1//===- SampleProfile.cpp - Incorporate sample profiles into the IR --------===// 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 the SampleProfileLoader transformation. This pass 11// reads a profile file generated by a sampling profiler (e.g. Linux Perf - 12// http://perf.wiki.kernel.org/) and generates IR metadata to reflect the 13// profile information in the given profile. 14// 15// This pass generates branch weight annotations on the IR: 16// 17// - prof: Represents branch weights. This annotation is added to branches 18// to indicate the weights of each edge coming out of the branch. 19// The weight of each edge is the weight of the target block for 20// that edge. The weight of a block B is computed as the maximum 21// number of samples found in B. 22// 23//===----------------------------------------------------------------------===// 24 25#include "llvm/Transforms/Scalar.h" 26#include "llvm/ADT/DenseMap.h" 27#include "llvm/ADT/SmallPtrSet.h" 28#include "llvm/ADT/SmallSet.h" 29#include "llvm/ADT/StringRef.h" 30#include "llvm/Analysis/LoopInfo.h" 31#include "llvm/Analysis/PostDominators.h" 32#include "llvm/IR/Constants.h" 33#include "llvm/IR/DebugInfo.h" 34#include "llvm/IR/DiagnosticInfo.h" 35#include "llvm/IR/Dominators.h" 36#include "llvm/IR/Function.h" 37#include "llvm/IR/InstIterator.h" 38#include "llvm/IR/Instructions.h" 39#include "llvm/IR/LLVMContext.h" 40#include "llvm/IR/MDBuilder.h" 41#include "llvm/IR/Metadata.h" 42#include "llvm/IR/Module.h" 43#include "llvm/Pass.h" 44#include "llvm/ProfileData/SampleProfReader.h" 45#include "llvm/Support/CommandLine.h" 46#include "llvm/Support/Debug.h" 47#include "llvm/Support/raw_ostream.h" 48#include <cctype> 49 50using namespace llvm; 51using namespace sampleprof; 52 53#define DEBUG_TYPE "sample-profile" 54 55// Command line option to specify the file to read samples from. This is 56// mainly used for debugging. 57static cl::opt<std::string> SampleProfileFile( 58 "sample-profile-file", cl::init(""), cl::value_desc("filename"), 59 cl::desc("Profile file loaded by -sample-profile"), cl::Hidden); 60static cl::opt<unsigned> SampleProfileMaxPropagateIterations( 61 "sample-profile-max-propagate-iterations", cl::init(100), 62 cl::desc("Maximum number of iterations to go through when propagating " 63 "sample block/edge weights through the CFG.")); 64 65namespace { 66typedef DenseMap<BasicBlock *, unsigned> BlockWeightMap; 67typedef DenseMap<BasicBlock *, BasicBlock *> EquivalenceClassMap; 68typedef std::pair<BasicBlock *, BasicBlock *> Edge; 69typedef DenseMap<Edge, unsigned> EdgeWeightMap; 70typedef DenseMap<BasicBlock *, SmallVector<BasicBlock *, 8>> BlockEdgeMap; 71 72/// \brief Sample profile pass. 73/// 74/// This pass reads profile data from the file specified by 75/// -sample-profile-file and annotates every affected function with the 76/// profile information found in that file. 77class SampleProfileLoader : public FunctionPass { 78public: 79 // Class identification, replacement for typeinfo 80 static char ID; 81 82 SampleProfileLoader(StringRef Name = SampleProfileFile) 83 : FunctionPass(ID), DT(nullptr), PDT(nullptr), LI(nullptr), Ctx(nullptr), 84 Reader(), Samples(nullptr), Filename(Name), ProfileIsValid(false) { 85 initializeSampleProfileLoaderPass(*PassRegistry::getPassRegistry()); 86 } 87 88 bool doInitialization(Module &M) override; 89 90 void dump() { Reader->dump(); } 91 92 const char *getPassName() const override { return "Sample profile pass"; } 93 94 bool runOnFunction(Function &F) override; 95 96 void getAnalysisUsage(AnalysisUsage &AU) const override { 97 AU.setPreservesCFG(); 98 AU.addRequired<LoopInfoWrapperPass>(); 99 AU.addRequired<DominatorTreeWrapperPass>(); 100 AU.addRequired<PostDominatorTree>(); 101 } 102 103protected: 104 unsigned getFunctionLoc(Function &F); 105 bool emitAnnotations(Function &F); 106 unsigned getInstWeight(Instruction &I); 107 unsigned getBlockWeight(BasicBlock *BB); 108 void printEdgeWeight(raw_ostream &OS, Edge E); 109 void printBlockWeight(raw_ostream &OS, BasicBlock *BB); 110 void printBlockEquivalence(raw_ostream &OS, BasicBlock *BB); 111 bool computeBlockWeights(Function &F); 112 void findEquivalenceClasses(Function &F); 113 void findEquivalencesFor(BasicBlock *BB1, 114 SmallVector<BasicBlock *, 8> Descendants, 115 DominatorTreeBase<BasicBlock> *DomTree); 116 void propagateWeights(Function &F); 117 unsigned visitEdge(Edge E, unsigned *NumUnknownEdges, Edge *UnknownEdge); 118 void buildEdges(Function &F); 119 bool propagateThroughEdges(Function &F); 120 121 /// \brief Line number for the function header. Used to compute absolute 122 /// line numbers from the relative line numbers found in the profile. 123 unsigned HeaderLineno; 124 125 /// \brief Map basic blocks to their computed weights. 126 /// 127 /// The weight of a basic block is defined to be the maximum 128 /// of all the instruction weights in that block. 129 BlockWeightMap BlockWeights; 130 131 /// \brief Map edges to their computed weights. 132 /// 133 /// Edge weights are computed by propagating basic block weights in 134 /// SampleProfile::propagateWeights. 135 EdgeWeightMap EdgeWeights; 136 137 /// \brief Set of visited blocks during propagation. 138 SmallPtrSet<BasicBlock *, 128> VisitedBlocks; 139 140 /// \brief Set of visited edges during propagation. 141 SmallSet<Edge, 128> VisitedEdges; 142 143 /// \brief Equivalence classes for block weights. 144 /// 145 /// Two blocks BB1 and BB2 are in the same equivalence class if they 146 /// dominate and post-dominate each other, and they are in the same loop 147 /// nest. When this happens, the two blocks are guaranteed to execute 148 /// the same number of times. 149 EquivalenceClassMap EquivalenceClass; 150 151 /// \brief Dominance, post-dominance and loop information. 152 DominatorTree *DT; 153 PostDominatorTree *PDT; 154 LoopInfo *LI; 155 156 /// \brief Predecessors for each basic block in the CFG. 157 BlockEdgeMap Predecessors; 158 159 /// \brief Successors for each basic block in the CFG. 160 BlockEdgeMap Successors; 161 162 /// \brief LLVM context holding the debug data we need. 163 LLVMContext *Ctx; 164 165 /// \brief Profile reader object. 166 std::unique_ptr<SampleProfileReader> Reader; 167 168 /// \brief Samples collected for the body of this function. 169 FunctionSamples *Samples; 170 171 /// \brief Name of the profile file to load. 172 StringRef Filename; 173 174 /// \brief Flag indicating whether the profile input loaded successfully. 175 bool ProfileIsValid; 176}; 177} 178 179/// \brief Print the weight of edge \p E on stream \p OS. 180/// 181/// \param OS Stream to emit the output to. 182/// \param E Edge to print. 183void SampleProfileLoader::printEdgeWeight(raw_ostream &OS, Edge E) { 184 OS << "weight[" << E.first->getName() << "->" << E.second->getName() 185 << "]: " << EdgeWeights[E] << "\n"; 186} 187 188/// \brief Print the equivalence class of block \p BB on stream \p OS. 189/// 190/// \param OS Stream to emit the output to. 191/// \param BB Block to print. 192void SampleProfileLoader::printBlockEquivalence(raw_ostream &OS, 193 BasicBlock *BB) { 194 BasicBlock *Equiv = EquivalenceClass[BB]; 195 OS << "equivalence[" << BB->getName() 196 << "]: " << ((Equiv) ? EquivalenceClass[BB]->getName() : "NONE") << "\n"; 197} 198 199/// \brief Print the weight of block \p BB on stream \p OS. 200/// 201/// \param OS Stream to emit the output to. 202/// \param BB Block to print. 203void SampleProfileLoader::printBlockWeight(raw_ostream &OS, BasicBlock *BB) { 204 OS << "weight[" << BB->getName() << "]: " << BlockWeights[BB] << "\n"; 205} 206 207/// \brief Get the weight for an instruction. 208/// 209/// The "weight" of an instruction \p Inst is the number of samples 210/// collected on that instruction at runtime. To retrieve it, we 211/// need to compute the line number of \p Inst relative to the start of its 212/// function. We use HeaderLineno to compute the offset. We then 213/// look up the samples collected for \p Inst using BodySamples. 214/// 215/// \param Inst Instruction to query. 216/// 217/// \returns The profiled weight of I. 218unsigned SampleProfileLoader::getInstWeight(Instruction &Inst) { 219 DebugLoc DLoc = Inst.getDebugLoc(); 220 if (!DLoc) 221 return 0; 222 223 unsigned Lineno = DLoc.getLine(); 224 if (Lineno < HeaderLineno) 225 return 0; 226 227 DILocation DIL = DLoc.get(); 228 int LOffset = Lineno - HeaderLineno; 229 unsigned Discriminator = DIL->getDiscriminator(); 230 unsigned Weight = Samples->samplesAt(LOffset, Discriminator); 231 DEBUG(dbgs() << " " << Lineno << "." << Discriminator << ":" << Inst 232 << " (line offset: " << LOffset << "." << Discriminator 233 << " - weight: " << Weight << ")\n"); 234 return Weight; 235} 236 237/// \brief Compute the weight of a basic block. 238/// 239/// The weight of basic block \p BB is the maximum weight of all the 240/// instructions in BB. The weight of \p BB is computed and cached in 241/// the BlockWeights map. 242/// 243/// \param BB The basic block to query. 244/// 245/// \returns The computed weight of BB. 246unsigned SampleProfileLoader::getBlockWeight(BasicBlock *BB) { 247 // If we've computed BB's weight before, return it. 248 std::pair<BlockWeightMap::iterator, bool> Entry = 249 BlockWeights.insert(std::make_pair(BB, 0)); 250 if (!Entry.second) 251 return Entry.first->second; 252 253 // Otherwise, compute and cache BB's weight. 254 unsigned Weight = 0; 255 for (auto &I : BB->getInstList()) { 256 unsigned InstWeight = getInstWeight(I); 257 if (InstWeight > Weight) 258 Weight = InstWeight; 259 } 260 Entry.first->second = Weight; 261 return Weight; 262} 263 264/// \brief Compute and store the weights of every basic block. 265/// 266/// This populates the BlockWeights map by computing 267/// the weights of every basic block in the CFG. 268/// 269/// \param F The function to query. 270bool SampleProfileLoader::computeBlockWeights(Function &F) { 271 bool Changed = false; 272 DEBUG(dbgs() << "Block weights\n"); 273 for (auto &BB : F) { 274 unsigned Weight = getBlockWeight(&BB); 275 Changed |= (Weight > 0); 276 DEBUG(printBlockWeight(dbgs(), &BB)); 277 } 278 279 return Changed; 280} 281 282/// \brief Find equivalence classes for the given block. 283/// 284/// This finds all the blocks that are guaranteed to execute the same 285/// number of times as \p BB1. To do this, it traverses all the the 286/// descendants of \p BB1 in the dominator or post-dominator tree. 287/// 288/// A block BB2 will be in the same equivalence class as \p BB1 if 289/// the following holds: 290/// 291/// 1- \p BB1 is a descendant of BB2 in the opposite tree. So, if BB2 292/// is a descendant of \p BB1 in the dominator tree, then BB2 should 293/// dominate BB1 in the post-dominator tree. 294/// 295/// 2- Both BB2 and \p BB1 must be in the same loop. 296/// 297/// For every block BB2 that meets those two requirements, we set BB2's 298/// equivalence class to \p BB1. 299/// 300/// \param BB1 Block to check. 301/// \param Descendants Descendants of \p BB1 in either the dom or pdom tree. 302/// \param DomTree Opposite dominator tree. If \p Descendants is filled 303/// with blocks from \p BB1's dominator tree, then 304/// this is the post-dominator tree, and vice versa. 305void SampleProfileLoader::findEquivalencesFor( 306 BasicBlock *BB1, SmallVector<BasicBlock *, 8> Descendants, 307 DominatorTreeBase<BasicBlock> *DomTree) { 308 for (auto *BB2 : Descendants) { 309 bool IsDomParent = DomTree->dominates(BB2, BB1); 310 bool IsInSameLoop = LI->getLoopFor(BB1) == LI->getLoopFor(BB2); 311 if (BB1 != BB2 && VisitedBlocks.insert(BB2).second && IsDomParent && 312 IsInSameLoop) { 313 EquivalenceClass[BB2] = BB1; 314 315 // If BB2 is heavier than BB1, make BB2 have the same weight 316 // as BB1. 317 // 318 // Note that we don't worry about the opposite situation here 319 // (when BB2 is lighter than BB1). We will deal with this 320 // during the propagation phase. Right now, we just want to 321 // make sure that BB1 has the largest weight of all the 322 // members of its equivalence set. 323 unsigned &BB1Weight = BlockWeights[BB1]; 324 unsigned &BB2Weight = BlockWeights[BB2]; 325 BB1Weight = std::max(BB1Weight, BB2Weight); 326 } 327 } 328} 329 330/// \brief Find equivalence classes. 331/// 332/// Since samples may be missing from blocks, we can fill in the gaps by setting 333/// the weights of all the blocks in the same equivalence class to the same 334/// weight. To compute the concept of equivalence, we use dominance and loop 335/// information. Two blocks B1 and B2 are in the same equivalence class if B1 336/// dominates B2, B2 post-dominates B1 and both are in the same loop. 337/// 338/// \param F The function to query. 339void SampleProfileLoader::findEquivalenceClasses(Function &F) { 340 SmallVector<BasicBlock *, 8> DominatedBBs; 341 DEBUG(dbgs() << "\nBlock equivalence classes\n"); 342 // Find equivalence sets based on dominance and post-dominance information. 343 for (auto &BB : F) { 344 BasicBlock *BB1 = &BB; 345 346 // Compute BB1's equivalence class once. 347 if (EquivalenceClass.count(BB1)) { 348 DEBUG(printBlockEquivalence(dbgs(), BB1)); 349 continue; 350 } 351 352 // By default, blocks are in their own equivalence class. 353 EquivalenceClass[BB1] = BB1; 354 355 // Traverse all the blocks dominated by BB1. We are looking for 356 // every basic block BB2 such that: 357 // 358 // 1- BB1 dominates BB2. 359 // 2- BB2 post-dominates BB1. 360 // 3- BB1 and BB2 are in the same loop nest. 361 // 362 // If all those conditions hold, it means that BB2 is executed 363 // as many times as BB1, so they are placed in the same equivalence 364 // class by making BB2's equivalence class be BB1. 365 DominatedBBs.clear(); 366 DT->getDescendants(BB1, DominatedBBs); 367 findEquivalencesFor(BB1, DominatedBBs, PDT->DT); 368 369 // Repeat the same logic for all the blocks post-dominated by BB1. 370 // We are looking for every basic block BB2 such that: 371 // 372 // 1- BB1 post-dominates BB2. 373 // 2- BB2 dominates BB1. 374 // 3- BB1 and BB2 are in the same loop nest. 375 // 376 // If all those conditions hold, BB2's equivalence class is BB1. 377 DominatedBBs.clear(); 378 PDT->getDescendants(BB1, DominatedBBs); 379 findEquivalencesFor(BB1, DominatedBBs, DT); 380 381 DEBUG(printBlockEquivalence(dbgs(), BB1)); 382 } 383 384 // Assign weights to equivalence classes. 385 // 386 // All the basic blocks in the same equivalence class will execute 387 // the same number of times. Since we know that the head block in 388 // each equivalence class has the largest weight, assign that weight 389 // to all the blocks in that equivalence class. 390 DEBUG(dbgs() << "\nAssign the same weight to all blocks in the same class\n"); 391 for (auto &BI : F) { 392 BasicBlock *BB = &BI; 393 BasicBlock *EquivBB = EquivalenceClass[BB]; 394 if (BB != EquivBB) 395 BlockWeights[BB] = BlockWeights[EquivBB]; 396 DEBUG(printBlockWeight(dbgs(), BB)); 397 } 398} 399 400/// \brief Visit the given edge to decide if it has a valid weight. 401/// 402/// If \p E has not been visited before, we copy to \p UnknownEdge 403/// and increment the count of unknown edges. 404/// 405/// \param E Edge to visit. 406/// \param NumUnknownEdges Current number of unknown edges. 407/// \param UnknownEdge Set if E has not been visited before. 408/// 409/// \returns E's weight, if known. Otherwise, return 0. 410unsigned SampleProfileLoader::visitEdge(Edge E, unsigned *NumUnknownEdges, 411 Edge *UnknownEdge) { 412 if (!VisitedEdges.count(E)) { 413 (*NumUnknownEdges)++; 414 *UnknownEdge = E; 415 return 0; 416 } 417 418 return EdgeWeights[E]; 419} 420 421/// \brief Propagate weights through incoming/outgoing edges. 422/// 423/// If the weight of a basic block is known, and there is only one edge 424/// with an unknown weight, we can calculate the weight of that edge. 425/// 426/// Similarly, if all the edges have a known count, we can calculate the 427/// count of the basic block, if needed. 428/// 429/// \param F Function to process. 430/// 431/// \returns True if new weights were assigned to edges or blocks. 432bool SampleProfileLoader::propagateThroughEdges(Function &F) { 433 bool Changed = false; 434 DEBUG(dbgs() << "\nPropagation through edges\n"); 435 for (auto &BI : F) { 436 BasicBlock *BB = &BI; 437 438 // Visit all the predecessor and successor edges to determine 439 // which ones have a weight assigned already. Note that it doesn't 440 // matter that we only keep track of a single unknown edge. The 441 // only case we are interested in handling is when only a single 442 // edge is unknown (see setEdgeOrBlockWeight). 443 for (unsigned i = 0; i < 2; i++) { 444 unsigned TotalWeight = 0; 445 unsigned NumUnknownEdges = 0; 446 Edge UnknownEdge, SelfReferentialEdge; 447 448 if (i == 0) { 449 // First, visit all predecessor edges. 450 for (auto *Pred : Predecessors[BB]) { 451 Edge E = std::make_pair(Pred, BB); 452 TotalWeight += visitEdge(E, &NumUnknownEdges, &UnknownEdge); 453 if (E.first == E.second) 454 SelfReferentialEdge = E; 455 } 456 } else { 457 // On the second round, visit all successor edges. 458 for (auto *Succ : Successors[BB]) { 459 Edge E = std::make_pair(BB, Succ); 460 TotalWeight += visitEdge(E, &NumUnknownEdges, &UnknownEdge); 461 } 462 } 463 464 // After visiting all the edges, there are three cases that we 465 // can handle immediately: 466 // 467 // - All the edge weights are known (i.e., NumUnknownEdges == 0). 468 // In this case, we simply check that the sum of all the edges 469 // is the same as BB's weight. If not, we change BB's weight 470 // to match. Additionally, if BB had not been visited before, 471 // we mark it visited. 472 // 473 // - Only one edge is unknown and BB has already been visited. 474 // In this case, we can compute the weight of the edge by 475 // subtracting the total block weight from all the known 476 // edge weights. If the edges weight more than BB, then the 477 // edge of the last remaining edge is set to zero. 478 // 479 // - There exists a self-referential edge and the weight of BB is 480 // known. In this case, this edge can be based on BB's weight. 481 // We add up all the other known edges and set the weight on 482 // the self-referential edge as we did in the previous case. 483 // 484 // In any other case, we must continue iterating. Eventually, 485 // all edges will get a weight, or iteration will stop when 486 // it reaches SampleProfileMaxPropagateIterations. 487 if (NumUnknownEdges <= 1) { 488 unsigned &BBWeight = BlockWeights[BB]; 489 if (NumUnknownEdges == 0) { 490 // If we already know the weight of all edges, the weight of the 491 // basic block can be computed. It should be no larger than the sum 492 // of all edge weights. 493 if (TotalWeight > BBWeight) { 494 BBWeight = TotalWeight; 495 Changed = true; 496 DEBUG(dbgs() << "All edge weights for " << BB->getName() 497 << " known. Set weight for block: "; 498 printBlockWeight(dbgs(), BB);); 499 } 500 if (VisitedBlocks.insert(BB).second) 501 Changed = true; 502 } else if (NumUnknownEdges == 1 && VisitedBlocks.count(BB)) { 503 // If there is a single unknown edge and the block has been 504 // visited, then we can compute E's weight. 505 if (BBWeight >= TotalWeight) 506 EdgeWeights[UnknownEdge] = BBWeight - TotalWeight; 507 else 508 EdgeWeights[UnknownEdge] = 0; 509 VisitedEdges.insert(UnknownEdge); 510 Changed = true; 511 DEBUG(dbgs() << "Set weight for edge: "; 512 printEdgeWeight(dbgs(), UnknownEdge)); 513 } 514 } else if (SelfReferentialEdge.first && VisitedBlocks.count(BB)) { 515 unsigned &BBWeight = BlockWeights[BB]; 516 // We have a self-referential edge and the weight of BB is known. 517 if (BBWeight >= TotalWeight) 518 EdgeWeights[SelfReferentialEdge] = BBWeight - TotalWeight; 519 else 520 EdgeWeights[SelfReferentialEdge] = 0; 521 VisitedEdges.insert(SelfReferentialEdge); 522 Changed = true; 523 DEBUG(dbgs() << "Set self-referential edge weight to: "; 524 printEdgeWeight(dbgs(), SelfReferentialEdge)); 525 } 526 } 527 } 528 529 return Changed; 530} 531 532/// \brief Build in/out edge lists for each basic block in the CFG. 533/// 534/// We are interested in unique edges. If a block B1 has multiple 535/// edges to another block B2, we only add a single B1->B2 edge. 536void SampleProfileLoader::buildEdges(Function &F) { 537 for (auto &BI : F) { 538 BasicBlock *B1 = &BI; 539 540 // Add predecessors for B1. 541 SmallPtrSet<BasicBlock *, 16> Visited; 542 if (!Predecessors[B1].empty()) 543 llvm_unreachable("Found a stale predecessors list in a basic block."); 544 for (pred_iterator PI = pred_begin(B1), PE = pred_end(B1); PI != PE; ++PI) { 545 BasicBlock *B2 = *PI; 546 if (Visited.insert(B2).second) 547 Predecessors[B1].push_back(B2); 548 } 549 550 // Add successors for B1. 551 Visited.clear(); 552 if (!Successors[B1].empty()) 553 llvm_unreachable("Found a stale successors list in a basic block."); 554 for (succ_iterator SI = succ_begin(B1), SE = succ_end(B1); SI != SE; ++SI) { 555 BasicBlock *B2 = *SI; 556 if (Visited.insert(B2).second) 557 Successors[B1].push_back(B2); 558 } 559 } 560} 561 562/// \brief Propagate weights into edges 563/// 564/// The following rules are applied to every block BB in the CFG: 565/// 566/// - If BB has a single predecessor/successor, then the weight 567/// of that edge is the weight of the block. 568/// 569/// - If all incoming or outgoing edges are known except one, and the 570/// weight of the block is already known, the weight of the unknown 571/// edge will be the weight of the block minus the sum of all the known 572/// edges. If the sum of all the known edges is larger than BB's weight, 573/// we set the unknown edge weight to zero. 574/// 575/// - If there is a self-referential edge, and the weight of the block is 576/// known, the weight for that edge is set to the weight of the block 577/// minus the weight of the other incoming edges to that block (if 578/// known). 579void SampleProfileLoader::propagateWeights(Function &F) { 580 bool Changed = true; 581 unsigned i = 0; 582 583 // Before propagation starts, build, for each block, a list of 584 // unique predecessors and successors. This is necessary to handle 585 // identical edges in multiway branches. Since we visit all blocks and all 586 // edges of the CFG, it is cleaner to build these lists once at the start 587 // of the pass. 588 buildEdges(F); 589 590 // Propagate until we converge or we go past the iteration limit. 591 while (Changed && i++ < SampleProfileMaxPropagateIterations) { 592 Changed = propagateThroughEdges(F); 593 } 594 595 // Generate MD_prof metadata for every branch instruction using the 596 // edge weights computed during propagation. 597 DEBUG(dbgs() << "\nPropagation complete. Setting branch weights\n"); 598 MDBuilder MDB(F.getContext()); 599 for (auto &BI : F) { 600 BasicBlock *BB = &BI; 601 TerminatorInst *TI = BB->getTerminator(); 602 if (TI->getNumSuccessors() == 1) 603 continue; 604 if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI)) 605 continue; 606 607 DEBUG(dbgs() << "\nGetting weights for branch at line " 608 << TI->getDebugLoc().getLine() << ".\n"); 609 SmallVector<unsigned, 4> Weights; 610 bool AllWeightsZero = true; 611 for (unsigned I = 0; I < TI->getNumSuccessors(); ++I) { 612 BasicBlock *Succ = TI->getSuccessor(I); 613 Edge E = std::make_pair(BB, Succ); 614 unsigned Weight = EdgeWeights[E]; 615 DEBUG(dbgs() << "\t"; printEdgeWeight(dbgs(), E)); 616 Weights.push_back(Weight); 617 if (Weight != 0) 618 AllWeightsZero = false; 619 } 620 621 // Only set weights if there is at least one non-zero weight. 622 // In any other case, let the analyzer set weights. 623 if (!AllWeightsZero) { 624 DEBUG(dbgs() << "SUCCESS. Found non-zero weights.\n"); 625 TI->setMetadata(llvm::LLVMContext::MD_prof, 626 MDB.createBranchWeights(Weights)); 627 } else { 628 DEBUG(dbgs() << "SKIPPED. All branch weights are zero.\n"); 629 } 630 } 631} 632 633/// \brief Get the line number for the function header. 634/// 635/// This looks up function \p F in the current compilation unit and 636/// retrieves the line number where the function is defined. This is 637/// line 0 for all the samples read from the profile file. Every line 638/// number is relative to this line. 639/// 640/// \param F Function object to query. 641/// 642/// \returns the line number where \p F is defined. If it returns 0, 643/// it means that there is no debug information available for \p F. 644unsigned SampleProfileLoader::getFunctionLoc(Function &F) { 645 if (MDSubprogram *S = getDISubprogram(&F)) 646 return S->getLine(); 647 648 // If could not find the start of \p F, emit a diagnostic to inform the user 649 // about the missed opportunity. 650 F.getContext().diagnose(DiagnosticInfoSampleProfile( 651 "No debug information found in function " + F.getName() + 652 ": Function profile not used", 653 DS_Warning)); 654 return 0; 655} 656 657/// \brief Generate branch weight metadata for all branches in \p F. 658/// 659/// Branch weights are computed out of instruction samples using a 660/// propagation heuristic. Propagation proceeds in 3 phases: 661/// 662/// 1- Assignment of block weights. All the basic blocks in the function 663/// are initial assigned the same weight as their most frequently 664/// executed instruction. 665/// 666/// 2- Creation of equivalence classes. Since samples may be missing from 667/// blocks, we can fill in the gaps by setting the weights of all the 668/// blocks in the same equivalence class to the same weight. To compute 669/// the concept of equivalence, we use dominance and loop information. 670/// Two blocks B1 and B2 are in the same equivalence class if B1 671/// dominates B2, B2 post-dominates B1 and both are in the same loop. 672/// 673/// 3- Propagation of block weights into edges. This uses a simple 674/// propagation heuristic. The following rules are applied to every 675/// block BB in the CFG: 676/// 677/// - If BB has a single predecessor/successor, then the weight 678/// of that edge is the weight of the block. 679/// 680/// - If all the edges are known except one, and the weight of the 681/// block is already known, the weight of the unknown edge will 682/// be the weight of the block minus the sum of all the known 683/// edges. If the sum of all the known edges is larger than BB's weight, 684/// we set the unknown edge weight to zero. 685/// 686/// - If there is a self-referential edge, and the weight of the block is 687/// known, the weight for that edge is set to the weight of the block 688/// minus the weight of the other incoming edges to that block (if 689/// known). 690/// 691/// Since this propagation is not guaranteed to finalize for every CFG, we 692/// only allow it to proceed for a limited number of iterations (controlled 693/// by -sample-profile-max-propagate-iterations). 694/// 695/// FIXME: Try to replace this propagation heuristic with a scheme 696/// that is guaranteed to finalize. A work-list approach similar to 697/// the standard value propagation algorithm used by SSA-CCP might 698/// work here. 699/// 700/// Once all the branch weights are computed, we emit the MD_prof 701/// metadata on BB using the computed values for each of its branches. 702/// 703/// \param F The function to query. 704/// 705/// \returns true if \p F was modified. Returns false, otherwise. 706bool SampleProfileLoader::emitAnnotations(Function &F) { 707 bool Changed = false; 708 709 // Initialize invariants used during computation and propagation. 710 HeaderLineno = getFunctionLoc(F); 711 if (HeaderLineno == 0) 712 return false; 713 714 DEBUG(dbgs() << "Line number for the first instruction in " << F.getName() 715 << ": " << HeaderLineno << "\n"); 716 717 // Compute basic block weights. 718 Changed |= computeBlockWeights(F); 719 720 if (Changed) { 721 // Find equivalence classes. 722 findEquivalenceClasses(F); 723 724 // Propagate weights to all edges. 725 propagateWeights(F); 726 } 727 728 return Changed; 729} 730 731char SampleProfileLoader::ID = 0; 732INITIALIZE_PASS_BEGIN(SampleProfileLoader, "sample-profile", 733 "Sample Profile loader", false, false) 734INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 735INITIALIZE_PASS_DEPENDENCY(PostDominatorTree) 736INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) 737INITIALIZE_PASS_DEPENDENCY(AddDiscriminators) 738INITIALIZE_PASS_END(SampleProfileLoader, "sample-profile", 739 "Sample Profile loader", false, false) 740 741bool SampleProfileLoader::doInitialization(Module &M) { 742 auto ReaderOrErr = SampleProfileReader::create(Filename, M.getContext()); 743 if (std::error_code EC = ReaderOrErr.getError()) { 744 std::string Msg = "Could not open profile: " + EC.message(); 745 M.getContext().diagnose(DiagnosticInfoSampleProfile(Filename.data(), Msg)); 746 return false; 747 } 748 Reader = std::move(ReaderOrErr.get()); 749 ProfileIsValid = (Reader->read() == sampleprof_error::success); 750 return true; 751} 752 753FunctionPass *llvm::createSampleProfileLoaderPass() { 754 return new SampleProfileLoader(SampleProfileFile); 755} 756 757FunctionPass *llvm::createSampleProfileLoaderPass(StringRef Name) { 758 return new SampleProfileLoader(Name); 759} 760 761bool SampleProfileLoader::runOnFunction(Function &F) { 762 if (!ProfileIsValid) 763 return false; 764 765 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 766 PDT = &getAnalysis<PostDominatorTree>(); 767 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); 768 Ctx = &F.getParent()->getContext(); 769 Samples = Reader->getSamplesFor(F); 770 if (!Samples->empty()) 771 return emitAnnotations(F); 772 return false; 773} 774