MachineBlockPlacement.cpp revision b0dadb9dd52aed7a82e24542be8adf881d91c929
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/Support/ErrorHandling.h" 40#include "llvm/ADT/DenseMap.h" 41#include "llvm/ADT/PostOrderIterator.h" 42#include "llvm/ADT/SCCIterator.h" 43#include "llvm/ADT/SmallPtrSet.h" 44#include "llvm/ADT/SmallVector.h" 45#include "llvm/ADT/Statistic.h" 46#include "llvm/Target/TargetInstrInfo.h" 47#include "llvm/Target/TargetLowering.h" 48#include <algorithm> 49using namespace llvm; 50 51STATISTIC(NumCondBranches, "Number of conditional branches"); 52STATISTIC(NumUncondBranches, "Number of uncondittional branches"); 53STATISTIC(CondBranchTakenFreq, 54 "Potential frequency of taking conditional branches"); 55STATISTIC(UncondBranchTakenFreq, 56 "Potential frequency of taking unconditional branches"); 57 58namespace { 59/// \brief A structure for storing a weighted edge. 60/// 61/// This stores an edge and its weight, computed as the product of the 62/// frequency that the starting block is entered with the probability of 63/// a particular exit block. 64struct WeightedEdge { 65 BlockFrequency EdgeFrequency; 66 MachineBasicBlock *From, *To; 67 68 bool operator<(const WeightedEdge &RHS) const { 69 return EdgeFrequency < RHS.EdgeFrequency; 70 } 71}; 72} 73 74namespace { 75class BlockChain; 76/// \brief Type for our function-wide basic block -> block chain mapping. 77typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType; 78} 79 80namespace { 81/// \brief A chain of blocks which will be laid out contiguously. 82/// 83/// This is the datastructure representing a chain of consecutive blocks that 84/// are profitable to layout together in order to maximize fallthrough 85/// probabilities. We also can use a block chain to represent a sequence of 86/// basic blocks which have some external (correctness) requirement for 87/// sequential layout. 88/// 89/// Eventually, the block chains will form a directed graph over the function. 90/// We provide an SCC-supporting-iterator in order to quicky build and walk the 91/// SCCs of block chains within a function. 92/// 93/// The block chains also have support for calculating and caching probability 94/// information related to the chain itself versus other chains. This is used 95/// for ranking during the final layout of block chains. 96class BlockChain { 97 /// \brief The sequence of blocks belonging to this chain. 98 /// 99 /// This is the sequence of blocks for a particular chain. These will be laid 100 /// out in-order within the function. 101 SmallVector<MachineBasicBlock *, 4> Blocks; 102 103 /// \brief A handle to the function-wide basic block to block chain mapping. 104 /// 105 /// This is retained in each block chain to simplify the computation of child 106 /// block chains for SCC-formation and iteration. We store the edges to child 107 /// basic blocks, and map them back to their associated chains using this 108 /// structure. 109 BlockToChainMapType &BlockToChain; 110 111public: 112 /// \brief Construct a new BlockChain. 113 /// 114 /// This builds a new block chain representing a single basic block in the 115 /// function. It also registers itself as the chain that block participates 116 /// in with the BlockToChain mapping. 117 BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB) 118 : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) { 119 assert(BB && "Cannot create a chain with a null basic block"); 120 BlockToChain[BB] = this; 121 } 122 123 /// \brief Iterator over blocks within the chain. 124 typedef SmallVectorImpl<MachineBasicBlock *>::const_iterator iterator; 125 126 /// \brief Beginning of blocks within the chain. 127 iterator begin() const { return Blocks.begin(); } 128 129 /// \brief End of blocks within the chain. 130 iterator end() const { return Blocks.end(); } 131 132 /// \brief Merge a block chain into this one. 133 /// 134 /// This routine merges a block chain into this one. It takes care of forming 135 /// a contiguous sequence of basic blocks, updating the edge list, and 136 /// updating the block -> chain mapping. It does not free or tear down the 137 /// old chain, but the old chain's block list is no longer valid. 138 void merge(MachineBasicBlock *BB, BlockChain *Chain) { 139 assert(BB); 140 assert(!Blocks.empty()); 141 142 // Fast path in case we don't have a chain already. 143 if (!Chain) { 144 assert(!BlockToChain[BB]); 145 Blocks.push_back(BB); 146 BlockToChain[BB] = this; 147 return; 148 } 149 150 assert(BB == *Chain->begin()); 151 assert(Chain->begin() != Chain->end()); 152 153 // Update the incoming blocks to point to this chain, and add them to the 154 // chain structure. 155 for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end(); 156 BI != BE; ++BI) { 157 Blocks.push_back(*BI); 158 assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain"); 159 BlockToChain[*BI] = this; 160 } 161 } 162 163 /// \brief Count of predecessors within the loop currently being processed. 164 /// 165 /// This count is updated at each loop we process to represent the number of 166 /// in-loop predecessors of this chain. 167 unsigned LoopPredecessors; 168}; 169} 170 171namespace { 172class MachineBlockPlacement : public MachineFunctionPass { 173 /// \brief A typedef for a block filter set. 174 typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet; 175 176 /// \brief A handle to the branch probability pass. 177 const MachineBranchProbabilityInfo *MBPI; 178 179 /// \brief A handle to the function-wide block frequency pass. 180 const MachineBlockFrequencyInfo *MBFI; 181 182 /// \brief A handle to the loop info. 183 const MachineLoopInfo *MLI; 184 185 /// \brief A handle to the target's instruction info. 186 const TargetInstrInfo *TII; 187 188 /// \brief A handle to the target's lowering info. 189 const TargetLowering *TLI; 190 191 /// \brief Allocator and owner of BlockChain structures. 192 /// 193 /// We build BlockChains lazily by merging together high probability BB 194 /// sequences acording to the "Algo2" in the paper mentioned at the top of 195 /// the file. To reduce malloc traffic, we allocate them using this slab-like 196 /// allocator, and destroy them after the pass completes. 197 SpecificBumpPtrAllocator<BlockChain> ChainAllocator; 198 199 /// \brief Function wide BasicBlock to BlockChain mapping. 200 /// 201 /// This mapping allows efficiently moving from any given basic block to the 202 /// BlockChain it participates in, if any. We use it to, among other things, 203 /// allow implicitly defining edges between chains as the existing edges 204 /// between basic blocks. 205 DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain; 206 207 void markChainSuccessors(BlockChain &Chain, 208 MachineBasicBlock *LoopHeaderBB, 209 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, 210 const BlockFilterSet *BlockFilter = 0); 211 MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB, 212 BlockChain &Chain, 213 const BlockFilterSet *BlockFilter); 214 MachineBasicBlock *selectBestCandidateBlock( 215 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList, 216 const BlockFilterSet *BlockFilter); 217 MachineBasicBlock *getFirstUnplacedBlock( 218 MachineFunction &F, 219 const BlockChain &PlacedChain, 220 MachineFunction::iterator &PrevUnplacedBlockIt, 221 const BlockFilterSet *BlockFilter); 222 void buildChain(MachineBasicBlock *BB, BlockChain &Chain, 223 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, 224 const BlockFilterSet *BlockFilter = 0); 225 void buildLoopChains(MachineFunction &F, MachineLoop &L); 226 void buildCFGChains(MachineFunction &F); 227 void AlignLoops(MachineFunction &F); 228 229public: 230 static char ID; // Pass identification, replacement for typeid 231 MachineBlockPlacement() : MachineFunctionPass(ID) { 232 initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry()); 233 } 234 235 bool runOnMachineFunction(MachineFunction &F); 236 237 void getAnalysisUsage(AnalysisUsage &AU) const { 238 AU.addRequired<MachineBranchProbabilityInfo>(); 239 AU.addRequired<MachineBlockFrequencyInfo>(); 240 AU.addRequired<MachineLoopInfo>(); 241 MachineFunctionPass::getAnalysisUsage(AU); 242 } 243 244 const char *getPassName() const { return "Block Placement"; } 245}; 246} 247 248char MachineBlockPlacement::ID = 0; 249INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2", 250 "Branch Probability Basic Block Placement", false, false) 251INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) 252INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) 253INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) 254INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2", 255 "Branch Probability Basic Block Placement", false, false) 256 257FunctionPass *llvm::createMachineBlockPlacementPass() { 258 return new MachineBlockPlacement(); 259} 260 261#ifndef NDEBUG 262/// \brief Helper to print the name of a MBB. 263/// 264/// Only used by debug logging. 265static std::string getBlockName(MachineBasicBlock *BB) { 266 std::string Result; 267 raw_string_ostream OS(Result); 268 OS << "BB#" << BB->getNumber() 269 << " (derived from LLVM BB '" << BB->getName() << "')"; 270 OS.flush(); 271 return Result; 272} 273 274/// \brief Helper to print the number of a MBB. 275/// 276/// Only used by debug logging. 277static std::string getBlockNum(MachineBasicBlock *BB) { 278 std::string Result; 279 raw_string_ostream OS(Result); 280 OS << "BB#" << BB->getNumber(); 281 OS.flush(); 282 return Result; 283} 284#endif 285 286/// \brief Mark a chain's successors as having one fewer preds. 287/// 288/// When a chain is being merged into the "placed" chain, this routine will 289/// quickly walk the successors of each block in the chain and mark them as 290/// having one fewer active predecessor. It also adds any successors of this 291/// chain which reach the zero-predecessor state to the worklist passed in. 292void MachineBlockPlacement::markChainSuccessors( 293 BlockChain &Chain, 294 MachineBasicBlock *LoopHeaderBB, 295 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, 296 const BlockFilterSet *BlockFilter) { 297 // Walk all the blocks in this chain, marking their successors as having 298 // a predecessor placed. 299 for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end(); 300 CBI != CBE; ++CBI) { 301 // Add any successors for which this is the only un-placed in-loop 302 // predecessor to the worklist as a viable candidate for CFG-neutral 303 // placement. No subsequent placement of this block will violate the CFG 304 // shape, so we get to use heuristics to choose a favorable placement. 305 for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(), 306 SE = (*CBI)->succ_end(); 307 SI != SE; ++SI) { 308 if (BlockFilter && !BlockFilter->count(*SI)) 309 continue; 310 BlockChain &SuccChain = *BlockToChain[*SI]; 311 // Disregard edges within a fixed chain, or edges to the loop header. 312 if (&Chain == &SuccChain || *SI == LoopHeaderBB) 313 continue; 314 315 // This is a cross-chain edge that is within the loop, so decrement the 316 // loop predecessor count of the destination chain. 317 if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0) 318 BlockWorkList.push_back(*SI); 319 } 320 } 321} 322 323/// \brief Select the best successor for a block. 324/// 325/// This looks across all successors of a particular block and attempts to 326/// select the "best" one to be the layout successor. It only considers direct 327/// successors which also pass the block filter. It will attempt to avoid 328/// breaking CFG structure, but cave and break such structures in the case of 329/// very hot successor edges. 330/// 331/// \returns The best successor block found, or null if none are viable. 332MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor( 333 MachineBasicBlock *BB, BlockChain &Chain, 334 const BlockFilterSet *BlockFilter) { 335 const BranchProbability HotProb(4, 5); // 80% 336 337 MachineBasicBlock *BestSucc = 0; 338 // FIXME: Due to the performance of the probability and weight routines in 339 // the MBPI analysis, we manually compute probabilities using the edge 340 // weights. This is suboptimal as it means that the somewhat subtle 341 // definition of edge weight semantics is encoded here as well. We should 342 // improve the MBPI interface to effeciently support query patterns such as 343 // this. 344 uint32_t BestWeight = 0; 345 uint32_t WeightScale = 0; 346 uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale); 347 DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n"); 348 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(), 349 SE = BB->succ_end(); 350 SI != SE; ++SI) { 351 if (BlockFilter && !BlockFilter->count(*SI)) 352 continue; 353 BlockChain &SuccChain = *BlockToChain[*SI]; 354 if (&SuccChain == &Chain) { 355 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n"); 356 continue; 357 } 358 if (*SI != *SuccChain.begin()) { 359 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n"); 360 continue; 361 } 362 363 uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI); 364 BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight); 365 366 // Only consider successors which are either "hot", or wouldn't violate 367 // any CFG constraints. 368 if (SuccChain.LoopPredecessors != 0) { 369 if (SuccProb < HotProb) { 370 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n"); 371 continue; 372 } 373 374 // Make sure that a hot successor doesn't have a globally more important 375 // predecessor. 376 BlockFrequency CandidateEdgeFreq 377 = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl(); 378 bool BadCFGConflict = false; 379 for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(), 380 PE = (*SI)->pred_end(); 381 PI != PE; ++PI) { 382 if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) || 383 BlockToChain[*PI] == &Chain) 384 continue; 385 BlockFrequency PredEdgeFreq 386 = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI); 387 if (PredEdgeFreq >= CandidateEdgeFreq) { 388 BadCFGConflict = true; 389 break; 390 } 391 } 392 if (BadCFGConflict) { 393 DEBUG(dbgs() << " " << getBlockName(*SI) 394 << " -> non-cold CFG conflict\n"); 395 continue; 396 } 397 } 398 399 DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb 400 << " (prob)" 401 << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "") 402 << "\n"); 403 if (BestSucc && BestWeight >= SuccWeight) 404 continue; 405 BestSucc = *SI; 406 BestWeight = SuccWeight; 407 } 408 return BestSucc; 409} 410 411namespace { 412/// \brief Predicate struct to detect blocks already placed. 413class IsBlockPlaced { 414 const BlockChain &PlacedChain; 415 const BlockToChainMapType &BlockToChain; 416 417public: 418 IsBlockPlaced(const BlockChain &PlacedChain, 419 const BlockToChainMapType &BlockToChain) 420 : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {} 421 422 bool operator()(MachineBasicBlock *BB) const { 423 return BlockToChain.lookup(BB) == &PlacedChain; 424 } 425}; 426} 427 428/// \brief Select the best block from a worklist. 429/// 430/// This looks through the provided worklist as a list of candidate basic 431/// blocks and select the most profitable one to place. The definition of 432/// profitable only really makes sense in the context of a loop. This returns 433/// the most frequently visited block in the worklist, which in the case of 434/// a loop, is the one most desirable to be physically close to the rest of the 435/// loop body in order to improve icache behavior. 436/// 437/// \returns The best block found, or null if none are viable. 438MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock( 439 BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList, 440 const BlockFilterSet *BlockFilter) { 441 // Once we need to walk the worklist looking for a candidate, cleanup the 442 // worklist of already placed entries. 443 // FIXME: If this shows up on profiles, it could be folded (at the cost of 444 // some code complexity) into the loop below. 445 WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(), 446 IsBlockPlaced(Chain, BlockToChain)), 447 WorkList.end()); 448 449 MachineBasicBlock *BestBlock = 0; 450 BlockFrequency BestFreq; 451 for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(), 452 WBE = WorkList.end(); 453 WBI != WBE; ++WBI) { 454 assert(!BlockFilter || BlockFilter->count(*WBI)); 455 BlockChain &SuccChain = *BlockToChain[*WBI]; 456 if (&SuccChain == &Chain) { 457 DEBUG(dbgs() << " " << getBlockName(*WBI) 458 << " -> Already merged!\n"); 459 continue; 460 } 461 assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block"); 462 463 BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI); 464 DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq 465 << " (freq)\n"); 466 if (BestBlock && BestFreq >= CandidateFreq) 467 continue; 468 BestBlock = *WBI; 469 BestFreq = CandidateFreq; 470 } 471 return BestBlock; 472} 473 474/// \brief Retrieve the first unplaced basic block. 475/// 476/// This routine is called when we are unable to use the CFG to walk through 477/// all of the basic blocks and form a chain due to unnatural loops in the CFG. 478/// We walk through the function's blocks in order, starting from the 479/// LastUnplacedBlockIt. We update this iterator on each call to avoid 480/// re-scanning the entire sequence on repeated calls to this routine. 481MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock( 482 MachineFunction &F, const BlockChain &PlacedChain, 483 MachineFunction::iterator &PrevUnplacedBlockIt, 484 const BlockFilterSet *BlockFilter) { 485 for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E; 486 ++I) { 487 if (BlockFilter && !BlockFilter->count(I)) 488 continue; 489 if (BlockToChain[I] != &PlacedChain) { 490 PrevUnplacedBlockIt = I; 491 return I; 492 } 493 } 494 return 0; 495} 496 497void MachineBlockPlacement::buildChain( 498 MachineBasicBlock *BB, 499 BlockChain &Chain, 500 SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, 501 const BlockFilterSet *BlockFilter) { 502 assert(BB); 503 assert(BlockToChain[BB] == &Chain); 504 assert(*Chain.begin() == BB); 505 MachineFunction &F = *BB->getParent(); 506 MachineFunction::iterator PrevUnplacedBlockIt = F.begin(); 507 508 MachineBasicBlock *LoopHeaderBB = BB; 509 markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter); 510 BB = *llvm::prior(Chain.end()); 511 for (;;) { 512 assert(BB); 513 assert(BlockToChain[BB] == &Chain); 514 assert(*llvm::prior(Chain.end()) == BB); 515 MachineBasicBlock *BestSucc = 0; 516 517 // Look for the best viable successor if there is one to place immediately 518 // after this block. 519 BestSucc = selectBestSuccessor(BB, Chain, BlockFilter); 520 521 // If an immediate successor isn't available, look for the best viable 522 // block among those we've identified as not violating the loop's CFG at 523 // this point. This won't be a fallthrough, but it will increase locality. 524 if (!BestSucc) 525 BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter); 526 527 if (!BestSucc) { 528 BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt, 529 BlockFilter); 530 if (!BestSucc) 531 break; 532 533 DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the " 534 "layout successor until the CFG reduces\n"); 535 } 536 537 // Place this block, updating the datastructures to reflect its placement. 538 BlockChain &SuccChain = *BlockToChain[BestSucc]; 539 // Zero out LoopPredecessors for the successor we're about to merge in case 540 // we selected a successor that didn't fit naturally into the CFG. 541 SuccChain.LoopPredecessors = 0; 542 DEBUG(dbgs() << "Merging from " << getBlockNum(BB) 543 << " to " << getBlockNum(BestSucc) << "\n"); 544 markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter); 545 Chain.merge(BestSucc, &SuccChain); 546 BB = *llvm::prior(Chain.end()); 547 }; 548 549 DEBUG(dbgs() << "Finished forming chain for header block " 550 << getBlockNum(*Chain.begin()) << "\n"); 551} 552 553/// \brief Forms basic block chains from the natural loop structures. 554/// 555/// These chains are designed to preserve the existing *structure* of the code 556/// as much as possible. We can then stitch the chains together in a way which 557/// both preserves the topological structure and minimizes taken conditional 558/// branches. 559void MachineBlockPlacement::buildLoopChains(MachineFunction &F, 560 MachineLoop &L) { 561 // First recurse through any nested loops, building chains for those inner 562 // loops. 563 for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI) 564 buildLoopChains(F, **LI); 565 566 SmallVector<MachineBasicBlock *, 16> BlockWorkList; 567 BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end()); 568 BlockChain &LoopChain = *BlockToChain[L.getHeader()]; 569 570 // FIXME: This is a really lame way of walking the chains in the loop: we 571 // walk the blocks, and use a set to prevent visiting a particular chain 572 // twice. 573 SmallPtrSet<BlockChain *, 4> UpdatedPreds; 574 for (MachineLoop::block_iterator BI = L.block_begin(), 575 BE = L.block_end(); 576 BI != BE; ++BI) { 577 BlockChain &Chain = *BlockToChain[*BI]; 578 if (!UpdatedPreds.insert(&Chain) || BI == L.block_begin()) 579 continue; 580 581 assert(Chain.LoopPredecessors == 0); 582 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end(); 583 BCI != BCE; ++BCI) { 584 assert(BlockToChain[*BCI] == &Chain); 585 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(), 586 PE = (*BCI)->pred_end(); 587 PI != PE; ++PI) { 588 if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI)) 589 continue; 590 ++Chain.LoopPredecessors; 591 } 592 } 593 594 if (Chain.LoopPredecessors == 0) 595 BlockWorkList.push_back(*BI); 596 } 597 598 buildChain(*L.block_begin(), LoopChain, BlockWorkList, &LoopBlockSet); 599 600 DEBUG({ 601 // Crash at the end so we get all of the debugging output first. 602 bool BadLoop = false; 603 if (LoopChain.LoopPredecessors) { 604 BadLoop = true; 605 dbgs() << "Loop chain contains a block without its preds placed!\n" 606 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 607 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"; 608 } 609 for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end(); 610 BCI != BCE; ++BCI) 611 if (!LoopBlockSet.erase(*BCI)) { 612 // We don't mark the loop as bad here because there are real situations 613 // where this can occur. For example, with an unanalyzable fallthrough 614 // from a loop block to a non-loop block. 615 // FIXME: Such constructs shouldn't exist. Track them down and fix them. 616 dbgs() << "Loop chain contains a block not contained by the loop!\n" 617 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 618 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n" 619 << " Bad block: " << getBlockName(*BCI) << "\n"; 620 } 621 622 if (!LoopBlockSet.empty()) { 623 BadLoop = true; 624 for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(), 625 LBE = LoopBlockSet.end(); 626 LBI != LBE; ++LBI) 627 dbgs() << "Loop contains blocks never placed into a chain!\n" 628 << " Loop header: " << getBlockName(*L.block_begin()) << "\n" 629 << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n" 630 << " Bad block: " << getBlockName(*LBI) << "\n"; 631 } 632 assert(!BadLoop && "Detected problems with the placement of this loop."); 633 }); 634} 635 636void MachineBlockPlacement::buildCFGChains(MachineFunction &F) { 637 // Ensure that every BB in the function has an associated chain to simplify 638 // the assumptions of the remaining algorithm. 639 SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch. 640 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) { 641 MachineBasicBlock *BB = FI; 642 BlockChain *&Chain = BlockToChain[BB]; 643 Chain = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB); 644 // Also, merge any blocks which we cannot reason about and must preserve 645 // the exact fallthrough behavior for. 646 for (;;) { 647 Cond.clear(); 648 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch. 649 if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough()) 650 break; 651 652 MachineFunction::iterator NextFI(llvm::next(FI)); 653 MachineBasicBlock *NextBB = NextFI; 654 // Ensure that the layout successor is a viable block, as we know that 655 // fallthrough is a possibility. 656 assert(NextFI != FE && "Can't fallthrough past the last block."); 657 DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: " 658 << getBlockName(BB) << " -> " << getBlockName(NextBB) 659 << "\n"); 660 Chain->merge(NextBB, 0); 661 FI = NextFI; 662 BB = NextBB; 663 } 664 } 665 666 // Build any loop-based chains. 667 for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE; 668 ++LI) 669 buildLoopChains(F, **LI); 670 671 SmallVector<MachineBasicBlock *, 16> BlockWorkList; 672 673 SmallPtrSet<BlockChain *, 4> UpdatedPreds; 674 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) { 675 MachineBasicBlock *BB = &*FI; 676 BlockChain &Chain = *BlockToChain[BB]; 677 if (!UpdatedPreds.insert(&Chain)) 678 continue; 679 680 assert(Chain.LoopPredecessors == 0); 681 for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end(); 682 BCI != BCE; ++BCI) { 683 assert(BlockToChain[*BCI] == &Chain); 684 for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(), 685 PE = (*BCI)->pred_end(); 686 PI != PE; ++PI) { 687 if (BlockToChain[*PI] == &Chain) 688 continue; 689 ++Chain.LoopPredecessors; 690 } 691 } 692 693 if (Chain.LoopPredecessors == 0) 694 BlockWorkList.push_back(BB); 695 } 696 697 BlockChain &FunctionChain = *BlockToChain[&F.front()]; 698 buildChain(&F.front(), FunctionChain, BlockWorkList); 699 700 typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType; 701 DEBUG({ 702 // Crash at the end so we get all of the debugging output first. 703 bool BadFunc = false; 704 FunctionBlockSetType FunctionBlockSet; 705 for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) 706 FunctionBlockSet.insert(FI); 707 708 for (BlockChain::iterator BCI = FunctionChain.begin(), 709 BCE = FunctionChain.end(); 710 BCI != BCE; ++BCI) 711 if (!FunctionBlockSet.erase(*BCI)) { 712 BadFunc = true; 713 dbgs() << "Function chain contains a block not in the function!\n" 714 << " Bad block: " << getBlockName(*BCI) << "\n"; 715 } 716 717 if (!FunctionBlockSet.empty()) { 718 BadFunc = true; 719 for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(), 720 FBE = FunctionBlockSet.end(); 721 FBI != FBE; ++FBI) 722 dbgs() << "Function contains blocks never placed into a chain!\n" 723 << " Bad block: " << getBlockName(*FBI) << "\n"; 724 } 725 assert(!BadFunc && "Detected problems with the block placement."); 726 }); 727 728 // Splice the blocks into place. 729 MachineFunction::iterator InsertPos = F.begin(); 730 for (BlockChain::iterator BI = FunctionChain.begin(), 731 BE = FunctionChain.end(); 732 BI != BE; ++BI) { 733 DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain " 734 : " ... ") 735 << getBlockName(*BI) << "\n"); 736 if (InsertPos != MachineFunction::iterator(*BI)) 737 F.splice(InsertPos, *BI); 738 else 739 ++InsertPos; 740 741 // Update the terminator of the previous block. 742 if (BI == FunctionChain.begin()) 743 continue; 744 MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI)); 745 746 // FIXME: It would be awesome of updateTerminator would just return rather 747 // than assert when the branch cannot be analyzed in order to remove this 748 // boiler plate. 749 Cond.clear(); 750 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch. 751 if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) 752 PrevBB->updateTerminator(); 753 } 754 755 // Fixup the last block. 756 Cond.clear(); 757 MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch. 758 if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond)) 759 F.back().updateTerminator(); 760} 761 762/// \brief Recursive helper to align a loop and any nested loops. 763static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) { 764 // Recurse through nested loops. 765 for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I) 766 AlignLoop(F, *I, Align); 767 768 L->getTopBlock()->setAlignment(Align); 769} 770 771/// \brief Align loop headers to target preferred alignments. 772void MachineBlockPlacement::AlignLoops(MachineFunction &F) { 773 if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize)) 774 return; 775 776 unsigned Align = TLI->getPrefLoopAlignment(); 777 if (!Align) 778 return; // Don't care about loop alignment. 779 780 for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I) 781 AlignLoop(F, *I, Align); 782} 783 784bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) { 785 // Check for single-block functions and skip them. 786 if (llvm::next(F.begin()) == F.end()) 787 return false; 788 789 MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); 790 MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 791 MLI = &getAnalysis<MachineLoopInfo>(); 792 TII = F.getTarget().getInstrInfo(); 793 TLI = F.getTarget().getTargetLowering(); 794 assert(BlockToChain.empty()); 795 796 buildCFGChains(F); 797 AlignLoops(F); 798 799 BlockToChain.clear(); 800 ChainAllocator.DestroyAll(); 801 802 // We always return true as we have no way to track whether the final order 803 // differs from the original order. 804 return true; 805} 806 807namespace { 808/// \brief A pass to compute block placement statistics. 809/// 810/// A separate pass to compute interesting statistics for evaluating block 811/// placement. This is separate from the actual placement pass so that they can 812/// be computed in the absense of any placement transformations or when using 813/// alternative placement strategies. 814class MachineBlockPlacementStats : public MachineFunctionPass { 815 /// \brief A handle to the branch probability pass. 816 const MachineBranchProbabilityInfo *MBPI; 817 818 /// \brief A handle to the function-wide block frequency pass. 819 const MachineBlockFrequencyInfo *MBFI; 820 821public: 822 static char ID; // Pass identification, replacement for typeid 823 MachineBlockPlacementStats() : MachineFunctionPass(ID) { 824 initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry()); 825 } 826 827 bool runOnMachineFunction(MachineFunction &F); 828 829 void getAnalysisUsage(AnalysisUsage &AU) const { 830 AU.addRequired<MachineBranchProbabilityInfo>(); 831 AU.addRequired<MachineBlockFrequencyInfo>(); 832 AU.setPreservesAll(); 833 MachineFunctionPass::getAnalysisUsage(AU); 834 } 835 836 const char *getPassName() const { return "Block Placement Stats"; } 837}; 838} 839 840char MachineBlockPlacementStats::ID = 0; 841INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats", 842 "Basic Block Placement Stats", false, false) 843INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) 844INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) 845INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats", 846 "Basic Block Placement Stats", false, false) 847 848FunctionPass *llvm::createMachineBlockPlacementStatsPass() { 849 return new MachineBlockPlacementStats(); 850} 851 852bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) { 853 // Check for single-block functions and skip them. 854 if (llvm::next(F.begin()) == F.end()) 855 return false; 856 857 MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); 858 MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 859 860 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { 861 BlockFrequency BlockFreq = MBFI->getBlockFreq(I); 862 Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches 863 : NumUncondBranches; 864 Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq 865 : UncondBranchTakenFreq; 866 for (MachineBasicBlock::succ_iterator SI = I->succ_begin(), 867 SE = I->succ_end(); 868 SI != SE; ++SI) { 869 // Skip if this successor is a fallthrough. 870 if (I->isLayoutSuccessor(*SI)) 871 continue; 872 873 BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI); 874 ++NumBranches; 875 BranchTakenFreq += EdgeFreq.getFrequency(); 876 } 877 } 878 879 return false; 880} 881 882