1 //===----------------- LoopRotationUtils.cpp -----------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file provides utilities to convert a loop into a loop with bottom test. 10 // 11 //===----------------------------------------------------------------------===// 12 13 #include "llvm/Transforms/Utils/LoopRotationUtils.h" 14 #include "llvm/ADT/Statistic.h" 15 #include "llvm/Analysis/AliasAnalysis.h" 16 #include "llvm/Analysis/AssumptionCache.h" 17 #include "llvm/Analysis/BasicAliasAnalysis.h" 18 #include "llvm/Analysis/CodeMetrics.h" 19 #include "llvm/Analysis/DomTreeUpdater.h" 20 #include "llvm/Analysis/GlobalsModRef.h" 21 #include "llvm/Analysis/InstructionSimplify.h" 22 #include "llvm/Analysis/LoopPass.h" 23 #include "llvm/Analysis/MemorySSA.h" 24 #include "llvm/Analysis/MemorySSAUpdater.h" 25 #include "llvm/Analysis/ScalarEvolution.h" 26 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h" 27 #include "llvm/Analysis/TargetTransformInfo.h" 28 #include "llvm/Analysis/ValueTracking.h" 29 #include "llvm/IR/CFG.h" 30 #include "llvm/IR/DebugInfoMetadata.h" 31 #include "llvm/IR/Dominators.h" 32 #include "llvm/IR/Function.h" 33 #include "llvm/IR/IntrinsicInst.h" 34 #include "llvm/IR/Module.h" 35 #include "llvm/Support/CommandLine.h" 36 #include "llvm/Support/Debug.h" 37 #include "llvm/Support/raw_ostream.h" 38 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 39 #include "llvm/Transforms/Utils/Local.h" 40 #include "llvm/Transforms/Utils/LoopUtils.h" 41 #include "llvm/Transforms/Utils/SSAUpdater.h" 42 #include "llvm/Transforms/Utils/ValueMapper.h" 43 using namespace llvm; 44 45 #define DEBUG_TYPE "loop-rotate" 46 47 STATISTIC(NumRotated, "Number of loops rotated"); 48 49 static cl::opt<bool> 50 MultiRotate("loop-rotate-multi", cl::init(false), cl::Hidden, 51 cl::desc("Allow loop rotation multiple times in order to reach " 52 "a better latch exit")); 53 54 namespace { 55 /// A simple loop rotation transformation. 56 class LoopRotate { 57 const unsigned MaxHeaderSize; 58 LoopInfo *LI; 59 const TargetTransformInfo *TTI; 60 AssumptionCache *AC; 61 DominatorTree *DT; 62 ScalarEvolution *SE; 63 MemorySSAUpdater *MSSAU; 64 const SimplifyQuery &SQ; 65 bool RotationOnly; 66 bool IsUtilMode; 67 68 public: 69 LoopRotate(unsigned MaxHeaderSize, LoopInfo *LI, 70 const TargetTransformInfo *TTI, AssumptionCache *AC, 71 DominatorTree *DT, ScalarEvolution *SE, MemorySSAUpdater *MSSAU, 72 const SimplifyQuery &SQ, bool RotationOnly, bool IsUtilMode) 73 : MaxHeaderSize(MaxHeaderSize), LI(LI), TTI(TTI), AC(AC), DT(DT), SE(SE), 74 MSSAU(MSSAU), SQ(SQ), RotationOnly(RotationOnly), 75 IsUtilMode(IsUtilMode) {} 76 bool processLoop(Loop *L); 77 78 private: 79 bool rotateLoop(Loop *L, bool SimplifiedLatch); 80 bool simplifyLoopLatch(Loop *L); 81 }; 82 } // end anonymous namespace 83 84 /// Insert (K, V) pair into the ValueToValueMap, and verify the key did not 85 /// previously exist in the map, and the value was inserted. 86 static void InsertNewValueIntoMap(ValueToValueMapTy &VM, Value *K, Value *V) { 87 bool Inserted = VM.insert({K, V}).second; 88 assert(Inserted); 89 (void)Inserted; 90 } 91 /// RewriteUsesOfClonedInstructions - We just cloned the instructions from the 92 /// old header into the preheader. If there were uses of the values produced by 93 /// these instruction that were outside of the loop, we have to insert PHI nodes 94 /// to merge the two values. Do this now. 95 static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader, 96 BasicBlock *OrigPreheader, 97 ValueToValueMapTy &ValueMap, 98 SmallVectorImpl<PHINode*> *InsertedPHIs) { 99 // Remove PHI node entries that are no longer live. 100 BasicBlock::iterator I, E = OrigHeader->end(); 101 for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I) 102 PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreheader)); 103 104 // Now fix up users of the instructions in OrigHeader, inserting PHI nodes 105 // as necessary. 106 SSAUpdater SSA(InsertedPHIs); 107 for (I = OrigHeader->begin(); I != E; ++I) { 108 Value *OrigHeaderVal = &*I; 109 110 // If there are no uses of the value (e.g. because it returns void), there 111 // is nothing to rewrite. 112 if (OrigHeaderVal->use_empty()) 113 continue; 114 115 Value *OrigPreHeaderVal = ValueMap.lookup(OrigHeaderVal); 116 117 // The value now exits in two versions: the initial value in the preheader 118 // and the loop "next" value in the original header. 119 SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName()); 120 SSA.AddAvailableValue(OrigHeader, OrigHeaderVal); 121 SSA.AddAvailableValue(OrigPreheader, OrigPreHeaderVal); 122 123 // Visit each use of the OrigHeader instruction. 124 for (Value::use_iterator UI = OrigHeaderVal->use_begin(), 125 UE = OrigHeaderVal->use_end(); 126 UI != UE;) { 127 // Grab the use before incrementing the iterator. 128 Use &U = *UI; 129 130 // Increment the iterator before removing the use from the list. 131 ++UI; 132 133 // SSAUpdater can't handle a non-PHI use in the same block as an 134 // earlier def. We can easily handle those cases manually. 135 Instruction *UserInst = cast<Instruction>(U.getUser()); 136 if (!isa<PHINode>(UserInst)) { 137 BasicBlock *UserBB = UserInst->getParent(); 138 139 // The original users in the OrigHeader are already using the 140 // original definitions. 141 if (UserBB == OrigHeader) 142 continue; 143 144 // Users in the OrigPreHeader need to use the value to which the 145 // original definitions are mapped. 146 if (UserBB == OrigPreheader) { 147 U = OrigPreHeaderVal; 148 continue; 149 } 150 } 151 152 // Anything else can be handled by SSAUpdater. 153 SSA.RewriteUse(U); 154 } 155 156 // Replace MetadataAsValue(ValueAsMetadata(OrigHeaderVal)) uses in debug 157 // intrinsics. 158 SmallVector<DbgValueInst *, 1> DbgValues; 159 llvm::findDbgValues(DbgValues, OrigHeaderVal); 160 for (auto &DbgValue : DbgValues) { 161 // The original users in the OrigHeader are already using the original 162 // definitions. 163 BasicBlock *UserBB = DbgValue->getParent(); 164 if (UserBB == OrigHeader) 165 continue; 166 167 // Users in the OrigPreHeader need to use the value to which the 168 // original definitions are mapped and anything else can be handled by 169 // the SSAUpdater. To avoid adding PHINodes, check if the value is 170 // available in UserBB, if not substitute undef. 171 Value *NewVal; 172 if (UserBB == OrigPreheader) 173 NewVal = OrigPreHeaderVal; 174 else if (SSA.HasValueForBlock(UserBB)) 175 NewVal = SSA.GetValueInMiddleOfBlock(UserBB); 176 else 177 NewVal = UndefValue::get(OrigHeaderVal->getType()); 178 DbgValue->setOperand(0, 179 MetadataAsValue::get(OrigHeaderVal->getContext(), 180 ValueAsMetadata::get(NewVal))); 181 } 182 } 183 } 184 185 // Assuming both header and latch are exiting, look for a phi which is only 186 // used outside the loop (via a LCSSA phi) in the exit from the header. 187 // This means that rotating the loop can remove the phi. 188 static bool profitableToRotateLoopExitingLatch(Loop *L) { 189 BasicBlock *Header = L->getHeader(); 190 BranchInst *BI = dyn_cast<BranchInst>(Header->getTerminator()); 191 assert(BI && BI->isConditional() && "need header with conditional exit"); 192 BasicBlock *HeaderExit = BI->getSuccessor(0); 193 if (L->contains(HeaderExit)) 194 HeaderExit = BI->getSuccessor(1); 195 196 for (auto &Phi : Header->phis()) { 197 // Look for uses of this phi in the loop/via exits other than the header. 198 if (llvm::any_of(Phi.users(), [HeaderExit](const User *U) { 199 return cast<Instruction>(U)->getParent() != HeaderExit; 200 })) 201 continue; 202 return true; 203 } 204 return false; 205 } 206 207 // Check that latch exit is deoptimizing (which means - very unlikely to happen) 208 // and there is another exit from the loop which is non-deoptimizing. 209 // If we rotate latch to that exit our loop has a better chance of being fully 210 // canonical. 211 // 212 // It can give false positives in some rare cases. 213 static bool canRotateDeoptimizingLatchExit(Loop *L) { 214 BasicBlock *Latch = L->getLoopLatch(); 215 assert(Latch && "need latch"); 216 BranchInst *BI = dyn_cast<BranchInst>(Latch->getTerminator()); 217 // Need normal exiting latch. 218 if (!BI || !BI->isConditional()) 219 return false; 220 221 BasicBlock *Exit = BI->getSuccessor(1); 222 if (L->contains(Exit)) 223 Exit = BI->getSuccessor(0); 224 225 // Latch exit is non-deoptimizing, no need to rotate. 226 if (!Exit->getPostdominatingDeoptimizeCall()) 227 return false; 228 229 SmallVector<BasicBlock *, 4> Exits; 230 L->getUniqueExitBlocks(Exits); 231 if (!Exits.empty()) { 232 // There is at least one non-deoptimizing exit. 233 // 234 // Note, that BasicBlock::getPostdominatingDeoptimizeCall is not exact, 235 // as it can conservatively return false for deoptimizing exits with 236 // complex enough control flow down to deoptimize call. 237 // 238 // That means here we can report success for a case where 239 // all exits are deoptimizing but one of them has complex enough 240 // control flow (e.g. with loops). 241 // 242 // That should be a very rare case and false positives for this function 243 // have compile-time effect only. 244 return any_of(Exits, [](const BasicBlock *BB) { 245 return !BB->getPostdominatingDeoptimizeCall(); 246 }); 247 } 248 return false; 249 } 250 251 /// Rotate loop LP. Return true if the loop is rotated. 252 /// 253 /// \param SimplifiedLatch is true if the latch was just folded into the final 254 /// loop exit. In this case we may want to rotate even though the new latch is 255 /// now an exiting branch. This rotation would have happened had the latch not 256 /// been simplified. However, if SimplifiedLatch is false, then we avoid 257 /// rotating loops in which the latch exits to avoid excessive or endless 258 /// rotation. LoopRotate should be repeatable and converge to a canonical 259 /// form. This property is satisfied because simplifying the loop latch can only 260 /// happen once across multiple invocations of the LoopRotate pass. 261 /// 262 /// If -loop-rotate-multi is enabled we can do multiple rotations in one go 263 /// so to reach a suitable (non-deoptimizing) exit. 264 bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) { 265 // If the loop has only one block then there is not much to rotate. 266 if (L->getBlocks().size() == 1) 267 return false; 268 269 bool Rotated = false; 270 do { 271 BasicBlock *OrigHeader = L->getHeader(); 272 BasicBlock *OrigLatch = L->getLoopLatch(); 273 274 BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator()); 275 if (!BI || BI->isUnconditional()) 276 return Rotated; 277 278 // If the loop header is not one of the loop exiting blocks then 279 // either this loop is already rotated or it is not 280 // suitable for loop rotation transformations. 281 if (!L->isLoopExiting(OrigHeader)) 282 return Rotated; 283 284 // If the loop latch already contains a branch that leaves the loop then the 285 // loop is already rotated. 286 if (!OrigLatch) 287 return Rotated; 288 289 // Rotate if either the loop latch does *not* exit the loop, or if the loop 290 // latch was just simplified. Or if we think it will be profitable. 291 if (L->isLoopExiting(OrigLatch) && !SimplifiedLatch && IsUtilMode == false && 292 !profitableToRotateLoopExitingLatch(L) && 293 !canRotateDeoptimizingLatchExit(L)) 294 return Rotated; 295 296 // Check size of original header and reject loop if it is very big or we can't 297 // duplicate blocks inside it. 298 { 299 SmallPtrSet<const Value *, 32> EphValues; 300 CodeMetrics::collectEphemeralValues(L, AC, EphValues); 301 302 CodeMetrics Metrics; 303 Metrics.analyzeBasicBlock(OrigHeader, *TTI, EphValues); 304 if (Metrics.notDuplicatable) { 305 LLVM_DEBUG( 306 dbgs() << "LoopRotation: NOT rotating - contains non-duplicatable" 307 << " instructions: "; 308 L->dump()); 309 return Rotated; 310 } 311 if (Metrics.convergent) { 312 LLVM_DEBUG(dbgs() << "LoopRotation: NOT rotating - contains convergent " 313 "instructions: "; 314 L->dump()); 315 return Rotated; 316 } 317 if (Metrics.NumInsts > MaxHeaderSize) { 318 LLVM_DEBUG(dbgs() << "LoopRotation: NOT rotating - contains " 319 << Metrics.NumInsts 320 << " instructions, which is more than the threshold (" 321 << MaxHeaderSize << " instructions): "; 322 L->dump()); 323 return Rotated; 324 } 325 } 326 327 // Now, this loop is suitable for rotation. 328 BasicBlock *OrigPreheader = L->getLoopPreheader(); 329 330 // If the loop could not be converted to canonical form, it must have an 331 // indirectbr in it, just give up. 332 if (!OrigPreheader || !L->hasDedicatedExits()) 333 return Rotated; 334 335 // Anything ScalarEvolution may know about this loop or the PHI nodes 336 // in its header will soon be invalidated. We should also invalidate 337 // all outer loops because insertion and deletion of blocks that happens 338 // during the rotation may violate invariants related to backedge taken 339 // infos in them. 340 if (SE) 341 SE->forgetTopmostLoop(L); 342 343 LLVM_DEBUG(dbgs() << "LoopRotation: rotating "; L->dump()); 344 if (MSSAU && VerifyMemorySSA) 345 MSSAU->getMemorySSA()->verifyMemorySSA(); 346 347 // Find new Loop header. NewHeader is a Header's one and only successor 348 // that is inside loop. Header's other successor is outside the 349 // loop. Otherwise loop is not suitable for rotation. 350 BasicBlock *Exit = BI->getSuccessor(0); 351 BasicBlock *NewHeader = BI->getSuccessor(1); 352 if (L->contains(Exit)) 353 std::swap(Exit, NewHeader); 354 assert(NewHeader && "Unable to determine new loop header"); 355 assert(L->contains(NewHeader) && !L->contains(Exit) && 356 "Unable to determine loop header and exit blocks"); 357 358 // This code assumes that the new header has exactly one predecessor. 359 // Remove any single-entry PHI nodes in it. 360 assert(NewHeader->getSinglePredecessor() && 361 "New header doesn't have one pred!"); 362 FoldSingleEntryPHINodes(NewHeader); 363 364 // Begin by walking OrigHeader and populating ValueMap with an entry for 365 // each Instruction. 366 BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end(); 367 ValueToValueMapTy ValueMap, ValueMapMSSA; 368 369 // For PHI nodes, the value available in OldPreHeader is just the 370 // incoming value from OldPreHeader. 371 for (; PHINode *PN = dyn_cast<PHINode>(I); ++I) 372 InsertNewValueIntoMap(ValueMap, PN, 373 PN->getIncomingValueForBlock(OrigPreheader)); 374 375 // For the rest of the instructions, either hoist to the OrigPreheader if 376 // possible or create a clone in the OldPreHeader if not. 377 Instruction *LoopEntryBranch = OrigPreheader->getTerminator(); 378 379 // Record all debug intrinsics preceding LoopEntryBranch to avoid duplication. 380 using DbgIntrinsicHash = 381 std::pair<std::pair<Value *, DILocalVariable *>, DIExpression *>; 382 auto makeHash = [](DbgVariableIntrinsic *D) -> DbgIntrinsicHash { 383 return {{D->getVariableLocation(), D->getVariable()}, D->getExpression()}; 384 }; 385 SmallDenseSet<DbgIntrinsicHash, 8> DbgIntrinsics; 386 for (auto I = std::next(OrigPreheader->rbegin()), E = OrigPreheader->rend(); 387 I != E; ++I) { 388 if (auto *DII = dyn_cast<DbgVariableIntrinsic>(&*I)) 389 DbgIntrinsics.insert(makeHash(DII)); 390 else 391 break; 392 } 393 394 while (I != E) { 395 Instruction *Inst = &*I++; 396 397 // If the instruction's operands are invariant and it doesn't read or write 398 // memory, then it is safe to hoist. Doing this doesn't change the order of 399 // execution in the preheader, but does prevent the instruction from 400 // executing in each iteration of the loop. This means it is safe to hoist 401 // something that might trap, but isn't safe to hoist something that reads 402 // memory (without proving that the loop doesn't write). 403 if (L->hasLoopInvariantOperands(Inst) && !Inst->mayReadFromMemory() && 404 !Inst->mayWriteToMemory() && !Inst->isTerminator() && 405 !isa<DbgInfoIntrinsic>(Inst) && !isa<AllocaInst>(Inst)) { 406 Inst->moveBefore(LoopEntryBranch); 407 continue; 408 } 409 410 // Otherwise, create a duplicate of the instruction. 411 Instruction *C = Inst->clone(); 412 413 // Eagerly remap the operands of the instruction. 414 RemapInstruction(C, ValueMap, 415 RF_NoModuleLevelChanges | RF_IgnoreMissingLocals); 416 417 // Avoid inserting the same intrinsic twice. 418 if (auto *DII = dyn_cast<DbgVariableIntrinsic>(C)) 419 if (DbgIntrinsics.count(makeHash(DII))) { 420 C->deleteValue(); 421 continue; 422 } 423 424 // With the operands remapped, see if the instruction constant folds or is 425 // otherwise simplifyable. This commonly occurs because the entry from PHI 426 // nodes allows icmps and other instructions to fold. 427 Value *V = SimplifyInstruction(C, SQ); 428 if (V && LI->replacementPreservesLCSSAForm(C, V)) { 429 // If so, then delete the temporary instruction and stick the folded value 430 // in the map. 431 InsertNewValueIntoMap(ValueMap, Inst, V); 432 if (!C->mayHaveSideEffects()) { 433 C->deleteValue(); 434 C = nullptr; 435 } 436 } else { 437 InsertNewValueIntoMap(ValueMap, Inst, C); 438 } 439 if (C) { 440 // Otherwise, stick the new instruction into the new block! 441 C->setName(Inst->getName()); 442 C->insertBefore(LoopEntryBranch); 443 444 if (auto *II = dyn_cast<IntrinsicInst>(C)) 445 if (II->getIntrinsicID() == Intrinsic::assume) 446 AC->registerAssumption(II); 447 // MemorySSA cares whether the cloned instruction was inserted or not, and 448 // not whether it can be remapped to a simplified value. 449 if (MSSAU) 450 InsertNewValueIntoMap(ValueMapMSSA, Inst, C); 451 } 452 } 453 454 // Along with all the other instructions, we just cloned OrigHeader's 455 // terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's 456 // successors by duplicating their incoming values for OrigHeader. 457 for (BasicBlock *SuccBB : successors(OrigHeader)) 458 for (BasicBlock::iterator BI = SuccBB->begin(); 459 PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 460 PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreheader); 461 462 // Now that OrigPreHeader has a clone of OrigHeader's terminator, remove 463 // OrigPreHeader's old terminator (the original branch into the loop), and 464 // remove the corresponding incoming values from the PHI nodes in OrigHeader. 465 LoopEntryBranch->eraseFromParent(); 466 467 // Update MemorySSA before the rewrite call below changes the 1:1 468 // instruction:cloned_instruction_or_value mapping. 469 if (MSSAU) { 470 InsertNewValueIntoMap(ValueMapMSSA, OrigHeader, OrigPreheader); 471 MSSAU->updateForClonedBlockIntoPred(OrigHeader, OrigPreheader, 472 ValueMapMSSA); 473 } 474 475 SmallVector<PHINode*, 2> InsertedPHIs; 476 // If there were any uses of instructions in the duplicated block outside the 477 // loop, update them, inserting PHI nodes as required 478 RewriteUsesOfClonedInstructions(OrigHeader, OrigPreheader, ValueMap, 479 &InsertedPHIs); 480 481 // Attach dbg.value intrinsics to the new phis if that phi uses a value that 482 // previously had debug metadata attached. This keeps the debug info 483 // up-to-date in the loop body. 484 if (!InsertedPHIs.empty()) 485 insertDebugValuesForPHIs(OrigHeader, InsertedPHIs); 486 487 // NewHeader is now the header of the loop. 488 L->moveToHeader(NewHeader); 489 assert(L->getHeader() == NewHeader && "Latch block is our new header"); 490 491 // Inform DT about changes to the CFG. 492 if (DT) { 493 // The OrigPreheader branches to the NewHeader and Exit now. Then, inform 494 // the DT about the removed edge to the OrigHeader (that got removed). 495 SmallVector<DominatorTree::UpdateType, 3> Updates; 496 Updates.push_back({DominatorTree::Insert, OrigPreheader, Exit}); 497 Updates.push_back({DominatorTree::Insert, OrigPreheader, NewHeader}); 498 Updates.push_back({DominatorTree::Delete, OrigPreheader, OrigHeader}); 499 DT->applyUpdates(Updates); 500 501 if (MSSAU) { 502 MSSAU->applyUpdates(Updates, *DT); 503 if (VerifyMemorySSA) 504 MSSAU->getMemorySSA()->verifyMemorySSA(); 505 } 506 } 507 508 // At this point, we've finished our major CFG changes. As part of cloning 509 // the loop into the preheader we've simplified instructions and the 510 // duplicated conditional branch may now be branching on a constant. If it is 511 // branching on a constant and if that constant means that we enter the loop, 512 // then we fold away the cond branch to an uncond branch. This simplifies the 513 // loop in cases important for nested loops, and it also means we don't have 514 // to split as many edges. 515 BranchInst *PHBI = cast<BranchInst>(OrigPreheader->getTerminator()); 516 assert(PHBI->isConditional() && "Should be clone of BI condbr!"); 517 if (!isa<ConstantInt>(PHBI->getCondition()) || 518 PHBI->getSuccessor(cast<ConstantInt>(PHBI->getCondition())->isZero()) != 519 NewHeader) { 520 // The conditional branch can't be folded, handle the general case. 521 // Split edges as necessary to preserve LoopSimplify form. 522 523 // Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and 524 // thus is not a preheader anymore. 525 // Split the edge to form a real preheader. 526 BasicBlock *NewPH = SplitCriticalEdge( 527 OrigPreheader, NewHeader, 528 CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA()); 529 NewPH->setName(NewHeader->getName() + ".lr.ph"); 530 531 // Preserve canonical loop form, which means that 'Exit' should have only 532 // one predecessor. Note that Exit could be an exit block for multiple 533 // nested loops, causing both of the edges to now be critical and need to 534 // be split. 535 SmallVector<BasicBlock *, 4> ExitPreds(pred_begin(Exit), pred_end(Exit)); 536 bool SplitLatchEdge = false; 537 for (BasicBlock *ExitPred : ExitPreds) { 538 // We only need to split loop exit edges. 539 Loop *PredLoop = LI->getLoopFor(ExitPred); 540 if (!PredLoop || PredLoop->contains(Exit) || 541 ExitPred->getTerminator()->isIndirectTerminator()) 542 continue; 543 SplitLatchEdge |= L->getLoopLatch() == ExitPred; 544 BasicBlock *ExitSplit = SplitCriticalEdge( 545 ExitPred, Exit, 546 CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA()); 547 ExitSplit->moveBefore(Exit); 548 } 549 assert(SplitLatchEdge && 550 "Despite splitting all preds, failed to split latch exit?"); 551 } else { 552 // We can fold the conditional branch in the preheader, this makes things 553 // simpler. The first step is to remove the extra edge to the Exit block. 554 Exit->removePredecessor(OrigPreheader, true /*preserve LCSSA*/); 555 BranchInst *NewBI = BranchInst::Create(NewHeader, PHBI); 556 NewBI->setDebugLoc(PHBI->getDebugLoc()); 557 PHBI->eraseFromParent(); 558 559 // With our CFG finalized, update DomTree if it is available. 560 if (DT) DT->deleteEdge(OrigPreheader, Exit); 561 562 // Update MSSA too, if available. 563 if (MSSAU) 564 MSSAU->removeEdge(OrigPreheader, Exit); 565 } 566 567 assert(L->getLoopPreheader() && "Invalid loop preheader after loop rotation"); 568 assert(L->getLoopLatch() && "Invalid loop latch after loop rotation"); 569 570 if (MSSAU && VerifyMemorySSA) 571 MSSAU->getMemorySSA()->verifyMemorySSA(); 572 573 // Now that the CFG and DomTree are in a consistent state again, try to merge 574 // the OrigHeader block into OrigLatch. This will succeed if they are 575 // connected by an unconditional branch. This is just a cleanup so the 576 // emitted code isn't too gross in this common case. 577 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); 578 MergeBlockIntoPredecessor(OrigHeader, &DTU, LI, MSSAU); 579 580 if (MSSAU && VerifyMemorySSA) 581 MSSAU->getMemorySSA()->verifyMemorySSA(); 582 583 LLVM_DEBUG(dbgs() << "LoopRotation: into "; L->dump()); 584 585 ++NumRotated; 586 587 Rotated = true; 588 SimplifiedLatch = false; 589 590 // Check that new latch is a deoptimizing exit and then repeat rotation if possible. 591 // Deoptimizing latch exit is not a generally typical case, so we just loop over. 592 // TODO: if it becomes a performance bottleneck extend rotation algorithm 593 // to handle multiple rotations in one go. 594 } while (MultiRotate && canRotateDeoptimizingLatchExit(L)); 595 596 597 return true; 598 } 599 600 /// Determine whether the instructions in this range may be safely and cheaply 601 /// speculated. This is not an important enough situation to develop complex 602 /// heuristics. We handle a single arithmetic instruction along with any type 603 /// conversions. 604 static bool shouldSpeculateInstrs(BasicBlock::iterator Begin, 605 BasicBlock::iterator End, Loop *L) { 606 bool seenIncrement = false; 607 bool MultiExitLoop = false; 608 609 if (!L->getExitingBlock()) 610 MultiExitLoop = true; 611 612 for (BasicBlock::iterator I = Begin; I != End; ++I) { 613 614 if (!isSafeToSpeculativelyExecute(&*I)) 615 return false; 616 617 if (isa<DbgInfoIntrinsic>(I)) 618 continue; 619 620 switch (I->getOpcode()) { 621 default: 622 return false; 623 case Instruction::GetElementPtr: 624 // GEPs are cheap if all indices are constant. 625 if (!cast<GEPOperator>(I)->hasAllConstantIndices()) 626 return false; 627 // fall-thru to increment case 628 LLVM_FALLTHROUGH; 629 case Instruction::Add: 630 case Instruction::Sub: 631 case Instruction::And: 632 case Instruction::Or: 633 case Instruction::Xor: 634 case Instruction::Shl: 635 case Instruction::LShr: 636 case Instruction::AShr: { 637 Value *IVOpnd = 638 !isa<Constant>(I->getOperand(0)) 639 ? I->getOperand(0) 640 : !isa<Constant>(I->getOperand(1)) ? I->getOperand(1) : nullptr; 641 if (!IVOpnd) 642 return false; 643 644 // If increment operand is used outside of the loop, this speculation 645 // could cause extra live range interference. 646 if (MultiExitLoop) { 647 for (User *UseI : IVOpnd->users()) { 648 auto *UserInst = cast<Instruction>(UseI); 649 if (!L->contains(UserInst)) 650 return false; 651 } 652 } 653 654 if (seenIncrement) 655 return false; 656 seenIncrement = true; 657 break; 658 } 659 case Instruction::Trunc: 660 case Instruction::ZExt: 661 case Instruction::SExt: 662 // ignore type conversions 663 break; 664 } 665 } 666 return true; 667 } 668 669 /// Fold the loop tail into the loop exit by speculating the loop tail 670 /// instructions. Typically, this is a single post-increment. In the case of a 671 /// simple 2-block loop, hoisting the increment can be much better than 672 /// duplicating the entire loop header. In the case of loops with early exits, 673 /// rotation will not work anyway, but simplifyLoopLatch will put the loop in 674 /// canonical form so downstream passes can handle it. 675 /// 676 /// I don't believe this invalidates SCEV. 677 bool LoopRotate::simplifyLoopLatch(Loop *L) { 678 BasicBlock *Latch = L->getLoopLatch(); 679 if (!Latch || Latch->hasAddressTaken()) 680 return false; 681 682 BranchInst *Jmp = dyn_cast<BranchInst>(Latch->getTerminator()); 683 if (!Jmp || !Jmp->isUnconditional()) 684 return false; 685 686 BasicBlock *LastExit = Latch->getSinglePredecessor(); 687 if (!LastExit || !L->isLoopExiting(LastExit)) 688 return false; 689 690 BranchInst *BI = dyn_cast<BranchInst>(LastExit->getTerminator()); 691 if (!BI) 692 return false; 693 694 if (!shouldSpeculateInstrs(Latch->begin(), Jmp->getIterator(), L)) 695 return false; 696 697 LLVM_DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into " 698 << LastExit->getName() << "\n"); 699 700 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); 701 MergeBlockIntoPredecessor(Latch, &DTU, LI, MSSAU, nullptr, 702 /*PredecessorWithTwoSuccessors=*/true); 703 704 if (MSSAU && VerifyMemorySSA) 705 MSSAU->getMemorySSA()->verifyMemorySSA(); 706 707 return true; 708 } 709 710 /// Rotate \c L, and return true if any modification was made. 711 bool LoopRotate::processLoop(Loop *L) { 712 // Save the loop metadata. 713 MDNode *LoopMD = L->getLoopID(); 714 715 bool SimplifiedLatch = false; 716 717 // Simplify the loop latch before attempting to rotate the header 718 // upward. Rotation may not be needed if the loop tail can be folded into the 719 // loop exit. 720 if (!RotationOnly) 721 SimplifiedLatch = simplifyLoopLatch(L); 722 723 bool MadeChange = rotateLoop(L, SimplifiedLatch); 724 assert((!MadeChange || L->isLoopExiting(L->getLoopLatch())) && 725 "Loop latch should be exiting after loop-rotate."); 726 727 // Restore the loop metadata. 728 // NB! We presume LoopRotation DOESN'T ADD its own metadata. 729 if ((MadeChange || SimplifiedLatch) && LoopMD) 730 L->setLoopID(LoopMD); 731 732 return MadeChange || SimplifiedLatch; 733 } 734 735 736 /// The utility to convert a loop into a loop with bottom test. 737 bool llvm::LoopRotation(Loop *L, LoopInfo *LI, const TargetTransformInfo *TTI, 738 AssumptionCache *AC, DominatorTree *DT, 739 ScalarEvolution *SE, MemorySSAUpdater *MSSAU, 740 const SimplifyQuery &SQ, bool RotationOnly = true, 741 unsigned Threshold = unsigned(-1), 742 bool IsUtilMode = true) { 743 if (MSSAU && VerifyMemorySSA) 744 MSSAU->getMemorySSA()->verifyMemorySSA(); 745 LoopRotate LR(Threshold, LI, TTI, AC, DT, SE, MSSAU, SQ, RotationOnly, 746 IsUtilMode); 747 if (MSSAU && VerifyMemorySSA) 748 MSSAU->getMemorySSA()->verifyMemorySSA(); 749 750 return LR.processLoop(L); 751 } 752