LoopInfo.cpp revision 360784 
1//===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file defines the LoopInfo class that is used to identify natural loops
10// and determine the loop depth of various nodes of the CFG.  Note that the
11// loops identified may actually be several natural loops that share the same
12// header node... not just a single natural loop.
13//
14//===----------------------------------------------------------------------===//
15
16#include "llvm/Analysis/LoopInfo.h"
17#include "llvm/ADT/DepthFirstIterator.h"
18#include "llvm/ADT/ScopeExit.h"
19#include "llvm/ADT/SmallPtrSet.h"
20#include "llvm/Analysis/IVDescriptors.h"
21#include "llvm/Analysis/LoopInfoImpl.h"
22#include "llvm/Analysis/LoopIterator.h"
23#include "llvm/Analysis/MemorySSA.h"
24#include "llvm/Analysis/MemorySSAUpdater.h"
25#include "llvm/Analysis/ScalarEvolutionExpressions.h"
26#include "llvm/Analysis/ValueTracking.h"
27#include "llvm/Config/llvm-config.h"
28#include "llvm/IR/CFG.h"
29#include "llvm/IR/Constants.h"
30#include "llvm/IR/DebugLoc.h"
31#include "llvm/IR/Dominators.h"
32#include "llvm/IR/IRPrintingPasses.h"
33#include "llvm/IR/Instructions.h"
34#include "llvm/IR/LLVMContext.h"
35#include "llvm/IR/Metadata.h"
36#include "llvm/IR/PassManager.h"
37#include "llvm/InitializePasses.h"
38#include "llvm/Support/CommandLine.h"
39#include "llvm/Support/Debug.h"
40#include "llvm/Support/raw_ostream.h"
41#include <algorithm>
42using namespace llvm;
43
44// Explicitly instantiate methods in LoopInfoImpl.h for IR-level Loops.
45template class llvm::LoopBase<BasicBlock, Loop>;
46template class llvm::LoopInfoBase<BasicBlock, Loop>;
47
48// Always verify loopinfo if expensive checking is enabled.
49#ifdef EXPENSIVE_CHECKS
50bool llvm::VerifyLoopInfo = true;
51#else
52bool llvm::VerifyLoopInfo = false;
53#endif
54static cl::opt<bool, true>
55    VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
56                    cl::Hidden, cl::desc("Verify loop info (time consuming)"));
57
58//===----------------------------------------------------------------------===//
59// Loop implementation
60//
61
62bool Loop::isLoopInvariant(const Value *V) const {
63  if (const Instruction *I = dyn_cast<Instruction>(V))
64    return !contains(I);
65  return true; // All non-instructions are loop invariant
66}
67
68bool Loop::hasLoopInvariantOperands(const Instruction *I) const {
69  return all_of(I->operands(), [this](Value *V) { return isLoopInvariant(V); });
70}
71
72bool Loop::makeLoopInvariant(Value *V, bool &Changed, Instruction *InsertPt,
73                             MemorySSAUpdater *MSSAU) const {
74  if (Instruction *I = dyn_cast<Instruction>(V))
75    return makeLoopInvariant(I, Changed, InsertPt, MSSAU);
76  return true; // All non-instructions are loop-invariant.
77}
78
79bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
80                             Instruction *InsertPt,
81                             MemorySSAUpdater *MSSAU) const {
82  // Test if the value is already loop-invariant.
83  if (isLoopInvariant(I))
84    return true;
85  if (!isSafeToSpeculativelyExecute(I))
86    return false;
87  if (I->mayReadFromMemory())
88    return false;
89  // EH block instructions are immobile.
90  if (I->isEHPad())
91    return false;
92  // Determine the insertion point, unless one was given.
93  if (!InsertPt) {
94    BasicBlock *Preheader = getLoopPreheader();
95    // Without a preheader, hoisting is not feasible.
96    if (!Preheader)
97      return false;
98    InsertPt = Preheader->getTerminator();
99  }
100  // Don't hoist instructions with loop-variant operands.
101  for (Value *Operand : I->operands())
102    if (!makeLoopInvariant(Operand, Changed, InsertPt, MSSAU))
103      return false;
104
105  // Hoist.
106  I->moveBefore(InsertPt);
107  if (MSSAU)
108    if (auto *MUD = MSSAU->getMemorySSA()->getMemoryAccess(I))
109      MSSAU->moveToPlace(MUD, InsertPt->getParent(),
110                         MemorySSA::BeforeTerminator);
111
112  // There is possibility of hoisting this instruction above some arbitrary
113  // condition. Any metadata defined on it can be control dependent on this
114  // condition. Conservatively strip it here so that we don't give any wrong
115  // information to the optimizer.
116  I->dropUnknownNonDebugMetadata();
117
118  Changed = true;
119  return true;
120}
121
122bool Loop::getIncomingAndBackEdge(BasicBlock *&Incoming,
123                                  BasicBlock *&Backedge) const {
124  BasicBlock *H = getHeader();
125
126  Incoming = nullptr;
127  Backedge = nullptr;
128  pred_iterator PI = pred_begin(H);
129  assert(PI != pred_end(H) && "Loop must have at least one backedge!");
130  Backedge = *PI++;
131  if (PI == pred_end(H))
132    return false; // dead loop
133  Incoming = *PI++;
134  if (PI != pred_end(H))
135    return false; // multiple backedges?
136
137  if (contains(Incoming)) {
138    if (contains(Backedge))
139      return false;
140    std::swap(Incoming, Backedge);
141  } else if (!contains(Backedge))
142    return false;
143
144  assert(Incoming && Backedge && "expected non-null incoming and backedges");
145  return true;
146}
147
148PHINode *Loop::getCanonicalInductionVariable() const {
149  BasicBlock *H = getHeader();
150
151  BasicBlock *Incoming = nullptr, *Backedge = nullptr;
152  if (!getIncomingAndBackEdge(Incoming, Backedge))
153    return nullptr;
154
155  // Loop over all of the PHI nodes, looking for a canonical indvar.
156  for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
157    PHINode *PN = cast<PHINode>(I);
158    if (ConstantInt *CI =
159            dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
160      if (CI->isZero())
161        if (Instruction *Inc =
162                dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
163          if (Inc->getOpcode() == Instruction::Add && Inc->getOperand(0) == PN)
164            if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
165              if (CI->isOne())
166                return PN;
167  }
168  return nullptr;
169}
170
171/// Get the latch condition instruction.
172static ICmpInst *getLatchCmpInst(const Loop &L) {
173  if (BasicBlock *Latch = L.getLoopLatch())
174    if (BranchInst *BI = dyn_cast_or_null<BranchInst>(Latch->getTerminator()))
175      if (BI->isConditional())
176        return dyn_cast<ICmpInst>(BI->getCondition());
177
178  return nullptr;
179}
180
181/// Return the final value of the loop induction variable if found.
182static Value *findFinalIVValue(const Loop &L, const PHINode &IndVar,
183                               const Instruction &StepInst) {
184  ICmpInst *LatchCmpInst = getLatchCmpInst(L);
185  if (!LatchCmpInst)
186    return nullptr;
187
188  Value *Op0 = LatchCmpInst->getOperand(0);
189  Value *Op1 = LatchCmpInst->getOperand(1);
190  if (Op0 == &IndVar || Op0 == &StepInst)
191    return Op1;
192
193  if (Op1 == &IndVar || Op1 == &StepInst)
194    return Op0;
195
196  return nullptr;
197}
198
199Optional<Loop::LoopBounds> Loop::LoopBounds::getBounds(const Loop &L,
200                                                       PHINode &IndVar,
201                                                       ScalarEvolution &SE) {
202  InductionDescriptor IndDesc;
203  if (!InductionDescriptor::isInductionPHI(&IndVar, &L, &SE, IndDesc))
204    return None;
205
206  Value *InitialIVValue = IndDesc.getStartValue();
207  Instruction *StepInst = IndDesc.getInductionBinOp();
208  if (!InitialIVValue || !StepInst)
209    return None;
210
211  const SCEV *Step = IndDesc.getStep();
212  Value *StepInstOp1 = StepInst->getOperand(1);
213  Value *StepInstOp0 = StepInst->getOperand(0);
214  Value *StepValue = nullptr;
215  if (SE.getSCEV(StepInstOp1) == Step)
216    StepValue = StepInstOp1;
217  else if (SE.getSCEV(StepInstOp0) == Step)
218    StepValue = StepInstOp0;
219
220  Value *FinalIVValue = findFinalIVValue(L, IndVar, *StepInst);
221  if (!FinalIVValue)
222    return None;
223
224  return LoopBounds(L, *InitialIVValue, *StepInst, StepValue, *FinalIVValue,
225                    SE);
226}
227
228using Direction = Loop::LoopBounds::Direction;
229
230ICmpInst::Predicate Loop::LoopBounds::getCanonicalPredicate() const {
231  BasicBlock *Latch = L.getLoopLatch();
232  assert(Latch && "Expecting valid latch");
233
234  BranchInst *BI = dyn_cast_or_null<BranchInst>(Latch->getTerminator());
235  assert(BI && BI->isConditional() && "Expecting conditional latch branch");
236
237  ICmpInst *LatchCmpInst = dyn_cast<ICmpInst>(BI->getCondition());
238  assert(LatchCmpInst &&
239         "Expecting the latch compare instruction to be a CmpInst");
240
241  // Need to inverse the predicate when first successor is not the loop
242  // header
243  ICmpInst::Predicate Pred = (BI->getSuccessor(0) == L.getHeader())
244                                 ? LatchCmpInst->getPredicate()
245                                 : LatchCmpInst->getInversePredicate();
246
247  if (LatchCmpInst->getOperand(0) == &getFinalIVValue())
248    Pred = ICmpInst::getSwappedPredicate(Pred);
249
250  // Need to flip strictness of the predicate when the latch compare instruction
251  // is not using StepInst
252  if (LatchCmpInst->getOperand(0) == &getStepInst() ||
253      LatchCmpInst->getOperand(1) == &getStepInst())
254    return Pred;
255
256  // Cannot flip strictness of NE and EQ
257  if (Pred != ICmpInst::ICMP_NE && Pred != ICmpInst::ICMP_EQ)
258    return ICmpInst::getFlippedStrictnessPredicate(Pred);
259
260  Direction D = getDirection();
261  if (D == Direction::Increasing)
262    return ICmpInst::ICMP_SLT;
263
264  if (D == Direction::Decreasing)
265    return ICmpInst::ICMP_SGT;
266
267  // If cannot determine the direction, then unable to find the canonical
268  // predicate
269  return ICmpInst::BAD_ICMP_PREDICATE;
270}
271
272Direction Loop::LoopBounds::getDirection() const {
273  if (const SCEVAddRecExpr *StepAddRecExpr =
274          dyn_cast<SCEVAddRecExpr>(SE.getSCEV(&getStepInst())))
275    if (const SCEV *StepRecur = StepAddRecExpr->getStepRecurrence(SE)) {
276      if (SE.isKnownPositive(StepRecur))
277        return Direction::Increasing;
278      if (SE.isKnownNegative(StepRecur))
279        return Direction::Decreasing;
280    }
281
282  return Direction::Unknown;
283}
284
285Optional<Loop::LoopBounds> Loop::getBounds(ScalarEvolution &SE) const {
286  if (PHINode *IndVar = getInductionVariable(SE))
287    return LoopBounds::getBounds(*this, *IndVar, SE);
288
289  return None;
290}
291
292PHINode *Loop::getInductionVariable(ScalarEvolution &SE) const {
293  if (!isLoopSimplifyForm())
294    return nullptr;
295
296  BasicBlock *Header = getHeader();
297  assert(Header && "Expected a valid loop header");
298  ICmpInst *CmpInst = getLatchCmpInst(*this);
299  if (!CmpInst)
300    return nullptr;
301
302  Instruction *LatchCmpOp0 = dyn_cast<Instruction>(CmpInst->getOperand(0));
303  Instruction *LatchCmpOp1 = dyn_cast<Instruction>(CmpInst->getOperand(1));
304
305  for (PHINode &IndVar : Header->phis()) {
306    InductionDescriptor IndDesc;
307    if (!InductionDescriptor::isInductionPHI(&IndVar, this, &SE, IndDesc))
308      continue;
309
310    Instruction *StepInst = IndDesc.getInductionBinOp();
311
312    // case 1:
313    // IndVar = phi[{InitialValue, preheader}, {StepInst, latch}]
314    // StepInst = IndVar + step
315    // cmp = StepInst < FinalValue
316    if (StepInst == LatchCmpOp0 || StepInst == LatchCmpOp1)
317      return &IndVar;
318
319    // case 2:
320    // IndVar = phi[{InitialValue, preheader}, {StepInst, latch}]
321    // StepInst = IndVar + step
322    // cmp = IndVar < FinalValue
323    if (&IndVar == LatchCmpOp0 || &IndVar == LatchCmpOp1)
324      return &IndVar;
325  }
326
327  return nullptr;
328}
329
330bool Loop::getInductionDescriptor(ScalarEvolution &SE,
331                                  InductionDescriptor &IndDesc) const {
332  if (PHINode *IndVar = getInductionVariable(SE))
333    return InductionDescriptor::isInductionPHI(IndVar, this, &SE, IndDesc);
334
335  return false;
336}
337
338bool Loop::isAuxiliaryInductionVariable(PHINode &AuxIndVar,
339                                        ScalarEvolution &SE) const {
340  // Located in the loop header
341  BasicBlock *Header = getHeader();
342  if (AuxIndVar.getParent() != Header)
343    return false;
344
345  // No uses outside of the loop
346  for (User *U : AuxIndVar.users())
347    if (const Instruction *I = dyn_cast<Instruction>(U))
348      if (!contains(I))
349        return false;
350
351  InductionDescriptor IndDesc;
352  if (!InductionDescriptor::isInductionPHI(&AuxIndVar, this, &SE, IndDesc))
353    return false;
354
355  // The step instruction opcode should be add or sub.
356  if (IndDesc.getInductionOpcode() != Instruction::Add &&
357      IndDesc.getInductionOpcode() != Instruction::Sub)
358    return false;
359
360  // Incremented by a loop invariant step for each loop iteration
361  return SE.isLoopInvariant(IndDesc.getStep(), this);
362}
363
364BranchInst *Loop::getLoopGuardBranch() const {
365  if (!isLoopSimplifyForm())
366    return nullptr;
367
368  BasicBlock *Preheader = getLoopPreheader();
369  assert(Preheader && getLoopLatch() &&
370         "Expecting a loop with valid preheader and latch");
371
372  // Loop should be in rotate form.
373  if (!isRotatedForm())
374    return nullptr;
375
376  // Disallow loops with more than one unique exit block, as we do not verify
377  // that GuardOtherSucc post dominates all exit blocks.
378  BasicBlock *ExitFromLatch = getUniqueExitBlock();
379  if (!ExitFromLatch)
380    return nullptr;
381
382  BasicBlock *ExitFromLatchSucc = ExitFromLatch->getUniqueSuccessor();
383  if (!ExitFromLatchSucc)
384    return nullptr;
385
386  BasicBlock *GuardBB = Preheader->getUniquePredecessor();
387  if (!GuardBB)
388    return nullptr;
389
390  assert(GuardBB->getTerminator() && "Expecting valid guard terminator");
391
392  BranchInst *GuardBI = dyn_cast<BranchInst>(GuardBB->getTerminator());
393  if (!GuardBI || GuardBI->isUnconditional())
394    return nullptr;
395
396  BasicBlock *GuardOtherSucc = (GuardBI->getSuccessor(0) == Preheader)
397                                   ? GuardBI->getSuccessor(1)
398                                   : GuardBI->getSuccessor(0);
399  return (GuardOtherSucc == ExitFromLatchSucc) ? GuardBI : nullptr;
400}
401
402bool Loop::isCanonical(ScalarEvolution &SE) const {
403  InductionDescriptor IndDesc;
404  if (!getInductionDescriptor(SE, IndDesc))
405    return false;
406
407  ConstantInt *Init = dyn_cast_or_null<ConstantInt>(IndDesc.getStartValue());
408  if (!Init || !Init->isZero())
409    return false;
410
411  if (IndDesc.getInductionOpcode() != Instruction::Add)
412    return false;
413
414  ConstantInt *Step = IndDesc.getConstIntStepValue();
415  if (!Step || !Step->isOne())
416    return false;
417
418  return true;
419}
420
421// Check that 'BB' doesn't have any uses outside of the 'L'
422static bool isBlockInLCSSAForm(const Loop &L, const BasicBlock &BB,
423                               DominatorTree &DT) {
424  for (const Instruction &I : BB) {
425    // Tokens can't be used in PHI nodes and live-out tokens prevent loop
426    // optimizations, so for the purposes of considered LCSSA form, we
427    // can ignore them.
428    if (I.getType()->isTokenTy())
429      continue;
430
431    for (const Use &U : I.uses()) {
432      const Instruction *UI = cast<Instruction>(U.getUser());
433      const BasicBlock *UserBB = UI->getParent();
434      if (const PHINode *P = dyn_cast<PHINode>(UI))
435        UserBB = P->getIncomingBlock(U);
436
437      // Check the current block, as a fast-path, before checking whether
438      // the use is anywhere in the loop.  Most values are used in the same
439      // block they are defined in.  Also, blocks not reachable from the
440      // entry are special; uses in them don't need to go through PHIs.
441      if (UserBB != &BB && !L.contains(UserBB) &&
442          DT.isReachableFromEntry(UserBB))
443        return false;
444    }
445  }
446  return true;
447}
448
449bool Loop::isLCSSAForm(DominatorTree &DT) const {
450  // For each block we check that it doesn't have any uses outside of this loop.
451  return all_of(this->blocks(), [&](const BasicBlock *BB) {
452    return isBlockInLCSSAForm(*this, *BB, DT);
453  });
454}
455
456bool Loop::isRecursivelyLCSSAForm(DominatorTree &DT, const LoopInfo &LI) const {
457  // For each block we check that it doesn't have any uses outside of its
458  // innermost loop. This process will transitively guarantee that the current
459  // loop and all of the nested loops are in LCSSA form.
460  return all_of(this->blocks(), [&](const BasicBlock *BB) {
461    return isBlockInLCSSAForm(*LI.getLoopFor(BB), *BB, DT);
462  });
463}
464
465bool Loop::isLoopSimplifyForm() const {
466  // Normal-form loops have a preheader, a single backedge, and all of their
467  // exits have all their predecessors inside the loop.
468  return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
469}
470
471// Routines that reform the loop CFG and split edges often fail on indirectbr.
472bool Loop::isSafeToClone() const {
473  // Return false if any loop blocks contain indirectbrs, or there are any calls
474  // to noduplicate functions.
475  // FIXME: it should be ok to clone CallBrInst's if we correctly update the
476  // operand list to reflect the newly cloned labels.
477  for (BasicBlock *BB : this->blocks()) {
478    if (isa<IndirectBrInst>(BB->getTerminator()) ||
479        isa<CallBrInst>(BB->getTerminator()))
480      return false;
481
482    for (Instruction &I : *BB)
483      if (auto CS = CallSite(&I))
484        if (CS.cannotDuplicate())
485          return false;
486  }
487  return true;
488}
489
490MDNode *Loop::getLoopID() const {
491  MDNode *LoopID = nullptr;
492
493  // Go through the latch blocks and check the terminator for the metadata.
494  SmallVector<BasicBlock *, 4> LatchesBlocks;
495  getLoopLatches(LatchesBlocks);
496  for (BasicBlock *BB : LatchesBlocks) {
497    Instruction *TI = BB->getTerminator();
498    MDNode *MD = TI->getMetadata(LLVMContext::MD_loop);
499
500    if (!MD)
501      return nullptr;
502
503    if (!LoopID)
504      LoopID = MD;
505    else if (MD != LoopID)
506      return nullptr;
507  }
508  if (!LoopID || LoopID->getNumOperands() == 0 ||
509      LoopID->getOperand(0) != LoopID)
510    return nullptr;
511  return LoopID;
512}
513
514void Loop::setLoopID(MDNode *LoopID) const {
515  assert((!LoopID || LoopID->getNumOperands() > 0) &&
516         "Loop ID needs at least one operand");
517  assert((!LoopID || LoopID->getOperand(0) == LoopID) &&
518         "Loop ID should refer to itself");
519
520  SmallVector<BasicBlock *, 4> LoopLatches;
521  getLoopLatches(LoopLatches);
522  for (BasicBlock *BB : LoopLatches)
523    BB->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopID);
524}
525
526void Loop::setLoopAlreadyUnrolled() {
527  LLVMContext &Context = getHeader()->getContext();
528
529  MDNode *DisableUnrollMD =
530      MDNode::get(Context, MDString::get(Context, "llvm.loop.unroll.disable"));
531  MDNode *LoopID = getLoopID();
532  MDNode *NewLoopID = makePostTransformationMetadata(
533      Context, LoopID, {"llvm.loop.unroll."}, {DisableUnrollMD});
534  setLoopID(NewLoopID);
535}
536
537bool Loop::isAnnotatedParallel() const {
538  MDNode *DesiredLoopIdMetadata = getLoopID();
539
540  if (!DesiredLoopIdMetadata)
541    return false;
542
543  MDNode *ParallelAccesses =
544      findOptionMDForLoop(this, "llvm.loop.parallel_accesses");
545  SmallPtrSet<MDNode *, 4>
546      ParallelAccessGroups; // For scalable 'contains' check.
547  if (ParallelAccesses) {
548    for (const MDOperand &MD : drop_begin(ParallelAccesses->operands(), 1)) {
549      MDNode *AccGroup = cast<MDNode>(MD.get());
550      assert(isValidAsAccessGroup(AccGroup) &&
551             "List item must be an access group");
552      ParallelAccessGroups.insert(AccGroup);
553    }
554  }
555
556  // The loop branch contains the parallel loop metadata. In order to ensure
557  // that any parallel-loop-unaware optimization pass hasn't added loop-carried
558  // dependencies (thus converted the loop back to a sequential loop), check
559  // that all the memory instructions in the loop belong to an access group that
560  // is parallel to this loop.
561  for (BasicBlock *BB : this->blocks()) {
562    for (Instruction &I : *BB) {
563      if (!I.mayReadOrWriteMemory())
564        continue;
565
566      if (MDNode *AccessGroup = I.getMetadata(LLVMContext::MD_access_group)) {
567        auto ContainsAccessGroup = [&ParallelAccessGroups](MDNode *AG) -> bool {
568          if (AG->getNumOperands() == 0) {
569            assert(isValidAsAccessGroup(AG) && "Item must be an access group");
570            return ParallelAccessGroups.count(AG);
571          }
572
573          for (const MDOperand &AccessListItem : AG->operands()) {
574            MDNode *AccGroup = cast<MDNode>(AccessListItem.get());
575            assert(isValidAsAccessGroup(AccGroup) &&
576                   "List item must be an access group");
577            if (ParallelAccessGroups.count(AccGroup))
578              return true;
579          }
580          return false;
581        };
582
583        if (ContainsAccessGroup(AccessGroup))
584          continue;
585      }
586
587      // The memory instruction can refer to the loop identifier metadata
588      // directly or indirectly through another list metadata (in case of
589      // nested parallel loops). The loop identifier metadata refers to
590      // itself so we can check both cases with the same routine.
591      MDNode *LoopIdMD =
592          I.getMetadata(LLVMContext::MD_mem_parallel_loop_access);
593
594      if (!LoopIdMD)
595        return false;
596
597      bool LoopIdMDFound = false;
598      for (const MDOperand &MDOp : LoopIdMD->operands()) {
599        if (MDOp == DesiredLoopIdMetadata) {
600          LoopIdMDFound = true;
601          break;
602        }
603      }
604
605      if (!LoopIdMDFound)
606        return false;
607    }
608  }
609  return true;
610}
611
612DebugLoc Loop::getStartLoc() const { return getLocRange().getStart(); }
613
614Loop::LocRange Loop::getLocRange() const {
615  // If we have a debug location in the loop ID, then use it.
616  if (MDNode *LoopID = getLoopID()) {
617    DebugLoc Start;
618    // We use the first DebugLoc in the header as the start location of the loop
619    // and if there is a second DebugLoc in the header we use it as end location
620    // of the loop.
621    for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
622      if (DILocation *L = dyn_cast<DILocation>(LoopID->getOperand(i))) {
623        if (!Start)
624          Start = DebugLoc(L);
625        else
626          return LocRange(Start, DebugLoc(L));
627      }
628    }
629
630    if (Start)
631      return LocRange(Start);
632  }
633
634  // Try the pre-header first.
635  if (BasicBlock *PHeadBB = getLoopPreheader())
636    if (DebugLoc DL = PHeadBB->getTerminator()->getDebugLoc())
637      return LocRange(DL);
638
639  // If we have no pre-header or there are no instructions with debug
640  // info in it, try the header.
641  if (BasicBlock *HeadBB = getHeader())
642    return LocRange(HeadBB->getTerminator()->getDebugLoc());
643
644  return LocRange();
645}
646
647#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
648LLVM_DUMP_METHOD void Loop::dump() const { print(dbgs()); }
649
650LLVM_DUMP_METHOD void Loop::dumpVerbose() const {
651  print(dbgs(), /*Depth=*/0, /*Verbose=*/true);
652}
653#endif
654
655//===----------------------------------------------------------------------===//
656// UnloopUpdater implementation
657//
658
659namespace {
660/// Find the new parent loop for all blocks within the "unloop" whose last
661/// backedges has just been removed.
662class UnloopUpdater {
663  Loop &Unloop;
664  LoopInfo *LI;
665
666  LoopBlocksDFS DFS;
667
668  // Map unloop's immediate subloops to their nearest reachable parents. Nested
669  // loops within these subloops will not change parents. However, an immediate
670  // subloop's new parent will be the nearest loop reachable from either its own
671  // exits *or* any of its nested loop's exits.
672  DenseMap<Loop *, Loop *> SubloopParents;
673
674  // Flag the presence of an irreducible backedge whose destination is a block
675  // directly contained by the original unloop.
676  bool FoundIB;
677
678public:
679  UnloopUpdater(Loop *UL, LoopInfo *LInfo)
680      : Unloop(*UL), LI(LInfo), DFS(UL), FoundIB(false) {}
681
682  void updateBlockParents();
683
684  void removeBlocksFromAncestors();
685
686  void updateSubloopParents();
687
688protected:
689  Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
690};
691} // end anonymous namespace
692
693/// Update the parent loop for all blocks that are directly contained within the
694/// original "unloop".
695void UnloopUpdater::updateBlockParents() {
696  if (Unloop.getNumBlocks()) {
697    // Perform a post order CFG traversal of all blocks within this loop,
698    // propagating the nearest loop from successors to predecessors.
699    LoopBlocksTraversal Traversal(DFS, LI);
700    for (BasicBlock *POI : Traversal) {
701
702      Loop *L = LI->getLoopFor(POI);
703      Loop *NL = getNearestLoop(POI, L);
704
705      if (NL != L) {
706        // For reducible loops, NL is now an ancestor of Unloop.
707        assert((NL != &Unloop && (!NL || NL->contains(&Unloop))) &&
708               "uninitialized successor");
709        LI->changeLoopFor(POI, NL);
710      } else {
711        // Or the current block is part of a subloop, in which case its parent
712        // is unchanged.
713        assert((FoundIB || Unloop.contains(L)) && "uninitialized successor");
714      }
715    }
716  }
717  // Each irreducible loop within the unloop induces a round of iteration using
718  // the DFS result cached by Traversal.
719  bool Changed = FoundIB;
720  for (unsigned NIters = 0; Changed; ++NIters) {
721    assert(NIters < Unloop.getNumBlocks() && "runaway iterative algorithm");
722
723    // Iterate over the postorder list of blocks, propagating the nearest loop
724    // from successors to predecessors as before.
725    Changed = false;
726    for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
727                                   POE = DFS.endPostorder();
728         POI != POE; ++POI) {
729
730      Loop *L = LI->getLoopFor(*POI);
731      Loop *NL = getNearestLoop(*POI, L);
732      if (NL != L) {
733        assert(NL != &Unloop && (!NL || NL->contains(&Unloop)) &&
734               "uninitialized successor");
735        LI->changeLoopFor(*POI, NL);
736        Changed = true;
737      }
738    }
739  }
740}
741
742/// Remove unloop's blocks from all ancestors below their new parents.
743void UnloopUpdater::removeBlocksFromAncestors() {
744  // Remove all unloop's blocks (including those in nested subloops) from
745  // ancestors below the new parent loop.
746  for (Loop::block_iterator BI = Unloop.block_begin(), BE = Unloop.block_end();
747       BI != BE; ++BI) {
748    Loop *OuterParent = LI->getLoopFor(*BI);
749    if (Unloop.contains(OuterParent)) {
750      while (OuterParent->getParentLoop() != &Unloop)
751        OuterParent = OuterParent->getParentLoop();
752      OuterParent = SubloopParents[OuterParent];
753    }
754    // Remove blocks from former Ancestors except Unloop itself which will be
755    // deleted.
756    for (Loop *OldParent = Unloop.getParentLoop(); OldParent != OuterParent;
757         OldParent = OldParent->getParentLoop()) {
758      assert(OldParent && "new loop is not an ancestor of the original");
759      OldParent->removeBlockFromLoop(*BI);
760    }
761  }
762}
763
764/// Update the parent loop for all subloops directly nested within unloop.
765void UnloopUpdater::updateSubloopParents() {
766  while (!Unloop.empty()) {
767    Loop *Subloop = *std::prev(Unloop.end());
768    Unloop.removeChildLoop(std::prev(Unloop.end()));
769
770    assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
771    if (Loop *Parent = SubloopParents[Subloop])
772      Parent->addChildLoop(Subloop);
773    else
774      LI->addTopLevelLoop(Subloop);
775  }
776}
777
778/// Return the nearest parent loop among this block's successors. If a successor
779/// is a subloop header, consider its parent to be the nearest parent of the
780/// subloop's exits.
781///
782/// For subloop blocks, simply update SubloopParents and return NULL.
783Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
784
785  // Initially for blocks directly contained by Unloop, NearLoop == Unloop and
786  // is considered uninitialized.
787  Loop *NearLoop = BBLoop;
788
789  Loop *Subloop = nullptr;
790  if (NearLoop != &Unloop && Unloop.contains(NearLoop)) {
791    Subloop = NearLoop;
792    // Find the subloop ancestor that is directly contained within Unloop.
793    while (Subloop->getParentLoop() != &Unloop) {
794      Subloop = Subloop->getParentLoop();
795      assert(Subloop && "subloop is not an ancestor of the original loop");
796    }
797    // Get the current nearest parent of the Subloop exits, initially Unloop.
798    NearLoop = SubloopParents.insert({Subloop, &Unloop}).first->second;
799  }
800
801  succ_iterator I = succ_begin(BB), E = succ_end(BB);
802  if (I == E) {
803    assert(!Subloop && "subloop blocks must have a successor");
804    NearLoop = nullptr; // unloop blocks may now exit the function.
805  }
806  for (; I != E; ++I) {
807    if (*I == BB)
808      continue; // self loops are uninteresting
809
810    Loop *L = LI->getLoopFor(*I);
811    if (L == &Unloop) {
812      // This successor has not been processed. This path must lead to an
813      // irreducible backedge.
814      assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
815      FoundIB = true;
816    }
817    if (L != &Unloop && Unloop.contains(L)) {
818      // Successor is in a subloop.
819      if (Subloop)
820        continue; // Branching within subloops. Ignore it.
821
822      // BB branches from the original into a subloop header.
823      assert(L->getParentLoop() == &Unloop && "cannot skip into nested loops");
824
825      // Get the current nearest parent of the Subloop's exits.
826      L = SubloopParents[L];
827      // L could be Unloop if the only exit was an irreducible backedge.
828    }
829    if (L == &Unloop) {
830      continue;
831    }
832    // Handle critical edges from Unloop into a sibling loop.
833    if (L && !L->contains(&Unloop)) {
834      L = L->getParentLoop();
835    }
836    // Remember the nearest parent loop among successors or subloop exits.
837    if (NearLoop == &Unloop || !NearLoop || NearLoop->contains(L))
838      NearLoop = L;
839  }
840  if (Subloop) {
841    SubloopParents[Subloop] = NearLoop;
842    return BBLoop;
843  }
844  return NearLoop;
845}
846
847LoopInfo::LoopInfo(const DomTreeBase<BasicBlock> &DomTree) { analyze(DomTree); }
848
849bool LoopInfo::invalidate(Function &F, const PreservedAnalyses &PA,
850                          FunctionAnalysisManager::Invalidator &) {
851  // Check whether the analysis, all analyses on functions, or the function's
852  // CFG have been preserved.
853  auto PAC = PA.getChecker<LoopAnalysis>();
854  return !(PAC.preserved() || PAC.preservedSet<AllAnalysesOn<Function>>() ||
855           PAC.preservedSet<CFGAnalyses>());
856}
857
858void LoopInfo::erase(Loop *Unloop) {
859  assert(!Unloop->isInvalid() && "Loop has already been erased!");
860
861  auto InvalidateOnExit = make_scope_exit([&]() { destroy(Unloop); });
862
863  // First handle the special case of no parent loop to simplify the algorithm.
864  if (!Unloop->getParentLoop()) {
865    // Since BBLoop had no parent, Unloop blocks are no longer in a loop.
866    for (Loop::block_iterator I = Unloop->block_begin(),
867                              E = Unloop->block_end();
868         I != E; ++I) {
869
870      // Don't reparent blocks in subloops.
871      if (getLoopFor(*I) != Unloop)
872        continue;
873
874      // Blocks no longer have a parent but are still referenced by Unloop until
875      // the Unloop object is deleted.
876      changeLoopFor(*I, nullptr);
877    }
878
879    // Remove the loop from the top-level LoopInfo object.
880    for (iterator I = begin();; ++I) {
881      assert(I != end() && "Couldn't find loop");
882      if (*I == Unloop) {
883        removeLoop(I);
884        break;
885      }
886    }
887
888    // Move all of the subloops to the top-level.
889    while (!Unloop->empty())
890      addTopLevelLoop(Unloop->removeChildLoop(std::prev(Unloop->end())));
891
892    return;
893  }
894
895  // Update the parent loop for all blocks within the loop. Blocks within
896  // subloops will not change parents.
897  UnloopUpdater Updater(Unloop, this);
898  Updater.updateBlockParents();
899
900  // Remove blocks from former ancestor loops.
901  Updater.removeBlocksFromAncestors();
902
903  // Add direct subloops as children in their new parent loop.
904  Updater.updateSubloopParents();
905
906  // Remove unloop from its parent loop.
907  Loop *ParentLoop = Unloop->getParentLoop();
908  for (Loop::iterator I = ParentLoop->begin();; ++I) {
909    assert(I != ParentLoop->end() && "Couldn't find loop");
910    if (*I == Unloop) {
911      ParentLoop->removeChildLoop(I);
912      break;
913    }
914  }
915}
916
917AnalysisKey LoopAnalysis::Key;
918
919LoopInfo LoopAnalysis::run(Function &F, FunctionAnalysisManager &AM) {
920  // FIXME: Currently we create a LoopInfo from scratch for every function.
921  // This may prove to be too wasteful due to deallocating and re-allocating
922  // memory each time for the underlying map and vector datastructures. At some
923  // point it may prove worthwhile to use a freelist and recycle LoopInfo
924  // objects. I don't want to add that kind of complexity until the scope of
925  // the problem is better understood.
926  LoopInfo LI;
927  LI.analyze(AM.getResult<DominatorTreeAnalysis>(F));
928  return LI;
929}
930
931PreservedAnalyses LoopPrinterPass::run(Function &F,
932                                       FunctionAnalysisManager &AM) {
933  AM.getResult<LoopAnalysis>(F).print(OS);
934  return PreservedAnalyses::all();
935}
936
937void llvm::printLoop(Loop &L, raw_ostream &OS, const std::string &Banner) {
938
939  if (forcePrintModuleIR()) {
940    // handling -print-module-scope
941    OS << Banner << " (loop: ";
942    L.getHeader()->printAsOperand(OS, false);
943    OS << ")\n";
944
945    // printing whole module
946    OS << *L.getHeader()->getModule();
947    return;
948  }
949
950  OS << Banner;
951
952  auto *PreHeader = L.getLoopPreheader();
953  if (PreHeader) {
954    OS << "\n; Preheader:";
955    PreHeader->print(OS);
956    OS << "\n; Loop:";
957  }
958
959  for (auto *Block : L.blocks())
960    if (Block)
961      Block->print(OS);
962    else
963      OS << "Printing <null> block";
964
965  SmallVector<BasicBlock *, 8> ExitBlocks;
966  L.getExitBlocks(ExitBlocks);
967  if (!ExitBlocks.empty()) {
968    OS << "\n; Exit blocks";
969    for (auto *Block : ExitBlocks)
970      if (Block)
971        Block->print(OS);
972      else
973        OS << "Printing <null> block";
974  }
975}
976
977MDNode *llvm::findOptionMDForLoopID(MDNode *LoopID, StringRef Name) {
978  // No loop metadata node, no loop properties.
979  if (!LoopID)
980    return nullptr;
981
982  // First operand should refer to the metadata node itself, for legacy reasons.
983  assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
984  assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
985
986  // Iterate over the metdata node operands and look for MDString metadata.
987  for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
988    MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
989    if (!MD || MD->getNumOperands() < 1)
990      continue;
991    MDString *S = dyn_cast<MDString>(MD->getOperand(0));
992    if (!S)
993      continue;
994    // Return the operand node if MDString holds expected metadata.
995    if (Name.equals(S->getString()))
996      return MD;
997  }
998
999  // Loop property not found.
1000  return nullptr;
1001}
1002
1003MDNode *llvm::findOptionMDForLoop(const Loop *TheLoop, StringRef Name) {
1004  return findOptionMDForLoopID(TheLoop->getLoopID(), Name);
1005}
1006
1007bool llvm::isValidAsAccessGroup(MDNode *Node) {
1008  return Node->getNumOperands() == 0 && Node->isDistinct();
1009}
1010
1011MDNode *llvm::makePostTransformationMetadata(LLVMContext &Context,
1012                                             MDNode *OrigLoopID,
1013                                             ArrayRef<StringRef> RemovePrefixes,
1014                                             ArrayRef<MDNode *> AddAttrs) {
1015  // First remove any existing loop metadata related to this transformation.
1016  SmallVector<Metadata *, 4> MDs;
1017
1018  // Reserve first location for self reference to the LoopID metadata node.
1019  TempMDTuple TempNode = MDNode::getTemporary(Context, None);
1020  MDs.push_back(TempNode.get());
1021
1022  // Remove metadata for the transformation that has been applied or that became
1023  // outdated.
1024  if (OrigLoopID) {
1025    for (unsigned i = 1, ie = OrigLoopID->getNumOperands(); i < ie; ++i) {
1026      bool IsVectorMetadata = false;
1027      Metadata *Op = OrigLoopID->getOperand(i);
1028      if (MDNode *MD = dyn_cast<MDNode>(Op)) {
1029        const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
1030        if (S)
1031          IsVectorMetadata =
1032              llvm::any_of(RemovePrefixes, [S](StringRef Prefix) -> bool {
1033                return S->getString().startswith(Prefix);
1034              });
1035      }
1036      if (!IsVectorMetadata)
1037        MDs.push_back(Op);
1038    }
1039  }
1040
1041  // Add metadata to avoid reapplying a transformation, such as
1042  // llvm.loop.unroll.disable and llvm.loop.isvectorized.
1043  MDs.append(AddAttrs.begin(), AddAttrs.end());
1044
1045  MDNode *NewLoopID = MDNode::getDistinct(Context, MDs);
1046  // Replace the temporary node with a self-reference.
1047  NewLoopID->replaceOperandWith(0, NewLoopID);
1048  return NewLoopID;
1049}
1050
1051//===----------------------------------------------------------------------===//
1052// LoopInfo implementation
1053//
1054
1055LoopInfoWrapperPass::LoopInfoWrapperPass() : FunctionPass(ID) {
1056  initializeLoopInfoWrapperPassPass(*PassRegistry::getPassRegistry());
1057}
1058
1059char LoopInfoWrapperPass::ID = 0;
1060INITIALIZE_PASS_BEGIN(LoopInfoWrapperPass, "loops", "Natural Loop Information",
1061                      true, true)
1062INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
1063INITIALIZE_PASS_END(LoopInfoWrapperPass, "loops", "Natural Loop Information",
1064                    true, true)
1065
1066bool LoopInfoWrapperPass::runOnFunction(Function &) {
1067  releaseMemory();
1068  LI.analyze(getAnalysis<DominatorTreeWrapperPass>().getDomTree());
1069  return false;
1070}
1071
1072void LoopInfoWrapperPass::verifyAnalysis() const {
1073  // LoopInfoWrapperPass is a FunctionPass, but verifying every loop in the
1074  // function each time verifyAnalysis is called is very expensive. The
1075  // -verify-loop-info option can enable this. In order to perform some
1076  // checking by default, LoopPass has been taught to call verifyLoop manually
1077  // during loop pass sequences.
1078  if (VerifyLoopInfo) {
1079    auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
1080    LI.verify(DT);
1081  }
1082}
1083
1084void LoopInfoWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
1085  AU.setPreservesAll();
1086  AU.addRequiredTransitive<DominatorTreeWrapperPass>();
1087}
1088
1089void LoopInfoWrapperPass::print(raw_ostream &OS, const Module *) const {
1090  LI.print(OS);
1091}
1092
1093PreservedAnalyses LoopVerifierPass::run(Function &F,
1094                                        FunctionAnalysisManager &AM) {
1095  LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
1096  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
1097  LI.verify(DT);
1098  return PreservedAnalyses::all();
1099}
1100
1101//===----------------------------------------------------------------------===//
1102// LoopBlocksDFS implementation
1103//
1104
1105/// Traverse the loop blocks and store the DFS result.
1106/// Useful for clients that just want the final DFS result and don't need to
1107/// visit blocks during the initial traversal.
1108void LoopBlocksDFS::perform(LoopInfo *LI) {
1109  LoopBlocksTraversal Traversal(*this, LI);
1110  for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
1111                                        POE = Traversal.end();
1112       POI != POE; ++POI)
1113    ;
1114}
1115