1 //===-- MachineBlockPlacement.cpp - Basic Block Code Layout optimization --===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements basic block placement transformations using the CFG 11 // structure and branch probability estimates. 12 // 13 // The pass strives to preserve the structure of the CFG (that is, retain 14 // a topological ordering of basic blocks) in the absense of a *strong* signal 15 // to the contrary from probabilities. However, within the CFG structure, it 16 // attempts to choose an ordering which favors placing more likely sequences of 17 // blocks adjacent to each other. 18 // 19 // The algorithm works from the inner-most loop within a function outward, and 20 // at each stage walks through the basic blocks, trying to coalesce them into 21 // sequential chains where allowed by the CFG (or demanded by heavy 22 // probabilities). Finally, it walks the blocks in topological order, and the 23 // first time it reaches a chain of basic blocks, it schedules them in the 24 // function in-order. 25 // 26 //===----------------------------------------------------------------------===// 27 28 #define DEBUG_TYPE "block-placement2" 29 #include "llvm/CodeGen/MachineBasicBlock.h" 30 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h" 31 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h" 32 #include "llvm/CodeGen/MachineFunction.h" 33 #include "llvm/CodeGen/MachineFunctionPass.h" 34 #include "llvm/CodeGen/MachineLoopInfo.h" 35 #include "llvm/CodeGen/MachineModuleInfo.h" 36 #include "llvm/CodeGen/Passes.h" 37 #include "llvm/Support/Allocator.h" 38 #include "llvm/Support/Debug.h" 39 #include "llvm/ADT/DenseMap.h" 40 #include "llvm/ADT/SmallPtrSet.h" 41 #include "llvm/ADT/SmallVector.h" 42 #include "llvm/ADT/Statistic.h" 43 #include "llvm/Target/TargetInstrInfo.h" 44 #include "llvm/Target/TargetLowering.h" 45 #include <algorithm> 46 using namespace llvm; 47 48 STATISTIC(NumCondBranches, "Number of conditional branches"); 49 STATISTIC(NumUncondBranches, "Number of uncondittional branches"); 50 STATISTIC(CondBranchTakenFreq, 51 "Potential frequency of taking conditional branches"); 52 STATISTIC(UncondBranchTakenFreq, 53 "Potential frequency of taking unconditional branches"); 54 55 namespace { 56 class BlockChain; 57 /// \brief Type for our function-wide basic block -> block chain mapping. 58 typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType; 59 } 60 61 namespace { 62 /// \brief A chain of blocks which will be laid out contiguously. 63 /// 64 /// This is the datastructure representing a chain of consecutive blocks that 65 /// are profitable to layout together in order to maximize fallthrough 66 /// probabilities. We also can use a block chain to represent a sequence of 67 /// basic blocks which have some external (correctness) requirement for 68 /// sequential layout. 69 /// 70 /// Eventually, the block chains will form a directed graph over the function. 71 /// We provide an SCC-supporting-iterator in order to quicky build and walk the 72 /// SCCs of block chains within a function. 73 /// 74 /// The block chains also have support for calculating and caching probability 75 /// information related to the chain itself versus other chains. This is used 76 /// for ranking during the final layout of block chains. 77 class BlockChain { 78 /// \brief The sequence of blocks belonging to this chain. 79 /// 80 /// This is the sequence of blocks for a particular chain. These will be laid 81 /// out in-order within the function. 82 SmallVector<MachineBasicBlock *, 4> Blocks; 83 84 /// \brief A handle to the function-wide basic block to block chain mapping. 85 /// 86 /// This is retained in each block chain to simplify the computation of child 87 /// block chains for SCC-formation and iteration. We store the edges to child 88 /// basic blocks, and map them back to their associated chains using this 89 /// structure. 90 BlockToChainMapType &BlockToChain; 91 92 public: 93 /// \brief Construct a new BlockChain. 94 /// 95 /// This builds a new block chain representing a single basic block in the 96 /// function. It also registers itself as the chain that block participates 97 /// in with the BlockToChain mapping. 98 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB) 99 : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) { 100 assert(BB && "Cannot create a chain with a null basic block"); 101 BlockToChain[BB] = this; 102 } 103 104 /// \brief Iterator over blocks within the chain. 105 typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator; 106 107 /// \brief Beginning of blocks within the chain. 108 iterator begin() { return Blocks.begin(); } 109 110 /// \brief End of blocks within the chain. 111 iterator end() { return Blocks.end(); } 112 113 /// \brief Merge a block chain into this one. 114 /// 115 /// This routine merges a block chain into this one. It takes care of forming 116 /// a contiguous sequence of basic blocks, updating the edge list, and 117 /// updating the block -> chain mapping. It does not free or tear down the 118 /// old chain, but the old chain's block list is no longer valid. 119 void merge(MachineBasicBlock *BB, BlockChain *Chain) { 120 assert(BB); 121 assert(!Blocks.empty()); 122 123 // Fast path in case we don't have a chain already. 124 if (!Chain) { 125 assert(!BlockToChain[BB]); 126 Blocks.push_back(BB); 127 BlockToChain[BB] = this; 128 return; 129 } 130 131 assert(BB == *Chain->begin()); 132 assert(Chain->begin() != Chain->end()); 133 134 // Update the incoming blocks to point to this chain, and add them to the 135 // chain structure. 136 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end(); 137 BI != BE; ++BI) { 138 Blocks.push_back(*BI); 139 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain"); 140 BlockToChain[*BI] = this; 141 } 142 } 143 144 #ifndef NDEBUG 145 /// \brief Dump the blocks in this chain. 146 void dump() LLVM_ATTRIBUTE_USED { 147 for (iterator I = begin(), E = end(); I != E; ++I) 148 (*I)->dump(); 149 } 150 #endif // NDEBUG 151 152 /// \brief Count of predecessors within the loop currently being processed. 153 /// 154 /// This count is updated at each loop we process to represent the number of 155 /// in-loop predecessors of this chain. 156 unsigned LoopPredecessors; 157 }; 158 } 159 160 namespace { 161 class MachineBlockPlacement : public MachineFunctionPass { 162 /// \brief A typedef for a block filter set. 163 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet; 164 165 /// \brief A handle to the branch probability pass. 166 const MachineBranchProbabilityInfo *MBPI; 167 168 /// \brief A handle to the function-wide block frequency pass. 169 const MachineBlockFrequencyInfo *MBFI; 170 171 /// \brief A handle to the loop info. 172 const MachineLoopInfo *MLI; 173 174 /// \brief A handle to the target's instruction info. 175 const TargetInstrInfo *TII; 176 177 /// \brief A handle to the target's lowering info. 178 const TargetLowering *TLI; 179 180 /// \brief Allocator and owner of BlockChain structures. 181 /// 182 /// We build BlockChains lazily by merging together high probability BB 183 /// sequences acording to the "Algo2" in the paper mentioned at the top of 184 /// the file. To reduce malloc traffic, we allocate them using this slab-like 185 /// allocator, and destroy them after the pass completes. 186 SpecificBumpPtrAllocator<BlockChain> ChainAllocator; 187 188 /// \brief Function wide BasicBlock to BlockChain mapping. 189 /// 190 /// This mapping allows efficiently moving from any given basic block to the 191 /// BlockChain it participates in, if any. We use it to, among other things, 192 /// allow implicitly defining edges between chains as the existing edges 193 /// between basic blocks. 194 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain; 195 196 void markChainSuccessors(BlockChain &Chain, 197 MachineBasicBlock *LoopHeaderBB, 198 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, 199 const BlockFilterSet *BlockFilter = 0); 200 MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB, 201 BlockChain &Chain, 202 const BlockFilterSet *BlockFilter); 203 MachineBasicBlock *selectBestCandidateBlock( 204 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList, 205 const BlockFilterSet *BlockFilter); 206 MachineBasicBlock *getFirstUnplacedBlock( 207 MachineFunction &F, 208 const BlockChain &PlacedChain, 209 MachineFunction::iterator &PrevUnplacedBlockIt, 210 const BlockFilterSet *BlockFilter); 211 void buildChain(MachineBasicBlock *BB, BlockChain &Chain, 212 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, 213 const BlockFilterSet *BlockFilter = 0); 214 MachineBasicBlock *findBestLoopTop(MachineLoop &L, 215 const BlockFilterSet &LoopBlockSet); 216 MachineBasicBlock *findBestLoopExit(MachineFunction &F, 217 MachineLoop &L, 218 const BlockFilterSet &LoopBlockSet); 219 void buildLoopChains(MachineFunction &F, MachineLoop &L); 220 void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB, 221 const BlockFilterSet &LoopBlockSet); 222 void buildCFGChains(MachineFunction &F); 223 224 public: 225 static char ID; // Pass identification, replacement for typeid 226 MachineBlockPlacement() : MachineFunctionPass(ID) { 227 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry()); 228 } 229 230 bool runOnMachineFunction(MachineFunction &F); 231 232 void getAnalysisUsage(AnalysisUsage &AU) const { 233 AU.addRequired<MachineBranchProbabilityInfo>(); 234 AU.addRequired<MachineBlockFrequencyInfo>(); 235 AU.addRequired<MachineLoopInfo>(); 236 MachineFunctionPass::getAnalysisUsage(AU); 237 } 238 }; 239 } 240 241 char MachineBlockPlacement::ID = 0; 242 char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID; 243 INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2", 244 "Branch Probability Basic Block Placement", false, false) 245 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) 246 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) 247 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) 248 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2", 249 "Branch Probability Basic Block Placement", false, false) 250 251 #ifndef NDEBUG 252 /// \brief Helper to print the name of a MBB. 253 /// 254 /// Only used by debug logging. 255 static std::string getBlockName(MachineBasicBlock *BB) { 256 std::string Result; 257 raw_string_ostream OS(Result); 258 OS << "BB#" << BB->getNumber() 259 << " (derived from LLVM BB '" << BB->getName() << "')"; 260 OS.flush(); 261 return Result; 262 } 263 264 /// \brief Helper to print the number of a MBB. 265 /// 266 /// Only used by debug logging. 267 static std::string getBlockNum(MachineBasicBlock *BB) { 268 std::string Result; 269 raw_string_ostream OS(Result); 270 OS << "BB#" << BB->getNumber(); 271 OS.flush(); 272 return Result; 273 } 274 #endif 275 276 /// \brief Mark a chain's successors as having one fewer preds. 277 /// 278 /// When a chain is being merged into the "placed" chain, this routine will 279 /// quickly walk the successors of each block in the chain and mark them as 280 /// having one fewer active predecessor. It also adds any successors of this 281 /// chain which reach the zero-predecessor state to the worklist passed in. 282 void MachineBlockPlacement::markChainSuccessors( 283 BlockChain &Chain, 284 MachineBasicBlock *LoopHeaderBB, 285 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, 286 const BlockFilterSet *BlockFilter) { 287 // Walk all the blocks in this chain, marking their successors as having 288 // a predecessor placed. 289 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end(); 290 CBI != CBE; ++CBI) { 291 // Add any successors for which this is the only un-placed in-loop 292 // predecessor to the worklist as a viable candidate for CFG-neutral 293 // placement. No subsequent placement of this block will violate the CFG 294 // shape, so we get to use heuristics to choose a favorable placement. 295 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(), 296 SE = (*CBI)->succ_end(); 297 SI != SE; ++SI) { 298 if (BlockFilter && !BlockFilter->count(*SI)) 299 continue; 300 BlockChain &SuccChain = *BlockToChain[*SI]; 301 // Disregard edges within a fixed chain, or edges to the loop header. 302 if (&Chain == &SuccChain || *SI == LoopHeaderBB) 303 continue; 304 305 // This is a cross-chain edge that is within the loop, so decrement the 306 // loop predecessor count of the destination chain. 307 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0) 308 BlockWorkList.push_back(*SuccChain.begin()); 309 } 310 } 311 } 312 313 /// \brief Select the best successor for a block. 314 /// 315 /// This looks across all successors of a particular block and attempts to 316 /// select the "best" one to be the layout successor. It only considers direct 317 /// successors which also pass the block filter. It will attempt to avoid 318 /// breaking CFG structure, but cave and break such structures in the case of 319 /// very hot successor edges. 320 /// 321 /// \returns The best successor block found, or null if none are viable. 322 MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor( 323 MachineBasicBlock *BB, BlockChain &Chain, 324 const BlockFilterSet *BlockFilter) { 325 const BranchProbability HotProb(4, 5); // 80% 326 327 MachineBasicBlock *BestSucc = 0; 328 // FIXME: Due to the performance of the probability and weight routines in 329 // the MBPI analysis, we manually compute probabilities using the edge 330 // weights. This is suboptimal as it means that the somewhat subtle 331 // definition of edge weight semantics is encoded here as well. We should 332 // improve the MBPI interface to effeciently support query patterns such as 333 // this. 334 uint32_t BestWeight = 0; 335 uint32_t WeightScale = 0; 336 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale); 337 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n"); 338 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(), 339 SE = BB->succ_end(); 340 SI != SE; ++SI) { 341 if (BlockFilter && !BlockFilter->count(*SI)) 342 continue; 343 BlockChain &SuccChain = *BlockToChain[*SI]; 344 if (&SuccChain == &Chain) { 345 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n"); 346 continue; 347 } 348 if (*SI != *SuccChain.begin()) { 349 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n"); 350 continue; 351 } 352 353 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI); 354 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight); 355 356 // Only consider successors which are either "hot", or wouldn't violate 357 // any CFG constraints. 358 if (SuccChain.LoopPredecessors != 0) { 359 if (SuccProb < HotProb) { 360 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n"); 361 continue; 362 } 363 364 // Make sure that a hot successor doesn't have a globally more important 365 // predecessor. 366 BlockFrequency CandidateEdgeFreq 367 = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl(); 368 bool BadCFGConflict = false; 369 for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(), 370 PE = (*SI)->pred_end(); 371 PI != PE; ++PI) { 372 if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) || 373 BlockToChain[*PI] == &Chain) 374 continue; 375 BlockFrequency PredEdgeFreq 376 = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI); 377 if (PredEdgeFreq >= CandidateEdgeFreq) { 378 BadCFGConflict = true; 379 break; 380 } 381 } 382 if (BadCFGConflict) { 383 DEBUG(dbgs() << " " << getBlockName(*SI) 384 << " -> non-cold CFG conflict\n"); 385 continue; 386 } 387 } 388 389 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb 390 << " (prob)" 391 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "") 392 << "\n"); 393 if (BestSucc && BestWeight >= SuccWeight) 394 continue; 395 BestSucc = *SI; 396 BestWeight = SuccWeight; 397 } 398 return BestSucc; 399 } 400 401 namespace { 402 /// \brief Predicate struct to detect blocks already placed. 403 class IsBlockPlaced { 404 const BlockChain &PlacedChain; 405 const BlockToChainMapType &BlockToChain; 406 407 public: 408 IsBlockPlaced(const BlockChain &PlacedChain, 409 const BlockToChainMapType &BlockToChain) 410 : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {} 411 412 bool operator()(MachineBasicBlock *BB) const { 413 return BlockToChain.lookup(BB) == &PlacedChain; 414 } 415 }; 416 } 417 418 /// \brief Select the best block from a worklist. 419 /// 420 /// This looks through the provided worklist as a list of candidate basic 421 /// blocks and select the most profitable one to place. The definition of 422 /// profitable only really makes sense in the context of a loop. This returns 423 /// the most frequently visited block in the worklist, which in the case of 424 /// a loop, is the one most desirable to be physically close to the rest of the 425 /// loop body in order to improve icache behavior. 426 /// 427 /// \returns The best block found, or null if none are viable. 428 MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock( 429 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList, 430 const BlockFilterSet *BlockFilter) { 431 // Once we need to walk the worklist looking for a candidate, cleanup the 432 // worklist of already placed entries. 433 // FIXME: If this shows up on profiles, it could be folded (at the cost of 434 // some code complexity) into the loop below. 435 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(), 436 IsBlockPlaced(Chain, BlockToChain)), 437 WorkList.end()); 438 439 MachineBasicBlock *BestBlock = 0; 440 BlockFrequency BestFreq; 441 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(), 442 WBE = WorkList.end(); 443 WBI != WBE; ++WBI) { 444 BlockChain &SuccChain = *BlockToChain[*WBI]; 445 if (&SuccChain == &Chain) { 446 DEBUG(dbgs() << " " << getBlockName(*WBI) 447 << " -> Already merged!\n"); 448 continue; 449 } 450 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block"); 451 452 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI); 453 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq 454 << " (freq)\n"); 455 if (BestBlock && BestFreq >= CandidateFreq) 456 continue; 457 BestBlock = *WBI; 458 BestFreq = CandidateFreq; 459 } 460 return BestBlock; 461 } 462 463 /// \brief Retrieve the first unplaced basic block. 464 /// 465 /// This routine is called when we are unable to use the CFG to walk through 466 /// all of the basic blocks and form a chain due to unnatural loops in the CFG. 467 /// We walk through the function's blocks in order, starting from the 468 /// LastUnplacedBlockIt. We update this iterator on each call to avoid 469 /// re-scanning the entire sequence on repeated calls to this routine. 470 MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock( 471 MachineFunction &F, const BlockChain &PlacedChain, 472 MachineFunction::iterator &PrevUnplacedBlockIt, 473 const BlockFilterSet *BlockFilter) { 474 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E; 475 ++I) { 476 if (BlockFilter && !BlockFilter->count(I)) 477 continue; 478 if (BlockToChain[I] != &PlacedChain) { 479 PrevUnplacedBlockIt = I; 480 // Now select the head of the chain to which the unplaced block belongs 481 // as the block to place. This will force the entire chain to be placed, 482 // and satisfies the requirements of merging chains. 483 return *BlockToChain[I]->begin(); 484 } 485 } 486 return 0; 487 } 488 489 void MachineBlockPlacement::buildChain( 490 MachineBasicBlock *BB, 491 BlockChain &Chain, 492 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, 493 const BlockFilterSet *BlockFilter) { 494 assert(BB); 495 assert(BlockToChain[BB] == &Chain); 496 MachineFunction &F = *BB->getParent(); 497 MachineFunction::iterator PrevUnplacedBlockIt = F.begin(); 498 499 MachineBasicBlock *LoopHeaderBB = BB; 500 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter); 501 BB = *llvm::prior(Chain.end()); 502 for (;;) { 503 assert(BB); 504 assert(BlockToChain[BB] == &Chain); 505 assert(*llvm::prior(Chain.end()) == BB); 506 MachineBasicBlock *BestSucc = 0; 507 508 // Look for the best viable successor if there is one to place immediately 509 // after this block. 510 BestSucc = selectBestSuccessor(BB, Chain, BlockFilter); 511 512 // If an immediate successor isn't available, look for the best viable 513 // block among those we've identified as not violating the loop's CFG at 514 // this point. This won't be a fallthrough, but it will increase locality. 515 if (!BestSucc) 516 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter); 517 518 if (!BestSucc) { 519 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt, 520 BlockFilter); 521 if (!BestSucc) 522 break; 523 524 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the " 525 "layout successor until the CFG reduces\n"); 526 } 527 528 // Place this block, updating the datastructures to reflect its placement. 529 BlockChain &SuccChain = *BlockToChain[BestSucc]; 530 // Zero out LoopPredecessors for the successor we're about to merge in case 531 // we selected a successor that didn't fit naturally into the CFG. 532 SuccChain.LoopPredecessors = 0; 533 DEBUG(dbgs() << "Merging from " << getBlockNum(BB) 534 << " to " << getBlockNum(BestSucc) << "\n"); 535 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter); 536 Chain.merge(BestSucc, &SuccChain); 537 BB = *llvm::prior(Chain.end()); 538 } 539 540 DEBUG(dbgs() << "Finished forming chain for header block " 541 << getBlockNum(*Chain.begin()) << "\n"); 542 } 543 544 /// \brief Find the best loop top block for layout. 545 /// 546 /// Look for a block which is strictly better than the loop header for laying 547 /// out at the top of the loop. This looks for one and only one pattern: 548 /// a latch block with no conditional exit. This block will cause a conditional 549 /// jump around it or will be the bottom of the loop if we lay it out in place, 550 /// but if it it doesn't end up at the bottom of the loop for any reason, 551 /// rotation alone won't fix it. Because such a block will always result in an 552 /// unconditional jump (for the backedge) rotating it in front of the loop 553 /// header is always profitable. 554 MachineBasicBlock * 555 MachineBlockPlacement::findBestLoopTop(MachineLoop &L, 556 const BlockFilterSet &LoopBlockSet) { 557 // Check that the header hasn't been fused with a preheader block due to 558 // crazy branches. If it has, we need to start with the header at the top to 559 // prevent pulling the preheader into the loop body. 560 BlockChain &HeaderChain = *BlockToChain[L.getHeader()]; 561 if (!LoopBlockSet.count(*HeaderChain.begin())) 562 return L.getHeader(); 563 564 DEBUG(dbgs() << "Finding best loop top for: " 565 << getBlockName(L.getHeader()) << "\n"); 566 567 BlockFrequency BestPredFreq; 568 MachineBasicBlock *BestPred = 0; 569 for (MachineBasicBlock::pred_iterator PI = L.getHeader()->pred_begin(), 570 PE = L.getHeader()->pred_end(); 571 PI != PE; ++PI) { 572 MachineBasicBlock *Pred = *PI; 573 if (!LoopBlockSet.count(Pred)) 574 continue; 575 DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", " 576 << Pred->succ_size() << " successors, " 577 << MBFI->getBlockFreq(Pred) << " freq\n"); 578 if (Pred->succ_size() > 1) 579 continue; 580 581 BlockFrequency PredFreq = MBFI->getBlockFreq(Pred); 582 if (!BestPred || PredFreq > BestPredFreq || 583 (!(PredFreq < BestPredFreq) && 584 Pred->isLayoutSuccessor(L.getHeader()))) { 585 BestPred = Pred; 586 BestPredFreq = PredFreq; 587 } 588 } 589 590 // If no direct predecessor is fine, just use the loop header. 591 if (!BestPred) 592 return L.getHeader(); 593 594 // Walk backwards through any straight line of predecessors. 595 while (BestPred->pred_size() == 1 && 596 (*BestPred->pred_begin())->succ_size() == 1 && 597 *BestPred->pred_begin() != L.getHeader()) 598 BestPred = *BestPred->pred_begin(); 599 600 DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n"); 601 return BestPred; 602 } 603 604 605 /// \brief Find the best loop exiting block for layout. 606 /// 607 /// This routine implements the logic to analyze the loop looking for the best 608 /// block to layout at the top of the loop. Typically this is done to maximize 609 /// fallthrough opportunities. 610 MachineBasicBlock * 611 MachineBlockPlacement::findBestLoopExit(MachineFunction &F, 612 MachineLoop &L, 613 const BlockFilterSet &LoopBlockSet) { 614 // We don't want to layout the loop linearly in all cases. If the loop header 615 // is just a normal basic block in the loop, we want to look for what block 616 // within the loop is the best one to layout at the top. However, if the loop 617 // header has be pre-merged into a chain due to predecessors not having 618 // analyzable branches, *and* the predecessor it is merged with is *not* part 619 // of the loop, rotating the header into the middle of the loop will create 620 // a non-contiguous range of blocks which is Very Bad. So start with the 621 // header and only rotate if safe. 622 BlockChain &HeaderChain = *BlockToChain[L.getHeader()]; 623 if (!LoopBlockSet.count(*HeaderChain.begin())) 624 return 0; 625 626 BlockFrequency BestExitEdgeFreq; 627 unsigned BestExitLoopDepth = 0; 628 MachineBasicBlock *ExitingBB = 0; 629 // If there are exits to outer loops, loop rotation can severely limit 630 // fallthrough opportunites unless it selects such an exit. Keep a set of 631 // blocks where rotating to exit with that block will reach an outer loop. 632 SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop; 633 634 DEBUG(dbgs() << "Finding best loop exit for: " 635 << getBlockName(L.getHeader()) << "\n"); 636 for (MachineLoop::block_iterator I = L.block_begin(), 637 E = L.block_end(); 638 I != E; ++I) { 639 BlockChain &Chain = *BlockToChain[*I]; 640 // Ensure that this block is at the end of a chain; otherwise it could be 641 // mid-way through an inner loop or a successor of an analyzable branch. 642 if (*I != *llvm::prior(Chain.end())) 643 continue; 644 645 // Now walk the successors. We need to establish whether this has a viable 646 // exiting successor and whether it has a viable non-exiting successor. 647 // We store the old exiting state and restore it if a viable looping 648 // successor isn't found. 649 MachineBasicBlock *OldExitingBB = ExitingBB; 650 BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq; 651 bool HasLoopingSucc = false; 652 // FIXME: Due to the performance of the probability and weight routines in 653 // the MBPI analysis, we use the internal weights and manually compute the 654 // probabilities to avoid quadratic behavior. 655 uint32_t WeightScale = 0; 656 uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale); 657 for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(), 658 SE = (*I)->succ_end(); 659 SI != SE; ++SI) { 660 if ((*SI)->isLandingPad()) 661 continue; 662 if (*SI == *I) 663 continue; 664 BlockChain &SuccChain = *BlockToChain[*SI]; 665 // Don't split chains, either this chain or the successor's chain. 666 if (&Chain == &SuccChain) { 667 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> " 668 << getBlockName(*SI) << " (chain conflict)\n"); 669 continue; 670 } 671 672 uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI); 673 if (LoopBlockSet.count(*SI)) { 674 DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> " 675 << getBlockName(*SI) << " (" << SuccWeight << ")\n"); 676 HasLoopingSucc = true; 677 continue; 678 } 679 680 unsigned SuccLoopDepth = 0; 681 if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI)) { 682 SuccLoopDepth = ExitLoop->getLoopDepth(); 683 if (ExitLoop->contains(&L)) 684 BlocksExitingToOuterLoop.insert(*I); 685 } 686 687 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight); 688 BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb; 689 DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> " 690 << getBlockName(*SI) << " [L:" << SuccLoopDepth 691 << "] (" << ExitEdgeFreq << ")\n"); 692 // Note that we slightly bias this toward an existing layout successor to 693 // retain incoming order in the absence of better information. 694 // FIXME: Should we bias this more strongly? It's pretty weak. 695 if (!ExitingBB || BestExitLoopDepth < SuccLoopDepth || 696 ExitEdgeFreq > BestExitEdgeFreq || 697 ((*I)->isLayoutSuccessor(*SI) && 698 !(ExitEdgeFreq < BestExitEdgeFreq))) { 699 BestExitEdgeFreq = ExitEdgeFreq; 700 ExitingBB = *I; 701 } 702 } 703 704 // Restore the old exiting state, no viable looping successor was found. 705 if (!HasLoopingSucc) { 706 ExitingBB = OldExitingBB; 707 BestExitEdgeFreq = OldBestExitEdgeFreq; 708 continue; 709 } 710 } 711 // Without a candidate exiting block or with only a single block in the 712 // loop, just use the loop header to layout the loop. 713 if (!ExitingBB || L.getNumBlocks() == 1) 714 return 0; 715 716 // Also, if we have exit blocks which lead to outer loops but didn't select 717 // one of them as the exiting block we are rotating toward, disable loop 718 // rotation altogether. 719 if (!BlocksExitingToOuterLoop.empty() && 720 !BlocksExitingToOuterLoop.count(ExitingBB)) 721 return 0; 722 723 DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n"); 724 return ExitingBB; 725 } 726 727 /// \brief Attempt to rotate an exiting block to the bottom of the loop. 728 /// 729 /// Once we have built a chain, try to rotate it to line up the hot exit block 730 /// with fallthrough out of the loop if doing so doesn't introduce unnecessary 731 /// branches. For example, if the loop has fallthrough into its header and out 732 /// of its bottom already, don't rotate it. 733 void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain, 734 MachineBasicBlock *ExitingBB, 735 const BlockFilterSet &LoopBlockSet) { 736 if (!ExitingBB) 737 return; 738 739 MachineBasicBlock *Top = *LoopChain.begin(); 740 bool ViableTopFallthrough = false; 741 for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(), 742 PE = Top->pred_end(); 743 PI != PE; ++PI) { 744 BlockChain *PredChain = BlockToChain[*PI]; 745 if (!LoopBlockSet.count(*PI) && 746 (!PredChain || *PI == *llvm::prior(PredChain->end()))) { 747 ViableTopFallthrough = true; 748 break; 749 } 750 } 751 752 // If the header has viable fallthrough, check whether the current loop 753 // bottom is a viable exiting block. If so, bail out as rotating will 754 // introduce an unnecessary branch. 755 if (ViableTopFallthrough) { 756 MachineBasicBlock *Bottom = *llvm::prior(LoopChain.end()); 757 for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(), 758 SE = Bottom->succ_end(); 759 SI != SE; ++SI) { 760 BlockChain *SuccChain = BlockToChain[*SI]; 761 if (!LoopBlockSet.count(*SI) && 762 (!SuccChain || *SI == *SuccChain->begin())) 763 return; 764 } 765 } 766 767 BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(), 768 ExitingBB); 769 if (ExitIt == LoopChain.end()) 770 return; 771 772 std::rotate(LoopChain.begin(), llvm::next(ExitIt), LoopChain.end()); 773 } 774 775 /// \brief Forms basic block chains from the natural loop structures. 776 /// 777 /// These chains are designed to preserve the existing *structure* of the code 778 /// as much as possible. We can then stitch the chains together in a way which 779 /// both preserves the topological structure and minimizes taken conditional 780 /// branches. 781 void MachineBlockPlacement::buildLoopChains(MachineFunction &F, 782 MachineLoop &L) { 783 // First recurse through any nested loops, building chains for those inner 784 // loops. 785 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI) 786 buildLoopChains(F, **LI); 787 788 SmallVector<MachineBasicBlock *, 16> BlockWorkList; 789 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end()); 790 791 // First check to see if there is an obviously preferable top block for the 792 // loop. This will default to the header, but may end up as one of the 793 // predecessors to the header if there is one which will result in strictly 794 // fewer branches in the loop body. 795 MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet); 796 797 // If we selected just the header for the loop top, look for a potentially 798 // profitable exit block in the event that rotating the loop can eliminate 799 // branches by placing an exit edge at the bottom. 800 MachineBasicBlock *ExitingBB = 0; 801 if (LoopTop == L.getHeader()) 802 ExitingBB = findBestLoopExit(F, L, LoopBlockSet); 803 804 BlockChain &LoopChain = *BlockToChain[LoopTop]; 805 806 // FIXME: This is a really lame way of walking the chains in the loop: we 807 // walk the blocks, and use a set to prevent visiting a particular chain 808 // twice. 809 SmallPtrSet<BlockChain *, 4> UpdatedPreds; 810 assert(LoopChain.LoopPredecessors == 0); 811 UpdatedPreds.insert(&LoopChain); 812 for (MachineLoop::block_iterator BI = L.block_begin(), 813 BE = L.block_end(); 814 BI != BE; ++BI) { 815 BlockChain &Chain = *BlockToChain[*BI]; 816 if (!UpdatedPreds.insert(&Chain)) 817 continue; 818 819 assert(Chain.LoopPredecessors == 0); 820 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end(); 821 BCI != BCE; ++BCI) { 822 assert(BlockToChain[*BCI] == &Chain); 823 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(), 824 PE = (*BCI)->pred_end(); 825 PI != PE; ++PI) { 826 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI)) 827 continue; 828 ++Chain.LoopPredecessors; 829 } 830 } 831 832 if (Chain.LoopPredecessors == 0) 833 BlockWorkList.push_back(*Chain.begin()); 834 } 835 836 buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet); 837 rotateLoop(LoopChain, ExitingBB, LoopBlockSet); 838 839 DEBUG({ 840 // Crash at the end so we get all of the debugging output first. 841 bool BadLoop = false; 842 if (LoopChain.LoopPredecessors) { 843 BadLoop = true; 844 dbgs() << "Loop chain contains a block without its preds placed!\n" 845 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 846 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"; 847 } 848 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end(); 849 BCI != BCE; ++BCI) { 850 dbgs() << " ... " << getBlockName(*BCI) << "\n"; 851 if (!LoopBlockSet.erase(*BCI)) { 852 // We don't mark the loop as bad here because there are real situations 853 // where this can occur. For example, with an unanalyzable fallthrough 854 // from a loop block to a non-loop block or vice versa. 855 dbgs() << "Loop chain contains a block not contained by the loop!\n" 856 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 857 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n" 858 << " Bad block: " << getBlockName(*BCI) << "\n"; 859 } 860 } 861 862 if (!LoopBlockSet.empty()) { 863 BadLoop = true; 864 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(), 865 LBE = LoopBlockSet.end(); 866 LBI != LBE; ++LBI) 867 dbgs() << "Loop contains blocks never placed into a chain!\n" 868 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 869 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n" 870 << " Bad block: " << getBlockName(*LBI) << "\n"; 871 } 872 assert(!BadLoop && "Detected problems with the placement of this loop."); 873 }); 874 } 875 876 void MachineBlockPlacement::buildCFGChains(MachineFunction &F) { 877 // Ensure that every BB in the function has an associated chain to simplify 878 // the assumptions of the remaining algorithm. 879 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch. 880 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) { 881 MachineBasicBlock *BB = FI; 882 BlockChain *Chain 883 = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB); 884 // Also, merge any blocks which we cannot reason about and must preserve 885 // the exact fallthrough behavior for. 886 for (;;) { 887 Cond.clear(); 888 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch. 889 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough()) 890 break; 891 892 MachineFunction::iterator NextFI(llvm::next(FI)); 893 MachineBasicBlock *NextBB = NextFI; 894 // Ensure that the layout successor is a viable block, as we know that 895 // fallthrough is a possibility. 896 assert(NextFI != FE && "Can't fallthrough past the last block."); 897 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: " 898 << getBlockName(BB) << " -> " << getBlockName(NextBB) 899 << "\n"); 900 Chain->merge(NextBB, 0); 901 FI = NextFI; 902 BB = NextBB; 903 } 904 } 905 906 // Build any loop-based chains. 907 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE; 908 ++LI) 909 buildLoopChains(F, **LI); 910 911 SmallVector<MachineBasicBlock *, 16> BlockWorkList; 912 913 SmallPtrSet<BlockChain *, 4> UpdatedPreds; 914 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) { 915 MachineBasicBlock *BB = &*FI; 916 BlockChain &Chain = *BlockToChain[BB]; 917 if (!UpdatedPreds.insert(&Chain)) 918 continue; 919 920 assert(Chain.LoopPredecessors == 0); 921 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end(); 922 BCI != BCE; ++BCI) { 923 assert(BlockToChain[*BCI] == &Chain); 924 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(), 925 PE = (*BCI)->pred_end(); 926 PI != PE; ++PI) { 927 if (BlockToChain[*PI] == &Chain) 928 continue; 929 ++Chain.LoopPredecessors; 930 } 931 } 932 933 if (Chain.LoopPredecessors == 0) 934 BlockWorkList.push_back(*Chain.begin()); 935 } 936 937 BlockChain &FunctionChain = *BlockToChain[&F.front()]; 938 buildChain(&F.front(), FunctionChain, BlockWorkList); 939 940 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType; 941 DEBUG({ 942 // Crash at the end so we get all of the debugging output first. 943 bool BadFunc = false; 944 FunctionBlockSetType FunctionBlockSet; 945 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) 946 FunctionBlockSet.insert(FI); 947 948 for (BlockChain::iterator BCI = FunctionChain.begin(), 949 BCE = FunctionChain.end(); 950 BCI != BCE; ++BCI) 951 if (!FunctionBlockSet.erase(*BCI)) { 952 BadFunc = true; 953 dbgs() << "Function chain contains a block not in the function!\n" 954 << " Bad block: " << getBlockName(*BCI) << "\n"; 955 } 956 957 if (!FunctionBlockSet.empty()) { 958 BadFunc = true; 959 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(), 960 FBE = FunctionBlockSet.end(); 961 FBI != FBE; ++FBI) 962 dbgs() << "Function contains blocks never placed into a chain!\n" 963 << " Bad block: " << getBlockName(*FBI) << "\n"; 964 } 965 assert(!BadFunc && "Detected problems with the block placement."); 966 }); 967 968 // Splice the blocks into place. 969 MachineFunction::iterator InsertPos = F.begin(); 970 for (BlockChain::iterator BI = FunctionChain.begin(), 971 BE = FunctionChain.end(); 972 BI != BE; ++BI) { 973 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain " 974 : " ... ") 975 << getBlockName(*BI) << "\n"); 976 if (InsertPos != MachineFunction::iterator(*BI)) 977 F.splice(InsertPos, *BI); 978 else 979 ++InsertPos; 980 981 // Update the terminator of the previous block. 982 if (BI == FunctionChain.begin()) 983 continue; 984 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI)); 985 986 // FIXME: It would be awesome of updateTerminator would just return rather 987 // than assert when the branch cannot be analyzed in order to remove this 988 // boiler plate. 989 Cond.clear(); 990 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch. 991 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) 992 PrevBB->updateTerminator(); 993 } 994 995 // Fixup the last block. 996 Cond.clear(); 997 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch. 998 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond)) 999 F.back().updateTerminator(); 1000 1001 // Walk through the backedges of the function now that we have fully laid out 1002 // the basic blocks and align the destination of each backedge. We don't rely 1003 // on the loop info here so that we can align backedges in unnatural CFGs and 1004 // backedges that were introduced purely because of the loop rotations done 1005 // during this layout pass. 1006 // FIXME: This isn't quite right, we shouldn't align backedges that result 1007 // from blocks being sunken below the exit block for the function. 1008 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize)) 1009 return; 1010 unsigned Align = TLI->getPrefLoopAlignment(); 1011 if (!Align) 1012 return; // Don't care about loop alignment. 1013 1014 SmallPtrSet<MachineBasicBlock *, 16> PreviousBlocks; 1015 for (BlockChain::iterator BI = FunctionChain.begin(), 1016 BE = FunctionChain.end(); 1017 BI != BE; ++BI) { 1018 PreviousBlocks.insert(*BI); 1019 // Set alignment on the destination of all the back edges in the new 1020 // ordering. 1021 for (MachineBasicBlock::succ_iterator SI = (*BI)->succ_begin(), 1022 SE = (*BI)->succ_end(); 1023 SI != SE; ++SI) 1024 if (PreviousBlocks.count(*SI)) 1025 (*SI)->setAlignment(Align); 1026 } 1027 } 1028 1029 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) { 1030 // Check for single-block functions and skip them. 1031 if (llvm::next(F.begin()) == F.end()) 1032 return false; 1033 1034 MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); 1035 MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 1036 MLI = &getAnalysis<MachineLoopInfo>(); 1037 TII = F.getTarget().getInstrInfo(); 1038 TLI = F.getTarget().getTargetLowering(); 1039 assert(BlockToChain.empty()); 1040 1041 buildCFGChains(F); 1042 1043 BlockToChain.clear(); 1044 ChainAllocator.DestroyAll(); 1045 1046 // We always return true as we have no way to track whether the final order 1047 // differs from the original order. 1048 return true; 1049 } 1050 1051 namespace { 1052 /// \brief A pass to compute block placement statistics. 1053 /// 1054 /// A separate pass to compute interesting statistics for evaluating block 1055 /// placement. This is separate from the actual placement pass so that they can 1056 /// be computed in the absense of any placement transformations or when using 1057 /// alternative placement strategies. 1058 class MachineBlockPlacementStats : public MachineFunctionPass { 1059 /// \brief A handle to the branch probability pass. 1060 const MachineBranchProbabilityInfo *MBPI; 1061 1062 /// \brief A handle to the function-wide block frequency pass. 1063 const MachineBlockFrequencyInfo *MBFI; 1064 1065 public: 1066 static char ID; // Pass identification, replacement for typeid 1067 MachineBlockPlacementStats() : MachineFunctionPass(ID) { 1068 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry()); 1069 } 1070 1071 bool runOnMachineFunction(MachineFunction &F); 1072 1073 void getAnalysisUsage(AnalysisUsage &AU) const { 1074 AU.addRequired<MachineBranchProbabilityInfo>(); 1075 AU.addRequired<MachineBlockFrequencyInfo>(); 1076 AU.setPreservesAll(); 1077 MachineFunctionPass::getAnalysisUsage(AU); 1078 } 1079 }; 1080 } 1081 1082 char MachineBlockPlacementStats::ID = 0; 1083 char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID; 1084 INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats", 1085 "Basic Block Placement Stats", false, false) 1086 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) 1087 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) 1088 INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats", 1089 "Basic Block Placement Stats", false, false) 1090 1091 bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) { 1092 // Check for single-block functions and skip them. 1093 if (llvm::next(F.begin()) == F.end()) 1094 return false; 1095 1096 MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); 1097 MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 1098 1099 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { 1100 BlockFrequency BlockFreq = MBFI->getBlockFreq(I); 1101 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches 1102 : NumUncondBranches; 1103 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq 1104 : UncondBranchTakenFreq; 1105 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(), 1106 SE = I->succ_end(); 1107 SI != SE; ++SI) { 1108 // Skip if this successor is a fallthrough. 1109 if (I->isLayoutSuccessor(*SI)) 1110 continue; 1111 1112 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI); 1113 ++NumBranches; 1114 BranchTakenFreq += EdgeFreq.getFrequency(); 1115 } 1116 } 1117 1118 return false; 1119 } 1120 1121