LLVM PASS 分析【1】：Rotate Loops

1. Introduction

1.1 循环相关概念

通常情况下，一个Natural Loop(自然循环)，有一个Entry Block(Header)和几个指向Header的回边的基本快（Latch),常常几个Latch共享一个Header. 实际上这种Loop结构是不方便后续的处理，常常需要简化成只有一个Latch的结构，在loop exit 之前增加加入一个PHI节点，这个loop形式又叫做Loop-closed SSA form，简称（LCSSA).

1.2 Loop Rotate 可视化

源代码

void main() {
     
  int ret = 0;
  for( int j = 0; j < 4; j++ ) {
     
    ret += j;
  }
}

for只是一个语法糖，实际是下面的执行顺序

void main() {
     
  int ret = 0;  | 基本块 0  PreHeader
  int j = 0;    | 基本块 0  PreHeader
  while(j < 4){
      | 基本块 1  Header
    ret += j;   | 基本块 2  
    j++;        | 基本块 3  Latch  
  }
  return ;      | 基本块 4  Exit
}

转换前CFG

Loop Rotate 将执行顺序改编为 if-do-while结构，从CFG图里面可以看出，直觉上可以看出，Rotate以后的，通过插入前后两个基本块，使得循环主体变得相对独立，也就是Loop-closed。

void main() {
     
  int ret = 0;  | 基本块 0  
  int j = 0;    | 基本块 0  
  if(j<4)       | 基本块 0
  {
     
    do{
     
    ret += j;   | 基本块 1 Header
    j++;        | 基本块 1 Header
   }while(j<4); | 基本块 2 Latch 
  }
  return 0;     | 基本块 3 Exit
}

转换后CFG

2. 源码分析

Rotate Loops相关文件有：
llvm-9.0.0.src\lib\Transforms\Scalar\LoopRotation.cpp
llvm-9.0.0.src\include\llvm\Transforms\Scalar\LoopRotation.h
llvm-9.0.0.src\lib\Transforms\Utils\LoopRotationUtils.cpp
llvm-9.0.0.src\include\llvm\Transforms\Utils\LoopRotationUtils.h

LoopRotation.cpp 是Pass相关代码，
LoopRotationUtils.cpp 是真正Loops Rotate 动作的代码。

2.1 Pass 相关代码

初始化：

char LoopRotateLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(LoopRotateLegacyPass, "loop-rotate", "Rotate Loops",false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(LoopPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_END(LoopRotateLegacyPass, "loop-rotate", "Rotate Loops", false,false)
Pass *llvm::createLoopRotatePass(int MaxHeaderSize) {
     
  return new LoopRotateLegacyPass(MaxHeaderSize);
}

从初始化相关代码可以看出，LoopRotateLegacyPass依赖于AssumptionCacheTracker，LoopPass，TargetTransformInfoWrapperPass，MemorySSAWrapperPass这几个Pass的分析结果。

LoopRotateLegacyPass：

class LoopRotateLegacyPass : public LoopPass {
     
  unsigned MaxHeaderSize;
public:
  static char ID; // Pass ID, replacement for typeid
  LoopRotateLegacyPass(int SpecifiedMaxHeaderSize = -1) : LoopPass(ID) {
     
    initializeLoopRotateLegacyPassPass(*PassRegistry::getPassRegistry());
    if (SpecifiedMaxHeaderSize == -1)
      MaxHeaderSize = DefaultRotationThreshold;
    else
      MaxHeaderSize = unsigned(SpecifiedMaxHeaderSize);
  }

  // LCSSA form makes instruction renaming easier.
  void getAnalysisUsage(AnalysisUsage &AU) const override {
     
    AU.addRequired<AssumptionCacheTracker>();
    AU.addRequired<TargetTransformInfoWrapperPass>();
    if (EnableMSSALoopDependency) {
     
      AU.addRequired<MemorySSAWrapperPass>();
      AU.addPreserved<MemorySSAWrapperPass>();
    }
    getLoopAnalysisUsage(AU);
  }

• LoopRotateLegacyPass 是一个Loop Pass.
• 分析强制依赖于AssumptionCacheTracker，TargetTransformInfoWrapperPass。
• 根据EnableMSSALoopDependency开关，决定需不需要MemorySSAWrapperPass。
• MaxHeaderSize： 代表

runOnLoop:

  bool runOnLoop(Loop *L, LPPassManager &LPM) override {
     
    if (skipLoop(L))
      return false;
    Function &F = *L->getHeader()->getParent();

    auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
    const auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
    auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
    auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
    auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
    auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
    auto *SE = SEWP ? &SEWP->getSE() : nullptr;
    const SimplifyQuery SQ = getBestSimplifyQuery(*this, F);
    Optional<MemorySSAUpdater> MSSAU;
    if (EnableMSSALoopDependency) {
     
      MemorySSA *MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
      MSSAU = MemorySSAUpdater(MSSA);
    }
    return LoopRotation(L, LI, TTI, AC, DT, SE,
                        MSSAU.hasValue() ? MSSAU.getPointer() : nullptr, SQ,
                        false, MaxHeaderSize, false);
  }
};

• runOnLoop（）,主要获取LoopRotation需要的相关依赖信息，包括LoopInfo,TargetTransformInfo,AssumptionCacheTracker,DominatorTree,ScalarEvolution,MemorySSA（可选）并调用LoopRotation（LoopRotationUtils.cpp）构造函数。

2.2 Loops Rotate 动作相关代码

LoopRotation 类：

class LoopRotate {
     
  const unsigned MaxHeaderSize;
  LoopInfo *LI;
  const TargetTransformInfo *TTI;
  AssumptionCache *AC;
  DominatorTree *DT;
  ScalarEvolution *SE;
  MemorySSAUpdater *MSSAU;
  const SimplifyQuery &SQ;
  bool RotationOnly;
  bool IsUtilMode;

public:
  LoopRotate(unsigned MaxHeaderSize, LoopInfo *LI,
             const TargetTransformInfo *TTI, AssumptionCache *AC,
             DominatorTree *DT, ScalarEvolution *SE, MemorySSAUpdater *MSSAU,
             const SimplifyQuery &SQ, bool RotationOnly, bool IsUtilMode)
      : MaxHeaderSize(MaxHeaderSize), LI(LI), TTI(TTI), AC(AC), DT(DT), SE(SE),
        MSSAU(MSSAU), SQ(SQ), RotationOnly(RotationOnly),
        IsUtilMode(IsUtilMode) {
     }
  bool processLoop(Loop *L);
private:
  bool rotateLoop(Loop *L, bool SimplifiedLatch);
  bool simplifyLoopLatch(Loop *L);
};

从类的public接口来看，processLoop应该真正的处理循环，决定整个rotate动作处理流程，Private的接口，应该是这个流程的分解。

processLoop：

bool LoopRotate::processLoop(Loop *L) {
     
  // Save the loop metadata.
  MDNode *LoopMD = L->getLoopID();

  bool SimplifiedLatch = false;

  // Simplify the loop latch before attempting to rotate the header
  // upward. Rotation may not be needed if the loop tail can be folded into the
  // loop exit.
  if (!RotationOnly)
    SimplifiedLatch = simplifyLoopLatch(L);

  bool MadeChange = rotateLoop(L, SimplifiedLatch);
  assert((!MadeChange || L->isLoopExiting(L->getLoopLatch())) &&
         "Loop latch should be exiting after loop-rotate.");

  // Restore the loop metadata.
  // NB! We presume LoopRotation DOESN'T ADD its own metadata.
  if ((MadeChange || SimplifiedLatch) && LoopMD)
    L->setLoopID(LoopMD);
    
  return MadeChange || SimplifiedLatch;
}

processLoop 先简化simplifyLoopLatch，最后真正的rotateLoop，将信息保存，还回值表示是否rotateLoop成功。

simplifyLoopLatch：

bool LoopRotate::simplifyLoopLatch(Loop *L) {
     
  BasicBlock *Latch = L->getLoopLatch();
  if (!Latch || Latch->hasAddressTaken())
    return false;
    
  BranchInst *Jmp = dyn_cast<BranchInst>(Latch->getTerminator());
  if (!Jmp || !Jmp->isUnconditional())
    return false;

  BasicBlock *LastExit = Latch->getSinglePredecessor();
  if (!LastExit || !L->isLoopExiting(LastExit))
    return false;

  BranchInst *BI = dyn_cast<BranchInst>(LastExit->getTerminator());
  if (!BI)
    return false;

  if (!shouldSpeculateInstrs(Latch->begin(), Jmp->getIterator(), L))
    return false;

  LLVM_DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into "
                    << LastExit->getName() << "\n");

  // Hoist the instructions from Latch into LastExit.
  Instruction *FirstLatchInst = &*(Latch->begin());
  LastExit->getInstList().splice(BI->getIterator(), Latch->getInstList(),
                                 Latch->begin(), Jmp->getIterator());

  // Update MemorySSA
  if (MSSAU)
    MSSAU->moveAllAfterMergeBlocks(Latch, LastExit, FirstLatchInst);

  unsigned FallThruPath = BI->getSuccessor(0) == Latch ? 0 : 1;
  BasicBlock *Header = Jmp->getSuccessor(0);
  assert(Header == L->getHeader() && "expected a backward branch");

  // Remove Latch from the CFG so that LastExit becomes the new Latch.
  BI->setSuccessor(FallThruPath, Header);
  Latch->replaceSuccessorsPhiUsesWith(LastExit);
  Jmp->eraseFromParent();

  // Nuke the Latch block.
  assert(Latch->empty() && "unable to evacuate Latch");
  LI->removeBlock(Latch);
  if (DT)
    DT->eraseNode(Latch);
  Latch->eraseFromParent();

  if (MSSAU && VerifyMemorySSA)
    MSSAU->getMemorySSA()->verifyMemorySSA();

  return true;
}

rotateLoop：

/// Rotate loop LP. Return true if the loop is rotated.
///
/// \param SimplifiedLatch is true if the latch was just folded into the final
/// loop exit. In this case we may want to rotate even though the new latch is
/// now an exiting branch. This rotation would have happened had the latch not
/// been simplified. However, if SimplifiedLatch is false, then we avoid
/// rotating loops in which the latch exits to avoid excessive or endless
/// rotation. LoopRotate should be repeatable and converge to a canonical
/// form. This property is satisfied because simplifying the loop latch can only
/// happen once across multiple invocations of the LoopRotate pass.
bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
     
  // If the loop has only one block then there is not much to rotate.
  if (L->getBlocks().size() == 1)
    return false;

  BasicBlock *OrigHeader = L->getHeader();
  BasicBlock *OrigLatch = L->getLoopLatch();

  BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
  if (!BI || BI->isUnconditional())
    return false;

  // If the loop header is not one of the loop exiting blocks then
  // either this loop is already rotated or it is not
  // suitable for loop rotation transformations.
  if (!L->isLoopExiting(OrigHeader))
    return false;

  // If the loop latch already contains a branch that leaves the loop then the
  // loop is already rotated.
  if (!OrigLatch)
    return false;

  // Rotate if either the loop latch does *not* exit the loop, or if the loop
  // latch was just simplified. Or if we think it will be profitable.
  if (L->isLoopExiting(OrigLatch) && !SimplifiedLatch && IsUtilMode == false &&
      !shouldRotateLoopExitingLatch(L))
    return false;

  // Check size of original header and reject loop if it is very big or we can't
  // duplicate blocks inside it.
  {
     
    SmallPtrSet<const Value *, 32> EphValues;
    CodeMetrics::collectEphemeralValues(L, AC, EphValues);

    CodeMetrics Metrics;
    Metrics.analyzeBasicBlock(OrigHeader, *TTI, EphValues);
    if (Metrics.notDuplicatable) {
     
      LLVM_DEBUG(
          dbgs() << "LoopRotation: NOT rotating - contains non-duplicatable"
                 << " instructions: ";
          L->dump());
      return false;
    }
    if (Metrics.convergent) {
     
      LLVM_DEBUG(dbgs() << "LoopRotation: NOT rotating - contains convergent "
                           "instructions: ";
                 L->dump());
      return false;
    }
    if (Metrics.NumInsts > MaxHeaderSize)
      return false;
  }

  // Now, this loop is suitable for rotation.
  BasicBlock *OrigPreheader = L->getLoopPreheader();

  // If the loop could not be converted to canonical form, it must have an
  // indirectbr in it, just give up.
  if (!OrigPreheader || !L->hasDedicatedExits())
    return false;

  // Anything ScalarEvolution may know about this loop or the PHI nodes
  // in its header will soon be invalidated. We should also invalidate
  // all outer loops because insertion and deletion of blocks that happens
  // during the rotation may violate invariants related to backedge taken
  // infos in them.
  if (SE)
    SE->forgetTopmostLoop(L);

  LLVM_DEBUG(dbgs() << "LoopRotation: rotating "; L->dump());
  if (MSSAU && VerifyMemorySSA)
    MSSAU->getMemorySSA()->verifyMemorySSA();

  // Find new Loop header. NewHeader is a Header's one and only successor
  // that is inside loop.  Header's other successor is outside the
  // loop.  Otherwise loop is not suitable for rotation.
  BasicBlock *Exit = BI->getSuccessor(0);
  BasicBlock *NewHeader = BI->getSuccessor(1);
  if (L->contains(Exit))
    std::swap(Exit, NewHeader);
  assert(NewHeader && "Unable to determine new loop header");
  assert(L->contains(NewHeader) && !L->contains(Exit) &&
         "Unable to determine loop header and exit blocks");

  // This code assumes that the new header has exactly one predecessor.
  // Remove any single-entry PHI nodes in it.
  assert(NewHeader->getSinglePredecessor() &&
         "New header doesn't have one pred!");
  FoldSingleEntryPHINodes(NewHeader);

  // Begin by walking OrigHeader and populating ValueMap with an entry for
  // each Instruction.
  BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
  ValueToValueMapTy ValueMap, ValueMapMSSA;

  // For PHI nodes, the value available in OldPreHeader is just the
  // incoming value from OldPreHeader.
  for (; PHINode *PN = dyn_cast<PHINode>(I); ++I)
    ValueMap[PN] = PN->getIncomingValueForBlock(OrigPreheader);

  // For the rest of the instructions, either hoist to the OrigPreheader if
  // possible or create a clone in the OldPreHeader if not.
  Instruction *LoopEntryBranch = OrigPreheader->getTerminator();

  // Record all debug intrinsics preceding LoopEntryBranch to avoid duplication.
  using DbgIntrinsicHash =
      std::pair<std::pair<Value *, DILocalVariable *>, DIExpression *>;
  auto makeHash = [](DbgVariableIntrinsic *D) -> DbgIntrinsicHash {
     
    return {
     {
     D->getVariableLocation(), D->getVariable()}, D->getExpression()};
  };
  SmallDenseSet<DbgIntrinsicHash, 8> DbgIntrinsics;
  for (auto I = std::next(OrigPreheader->rbegin()), E = OrigPreheader->rend();
       I != E; ++I) {
     
    if (auto *DII = dyn_cast<DbgVariableIntrinsic>(&*I))
      DbgIntrinsics.insert(makeHash(DII));
    else
      break;
  }

  while (I != E) {
     
    Instruction *Inst = &*I++;

    // If the instruction's operands are invariant and it doesn't read or write
    // memory, then it is safe to hoist.  Doing this doesn't change the order of
    // execution in the preheader, but does prevent the instruction from
    // executing in each iteration of the loop.  This means it is safe to hoist
    // something that might trap, but isn't safe to hoist something that reads
    // memory (without proving that the loop doesn't write).
    if (L->hasLoopInvariantOperands(Inst) && !Inst->mayReadFromMemory() &&
        !Inst->mayWriteToMemory() && !Inst->isTerminator() &&
        !isa<DbgInfoIntrinsic>(Inst) && !isa<AllocaInst>(Inst)) {
     
      Inst->moveBefore(LoopEntryBranch);
      continue;
    }

    // Otherwise, create a duplicate of the instruction.
    Instruction *C = Inst->clone();

    // Eagerly remap the operands of the instruction.
    RemapInstruction(C, ValueMap,
                     RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);

    // Avoid inserting the same intrinsic twice.
    if (auto *DII = dyn_cast<DbgVariableIntrinsic>(C))
      if (DbgIntrinsics.count(makeHash(DII))) {
     
        C->deleteValue();
        continue;
      }

    // With the operands remapped, see if the instruction constant folds or is
    // otherwise simplifyable.  This commonly occurs because the entry from PHI
    // nodes allows icmps and other instructions to fold.
    Value *V = SimplifyInstruction(C, SQ);
    if (V && LI->replacementPreservesLCSSAForm(C, V)) {
     
      // If so, then delete the temporary instruction and stick the folded value
      // in the map.
      ValueMap[Inst] = V;
      if (!C->mayHaveSideEffects()) {
     
        C->deleteValue();
        C = nullptr;
      }
    } else {
     
      ValueMap[Inst] = C;
    }
    if (C) {
     
      // Otherwise, stick the new instruction into the new block!
      C->setName(Inst->getName());
      C->insertBefore(LoopEntryBranch);

      if (auto *II = dyn_cast<IntrinsicInst>(C))
        if (II->getIntrinsicID() == Intrinsic::assume)
          AC->registerAssumption(II);
      // MemorySSA cares whether the cloned instruction was inserted or not, and
      // not whether it can be remapped to a simplified value.
      ValueMapMSSA[Inst] = C;
    }
  }

  // Along with all the other instructions, we just cloned OrigHeader's
  // terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's
  // successors by duplicating their incoming values for OrigHeader.
  for (BasicBlock *SuccBB : successors(OrigHeader))
    for (BasicBlock::iterator BI = SuccBB->begin();
         PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
      PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreheader);

  // Now that OrigPreHeader has a clone of OrigHeader's terminator, remove
  // OrigPreHeader's old terminator (the original branch into the loop), and
  // remove the corresponding incoming values from the PHI nodes in OrigHeader.
  LoopEntryBranch->eraseFromParent();

  // Update MemorySSA before the rewrite call below changes the 1:1
  // instruction:cloned_instruction_or_value mapping.
  if (MSSAU) {
     
    ValueMapMSSA[OrigHeader] = OrigPreheader;
    MSSAU->updateForClonedBlockIntoPred(OrigHeader, OrigPreheader,
                                        ValueMapMSSA);
  }

  SmallVector<PHINode*, 2> InsertedPHIs;
  // If there were any uses of instructions in the duplicated block outside the
  // loop, update them, inserting PHI nodes as required
  RewriteUsesOfClonedInstructions(OrigHeader, OrigPreheader, ValueMap,
                                  &InsertedPHIs);

  // Attach dbg.value intrinsics to the new phis if that phi uses a value that
  // previously had debug metadata attached. This keeps the debug info
  // up-to-date in the loop body.
  if (!InsertedPHIs.empty())
    insertDebugValuesForPHIs(OrigHeader, InsertedPHIs);

  // NewHeader is now the header of the loop.
  L->moveToHeader(NewHeader);
  assert(L->getHeader() == NewHeader && "Latch block is our new header");

  // Inform DT about changes to the CFG.
  if (DT) {
     
    // The OrigPreheader branches to the NewHeader and Exit now. Then, inform
    // the DT about the removed edge to the OrigHeader (that got removed).
    SmallVector<DominatorTree::UpdateType, 3> Updates;
    Updates.push_back({
     DominatorTree::Insert, OrigPreheader, Exit});
    Updates.push_back({
     DominatorTree::Insert, OrigPreheader, NewHeader});
    Updates.push_back({
     DominatorTree::Delete, OrigPreheader, OrigHeader});
    DT->applyUpdates(Updates);

    if (MSSAU) {
     
      MSSAU->applyUpdates(Updates, *DT);
      if (VerifyMemorySSA)
        MSSAU->getMemorySSA()->verifyMemorySSA();
    }
  }

  // At this point, we've finished our major CFG changes.  As part of cloning
  // the loop into the preheader we've simplified instructions and the
  // duplicated conditional branch may now be branching on a constant.  If it is
  // branching on a constant and if that constant means that we enter the loop,
  // then we fold away the cond branch to an uncond branch.  This simplifies the
  // loop in cases important for nested loops, and it also means we don't have
  // to split as many edges.
  BranchInst *PHBI = cast<BranchInst>(OrigPreheader->getTerminator());
  assert(PHBI->isConditional() && "Should be clone of BI condbr!");
  if (!isa<ConstantInt>(PHBI->getCondition()) ||
      PHBI->getSuccessor(cast<ConstantInt>(PHBI->getCondition())->isZero()) !=
          NewHeader) {
     
    // The conditional branch can't be folded, handle the general case.
    // Split edges as necessary to preserve LoopSimplify form.

    // Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and
    // thus is not a preheader anymore.
    // Split the edge to form a real preheader.
    BasicBlock *NewPH = SplitCriticalEdge(
        OrigPreheader, NewHeader,
        CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA());
    NewPH->setName(NewHeader->getName() + ".lr.ph");

    // Preserve canonical loop form, which means that 'Exit' should have only
    // one predecessor. Note that Exit could be an exit block for multiple
    // nested loops, causing both of the edges to now be critical and need to
    // be split.
    SmallVector<BasicBlock *, 4> ExitPreds(pred_begin(Exit), pred_end(Exit));
    bool SplitLatchEdge = false;
    for (BasicBlock *ExitPred : ExitPreds) {
     
      // We only need to split loop exit edges.
      Loop *PredLoop = LI->getLoopFor(ExitPred);
      if (!PredLoop || PredLoop->contains(Exit) ||
          ExitPred->getTerminator()->isIndirectTerminator())
        continue;
      SplitLatchEdge |= L->getLoopLatch() == ExitPred;
      BasicBlock *ExitSplit = SplitCriticalEdge(
          ExitPred, Exit,
          CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA());
      ExitSplit->moveBefore(Exit);
    }
    assert(SplitLatchEdge &&
           "Despite splitting all preds, failed to split latch exit?");
  } else {
     
    // We can fold the conditional branch in the preheader, this makes things
    // simpler. The first step is to remove the extra edge to the Exit block.
    Exit->removePredecessor(OrigPreheader, true /*preserve LCSSA*/);
    BranchInst *NewBI = BranchInst::Create(NewHeader, PHBI);
    NewBI->setDebugLoc(PHBI->getDebugLoc());
    PHBI->eraseFromParent();

    // With our CFG finalized, update DomTree if it is available.
    if (DT) DT->deleteEdge(OrigPreheader, Exit);

    // Update MSSA too, if available.
    if (MSSAU)
      MSSAU->removeEdge(OrigPreheader, Exit);
  }

  assert(L->getLoopPreheader() && "Invalid loop preheader after loop rotation");
  assert(L->getLoopLatch() && "Invalid loop latch after loop rotation");

  if (MSSAU && VerifyMemorySSA)
    MSSAU->getMemorySSA()->verifyMemorySSA();

  // Now that the CFG and DomTree are in a consistent state again, try to merge
  // the OrigHeader block into OrigLatch.  This will succeed if they are
  // connected by an unconditional branch.  This is just a cleanup so the
  // emitted code isn't too gross in this common case.
  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
  MergeBlockIntoPredecessor(OrigHeader, &DTU, LI, MSSAU);

  if (MSSAU && VerifyMemorySSA)
    MSSAU->getMemorySSA()->verifyMemorySSA();

  LLVM_DEBUG(dbgs() << "LoopRotation: into "; L->dump());

  ++NumRotated;  // NumRotated这个是STATICS宏表达的次数，用于统计信息
  return true;

3. 参考资料

[1] llvm-9.0.0源码
[2] https://www.llvm.org/devmtg/2009-10/ScalarEvolutionAndLoopOptimization.pdf
[3] https://reviews.llvm.org/D22630