LoopRotation.cpp revision 198892
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/Transforms/Utils/SSAUpdater.h" 25#include "llvm/Support/CommandLine.h" 26#include "llvm/Support/Debug.h" 27#include "llvm/ADT/Statistic.h" 28#include "llvm/ADT/SmallVector.h" 29using namespace llvm; 30 31#define MAX_HEADER_SIZE 16 32 33STATISTIC(NumRotated, "Number of loops rotated"); 34namespace { 35 36 class LoopRotate : public LoopPass { 37 public: 38 static char ID; // Pass ID, replacement for typeid 39 LoopRotate() : LoopPass(&ID) {} 40 41 // Rotate Loop L as many times as possible. Return true if 42 // loop is rotated at least once. 43 bool runOnLoop(Loop *L, LPPassManager &LPM); 44 45 // LCSSA form makes instruction renaming easier. 46 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 47 AU.addRequiredID(LoopSimplifyID); 48 AU.addPreservedID(LoopSimplifyID); 49 AU.addRequiredID(LCSSAID); 50 AU.addPreservedID(LCSSAID); 51 AU.addPreserved<ScalarEvolution>(); 52 AU.addPreserved<LoopInfo>(); 53 AU.addPreserved<DominatorTree>(); 54 AU.addPreserved<DominanceFrontier>(); 55 } 56 57 // Helper functions 58 59 /// Do actual work 60 bool rotateLoop(Loop *L, LPPassManager &LPM); 61 62 /// Initialize local data 63 void initialize(); 64 65 /// After loop rotation, loop pre-header has multiple sucessors. 66 /// Insert one forwarding basic block to ensure that loop pre-header 67 /// has only one successor. 68 void preserveCanonicalLoopForm(LPPassManager &LPM); 69 70 private: 71 Loop *L; 72 BasicBlock *OrigHeader; 73 BasicBlock *OrigPreHeader; 74 BasicBlock *OrigLatch; 75 BasicBlock *NewHeader; 76 BasicBlock *Exit; 77 LPPassManager *LPM_Ptr; 78 }; 79} 80 81char LoopRotate::ID = 0; 82static RegisterPass<LoopRotate> X("loop-rotate", "Rotate Loops"); 83 84Pass *llvm::createLoopRotatePass() { return new LoopRotate(); } 85 86/// Rotate Loop L as many times as possible. Return true if 87/// the loop is rotated at least once. 88bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) { 89 90 bool RotatedOneLoop = false; 91 initialize(); 92 LPM_Ptr = &LPM; 93 94 // One loop can be rotated multiple times. 95 while (rotateLoop(Lp,LPM)) { 96 RotatedOneLoop = true; 97 initialize(); 98 } 99 100 return RotatedOneLoop; 101} 102 103/// Rotate loop LP. Return true if the loop is rotated. 104bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) { 105 L = Lp; 106 107 OrigHeader = L->getHeader(); 108 OrigPreHeader = L->getLoopPreheader(); 109 OrigLatch = L->getLoopLatch(); 110 111 // If the loop has only one block then there is not much to rotate. 112 if (L->getBlocks().size() == 1) 113 return false; 114 115 assert(OrigHeader && OrigLatch && OrigPreHeader && 116 "Loop is not in canonical form"); 117 118 // If the loop header is not one of the loop exiting blocks then 119 // either this loop is already rotated or it is not 120 // suitable for loop rotation transformations. 121 if (!L->isLoopExiting(OrigHeader)) 122 return false; 123 124 BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator()); 125 if (!BI) 126 return false; 127 assert(BI->isConditional() && "Branch Instruction is not conditional"); 128 129 // Updating PHInodes in loops with multiple exits adds complexity. 130 // Keep it simple, and restrict loop rotation to loops with one exit only. 131 // In future, lift this restriction and support for multiple exits if 132 // required. 133 SmallVector<BasicBlock*, 8> ExitBlocks; 134 L->getExitBlocks(ExitBlocks); 135 if (ExitBlocks.size() > 1) 136 return false; 137 138 // Check size of original header and reject 139 // loop if it is very big. 140 unsigned Size = 0; 141 142 // FIXME: Use common api to estimate size. 143 for (BasicBlock::const_iterator OI = OrigHeader->begin(), 144 OE = OrigHeader->end(); OI != OE; ++OI) { 145 if (isa<PHINode>(OI)) 146 continue; // PHI nodes don't count. 147 if (isa<DbgInfoIntrinsic>(OI)) 148 continue; // Debug intrinsics don't count as size. 149 Size++; 150 } 151 152 if (Size > MAX_HEADER_SIZE) 153 return false; 154 155 // Now, this loop is suitable for rotation. 156 157 // Anything ScalarEvolution may know about this loop or the PHI nodes 158 // in its header will soon be invalidated. 159 if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>()) 160 SE->forgetLoop(L); 161 162 // Find new Loop header. NewHeader is a Header's one and only successor 163 // that is inside loop. Header's other successor is outside the 164 // loop. Otherwise loop is not suitable for rotation. 165 Exit = BI->getSuccessor(0); 166 NewHeader = BI->getSuccessor(1); 167 if (L->contains(Exit)) 168 std::swap(Exit, NewHeader); 169 assert(NewHeader && "Unable to determine new loop header"); 170 assert(L->contains(NewHeader) && !L->contains(Exit) && 171 "Unable to determine loop header and exit blocks"); 172 173 // This code assumes that the new header has exactly one predecessor. 174 // Remove any single-entry PHI nodes in it. 175 assert(NewHeader->getSinglePredecessor() && 176 "New header doesn't have one pred!"); 177 FoldSingleEntryPHINodes(NewHeader); 178 179 // Begin by walking OrigHeader and populating ValueMap with an entry for 180 // each Instruction. 181 BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end(); 182 DenseMap<const Value *, Value *> ValueMap; 183 184 // For PHI nodes, the value available in OldPreHeader is just the 185 // incoming value from OldPreHeader. 186 for (; PHINode *PN = dyn_cast<PHINode>(I); ++I) 187 ValueMap[PN] = PN->getIncomingValue(PN->getBasicBlockIndex(OrigPreHeader)); 188 189 // For the rest of the instructions, create a clone in the OldPreHeader. 190 TerminatorInst *LoopEntryBranch = OrigPreHeader->getTerminator(); 191 for (; I != E; ++I) { 192 Instruction *C = I->clone(); 193 C->setName(I->getName()); 194 C->insertBefore(LoopEntryBranch); 195 ValueMap[I] = C; 196 } 197 198 // Along with all the other instructions, we just cloned OrigHeader's 199 // terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's 200 // successors by duplicating their incoming values for OrigHeader. 201 TerminatorInst *TI = OrigHeader->getTerminator(); 202 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) 203 for (BasicBlock::iterator BI = TI->getSuccessor(i)->begin(); 204 PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 205 PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreHeader); 206 207 // Now that OrigPreHeader has a clone of OrigHeader's terminator, remove 208 // OrigPreHeader's old terminator (the original branch into the loop), and 209 // remove the corresponding incoming values from the PHI nodes in OrigHeader. 210 LoopEntryBranch->eraseFromParent(); 211 for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I) 212 PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreHeader)); 213 214 // Now fix up users of the instructions in OrigHeader, inserting PHI nodes 215 // as necessary. 216 SSAUpdater SSA; 217 for (I = OrigHeader->begin(); I != E; ++I) { 218 Value *OrigHeaderVal = I; 219 Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal]; 220 221 // The value now exits in two versions: the initial value in the preheader 222 // and the loop "next" value in the original header. 223 SSA.Initialize(OrigHeaderVal); 224 SSA.AddAvailableValue(OrigHeader, OrigHeaderVal); 225 SSA.AddAvailableValue(OrigPreHeader, OrigPreHeaderVal); 226 227 // Visit each use of the OrigHeader instruction. 228 for (Value::use_iterator UI = OrigHeaderVal->use_begin(), 229 UE = OrigHeaderVal->use_end(); UI != UE; ) { 230 // Grab the use before incrementing the iterator. 231 Use &U = UI.getUse(); 232 233 // Increment the iterator before removing the use from the list. 234 ++UI; 235 236 // SSAUpdater can't handle a non-PHI use in the same block as an 237 // earlier def. We can easily handle those cases manually. 238 Instruction *UserInst = cast<Instruction>(U.getUser()); 239 if (!isa<PHINode>(UserInst)) { 240 BasicBlock *UserBB = UserInst->getParent(); 241 242 // The original users in the OrigHeader are already using the 243 // original definitions. 244 if (UserBB == OrigHeader) 245 continue; 246 247 // Users in the OrigPreHeader need to use the value to which the 248 // original definitions are mapped. 249 if (UserBB == OrigPreHeader) { 250 U = OrigPreHeaderVal; 251 continue; 252 } 253 } 254 255 // Anything else can be handled by SSAUpdater. 256 SSA.RewriteUse(U); 257 } 258 } 259 260 // NewHeader is now the header of the loop. 261 L->moveToHeader(NewHeader); 262 263 preserveCanonicalLoopForm(LPM); 264 265 NumRotated++; 266 return true; 267} 268 269/// Initialize local data 270void LoopRotate::initialize() { 271 L = NULL; 272 OrigHeader = NULL; 273 OrigPreHeader = NULL; 274 NewHeader = NULL; 275 Exit = NULL; 276} 277 278/// After loop rotation, loop pre-header has multiple sucessors. 279/// Insert one forwarding basic block to ensure that loop pre-header 280/// has only one successor. 281void LoopRotate::preserveCanonicalLoopForm(LPPassManager &LPM) { 282 283 // Right now original pre-header has two successors, new header and 284 // exit block. Insert new block between original pre-header and 285 // new header such that loop's new pre-header has only one successor. 286 BasicBlock *NewPreHeader = BasicBlock::Create(OrigHeader->getContext(), 287 "bb.nph", 288 OrigHeader->getParent(), 289 NewHeader); 290 LoopInfo &LI = LPM.getAnalysis<LoopInfo>(); 291 if (Loop *PL = LI.getLoopFor(OrigPreHeader)) 292 PL->addBasicBlockToLoop(NewPreHeader, LI.getBase()); 293 BranchInst::Create(NewHeader, NewPreHeader); 294 295 BranchInst *OrigPH_BI = cast<BranchInst>(OrigPreHeader->getTerminator()); 296 if (OrigPH_BI->getSuccessor(0) == NewHeader) 297 OrigPH_BI->setSuccessor(0, NewPreHeader); 298 else { 299 assert(OrigPH_BI->getSuccessor(1) == NewHeader && 300 "Unexpected original pre-header terminator"); 301 OrigPH_BI->setSuccessor(1, NewPreHeader); 302 } 303 304 PHINode *PN; 305 for (BasicBlock::iterator I = NewHeader->begin(); 306 (PN = dyn_cast<PHINode>(I)); ++I) { 307 int index = PN->getBasicBlockIndex(OrigPreHeader); 308 assert(index != -1 && "Expected incoming value from Original PreHeader"); 309 PN->setIncomingBlock(index, NewPreHeader); 310 assert(PN->getBasicBlockIndex(OrigPreHeader) == -1 && 311 "Expected only one incoming value from Original PreHeader"); 312 } 313 314 if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) { 315 DT->addNewBlock(NewPreHeader, OrigPreHeader); 316 DT->changeImmediateDominator(L->getHeader(), NewPreHeader); 317 DT->changeImmediateDominator(Exit, OrigPreHeader); 318 for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end(); 319 BI != BE; ++BI) { 320 BasicBlock *B = *BI; 321 if (L->getHeader() != B) { 322 DomTreeNode *Node = DT->getNode(B); 323 if (Node && Node->getBlock() == OrigHeader) 324 DT->changeImmediateDominator(*BI, L->getHeader()); 325 } 326 } 327 DT->changeImmediateDominator(OrigHeader, OrigLatch); 328 } 329 330 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>()) { 331 // New Preheader's dominance frontier is Exit block. 332 DominanceFrontier::DomSetType NewPHSet; 333 NewPHSet.insert(Exit); 334 DF->addBasicBlock(NewPreHeader, NewPHSet); 335 336 // New Header's dominance frontier now includes itself and Exit block 337 DominanceFrontier::iterator HeadI = DF->find(L->getHeader()); 338 if (HeadI != DF->end()) { 339 DominanceFrontier::DomSetType & HeaderSet = HeadI->second; 340 HeaderSet.clear(); 341 HeaderSet.insert(L->getHeader()); 342 HeaderSet.insert(Exit); 343 } else { 344 DominanceFrontier::DomSetType HeaderSet; 345 HeaderSet.insert(L->getHeader()); 346 HeaderSet.insert(Exit); 347 DF->addBasicBlock(L->getHeader(), HeaderSet); 348 } 349 350 // Original header (new Loop Latch)'s dominance frontier is Exit. 351 DominanceFrontier::iterator LatchI = DF->find(L->getLoopLatch()); 352 if (LatchI != DF->end()) { 353 DominanceFrontier::DomSetType &LatchSet = LatchI->second; 354 LatchSet = LatchI->second; 355 LatchSet.clear(); 356 LatchSet.insert(Exit); 357 } else { 358 DominanceFrontier::DomSetType LatchSet; 359 LatchSet.insert(Exit); 360 DF->addBasicBlock(L->getHeader(), LatchSet); 361 } 362 363 // If a loop block dominates new loop latch then add to its frontiers 364 // new header and Exit and remove new latch (which is equal to original 365 // header). 366 BasicBlock *NewLatch = L->getLoopLatch(); 367 368 assert(NewLatch == OrigHeader && "NewLatch is inequal to OrigHeader"); 369 370 if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) { 371 for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end(); 372 BI != BE; ++BI) { 373 BasicBlock *B = *BI; 374 if (DT->dominates(B, NewLatch)) { 375 DominanceFrontier::iterator BDFI = DF->find(B); 376 if (BDFI != DF->end()) { 377 DominanceFrontier::DomSetType &BSet = BDFI->second; 378 BSet.erase(NewLatch); 379 BSet.insert(L->getHeader()); 380 BSet.insert(Exit); 381 } else { 382 DominanceFrontier::DomSetType BSet; 383 BSet.insert(L->getHeader()); 384 BSet.insert(Exit); 385 DF->addBasicBlock(B, BSet); 386 } 387 } 388 } 389 } 390 } 391 392 // Preserve canonical loop form, which means Exit block should 393 // have only one predecessor. 394 SplitEdge(L->getLoopLatch(), Exit, this); 395 396 assert(NewHeader && L->getHeader() == NewHeader && 397 "Invalid loop header after loop rotation"); 398 assert(NewPreHeader && L->getLoopPreheader() == NewPreHeader && 399 "Invalid loop preheader after loop rotation"); 400 assert(L->getLoopLatch() && 401 "Invalid loop latch after loop rotation"); 402} 403