src/llvm/lib/Transforms/Scalar/LoopRotation.cpp

   1 //===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
   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 //
  10 // This file implements Loop Rotation Pass.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Transforms/Scalar.h"
  15 #include "llvm/ADT/Statistic.h"
  16 #include "llvm/Analysis/AssumptionCache.h"
  17 #include "llvm/Analysis/CodeMetrics.h"
  18 #include "llvm/Analysis/InstructionSimplify.h"
  19 #include "llvm/Analysis/LoopPass.h"
  20 #include "llvm/Analysis/ScalarEvolution.h"
  21 #include "llvm/Analysis/TargetTransformInfo.h"
  22 #include "llvm/Analysis/ValueTracking.h"
  23 #include "llvm/IR/CFG.h"
  24 #include "llvm/IR/Dominators.h"
  25 #include "llvm/IR/Function.h"
  26 #include "llvm/IR/IntrinsicInst.h"
  27 #include "llvm/Support/CommandLine.h"
  28 #include "llvm/Support/Debug.h"
  29 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  30 #include "llvm/Transforms/Utils/Local.h"
  31 #include "llvm/Transforms/Utils/SSAUpdater.h"
  32 #include "llvm/Transforms/Utils/ValueMapper.h"
  33 using namespace llvm;
  34
  35 #define DEBUG_TYPE "loop-rotate"
  36
  37 static cl::opt<unsigned>
  38 DefaultRotationThreshold("rotation-max-header-size", cl::init(16), cl::Hidden,
  39        cl::desc("The default maximum header size for automatic loop rotation"));
  40
  41 STATISTIC(NumRotated, "Number of loops rotated");
  42 namespace {
  43
  44   class LoopRotate : public LoopPass {
  45   public:
  46     static char ID; // Pass ID, replacement for typeid
  47     LoopRotate(int SpecifiedMaxHeaderSize = -1) : LoopPass(ID) {
  48       initializeLoopRotatePass(*PassRegistry::getPassRegistry());
  49       if (SpecifiedMaxHeaderSize == -1)
  50         MaxHeaderSize = DefaultRotationThreshold;
  51       else
  52         MaxHeaderSize = unsigned(SpecifiedMaxHeaderSize);
  53     }
  54
  55     // LCSSA form makes instruction renaming easier.
  56     void getAnalysisUsage(AnalysisUsage &AU) const override {
  57       AU.addRequired<AssumptionCacheTracker>();
  58       AU.addPreserved<DominatorTreeWrapperPass>();
  59       AU.addRequired<LoopInfo>();
  60       AU.addPreserved<LoopInfo>();
  61       AU.addRequiredID(LoopSimplifyID);
  62       AU.addPreservedID(LoopSimplifyID);
  63       AU.addRequiredID(LCSSAID);
  64       AU.addPreservedID(LCSSAID);
  65       AU.addPreserved<ScalarEvolution>();
  66       AU.addRequired<TargetTransformInfo>();
  67     }
  68
  69     bool runOnLoop(Loop *L, LPPassManager &LPM) override;
  70     bool simplifyLoopLatch(Loop *L);
  71     bool rotateLoop(Loop *L, bool SimplifiedLatch);
  72
  73   private:
  74     unsigned MaxHeaderSize;
  75     LoopInfo *LI;
  76     const TargetTransformInfo *TTI;
  77     AssumptionCache *AC;
  78   };
  79 }
  80
  81 char LoopRotate::ID = 0;
  82 INITIALIZE_PASS_BEGIN(LoopRotate, "loop-rotate", "Rotate Loops", false, false)
  83 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
  84 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
  85 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  86 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
  87 INITIALIZE_PASS_DEPENDENCY(LCSSA)
  88 INITIALIZE_PASS_END(LoopRotate, "loop-rotate", "Rotate Loops", false, false)
  89
  90 Pass *llvm::createLoopRotatePass(int MaxHeaderSize) {
  91   return new LoopRotate(MaxHeaderSize);
  92 }
  93
  94 /// Rotate Loop L as many times as possible. Return true if
  95 /// the loop is rotated at least once.
  96 bool LoopRotate::runOnLoop(Loop *L, LPPassManager &LPM) {
  97   if (skipOptnoneFunction(L))
  98     return false;
  99
 100   // Save the loop metadata.
 101   MDNode *LoopMD = L->getLoopID();
 102
 103   LI = &getAnalysis<LoopInfo>();
 104   TTI = &getAnalysis<TargetTransformInfo>();
 105   AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
 106       *L->getHeader()->getParent());
 107
 108   // Simplify the loop latch before attempting to rotate the header
 109   // upward. Rotation may not be needed if the loop tail can be folded into the
 110   // loop exit.
 111   bool SimplifiedLatch = simplifyLoopLatch(L);
 112
 113   // One loop can be rotated multiple times.
 114   bool MadeChange = false;
 115   while (rotateLoop(L, SimplifiedLatch)) {
 116     MadeChange = true;
 117     SimplifiedLatch = false;
 118   }
 119
 120   // Restore the loop metadata.
 121   // NB! We presume LoopRotation DOESN'T ADD its own metadata.
 122   if ((MadeChange || SimplifiedLatch) && LoopMD)
 123     L->setLoopID(LoopMD);
 124
 125   return MadeChange;
 126 }
 127
 128 /// RewriteUsesOfClonedInstructions - We just cloned the instructions from the
 129 /// old header into the preheader.  If there were uses of the values produced by
 130 /// these instruction that were outside of the loop, we have to insert PHI nodes
 131 /// to merge the two values.  Do this now.
 132 static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
 133                                             BasicBlock *OrigPreheader,
 134                                             ValueToValueMapTy &ValueMap) {
 135   // Remove PHI node entries that are no longer live.
 136   BasicBlock::iterator I, E = OrigHeader->end();
 137   for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I)
 138     PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreheader));
 139
 140   // Now fix up users of the instructions in OrigHeader, inserting PHI nodes
 141   // as necessary.
 142   SSAUpdater SSA;
 143   for (I = OrigHeader->begin(); I != E; ++I) {
 144     Value *OrigHeaderVal = I;
 145
 146     // If there are no uses of the value (e.g. because it returns void), there
 147     // is nothing to rewrite.
 148     if (OrigHeaderVal->use_empty())
 149       continue;
 150
 151     Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal];
 152
 153     // The value now exits in two versions: the initial value in the preheader
 154     // and the loop "next" value in the original header.
 155     SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName());
 156     SSA.AddAvailableValue(OrigHeader, OrigHeaderVal);
 157     SSA.AddAvailableValue(OrigPreheader, OrigPreHeaderVal);
 158
 159     // Visit each use of the OrigHeader instruction.
 160     for (Value::use_iterator UI = OrigHeaderVal->use_begin(),
 161          UE = OrigHeaderVal->use_end(); UI != UE; ) {
 162       // Grab the use before incrementing the iterator.
 163       Use &U = *UI;
 164
 165       // Increment the iterator before removing the use from the list.
 166       ++UI;
 167
 168       // SSAUpdater can't handle a non-PHI use in the same block as an
 169       // earlier def. We can easily handle those cases manually.
 170       Instruction *UserInst = cast<Instruction>(U.getUser());
 171       if (!isa<PHINode>(UserInst)) {
 172         BasicBlock *UserBB = UserInst->getParent();
 173
 174         // The original users in the OrigHeader are already using the
 175         // original definitions.
 176         if (UserBB == OrigHeader)
 177           continue;
 178
 179         // Users in the OrigPreHeader need to use the value to which the
 180         // original definitions are mapped.
 181         if (UserBB == OrigPreheader) {
 182           U = OrigPreHeaderVal;
 183           continue;
 184         }
 185       }
 186
 187       // Anything else can be handled by SSAUpdater.
 188       SSA.RewriteUse(U);
 189     }
 190   }
 191 }
 192
 193 /// Determine whether the instructions in this range may be safely and cheaply
 194 /// speculated. This is not an important enough situation to develop complex
 195 /// heuristics. We handle a single arithmetic instruction along with any type
 196 /// conversions.
 197 static bool shouldSpeculateInstrs(BasicBlock::iterator Begin,
 198                                   BasicBlock::iterator End, Loop *L) {
 199   bool seenIncrement = false;
 200   bool MultiExitLoop = false;
 201
 202   if (!L->getExitingBlock())
 203     MultiExitLoop = true;
 204
 205   for (BasicBlock::iterator I = Begin; I != End; ++I) {
 206
 207     if (!isSafeToSpeculativelyExecute(I))
 208       return false;
 209
 210     if (isa<DbgInfoIntrinsic>(I))
 211       continue;
 212
 213     switch (I->getOpcode()) {
 214     default:
 215       return false;
 216     case Instruction::GetElementPtr:
 217       // GEPs are cheap if all indices are constant.
 218       if (!cast<GEPOperator>(I)->hasAllConstantIndices())
 219         return false;
 220       // fall-thru to increment case
 221     case Instruction::Add:
 222     case Instruction::Sub:
 223     case Instruction::And:
 224     case Instruction::Or:
 225     case Instruction::Xor:
 226     case Instruction::Shl:
 227     case Instruction::LShr:
 228     case Instruction::AShr: {
 229       Value *IVOpnd = nullptr;
 230       if (isa<ConstantInt>(I->getOperand(0)))
 231         IVOpnd = I->getOperand(1);
 232
 233       if (isa<ConstantInt>(I->getOperand(1))) {
 234         if (IVOpnd)
 235           return false;
 236
 237         IVOpnd = I->getOperand(0);
 238       }
 239
 240       // If increment operand is used outside of the loop, this speculation
 241       // could cause extra live range interference.
 242       if (MultiExitLoop && IVOpnd) {
 243         for (User *UseI : IVOpnd->users()) {
 244           auto *UserInst = cast<Instruction>(UseI);
 245           if (!L->contains(UserInst))
 246             return false;
 247         }
 248       }
 249
 250       if (seenIncrement)
 251         return false;
 252       seenIncrement = true;
 253       break;
 254     }
 255     case Instruction::Trunc:
 256     case Instruction::ZExt:
 257     case Instruction::SExt:
 258       // ignore type conversions
 259       break;
 260     }
 261   }
 262   return true;
 263 }
 264
 265 /// Fold the loop tail into the loop exit by speculating the loop tail
 266 /// instructions. Typically, this is a single post-increment. In the case of a
 267 /// simple 2-block loop, hoisting the increment can be much better than
 268 /// duplicating the entire loop header. In the case of loops with early exits,
 269 /// rotation will not work anyway, but simplifyLoopLatch will put the loop in
 270 /// canonical form so downstream passes can handle it.
 271 ///
 272 /// I don't believe this invalidates SCEV.
 273 bool LoopRotate::simplifyLoopLatch(Loop *L) {
 274   BasicBlock *Latch = L->getLoopLatch();
 275   if (!Latch || Latch->hasAddressTaken())
 276     return false;
 277
 278   BranchInst *Jmp = dyn_cast<BranchInst>(Latch->getTerminator());
 279   if (!Jmp || !Jmp->isUnconditional())
 280     return false;
 281
 282   BasicBlock *LastExit = Latch->getSinglePredecessor();
 283   if (!LastExit || !L->isLoopExiting(LastExit))
 284     return false;
 285
 286   BranchInst *BI = dyn_cast<BranchInst>(LastExit->getTerminator());
 287   if (!BI)
 288     return false;
 289
 290   if (!shouldSpeculateInstrs(Latch->begin(), Jmp, L))
 291     return false;
 292
 293   DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into "
 294         << LastExit->getName() << "\n");
 295
 296   // Hoist the instructions from Latch into LastExit.
 297   LastExit->getInstList().splice(BI, Latch->getInstList(), Latch->begin(), Jmp);
 298
 299   unsigned FallThruPath = BI->getSuccessor(0) == Latch ? 0 : 1;
 300   BasicBlock *Header = Jmp->getSuccessor(0);
 301   assert(Header == L->getHeader() && "expected a backward branch");
 302
 303   // Remove Latch from the CFG so that LastExit becomes the new Latch.
 304   BI->setSuccessor(FallThruPath, Header);
 305   Latch->replaceSuccessorsPhiUsesWith(LastExit);
 306   Jmp->eraseFromParent();
 307
 308   // Nuke the Latch block.
 309   assert(Latch->empty() && "unable to evacuate Latch");
 310   LI->removeBlock(Latch);
 311   if (DominatorTreeWrapperPass *DTWP =
 312           getAnalysisIfAvailable<DominatorTreeWrapperPass>())
 313     DTWP->getDomTree().eraseNode(Latch);
 314   Latch->eraseFromParent();
 315   return true;
 316 }
 317
 318 /// Rotate loop LP. Return true if the loop is rotated.
 319 ///
 320 /// \param SimplifiedLatch is true if the latch was just folded into the final
 321 /// loop exit. In this case we may want to rotate even though the new latch is
 322 /// now an exiting branch. This rotation would have happened had the latch not
 323 /// been simplified. However, if SimplifiedLatch is false, then we avoid
 324 /// rotating loops in which the latch exits to avoid excessive or endless
 325 /// rotation. LoopRotate should be repeatable and converge to a canonical
 326 /// form. This property is satisfied because simplifying the loop latch can only
 327 /// happen once across multiple invocations of the LoopRotate pass.
 328 bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
 329   // If the loop has only one block then there is not much to rotate.
 330   if (L->getBlocks().size() == 1)
 331     return false;
 332
 333   BasicBlock *OrigHeader = L->getHeader();
 334   BasicBlock *OrigLatch = L->getLoopLatch();
 335
 336   BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
 337   if (!BI || BI->isUnconditional())
 338     return false;
 339
 340   // If the loop header is not one of the loop exiting blocks then
 341   // either this loop is already rotated or it is not
 342   // suitable for loop rotation transformations.
 343   if (!L->isLoopExiting(OrigHeader))
 344     return false;
 345
 346   // If the loop latch already contains a branch that leaves the loop then the
 347   // loop is already rotated.
 348   if (!OrigLatch)
 349     return false;
 350
 351   // Rotate if either the loop latch does *not* exit the loop, or if the loop
 352   // latch was just simplified.
 353   if (L->isLoopExiting(OrigLatch) && !SimplifiedLatch)
 354     return false;
 355
 356   // Check size of original header and reject loop if it is very big or we can't
 357   // duplicate blocks inside it.
 358   {
 359     SmallPtrSet<const Value *, 32> EphValues;
 360     CodeMetrics::collectEphemeralValues(L, AC, EphValues);
 361
 362     CodeMetrics Metrics;
 363     Metrics.analyzeBasicBlock(OrigHeader, *TTI, EphValues);
 364     if (Metrics.notDuplicatable) {
 365       DEBUG(dbgs() << "LoopRotation: NOT rotating - contains non-duplicatable"
 366             << " instructions: "; L->dump());
 367       return false;
 368     }
 369     if (Metrics.NumInsts > MaxHeaderSize)
 370       return false;
 371   }
 372
 373   // Now, this loop is suitable for rotation.
 374   BasicBlock *OrigPreheader = L->getLoopPreheader();
 375
 376   // If the loop could not be converted to canonical form, it must have an
 377   // indirectbr in it, just give up.
 378   if (!OrigPreheader)
 379     return false;
 380
 381   // Anything ScalarEvolution may know about this loop or the PHI nodes
 382   // in its header will soon be invalidated.
 383   if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
 384     SE->forgetLoop(L);
 385
 386   DEBUG(dbgs() << "LoopRotation: rotating "; L->dump());
 387
 388   // Find new Loop header. NewHeader is a Header's one and only successor
 389   // that is inside loop.  Header's other successor is outside the
 390   // loop.  Otherwise loop is not suitable for rotation.
 391   BasicBlock *Exit = BI->getSuccessor(0);
 392   BasicBlock *NewHeader = BI->getSuccessor(1);
 393   if (L->contains(Exit))
 394     std::swap(Exit, NewHeader);
 395   assert(NewHeader && "Unable to determine new loop header");
 396   assert(L->contains(NewHeader) && !L->contains(Exit) &&
 397          "Unable to determine loop header and exit blocks");
 398
 399   // This code assumes that the new header has exactly one predecessor.
 400   // Remove any single-entry PHI nodes in it.
 401   assert(NewHeader->getSinglePredecessor() &&
 402          "New header doesn't have one pred!");
 403   FoldSingleEntryPHINodes(NewHeader);
 404
 405   // Begin by walking OrigHeader and populating ValueMap with an entry for
 406   // each Instruction.
 407   BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
 408   ValueToValueMapTy ValueMap;
 409
 410   // For PHI nodes, the value available in OldPreHeader is just the
 411   // incoming value from OldPreHeader.
 412   for (; PHINode *PN = dyn_cast<PHINode>(I); ++I)
 413     ValueMap[PN] = PN->getIncomingValueForBlock(OrigPreheader);
 414
 415   // For the rest of the instructions, either hoist to the OrigPreheader if
 416   // possible or create a clone in the OldPreHeader if not.
 417   TerminatorInst *LoopEntryBranch = OrigPreheader->getTerminator();
 418   while (I != E) {
 419     Instruction *Inst = I++;
 420
 421     // If the instruction's operands are invariant and it doesn't read or write
 422     // memory, then it is safe to hoist.  Doing this doesn't change the order of
 423     // execution in the preheader, but does prevent the instruction from
 424     // executing in each iteration of the loop.  This means it is safe to hoist
 425     // something that might trap, but isn't safe to hoist something that reads
 426     // memory (without proving that the loop doesn't write).
 427     if (L->hasLoopInvariantOperands(Inst) &&
 428         !Inst->mayReadFromMemory() && !Inst->mayWriteToMemory() &&
 429         !isa<TerminatorInst>(Inst) && !isa<DbgInfoIntrinsic>(Inst) &&
 430         !isa<AllocaInst>(Inst)) {
 431       Inst->moveBefore(LoopEntryBranch);
 432       continue;
 433     }
 434
 435     // Otherwise, create a duplicate of the instruction.
 436     Instruction *C = Inst->clone();
 437
 438     // Eagerly remap the operands of the instruction.
 439     RemapInstruction(C, ValueMap,
 440                      RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
 441
 442     // With the operands remapped, see if the instruction constant folds or is
 443     // otherwise simplifyable.  This commonly occurs because the entry from PHI
 444     // nodes allows icmps and other instructions to fold.
 445     // FIXME: Provide DL, TLI, DT, AC to SimplifyInstruction.
 446     Value *V = SimplifyInstruction(C);
 447     if (V && LI->replacementPreservesLCSSAForm(C, V)) {
 448       // If so, then delete the temporary instruction and stick the folded value
 449       // in the map.
 450       delete C;
 451       ValueMap[Inst] = V;
 452     } else {
 453       // Otherwise, stick the new instruction into the new block!
 454       C->setName(Inst->getName());
 455       C->insertBefore(LoopEntryBranch);
 456       ValueMap[Inst] = C;
 457     }
 458   }
 459
 460   // Along with all the other instructions, we just cloned OrigHeader's
 461   // terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's
 462   // successors by duplicating their incoming values for OrigHeader.
 463   TerminatorInst *TI = OrigHeader->getTerminator();
 464   for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
 465     for (BasicBlock::iterator BI = TI->getSuccessor(i)->begin();
 466          PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
 467       PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreheader);
 468
 469   // Now that OrigPreHeader has a clone of OrigHeader's terminator, remove
 470   // OrigPreHeader's old terminator (the original branch into the loop), and
 471   // remove the corresponding incoming values from the PHI nodes in OrigHeader.
 472   LoopEntryBranch->eraseFromParent();
 473
 474   // If there were any uses of instructions in the duplicated block outside the
 475   // loop, update them, inserting PHI nodes as required
 476   RewriteUsesOfClonedInstructions(OrigHeader, OrigPreheader, ValueMap);
 477
 478   // NewHeader is now the header of the loop.
 479   L->moveToHeader(NewHeader);
 480   assert(L->getHeader() == NewHeader && "Latch block is our new header");
 481
 482
 483   // At this point, we've finished our major CFG changes.  As part of cloning
 484   // the loop into the preheader we've simplified instructions and the
 485   // duplicated conditional branch may now be branching on a constant.  If it is
 486   // branching on a constant and if that constant means that we enter the loop,
 487   // then we fold away the cond branch to an uncond branch.  This simplifies the
 488   // loop in cases important for nested loops, and it also means we don't have
 489   // to split as many edges.
 490   BranchInst *PHBI = cast<BranchInst>(OrigPreheader->getTerminator());
 491   assert(PHBI->isConditional() && "Should be clone of BI condbr!");
 492   if (!isa<ConstantInt>(PHBI->getCondition()) ||
 493       PHBI->getSuccessor(cast<ConstantInt>(PHBI->getCondition())->isZero())
 494           != NewHeader) {
 495     // The conditional branch can't be folded, handle the general case.
 496     // Update DominatorTree to reflect the CFG change we just made.  Then split
 497     // edges as necessary to preserve LoopSimplify form.
 498     if (DominatorTreeWrapperPass *DTWP =
 499             getAnalysisIfAvailable<DominatorTreeWrapperPass>()) {
 500       DominatorTree &DT = DTWP->getDomTree();
 501       // Everything that was dominated by the old loop header is now dominated
 502       // by the original loop preheader. Conceptually the header was merged
 503       // into the preheader, even though we reuse the actual block as a new
 504       // loop latch.
 505       DomTreeNode *OrigHeaderNode = DT.getNode(OrigHeader);
 506       SmallVector<DomTreeNode *, 8> HeaderChildren(OrigHeaderNode->begin(),
 507                                                    OrigHeaderNode->end());
 508       DomTreeNode *OrigPreheaderNode = DT.getNode(OrigPreheader);
 509       for (unsigned I = 0, E = HeaderChildren.size(); I != E; ++I)
 510         DT.changeImmediateDominator(HeaderChildren[I], OrigPreheaderNode);
 511
 512       assert(DT.getNode(Exit)->getIDom() == OrigPreheaderNode);
 513       assert(DT.getNode(NewHeader)->getIDom() == OrigPreheaderNode);
 514
 515       // Update OrigHeader to be dominated by the new header block.
 516       DT.changeImmediateDominator(OrigHeader, OrigLatch);
 517     }
 518
 519     // Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and
 520     // thus is not a preheader anymore.
 521     // Split the edge to form a real preheader.
 522     BasicBlock *NewPH = SplitCriticalEdge(OrigPreheader, NewHeader, this);
 523     NewPH->setName(NewHeader->getName() + ".lr.ph");
 524
 525     // Preserve canonical loop form, which means that 'Exit' should have only
 526     // one predecessor. Note that Exit could be an exit block for multiple
 527     // nested loops, causing both of the edges to now be critical and need to
 528     // be split.
 529     SmallVector<BasicBlock *, 4> ExitPreds(pred_begin(Exit), pred_end(Exit));
 530     bool SplitLatchEdge = false;
 531     for (SmallVectorImpl<BasicBlock *>::iterator PI = ExitPreds.begin(),
 532                                                  PE = ExitPreds.end();
 533          PI != PE; ++PI) {
 534       // We only need to split loop exit edges.
 535       Loop *PredLoop = LI->getLoopFor(*PI);
 536       if (!PredLoop || PredLoop->contains(Exit))
 537         continue;
 538       SplitLatchEdge |= L->getLoopLatch() == *PI;
 539       BasicBlock *ExitSplit = SplitCriticalEdge(*PI, Exit, this);
 540       ExitSplit->moveBefore(Exit);
 541     }
 542     assert(SplitLatchEdge &&
 543            "Despite splitting all preds, failed to split latch exit?");
 544   } else {
 545     // We can fold the conditional branch in the preheader, this makes things
 546     // simpler. The first step is to remove the extra edge to the Exit block.
 547     Exit->removePredecessor(OrigPreheader, true /*preserve LCSSA*/);
 548     BranchInst *NewBI = BranchInst::Create(NewHeader, PHBI);
 549     NewBI->setDebugLoc(PHBI->getDebugLoc());
 550     PHBI->eraseFromParent();
 551
 552     // With our CFG finalized, update DomTree if it is available.
 553     if (DominatorTreeWrapperPass *DTWP =
 554             getAnalysisIfAvailable<DominatorTreeWrapperPass>()) {
 555       DominatorTree &DT = DTWP->getDomTree();
 556       // Update OrigHeader to be dominated by the new header block.
 557       DT.changeImmediateDominator(NewHeader, OrigPreheader);
 558       DT.changeImmediateDominator(OrigHeader, OrigLatch);
 559
 560       // Brute force incremental dominator tree update. Call
 561       // findNearestCommonDominator on all CFG predecessors of each child of the
 562       // original header.
 563       DomTreeNode *OrigHeaderNode = DT.getNode(OrigHeader);
 564       SmallVector<DomTreeNode *, 8> HeaderChildren(OrigHeaderNode->begin(),
 565                                                    OrigHeaderNode->end());
 566       bool Changed;
 567       do {
 568         Changed = false;
 569         for (unsigned I = 0, E = HeaderChildren.size(); I != E; ++I) {
 570           DomTreeNode *Node = HeaderChildren[I];
 571           BasicBlock *BB = Node->getBlock();
 572
 573           pred_iterator PI = pred_begin(BB);
 574           BasicBlock *NearestDom = *PI;
 575           for (pred_iterator PE = pred_end(BB); PI != PE; ++PI)
 576             NearestDom = DT.findNearestCommonDominator(NearestDom, *PI);
 577
 578           // Remember if this changes the DomTree.
 579           if (Node->getIDom()->getBlock() != NearestDom) {
 580             DT.changeImmediateDominator(BB, NearestDom);
 581             Changed = true;
 582           }
 583         }
 584
 585       // If the dominator changed, this may have an effect on other
 586       // predecessors, continue until we reach a fixpoint.
 587       } while (Changed);
 588     }
 589   }
 590
 591   assert(L->getLoopPreheader() && "Invalid loop preheader after loop rotation");
 592   assert(L->getLoopLatch() && "Invalid loop latch after loop rotation");
 593
 594   // Now that the CFG and DomTree are in a consistent state again, try to merge
 595   // the OrigHeader block into OrigLatch.  This will succeed if they are
 596   // connected by an unconditional branch.  This is just a cleanup so the
 597   // emitted code isn't too gross in this common case.
 598   MergeBlockIntoPredecessor(OrigHeader, this);
 599
 600   DEBUG(dbgs() << "LoopRotation: into "; L->dump());
 601
 602   ++NumRotated;
 603   return true;
 604 }