1//===-- InductiveRangeCheckElimination.cpp - ------------------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9// The InductiveRangeCheckElimination pass splits a loop's iteration space into
10// three disjoint ranges.  It does that in a way such that the loop running in
11// the middle loop provably does not need range checks. As an example, it will
12// convert
13//
14//   len = < known positive >
15//   for (i = 0; i < n; i++) {
16//     if (0 <= i && i < len) {
17//       do_something();
18//     } else {
19//       throw_out_of_bounds();
20//     }
21//   }
22//
23// to
24//
25//   len = < known positive >
26//   limit = smin(n, len)
27//   // no first segment
28//   for (i = 0; i < limit; i++) {
29//     if (0 <= i && i < len) { // this check is fully redundant
30//       do_something();
31//     } else {
32//       throw_out_of_bounds();
33//     }
34//   }
35//   for (i = limit; i < n; i++) {
36//     if (0 <= i && i < len) {
37//       do_something();
38//     } else {
39//       throw_out_of_bounds();
40//     }
41//   }
42//===----------------------------------------------------------------------===//
43
44#include "llvm/ADT/Optional.h"
45#include "llvm/Analysis/BranchProbabilityInfo.h"
46#include "llvm/Analysis/InstructionSimplify.h"
47#include "llvm/Analysis/LoopInfo.h"
48#include "llvm/Analysis/LoopPass.h"
49#include "llvm/Analysis/ScalarEvolution.h"
50#include "llvm/Analysis/ScalarEvolutionExpander.h"
51#include "llvm/Analysis/ScalarEvolutionExpressions.h"
52#include "llvm/Analysis/ValueTracking.h"
53#include "llvm/IR/Dominators.h"
54#include "llvm/IR/Function.h"
55#include "llvm/IR/IRBuilder.h"
56#include "llvm/IR/Instructions.h"
57#include "llvm/IR/Module.h"
58#include "llvm/IR/PatternMatch.h"
59#include "llvm/IR/ValueHandle.h"
60#include "llvm/IR/Verifier.h"
61#include "llvm/Pass.h"
62#include "llvm/Support/Debug.h"
63#include "llvm/Support/raw_ostream.h"
64#include "llvm/Transforms/Scalar.h"
65#include "llvm/Transforms/Utils/BasicBlockUtils.h"
66#include "llvm/Transforms/Utils/Cloning.h"
67#include "llvm/Transforms/Utils/LoopUtils.h"
68#include "llvm/Transforms/Utils/SimplifyIndVar.h"
69#include "llvm/Transforms/Utils/UnrollLoop.h"
70#include <array>
71
72using namespace llvm;
73
74static cl::opt<unsigned> LoopSizeCutoff("irce-loop-size-cutoff", cl::Hidden,
75                                        cl::init(64));
76
77static cl::opt<bool> PrintChangedLoops("irce-print-changed-loops", cl::Hidden,
78                                       cl::init(false));
79
80static cl::opt<bool> PrintRangeChecks("irce-print-range-checks", cl::Hidden,
81                                      cl::init(false));
82
83static cl::opt<int> MaxExitProbReciprocal("irce-max-exit-prob-reciprocal",
84                                          cl::Hidden, cl::init(10));
85
86#define DEBUG_TYPE "irce"
87
88namespace {
89
90/// An inductive range check is conditional branch in a loop with
91///
92///  1. a very cold successor (i.e. the branch jumps to that successor very
93///     rarely)
94///
95///  and
96///
97///  2. a condition that is provably true for some contiguous range of values
98///     taken by the containing loop's induction variable.
99///
100class InductiveRangeCheck {
101  // Classifies a range check
102  enum RangeCheckKind : unsigned {
103    // Range check of the form "0 <= I".
104    RANGE_CHECK_LOWER = 1,
105
106    // Range check of the form "I < L" where L is known positive.
107    RANGE_CHECK_UPPER = 2,
108
109    // The logical and of the RANGE_CHECK_LOWER and RANGE_CHECK_UPPER
110    // conditions.
111    RANGE_CHECK_BOTH = RANGE_CHECK_LOWER | RANGE_CHECK_UPPER,
112
113    // Unrecognized range check condition.
114    RANGE_CHECK_UNKNOWN = (unsigned)-1
115  };
116
117  static const char *rangeCheckKindToStr(RangeCheckKind);
118
119  const SCEV *Offset;
120  const SCEV *Scale;
121  Value *Length;
122  BranchInst *Branch;
123  RangeCheckKind Kind;
124
125  static RangeCheckKind parseRangeCheckICmp(Loop *L, ICmpInst *ICI,
126                                            ScalarEvolution &SE, Value *&Index,
127                                            Value *&Length);
128
129  static InductiveRangeCheck::RangeCheckKind
130  parseRangeCheck(Loop *L, ScalarEvolution &SE, Value *Condition,
131                  const SCEV *&Index, Value *&UpperLimit);
132
133  InductiveRangeCheck() :
134    Offset(nullptr), Scale(nullptr), Length(nullptr), Branch(nullptr) { }
135
136public:
137  const SCEV *getOffset() const { return Offset; }
138  const SCEV *getScale() const { return Scale; }
139  Value *getLength() const { return Length; }
140
141  void print(raw_ostream &OS) const {
142    OS << "InductiveRangeCheck:\n";
143    OS << "  Kind: " << rangeCheckKindToStr(Kind) << "\n";
144    OS << "  Offset: ";
145    Offset->print(OS);
146    OS << "  Scale: ";
147    Scale->print(OS);
148    OS << "  Length: ";
149    if (Length)
150      Length->print(OS);
151    else
152      OS << "(null)";
153    OS << "\n  Branch: ";
154    getBranch()->print(OS);
155    OS << "\n";
156  }
157
158#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
159  void dump() {
160    print(dbgs());
161  }
162#endif
163
164  BranchInst *getBranch() const { return Branch; }
165
166  /// Represents an signed integer range [Range.getBegin(), Range.getEnd()).  If
167  /// R.getEnd() sle R.getBegin(), then R denotes the empty range.
168
169  class Range {
170    const SCEV *Begin;
171    const SCEV *End;
172
173  public:
174    Range(const SCEV *Begin, const SCEV *End) : Begin(Begin), End(End) {
175      assert(Begin->getType() == End->getType() && "ill-typed range!");
176    }
177
178    Type *getType() const { return Begin->getType(); }
179    const SCEV *getBegin() const { return Begin; }
180    const SCEV *getEnd() const { return End; }
181  };
182
183  typedef SpecificBumpPtrAllocator<InductiveRangeCheck> AllocatorTy;
184
185  /// This is the value the condition of the branch needs to evaluate to for the
186  /// branch to take the hot successor (see (1) above).
187  bool getPassingDirection() { return true; }
188
189  /// Computes a range for the induction variable (IndVar) in which the range
190  /// check is redundant and can be constant-folded away.  The induction
191  /// variable is not required to be the canonical {0,+,1} induction variable.
192  Optional<Range> computeSafeIterationSpace(ScalarEvolution &SE,
193                                            const SCEVAddRecExpr *IndVar,
194                                            IRBuilder<> &B) const;
195
196  /// Create an inductive range check out of BI if possible, else return
197  /// nullptr.
198  static InductiveRangeCheck *create(AllocatorTy &Alloc, BranchInst *BI,
199                                     Loop *L, ScalarEvolution &SE,
200                                     BranchProbabilityInfo &BPI);
201};
202
203class InductiveRangeCheckElimination : public LoopPass {
204  InductiveRangeCheck::AllocatorTy Allocator;
205
206public:
207  static char ID;
208  InductiveRangeCheckElimination() : LoopPass(ID) {
209    initializeInductiveRangeCheckEliminationPass(
210        *PassRegistry::getPassRegistry());
211  }
212
213  void getAnalysisUsage(AnalysisUsage &AU) const override {
214    AU.addRequired<LoopInfoWrapperPass>();
215    AU.addRequiredID(LoopSimplifyID);
216    AU.addRequiredID(LCSSAID);
217    AU.addRequired<ScalarEvolution>();
218    AU.addRequired<BranchProbabilityInfo>();
219  }
220
221  bool runOnLoop(Loop *L, LPPassManager &LPM) override;
222};
223
224char InductiveRangeCheckElimination::ID = 0;
225}
226
227INITIALIZE_PASS(InductiveRangeCheckElimination, "irce",
228                "Inductive range check elimination", false, false)
229
230const char *InductiveRangeCheck::rangeCheckKindToStr(
231    InductiveRangeCheck::RangeCheckKind RCK) {
232  switch (RCK) {
233  case InductiveRangeCheck::RANGE_CHECK_UNKNOWN:
234    return "RANGE_CHECK_UNKNOWN";
235
236  case InductiveRangeCheck::RANGE_CHECK_UPPER:
237    return "RANGE_CHECK_UPPER";
238
239  case InductiveRangeCheck::RANGE_CHECK_LOWER:
240    return "RANGE_CHECK_LOWER";
241
242  case InductiveRangeCheck::RANGE_CHECK_BOTH:
243    return "RANGE_CHECK_BOTH";
244  }
245
246  llvm_unreachable("unknown range check type!");
247}
248
249/// Parse a single ICmp instruction, `ICI`, into a range check.  If `ICI`
250/// cannot
251/// be interpreted as a range check, return `RANGE_CHECK_UNKNOWN` and set
252/// `Index` and `Length` to `nullptr`.  Otherwise set `Index` to the value
253/// being
254/// range checked, and set `Length` to the upper limit `Index` is being range
255/// checked with if (and only if) the range check type is stronger or equal to
256/// RANGE_CHECK_UPPER.
257///
258InductiveRangeCheck::RangeCheckKind
259InductiveRangeCheck::parseRangeCheckICmp(Loop *L, ICmpInst *ICI,
260                                         ScalarEvolution &SE, Value *&Index,
261                                         Value *&Length) {
262
263  auto IsNonNegativeAndNotLoopVarying = [&SE, L](Value *V) {
264    const SCEV *S = SE.getSCEV(V);
265    if (isa<SCEVCouldNotCompute>(S))
266      return false;
267
268    return SE.getLoopDisposition(S, L) == ScalarEvolution::LoopInvariant &&
269           SE.isKnownNonNegative(S);
270  };
271
272  using namespace llvm::PatternMatch;
273
274  ICmpInst::Predicate Pred = ICI->getPredicate();
275  Value *LHS = ICI->getOperand(0);
276  Value *RHS = ICI->getOperand(1);
277
278  switch (Pred) {
279  default:
280    return RANGE_CHECK_UNKNOWN;
281
282  case ICmpInst::ICMP_SLE:
283    std::swap(LHS, RHS);
284  // fallthrough
285  case ICmpInst::ICMP_SGE:
286    if (match(RHS, m_ConstantInt<0>())) {
287      Index = LHS;
288      return RANGE_CHECK_LOWER;
289    }
290    return RANGE_CHECK_UNKNOWN;
291
292  case ICmpInst::ICMP_SLT:
293    std::swap(LHS, RHS);
294  // fallthrough
295  case ICmpInst::ICMP_SGT:
296    if (match(RHS, m_ConstantInt<-1>())) {
297      Index = LHS;
298      return RANGE_CHECK_LOWER;
299    }
300
301    if (IsNonNegativeAndNotLoopVarying(LHS)) {
302      Index = RHS;
303      Length = LHS;
304      return RANGE_CHECK_UPPER;
305    }
306    return RANGE_CHECK_UNKNOWN;
307
308  case ICmpInst::ICMP_ULT:
309    std::swap(LHS, RHS);
310  // fallthrough
311  case ICmpInst::ICMP_UGT:
312    if (IsNonNegativeAndNotLoopVarying(LHS)) {
313      Index = RHS;
314      Length = LHS;
315      return RANGE_CHECK_BOTH;
316    }
317    return RANGE_CHECK_UNKNOWN;
318  }
319
320  llvm_unreachable("default clause returns!");
321}
322
323/// Parses an arbitrary condition into a range check.  `Length` is set only if
324/// the range check is recognized to be `RANGE_CHECK_UPPER` or stronger.
325InductiveRangeCheck::RangeCheckKind
326InductiveRangeCheck::parseRangeCheck(Loop *L, ScalarEvolution &SE,
327                                     Value *Condition, const SCEV *&Index,
328                                     Value *&Length) {
329  using namespace llvm::PatternMatch;
330
331  Value *A = nullptr;
332  Value *B = nullptr;
333
334  if (match(Condition, m_And(m_Value(A), m_Value(B)))) {
335    Value *IndexA = nullptr, *IndexB = nullptr;
336    Value *LengthA = nullptr, *LengthB = nullptr;
337    ICmpInst *ICmpA = dyn_cast<ICmpInst>(A), *ICmpB = dyn_cast<ICmpInst>(B);
338
339    if (!ICmpA || !ICmpB)
340      return InductiveRangeCheck::RANGE_CHECK_UNKNOWN;
341
342    auto RCKindA = parseRangeCheckICmp(L, ICmpA, SE, IndexA, LengthA);
343    auto RCKindB = parseRangeCheckICmp(L, ICmpB, SE, IndexB, LengthB);
344
345    if (RCKindA == InductiveRangeCheck::RANGE_CHECK_UNKNOWN ||
346        RCKindB == InductiveRangeCheck::RANGE_CHECK_UNKNOWN)
347      return InductiveRangeCheck::RANGE_CHECK_UNKNOWN;
348
349    if (IndexA != IndexB)
350      return InductiveRangeCheck::RANGE_CHECK_UNKNOWN;
351
352    if (LengthA != nullptr && LengthB != nullptr && LengthA != LengthB)
353      return InductiveRangeCheck::RANGE_CHECK_UNKNOWN;
354
355    Index = SE.getSCEV(IndexA);
356    if (isa<SCEVCouldNotCompute>(Index))
357      return InductiveRangeCheck::RANGE_CHECK_UNKNOWN;
358
359    Length = LengthA == nullptr ? LengthB : LengthA;
360
361    return (InductiveRangeCheck::RangeCheckKind)(RCKindA | RCKindB);
362  }
363
364  if (ICmpInst *ICI = dyn_cast<ICmpInst>(Condition)) {
365    Value *IndexVal = nullptr;
366
367    auto RCKind = parseRangeCheckICmp(L, ICI, SE, IndexVal, Length);
368
369    if (RCKind == InductiveRangeCheck::RANGE_CHECK_UNKNOWN)
370      return InductiveRangeCheck::RANGE_CHECK_UNKNOWN;
371
372    Index = SE.getSCEV(IndexVal);
373    if (isa<SCEVCouldNotCompute>(Index))
374      return InductiveRangeCheck::RANGE_CHECK_UNKNOWN;
375
376    return RCKind;
377  }
378
379  return InductiveRangeCheck::RANGE_CHECK_UNKNOWN;
380}
381
382
383InductiveRangeCheck *
384InductiveRangeCheck::create(InductiveRangeCheck::AllocatorTy &A, BranchInst *BI,
385                            Loop *L, ScalarEvolution &SE,
386                            BranchProbabilityInfo &BPI) {
387
388  if (BI->isUnconditional() || BI->getParent() == L->getLoopLatch())
389    return nullptr;
390
391  BranchProbability LikelyTaken(15, 16);
392
393  if (BPI.getEdgeProbability(BI->getParent(), (unsigned) 0) < LikelyTaken)
394    return nullptr;
395
396  Value *Length = nullptr;
397  const SCEV *IndexSCEV = nullptr;
398
399  auto RCKind = InductiveRangeCheck::parseRangeCheck(L, SE, BI->getCondition(),
400                                                     IndexSCEV, Length);
401
402  if (RCKind == InductiveRangeCheck::RANGE_CHECK_UNKNOWN)
403    return nullptr;
404
405  assert(IndexSCEV && "contract with SplitRangeCheckCondition!");
406  assert((!(RCKind & InductiveRangeCheck::RANGE_CHECK_UPPER) || Length) &&
407         "contract with SplitRangeCheckCondition!");
408
409  const SCEVAddRecExpr *IndexAddRec = dyn_cast<SCEVAddRecExpr>(IndexSCEV);
410  bool IsAffineIndex =
411      IndexAddRec && (IndexAddRec->getLoop() == L) && IndexAddRec->isAffine();
412
413  if (!IsAffineIndex)
414    return nullptr;
415
416  InductiveRangeCheck *IRC = new (A.Allocate()) InductiveRangeCheck;
417  IRC->Length = Length;
418  IRC->Offset = IndexAddRec->getStart();
419  IRC->Scale = IndexAddRec->getStepRecurrence(SE);
420  IRC->Branch = BI;
421  IRC->Kind = RCKind;
422  return IRC;
423}
424
425namespace {
426
427// Keeps track of the structure of a loop.  This is similar to llvm::Loop,
428// except that it is more lightweight and can track the state of a loop through
429// changing and potentially invalid IR.  This structure also formalizes the
430// kinds of loops we can deal with -- ones that have a single latch that is also
431// an exiting block *and* have a canonical induction variable.
432struct LoopStructure {
433  const char *Tag;
434
435  BasicBlock *Header;
436  BasicBlock *Latch;
437
438  // `Latch's terminator instruction is `LatchBr', and it's `LatchBrExitIdx'th
439  // successor is `LatchExit', the exit block of the loop.
440  BranchInst *LatchBr;
441  BasicBlock *LatchExit;
442  unsigned LatchBrExitIdx;
443
444  Value *IndVarNext;
445  Value *IndVarStart;
446  Value *LoopExitAt;
447  bool IndVarIncreasing;
448
449  LoopStructure()
450      : Tag(""), Header(nullptr), Latch(nullptr), LatchBr(nullptr),
451        LatchExit(nullptr), LatchBrExitIdx(-1), IndVarNext(nullptr),
452        IndVarStart(nullptr), LoopExitAt(nullptr), IndVarIncreasing(false) {}
453
454  template <typename M> LoopStructure map(M Map) const {
455    LoopStructure Result;
456    Result.Tag = Tag;
457    Result.Header = cast<BasicBlock>(Map(Header));
458    Result.Latch = cast<BasicBlock>(Map(Latch));
459    Result.LatchBr = cast<BranchInst>(Map(LatchBr));
460    Result.LatchExit = cast<BasicBlock>(Map(LatchExit));
461    Result.LatchBrExitIdx = LatchBrExitIdx;
462    Result.IndVarNext = Map(IndVarNext);
463    Result.IndVarStart = Map(IndVarStart);
464    Result.LoopExitAt = Map(LoopExitAt);
465    Result.IndVarIncreasing = IndVarIncreasing;
466    return Result;
467  }
468
469  static Optional<LoopStructure> parseLoopStructure(ScalarEvolution &,
470                                                    BranchProbabilityInfo &BPI,
471                                                    Loop &,
472                                                    const char *&);
473};
474
475/// This class is used to constrain loops to run within a given iteration space.
476/// The algorithm this class implements is given a Loop and a range [Begin,
477/// End).  The algorithm then tries to break out a "main loop" out of the loop
478/// it is given in a way that the "main loop" runs with the induction variable
479/// in a subset of [Begin, End).  The algorithm emits appropriate pre and post
480/// loops to run any remaining iterations.  The pre loop runs any iterations in
481/// which the induction variable is < Begin, and the post loop runs any
482/// iterations in which the induction variable is >= End.
483///
484class LoopConstrainer {
485  // The representation of a clone of the original loop we started out with.
486  struct ClonedLoop {
487    // The cloned blocks
488    std::vector<BasicBlock *> Blocks;
489
490    // `Map` maps values in the clonee into values in the cloned version
491    ValueToValueMapTy Map;
492
493    // An instance of `LoopStructure` for the cloned loop
494    LoopStructure Structure;
495  };
496
497  // Result of rewriting the range of a loop.  See changeIterationSpaceEnd for
498  // more details on what these fields mean.
499  struct RewrittenRangeInfo {
500    BasicBlock *PseudoExit;
501    BasicBlock *ExitSelector;
502    std::vector<PHINode *> PHIValuesAtPseudoExit;
503    PHINode *IndVarEnd;
504
505    RewrittenRangeInfo()
506        : PseudoExit(nullptr), ExitSelector(nullptr), IndVarEnd(nullptr) {}
507  };
508
509  // Calculated subranges we restrict the iteration space of the main loop to.
510  // See the implementation of `calculateSubRanges' for more details on how
511  // these fields are computed.  `LowLimit` is None if there is no restriction
512  // on low end of the restricted iteration space of the main loop.  `HighLimit`
513  // is None if there is no restriction on high end of the restricted iteration
514  // space of the main loop.
515
516  struct SubRanges {
517    Optional<const SCEV *> LowLimit;
518    Optional<const SCEV *> HighLimit;
519  };
520
521  // A utility function that does a `replaceUsesOfWith' on the incoming block
522  // set of a `PHINode' -- replaces instances of `Block' in the `PHINode's
523  // incoming block list with `ReplaceBy'.
524  static void replacePHIBlock(PHINode *PN, BasicBlock *Block,
525                              BasicBlock *ReplaceBy);
526
527  // Compute a safe set of limits for the main loop to run in -- effectively the
528  // intersection of `Range' and the iteration space of the original loop.
529  // Return None if unable to compute the set of subranges.
530  //
531  Optional<SubRanges> calculateSubRanges() const;
532
533  // Clone `OriginalLoop' and return the result in CLResult.  The IR after
534  // running `cloneLoop' is well formed except for the PHI nodes in CLResult --
535  // the PHI nodes say that there is an incoming edge from `OriginalPreheader`
536  // but there is no such edge.
537  //
538  void cloneLoop(ClonedLoop &CLResult, const char *Tag) const;
539
540  // Rewrite the iteration space of the loop denoted by (LS, Preheader). The
541  // iteration space of the rewritten loop ends at ExitLoopAt.  The start of the
542  // iteration space is not changed.  `ExitLoopAt' is assumed to be slt
543  // `OriginalHeaderCount'.
544  //
545  // If there are iterations left to execute, control is made to jump to
546  // `ContinuationBlock', otherwise they take the normal loop exit.  The
547  // returned `RewrittenRangeInfo' object is populated as follows:
548  //
549  //  .PseudoExit is a basic block that unconditionally branches to
550  //      `ContinuationBlock'.
551  //
552  //  .ExitSelector is a basic block that decides, on exit from the loop,
553  //      whether to branch to the "true" exit or to `PseudoExit'.
554  //
555  //  .PHIValuesAtPseudoExit are PHINodes in `PseudoExit' that compute the value
556  //      for each PHINode in the loop header on taking the pseudo exit.
557  //
558  // After changeIterationSpaceEnd, `Preheader' is no longer a legitimate
559  // preheader because it is made to branch to the loop header only
560  // conditionally.
561  //
562  RewrittenRangeInfo
563  changeIterationSpaceEnd(const LoopStructure &LS, BasicBlock *Preheader,
564                          Value *ExitLoopAt,
565                          BasicBlock *ContinuationBlock) const;
566
567  // The loop denoted by `LS' has `OldPreheader' as its preheader.  This
568  // function creates a new preheader for `LS' and returns it.
569  //
570  BasicBlock *createPreheader(const LoopStructure &LS, BasicBlock *OldPreheader,
571                              const char *Tag) const;
572
573  // `ContinuationBlockAndPreheader' was the continuation block for some call to
574  // `changeIterationSpaceEnd' and is the preheader to the loop denoted by `LS'.
575  // This function rewrites the PHI nodes in `LS.Header' to start with the
576  // correct value.
577  void rewriteIncomingValuesForPHIs(
578      LoopStructure &LS, BasicBlock *ContinuationBlockAndPreheader,
579      const LoopConstrainer::RewrittenRangeInfo &RRI) const;
580
581  // Even though we do not preserve any passes at this time, we at least need to
582  // keep the parent loop structure consistent.  The `LPPassManager' seems to
583  // verify this after running a loop pass.  This function adds the list of
584  // blocks denoted by BBs to this loops parent loop if required.
585  void addToParentLoopIfNeeded(ArrayRef<BasicBlock *> BBs);
586
587  // Some global state.
588  Function &F;
589  LLVMContext &Ctx;
590  ScalarEvolution &SE;
591
592  // Information about the original loop we started out with.
593  Loop &OriginalLoop;
594  LoopInfo &OriginalLoopInfo;
595  const SCEV *LatchTakenCount;
596  BasicBlock *OriginalPreheader;
597
598  // The preheader of the main loop.  This may or may not be different from
599  // `OriginalPreheader'.
600  BasicBlock *MainLoopPreheader;
601
602  // The range we need to run the main loop in.
603  InductiveRangeCheck::Range Range;
604
605  // The structure of the main loop (see comment at the beginning of this class
606  // for a definition)
607  LoopStructure MainLoopStructure;
608
609public:
610  LoopConstrainer(Loop &L, LoopInfo &LI, const LoopStructure &LS,
611                  ScalarEvolution &SE, InductiveRangeCheck::Range R)
612      : F(*L.getHeader()->getParent()), Ctx(L.getHeader()->getContext()),
613        SE(SE), OriginalLoop(L), OriginalLoopInfo(LI), LatchTakenCount(nullptr),
614        OriginalPreheader(nullptr), MainLoopPreheader(nullptr), Range(R),
615        MainLoopStructure(LS) {}
616
617  // Entry point for the algorithm.  Returns true on success.
618  bool run();
619};
620
621}
622
623void LoopConstrainer::replacePHIBlock(PHINode *PN, BasicBlock *Block,
624                                      BasicBlock *ReplaceBy) {
625  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
626    if (PN->getIncomingBlock(i) == Block)
627      PN->setIncomingBlock(i, ReplaceBy);
628}
629
630static bool CanBeSMax(ScalarEvolution &SE, const SCEV *S) {
631  APInt SMax =
632      APInt::getSignedMaxValue(cast<IntegerType>(S->getType())->getBitWidth());
633  return SE.getSignedRange(S).contains(SMax) &&
634         SE.getUnsignedRange(S).contains(SMax);
635}
636
637static bool CanBeSMin(ScalarEvolution &SE, const SCEV *S) {
638  APInt SMin =
639      APInt::getSignedMinValue(cast<IntegerType>(S->getType())->getBitWidth());
640  return SE.getSignedRange(S).contains(SMin) &&
641         SE.getUnsignedRange(S).contains(SMin);
642}
643
644Optional<LoopStructure>
645LoopStructure::parseLoopStructure(ScalarEvolution &SE, BranchProbabilityInfo &BPI,
646                                  Loop &L, const char *&FailureReason) {
647  assert(L.isLoopSimplifyForm() && "should follow from addRequired<>");
648
649  BasicBlock *Latch = L.getLoopLatch();
650  if (!L.isLoopExiting(Latch)) {
651    FailureReason = "no loop latch";
652    return None;
653  }
654
655  BasicBlock *Header = L.getHeader();
656  BasicBlock *Preheader = L.getLoopPreheader();
657  if (!Preheader) {
658    FailureReason = "no preheader";
659    return None;
660  }
661
662  BranchInst *LatchBr = dyn_cast<BranchInst>(&*Latch->rbegin());
663  if (!LatchBr || LatchBr->isUnconditional()) {
664    FailureReason = "latch terminator not conditional branch";
665    return None;
666  }
667
668  unsigned LatchBrExitIdx = LatchBr->getSuccessor(0) == Header ? 1 : 0;
669
670  BranchProbability ExitProbability =
671    BPI.getEdgeProbability(LatchBr->getParent(), LatchBrExitIdx);
672
673  if (ExitProbability > BranchProbability(1, MaxExitProbReciprocal)) {
674    FailureReason = "short running loop, not profitable";
675    return None;
676  }
677
678  ICmpInst *ICI = dyn_cast<ICmpInst>(LatchBr->getCondition());
679  if (!ICI || !isa<IntegerType>(ICI->getOperand(0)->getType())) {
680    FailureReason = "latch terminator branch not conditional on integral icmp";
681    return None;
682  }
683
684  const SCEV *LatchCount = SE.getExitCount(&L, Latch);
685  if (isa<SCEVCouldNotCompute>(LatchCount)) {
686    FailureReason = "could not compute latch count";
687    return None;
688  }
689
690  ICmpInst::Predicate Pred = ICI->getPredicate();
691  Value *LeftValue = ICI->getOperand(0);
692  const SCEV *LeftSCEV = SE.getSCEV(LeftValue);
693  IntegerType *IndVarTy = cast<IntegerType>(LeftValue->getType());
694
695  Value *RightValue = ICI->getOperand(1);
696  const SCEV *RightSCEV = SE.getSCEV(RightValue);
697
698  // We canonicalize `ICI` such that `LeftSCEV` is an add recurrence.
699  if (!isa<SCEVAddRecExpr>(LeftSCEV)) {
700    if (isa<SCEVAddRecExpr>(RightSCEV)) {
701      std::swap(LeftSCEV, RightSCEV);
702      std::swap(LeftValue, RightValue);
703      Pred = ICmpInst::getSwappedPredicate(Pred);
704    } else {
705      FailureReason = "no add recurrences in the icmp";
706      return None;
707    }
708  }
709
710  auto HasNoSignedWrap = [&](const SCEVAddRecExpr *AR) {
711    if (AR->getNoWrapFlags(SCEV::FlagNSW))
712      return true;
713
714    IntegerType *Ty = cast<IntegerType>(AR->getType());
715    IntegerType *WideTy =
716        IntegerType::get(Ty->getContext(), Ty->getBitWidth() * 2);
717
718    const SCEVAddRecExpr *ExtendAfterOp =
719        dyn_cast<SCEVAddRecExpr>(SE.getSignExtendExpr(AR, WideTy));
720    if (ExtendAfterOp) {
721      const SCEV *ExtendedStart = SE.getSignExtendExpr(AR->getStart(), WideTy);
722      const SCEV *ExtendedStep =
723          SE.getSignExtendExpr(AR->getStepRecurrence(SE), WideTy);
724
725      bool NoSignedWrap = ExtendAfterOp->getStart() == ExtendedStart &&
726                          ExtendAfterOp->getStepRecurrence(SE) == ExtendedStep;
727
728      if (NoSignedWrap)
729        return true;
730    }
731
732    // We may have proved this when computing the sign extension above.
733    return AR->getNoWrapFlags(SCEV::FlagNSW) != SCEV::FlagAnyWrap;
734  };
735
736  auto IsInductionVar = [&](const SCEVAddRecExpr *AR, bool &IsIncreasing) {
737    if (!AR->isAffine())
738      return false;
739
740    // Currently we only work with induction variables that have been proved to
741    // not wrap.  This restriction can potentially be lifted in the future.
742
743    if (!HasNoSignedWrap(AR))
744      return false;
745
746    if (const SCEVConstant *StepExpr =
747            dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE))) {
748      ConstantInt *StepCI = StepExpr->getValue();
749      if (StepCI->isOne() || StepCI->isMinusOne()) {
750        IsIncreasing = StepCI->isOne();
751        return true;
752      }
753    }
754
755    return false;
756  };
757
758  // `ICI` is interpreted as taking the backedge if the *next* value of the
759  // induction variable satisfies some constraint.
760
761  const SCEVAddRecExpr *IndVarNext = cast<SCEVAddRecExpr>(LeftSCEV);
762  bool IsIncreasing = false;
763  if (!IsInductionVar(IndVarNext, IsIncreasing)) {
764    FailureReason = "LHS in icmp not induction variable";
765    return None;
766  }
767
768  ConstantInt *One = ConstantInt::get(IndVarTy, 1);
769  // TODO: generalize the predicates here to also match their unsigned variants.
770  if (IsIncreasing) {
771    bool FoundExpectedPred =
772        (Pred == ICmpInst::ICMP_SLT && LatchBrExitIdx == 1) ||
773        (Pred == ICmpInst::ICMP_SGT && LatchBrExitIdx == 0);
774
775    if (!FoundExpectedPred) {
776      FailureReason = "expected icmp slt semantically, found something else";
777      return None;
778    }
779
780    if (LatchBrExitIdx == 0) {
781      if (CanBeSMax(SE, RightSCEV)) {
782        // TODO: this restriction is easily removable -- we just have to
783        // remember that the icmp was an slt and not an sle.
784        FailureReason = "limit may overflow when coercing sle to slt";
785        return None;
786      }
787
788      IRBuilder<> B(&*Preheader->rbegin());
789      RightValue = B.CreateAdd(RightValue, One);
790    }
791
792  } else {
793    bool FoundExpectedPred =
794        (Pred == ICmpInst::ICMP_SGT && LatchBrExitIdx == 1) ||
795        (Pred == ICmpInst::ICMP_SLT && LatchBrExitIdx == 0);
796
797    if (!FoundExpectedPred) {
798      FailureReason = "expected icmp sgt semantically, found something else";
799      return None;
800    }
801
802    if (LatchBrExitIdx == 0) {
803      if (CanBeSMin(SE, RightSCEV)) {
804        // TODO: this restriction is easily removable -- we just have to
805        // remember that the icmp was an sgt and not an sge.
806        FailureReason = "limit may overflow when coercing sge to sgt";
807        return None;
808      }
809
810      IRBuilder<> B(&*Preheader->rbegin());
811      RightValue = B.CreateSub(RightValue, One);
812    }
813  }
814
815  const SCEV *StartNext = IndVarNext->getStart();
816  const SCEV *Addend = SE.getNegativeSCEV(IndVarNext->getStepRecurrence(SE));
817  const SCEV *IndVarStart = SE.getAddExpr(StartNext, Addend);
818
819  BasicBlock *LatchExit = LatchBr->getSuccessor(LatchBrExitIdx);
820
821  assert(SE.getLoopDisposition(LatchCount, &L) ==
822             ScalarEvolution::LoopInvariant &&
823         "loop variant exit count doesn't make sense!");
824
825  assert(!L.contains(LatchExit) && "expected an exit block!");
826  const DataLayout &DL = Preheader->getModule()->getDataLayout();
827  Value *IndVarStartV =
828      SCEVExpander(SE, DL, "irce")
829          .expandCodeFor(IndVarStart, IndVarTy, &*Preheader->rbegin());
830  IndVarStartV->setName("indvar.start");
831
832  LoopStructure Result;
833
834  Result.Tag = "main";
835  Result.Header = Header;
836  Result.Latch = Latch;
837  Result.LatchBr = LatchBr;
838  Result.LatchExit = LatchExit;
839  Result.LatchBrExitIdx = LatchBrExitIdx;
840  Result.IndVarStart = IndVarStartV;
841  Result.IndVarNext = LeftValue;
842  Result.IndVarIncreasing = IsIncreasing;
843  Result.LoopExitAt = RightValue;
844
845  FailureReason = nullptr;
846
847  return Result;
848}
849
850Optional<LoopConstrainer::SubRanges>
851LoopConstrainer::calculateSubRanges() const {
852  IntegerType *Ty = cast<IntegerType>(LatchTakenCount->getType());
853
854  if (Range.getType() != Ty)
855    return None;
856
857  LoopConstrainer::SubRanges Result;
858
859  // I think we can be more aggressive here and make this nuw / nsw if the
860  // addition that feeds into the icmp for the latch's terminating branch is nuw
861  // / nsw.  In any case, a wrapping 2's complement addition is safe.
862  ConstantInt *One = ConstantInt::get(Ty, 1);
863  const SCEV *Start = SE.getSCEV(MainLoopStructure.IndVarStart);
864  const SCEV *End = SE.getSCEV(MainLoopStructure.LoopExitAt);
865
866  bool Increasing = MainLoopStructure.IndVarIncreasing;
867
868  // We compute `Smallest` and `Greatest` such that [Smallest, Greatest) is the
869  // range of values the induction variable takes.
870
871  const SCEV *Smallest = nullptr, *Greatest = nullptr;
872
873  if (Increasing) {
874    Smallest = Start;
875    Greatest = End;
876  } else {
877    // These two computations may sign-overflow.  Here is why that is okay:
878    //
879    // We know that the induction variable does not sign-overflow on any
880    // iteration except the last one, and it starts at `Start` and ends at
881    // `End`, decrementing by one every time.
882    //
883    //  * if `Smallest` sign-overflows we know `End` is `INT_SMAX`. Since the
884    //    induction variable is decreasing we know that that the smallest value
885    //    the loop body is actually executed with is `INT_SMIN` == `Smallest`.
886    //
887    //  * if `Greatest` sign-overflows, we know it can only be `INT_SMIN`.  In
888    //    that case, `Clamp` will always return `Smallest` and
889    //    [`Result.LowLimit`, `Result.HighLimit`) = [`Smallest`, `Smallest`)
890    //    will be an empty range.  Returning an empty range is always safe.
891    //
892
893    Smallest = SE.getAddExpr(End, SE.getSCEV(One));
894    Greatest = SE.getAddExpr(Start, SE.getSCEV(One));
895  }
896
897  auto Clamp = [this, Smallest, Greatest](const SCEV *S) {
898    return SE.getSMaxExpr(Smallest, SE.getSMinExpr(Greatest, S));
899  };
900
901  // In some cases we can prove that we don't need a pre or post loop
902
903  bool ProvablyNoPreloop =
904      SE.isKnownPredicate(ICmpInst::ICMP_SLE, Range.getBegin(), Smallest);
905  if (!ProvablyNoPreloop)
906    Result.LowLimit = Clamp(Range.getBegin());
907
908  bool ProvablyNoPostLoop =
909      SE.isKnownPredicate(ICmpInst::ICMP_SLE, Greatest, Range.getEnd());
910  if (!ProvablyNoPostLoop)
911    Result.HighLimit = Clamp(Range.getEnd());
912
913  return Result;
914}
915
916void LoopConstrainer::cloneLoop(LoopConstrainer::ClonedLoop &Result,
917                                const char *Tag) const {
918  for (BasicBlock *BB : OriginalLoop.getBlocks()) {
919    BasicBlock *Clone = CloneBasicBlock(BB, Result.Map, Twine(".") + Tag, &F);
920    Result.Blocks.push_back(Clone);
921    Result.Map[BB] = Clone;
922  }
923
924  auto GetClonedValue = [&Result](Value *V) {
925    assert(V && "null values not in domain!");
926    auto It = Result.Map.find(V);
927    if (It == Result.Map.end())
928      return V;
929    return static_cast<Value *>(It->second);
930  };
931
932  Result.Structure = MainLoopStructure.map(GetClonedValue);
933  Result.Structure.Tag = Tag;
934
935  for (unsigned i = 0, e = Result.Blocks.size(); i != e; ++i) {
936    BasicBlock *ClonedBB = Result.Blocks[i];
937    BasicBlock *OriginalBB = OriginalLoop.getBlocks()[i];
938
939    assert(Result.Map[OriginalBB] == ClonedBB && "invariant!");
940
941    for (Instruction &I : *ClonedBB)
942      RemapInstruction(&I, Result.Map,
943                       RF_NoModuleLevelChanges | RF_IgnoreMissingEntries);
944
945    // Exit blocks will now have one more predecessor and their PHI nodes need
946    // to be edited to reflect that.  No phi nodes need to be introduced because
947    // the loop is in LCSSA.
948
949    for (auto SBBI = succ_begin(OriginalBB), SBBE = succ_end(OriginalBB);
950         SBBI != SBBE; ++SBBI) {
951
952      if (OriginalLoop.contains(*SBBI))
953        continue; // not an exit block
954
955      for (Instruction &I : **SBBI) {
956        if (!isa<PHINode>(&I))
957          break;
958
959        PHINode *PN = cast<PHINode>(&I);
960        Value *OldIncoming = PN->getIncomingValueForBlock(OriginalBB);
961        PN->addIncoming(GetClonedValue(OldIncoming), ClonedBB);
962      }
963    }
964  }
965}
966
967LoopConstrainer::RewrittenRangeInfo LoopConstrainer::changeIterationSpaceEnd(
968    const LoopStructure &LS, BasicBlock *Preheader, Value *ExitSubloopAt,
969    BasicBlock *ContinuationBlock) const {
970
971  // We start with a loop with a single latch:
972  //
973  //    +--------------------+
974  //    |                    |
975  //    |     preheader      |
976  //    |                    |
977  //    +--------+-----------+
978  //             |      ----------------\
979  //             |     /                |
980  //    +--------v----v------+          |
981  //    |                    |          |
982  //    |      header        |          |
983  //    |                    |          |
984  //    +--------------------+          |
985  //                                    |
986  //            .....                   |
987  //                                    |
988  //    +--------------------+          |
989  //    |                    |          |
990  //    |       latch        >----------/
991  //    |                    |
992  //    +-------v------------+
993  //            |
994  //            |
995  //            |   +--------------------+
996  //            |   |                    |
997  //            +--->   original exit    |
998  //                |                    |
999  //                +--------------------+
1000  //
1001  // We change the control flow to look like
1002  //
1003  //
1004  //    +--------------------+
1005  //    |                    |
1006  //    |     preheader      >-------------------------+
1007  //    |                    |                         |
1008  //    +--------v-----------+                         |
1009  //             |    /-------------+                  |
1010  //             |   /              |                  |
1011  //    +--------v--v--------+      |                  |
1012  //    |                    |      |                  |
1013  //    |      header        |      |   +--------+     |
1014  //    |                    |      |   |        |     |
1015  //    +--------------------+      |   |  +-----v-----v-----------+
1016  //                                |   |  |                       |
1017  //                                |   |  |     .pseudo.exit      |
1018  //                                |   |  |                       |
1019  //                                |   |  +-----------v-----------+
1020  //                                |   |              |
1021  //            .....               |   |              |
1022  //                                |   |     +--------v-------------+
1023  //    +--------------------+      |   |     |                      |
1024  //    |                    |      |   |     |   ContinuationBlock  |
1025  //    |       latch        >------+   |     |                      |
1026  //    |                    |          |     +----------------------+
1027  //    +---------v----------+          |
1028  //              |                     |
1029  //              |                     |
1030  //              |     +---------------^-----+
1031  //              |     |                     |
1032  //              +----->    .exit.selector   |
1033  //                    |                     |
1034  //                    +----------v----------+
1035  //                               |
1036  //     +--------------------+    |
1037  //     |                    |    |
1038  //     |   original exit    <----+
1039  //     |                    |
1040  //     +--------------------+
1041  //
1042
1043  RewrittenRangeInfo RRI;
1044
1045  auto BBInsertLocation = std::next(Function::iterator(LS.Latch));
1046  RRI.ExitSelector = BasicBlock::Create(Ctx, Twine(LS.Tag) + ".exit.selector",
1047                                        &F, BBInsertLocation);
1048  RRI.PseudoExit = BasicBlock::Create(Ctx, Twine(LS.Tag) + ".pseudo.exit", &F,
1049                                      BBInsertLocation);
1050
1051  BranchInst *PreheaderJump = cast<BranchInst>(&*Preheader->rbegin());
1052  bool Increasing = LS.IndVarIncreasing;
1053
1054  IRBuilder<> B(PreheaderJump);
1055
1056  // EnterLoopCond - is it okay to start executing this `LS'?
1057  Value *EnterLoopCond = Increasing
1058                             ? B.CreateICmpSLT(LS.IndVarStart, ExitSubloopAt)
1059                             : B.CreateICmpSGT(LS.IndVarStart, ExitSubloopAt);
1060
1061  B.CreateCondBr(EnterLoopCond, LS.Header, RRI.PseudoExit);
1062  PreheaderJump->eraseFromParent();
1063
1064  LS.LatchBr->setSuccessor(LS.LatchBrExitIdx, RRI.ExitSelector);
1065  B.SetInsertPoint(LS.LatchBr);
1066  Value *TakeBackedgeLoopCond =
1067      Increasing ? B.CreateICmpSLT(LS.IndVarNext, ExitSubloopAt)
1068                 : B.CreateICmpSGT(LS.IndVarNext, ExitSubloopAt);
1069  Value *CondForBranch = LS.LatchBrExitIdx == 1
1070                             ? TakeBackedgeLoopCond
1071                             : B.CreateNot(TakeBackedgeLoopCond);
1072
1073  LS.LatchBr->setCondition(CondForBranch);
1074
1075  B.SetInsertPoint(RRI.ExitSelector);
1076
1077  // IterationsLeft - are there any more iterations left, given the original
1078  // upper bound on the induction variable?  If not, we branch to the "real"
1079  // exit.
1080  Value *IterationsLeft = Increasing
1081                              ? B.CreateICmpSLT(LS.IndVarNext, LS.LoopExitAt)
1082                              : B.CreateICmpSGT(LS.IndVarNext, LS.LoopExitAt);
1083  B.CreateCondBr(IterationsLeft, RRI.PseudoExit, LS.LatchExit);
1084
1085  BranchInst *BranchToContinuation =
1086      BranchInst::Create(ContinuationBlock, RRI.PseudoExit);
1087
1088  // We emit PHI nodes into `RRI.PseudoExit' that compute the "latest" value of
1089  // each of the PHI nodes in the loop header.  This feeds into the initial
1090  // value of the same PHI nodes if/when we continue execution.
1091  for (Instruction &I : *LS.Header) {
1092    if (!isa<PHINode>(&I))
1093      break;
1094
1095    PHINode *PN = cast<PHINode>(&I);
1096
1097    PHINode *NewPHI = PHINode::Create(PN->getType(), 2, PN->getName() + ".copy",
1098                                      BranchToContinuation);
1099
1100    NewPHI->addIncoming(PN->getIncomingValueForBlock(Preheader), Preheader);
1101    NewPHI->addIncoming(PN->getIncomingValueForBlock(LS.Latch),
1102                        RRI.ExitSelector);
1103    RRI.PHIValuesAtPseudoExit.push_back(NewPHI);
1104  }
1105
1106  RRI.IndVarEnd = PHINode::Create(LS.IndVarNext->getType(), 2, "indvar.end",
1107                                  BranchToContinuation);
1108  RRI.IndVarEnd->addIncoming(LS.IndVarStart, Preheader);
1109  RRI.IndVarEnd->addIncoming(LS.IndVarNext, RRI.ExitSelector);
1110
1111  // The latch exit now has a branch from `RRI.ExitSelector' instead of
1112  // `LS.Latch'.  The PHI nodes need to be updated to reflect that.
1113  for (Instruction &I : *LS.LatchExit) {
1114    if (PHINode *PN = dyn_cast<PHINode>(&I))
1115      replacePHIBlock(PN, LS.Latch, RRI.ExitSelector);
1116    else
1117      break;
1118  }
1119
1120  return RRI;
1121}
1122
1123void LoopConstrainer::rewriteIncomingValuesForPHIs(
1124    LoopStructure &LS, BasicBlock *ContinuationBlock,
1125    const LoopConstrainer::RewrittenRangeInfo &RRI) const {
1126
1127  unsigned PHIIndex = 0;
1128  for (Instruction &I : *LS.Header) {
1129    if (!isa<PHINode>(&I))
1130      break;
1131
1132    PHINode *PN = cast<PHINode>(&I);
1133
1134    for (unsigned i = 0, e = PN->getNumIncomingValues(); i < e; ++i)
1135      if (PN->getIncomingBlock(i) == ContinuationBlock)
1136        PN->setIncomingValue(i, RRI.PHIValuesAtPseudoExit[PHIIndex++]);
1137  }
1138
1139  LS.IndVarStart = RRI.IndVarEnd;
1140}
1141
1142BasicBlock *LoopConstrainer::createPreheader(const LoopStructure &LS,
1143                                             BasicBlock *OldPreheader,
1144                                             const char *Tag) const {
1145
1146  BasicBlock *Preheader = BasicBlock::Create(Ctx, Tag, &F, LS.Header);
1147  BranchInst::Create(LS.Header, Preheader);
1148
1149  for (Instruction &I : *LS.Header) {
1150    if (!isa<PHINode>(&I))
1151      break;
1152
1153    PHINode *PN = cast<PHINode>(&I);
1154    for (unsigned i = 0, e = PN->getNumIncomingValues(); i < e; ++i)
1155      replacePHIBlock(PN, OldPreheader, Preheader);
1156  }
1157
1158  return Preheader;
1159}
1160
1161void LoopConstrainer::addToParentLoopIfNeeded(ArrayRef<BasicBlock *> BBs) {
1162  Loop *ParentLoop = OriginalLoop.getParentLoop();
1163  if (!ParentLoop)
1164    return;
1165
1166  for (BasicBlock *BB : BBs)
1167    ParentLoop->addBasicBlockToLoop(BB, OriginalLoopInfo);
1168}
1169
1170bool LoopConstrainer::run() {
1171  BasicBlock *Preheader = nullptr;
1172  LatchTakenCount = SE.getExitCount(&OriginalLoop, MainLoopStructure.Latch);
1173  Preheader = OriginalLoop.getLoopPreheader();
1174  assert(!isa<SCEVCouldNotCompute>(LatchTakenCount) && Preheader != nullptr &&
1175         "preconditions!");
1176
1177  OriginalPreheader = Preheader;
1178  MainLoopPreheader = Preheader;
1179
1180  Optional<SubRanges> MaybeSR = calculateSubRanges();
1181  if (!MaybeSR.hasValue()) {
1182    DEBUG(dbgs() << "irce: could not compute subranges\n");
1183    return false;
1184  }
1185
1186  SubRanges SR = MaybeSR.getValue();
1187  bool Increasing = MainLoopStructure.IndVarIncreasing;
1188  IntegerType *IVTy =
1189      cast<IntegerType>(MainLoopStructure.IndVarNext->getType());
1190
1191  SCEVExpander Expander(SE, F.getParent()->getDataLayout(), "irce");
1192  Instruction *InsertPt = OriginalPreheader->getTerminator();
1193
1194  // It would have been better to make `PreLoop' and `PostLoop'
1195  // `Optional<ClonedLoop>'s, but `ValueToValueMapTy' does not have a copy
1196  // constructor.
1197  ClonedLoop PreLoop, PostLoop;
1198  bool NeedsPreLoop =
1199      Increasing ? SR.LowLimit.hasValue() : SR.HighLimit.hasValue();
1200  bool NeedsPostLoop =
1201      Increasing ? SR.HighLimit.hasValue() : SR.LowLimit.hasValue();
1202
1203  Value *ExitPreLoopAt = nullptr;
1204  Value *ExitMainLoopAt = nullptr;
1205  const SCEVConstant *MinusOneS =
1206      cast<SCEVConstant>(SE.getConstant(IVTy, -1, true /* isSigned */));
1207
1208  if (NeedsPreLoop) {
1209    const SCEV *ExitPreLoopAtSCEV = nullptr;
1210
1211    if (Increasing)
1212      ExitPreLoopAtSCEV = *SR.LowLimit;
1213    else {
1214      if (CanBeSMin(SE, *SR.HighLimit)) {
1215        DEBUG(dbgs() << "irce: could not prove no-overflow when computing "
1216                     << "preloop exit limit.  HighLimit = " << *(*SR.HighLimit)
1217                     << "\n");
1218        return false;
1219      }
1220      ExitPreLoopAtSCEV = SE.getAddExpr(*SR.HighLimit, MinusOneS);
1221    }
1222
1223    ExitPreLoopAt = Expander.expandCodeFor(ExitPreLoopAtSCEV, IVTy, InsertPt);
1224    ExitPreLoopAt->setName("exit.preloop.at");
1225  }
1226
1227  if (NeedsPostLoop) {
1228    const SCEV *ExitMainLoopAtSCEV = nullptr;
1229
1230    if (Increasing)
1231      ExitMainLoopAtSCEV = *SR.HighLimit;
1232    else {
1233      if (CanBeSMin(SE, *SR.LowLimit)) {
1234        DEBUG(dbgs() << "irce: could not prove no-overflow when computing "
1235                     << "mainloop exit limit.  LowLimit = " << *(*SR.LowLimit)
1236                     << "\n");
1237        return false;
1238      }
1239      ExitMainLoopAtSCEV = SE.getAddExpr(*SR.LowLimit, MinusOneS);
1240    }
1241
1242    ExitMainLoopAt = Expander.expandCodeFor(ExitMainLoopAtSCEV, IVTy, InsertPt);
1243    ExitMainLoopAt->setName("exit.mainloop.at");
1244  }
1245
1246  // We clone these ahead of time so that we don't have to deal with changing
1247  // and temporarily invalid IR as we transform the loops.
1248  if (NeedsPreLoop)
1249    cloneLoop(PreLoop, "preloop");
1250  if (NeedsPostLoop)
1251    cloneLoop(PostLoop, "postloop");
1252
1253  RewrittenRangeInfo PreLoopRRI;
1254
1255  if (NeedsPreLoop) {
1256    Preheader->getTerminator()->replaceUsesOfWith(MainLoopStructure.Header,
1257                                                  PreLoop.Structure.Header);
1258
1259    MainLoopPreheader =
1260        createPreheader(MainLoopStructure, Preheader, "mainloop");
1261    PreLoopRRI = changeIterationSpaceEnd(PreLoop.Structure, Preheader,
1262                                         ExitPreLoopAt, MainLoopPreheader);
1263    rewriteIncomingValuesForPHIs(MainLoopStructure, MainLoopPreheader,
1264                                 PreLoopRRI);
1265  }
1266
1267  BasicBlock *PostLoopPreheader = nullptr;
1268  RewrittenRangeInfo PostLoopRRI;
1269
1270  if (NeedsPostLoop) {
1271    PostLoopPreheader =
1272        createPreheader(PostLoop.Structure, Preheader, "postloop");
1273    PostLoopRRI = changeIterationSpaceEnd(MainLoopStructure, MainLoopPreheader,
1274                                          ExitMainLoopAt, PostLoopPreheader);
1275    rewriteIncomingValuesForPHIs(PostLoop.Structure, PostLoopPreheader,
1276                                 PostLoopRRI);
1277  }
1278
1279  BasicBlock *NewMainLoopPreheader =
1280      MainLoopPreheader != Preheader ? MainLoopPreheader : nullptr;
1281  BasicBlock *NewBlocks[] = {PostLoopPreheader,        PreLoopRRI.PseudoExit,
1282                             PreLoopRRI.ExitSelector,  PostLoopRRI.PseudoExit,
1283                             PostLoopRRI.ExitSelector, NewMainLoopPreheader};
1284
1285  // Some of the above may be nullptr, filter them out before passing to
1286  // addToParentLoopIfNeeded.
1287  auto NewBlocksEnd =
1288      std::remove(std::begin(NewBlocks), std::end(NewBlocks), nullptr);
1289
1290  addToParentLoopIfNeeded(makeArrayRef(std::begin(NewBlocks), NewBlocksEnd));
1291  addToParentLoopIfNeeded(PreLoop.Blocks);
1292  addToParentLoopIfNeeded(PostLoop.Blocks);
1293
1294  return true;
1295}
1296
1297/// Computes and returns a range of values for the induction variable (IndVar)
1298/// in which the range check can be safely elided.  If it cannot compute such a
1299/// range, returns None.
1300Optional<InductiveRangeCheck::Range>
1301InductiveRangeCheck::computeSafeIterationSpace(ScalarEvolution &SE,
1302                                               const SCEVAddRecExpr *IndVar,
1303                                               IRBuilder<> &) const {
1304  // IndVar is of the form "A + B * I" (where "I" is the canonical induction
1305  // variable, that may or may not exist as a real llvm::Value in the loop) and
1306  // this inductive range check is a range check on the "C + D * I" ("C" is
1307  // getOffset() and "D" is getScale()).  We rewrite the value being range
1308  // checked to "M + N * IndVar" where "N" = "D * B^(-1)" and "M" = "C - NA".
1309  // Currently we support this only for "B" = "D" = { 1 or -1 }, but the code
1310  // can be generalized as needed.
1311  //
1312  // The actual inequalities we solve are of the form
1313  //
1314  //   0 <= M + 1 * IndVar < L given L >= 0  (i.e. N == 1)
1315  //
1316  // The inequality is satisfied by -M <= IndVar < (L - M) [^1].  All additions
1317  // and subtractions are twos-complement wrapping and comparisons are signed.
1318  //
1319  // Proof:
1320  //
1321  //   If there exists IndVar such that -M <= IndVar < (L - M) then it follows
1322  //   that -M <= (-M + L) [== Eq. 1].  Since L >= 0, if (-M + L) sign-overflows
1323  //   then (-M + L) < (-M).  Hence by [Eq. 1], (-M + L) could not have
1324  //   overflown.
1325  //
1326  //   This means IndVar = t + (-M) for t in [0, L).  Hence (IndVar + M) = t.
1327  //   Hence 0 <= (IndVar + M) < L
1328
1329  // [^1]: Note that the solution does _not_ apply if L < 0; consider values M =
1330  // 127, IndVar = 126 and L = -2 in an i8 world.
1331
1332  if (!IndVar->isAffine())
1333    return None;
1334
1335  const SCEV *A = IndVar->getStart();
1336  const SCEVConstant *B = dyn_cast<SCEVConstant>(IndVar->getStepRecurrence(SE));
1337  if (!B)
1338    return None;
1339
1340  const SCEV *C = getOffset();
1341  const SCEVConstant *D = dyn_cast<SCEVConstant>(getScale());
1342  if (D != B)
1343    return None;
1344
1345  ConstantInt *ConstD = D->getValue();
1346  if (!(ConstD->isMinusOne() || ConstD->isOne()))
1347    return None;
1348
1349  const SCEV *M = SE.getMinusSCEV(C, A);
1350
1351  const SCEV *Begin = SE.getNegativeSCEV(M);
1352  const SCEV *UpperLimit = nullptr;
1353
1354  // We strengthen "0 <= I" to "0 <= I < INT_SMAX" and "I < L" to "0 <= I < L".
1355  // We can potentially do much better here.
1356  if (Value *V = getLength()) {
1357    UpperLimit = SE.getSCEV(V);
1358  } else {
1359    assert(Kind == InductiveRangeCheck::RANGE_CHECK_LOWER && "invariant!");
1360    unsigned BitWidth = cast<IntegerType>(IndVar->getType())->getBitWidth();
1361    UpperLimit = SE.getConstant(APInt::getSignedMaxValue(BitWidth));
1362  }
1363
1364  const SCEV *End = SE.getMinusSCEV(UpperLimit, M);
1365  return InductiveRangeCheck::Range(Begin, End);
1366}
1367
1368static Optional<InductiveRangeCheck::Range>
1369IntersectRange(ScalarEvolution &SE,
1370               const Optional<InductiveRangeCheck::Range> &R1,
1371               const InductiveRangeCheck::Range &R2, IRBuilder<> &B) {
1372  if (!R1.hasValue())
1373    return R2;
1374  auto &R1Value = R1.getValue();
1375
1376  // TODO: we could widen the smaller range and have this work; but for now we
1377  // bail out to keep things simple.
1378  if (R1Value.getType() != R2.getType())
1379    return None;
1380
1381  const SCEV *NewBegin = SE.getSMaxExpr(R1Value.getBegin(), R2.getBegin());
1382  const SCEV *NewEnd = SE.getSMinExpr(R1Value.getEnd(), R2.getEnd());
1383
1384  return InductiveRangeCheck::Range(NewBegin, NewEnd);
1385}
1386
1387bool InductiveRangeCheckElimination::runOnLoop(Loop *L, LPPassManager &LPM) {
1388  if (L->getBlocks().size() >= LoopSizeCutoff) {
1389    DEBUG(dbgs() << "irce: giving up constraining loop, too large\n";);
1390    return false;
1391  }
1392
1393  BasicBlock *Preheader = L->getLoopPreheader();
1394  if (!Preheader) {
1395    DEBUG(dbgs() << "irce: loop has no preheader, leaving\n");
1396    return false;
1397  }
1398
1399  LLVMContext &Context = Preheader->getContext();
1400  InductiveRangeCheck::AllocatorTy IRCAlloc;
1401  SmallVector<InductiveRangeCheck *, 16> RangeChecks;
1402  ScalarEvolution &SE = getAnalysis<ScalarEvolution>();
1403  BranchProbabilityInfo &BPI = getAnalysis<BranchProbabilityInfo>();
1404
1405  for (auto BBI : L->getBlocks())
1406    if (BranchInst *TBI = dyn_cast<BranchInst>(BBI->getTerminator()))
1407      if (InductiveRangeCheck *IRC =
1408          InductiveRangeCheck::create(IRCAlloc, TBI, L, SE, BPI))
1409        RangeChecks.push_back(IRC);
1410
1411  if (RangeChecks.empty())
1412    return false;
1413
1414  auto PrintRecognizedRangeChecks = [&](raw_ostream &OS) {
1415    OS << "irce: looking at loop "; L->print(OS);
1416    OS << "irce: loop has " << RangeChecks.size()
1417       << " inductive range checks: \n";
1418    for (InductiveRangeCheck *IRC : RangeChecks)
1419      IRC->print(OS);
1420  };
1421
1422  DEBUG(PrintRecognizedRangeChecks(dbgs()));
1423
1424  if (PrintRangeChecks)
1425    PrintRecognizedRangeChecks(errs());
1426
1427  const char *FailureReason = nullptr;
1428  Optional<LoopStructure> MaybeLoopStructure =
1429      LoopStructure::parseLoopStructure(SE, BPI, *L, FailureReason);
1430  if (!MaybeLoopStructure.hasValue()) {
1431    DEBUG(dbgs() << "irce: could not parse loop structure: " << FailureReason
1432                 << "\n";);
1433    return false;
1434  }
1435  LoopStructure LS = MaybeLoopStructure.getValue();
1436  bool Increasing = LS.IndVarIncreasing;
1437  const SCEV *MinusOne =
1438      SE.getConstant(LS.IndVarNext->getType(), Increasing ? -1 : 1, true);
1439  const SCEVAddRecExpr *IndVar =
1440      cast<SCEVAddRecExpr>(SE.getAddExpr(SE.getSCEV(LS.IndVarNext), MinusOne));
1441
1442  Optional<InductiveRangeCheck::Range> SafeIterRange;
1443  Instruction *ExprInsertPt = Preheader->getTerminator();
1444
1445  SmallVector<InductiveRangeCheck *, 4> RangeChecksToEliminate;
1446
1447  IRBuilder<> B(ExprInsertPt);
1448  for (InductiveRangeCheck *IRC : RangeChecks) {
1449    auto Result = IRC->computeSafeIterationSpace(SE, IndVar, B);
1450    if (Result.hasValue()) {
1451      auto MaybeSafeIterRange =
1452        IntersectRange(SE, SafeIterRange, Result.getValue(), B);
1453      if (MaybeSafeIterRange.hasValue()) {
1454        RangeChecksToEliminate.push_back(IRC);
1455        SafeIterRange = MaybeSafeIterRange.getValue();
1456      }
1457    }
1458  }
1459
1460  if (!SafeIterRange.hasValue())
1461    return false;
1462
1463  LoopConstrainer LC(*L, getAnalysis<LoopInfoWrapperPass>().getLoopInfo(), LS,
1464                     SE, SafeIterRange.getValue());
1465  bool Changed = LC.run();
1466
1467  if (Changed) {
1468    auto PrintConstrainedLoopInfo = [L]() {
1469      dbgs() << "irce: in function ";
1470      dbgs() << L->getHeader()->getParent()->getName() << ": ";
1471      dbgs() << "constrained ";
1472      L->print(dbgs());
1473    };
1474
1475    DEBUG(PrintConstrainedLoopInfo());
1476
1477    if (PrintChangedLoops)
1478      PrintConstrainedLoopInfo();
1479
1480    // Optimize away the now-redundant range checks.
1481
1482    for (InductiveRangeCheck *IRC : RangeChecksToEliminate) {
1483      ConstantInt *FoldedRangeCheck = IRC->getPassingDirection()
1484                                          ? ConstantInt::getTrue(Context)
1485                                          : ConstantInt::getFalse(Context);
1486      IRC->getBranch()->setCondition(FoldedRangeCheck);
1487    }
1488  }
1489
1490  return Changed;
1491}
1492
1493Pass *llvm::createInductiveRangeCheckEliminationPass() {
1494  return new InductiveRangeCheckElimination;
1495}
1496