1 ///===- SimpleLoopUnswitch.cpp - Hoist loop-invariant control flow ---------===// 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 #include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h" 10 #include "llvm/ADT/DenseMap.h" 11 #include "llvm/ADT/STLExtras.h" 12 #include "llvm/ADT/Sequence.h" 13 #include "llvm/ADT/SetVector.h" 14 #include "llvm/ADT/SmallPtrSet.h" 15 #include "llvm/ADT/SmallVector.h" 16 #include "llvm/ADT/Statistic.h" 17 #include "llvm/ADT/Twine.h" 18 #include "llvm/Analysis/AssumptionCache.h" 19 #include "llvm/Analysis/BlockFrequencyInfo.h" 20 #include "llvm/Analysis/CFG.h" 21 #include "llvm/Analysis/CodeMetrics.h" 22 #include "llvm/Analysis/DomTreeUpdater.h" 23 #include "llvm/Analysis/GuardUtils.h" 24 #include "llvm/Analysis/LoopAnalysisManager.h" 25 #include "llvm/Analysis/LoopInfo.h" 26 #include "llvm/Analysis/LoopIterator.h" 27 #include "llvm/Analysis/MemorySSA.h" 28 #include "llvm/Analysis/MemorySSAUpdater.h" 29 #include "llvm/Analysis/MustExecute.h" 30 #include "llvm/Analysis/ProfileSummaryInfo.h" 31 #include "llvm/Analysis/ScalarEvolution.h" 32 #include "llvm/Analysis/TargetTransformInfo.h" 33 #include "llvm/Analysis/ValueTracking.h" 34 #include "llvm/IR/BasicBlock.h" 35 #include "llvm/IR/Constant.h" 36 #include "llvm/IR/Constants.h" 37 #include "llvm/IR/Dominators.h" 38 #include "llvm/IR/Function.h" 39 #include "llvm/IR/IRBuilder.h" 40 #include "llvm/IR/InstrTypes.h" 41 #include "llvm/IR/Instruction.h" 42 #include "llvm/IR/Instructions.h" 43 #include "llvm/IR/IntrinsicInst.h" 44 #include "llvm/IR/PatternMatch.h" 45 #include "llvm/IR/ProfDataUtils.h" 46 #include "llvm/IR/Use.h" 47 #include "llvm/IR/Value.h" 48 #include "llvm/Support/Casting.h" 49 #include "llvm/Support/CommandLine.h" 50 #include "llvm/Support/Debug.h" 51 #include "llvm/Support/ErrorHandling.h" 52 #include "llvm/Support/GenericDomTree.h" 53 #include "llvm/Support/InstructionCost.h" 54 #include "llvm/Support/raw_ostream.h" 55 #include "llvm/Transforms/Scalar/LoopPassManager.h" 56 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 57 #include "llvm/Transforms/Utils/Cloning.h" 58 #include "llvm/Transforms/Utils/Local.h" 59 #include "llvm/Transforms/Utils/LoopUtils.h" 60 #include "llvm/Transforms/Utils/ValueMapper.h" 61 #include <algorithm> 62 #include <cassert> 63 #include <iterator> 64 #include <numeric> 65 #include <optional> 66 #include <utility> 67 68 #define DEBUG_TYPE "simple-loop-unswitch" 69 70 using namespace llvm; 71 using namespace llvm::PatternMatch; 72 73 STATISTIC(NumBranches, "Number of branches unswitched"); 74 STATISTIC(NumSwitches, "Number of switches unswitched"); 75 STATISTIC(NumSelects, "Number of selects turned into branches for unswitching"); 76 STATISTIC(NumGuards, "Number of guards turned into branches for unswitching"); 77 STATISTIC(NumTrivial, "Number of unswitches that are trivial"); 78 STATISTIC( 79 NumCostMultiplierSkipped, 80 "Number of unswitch candidates that had their cost multiplier skipped"); 81 STATISTIC(NumInvariantConditionsInjected, 82 "Number of invariant conditions injected and unswitched"); 83 84 static cl::opt<bool> EnableNonTrivialUnswitch( 85 "enable-nontrivial-unswitch", cl::init(false), cl::Hidden, 86 cl::desc("Forcibly enables non-trivial loop unswitching rather than " 87 "following the configuration passed into the pass.")); 88 89 static cl::opt<int> 90 UnswitchThreshold("unswitch-threshold", cl::init(50), cl::Hidden, 91 cl::desc("The cost threshold for unswitching a loop.")); 92 93 static cl::opt<bool> EnableUnswitchCostMultiplier( 94 "enable-unswitch-cost-multiplier", cl::init(true), cl::Hidden, 95 cl::desc("Enable unswitch cost multiplier that prohibits exponential " 96 "explosion in nontrivial unswitch.")); 97 static cl::opt<int> UnswitchSiblingsToplevelDiv( 98 "unswitch-siblings-toplevel-div", cl::init(2), cl::Hidden, 99 cl::desc("Toplevel siblings divisor for cost multiplier.")); 100 static cl::opt<int> UnswitchNumInitialUnscaledCandidates( 101 "unswitch-num-initial-unscaled-candidates", cl::init(8), cl::Hidden, 102 cl::desc("Number of unswitch candidates that are ignored when calculating " 103 "cost multiplier.")); 104 static cl::opt<bool> UnswitchGuards( 105 "simple-loop-unswitch-guards", cl::init(true), cl::Hidden, 106 cl::desc("If enabled, simple loop unswitching will also consider " 107 "llvm.experimental.guard intrinsics as unswitch candidates.")); 108 static cl::opt<bool> DropNonTrivialImplicitNullChecks( 109 "simple-loop-unswitch-drop-non-trivial-implicit-null-checks", 110 cl::init(false), cl::Hidden, 111 cl::desc("If enabled, drop make.implicit metadata in unswitched implicit " 112 "null checks to save time analyzing if we can keep it.")); 113 static cl::opt<unsigned> 114 MSSAThreshold("simple-loop-unswitch-memoryssa-threshold", 115 cl::desc("Max number of memory uses to explore during " 116 "partial unswitching analysis"), 117 cl::init(100), cl::Hidden); 118 static cl::opt<bool> FreezeLoopUnswitchCond( 119 "freeze-loop-unswitch-cond", cl::init(true), cl::Hidden, 120 cl::desc("If enabled, the freeze instruction will be added to condition " 121 "of loop unswitch to prevent miscompilation.")); 122 123 static cl::opt<bool> InjectInvariantConditions( 124 "simple-loop-unswitch-inject-invariant-conditions", cl::Hidden, 125 cl::desc("Whether we should inject new invariants and unswitch them to " 126 "eliminate some existing (non-invariant) conditions."), 127 cl::init(true)); 128 129 static cl::opt<unsigned> InjectInvariantConditionHotnesThreshold( 130 "simple-loop-unswitch-inject-invariant-condition-hotness-threshold", 131 cl::Hidden, cl::desc("Only try to inject loop invariant conditions and " 132 "unswitch on them to eliminate branches that are " 133 "not-taken 1/<this option> times or less."), 134 cl::init(16)); 135 136 AnalysisKey ShouldRunExtraSimpleLoopUnswitch::Key; 137 namespace { 138 struct CompareDesc { 139 BranchInst *Term; 140 Value *Invariant; 141 BasicBlock *InLoopSucc; 142 143 CompareDesc(BranchInst *Term, Value *Invariant, BasicBlock *InLoopSucc) 144 : Term(Term), Invariant(Invariant), InLoopSucc(InLoopSucc) {} 145 }; 146 147 struct InjectedInvariant { 148 ICmpInst::Predicate Pred; 149 Value *LHS; 150 Value *RHS; 151 BasicBlock *InLoopSucc; 152 153 InjectedInvariant(ICmpInst::Predicate Pred, Value *LHS, Value *RHS, 154 BasicBlock *InLoopSucc) 155 : Pred(Pred), LHS(LHS), RHS(RHS), InLoopSucc(InLoopSucc) {} 156 }; 157 158 struct NonTrivialUnswitchCandidate { 159 Instruction *TI = nullptr; 160 TinyPtrVector<Value *> Invariants; 161 std::optional<InstructionCost> Cost; 162 std::optional<InjectedInvariant> PendingInjection; 163 NonTrivialUnswitchCandidate( 164 Instruction *TI, ArrayRef<Value *> Invariants, 165 std::optional<InstructionCost> Cost = std::nullopt, 166 std::optional<InjectedInvariant> PendingInjection = std::nullopt) 167 : TI(TI), Invariants(Invariants), Cost(Cost), 168 PendingInjection(PendingInjection) {}; 169 170 bool hasPendingInjection() const { return PendingInjection.has_value(); } 171 }; 172 } // end anonymous namespace. 173 174 // Helper to skip (select x, true, false), which matches both a logical AND and 175 // OR and can confuse code that tries to determine if \p Cond is either a 176 // logical AND or OR but not both. 177 static Value *skipTrivialSelect(Value *Cond) { 178 Value *CondNext; 179 while (match(Cond, m_Select(m_Value(CondNext), m_One(), m_Zero()))) 180 Cond = CondNext; 181 return Cond; 182 } 183 184 /// Collect all of the loop invariant input values transitively used by the 185 /// homogeneous instruction graph from a given root. 186 /// 187 /// This essentially walks from a root recursively through loop variant operands 188 /// which have perform the same logical operation (AND or OR) and finds all 189 /// inputs which are loop invariant. For some operations these can be 190 /// re-associated and unswitched out of the loop entirely. 191 static TinyPtrVector<Value *> 192 collectHomogenousInstGraphLoopInvariants(const Loop &L, Instruction &Root, 193 const LoopInfo &LI) { 194 assert(!L.isLoopInvariant(&Root) && 195 "Only need to walk the graph if root itself is not invariant."); 196 TinyPtrVector<Value *> Invariants; 197 198 bool IsRootAnd = match(&Root, m_LogicalAnd()); 199 bool IsRootOr = match(&Root, m_LogicalOr()); 200 201 // Build a worklist and recurse through operators collecting invariants. 202 SmallVector<Instruction *, 4> Worklist; 203 SmallPtrSet<Instruction *, 8> Visited; 204 Worklist.push_back(&Root); 205 Visited.insert(&Root); 206 do { 207 Instruction &I = *Worklist.pop_back_val(); 208 for (Value *OpV : I.operand_values()) { 209 // Skip constants as unswitching isn't interesting for them. 210 if (isa<Constant>(OpV)) 211 continue; 212 213 // Add it to our result if loop invariant. 214 if (L.isLoopInvariant(OpV)) { 215 Invariants.push_back(OpV); 216 continue; 217 } 218 219 // If not an instruction with the same opcode, nothing we can do. 220 Instruction *OpI = dyn_cast<Instruction>(skipTrivialSelect(OpV)); 221 222 if (OpI && ((IsRootAnd && match(OpI, m_LogicalAnd())) || 223 (IsRootOr && match(OpI, m_LogicalOr())))) { 224 // Visit this operand. 225 if (Visited.insert(OpI).second) 226 Worklist.push_back(OpI); 227 } 228 } 229 } while (!Worklist.empty()); 230 231 return Invariants; 232 } 233 234 static void replaceLoopInvariantUses(const Loop &L, Value *Invariant, 235 Constant &Replacement) { 236 assert(!isa<Constant>(Invariant) && "Why are we unswitching on a constant?"); 237 238 // Replace uses of LIC in the loop with the given constant. 239 // We use make_early_inc_range as set invalidates the iterator. 240 for (Use &U : llvm::make_early_inc_range(Invariant->uses())) { 241 Instruction *UserI = dyn_cast<Instruction>(U.getUser()); 242 243 // Replace this use within the loop body. 244 if (UserI && L.contains(UserI)) 245 U.set(&Replacement); 246 } 247 } 248 249 /// Check that all the LCSSA PHI nodes in the loop exit block have trivial 250 /// incoming values along this edge. 251 static bool areLoopExitPHIsLoopInvariant(const Loop &L, 252 const BasicBlock &ExitingBB, 253 const BasicBlock &ExitBB) { 254 for (const Instruction &I : ExitBB) { 255 auto *PN = dyn_cast<PHINode>(&I); 256 if (!PN) 257 // No more PHIs to check. 258 return true; 259 260 // If the incoming value for this edge isn't loop invariant the unswitch 261 // won't be trivial. 262 if (!L.isLoopInvariant(PN->getIncomingValueForBlock(&ExitingBB))) 263 return false; 264 } 265 llvm_unreachable("Basic blocks should never be empty!"); 266 } 267 268 /// Copy a set of loop invariant values \p ToDuplicate and insert them at the 269 /// end of \p BB and conditionally branch on the copied condition. We only 270 /// branch on a single value. 271 static void buildPartialUnswitchConditionalBranch( 272 BasicBlock &BB, ArrayRef<Value *> Invariants, bool Direction, 273 BasicBlock &UnswitchedSucc, BasicBlock &NormalSucc, bool InsertFreeze, 274 const Instruction *I, AssumptionCache *AC, const DominatorTree &DT) { 275 IRBuilder<> IRB(&BB); 276 277 SmallVector<Value *> FrozenInvariants; 278 for (Value *Inv : Invariants) { 279 if (InsertFreeze && !isGuaranteedNotToBeUndefOrPoison(Inv, AC, I, &DT)) 280 Inv = IRB.CreateFreeze(Inv, Inv->getName() + ".fr"); 281 FrozenInvariants.push_back(Inv); 282 } 283 284 Value *Cond = Direction ? IRB.CreateOr(FrozenInvariants) 285 : IRB.CreateAnd(FrozenInvariants); 286 IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc, 287 Direction ? &NormalSucc : &UnswitchedSucc); 288 } 289 290 /// Copy a set of loop invariant values, and conditionally branch on them. 291 static void buildPartialInvariantUnswitchConditionalBranch( 292 BasicBlock &BB, ArrayRef<Value *> ToDuplicate, bool Direction, 293 BasicBlock &UnswitchedSucc, BasicBlock &NormalSucc, Loop &L, 294 MemorySSAUpdater *MSSAU) { 295 ValueToValueMapTy VMap; 296 for (auto *Val : reverse(ToDuplicate)) { 297 Instruction *Inst = cast<Instruction>(Val); 298 Instruction *NewInst = Inst->clone(); 299 NewInst->insertInto(&BB, BB.end()); 300 RemapInstruction(NewInst, VMap, 301 RF_NoModuleLevelChanges | RF_IgnoreMissingLocals); 302 VMap[Val] = NewInst; 303 304 if (!MSSAU) 305 continue; 306 307 MemorySSA *MSSA = MSSAU->getMemorySSA(); 308 if (auto *MemUse = 309 dyn_cast_or_null<MemoryUse>(MSSA->getMemoryAccess(Inst))) { 310 auto *DefiningAccess = MemUse->getDefiningAccess(); 311 // Get the first defining access before the loop. 312 while (L.contains(DefiningAccess->getBlock())) { 313 // If the defining access is a MemoryPhi, get the incoming 314 // value for the pre-header as defining access. 315 if (auto *MemPhi = dyn_cast<MemoryPhi>(DefiningAccess)) 316 DefiningAccess = 317 MemPhi->getIncomingValueForBlock(L.getLoopPreheader()); 318 else 319 DefiningAccess = cast<MemoryDef>(DefiningAccess)->getDefiningAccess(); 320 } 321 MSSAU->createMemoryAccessInBB(NewInst, DefiningAccess, 322 NewInst->getParent(), 323 MemorySSA::BeforeTerminator); 324 } 325 } 326 327 IRBuilder<> IRB(&BB); 328 Value *Cond = VMap[ToDuplicate[0]]; 329 IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc, 330 Direction ? &NormalSucc : &UnswitchedSucc); 331 } 332 333 /// Rewrite the PHI nodes in an unswitched loop exit basic block. 334 /// 335 /// Requires that the loop exit and unswitched basic block are the same, and 336 /// that the exiting block was a unique predecessor of that block. Rewrites the 337 /// PHI nodes in that block such that what were LCSSA PHI nodes become trivial 338 /// PHI nodes from the old preheader that now contains the unswitched 339 /// terminator. 340 static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB, 341 BasicBlock &OldExitingBB, 342 BasicBlock &OldPH) { 343 for (PHINode &PN : UnswitchedBB.phis()) { 344 // When the loop exit is directly unswitched we just need to update the 345 // incoming basic block. We loop to handle weird cases with repeated 346 // incoming blocks, but expect to typically only have one operand here. 347 for (auto i : seq<int>(0, PN.getNumOperands())) { 348 assert(PN.getIncomingBlock(i) == &OldExitingBB && 349 "Found incoming block different from unique predecessor!"); 350 PN.setIncomingBlock(i, &OldPH); 351 } 352 } 353 } 354 355 /// Rewrite the PHI nodes in the loop exit basic block and the split off 356 /// unswitched block. 357 /// 358 /// Because the exit block remains an exit from the loop, this rewrites the 359 /// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI 360 /// nodes into the unswitched basic block to select between the value in the 361 /// old preheader and the loop exit. 362 static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB, 363 BasicBlock &UnswitchedBB, 364 BasicBlock &OldExitingBB, 365 BasicBlock &OldPH, 366 bool FullUnswitch) { 367 assert(&ExitBB != &UnswitchedBB && 368 "Must have different loop exit and unswitched blocks!"); 369 BasicBlock::iterator InsertPt = UnswitchedBB.begin(); 370 for (PHINode &PN : ExitBB.phis()) { 371 auto *NewPN = PHINode::Create(PN.getType(), /*NumReservedValues*/ 2, 372 PN.getName() + ".split"); 373 NewPN->insertBefore(InsertPt); 374 375 // Walk backwards over the old PHI node's inputs to minimize the cost of 376 // removing each one. We have to do this weird loop manually so that we 377 // create the same number of new incoming edges in the new PHI as we expect 378 // each case-based edge to be included in the unswitched switch in some 379 // cases. 380 // FIXME: This is really, really gross. It would be much cleaner if LLVM 381 // allowed us to create a single entry for a predecessor block without 382 // having separate entries for each "edge" even though these edges are 383 // required to produce identical results. 384 for (int i = PN.getNumIncomingValues() - 1; i >= 0; --i) { 385 if (PN.getIncomingBlock(i) != &OldExitingBB) 386 continue; 387 388 Value *Incoming = PN.getIncomingValue(i); 389 if (FullUnswitch) 390 // No more edge from the old exiting block to the exit block. 391 PN.removeIncomingValue(i); 392 393 NewPN->addIncoming(Incoming, &OldPH); 394 } 395 396 // Now replace the old PHI with the new one and wire the old one in as an 397 // input to the new one. 398 PN.replaceAllUsesWith(NewPN); 399 NewPN->addIncoming(&PN, &ExitBB); 400 } 401 } 402 403 /// Hoist the current loop up to the innermost loop containing a remaining exit. 404 /// 405 /// Because we've removed an exit from the loop, we may have changed the set of 406 /// loops reachable and need to move the current loop up the loop nest or even 407 /// to an entirely separate nest. 408 static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader, 409 DominatorTree &DT, LoopInfo &LI, 410 MemorySSAUpdater *MSSAU, ScalarEvolution *SE) { 411 // If the loop is already at the top level, we can't hoist it anywhere. 412 Loop *OldParentL = L.getParentLoop(); 413 if (!OldParentL) 414 return; 415 416 SmallVector<BasicBlock *, 4> Exits; 417 L.getExitBlocks(Exits); 418 Loop *NewParentL = nullptr; 419 for (auto *ExitBB : Exits) 420 if (Loop *ExitL = LI.getLoopFor(ExitBB)) 421 if (!NewParentL || NewParentL->contains(ExitL)) 422 NewParentL = ExitL; 423 424 if (NewParentL == OldParentL) 425 return; 426 427 // The new parent loop (if different) should always contain the old one. 428 if (NewParentL) 429 assert(NewParentL->contains(OldParentL) && 430 "Can only hoist this loop up the nest!"); 431 432 // The preheader will need to move with the body of this loop. However, 433 // because it isn't in this loop we also need to update the primary loop map. 434 assert(OldParentL == LI.getLoopFor(&Preheader) && 435 "Parent loop of this loop should contain this loop's preheader!"); 436 LI.changeLoopFor(&Preheader, NewParentL); 437 438 // Remove this loop from its old parent. 439 OldParentL->removeChildLoop(&L); 440 441 // Add the loop either to the new parent or as a top-level loop. 442 if (NewParentL) 443 NewParentL->addChildLoop(&L); 444 else 445 LI.addTopLevelLoop(&L); 446 447 // Remove this loops blocks from the old parent and every other loop up the 448 // nest until reaching the new parent. Also update all of these 449 // no-longer-containing loops to reflect the nesting change. 450 for (Loop *OldContainingL = OldParentL; OldContainingL != NewParentL; 451 OldContainingL = OldContainingL->getParentLoop()) { 452 llvm::erase_if(OldContainingL->getBlocksVector(), 453 [&](const BasicBlock *BB) { 454 return BB == &Preheader || L.contains(BB); 455 }); 456 457 OldContainingL->getBlocksSet().erase(&Preheader); 458 for (BasicBlock *BB : L.blocks()) 459 OldContainingL->getBlocksSet().erase(BB); 460 461 // Because we just hoisted a loop out of this one, we have essentially 462 // created new exit paths from it. That means we need to form LCSSA PHI 463 // nodes for values used in the no-longer-nested loop. 464 formLCSSA(*OldContainingL, DT, &LI, SE); 465 466 // We shouldn't need to form dedicated exits because the exit introduced 467 // here is the (just split by unswitching) preheader. However, after trivial 468 // unswitching it is possible to get new non-dedicated exits out of parent 469 // loop so let's conservatively form dedicated exit blocks and figure out 470 // if we can optimize later. 471 formDedicatedExitBlocks(OldContainingL, &DT, &LI, MSSAU, 472 /*PreserveLCSSA*/ true); 473 } 474 } 475 476 // Return the top-most loop containing ExitBB and having ExitBB as exiting block 477 // or the loop containing ExitBB, if there is no parent loop containing ExitBB 478 // as exiting block. 479 static Loop *getTopMostExitingLoop(const BasicBlock *ExitBB, 480 const LoopInfo &LI) { 481 Loop *TopMost = LI.getLoopFor(ExitBB); 482 Loop *Current = TopMost; 483 while (Current) { 484 if (Current->isLoopExiting(ExitBB)) 485 TopMost = Current; 486 Current = Current->getParentLoop(); 487 } 488 return TopMost; 489 } 490 491 /// Unswitch a trivial branch if the condition is loop invariant. 492 /// 493 /// This routine should only be called when loop code leading to the branch has 494 /// been validated as trivial (no side effects). This routine checks if the 495 /// condition is invariant and one of the successors is a loop exit. This 496 /// allows us to unswitch without duplicating the loop, making it trivial. 497 /// 498 /// If this routine fails to unswitch the branch it returns false. 499 /// 500 /// If the branch can be unswitched, this routine splits the preheader and 501 /// hoists the branch above that split. Preserves loop simplified form 502 /// (splitting the exit block as necessary). It simplifies the branch within 503 /// the loop to an unconditional branch but doesn't remove it entirely. Further 504 /// cleanup can be done with some simplifycfg like pass. 505 /// 506 /// If `SE` is not null, it will be updated based on the potential loop SCEVs 507 /// invalidated by this. 508 static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT, 509 LoopInfo &LI, ScalarEvolution *SE, 510 MemorySSAUpdater *MSSAU) { 511 assert(BI.isConditional() && "Can only unswitch a conditional branch!"); 512 LLVM_DEBUG(dbgs() << " Trying to unswitch branch: " << BI << "\n"); 513 514 // The loop invariant values that we want to unswitch. 515 TinyPtrVector<Value *> Invariants; 516 517 // When true, we're fully unswitching the branch rather than just unswitching 518 // some input conditions to the branch. 519 bool FullUnswitch = false; 520 521 Value *Cond = skipTrivialSelect(BI.getCondition()); 522 if (L.isLoopInvariant(Cond)) { 523 Invariants.push_back(Cond); 524 FullUnswitch = true; 525 } else { 526 if (auto *CondInst = dyn_cast<Instruction>(Cond)) 527 Invariants = collectHomogenousInstGraphLoopInvariants(L, *CondInst, LI); 528 if (Invariants.empty()) { 529 LLVM_DEBUG(dbgs() << " Couldn't find invariant inputs!\n"); 530 return false; 531 } 532 } 533 534 // Check that one of the branch's successors exits, and which one. 535 bool ExitDirection = true; 536 int LoopExitSuccIdx = 0; 537 auto *LoopExitBB = BI.getSuccessor(0); 538 if (L.contains(LoopExitBB)) { 539 ExitDirection = false; 540 LoopExitSuccIdx = 1; 541 LoopExitBB = BI.getSuccessor(1); 542 if (L.contains(LoopExitBB)) { 543 LLVM_DEBUG(dbgs() << " Branch doesn't exit the loop!\n"); 544 return false; 545 } 546 } 547 auto *ContinueBB = BI.getSuccessor(1 - LoopExitSuccIdx); 548 auto *ParentBB = BI.getParent(); 549 if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, *LoopExitBB)) { 550 LLVM_DEBUG(dbgs() << " Loop exit PHI's aren't loop-invariant!\n"); 551 return false; 552 } 553 554 // When unswitching only part of the branch's condition, we need the exit 555 // block to be reached directly from the partially unswitched input. This can 556 // be done when the exit block is along the true edge and the branch condition 557 // is a graph of `or` operations, or the exit block is along the false edge 558 // and the condition is a graph of `and` operations. 559 if (!FullUnswitch) { 560 if (ExitDirection ? !match(Cond, m_LogicalOr()) 561 : !match(Cond, m_LogicalAnd())) { 562 LLVM_DEBUG(dbgs() << " Branch condition is in improper form for " 563 "non-full unswitch!\n"); 564 return false; 565 } 566 } 567 568 LLVM_DEBUG({ 569 dbgs() << " unswitching trivial invariant conditions for: " << BI 570 << "\n"; 571 for (Value *Invariant : Invariants) { 572 dbgs() << " " << *Invariant << " == true"; 573 if (Invariant != Invariants.back()) 574 dbgs() << " ||"; 575 dbgs() << "\n"; 576 } 577 }); 578 579 // If we have scalar evolutions, we need to invalidate them including this 580 // loop, the loop containing the exit block and the topmost parent loop 581 // exiting via LoopExitBB. 582 if (SE) { 583 if (const Loop *ExitL = getTopMostExitingLoop(LoopExitBB, LI)) 584 SE->forgetLoop(ExitL); 585 else 586 // Forget the entire nest as this exits the entire nest. 587 SE->forgetTopmostLoop(&L); 588 SE->forgetBlockAndLoopDispositions(); 589 } 590 591 if (MSSAU && VerifyMemorySSA) 592 MSSAU->getMemorySSA()->verifyMemorySSA(); 593 594 // Split the preheader, so that we know that there is a safe place to insert 595 // the conditional branch. We will change the preheader to have a conditional 596 // branch on LoopCond. 597 BasicBlock *OldPH = L.getLoopPreheader(); 598 BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU); 599 600 // Now that we have a place to insert the conditional branch, create a place 601 // to branch to: this is the exit block out of the loop that we are 602 // unswitching. We need to split this if there are other loop predecessors. 603 // Because the loop is in simplified form, *any* other predecessor is enough. 604 BasicBlock *UnswitchedBB; 605 if (FullUnswitch && LoopExitBB->getUniquePredecessor()) { 606 assert(LoopExitBB->getUniquePredecessor() == BI.getParent() && 607 "A branch's parent isn't a predecessor!"); 608 UnswitchedBB = LoopExitBB; 609 } else { 610 UnswitchedBB = 611 SplitBlock(LoopExitBB, LoopExitBB->begin(), &DT, &LI, MSSAU, "", false); 612 } 613 614 if (MSSAU && VerifyMemorySSA) 615 MSSAU->getMemorySSA()->verifyMemorySSA(); 616 617 // Actually move the invariant uses into the unswitched position. If possible, 618 // we do this by moving the instructions, but when doing partial unswitching 619 // we do it by building a new merge of the values in the unswitched position. 620 OldPH->getTerminator()->eraseFromParent(); 621 if (FullUnswitch) { 622 // If fully unswitching, we can use the existing branch instruction. 623 // Splice it into the old PH to gate reaching the new preheader and re-point 624 // its successors. 625 BI.moveBefore(*OldPH, OldPH->end()); 626 BI.setCondition(Cond); 627 if (MSSAU) { 628 // Temporarily clone the terminator, to make MSSA update cheaper by 629 // separating "insert edge" updates from "remove edge" ones. 630 BI.clone()->insertInto(ParentBB, ParentBB->end()); 631 } else { 632 // Create a new unconditional branch that will continue the loop as a new 633 // terminator. 634 BranchInst::Create(ContinueBB, ParentBB); 635 } 636 BI.setSuccessor(LoopExitSuccIdx, UnswitchedBB); 637 BI.setSuccessor(1 - LoopExitSuccIdx, NewPH); 638 } else { 639 // Only unswitching a subset of inputs to the condition, so we will need to 640 // build a new branch that merges the invariant inputs. 641 if (ExitDirection) 642 assert(match(skipTrivialSelect(BI.getCondition()), m_LogicalOr()) && 643 "Must have an `or` of `i1`s or `select i1 X, true, Y`s for the " 644 "condition!"); 645 else 646 assert(match(skipTrivialSelect(BI.getCondition()), m_LogicalAnd()) && 647 "Must have an `and` of `i1`s or `select i1 X, Y, false`s for the" 648 " condition!"); 649 buildPartialUnswitchConditionalBranch( 650 *OldPH, Invariants, ExitDirection, *UnswitchedBB, *NewPH, 651 FreezeLoopUnswitchCond, OldPH->getTerminator(), nullptr, DT); 652 } 653 654 // Update the dominator tree with the added edge. 655 DT.insertEdge(OldPH, UnswitchedBB); 656 657 // After the dominator tree was updated with the added edge, update MemorySSA 658 // if available. 659 if (MSSAU) { 660 SmallVector<CFGUpdate, 1> Updates; 661 Updates.push_back({cfg::UpdateKind::Insert, OldPH, UnswitchedBB}); 662 MSSAU->applyInsertUpdates(Updates, DT); 663 } 664 665 // Finish updating dominator tree and memory ssa for full unswitch. 666 if (FullUnswitch) { 667 if (MSSAU) { 668 // Remove the cloned branch instruction. 669 ParentBB->getTerminator()->eraseFromParent(); 670 // Create unconditional branch now. 671 BranchInst::Create(ContinueBB, ParentBB); 672 MSSAU->removeEdge(ParentBB, LoopExitBB); 673 } 674 DT.deleteEdge(ParentBB, LoopExitBB); 675 } 676 677 if (MSSAU && VerifyMemorySSA) 678 MSSAU->getMemorySSA()->verifyMemorySSA(); 679 680 // Rewrite the relevant PHI nodes. 681 if (UnswitchedBB == LoopExitBB) 682 rewritePHINodesForUnswitchedExitBlock(*UnswitchedBB, *ParentBB, *OldPH); 683 else 684 rewritePHINodesForExitAndUnswitchedBlocks(*LoopExitBB, *UnswitchedBB, 685 *ParentBB, *OldPH, FullUnswitch); 686 687 // The constant we can replace all of our invariants with inside the loop 688 // body. If any of the invariants have a value other than this the loop won't 689 // be entered. 690 ConstantInt *Replacement = ExitDirection 691 ? ConstantInt::getFalse(BI.getContext()) 692 : ConstantInt::getTrue(BI.getContext()); 693 694 // Since this is an i1 condition we can also trivially replace uses of it 695 // within the loop with a constant. 696 for (Value *Invariant : Invariants) 697 replaceLoopInvariantUses(L, Invariant, *Replacement); 698 699 // If this was full unswitching, we may have changed the nesting relationship 700 // for this loop so hoist it to its correct parent if needed. 701 if (FullUnswitch) 702 hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU, SE); 703 704 if (MSSAU && VerifyMemorySSA) 705 MSSAU->getMemorySSA()->verifyMemorySSA(); 706 707 LLVM_DEBUG(dbgs() << " done: unswitching trivial branch...\n"); 708 ++NumTrivial; 709 ++NumBranches; 710 return true; 711 } 712 713 /// Unswitch a trivial switch if the condition is loop invariant. 714 /// 715 /// This routine should only be called when loop code leading to the switch has 716 /// been validated as trivial (no side effects). This routine checks if the 717 /// condition is invariant and that at least one of the successors is a loop 718 /// exit. This allows us to unswitch without duplicating the loop, making it 719 /// trivial. 720 /// 721 /// If this routine fails to unswitch the switch it returns false. 722 /// 723 /// If the switch can be unswitched, this routine splits the preheader and 724 /// copies the switch above that split. If the default case is one of the 725 /// exiting cases, it copies the non-exiting cases and points them at the new 726 /// preheader. If the default case is not exiting, it copies the exiting cases 727 /// and points the default at the preheader. It preserves loop simplified form 728 /// (splitting the exit blocks as necessary). It simplifies the switch within 729 /// the loop by removing now-dead cases. If the default case is one of those 730 /// unswitched, it replaces its destination with a new basic block containing 731 /// only unreachable. Such basic blocks, while technically loop exits, are not 732 /// considered for unswitching so this is a stable transform and the same 733 /// switch will not be revisited. If after unswitching there is only a single 734 /// in-loop successor, the switch is further simplified to an unconditional 735 /// branch. Still more cleanup can be done with some simplifycfg like pass. 736 /// 737 /// If `SE` is not null, it will be updated based on the potential loop SCEVs 738 /// invalidated by this. 739 static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT, 740 LoopInfo &LI, ScalarEvolution *SE, 741 MemorySSAUpdater *MSSAU) { 742 LLVM_DEBUG(dbgs() << " Trying to unswitch switch: " << SI << "\n"); 743 Value *LoopCond = SI.getCondition(); 744 745 // If this isn't switching on an invariant condition, we can't unswitch it. 746 if (!L.isLoopInvariant(LoopCond)) 747 return false; 748 749 auto *ParentBB = SI.getParent(); 750 751 // The same check must be used both for the default and the exit cases. We 752 // should never leave edges from the switch instruction to a basic block that 753 // we are unswitching, hence the condition used to determine the default case 754 // needs to also be used to populate ExitCaseIndices, which is then used to 755 // remove cases from the switch. 756 auto IsTriviallyUnswitchableExitBlock = [&](BasicBlock &BBToCheck) { 757 // BBToCheck is not an exit block if it is inside loop L. 758 if (L.contains(&BBToCheck)) 759 return false; 760 // BBToCheck is not trivial to unswitch if its phis aren't loop invariant. 761 if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, BBToCheck)) 762 return false; 763 // We do not unswitch a block that only has an unreachable statement, as 764 // it's possible this is a previously unswitched block. Only unswitch if 765 // either the terminator is not unreachable, or, if it is, it's not the only 766 // instruction in the block. 767 auto *TI = BBToCheck.getTerminator(); 768 bool isUnreachable = isa<UnreachableInst>(TI); 769 return !isUnreachable || 770 (isUnreachable && (BBToCheck.getFirstNonPHIOrDbg() != TI)); 771 }; 772 773 SmallVector<int, 4> ExitCaseIndices; 774 for (auto Case : SI.cases()) 775 if (IsTriviallyUnswitchableExitBlock(*Case.getCaseSuccessor())) 776 ExitCaseIndices.push_back(Case.getCaseIndex()); 777 BasicBlock *DefaultExitBB = nullptr; 778 SwitchInstProfUpdateWrapper::CaseWeightOpt DefaultCaseWeight = 779 SwitchInstProfUpdateWrapper::getSuccessorWeight(SI, 0); 780 if (IsTriviallyUnswitchableExitBlock(*SI.getDefaultDest())) { 781 DefaultExitBB = SI.getDefaultDest(); 782 } else if (ExitCaseIndices.empty()) 783 return false; 784 785 LLVM_DEBUG(dbgs() << " unswitching trivial switch...\n"); 786 787 if (MSSAU && VerifyMemorySSA) 788 MSSAU->getMemorySSA()->verifyMemorySSA(); 789 790 // We may need to invalidate SCEVs for the outermost loop reached by any of 791 // the exits. 792 Loop *OuterL = &L; 793 794 if (DefaultExitBB) { 795 // Check the loop containing this exit. 796 Loop *ExitL = getTopMostExitingLoop(DefaultExitBB, LI); 797 if (!ExitL || ExitL->contains(OuterL)) 798 OuterL = ExitL; 799 } 800 for (unsigned Index : ExitCaseIndices) { 801 auto CaseI = SI.case_begin() + Index; 802 // Compute the outer loop from this exit. 803 Loop *ExitL = getTopMostExitingLoop(CaseI->getCaseSuccessor(), LI); 804 if (!ExitL || ExitL->contains(OuterL)) 805 OuterL = ExitL; 806 } 807 808 if (SE) { 809 if (OuterL) 810 SE->forgetLoop(OuterL); 811 else 812 SE->forgetTopmostLoop(&L); 813 } 814 815 if (DefaultExitBB) { 816 // Clear out the default destination temporarily to allow accurate 817 // predecessor lists to be examined below. 818 SI.setDefaultDest(nullptr); 819 } 820 821 // Store the exit cases into a separate data structure and remove them from 822 // the switch. 823 SmallVector<std::tuple<ConstantInt *, BasicBlock *, 824 SwitchInstProfUpdateWrapper::CaseWeightOpt>, 825 4> ExitCases; 826 ExitCases.reserve(ExitCaseIndices.size()); 827 SwitchInstProfUpdateWrapper SIW(SI); 828 // We walk the case indices backwards so that we remove the last case first 829 // and don't disrupt the earlier indices. 830 for (unsigned Index : reverse(ExitCaseIndices)) { 831 auto CaseI = SI.case_begin() + Index; 832 // Save the value of this case. 833 auto W = SIW.getSuccessorWeight(CaseI->getSuccessorIndex()); 834 ExitCases.emplace_back(CaseI->getCaseValue(), CaseI->getCaseSuccessor(), W); 835 // Delete the unswitched cases. 836 SIW.removeCase(CaseI); 837 } 838 839 // Check if after this all of the remaining cases point at the same 840 // successor. 841 BasicBlock *CommonSuccBB = nullptr; 842 if (SI.getNumCases() > 0 && 843 all_of(drop_begin(SI.cases()), [&SI](const SwitchInst::CaseHandle &Case) { 844 return Case.getCaseSuccessor() == SI.case_begin()->getCaseSuccessor(); 845 })) 846 CommonSuccBB = SI.case_begin()->getCaseSuccessor(); 847 if (!DefaultExitBB) { 848 // If we're not unswitching the default, we need it to match any cases to 849 // have a common successor or if we have no cases it is the common 850 // successor. 851 if (SI.getNumCases() == 0) 852 CommonSuccBB = SI.getDefaultDest(); 853 else if (SI.getDefaultDest() != CommonSuccBB) 854 CommonSuccBB = nullptr; 855 } 856 857 // Split the preheader, so that we know that there is a safe place to insert 858 // the switch. 859 BasicBlock *OldPH = L.getLoopPreheader(); 860 BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU); 861 OldPH->getTerminator()->eraseFromParent(); 862 863 // Now add the unswitched switch. 864 auto *NewSI = SwitchInst::Create(LoopCond, NewPH, ExitCases.size(), OldPH); 865 SwitchInstProfUpdateWrapper NewSIW(*NewSI); 866 867 // Rewrite the IR for the unswitched basic blocks. This requires two steps. 868 // First, we split any exit blocks with remaining in-loop predecessors. Then 869 // we update the PHIs in one of two ways depending on if there was a split. 870 // We walk in reverse so that we split in the same order as the cases 871 // appeared. This is purely for convenience of reading the resulting IR, but 872 // it doesn't cost anything really. 873 SmallPtrSet<BasicBlock *, 2> UnswitchedExitBBs; 874 SmallDenseMap<BasicBlock *, BasicBlock *, 2> SplitExitBBMap; 875 // Handle the default exit if necessary. 876 // FIXME: It'd be great if we could merge this with the loop below but LLVM's 877 // ranges aren't quite powerful enough yet. 878 if (DefaultExitBB) { 879 if (pred_empty(DefaultExitBB)) { 880 UnswitchedExitBBs.insert(DefaultExitBB); 881 rewritePHINodesForUnswitchedExitBlock(*DefaultExitBB, *ParentBB, *OldPH); 882 } else { 883 auto *SplitBB = 884 SplitBlock(DefaultExitBB, DefaultExitBB->begin(), &DT, &LI, MSSAU); 885 rewritePHINodesForExitAndUnswitchedBlocks(*DefaultExitBB, *SplitBB, 886 *ParentBB, *OldPH, 887 /*FullUnswitch*/ true); 888 DefaultExitBB = SplitExitBBMap[DefaultExitBB] = SplitBB; 889 } 890 } 891 // Note that we must use a reference in the for loop so that we update the 892 // container. 893 for (auto &ExitCase : reverse(ExitCases)) { 894 // Grab a reference to the exit block in the pair so that we can update it. 895 BasicBlock *ExitBB = std::get<1>(ExitCase); 896 897 // If this case is the last edge into the exit block, we can simply reuse it 898 // as it will no longer be a loop exit. No mapping necessary. 899 if (pred_empty(ExitBB)) { 900 // Only rewrite once. 901 if (UnswitchedExitBBs.insert(ExitBB).second) 902 rewritePHINodesForUnswitchedExitBlock(*ExitBB, *ParentBB, *OldPH); 903 continue; 904 } 905 906 // Otherwise we need to split the exit block so that we retain an exit 907 // block from the loop and a target for the unswitched condition. 908 BasicBlock *&SplitExitBB = SplitExitBBMap[ExitBB]; 909 if (!SplitExitBB) { 910 // If this is the first time we see this, do the split and remember it. 911 SplitExitBB = SplitBlock(ExitBB, ExitBB->begin(), &DT, &LI, MSSAU); 912 rewritePHINodesForExitAndUnswitchedBlocks(*ExitBB, *SplitExitBB, 913 *ParentBB, *OldPH, 914 /*FullUnswitch*/ true); 915 } 916 // Update the case pair to point to the split block. 917 std::get<1>(ExitCase) = SplitExitBB; 918 } 919 920 // Now add the unswitched cases. We do this in reverse order as we built them 921 // in reverse order. 922 for (auto &ExitCase : reverse(ExitCases)) { 923 ConstantInt *CaseVal = std::get<0>(ExitCase); 924 BasicBlock *UnswitchedBB = std::get<1>(ExitCase); 925 926 NewSIW.addCase(CaseVal, UnswitchedBB, std::get<2>(ExitCase)); 927 } 928 929 // If the default was unswitched, re-point it and add explicit cases for 930 // entering the loop. 931 if (DefaultExitBB) { 932 NewSIW->setDefaultDest(DefaultExitBB); 933 NewSIW.setSuccessorWeight(0, DefaultCaseWeight); 934 935 // We removed all the exit cases, so we just copy the cases to the 936 // unswitched switch. 937 for (const auto &Case : SI.cases()) 938 NewSIW.addCase(Case.getCaseValue(), NewPH, 939 SIW.getSuccessorWeight(Case.getSuccessorIndex())); 940 } else if (DefaultCaseWeight) { 941 // We have to set branch weight of the default case. 942 uint64_t SW = *DefaultCaseWeight; 943 for (const auto &Case : SI.cases()) { 944 auto W = SIW.getSuccessorWeight(Case.getSuccessorIndex()); 945 assert(W && 946 "case weight must be defined as default case weight is defined"); 947 SW += *W; 948 } 949 NewSIW.setSuccessorWeight(0, SW); 950 } 951 952 // If we ended up with a common successor for every path through the switch 953 // after unswitching, rewrite it to an unconditional branch to make it easy 954 // to recognize. Otherwise we potentially have to recognize the default case 955 // pointing at unreachable and other complexity. 956 if (CommonSuccBB) { 957 BasicBlock *BB = SI.getParent(); 958 // We may have had multiple edges to this common successor block, so remove 959 // them as predecessors. We skip the first one, either the default or the 960 // actual first case. 961 bool SkippedFirst = DefaultExitBB == nullptr; 962 for (auto Case : SI.cases()) { 963 assert(Case.getCaseSuccessor() == CommonSuccBB && 964 "Non-common successor!"); 965 (void)Case; 966 if (!SkippedFirst) { 967 SkippedFirst = true; 968 continue; 969 } 970 CommonSuccBB->removePredecessor(BB, 971 /*KeepOneInputPHIs*/ true); 972 } 973 // Now nuke the switch and replace it with a direct branch. 974 SIW.eraseFromParent(); 975 BranchInst::Create(CommonSuccBB, BB); 976 } else if (DefaultExitBB) { 977 assert(SI.getNumCases() > 0 && 978 "If we had no cases we'd have a common successor!"); 979 // Move the last case to the default successor. This is valid as if the 980 // default got unswitched it cannot be reached. This has the advantage of 981 // being simple and keeping the number of edges from this switch to 982 // successors the same, and avoiding any PHI update complexity. 983 auto LastCaseI = std::prev(SI.case_end()); 984 985 SI.setDefaultDest(LastCaseI->getCaseSuccessor()); 986 SIW.setSuccessorWeight( 987 0, SIW.getSuccessorWeight(LastCaseI->getSuccessorIndex())); 988 SIW.removeCase(LastCaseI); 989 } 990 991 // Walk the unswitched exit blocks and the unswitched split blocks and update 992 // the dominator tree based on the CFG edits. While we are walking unordered 993 // containers here, the API for applyUpdates takes an unordered list of 994 // updates and requires them to not contain duplicates. 995 SmallVector<DominatorTree::UpdateType, 4> DTUpdates; 996 for (auto *UnswitchedExitBB : UnswitchedExitBBs) { 997 DTUpdates.push_back({DT.Delete, ParentBB, UnswitchedExitBB}); 998 DTUpdates.push_back({DT.Insert, OldPH, UnswitchedExitBB}); 999 } 1000 for (auto SplitUnswitchedPair : SplitExitBBMap) { 1001 DTUpdates.push_back({DT.Delete, ParentBB, SplitUnswitchedPair.first}); 1002 DTUpdates.push_back({DT.Insert, OldPH, SplitUnswitchedPair.second}); 1003 } 1004 1005 if (MSSAU) { 1006 MSSAU->applyUpdates(DTUpdates, DT, /*UpdateDT=*/true); 1007 if (VerifyMemorySSA) 1008 MSSAU->getMemorySSA()->verifyMemorySSA(); 1009 } else { 1010 DT.applyUpdates(DTUpdates); 1011 } 1012 1013 assert(DT.verify(DominatorTree::VerificationLevel::Fast)); 1014 1015 // We may have changed the nesting relationship for this loop so hoist it to 1016 // its correct parent if needed. 1017 hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU, SE); 1018 1019 if (MSSAU && VerifyMemorySSA) 1020 MSSAU->getMemorySSA()->verifyMemorySSA(); 1021 1022 ++NumTrivial; 1023 ++NumSwitches; 1024 LLVM_DEBUG(dbgs() << " done: unswitching trivial switch...\n"); 1025 return true; 1026 } 1027 1028 /// This routine scans the loop to find a branch or switch which occurs before 1029 /// any side effects occur. These can potentially be unswitched without 1030 /// duplicating the loop. If a branch or switch is successfully unswitched the 1031 /// scanning continues to see if subsequent branches or switches have become 1032 /// trivial. Once all trivial candidates have been unswitched, this routine 1033 /// returns. 1034 /// 1035 /// The return value indicates whether anything was unswitched (and therefore 1036 /// changed). 1037 /// 1038 /// If `SE` is not null, it will be updated based on the potential loop SCEVs 1039 /// invalidated by this. 1040 static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT, 1041 LoopInfo &LI, ScalarEvolution *SE, 1042 MemorySSAUpdater *MSSAU) { 1043 bool Changed = false; 1044 1045 // If loop header has only one reachable successor we should keep looking for 1046 // trivial condition candidates in the successor as well. An alternative is 1047 // to constant fold conditions and merge successors into loop header (then we 1048 // only need to check header's terminator). The reason for not doing this in 1049 // LoopUnswitch pass is that it could potentially break LoopPassManager's 1050 // invariants. Folding dead branches could either eliminate the current loop 1051 // or make other loops unreachable. LCSSA form might also not be preserved 1052 // after deleting branches. The following code keeps traversing loop header's 1053 // successors until it finds the trivial condition candidate (condition that 1054 // is not a constant). Since unswitching generates branches with constant 1055 // conditions, this scenario could be very common in practice. 1056 BasicBlock *CurrentBB = L.getHeader(); 1057 SmallPtrSet<BasicBlock *, 8> Visited; 1058 Visited.insert(CurrentBB); 1059 do { 1060 // Check if there are any side-effecting instructions (e.g. stores, calls, 1061 // volatile loads) in the part of the loop that the code *would* execute 1062 // without unswitching. 1063 if (MSSAU) // Possible early exit with MSSA 1064 if (auto *Defs = MSSAU->getMemorySSA()->getBlockDefs(CurrentBB)) 1065 if (!isa<MemoryPhi>(*Defs->begin()) || (++Defs->begin() != Defs->end())) 1066 return Changed; 1067 if (llvm::any_of(*CurrentBB, 1068 [](Instruction &I) { return I.mayHaveSideEffects(); })) 1069 return Changed; 1070 1071 Instruction *CurrentTerm = CurrentBB->getTerminator(); 1072 1073 if (auto *SI = dyn_cast<SwitchInst>(CurrentTerm)) { 1074 // Don't bother trying to unswitch past a switch with a constant 1075 // condition. This should be removed prior to running this pass by 1076 // simplifycfg. 1077 if (isa<Constant>(SI->getCondition())) 1078 return Changed; 1079 1080 if (!unswitchTrivialSwitch(L, *SI, DT, LI, SE, MSSAU)) 1081 // Couldn't unswitch this one so we're done. 1082 return Changed; 1083 1084 // Mark that we managed to unswitch something. 1085 Changed = true; 1086 1087 // If unswitching turned the terminator into an unconditional branch then 1088 // we can continue. The unswitching logic specifically works to fold any 1089 // cases it can into an unconditional branch to make it easier to 1090 // recognize here. 1091 auto *BI = dyn_cast<BranchInst>(CurrentBB->getTerminator()); 1092 if (!BI || BI->isConditional()) 1093 return Changed; 1094 1095 CurrentBB = BI->getSuccessor(0); 1096 continue; 1097 } 1098 1099 auto *BI = dyn_cast<BranchInst>(CurrentTerm); 1100 if (!BI) 1101 // We do not understand other terminator instructions. 1102 return Changed; 1103 1104 // Don't bother trying to unswitch past an unconditional branch or a branch 1105 // with a constant value. These should be removed by simplifycfg prior to 1106 // running this pass. 1107 if (!BI->isConditional() || 1108 isa<Constant>(skipTrivialSelect(BI->getCondition()))) 1109 return Changed; 1110 1111 // Found a trivial condition candidate: non-foldable conditional branch. If 1112 // we fail to unswitch this, we can't do anything else that is trivial. 1113 if (!unswitchTrivialBranch(L, *BI, DT, LI, SE, MSSAU)) 1114 return Changed; 1115 1116 // Mark that we managed to unswitch something. 1117 Changed = true; 1118 1119 // If we only unswitched some of the conditions feeding the branch, we won't 1120 // have collapsed it to a single successor. 1121 BI = cast<BranchInst>(CurrentBB->getTerminator()); 1122 if (BI->isConditional()) 1123 return Changed; 1124 1125 // Follow the newly unconditional branch into its successor. 1126 CurrentBB = BI->getSuccessor(0); 1127 1128 // When continuing, if we exit the loop or reach a previous visited block, 1129 // then we can not reach any trivial condition candidates (unfoldable 1130 // branch instructions or switch instructions) and no unswitch can happen. 1131 } while (L.contains(CurrentBB) && Visited.insert(CurrentBB).second); 1132 1133 return Changed; 1134 } 1135 1136 /// Build the cloned blocks for an unswitched copy of the given loop. 1137 /// 1138 /// The cloned blocks are inserted before the loop preheader (`LoopPH`) and 1139 /// after the split block (`SplitBB`) that will be used to select between the 1140 /// cloned and original loop. 1141 /// 1142 /// This routine handles cloning all of the necessary loop blocks and exit 1143 /// blocks including rewriting their instructions and the relevant PHI nodes. 1144 /// Any loop blocks or exit blocks which are dominated by a different successor 1145 /// than the one for this clone of the loop blocks can be trivially skipped. We 1146 /// use the `DominatingSucc` map to determine whether a block satisfies that 1147 /// property with a simple map lookup. 1148 /// 1149 /// It also correctly creates the unconditional branch in the cloned 1150 /// unswitched parent block to only point at the unswitched successor. 1151 /// 1152 /// This does not handle most of the necessary updates to `LoopInfo`. Only exit 1153 /// block splitting is correctly reflected in `LoopInfo`, essentially all of 1154 /// the cloned blocks (and their loops) are left without full `LoopInfo` 1155 /// updates. This also doesn't fully update `DominatorTree`. It adds the cloned 1156 /// blocks to them but doesn't create the cloned `DominatorTree` structure and 1157 /// instead the caller must recompute an accurate DT. It *does* correctly 1158 /// update the `AssumptionCache` provided in `AC`. 1159 static BasicBlock *buildClonedLoopBlocks( 1160 Loop &L, BasicBlock *LoopPH, BasicBlock *SplitBB, 1161 ArrayRef<BasicBlock *> ExitBlocks, BasicBlock *ParentBB, 1162 BasicBlock *UnswitchedSuccBB, BasicBlock *ContinueSuccBB, 1163 const SmallDenseMap<BasicBlock *, BasicBlock *, 16> &DominatingSucc, 1164 ValueToValueMapTy &VMap, 1165 SmallVectorImpl<DominatorTree::UpdateType> &DTUpdates, AssumptionCache &AC, 1166 DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU, 1167 ScalarEvolution *SE) { 1168 SmallVector<BasicBlock *, 4> NewBlocks; 1169 NewBlocks.reserve(L.getNumBlocks() + ExitBlocks.size()); 1170 1171 // We will need to clone a bunch of blocks, wrap up the clone operation in 1172 // a helper. 1173 auto CloneBlock = [&](BasicBlock *OldBB) { 1174 // Clone the basic block and insert it before the new preheader. 1175 BasicBlock *NewBB = CloneBasicBlock(OldBB, VMap, ".us", OldBB->getParent()); 1176 NewBB->moveBefore(LoopPH); 1177 1178 // Record this block and the mapping. 1179 NewBlocks.push_back(NewBB); 1180 VMap[OldBB] = NewBB; 1181 1182 return NewBB; 1183 }; 1184 1185 // We skip cloning blocks when they have a dominating succ that is not the 1186 // succ we are cloning for. 1187 auto SkipBlock = [&](BasicBlock *BB) { 1188 auto It = DominatingSucc.find(BB); 1189 return It != DominatingSucc.end() && It->second != UnswitchedSuccBB; 1190 }; 1191 1192 // First, clone the preheader. 1193 auto *ClonedPH = CloneBlock(LoopPH); 1194 1195 // Then clone all the loop blocks, skipping the ones that aren't necessary. 1196 for (auto *LoopBB : L.blocks()) 1197 if (!SkipBlock(LoopBB)) 1198 CloneBlock(LoopBB); 1199 1200 // Split all the loop exit edges so that when we clone the exit blocks, if 1201 // any of the exit blocks are *also* a preheader for some other loop, we 1202 // don't create multiple predecessors entering the loop header. 1203 for (auto *ExitBB : ExitBlocks) { 1204 if (SkipBlock(ExitBB)) 1205 continue; 1206 1207 // When we are going to clone an exit, we don't need to clone all the 1208 // instructions in the exit block and we want to ensure we have an easy 1209 // place to merge the CFG, so split the exit first. This is always safe to 1210 // do because there cannot be any non-loop predecessors of a loop exit in 1211 // loop simplified form. 1212 auto *MergeBB = SplitBlock(ExitBB, ExitBB->begin(), &DT, &LI, MSSAU); 1213 1214 // Rearrange the names to make it easier to write test cases by having the 1215 // exit block carry the suffix rather than the merge block carrying the 1216 // suffix. 1217 MergeBB->takeName(ExitBB); 1218 ExitBB->setName(Twine(MergeBB->getName()) + ".split"); 1219 1220 // Now clone the original exit block. 1221 auto *ClonedExitBB = CloneBlock(ExitBB); 1222 assert(ClonedExitBB->getTerminator()->getNumSuccessors() == 1 && 1223 "Exit block should have been split to have one successor!"); 1224 assert(ClonedExitBB->getTerminator()->getSuccessor(0) == MergeBB && 1225 "Cloned exit block has the wrong successor!"); 1226 1227 // Remap any cloned instructions and create a merge phi node for them. 1228 for (auto ZippedInsts : llvm::zip_first( 1229 llvm::make_range(ExitBB->begin(), std::prev(ExitBB->end())), 1230 llvm::make_range(ClonedExitBB->begin(), 1231 std::prev(ClonedExitBB->end())))) { 1232 Instruction &I = std::get<0>(ZippedInsts); 1233 Instruction &ClonedI = std::get<1>(ZippedInsts); 1234 1235 // The only instructions in the exit block should be PHI nodes and 1236 // potentially a landing pad. 1237 assert( 1238 (isa<PHINode>(I) || isa<LandingPadInst>(I) || isa<CatchPadInst>(I)) && 1239 "Bad instruction in exit block!"); 1240 // We should have a value map between the instruction and its clone. 1241 assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!"); 1242 1243 // Forget SCEVs based on exit phis in case SCEV looked through the phi. 1244 if (SE && isa<PHINode>(I)) 1245 SE->forgetValue(&I); 1246 1247 auto *MergePN = 1248 PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi"); 1249 MergePN->insertBefore(MergeBB->getFirstInsertionPt()); 1250 I.replaceAllUsesWith(MergePN); 1251 MergePN->addIncoming(&I, ExitBB); 1252 MergePN->addIncoming(&ClonedI, ClonedExitBB); 1253 } 1254 } 1255 1256 // Rewrite the instructions in the cloned blocks to refer to the instructions 1257 // in the cloned blocks. We have to do this as a second pass so that we have 1258 // everything available. Also, we have inserted new instructions which may 1259 // include assume intrinsics, so we update the assumption cache while 1260 // processing this. 1261 Module *M = ClonedPH->getParent()->getParent(); 1262 for (auto *ClonedBB : NewBlocks) 1263 for (Instruction &I : *ClonedBB) { 1264 RemapDbgVariableRecordRange(M, I.getDbgRecordRange(), VMap, 1265 RF_NoModuleLevelChanges | 1266 RF_IgnoreMissingLocals); 1267 RemapInstruction(&I, VMap, 1268 RF_NoModuleLevelChanges | RF_IgnoreMissingLocals); 1269 if (auto *II = dyn_cast<AssumeInst>(&I)) 1270 AC.registerAssumption(II); 1271 } 1272 1273 // Update any PHI nodes in the cloned successors of the skipped blocks to not 1274 // have spurious incoming values. 1275 for (auto *LoopBB : L.blocks()) 1276 if (SkipBlock(LoopBB)) 1277 for (auto *SuccBB : successors(LoopBB)) 1278 if (auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB))) 1279 for (PHINode &PN : ClonedSuccBB->phis()) 1280 PN.removeIncomingValue(LoopBB, /*DeletePHIIfEmpty*/ false); 1281 1282 // Remove the cloned parent as a predecessor of any successor we ended up 1283 // cloning other than the unswitched one. 1284 auto *ClonedParentBB = cast<BasicBlock>(VMap.lookup(ParentBB)); 1285 for (auto *SuccBB : successors(ParentBB)) { 1286 if (SuccBB == UnswitchedSuccBB) 1287 continue; 1288 1289 auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB)); 1290 if (!ClonedSuccBB) 1291 continue; 1292 1293 ClonedSuccBB->removePredecessor(ClonedParentBB, 1294 /*KeepOneInputPHIs*/ true); 1295 } 1296 1297 // Replace the cloned branch with an unconditional branch to the cloned 1298 // unswitched successor. 1299 auto *ClonedSuccBB = cast<BasicBlock>(VMap.lookup(UnswitchedSuccBB)); 1300 Instruction *ClonedTerminator = ClonedParentBB->getTerminator(); 1301 // Trivial Simplification. If Terminator is a conditional branch and 1302 // condition becomes dead - erase it. 1303 Value *ClonedConditionToErase = nullptr; 1304 if (auto *BI = dyn_cast<BranchInst>(ClonedTerminator)) 1305 ClonedConditionToErase = BI->getCondition(); 1306 else if (auto *SI = dyn_cast<SwitchInst>(ClonedTerminator)) 1307 ClonedConditionToErase = SI->getCondition(); 1308 1309 ClonedTerminator->eraseFromParent(); 1310 BranchInst::Create(ClonedSuccBB, ClonedParentBB); 1311 1312 if (ClonedConditionToErase) 1313 RecursivelyDeleteTriviallyDeadInstructions(ClonedConditionToErase, nullptr, 1314 MSSAU); 1315 1316 // If there are duplicate entries in the PHI nodes because of multiple edges 1317 // to the unswitched successor, we need to nuke all but one as we replaced it 1318 // with a direct branch. 1319 for (PHINode &PN : ClonedSuccBB->phis()) { 1320 bool Found = false; 1321 // Loop over the incoming operands backwards so we can easily delete as we 1322 // go without invalidating the index. 1323 for (int i = PN.getNumOperands() - 1; i >= 0; --i) { 1324 if (PN.getIncomingBlock(i) != ClonedParentBB) 1325 continue; 1326 if (!Found) { 1327 Found = true; 1328 continue; 1329 } 1330 PN.removeIncomingValue(i, /*DeletePHIIfEmpty*/ false); 1331 } 1332 } 1333 1334 // Record the domtree updates for the new blocks. 1335 SmallPtrSet<BasicBlock *, 4> SuccSet; 1336 for (auto *ClonedBB : NewBlocks) { 1337 for (auto *SuccBB : successors(ClonedBB)) 1338 if (SuccSet.insert(SuccBB).second) 1339 DTUpdates.push_back({DominatorTree::Insert, ClonedBB, SuccBB}); 1340 SuccSet.clear(); 1341 } 1342 1343 return ClonedPH; 1344 } 1345 1346 /// Recursively clone the specified loop and all of its children. 1347 /// 1348 /// The target parent loop for the clone should be provided, or can be null if 1349 /// the clone is a top-level loop. While cloning, all the blocks are mapped 1350 /// with the provided value map. The entire original loop must be present in 1351 /// the value map. The cloned loop is returned. 1352 static Loop *cloneLoopNest(Loop &OrigRootL, Loop *RootParentL, 1353 const ValueToValueMapTy &VMap, LoopInfo &LI) { 1354 auto AddClonedBlocksToLoop = [&](Loop &OrigL, Loop &ClonedL) { 1355 assert(ClonedL.getBlocks().empty() && "Must start with an empty loop!"); 1356 ClonedL.reserveBlocks(OrigL.getNumBlocks()); 1357 for (auto *BB : OrigL.blocks()) { 1358 auto *ClonedBB = cast<BasicBlock>(VMap.lookup(BB)); 1359 ClonedL.addBlockEntry(ClonedBB); 1360 if (LI.getLoopFor(BB) == &OrigL) 1361 LI.changeLoopFor(ClonedBB, &ClonedL); 1362 } 1363 }; 1364 1365 // We specially handle the first loop because it may get cloned into 1366 // a different parent and because we most commonly are cloning leaf loops. 1367 Loop *ClonedRootL = LI.AllocateLoop(); 1368 if (RootParentL) 1369 RootParentL->addChildLoop(ClonedRootL); 1370 else 1371 LI.addTopLevelLoop(ClonedRootL); 1372 AddClonedBlocksToLoop(OrigRootL, *ClonedRootL); 1373 1374 if (OrigRootL.isInnermost()) 1375 return ClonedRootL; 1376 1377 // If we have a nest, we can quickly clone the entire loop nest using an 1378 // iterative approach because it is a tree. We keep the cloned parent in the 1379 // data structure to avoid repeatedly querying through a map to find it. 1380 SmallVector<std::pair<Loop *, Loop *>, 16> LoopsToClone; 1381 // Build up the loops to clone in reverse order as we'll clone them from the 1382 // back. 1383 for (Loop *ChildL : llvm::reverse(OrigRootL)) 1384 LoopsToClone.push_back({ClonedRootL, ChildL}); 1385 do { 1386 Loop *ClonedParentL, *L; 1387 std::tie(ClonedParentL, L) = LoopsToClone.pop_back_val(); 1388 Loop *ClonedL = LI.AllocateLoop(); 1389 ClonedParentL->addChildLoop(ClonedL); 1390 AddClonedBlocksToLoop(*L, *ClonedL); 1391 for (Loop *ChildL : llvm::reverse(*L)) 1392 LoopsToClone.push_back({ClonedL, ChildL}); 1393 } while (!LoopsToClone.empty()); 1394 1395 return ClonedRootL; 1396 } 1397 1398 /// Build the cloned loops of an original loop from unswitching. 1399 /// 1400 /// Because unswitching simplifies the CFG of the loop, this isn't a trivial 1401 /// operation. We need to re-verify that there even is a loop (as the backedge 1402 /// may not have been cloned), and even if there are remaining backedges the 1403 /// backedge set may be different. However, we know that each child loop is 1404 /// undisturbed, we only need to find where to place each child loop within 1405 /// either any parent loop or within a cloned version of the original loop. 1406 /// 1407 /// Because child loops may end up cloned outside of any cloned version of the 1408 /// original loop, multiple cloned sibling loops may be created. All of them 1409 /// are returned so that the newly introduced loop nest roots can be 1410 /// identified. 1411 static void buildClonedLoops(Loop &OrigL, ArrayRef<BasicBlock *> ExitBlocks, 1412 const ValueToValueMapTy &VMap, LoopInfo &LI, 1413 SmallVectorImpl<Loop *> &NonChildClonedLoops) { 1414 Loop *ClonedL = nullptr; 1415 1416 auto *OrigPH = OrigL.getLoopPreheader(); 1417 auto *OrigHeader = OrigL.getHeader(); 1418 1419 auto *ClonedPH = cast<BasicBlock>(VMap.lookup(OrigPH)); 1420 auto *ClonedHeader = cast<BasicBlock>(VMap.lookup(OrigHeader)); 1421 1422 // We need to know the loops of the cloned exit blocks to even compute the 1423 // accurate parent loop. If we only clone exits to some parent of the 1424 // original parent, we want to clone into that outer loop. We also keep track 1425 // of the loops that our cloned exit blocks participate in. 1426 Loop *ParentL = nullptr; 1427 SmallVector<BasicBlock *, 4> ClonedExitsInLoops; 1428 SmallDenseMap<BasicBlock *, Loop *, 16> ExitLoopMap; 1429 ClonedExitsInLoops.reserve(ExitBlocks.size()); 1430 for (auto *ExitBB : ExitBlocks) 1431 if (auto *ClonedExitBB = cast_or_null<BasicBlock>(VMap.lookup(ExitBB))) 1432 if (Loop *ExitL = LI.getLoopFor(ExitBB)) { 1433 ExitLoopMap[ClonedExitBB] = ExitL; 1434 ClonedExitsInLoops.push_back(ClonedExitBB); 1435 if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL))) 1436 ParentL = ExitL; 1437 } 1438 assert((!ParentL || ParentL == OrigL.getParentLoop() || 1439 ParentL->contains(OrigL.getParentLoop())) && 1440 "The computed parent loop should always contain (or be) the parent of " 1441 "the original loop."); 1442 1443 // We build the set of blocks dominated by the cloned header from the set of 1444 // cloned blocks out of the original loop. While not all of these will 1445 // necessarily be in the cloned loop, it is enough to establish that they 1446 // aren't in unreachable cycles, etc. 1447 SmallSetVector<BasicBlock *, 16> ClonedLoopBlocks; 1448 for (auto *BB : OrigL.blocks()) 1449 if (auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB))) 1450 ClonedLoopBlocks.insert(ClonedBB); 1451 1452 // Rebuild the set of blocks that will end up in the cloned loop. We may have 1453 // skipped cloning some region of this loop which can in turn skip some of 1454 // the backedges so we have to rebuild the blocks in the loop based on the 1455 // backedges that remain after cloning. 1456 SmallVector<BasicBlock *, 16> Worklist; 1457 SmallPtrSet<BasicBlock *, 16> BlocksInClonedLoop; 1458 for (auto *Pred : predecessors(ClonedHeader)) { 1459 // The only possible non-loop header predecessor is the preheader because 1460 // we know we cloned the loop in simplified form. 1461 if (Pred == ClonedPH) 1462 continue; 1463 1464 // Because the loop was in simplified form, the only non-loop predecessor 1465 // should be the preheader. 1466 assert(ClonedLoopBlocks.count(Pred) && "Found a predecessor of the loop " 1467 "header other than the preheader " 1468 "that is not part of the loop!"); 1469 1470 // Insert this block into the loop set and on the first visit (and if it 1471 // isn't the header we're currently walking) put it into the worklist to 1472 // recurse through. 1473 if (BlocksInClonedLoop.insert(Pred).second && Pred != ClonedHeader) 1474 Worklist.push_back(Pred); 1475 } 1476 1477 // If we had any backedges then there *is* a cloned loop. Put the header into 1478 // the loop set and then walk the worklist backwards to find all the blocks 1479 // that remain within the loop after cloning. 1480 if (!BlocksInClonedLoop.empty()) { 1481 BlocksInClonedLoop.insert(ClonedHeader); 1482 1483 while (!Worklist.empty()) { 1484 BasicBlock *BB = Worklist.pop_back_val(); 1485 assert(BlocksInClonedLoop.count(BB) && 1486 "Didn't put block into the loop set!"); 1487 1488 // Insert any predecessors that are in the possible set into the cloned 1489 // set, and if the insert is successful, add them to the worklist. Note 1490 // that we filter on the blocks that are definitely reachable via the 1491 // backedge to the loop header so we may prune out dead code within the 1492 // cloned loop. 1493 for (auto *Pred : predecessors(BB)) 1494 if (ClonedLoopBlocks.count(Pred) && 1495 BlocksInClonedLoop.insert(Pred).second) 1496 Worklist.push_back(Pred); 1497 } 1498 1499 ClonedL = LI.AllocateLoop(); 1500 if (ParentL) { 1501 ParentL->addBasicBlockToLoop(ClonedPH, LI); 1502 ParentL->addChildLoop(ClonedL); 1503 } else { 1504 LI.addTopLevelLoop(ClonedL); 1505 } 1506 NonChildClonedLoops.push_back(ClonedL); 1507 1508 ClonedL->reserveBlocks(BlocksInClonedLoop.size()); 1509 // We don't want to just add the cloned loop blocks based on how we 1510 // discovered them. The original order of blocks was carefully built in 1511 // a way that doesn't rely on predecessor ordering. Rather than re-invent 1512 // that logic, we just re-walk the original blocks (and those of the child 1513 // loops) and filter them as we add them into the cloned loop. 1514 for (auto *BB : OrigL.blocks()) { 1515 auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB)); 1516 if (!ClonedBB || !BlocksInClonedLoop.count(ClonedBB)) 1517 continue; 1518 1519 // Directly add the blocks that are only in this loop. 1520 if (LI.getLoopFor(BB) == &OrigL) { 1521 ClonedL->addBasicBlockToLoop(ClonedBB, LI); 1522 continue; 1523 } 1524 1525 // We want to manually add it to this loop and parents. 1526 // Registering it with LoopInfo will happen when we clone the top 1527 // loop for this block. 1528 for (Loop *PL = ClonedL; PL; PL = PL->getParentLoop()) 1529 PL->addBlockEntry(ClonedBB); 1530 } 1531 1532 // Now add each child loop whose header remains within the cloned loop. All 1533 // of the blocks within the loop must satisfy the same constraints as the 1534 // header so once we pass the header checks we can just clone the entire 1535 // child loop nest. 1536 for (Loop *ChildL : OrigL) { 1537 auto *ClonedChildHeader = 1538 cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader())); 1539 if (!ClonedChildHeader || !BlocksInClonedLoop.count(ClonedChildHeader)) 1540 continue; 1541 1542 #ifndef NDEBUG 1543 // We should never have a cloned child loop header but fail to have 1544 // all of the blocks for that child loop. 1545 for (auto *ChildLoopBB : ChildL->blocks()) 1546 assert(BlocksInClonedLoop.count( 1547 cast<BasicBlock>(VMap.lookup(ChildLoopBB))) && 1548 "Child cloned loop has a header within the cloned outer " 1549 "loop but not all of its blocks!"); 1550 #endif 1551 1552 cloneLoopNest(*ChildL, ClonedL, VMap, LI); 1553 } 1554 } 1555 1556 // Now that we've handled all the components of the original loop that were 1557 // cloned into a new loop, we still need to handle anything from the original 1558 // loop that wasn't in a cloned loop. 1559 1560 // Figure out what blocks are left to place within any loop nest containing 1561 // the unswitched loop. If we never formed a loop, the cloned PH is one of 1562 // them. 1563 SmallPtrSet<BasicBlock *, 16> UnloopedBlockSet; 1564 if (BlocksInClonedLoop.empty()) 1565 UnloopedBlockSet.insert(ClonedPH); 1566 for (auto *ClonedBB : ClonedLoopBlocks) 1567 if (!BlocksInClonedLoop.count(ClonedBB)) 1568 UnloopedBlockSet.insert(ClonedBB); 1569 1570 // Copy the cloned exits and sort them in ascending loop depth, we'll work 1571 // backwards across these to process them inside out. The order shouldn't 1572 // matter as we're just trying to build up the map from inside-out; we use 1573 // the map in a more stably ordered way below. 1574 auto OrderedClonedExitsInLoops = ClonedExitsInLoops; 1575 llvm::sort(OrderedClonedExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) { 1576 return ExitLoopMap.lookup(LHS)->getLoopDepth() < 1577 ExitLoopMap.lookup(RHS)->getLoopDepth(); 1578 }); 1579 1580 // Populate the existing ExitLoopMap with everything reachable from each 1581 // exit, starting from the inner most exit. 1582 while (!UnloopedBlockSet.empty() && !OrderedClonedExitsInLoops.empty()) { 1583 assert(Worklist.empty() && "Didn't clear worklist!"); 1584 1585 BasicBlock *ExitBB = OrderedClonedExitsInLoops.pop_back_val(); 1586 Loop *ExitL = ExitLoopMap.lookup(ExitBB); 1587 1588 // Walk the CFG back until we hit the cloned PH adding everything reachable 1589 // and in the unlooped set to this exit block's loop. 1590 Worklist.push_back(ExitBB); 1591 do { 1592 BasicBlock *BB = Worklist.pop_back_val(); 1593 // We can stop recursing at the cloned preheader (if we get there). 1594 if (BB == ClonedPH) 1595 continue; 1596 1597 for (BasicBlock *PredBB : predecessors(BB)) { 1598 // If this pred has already been moved to our set or is part of some 1599 // (inner) loop, no update needed. 1600 if (!UnloopedBlockSet.erase(PredBB)) { 1601 assert( 1602 (BlocksInClonedLoop.count(PredBB) || ExitLoopMap.count(PredBB)) && 1603 "Predecessor not mapped to a loop!"); 1604 continue; 1605 } 1606 1607 // We just insert into the loop set here. We'll add these blocks to the 1608 // exit loop after we build up the set in an order that doesn't rely on 1609 // predecessor order (which in turn relies on use list order). 1610 bool Inserted = ExitLoopMap.insert({PredBB, ExitL}).second; 1611 (void)Inserted; 1612 assert(Inserted && "Should only visit an unlooped block once!"); 1613 1614 // And recurse through to its predecessors. 1615 Worklist.push_back(PredBB); 1616 } 1617 } while (!Worklist.empty()); 1618 } 1619 1620 // Now that the ExitLoopMap gives as mapping for all the non-looping cloned 1621 // blocks to their outer loops, walk the cloned blocks and the cloned exits 1622 // in their original order adding them to the correct loop. 1623 1624 // We need a stable insertion order. We use the order of the original loop 1625 // order and map into the correct parent loop. 1626 for (auto *BB : llvm::concat<BasicBlock *const>( 1627 ArrayRef(ClonedPH), ClonedLoopBlocks, ClonedExitsInLoops)) 1628 if (Loop *OuterL = ExitLoopMap.lookup(BB)) 1629 OuterL->addBasicBlockToLoop(BB, LI); 1630 1631 #ifndef NDEBUG 1632 for (auto &BBAndL : ExitLoopMap) { 1633 auto *BB = BBAndL.first; 1634 auto *OuterL = BBAndL.second; 1635 assert(LI.getLoopFor(BB) == OuterL && 1636 "Failed to put all blocks into outer loops!"); 1637 } 1638 #endif 1639 1640 // Now that all the blocks are placed into the correct containing loop in the 1641 // absence of child loops, find all the potentially cloned child loops and 1642 // clone them into whatever outer loop we placed their header into. 1643 for (Loop *ChildL : OrigL) { 1644 auto *ClonedChildHeader = 1645 cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader())); 1646 if (!ClonedChildHeader || BlocksInClonedLoop.count(ClonedChildHeader)) 1647 continue; 1648 1649 #ifndef NDEBUG 1650 for (auto *ChildLoopBB : ChildL->blocks()) 1651 assert(VMap.count(ChildLoopBB) && 1652 "Cloned a child loop header but not all of that loops blocks!"); 1653 #endif 1654 1655 NonChildClonedLoops.push_back(cloneLoopNest( 1656 *ChildL, ExitLoopMap.lookup(ClonedChildHeader), VMap, LI)); 1657 } 1658 } 1659 1660 static void 1661 deleteDeadClonedBlocks(Loop &L, ArrayRef<BasicBlock *> ExitBlocks, 1662 ArrayRef<std::unique_ptr<ValueToValueMapTy>> VMaps, 1663 DominatorTree &DT, MemorySSAUpdater *MSSAU) { 1664 // Find all the dead clones, and remove them from their successors. 1665 SmallVector<BasicBlock *, 16> DeadBlocks; 1666 for (BasicBlock *BB : llvm::concat<BasicBlock *const>(L.blocks(), ExitBlocks)) 1667 for (const auto &VMap : VMaps) 1668 if (BasicBlock *ClonedBB = cast_or_null<BasicBlock>(VMap->lookup(BB))) 1669 if (!DT.isReachableFromEntry(ClonedBB)) { 1670 for (BasicBlock *SuccBB : successors(ClonedBB)) 1671 SuccBB->removePredecessor(ClonedBB); 1672 DeadBlocks.push_back(ClonedBB); 1673 } 1674 1675 // Remove all MemorySSA in the dead blocks 1676 if (MSSAU) { 1677 SmallSetVector<BasicBlock *, 8> DeadBlockSet(DeadBlocks.begin(), 1678 DeadBlocks.end()); 1679 MSSAU->removeBlocks(DeadBlockSet); 1680 } 1681 1682 // Drop any remaining references to break cycles. 1683 for (BasicBlock *BB : DeadBlocks) 1684 BB->dropAllReferences(); 1685 // Erase them from the IR. 1686 for (BasicBlock *BB : DeadBlocks) 1687 BB->eraseFromParent(); 1688 } 1689 1690 static void deleteDeadBlocksFromLoop(Loop &L, 1691 SmallVectorImpl<BasicBlock *> &ExitBlocks, 1692 DominatorTree &DT, LoopInfo &LI, 1693 MemorySSAUpdater *MSSAU, 1694 ScalarEvolution *SE, 1695 LPMUpdater &LoopUpdater) { 1696 // Find all the dead blocks tied to this loop, and remove them from their 1697 // successors. 1698 SmallSetVector<BasicBlock *, 8> DeadBlockSet; 1699 1700 // Start with loop/exit blocks and get a transitive closure of reachable dead 1701 // blocks. 1702 SmallVector<BasicBlock *, 16> DeathCandidates(ExitBlocks.begin(), 1703 ExitBlocks.end()); 1704 DeathCandidates.append(L.blocks().begin(), L.blocks().end()); 1705 while (!DeathCandidates.empty()) { 1706 auto *BB = DeathCandidates.pop_back_val(); 1707 if (!DeadBlockSet.count(BB) && !DT.isReachableFromEntry(BB)) { 1708 for (BasicBlock *SuccBB : successors(BB)) { 1709 SuccBB->removePredecessor(BB); 1710 DeathCandidates.push_back(SuccBB); 1711 } 1712 DeadBlockSet.insert(BB); 1713 } 1714 } 1715 1716 // Remove all MemorySSA in the dead blocks 1717 if (MSSAU) 1718 MSSAU->removeBlocks(DeadBlockSet); 1719 1720 // Filter out the dead blocks from the exit blocks list so that it can be 1721 // used in the caller. 1722 llvm::erase_if(ExitBlocks, 1723 [&](BasicBlock *BB) { return DeadBlockSet.count(BB); }); 1724 1725 // Walk from this loop up through its parents removing all of the dead blocks. 1726 for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) { 1727 for (auto *BB : DeadBlockSet) 1728 ParentL->getBlocksSet().erase(BB); 1729 llvm::erase_if(ParentL->getBlocksVector(), 1730 [&](BasicBlock *BB) { return DeadBlockSet.count(BB); }); 1731 } 1732 1733 // Now delete the dead child loops. This raw delete will clear them 1734 // recursively. 1735 llvm::erase_if(L.getSubLoopsVector(), [&](Loop *ChildL) { 1736 if (!DeadBlockSet.count(ChildL->getHeader())) 1737 return false; 1738 1739 assert(llvm::all_of(ChildL->blocks(), 1740 [&](BasicBlock *ChildBB) { 1741 return DeadBlockSet.count(ChildBB); 1742 }) && 1743 "If the child loop header is dead all blocks in the child loop must " 1744 "be dead as well!"); 1745 LoopUpdater.markLoopAsDeleted(*ChildL, ChildL->getName()); 1746 if (SE) 1747 SE->forgetBlockAndLoopDispositions(); 1748 LI.destroy(ChildL); 1749 return true; 1750 }); 1751 1752 // Remove the loop mappings for the dead blocks and drop all the references 1753 // from these blocks to others to handle cyclic references as we start 1754 // deleting the blocks themselves. 1755 for (auto *BB : DeadBlockSet) { 1756 // Check that the dominator tree has already been updated. 1757 assert(!DT.getNode(BB) && "Should already have cleared domtree!"); 1758 LI.changeLoopFor(BB, nullptr); 1759 // Drop all uses of the instructions to make sure we won't have dangling 1760 // uses in other blocks. 1761 for (auto &I : *BB) 1762 if (!I.use_empty()) 1763 I.replaceAllUsesWith(PoisonValue::get(I.getType())); 1764 BB->dropAllReferences(); 1765 } 1766 1767 // Actually delete the blocks now that they've been fully unhooked from the 1768 // IR. 1769 for (auto *BB : DeadBlockSet) 1770 BB->eraseFromParent(); 1771 } 1772 1773 /// Recompute the set of blocks in a loop after unswitching. 1774 /// 1775 /// This walks from the original headers predecessors to rebuild the loop. We 1776 /// take advantage of the fact that new blocks can't have been added, and so we 1777 /// filter by the original loop's blocks. This also handles potentially 1778 /// unreachable code that we don't want to explore but might be found examining 1779 /// the predecessors of the header. 1780 /// 1781 /// If the original loop is no longer a loop, this will return an empty set. If 1782 /// it remains a loop, all the blocks within it will be added to the set 1783 /// (including those blocks in inner loops). 1784 static SmallPtrSet<const BasicBlock *, 16> recomputeLoopBlockSet(Loop &L, 1785 LoopInfo &LI) { 1786 SmallPtrSet<const BasicBlock *, 16> LoopBlockSet; 1787 1788 auto *PH = L.getLoopPreheader(); 1789 auto *Header = L.getHeader(); 1790 1791 // A worklist to use while walking backwards from the header. 1792 SmallVector<BasicBlock *, 16> Worklist; 1793 1794 // First walk the predecessors of the header to find the backedges. This will 1795 // form the basis of our walk. 1796 for (auto *Pred : predecessors(Header)) { 1797 // Skip the preheader. 1798 if (Pred == PH) 1799 continue; 1800 1801 // Because the loop was in simplified form, the only non-loop predecessor 1802 // is the preheader. 1803 assert(L.contains(Pred) && "Found a predecessor of the loop header other " 1804 "than the preheader that is not part of the " 1805 "loop!"); 1806 1807 // Insert this block into the loop set and on the first visit and, if it 1808 // isn't the header we're currently walking, put it into the worklist to 1809 // recurse through. 1810 if (LoopBlockSet.insert(Pred).second && Pred != Header) 1811 Worklist.push_back(Pred); 1812 } 1813 1814 // If no backedges were found, we're done. 1815 if (LoopBlockSet.empty()) 1816 return LoopBlockSet; 1817 1818 // We found backedges, recurse through them to identify the loop blocks. 1819 while (!Worklist.empty()) { 1820 BasicBlock *BB = Worklist.pop_back_val(); 1821 assert(LoopBlockSet.count(BB) && "Didn't put block into the loop set!"); 1822 1823 // No need to walk past the header. 1824 if (BB == Header) 1825 continue; 1826 1827 // Because we know the inner loop structure remains valid we can use the 1828 // loop structure to jump immediately across the entire nested loop. 1829 // Further, because it is in loop simplified form, we can directly jump 1830 // to its preheader afterward. 1831 if (Loop *InnerL = LI.getLoopFor(BB)) 1832 if (InnerL != &L) { 1833 assert(L.contains(InnerL) && 1834 "Should not reach a loop *outside* this loop!"); 1835 // The preheader is the only possible predecessor of the loop so 1836 // insert it into the set and check whether it was already handled. 1837 auto *InnerPH = InnerL->getLoopPreheader(); 1838 assert(L.contains(InnerPH) && "Cannot contain an inner loop block " 1839 "but not contain the inner loop " 1840 "preheader!"); 1841 if (!LoopBlockSet.insert(InnerPH).second) 1842 // The only way to reach the preheader is through the loop body 1843 // itself so if it has been visited the loop is already handled. 1844 continue; 1845 1846 // Insert all of the blocks (other than those already present) into 1847 // the loop set. We expect at least the block that led us to find the 1848 // inner loop to be in the block set, but we may also have other loop 1849 // blocks if they were already enqueued as predecessors of some other 1850 // outer loop block. 1851 for (auto *InnerBB : InnerL->blocks()) { 1852 if (InnerBB == BB) { 1853 assert(LoopBlockSet.count(InnerBB) && 1854 "Block should already be in the set!"); 1855 continue; 1856 } 1857 1858 LoopBlockSet.insert(InnerBB); 1859 } 1860 1861 // Add the preheader to the worklist so we will continue past the 1862 // loop body. 1863 Worklist.push_back(InnerPH); 1864 continue; 1865 } 1866 1867 // Insert any predecessors that were in the original loop into the new 1868 // set, and if the insert is successful, add them to the worklist. 1869 for (auto *Pred : predecessors(BB)) 1870 if (L.contains(Pred) && LoopBlockSet.insert(Pred).second) 1871 Worklist.push_back(Pred); 1872 } 1873 1874 assert(LoopBlockSet.count(Header) && "Cannot fail to add the header!"); 1875 1876 // We've found all the blocks participating in the loop, return our completed 1877 // set. 1878 return LoopBlockSet; 1879 } 1880 1881 /// Rebuild a loop after unswitching removes some subset of blocks and edges. 1882 /// 1883 /// The removal may have removed some child loops entirely but cannot have 1884 /// disturbed any remaining child loops. However, they may need to be hoisted 1885 /// to the parent loop (or to be top-level loops). The original loop may be 1886 /// completely removed. 1887 /// 1888 /// The sibling loops resulting from this update are returned. If the original 1889 /// loop remains a valid loop, it will be the first entry in this list with all 1890 /// of the newly sibling loops following it. 1891 /// 1892 /// Returns true if the loop remains a loop after unswitching, and false if it 1893 /// is no longer a loop after unswitching (and should not continue to be 1894 /// referenced). 1895 static bool rebuildLoopAfterUnswitch(Loop &L, ArrayRef<BasicBlock *> ExitBlocks, 1896 LoopInfo &LI, 1897 SmallVectorImpl<Loop *> &HoistedLoops, 1898 ScalarEvolution *SE) { 1899 auto *PH = L.getLoopPreheader(); 1900 1901 // Compute the actual parent loop from the exit blocks. Because we may have 1902 // pruned some exits the loop may be different from the original parent. 1903 Loop *ParentL = nullptr; 1904 SmallVector<Loop *, 4> ExitLoops; 1905 SmallVector<BasicBlock *, 4> ExitsInLoops; 1906 ExitsInLoops.reserve(ExitBlocks.size()); 1907 for (auto *ExitBB : ExitBlocks) 1908 if (Loop *ExitL = LI.getLoopFor(ExitBB)) { 1909 ExitLoops.push_back(ExitL); 1910 ExitsInLoops.push_back(ExitBB); 1911 if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL))) 1912 ParentL = ExitL; 1913 } 1914 1915 // Recompute the blocks participating in this loop. This may be empty if it 1916 // is no longer a loop. 1917 auto LoopBlockSet = recomputeLoopBlockSet(L, LI); 1918 1919 // If we still have a loop, we need to re-set the loop's parent as the exit 1920 // block set changing may have moved it within the loop nest. Note that this 1921 // can only happen when this loop has a parent as it can only hoist the loop 1922 // *up* the nest. 1923 if (!LoopBlockSet.empty() && L.getParentLoop() != ParentL) { 1924 // Remove this loop's (original) blocks from all of the intervening loops. 1925 for (Loop *IL = L.getParentLoop(); IL != ParentL; 1926 IL = IL->getParentLoop()) { 1927 IL->getBlocksSet().erase(PH); 1928 for (auto *BB : L.blocks()) 1929 IL->getBlocksSet().erase(BB); 1930 llvm::erase_if(IL->getBlocksVector(), [&](BasicBlock *BB) { 1931 return BB == PH || L.contains(BB); 1932 }); 1933 } 1934 1935 LI.changeLoopFor(PH, ParentL); 1936 L.getParentLoop()->removeChildLoop(&L); 1937 if (ParentL) 1938 ParentL->addChildLoop(&L); 1939 else 1940 LI.addTopLevelLoop(&L); 1941 } 1942 1943 // Now we update all the blocks which are no longer within the loop. 1944 auto &Blocks = L.getBlocksVector(); 1945 auto BlocksSplitI = 1946 LoopBlockSet.empty() 1947 ? Blocks.begin() 1948 : std::stable_partition( 1949 Blocks.begin(), Blocks.end(), 1950 [&](BasicBlock *BB) { return LoopBlockSet.count(BB); }); 1951 1952 // Before we erase the list of unlooped blocks, build a set of them. 1953 SmallPtrSet<BasicBlock *, 16> UnloopedBlocks(BlocksSplitI, Blocks.end()); 1954 if (LoopBlockSet.empty()) 1955 UnloopedBlocks.insert(PH); 1956 1957 // Now erase these blocks from the loop. 1958 for (auto *BB : make_range(BlocksSplitI, Blocks.end())) 1959 L.getBlocksSet().erase(BB); 1960 Blocks.erase(BlocksSplitI, Blocks.end()); 1961 1962 // Sort the exits in ascending loop depth, we'll work backwards across these 1963 // to process them inside out. 1964 llvm::stable_sort(ExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) { 1965 return LI.getLoopDepth(LHS) < LI.getLoopDepth(RHS); 1966 }); 1967 1968 // We'll build up a set for each exit loop. 1969 SmallPtrSet<BasicBlock *, 16> NewExitLoopBlocks; 1970 Loop *PrevExitL = L.getParentLoop(); // The deepest possible exit loop. 1971 1972 auto RemoveUnloopedBlocksFromLoop = 1973 [](Loop &L, SmallPtrSetImpl<BasicBlock *> &UnloopedBlocks) { 1974 for (auto *BB : UnloopedBlocks) 1975 L.getBlocksSet().erase(BB); 1976 llvm::erase_if(L.getBlocksVector(), [&](BasicBlock *BB) { 1977 return UnloopedBlocks.count(BB); 1978 }); 1979 }; 1980 1981 SmallVector<BasicBlock *, 16> Worklist; 1982 while (!UnloopedBlocks.empty() && !ExitsInLoops.empty()) { 1983 assert(Worklist.empty() && "Didn't clear worklist!"); 1984 assert(NewExitLoopBlocks.empty() && "Didn't clear loop set!"); 1985 1986 // Grab the next exit block, in decreasing loop depth order. 1987 BasicBlock *ExitBB = ExitsInLoops.pop_back_val(); 1988 Loop &ExitL = *LI.getLoopFor(ExitBB); 1989 assert(ExitL.contains(&L) && "Exit loop must contain the inner loop!"); 1990 1991 // Erase all of the unlooped blocks from the loops between the previous 1992 // exit loop and this exit loop. This works because the ExitInLoops list is 1993 // sorted in increasing order of loop depth and thus we visit loops in 1994 // decreasing order of loop depth. 1995 for (; PrevExitL != &ExitL; PrevExitL = PrevExitL->getParentLoop()) 1996 RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks); 1997 1998 // Walk the CFG back until we hit the cloned PH adding everything reachable 1999 // and in the unlooped set to this exit block's loop. 2000 Worklist.push_back(ExitBB); 2001 do { 2002 BasicBlock *BB = Worklist.pop_back_val(); 2003 // We can stop recursing at the cloned preheader (if we get there). 2004 if (BB == PH) 2005 continue; 2006 2007 for (BasicBlock *PredBB : predecessors(BB)) { 2008 // If this pred has already been moved to our set or is part of some 2009 // (inner) loop, no update needed. 2010 if (!UnloopedBlocks.erase(PredBB)) { 2011 assert((NewExitLoopBlocks.count(PredBB) || 2012 ExitL.contains(LI.getLoopFor(PredBB))) && 2013 "Predecessor not in a nested loop (or already visited)!"); 2014 continue; 2015 } 2016 2017 // We just insert into the loop set here. We'll add these blocks to the 2018 // exit loop after we build up the set in a deterministic order rather 2019 // than the predecessor-influenced visit order. 2020 bool Inserted = NewExitLoopBlocks.insert(PredBB).second; 2021 (void)Inserted; 2022 assert(Inserted && "Should only visit an unlooped block once!"); 2023 2024 // And recurse through to its predecessors. 2025 Worklist.push_back(PredBB); 2026 } 2027 } while (!Worklist.empty()); 2028 2029 // If blocks in this exit loop were directly part of the original loop (as 2030 // opposed to a child loop) update the map to point to this exit loop. This 2031 // just updates a map and so the fact that the order is unstable is fine. 2032 for (auto *BB : NewExitLoopBlocks) 2033 if (Loop *BBL = LI.getLoopFor(BB)) 2034 if (BBL == &L || !L.contains(BBL)) 2035 LI.changeLoopFor(BB, &ExitL); 2036 2037 // We will remove the remaining unlooped blocks from this loop in the next 2038 // iteration or below. 2039 NewExitLoopBlocks.clear(); 2040 } 2041 2042 // Any remaining unlooped blocks are no longer part of any loop unless they 2043 // are part of some child loop. 2044 for (; PrevExitL; PrevExitL = PrevExitL->getParentLoop()) 2045 RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks); 2046 for (auto *BB : UnloopedBlocks) 2047 if (Loop *BBL = LI.getLoopFor(BB)) 2048 if (BBL == &L || !L.contains(BBL)) 2049 LI.changeLoopFor(BB, nullptr); 2050 2051 // Sink all the child loops whose headers are no longer in the loop set to 2052 // the parent (or to be top level loops). We reach into the loop and directly 2053 // update its subloop vector to make this batch update efficient. 2054 auto &SubLoops = L.getSubLoopsVector(); 2055 auto SubLoopsSplitI = 2056 LoopBlockSet.empty() 2057 ? SubLoops.begin() 2058 : std::stable_partition( 2059 SubLoops.begin(), SubLoops.end(), [&](Loop *SubL) { 2060 return LoopBlockSet.count(SubL->getHeader()); 2061 }); 2062 for (auto *HoistedL : make_range(SubLoopsSplitI, SubLoops.end())) { 2063 HoistedLoops.push_back(HoistedL); 2064 HoistedL->setParentLoop(nullptr); 2065 2066 // To compute the new parent of this hoisted loop we look at where we 2067 // placed the preheader above. We can't lookup the header itself because we 2068 // retained the mapping from the header to the hoisted loop. But the 2069 // preheader and header should have the exact same new parent computed 2070 // based on the set of exit blocks from the original loop as the preheader 2071 // is a predecessor of the header and so reached in the reverse walk. And 2072 // because the loops were all in simplified form the preheader of the 2073 // hoisted loop can't be part of some *other* loop. 2074 if (auto *NewParentL = LI.getLoopFor(HoistedL->getLoopPreheader())) 2075 NewParentL->addChildLoop(HoistedL); 2076 else 2077 LI.addTopLevelLoop(HoistedL); 2078 } 2079 SubLoops.erase(SubLoopsSplitI, SubLoops.end()); 2080 2081 // Actually delete the loop if nothing remained within it. 2082 if (Blocks.empty()) { 2083 assert(SubLoops.empty() && 2084 "Failed to remove all subloops from the original loop!"); 2085 if (Loop *ParentL = L.getParentLoop()) 2086 ParentL->removeChildLoop(llvm::find(*ParentL, &L)); 2087 else 2088 LI.removeLoop(llvm::find(LI, &L)); 2089 // markLoopAsDeleted for L should be triggered by the caller (it is 2090 // typically done within postUnswitch). 2091 if (SE) 2092 SE->forgetBlockAndLoopDispositions(); 2093 LI.destroy(&L); 2094 return false; 2095 } 2096 2097 return true; 2098 } 2099 2100 /// Helper to visit a dominator subtree, invoking a callable on each node. 2101 /// 2102 /// Returning false at any point will stop walking past that node of the tree. 2103 template <typename CallableT> 2104 void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) { 2105 SmallVector<DomTreeNode *, 4> DomWorklist; 2106 DomWorklist.push_back(DT[BB]); 2107 #ifndef NDEBUG 2108 SmallPtrSet<DomTreeNode *, 4> Visited; 2109 Visited.insert(DT[BB]); 2110 #endif 2111 do { 2112 DomTreeNode *N = DomWorklist.pop_back_val(); 2113 2114 // Visit this node. 2115 if (!Callable(N->getBlock())) 2116 continue; 2117 2118 // Accumulate the child nodes. 2119 for (DomTreeNode *ChildN : *N) { 2120 assert(Visited.insert(ChildN).second && 2121 "Cannot visit a node twice when walking a tree!"); 2122 DomWorklist.push_back(ChildN); 2123 } 2124 } while (!DomWorklist.empty()); 2125 } 2126 2127 void postUnswitch(Loop &L, LPMUpdater &U, StringRef LoopName, 2128 bool CurrentLoopValid, bool PartiallyInvariant, 2129 bool InjectedCondition, ArrayRef<Loop *> NewLoops) { 2130 // If we did a non-trivial unswitch, we have added new (cloned) loops. 2131 if (!NewLoops.empty()) 2132 U.addSiblingLoops(NewLoops); 2133 2134 // If the current loop remains valid, we should revisit it to catch any 2135 // other unswitch opportunities. Otherwise, we need to mark it as deleted. 2136 if (CurrentLoopValid) { 2137 if (PartiallyInvariant) { 2138 // Mark the new loop as partially unswitched, to avoid unswitching on 2139 // the same condition again. 2140 auto &Context = L.getHeader()->getContext(); 2141 MDNode *DisableUnswitchMD = MDNode::get( 2142 Context, 2143 MDString::get(Context, "llvm.loop.unswitch.partial.disable")); 2144 MDNode *NewLoopID = makePostTransformationMetadata( 2145 Context, L.getLoopID(), {"llvm.loop.unswitch.partial"}, 2146 {DisableUnswitchMD}); 2147 L.setLoopID(NewLoopID); 2148 } else if (InjectedCondition) { 2149 // Do the same for injection of invariant conditions. 2150 auto &Context = L.getHeader()->getContext(); 2151 MDNode *DisableUnswitchMD = MDNode::get( 2152 Context, 2153 MDString::get(Context, "llvm.loop.unswitch.injection.disable")); 2154 MDNode *NewLoopID = makePostTransformationMetadata( 2155 Context, L.getLoopID(), {"llvm.loop.unswitch.injection"}, 2156 {DisableUnswitchMD}); 2157 L.setLoopID(NewLoopID); 2158 } else 2159 U.revisitCurrentLoop(); 2160 } else 2161 U.markLoopAsDeleted(L, LoopName); 2162 } 2163 2164 static void unswitchNontrivialInvariants( 2165 Loop &L, Instruction &TI, ArrayRef<Value *> Invariants, 2166 IVConditionInfo &PartialIVInfo, DominatorTree &DT, LoopInfo &LI, 2167 AssumptionCache &AC, ScalarEvolution *SE, MemorySSAUpdater *MSSAU, 2168 LPMUpdater &LoopUpdater, bool InsertFreeze, bool InjectedCondition) { 2169 auto *ParentBB = TI.getParent(); 2170 BranchInst *BI = dyn_cast<BranchInst>(&TI); 2171 SwitchInst *SI = BI ? nullptr : cast<SwitchInst>(&TI); 2172 2173 // Save the current loop name in a variable so that we can report it even 2174 // after it has been deleted. 2175 std::string LoopName(L.getName()); 2176 2177 // We can only unswitch switches, conditional branches with an invariant 2178 // condition, or combining invariant conditions with an instruction or 2179 // partially invariant instructions. 2180 assert((SI || (BI && BI->isConditional())) && 2181 "Can only unswitch switches and conditional branch!"); 2182 bool PartiallyInvariant = !PartialIVInfo.InstToDuplicate.empty(); 2183 bool FullUnswitch = 2184 SI || (skipTrivialSelect(BI->getCondition()) == Invariants[0] && 2185 !PartiallyInvariant); 2186 if (FullUnswitch) 2187 assert(Invariants.size() == 1 && 2188 "Cannot have other invariants with full unswitching!"); 2189 else 2190 assert(isa<Instruction>(skipTrivialSelect(BI->getCondition())) && 2191 "Partial unswitching requires an instruction as the condition!"); 2192 2193 if (MSSAU && VerifyMemorySSA) 2194 MSSAU->getMemorySSA()->verifyMemorySSA(); 2195 2196 // Constant and BBs tracking the cloned and continuing successor. When we are 2197 // unswitching the entire condition, this can just be trivially chosen to 2198 // unswitch towards `true`. However, when we are unswitching a set of 2199 // invariants combined with `and` or `or` or partially invariant instructions, 2200 // the combining operation determines the best direction to unswitch: we want 2201 // to unswitch the direction that will collapse the branch. 2202 bool Direction = true; 2203 int ClonedSucc = 0; 2204 if (!FullUnswitch) { 2205 Value *Cond = skipTrivialSelect(BI->getCondition()); 2206 (void)Cond; 2207 assert(((match(Cond, m_LogicalAnd()) ^ match(Cond, m_LogicalOr())) || 2208 PartiallyInvariant) && 2209 "Only `or`, `and`, an `select`, partially invariant instructions " 2210 "can combine invariants being unswitched."); 2211 if (!match(Cond, m_LogicalOr())) { 2212 if (match(Cond, m_LogicalAnd()) || 2213 (PartiallyInvariant && !PartialIVInfo.KnownValue->isOneValue())) { 2214 Direction = false; 2215 ClonedSucc = 1; 2216 } 2217 } 2218 } 2219 2220 BasicBlock *RetainedSuccBB = 2221 BI ? BI->getSuccessor(1 - ClonedSucc) : SI->getDefaultDest(); 2222 SmallSetVector<BasicBlock *, 4> UnswitchedSuccBBs; 2223 if (BI) 2224 UnswitchedSuccBBs.insert(BI->getSuccessor(ClonedSucc)); 2225 else 2226 for (auto Case : SI->cases()) 2227 if (Case.getCaseSuccessor() != RetainedSuccBB) 2228 UnswitchedSuccBBs.insert(Case.getCaseSuccessor()); 2229 2230 assert(!UnswitchedSuccBBs.count(RetainedSuccBB) && 2231 "Should not unswitch the same successor we are retaining!"); 2232 2233 // The branch should be in this exact loop. Any inner loop's invariant branch 2234 // should be handled by unswitching that inner loop. The caller of this 2235 // routine should filter out any candidates that remain (but were skipped for 2236 // whatever reason). 2237 assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!"); 2238 2239 // Compute the parent loop now before we start hacking on things. 2240 Loop *ParentL = L.getParentLoop(); 2241 // Get blocks in RPO order for MSSA update, before changing the CFG. 2242 LoopBlocksRPO LBRPO(&L); 2243 if (MSSAU) 2244 LBRPO.perform(&LI); 2245 2246 // Compute the outer-most loop containing one of our exit blocks. This is the 2247 // furthest up our loopnest which can be mutated, which we will use below to 2248 // update things. 2249 Loop *OuterExitL = &L; 2250 SmallVector<BasicBlock *, 4> ExitBlocks; 2251 L.getUniqueExitBlocks(ExitBlocks); 2252 for (auto *ExitBB : ExitBlocks) { 2253 // ExitBB can be an exit block for several levels in the loop nest. Make 2254 // sure we find the top most. 2255 Loop *NewOuterExitL = getTopMostExitingLoop(ExitBB, LI); 2256 if (!NewOuterExitL) { 2257 // We exited the entire nest with this block, so we're done. 2258 OuterExitL = nullptr; 2259 break; 2260 } 2261 if (NewOuterExitL != OuterExitL && NewOuterExitL->contains(OuterExitL)) 2262 OuterExitL = NewOuterExitL; 2263 } 2264 2265 // At this point, we're definitely going to unswitch something so invalidate 2266 // any cached information in ScalarEvolution for the outer most loop 2267 // containing an exit block and all nested loops. 2268 if (SE) { 2269 if (OuterExitL) 2270 SE->forgetLoop(OuterExitL); 2271 else 2272 SE->forgetTopmostLoop(&L); 2273 SE->forgetBlockAndLoopDispositions(); 2274 } 2275 2276 // If the edge from this terminator to a successor dominates that successor, 2277 // store a map from each block in its dominator subtree to it. This lets us 2278 // tell when cloning for a particular successor if a block is dominated by 2279 // some *other* successor with a single data structure. We use this to 2280 // significantly reduce cloning. 2281 SmallDenseMap<BasicBlock *, BasicBlock *, 16> DominatingSucc; 2282 for (auto *SuccBB : llvm::concat<BasicBlock *const>(ArrayRef(RetainedSuccBB), 2283 UnswitchedSuccBBs)) 2284 if (SuccBB->getUniquePredecessor() || 2285 llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) { 2286 return PredBB == ParentBB || DT.dominates(SuccBB, PredBB); 2287 })) 2288 visitDomSubTree(DT, SuccBB, [&](BasicBlock *BB) { 2289 DominatingSucc[BB] = SuccBB; 2290 return true; 2291 }); 2292 2293 // Split the preheader, so that we know that there is a safe place to insert 2294 // the conditional branch. We will change the preheader to have a conditional 2295 // branch on LoopCond. The original preheader will become the split point 2296 // between the unswitched versions, and we will have a new preheader for the 2297 // original loop. 2298 BasicBlock *SplitBB = L.getLoopPreheader(); 2299 BasicBlock *LoopPH = SplitEdge(SplitBB, L.getHeader(), &DT, &LI, MSSAU); 2300 2301 // Keep track of the dominator tree updates needed. 2302 SmallVector<DominatorTree::UpdateType, 4> DTUpdates; 2303 2304 // Clone the loop for each unswitched successor. 2305 SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps; 2306 VMaps.reserve(UnswitchedSuccBBs.size()); 2307 SmallDenseMap<BasicBlock *, BasicBlock *, 4> ClonedPHs; 2308 for (auto *SuccBB : UnswitchedSuccBBs) { 2309 VMaps.emplace_back(new ValueToValueMapTy()); 2310 ClonedPHs[SuccBB] = buildClonedLoopBlocks( 2311 L, LoopPH, SplitBB, ExitBlocks, ParentBB, SuccBB, RetainedSuccBB, 2312 DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU, SE); 2313 } 2314 2315 // Drop metadata if we may break its semantics by moving this instr into the 2316 // split block. 2317 if (TI.getMetadata(LLVMContext::MD_make_implicit)) { 2318 if (DropNonTrivialImplicitNullChecks) 2319 // Do not spend time trying to understand if we can keep it, just drop it 2320 // to save compile time. 2321 TI.setMetadata(LLVMContext::MD_make_implicit, nullptr); 2322 else { 2323 // It is only legal to preserve make.implicit metadata if we are 2324 // guaranteed no reach implicit null check after following this branch. 2325 ICFLoopSafetyInfo SafetyInfo; 2326 SafetyInfo.computeLoopSafetyInfo(&L); 2327 if (!SafetyInfo.isGuaranteedToExecute(TI, &DT, &L)) 2328 TI.setMetadata(LLVMContext::MD_make_implicit, nullptr); 2329 } 2330 } 2331 2332 // The stitching of the branched code back together depends on whether we're 2333 // doing full unswitching or not with the exception that we always want to 2334 // nuke the initial terminator placed in the split block. 2335 SplitBB->getTerminator()->eraseFromParent(); 2336 if (FullUnswitch) { 2337 // Splice the terminator from the original loop and rewrite its 2338 // successors. 2339 TI.moveBefore(*SplitBB, SplitBB->end()); 2340 2341 // Keep a clone of the terminator for MSSA updates. 2342 Instruction *NewTI = TI.clone(); 2343 NewTI->insertInto(ParentBB, ParentBB->end()); 2344 2345 // First wire up the moved terminator to the preheaders. 2346 if (BI) { 2347 BasicBlock *ClonedPH = ClonedPHs.begin()->second; 2348 BI->setSuccessor(ClonedSucc, ClonedPH); 2349 BI->setSuccessor(1 - ClonedSucc, LoopPH); 2350 Value *Cond = skipTrivialSelect(BI->getCondition()); 2351 if (InsertFreeze) 2352 Cond = new FreezeInst(Cond, Cond->getName() + ".fr", BI->getIterator()); 2353 BI->setCondition(Cond); 2354 DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH}); 2355 } else { 2356 assert(SI && "Must either be a branch or switch!"); 2357 2358 // Walk the cases and directly update their successors. 2359 assert(SI->getDefaultDest() == RetainedSuccBB && 2360 "Not retaining default successor!"); 2361 SI->setDefaultDest(LoopPH); 2362 for (const auto &Case : SI->cases()) 2363 if (Case.getCaseSuccessor() == RetainedSuccBB) 2364 Case.setSuccessor(LoopPH); 2365 else 2366 Case.setSuccessor(ClonedPHs.find(Case.getCaseSuccessor())->second); 2367 2368 if (InsertFreeze) 2369 SI->setCondition(new FreezeInst(SI->getCondition(), 2370 SI->getCondition()->getName() + ".fr", 2371 SI->getIterator())); 2372 2373 // We need to use the set to populate domtree updates as even when there 2374 // are multiple cases pointing at the same successor we only want to 2375 // remove and insert one edge in the domtree. 2376 for (BasicBlock *SuccBB : UnswitchedSuccBBs) 2377 DTUpdates.push_back( 2378 {DominatorTree::Insert, SplitBB, ClonedPHs.find(SuccBB)->second}); 2379 } 2380 2381 if (MSSAU) { 2382 DT.applyUpdates(DTUpdates); 2383 DTUpdates.clear(); 2384 2385 // Remove all but one edge to the retained block and all unswitched 2386 // blocks. This is to avoid having duplicate entries in the cloned Phis, 2387 // when we know we only keep a single edge for each case. 2388 MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, RetainedSuccBB); 2389 for (BasicBlock *SuccBB : UnswitchedSuccBBs) 2390 MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, SuccBB); 2391 2392 for (auto &VMap : VMaps) 2393 MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap, 2394 /*IgnoreIncomingWithNoClones=*/true); 2395 MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT); 2396 2397 // Remove all edges to unswitched blocks. 2398 for (BasicBlock *SuccBB : UnswitchedSuccBBs) 2399 MSSAU->removeEdge(ParentBB, SuccBB); 2400 } 2401 2402 // Now unhook the successor relationship as we'll be replacing 2403 // the terminator with a direct branch. This is much simpler for branches 2404 // than switches so we handle those first. 2405 if (BI) { 2406 // Remove the parent as a predecessor of the unswitched successor. 2407 assert(UnswitchedSuccBBs.size() == 1 && 2408 "Only one possible unswitched block for a branch!"); 2409 BasicBlock *UnswitchedSuccBB = *UnswitchedSuccBBs.begin(); 2410 UnswitchedSuccBB->removePredecessor(ParentBB, 2411 /*KeepOneInputPHIs*/ true); 2412 DTUpdates.push_back({DominatorTree::Delete, ParentBB, UnswitchedSuccBB}); 2413 } else { 2414 // Note that we actually want to remove the parent block as a predecessor 2415 // of *every* case successor. The case successor is either unswitched, 2416 // completely eliminating an edge from the parent to that successor, or it 2417 // is a duplicate edge to the retained successor as the retained successor 2418 // is always the default successor and as we'll replace this with a direct 2419 // branch we no longer need the duplicate entries in the PHI nodes. 2420 SwitchInst *NewSI = cast<SwitchInst>(NewTI); 2421 assert(NewSI->getDefaultDest() == RetainedSuccBB && 2422 "Not retaining default successor!"); 2423 for (const auto &Case : NewSI->cases()) 2424 Case.getCaseSuccessor()->removePredecessor( 2425 ParentBB, 2426 /*KeepOneInputPHIs*/ true); 2427 2428 // We need to use the set to populate domtree updates as even when there 2429 // are multiple cases pointing at the same successor we only want to 2430 // remove and insert one edge in the domtree. 2431 for (BasicBlock *SuccBB : UnswitchedSuccBBs) 2432 DTUpdates.push_back({DominatorTree::Delete, ParentBB, SuccBB}); 2433 } 2434 2435 // After MSSAU update, remove the cloned terminator instruction NewTI. 2436 ParentBB->getTerminator()->eraseFromParent(); 2437 2438 // Create a new unconditional branch to the continuing block (as opposed to 2439 // the one cloned). 2440 BranchInst::Create(RetainedSuccBB, ParentBB); 2441 } else { 2442 assert(BI && "Only branches have partial unswitching."); 2443 assert(UnswitchedSuccBBs.size() == 1 && 2444 "Only one possible unswitched block for a branch!"); 2445 BasicBlock *ClonedPH = ClonedPHs.begin()->second; 2446 // When doing a partial unswitch, we have to do a bit more work to build up 2447 // the branch in the split block. 2448 if (PartiallyInvariant) 2449 buildPartialInvariantUnswitchConditionalBranch( 2450 *SplitBB, Invariants, Direction, *ClonedPH, *LoopPH, L, MSSAU); 2451 else { 2452 buildPartialUnswitchConditionalBranch( 2453 *SplitBB, Invariants, Direction, *ClonedPH, *LoopPH, 2454 FreezeLoopUnswitchCond, BI, &AC, DT); 2455 } 2456 DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH}); 2457 2458 if (MSSAU) { 2459 DT.applyUpdates(DTUpdates); 2460 DTUpdates.clear(); 2461 2462 // Perform MSSA cloning updates. 2463 for (auto &VMap : VMaps) 2464 MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap, 2465 /*IgnoreIncomingWithNoClones=*/true); 2466 MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT); 2467 } 2468 } 2469 2470 // Apply the updates accumulated above to get an up-to-date dominator tree. 2471 DT.applyUpdates(DTUpdates); 2472 2473 // Now that we have an accurate dominator tree, first delete the dead cloned 2474 // blocks so that we can accurately build any cloned loops. It is important to 2475 // not delete the blocks from the original loop yet because we still want to 2476 // reference the original loop to understand the cloned loop's structure. 2477 deleteDeadClonedBlocks(L, ExitBlocks, VMaps, DT, MSSAU); 2478 2479 // Build the cloned loop structure itself. This may be substantially 2480 // different from the original structure due to the simplified CFG. This also 2481 // handles inserting all the cloned blocks into the correct loops. 2482 SmallVector<Loop *, 4> NonChildClonedLoops; 2483 for (std::unique_ptr<ValueToValueMapTy> &VMap : VMaps) 2484 buildClonedLoops(L, ExitBlocks, *VMap, LI, NonChildClonedLoops); 2485 2486 // Now that our cloned loops have been built, we can update the original loop. 2487 // First we delete the dead blocks from it and then we rebuild the loop 2488 // structure taking these deletions into account. 2489 deleteDeadBlocksFromLoop(L, ExitBlocks, DT, LI, MSSAU, SE, LoopUpdater); 2490 2491 if (MSSAU && VerifyMemorySSA) 2492 MSSAU->getMemorySSA()->verifyMemorySSA(); 2493 2494 SmallVector<Loop *, 4> HoistedLoops; 2495 bool IsStillLoop = 2496 rebuildLoopAfterUnswitch(L, ExitBlocks, LI, HoistedLoops, SE); 2497 2498 if (MSSAU && VerifyMemorySSA) 2499 MSSAU->getMemorySSA()->verifyMemorySSA(); 2500 2501 // This transformation has a high risk of corrupting the dominator tree, and 2502 // the below steps to rebuild loop structures will result in hard to debug 2503 // errors in that case so verify that the dominator tree is sane first. 2504 // FIXME: Remove this when the bugs stop showing up and rely on existing 2505 // verification steps. 2506 assert(DT.verify(DominatorTree::VerificationLevel::Fast)); 2507 2508 if (BI && !PartiallyInvariant) { 2509 // If we unswitched a branch which collapses the condition to a known 2510 // constant we want to replace all the uses of the invariants within both 2511 // the original and cloned blocks. We do this here so that we can use the 2512 // now updated dominator tree to identify which side the users are on. 2513 assert(UnswitchedSuccBBs.size() == 1 && 2514 "Only one possible unswitched block for a branch!"); 2515 BasicBlock *ClonedPH = ClonedPHs.begin()->second; 2516 2517 // When considering multiple partially-unswitched invariants 2518 // we cant just go replace them with constants in both branches. 2519 // 2520 // For 'AND' we infer that true branch ("continue") means true 2521 // for each invariant operand. 2522 // For 'OR' we can infer that false branch ("continue") means false 2523 // for each invariant operand. 2524 // So it happens that for multiple-partial case we dont replace 2525 // in the unswitched branch. 2526 bool ReplaceUnswitched = 2527 FullUnswitch || (Invariants.size() == 1) || PartiallyInvariant; 2528 2529 ConstantInt *UnswitchedReplacement = 2530 Direction ? ConstantInt::getTrue(BI->getContext()) 2531 : ConstantInt::getFalse(BI->getContext()); 2532 ConstantInt *ContinueReplacement = 2533 Direction ? ConstantInt::getFalse(BI->getContext()) 2534 : ConstantInt::getTrue(BI->getContext()); 2535 for (Value *Invariant : Invariants) { 2536 assert(!isa<Constant>(Invariant) && 2537 "Should not be replacing constant values!"); 2538 // Use make_early_inc_range here as set invalidates the iterator. 2539 for (Use &U : llvm::make_early_inc_range(Invariant->uses())) { 2540 Instruction *UserI = dyn_cast<Instruction>(U.getUser()); 2541 if (!UserI) 2542 continue; 2543 2544 // Replace it with the 'continue' side if in the main loop body, and the 2545 // unswitched if in the cloned blocks. 2546 if (DT.dominates(LoopPH, UserI->getParent())) 2547 U.set(ContinueReplacement); 2548 else if (ReplaceUnswitched && 2549 DT.dominates(ClonedPH, UserI->getParent())) 2550 U.set(UnswitchedReplacement); 2551 } 2552 } 2553 } 2554 2555 // We can change which blocks are exit blocks of all the cloned sibling 2556 // loops, the current loop, and any parent loops which shared exit blocks 2557 // with the current loop. As a consequence, we need to re-form LCSSA for 2558 // them. But we shouldn't need to re-form LCSSA for any child loops. 2559 // FIXME: This could be made more efficient by tracking which exit blocks are 2560 // new, and focusing on them, but that isn't likely to be necessary. 2561 // 2562 // In order to reasonably rebuild LCSSA we need to walk inside-out across the 2563 // loop nest and update every loop that could have had its exits changed. We 2564 // also need to cover any intervening loops. We add all of these loops to 2565 // a list and sort them by loop depth to achieve this without updating 2566 // unnecessary loops. 2567 auto UpdateLoop = [&](Loop &UpdateL) { 2568 #ifndef NDEBUG 2569 UpdateL.verifyLoop(); 2570 for (Loop *ChildL : UpdateL) { 2571 ChildL->verifyLoop(); 2572 assert(ChildL->isRecursivelyLCSSAForm(DT, LI) && 2573 "Perturbed a child loop's LCSSA form!"); 2574 } 2575 #endif 2576 // First build LCSSA for this loop so that we can preserve it when 2577 // forming dedicated exits. We don't want to perturb some other loop's 2578 // LCSSA while doing that CFG edit. 2579 formLCSSA(UpdateL, DT, &LI, SE); 2580 2581 // For loops reached by this loop's original exit blocks we may 2582 // introduced new, non-dedicated exits. At least try to re-form dedicated 2583 // exits for these loops. This may fail if they couldn't have dedicated 2584 // exits to start with. 2585 formDedicatedExitBlocks(&UpdateL, &DT, &LI, MSSAU, /*PreserveLCSSA*/ true); 2586 }; 2587 2588 // For non-child cloned loops and hoisted loops, we just need to update LCSSA 2589 // and we can do it in any order as they don't nest relative to each other. 2590 // 2591 // Also check if any of the loops we have updated have become top-level loops 2592 // as that will necessitate widening the outer loop scope. 2593 for (Loop *UpdatedL : 2594 llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops)) { 2595 UpdateLoop(*UpdatedL); 2596 if (UpdatedL->isOutermost()) 2597 OuterExitL = nullptr; 2598 } 2599 if (IsStillLoop) { 2600 UpdateLoop(L); 2601 if (L.isOutermost()) 2602 OuterExitL = nullptr; 2603 } 2604 2605 // If the original loop had exit blocks, walk up through the outer most loop 2606 // of those exit blocks to update LCSSA and form updated dedicated exits. 2607 if (OuterExitL != &L) 2608 for (Loop *OuterL = ParentL; OuterL != OuterExitL; 2609 OuterL = OuterL->getParentLoop()) 2610 UpdateLoop(*OuterL); 2611 2612 #ifndef NDEBUG 2613 // Verify the entire loop structure to catch any incorrect updates before we 2614 // progress in the pass pipeline. 2615 LI.verify(DT); 2616 #endif 2617 2618 // Now that we've unswitched something, make callbacks to report the changes. 2619 // For that we need to merge together the updated loops and the cloned loops 2620 // and check whether the original loop survived. 2621 SmallVector<Loop *, 4> SibLoops; 2622 for (Loop *UpdatedL : llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops)) 2623 if (UpdatedL->getParentLoop() == ParentL) 2624 SibLoops.push_back(UpdatedL); 2625 postUnswitch(L, LoopUpdater, LoopName, IsStillLoop, PartiallyInvariant, 2626 InjectedCondition, SibLoops); 2627 2628 if (MSSAU && VerifyMemorySSA) 2629 MSSAU->getMemorySSA()->verifyMemorySSA(); 2630 2631 if (BI) 2632 ++NumBranches; 2633 else 2634 ++NumSwitches; 2635 } 2636 2637 /// Recursively compute the cost of a dominator subtree based on the per-block 2638 /// cost map provided. 2639 /// 2640 /// The recursive computation is memozied into the provided DT-indexed cost map 2641 /// to allow querying it for most nodes in the domtree without it becoming 2642 /// quadratic. 2643 static InstructionCost computeDomSubtreeCost( 2644 DomTreeNode &N, 2645 const SmallDenseMap<BasicBlock *, InstructionCost, 4> &BBCostMap, 2646 SmallDenseMap<DomTreeNode *, InstructionCost, 4> &DTCostMap) { 2647 // Don't accumulate cost (or recurse through) blocks not in our block cost 2648 // map and thus not part of the duplication cost being considered. 2649 auto BBCostIt = BBCostMap.find(N.getBlock()); 2650 if (BBCostIt == BBCostMap.end()) 2651 return 0; 2652 2653 // Lookup this node to see if we already computed its cost. 2654 auto DTCostIt = DTCostMap.find(&N); 2655 if (DTCostIt != DTCostMap.end()) 2656 return DTCostIt->second; 2657 2658 // If not, we have to compute it. We can't use insert above and update 2659 // because computing the cost may insert more things into the map. 2660 InstructionCost Cost = std::accumulate( 2661 N.begin(), N.end(), BBCostIt->second, 2662 [&](InstructionCost Sum, DomTreeNode *ChildN) -> InstructionCost { 2663 return Sum + computeDomSubtreeCost(*ChildN, BBCostMap, DTCostMap); 2664 }); 2665 bool Inserted = DTCostMap.insert({&N, Cost}).second; 2666 (void)Inserted; 2667 assert(Inserted && "Should not insert a node while visiting children!"); 2668 return Cost; 2669 } 2670 2671 /// Turns a select instruction into implicit control flow branch, 2672 /// making the following replacement: 2673 /// 2674 /// head: 2675 /// --code before select-- 2676 /// select %cond, %trueval, %falseval 2677 /// --code after select-- 2678 /// 2679 /// into 2680 /// 2681 /// head: 2682 /// --code before select-- 2683 /// br i1 %cond, label %then, label %tail 2684 /// 2685 /// then: 2686 /// br %tail 2687 /// 2688 /// tail: 2689 /// phi [ %trueval, %then ], [ %falseval, %head] 2690 /// unreachable 2691 /// 2692 /// It also makes all relevant DT and LI updates, so that all structures are in 2693 /// valid state after this transform. 2694 static BranchInst *turnSelectIntoBranch(SelectInst *SI, DominatorTree &DT, 2695 LoopInfo &LI, MemorySSAUpdater *MSSAU, 2696 AssumptionCache *AC) { 2697 LLVM_DEBUG(dbgs() << "Turning " << *SI << " into a branch.\n"); 2698 BasicBlock *HeadBB = SI->getParent(); 2699 2700 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); 2701 SplitBlockAndInsertIfThen(SI->getCondition(), SI, false, 2702 SI->getMetadata(LLVMContext::MD_prof), &DTU, &LI); 2703 auto *CondBr = cast<BranchInst>(HeadBB->getTerminator()); 2704 BasicBlock *ThenBB = CondBr->getSuccessor(0), 2705 *TailBB = CondBr->getSuccessor(1); 2706 if (MSSAU) 2707 MSSAU->moveAllAfterSpliceBlocks(HeadBB, TailBB, SI); 2708 2709 PHINode *Phi = 2710 PHINode::Create(SI->getType(), 2, "unswitched.select", SI->getIterator()); 2711 Phi->addIncoming(SI->getTrueValue(), ThenBB); 2712 Phi->addIncoming(SI->getFalseValue(), HeadBB); 2713 SI->replaceAllUsesWith(Phi); 2714 SI->eraseFromParent(); 2715 2716 if (MSSAU && VerifyMemorySSA) 2717 MSSAU->getMemorySSA()->verifyMemorySSA(); 2718 2719 ++NumSelects; 2720 return CondBr; 2721 } 2722 2723 /// Turns a llvm.experimental.guard intrinsic into implicit control flow branch, 2724 /// making the following replacement: 2725 /// 2726 /// --code before guard-- 2727 /// call void (i1, ...) @llvm.experimental.guard(i1 %cond) [ "deopt"() ] 2728 /// --code after guard-- 2729 /// 2730 /// into 2731 /// 2732 /// --code before guard-- 2733 /// br i1 %cond, label %guarded, label %deopt 2734 /// 2735 /// guarded: 2736 /// --code after guard-- 2737 /// 2738 /// deopt: 2739 /// call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ] 2740 /// unreachable 2741 /// 2742 /// It also makes all relevant DT and LI updates, so that all structures are in 2743 /// valid state after this transform. 2744 static BranchInst *turnGuardIntoBranch(IntrinsicInst *GI, Loop &L, 2745 DominatorTree &DT, LoopInfo &LI, 2746 MemorySSAUpdater *MSSAU) { 2747 SmallVector<DominatorTree::UpdateType, 4> DTUpdates; 2748 LLVM_DEBUG(dbgs() << "Turning " << *GI << " into a branch.\n"); 2749 BasicBlock *CheckBB = GI->getParent(); 2750 2751 if (MSSAU && VerifyMemorySSA) 2752 MSSAU->getMemorySSA()->verifyMemorySSA(); 2753 2754 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); 2755 Instruction *DeoptBlockTerm = 2756 SplitBlockAndInsertIfThen(GI->getArgOperand(0), GI, true, 2757 GI->getMetadata(LLVMContext::MD_prof), &DTU, &LI); 2758 BranchInst *CheckBI = cast<BranchInst>(CheckBB->getTerminator()); 2759 // SplitBlockAndInsertIfThen inserts control flow that branches to 2760 // DeoptBlockTerm if the condition is true. We want the opposite. 2761 CheckBI->swapSuccessors(); 2762 2763 BasicBlock *GuardedBlock = CheckBI->getSuccessor(0); 2764 GuardedBlock->setName("guarded"); 2765 CheckBI->getSuccessor(1)->setName("deopt"); 2766 BasicBlock *DeoptBlock = CheckBI->getSuccessor(1); 2767 2768 if (MSSAU) 2769 MSSAU->moveAllAfterSpliceBlocks(CheckBB, GuardedBlock, GI); 2770 2771 GI->moveBefore(DeoptBlockTerm); 2772 GI->setArgOperand(0, ConstantInt::getFalse(GI->getContext())); 2773 2774 if (MSSAU) { 2775 MemoryDef *MD = cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(GI)); 2776 MSSAU->moveToPlace(MD, DeoptBlock, MemorySSA::BeforeTerminator); 2777 if (VerifyMemorySSA) 2778 MSSAU->getMemorySSA()->verifyMemorySSA(); 2779 } 2780 2781 if (VerifyLoopInfo) 2782 LI.verify(DT); 2783 ++NumGuards; 2784 return CheckBI; 2785 } 2786 2787 /// Cost multiplier is a way to limit potentially exponential behavior 2788 /// of loop-unswitch. Cost is multipied in proportion of 2^number of unswitch 2789 /// candidates available. Also accounting for the number of "sibling" loops with 2790 /// the idea to account for previous unswitches that already happened on this 2791 /// cluster of loops. There was an attempt to keep this formula simple, 2792 /// just enough to limit the worst case behavior. Even if it is not that simple 2793 /// now it is still not an attempt to provide a detailed heuristic size 2794 /// prediction. 2795 /// 2796 /// TODO: Make a proper accounting of "explosion" effect for all kinds of 2797 /// unswitch candidates, making adequate predictions instead of wild guesses. 2798 /// That requires knowing not just the number of "remaining" candidates but 2799 /// also costs of unswitching for each of these candidates. 2800 static int CalculateUnswitchCostMultiplier( 2801 const Instruction &TI, const Loop &L, const LoopInfo &LI, 2802 const DominatorTree &DT, 2803 ArrayRef<NonTrivialUnswitchCandidate> UnswitchCandidates) { 2804 2805 // Guards and other exiting conditions do not contribute to exponential 2806 // explosion as soon as they dominate the latch (otherwise there might be 2807 // another path to the latch remaining that does not allow to eliminate the 2808 // loop copy on unswitch). 2809 const BasicBlock *Latch = L.getLoopLatch(); 2810 const BasicBlock *CondBlock = TI.getParent(); 2811 if (DT.dominates(CondBlock, Latch) && 2812 (isGuard(&TI) || 2813 (TI.isTerminator() && 2814 llvm::count_if(successors(&TI), [&L](const BasicBlock *SuccBB) { 2815 return L.contains(SuccBB); 2816 }) <= 1))) { 2817 NumCostMultiplierSkipped++; 2818 return 1; 2819 } 2820 2821 auto *ParentL = L.getParentLoop(); 2822 int SiblingsCount = (ParentL ? ParentL->getSubLoopsVector().size() 2823 : std::distance(LI.begin(), LI.end())); 2824 // Count amount of clones that all the candidates might cause during 2825 // unswitching. Branch/guard/select counts as 1, switch counts as log2 of its 2826 // cases. 2827 int UnswitchedClones = 0; 2828 for (const auto &Candidate : UnswitchCandidates) { 2829 const Instruction *CI = Candidate.TI; 2830 const BasicBlock *CondBlock = CI->getParent(); 2831 bool SkipExitingSuccessors = DT.dominates(CondBlock, Latch); 2832 if (isa<SelectInst>(CI)) { 2833 UnswitchedClones++; 2834 continue; 2835 } 2836 if (isGuard(CI)) { 2837 if (!SkipExitingSuccessors) 2838 UnswitchedClones++; 2839 continue; 2840 } 2841 int NonExitingSuccessors = 2842 llvm::count_if(successors(CondBlock), 2843 [SkipExitingSuccessors, &L](const BasicBlock *SuccBB) { 2844 return !SkipExitingSuccessors || L.contains(SuccBB); 2845 }); 2846 UnswitchedClones += Log2_32(NonExitingSuccessors); 2847 } 2848 2849 // Ignore up to the "unscaled candidates" number of unswitch candidates 2850 // when calculating the power-of-two scaling of the cost. The main idea 2851 // with this control is to allow a small number of unswitches to happen 2852 // and rely more on siblings multiplier (see below) when the number 2853 // of candidates is small. 2854 unsigned ClonesPower = 2855 std::max(UnswitchedClones - (int)UnswitchNumInitialUnscaledCandidates, 0); 2856 2857 // Allowing top-level loops to spread a bit more than nested ones. 2858 int SiblingsMultiplier = 2859 std::max((ParentL ? SiblingsCount 2860 : SiblingsCount / (int)UnswitchSiblingsToplevelDiv), 2861 1); 2862 // Compute the cost multiplier in a way that won't overflow by saturating 2863 // at an upper bound. 2864 int CostMultiplier; 2865 if (ClonesPower > Log2_32(UnswitchThreshold) || 2866 SiblingsMultiplier > UnswitchThreshold) 2867 CostMultiplier = UnswitchThreshold; 2868 else 2869 CostMultiplier = std::min(SiblingsMultiplier * (1 << ClonesPower), 2870 (int)UnswitchThreshold); 2871 2872 LLVM_DEBUG(dbgs() << " Computed multiplier " << CostMultiplier 2873 << " (siblings " << SiblingsMultiplier << " * clones " 2874 << (1 << ClonesPower) << ")" 2875 << " for unswitch candidate: " << TI << "\n"); 2876 return CostMultiplier; 2877 } 2878 2879 static bool collectUnswitchCandidates( 2880 SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates, 2881 IVConditionInfo &PartialIVInfo, Instruction *&PartialIVCondBranch, 2882 const Loop &L, const LoopInfo &LI, AAResults &AA, 2883 const MemorySSAUpdater *MSSAU) { 2884 assert(UnswitchCandidates.empty() && "Should be!"); 2885 2886 auto AddUnswitchCandidatesForInst = [&](Instruction *I, Value *Cond) { 2887 Cond = skipTrivialSelect(Cond); 2888 if (isa<Constant>(Cond)) 2889 return; 2890 if (L.isLoopInvariant(Cond)) { 2891 UnswitchCandidates.push_back({I, {Cond}}); 2892 return; 2893 } 2894 if (match(Cond, m_CombineOr(m_LogicalAnd(), m_LogicalOr()))) { 2895 TinyPtrVector<Value *> Invariants = 2896 collectHomogenousInstGraphLoopInvariants( 2897 L, *static_cast<Instruction *>(Cond), LI); 2898 if (!Invariants.empty()) 2899 UnswitchCandidates.push_back({I, std::move(Invariants)}); 2900 } 2901 }; 2902 2903 // Whether or not we should also collect guards in the loop. 2904 bool CollectGuards = false; 2905 if (UnswitchGuards) { 2906 auto *GuardDecl = L.getHeader()->getParent()->getParent()->getFunction( 2907 Intrinsic::getName(Intrinsic::experimental_guard)); 2908 if (GuardDecl && !GuardDecl->use_empty()) 2909 CollectGuards = true; 2910 } 2911 2912 for (auto *BB : L.blocks()) { 2913 if (LI.getLoopFor(BB) != &L) 2914 continue; 2915 2916 for (auto &I : *BB) { 2917 if (auto *SI = dyn_cast<SelectInst>(&I)) { 2918 auto *Cond = SI->getCondition(); 2919 // Do not unswitch vector selects and logical and/or selects 2920 if (Cond->getType()->isIntegerTy(1) && !SI->getType()->isIntegerTy(1)) 2921 AddUnswitchCandidatesForInst(SI, Cond); 2922 } else if (CollectGuards && isGuard(&I)) { 2923 auto *Cond = 2924 skipTrivialSelect(cast<IntrinsicInst>(&I)->getArgOperand(0)); 2925 // TODO: Support AND, OR conditions and partial unswitching. 2926 if (!isa<Constant>(Cond) && L.isLoopInvariant(Cond)) 2927 UnswitchCandidates.push_back({&I, {Cond}}); 2928 } 2929 } 2930 2931 if (auto *SI = dyn_cast<SwitchInst>(BB->getTerminator())) { 2932 // We can only consider fully loop-invariant switch conditions as we need 2933 // to completely eliminate the switch after unswitching. 2934 if (!isa<Constant>(SI->getCondition()) && 2935 L.isLoopInvariant(SI->getCondition()) && !BB->getUniqueSuccessor()) 2936 UnswitchCandidates.push_back({SI, {SI->getCondition()}}); 2937 continue; 2938 } 2939 2940 auto *BI = dyn_cast<BranchInst>(BB->getTerminator()); 2941 if (!BI || !BI->isConditional() || 2942 BI->getSuccessor(0) == BI->getSuccessor(1)) 2943 continue; 2944 2945 AddUnswitchCandidatesForInst(BI, BI->getCondition()); 2946 } 2947 2948 if (MSSAU && !findOptionMDForLoop(&L, "llvm.loop.unswitch.partial.disable") && 2949 !any_of(UnswitchCandidates, [&L](auto &TerminatorAndInvariants) { 2950 return TerminatorAndInvariants.TI == L.getHeader()->getTerminator(); 2951 })) { 2952 MemorySSA *MSSA = MSSAU->getMemorySSA(); 2953 if (auto Info = hasPartialIVCondition(L, MSSAThreshold, *MSSA, AA)) { 2954 LLVM_DEBUG( 2955 dbgs() << "simple-loop-unswitch: Found partially invariant condition " 2956 << *Info->InstToDuplicate[0] << "\n"); 2957 PartialIVInfo = *Info; 2958 PartialIVCondBranch = L.getHeader()->getTerminator(); 2959 TinyPtrVector<Value *> ValsToDuplicate; 2960 llvm::append_range(ValsToDuplicate, Info->InstToDuplicate); 2961 UnswitchCandidates.push_back( 2962 {L.getHeader()->getTerminator(), std::move(ValsToDuplicate)}); 2963 } 2964 } 2965 return !UnswitchCandidates.empty(); 2966 } 2967 2968 /// Tries to canonicalize condition described by: 2969 /// 2970 /// br (LHS pred RHS), label IfTrue, label IfFalse 2971 /// 2972 /// into its equivalent where `Pred` is something that we support for injected 2973 /// invariants (so far it is limited to ult), LHS in canonicalized form is 2974 /// non-invariant and RHS is an invariant. 2975 static void canonicalizeForInvariantConditionInjection( 2976 ICmpInst::Predicate &Pred, Value *&LHS, Value *&RHS, BasicBlock *&IfTrue, 2977 BasicBlock *&IfFalse, const Loop &L) { 2978 if (!L.contains(IfTrue)) { 2979 Pred = ICmpInst::getInversePredicate(Pred); 2980 std::swap(IfTrue, IfFalse); 2981 } 2982 2983 // Move loop-invariant argument to RHS position. 2984 if (L.isLoopInvariant(LHS)) { 2985 Pred = ICmpInst::getSwappedPredicate(Pred); 2986 std::swap(LHS, RHS); 2987 } 2988 2989 if (Pred == ICmpInst::ICMP_SGE && match(RHS, m_Zero())) { 2990 // Turn "x >=s 0" into "x <u UMIN_INT" 2991 Pred = ICmpInst::ICMP_ULT; 2992 RHS = ConstantInt::get( 2993 RHS->getContext(), 2994 APInt::getSignedMinValue(RHS->getType()->getIntegerBitWidth())); 2995 } 2996 } 2997 2998 /// Returns true, if predicate described by ( \p Pred, \p LHS, \p RHS ) 2999 /// succeeding into blocks ( \p IfTrue, \p IfFalse) can be optimized by 3000 /// injecting a loop-invariant condition. 3001 static bool shouldTryInjectInvariantCondition( 3002 const ICmpInst::Predicate Pred, const Value *LHS, const Value *RHS, 3003 const BasicBlock *IfTrue, const BasicBlock *IfFalse, const Loop &L) { 3004 if (L.isLoopInvariant(LHS) || !L.isLoopInvariant(RHS)) 3005 return false; 3006 // TODO: Support other predicates. 3007 if (Pred != ICmpInst::ICMP_ULT) 3008 return false; 3009 // TODO: Support non-loop-exiting branches? 3010 if (!L.contains(IfTrue) || L.contains(IfFalse)) 3011 return false; 3012 // FIXME: For some reason this causes problems with MSSA updates, need to 3013 // investigate why. So far, just don't unswitch latch. 3014 if (L.getHeader() == IfTrue) 3015 return false; 3016 return true; 3017 } 3018 3019 /// Returns true, if metadata on \p BI allows us to optimize branching into \p 3020 /// TakenSucc via injection of invariant conditions. The branch should be not 3021 /// enough and not previously unswitched, the information about this comes from 3022 /// the metadata. 3023 bool shouldTryInjectBasingOnMetadata(const BranchInst *BI, 3024 const BasicBlock *TakenSucc) { 3025 SmallVector<uint32_t> Weights; 3026 if (!extractBranchWeights(*BI, Weights)) 3027 return false; 3028 unsigned T = InjectInvariantConditionHotnesThreshold; 3029 BranchProbability LikelyTaken(T - 1, T); 3030 3031 assert(Weights.size() == 2 && "Unexpected profile data!"); 3032 size_t Idx = BI->getSuccessor(0) == TakenSucc ? 0 : 1; 3033 auto Num = Weights[Idx]; 3034 auto Denom = Weights[0] + Weights[1]; 3035 // Degenerate or overflowed metadata. 3036 if (Denom == 0 || Num > Denom) 3037 return false; 3038 BranchProbability ActualTaken(Num, Denom); 3039 if (LikelyTaken > ActualTaken) 3040 return false; 3041 return true; 3042 } 3043 3044 /// Materialize pending invariant condition of the given candidate into IR. The 3045 /// injected loop-invariant condition implies the original loop-variant branch 3046 /// condition, so the materialization turns 3047 /// 3048 /// loop_block: 3049 /// ... 3050 /// br i1 %variant_cond, label InLoopSucc, label OutOfLoopSucc 3051 /// 3052 /// into 3053 /// 3054 /// preheader: 3055 /// %invariant_cond = LHS pred RHS 3056 /// ... 3057 /// loop_block: 3058 /// br i1 %invariant_cond, label InLoopSucc, label OriginalCheck 3059 /// OriginalCheck: 3060 /// br i1 %variant_cond, label InLoopSucc, label OutOfLoopSucc 3061 /// ... 3062 static NonTrivialUnswitchCandidate 3063 injectPendingInvariantConditions(NonTrivialUnswitchCandidate Candidate, Loop &L, 3064 DominatorTree &DT, LoopInfo &LI, 3065 AssumptionCache &AC, MemorySSAUpdater *MSSAU) { 3066 assert(Candidate.hasPendingInjection() && "Nothing to inject!"); 3067 BasicBlock *Preheader = L.getLoopPreheader(); 3068 assert(Preheader && "Loop is not in simplified form?"); 3069 assert(LI.getLoopFor(Candidate.TI->getParent()) == &L && 3070 "Unswitching branch of inner loop!"); 3071 3072 auto Pred = Candidate.PendingInjection->Pred; 3073 auto *LHS = Candidate.PendingInjection->LHS; 3074 auto *RHS = Candidate.PendingInjection->RHS; 3075 auto *InLoopSucc = Candidate.PendingInjection->InLoopSucc; 3076 auto *TI = cast<BranchInst>(Candidate.TI); 3077 auto *BB = Candidate.TI->getParent(); 3078 auto *OutOfLoopSucc = InLoopSucc == TI->getSuccessor(0) ? TI->getSuccessor(1) 3079 : TI->getSuccessor(0); 3080 // FIXME: Remove this once limitation on successors is lifted. 3081 assert(L.contains(InLoopSucc) && "Not supported yet!"); 3082 assert(!L.contains(OutOfLoopSucc) && "Not supported yet!"); 3083 auto &Ctx = BB->getContext(); 3084 3085 IRBuilder<> Builder(Preheader->getTerminator()); 3086 assert(ICmpInst::isUnsigned(Pred) && "Not supported yet!"); 3087 if (LHS->getType() != RHS->getType()) { 3088 if (LHS->getType()->getIntegerBitWidth() < 3089 RHS->getType()->getIntegerBitWidth()) 3090 LHS = Builder.CreateZExt(LHS, RHS->getType(), LHS->getName() + ".wide"); 3091 else 3092 RHS = Builder.CreateZExt(RHS, LHS->getType(), RHS->getName() + ".wide"); 3093 } 3094 // Do not use builder here: CreateICmp may simplify this into a constant and 3095 // unswitching will break. Better optimize it away later. 3096 auto *InjectedCond = 3097 ICmpInst::Create(Instruction::ICmp, Pred, LHS, RHS, "injected.cond", 3098 Preheader->getTerminator()->getIterator()); 3099 3100 BasicBlock *CheckBlock = BasicBlock::Create(Ctx, BB->getName() + ".check", 3101 BB->getParent(), InLoopSucc); 3102 Builder.SetInsertPoint(TI); 3103 auto *InvariantBr = 3104 Builder.CreateCondBr(InjectedCond, InLoopSucc, CheckBlock); 3105 3106 Builder.SetInsertPoint(CheckBlock); 3107 Builder.CreateCondBr(TI->getCondition(), TI->getSuccessor(0), 3108 TI->getSuccessor(1)); 3109 TI->eraseFromParent(); 3110 3111 // Fixup phis. 3112 for (auto &I : *InLoopSucc) { 3113 auto *PN = dyn_cast<PHINode>(&I); 3114 if (!PN) 3115 break; 3116 auto *Inc = PN->getIncomingValueForBlock(BB); 3117 PN->addIncoming(Inc, CheckBlock); 3118 } 3119 OutOfLoopSucc->replacePhiUsesWith(BB, CheckBlock); 3120 3121 SmallVector<DominatorTree::UpdateType, 4> DTUpdates = { 3122 { DominatorTree::Insert, BB, CheckBlock }, 3123 { DominatorTree::Insert, CheckBlock, InLoopSucc }, 3124 { DominatorTree::Insert, CheckBlock, OutOfLoopSucc }, 3125 { DominatorTree::Delete, BB, OutOfLoopSucc } 3126 }; 3127 3128 DT.applyUpdates(DTUpdates); 3129 if (MSSAU) 3130 MSSAU->applyUpdates(DTUpdates, DT); 3131 L.addBasicBlockToLoop(CheckBlock, LI); 3132 3133 #ifndef NDEBUG 3134 DT.verify(); 3135 LI.verify(DT); 3136 if (MSSAU && VerifyMemorySSA) 3137 MSSAU->getMemorySSA()->verifyMemorySSA(); 3138 #endif 3139 3140 // TODO: In fact, cost of unswitching a new invariant candidate is *slightly* 3141 // higher because we have just inserted a new block. Need to think how to 3142 // adjust the cost of injected candidates when it was first computed. 3143 LLVM_DEBUG(dbgs() << "Injected a new loop-invariant branch " << *InvariantBr 3144 << " and considering it for unswitching."); 3145 ++NumInvariantConditionsInjected; 3146 return NonTrivialUnswitchCandidate(InvariantBr, { InjectedCond }, 3147 Candidate.Cost); 3148 } 3149 3150 /// Given chain of loop branch conditions looking like: 3151 /// br (Variant < Invariant1) 3152 /// br (Variant < Invariant2) 3153 /// br (Variant < Invariant3) 3154 /// ... 3155 /// collect set of invariant conditions on which we want to unswitch, which 3156 /// look like: 3157 /// Invariant1 <= Invariant2 3158 /// Invariant2 <= Invariant3 3159 /// ... 3160 /// Though they might not immediately exist in the IR, we can still inject them. 3161 static bool insertCandidatesWithPendingInjections( 3162 SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates, Loop &L, 3163 ICmpInst::Predicate Pred, ArrayRef<CompareDesc> Compares, 3164 const DominatorTree &DT) { 3165 3166 assert(ICmpInst::isRelational(Pred)); 3167 assert(ICmpInst::isStrictPredicate(Pred)); 3168 if (Compares.size() < 2) 3169 return false; 3170 ICmpInst::Predicate NonStrictPred = ICmpInst::getNonStrictPredicate(Pred); 3171 for (auto Prev = Compares.begin(), Next = Compares.begin() + 1; 3172 Next != Compares.end(); ++Prev, ++Next) { 3173 Value *LHS = Next->Invariant; 3174 Value *RHS = Prev->Invariant; 3175 BasicBlock *InLoopSucc = Prev->InLoopSucc; 3176 InjectedInvariant ToInject(NonStrictPred, LHS, RHS, InLoopSucc); 3177 NonTrivialUnswitchCandidate Candidate(Prev->Term, { LHS, RHS }, 3178 std::nullopt, std::move(ToInject)); 3179 UnswitchCandidates.push_back(std::move(Candidate)); 3180 } 3181 return true; 3182 } 3183 3184 /// Collect unswitch candidates by invariant conditions that are not immediately 3185 /// present in the loop. However, they can be injected into the code if we 3186 /// decide it's profitable. 3187 /// An example of such conditions is following: 3188 /// 3189 /// for (...) { 3190 /// x = load ... 3191 /// if (! x <u C1) break; 3192 /// if (! x <u C2) break; 3193 /// <do something> 3194 /// } 3195 /// 3196 /// We can unswitch by condition "C1 <=u C2". If that is true, then "x <u C1 <= 3197 /// C2" automatically implies "x <u C2", so we can get rid of one of 3198 /// loop-variant checks in unswitched loop version. 3199 static bool collectUnswitchCandidatesWithInjections( 3200 SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates, 3201 IVConditionInfo &PartialIVInfo, Instruction *&PartialIVCondBranch, Loop &L, 3202 const DominatorTree &DT, const LoopInfo &LI, AAResults &AA, 3203 const MemorySSAUpdater *MSSAU) { 3204 if (!InjectInvariantConditions) 3205 return false; 3206 3207 if (!DT.isReachableFromEntry(L.getHeader())) 3208 return false; 3209 auto *Latch = L.getLoopLatch(); 3210 // Need to have a single latch and a preheader. 3211 if (!Latch) 3212 return false; 3213 assert(L.getLoopPreheader() && "Must have a preheader!"); 3214 3215 DenseMap<Value *, SmallVector<CompareDesc, 4> > CandidatesULT; 3216 // Traverse the conditions that dominate latch (and therefore dominate each 3217 // other). 3218 for (auto *DTN = DT.getNode(Latch); L.contains(DTN->getBlock()); 3219 DTN = DTN->getIDom()) { 3220 ICmpInst::Predicate Pred; 3221 Value *LHS = nullptr, *RHS = nullptr; 3222 BasicBlock *IfTrue = nullptr, *IfFalse = nullptr; 3223 auto *BB = DTN->getBlock(); 3224 // Ignore inner loops. 3225 if (LI.getLoopFor(BB) != &L) 3226 continue; 3227 auto *Term = BB->getTerminator(); 3228 if (!match(Term, m_Br(m_ICmp(Pred, m_Value(LHS), m_Value(RHS)), 3229 m_BasicBlock(IfTrue), m_BasicBlock(IfFalse)))) 3230 continue; 3231 if (!LHS->getType()->isIntegerTy()) 3232 continue; 3233 canonicalizeForInvariantConditionInjection(Pred, LHS, RHS, IfTrue, IfFalse, 3234 L); 3235 if (!shouldTryInjectInvariantCondition(Pred, LHS, RHS, IfTrue, IfFalse, L)) 3236 continue; 3237 if (!shouldTryInjectBasingOnMetadata(cast<BranchInst>(Term), IfTrue)) 3238 continue; 3239 // Strip ZEXT for unsigned predicate. 3240 // TODO: once signed predicates are supported, also strip SEXT. 3241 CompareDesc Desc(cast<BranchInst>(Term), RHS, IfTrue); 3242 while (auto *Zext = dyn_cast<ZExtInst>(LHS)) 3243 LHS = Zext->getOperand(0); 3244 CandidatesULT[LHS].push_back(Desc); 3245 } 3246 3247 bool Found = false; 3248 for (auto &It : CandidatesULT) 3249 Found |= insertCandidatesWithPendingInjections( 3250 UnswitchCandidates, L, ICmpInst::ICMP_ULT, It.second, DT); 3251 return Found; 3252 } 3253 3254 static bool isSafeForNoNTrivialUnswitching(Loop &L, LoopInfo &LI) { 3255 if (!L.isSafeToClone()) 3256 return false; 3257 for (auto *BB : L.blocks()) 3258 for (auto &I : *BB) { 3259 if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB)) 3260 return false; 3261 if (auto *CB = dyn_cast<CallBase>(&I)) { 3262 assert(!CB->cannotDuplicate() && "Checked by L.isSafeToClone()."); 3263 if (CB->isConvergent()) 3264 return false; 3265 } 3266 } 3267 3268 // Check if there are irreducible CFG cycles in this loop. If so, we cannot 3269 // easily unswitch non-trivial edges out of the loop. Doing so might turn the 3270 // irreducible control flow into reducible control flow and introduce new 3271 // loops "out of thin air". If we ever discover important use cases for doing 3272 // this, we can add support to loop unswitch, but it is a lot of complexity 3273 // for what seems little or no real world benefit. 3274 LoopBlocksRPO RPOT(&L); 3275 RPOT.perform(&LI); 3276 if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI)) 3277 return false; 3278 3279 SmallVector<BasicBlock *, 4> ExitBlocks; 3280 L.getUniqueExitBlocks(ExitBlocks); 3281 // We cannot unswitch if exit blocks contain a cleanuppad/catchswitch 3282 // instruction as we don't know how to split those exit blocks. 3283 // FIXME: We should teach SplitBlock to handle this and remove this 3284 // restriction. 3285 for (auto *ExitBB : ExitBlocks) { 3286 auto *I = ExitBB->getFirstNonPHI(); 3287 if (isa<CleanupPadInst>(I) || isa<CatchSwitchInst>(I)) { 3288 LLVM_DEBUG(dbgs() << "Cannot unswitch because of cleanuppad/catchswitch " 3289 "in exit block\n"); 3290 return false; 3291 } 3292 } 3293 3294 return true; 3295 } 3296 3297 static NonTrivialUnswitchCandidate findBestNonTrivialUnswitchCandidate( 3298 ArrayRef<NonTrivialUnswitchCandidate> UnswitchCandidates, const Loop &L, 3299 const DominatorTree &DT, const LoopInfo &LI, AssumptionCache &AC, 3300 const TargetTransformInfo &TTI, const IVConditionInfo &PartialIVInfo) { 3301 // Given that unswitching these terminators will require duplicating parts of 3302 // the loop, so we need to be able to model that cost. Compute the ephemeral 3303 // values and set up a data structure to hold per-BB costs. We cache each 3304 // block's cost so that we don't recompute this when considering different 3305 // subsets of the loop for duplication during unswitching. 3306 SmallPtrSet<const Value *, 4> EphValues; 3307 CodeMetrics::collectEphemeralValues(&L, &AC, EphValues); 3308 SmallDenseMap<BasicBlock *, InstructionCost, 4> BBCostMap; 3309 3310 // Compute the cost of each block, as well as the total loop cost. Also, bail 3311 // out if we see instructions which are incompatible with loop unswitching 3312 // (convergent, noduplicate, or cross-basic-block tokens). 3313 // FIXME: We might be able to safely handle some of these in non-duplicated 3314 // regions. 3315 TargetTransformInfo::TargetCostKind CostKind = 3316 L.getHeader()->getParent()->hasMinSize() 3317 ? TargetTransformInfo::TCK_CodeSize 3318 : TargetTransformInfo::TCK_SizeAndLatency; 3319 InstructionCost LoopCost = 0; 3320 for (auto *BB : L.blocks()) { 3321 InstructionCost Cost = 0; 3322 for (auto &I : *BB) { 3323 if (EphValues.count(&I)) 3324 continue; 3325 Cost += TTI.getInstructionCost(&I, CostKind); 3326 } 3327 assert(Cost >= 0 && "Must not have negative costs!"); 3328 LoopCost += Cost; 3329 assert(LoopCost >= 0 && "Must not have negative loop costs!"); 3330 BBCostMap[BB] = Cost; 3331 } 3332 LLVM_DEBUG(dbgs() << " Total loop cost: " << LoopCost << "\n"); 3333 3334 // Now we find the best candidate by searching for the one with the following 3335 // properties in order: 3336 // 3337 // 1) An unswitching cost below the threshold 3338 // 2) The smallest number of duplicated unswitch candidates (to avoid 3339 // creating redundant subsequent unswitching) 3340 // 3) The smallest cost after unswitching. 3341 // 3342 // We prioritize reducing fanout of unswitch candidates provided the cost 3343 // remains below the threshold because this has a multiplicative effect. 3344 // 3345 // This requires memoizing each dominator subtree to avoid redundant work. 3346 // 3347 // FIXME: Need to actually do the number of candidates part above. 3348 SmallDenseMap<DomTreeNode *, InstructionCost, 4> DTCostMap; 3349 // Given a terminator which might be unswitched, computes the non-duplicated 3350 // cost for that terminator. 3351 auto ComputeUnswitchedCost = [&](Instruction &TI, 3352 bool FullUnswitch) -> InstructionCost { 3353 // Unswitching selects unswitches the entire loop. 3354 if (isa<SelectInst>(TI)) 3355 return LoopCost; 3356 3357 BasicBlock &BB = *TI.getParent(); 3358 SmallPtrSet<BasicBlock *, 4> Visited; 3359 3360 InstructionCost Cost = 0; 3361 for (BasicBlock *SuccBB : successors(&BB)) { 3362 // Don't count successors more than once. 3363 if (!Visited.insert(SuccBB).second) 3364 continue; 3365 3366 // If this is a partial unswitch candidate, then it must be a conditional 3367 // branch with a condition of either `or`, `and`, their corresponding 3368 // select forms or partially invariant instructions. In that case, one of 3369 // the successors is necessarily duplicated, so don't even try to remove 3370 // its cost. 3371 if (!FullUnswitch) { 3372 auto &BI = cast<BranchInst>(TI); 3373 Value *Cond = skipTrivialSelect(BI.getCondition()); 3374 if (match(Cond, m_LogicalAnd())) { 3375 if (SuccBB == BI.getSuccessor(1)) 3376 continue; 3377 } else if (match(Cond, m_LogicalOr())) { 3378 if (SuccBB == BI.getSuccessor(0)) 3379 continue; 3380 } else if ((PartialIVInfo.KnownValue->isOneValue() && 3381 SuccBB == BI.getSuccessor(0)) || 3382 (!PartialIVInfo.KnownValue->isOneValue() && 3383 SuccBB == BI.getSuccessor(1))) 3384 continue; 3385 } 3386 3387 // This successor's domtree will not need to be duplicated after 3388 // unswitching if the edge to the successor dominates it (and thus the 3389 // entire tree). This essentially means there is no other path into this 3390 // subtree and so it will end up live in only one clone of the loop. 3391 if (SuccBB->getUniquePredecessor() || 3392 llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) { 3393 return PredBB == &BB || DT.dominates(SuccBB, PredBB); 3394 })) { 3395 Cost += computeDomSubtreeCost(*DT[SuccBB], BBCostMap, DTCostMap); 3396 assert(Cost <= LoopCost && 3397 "Non-duplicated cost should never exceed total loop cost!"); 3398 } 3399 } 3400 3401 // Now scale the cost by the number of unique successors minus one. We 3402 // subtract one because there is already at least one copy of the entire 3403 // loop. This is computing the new cost of unswitching a condition. 3404 // Note that guards always have 2 unique successors that are implicit and 3405 // will be materialized if we decide to unswitch it. 3406 int SuccessorsCount = isGuard(&TI) ? 2 : Visited.size(); 3407 assert(SuccessorsCount > 1 && 3408 "Cannot unswitch a condition without multiple distinct successors!"); 3409 return (LoopCost - Cost) * (SuccessorsCount - 1); 3410 }; 3411 3412 std::optional<NonTrivialUnswitchCandidate> Best; 3413 for (auto &Candidate : UnswitchCandidates) { 3414 Instruction &TI = *Candidate.TI; 3415 ArrayRef<Value *> Invariants = Candidate.Invariants; 3416 BranchInst *BI = dyn_cast<BranchInst>(&TI); 3417 bool FullUnswitch = 3418 !BI || Candidate.hasPendingInjection() || 3419 (Invariants.size() == 1 && 3420 Invariants[0] == skipTrivialSelect(BI->getCondition())); 3421 InstructionCost CandidateCost = ComputeUnswitchedCost(TI, FullUnswitch); 3422 // Calculate cost multiplier which is a tool to limit potentially 3423 // exponential behavior of loop-unswitch. 3424 if (EnableUnswitchCostMultiplier) { 3425 int CostMultiplier = 3426 CalculateUnswitchCostMultiplier(TI, L, LI, DT, UnswitchCandidates); 3427 assert( 3428 (CostMultiplier > 0 && CostMultiplier <= UnswitchThreshold) && 3429 "cost multiplier needs to be in the range of 1..UnswitchThreshold"); 3430 CandidateCost *= CostMultiplier; 3431 LLVM_DEBUG(dbgs() << " Computed cost of " << CandidateCost 3432 << " (multiplier: " << CostMultiplier << ")" 3433 << " for unswitch candidate: " << TI << "\n"); 3434 } else { 3435 LLVM_DEBUG(dbgs() << " Computed cost of " << CandidateCost 3436 << " for unswitch candidate: " << TI << "\n"); 3437 } 3438 3439 if (!Best || CandidateCost < Best->Cost) { 3440 Best = Candidate; 3441 Best->Cost = CandidateCost; 3442 } 3443 } 3444 assert(Best && "Must be!"); 3445 return *Best; 3446 } 3447 3448 // Insert a freeze on an unswitched branch if all is true: 3449 // 1. freeze-loop-unswitch-cond option is true 3450 // 2. The branch may not execute in the loop pre-transformation. If a branch may 3451 // not execute and could cause UB, it would always cause UB if it is hoisted outside 3452 // of the loop. Insert a freeze to prevent this case. 3453 // 3. The branch condition may be poison or undef 3454 static bool shouldInsertFreeze(Loop &L, Instruction &TI, DominatorTree &DT, 3455 AssumptionCache &AC) { 3456 assert(isa<BranchInst>(TI) || isa<SwitchInst>(TI)); 3457 if (!FreezeLoopUnswitchCond) 3458 return false; 3459 3460 ICFLoopSafetyInfo SafetyInfo; 3461 SafetyInfo.computeLoopSafetyInfo(&L); 3462 if (SafetyInfo.isGuaranteedToExecute(TI, &DT, &L)) 3463 return false; 3464 3465 Value *Cond; 3466 if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) 3467 Cond = skipTrivialSelect(BI->getCondition()); 3468 else 3469 Cond = skipTrivialSelect(cast<SwitchInst>(&TI)->getCondition()); 3470 return !isGuaranteedNotToBeUndefOrPoison( 3471 Cond, &AC, L.getLoopPreheader()->getTerminator(), &DT); 3472 } 3473 3474 static bool unswitchBestCondition(Loop &L, DominatorTree &DT, LoopInfo &LI, 3475 AssumptionCache &AC, AAResults &AA, 3476 TargetTransformInfo &TTI, ScalarEvolution *SE, 3477 MemorySSAUpdater *MSSAU, 3478 LPMUpdater &LoopUpdater) { 3479 // Collect all invariant conditions within this loop (as opposed to an inner 3480 // loop which would be handled when visiting that inner loop). 3481 SmallVector<NonTrivialUnswitchCandidate, 4> UnswitchCandidates; 3482 IVConditionInfo PartialIVInfo; 3483 Instruction *PartialIVCondBranch = nullptr; 3484 collectUnswitchCandidates(UnswitchCandidates, PartialIVInfo, 3485 PartialIVCondBranch, L, LI, AA, MSSAU); 3486 if (!findOptionMDForLoop(&L, "llvm.loop.unswitch.injection.disable")) 3487 collectUnswitchCandidatesWithInjections(UnswitchCandidates, PartialIVInfo, 3488 PartialIVCondBranch, L, DT, LI, AA, 3489 MSSAU); 3490 // If we didn't find any candidates, we're done. 3491 if (UnswitchCandidates.empty()) 3492 return false; 3493 3494 LLVM_DEBUG( 3495 dbgs() << "Considering " << UnswitchCandidates.size() 3496 << " non-trivial loop invariant conditions for unswitching.\n"); 3497 3498 NonTrivialUnswitchCandidate Best = findBestNonTrivialUnswitchCandidate( 3499 UnswitchCandidates, L, DT, LI, AC, TTI, PartialIVInfo); 3500 3501 assert(Best.TI && "Failed to find loop unswitch candidate"); 3502 assert(Best.Cost && "Failed to compute cost"); 3503 3504 if (*Best.Cost >= UnswitchThreshold) { 3505 LLVM_DEBUG(dbgs() << "Cannot unswitch, lowest cost found: " << *Best.Cost 3506 << "\n"); 3507 return false; 3508 } 3509 3510 bool InjectedCondition = false; 3511 if (Best.hasPendingInjection()) { 3512 Best = injectPendingInvariantConditions(Best, L, DT, LI, AC, MSSAU); 3513 InjectedCondition = true; 3514 } 3515 assert(!Best.hasPendingInjection() && 3516 "All injections should have been done by now!"); 3517 3518 if (Best.TI != PartialIVCondBranch) 3519 PartialIVInfo.InstToDuplicate.clear(); 3520 3521 bool InsertFreeze; 3522 if (auto *SI = dyn_cast<SelectInst>(Best.TI)) { 3523 // If the best candidate is a select, turn it into a branch. Select 3524 // instructions with a poison conditional do not propagate poison, but 3525 // branching on poison causes UB. Insert a freeze on the select 3526 // conditional to prevent UB after turning the select into a branch. 3527 InsertFreeze = !isGuaranteedNotToBeUndefOrPoison( 3528 SI->getCondition(), &AC, L.getLoopPreheader()->getTerminator(), &DT); 3529 Best.TI = turnSelectIntoBranch(SI, DT, LI, MSSAU, &AC); 3530 } else { 3531 // If the best candidate is a guard, turn it into a branch. 3532 if (isGuard(Best.TI)) 3533 Best.TI = 3534 turnGuardIntoBranch(cast<IntrinsicInst>(Best.TI), L, DT, LI, MSSAU); 3535 InsertFreeze = shouldInsertFreeze(L, *Best.TI, DT, AC); 3536 } 3537 3538 LLVM_DEBUG(dbgs() << " Unswitching non-trivial (cost = " << Best.Cost 3539 << ") terminator: " << *Best.TI << "\n"); 3540 unswitchNontrivialInvariants(L, *Best.TI, Best.Invariants, PartialIVInfo, DT, 3541 LI, AC, SE, MSSAU, LoopUpdater, InsertFreeze, 3542 InjectedCondition); 3543 return true; 3544 } 3545 3546 /// Unswitch control flow predicated on loop invariant conditions. 3547 /// 3548 /// This first hoists all branches or switches which are trivial (IE, do not 3549 /// require duplicating any part of the loop) out of the loop body. It then 3550 /// looks at other loop invariant control flows and tries to unswitch those as 3551 /// well by cloning the loop if the result is small enough. 3552 /// 3553 /// The `DT`, `LI`, `AC`, `AA`, `TTI` parameters are required analyses that are 3554 /// also updated based on the unswitch. The `MSSA` analysis is also updated if 3555 /// valid (i.e. its use is enabled). 3556 /// 3557 /// If either `NonTrivial` is true or the flag `EnableNonTrivialUnswitch` is 3558 /// true, we will attempt to do non-trivial unswitching as well as trivial 3559 /// unswitching. 3560 /// 3561 /// The `postUnswitch` function will be run after unswitching is complete 3562 /// with information on whether or not the provided loop remains a loop and 3563 /// a list of new sibling loops created. 3564 /// 3565 /// If `SE` is non-null, we will update that analysis based on the unswitching 3566 /// done. 3567 static bool unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI, 3568 AssumptionCache &AC, AAResults &AA, 3569 TargetTransformInfo &TTI, bool Trivial, 3570 bool NonTrivial, ScalarEvolution *SE, 3571 MemorySSAUpdater *MSSAU, ProfileSummaryInfo *PSI, 3572 BlockFrequencyInfo *BFI, LPMUpdater &LoopUpdater) { 3573 assert(L.isRecursivelyLCSSAForm(DT, LI) && 3574 "Loops must be in LCSSA form before unswitching."); 3575 3576 // Must be in loop simplified form: we need a preheader and dedicated exits. 3577 if (!L.isLoopSimplifyForm()) 3578 return false; 3579 3580 // Try trivial unswitch first before loop over other basic blocks in the loop. 3581 if (Trivial && unswitchAllTrivialConditions(L, DT, LI, SE, MSSAU)) { 3582 // If we unswitched successfully we will want to clean up the loop before 3583 // processing it further so just mark it as unswitched and return. 3584 postUnswitch(L, LoopUpdater, L.getName(), 3585 /*CurrentLoopValid*/ true, /*PartiallyInvariant*/ false, 3586 /*InjectedCondition*/ false, {}); 3587 return true; 3588 } 3589 3590 const Function *F = L.getHeader()->getParent(); 3591 3592 // Check whether we should continue with non-trivial conditions. 3593 // EnableNonTrivialUnswitch: Global variable that forces non-trivial 3594 // unswitching for testing and debugging. 3595 // NonTrivial: Parameter that enables non-trivial unswitching for this 3596 // invocation of the transform. But this should be allowed only 3597 // for targets without branch divergence. 3598 // 3599 // FIXME: If divergence analysis becomes available to a loop 3600 // transform, we should allow unswitching for non-trivial uniform 3601 // branches even on targets that have divergence. 3602 // https://bugs.llvm.org/show_bug.cgi?id=48819 3603 bool ContinueWithNonTrivial = 3604 EnableNonTrivialUnswitch || (NonTrivial && !TTI.hasBranchDivergence(F)); 3605 if (!ContinueWithNonTrivial) 3606 return false; 3607 3608 // Skip non-trivial unswitching for optsize functions. 3609 if (F->hasOptSize()) 3610 return false; 3611 3612 // Returns true if Loop L's loop nest is cold, i.e. if the headers of L, 3613 // of the loops L is nested in, and of the loops nested in L are all cold. 3614 auto IsLoopNestCold = [&](const Loop *L) { 3615 // Check L and all of its parent loops. 3616 auto *Parent = L; 3617 while (Parent) { 3618 if (!PSI->isColdBlock(Parent->getHeader(), BFI)) 3619 return false; 3620 Parent = Parent->getParentLoop(); 3621 } 3622 // Next check all loops nested within L. 3623 SmallVector<const Loop *, 4> Worklist; 3624 Worklist.insert(Worklist.end(), L->getSubLoops().begin(), 3625 L->getSubLoops().end()); 3626 while (!Worklist.empty()) { 3627 auto *CurLoop = Worklist.pop_back_val(); 3628 if (!PSI->isColdBlock(CurLoop->getHeader(), BFI)) 3629 return false; 3630 Worklist.insert(Worklist.end(), CurLoop->getSubLoops().begin(), 3631 CurLoop->getSubLoops().end()); 3632 } 3633 return true; 3634 }; 3635 3636 // Skip cold loops in cold loop nests, as unswitching them brings little 3637 // benefit but increases the code size 3638 if (PSI && PSI->hasProfileSummary() && BFI && IsLoopNestCold(&L)) { 3639 LLVM_DEBUG(dbgs() << " Skip cold loop: " << L << "\n"); 3640 return false; 3641 } 3642 3643 // Perform legality checks. 3644 if (!isSafeForNoNTrivialUnswitching(L, LI)) 3645 return false; 3646 3647 // For non-trivial unswitching, because it often creates new loops, we rely on 3648 // the pass manager to iterate on the loops rather than trying to immediately 3649 // reach a fixed point. There is no substantial advantage to iterating 3650 // internally, and if any of the new loops are simplified enough to contain 3651 // trivial unswitching we want to prefer those. 3652 3653 // Try to unswitch the best invariant condition. We prefer this full unswitch to 3654 // a partial unswitch when possible below the threshold. 3655 if (unswitchBestCondition(L, DT, LI, AC, AA, TTI, SE, MSSAU, LoopUpdater)) 3656 return true; 3657 3658 // No other opportunities to unswitch. 3659 return false; 3660 } 3661 3662 PreservedAnalyses SimpleLoopUnswitchPass::run(Loop &L, LoopAnalysisManager &AM, 3663 LoopStandardAnalysisResults &AR, 3664 LPMUpdater &U) { 3665 Function &F = *L.getHeader()->getParent(); 3666 (void)F; 3667 ProfileSummaryInfo *PSI = nullptr; 3668 if (auto OuterProxy = 3669 AM.getResult<FunctionAnalysisManagerLoopProxy>(L, AR) 3670 .getCachedResult<ModuleAnalysisManagerFunctionProxy>(F)) 3671 PSI = OuterProxy->getCachedResult<ProfileSummaryAnalysis>(*F.getParent()); 3672 LLVM_DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << L 3673 << "\n"); 3674 3675 std::optional<MemorySSAUpdater> MSSAU; 3676 if (AR.MSSA) { 3677 MSSAU = MemorySSAUpdater(AR.MSSA); 3678 if (VerifyMemorySSA) 3679 AR.MSSA->verifyMemorySSA(); 3680 } 3681 if (!unswitchLoop(L, AR.DT, AR.LI, AR.AC, AR.AA, AR.TTI, Trivial, NonTrivial, 3682 &AR.SE, MSSAU ? &*MSSAU : nullptr, PSI, AR.BFI, U)) 3683 return PreservedAnalyses::all(); 3684 3685 if (AR.MSSA && VerifyMemorySSA) 3686 AR.MSSA->verifyMemorySSA(); 3687 3688 // Historically this pass has had issues with the dominator tree so verify it 3689 // in asserts builds. 3690 assert(AR.DT.verify(DominatorTree::VerificationLevel::Fast)); 3691 3692 auto PA = getLoopPassPreservedAnalyses(); 3693 if (AR.MSSA) 3694 PA.preserve<MemorySSAAnalysis>(); 3695 return PA; 3696 } 3697 3698 void SimpleLoopUnswitchPass::printPipeline( 3699 raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) { 3700 static_cast<PassInfoMixin<SimpleLoopUnswitchPass> *>(this)->printPipeline( 3701 OS, MapClassName2PassName); 3702 3703 OS << '<'; 3704 OS << (NonTrivial ? "" : "no-") << "nontrivial;"; 3705 OS << (Trivial ? "" : "no-") << "trivial"; 3706 OS << '>'; 3707 } 3708