LoopRotation.cpp revision 6c36157f3d0b27c05f5fbb599de0d992ea2f04ef
1//===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by Devang Patel and is distributed under 6// the University of Illinois Open Source 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 16#include "llvm/Transforms/Scalar.h" 17#include "llvm/Function.h" 18#include "llvm/Instructions.h" 19#include "llvm/Analysis/LoopInfo.h" 20#include "llvm/Analysis/LoopPass.h" 21#include "llvm/Transforms/Utils/Local.h" 22#include "llvm/Support/CommandLine.h" 23#include "llvm/Support/Debug.h" 24#include "llvm/ADT/Statistic.h" 25#include "llvm/ADT/SmallVector.h" 26 27using namespace llvm; 28 29#define MAX_HEADER_SIZE 16 30 31STATISTIC(NumRotated, "Number of loops rotated"); 32namespace { 33 34 class VISIBILITY_HIDDEN RenameData { 35 public: 36 RenameData(Instruction *O, Value *P, Instruction *H) 37 : Original(O), PreHeader(P), Header(H) { } 38 public: 39 Instruction *Original; // Original instruction 40 Value *PreHeader; // Original pre-header replacement 41 Instruction *Header; // New header replacement 42 }; 43 44 class VISIBILITY_HIDDEN LoopRotate : public LoopPass { 45 46 public: 47 48 // Rotate Loop L as many times as possible. Return true if 49 // loop is rotated at least once. 50 bool runOnLoop(Loop *L, LPPassManager &LPM); 51 52 // LCSSA form makes instruction renaming easier. 53 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 54 AU.addRequiredID(LCSSAID); 55 AU.addPreservedID(LCSSAID); 56 } 57 58 // Helper functions 59 60 /// Do actual work 61 bool rotateLoop(Loop *L, LPPassManager &LPM); 62 63 /// Initialize local data 64 void initialize(); 65 66 /// Make sure all Exit block PHINodes have required incoming values. 67 /// If incoming value is constant or defined outside the loop then 68 /// PHINode may not have an entry for original pre-header. 69 void updateExitBlock(); 70 71 /// Return true if this instruction is used outside original header. 72 bool usedOutsideOriginalHeader(Instruction *In); 73 74 /// Find Replacement information for instruction. Return NULL if it is 75 /// not available. 76 const RenameData *findReplacementData(Instruction *I); 77 78 /// After loop rotation, loop pre-header has multiple sucessors. 79 /// Insert one forwarding basic block to ensure that loop pre-header 80 /// has only one successor. 81 void preserveCanonicalLoopForm(LPPassManager &LPM); 82 83 private: 84 85 Loop *L; 86 BasicBlock *OrigHeader; 87 BasicBlock *OrigPreHeader; 88 BasicBlock *OrigLatch; 89 BasicBlock *NewHeader; 90 BasicBlock *Exit; 91 92 SmallVector<RenameData, MAX_HEADER_SIZE> LoopHeaderInfo; 93 }; 94 95 RegisterPass<LoopRotate> X ("loop-rotate", "Rotate Loops"); 96} 97 98LoopPass *llvm::createLoopRotatePass() { return new LoopRotate(); } 99 100/// Rotate Loop L as many times as possible. Return true if 101/// loop is rotated at least once. 102bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) { 103 104 bool RotatedOneLoop = false; 105 initialize(); 106 107 // One loop can be rotated multiple times. 108 while (rotateLoop(Lp,LPM)) { 109 RotatedOneLoop = true; 110 initialize(); 111 } 112 113 return RotatedOneLoop; 114} 115 116/// Rotate loop LP. Return true if it loop is rotated. 117bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) { 118 119 L = Lp; 120 121 OrigHeader = L->getHeader(); 122 OrigPreHeader = L->getLoopPreheader(); 123 OrigLatch = L->getLoopLatch(); 124 125 // If loop has only one block then there is not much to rotate. 126 if (L->getBlocks().size() == 1) 127 return false; 128 129 assert (OrigHeader && OrigLatch && OrigPreHeader && 130 "Loop is not in cannocial form"); 131 132 // If loop header is not one of the loop exit block then 133 // either this loop is already rotated or it is not 134 // suitable for loop rotation transformations. 135 if (!L->isLoopExit(OrigHeader)) 136 return false; 137 138 BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator()); 139 if (!BI) 140 return false; 141 assert (BI->isConditional() && "Branch Instruction is not condiitional"); 142 143 // Updating PHInodes in loops with multiple exits adds complexity. 144 // Keep it simple, and restrict loop rotation to loops with one exit only. 145 // In future, lift this restriction and support for multiple exits if 146 // required. 147 std::vector<BasicBlock *> ExitBlocks; 148 L->getExitBlocks(ExitBlocks); 149 if (ExitBlocks.size() > 1) 150 return false; 151 152 // Find new Loop header. NewHeader is a Header's one and only successor 153 // that is inside loop. Header's other successor is out side the 154 // loop. Otherwise loop is not suitable for rotation. 155 Exit = BI->getSuccessor(0); 156 NewHeader = BI->getSuccessor(1); 157 if (L->contains(Exit)) 158 std::swap(Exit, NewHeader); 159 assert (NewHeader && "Unable to determine new loop header"); 160 assert(L->contains(NewHeader) && !L->contains(Exit) && 161 "Unable to determine loop header and exit blocks"); 162 163 // Check size of original header and reject 164 // loop if it is very big. 165 if (OrigHeader->getInstList().size() > MAX_HEADER_SIZE) 166 return false; 167 168 // Now, this loop is suitable for rotation. 169 170 // Copy PHI nodes and other instructions from original header 171 // into original pre-header. Unlike original header, original pre-header is 172 // not a member of loop. 173 // 174 // New loop header is one and only successor of original header that 175 // is inside the loop. All other original header successors are outside 176 // the loop. Copy PHI Nodes from original header into new loop header. 177 // Add second incoming value, from original loop pre-header into these phi 178 // nodes. If a value defined in original header is used outside original 179 // header then new loop header will need new phi nodes with two incoming 180 // values, one definition from original header and second definition is 181 // from original loop pre-header. 182 183 // Remove terminator from Original pre-header. Original pre-header will 184 // receive a clone of original header terminator as a new terminator. 185 OrigPreHeader->getInstList().pop_back(); 186 BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end(); 187 PHINode *PN = NULL; 188 for (; (PN = dyn_cast<PHINode>(I)); ++I) { 189 Instruction *In = I; 190 191 // PHI nodes are not copied into original pre-header. Instead their values 192 // are directly propagated. 193 Value * NPV = PN->getIncomingValueForBlock(OrigPreHeader); 194 195 // Create new PHI node with two incoming values for NewHeader. 196 // One incoming value is from OrigLatch (through OrigHeader) and 197 // second incoming value is from original pre-header. 198 PHINode *NH = new PHINode(In->getType(), In->getName()); 199 NH->addIncoming(PN->getIncomingValueForBlock(OrigLatch), OrigHeader); 200 NH->addIncoming(NPV, OrigPreHeader); 201 NewHeader->getInstList().push_front(NH); 202 203 // "In" can be replaced by NH at various places. 204 LoopHeaderInfo.push_back(RenameData(In, NPV, NH)); 205 } 206 207 // Now, handle non-phi instructions. 208 for (; I != E; ++I) { 209 Instruction *In = I; 210 211 assert (!isa<PHINode>(In) && "PHINode is not expected here"); 212 // This is not a PHI instruction. Insert its clone into original pre-header. 213 // If this instruction is using a value from same basic block then 214 // update it to use value from cloned instruction. 215 Instruction *C = In->clone(); 216 C->setName(In->getName()); 217 OrigPreHeader->getInstList().push_back(C); 218 219 for (unsigned opi = 0, e = In->getNumOperands(); opi != e; ++opi) { 220 if (Instruction *OpPhi = dyn_cast<PHINode>(In->getOperand(opi))) { 221 if (const RenameData *D = findReplacementData(OpPhi)) { 222 // This is using values from original header PHI node. 223 // Here, directly used incoming value from original pre-header. 224 C->setOperand(opi, D->PreHeader); 225 } 226 } 227 else if (Instruction *OpInsn = 228 dyn_cast<Instruction>(In->getOperand(opi))) { 229 if (const RenameData *D = findReplacementData(OpInsn)) 230 C->setOperand(opi, D->PreHeader); 231 } 232 } 233 234 235 // If this instruction is used outside this basic block then 236 // create new PHINode for this instruction. 237 Instruction *NewHeaderReplacement = NULL; 238 if (usedOutsideOriginalHeader(In)) { 239 PHINode *PN = new PHINode(In->getType(), In->getName()); 240 PN->addIncoming(In, OrigHeader); 241 PN->addIncoming(C, OrigPreHeader); 242 NewHeader->getInstList().push_front(PN); 243 NewHeaderReplacement = PN; 244 } 245 246 // "In" can be replaced by NPH or NH at various places. 247 LoopHeaderInfo.push_back(RenameData(In, C, NewHeaderReplacement)); 248 } 249 250 // Rename uses of original header instructions to reflect their new 251 // definitions (either from original pre-header node or from newly created 252 // new header PHINodes. 253 // 254 // Original header instructions are used in 255 // 1) Original header: 256 // 257 // If instruction is used in non-phi instructions then it is using 258 // defintion from original heder iteself. Do not replace this use 259 // with definition from new header or original pre-header. 260 // 261 // If instruction is used in phi node then it is an incoming 262 // value. Rename its use to reflect new definition from new-preheader 263 // or new header. 264 // 265 // 2) Inside loop but not in original header 266 // 267 // Replace this use to reflect definition from new header. 268 for(unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) { 269 const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI]; 270 271 if (!ILoopHeaderInfo.Header) 272 continue; 273 274 Instruction *OldPhi = ILoopHeaderInfo.Original; 275 Instruction *NewPhi = ILoopHeaderInfo.Header; 276 277 // Before replacing uses, collect them first, so that iterator is 278 // not invalidated. 279 SmallVector<Instruction *, 16> AllUses; 280 for (Value::use_iterator UI = OldPhi->use_begin(), UE = OldPhi->use_end(); 281 UI != UE; ++UI) { 282 Instruction *U = cast<Instruction>(UI); 283 AllUses.push_back(U); 284 } 285 286 for (SmallVector<Instruction *, 16>::iterator UI = AllUses.begin(), 287 UE = AllUses.end(); UI != UE; ++UI) { 288 Instruction *U = *UI; 289 BasicBlock *Parent = U->getParent(); 290 291 // Used inside original header 292 if (Parent == OrigHeader) { 293 // Do not rename uses inside original header non-phi instructions. 294 PHINode *PU = dyn_cast<PHINode>(U); 295 if (!PU) 296 continue; 297 298 // Do not rename uses inside original header phi nodes, if the 299 // incoming value is for new header. 300 if (PU->getBasicBlockIndex(NewHeader) != -1 301 && PU->getIncomingValueForBlock(NewHeader) == U) 302 continue; 303 304 U->replaceUsesOfWith(OldPhi, NewPhi); 305 continue; 306 } 307 308 // Used inside loop, but not in original header. 309 if (L->contains(U->getParent())) { 310 if (U != NewPhi) 311 U->replaceUsesOfWith(OldPhi, NewPhi); 312 continue; 313 } 314 315 // Used inside Exit Block. Since we are in LCSSA form, U must be PHINode. 316 assert (U->getParent() == Exit 317 && "Need to propagate new PHI into Exit blocks"); 318 assert (isa<PHINode>(U) && "Use in Exit Block that is not PHINode"); 319 320 PHINode *UPhi = cast<PHINode>(U); 321 322 // UPhi already has one incoming argument from original header. 323 // Add second incoming argument from new Pre header. 324 325 UPhi->addIncoming(ILoopHeaderInfo.PreHeader, OrigPreHeader); 326 } 327 } 328 329 /// Make sure all Exit block PHINodes have required incoming values. 330 updateExitBlock(); 331 332 // Update CFG 333 334 // Removing incoming branch from loop preheader to original header. 335 // Now original header is inside the loop. 336 for (BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end(); 337 I != E; ++I) { 338 Instruction *In = I; 339 PHINode *PN = dyn_cast<PHINode>(In); 340 if (!PN) 341 break; 342 343 PN->removeIncomingValue(OrigPreHeader); 344 } 345 346 // Make NewHeader as the new header for the loop. 347 L->moveToHeader(NewHeader); 348 349 preserveCanonicalLoopForm(LPM); 350 351 NumRotated++; 352 return true; 353} 354 355/// Make sure all Exit block PHINodes have required incoming values. 356/// If incoming value is constant or defined outside the loop then 357/// PHINode may not have an entry for original pre-header. 358void LoopRotate::updateExitBlock() { 359 360 for (BasicBlock::iterator I = Exit->begin(), E = Exit->end(); 361 I != E; ++I) { 362 363 PHINode *PN = dyn_cast<PHINode>(I); 364 if (!PN) 365 break; 366 367 // There is already one incoming value from original pre-header block. 368 if (PN->getBasicBlockIndex(OrigPreHeader) != -1) 369 continue; 370 371 const RenameData *ILoopHeaderInfo; 372 Value *V = PN->getIncomingValueForBlock(OrigHeader); 373 if (isa<Instruction>(V) && 374 (ILoopHeaderInfo = findReplacementData(cast<Instruction>(V)))) { 375 assert (ILoopHeaderInfo->PreHeader && "Missing New Preheader Instruction"); 376 PN->addIncoming(ILoopHeaderInfo->PreHeader, OrigPreHeader); 377 } else { 378 PN->addIncoming(V, OrigPreHeader); 379 } 380 } 381} 382 383/// Initialize local data 384void LoopRotate::initialize() { 385 L = NULL; 386 OrigHeader = NULL; 387 OrigPreHeader = NULL; 388 NewHeader = NULL; 389 Exit = NULL; 390 391 LoopHeaderInfo.clear(); 392} 393 394/// Return true if this instruction is used by any instructions in the loop that 395/// aren't in original header. 396bool LoopRotate::usedOutsideOriginalHeader(Instruction *In) { 397 398 for (Value::use_iterator UI = In->use_begin(), UE = In->use_end(); 399 UI != UE; ++UI) { 400 Instruction *U = cast<Instruction>(UI); 401 if (U->getParent() != OrigHeader) { 402 if (L->contains(U->getParent())) 403 return true; 404 } 405 } 406 407 return false; 408} 409 410/// Find Replacement information for instruction. Return NULL if it is 411/// not available. 412const RenameData *LoopRotate::findReplacementData(Instruction *In) { 413 414 // Since LoopHeaderInfo is small, linear walk is OK. 415 for(unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) { 416 const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI]; 417 if (ILoopHeaderInfo.Original == In) 418 return &ILoopHeaderInfo; 419 } 420 return NULL; 421} 422 423/// After loop rotation, loop pre-header has multiple sucessors. 424/// Insert one forwarding basic block to ensure that loop pre-header 425/// has only one successor. 426void LoopRotate::preserveCanonicalLoopForm(LPPassManager &LPM) { 427 428 // Right now original pre-header has two successors, new header and 429 // exit block. Insert new block between original pre-header and 430 // new header such that loop's new pre-header has only one successor. 431 BasicBlock *NewPreHeader = new BasicBlock("bb.nph", OrigHeader->getParent(), 432 NewHeader); 433 LoopInfo &LI = LPM.getAnalysis<LoopInfo>(); 434 if (Loop *PL = LI.getLoopFor(OrigPreHeader)) 435 PL->addBasicBlockToLoop(NewPreHeader, LI); 436 new BranchInst(NewHeader, NewPreHeader); 437 438 BranchInst *OrigPH_BI = cast<BranchInst>(OrigPreHeader->getTerminator()); 439 if (OrigPH_BI->getSuccessor(0) == NewHeader) 440 OrigPH_BI->setSuccessor(0, NewPreHeader); 441 else { 442 assert (OrigPH_BI->getSuccessor(1) == NewHeader && 443 "Unexpected original pre-header terminator"); 444 OrigPH_BI->setSuccessor(1, NewPreHeader); 445 } 446 447 for (BasicBlock::iterator I = NewHeader->begin(), E = NewHeader->end(); 448 I != E; ++I) { 449 Instruction *In = I; 450 PHINode *PN = dyn_cast<PHINode>(In); 451 if (!PN) 452 break; 453 454 int index = PN->getBasicBlockIndex(OrigPreHeader); 455 assert (index != -1 && "Expected incoming value from Original PreHeader"); 456 PN->setIncomingBlock(index, NewPreHeader); 457 assert (PN->getBasicBlockIndex(OrigPreHeader) == -1 && 458 "Expected only one incoming value from Original PreHeader"); 459 } 460 461 assert (NewHeader && L->getHeader() == NewHeader 462 && "Invalid loop header after loop rotation"); 463 assert (NewPreHeader && L->getLoopPreheader() == NewPreHeader 464 && "Invalid loop preheader after loop rotation"); 465 assert (L->getLoopLatch() 466 && "Invalid loop latch after loop rotation"); 467} 468