1 //===- MachineBlockPlacement.cpp - Basic Block Code Layout optimization ---===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file implements basic block placement transformations using the CFG 10 // structure and branch probability estimates. 11 // 12 // The pass strives to preserve the structure of the CFG (that is, retain 13 // a topological ordering of basic blocks) in the absence of a *strong* signal 14 // to the contrary from probabilities. However, within the CFG structure, it 15 // attempts to choose an ordering which favors placing more likely sequences of 16 // blocks adjacent to each other. 17 // 18 // The algorithm works from the inner-most loop within a function outward, and 19 // at each stage walks through the basic blocks, trying to coalesce them into 20 // sequential chains where allowed by the CFG (or demanded by heavy 21 // probabilities). Finally, it walks the blocks in topological order, and the 22 // first time it reaches a chain of basic blocks, it schedules them in the 23 // function in-order. 24 // 25 //===----------------------------------------------------------------------===// 26 27 #include "BranchFolding.h" 28 #include "llvm/ADT/ArrayRef.h" 29 #include "llvm/ADT/DenseMap.h" 30 #include "llvm/ADT/STLExtras.h" 31 #include "llvm/ADT/SetVector.h" 32 #include "llvm/ADT/SmallPtrSet.h" 33 #include "llvm/ADT/SmallVector.h" 34 #include "llvm/ADT/Statistic.h" 35 #include "llvm/Analysis/BlockFrequencyInfoImpl.h" 36 #include "llvm/Analysis/ProfileSummaryInfo.h" 37 #include "llvm/CodeGen/MachineBasicBlock.h" 38 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h" 39 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h" 40 #include "llvm/CodeGen/MachineFunction.h" 41 #include "llvm/CodeGen/MachineFunctionPass.h" 42 #include "llvm/CodeGen/MachineLoopInfo.h" 43 #include "llvm/CodeGen/MachineModuleInfo.h" 44 #include "llvm/CodeGen/MachinePostDominators.h" 45 #include "llvm/CodeGen/MachineSizeOpts.h" 46 #include "llvm/CodeGen/TailDuplicator.h" 47 #include "llvm/CodeGen/TargetInstrInfo.h" 48 #include "llvm/CodeGen/TargetLowering.h" 49 #include "llvm/CodeGen/TargetPassConfig.h" 50 #include "llvm/CodeGen/TargetSubtargetInfo.h" 51 #include "llvm/IR/DebugLoc.h" 52 #include "llvm/IR/Function.h" 53 #include "llvm/InitializePasses.h" 54 #include "llvm/Pass.h" 55 #include "llvm/Support/Allocator.h" 56 #include "llvm/Support/BlockFrequency.h" 57 #include "llvm/Support/BranchProbability.h" 58 #include "llvm/Support/CodeGen.h" 59 #include "llvm/Support/CommandLine.h" 60 #include "llvm/Support/Compiler.h" 61 #include "llvm/Support/Debug.h" 62 #include "llvm/Support/raw_ostream.h" 63 #include "llvm/Target/TargetMachine.h" 64 #include <algorithm> 65 #include <cassert> 66 #include <cstdint> 67 #include <iterator> 68 #include <memory> 69 #include <string> 70 #include <tuple> 71 #include <utility> 72 #include <vector> 73 74 using namespace llvm; 75 76 #define DEBUG_TYPE "block-placement" 77 78 STATISTIC(NumCondBranches, "Number of conditional branches"); 79 STATISTIC(NumUncondBranches, "Number of unconditional branches"); 80 STATISTIC(CondBranchTakenFreq, 81 "Potential frequency of taking conditional branches"); 82 STATISTIC(UncondBranchTakenFreq, 83 "Potential frequency of taking unconditional branches"); 84 85 static cl::opt<unsigned> AlignAllBlock( 86 "align-all-blocks", 87 cl::desc("Force the alignment of all blocks in the function in log2 format " 88 "(e.g 4 means align on 16B boundaries)."), 89 cl::init(0), cl::Hidden); 90 91 static cl::opt<unsigned> AlignAllNonFallThruBlocks( 92 "align-all-nofallthru-blocks", 93 cl::desc("Force the alignment of all blocks that have no fall-through " 94 "predecessors (i.e. don't add nops that are executed). In log2 " 95 "format (e.g 4 means align on 16B boundaries)."), 96 cl::init(0), cl::Hidden); 97 98 // FIXME: Find a good default for this flag and remove the flag. 99 static cl::opt<unsigned> ExitBlockBias( 100 "block-placement-exit-block-bias", 101 cl::desc("Block frequency percentage a loop exit block needs " 102 "over the original exit to be considered the new exit."), 103 cl::init(0), cl::Hidden); 104 105 // Definition: 106 // - Outlining: placement of a basic block outside the chain or hot path. 107 108 static cl::opt<unsigned> LoopToColdBlockRatio( 109 "loop-to-cold-block-ratio", 110 cl::desc("Outline loop blocks from loop chain if (frequency of loop) / " 111 "(frequency of block) is greater than this ratio"), 112 cl::init(5), cl::Hidden); 113 114 static cl::opt<bool> ForceLoopColdBlock( 115 "force-loop-cold-block", 116 cl::desc("Force outlining cold blocks from loops."), 117 cl::init(false), cl::Hidden); 118 119 static cl::opt<bool> 120 PreciseRotationCost("precise-rotation-cost", 121 cl::desc("Model the cost of loop rotation more " 122 "precisely by using profile data."), 123 cl::init(false), cl::Hidden); 124 125 static cl::opt<bool> 126 ForcePreciseRotationCost("force-precise-rotation-cost", 127 cl::desc("Force the use of precise cost " 128 "loop rotation strategy."), 129 cl::init(false), cl::Hidden); 130 131 static cl::opt<unsigned> MisfetchCost( 132 "misfetch-cost", 133 cl::desc("Cost that models the probabilistic risk of an instruction " 134 "misfetch due to a jump comparing to falling through, whose cost " 135 "is zero."), 136 cl::init(1), cl::Hidden); 137 138 static cl::opt<unsigned> JumpInstCost("jump-inst-cost", 139 cl::desc("Cost of jump instructions."), 140 cl::init(1), cl::Hidden); 141 static cl::opt<bool> 142 TailDupPlacement("tail-dup-placement", 143 cl::desc("Perform tail duplication during placement. " 144 "Creates more fallthrough opportunites in " 145 "outline branches."), 146 cl::init(true), cl::Hidden); 147 148 static cl::opt<bool> 149 BranchFoldPlacement("branch-fold-placement", 150 cl::desc("Perform branch folding during placement. " 151 "Reduces code size."), 152 cl::init(true), cl::Hidden); 153 154 // Heuristic for tail duplication. 155 static cl::opt<unsigned> TailDupPlacementThreshold( 156 "tail-dup-placement-threshold", 157 cl::desc("Instruction cutoff for tail duplication during layout. " 158 "Tail merging during layout is forced to have a threshold " 159 "that won't conflict."), cl::init(2), 160 cl::Hidden); 161 162 // Heuristic for aggressive tail duplication. 163 static cl::opt<unsigned> TailDupPlacementAggressiveThreshold( 164 "tail-dup-placement-aggressive-threshold", 165 cl::desc("Instruction cutoff for aggressive tail duplication during " 166 "layout. Used at -O3. Tail merging during layout is forced to " 167 "have a threshold that won't conflict."), cl::init(4), 168 cl::Hidden); 169 170 // Heuristic for tail duplication. 171 static cl::opt<unsigned> TailDupPlacementPenalty( 172 "tail-dup-placement-penalty", 173 cl::desc("Cost penalty for blocks that can avoid breaking CFG by copying. " 174 "Copying can increase fallthrough, but it also increases icache " 175 "pressure. This parameter controls the penalty to account for that. " 176 "Percent as integer."), 177 cl::init(2), 178 cl::Hidden); 179 180 // Heuristic for triangle chains. 181 static cl::opt<unsigned> TriangleChainCount( 182 "triangle-chain-count", 183 cl::desc("Number of triangle-shaped-CFG's that need to be in a row for the " 184 "triangle tail duplication heuristic to kick in. 0 to disable."), 185 cl::init(2), 186 cl::Hidden); 187 188 extern cl::opt<unsigned> StaticLikelyProb; 189 extern cl::opt<unsigned> ProfileLikelyProb; 190 191 // Internal option used to control BFI display only after MBP pass. 192 // Defined in CodeGen/MachineBlockFrequencyInfo.cpp: 193 // -view-block-layout-with-bfi= 194 extern cl::opt<GVDAGType> ViewBlockLayoutWithBFI; 195 196 // Command line option to specify the name of the function for CFG dump 197 // Defined in Analysis/BlockFrequencyInfo.cpp: -view-bfi-func-name= 198 extern cl::opt<std::string> ViewBlockFreqFuncName; 199 200 namespace { 201 202 class BlockChain; 203 204 /// Type for our function-wide basic block -> block chain mapping. 205 using BlockToChainMapType = DenseMap<const MachineBasicBlock *, BlockChain *>; 206 207 /// A chain of blocks which will be laid out contiguously. 208 /// 209 /// This is the datastructure representing a chain of consecutive blocks that 210 /// are profitable to layout together in order to maximize fallthrough 211 /// probabilities and code locality. We also can use a block chain to represent 212 /// a sequence of basic blocks which have some external (correctness) 213 /// requirement for sequential layout. 214 /// 215 /// Chains can be built around a single basic block and can be merged to grow 216 /// them. They participate in a block-to-chain mapping, which is updated 217 /// automatically as chains are merged together. 218 class BlockChain { 219 /// The sequence of blocks belonging to this chain. 220 /// 221 /// This is the sequence of blocks for a particular chain. These will be laid 222 /// out in-order within the function. 223 SmallVector<MachineBasicBlock *, 4> Blocks; 224 225 /// A handle to the function-wide basic block to block chain mapping. 226 /// 227 /// This is retained in each block chain to simplify the computation of child 228 /// block chains for SCC-formation and iteration. We store the edges to child 229 /// basic blocks, and map them back to their associated chains using this 230 /// structure. 231 BlockToChainMapType &BlockToChain; 232 233 public: 234 /// Construct a new BlockChain. 235 /// 236 /// This builds a new block chain representing a single basic block in the 237 /// function. It also registers itself as the chain that block participates 238 /// in with the BlockToChain mapping. 239 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB) 240 : Blocks(1, BB), BlockToChain(BlockToChain) { 241 assert(BB && "Cannot create a chain with a null basic block"); 242 BlockToChain[BB] = this; 243 } 244 245 /// Iterator over blocks within the chain. 246 using iterator = SmallVectorImpl<MachineBasicBlock *>::iterator; 247 using const_iterator = SmallVectorImpl<MachineBasicBlock *>::const_iterator; 248 249 /// Beginning of blocks within the chain. 250 iterator begin() { return Blocks.begin(); } 251 const_iterator begin() const { return Blocks.begin(); } 252 253 /// End of blocks within the chain. 254 iterator end() { return Blocks.end(); } 255 const_iterator end() const { return Blocks.end(); } 256 257 bool remove(MachineBasicBlock* BB) { 258 for(iterator i = begin(); i != end(); ++i) { 259 if (*i == BB) { 260 Blocks.erase(i); 261 return true; 262 } 263 } 264 return false; 265 } 266 267 /// Merge a block chain into this one. 268 /// 269 /// This routine merges a block chain into this one. It takes care of forming 270 /// a contiguous sequence of basic blocks, updating the edge list, and 271 /// updating the block -> chain mapping. It does not free or tear down the 272 /// old chain, but the old chain's block list is no longer valid. 273 void merge(MachineBasicBlock *BB, BlockChain *Chain) { 274 assert(BB && "Can't merge a null block."); 275 assert(!Blocks.empty() && "Can't merge into an empty chain."); 276 277 // Fast path in case we don't have a chain already. 278 if (!Chain) { 279 assert(!BlockToChain[BB] && 280 "Passed chain is null, but BB has entry in BlockToChain."); 281 Blocks.push_back(BB); 282 BlockToChain[BB] = this; 283 return; 284 } 285 286 assert(BB == *Chain->begin() && "Passed BB is not head of Chain."); 287 assert(Chain->begin() != Chain->end()); 288 289 // Update the incoming blocks to point to this chain, and add them to the 290 // chain structure. 291 for (MachineBasicBlock *ChainBB : *Chain) { 292 Blocks.push_back(ChainBB); 293 assert(BlockToChain[ChainBB] == Chain && "Incoming blocks not in chain."); 294 BlockToChain[ChainBB] = this; 295 } 296 } 297 298 #ifndef NDEBUG 299 /// Dump the blocks in this chain. 300 LLVM_DUMP_METHOD void dump() { 301 for (MachineBasicBlock *MBB : *this) 302 MBB->dump(); 303 } 304 #endif // NDEBUG 305 306 /// Count of predecessors of any block within the chain which have not 307 /// yet been scheduled. In general, we will delay scheduling this chain 308 /// until those predecessors are scheduled (or we find a sufficiently good 309 /// reason to override this heuristic.) Note that when forming loop chains, 310 /// blocks outside the loop are ignored and treated as if they were already 311 /// scheduled. 312 /// 313 /// Note: This field is reinitialized multiple times - once for each loop, 314 /// and then once for the function as a whole. 315 unsigned UnscheduledPredecessors = 0; 316 }; 317 318 class MachineBlockPlacement : public MachineFunctionPass { 319 /// A type for a block filter set. 320 using BlockFilterSet = SmallSetVector<const MachineBasicBlock *, 16>; 321 322 /// Pair struct containing basic block and taildup profitability 323 struct BlockAndTailDupResult { 324 MachineBasicBlock *BB; 325 bool ShouldTailDup; 326 }; 327 328 /// Triple struct containing edge weight and the edge. 329 struct WeightedEdge { 330 BlockFrequency Weight; 331 MachineBasicBlock *Src; 332 MachineBasicBlock *Dest; 333 }; 334 335 /// work lists of blocks that are ready to be laid out 336 SmallVector<MachineBasicBlock *, 16> BlockWorkList; 337 SmallVector<MachineBasicBlock *, 16> EHPadWorkList; 338 339 /// Edges that have already been computed as optimal. 340 DenseMap<const MachineBasicBlock *, BlockAndTailDupResult> ComputedEdges; 341 342 /// Machine Function 343 MachineFunction *F; 344 345 /// A handle to the branch probability pass. 346 const MachineBranchProbabilityInfo *MBPI; 347 348 /// A handle to the function-wide block frequency pass. 349 std::unique_ptr<MBFIWrapper> MBFI; 350 351 /// A handle to the loop info. 352 MachineLoopInfo *MLI; 353 354 /// Preferred loop exit. 355 /// Member variable for convenience. It may be removed by duplication deep 356 /// in the call stack. 357 MachineBasicBlock *PreferredLoopExit; 358 359 /// A handle to the target's instruction info. 360 const TargetInstrInfo *TII; 361 362 /// A handle to the target's lowering info. 363 const TargetLoweringBase *TLI; 364 365 /// A handle to the post dominator tree. 366 MachinePostDominatorTree *MPDT; 367 368 ProfileSummaryInfo *PSI; 369 370 /// Duplicator used to duplicate tails during placement. 371 /// 372 /// Placement decisions can open up new tail duplication opportunities, but 373 /// since tail duplication affects placement decisions of later blocks, it 374 /// must be done inline. 375 TailDuplicator TailDup; 376 377 /// Partial tail duplication threshold. 378 BlockFrequency DupThreshold; 379 380 /// Allocator and owner of BlockChain structures. 381 /// 382 /// We build BlockChains lazily while processing the loop structure of 383 /// a function. To reduce malloc traffic, we allocate them using this 384 /// slab-like allocator, and destroy them after the pass completes. An 385 /// important guarantee is that this allocator produces stable pointers to 386 /// the chains. 387 SpecificBumpPtrAllocator<BlockChain> ChainAllocator; 388 389 /// Function wide BasicBlock to BlockChain mapping. 390 /// 391 /// This mapping allows efficiently moving from any given basic block to the 392 /// BlockChain it participates in, if any. We use it to, among other things, 393 /// allow implicitly defining edges between chains as the existing edges 394 /// between basic blocks. 395 DenseMap<const MachineBasicBlock *, BlockChain *> BlockToChain; 396 397 #ifndef NDEBUG 398 /// The set of basic blocks that have terminators that cannot be fully 399 /// analyzed. These basic blocks cannot be re-ordered safely by 400 /// MachineBlockPlacement, and we must preserve physical layout of these 401 /// blocks and their successors through the pass. 402 SmallPtrSet<MachineBasicBlock *, 4> BlocksWithUnanalyzableExits; 403 #endif 404 405 /// Scale the DupThreshold according to basic block size. 406 BlockFrequency scaleThreshold(MachineBasicBlock *BB); 407 void initDupThreshold(); 408 409 /// Decrease the UnscheduledPredecessors count for all blocks in chain, and 410 /// if the count goes to 0, add them to the appropriate work list. 411 void markChainSuccessors( 412 const BlockChain &Chain, const MachineBasicBlock *LoopHeaderBB, 413 const BlockFilterSet *BlockFilter = nullptr); 414 415 /// Decrease the UnscheduledPredecessors count for a single block, and 416 /// if the count goes to 0, add them to the appropriate work list. 417 void markBlockSuccessors( 418 const BlockChain &Chain, const MachineBasicBlock *BB, 419 const MachineBasicBlock *LoopHeaderBB, 420 const BlockFilterSet *BlockFilter = nullptr); 421 422 BranchProbability 423 collectViableSuccessors( 424 const MachineBasicBlock *BB, const BlockChain &Chain, 425 const BlockFilterSet *BlockFilter, 426 SmallVector<MachineBasicBlock *, 4> &Successors); 427 bool shouldPredBlockBeOutlined( 428 const MachineBasicBlock *BB, const MachineBasicBlock *Succ, 429 const BlockChain &Chain, const BlockFilterSet *BlockFilter, 430 BranchProbability SuccProb, BranchProbability HotProb); 431 bool isBestSuccessor(MachineBasicBlock *BB, MachineBasicBlock *Pred, 432 BlockFilterSet *BlockFilter); 433 void findDuplicateCandidates(SmallVectorImpl<MachineBasicBlock *> &Candidates, 434 MachineBasicBlock *BB, 435 BlockFilterSet *BlockFilter); 436 bool repeatedlyTailDuplicateBlock( 437 MachineBasicBlock *BB, MachineBasicBlock *&LPred, 438 const MachineBasicBlock *LoopHeaderBB, 439 BlockChain &Chain, BlockFilterSet *BlockFilter, 440 MachineFunction::iterator &PrevUnplacedBlockIt); 441 bool maybeTailDuplicateBlock( 442 MachineBasicBlock *BB, MachineBasicBlock *LPred, 443 BlockChain &Chain, BlockFilterSet *BlockFilter, 444 MachineFunction::iterator &PrevUnplacedBlockIt, 445 bool &DuplicatedToLPred); 446 bool hasBetterLayoutPredecessor( 447 const MachineBasicBlock *BB, const MachineBasicBlock *Succ, 448 const BlockChain &SuccChain, BranchProbability SuccProb, 449 BranchProbability RealSuccProb, const BlockChain &Chain, 450 const BlockFilterSet *BlockFilter); 451 BlockAndTailDupResult selectBestSuccessor( 452 const MachineBasicBlock *BB, const BlockChain &Chain, 453 const BlockFilterSet *BlockFilter); 454 MachineBasicBlock *selectBestCandidateBlock( 455 const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList); 456 MachineBasicBlock *getFirstUnplacedBlock( 457 const BlockChain &PlacedChain, 458 MachineFunction::iterator &PrevUnplacedBlockIt, 459 const BlockFilterSet *BlockFilter); 460 461 /// Add a basic block to the work list if it is appropriate. 462 /// 463 /// If the optional parameter BlockFilter is provided, only MBB 464 /// present in the set will be added to the worklist. If nullptr 465 /// is provided, no filtering occurs. 466 void fillWorkLists(const MachineBasicBlock *MBB, 467 SmallPtrSetImpl<BlockChain *> &UpdatedPreds, 468 const BlockFilterSet *BlockFilter); 469 470 void buildChain(const MachineBasicBlock *BB, BlockChain &Chain, 471 BlockFilterSet *BlockFilter = nullptr); 472 bool canMoveBottomBlockToTop(const MachineBasicBlock *BottomBlock, 473 const MachineBasicBlock *OldTop); 474 bool hasViableTopFallthrough(const MachineBasicBlock *Top, 475 const BlockFilterSet &LoopBlockSet); 476 BlockFrequency TopFallThroughFreq(const MachineBasicBlock *Top, 477 const BlockFilterSet &LoopBlockSet); 478 BlockFrequency FallThroughGains(const MachineBasicBlock *NewTop, 479 const MachineBasicBlock *OldTop, 480 const MachineBasicBlock *ExitBB, 481 const BlockFilterSet &LoopBlockSet); 482 MachineBasicBlock *findBestLoopTopHelper(MachineBasicBlock *OldTop, 483 const MachineLoop &L, const BlockFilterSet &LoopBlockSet); 484 MachineBasicBlock *findBestLoopTop( 485 const MachineLoop &L, const BlockFilterSet &LoopBlockSet); 486 MachineBasicBlock *findBestLoopExit( 487 const MachineLoop &L, const BlockFilterSet &LoopBlockSet, 488 BlockFrequency &ExitFreq); 489 BlockFilterSet collectLoopBlockSet(const MachineLoop &L); 490 void buildLoopChains(const MachineLoop &L); 491 void rotateLoop( 492 BlockChain &LoopChain, const MachineBasicBlock *ExitingBB, 493 BlockFrequency ExitFreq, const BlockFilterSet &LoopBlockSet); 494 void rotateLoopWithProfile( 495 BlockChain &LoopChain, const MachineLoop &L, 496 const BlockFilterSet &LoopBlockSet); 497 void buildCFGChains(); 498 void optimizeBranches(); 499 void alignBlocks(); 500 /// Returns true if a block should be tail-duplicated to increase fallthrough 501 /// opportunities. 502 bool shouldTailDuplicate(MachineBasicBlock *BB); 503 /// Check the edge frequencies to see if tail duplication will increase 504 /// fallthroughs. 505 bool isProfitableToTailDup( 506 const MachineBasicBlock *BB, const MachineBasicBlock *Succ, 507 BranchProbability QProb, 508 const BlockChain &Chain, const BlockFilterSet *BlockFilter); 509 510 /// Check for a trellis layout. 511 bool isTrellis(const MachineBasicBlock *BB, 512 const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs, 513 const BlockChain &Chain, const BlockFilterSet *BlockFilter); 514 515 /// Get the best successor given a trellis layout. 516 BlockAndTailDupResult getBestTrellisSuccessor( 517 const MachineBasicBlock *BB, 518 const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs, 519 BranchProbability AdjustedSumProb, const BlockChain &Chain, 520 const BlockFilterSet *BlockFilter); 521 522 /// Get the best pair of non-conflicting edges. 523 static std::pair<WeightedEdge, WeightedEdge> getBestNonConflictingEdges( 524 const MachineBasicBlock *BB, 525 MutableArrayRef<SmallVector<WeightedEdge, 8>> Edges); 526 527 /// Returns true if a block can tail duplicate into all unplaced 528 /// predecessors. Filters based on loop. 529 bool canTailDuplicateUnplacedPreds( 530 const MachineBasicBlock *BB, MachineBasicBlock *Succ, 531 const BlockChain &Chain, const BlockFilterSet *BlockFilter); 532 533 /// Find chains of triangles to tail-duplicate where a global analysis works, 534 /// but a local analysis would not find them. 535 void precomputeTriangleChains(); 536 537 public: 538 static char ID; // Pass identification, replacement for typeid 539 540 MachineBlockPlacement() : MachineFunctionPass(ID) { 541 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry()); 542 } 543 544 bool runOnMachineFunction(MachineFunction &F) override; 545 546 bool allowTailDupPlacement() const { 547 assert(F); 548 return TailDupPlacement && !F->getTarget().requiresStructuredCFG(); 549 } 550 551 void getAnalysisUsage(AnalysisUsage &AU) const override { 552 AU.addRequired<MachineBranchProbabilityInfo>(); 553 AU.addRequired<MachineBlockFrequencyInfo>(); 554 if (TailDupPlacement) 555 AU.addRequired<MachinePostDominatorTree>(); 556 AU.addRequired<MachineLoopInfo>(); 557 AU.addRequired<ProfileSummaryInfoWrapperPass>(); 558 AU.addRequired<TargetPassConfig>(); 559 MachineFunctionPass::getAnalysisUsage(AU); 560 } 561 }; 562 563 } // end anonymous namespace 564 565 char MachineBlockPlacement::ID = 0; 566 567 char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID; 568 569 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, DEBUG_TYPE, 570 "Branch Probability Basic Block Placement", false, false) 571 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) 572 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) 573 INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTree) 574 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) 575 INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass) 576 INITIALIZE_PASS_END(MachineBlockPlacement, DEBUG_TYPE, 577 "Branch Probability Basic Block Placement", false, false) 578 579 #ifndef NDEBUG 580 /// Helper to print the name of a MBB. 581 /// 582 /// Only used by debug logging. 583 static std::string getBlockName(const MachineBasicBlock *BB) { 584 std::string Result; 585 raw_string_ostream OS(Result); 586 OS << printMBBReference(*BB); 587 OS << " ('" << BB->getName() << "')"; 588 OS.flush(); 589 return Result; 590 } 591 #endif 592 593 /// Mark a chain's successors as having one fewer preds. 594 /// 595 /// When a chain is being merged into the "placed" chain, this routine will 596 /// quickly walk the successors of each block in the chain and mark them as 597 /// having one fewer active predecessor. It also adds any successors of this 598 /// chain which reach the zero-predecessor state to the appropriate worklist. 599 void MachineBlockPlacement::markChainSuccessors( 600 const BlockChain &Chain, const MachineBasicBlock *LoopHeaderBB, 601 const BlockFilterSet *BlockFilter) { 602 // Walk all the blocks in this chain, marking their successors as having 603 // a predecessor placed. 604 for (MachineBasicBlock *MBB : Chain) { 605 markBlockSuccessors(Chain, MBB, LoopHeaderBB, BlockFilter); 606 } 607 } 608 609 /// Mark a single block's successors as having one fewer preds. 610 /// 611 /// Under normal circumstances, this is only called by markChainSuccessors, 612 /// but if a block that was to be placed is completely tail-duplicated away, 613 /// and was duplicated into the chain end, we need to redo markBlockSuccessors 614 /// for just that block. 615 void MachineBlockPlacement::markBlockSuccessors( 616 const BlockChain &Chain, const MachineBasicBlock *MBB, 617 const MachineBasicBlock *LoopHeaderBB, const BlockFilterSet *BlockFilter) { 618 // Add any successors for which this is the only un-placed in-loop 619 // predecessor to the worklist as a viable candidate for CFG-neutral 620 // placement. No subsequent placement of this block will violate the CFG 621 // shape, so we get to use heuristics to choose a favorable placement. 622 for (MachineBasicBlock *Succ : MBB->successors()) { 623 if (BlockFilter && !BlockFilter->count(Succ)) 624 continue; 625 BlockChain &SuccChain = *BlockToChain[Succ]; 626 // Disregard edges within a fixed chain, or edges to the loop header. 627 if (&Chain == &SuccChain || Succ == LoopHeaderBB) 628 continue; 629 630 // This is a cross-chain edge that is within the loop, so decrement the 631 // loop predecessor count of the destination chain. 632 if (SuccChain.UnscheduledPredecessors == 0 || 633 --SuccChain.UnscheduledPredecessors > 0) 634 continue; 635 636 auto *NewBB = *SuccChain.begin(); 637 if (NewBB->isEHPad()) 638 EHPadWorkList.push_back(NewBB); 639 else 640 BlockWorkList.push_back(NewBB); 641 } 642 } 643 644 /// This helper function collects the set of successors of block 645 /// \p BB that are allowed to be its layout successors, and return 646 /// the total branch probability of edges from \p BB to those 647 /// blocks. 648 BranchProbability MachineBlockPlacement::collectViableSuccessors( 649 const MachineBasicBlock *BB, const BlockChain &Chain, 650 const BlockFilterSet *BlockFilter, 651 SmallVector<MachineBasicBlock *, 4> &Successors) { 652 // Adjust edge probabilities by excluding edges pointing to blocks that is 653 // either not in BlockFilter or is already in the current chain. Consider the 654 // following CFG: 655 // 656 // --->A 657 // | / \ 658 // | B C 659 // | \ / \ 660 // ----D E 661 // 662 // Assume A->C is very hot (>90%), and C->D has a 50% probability, then after 663 // A->C is chosen as a fall-through, D won't be selected as a successor of C 664 // due to CFG constraint (the probability of C->D is not greater than 665 // HotProb to break topo-order). If we exclude E that is not in BlockFilter 666 // when calculating the probability of C->D, D will be selected and we 667 // will get A C D B as the layout of this loop. 668 auto AdjustedSumProb = BranchProbability::getOne(); 669 for (MachineBasicBlock *Succ : BB->successors()) { 670 bool SkipSucc = false; 671 if (Succ->isEHPad() || (BlockFilter && !BlockFilter->count(Succ))) { 672 SkipSucc = true; 673 } else { 674 BlockChain *SuccChain = BlockToChain[Succ]; 675 if (SuccChain == &Chain) { 676 SkipSucc = true; 677 } else if (Succ != *SuccChain->begin()) { 678 LLVM_DEBUG(dbgs() << " " << getBlockName(Succ) 679 << " -> Mid chain!\n"); 680 continue; 681 } 682 } 683 if (SkipSucc) 684 AdjustedSumProb -= MBPI->getEdgeProbability(BB, Succ); 685 else 686 Successors.push_back(Succ); 687 } 688 689 return AdjustedSumProb; 690 } 691 692 /// The helper function returns the branch probability that is adjusted 693 /// or normalized over the new total \p AdjustedSumProb. 694 static BranchProbability 695 getAdjustedProbability(BranchProbability OrigProb, 696 BranchProbability AdjustedSumProb) { 697 BranchProbability SuccProb; 698 uint32_t SuccProbN = OrigProb.getNumerator(); 699 uint32_t SuccProbD = AdjustedSumProb.getNumerator(); 700 if (SuccProbN >= SuccProbD) 701 SuccProb = BranchProbability::getOne(); 702 else 703 SuccProb = BranchProbability(SuccProbN, SuccProbD); 704 705 return SuccProb; 706 } 707 708 /// Check if \p BB has exactly the successors in \p Successors. 709 static bool 710 hasSameSuccessors(MachineBasicBlock &BB, 711 SmallPtrSetImpl<const MachineBasicBlock *> &Successors) { 712 if (BB.succ_size() != Successors.size()) 713 return false; 714 // We don't want to count self-loops 715 if (Successors.count(&BB)) 716 return false; 717 for (MachineBasicBlock *Succ : BB.successors()) 718 if (!Successors.count(Succ)) 719 return false; 720 return true; 721 } 722 723 /// Check if a block should be tail duplicated to increase fallthrough 724 /// opportunities. 725 /// \p BB Block to check. 726 bool MachineBlockPlacement::shouldTailDuplicate(MachineBasicBlock *BB) { 727 // Blocks with single successors don't create additional fallthrough 728 // opportunities. Don't duplicate them. TODO: When conditional exits are 729 // analyzable, allow them to be duplicated. 730 bool IsSimple = TailDup.isSimpleBB(BB); 731 732 if (BB->succ_size() == 1) 733 return false; 734 return TailDup.shouldTailDuplicate(IsSimple, *BB); 735 } 736 737 /// Compare 2 BlockFrequency's with a small penalty for \p A. 738 /// In order to be conservative, we apply a X% penalty to account for 739 /// increased icache pressure and static heuristics. For small frequencies 740 /// we use only the numerators to improve accuracy. For simplicity, we assume the 741 /// penalty is less than 100% 742 /// TODO(iteratee): Use 64-bit fixed point edge frequencies everywhere. 743 static bool greaterWithBias(BlockFrequency A, BlockFrequency B, 744 uint64_t EntryFreq) { 745 BranchProbability ThresholdProb(TailDupPlacementPenalty, 100); 746 BlockFrequency Gain = A - B; 747 return (Gain / ThresholdProb).getFrequency() >= EntryFreq; 748 } 749 750 /// Check the edge frequencies to see if tail duplication will increase 751 /// fallthroughs. It only makes sense to call this function when 752 /// \p Succ would not be chosen otherwise. Tail duplication of \p Succ is 753 /// always locally profitable if we would have picked \p Succ without 754 /// considering duplication. 755 bool MachineBlockPlacement::isProfitableToTailDup( 756 const MachineBasicBlock *BB, const MachineBasicBlock *Succ, 757 BranchProbability QProb, 758 const BlockChain &Chain, const BlockFilterSet *BlockFilter) { 759 // We need to do a probability calculation to make sure this is profitable. 760 // First: does succ have a successor that post-dominates? This affects the 761 // calculation. The 2 relevant cases are: 762 // BB BB 763 // | \Qout | \Qout 764 // P| C |P C 765 // = C' = C' 766 // | /Qin | /Qin 767 // | / | / 768 // Succ Succ 769 // / \ | \ V 770 // U/ =V |U \ 771 // / \ = D 772 // D E | / 773 // | / 774 // |/ 775 // PDom 776 // '=' : Branch taken for that CFG edge 777 // In the second case, Placing Succ while duplicating it into C prevents the 778 // fallthrough of Succ into either D or PDom, because they now have C as an 779 // unplaced predecessor 780 781 // Start by figuring out which case we fall into 782 MachineBasicBlock *PDom = nullptr; 783 SmallVector<MachineBasicBlock *, 4> SuccSuccs; 784 // Only scan the relevant successors 785 auto AdjustedSuccSumProb = 786 collectViableSuccessors(Succ, Chain, BlockFilter, SuccSuccs); 787 BranchProbability PProb = MBPI->getEdgeProbability(BB, Succ); 788 auto BBFreq = MBFI->getBlockFreq(BB); 789 auto SuccFreq = MBFI->getBlockFreq(Succ); 790 BlockFrequency P = BBFreq * PProb; 791 BlockFrequency Qout = BBFreq * QProb; 792 uint64_t EntryFreq = MBFI->getEntryFreq(); 793 // If there are no more successors, it is profitable to copy, as it strictly 794 // increases fallthrough. 795 if (SuccSuccs.size() == 0) 796 return greaterWithBias(P, Qout, EntryFreq); 797 798 auto BestSuccSucc = BranchProbability::getZero(); 799 // Find the PDom or the best Succ if no PDom exists. 800 for (MachineBasicBlock *SuccSucc : SuccSuccs) { 801 auto Prob = MBPI->getEdgeProbability(Succ, SuccSucc); 802 if (Prob > BestSuccSucc) 803 BestSuccSucc = Prob; 804 if (PDom == nullptr) 805 if (MPDT->dominates(SuccSucc, Succ)) { 806 PDom = SuccSucc; 807 break; 808 } 809 } 810 // For the comparisons, we need to know Succ's best incoming edge that isn't 811 // from BB. 812 auto SuccBestPred = BlockFrequency(0); 813 for (MachineBasicBlock *SuccPred : Succ->predecessors()) { 814 if (SuccPred == Succ || SuccPred == BB 815 || BlockToChain[SuccPred] == &Chain 816 || (BlockFilter && !BlockFilter->count(SuccPred))) 817 continue; 818 auto Freq = MBFI->getBlockFreq(SuccPred) 819 * MBPI->getEdgeProbability(SuccPred, Succ); 820 if (Freq > SuccBestPred) 821 SuccBestPred = Freq; 822 } 823 // Qin is Succ's best unplaced incoming edge that isn't BB 824 BlockFrequency Qin = SuccBestPred; 825 // If it doesn't have a post-dominating successor, here is the calculation: 826 // BB BB 827 // | \Qout | \ 828 // P| C | = 829 // = C' | C 830 // | /Qin | | 831 // | / | C' (+Succ) 832 // Succ Succ /| 833 // / \ | \/ | 834 // U/ =V | == | 835 // / \ | / \| 836 // D E D E 837 // '=' : Branch taken for that CFG edge 838 // Cost in the first case is: P + V 839 // For this calculation, we always assume P > Qout. If Qout > P 840 // The result of this function will be ignored at the caller. 841 // Let F = SuccFreq - Qin 842 // Cost in the second case is: Qout + min(Qin, F) * U + max(Qin, F) * V 843 844 if (PDom == nullptr || !Succ->isSuccessor(PDom)) { 845 BranchProbability UProb = BestSuccSucc; 846 BranchProbability VProb = AdjustedSuccSumProb - UProb; 847 BlockFrequency F = SuccFreq - Qin; 848 BlockFrequency V = SuccFreq * VProb; 849 BlockFrequency QinU = std::min(Qin, F) * UProb; 850 BlockFrequency BaseCost = P + V; 851 BlockFrequency DupCost = Qout + QinU + std::max(Qin, F) * VProb; 852 return greaterWithBias(BaseCost, DupCost, EntryFreq); 853 } 854 BranchProbability UProb = MBPI->getEdgeProbability(Succ, PDom); 855 BranchProbability VProb = AdjustedSuccSumProb - UProb; 856 BlockFrequency U = SuccFreq * UProb; 857 BlockFrequency V = SuccFreq * VProb; 858 BlockFrequency F = SuccFreq - Qin; 859 // If there is a post-dominating successor, here is the calculation: 860 // BB BB BB BB 861 // | \Qout | \ | \Qout | \ 862 // |P C | = |P C | = 863 // = C' |P C = C' |P C 864 // | /Qin | | | /Qin | | 865 // | / | C' (+Succ) | / | C' (+Succ) 866 // Succ Succ /| Succ Succ /| 867 // | \ V | \/ | | \ V | \/ | 868 // |U \ |U /\ =? |U = |U /\ | 869 // = D = = =?| | D | = =| 870 // | / |/ D | / |/ D 871 // | / | / | = | / 872 // |/ | / |/ | = 873 // Dom Dom Dom Dom 874 // '=' : Branch taken for that CFG edge 875 // The cost for taken branches in the first case is P + U 876 // Let F = SuccFreq - Qin 877 // The cost in the second case (assuming independence), given the layout: 878 // BB, Succ, (C+Succ), D, Dom or the layout: 879 // BB, Succ, D, Dom, (C+Succ) 880 // is Qout + max(F, Qin) * U + min(F, Qin) 881 // compare P + U vs Qout + P * U + Qin. 882 // 883 // The 3rd and 4th cases cover when Dom would be chosen to follow Succ. 884 // 885 // For the 3rd case, the cost is P + 2 * V 886 // For the 4th case, the cost is Qout + min(Qin, F) * U + max(Qin, F) * V + V 887 // We choose 4 over 3 when (P + V) > Qout + min(Qin, F) * U + max(Qin, F) * V 888 if (UProb > AdjustedSuccSumProb / 2 && 889 !hasBetterLayoutPredecessor(Succ, PDom, *BlockToChain[PDom], UProb, UProb, 890 Chain, BlockFilter)) 891 // Cases 3 & 4 892 return greaterWithBias( 893 (P + V), (Qout + std::max(Qin, F) * VProb + std::min(Qin, F) * UProb), 894 EntryFreq); 895 // Cases 1 & 2 896 return greaterWithBias((P + U), 897 (Qout + std::min(Qin, F) * AdjustedSuccSumProb + 898 std::max(Qin, F) * UProb), 899 EntryFreq); 900 } 901 902 /// Check for a trellis layout. \p BB is the upper part of a trellis if its 903 /// successors form the lower part of a trellis. A successor set S forms the 904 /// lower part of a trellis if all of the predecessors of S are either in S or 905 /// have all of S as successors. We ignore trellises where BB doesn't have 2 906 /// successors because for fewer than 2, it's trivial, and for 3 or greater they 907 /// are very uncommon and complex to compute optimally. Allowing edges within S 908 /// is not strictly a trellis, but the same algorithm works, so we allow it. 909 bool MachineBlockPlacement::isTrellis( 910 const MachineBasicBlock *BB, 911 const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs, 912 const BlockChain &Chain, const BlockFilterSet *BlockFilter) { 913 // Technically BB could form a trellis with branching factor higher than 2. 914 // But that's extremely uncommon. 915 if (BB->succ_size() != 2 || ViableSuccs.size() != 2) 916 return false; 917 918 SmallPtrSet<const MachineBasicBlock *, 2> Successors(BB->succ_begin(), 919 BB->succ_end()); 920 // To avoid reviewing the same predecessors twice. 921 SmallPtrSet<const MachineBasicBlock *, 8> SeenPreds; 922 923 for (MachineBasicBlock *Succ : ViableSuccs) { 924 int PredCount = 0; 925 for (auto SuccPred : Succ->predecessors()) { 926 // Allow triangle successors, but don't count them. 927 if (Successors.count(SuccPred)) { 928 // Make sure that it is actually a triangle. 929 for (MachineBasicBlock *CheckSucc : SuccPred->successors()) 930 if (!Successors.count(CheckSucc)) 931 return false; 932 continue; 933 } 934 const BlockChain *PredChain = BlockToChain[SuccPred]; 935 if (SuccPred == BB || (BlockFilter && !BlockFilter->count(SuccPred)) || 936 PredChain == &Chain || PredChain == BlockToChain[Succ]) 937 continue; 938 ++PredCount; 939 // Perform the successor check only once. 940 if (!SeenPreds.insert(SuccPred).second) 941 continue; 942 if (!hasSameSuccessors(*SuccPred, Successors)) 943 return false; 944 } 945 // If one of the successors has only BB as a predecessor, it is not a 946 // trellis. 947 if (PredCount < 1) 948 return false; 949 } 950 return true; 951 } 952 953 /// Pick the highest total weight pair of edges that can both be laid out. 954 /// The edges in \p Edges[0] are assumed to have a different destination than 955 /// the edges in \p Edges[1]. Simple counting shows that the best pair is either 956 /// the individual highest weight edges to the 2 different destinations, or in 957 /// case of a conflict, one of them should be replaced with a 2nd best edge. 958 std::pair<MachineBlockPlacement::WeightedEdge, 959 MachineBlockPlacement::WeightedEdge> 960 MachineBlockPlacement::getBestNonConflictingEdges( 961 const MachineBasicBlock *BB, 962 MutableArrayRef<SmallVector<MachineBlockPlacement::WeightedEdge, 8>> 963 Edges) { 964 // Sort the edges, and then for each successor, find the best incoming 965 // predecessor. If the best incoming predecessors aren't the same, 966 // then that is clearly the best layout. If there is a conflict, one of the 967 // successors will have to fallthrough from the second best predecessor. We 968 // compare which combination is better overall. 969 970 // Sort for highest frequency. 971 auto Cmp = [](WeightedEdge A, WeightedEdge B) { return A.Weight > B.Weight; }; 972 973 llvm::stable_sort(Edges[0], Cmp); 974 llvm::stable_sort(Edges[1], Cmp); 975 auto BestA = Edges[0].begin(); 976 auto BestB = Edges[1].begin(); 977 // Arrange for the correct answer to be in BestA and BestB 978 // If the 2 best edges don't conflict, the answer is already there. 979 if (BestA->Src == BestB->Src) { 980 // Compare the total fallthrough of (Best + Second Best) for both pairs 981 auto SecondBestA = std::next(BestA); 982 auto SecondBestB = std::next(BestB); 983 BlockFrequency BestAScore = BestA->Weight + SecondBestB->Weight; 984 BlockFrequency BestBScore = BestB->Weight + SecondBestA->Weight; 985 if (BestAScore < BestBScore) 986 BestA = SecondBestA; 987 else 988 BestB = SecondBestB; 989 } 990 // Arrange for the BB edge to be in BestA if it exists. 991 if (BestB->Src == BB) 992 std::swap(BestA, BestB); 993 return std::make_pair(*BestA, *BestB); 994 } 995 996 /// Get the best successor from \p BB based on \p BB being part of a trellis. 997 /// We only handle trellises with 2 successors, so the algorithm is 998 /// straightforward: Find the best pair of edges that don't conflict. We find 999 /// the best incoming edge for each successor in the trellis. If those conflict, 1000 /// we consider which of them should be replaced with the second best. 1001 /// Upon return the two best edges will be in \p BestEdges. If one of the edges 1002 /// comes from \p BB, it will be in \p BestEdges[0] 1003 MachineBlockPlacement::BlockAndTailDupResult 1004 MachineBlockPlacement::getBestTrellisSuccessor( 1005 const MachineBasicBlock *BB, 1006 const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs, 1007 BranchProbability AdjustedSumProb, const BlockChain &Chain, 1008 const BlockFilterSet *BlockFilter) { 1009 1010 BlockAndTailDupResult Result = {nullptr, false}; 1011 SmallPtrSet<const MachineBasicBlock *, 4> Successors(BB->succ_begin(), 1012 BB->succ_end()); 1013 1014 // We assume size 2 because it's common. For general n, we would have to do 1015 // the Hungarian algorithm, but it's not worth the complexity because more 1016 // than 2 successors is fairly uncommon, and a trellis even more so. 1017 if (Successors.size() != 2 || ViableSuccs.size() != 2) 1018 return Result; 1019 1020 // Collect the edge frequencies of all edges that form the trellis. 1021 SmallVector<WeightedEdge, 8> Edges[2]; 1022 int SuccIndex = 0; 1023 for (auto Succ : ViableSuccs) { 1024 for (MachineBasicBlock *SuccPred : Succ->predecessors()) { 1025 // Skip any placed predecessors that are not BB 1026 if (SuccPred != BB) 1027 if ((BlockFilter && !BlockFilter->count(SuccPred)) || 1028 BlockToChain[SuccPred] == &Chain || 1029 BlockToChain[SuccPred] == BlockToChain[Succ]) 1030 continue; 1031 BlockFrequency EdgeFreq = MBFI->getBlockFreq(SuccPred) * 1032 MBPI->getEdgeProbability(SuccPred, Succ); 1033 Edges[SuccIndex].push_back({EdgeFreq, SuccPred, Succ}); 1034 } 1035 ++SuccIndex; 1036 } 1037 1038 // Pick the best combination of 2 edges from all the edges in the trellis. 1039 WeightedEdge BestA, BestB; 1040 std::tie(BestA, BestB) = getBestNonConflictingEdges(BB, Edges); 1041 1042 if (BestA.Src != BB) { 1043 // If we have a trellis, and BB doesn't have the best fallthrough edges, 1044 // we shouldn't choose any successor. We've already looked and there's a 1045 // better fallthrough edge for all the successors. 1046 LLVM_DEBUG(dbgs() << "Trellis, but not one of the chosen edges.\n"); 1047 return Result; 1048 } 1049 1050 // Did we pick the triangle edge? If tail-duplication is profitable, do 1051 // that instead. Otherwise merge the triangle edge now while we know it is 1052 // optimal. 1053 if (BestA.Dest == BestB.Src) { 1054 // The edges are BB->Succ1->Succ2, and we're looking to see if BB->Succ2 1055 // would be better. 1056 MachineBasicBlock *Succ1 = BestA.Dest; 1057 MachineBasicBlock *Succ2 = BestB.Dest; 1058 // Check to see if tail-duplication would be profitable. 1059 if (allowTailDupPlacement() && shouldTailDuplicate(Succ2) && 1060 canTailDuplicateUnplacedPreds(BB, Succ2, Chain, BlockFilter) && 1061 isProfitableToTailDup(BB, Succ2, MBPI->getEdgeProbability(BB, Succ1), 1062 Chain, BlockFilter)) { 1063 LLVM_DEBUG(BranchProbability Succ2Prob = getAdjustedProbability( 1064 MBPI->getEdgeProbability(BB, Succ2), AdjustedSumProb); 1065 dbgs() << " Selected: " << getBlockName(Succ2) 1066 << ", probability: " << Succ2Prob 1067 << " (Tail Duplicate)\n"); 1068 Result.BB = Succ2; 1069 Result.ShouldTailDup = true; 1070 return Result; 1071 } 1072 } 1073 // We have already computed the optimal edge for the other side of the 1074 // trellis. 1075 ComputedEdges[BestB.Src] = { BestB.Dest, false }; 1076 1077 auto TrellisSucc = BestA.Dest; 1078 LLVM_DEBUG(BranchProbability SuccProb = getAdjustedProbability( 1079 MBPI->getEdgeProbability(BB, TrellisSucc), AdjustedSumProb); 1080 dbgs() << " Selected: " << getBlockName(TrellisSucc) 1081 << ", probability: " << SuccProb << " (Trellis)\n"); 1082 Result.BB = TrellisSucc; 1083 return Result; 1084 } 1085 1086 /// When the option allowTailDupPlacement() is on, this method checks if the 1087 /// fallthrough candidate block \p Succ (of block \p BB) can be tail-duplicated 1088 /// into all of its unplaced, unfiltered predecessors, that are not BB. 1089 bool MachineBlockPlacement::canTailDuplicateUnplacedPreds( 1090 const MachineBasicBlock *BB, MachineBasicBlock *Succ, 1091 const BlockChain &Chain, const BlockFilterSet *BlockFilter) { 1092 if (!shouldTailDuplicate(Succ)) 1093 return false; 1094 1095 // The result of canTailDuplicate. 1096 bool Duplicate = true; 1097 // Number of possible duplication. 1098 unsigned int NumDup = 0; 1099 1100 // For CFG checking. 1101 SmallPtrSet<const MachineBasicBlock *, 4> Successors(BB->succ_begin(), 1102 BB->succ_end()); 1103 for (MachineBasicBlock *Pred : Succ->predecessors()) { 1104 // Make sure all unplaced and unfiltered predecessors can be 1105 // tail-duplicated into. 1106 // Skip any blocks that are already placed or not in this loop. 1107 if (Pred == BB || (BlockFilter && !BlockFilter->count(Pred)) 1108 || BlockToChain[Pred] == &Chain) 1109 continue; 1110 if (!TailDup.canTailDuplicate(Succ, Pred)) { 1111 if (Successors.size() > 1 && hasSameSuccessors(*Pred, Successors)) 1112 // This will result in a trellis after tail duplication, so we don't 1113 // need to copy Succ into this predecessor. In the presence 1114 // of a trellis tail duplication can continue to be profitable. 1115 // For example: 1116 // A A 1117 // |\ |\ 1118 // | \ | \ 1119 // | C | C+BB 1120 // | / | | 1121 // |/ | | 1122 // BB => BB | 1123 // |\ |\/| 1124 // | \ |/\| 1125 // | D | D 1126 // | / | / 1127 // |/ |/ 1128 // Succ Succ 1129 // 1130 // After BB was duplicated into C, the layout looks like the one on the 1131 // right. BB and C now have the same successors. When considering 1132 // whether Succ can be duplicated into all its unplaced predecessors, we 1133 // ignore C. 1134 // We can do this because C already has a profitable fallthrough, namely 1135 // D. TODO(iteratee): ignore sufficiently cold predecessors for 1136 // duplication and for this test. 1137 // 1138 // This allows trellises to be laid out in 2 separate chains 1139 // (A,B,Succ,...) and later (C,D,...) This is a reasonable heuristic 1140 // because it allows the creation of 2 fallthrough paths with links 1141 // between them, and we correctly identify the best layout for these 1142 // CFGs. We want to extend trellises that the user created in addition 1143 // to trellises created by tail-duplication, so we just look for the 1144 // CFG. 1145 continue; 1146 Duplicate = false; 1147 continue; 1148 } 1149 NumDup++; 1150 } 1151 1152 // No possible duplication in current filter set. 1153 if (NumDup == 0) 1154 return false; 1155 1156 // If profile information is available, findDuplicateCandidates can do more 1157 // precise benefit analysis. 1158 if (F->getFunction().hasProfileData()) 1159 return true; 1160 1161 // This is mainly for function exit BB. 1162 // The integrated tail duplication is really designed for increasing 1163 // fallthrough from predecessors from Succ to its successors. We may need 1164 // other machanism to handle different cases. 1165 if (Succ->succ_size() == 0) 1166 return true; 1167 1168 // Plus the already placed predecessor. 1169 NumDup++; 1170 1171 // If the duplication candidate has more unplaced predecessors than 1172 // successors, the extra duplication can't bring more fallthrough. 1173 // 1174 // Pred1 Pred2 Pred3 1175 // \ | / 1176 // \ | / 1177 // \ | / 1178 // Dup 1179 // / \ 1180 // / \ 1181 // Succ1 Succ2 1182 // 1183 // In this example Dup has 2 successors and 3 predecessors, duplication of Dup 1184 // can increase the fallthrough from Pred1 to Succ1 and from Pred2 to Succ2, 1185 // but the duplication into Pred3 can't increase fallthrough. 1186 // 1187 // A small number of extra duplication may not hurt too much. We need a better 1188 // heuristic to handle it. 1189 if ((NumDup > Succ->succ_size()) || !Duplicate) 1190 return false; 1191 1192 return true; 1193 } 1194 1195 /// Find chains of triangles where we believe it would be profitable to 1196 /// tail-duplicate them all, but a local analysis would not find them. 1197 /// There are 3 ways this can be profitable: 1198 /// 1) The post-dominators marked 50% are actually taken 55% (This shrinks with 1199 /// longer chains) 1200 /// 2) The chains are statically correlated. Branch probabilities have a very 1201 /// U-shaped distribution. 1202 /// [http://nrs.harvard.edu/urn-3:HUL.InstRepos:24015805] 1203 /// If the branches in a chain are likely to be from the same side of the 1204 /// distribution as their predecessor, but are independent at runtime, this 1205 /// transformation is profitable. (Because the cost of being wrong is a small 1206 /// fixed cost, unlike the standard triangle layout where the cost of being 1207 /// wrong scales with the # of triangles.) 1208 /// 3) The chains are dynamically correlated. If the probability that a previous 1209 /// branch was taken positively influences whether the next branch will be 1210 /// taken 1211 /// We believe that 2 and 3 are common enough to justify the small margin in 1. 1212 void MachineBlockPlacement::precomputeTriangleChains() { 1213 struct TriangleChain { 1214 std::vector<MachineBasicBlock *> Edges; 1215 1216 TriangleChain(MachineBasicBlock *src, MachineBasicBlock *dst) 1217 : Edges({src, dst}) {} 1218 1219 void append(MachineBasicBlock *dst) { 1220 assert(getKey()->isSuccessor(dst) && 1221 "Attempting to append a block that is not a successor."); 1222 Edges.push_back(dst); 1223 } 1224 1225 unsigned count() const { return Edges.size() - 1; } 1226 1227 MachineBasicBlock *getKey() const { 1228 return Edges.back(); 1229 } 1230 }; 1231 1232 if (TriangleChainCount == 0) 1233 return; 1234 1235 LLVM_DEBUG(dbgs() << "Pre-computing triangle chains.\n"); 1236 // Map from last block to the chain that contains it. This allows us to extend 1237 // chains as we find new triangles. 1238 DenseMap<const MachineBasicBlock *, TriangleChain> TriangleChainMap; 1239 for (MachineBasicBlock &BB : *F) { 1240 // If BB doesn't have 2 successors, it doesn't start a triangle. 1241 if (BB.succ_size() != 2) 1242 continue; 1243 MachineBasicBlock *PDom = nullptr; 1244 for (MachineBasicBlock *Succ : BB.successors()) { 1245 if (!MPDT->dominates(Succ, &BB)) 1246 continue; 1247 PDom = Succ; 1248 break; 1249 } 1250 // If BB doesn't have a post-dominating successor, it doesn't form a 1251 // triangle. 1252 if (PDom == nullptr) 1253 continue; 1254 // If PDom has a hint that it is low probability, skip this triangle. 1255 if (MBPI->getEdgeProbability(&BB, PDom) < BranchProbability(50, 100)) 1256 continue; 1257 // If PDom isn't eligible for duplication, this isn't the kind of triangle 1258 // we're looking for. 1259 if (!shouldTailDuplicate(PDom)) 1260 continue; 1261 bool CanTailDuplicate = true; 1262 // If PDom can't tail-duplicate into it's non-BB predecessors, then this 1263 // isn't the kind of triangle we're looking for. 1264 for (MachineBasicBlock* Pred : PDom->predecessors()) { 1265 if (Pred == &BB) 1266 continue; 1267 if (!TailDup.canTailDuplicate(PDom, Pred)) { 1268 CanTailDuplicate = false; 1269 break; 1270 } 1271 } 1272 // If we can't tail-duplicate PDom to its predecessors, then skip this 1273 // triangle. 1274 if (!CanTailDuplicate) 1275 continue; 1276 1277 // Now we have an interesting triangle. Insert it if it's not part of an 1278 // existing chain. 1279 // Note: This cannot be replaced with a call insert() or emplace() because 1280 // the find key is BB, but the insert/emplace key is PDom. 1281 auto Found = TriangleChainMap.find(&BB); 1282 // If it is, remove the chain from the map, grow it, and put it back in the 1283 // map with the end as the new key. 1284 if (Found != TriangleChainMap.end()) { 1285 TriangleChain Chain = std::move(Found->second); 1286 TriangleChainMap.erase(Found); 1287 Chain.append(PDom); 1288 TriangleChainMap.insert(std::make_pair(Chain.getKey(), std::move(Chain))); 1289 } else { 1290 auto InsertResult = TriangleChainMap.try_emplace(PDom, &BB, PDom); 1291 assert(InsertResult.second && "Block seen twice."); 1292 (void)InsertResult; 1293 } 1294 } 1295 1296 // Iterating over a DenseMap is safe here, because the only thing in the body 1297 // of the loop is inserting into another DenseMap (ComputedEdges). 1298 // ComputedEdges is never iterated, so this doesn't lead to non-determinism. 1299 for (auto &ChainPair : TriangleChainMap) { 1300 TriangleChain &Chain = ChainPair.second; 1301 // Benchmarking has shown that due to branch correlation duplicating 2 or 1302 // more triangles is profitable, despite the calculations assuming 1303 // independence. 1304 if (Chain.count() < TriangleChainCount) 1305 continue; 1306 MachineBasicBlock *dst = Chain.Edges.back(); 1307 Chain.Edges.pop_back(); 1308 for (MachineBasicBlock *src : reverse(Chain.Edges)) { 1309 LLVM_DEBUG(dbgs() << "Marking edge: " << getBlockName(src) << "->" 1310 << getBlockName(dst) 1311 << " as pre-computed based on triangles.\n"); 1312 1313 auto InsertResult = ComputedEdges.insert({src, {dst, true}}); 1314 assert(InsertResult.second && "Block seen twice."); 1315 (void)InsertResult; 1316 1317 dst = src; 1318 } 1319 } 1320 } 1321 1322 // When profile is not present, return the StaticLikelyProb. 1323 // When profile is available, we need to handle the triangle-shape CFG. 1324 static BranchProbability getLayoutSuccessorProbThreshold( 1325 const MachineBasicBlock *BB) { 1326 if (!BB->getParent()->getFunction().hasProfileData()) 1327 return BranchProbability(StaticLikelyProb, 100); 1328 if (BB->succ_size() == 2) { 1329 const MachineBasicBlock *Succ1 = *BB->succ_begin(); 1330 const MachineBasicBlock *Succ2 = *(BB->succ_begin() + 1); 1331 if (Succ1->isSuccessor(Succ2) || Succ2->isSuccessor(Succ1)) { 1332 /* See case 1 below for the cost analysis. For BB->Succ to 1333 * be taken with smaller cost, the following needs to hold: 1334 * Prob(BB->Succ) > 2 * Prob(BB->Pred) 1335 * So the threshold T in the calculation below 1336 * (1-T) * Prob(BB->Succ) > T * Prob(BB->Pred) 1337 * So T / (1 - T) = 2, Yielding T = 2/3 1338 * Also adding user specified branch bias, we have 1339 * T = (2/3)*(ProfileLikelyProb/50) 1340 * = (2*ProfileLikelyProb)/150) 1341 */ 1342 return BranchProbability(2 * ProfileLikelyProb, 150); 1343 } 1344 } 1345 return BranchProbability(ProfileLikelyProb, 100); 1346 } 1347 1348 /// Checks to see if the layout candidate block \p Succ has a better layout 1349 /// predecessor than \c BB. If yes, returns true. 1350 /// \p SuccProb: The probability adjusted for only remaining blocks. 1351 /// Only used for logging 1352 /// \p RealSuccProb: The un-adjusted probability. 1353 /// \p Chain: The chain that BB belongs to and Succ is being considered for. 1354 /// \p BlockFilter: if non-null, the set of blocks that make up the loop being 1355 /// considered 1356 bool MachineBlockPlacement::hasBetterLayoutPredecessor( 1357 const MachineBasicBlock *BB, const MachineBasicBlock *Succ, 1358 const BlockChain &SuccChain, BranchProbability SuccProb, 1359 BranchProbability RealSuccProb, const BlockChain &Chain, 1360 const BlockFilterSet *BlockFilter) { 1361 1362 // There isn't a better layout when there are no unscheduled predecessors. 1363 if (SuccChain.UnscheduledPredecessors == 0) 1364 return false; 1365 1366 // There are two basic scenarios here: 1367 // ------------------------------------- 1368 // Case 1: triangular shape CFG (if-then): 1369 // BB 1370 // | \ 1371 // | \ 1372 // | Pred 1373 // | / 1374 // Succ 1375 // In this case, we are evaluating whether to select edge -> Succ, e.g. 1376 // set Succ as the layout successor of BB. Picking Succ as BB's 1377 // successor breaks the CFG constraints (FIXME: define these constraints). 1378 // With this layout, Pred BB 1379 // is forced to be outlined, so the overall cost will be cost of the 1380 // branch taken from BB to Pred, plus the cost of back taken branch 1381 // from Pred to Succ, as well as the additional cost associated 1382 // with the needed unconditional jump instruction from Pred To Succ. 1383 1384 // The cost of the topological order layout is the taken branch cost 1385 // from BB to Succ, so to make BB->Succ a viable candidate, the following 1386 // must hold: 1387 // 2 * freq(BB->Pred) * taken_branch_cost + unconditional_jump_cost 1388 // < freq(BB->Succ) * taken_branch_cost. 1389 // Ignoring unconditional jump cost, we get 1390 // freq(BB->Succ) > 2 * freq(BB->Pred), i.e., 1391 // prob(BB->Succ) > 2 * prob(BB->Pred) 1392 // 1393 // When real profile data is available, we can precisely compute the 1394 // probability threshold that is needed for edge BB->Succ to be considered. 1395 // Without profile data, the heuristic requires the branch bias to be 1396 // a lot larger to make sure the signal is very strong (e.g. 80% default). 1397 // ----------------------------------------------------------------- 1398 // Case 2: diamond like CFG (if-then-else): 1399 // S 1400 // / \ 1401 // | \ 1402 // BB Pred 1403 // \ / 1404 // Succ 1405 // .. 1406 // 1407 // The current block is BB and edge BB->Succ is now being evaluated. 1408 // Note that edge S->BB was previously already selected because 1409 // prob(S->BB) > prob(S->Pred). 1410 // At this point, 2 blocks can be placed after BB: Pred or Succ. If we 1411 // choose Pred, we will have a topological ordering as shown on the left 1412 // in the picture below. If we choose Succ, we have the solution as shown 1413 // on the right: 1414 // 1415 // topo-order: 1416 // 1417 // S----- ---S 1418 // | | | | 1419 // ---BB | | BB 1420 // | | | | 1421 // | Pred-- | Succ-- 1422 // | | | | 1423 // ---Succ ---Pred-- 1424 // 1425 // cost = freq(S->Pred) + freq(BB->Succ) cost = 2 * freq (S->Pred) 1426 // = freq(S->Pred) + freq(S->BB) 1427 // 1428 // If we have profile data (i.e, branch probabilities can be trusted), the 1429 // cost (number of taken branches) with layout S->BB->Succ->Pred is 2 * 1430 // freq(S->Pred) while the cost of topo order is freq(S->Pred) + freq(S->BB). 1431 // We know Prob(S->BB) > Prob(S->Pred), so freq(S->BB) > freq(S->Pred), which 1432 // means the cost of topological order is greater. 1433 // When profile data is not available, however, we need to be more 1434 // conservative. If the branch prediction is wrong, breaking the topo-order 1435 // will actually yield a layout with large cost. For this reason, we need 1436 // strong biased branch at block S with Prob(S->BB) in order to select 1437 // BB->Succ. This is equivalent to looking the CFG backward with backward 1438 // edge: Prob(Succ->BB) needs to >= HotProb in order to be selected (without 1439 // profile data). 1440 // -------------------------------------------------------------------------- 1441 // Case 3: forked diamond 1442 // S 1443 // / \ 1444 // / \ 1445 // BB Pred 1446 // | \ / | 1447 // | \ / | 1448 // | X | 1449 // | / \ | 1450 // | / \ | 1451 // S1 S2 1452 // 1453 // The current block is BB and edge BB->S1 is now being evaluated. 1454 // As above S->BB was already selected because 1455 // prob(S->BB) > prob(S->Pred). Assume that prob(BB->S1) >= prob(BB->S2). 1456 // 1457 // topo-order: 1458 // 1459 // S-------| ---S 1460 // | | | | 1461 // ---BB | | BB 1462 // | | | | 1463 // | Pred----| | S1---- 1464 // | | | | 1465 // --(S1 or S2) ---Pred-- 1466 // | 1467 // S2 1468 // 1469 // topo-cost = freq(S->Pred) + freq(BB->S1) + freq(BB->S2) 1470 // + min(freq(Pred->S1), freq(Pred->S2)) 1471 // Non-topo-order cost: 1472 // non-topo-cost = 2 * freq(S->Pred) + freq(BB->S2). 1473 // To be conservative, we can assume that min(freq(Pred->S1), freq(Pred->S2)) 1474 // is 0. Then the non topo layout is better when 1475 // freq(S->Pred) < freq(BB->S1). 1476 // This is exactly what is checked below. 1477 // Note there are other shapes that apply (Pred may not be a single block, 1478 // but they all fit this general pattern.) 1479 BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB); 1480 1481 // Make sure that a hot successor doesn't have a globally more 1482 // important predecessor. 1483 BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(BB) * RealSuccProb; 1484 bool BadCFGConflict = false; 1485 1486 for (MachineBasicBlock *Pred : Succ->predecessors()) { 1487 BlockChain *PredChain = BlockToChain[Pred]; 1488 if (Pred == Succ || PredChain == &SuccChain || 1489 (BlockFilter && !BlockFilter->count(Pred)) || 1490 PredChain == &Chain || Pred != *std::prev(PredChain->end()) || 1491 // This check is redundant except for look ahead. This function is 1492 // called for lookahead by isProfitableToTailDup when BB hasn't been 1493 // placed yet. 1494 (Pred == BB)) 1495 continue; 1496 // Do backward checking. 1497 // For all cases above, we need a backward checking to filter out edges that 1498 // are not 'strongly' biased. 1499 // BB Pred 1500 // \ / 1501 // Succ 1502 // We select edge BB->Succ if 1503 // freq(BB->Succ) > freq(Succ) * HotProb 1504 // i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) * 1505 // HotProb 1506 // i.e. freq((BB->Succ) * (1 - HotProb) > freq(Pred->Succ) * HotProb 1507 // Case 1 is covered too, because the first equation reduces to: 1508 // prob(BB->Succ) > HotProb. (freq(Succ) = freq(BB) for a triangle) 1509 BlockFrequency PredEdgeFreq = 1510 MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ); 1511 if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) { 1512 BadCFGConflict = true; 1513 break; 1514 } 1515 } 1516 1517 if (BadCFGConflict) { 1518 LLVM_DEBUG(dbgs() << " Not a candidate: " << getBlockName(Succ) << " -> " 1519 << SuccProb << " (prob) (non-cold CFG conflict)\n"); 1520 return true; 1521 } 1522 1523 return false; 1524 } 1525 1526 /// Select the best successor for a block. 1527 /// 1528 /// This looks across all successors of a particular block and attempts to 1529 /// select the "best" one to be the layout successor. It only considers direct 1530 /// successors which also pass the block filter. It will attempt to avoid 1531 /// breaking CFG structure, but cave and break such structures in the case of 1532 /// very hot successor edges. 1533 /// 1534 /// \returns The best successor block found, or null if none are viable, along 1535 /// with a boolean indicating if tail duplication is necessary. 1536 MachineBlockPlacement::BlockAndTailDupResult 1537 MachineBlockPlacement::selectBestSuccessor( 1538 const MachineBasicBlock *BB, const BlockChain &Chain, 1539 const BlockFilterSet *BlockFilter) { 1540 const BranchProbability HotProb(StaticLikelyProb, 100); 1541 1542 BlockAndTailDupResult BestSucc = { nullptr, false }; 1543 auto BestProb = BranchProbability::getZero(); 1544 1545 SmallVector<MachineBasicBlock *, 4> Successors; 1546 auto AdjustedSumProb = 1547 collectViableSuccessors(BB, Chain, BlockFilter, Successors); 1548 1549 LLVM_DEBUG(dbgs() << "Selecting best successor for: " << getBlockName(BB) 1550 << "\n"); 1551 1552 // if we already precomputed the best successor for BB, return that if still 1553 // applicable. 1554 auto FoundEdge = ComputedEdges.find(BB); 1555 if (FoundEdge != ComputedEdges.end()) { 1556 MachineBasicBlock *Succ = FoundEdge->second.BB; 1557 ComputedEdges.erase(FoundEdge); 1558 BlockChain *SuccChain = BlockToChain[Succ]; 1559 if (BB->isSuccessor(Succ) && (!BlockFilter || BlockFilter->count(Succ)) && 1560 SuccChain != &Chain && Succ == *SuccChain->begin()) 1561 return FoundEdge->second; 1562 } 1563 1564 // if BB is part of a trellis, Use the trellis to determine the optimal 1565 // fallthrough edges 1566 if (isTrellis(BB, Successors, Chain, BlockFilter)) 1567 return getBestTrellisSuccessor(BB, Successors, AdjustedSumProb, Chain, 1568 BlockFilter); 1569 1570 // For blocks with CFG violations, we may be able to lay them out anyway with 1571 // tail-duplication. We keep this vector so we can perform the probability 1572 // calculations the minimum number of times. 1573 SmallVector<std::pair<BranchProbability, MachineBasicBlock *>, 4> 1574 DupCandidates; 1575 for (MachineBasicBlock *Succ : Successors) { 1576 auto RealSuccProb = MBPI->getEdgeProbability(BB, Succ); 1577 BranchProbability SuccProb = 1578 getAdjustedProbability(RealSuccProb, AdjustedSumProb); 1579 1580 BlockChain &SuccChain = *BlockToChain[Succ]; 1581 // Skip the edge \c BB->Succ if block \c Succ has a better layout 1582 // predecessor that yields lower global cost. 1583 if (hasBetterLayoutPredecessor(BB, Succ, SuccChain, SuccProb, RealSuccProb, 1584 Chain, BlockFilter)) { 1585 // If tail duplication would make Succ profitable, place it. 1586 if (allowTailDupPlacement() && shouldTailDuplicate(Succ)) 1587 DupCandidates.emplace_back(SuccProb, Succ); 1588 continue; 1589 } 1590 1591 LLVM_DEBUG( 1592 dbgs() << " Candidate: " << getBlockName(Succ) 1593 << ", probability: " << SuccProb 1594 << (SuccChain.UnscheduledPredecessors != 0 ? " (CFG break)" : "") 1595 << "\n"); 1596 1597 if (BestSucc.BB && BestProb >= SuccProb) { 1598 LLVM_DEBUG(dbgs() << " Not the best candidate, continuing\n"); 1599 continue; 1600 } 1601 1602 LLVM_DEBUG(dbgs() << " Setting it as best candidate\n"); 1603 BestSucc.BB = Succ; 1604 BestProb = SuccProb; 1605 } 1606 // Handle the tail duplication candidates in order of decreasing probability. 1607 // Stop at the first one that is profitable. Also stop if they are less 1608 // profitable than BestSucc. Position is important because we preserve it and 1609 // prefer first best match. Here we aren't comparing in order, so we capture 1610 // the position instead. 1611 llvm::stable_sort(DupCandidates, 1612 [](std::tuple<BranchProbability, MachineBasicBlock *> L, 1613 std::tuple<BranchProbability, MachineBasicBlock *> R) { 1614 return std::get<0>(L) > std::get<0>(R); 1615 }); 1616 for (auto &Tup : DupCandidates) { 1617 BranchProbability DupProb; 1618 MachineBasicBlock *Succ; 1619 std::tie(DupProb, Succ) = Tup; 1620 if (DupProb < BestProb) 1621 break; 1622 if (canTailDuplicateUnplacedPreds(BB, Succ, Chain, BlockFilter) 1623 && (isProfitableToTailDup(BB, Succ, BestProb, Chain, BlockFilter))) { 1624 LLVM_DEBUG(dbgs() << " Candidate: " << getBlockName(Succ) 1625 << ", probability: " << DupProb 1626 << " (Tail Duplicate)\n"); 1627 BestSucc.BB = Succ; 1628 BestSucc.ShouldTailDup = true; 1629 break; 1630 } 1631 } 1632 1633 if (BestSucc.BB) 1634 LLVM_DEBUG(dbgs() << " Selected: " << getBlockName(BestSucc.BB) << "\n"); 1635 1636 return BestSucc; 1637 } 1638 1639 /// Select the best block from a worklist. 1640 /// 1641 /// This looks through the provided worklist as a list of candidate basic 1642 /// blocks and select the most profitable one to place. The definition of 1643 /// profitable only really makes sense in the context of a loop. This returns 1644 /// the most frequently visited block in the worklist, which in the case of 1645 /// a loop, is the one most desirable to be physically close to the rest of the 1646 /// loop body in order to improve i-cache behavior. 1647 /// 1648 /// \returns The best block found, or null if none are viable. 1649 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock( 1650 const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList) { 1651 // Once we need to walk the worklist looking for a candidate, cleanup the 1652 // worklist of already placed entries. 1653 // FIXME: If this shows up on profiles, it could be folded (at the cost of 1654 // some code complexity) into the loop below. 1655 WorkList.erase(llvm::remove_if(WorkList, 1656 [&](MachineBasicBlock *BB) { 1657 return BlockToChain.lookup(BB) == &Chain; 1658 }), 1659 WorkList.end()); 1660 1661 if (WorkList.empty()) 1662 return nullptr; 1663 1664 bool IsEHPad = WorkList[0]->isEHPad(); 1665 1666 MachineBasicBlock *BestBlock = nullptr; 1667 BlockFrequency BestFreq; 1668 for (MachineBasicBlock *MBB : WorkList) { 1669 assert(MBB->isEHPad() == IsEHPad && 1670 "EHPad mismatch between block and work list."); 1671 1672 BlockChain &SuccChain = *BlockToChain[MBB]; 1673 if (&SuccChain == &Chain) 1674 continue; 1675 1676 assert(SuccChain.UnscheduledPredecessors == 0 && 1677 "Found CFG-violating block"); 1678 1679 BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB); 1680 LLVM_DEBUG(dbgs() << " " << getBlockName(MBB) << " -> "; 1681 MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n"); 1682 1683 // For ehpad, we layout the least probable first as to avoid jumping back 1684 // from least probable landingpads to more probable ones. 1685 // 1686 // FIXME: Using probability is probably (!) not the best way to achieve 1687 // this. We should probably have a more principled approach to layout 1688 // cleanup code. 1689 // 1690 // The goal is to get: 1691 // 1692 // +--------------------------+ 1693 // | V 1694 // InnerLp -> InnerCleanup OuterLp -> OuterCleanup -> Resume 1695 // 1696 // Rather than: 1697 // 1698 // +-------------------------------------+ 1699 // V | 1700 // OuterLp -> OuterCleanup -> Resume InnerLp -> InnerCleanup 1701 if (BestBlock && (IsEHPad ^ (BestFreq >= CandidateFreq))) 1702 continue; 1703 1704 BestBlock = MBB; 1705 BestFreq = CandidateFreq; 1706 } 1707 1708 return BestBlock; 1709 } 1710 1711 /// Retrieve the first unplaced basic block. 1712 /// 1713 /// This routine is called when we are unable to use the CFG to walk through 1714 /// all of the basic blocks and form a chain due to unnatural loops in the CFG. 1715 /// We walk through the function's blocks in order, starting from the 1716 /// LastUnplacedBlockIt. We update this iterator on each call to avoid 1717 /// re-scanning the entire sequence on repeated calls to this routine. 1718 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock( 1719 const BlockChain &PlacedChain, 1720 MachineFunction::iterator &PrevUnplacedBlockIt, 1721 const BlockFilterSet *BlockFilter) { 1722 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F->end(); I != E; 1723 ++I) { 1724 if (BlockFilter && !BlockFilter->count(&*I)) 1725 continue; 1726 if (BlockToChain[&*I] != &PlacedChain) { 1727 PrevUnplacedBlockIt = I; 1728 // Now select the head of the chain to which the unplaced block belongs 1729 // as the block to place. This will force the entire chain to be placed, 1730 // and satisfies the requirements of merging chains. 1731 return *BlockToChain[&*I]->begin(); 1732 } 1733 } 1734 return nullptr; 1735 } 1736 1737 void MachineBlockPlacement::fillWorkLists( 1738 const MachineBasicBlock *MBB, 1739 SmallPtrSetImpl<BlockChain *> &UpdatedPreds, 1740 const BlockFilterSet *BlockFilter = nullptr) { 1741 BlockChain &Chain = *BlockToChain[MBB]; 1742 if (!UpdatedPreds.insert(&Chain).second) 1743 return; 1744 1745 assert( 1746 Chain.UnscheduledPredecessors == 0 && 1747 "Attempting to place block with unscheduled predecessors in worklist."); 1748 for (MachineBasicBlock *ChainBB : Chain) { 1749 assert(BlockToChain[ChainBB] == &Chain && 1750 "Block in chain doesn't match BlockToChain map."); 1751 for (MachineBasicBlock *Pred : ChainBB->predecessors()) { 1752 if (BlockFilter && !BlockFilter->count(Pred)) 1753 continue; 1754 if (BlockToChain[Pred] == &Chain) 1755 continue; 1756 ++Chain.UnscheduledPredecessors; 1757 } 1758 } 1759 1760 if (Chain.UnscheduledPredecessors != 0) 1761 return; 1762 1763 MachineBasicBlock *BB = *Chain.begin(); 1764 if (BB->isEHPad()) 1765 EHPadWorkList.push_back(BB); 1766 else 1767 BlockWorkList.push_back(BB); 1768 } 1769 1770 void MachineBlockPlacement::buildChain( 1771 const MachineBasicBlock *HeadBB, BlockChain &Chain, 1772 BlockFilterSet *BlockFilter) { 1773 assert(HeadBB && "BB must not be null.\n"); 1774 assert(BlockToChain[HeadBB] == &Chain && "BlockToChainMap mis-match.\n"); 1775 MachineFunction::iterator PrevUnplacedBlockIt = F->begin(); 1776 1777 const MachineBasicBlock *LoopHeaderBB = HeadBB; 1778 markChainSuccessors(Chain, LoopHeaderBB, BlockFilter); 1779 MachineBasicBlock *BB = *std::prev(Chain.end()); 1780 while (true) { 1781 assert(BB && "null block found at end of chain in loop."); 1782 assert(BlockToChain[BB] == &Chain && "BlockToChainMap mis-match in loop."); 1783 assert(*std::prev(Chain.end()) == BB && "BB Not found at end of chain."); 1784 1785 1786 // Look for the best viable successor if there is one to place immediately 1787 // after this block. 1788 auto Result = selectBestSuccessor(BB, Chain, BlockFilter); 1789 MachineBasicBlock* BestSucc = Result.BB; 1790 bool ShouldTailDup = Result.ShouldTailDup; 1791 if (allowTailDupPlacement()) 1792 ShouldTailDup |= (BestSucc && canTailDuplicateUnplacedPreds(BB, BestSucc, 1793 Chain, 1794 BlockFilter)); 1795 1796 // If an immediate successor isn't available, look for the best viable 1797 // block among those we've identified as not violating the loop's CFG at 1798 // this point. This won't be a fallthrough, but it will increase locality. 1799 if (!BestSucc) 1800 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList); 1801 if (!BestSucc) 1802 BestSucc = selectBestCandidateBlock(Chain, EHPadWorkList); 1803 1804 if (!BestSucc) { 1805 BestSucc = getFirstUnplacedBlock(Chain, PrevUnplacedBlockIt, BlockFilter); 1806 if (!BestSucc) 1807 break; 1808 1809 LLVM_DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the " 1810 "layout successor until the CFG reduces\n"); 1811 } 1812 1813 // Placement may have changed tail duplication opportunities. 1814 // Check for that now. 1815 if (allowTailDupPlacement() && BestSucc && ShouldTailDup) { 1816 repeatedlyTailDuplicateBlock(BestSucc, BB, LoopHeaderBB, Chain, 1817 BlockFilter, PrevUnplacedBlockIt); 1818 // If the chosen successor was duplicated into BB, don't bother laying 1819 // it out, just go round the loop again with BB as the chain end. 1820 if (!BB->isSuccessor(BestSucc)) 1821 continue; 1822 } 1823 1824 // Place this block, updating the datastructures to reflect its placement. 1825 BlockChain &SuccChain = *BlockToChain[BestSucc]; 1826 // Zero out UnscheduledPredecessors for the successor we're about to merge in case 1827 // we selected a successor that didn't fit naturally into the CFG. 1828 SuccChain.UnscheduledPredecessors = 0; 1829 LLVM_DEBUG(dbgs() << "Merging from " << getBlockName(BB) << " to " 1830 << getBlockName(BestSucc) << "\n"); 1831 markChainSuccessors(SuccChain, LoopHeaderBB, BlockFilter); 1832 Chain.merge(BestSucc, &SuccChain); 1833 BB = *std::prev(Chain.end()); 1834 } 1835 1836 LLVM_DEBUG(dbgs() << "Finished forming chain for header block " 1837 << getBlockName(*Chain.begin()) << "\n"); 1838 } 1839 1840 // If bottom of block BB has only one successor OldTop, in most cases it is 1841 // profitable to move it before OldTop, except the following case: 1842 // 1843 // -->OldTop<- 1844 // | . | 1845 // | . | 1846 // | . | 1847 // ---Pred | 1848 // | | 1849 // BB----- 1850 // 1851 // If BB is moved before OldTop, Pred needs a taken branch to BB, and it can't 1852 // layout the other successor below it, so it can't reduce taken branch. 1853 // In this case we keep its original layout. 1854 bool 1855 MachineBlockPlacement::canMoveBottomBlockToTop( 1856 const MachineBasicBlock *BottomBlock, 1857 const MachineBasicBlock *OldTop) { 1858 if (BottomBlock->pred_size() != 1) 1859 return true; 1860 MachineBasicBlock *Pred = *BottomBlock->pred_begin(); 1861 if (Pred->succ_size() != 2) 1862 return true; 1863 1864 MachineBasicBlock *OtherBB = *Pred->succ_begin(); 1865 if (OtherBB == BottomBlock) 1866 OtherBB = *Pred->succ_rbegin(); 1867 if (OtherBB == OldTop) 1868 return false; 1869 1870 return true; 1871 } 1872 1873 // Find out the possible fall through frequence to the top of a loop. 1874 BlockFrequency 1875 MachineBlockPlacement::TopFallThroughFreq( 1876 const MachineBasicBlock *Top, 1877 const BlockFilterSet &LoopBlockSet) { 1878 BlockFrequency MaxFreq = 0; 1879 for (MachineBasicBlock *Pred : Top->predecessors()) { 1880 BlockChain *PredChain = BlockToChain[Pred]; 1881 if (!LoopBlockSet.count(Pred) && 1882 (!PredChain || Pred == *std::prev(PredChain->end()))) { 1883 // Found a Pred block can be placed before Top. 1884 // Check if Top is the best successor of Pred. 1885 auto TopProb = MBPI->getEdgeProbability(Pred, Top); 1886 bool TopOK = true; 1887 for (MachineBasicBlock *Succ : Pred->successors()) { 1888 auto SuccProb = MBPI->getEdgeProbability(Pred, Succ); 1889 BlockChain *SuccChain = BlockToChain[Succ]; 1890 // Check if Succ can be placed after Pred. 1891 // Succ should not be in any chain, or it is the head of some chain. 1892 if (!LoopBlockSet.count(Succ) && (SuccProb > TopProb) && 1893 (!SuccChain || Succ == *SuccChain->begin())) { 1894 TopOK = false; 1895 break; 1896 } 1897 } 1898 if (TopOK) { 1899 BlockFrequency EdgeFreq = MBFI->getBlockFreq(Pred) * 1900 MBPI->getEdgeProbability(Pred, Top); 1901 if (EdgeFreq > MaxFreq) 1902 MaxFreq = EdgeFreq; 1903 } 1904 } 1905 } 1906 return MaxFreq; 1907 } 1908 1909 // Compute the fall through gains when move NewTop before OldTop. 1910 // 1911 // In following diagram, edges marked as "-" are reduced fallthrough, edges 1912 // marked as "+" are increased fallthrough, this function computes 1913 // 1914 // SUM(increased fallthrough) - SUM(decreased fallthrough) 1915 // 1916 // | 1917 // | - 1918 // V 1919 // --->OldTop 1920 // | . 1921 // | . 1922 // +| . + 1923 // | Pred ---> 1924 // | |- 1925 // | V 1926 // --- NewTop <--- 1927 // |- 1928 // V 1929 // 1930 BlockFrequency 1931 MachineBlockPlacement::FallThroughGains( 1932 const MachineBasicBlock *NewTop, 1933 const MachineBasicBlock *OldTop, 1934 const MachineBasicBlock *ExitBB, 1935 const BlockFilterSet &LoopBlockSet) { 1936 BlockFrequency FallThrough2Top = TopFallThroughFreq(OldTop, LoopBlockSet); 1937 BlockFrequency FallThrough2Exit = 0; 1938 if (ExitBB) 1939 FallThrough2Exit = MBFI->getBlockFreq(NewTop) * 1940 MBPI->getEdgeProbability(NewTop, ExitBB); 1941 BlockFrequency BackEdgeFreq = MBFI->getBlockFreq(NewTop) * 1942 MBPI->getEdgeProbability(NewTop, OldTop); 1943 1944 // Find the best Pred of NewTop. 1945 MachineBasicBlock *BestPred = nullptr; 1946 BlockFrequency FallThroughFromPred = 0; 1947 for (MachineBasicBlock *Pred : NewTop->predecessors()) { 1948 if (!LoopBlockSet.count(Pred)) 1949 continue; 1950 BlockChain *PredChain = BlockToChain[Pred]; 1951 if (!PredChain || Pred == *std::prev(PredChain->end())) { 1952 BlockFrequency EdgeFreq = MBFI->getBlockFreq(Pred) * 1953 MBPI->getEdgeProbability(Pred, NewTop); 1954 if (EdgeFreq > FallThroughFromPred) { 1955 FallThroughFromPred = EdgeFreq; 1956 BestPred = Pred; 1957 } 1958 } 1959 } 1960 1961 // If NewTop is not placed after Pred, another successor can be placed 1962 // after Pred. 1963 BlockFrequency NewFreq = 0; 1964 if (BestPred) { 1965 for (MachineBasicBlock *Succ : BestPred->successors()) { 1966 if ((Succ == NewTop) || (Succ == BestPred) || !LoopBlockSet.count(Succ)) 1967 continue; 1968 if (ComputedEdges.find(Succ) != ComputedEdges.end()) 1969 continue; 1970 BlockChain *SuccChain = BlockToChain[Succ]; 1971 if ((SuccChain && (Succ != *SuccChain->begin())) || 1972 (SuccChain == BlockToChain[BestPred])) 1973 continue; 1974 BlockFrequency EdgeFreq = MBFI->getBlockFreq(BestPred) * 1975 MBPI->getEdgeProbability(BestPred, Succ); 1976 if (EdgeFreq > NewFreq) 1977 NewFreq = EdgeFreq; 1978 } 1979 BlockFrequency OrigEdgeFreq = MBFI->getBlockFreq(BestPred) * 1980 MBPI->getEdgeProbability(BestPred, NewTop); 1981 if (NewFreq > OrigEdgeFreq) { 1982 // If NewTop is not the best successor of Pred, then Pred doesn't 1983 // fallthrough to NewTop. So there is no FallThroughFromPred and 1984 // NewFreq. 1985 NewFreq = 0; 1986 FallThroughFromPred = 0; 1987 } 1988 } 1989 1990 BlockFrequency Result = 0; 1991 BlockFrequency Gains = BackEdgeFreq + NewFreq; 1992 BlockFrequency Lost = FallThrough2Top + FallThrough2Exit + 1993 FallThroughFromPred; 1994 if (Gains > Lost) 1995 Result = Gains - Lost; 1996 return Result; 1997 } 1998 1999 /// Helper function of findBestLoopTop. Find the best loop top block 2000 /// from predecessors of old top. 2001 /// 2002 /// Look for a block which is strictly better than the old top for laying 2003 /// out before the old top of the loop. This looks for only two patterns: 2004 /// 2005 /// 1. a block has only one successor, the old loop top 2006 /// 2007 /// Because such a block will always result in an unconditional jump, 2008 /// rotating it in front of the old top is always profitable. 2009 /// 2010 /// 2. a block has two successors, one is old top, another is exit 2011 /// and it has more than one predecessors 2012 /// 2013 /// If it is below one of its predecessors P, only P can fall through to 2014 /// it, all other predecessors need a jump to it, and another conditional 2015 /// jump to loop header. If it is moved before loop header, all its 2016 /// predecessors jump to it, then fall through to loop header. So all its 2017 /// predecessors except P can reduce one taken branch. 2018 /// At the same time, move it before old top increases the taken branch 2019 /// to loop exit block, so the reduced taken branch will be compared with 2020 /// the increased taken branch to the loop exit block. 2021 MachineBasicBlock * 2022 MachineBlockPlacement::findBestLoopTopHelper( 2023 MachineBasicBlock *OldTop, 2024 const MachineLoop &L, 2025 const BlockFilterSet &LoopBlockSet) { 2026 // Check that the header hasn't been fused with a preheader block due to 2027 // crazy branches. If it has, we need to start with the header at the top to 2028 // prevent pulling the preheader into the loop body. 2029 BlockChain &HeaderChain = *BlockToChain[OldTop]; 2030 if (!LoopBlockSet.count(*HeaderChain.begin())) 2031 return OldTop; 2032 2033 LLVM_DEBUG(dbgs() << "Finding best loop top for: " << getBlockName(OldTop) 2034 << "\n"); 2035 2036 BlockFrequency BestGains = 0; 2037 MachineBasicBlock *BestPred = nullptr; 2038 for (MachineBasicBlock *Pred : OldTop->predecessors()) { 2039 if (!LoopBlockSet.count(Pred)) 2040 continue; 2041 if (Pred == L.getHeader()) 2042 continue; 2043 LLVM_DEBUG(dbgs() << " old top pred: " << getBlockName(Pred) << ", has " 2044 << Pred->succ_size() << " successors, "; 2045 MBFI->printBlockFreq(dbgs(), Pred) << " freq\n"); 2046 if (Pred->succ_size() > 2) 2047 continue; 2048 2049 MachineBasicBlock *OtherBB = nullptr; 2050 if (Pred->succ_size() == 2) { 2051 OtherBB = *Pred->succ_begin(); 2052 if (OtherBB == OldTop) 2053 OtherBB = *Pred->succ_rbegin(); 2054 } 2055 2056 if (!canMoveBottomBlockToTop(Pred, OldTop)) 2057 continue; 2058 2059 BlockFrequency Gains = FallThroughGains(Pred, OldTop, OtherBB, 2060 LoopBlockSet); 2061 if ((Gains > 0) && (Gains > BestGains || 2062 ((Gains == BestGains) && Pred->isLayoutSuccessor(OldTop)))) { 2063 BestPred = Pred; 2064 BestGains = Gains; 2065 } 2066 } 2067 2068 // If no direct predecessor is fine, just use the loop header. 2069 if (!BestPred) { 2070 LLVM_DEBUG(dbgs() << " final top unchanged\n"); 2071 return OldTop; 2072 } 2073 2074 // Walk backwards through any straight line of predecessors. 2075 while (BestPred->pred_size() == 1 && 2076 (*BestPred->pred_begin())->succ_size() == 1 && 2077 *BestPred->pred_begin() != L.getHeader()) 2078 BestPred = *BestPred->pred_begin(); 2079 2080 LLVM_DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n"); 2081 return BestPred; 2082 } 2083 2084 /// Find the best loop top block for layout. 2085 /// 2086 /// This function iteratively calls findBestLoopTopHelper, until no new better 2087 /// BB can be found. 2088 MachineBasicBlock * 2089 MachineBlockPlacement::findBestLoopTop(const MachineLoop &L, 2090 const BlockFilterSet &LoopBlockSet) { 2091 // Placing the latch block before the header may introduce an extra branch 2092 // that skips this block the first time the loop is executed, which we want 2093 // to avoid when optimising for size. 2094 // FIXME: in theory there is a case that does not introduce a new branch, 2095 // i.e. when the layout predecessor does not fallthrough to the loop header. 2096 // In practice this never happens though: there always seems to be a preheader 2097 // that can fallthrough and that is also placed before the header. 2098 bool OptForSize = F->getFunction().hasOptSize() || 2099 llvm::shouldOptimizeForSize(L.getHeader(), PSI, MBFI.get()); 2100 if (OptForSize) 2101 return L.getHeader(); 2102 2103 MachineBasicBlock *OldTop = nullptr; 2104 MachineBasicBlock *NewTop = L.getHeader(); 2105 while (NewTop != OldTop) { 2106 OldTop = NewTop; 2107 NewTop = findBestLoopTopHelper(OldTop, L, LoopBlockSet); 2108 if (NewTop != OldTop) 2109 ComputedEdges[NewTop] = { OldTop, false }; 2110 } 2111 return NewTop; 2112 } 2113 2114 /// Find the best loop exiting block for layout. 2115 /// 2116 /// This routine implements the logic to analyze the loop looking for the best 2117 /// block to layout at the top of the loop. Typically this is done to maximize 2118 /// fallthrough opportunities. 2119 MachineBasicBlock * 2120 MachineBlockPlacement::findBestLoopExit(const MachineLoop &L, 2121 const BlockFilterSet &LoopBlockSet, 2122 BlockFrequency &ExitFreq) { 2123 // We don't want to layout the loop linearly in all cases. If the loop header 2124 // is just a normal basic block in the loop, we want to look for what block 2125 // within the loop is the best one to layout at the top. However, if the loop 2126 // header has be pre-merged into a chain due to predecessors not having 2127 // analyzable branches, *and* the predecessor it is merged with is *not* part 2128 // of the loop, rotating the header into the middle of the loop will create 2129 // a non-contiguous range of blocks which is Very Bad. So start with the 2130 // header and only rotate if safe. 2131 BlockChain &HeaderChain = *BlockToChain[L.getHeader()]; 2132 if (!LoopBlockSet.count(*HeaderChain.begin())) 2133 return nullptr; 2134 2135 BlockFrequency BestExitEdgeFreq; 2136 unsigned BestExitLoopDepth = 0; 2137 MachineBasicBlock *ExitingBB = nullptr; 2138 // If there are exits to outer loops, loop rotation can severely limit 2139 // fallthrough opportunities unless it selects such an exit. Keep a set of 2140 // blocks where rotating to exit with that block will reach an outer loop. 2141 SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop; 2142 2143 LLVM_DEBUG(dbgs() << "Finding best loop exit for: " 2144 << getBlockName(L.getHeader()) << "\n"); 2145 for (MachineBasicBlock *MBB : L.getBlocks()) { 2146 BlockChain &Chain = *BlockToChain[MBB]; 2147 // Ensure that this block is at the end of a chain; otherwise it could be 2148 // mid-way through an inner loop or a successor of an unanalyzable branch. 2149 if (MBB != *std::prev(Chain.end())) 2150 continue; 2151 2152 // Now walk the successors. We need to establish whether this has a viable 2153 // exiting successor and whether it has a viable non-exiting successor. 2154 // We store the old exiting state and restore it if a viable looping 2155 // successor isn't found. 2156 MachineBasicBlock *OldExitingBB = ExitingBB; 2157 BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq; 2158 bool HasLoopingSucc = false; 2159 for (MachineBasicBlock *Succ : MBB->successors()) { 2160 if (Succ->isEHPad()) 2161 continue; 2162 if (Succ == MBB) 2163 continue; 2164 BlockChain &SuccChain = *BlockToChain[Succ]; 2165 // Don't split chains, either this chain or the successor's chain. 2166 if (&Chain == &SuccChain) { 2167 LLVM_DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> " 2168 << getBlockName(Succ) << " (chain conflict)\n"); 2169 continue; 2170 } 2171 2172 auto SuccProb = MBPI->getEdgeProbability(MBB, Succ); 2173 if (LoopBlockSet.count(Succ)) { 2174 LLVM_DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> " 2175 << getBlockName(Succ) << " (" << SuccProb << ")\n"); 2176 HasLoopingSucc = true; 2177 continue; 2178 } 2179 2180 unsigned SuccLoopDepth = 0; 2181 if (MachineLoop *ExitLoop = MLI->getLoopFor(Succ)) { 2182 SuccLoopDepth = ExitLoop->getLoopDepth(); 2183 if (ExitLoop->contains(&L)) 2184 BlocksExitingToOuterLoop.insert(MBB); 2185 } 2186 2187 BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(MBB) * SuccProb; 2188 LLVM_DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> " 2189 << getBlockName(Succ) << " [L:" << SuccLoopDepth 2190 << "] ("; 2191 MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n"); 2192 // Note that we bias this toward an existing layout successor to retain 2193 // incoming order in the absence of better information. The exit must have 2194 // a frequency higher than the current exit before we consider breaking 2195 // the layout. 2196 BranchProbability Bias(100 - ExitBlockBias, 100); 2197 if (!ExitingBB || SuccLoopDepth > BestExitLoopDepth || 2198 ExitEdgeFreq > BestExitEdgeFreq || 2199 (MBB->isLayoutSuccessor(Succ) && 2200 !(ExitEdgeFreq < BestExitEdgeFreq * Bias))) { 2201 BestExitEdgeFreq = ExitEdgeFreq; 2202 ExitingBB = MBB; 2203 } 2204 } 2205 2206 if (!HasLoopingSucc) { 2207 // Restore the old exiting state, no viable looping successor was found. 2208 ExitingBB = OldExitingBB; 2209 BestExitEdgeFreq = OldBestExitEdgeFreq; 2210 } 2211 } 2212 // Without a candidate exiting block or with only a single block in the 2213 // loop, just use the loop header to layout the loop. 2214 if (!ExitingBB) { 2215 LLVM_DEBUG( 2216 dbgs() << " No other candidate exit blocks, using loop header\n"); 2217 return nullptr; 2218 } 2219 if (L.getNumBlocks() == 1) { 2220 LLVM_DEBUG(dbgs() << " Loop has 1 block, using loop header as exit\n"); 2221 return nullptr; 2222 } 2223 2224 // Also, if we have exit blocks which lead to outer loops but didn't select 2225 // one of them as the exiting block we are rotating toward, disable loop 2226 // rotation altogether. 2227 if (!BlocksExitingToOuterLoop.empty() && 2228 !BlocksExitingToOuterLoop.count(ExitingBB)) 2229 return nullptr; 2230 2231 LLVM_DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) 2232 << "\n"); 2233 ExitFreq = BestExitEdgeFreq; 2234 return ExitingBB; 2235 } 2236 2237 /// Check if there is a fallthrough to loop header Top. 2238 /// 2239 /// 1. Look for a Pred that can be layout before Top. 2240 /// 2. Check if Top is the most possible successor of Pred. 2241 bool 2242 MachineBlockPlacement::hasViableTopFallthrough( 2243 const MachineBasicBlock *Top, 2244 const BlockFilterSet &LoopBlockSet) { 2245 for (MachineBasicBlock *Pred : Top->predecessors()) { 2246 BlockChain *PredChain = BlockToChain[Pred]; 2247 if (!LoopBlockSet.count(Pred) && 2248 (!PredChain || Pred == *std::prev(PredChain->end()))) { 2249 // Found a Pred block can be placed before Top. 2250 // Check if Top is the best successor of Pred. 2251 auto TopProb = MBPI->getEdgeProbability(Pred, Top); 2252 bool TopOK = true; 2253 for (MachineBasicBlock *Succ : Pred->successors()) { 2254 auto SuccProb = MBPI->getEdgeProbability(Pred, Succ); 2255 BlockChain *SuccChain = BlockToChain[Succ]; 2256 // Check if Succ can be placed after Pred. 2257 // Succ should not be in any chain, or it is the head of some chain. 2258 if ((!SuccChain || Succ == *SuccChain->begin()) && SuccProb > TopProb) { 2259 TopOK = false; 2260 break; 2261 } 2262 } 2263 if (TopOK) 2264 return true; 2265 } 2266 } 2267 return false; 2268 } 2269 2270 /// Attempt to rotate an exiting block to the bottom of the loop. 2271 /// 2272 /// Once we have built a chain, try to rotate it to line up the hot exit block 2273 /// with fallthrough out of the loop if doing so doesn't introduce unnecessary 2274 /// branches. For example, if the loop has fallthrough into its header and out 2275 /// of its bottom already, don't rotate it. 2276 void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain, 2277 const MachineBasicBlock *ExitingBB, 2278 BlockFrequency ExitFreq, 2279 const BlockFilterSet &LoopBlockSet) { 2280 if (!ExitingBB) 2281 return; 2282 2283 MachineBasicBlock *Top = *LoopChain.begin(); 2284 MachineBasicBlock *Bottom = *std::prev(LoopChain.end()); 2285 2286 // If ExitingBB is already the last one in a chain then nothing to do. 2287 if (Bottom == ExitingBB) 2288 return; 2289 2290 bool ViableTopFallthrough = hasViableTopFallthrough(Top, LoopBlockSet); 2291 2292 // If the header has viable fallthrough, check whether the current loop 2293 // bottom is a viable exiting block. If so, bail out as rotating will 2294 // introduce an unnecessary branch. 2295 if (ViableTopFallthrough) { 2296 for (MachineBasicBlock *Succ : Bottom->successors()) { 2297 BlockChain *SuccChain = BlockToChain[Succ]; 2298 if (!LoopBlockSet.count(Succ) && 2299 (!SuccChain || Succ == *SuccChain->begin())) 2300 return; 2301 } 2302 2303 // Rotate will destroy the top fallthrough, we need to ensure the new exit 2304 // frequency is larger than top fallthrough. 2305 BlockFrequency FallThrough2Top = TopFallThroughFreq(Top, LoopBlockSet); 2306 if (FallThrough2Top >= ExitFreq) 2307 return; 2308 } 2309 2310 BlockChain::iterator ExitIt = llvm::find(LoopChain, ExitingBB); 2311 if (ExitIt == LoopChain.end()) 2312 return; 2313 2314 // Rotating a loop exit to the bottom when there is a fallthrough to top 2315 // trades the entry fallthrough for an exit fallthrough. 2316 // If there is no bottom->top edge, but the chosen exit block does have 2317 // a fallthrough, we break that fallthrough for nothing in return. 2318 2319 // Let's consider an example. We have a built chain of basic blocks 2320 // B1, B2, ..., Bn, where Bk is a ExitingBB - chosen exit block. 2321 // By doing a rotation we get 2322 // Bk+1, ..., Bn, B1, ..., Bk 2323 // Break of fallthrough to B1 is compensated by a fallthrough from Bk. 2324 // If we had a fallthrough Bk -> Bk+1 it is broken now. 2325 // It might be compensated by fallthrough Bn -> B1. 2326 // So we have a condition to avoid creation of extra branch by loop rotation. 2327 // All below must be true to avoid loop rotation: 2328 // If there is a fallthrough to top (B1) 2329 // There was fallthrough from chosen exit block (Bk) to next one (Bk+1) 2330 // There is no fallthrough from bottom (Bn) to top (B1). 2331 // Please note that there is no exit fallthrough from Bn because we checked it 2332 // above. 2333 if (ViableTopFallthrough) { 2334 assert(std::next(ExitIt) != LoopChain.end() && 2335 "Exit should not be last BB"); 2336 MachineBasicBlock *NextBlockInChain = *std::next(ExitIt); 2337 if (ExitingBB->isSuccessor(NextBlockInChain)) 2338 if (!Bottom->isSuccessor(Top)) 2339 return; 2340 } 2341 2342 LLVM_DEBUG(dbgs() << "Rotating loop to put exit " << getBlockName(ExitingBB) 2343 << " at bottom\n"); 2344 std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end()); 2345 } 2346 2347 /// Attempt to rotate a loop based on profile data to reduce branch cost. 2348 /// 2349 /// With profile data, we can determine the cost in terms of missed fall through 2350 /// opportunities when rotating a loop chain and select the best rotation. 2351 /// Basically, there are three kinds of cost to consider for each rotation: 2352 /// 1. The possibly missed fall through edge (if it exists) from BB out of 2353 /// the loop to the loop header. 2354 /// 2. The possibly missed fall through edges (if they exist) from the loop 2355 /// exits to BB out of the loop. 2356 /// 3. The missed fall through edge (if it exists) from the last BB to the 2357 /// first BB in the loop chain. 2358 /// Therefore, the cost for a given rotation is the sum of costs listed above. 2359 /// We select the best rotation with the smallest cost. 2360 void MachineBlockPlacement::rotateLoopWithProfile( 2361 BlockChain &LoopChain, const MachineLoop &L, 2362 const BlockFilterSet &LoopBlockSet) { 2363 auto RotationPos = LoopChain.end(); 2364 2365 BlockFrequency SmallestRotationCost = BlockFrequency::getMaxFrequency(); 2366 2367 // A utility lambda that scales up a block frequency by dividing it by a 2368 // branch probability which is the reciprocal of the scale. 2369 auto ScaleBlockFrequency = [](BlockFrequency Freq, 2370 unsigned Scale) -> BlockFrequency { 2371 if (Scale == 0) 2372 return 0; 2373 // Use operator / between BlockFrequency and BranchProbability to implement 2374 // saturating multiplication. 2375 return Freq / BranchProbability(1, Scale); 2376 }; 2377 2378 // Compute the cost of the missed fall-through edge to the loop header if the 2379 // chain head is not the loop header. As we only consider natural loops with 2380 // single header, this computation can be done only once. 2381 BlockFrequency HeaderFallThroughCost(0); 2382 MachineBasicBlock *ChainHeaderBB = *LoopChain.begin(); 2383 for (auto *Pred : ChainHeaderBB->predecessors()) { 2384 BlockChain *PredChain = BlockToChain[Pred]; 2385 if (!LoopBlockSet.count(Pred) && 2386 (!PredChain || Pred == *std::prev(PredChain->end()))) { 2387 auto EdgeFreq = MBFI->getBlockFreq(Pred) * 2388 MBPI->getEdgeProbability(Pred, ChainHeaderBB); 2389 auto FallThruCost = ScaleBlockFrequency(EdgeFreq, MisfetchCost); 2390 // If the predecessor has only an unconditional jump to the header, we 2391 // need to consider the cost of this jump. 2392 if (Pred->succ_size() == 1) 2393 FallThruCost += ScaleBlockFrequency(EdgeFreq, JumpInstCost); 2394 HeaderFallThroughCost = std::max(HeaderFallThroughCost, FallThruCost); 2395 } 2396 } 2397 2398 // Here we collect all exit blocks in the loop, and for each exit we find out 2399 // its hottest exit edge. For each loop rotation, we define the loop exit cost 2400 // as the sum of frequencies of exit edges we collect here, excluding the exit 2401 // edge from the tail of the loop chain. 2402 SmallVector<std::pair<MachineBasicBlock *, BlockFrequency>, 4> ExitsWithFreq; 2403 for (auto BB : LoopChain) { 2404 auto LargestExitEdgeProb = BranchProbability::getZero(); 2405 for (auto *Succ : BB->successors()) { 2406 BlockChain *SuccChain = BlockToChain[Succ]; 2407 if (!LoopBlockSet.count(Succ) && 2408 (!SuccChain || Succ == *SuccChain->begin())) { 2409 auto SuccProb = MBPI->getEdgeProbability(BB, Succ); 2410 LargestExitEdgeProb = std::max(LargestExitEdgeProb, SuccProb); 2411 } 2412 } 2413 if (LargestExitEdgeProb > BranchProbability::getZero()) { 2414 auto ExitFreq = MBFI->getBlockFreq(BB) * LargestExitEdgeProb; 2415 ExitsWithFreq.emplace_back(BB, ExitFreq); 2416 } 2417 } 2418 2419 // In this loop we iterate every block in the loop chain and calculate the 2420 // cost assuming the block is the head of the loop chain. When the loop ends, 2421 // we should have found the best candidate as the loop chain's head. 2422 for (auto Iter = LoopChain.begin(), TailIter = std::prev(LoopChain.end()), 2423 EndIter = LoopChain.end(); 2424 Iter != EndIter; Iter++, TailIter++) { 2425 // TailIter is used to track the tail of the loop chain if the block we are 2426 // checking (pointed by Iter) is the head of the chain. 2427 if (TailIter == LoopChain.end()) 2428 TailIter = LoopChain.begin(); 2429 2430 auto TailBB = *TailIter; 2431 2432 // Calculate the cost by putting this BB to the top. 2433 BlockFrequency Cost = 0; 2434 2435 // If the current BB is the loop header, we need to take into account the 2436 // cost of the missed fall through edge from outside of the loop to the 2437 // header. 2438 if (Iter != LoopChain.begin()) 2439 Cost += HeaderFallThroughCost; 2440 2441 // Collect the loop exit cost by summing up frequencies of all exit edges 2442 // except the one from the chain tail. 2443 for (auto &ExitWithFreq : ExitsWithFreq) 2444 if (TailBB != ExitWithFreq.first) 2445 Cost += ExitWithFreq.second; 2446 2447 // The cost of breaking the once fall-through edge from the tail to the top 2448 // of the loop chain. Here we need to consider three cases: 2449 // 1. If the tail node has only one successor, then we will get an 2450 // additional jmp instruction. So the cost here is (MisfetchCost + 2451 // JumpInstCost) * tail node frequency. 2452 // 2. If the tail node has two successors, then we may still get an 2453 // additional jmp instruction if the layout successor after the loop 2454 // chain is not its CFG successor. Note that the more frequently executed 2455 // jmp instruction will be put ahead of the other one. Assume the 2456 // frequency of those two branches are x and y, where x is the frequency 2457 // of the edge to the chain head, then the cost will be 2458 // (x * MisfetechCost + min(x, y) * JumpInstCost) * tail node frequency. 2459 // 3. If the tail node has more than two successors (this rarely happens), 2460 // we won't consider any additional cost. 2461 if (TailBB->isSuccessor(*Iter)) { 2462 auto TailBBFreq = MBFI->getBlockFreq(TailBB); 2463 if (TailBB->succ_size() == 1) 2464 Cost += ScaleBlockFrequency(TailBBFreq.getFrequency(), 2465 MisfetchCost + JumpInstCost); 2466 else if (TailBB->succ_size() == 2) { 2467 auto TailToHeadProb = MBPI->getEdgeProbability(TailBB, *Iter); 2468 auto TailToHeadFreq = TailBBFreq * TailToHeadProb; 2469 auto ColderEdgeFreq = TailToHeadProb > BranchProbability(1, 2) 2470 ? TailBBFreq * TailToHeadProb.getCompl() 2471 : TailToHeadFreq; 2472 Cost += ScaleBlockFrequency(TailToHeadFreq, MisfetchCost) + 2473 ScaleBlockFrequency(ColderEdgeFreq, JumpInstCost); 2474 } 2475 } 2476 2477 LLVM_DEBUG(dbgs() << "The cost of loop rotation by making " 2478 << getBlockName(*Iter) 2479 << " to the top: " << Cost.getFrequency() << "\n"); 2480 2481 if (Cost < SmallestRotationCost) { 2482 SmallestRotationCost = Cost; 2483 RotationPos = Iter; 2484 } 2485 } 2486 2487 if (RotationPos != LoopChain.end()) { 2488 LLVM_DEBUG(dbgs() << "Rotate loop by making " << getBlockName(*RotationPos) 2489 << " to the top\n"); 2490 std::rotate(LoopChain.begin(), RotationPos, LoopChain.end()); 2491 } 2492 } 2493 2494 /// Collect blocks in the given loop that are to be placed. 2495 /// 2496 /// When profile data is available, exclude cold blocks from the returned set; 2497 /// otherwise, collect all blocks in the loop. 2498 MachineBlockPlacement::BlockFilterSet 2499 MachineBlockPlacement::collectLoopBlockSet(const MachineLoop &L) { 2500 BlockFilterSet LoopBlockSet; 2501 2502 // Filter cold blocks off from LoopBlockSet when profile data is available. 2503 // Collect the sum of frequencies of incoming edges to the loop header from 2504 // outside. If we treat the loop as a super block, this is the frequency of 2505 // the loop. Then for each block in the loop, we calculate the ratio between 2506 // its frequency and the frequency of the loop block. When it is too small, 2507 // don't add it to the loop chain. If there are outer loops, then this block 2508 // will be merged into the first outer loop chain for which this block is not 2509 // cold anymore. This needs precise profile data and we only do this when 2510 // profile data is available. 2511 if (F->getFunction().hasProfileData() || ForceLoopColdBlock) { 2512 BlockFrequency LoopFreq(0); 2513 for (auto LoopPred : L.getHeader()->predecessors()) 2514 if (!L.contains(LoopPred)) 2515 LoopFreq += MBFI->getBlockFreq(LoopPred) * 2516 MBPI->getEdgeProbability(LoopPred, L.getHeader()); 2517 2518 for (MachineBasicBlock *LoopBB : L.getBlocks()) { 2519 auto Freq = MBFI->getBlockFreq(LoopBB).getFrequency(); 2520 if (Freq == 0 || LoopFreq.getFrequency() / Freq > LoopToColdBlockRatio) 2521 continue; 2522 LoopBlockSet.insert(LoopBB); 2523 } 2524 } else 2525 LoopBlockSet.insert(L.block_begin(), L.block_end()); 2526 2527 return LoopBlockSet; 2528 } 2529 2530 /// Forms basic block chains from the natural loop structures. 2531 /// 2532 /// These chains are designed to preserve the existing *structure* of the code 2533 /// as much as possible. We can then stitch the chains together in a way which 2534 /// both preserves the topological structure and minimizes taken conditional 2535 /// branches. 2536 void MachineBlockPlacement::buildLoopChains(const MachineLoop &L) { 2537 // First recurse through any nested loops, building chains for those inner 2538 // loops. 2539 for (const MachineLoop *InnerLoop : L) 2540 buildLoopChains(*InnerLoop); 2541 2542 assert(BlockWorkList.empty() && 2543 "BlockWorkList not empty when starting to build loop chains."); 2544 assert(EHPadWorkList.empty() && 2545 "EHPadWorkList not empty when starting to build loop chains."); 2546 BlockFilterSet LoopBlockSet = collectLoopBlockSet(L); 2547 2548 // Check if we have profile data for this function. If yes, we will rotate 2549 // this loop by modeling costs more precisely which requires the profile data 2550 // for better layout. 2551 bool RotateLoopWithProfile = 2552 ForcePreciseRotationCost || 2553 (PreciseRotationCost && F->getFunction().hasProfileData()); 2554 2555 // First check to see if there is an obviously preferable top block for the 2556 // loop. This will default to the header, but may end up as one of the 2557 // predecessors to the header if there is one which will result in strictly 2558 // fewer branches in the loop body. 2559 MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet); 2560 2561 // If we selected just the header for the loop top, look for a potentially 2562 // profitable exit block in the event that rotating the loop can eliminate 2563 // branches by placing an exit edge at the bottom. 2564 // 2565 // Loops are processed innermost to uttermost, make sure we clear 2566 // PreferredLoopExit before processing a new loop. 2567 PreferredLoopExit = nullptr; 2568 BlockFrequency ExitFreq; 2569 if (!RotateLoopWithProfile && LoopTop == L.getHeader()) 2570 PreferredLoopExit = findBestLoopExit(L, LoopBlockSet, ExitFreq); 2571 2572 BlockChain &LoopChain = *BlockToChain[LoopTop]; 2573 2574 // FIXME: This is a really lame way of walking the chains in the loop: we 2575 // walk the blocks, and use a set to prevent visiting a particular chain 2576 // twice. 2577 SmallPtrSet<BlockChain *, 4> UpdatedPreds; 2578 assert(LoopChain.UnscheduledPredecessors == 0 && 2579 "LoopChain should not have unscheduled predecessors."); 2580 UpdatedPreds.insert(&LoopChain); 2581 2582 for (const MachineBasicBlock *LoopBB : LoopBlockSet) 2583 fillWorkLists(LoopBB, UpdatedPreds, &LoopBlockSet); 2584 2585 buildChain(LoopTop, LoopChain, &LoopBlockSet); 2586 2587 if (RotateLoopWithProfile) 2588 rotateLoopWithProfile(LoopChain, L, LoopBlockSet); 2589 else 2590 rotateLoop(LoopChain, PreferredLoopExit, ExitFreq, LoopBlockSet); 2591 2592 LLVM_DEBUG({ 2593 // Crash at the end so we get all of the debugging output first. 2594 bool BadLoop = false; 2595 if (LoopChain.UnscheduledPredecessors) { 2596 BadLoop = true; 2597 dbgs() << "Loop chain contains a block without its preds placed!\n" 2598 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 2599 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"; 2600 } 2601 for (MachineBasicBlock *ChainBB : LoopChain) { 2602 dbgs() << " ... " << getBlockName(ChainBB) << "\n"; 2603 if (!LoopBlockSet.remove(ChainBB)) { 2604 // We don't mark the loop as bad here because there are real situations 2605 // where this can occur. For example, with an unanalyzable fallthrough 2606 // from a loop block to a non-loop block or vice versa. 2607 dbgs() << "Loop chain contains a block not contained by the loop!\n" 2608 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 2609 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n" 2610 << " Bad block: " << getBlockName(ChainBB) << "\n"; 2611 } 2612 } 2613 2614 if (!LoopBlockSet.empty()) { 2615 BadLoop = true; 2616 for (const MachineBasicBlock *LoopBB : LoopBlockSet) 2617 dbgs() << "Loop contains blocks never placed into a chain!\n" 2618 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 2619 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n" 2620 << " Bad block: " << getBlockName(LoopBB) << "\n"; 2621 } 2622 assert(!BadLoop && "Detected problems with the placement of this loop."); 2623 }); 2624 2625 BlockWorkList.clear(); 2626 EHPadWorkList.clear(); 2627 } 2628 2629 void MachineBlockPlacement::buildCFGChains() { 2630 // Ensure that every BB in the function has an associated chain to simplify 2631 // the assumptions of the remaining algorithm. 2632 SmallVector<MachineOperand, 4> Cond; // For analyzeBranch. 2633 for (MachineFunction::iterator FI = F->begin(), FE = F->end(); FI != FE; 2634 ++FI) { 2635 MachineBasicBlock *BB = &*FI; 2636 BlockChain *Chain = 2637 new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB); 2638 // Also, merge any blocks which we cannot reason about and must preserve 2639 // the exact fallthrough behavior for. 2640 while (true) { 2641 Cond.clear(); 2642 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch. 2643 if (!TII->analyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough()) 2644 break; 2645 2646 MachineFunction::iterator NextFI = std::next(FI); 2647 MachineBasicBlock *NextBB = &*NextFI; 2648 // Ensure that the layout successor is a viable block, as we know that 2649 // fallthrough is a possibility. 2650 assert(NextFI != FE && "Can't fallthrough past the last block."); 2651 LLVM_DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: " 2652 << getBlockName(BB) << " -> " << getBlockName(NextBB) 2653 << "\n"); 2654 Chain->merge(NextBB, nullptr); 2655 #ifndef NDEBUG 2656 BlocksWithUnanalyzableExits.insert(&*BB); 2657 #endif 2658 FI = NextFI; 2659 BB = NextBB; 2660 } 2661 } 2662 2663 // Build any loop-based chains. 2664 PreferredLoopExit = nullptr; 2665 for (MachineLoop *L : *MLI) 2666 buildLoopChains(*L); 2667 2668 assert(BlockWorkList.empty() && 2669 "BlockWorkList should be empty before building final chain."); 2670 assert(EHPadWorkList.empty() && 2671 "EHPadWorkList should be empty before building final chain."); 2672 2673 SmallPtrSet<BlockChain *, 4> UpdatedPreds; 2674 for (MachineBasicBlock &MBB : *F) 2675 fillWorkLists(&MBB, UpdatedPreds); 2676 2677 BlockChain &FunctionChain = *BlockToChain[&F->front()]; 2678 buildChain(&F->front(), FunctionChain); 2679 2680 #ifndef NDEBUG 2681 using FunctionBlockSetType = SmallPtrSet<MachineBasicBlock *, 16>; 2682 #endif 2683 LLVM_DEBUG({ 2684 // Crash at the end so we get all of the debugging output first. 2685 bool BadFunc = false; 2686 FunctionBlockSetType FunctionBlockSet; 2687 for (MachineBasicBlock &MBB : *F) 2688 FunctionBlockSet.insert(&MBB); 2689 2690 for (MachineBasicBlock *ChainBB : FunctionChain) 2691 if (!FunctionBlockSet.erase(ChainBB)) { 2692 BadFunc = true; 2693 dbgs() << "Function chain contains a block not in the function!\n" 2694 << " Bad block: " << getBlockName(ChainBB) << "\n"; 2695 } 2696 2697 if (!FunctionBlockSet.empty()) { 2698 BadFunc = true; 2699 for (MachineBasicBlock *RemainingBB : FunctionBlockSet) 2700 dbgs() << "Function contains blocks never placed into a chain!\n" 2701 << " Bad block: " << getBlockName(RemainingBB) << "\n"; 2702 } 2703 assert(!BadFunc && "Detected problems with the block placement."); 2704 }); 2705 2706 // Remember original layout ordering, so we can update terminators after 2707 // reordering to point to the original layout successor. 2708 SmallVector<MachineBasicBlock *, 4> OriginalLayoutSuccessors( 2709 F->getNumBlockIDs()); 2710 { 2711 MachineBasicBlock *LastMBB = nullptr; 2712 for (auto &MBB : *F) { 2713 if (LastMBB != nullptr) 2714 OriginalLayoutSuccessors[LastMBB->getNumber()] = &MBB; 2715 LastMBB = &MBB; 2716 } 2717 OriginalLayoutSuccessors[F->back().getNumber()] = nullptr; 2718 } 2719 2720 // Splice the blocks into place. 2721 MachineFunction::iterator InsertPos = F->begin(); 2722 LLVM_DEBUG(dbgs() << "[MBP] Function: " << F->getName() << "\n"); 2723 for (MachineBasicBlock *ChainBB : FunctionChain) { 2724 LLVM_DEBUG(dbgs() << (ChainBB == *FunctionChain.begin() ? "Placing chain " 2725 : " ... ") 2726 << getBlockName(ChainBB) << "\n"); 2727 if (InsertPos != MachineFunction::iterator(ChainBB)) 2728 F->splice(InsertPos, ChainBB); 2729 else 2730 ++InsertPos; 2731 2732 // Update the terminator of the previous block. 2733 if (ChainBB == *FunctionChain.begin()) 2734 continue; 2735 MachineBasicBlock *PrevBB = &*std::prev(MachineFunction::iterator(ChainBB)); 2736 2737 // FIXME: It would be awesome of updateTerminator would just return rather 2738 // than assert when the branch cannot be analyzed in order to remove this 2739 // boiler plate. 2740 Cond.clear(); 2741 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch. 2742 2743 #ifndef NDEBUG 2744 if (!BlocksWithUnanalyzableExits.count(PrevBB)) { 2745 // Given the exact block placement we chose, we may actually not _need_ to 2746 // be able to edit PrevBB's terminator sequence, but not being _able_ to 2747 // do that at this point is a bug. 2748 assert((!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) || 2749 !PrevBB->canFallThrough()) && 2750 "Unexpected block with un-analyzable fallthrough!"); 2751 Cond.clear(); 2752 TBB = FBB = nullptr; 2753 } 2754 #endif 2755 2756 // The "PrevBB" is not yet updated to reflect current code layout, so, 2757 // o. it may fall-through to a block without explicit "goto" instruction 2758 // before layout, and no longer fall-through it after layout; or 2759 // o. just opposite. 2760 // 2761 // analyzeBranch() may return erroneous value for FBB when these two 2762 // situations take place. For the first scenario FBB is mistakenly set NULL; 2763 // for the 2nd scenario, the FBB, which is expected to be NULL, is 2764 // mistakenly pointing to "*BI". 2765 // Thus, if the future change needs to use FBB before the layout is set, it 2766 // has to correct FBB first by using the code similar to the following: 2767 // 2768 // if (!Cond.empty() && (!FBB || FBB == ChainBB)) { 2769 // PrevBB->updateTerminator(); 2770 // Cond.clear(); 2771 // TBB = FBB = nullptr; 2772 // if (TII->analyzeBranch(*PrevBB, TBB, FBB, Cond)) { 2773 // // FIXME: This should never take place. 2774 // TBB = FBB = nullptr; 2775 // } 2776 // } 2777 if (!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond)) { 2778 PrevBB->updateTerminator(OriginalLayoutSuccessors[PrevBB->getNumber()]); 2779 } 2780 } 2781 2782 // Fixup the last block. 2783 Cond.clear(); 2784 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch. 2785 if (!TII->analyzeBranch(F->back(), TBB, FBB, Cond)) { 2786 MachineBasicBlock *PrevBB = &F->back(); 2787 PrevBB->updateTerminator(OriginalLayoutSuccessors[PrevBB->getNumber()]); 2788 } 2789 2790 BlockWorkList.clear(); 2791 EHPadWorkList.clear(); 2792 } 2793 2794 void MachineBlockPlacement::optimizeBranches() { 2795 BlockChain &FunctionChain = *BlockToChain[&F->front()]; 2796 SmallVector<MachineOperand, 4> Cond; // For analyzeBranch. 2797 2798 // Now that all the basic blocks in the chain have the proper layout, 2799 // make a final call to analyzeBranch with AllowModify set. 2800 // Indeed, the target may be able to optimize the branches in a way we 2801 // cannot because all branches may not be analyzable. 2802 // E.g., the target may be able to remove an unconditional branch to 2803 // a fallthrough when it occurs after predicated terminators. 2804 for (MachineBasicBlock *ChainBB : FunctionChain) { 2805 Cond.clear(); 2806 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch. 2807 if (!TII->analyzeBranch(*ChainBB, TBB, FBB, Cond, /*AllowModify*/ true)) { 2808 // If PrevBB has a two-way branch, try to re-order the branches 2809 // such that we branch to the successor with higher probability first. 2810 if (TBB && !Cond.empty() && FBB && 2811 MBPI->getEdgeProbability(ChainBB, FBB) > 2812 MBPI->getEdgeProbability(ChainBB, TBB) && 2813 !TII->reverseBranchCondition(Cond)) { 2814 LLVM_DEBUG(dbgs() << "Reverse order of the two branches: " 2815 << getBlockName(ChainBB) << "\n"); 2816 LLVM_DEBUG(dbgs() << " Edge probability: " 2817 << MBPI->getEdgeProbability(ChainBB, FBB) << " vs " 2818 << MBPI->getEdgeProbability(ChainBB, TBB) << "\n"); 2819 DebugLoc dl; // FIXME: this is nowhere 2820 TII->removeBranch(*ChainBB); 2821 TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl); 2822 } 2823 } 2824 } 2825 } 2826 2827 void MachineBlockPlacement::alignBlocks() { 2828 // Walk through the backedges of the function now that we have fully laid out 2829 // the basic blocks and align the destination of each backedge. We don't rely 2830 // exclusively on the loop info here so that we can align backedges in 2831 // unnatural CFGs and backedges that were introduced purely because of the 2832 // loop rotations done during this layout pass. 2833 if (F->getFunction().hasMinSize() || 2834 (F->getFunction().hasOptSize() && !TLI->alignLoopsWithOptSize())) 2835 return; 2836 BlockChain &FunctionChain = *BlockToChain[&F->front()]; 2837 if (FunctionChain.begin() == FunctionChain.end()) 2838 return; // Empty chain. 2839 2840 const BranchProbability ColdProb(1, 5); // 20% 2841 BlockFrequency EntryFreq = MBFI->getBlockFreq(&F->front()); 2842 BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb; 2843 for (MachineBasicBlock *ChainBB : FunctionChain) { 2844 if (ChainBB == *FunctionChain.begin()) 2845 continue; 2846 2847 // Don't align non-looping basic blocks. These are unlikely to execute 2848 // enough times to matter in practice. Note that we'll still handle 2849 // unnatural CFGs inside of a natural outer loop (the common case) and 2850 // rotated loops. 2851 MachineLoop *L = MLI->getLoopFor(ChainBB); 2852 if (!L) 2853 continue; 2854 2855 const Align Align = TLI->getPrefLoopAlignment(L); 2856 if (Align == 1) 2857 continue; // Don't care about loop alignment. 2858 2859 // If the block is cold relative to the function entry don't waste space 2860 // aligning it. 2861 BlockFrequency Freq = MBFI->getBlockFreq(ChainBB); 2862 if (Freq < WeightedEntryFreq) 2863 continue; 2864 2865 // If the block is cold relative to its loop header, don't align it 2866 // regardless of what edges into the block exist. 2867 MachineBasicBlock *LoopHeader = L->getHeader(); 2868 BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader); 2869 if (Freq < (LoopHeaderFreq * ColdProb)) 2870 continue; 2871 2872 // If the global profiles indicates so, don't align it. 2873 if (llvm::shouldOptimizeForSize(ChainBB, PSI, MBFI.get()) && 2874 !TLI->alignLoopsWithOptSize()) 2875 continue; 2876 2877 // Check for the existence of a non-layout predecessor which would benefit 2878 // from aligning this block. 2879 MachineBasicBlock *LayoutPred = 2880 &*std::prev(MachineFunction::iterator(ChainBB)); 2881 2882 // Force alignment if all the predecessors are jumps. We already checked 2883 // that the block isn't cold above. 2884 if (!LayoutPred->isSuccessor(ChainBB)) { 2885 ChainBB->setAlignment(Align); 2886 continue; 2887 } 2888 2889 // Align this block if the layout predecessor's edge into this block is 2890 // cold relative to the block. When this is true, other predecessors make up 2891 // all of the hot entries into the block and thus alignment is likely to be 2892 // important. 2893 BranchProbability LayoutProb = 2894 MBPI->getEdgeProbability(LayoutPred, ChainBB); 2895 BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb; 2896 if (LayoutEdgeFreq <= (Freq * ColdProb)) 2897 ChainBB->setAlignment(Align); 2898 } 2899 } 2900 2901 /// Tail duplicate \p BB into (some) predecessors if profitable, repeating if 2902 /// it was duplicated into its chain predecessor and removed. 2903 /// \p BB - Basic block that may be duplicated. 2904 /// 2905 /// \p LPred - Chosen layout predecessor of \p BB. 2906 /// Updated to be the chain end if LPred is removed. 2907 /// \p Chain - Chain to which \p LPred belongs, and \p BB will belong. 2908 /// \p BlockFilter - Set of blocks that belong to the loop being laid out. 2909 /// Used to identify which blocks to update predecessor 2910 /// counts. 2911 /// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was 2912 /// chosen in the given order due to unnatural CFG 2913 /// only needed if \p BB is removed and 2914 /// \p PrevUnplacedBlockIt pointed to \p BB. 2915 /// @return true if \p BB was removed. 2916 bool MachineBlockPlacement::repeatedlyTailDuplicateBlock( 2917 MachineBasicBlock *BB, MachineBasicBlock *&LPred, 2918 const MachineBasicBlock *LoopHeaderBB, 2919 BlockChain &Chain, BlockFilterSet *BlockFilter, 2920 MachineFunction::iterator &PrevUnplacedBlockIt) { 2921 bool Removed, DuplicatedToLPred; 2922 bool DuplicatedToOriginalLPred; 2923 Removed = maybeTailDuplicateBlock(BB, LPred, Chain, BlockFilter, 2924 PrevUnplacedBlockIt, 2925 DuplicatedToLPred); 2926 if (!Removed) 2927 return false; 2928 DuplicatedToOriginalLPred = DuplicatedToLPred; 2929 // Iteratively try to duplicate again. It can happen that a block that is 2930 // duplicated into is still small enough to be duplicated again. 2931 // No need to call markBlockSuccessors in this case, as the blocks being 2932 // duplicated from here on are already scheduled. 2933 while (DuplicatedToLPred && Removed) { 2934 MachineBasicBlock *DupBB, *DupPred; 2935 // The removal callback causes Chain.end() to be updated when a block is 2936 // removed. On the first pass through the loop, the chain end should be the 2937 // same as it was on function entry. On subsequent passes, because we are 2938 // duplicating the block at the end of the chain, if it is removed the 2939 // chain will have shrunk by one block. 2940 BlockChain::iterator ChainEnd = Chain.end(); 2941 DupBB = *(--ChainEnd); 2942 // Now try to duplicate again. 2943 if (ChainEnd == Chain.begin()) 2944 break; 2945 DupPred = *std::prev(ChainEnd); 2946 Removed = maybeTailDuplicateBlock(DupBB, DupPred, Chain, BlockFilter, 2947 PrevUnplacedBlockIt, 2948 DuplicatedToLPred); 2949 } 2950 // If BB was duplicated into LPred, it is now scheduled. But because it was 2951 // removed, markChainSuccessors won't be called for its chain. Instead we 2952 // call markBlockSuccessors for LPred to achieve the same effect. This must go 2953 // at the end because repeating the tail duplication can increase the number 2954 // of unscheduled predecessors. 2955 LPred = *std::prev(Chain.end()); 2956 if (DuplicatedToOriginalLPred) 2957 markBlockSuccessors(Chain, LPred, LoopHeaderBB, BlockFilter); 2958 return true; 2959 } 2960 2961 /// Tail duplicate \p BB into (some) predecessors if profitable. 2962 /// \p BB - Basic block that may be duplicated 2963 /// \p LPred - Chosen layout predecessor of \p BB 2964 /// \p Chain - Chain to which \p LPred belongs, and \p BB will belong. 2965 /// \p BlockFilter - Set of blocks that belong to the loop being laid out. 2966 /// Used to identify which blocks to update predecessor 2967 /// counts. 2968 /// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was 2969 /// chosen in the given order due to unnatural CFG 2970 /// only needed if \p BB is removed and 2971 /// \p PrevUnplacedBlockIt pointed to \p BB. 2972 /// \p DuplicatedToLPred - True if the block was duplicated into LPred. 2973 /// \return - True if the block was duplicated into all preds and removed. 2974 bool MachineBlockPlacement::maybeTailDuplicateBlock( 2975 MachineBasicBlock *BB, MachineBasicBlock *LPred, 2976 BlockChain &Chain, BlockFilterSet *BlockFilter, 2977 MachineFunction::iterator &PrevUnplacedBlockIt, 2978 bool &DuplicatedToLPred) { 2979 DuplicatedToLPred = false; 2980 if (!shouldTailDuplicate(BB)) 2981 return false; 2982 2983 LLVM_DEBUG(dbgs() << "Redoing tail duplication for Succ#" << BB->getNumber() 2984 << "\n"); 2985 2986 // This has to be a callback because none of it can be done after 2987 // BB is deleted. 2988 bool Removed = false; 2989 auto RemovalCallback = 2990 [&](MachineBasicBlock *RemBB) { 2991 // Signal to outer function 2992 Removed = true; 2993 2994 // Conservative default. 2995 bool InWorkList = true; 2996 // Remove from the Chain and Chain Map 2997 if (BlockToChain.count(RemBB)) { 2998 BlockChain *Chain = BlockToChain[RemBB]; 2999 InWorkList = Chain->UnscheduledPredecessors == 0; 3000 Chain->remove(RemBB); 3001 BlockToChain.erase(RemBB); 3002 } 3003 3004 // Handle the unplaced block iterator 3005 if (&(*PrevUnplacedBlockIt) == RemBB) { 3006 PrevUnplacedBlockIt++; 3007 } 3008 3009 // Handle the Work Lists 3010 if (InWorkList) { 3011 SmallVectorImpl<MachineBasicBlock *> &RemoveList = BlockWorkList; 3012 if (RemBB->isEHPad()) 3013 RemoveList = EHPadWorkList; 3014 RemoveList.erase( 3015 llvm::remove_if(RemoveList, 3016 [RemBB](MachineBasicBlock *BB) { 3017 return BB == RemBB; 3018 }), 3019 RemoveList.end()); 3020 } 3021 3022 // Handle the filter set 3023 if (BlockFilter) { 3024 BlockFilter->remove(RemBB); 3025 } 3026 3027 // Remove the block from loop info. 3028 MLI->removeBlock(RemBB); 3029 if (RemBB == PreferredLoopExit) 3030 PreferredLoopExit = nullptr; 3031 3032 LLVM_DEBUG(dbgs() << "TailDuplicator deleted block: " 3033 << getBlockName(RemBB) << "\n"); 3034 }; 3035 auto RemovalCallbackRef = 3036 function_ref<void(MachineBasicBlock*)>(RemovalCallback); 3037 3038 SmallVector<MachineBasicBlock *, 8> DuplicatedPreds; 3039 bool IsSimple = TailDup.isSimpleBB(BB); 3040 SmallVector<MachineBasicBlock *, 8> CandidatePreds; 3041 SmallVectorImpl<MachineBasicBlock *> *CandidatePtr = nullptr; 3042 if (F->getFunction().hasProfileData()) { 3043 // We can do partial duplication with precise profile information. 3044 findDuplicateCandidates(CandidatePreds, BB, BlockFilter); 3045 if (CandidatePreds.size() == 0) 3046 return false; 3047 if (CandidatePreds.size() < BB->pred_size()) 3048 CandidatePtr = &CandidatePreds; 3049 } 3050 TailDup.tailDuplicateAndUpdate(IsSimple, BB, LPred, &DuplicatedPreds, 3051 &RemovalCallbackRef, CandidatePtr); 3052 3053 // Update UnscheduledPredecessors to reflect tail-duplication. 3054 DuplicatedToLPred = false; 3055 for (MachineBasicBlock *Pred : DuplicatedPreds) { 3056 // We're only looking for unscheduled predecessors that match the filter. 3057 BlockChain* PredChain = BlockToChain[Pred]; 3058 if (Pred == LPred) 3059 DuplicatedToLPred = true; 3060 if (Pred == LPred || (BlockFilter && !BlockFilter->count(Pred)) 3061 || PredChain == &Chain) 3062 continue; 3063 for (MachineBasicBlock *NewSucc : Pred->successors()) { 3064 if (BlockFilter && !BlockFilter->count(NewSucc)) 3065 continue; 3066 BlockChain *NewChain = BlockToChain[NewSucc]; 3067 if (NewChain != &Chain && NewChain != PredChain) 3068 NewChain->UnscheduledPredecessors++; 3069 } 3070 } 3071 return Removed; 3072 } 3073 3074 // Count the number of actual machine instructions. 3075 static uint64_t countMBBInstruction(MachineBasicBlock *MBB) { 3076 uint64_t InstrCount = 0; 3077 for (MachineInstr &MI : *MBB) { 3078 if (!MI.isPHI() && !MI.isMetaInstruction()) 3079 InstrCount += 1; 3080 } 3081 return InstrCount; 3082 } 3083 3084 // The size cost of duplication is the instruction size of the duplicated block. 3085 // So we should scale the threshold accordingly. But the instruction size is not 3086 // available on all targets, so we use the number of instructions instead. 3087 BlockFrequency MachineBlockPlacement::scaleThreshold(MachineBasicBlock *BB) { 3088 return DupThreshold.getFrequency() * countMBBInstruction(BB); 3089 } 3090 3091 // Returns true if BB is Pred's best successor. 3092 bool MachineBlockPlacement::isBestSuccessor(MachineBasicBlock *BB, 3093 MachineBasicBlock *Pred, 3094 BlockFilterSet *BlockFilter) { 3095 if (BB == Pred) 3096 return false; 3097 if (BlockFilter && !BlockFilter->count(Pred)) 3098 return false; 3099 BlockChain *PredChain = BlockToChain[Pred]; 3100 if (PredChain && (Pred != *std::prev(PredChain->end()))) 3101 return false; 3102 3103 // Find the successor with largest probability excluding BB. 3104 BranchProbability BestProb = BranchProbability::getZero(); 3105 for (MachineBasicBlock *Succ : Pred->successors()) 3106 if (Succ != BB) { 3107 if (BlockFilter && !BlockFilter->count(Succ)) 3108 continue; 3109 BlockChain *SuccChain = BlockToChain[Succ]; 3110 if (SuccChain && (Succ != *SuccChain->begin())) 3111 continue; 3112 BranchProbability SuccProb = MBPI->getEdgeProbability(Pred, Succ); 3113 if (SuccProb > BestProb) 3114 BestProb = SuccProb; 3115 } 3116 3117 BranchProbability BBProb = MBPI->getEdgeProbability(Pred, BB); 3118 if (BBProb <= BestProb) 3119 return false; 3120 3121 // Compute the number of reduced taken branches if Pred falls through to BB 3122 // instead of another successor. Then compare it with threshold. 3123 BlockFrequency PredFreq = MBFI->getBlockFreq(Pred); 3124 BlockFrequency Gain = PredFreq * (BBProb - BestProb); 3125 return Gain > scaleThreshold(BB); 3126 } 3127 3128 // Find out the predecessors of BB and BB can be beneficially duplicated into 3129 // them. 3130 void MachineBlockPlacement::findDuplicateCandidates( 3131 SmallVectorImpl<MachineBasicBlock *> &Candidates, 3132 MachineBasicBlock *BB, 3133 BlockFilterSet *BlockFilter) { 3134 MachineBasicBlock *Fallthrough = nullptr; 3135 BranchProbability DefaultBranchProb = BranchProbability::getZero(); 3136 BlockFrequency BBDupThreshold(scaleThreshold(BB)); 3137 SmallVector<MachineBasicBlock *, 8> Preds(BB->pred_begin(), BB->pred_end()); 3138 SmallVector<MachineBasicBlock *, 8> Succs(BB->succ_begin(), BB->succ_end()); 3139 3140 // Sort for highest frequency. 3141 auto CmpSucc = [&](MachineBasicBlock *A, MachineBasicBlock *B) { 3142 return MBPI->getEdgeProbability(BB, A) > MBPI->getEdgeProbability(BB, B); 3143 }; 3144 auto CmpPred = [&](MachineBasicBlock *A, MachineBasicBlock *B) { 3145 return MBFI->getBlockFreq(A) > MBFI->getBlockFreq(B); 3146 }; 3147 llvm::stable_sort(Succs, CmpSucc); 3148 llvm::stable_sort(Preds, CmpPred); 3149 3150 auto SuccIt = Succs.begin(); 3151 if (SuccIt != Succs.end()) { 3152 DefaultBranchProb = MBPI->getEdgeProbability(BB, *SuccIt).getCompl(); 3153 } 3154 3155 // For each predecessors of BB, compute the benefit of duplicating BB, 3156 // if it is larger than the threshold, add it into Candidates. 3157 // 3158 // If we have following control flow. 3159 // 3160 // PB1 PB2 PB3 PB4 3161 // \ | / /\ 3162 // \ | / / \ 3163 // \ |/ / \ 3164 // BB----/ OB 3165 // /\ 3166 // / \ 3167 // SB1 SB2 3168 // 3169 // And it can be partially duplicated as 3170 // 3171 // PB2+BB 3172 // | PB1 PB3 PB4 3173 // | | / /\ 3174 // | | / / \ 3175 // | |/ / \ 3176 // | BB----/ OB 3177 // |\ /| 3178 // | X | 3179 // |/ \| 3180 // SB2 SB1 3181 // 3182 // The benefit of duplicating into a predecessor is defined as 3183 // Orig_taken_branch - Duplicated_taken_branch 3184 // 3185 // The Orig_taken_branch is computed with the assumption that predecessor 3186 // jumps to BB and the most possible successor is laid out after BB. 3187 // 3188 // The Duplicated_taken_branch is computed with the assumption that BB is 3189 // duplicated into PB, and one successor is layout after it (SB1 for PB1 and 3190 // SB2 for PB2 in our case). If there is no available successor, the combined 3191 // block jumps to all BB's successor, like PB3 in this example. 3192 // 3193 // If a predecessor has multiple successors, so BB can't be duplicated into 3194 // it. But it can beneficially fall through to BB, and duplicate BB into other 3195 // predecessors. 3196 for (MachineBasicBlock *Pred : Preds) { 3197 BlockFrequency PredFreq = MBFI->getBlockFreq(Pred); 3198 3199 if (!TailDup.canTailDuplicate(BB, Pred)) { 3200 // BB can't be duplicated into Pred, but it is possible to be layout 3201 // below Pred. 3202 if (!Fallthrough && isBestSuccessor(BB, Pred, BlockFilter)) { 3203 Fallthrough = Pred; 3204 if (SuccIt != Succs.end()) 3205 SuccIt++; 3206 } 3207 continue; 3208 } 3209 3210 BlockFrequency OrigCost = PredFreq + PredFreq * DefaultBranchProb; 3211 BlockFrequency DupCost; 3212 if (SuccIt == Succs.end()) { 3213 // Jump to all successors; 3214 if (Succs.size() > 0) 3215 DupCost += PredFreq; 3216 } else { 3217 // Fallthrough to *SuccIt, jump to all other successors; 3218 DupCost += PredFreq; 3219 DupCost -= PredFreq * MBPI->getEdgeProbability(BB, *SuccIt); 3220 } 3221 3222 assert(OrigCost >= DupCost); 3223 OrigCost -= DupCost; 3224 if (OrigCost > BBDupThreshold) { 3225 Candidates.push_back(Pred); 3226 if (SuccIt != Succs.end()) 3227 SuccIt++; 3228 } 3229 } 3230 3231 // No predecessors can optimally fallthrough to BB. 3232 // So we can change one duplication into fallthrough. 3233 if (!Fallthrough) { 3234 if ((Candidates.size() < Preds.size()) && (Candidates.size() > 0)) { 3235 Candidates[0] = Candidates.back(); 3236 Candidates.pop_back(); 3237 } 3238 } 3239 } 3240 3241 void MachineBlockPlacement::initDupThreshold() { 3242 DupThreshold = 0; 3243 if (!F->getFunction().hasProfileData()) 3244 return; 3245 3246 BlockFrequency MaxFreq = 0; 3247 for (MachineBasicBlock &MBB : *F) { 3248 BlockFrequency Freq = MBFI->getBlockFreq(&MBB); 3249 if (Freq > MaxFreq) 3250 MaxFreq = Freq; 3251 } 3252 3253 // FIXME: we may use profile count instead of frequency, 3254 // and need more fine tuning. 3255 BranchProbability ThresholdProb(TailDupPlacementPenalty, 100); 3256 DupThreshold = MaxFreq * ThresholdProb; 3257 } 3258 3259 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) { 3260 if (skipFunction(MF.getFunction())) 3261 return false; 3262 3263 // Check for single-block functions and skip them. 3264 if (std::next(MF.begin()) == MF.end()) 3265 return false; 3266 3267 F = &MF; 3268 MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); 3269 MBFI = std::make_unique<MBFIWrapper>( 3270 getAnalysis<MachineBlockFrequencyInfo>()); 3271 MLI = &getAnalysis<MachineLoopInfo>(); 3272 TII = MF.getSubtarget().getInstrInfo(); 3273 TLI = MF.getSubtarget().getTargetLowering(); 3274 MPDT = nullptr; 3275 PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI(); 3276 3277 initDupThreshold(); 3278 3279 // Initialize PreferredLoopExit to nullptr here since it may never be set if 3280 // there are no MachineLoops. 3281 PreferredLoopExit = nullptr; 3282 3283 assert(BlockToChain.empty() && 3284 "BlockToChain map should be empty before starting placement."); 3285 assert(ComputedEdges.empty() && 3286 "Computed Edge map should be empty before starting placement."); 3287 3288 unsigned TailDupSize = TailDupPlacementThreshold; 3289 // If only the aggressive threshold is explicitly set, use it. 3290 if (TailDupPlacementAggressiveThreshold.getNumOccurrences() != 0 && 3291 TailDupPlacementThreshold.getNumOccurrences() == 0) 3292 TailDupSize = TailDupPlacementAggressiveThreshold; 3293 3294 TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>(); 3295 // For aggressive optimization, we can adjust some thresholds to be less 3296 // conservative. 3297 if (PassConfig->getOptLevel() >= CodeGenOpt::Aggressive) { 3298 // At O3 we should be more willing to copy blocks for tail duplication. This 3299 // increases size pressure, so we only do it at O3 3300 // Do this unless only the regular threshold is explicitly set. 3301 if (TailDupPlacementThreshold.getNumOccurrences() == 0 || 3302 TailDupPlacementAggressiveThreshold.getNumOccurrences() != 0) 3303 TailDupSize = TailDupPlacementAggressiveThreshold; 3304 } 3305 3306 if (allowTailDupPlacement()) { 3307 MPDT = &getAnalysis<MachinePostDominatorTree>(); 3308 bool OptForSize = MF.getFunction().hasOptSize() || 3309 llvm::shouldOptimizeForSize(&MF, PSI, &MBFI->getMBFI()); 3310 if (OptForSize) 3311 TailDupSize = 1; 3312 bool PreRegAlloc = false; 3313 TailDup.initMF(MF, PreRegAlloc, MBPI, MBFI.get(), PSI, 3314 /* LayoutMode */ true, TailDupSize); 3315 precomputeTriangleChains(); 3316 } 3317 3318 buildCFGChains(); 3319 3320 // Changing the layout can create new tail merging opportunities. 3321 // TailMerge can create jump into if branches that make CFG irreducible for 3322 // HW that requires structured CFG. 3323 bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() && 3324 PassConfig->getEnableTailMerge() && 3325 BranchFoldPlacement; 3326 // No tail merging opportunities if the block number is less than four. 3327 if (MF.size() > 3 && EnableTailMerge) { 3328 unsigned TailMergeSize = TailDupSize + 1; 3329 BranchFolder BF(/*EnableTailMerge=*/true, /*CommonHoist=*/false, *MBFI, 3330 *MBPI, PSI, TailMergeSize); 3331 3332 if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(), MLI, 3333 /*AfterPlacement=*/true)) { 3334 // Redo the layout if tail merging creates/removes/moves blocks. 3335 BlockToChain.clear(); 3336 ComputedEdges.clear(); 3337 // Must redo the post-dominator tree if blocks were changed. 3338 if (MPDT) 3339 MPDT->runOnMachineFunction(MF); 3340 ChainAllocator.DestroyAll(); 3341 buildCFGChains(); 3342 } 3343 } 3344 3345 optimizeBranches(); 3346 alignBlocks(); 3347 3348 BlockToChain.clear(); 3349 ComputedEdges.clear(); 3350 ChainAllocator.DestroyAll(); 3351 3352 if (AlignAllBlock) 3353 // Align all of the blocks in the function to a specific alignment. 3354 for (MachineBasicBlock &MBB : MF) 3355 MBB.setAlignment(Align(1ULL << AlignAllBlock)); 3356 else if (AlignAllNonFallThruBlocks) { 3357 // Align all of the blocks that have no fall-through predecessors to a 3358 // specific alignment. 3359 for (auto MBI = std::next(MF.begin()), MBE = MF.end(); MBI != MBE; ++MBI) { 3360 auto LayoutPred = std::prev(MBI); 3361 if (!LayoutPred->isSuccessor(&*MBI)) 3362 MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks)); 3363 } 3364 } 3365 if (ViewBlockLayoutWithBFI != GVDT_None && 3366 (ViewBlockFreqFuncName.empty() || 3367 F->getFunction().getName().equals(ViewBlockFreqFuncName))) { 3368 MBFI->view("MBP." + MF.getName(), false); 3369 } 3370 3371 3372 // We always return true as we have no way to track whether the final order 3373 // differs from the original order. 3374 return true; 3375 } 3376 3377 namespace { 3378 3379 /// A pass to compute block placement statistics. 3380 /// 3381 /// A separate pass to compute interesting statistics for evaluating block 3382 /// placement. This is separate from the actual placement pass so that they can 3383 /// be computed in the absence of any placement transformations or when using 3384 /// alternative placement strategies. 3385 class MachineBlockPlacementStats : public MachineFunctionPass { 3386 /// A handle to the branch probability pass. 3387 const MachineBranchProbabilityInfo *MBPI; 3388 3389 /// A handle to the function-wide block frequency pass. 3390 const MachineBlockFrequencyInfo *MBFI; 3391 3392 public: 3393 static char ID; // Pass identification, replacement for typeid 3394 3395 MachineBlockPlacementStats() : MachineFunctionPass(ID) { 3396 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry()); 3397 } 3398 3399 bool runOnMachineFunction(MachineFunction &F) override; 3400 3401 void getAnalysisUsage(AnalysisUsage &AU) const override { 3402 AU.addRequired<MachineBranchProbabilityInfo>(); 3403 AU.addRequired<MachineBlockFrequencyInfo>(); 3404 AU.setPreservesAll(); 3405 MachineFunctionPass::getAnalysisUsage(AU); 3406 } 3407 }; 3408 3409 } // end anonymous namespace 3410 3411 char MachineBlockPlacementStats::ID = 0; 3412 3413 char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID; 3414 3415 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats", 3416 "Basic Block Placement Stats", false, false) 3417 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) 3418 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) 3419 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats", 3420 "Basic Block Placement Stats", false, false) 3421 3422 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) { 3423 // Check for single-block functions and skip them. 3424 if (std::next(F.begin()) == F.end()) 3425 return false; 3426 3427 MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); 3428 MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 3429 3430 for (MachineBasicBlock &MBB : F) { 3431 BlockFrequency BlockFreq = MBFI->getBlockFreq(&MBB); 3432 Statistic &NumBranches = 3433 (MBB.succ_size() > 1) ? NumCondBranches : NumUncondBranches; 3434 Statistic &BranchTakenFreq = 3435 (MBB.succ_size() > 1) ? CondBranchTakenFreq : UncondBranchTakenFreq; 3436 for (MachineBasicBlock *Succ : MBB.successors()) { 3437 // Skip if this successor is a fallthrough. 3438 if (MBB.isLayoutSuccessor(Succ)) 3439 continue; 3440 3441 BlockFrequency EdgeFreq = 3442 BlockFreq * MBPI->getEdgeProbability(&MBB, Succ); 3443 ++NumBranches; 3444 BranchTakenFreq += EdgeFreq.getFrequency(); 3445 } 3446 } 3447 3448 return false; 3449 } 3450