");

  assert(!L.contains(LatchExit) && "expected an exit block!");
  const DataLayout &DL = Preheader->getModule()->getDataLayout();
  SCEVExpander Expander(SE, DL, "irce");
  Instruction *Ins = Preheader->getTerminator();

  if (FixedRightSCEV)
    RightValue =
        Expander.expandCodeFor(FixedRightSCEV, FixedRightSCEV->getType(), Ins);

  Value *IndVarStartV = Expander.expandCodeFor(IndVarStart, IndVarTy, Ins);
  IndVarStartV->setName("indvar.start");

  LoopStructure Result;

  Result.Tag = "main";
  Result.Header = Header;
  Result.Latch = Latch;
  Result.LatchBr = LatchBr;
  Result.LatchExit = LatchExit;
  Result.LatchBrExitIdx = LatchBrExitIdx;
  Result.IndVarStart = IndVarStartV;
  Result.IndVarStep = StepCI;
  Result.IndVarBase = LeftValue;
  Result.IndVarIncreasing = IsIncreasing;
  Result.LoopExitAt = RightValue;
  Result.IsSignedPredicate = IsSignedPredicate;

  FailureReason = nullptr;

  return Result;
}

/// If the type of \p S matches with \p Ty, return \p S. Otherwise, return
/// signed or unsigned extension of \p S to type \p Ty.
static const SCEV *NoopOrExtend(const SCEV *S, Type *Ty, ScalarEvolution &SE,
                                bool Signed) {
  return Signed ? SE.getNoopOrSignExtend(S, Ty) : SE.getNoopOrZeroExtend(S, Ty);
}

Optional<LoopConstrainer::SubRanges>
LoopConstrainer::calculateSubRanges(bool IsSignedPredicate) const {
  IntegerType *Ty = cast<IntegerType>(LatchTakenCount->getType());

  auto *RTy = cast<IntegerType>(Range.getType());

  // We only support wide range checks and narrow latches.
  if (!AllowNarrowLatchCondition && RTy != Ty)
    return None;
  if (RTy->getBitWidth() < Ty->getBitWidth())
    return None;

  LoopConstrainer::SubRanges Result;

  // I think we can be more aggressive here and make this nuw / nsw if the
  // addition that feeds into the icmp for the latch's terminating branch is nuw
  // / nsw.  In any case, a wrapping 2's complement addition is safe.
  const SCEV *Start = NoopOrExtend(SE.getSCEV(MainLoopStructure.IndVarStart),
                                   RTy, SE, IsSignedPredicate);
  const SCEV *End = NoopOrExtend(SE.getSCEV(MainLoopStructure.LoopExitAt), RTy,
                                 SE, IsSignedPredicate);

  bool Increasing = MainLoopStructure.IndVarIncreasing;

  // We compute `Smallest` and `Greatest` such that [Smallest, Greatest), or
  // [Smallest, GreatestSeen] is the range of values the induction variable
  // takes.

  const SCEV *Smallest = nullptr, *Greatest = nullptr, *GreatestSeen = nullptr;

  const SCEV *One = SE.getOne(RTy);
  if (Increasing) {
    Smallest = Start;
    Greatest = End;
    // No overflow, because the range [Smallest, GreatestSeen] is not empty.
    GreatestSeen = SE.getMinusSCEV(End, One);
  } else {
    // These two computations may sign-overflow.  Here is why that is okay:
    //
    // We know that the induction variable does not sign-overflow on any
    // iteration except the last one, and it starts at `Start` and ends at
    // `End`, decrementing by one every time.
    //
    //  * if `Smallest` sign-overflows we know `End` is `INT_SMAX`. Since the
    //    induction variable is decreasing we know that that the smallest value
    //    the loop body is actually executed with is `INT_SMIN` == `Smallest`.
    //
    //  * if `Greatest` sign-overflows, we know it can only be `INT_SMIN`.  In
    //    that case, `Clamp` will always return `Smallest` and
    //    [`Result.LowLimit`, `Result.HighLimit`) = [`Smallest`, `Smallest`)
    //    will be an empty range.  Returning an empty range is always safe.

    Smallest = SE.getAddExpr(End, One);
    Greatest = SE.getAddExpr(Start, One);
    GreatestSeen = Start;
  }

  auto Clamp = [this, Smallest, Greatest, IsSignedPredicate](const SCEV *S) {
    return IsSignedPredicate
               ? SE.getSMaxExpr(Smallest, SE.getSMinExpr(Greatest, S))
               : SE.getUMaxExpr(Smallest, SE.getUMinExpr(Greatest, S));
  };

  // In some cases we can prove that we don't need a pre or post loop.
  ICmpInst::Predicate PredLE =
      IsSignedPredicate ? ICmpInst::ICMP_SLE : ICmpInst::ICMP_ULE;
  ICmpInst::Predicate PredLT =
      IsSignedPredicate ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;

  bool ProvablyNoPreloop =
      SE.isKnownPredicate(PredLE, Range.getBegin(), Smallest);
  if (!ProvablyNoPreloop)
    Result.LowLimit = Clamp(Range.getBegin());

  bool ProvablyNoPostLoop =
      SE.isKnownPredicate(PredLT, GreatestSeen, Range.getEnd());
  if (!ProvablyNoPostLoop)
    Result.HighLimit = Clamp(Range.getEnd());

  return Result;
}

void LoopConstrainer::cloneLoop(LoopConstrainer::ClonedLoop &Result,
                                const char *Tag) const {
  for (BasicBlock *BB : OriginalLoop.getBlocks()) {
    BasicBlock *Clone = CloneBasicBlock(BB, Result.Map, Twine(".") + Tag, &F);
    Result.Blocks.push_back(Clone);
    Result.Map[BB] = Clone;
  }

  auto GetClonedValue = [&Result](Value *V) {
    assert(V && "null values not in domain!");
    auto It = Result.Map.find(V);
    if (It == Result.Map.end())
      return V;
    return static_cast<Value *>(It->second);
  };

  auto *ClonedLatch =
      cast<BasicBlock>(GetClonedValue(OriginalLoop.getLoopLatch()));
  ClonedLatch->getTerminator()->setMetadata(ClonedLoopTag,
                                            MDNode::get(Ctx, {}));

  Result.Structure = MainLoopStructure.map(GetClonedValue);
  Result.Structure.Tag = Tag;

  for (unsigned i = 0, e = Result.Blocks.size(); i != e; ++i) {
    BasicBlock *ClonedBB = Result.Blocks[i];
    BasicBlock *OriginalBB = OriginalLoop.getBlocks()[i];

    assert(Result.Map[OriginalBB] == ClonedBB && "invariant!");

    for (Instruction &I : *ClonedBB)
      RemapInstruction(&I, Result.Map,
                       RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);

    // Exit blocks will now have one more predecessor and their PHI nodes need
    // to be edited to reflect that.  No phi nodes need to be introduced because
    // the loop is in LCSSA.

    for (auto *SBB : successors(OriginalBB)) {
      if (OriginalLoop.contains(SBB))
        continue; // not an exit block

      for (PHINode &PN : SBB->phis()) {
        Value *OldIncoming = PN.getIncomingValueForBlock(OriginalBB);
        PN.addIncoming(GetClonedValue(OldIncoming), ClonedBB);
      }
    }
  }
}

LoopConstrainer::RewrittenRangeInfo LoopConstrainer::changeIterationSpaceEnd(
    const LoopStructure &LS, BasicBlock *Preheader, Value *ExitSubloopAt,
    BasicBlock *ContinuationBlock) const {
  // We start with a loop with a single latch:
  //
  //    +--------------------+
  //    |                    |
  //    |     preheader      |
  //    |                    |
  //    +--------+-----------+
  //             |      ----------------\
  //             |     /                |
  //    +--------v----v------+          |
  //    |                    |          |
  //    |      header        |          |
  //    |                    |          |
  //    +--------------------+          |
  //                                    |
  //            .....                   |
  //                                    |
  //    +--------------------+          |
  //    |                    |          |
  //    |       latch        >----------/
  //    |                    |
  //    +-------v------------+
  //            |
  //            |
  //            |   +--------------------+
  //            |   |                    |
  //            +--->   original exit    |
  //                |                    |
  //                +--------------------+
  //
  // We change the control flow to look like
  //
  //
  //    +--------------------+
  //    |                    |
  //    |     preheader      >-------------------------+
  //    |                    |                         |
  //    +--------v-----------+                         |
  //             |    /-------------+                  |
  //             |   /              |                  |
  //    +--------v--v--------+      |                  |
  //    |                    |      |                  |
  //    |      header        |      |   +--------+     |
  //    |                    |      |   |        |     |
  //    +--------------------+      |   |  +-----v-----v-----------+
  //                                |   |  |                       |
  //                                |   |  |     .pseudo.exit      |
  //                                |   |  |                       |
  //                                |   |  +-----------v-----------+
  //                                |   |              |
  //            .....               |   |              |
  //                                |   |     +--------v-------------+
  //    +--------------------+      |   |     |                      |
  //    |                    |      |   |     |   ContinuationBlock  |
  //    |       latch        >------+   |     |                      |
  //    |                    |          |     +----------------------+
  //    +---------v----------+          |
  //              |                     |
  //              |                     |
  //              |     +---------------^-----+
  //              |     |                     |
  //              +----->    .exit.selector   |
  //                    |                     |
  //                    +----------v----------+
  //                               |
  //     +--------------------+    |
  //     |                    |    |
  //     |   original exit    <----+
  //     |                    |
  //     +--------------------+

  RewrittenRangeInfo RRI;

  BasicBlock *BBInsertLocation = LS.Latch->getNextNode();
  RRI.ExitSelector = BasicBlock::Create(Ctx, Twine(LS.Tag) + ".exit.selector",
                                        &F, BBInsertLocation);
  RRI.PseudoExit = BasicBlock::Create(Ctx, Twine(LS.Tag) + ".pseudo.exit", &F,
                                      BBInsertLocation);

  BranchInst *PreheaderJump = cast<BranchInst>(Preheader->getTerminator());
  bool Increasing = LS.IndVarIncreasing;
  bool IsSignedPredicate = LS.IsSignedPredicate;

  IRBuilder<> B(PreheaderJump);
  auto *RangeTy = Range.getBegin()->getType();
  auto NoopOrExt = [&](Value *V) {
    if (V->getType() == RangeTy)
      return V;
    return IsSignedPredicate ? B.CreateSExt(V, RangeTy, "wide." + V->getName())
                             : B.CreateZExt(V, RangeTy, "wide." + V->getName());
  };

  // EnterLoopCond - is it okay to start executing this `LS'?
  Value *EnterLoopCond = nullptr;
  auto Pred =
      Increasing
          ? (IsSignedPredicate ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT)
          : (IsSignedPredicate ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT);
  Value *IndVarStart = NoopOrExt(LS.IndVarStart);
  EnterLoopCond = B.CreateICmp(Pred, IndVarStart, ExitSubloopAt);

  B.CreateCondBr(EnterLoopCond, LS.Header, RRI.PseudoExit);
  PreheaderJump->eraseFromParent();

  LS.LatchBr->setSuccessor(LS.LatchBrExitIdx, RRI.ExitSelector);
  B.SetInsertPoint(LS.LatchBr);
  Value *IndVarBase = NoopOrExt(LS.IndVarBase);
  Value *TakeBackedgeLoopCond = B.CreateICmp(Pred, IndVarBase, ExitSubloopAt);

  Value *CondForBranch = LS.LatchBrExitIdx == 1
                             ? TakeBackedgeLoopCond
                             : B.CreateNot(TakeBackedgeLoopCond);

  LS.LatchBr->setCondition(CondForBranch);

  B.SetInsertPoint(RRI.ExitSelector);

  // IterationsLeft - are there any more iterations left, given the original
  // upper bound on the induction variable?  If not, we branch to the "real"
  // exit.
  Value *LoopExitAt = NoopOrExt(LS.LoopExitAt);
  Value *IterationsLeft = B.CreateICmp(Pred, IndVarBase, LoopExitAt);
  B.CreateCondBr(IterationsLeft, RRI.PseudoExit, LS.LatchExit);

  BranchInst *BranchToContinuation =
      BranchInst::Create(ContinuationBlock, RRI.PseudoExit);

  // We emit PHI nodes into `RRI.PseudoExit' that compute the "latest" value of
  // each of the PHI nodes in the loop header.  This feeds into the initial
  // value of the same PHI nodes if/when we continue execution.
  for (PHINode &PN : LS.Header->phis()) {
    PHINode *NewPHI = PHINode::Create(PN.getType(), 2, PN.getName() + ".copy",
                                      BranchToContinuation);

    NewPHI->addIncoming(PN.getIncomingValueForBlock(Preheader), Preheader);
    NewPHI->addIncoming(PN.getIncomingValueForBlock(LS.Latch),
                        RRI.ExitSelector);
    RRI.PHIValuesAtPseudoExit.push_back(NewPHI);
  }

  RRI.IndVarEnd = PHINode::Create(IndVarBase->getType(), 2, "indvar.end",
                                  BranchToContinuation);
  RRI.IndVarEnd->addIncoming(IndVarStart, Preheader);
  RRI.IndVarEnd->addIncoming(IndVarBase, RRI.ExitSelector);

  // The latch exit now has a branch from `RRI.ExitSelector' instead of
  // `LS.Latch'.  The PHI nodes need to be updated to reflect that.
  LS.LatchExit->replacePhiUsesWith(LS.Latch, RRI.ExitSelector);

  return RRI;
}

void LoopConstrainer::rewriteIncomingValuesForPHIs(
    LoopStructure &LS, BasicBlock *ContinuationBlock,
    const LoopConstrainer::RewrittenRangeInfo &RRI) const {
  unsigned PHIIndex = 0;
  for (PHINode &PN : LS.Header->phis())
    PN.setIncomingValueForBlock(ContinuationBlock,
                                RRI.PHIValuesAtPseudoExit[PHIIndex++]);

  LS.IndVarStart = RRI.IndVarEnd;
}

BasicBlock *LoopConstrainer::createPreheader(const LoopStructure &LS,
                                             BasicBlock *OldPreheader,
                                             const char *Tag) const {
  BasicBlock *Preheader = BasicBlock::Create(Ctx, Tag, &F, LS.Header);
  BranchInst::Create(LS.Header, Preheader);

  LS.Header->replacePhiUsesWith(OldPreheader, Preheader);

  return Preheader;
}

void LoopConstrainer::addToParentLoopIfNeeded(ArrayRef<BasicBlock *> BBs) {
  Loop *ParentLoop = OriginalLoop.getParentLoop();
  if (!ParentLoop)
    return;

  for (BasicBlock *BB : BBs)
    ParentLoop->addBasicBlockToLoop(BB, LI);
}

Loop *LoopConstrainer::createClonedLoopStructure(Loop *Original, Loop *Parent,
                                                 ValueToValueMapTy &VM,
                                                 bool IsSubloop) {
  Loop &New = *LI.AllocateLoop();
  if (Parent)
    Parent->addChildLoop(&New);
  else
    LI.addTopLevelLoop(&New);
  LPMAddNewLoop(&New, IsSubloop);

  // Add all of the blocks in Original to the new loop.
  for (auto *BB : Original->blocks())
    if (LI.getLoopFor(BB) == Original)
      New.addBasicBlockToLoop(cast<BasicBlock>(VM[BB]), LI);

  // Add all of the subloops to the new loop.
  for (Loop *SubLoop : *Original)
    createClonedLoopStructure(SubLoop, &New, VM, /* IsSubloop */ true);

  return &New;
}

bool LoopConstrainer::run() {
  BasicBlock *Preheader = nullptr;
  LatchTakenCount = SE.getExitCount(&OriginalLoop, MainLoopStructure.Latch);
  Preheader = OriginalLoop.getLoopPreheader();
  assert(!isa<SCEVCouldNotCompute>(LatchTakenCount) && Preheader != nullptr &&
         "preconditions!");

  OriginalPreheader = Preheader;
  MainLoopPreheader = Preheader;

  bool IsSignedPredicate = MainLoopStructure.IsSignedPredicate;
  Optional<SubRanges> MaybeSR = calculateSubRanges(IsSignedPredicate);
  if (!MaybeSR.hasValue()) {
    LLVM_DEBUG(dbgs() << "irce: could not compute subranges\n");
    return false;
  }

  SubRanges SR = MaybeSR.getValue();
  bool Increasing = MainLoopStructure.IndVarIncreasing;
  IntegerType *IVTy =
      cast<IntegerType>(Range.getBegin()->getType());

  SCEVExpander Expander(SE, F.getParent()->getDataLayout(), "irce");
  Instruction *InsertPt = OriginalPreheader->getTerminator();

  // It would have been better to make `PreLoop' and `PostLoop'
  // `Optional<ClonedLoop>'s, but `ValueToValueMapTy' does not have a copy
  // constructor.
  ClonedLoop PreLoop, PostLoop;
  bool NeedsPreLoop =
      Increasing ? SR.LowLimit.hasValue() : SR.HighLimit.hasValue();
  bool NeedsPostLoop =
      Increasing ? SR.HighLimit.hasValue() : SR.LowLimit.hasValue();

  Value *ExitPreLoopAt = nullptr;
  Value *ExitMainLoopAt = nullptr;
  const SCEVConstant *MinusOneS =
      cast<SCEVConstant>(SE.getConstant(IVTy, -1, true /* isSigned */));

  if (NeedsPreLoop) {
    const SCEV *ExitPreLoopAtSCEV = nullptr;

    if (Increasing)
      ExitPreLoopAtSCEV = *SR.LowLimit;
    else if (cannotBeMinInLoop(*SR.HighLimit, &OriginalLoop, SE,
                               IsSignedPredicate))
      ExitPreLoopAtSCEV = SE.getAddExpr(*SR.HighLimit, MinusOneS);
    else {
      LLVM_DEBUG(dbgs() << "irce: could not prove no-overflow when computing "
                        << "preloop exit limit.  HighLimit = "
                        << *(*SR.HighLimit) << "\n");
      return false;
    }

    if (!isSafeToExpandAt(ExitPreLoopAtSCEV, InsertPt, SE)) {
      LLVM_DEBUG(dbgs() << "irce: could not prove that it is safe to expand the"
                        << " preloop exit limit " << *ExitPreLoopAtSCEV
                        << " at block " << InsertPt->getParent()->getName()
                        << "\n");
      return false;
    }

    ExitPreLoopAt = Expander.expandCodeFor(ExitPreLoopAtSCEV, IVTy, InsertPt);
    ExitPreLoopAt->setName("exit.preloop.at");
  }

  if (NeedsPostLoop) {
    const SCEV *ExitMainLoopAtSCEV = nullptr;

    if (Increasing)
      ExitMainLoopAtSCEV = *SR.HighLimit;
    else if (cannotBeMinInLoop(*SR.LowLimit, &OriginalLoop, SE,
                               IsSignedPredicate))
      ExitMainLoopAtSCEV = SE.getAddExpr(*SR.LowLimit, MinusOneS);
    else {
      LLVM_DEBUG(dbgs() << "irce: could not prove no-overflow when computing "
                        << "mainloop exit limit.  LowLimit = "
                        << *(*SR.LowLimit) << "\n");
      return false;
    }

    if (!isSafeToExpandAt(ExitMainLoopAtSCEV, InsertPt, SE)) {
      LLVM_DEBUG(dbgs() << "irce: could not prove that it is safe to expand the"
                        << " main loop exit limit " << *ExitMainLoopAtSCEV
                        << " at block " << InsertPt->getParent()->getName()
                        << "\n");
      return false;
    }

    ExitMainLoopAt = Expander.expandCodeFor(ExitMainLoopAtSCEV, IVTy, InsertPt);
    ExitMainLoopAt->setName("exit.mainloop.at");
  }

  // We clone these ahead of time so that we don't have to deal with changing
  // and temporarily invalid IR as we transform the loops.
  if (NeedsPreLoop)
    cloneLoop(PreLoop, "preloop");
  if (NeedsPostLoop)
    cloneLoop(PostLoop, "postloop");

  RewrittenRangeInfo PreLoopRRI;

  if (NeedsPreLoop) {
    Preheader->getTerminator()->replaceUsesOfWith(MainLoopStructure.Header,
                                                  PreLoop.Structure.Header);

    MainLoopPreheader =
        createPreheader(MainLoopStructure, Preheader, "mainloop");
    PreLoopRRI = changeIterationSpaceEnd(PreLoop.Structure, Preheader,
                                         ExitPreLoopAt, MainLoopPreheader);
    rewriteIncomingValuesForPHIs(MainLoopStructure, MainLoopPreheader,
                                 PreLoopRRI);
  }

  BasicBlock *PostLoopPreheader = nullptr;
  RewrittenRangeInfo PostLoopRRI;

  if (NeedsPostLoop) {
    PostLoopPreheader =
        createPreheader(PostLoop.Structure, Preheader, "postloop");
    PostLoopRRI = changeIterationSpaceEnd(MainLoopStructure, MainLoopPreheader,
                                          ExitMainLoopAt, PostLoopPreheader);
    rewriteIncomingValuesForPHIs(PostLoop.Structure, PostLoopPreheader,
                                 PostLoopRRI);
  }

  BasicBlock *NewMainLoopPreheader =
      MainLoopPreheader != Preheader ? MainLoopPreheader : nullptr;
  BasicBlock *NewBlocks[] = {PostLoopPreheader,        PreLoopRRI.PseudoExit,
                             PreLoopRRI.ExitSelector,  PostLoopRRI.PseudoExit,
                             PostLoopRRI.ExitSelector, NewMainLoopPreheader};

  // Some of the above may be nullptr, filter them out before passing to
  // addToParentLoopIfNeeded.
  auto NewBlocksEnd =
      std::remove(std::begin(NewBlocks), std::end(NewBlocks), nullptr);

  addToParentLoopIfNeeded(makeArrayRef(std::begin(NewBlocks), NewBlocksEnd));

  DT.recalculate(F);

  // We need to first add all the pre and post loop blocks into the loop
  // structures (as part of createClonedLoopStructure), and then update the
  // LCSSA form and LoopSimplifyForm. This is necessary for correctly updating
  // LI when LoopSimplifyForm is generated.
  Loop *PreL = nullptr, *PostL = nullptr;
  if (!PreLoop.Blocks.empty()) {
    PreL = createClonedLoopStructure(&OriginalLoop,
                                     OriginalLoop.getParentLoop(), PreLoop.Map,
                                     /* IsSubLoop */ false);
  }

  if (!PostLoop.Blocks.empty()) {
    PostL =
        createClonedLoopStructure(&OriginalLoop, OriginalLoop.getParentLoop(),
                                  PostLoop.Map, /* IsSubLoop */ false);
  }

  // This function canonicalizes the loop into Loop-Simplify and LCSSA forms.
  auto CanonicalizeLoop = [&] (Loop *L, bool IsOriginalLoop) {
    formLCSSARecursively(*L, DT, &LI, &SE);
    simplifyLoop(L, &DT, &LI, &SE, nullptr, nullptr, true);
    // Pre/post loops are slow paths, we do not need to perform any loop
    // optimizations on them.
    if (!IsOriginalLoop)
      DisableAllLoopOptsOnLoop(*L);
  };
  if (PreL)
    CanonicalizeLoop(PreL, false);
  if (PostL)
    CanonicalizeLoop(PostL, false);
  CanonicalizeLoop(&OriginalLoop, true);

  return true;
}

/// Computes and returns a range of values for the induction variable (IndVar)
/// in which the range check can be safely elided.  If it cannot compute such a
/// range, returns None.
Optional<InductiveRangeCheck::Range>
InductiveRangeCheck::computeSafeIterationSpace(
    ScalarEvolution &SE, const SCEVAddRecExpr *IndVar,
    bool IsLatchSigned) const {
  // We can deal when types of latch check and range checks don't match in case
  // if latch check is more narrow.
  auto *IVType = cast<IntegerType>(IndVar->getType());
  auto *RCType = cast<IntegerType>(getBegin()->getType());
  if (IVType->getBitWidth() > RCType->getBitWidth())
    return None;
  // IndVar is of the form "A + B * I" (where "I" is the canonical induction
  // variable, that may or may not exist as a real llvm::Value in the loop) and
  // this inductive range check is a range check on the "C + D * I" ("C" is
  // getBegin() and "D" is getStep()).  We rewrite the value being range
  // checked to "M + N * IndVar" where "N" = "D * B^(-1)" and "M" = "C - NA".
  //
  // The actual inequalities we solve are of the form
  //
  //   0 <= M + 1 * IndVar < L given L >= 0  (i.e. N == 1)
  //
  // Here L stands for upper limit of the safe iteration space.
  // The inequality is satisfied by (0 - M) <= IndVar < (L - M). To avoid
  // overflows when calculating (0 - M) and (L - M) we, depending on type of
  // IV's iteration space, limit the calculations by borders of the iteration
  // space. For example, if IndVar is unsigned, (0 - M) overflows for any M > 0.
  // If we figured out that "anything greater than (-M) is safe", we strengthen
  // this to "everything greater than 0 is safe", assuming that values between
  // -M and 0 just do not exist in unsigned iteration space, and we don't want
  // to deal with overflown values.

  if (!IndVar->isAffine())
    return None;

  const SCEV *A = NoopOrExtend(IndVar->getStart(), RCType, SE, IsLatchSigned);
  const SCEVConstant *B = dyn_cast<SCEVConstant>(
      NoopOrExtend(IndVar->getStepRecurrence(SE), RCType, SE, IsLatchSigned));
  if (!B)
    return None;
  assert(!B->isZero() && "Recurrence with zero step?");

  const SCEV *C = getBegin();
  const SCEVConstant *D = dyn_cast<SCEVConstant>(getStep());
  if (D != B)
    return None;

  assert(!D->getValue()->isZero() && "Recurrence with zero step?");
  unsigned BitWidth = RCType->getBitWidth();
  const SCEV *SIntMax = SE.getConstant(APInt::getSignedMaxValue(BitWidth));

  // Subtract Y from X so that it does not go through border of the IV
  // iteration space. Mathematically, it is equivalent to:
  //
  //    ClampedSubtract(X, Y) = min(max(X - Y, INT_MIN), INT_MAX).        [1]
  //
  // In [1], 'X - Y' is a mathematical subtraction (result is not bounded to
  // any width of bit grid). But after we take min/max, the result is
  // guaranteed to be within [INT_MIN, INT_MAX].
  //
  // In [1], INT_MAX and INT_MIN are respectively signed and unsigned max/min
  // values, depending on type of latch condition that defines IV iteration
  // space.
  auto ClampedSubtract = [&](const SCEV *X, const SCEV *Y) {
    // FIXME: The current implementation assumes that X is in [0, SINT_MAX].
    // This is required to ensure that SINT_MAX - X does not overflow signed and
    // that X - Y does not overflow unsigned if Y is negative. Can we lift this
    // restriction and make it work for negative X either?
    if (IsLatchSigned) {
      // X is a number from signed range, Y is interpreted as signed.
      // Even if Y is SINT_MAX, (X - Y) does not reach SINT_MIN. So the only
      // thing we should care about is that we didn't cross SINT_MAX.
      // So, if Y is positive, we subtract Y safely.
      //   Rule 1: Y > 0 ---> Y.
      // If 0 <= -Y <= (SINT_MAX - X), we subtract Y safely.
      //   Rule 2: Y >=s (X - SINT_MAX) ---> Y.
      // If 0 <= (SINT_MAX - X) < -Y, we can only subtract (X - SINT_MAX).
      //   Rule 3: Y <s (X - SINT_MAX) ---> (X - SINT_MAX).
      // It gives us smax(Y, X - SINT_MAX) to subtract in all cases.
      const SCEV *XMinusSIntMax = SE.getMinusSCEV(X, SIntMax);
      return SE.getMinusSCEV(X, SE.getSMaxExpr(Y, XMinusSIntMax),
                             SCEV::FlagNSW);
    } else
      // X is a number from unsigned range, Y is interpreted as signed.
      // Even if Y is SINT_MIN, (X - Y) does not reach UINT_MAX. So the only
      // thing we should care about is that we didn't cross zero.
      // So, if Y is negative, we subtract Y safely.
      //   Rule 1: Y <s 0 ---> Y.
      // If 0 <= Y <= X, we subtract Y safely.
      //   Rule 2: Y <=s X ---> Y.
      // If 0 <= X < Y, we should stop at 0 and can only subtract X.
      //   Rule 3: Y >s X ---> X.
      // It gives us smin(X, Y) to subtract in all cases.
      return SE.getMinusSCEV(X, SE.getSMinExpr(X, Y), SCEV::FlagNUW);
  };
  const SCEV *M = SE.getMinusSCEV(C, A);
  const SCEV *Zero = SE.getZero(M->getType());

  // This function returns SCEV equal to 1 if X is non-negative 0 otherwise.
  auto SCEVCheckNonNegative = [&](const SCEV *X) {
    const Loop *L = IndVar->getLoop();
    const SCEV *One = SE.getOne(X->getType());
    // Can we trivially prove that X is a non-negative or negative value?
    if (isKnownNonNegativeInLoop(X, L, SE))
      return One;
    else if (isKnownNegativeInLoop(X, L, SE))
      return Zero;
    // If not, we will have to figure it out during the execution.
    // Function smax(smin(X, 0), -1) + 1 equals to 1 if X >= 0 and 0 if X < 0.
    const SCEV *NegOne = SE.getNegativeSCEV(One);
    return SE.getAddExpr(SE.getSMaxExpr(SE.getSMinExpr(X, Zero), NegOne), One);
  };
  // FIXME: Current implementation of ClampedSubtract implicitly assumes that
  // X is non-negative (in sense of a signed value). We need to re-implement
  // this function in a way that it will correctly handle negative X as well.
  // We use it twice: for X = 0 everything is fine, but for X = getEnd() we can
  // end up with a negative X and produce wrong results. So currently we ensure
  // that if getEnd() is negative then both ends of the safe range are zero.
  // Note that this may pessimize elimination of unsigned range checks against
  // negative values.
  const SCEV *REnd = getEnd();
  const SCEV *EndIsNonNegative = SCEVCheckNonNegative(REnd);

  const SCEV *Begin = SE.getMulExpr(ClampedSubtract(Zero, M), EndIsNonNegative);
  const SCEV *End = SE.getMulExpr(ClampedSubtract(REnd, M), EndIsNonNegative);
  return InductiveRangeCheck::Range(Begin, End);
}

static Optional<InductiveRangeCheck::Range>
IntersectSignedRange(ScalarEvolution &SE,
                     const Optional<InductiveRangeCheck::Range> &R1,
                     const InductiveRangeCheck::Range &R2) {
  if (R2.isEmpty(SE, /* IsSigned */ true))
    return None;
  if (!R1.hasValue())
    return R2;
  auto &R1Value = R1.getValue();
  // We never return empty ranges from this function, and R1 is supposed to be
  // a result of intersection. Thus, R1 is never empty.
  assert(!R1Value.isEmpty(SE, /* IsSigned */ true) &&
         "We should never have empty R1!");

  // TODO: we could widen the smaller range and have this work; but for now we
  // bail out to keep things simple.
  if (R1Value.getType() != R2.getType())
    return None;

  const SCEV *NewBegin = SE.getSMaxExpr(R1Value.getBegin(), R2.getBegin());
  const SCEV *NewEnd = SE.getSMinExpr(R1Value.getEnd(), R2.getEnd());

  // If the resulting range is empty, just return None.
  auto Ret = InductiveRangeCheck::Range(NewBegin, NewEnd);
  if (Ret.isEmpty(SE, /* IsSigned */ true))
    return None;
  return Ret;
}

static Optional<InductiveRangeCheck::Range>
IntersectUnsignedRange(ScalarEvolution &SE,
                       const Optional<InductiveRangeCheck::Range> &R1,
                       const InductiveRangeCheck::Range &R2) {
  if (R2.isEmpty(SE, /* IsSigned */ false))
    return None;
  if (!R1.hasValue())
    return R2;
  auto &R1Value = R1.getValue();
  // We never return empty ranges from this function, and R1 is supposed to be
  // a result of intersection. Thus, R1 is never empty.
  assert(!R1Value.isEmpty(SE, /* IsSigned */ false) &&
         "We should never have empty R1!");

  // TODO: we could widen the smaller range and have this work; but for now we
  // bail out to keep things simple.
  if (R1Value.getType() != R2.getType())
    return None;

  const SCEV *NewBegin = SE.getUMaxExpr(R1Value.getBegin(), R2.getBegin());
  const SCEV *NewEnd = SE.getUMinExpr(R1Value.getEnd(), R2.getEnd());

  // If the resulting range is empty, just return None.
  auto Ret = InductiveRangeCheck::Range(NewBegin, NewEnd);
  if (Ret.isEmpty(SE, /* IsSigned */ false))
    return None;
  return Ret;
}

PreservedAnalyses IRCEPass::run(Function &F, FunctionAnalysisManager &AM) {
  auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
  auto &BPI = AM.getResult<BranchProbabilityAnalysis>(F);
  LoopInfo &LI = AM.getResult<LoopAnalysis>(F);

  // Get BFI analysis result on demand. Please note that modification of
  // CFG invalidates this analysis and we should handle it.
  auto getBFI = [&F, &AM ]()->BlockFrequencyInfo & {
    return AM.getResult<BlockFrequencyAnalysis>(F);
  };
  InductiveRangeCheckElimination IRCE(SE, &BPI, DT, LI, { getBFI });

  bool Changed = false;
  {
    bool CFGChanged = false;
    for (const auto &L : LI) {
      CFGChanged |= simplifyLoop(L, &DT, &LI, &SE, nullptr, nullptr,
                                 /*PreserveLCSSA=*/false);
      Changed |= formLCSSARecursively(*L, DT, &LI, &SE);
    }
    Changed |= CFGChanged;

    if (CFGChanged && !SkipProfitabilityChecks)
      AM.invalidate<BlockFrequencyAnalysis>(F);
  }

  SmallPriorityWorklist<Loop *, 4> Worklist;
  appendLoopsToWorklist(LI, Worklist);
  auto LPMAddNewLoop = [&Worklist](Loop *NL, bool IsSubloop) {
    if (!IsSubloop)
      appendLoopsToWorklist(*NL, Worklist);
  };

  while (!Worklist.empty()) {
    Loop *L = Worklist.pop_back_val();
    if (IRCE.run(L, LPMAddNewLoop)) {
      Changed = true;
      if (!SkipProfitabilityChecks)
        AM.invalidate<BlockFrequencyAnalysis>(F);
    }
  }

  if (!Changed)
    return PreservedAnalyses::all();
  return getLoopPassPreservedAnalyses();
}

bool IRCELegacyPass::runOnFunction(Function &F) {
  if (skipFunction(F))
    return false;

  ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
  BranchProbabilityInfo &BPI =
      getAnalysis<BranchProbabilityInfoWrapperPass>().getBPI();
  auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
  auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
  InductiveRangeCheckElimination IRCE(SE, &BPI, DT, LI);

  bool Changed = false;

  for (const auto &L : LI) {
    Changed |= simplifyLoop(L, &DT, &LI, &SE, nullptr, nullptr,
                            /*PreserveLCSSA=*/false);
    Changed |= formLCSSARecursively(*L, DT, &LI, &SE);
  }

  SmallPriorityWorklist<Loop *, 4> Worklist;
  appendLoopsToWorklist(LI, Worklist);
  auto LPMAddNewLoop = [&](Loop *NL, bool IsSubloop) {
    if (!IsSubloop)
      appendLoopsToWorklist(*NL, Worklist);
  };

  while (!Worklist.empty()) {
    Loop *L = Worklist.pop_back_val();
    Changed |= IRCE.run(L, LPMAddNewLoop);
  }
  return Changed;
}

bool
InductiveRangeCheckElimination::isProfitableToTransform(const Loop &L,
                                                        LoopStructure &LS) {
  if (SkipProfitabilityChecks)
    return true;
  if (GetBFI.hasValue()) {
    BlockFrequencyInfo &BFI = (*GetBFI)();
    uint64_t hFreq = BFI.getBlockFreq(LS.Header).getFrequency();
    uint64_t phFreq = BFI.getBlockFreq(L.getLoopPreheader()).getFrequency();
    if (phFreq != 0 && hFreq != 0 && (hFreq / phFreq < MinRuntimeIterations)) {
      LLVM_DEBUG(dbgs() << "irce: could not prove profitability: "
                        << "the estimated number of iterations basing on "
                           "frequency info is " << (hFreq / phFreq) << "\n";);
      return false;
    }
    return true;
  }

  if (!BPI)
    return true;
  BranchProbability ExitProbability =
      BPI->getEdgeProbability(LS.Latch, LS.LatchBrExitIdx);
  if (ExitProbability > BranchProbability(1, MinRuntimeIterations)) {
    LLVM_DEBUG(dbgs() << "irce: could not prove profitability: "
                      << "the exit probability is too big " << ExitProbability
                      << "\n";);
    return false;
  }
  return true;
}

bool InductiveRangeCheckElimination::run(
    Loop *L, function_ref<void(Loop *, bool)> LPMAddNewLoop) {
  if (L->getBlocks().size() >= LoopSizeCutoff) {
    LLVM_DEBUG(dbgs() << "irce: giving up constraining loop, too large\n");
    return false;
  }

  BasicBlock *Preheader = L->getLoopPreheader();
  if (!Preheader) {
    LLVM_DEBUG(dbgs() << "irce: loop has no preheader, leaving\n");
    return false;
  }

  LLVMContext &Context = Preheader->getContext();
  SmallVector<InductiveRangeCheck, 16> RangeChecks;

  for (auto BBI : L->getBlocks())
    if (BranchInst *TBI = dyn_cast<BranchInst>(BBI->getTerminator()))
      InductiveRangeCheck::extractRangeChecksFromBranch(TBI, L, SE, BPI,
                                                        RangeChecks);

  if (RangeChecks.empty())
    return false;

  auto PrintRecognizedRangeChecks = [&](raw_ostream &OS) {
    OS << "irce: looking at loop "; L->print(OS);
    OS << "irce: loop has " << RangeChecks.size()
       << " inductive range checks: \n";
    for (InductiveRangeCheck &IRC : RangeChecks)
      IRC.print(OS);
  };

  LLVM_DEBUG(PrintRecognizedRangeChecks(dbgs()));

  if (PrintRangeChecks)
    PrintRecognizedRangeChecks(errs());

  const char *FailureReason = nullptr;
  Optional<LoopStructure> MaybeLoopStructure =
      LoopStructure::parseLoopStructure(SE, *L, FailureReason);
  if (!MaybeLoopStructure.hasValue()) {
    LLVM_DEBUG(dbgs() << "irce: could not parse loop structure: "
                      << FailureReason << "\n";);
    return false;
  }
  LoopStructure LS = MaybeLoopStructure.getValue();
  if (!isProfitableToTransform(*L, LS))
    return false;
  const SCEVAddRecExpr *IndVar =
      cast<SCEVAddRecExpr>(SE.getMinusSCEV(SE.getSCEV(LS.IndVarBase), SE.getSCEV(LS.IndVarStep)));

  Optional<InductiveRangeCheck::Range> SafeIterRange;
  Instruction *ExprInsertPt = Preheader->getTerminator();

  SmallVector<InductiveRangeCheck, 4> RangeChecksToEliminate;
  // Basing on the type of latch predicate, we interpret the IV iteration range
  // as signed or unsigned range. We use different min/max functions (signed or
  // unsigned) when intersecting this range with safe iteration ranges implied
  // by range checks.
  auto IntersectRange =
      LS.IsSignedPredicate ? IntersectSignedRange : IntersectUnsignedRange;

  IRBuilder<> B(ExprInsertPt);
  for (InductiveRangeCheck &IRC : RangeChecks) {
    auto Result = IRC.computeSafeIterationSpace(SE, IndVar,
                                                LS.IsSignedPredicate);
    if (Result.hasValue()) {
      auto MaybeSafeIterRange =
          IntersectRange(SE, SafeIterRange, Result.getValue());
      if (MaybeSafeIterRange.hasValue()) {
        assert(
            !MaybeSafeIterRange.getValue().isEmpty(SE, LS.IsSignedPredicate) &&
            "We should never return empty ranges!");
        RangeChecksToEliminate.push_back(IRC);
        SafeIterRange = MaybeSafeIterRange.getValue();
      }
    }
  }

  if (!SafeIterRange.hasValue())
    return false;

  LoopConstrainer LC(*L, LI, LPMAddNewLoop, LS, SE, DT,
                     SafeIterRange.getValue());
  bool Changed = LC.run();

  if (Changed) {
    auto PrintConstrainedLoopInfo = [L]() {
      dbgs() << "irce: in function ";
      dbgs() << L->getHeader()->getParent()->getName() << ": ";
      dbgs() << "constrained ";
      L->print(dbgs());
    };

    LLVM_DEBUG(PrintConstrainedLoopInfo());

    if (PrintChangedLoops)
      PrintConstrainedLoopInfo();

    // Optimize away the now-redundant range checks.

    for (InductiveRangeCheck &IRC : RangeChecksToEliminate) {
      ConstantInt *FoldedRangeCheck = IRC.getPassingDirection()
                                          ? ConstantInt::getTrue(Context)
                                          : ConstantInt::getFalse(Context);
      IRC.getCheckUse()->set(FoldedRangeCheck);
    }
  }

  return Changed;
}

Pass *llvm::createInductiveRangeCheckEliminationPass() {
  return new IRCELegacyPass();
}