lib/Transforms/IPO/HotColdSplitting.cpp

   1 //===- HotColdSplitting.cpp -- Outline Cold Regions -------------*- C++ -*-===//
   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 /// \file
  10 /// The goal of hot/cold splitting is to improve the memory locality of code.
  11 /// The splitting pass does this by identifying cold blocks and moving them into
  12 /// separate functions.
  13 ///
  14 /// When the splitting pass finds a cold block (referred to as "the sink"), it
  15 /// grows a maximal cold region around that block. The maximal region contains
  16 /// all blocks (post-)dominated by the sink [*]. In theory, these blocks are as
  17 /// cold as the sink. Once a region is found, it's split out of the original
  18 /// function provided it's profitable to do so.
  19 ///
  20 /// [*] In practice, there is some added complexity because some blocks are not
  21 /// safe to extract.
  22 ///
  23 /// TODO: Use the PM to get domtrees, and preserve BFI/BPI.
  24 /// TODO: Reorder outlined functions.
  25 ///
  26 //===----------------------------------------------------------------------===//
  27
  28 #include "llvm/ADT/PostOrderIterator.h"
  29 #include "llvm/ADT/SmallVector.h"
  30 #include "llvm/ADT/Statistic.h"
  31 #include "llvm/Analysis/AliasAnalysis.h"
  32 #include "llvm/Analysis/BlockFrequencyInfo.h"
  33 #include "llvm/Analysis/BranchProbabilityInfo.h"
  34 #include "llvm/Analysis/CFG.h"
  35 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
  36 #include "llvm/Analysis/PostDominators.h"
  37 #include "llvm/Analysis/ProfileSummaryInfo.h"
  38 #include "llvm/Analysis/TargetTransformInfo.h"
  39 #include "llvm/IR/BasicBlock.h"
  40 #include "llvm/IR/CFG.h"
  41 #include "llvm/IR/CallSite.h"
  42 #include "llvm/IR/DataLayout.h"
  43 #include "llvm/IR/DiagnosticInfo.h"
  44 #include "llvm/IR/Dominators.h"
  45 #include "llvm/IR/Function.h"
  46 #include "llvm/IR/Instruction.h"
  47 #include "llvm/IR/Instructions.h"
  48 #include "llvm/IR/IntrinsicInst.h"
  49 #include "llvm/IR/Metadata.h"
  50 #include "llvm/IR/Module.h"
  51 #include "llvm/IR/PassManager.h"
  52 #include "llvm/IR/Type.h"
  53 #include "llvm/IR/Use.h"
  54 #include "llvm/IR/User.h"
  55 #include "llvm/IR/Value.h"
  56 #include "llvm/Pass.h"
  57 #include "llvm/Support/BlockFrequency.h"
  58 #include "llvm/Support/BranchProbability.h"
  59 #include "llvm/Support/Debug.h"
  60 #include "llvm/Support/raw_ostream.h"
  61 #include "llvm/Transforms/IPO.h"
  62 #include "llvm/Transforms/IPO/HotColdSplitting.h"
  63 #include "llvm/Transforms/Scalar.h"
  64 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  65 #include "llvm/Transforms/Utils/Cloning.h"
  66 #include "llvm/Transforms/Utils/CodeExtractor.h"
  67 #include "llvm/Transforms/Utils/Local.h"
  68 #include "llvm/Transforms/Utils/ValueMapper.h"
  69 #include <algorithm>
  70 #include <cassert>
  71
  72 #define DEBUG_TYPE "hotcoldsplit"
  73
  74 STATISTIC(NumColdRegionsFound, "Number of cold regions found.");
  75 STATISTIC(NumColdRegionsOutlined, "Number of cold regions outlined.");
  76
  77 using namespace llvm;
  78
  79 static cl::opt<bool> EnableStaticAnalyis("hot-cold-static-analysis",
  80                               cl::init(true), cl::Hidden);
  81
  82 static cl::opt<int>
  83     SplittingThreshold("hotcoldsplit-threshold", cl::init(2), cl::Hidden,
  84                        cl::desc("Base penalty for splitting cold code (as a "
  85                                 "multiple of TCC_Basic)"));
  86
  87 namespace {
  88
  89 /// A sequence of basic blocks.
  90 ///
  91 /// A 0-sized SmallVector is slightly cheaper to move than a std::vector.
  92 using BlockSequence = SmallVector<BasicBlock *, 0>;
  93
  94 // Same as blockEndsInUnreachable in CodeGen/BranchFolding.cpp. Do not modify
  95 // this function unless you modify the MBB version as well.
  96 //
  97 /// A no successor, non-return block probably ends in unreachable and is cold.
  98 /// Also consider a block that ends in an indirect branch to be a return block,
  99 /// since many targets use plain indirect branches to return.
 100 bool blockEndsInUnreachable(const BasicBlock &BB) {
 101   if (!succ_empty(&BB))
 102     return false;
 103   if (BB.empty())
 104     return true;
 105   const Instruction *I = BB.getTerminator();
 106   return !(isa<ReturnInst>(I) || isa<IndirectBrInst>(I));
 107 }
 108
 109 bool unlikelyExecuted(BasicBlock &BB) {
 110   // Exception handling blocks are unlikely executed.
 111   if (BB.isEHPad() || isa<ResumeInst>(BB.getTerminator()))
 112     return true;
 113
 114   // The block is cold if it calls/invokes a cold function.
 115   for (Instruction &I : BB)
 116     if (auto CS = CallSite(&I))
 117       if (CS.hasFnAttr(Attribute::Cold))
 118         return true;
 119
 120   // The block is cold if it has an unreachable terminator, unless it's
 121   // preceded by a call to a (possibly warm) noreturn call (e.g. longjmp).
 122   if (blockEndsInUnreachable(BB)) {
 123     if (auto *CI =
 124             dyn_cast_or_null<CallInst>(BB.getTerminator()->getPrevNode()))
 125       if (CI->hasFnAttr(Attribute::NoReturn))
 126         return false;
 127     return true;
 128   }
 129
 130   return false;
 131 }
 132
 133 /// Check whether it's safe to outline \p BB.
 134 static bool mayExtractBlock(const BasicBlock &BB) {
 135   // EH pads are unsafe to outline because doing so breaks EH type tables. It
 136   // follows that invoke instructions cannot be extracted, because CodeExtractor
 137   // requires unwind destinations to be within the extraction region.
 138   //
 139   // Resumes that are not reachable from a cleanup landing pad are considered to
 140   // be unreachable. It’s not safe to split them out either.
 141   auto Term = BB.getTerminator();
 142   return !BB.hasAddressTaken() && !BB.isEHPad() && !isa<InvokeInst>(Term) &&
 143          !isa<ResumeInst>(Term);
 144 }
 145
 146 /// Mark \p F cold. Based on this assumption, also optimize it for minimum size.
 147 /// If \p UpdateEntryCount is true (set when this is a new split function and
 148 /// module has profile data), set entry count to 0 to ensure treated as cold.
 149 /// Return true if the function is changed.
 150 static bool markFunctionCold(Function &F, bool UpdateEntryCount = false) {
 151   assert(!F.hasFnAttribute(Attribute::OptimizeNone) && "Can't mark this cold");
 152   bool Changed = false;
 153   if (!F.hasFnAttribute(Attribute::Cold)) {
 154     F.addFnAttr(Attribute::Cold);
 155     Changed = true;
 156   }
 157   if (!F.hasFnAttribute(Attribute::MinSize)) {
 158     F.addFnAttr(Attribute::MinSize);
 159     Changed = true;
 160   }
 161   if (UpdateEntryCount) {
 162     // Set the entry count to 0 to ensure it is placed in the unlikely text
 163     // section when function sections are enabled.
 164     F.setEntryCount(0);
 165     Changed = true;
 166   }
 167
 168   return Changed;
 169 }
 170
 171 class HotColdSplitting {
 172 public:
 173   HotColdSplitting(ProfileSummaryInfo *ProfSI,
 174                    function_ref<BlockFrequencyInfo *(Function &)> GBFI,
 175                    function_ref<TargetTransformInfo &(Function &)> GTTI,
 176                    std::function<OptimizationRemarkEmitter &(Function &)> *GORE,
 177                    function_ref<AssumptionCache *(Function &)> LAC)
 178       : PSI(ProfSI), GetBFI(GBFI), GetTTI(GTTI), GetORE(GORE), LookupAC(LAC) {}
 179   bool run(Module &M);
 180
 181 private:
 182   bool isFunctionCold(const Function &F) const;
 183   bool shouldOutlineFrom(const Function &F) const;
 184   bool outlineColdRegions(Function &F, bool HasProfileSummary);
 185   Function *extractColdRegion(const BlockSequence &Region, DominatorTree &DT,
 186                               BlockFrequencyInfo *BFI, TargetTransformInfo &TTI,
 187                               OptimizationRemarkEmitter &ORE,
 188                               AssumptionCache *AC, unsigned Count);
 189   ProfileSummaryInfo *PSI;
 190   function_ref<BlockFrequencyInfo *(Function &)> GetBFI;
 191   function_ref<TargetTransformInfo &(Function &)> GetTTI;
 192   std::function<OptimizationRemarkEmitter &(Function &)> *GetORE;
 193   function_ref<AssumptionCache *(Function &)> LookupAC;
 194 };
 195
 196 class HotColdSplittingLegacyPass : public ModulePass {
 197 public:
 198   static char ID;
 199   HotColdSplittingLegacyPass() : ModulePass(ID) {
 200     initializeHotColdSplittingLegacyPassPass(*PassRegistry::getPassRegistry());
 201   }
 202
 203   void getAnalysisUsage(AnalysisUsage &AU) const override {
 204     AU.addRequired<BlockFrequencyInfoWrapperPass>();
 205     AU.addRequired<ProfileSummaryInfoWrapperPass>();
 206     AU.addRequired<TargetTransformInfoWrapperPass>();
 207     AU.addUsedIfAvailable<AssumptionCacheTracker>();
 208   }
 209
 210   bool runOnModule(Module &M) override;
 211 };
 212
 213 } // end anonymous namespace
 214
 215 /// Check whether \p F is inherently cold.
 216 bool HotColdSplitting::isFunctionCold(const Function &F) const {
 217   if (F.hasFnAttribute(Attribute::Cold))
 218     return true;
 219
 220   if (F.getCallingConv() == CallingConv::Cold)
 221     return true;
 222
 223   if (PSI->isFunctionEntryCold(&F))
 224     return true;
 225
 226   return false;
 227 }
 228
 229 // Returns false if the function should not be considered for hot-cold split
 230 // optimization.
 231 bool HotColdSplitting::shouldOutlineFrom(const Function &F) const {
 232   if (F.hasFnAttribute(Attribute::AlwaysInline))
 233     return false;
 234
 235   if (F.hasFnAttribute(Attribute::NoInline))
 236     return false;
 237
 238   return true;
 239 }
 240
 241 /// Get the benefit score of outlining \p Region.
 242 static int getOutliningBenefit(ArrayRef<BasicBlock *> Region,
 243                                TargetTransformInfo &TTI) {
 244   // Sum up the code size costs of non-terminator instructions. Tight coupling
 245   // with \ref getOutliningPenalty is needed to model the costs of terminators.
 246   int Benefit = 0;
 247   for (BasicBlock *BB : Region)
 248     for (Instruction &I : BB->instructionsWithoutDebug())
 249       if (&I != BB->getTerminator())
 250         Benefit +=
 251             TTI.getInstructionCost(&I, TargetTransformInfo::TCK_CodeSize);
 252
 253   return Benefit;
 254 }
 255
 256 /// Get the penalty score for outlining \p Region.
 257 static int getOutliningPenalty(ArrayRef<BasicBlock *> Region,
 258                                unsigned NumInputs, unsigned NumOutputs) {
 259   int Penalty = SplittingThreshold;
 260   LLVM_DEBUG(dbgs() << "Applying penalty for splitting: " << Penalty << "\n");
 261
 262   // If the splitting threshold is set at or below zero, skip the usual
 263   // profitability check.
 264   if (SplittingThreshold <= 0)
 265     return Penalty;
 266
 267   // The typical code size cost for materializing an argument for the outlined
 268   // call.
 269   LLVM_DEBUG(dbgs() << "Applying penalty for: " << NumInputs << " inputs\n");
 270   const int CostForArgMaterialization = TargetTransformInfo::TCC_Basic;
 271   Penalty += CostForArgMaterialization * NumInputs;
 272
 273   // The typical code size cost for an output alloca, its associated store, and
 274   // its associated reload.
 275   LLVM_DEBUG(dbgs() << "Applying penalty for: " << NumOutputs << " outputs\n");
 276   const int CostForRegionOutput = 3 * TargetTransformInfo::TCC_Basic;
 277   Penalty += CostForRegionOutput * NumOutputs;
 278
 279   // Find the number of distinct exit blocks for the region. Use a conservative
 280   // check to determine whether control returns from the region.
 281   bool NoBlocksReturn = true;
 282   SmallPtrSet<BasicBlock *, 2> SuccsOutsideRegion;
 283   for (BasicBlock *BB : Region) {
 284     // If a block has no successors, only assume it does not return if it's
 285     // unreachable.
 286     if (succ_empty(BB)) {
 287       NoBlocksReturn &= isa<UnreachableInst>(BB->getTerminator());
 288       continue;
 289     }
 290
 291     for (BasicBlock *SuccBB : successors(BB)) {
 292       if (find(Region, SuccBB) == Region.end()) {
 293         NoBlocksReturn = false;
 294         SuccsOutsideRegion.insert(SuccBB);
 295       }
 296     }
 297   }
 298
 299   // Apply a `noreturn` bonus.
 300   if (NoBlocksReturn) {
 301     LLVM_DEBUG(dbgs() << "Applying bonus for: " << Region.size()
 302                       << " non-returning terminators\n");
 303     Penalty -= Region.size();
 304   }
 305
 306   // Apply a penalty for having more than one successor outside of the region.
 307   // This penalty accounts for the switch needed in the caller.
 308   if (!SuccsOutsideRegion.empty()) {
 309     LLVM_DEBUG(dbgs() << "Applying penalty for: " << SuccsOutsideRegion.size()
 310                       << " non-region successors\n");
 311     Penalty += (SuccsOutsideRegion.size() - 1) * TargetTransformInfo::TCC_Basic;
 312   }
 313
 314   return Penalty;
 315 }
 316
 317 Function *HotColdSplitting::extractColdRegion(const BlockSequence &Region,
 318                                               DominatorTree &DT,
 319                                               BlockFrequencyInfo *BFI,
 320                                               TargetTransformInfo &TTI,
 321                                               OptimizationRemarkEmitter &ORE,
 322                                               AssumptionCache *AC,
 323                                               unsigned Count) {
 324   assert(!Region.empty());
 325
 326   // TODO: Pass BFI and BPI to update profile information.
 327   CodeExtractor CE(Region, &DT, /* AggregateArgs */ false, /* BFI */ nullptr,
 328                    /* BPI */ nullptr, AC, /* AllowVarArgs */ false,
 329                    /* AllowAlloca */ false,
 330                    /* Suffix */ "cold." + std::to_string(Count));
 331
 332   // Perform a simple cost/benefit analysis to decide whether or not to permit
 333   // splitting.
 334   SetVector<Value *> Inputs, Outputs, Sinks;
 335   CE.findInputsOutputs(Inputs, Outputs, Sinks);
 336   int OutliningBenefit = getOutliningBenefit(Region, TTI);
 337   int OutliningPenalty =
 338       getOutliningPenalty(Region, Inputs.size(), Outputs.size());
 339   LLVM_DEBUG(dbgs() << "Split profitability: benefit = " << OutliningBenefit
 340                     << ", penalty = " << OutliningPenalty << "\n");
 341   if (OutliningBenefit <= OutliningPenalty)
 342     return nullptr;
 343
 344   Function *OrigF = Region[0]->getParent();
 345   if (Function *OutF = CE.extractCodeRegion()) {
 346     User *U = *OutF->user_begin();
 347     CallInst *CI = cast<CallInst>(U);
 348     CallSite CS(CI);
 349     NumColdRegionsOutlined++;
 350     if (TTI.useColdCCForColdCall(*OutF)) {
 351       OutF->setCallingConv(CallingConv::Cold);
 352       CS.setCallingConv(CallingConv::Cold);
 353     }
 354     CI->setIsNoInline();
 355
 356     markFunctionCold(*OutF, BFI != nullptr);
 357
 358     LLVM_DEBUG(llvm::dbgs() << "Outlined Region: " << *OutF);
 359     ORE.emit([&]() {
 360       return OptimizationRemark(DEBUG_TYPE, "HotColdSplit",
 361                                 &*Region[0]->begin())
 362              << ore::NV("Original", OrigF) << " split cold code into "
 363              << ore::NV("Split", OutF);
 364     });
 365     return OutF;
 366   }
 367
 368   ORE.emit([&]() {
 369     return OptimizationRemarkMissed(DEBUG_TYPE, "ExtractFailed",
 370                                     &*Region[0]->begin())
 371            << "Failed to extract region at block "
 372            << ore::NV("Block", Region.front());
 373   });
 374   return nullptr;
 375 }
 376
 377 /// A pair of (basic block, score).
 378 using BlockTy = std::pair<BasicBlock *, unsigned>;
 379
 380 namespace {
 381 /// A maximal outlining region. This contains all blocks post-dominated by a
 382 /// sink block, the sink block itself, and all blocks dominated by the sink.
 383 /// If sink-predecessors and sink-successors cannot be extracted in one region,
 384 /// the static constructor returns a list of suitable extraction regions.
 385 class OutliningRegion {
 386   /// A list of (block, score) pairs. A block's score is non-zero iff it's a
 387   /// viable sub-region entry point. Blocks with higher scores are better entry
 388   /// points (i.e. they are more distant ancestors of the sink block).
 389   SmallVector<BlockTy, 0> Blocks = {};
 390
 391   /// The suggested entry point into the region. If the region has multiple
 392   /// entry points, all blocks within the region may not be reachable from this
 393   /// entry point.
 394   BasicBlock *SuggestedEntryPoint = nullptr;
 395
 396   /// Whether the entire function is cold.
 397   bool EntireFunctionCold = false;
 398
 399   /// If \p BB is a viable entry point, return \p Score. Return 0 otherwise.
 400   static unsigned getEntryPointScore(BasicBlock &BB, unsigned Score) {
 401     return mayExtractBlock(BB) ? Score : 0;
 402   }
 403
 404   /// These scores should be lower than the score for predecessor blocks,
 405   /// because regions starting at predecessor blocks are typically larger.
 406   static constexpr unsigned ScoreForSuccBlock = 1;
 407   static constexpr unsigned ScoreForSinkBlock = 1;
 408
 409   OutliningRegion(const OutliningRegion &) = delete;
 410   OutliningRegion &operator=(const OutliningRegion &) = delete;
 411
 412 public:
 413   OutliningRegion() = default;
 414   OutliningRegion(OutliningRegion &&) = default;
 415   OutliningRegion &operator=(OutliningRegion &&) = default;
 416
 417   static std::vector<OutliningRegion> create(BasicBlock &SinkBB,
 418                                              const DominatorTree &DT,
 419                                              const PostDominatorTree &PDT) {
 420     std::vector<OutliningRegion> Regions;
 421     SmallPtrSet<BasicBlock *, 4> RegionBlocks;
 422
 423     Regions.emplace_back();
 424     OutliningRegion *ColdRegion = &Regions.back();
 425
 426     auto addBlockToRegion = [&](BasicBlock *BB, unsigned Score) {
 427       RegionBlocks.insert(BB);
 428       ColdRegion->Blocks.emplace_back(BB, Score);
 429     };
 430
 431     // The ancestor farthest-away from SinkBB, and also post-dominated by it.
 432     unsigned SinkScore = getEntryPointScore(SinkBB, ScoreForSinkBlock);
 433     ColdRegion->SuggestedEntryPoint = (SinkScore > 0) ? &SinkBB : nullptr;
 434     unsigned BestScore = SinkScore;
 435
 436     // Visit SinkBB's ancestors using inverse DFS.
 437     auto PredIt = ++idf_begin(&SinkBB);
 438     auto PredEnd = idf_end(&SinkBB);
 439     while (PredIt != PredEnd) {
 440       BasicBlock &PredBB = **PredIt;
 441       bool SinkPostDom = PDT.dominates(&SinkBB, &PredBB);
 442
 443       // If the predecessor is cold and has no predecessors, the entire
 444       // function must be cold.
 445       if (SinkPostDom && pred_empty(&PredBB)) {
 446         ColdRegion->EntireFunctionCold = true;
 447         return Regions;
 448       }
 449
 450       // If SinkBB does not post-dominate a predecessor, do not mark the
 451       // predecessor (or any of its predecessors) cold.
 452       if (!SinkPostDom || !mayExtractBlock(PredBB)) {
 453         PredIt.skipChildren();
 454         continue;
 455       }
 456
 457       // Keep track of the post-dominated ancestor farthest away from the sink.
 458       // The path length is always >= 2, ensuring that predecessor blocks are
 459       // considered as entry points before the sink block.
 460       unsigned PredScore = getEntryPointScore(PredBB, PredIt.getPathLength());
 461       if (PredScore > BestScore) {
 462         ColdRegion->SuggestedEntryPoint = &PredBB;
 463         BestScore = PredScore;
 464       }
 465
 466       addBlockToRegion(&PredBB, PredScore);
 467       ++PredIt;
 468     }
 469
 470     // If the sink can be added to the cold region, do so. It's considered as
 471     // an entry point before any sink-successor blocks.
 472     //
 473     // Otherwise, split cold sink-successor blocks using a separate region.
 474     // This satisfies the requirement that all extraction blocks other than the
 475     // first have predecessors within the extraction region.
 476     if (mayExtractBlock(SinkBB)) {
 477       addBlockToRegion(&SinkBB, SinkScore);
 478     } else {
 479       Regions.emplace_back();
 480       ColdRegion = &Regions.back();
 481       BestScore = 0;
 482     }
 483
 484     // Find all successors of SinkBB dominated by SinkBB using DFS.
 485     auto SuccIt = ++df_begin(&SinkBB);
 486     auto SuccEnd = df_end(&SinkBB);
 487     while (SuccIt != SuccEnd) {
 488       BasicBlock &SuccBB = **SuccIt;
 489       bool SinkDom = DT.dominates(&SinkBB, &SuccBB);
 490
 491       // Don't allow the backwards & forwards DFSes to mark the same block.
 492       bool DuplicateBlock = RegionBlocks.count(&SuccBB);
 493
 494       // If SinkBB does not dominate a successor, do not mark the successor (or
 495       // any of its successors) cold.
 496       if (DuplicateBlock || !SinkDom || !mayExtractBlock(SuccBB)) {
 497         SuccIt.skipChildren();
 498         continue;
 499       }
 500
 501       unsigned SuccScore = getEntryPointScore(SuccBB, ScoreForSuccBlock);
 502       if (SuccScore > BestScore) {
 503         ColdRegion->SuggestedEntryPoint = &SuccBB;
 504         BestScore = SuccScore;
 505       }
 506
 507       addBlockToRegion(&SuccBB, SuccScore);
 508       ++SuccIt;
 509     }
 510
 511     return Regions;
 512   }
 513
 514   /// Whether this region has nothing to extract.
 515   bool empty() const { return !SuggestedEntryPoint; }
 516
 517   /// The blocks in this region.
 518   ArrayRef<std::pair<BasicBlock *, unsigned>> blocks() const { return Blocks; }
 519
 520   /// Whether the entire function containing this region is cold.
 521   bool isEntireFunctionCold() const { return EntireFunctionCold; }
 522
 523   /// Remove a sub-region from this region and return it as a block sequence.
 524   BlockSequence takeSingleEntrySubRegion(DominatorTree &DT) {
 525     assert(!empty() && !isEntireFunctionCold() && "Nothing to extract");
 526
 527     // Remove blocks dominated by the suggested entry point from this region.
 528     // During the removal, identify the next best entry point into the region.
 529     // Ensure that the first extracted block is the suggested entry point.
 530     BlockSequence SubRegion = {SuggestedEntryPoint};
 531     BasicBlock *NextEntryPoint = nullptr;
 532     unsigned NextScore = 0;
 533     auto RegionEndIt = Blocks.end();
 534     auto RegionStartIt = remove_if(Blocks, [&](const BlockTy &Block) {
 535       BasicBlock *BB = Block.first;
 536       unsigned Score = Block.second;
 537       bool InSubRegion =
 538           BB == SuggestedEntryPoint || DT.dominates(SuggestedEntryPoint, BB);
 539       if (!InSubRegion && Score > NextScore) {
 540         NextEntryPoint = BB;
 541         NextScore = Score;
 542       }
 543       if (InSubRegion && BB != SuggestedEntryPoint)
 544         SubRegion.push_back(BB);
 545       return InSubRegion;
 546     });
 547     Blocks.erase(RegionStartIt, RegionEndIt);
 548
 549     // Update the suggested entry point.
 550     SuggestedEntryPoint = NextEntryPoint;
 551
 552     return SubRegion;
 553   }
 554 };
 555 } // namespace
 556
 557 bool HotColdSplitting::outlineColdRegions(Function &F, bool HasProfileSummary) {
 558   bool Changed = false;
 559
 560   // The set of cold blocks.
 561   SmallPtrSet<BasicBlock *, 4> ColdBlocks;
 562
 563   // The worklist of non-intersecting regions left to outline.
 564   SmallVector<OutliningRegion, 2> OutliningWorklist;
 565
 566   // Set up an RPO traversal. Experimentally, this performs better (outlines
 567   // more) than a PO traversal, because we prevent region overlap by keeping
 568   // the first region to contain a block.
 569   ReversePostOrderTraversal<Function *> RPOT(&F);
 570
 571   // Calculate domtrees lazily. This reduces compile-time significantly.
 572   std::unique_ptr<DominatorTree> DT;
 573   std::unique_ptr<PostDominatorTree> PDT;
 574
 575   // Calculate BFI lazily (it's only used to query ProfileSummaryInfo). This
 576   // reduces compile-time significantly. TODO: When we *do* use BFI, we should
 577   // be able to salvage its domtrees instead of recomputing them.
 578   BlockFrequencyInfo *BFI = nullptr;
 579   if (HasProfileSummary)
 580     BFI = GetBFI(F);
 581
 582   TargetTransformInfo &TTI = GetTTI(F);
 583   OptimizationRemarkEmitter &ORE = (*GetORE)(F);
 584   AssumptionCache *AC = LookupAC(F);
 585
 586   // Find all cold regions.
 587   for (BasicBlock *BB : RPOT) {
 588     // This block is already part of some outlining region.
 589     if (ColdBlocks.count(BB))
 590       continue;
 591
 592     bool Cold = (BFI && PSI->isColdBlock(BB, BFI)) ||
 593                 (EnableStaticAnalyis && unlikelyExecuted(*BB));
 594     if (!Cold)
 595       continue;
 596
 597     LLVM_DEBUG({
 598       dbgs() << "Found a cold block:\n";
 599       BB->dump();
 600     });
 601
 602     if (!DT)
 603       DT = make_unique<DominatorTree>(F);
 604     if (!PDT)
 605       PDT = make_unique<PostDominatorTree>(F);
 606
 607     auto Regions = OutliningRegion::create(*BB, *DT, *PDT);
 608     for (OutliningRegion &Region : Regions) {
 609       if (Region.empty())
 610         continue;
 611
 612       if (Region.isEntireFunctionCold()) {
 613         LLVM_DEBUG(dbgs() << "Entire function is cold\n");
 614         return markFunctionCold(F);
 615       }
 616
 617       // If this outlining region intersects with another, drop the new region.
 618       //
 619       // TODO: It's theoretically possible to outline more by only keeping the
 620       // largest region which contains a block, but the extra bookkeeping to do
 621       // this is tricky/expensive.
 622       bool RegionsOverlap = any_of(Region.blocks(), [&](const BlockTy &Block) {
 623         return !ColdBlocks.insert(Block.first).second;
 624       });
 625       if (RegionsOverlap)
 626         continue;
 627
 628       OutliningWorklist.emplace_back(std::move(Region));
 629       ++NumColdRegionsFound;
 630     }
 631   }
 632
 633   // Outline single-entry cold regions, splitting up larger regions as needed.
 634   unsigned OutlinedFunctionID = 1;
 635   while (!OutliningWorklist.empty()) {
 636     OutliningRegion Region = OutliningWorklist.pop_back_val();
 637     assert(!Region.empty() && "Empty outlining region in worklist");
 638     do {
 639       BlockSequence SubRegion = Region.takeSingleEntrySubRegion(*DT);
 640       LLVM_DEBUG({
 641         dbgs() << "Hot/cold splitting attempting to outline these blocks:\n";
 642         for (BasicBlock *BB : SubRegion)
 643           BB->dump();
 644       });
 645
 646       Function *Outlined = extractColdRegion(SubRegion, *DT, BFI, TTI, ORE, AC,
 647                                              OutlinedFunctionID);
 648       if (Outlined) {
 649         ++OutlinedFunctionID;
 650         Changed = true;
 651       }
 652     } while (!Region.empty());
 653   }
 654
 655   return Changed;
 656 }
 657
 658 bool HotColdSplitting::run(Module &M) {
 659   bool Changed = false;
 660   bool HasProfileSummary = M.getProfileSummary();
 661   for (auto It = M.begin(), End = M.end(); It != End; ++It) {
 662     Function &F = *It;
 663
 664     // Do not touch declarations.
 665     if (F.isDeclaration())
 666       continue;
 667
 668     // Do not modify `optnone` functions.
 669     if (F.hasFnAttribute(Attribute::OptimizeNone))
 670       continue;
 671
 672     // Detect inherently cold functions and mark them as such.
 673     if (isFunctionCold(F)) {
 674       Changed |= markFunctionCold(F);
 675       continue;
 676     }
 677
 678     if (!shouldOutlineFrom(F)) {
 679       LLVM_DEBUG(llvm::dbgs() << "Skipping " << F.getName() << "\n");
 680       continue;
 681     }
 682
 683     LLVM_DEBUG(llvm::dbgs() << "Outlining in " << F.getName() << "\n");
 684     Changed |= outlineColdRegions(F, HasProfileSummary);
 685   }
 686   return Changed;
 687 }
 688
 689 bool HotColdSplittingLegacyPass::runOnModule(Module &M) {
 690   if (skipModule(M))
 691     return false;
 692   ProfileSummaryInfo *PSI =
 693       &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
 694   auto GTTI = [this](Function &F) -> TargetTransformInfo & {
 695     return this->getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
 696   };
 697   auto GBFI = [this](Function &F) {
 698     return &this->getAnalysis<BlockFrequencyInfoWrapperPass>(F).getBFI();
 699   };
 700   std::unique_ptr<OptimizationRemarkEmitter> ORE;
 701   std::function<OptimizationRemarkEmitter &(Function &)> GetORE =
 702       [&ORE](Function &F) -> OptimizationRemarkEmitter & {
 703     ORE.reset(new OptimizationRemarkEmitter(&F));
 704     return *ORE.get();
 705   };
 706   auto LookupAC = [this](Function &F) -> AssumptionCache * {
 707     if (auto *ACT = getAnalysisIfAvailable<AssumptionCacheTracker>())
 708       return ACT->lookupAssumptionCache(F);
 709     return nullptr;
 710   };
 711
 712   return HotColdSplitting(PSI, GBFI, GTTI, &GetORE, LookupAC).run(M);
 713 }
 714
 715 PreservedAnalyses
 716 HotColdSplittingPass::run(Module &M, ModuleAnalysisManager &AM) {
 717   auto &FAM = AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
 718
 719   auto LookupAC = [&FAM](Function &F) -> AssumptionCache * {
 720     return FAM.getCachedResult<AssumptionAnalysis>(F);
 721   };
 722
 723   auto GBFI = [&FAM](Function &F) {
 724     return &FAM.getResult<BlockFrequencyAnalysis>(F);
 725   };
 726
 727   std::function<TargetTransformInfo &(Function &)> GTTI =
 728       [&FAM](Function &F) -> TargetTransformInfo & {
 729     return FAM.getResult<TargetIRAnalysis>(F);
 730   };
 731
 732   std::unique_ptr<OptimizationRemarkEmitter> ORE;
 733   std::function<OptimizationRemarkEmitter &(Function &)> GetORE =
 734       [&ORE](Function &F) -> OptimizationRemarkEmitter & {
 735     ORE.reset(new OptimizationRemarkEmitter(&F));
 736     return *ORE.get();
 737   };
 738
 739   ProfileSummaryInfo *PSI = &AM.getResult<ProfileSummaryAnalysis>(M);
 740
 741   if (HotColdSplitting(PSI, GBFI, GTTI, &GetORE, LookupAC).run(M))
 742     return PreservedAnalyses::none();
 743   return PreservedAnalyses::all();
 744 }
 745
 746 char HotColdSplittingLegacyPass::ID = 0;
 747 INITIALIZE_PASS_BEGIN(HotColdSplittingLegacyPass, "hotcoldsplit",
 748                       "Hot Cold Splitting", false, false)
 749 INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
 750 INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)
 751 INITIALIZE_PASS_END(HotColdSplittingLegacyPass, "hotcoldsplit",
 752                     "Hot Cold Splitting", false, false)
 753
 754 ModulePass *llvm::createHotColdSplittingPass() {
 755   return new HotColdSplittingLegacyPass();
 756 }