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 #define DEBUG_TYPE "loop-rotate"
  15 #include "llvm/Transforms/Scalar.h"
  16 #include "llvm/Function.h"
  17 #include "llvm/IntrinsicInst.h"
  18 #include "llvm/Analysis/LoopInfo.h"
  19 #include "llvm/Analysis/LoopPass.h"
  20 #include "llvm/Analysis/Dominators.h"
  21 #include "llvm/Analysis/ScalarEvolution.h"
  22 #include "llvm/Transforms/Utils/Local.h"
  23 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  24 #include "llvm/Support/CommandLine.h"
  25 #include "llvm/Support/Debug.h"
  26 #include "llvm/ADT/Statistic.h"
  27 #include "llvm/ADT/SmallVector.h"
  28 using namespace llvm;
  29
  30 #define MAX_HEADER_SIZE 16
  31
  32 STATISTIC(NumRotated, "Number of loops rotated");
  33 namespace {
  34
  35   class VISIBILITY_HIDDEN RenameData {
  36   public:
  37     RenameData(Instruction *O, Value *P, Instruction *H)
  38       : Original(O), PreHeader(P), Header(H) { }
  39   public:
  40     Instruction *Original; // Original instruction
  41     Value *PreHeader; // Original pre-header replacement
  42     Instruction *Header; // New header replacement
  43   };
  44
  45   class VISIBILITY_HIDDEN LoopRotate : public LoopPass {
  46
  47   public:
  48     static char ID; // Pass ID, replacement for typeid
  49     LoopRotate() : LoopPass(&ID) {}
  50
  51     // Rotate Loop L as many times as possible. Return true if
  52     // loop is rotated at least once.
  53     bool runOnLoop(Loop *L, LPPassManager &LPM);
  54
  55     // LCSSA form makes instruction renaming easier.
  56     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  57       AU.addRequiredID(LoopSimplifyID);
  58       AU.addPreservedID(LoopSimplifyID);
  59       AU.addRequiredID(LCSSAID);
  60       AU.addPreservedID(LCSSAID);
  61       AU.addPreserved<ScalarEvolution>();
  62       AU.addPreserved<LoopInfo>();
  63       AU.addPreserved<DominatorTree>();
  64       AU.addPreserved<DominanceFrontier>();
  65     }
  66
  67     // Helper functions
  68
  69     /// Do actual work
  70     bool rotateLoop(Loop *L, LPPassManager &LPM);
  71
  72     /// Initialize local data
  73     void initialize();
  74
  75     /// Make sure all Exit block PHINodes have required incoming values.
  76     /// If incoming value is constant or defined outside the loop then
  77     /// PHINode may not have an entry for original pre-header.
  78     void  updateExitBlock();
  79
  80     /// Return true if this instruction is used outside original header.
  81     bool usedOutsideOriginalHeader(Instruction *In);
  82
  83     /// Find Replacement information for instruction. Return NULL if it is
  84     /// not available.
  85     const RenameData *findReplacementData(Instruction *I);
  86
  87     /// After loop rotation, loop pre-header has multiple sucessors.
  88     /// Insert one forwarding basic block to ensure that loop pre-header
  89     /// has only one successor.
  90     void preserveCanonicalLoopForm(LPPassManager &LPM);
  91
  92   private:
  93
  94     Loop *L;
  95     BasicBlock *OrigHeader;
  96     BasicBlock *OrigPreHeader;
  97     BasicBlock *OrigLatch;
  98     BasicBlock *NewHeader;
  99     BasicBlock *Exit;
 100     LPPassManager *LPM_Ptr;
 101     SmallVector<RenameData, MAX_HEADER_SIZE> LoopHeaderInfo;
 102   };
 103 }
 104
 105 char LoopRotate::ID = 0;
 106 static RegisterPass<LoopRotate> X("loop-rotate", "Rotate Loops");
 107
 108 Pass *llvm::createLoopRotatePass() { return new LoopRotate(); }
 109
 110 /// Rotate Loop L as many times as possible. Return true if
 111 /// the loop is rotated at least once.
 112 bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) {
 113
 114   bool RotatedOneLoop = false;
 115   initialize();
 116   LPM_Ptr = &LPM;
 117
 118   // One loop can be rotated multiple times.
 119   while (rotateLoop(Lp,LPM)) {
 120     RotatedOneLoop = true;
 121     initialize();
 122   }
 123
 124   return RotatedOneLoop;
 125 }
 126
 127 /// Rotate loop LP. Return true if the loop is rotated.
 128 bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
 129   L = Lp;
 130
 131   OrigHeader =  L->getHeader();
 132   OrigPreHeader = L->getLoopPreheader();
 133   OrigLatch = L->getLoopLatch();
 134
 135   // If the loop has only one block then there is not much to rotate.
 136   if (L->getBlocks().size() == 1)
 137     return false;
 138
 139   assert(OrigHeader && OrigLatch && OrigPreHeader &&
 140          "Loop is not in canonical form");
 141
 142   // If the loop header is not one of the loop exiting blocks then
 143   // either this loop is already rotated or it is not
 144   // suitable for loop rotation transformations.
 145   if (!L->isLoopExit(OrigHeader))
 146     return false;
 147
 148   BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
 149   if (!BI)
 150     return false;
 151   assert(BI->isConditional() && "Branch Instruction is not conditional");
 152
 153   // Updating PHInodes in loops with multiple exits adds complexity.
 154   // Keep it simple, and restrict loop rotation to loops with one exit only.
 155   // In future, lift this restriction and support for multiple exits if
 156   // required.
 157   SmallVector<BasicBlock*, 8> ExitBlocks;
 158   L->getExitBlocks(ExitBlocks);
 159   if (ExitBlocks.size() > 1)
 160     return false;
 161
 162   // Check size of original header and reject
 163   // loop if it is very big.
 164   unsigned Size = 0;
 165
 166   // FIXME: Use common api to estimate size.
 167   for (BasicBlock::const_iterator OI = OrigHeader->begin(),
 168          OE = OrigHeader->end(); OI != OE; ++OI) {
 169       if (isa<PHINode>(OI))
 170         continue;           // PHI nodes don't count.
 171       if (isa<DbgInfoIntrinsic>(OI))
 172         continue;  // Debug intrinsics don't count as size.
 173       Size++;
 174   }
 175
 176   if (Size > MAX_HEADER_SIZE)
 177     return false;
 178
 179   // Now, this loop is suitable for rotation.
 180
 181   // Find new Loop header. NewHeader is a Header's one and only successor
 182   // that is inside loop.  Header's other successor is outside the
 183   // loop.  Otherwise loop is not suitable for rotation.
 184   Exit = BI->getSuccessor(0);
 185   NewHeader = BI->getSuccessor(1);
 186   if (L->contains(Exit))
 187     std::swap(Exit, NewHeader);
 188   assert(NewHeader && "Unable to determine new loop header");
 189   assert(L->contains(NewHeader) && !L->contains(Exit) &&
 190          "Unable to determine loop header and exit blocks");
 191
 192   // This code assumes that the new header has exactly one predecessor.
 193   // Remove any single-entry PHI nodes in it.
 194   assert(NewHeader->getSinglePredecessor() &&
 195          "New header doesn't have one pred!");
 196   FoldSingleEntryPHINodes(NewHeader);
 197
 198   // Copy PHI nodes and other instructions from the original header
 199   // into the original pre-header. Unlike the original header, the original
 200   // pre-header is not a member of the loop.
 201   //
 202   // The new loop header is the one and only successor of original header that
 203   // is inside the loop. All other original header successors are outside
 204   // the loop. Copy PHI Nodes from the original header into the new loop header.
 205   // Add second incoming value, from original loop pre-header into these phi
 206   // nodes. If a value defined in original header is used outside original
 207   // header then new loop header will need new phi nodes with two incoming
 208   // values, one definition from original header and second definition is
 209   // from original loop pre-header.
 210
 211   // Remove terminator from Original pre-header. Original pre-header will
 212   // receive a clone of original header terminator as a new terminator.
 213   OrigPreHeader->getInstList().pop_back();
 214   BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
 215   PHINode *PN = 0;
 216   for (; (PN = dyn_cast<PHINode>(I)); ++I) {
 217     // PHI nodes are not copied into original pre-header. Instead their values
 218     // are directly propagated.
 219     Value *NPV = PN->getIncomingValueForBlock(OrigPreHeader);
 220
 221     // Create a new PHI node with two incoming values for NewHeader.
 222     // One incoming value is from OrigLatch (through OrigHeader) and the
 223     // second incoming value is from original pre-header.
 224     PHINode *NH = PHINode::Create(PN->getType(), PN->getName(),
 225                                   NewHeader->begin());
 226     NH->addIncoming(PN->getIncomingValueForBlock(OrigLatch), OrigHeader);
 227     NH->addIncoming(NPV, OrigPreHeader);
 228
 229     // "In" can be replaced by NH at various places.
 230     LoopHeaderInfo.push_back(RenameData(PN, NPV, NH));
 231   }
 232
 233   // Now, handle non-phi instructions.
 234   for (; I != E; ++I) {
 235     Instruction *In = I;
 236     assert(!isa<PHINode>(In) && "PHINode is not expected here");
 237
 238     // This is not a PHI instruction. Insert its clone into original pre-header.
 239     // If this instruction is using a value from same basic block then
 240     // update it to use value from cloned instruction.
 241     Instruction *C = In->clone(In->getContext());
 242     C->setName(In->getName());
 243     OrigPreHeader->getInstList().push_back(C);
 244
 245     for (unsigned opi = 0, e = In->getNumOperands(); opi != e; ++opi) {
 246       Instruction *OpInsn = dyn_cast<Instruction>(In->getOperand(opi));
 247       if (!OpInsn) continue;  // Ignore non-instruction values.
 248       if (const RenameData *D = findReplacementData(OpInsn))
 249         C->setOperand(opi, D->PreHeader);
 250     }
 251
 252     // If this instruction is used outside this basic block then
 253     // create new PHINode for this instruction.
 254     Instruction *NewHeaderReplacement = NULL;
 255     if (usedOutsideOriginalHeader(In)) {
 256       PHINode *PN = PHINode::Create(In->getType(), In->getName(),
 257                                     NewHeader->begin());
 258       PN->addIncoming(In, OrigHeader);
 259       PN->addIncoming(C, OrigPreHeader);
 260       NewHeaderReplacement = PN;
 261     }
 262     LoopHeaderInfo.push_back(RenameData(In, C, NewHeaderReplacement));
 263   }
 264
 265   // Rename uses of original header instructions to reflect their new
 266   // definitions (either from original pre-header node or from newly created
 267   // new header PHINodes.
 268   //
 269   // Original header instructions are used in
 270   // 1) Original header:
 271   //
 272   //    If instruction is used in non-phi instructions then it is using
 273   //    defintion from original heder iteself. Do not replace this use
 274   //    with definition from new header or original pre-header.
 275   //
 276   //    If instruction is used in phi node then it is an incoming
 277   //    value. Rename its use to reflect new definition from new-preheader
 278   //    or new header.
 279   //
 280   // 2) Inside loop but not in original header
 281   //
 282   //    Replace this use to reflect definition from new header.
 283   for (unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
 284     const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
 285
 286     if (!ILoopHeaderInfo.Header)
 287       continue;
 288
 289     Instruction *OldPhi = ILoopHeaderInfo.Original;
 290     Instruction *NewPhi = ILoopHeaderInfo.Header;
 291
 292     // Before replacing uses, collect them first, so that iterator is
 293     // not invalidated.
 294     SmallVector<Instruction *, 16> AllUses;
 295     for (Value::use_iterator UI = OldPhi->use_begin(), UE = OldPhi->use_end();
 296          UI != UE; ++UI)
 297       AllUses.push_back(cast<Instruction>(UI));
 298
 299     for (SmallVector<Instruction *, 16>::iterator UI = AllUses.begin(),
 300            UE = AllUses.end(); UI != UE; ++UI) {
 301       Instruction *U = *UI;
 302       BasicBlock *Parent = U->getParent();
 303
 304       // Used inside original header
 305       if (Parent == OrigHeader) {
 306         // Do not rename uses inside original header non-phi instructions.
 307         PHINode *PU = dyn_cast<PHINode>(U);
 308         if (!PU)
 309           continue;
 310
 311         // Do not rename uses inside original header phi nodes, if the
 312         // incoming value is for new header.
 313         if (PU->getBasicBlockIndex(NewHeader) != -1
 314             && PU->getIncomingValueForBlock(NewHeader) == U)
 315           continue;
 316
 317        U->replaceUsesOfWith(OldPhi, NewPhi);
 318        continue;
 319       }
 320
 321       // Used inside loop, but not in original header.
 322       if (L->contains(U->getParent())) {
 323         if (U != NewPhi)
 324           U->replaceUsesOfWith(OldPhi, NewPhi);
 325         continue;
 326       }
 327
 328       // Used inside Exit Block. Since we are in LCSSA form, U must be PHINode.
 329       if (U->getParent() == Exit) {
 330         assert(isa<PHINode>(U) && "Use in Exit Block that is not PHINode");
 331
 332         PHINode *UPhi = cast<PHINode>(U);
 333         // UPhi already has one incoming argument from original header.
 334         // Add second incoming argument from new Pre header.
 335         UPhi->addIncoming(ILoopHeaderInfo.PreHeader, OrigPreHeader);
 336       } else {
 337         // Used outside Exit block. Create a new PHI node in the exit block
 338         // to receive the value from the new header and pre-header.
 339         PHINode *PN = PHINode::Create(U->getType(), U->getName(),
 340                                       Exit->begin());
 341         PN->addIncoming(ILoopHeaderInfo.PreHeader, OrigPreHeader);
 342         PN->addIncoming(OldPhi, OrigHeader);
 343         U->replaceUsesOfWith(OldPhi, PN);
 344       }
 345     }
 346   }
 347
 348   /// Make sure all Exit block PHINodes have required incoming values.
 349   updateExitBlock();
 350
 351   // Update CFG
 352
 353   // Removing incoming branch from loop preheader to original header.
 354   // Now original header is inside the loop.
 355   for (BasicBlock::iterator I = OrigHeader->begin();
 356        (PN = dyn_cast<PHINode>(I)); ++I)
 357     PN->removeIncomingValue(OrigPreHeader);
 358
 359   // Make NewHeader as the new header for the loop.
 360   L->moveToHeader(NewHeader);
 361
 362   preserveCanonicalLoopForm(LPM);
 363
 364   NumRotated++;
 365   return true;
 366 }
 367
 368 /// Make sure all Exit block PHINodes have required incoming values.
 369 /// If an incoming value is constant or defined outside the loop then
 370 /// PHINode may not have an entry for the original pre-header.
 371 void LoopRotate::updateExitBlock() {
 372
 373   PHINode *PN;
 374   for (BasicBlock::iterator I = Exit->begin();
 375        (PN = dyn_cast<PHINode>(I)); ++I) {
 376
 377     // There is already one incoming value from original pre-header block.
 378     if (PN->getBasicBlockIndex(OrigPreHeader) != -1)
 379       continue;
 380
 381     const RenameData *ILoopHeaderInfo;
 382     Value *V = PN->getIncomingValueForBlock(OrigHeader);
 383     if (isa<Instruction>(V) &&
 384         (ILoopHeaderInfo = findReplacementData(cast<Instruction>(V)))) {
 385       assert(ILoopHeaderInfo->PreHeader && "Missing New Preheader Instruction");
 386       PN->addIncoming(ILoopHeaderInfo->PreHeader, OrigPreHeader);
 387     } else {
 388       PN->addIncoming(V, OrigPreHeader);
 389     }
 390   }
 391 }
 392
 393 /// Initialize local data
 394 void LoopRotate::initialize() {
 395   L = NULL;
 396   OrigHeader = NULL;
 397   OrigPreHeader = NULL;
 398   NewHeader = NULL;
 399   Exit = NULL;
 400
 401   LoopHeaderInfo.clear();
 402 }
 403
 404 /// Return true if this instruction is used by any instructions in the loop that
 405 /// aren't in original header.
 406 bool LoopRotate::usedOutsideOriginalHeader(Instruction *In) {
 407   for (Value::use_iterator UI = In->use_begin(), UE = In->use_end();
 408        UI != UE; ++UI) {
 409     BasicBlock *UserBB = cast<Instruction>(UI)->getParent();
 410     if (UserBB != OrigHeader && L->contains(UserBB))
 411       return true;
 412   }
 413
 414   return false;
 415 }
 416
 417 /// Find Replacement information for instruction. Return NULL if it is
 418 /// not available.
 419 const RenameData *LoopRotate::findReplacementData(Instruction *In) {
 420
 421   // Since LoopHeaderInfo is small, linear walk is OK.
 422   for (unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
 423     const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
 424     if (ILoopHeaderInfo.Original == In)
 425       return &ILoopHeaderInfo;
 426   }
 427   return NULL;
 428 }
 429
 430 /// After loop rotation, loop pre-header has multiple sucessors.
 431 /// Insert one forwarding basic block to ensure that loop pre-header
 432 /// has only one successor.
 433 void LoopRotate::preserveCanonicalLoopForm(LPPassManager &LPM) {
 434
 435   // Right now original pre-header has two successors, new header and
 436   // exit block. Insert new block between original pre-header and
 437   // new header such that loop's new pre-header has only one successor.
 438   BasicBlock *NewPreHeader = BasicBlock::Create("bb.nph",
 439                                                 OrigHeader->getParent(),
 440                                                 NewHeader);
 441   LoopInfo &LI = LPM.getAnalysis<LoopInfo>();
 442   if (Loop *PL = LI.getLoopFor(OrigPreHeader))
 443     PL->addBasicBlockToLoop(NewPreHeader, LI.getBase());
 444   BranchInst::Create(NewHeader, NewPreHeader);
 445
 446   BranchInst *OrigPH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
 447   if (OrigPH_BI->getSuccessor(0) == NewHeader)
 448     OrigPH_BI->setSuccessor(0, NewPreHeader);
 449   else {
 450     assert(OrigPH_BI->getSuccessor(1) == NewHeader &&
 451            "Unexpected original pre-header terminator");
 452     OrigPH_BI->setSuccessor(1, NewPreHeader);
 453   }
 454
 455   PHINode *PN;
 456   for (BasicBlock::iterator I = NewHeader->begin();
 457        (PN = dyn_cast<PHINode>(I)); ++I) {
 458     int index = PN->getBasicBlockIndex(OrigPreHeader);
 459     assert(index != -1 && "Expected incoming value from Original PreHeader");
 460     PN->setIncomingBlock(index, NewPreHeader);
 461     assert(PN->getBasicBlockIndex(OrigPreHeader) == -1 &&
 462            "Expected only one incoming value from Original PreHeader");
 463   }
 464
 465   if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
 466     DT->addNewBlock(NewPreHeader, OrigPreHeader);
 467     DT->changeImmediateDominator(L->getHeader(), NewPreHeader);
 468     DT->changeImmediateDominator(Exit, OrigPreHeader);
 469     for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
 470          BI != BE; ++BI) {
 471       BasicBlock *B = *BI;
 472       if (L->getHeader() != B) {
 473         DomTreeNode *Node = DT->getNode(B);
 474         if (Node && Node->getBlock() == OrigHeader)
 475           DT->changeImmediateDominator(*BI, L->getHeader());
 476       }
 477     }
 478     DT->changeImmediateDominator(OrigHeader, OrigLatch);
 479   }
 480
 481   if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>()) {
 482     // New Preheader's dominance frontier is Exit block.
 483     DominanceFrontier::DomSetType NewPHSet;
 484     NewPHSet.insert(Exit);
 485     DF->addBasicBlock(NewPreHeader, NewPHSet);
 486
 487     // New Header's dominance frontier now includes itself and Exit block
 488     DominanceFrontier::iterator HeadI = DF->find(L->getHeader());
 489     if (HeadI != DF->end()) {
 490       DominanceFrontier::DomSetType & HeaderSet = HeadI->second;
 491       HeaderSet.clear();
 492       HeaderSet.insert(L->getHeader());
 493       HeaderSet.insert(Exit);
 494     } else {
 495       DominanceFrontier::DomSetType HeaderSet;
 496       HeaderSet.insert(L->getHeader());
 497       HeaderSet.insert(Exit);
 498       DF->addBasicBlock(L->getHeader(), HeaderSet);
 499     }
 500
 501     // Original header (new Loop Latch)'s dominance frontier is Exit.
 502     DominanceFrontier::iterator LatchI = DF->find(L->getLoopLatch());
 503     if (LatchI != DF->end()) {
 504       DominanceFrontier::DomSetType &LatchSet = LatchI->second;
 505       LatchSet = LatchI->second;
 506       LatchSet.clear();
 507       LatchSet.insert(Exit);
 508     } else {
 509       DominanceFrontier::DomSetType LatchSet;
 510       LatchSet.insert(Exit);
 511       DF->addBasicBlock(L->getHeader(), LatchSet);
 512     }
 513
 514     // If a loop block dominates new loop latch then its frontier is
 515     // new header and Exit.
 516     BasicBlock *NewLatch = L->getLoopLatch();
 517     DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
 518     for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
 519          BI != BE; ++BI) {
 520       BasicBlock *B = *BI;
 521       if (DT->dominates(B, NewLatch)) {
 522         DominanceFrontier::iterator BDFI = DF->find(B);
 523         if (BDFI != DF->end()) {
 524           DominanceFrontier::DomSetType &BSet = BDFI->second;
 525           BSet = BDFI->second;
 526           BSet.clear();
 527           BSet.insert(L->getHeader());
 528           BSet.insert(Exit);
 529         } else {
 530           DominanceFrontier::DomSetType BSet;
 531           BSet.insert(L->getHeader());
 532           BSet.insert(Exit);
 533           DF->addBasicBlock(B, BSet);
 534         }
 535       }
 536     }
 537   }
 538
 539   // Preserve canonical loop form, which means Exit block should
 540   // have only one predecessor.
 541   BasicBlock *NExit = SplitEdge(L->getLoopLatch(), Exit, this);
 542
 543   // Preserve LCSSA.
 544   for (BasicBlock::iterator I = Exit->begin();
 545        (PN = dyn_cast<PHINode>(I)); ++I) {
 546     unsigned N = PN->getNumIncomingValues();
 547     for (unsigned index = 0; index != N; ++index)
 548       if (PN->getIncomingBlock(index) == NExit) {
 549         PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName(),
 550                                          NExit->begin());
 551         NewPN->addIncoming(PN->getIncomingValue(index), L->getLoopLatch());
 552         PN->setIncomingValue(index, NewPN);
 553         PN->setIncomingBlock(index, NExit);
 554         break;
 555       }
 556   }
 557
 558   assert(NewHeader && L->getHeader() == NewHeader &&
 559          "Invalid loop header after loop rotation");
 560   assert(NewPreHeader && L->getLoopPreheader() == NewPreHeader &&
 561          "Invalid loop preheader after loop rotation");
 562   assert(L->getLoopLatch() &&
 563          "Invalid loop latch after loop rotation");
 564 }