| //===- SampleProfile.cpp - Incorporate sample profiles into the IR --------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements the SampleProfileLoader transformation. This pass |
| // reads a profile file generated by a sampling profiler (e.g. Linux Perf - |
| // http://perf.wiki.kernel.org/) and generates IR metadata to reflect the |
| // profile information in the given profile. |
| // |
| // This pass generates branch weight annotations on the IR: |
| // |
| // - prof: Represents branch weights. This annotation is added to branches |
| // to indicate the weights of each edge coming out of the branch. |
| // The weight of each edge is the weight of the target block for |
| // that edge. The weight of a block B is computed as the maximum |
| // number of samples found in B. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/ADT/SmallSet.h" |
| #include "llvm/ADT/StringRef.h" |
| #include "llvm/Analysis/LoopInfo.h" |
| #include "llvm/Analysis/PostDominators.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/DebugInfo.h" |
| #include "llvm/IR/DiagnosticInfo.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/InstIterator.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/LLVMContext.h" |
| #include "llvm/IR/MDBuilder.h" |
| #include "llvm/IR/Metadata.h" |
| #include "llvm/IR/Module.h" |
| #include "llvm/Pass.h" |
| #include "llvm/ProfileData/SampleProfReader.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include <cctype> |
| |
| using namespace llvm; |
| using namespace sampleprof; |
| |
| #define DEBUG_TYPE "sample-profile" |
| |
| // Command line option to specify the file to read samples from. This is |
| // mainly used for debugging. |
| static cl::opt<std::string> SampleProfileFile( |
| "sample-profile-file", cl::init(""), cl::value_desc("filename"), |
| cl::desc("Profile file loaded by -sample-profile"), cl::Hidden); |
| static cl::opt<unsigned> SampleProfileMaxPropagateIterations( |
| "sample-profile-max-propagate-iterations", cl::init(100), |
| cl::desc("Maximum number of iterations to go through when propagating " |
| "sample block/edge weights through the CFG.")); |
| |
| namespace { |
| typedef DenseMap<BasicBlock *, unsigned> BlockWeightMap; |
| typedef DenseMap<BasicBlock *, BasicBlock *> EquivalenceClassMap; |
| typedef std::pair<BasicBlock *, BasicBlock *> Edge; |
| typedef DenseMap<Edge, unsigned> EdgeWeightMap; |
| typedef DenseMap<BasicBlock *, SmallVector<BasicBlock *, 8>> BlockEdgeMap; |
| |
| /// \brief Sample profile pass. |
| /// |
| /// This pass reads profile data from the file specified by |
| /// -sample-profile-file and annotates every affected function with the |
| /// profile information found in that file. |
| class SampleProfileLoader : public FunctionPass { |
| public: |
| // Class identification, replacement for typeinfo |
| static char ID; |
| |
| SampleProfileLoader(StringRef Name = SampleProfileFile) |
| : FunctionPass(ID), DT(nullptr), PDT(nullptr), LI(nullptr), Ctx(nullptr), |
| Reader(), Samples(nullptr), Filename(Name), ProfileIsValid(false) { |
| initializeSampleProfileLoaderPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool doInitialization(Module &M) override; |
| |
| void dump() { Reader->dump(); } |
| |
| const char *getPassName() const override { return "Sample profile pass"; } |
| |
| bool runOnFunction(Function &F) override; |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.setPreservesCFG(); |
| AU.addRequired<LoopInfoWrapperPass>(); |
| AU.addRequired<DominatorTreeWrapperPass>(); |
| AU.addRequired<PostDominatorTree>(); |
| } |
| |
| protected: |
| unsigned getFunctionLoc(Function &F); |
| bool emitAnnotations(Function &F); |
| unsigned getInstWeight(Instruction &I); |
| unsigned getBlockWeight(BasicBlock *BB); |
| void printEdgeWeight(raw_ostream &OS, Edge E); |
| void printBlockWeight(raw_ostream &OS, BasicBlock *BB); |
| void printBlockEquivalence(raw_ostream &OS, BasicBlock *BB); |
| bool computeBlockWeights(Function &F); |
| void findEquivalenceClasses(Function &F); |
| void findEquivalencesFor(BasicBlock *BB1, |
| SmallVector<BasicBlock *, 8> Descendants, |
| DominatorTreeBase<BasicBlock> *DomTree); |
| void propagateWeights(Function &F); |
| unsigned visitEdge(Edge E, unsigned *NumUnknownEdges, Edge *UnknownEdge); |
| void buildEdges(Function &F); |
| bool propagateThroughEdges(Function &F); |
| |
| /// \brief Line number for the function header. Used to compute absolute |
| /// line numbers from the relative line numbers found in the profile. |
| unsigned HeaderLineno; |
| |
| /// \brief Map basic blocks to their computed weights. |
| /// |
| /// The weight of a basic block is defined to be the maximum |
| /// of all the instruction weights in that block. |
| BlockWeightMap BlockWeights; |
| |
| /// \brief Map edges to their computed weights. |
| /// |
| /// Edge weights are computed by propagating basic block weights in |
| /// SampleProfile::propagateWeights. |
| EdgeWeightMap EdgeWeights; |
| |
| /// \brief Set of visited blocks during propagation. |
| SmallPtrSet<BasicBlock *, 128> VisitedBlocks; |
| |
| /// \brief Set of visited edges during propagation. |
| SmallSet<Edge, 128> VisitedEdges; |
| |
| /// \brief Equivalence classes for block weights. |
| /// |
| /// Two blocks BB1 and BB2 are in the same equivalence class if they |
| /// dominate and post-dominate each other, and they are in the same loop |
| /// nest. When this happens, the two blocks are guaranteed to execute |
| /// the same number of times. |
| EquivalenceClassMap EquivalenceClass; |
| |
| /// \brief Dominance, post-dominance and loop information. |
| DominatorTree *DT; |
| PostDominatorTree *PDT; |
| LoopInfo *LI; |
| |
| /// \brief Predecessors for each basic block in the CFG. |
| BlockEdgeMap Predecessors; |
| |
| /// \brief Successors for each basic block in the CFG. |
| BlockEdgeMap Successors; |
| |
| /// \brief LLVM context holding the debug data we need. |
| LLVMContext *Ctx; |
| |
| /// \brief Profile reader object. |
| std::unique_ptr<SampleProfileReader> Reader; |
| |
| /// \brief Samples collected for the body of this function. |
| FunctionSamples *Samples; |
| |
| /// \brief Name of the profile file to load. |
| StringRef Filename; |
| |
| /// \brief Flag indicating whether the profile input loaded successfully. |
| bool ProfileIsValid; |
| }; |
| } |
| |
| /// \brief Print the weight of edge \p E on stream \p OS. |
| /// |
| /// \param OS Stream to emit the output to. |
| /// \param E Edge to print. |
| void SampleProfileLoader::printEdgeWeight(raw_ostream &OS, Edge E) { |
| OS << "weight[" << E.first->getName() << "->" << E.second->getName() |
| << "]: " << EdgeWeights[E] << "\n"; |
| } |
| |
| /// \brief Print the equivalence class of block \p BB on stream \p OS. |
| /// |
| /// \param OS Stream to emit the output to. |
| /// \param BB Block to print. |
| void SampleProfileLoader::printBlockEquivalence(raw_ostream &OS, |
| BasicBlock *BB) { |
| BasicBlock *Equiv = EquivalenceClass[BB]; |
| OS << "equivalence[" << BB->getName() |
| << "]: " << ((Equiv) ? EquivalenceClass[BB]->getName() : "NONE") << "\n"; |
| } |
| |
| /// \brief Print the weight of block \p BB on stream \p OS. |
| /// |
| /// \param OS Stream to emit the output to. |
| /// \param BB Block to print. |
| void SampleProfileLoader::printBlockWeight(raw_ostream &OS, BasicBlock *BB) { |
| OS << "weight[" << BB->getName() << "]: " << BlockWeights[BB] << "\n"; |
| } |
| |
| /// \brief Get the weight for an instruction. |
| /// |
| /// The "weight" of an instruction \p Inst is the number of samples |
| /// collected on that instruction at runtime. To retrieve it, we |
| /// need to compute the line number of \p Inst relative to the start of its |
| /// function. We use HeaderLineno to compute the offset. We then |
| /// look up the samples collected for \p Inst using BodySamples. |
| /// |
| /// \param Inst Instruction to query. |
| /// |
| /// \returns The profiled weight of I. |
| unsigned SampleProfileLoader::getInstWeight(Instruction &Inst) { |
| DebugLoc DLoc = Inst.getDebugLoc(); |
| if (!DLoc) |
| return 0; |
| |
| unsigned Lineno = DLoc.getLine(); |
| if (Lineno < HeaderLineno) |
| return 0; |
| |
| const DILocation *DIL = DLoc; |
| int LOffset = Lineno - HeaderLineno; |
| unsigned Discriminator = DIL->getDiscriminator(); |
| unsigned Weight = Samples->samplesAt(LOffset, Discriminator); |
| DEBUG(dbgs() << " " << Lineno << "." << Discriminator << ":" << Inst |
| << " (line offset: " << LOffset << "." << Discriminator |
| << " - weight: " << Weight << ")\n"); |
| return Weight; |
| } |
| |
| /// \brief Compute the weight of a basic block. |
| /// |
| /// The weight of basic block \p BB is the maximum weight of all the |
| /// instructions in BB. The weight of \p BB is computed and cached in |
| /// the BlockWeights map. |
| /// |
| /// \param BB The basic block to query. |
| /// |
| /// \returns The computed weight of BB. |
| unsigned SampleProfileLoader::getBlockWeight(BasicBlock *BB) { |
| // If we've computed BB's weight before, return it. |
| std::pair<BlockWeightMap::iterator, bool> Entry = |
| BlockWeights.insert(std::make_pair(BB, 0)); |
| if (!Entry.second) |
| return Entry.first->second; |
| |
| // Otherwise, compute and cache BB's weight. |
| unsigned Weight = 0; |
| for (auto &I : BB->getInstList()) { |
| unsigned InstWeight = getInstWeight(I); |
| if (InstWeight > Weight) |
| Weight = InstWeight; |
| } |
| Entry.first->second = Weight; |
| return Weight; |
| } |
| |
| /// \brief Compute and store the weights of every basic block. |
| /// |
| /// This populates the BlockWeights map by computing |
| /// the weights of every basic block in the CFG. |
| /// |
| /// \param F The function to query. |
| bool SampleProfileLoader::computeBlockWeights(Function &F) { |
| bool Changed = false; |
| DEBUG(dbgs() << "Block weights\n"); |
| for (auto &BB : F) { |
| unsigned Weight = getBlockWeight(&BB); |
| Changed |= (Weight > 0); |
| DEBUG(printBlockWeight(dbgs(), &BB)); |
| } |
| |
| return Changed; |
| } |
| |
| /// \brief Find equivalence classes for the given block. |
| /// |
| /// This finds all the blocks that are guaranteed to execute the same |
| /// number of times as \p BB1. To do this, it traverses all the |
| /// descendants of \p BB1 in the dominator or post-dominator tree. |
| /// |
| /// A block BB2 will be in the same equivalence class as \p BB1 if |
| /// the following holds: |
| /// |
| /// 1- \p BB1 is a descendant of BB2 in the opposite tree. So, if BB2 |
| /// is a descendant of \p BB1 in the dominator tree, then BB2 should |
| /// dominate BB1 in the post-dominator tree. |
| /// |
| /// 2- Both BB2 and \p BB1 must be in the same loop. |
| /// |
| /// For every block BB2 that meets those two requirements, we set BB2's |
| /// equivalence class to \p BB1. |
| /// |
| /// \param BB1 Block to check. |
| /// \param Descendants Descendants of \p BB1 in either the dom or pdom tree. |
| /// \param DomTree Opposite dominator tree. If \p Descendants is filled |
| /// with blocks from \p BB1's dominator tree, then |
| /// this is the post-dominator tree, and vice versa. |
| void SampleProfileLoader::findEquivalencesFor( |
| BasicBlock *BB1, SmallVector<BasicBlock *, 8> Descendants, |
| DominatorTreeBase<BasicBlock> *DomTree) { |
| for (auto *BB2 : Descendants) { |
| bool IsDomParent = DomTree->dominates(BB2, BB1); |
| bool IsInSameLoop = LI->getLoopFor(BB1) == LI->getLoopFor(BB2); |
| if (BB1 != BB2 && VisitedBlocks.insert(BB2).second && IsDomParent && |
| IsInSameLoop) { |
| EquivalenceClass[BB2] = BB1; |
| |
| // If BB2 is heavier than BB1, make BB2 have the same weight |
| // as BB1. |
| // |
| // Note that we don't worry about the opposite situation here |
| // (when BB2 is lighter than BB1). We will deal with this |
| // during the propagation phase. Right now, we just want to |
| // make sure that BB1 has the largest weight of all the |
| // members of its equivalence set. |
| unsigned &BB1Weight = BlockWeights[BB1]; |
| unsigned &BB2Weight = BlockWeights[BB2]; |
| BB1Weight = std::max(BB1Weight, BB2Weight); |
| } |
| } |
| } |
| |
| /// \brief Find equivalence classes. |
| /// |
| /// Since samples may be missing from blocks, we can fill in the gaps by setting |
| /// the weights of all the blocks in the same equivalence class to the same |
| /// weight. To compute the concept of equivalence, we use dominance and loop |
| /// information. Two blocks B1 and B2 are in the same equivalence class if B1 |
| /// dominates B2, B2 post-dominates B1 and both are in the same loop. |
| /// |
| /// \param F The function to query. |
| void SampleProfileLoader::findEquivalenceClasses(Function &F) { |
| SmallVector<BasicBlock *, 8> DominatedBBs; |
| DEBUG(dbgs() << "\nBlock equivalence classes\n"); |
| // Find equivalence sets based on dominance and post-dominance information. |
| for (auto &BB : F) { |
| BasicBlock *BB1 = &BB; |
| |
| // Compute BB1's equivalence class once. |
| if (EquivalenceClass.count(BB1)) { |
| DEBUG(printBlockEquivalence(dbgs(), BB1)); |
| continue; |
| } |
| |
| // By default, blocks are in their own equivalence class. |
| EquivalenceClass[BB1] = BB1; |
| |
| // Traverse all the blocks dominated by BB1. We are looking for |
| // every basic block BB2 such that: |
| // |
| // 1- BB1 dominates BB2. |
| // 2- BB2 post-dominates BB1. |
| // 3- BB1 and BB2 are in the same loop nest. |
| // |
| // If all those conditions hold, it means that BB2 is executed |
| // as many times as BB1, so they are placed in the same equivalence |
| // class by making BB2's equivalence class be BB1. |
| DominatedBBs.clear(); |
| DT->getDescendants(BB1, DominatedBBs); |
| findEquivalencesFor(BB1, DominatedBBs, PDT->DT); |
| |
| // Repeat the same logic for all the blocks post-dominated by BB1. |
| // We are looking for every basic block BB2 such that: |
| // |
| // 1- BB1 post-dominates BB2. |
| // 2- BB2 dominates BB1. |
| // 3- BB1 and BB2 are in the same loop nest. |
| // |
| // If all those conditions hold, BB2's equivalence class is BB1. |
| DominatedBBs.clear(); |
| PDT->getDescendants(BB1, DominatedBBs); |
| findEquivalencesFor(BB1, DominatedBBs, DT); |
| |
| DEBUG(printBlockEquivalence(dbgs(), BB1)); |
| } |
| |
| // Assign weights to equivalence classes. |
| // |
| // All the basic blocks in the same equivalence class will execute |
| // the same number of times. Since we know that the head block in |
| // each equivalence class has the largest weight, assign that weight |
| // to all the blocks in that equivalence class. |
| DEBUG(dbgs() << "\nAssign the same weight to all blocks in the same class\n"); |
| for (auto &BI : F) { |
| BasicBlock *BB = &BI; |
| BasicBlock *EquivBB = EquivalenceClass[BB]; |
| if (BB != EquivBB) |
| BlockWeights[BB] = BlockWeights[EquivBB]; |
| DEBUG(printBlockWeight(dbgs(), BB)); |
| } |
| } |
| |
| /// \brief Visit the given edge to decide if it has a valid weight. |
| /// |
| /// If \p E has not been visited before, we copy to \p UnknownEdge |
| /// and increment the count of unknown edges. |
| /// |
| /// \param E Edge to visit. |
| /// \param NumUnknownEdges Current number of unknown edges. |
| /// \param UnknownEdge Set if E has not been visited before. |
| /// |
| /// \returns E's weight, if known. Otherwise, return 0. |
| unsigned SampleProfileLoader::visitEdge(Edge E, unsigned *NumUnknownEdges, |
| Edge *UnknownEdge) { |
| if (!VisitedEdges.count(E)) { |
| (*NumUnknownEdges)++; |
| *UnknownEdge = E; |
| return 0; |
| } |
| |
| return EdgeWeights[E]; |
| } |
| |
| /// \brief Propagate weights through incoming/outgoing edges. |
| /// |
| /// If the weight of a basic block is known, and there is only one edge |
| /// with an unknown weight, we can calculate the weight of that edge. |
| /// |
| /// Similarly, if all the edges have a known count, we can calculate the |
| /// count of the basic block, if needed. |
| /// |
| /// \param F Function to process. |
| /// |
| /// \returns True if new weights were assigned to edges or blocks. |
| bool SampleProfileLoader::propagateThroughEdges(Function &F) { |
| bool Changed = false; |
| DEBUG(dbgs() << "\nPropagation through edges\n"); |
| for (auto &BI : F) { |
| BasicBlock *BB = &BI; |
| |
| // Visit all the predecessor and successor edges to determine |
| // which ones have a weight assigned already. Note that it doesn't |
| // matter that we only keep track of a single unknown edge. The |
| // only case we are interested in handling is when only a single |
| // edge is unknown (see setEdgeOrBlockWeight). |
| for (unsigned i = 0; i < 2; i++) { |
| unsigned TotalWeight = 0; |
| unsigned NumUnknownEdges = 0; |
| Edge UnknownEdge, SelfReferentialEdge; |
| |
| if (i == 0) { |
| // First, visit all predecessor edges. |
| for (auto *Pred : Predecessors[BB]) { |
| Edge E = std::make_pair(Pred, BB); |
| TotalWeight += visitEdge(E, &NumUnknownEdges, &UnknownEdge); |
| if (E.first == E.second) |
| SelfReferentialEdge = E; |
| } |
| } else { |
| // On the second round, visit all successor edges. |
| for (auto *Succ : Successors[BB]) { |
| Edge E = std::make_pair(BB, Succ); |
| TotalWeight += visitEdge(E, &NumUnknownEdges, &UnknownEdge); |
| } |
| } |
| |
| // After visiting all the edges, there are three cases that we |
| // can handle immediately: |
| // |
| // - All the edge weights are known (i.e., NumUnknownEdges == 0). |
| // In this case, we simply check that the sum of all the edges |
| // is the same as BB's weight. If not, we change BB's weight |
| // to match. Additionally, if BB had not been visited before, |
| // we mark it visited. |
| // |
| // - Only one edge is unknown and BB has already been visited. |
| // In this case, we can compute the weight of the edge by |
| // subtracting the total block weight from all the known |
| // edge weights. If the edges weight more than BB, then the |
| // edge of the last remaining edge is set to zero. |
| // |
| // - There exists a self-referential edge and the weight of BB is |
| // known. In this case, this edge can be based on BB's weight. |
| // We add up all the other known edges and set the weight on |
| // the self-referential edge as we did in the previous case. |
| // |
| // In any other case, we must continue iterating. Eventually, |
| // all edges will get a weight, or iteration will stop when |
| // it reaches SampleProfileMaxPropagateIterations. |
| if (NumUnknownEdges <= 1) { |
| unsigned &BBWeight = BlockWeights[BB]; |
| if (NumUnknownEdges == 0) { |
| // If we already know the weight of all edges, the weight of the |
| // basic block can be computed. It should be no larger than the sum |
| // of all edge weights. |
| if (TotalWeight > BBWeight) { |
| BBWeight = TotalWeight; |
| Changed = true; |
| DEBUG(dbgs() << "All edge weights for " << BB->getName() |
| << " known. Set weight for block: "; |
| printBlockWeight(dbgs(), BB);); |
| } |
| if (VisitedBlocks.insert(BB).second) |
| Changed = true; |
| } else if (NumUnknownEdges == 1 && VisitedBlocks.count(BB)) { |
| // If there is a single unknown edge and the block has been |
| // visited, then we can compute E's weight. |
| if (BBWeight >= TotalWeight) |
| EdgeWeights[UnknownEdge] = BBWeight - TotalWeight; |
| else |
| EdgeWeights[UnknownEdge] = 0; |
| VisitedEdges.insert(UnknownEdge); |
| Changed = true; |
| DEBUG(dbgs() << "Set weight for edge: "; |
| printEdgeWeight(dbgs(), UnknownEdge)); |
| } |
| } else if (SelfReferentialEdge.first && VisitedBlocks.count(BB)) { |
| unsigned &BBWeight = BlockWeights[BB]; |
| // We have a self-referential edge and the weight of BB is known. |
| if (BBWeight >= TotalWeight) |
| EdgeWeights[SelfReferentialEdge] = BBWeight - TotalWeight; |
| else |
| EdgeWeights[SelfReferentialEdge] = 0; |
| VisitedEdges.insert(SelfReferentialEdge); |
| Changed = true; |
| DEBUG(dbgs() << "Set self-referential edge weight to: "; |
| printEdgeWeight(dbgs(), SelfReferentialEdge)); |
| } |
| } |
| } |
| |
| return Changed; |
| } |
| |
| /// \brief Build in/out edge lists for each basic block in the CFG. |
| /// |
| /// We are interested in unique edges. If a block B1 has multiple |
| /// edges to another block B2, we only add a single B1->B2 edge. |
| void SampleProfileLoader::buildEdges(Function &F) { |
| for (auto &BI : F) { |
| BasicBlock *B1 = &BI; |
| |
| // Add predecessors for B1. |
| SmallPtrSet<BasicBlock *, 16> Visited; |
| if (!Predecessors[B1].empty()) |
| llvm_unreachable("Found a stale predecessors list in a basic block."); |
| for (pred_iterator PI = pred_begin(B1), PE = pred_end(B1); PI != PE; ++PI) { |
| BasicBlock *B2 = *PI; |
| if (Visited.insert(B2).second) |
| Predecessors[B1].push_back(B2); |
| } |
| |
| // Add successors for B1. |
| Visited.clear(); |
| if (!Successors[B1].empty()) |
| llvm_unreachable("Found a stale successors list in a basic block."); |
| for (succ_iterator SI = succ_begin(B1), SE = succ_end(B1); SI != SE; ++SI) { |
| BasicBlock *B2 = *SI; |
| if (Visited.insert(B2).second) |
| Successors[B1].push_back(B2); |
| } |
| } |
| } |
| |
| /// \brief Propagate weights into edges |
| /// |
| /// The following rules are applied to every block BB in the CFG: |
| /// |
| /// - If BB has a single predecessor/successor, then the weight |
| /// of that edge is the weight of the block. |
| /// |
| /// - If all incoming or outgoing edges are known except one, and the |
| /// weight of the block is already known, the weight of the unknown |
| /// edge will be the weight of the block minus the sum of all the known |
| /// edges. If the sum of all the known edges is larger than BB's weight, |
| /// we set the unknown edge weight to zero. |
| /// |
| /// - If there is a self-referential edge, and the weight of the block is |
| /// known, the weight for that edge is set to the weight of the block |
| /// minus the weight of the other incoming edges to that block (if |
| /// known). |
| void SampleProfileLoader::propagateWeights(Function &F) { |
| bool Changed = true; |
| unsigned i = 0; |
| |
| // Add an entry count to the function using the samples gathered |
| // at the function entry. |
| F.setEntryCount(Samples->getHeadSamples()); |
| |
| // Before propagation starts, build, for each block, a list of |
| // unique predecessors and successors. This is necessary to handle |
| // identical edges in multiway branches. Since we visit all blocks and all |
| // edges of the CFG, it is cleaner to build these lists once at the start |
| // of the pass. |
| buildEdges(F); |
| |
| // Propagate until we converge or we go past the iteration limit. |
| while (Changed && i++ < SampleProfileMaxPropagateIterations) { |
| Changed = propagateThroughEdges(F); |
| } |
| |
| // Generate MD_prof metadata for every branch instruction using the |
| // edge weights computed during propagation. |
| DEBUG(dbgs() << "\nPropagation complete. Setting branch weights\n"); |
| MDBuilder MDB(F.getContext()); |
| for (auto &BI : F) { |
| BasicBlock *BB = &BI; |
| TerminatorInst *TI = BB->getTerminator(); |
| if (TI->getNumSuccessors() == 1) |
| continue; |
| if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI)) |
| continue; |
| |
| DEBUG(dbgs() << "\nGetting weights for branch at line " |
| << TI->getDebugLoc().getLine() << ".\n"); |
| SmallVector<unsigned, 4> Weights; |
| bool AllWeightsZero = true; |
| for (unsigned I = 0; I < TI->getNumSuccessors(); ++I) { |
| BasicBlock *Succ = TI->getSuccessor(I); |
| Edge E = std::make_pair(BB, Succ); |
| unsigned Weight = EdgeWeights[E]; |
| DEBUG(dbgs() << "\t"; printEdgeWeight(dbgs(), E)); |
| Weights.push_back(Weight); |
| if (Weight != 0) |
| AllWeightsZero = false; |
| } |
| |
| // Only set weights if there is at least one non-zero weight. |
| // In any other case, let the analyzer set weights. |
| if (!AllWeightsZero) { |
| DEBUG(dbgs() << "SUCCESS. Found non-zero weights.\n"); |
| TI->setMetadata(llvm::LLVMContext::MD_prof, |
| MDB.createBranchWeights(Weights)); |
| } else { |
| DEBUG(dbgs() << "SKIPPED. All branch weights are zero.\n"); |
| } |
| } |
| } |
| |
| /// \brief Get the line number for the function header. |
| /// |
| /// This looks up function \p F in the current compilation unit and |
| /// retrieves the line number where the function is defined. This is |
| /// line 0 for all the samples read from the profile file. Every line |
| /// number is relative to this line. |
| /// |
| /// \param F Function object to query. |
| /// |
| /// \returns the line number where \p F is defined. If it returns 0, |
| /// it means that there is no debug information available for \p F. |
| unsigned SampleProfileLoader::getFunctionLoc(Function &F) { |
| if (DISubprogram *S = getDISubprogram(&F)) |
| return S->getLine(); |
| |
| // If could not find the start of \p F, emit a diagnostic to inform the user |
| // about the missed opportunity. |
| F.getContext().diagnose(DiagnosticInfoSampleProfile( |
| "No debug information found in function " + F.getName() + |
| ": Function profile not used", |
| DS_Warning)); |
| return 0; |
| } |
| |
| /// \brief Generate branch weight metadata for all branches in \p F. |
| /// |
| /// Branch weights are computed out of instruction samples using a |
| /// propagation heuristic. Propagation proceeds in 3 phases: |
| /// |
| /// 1- Assignment of block weights. All the basic blocks in the function |
| /// are initial assigned the same weight as their most frequently |
| /// executed instruction. |
| /// |
| /// 2- Creation of equivalence classes. Since samples may be missing from |
| /// blocks, we can fill in the gaps by setting the weights of all the |
| /// blocks in the same equivalence class to the same weight. To compute |
| /// the concept of equivalence, we use dominance and loop information. |
| /// Two blocks B1 and B2 are in the same equivalence class if B1 |
| /// dominates B2, B2 post-dominates B1 and both are in the same loop. |
| /// |
| /// 3- Propagation of block weights into edges. This uses a simple |
| /// propagation heuristic. The following rules are applied to every |
| /// block BB in the CFG: |
| /// |
| /// - If BB has a single predecessor/successor, then the weight |
| /// of that edge is the weight of the block. |
| /// |
| /// - If all the edges are known except one, and the weight of the |
| /// block is already known, the weight of the unknown edge will |
| /// be the weight of the block minus the sum of all the known |
| /// edges. If the sum of all the known edges is larger than BB's weight, |
| /// we set the unknown edge weight to zero. |
| /// |
| /// - If there is a self-referential edge, and the weight of the block is |
| /// known, the weight for that edge is set to the weight of the block |
| /// minus the weight of the other incoming edges to that block (if |
| /// known). |
| /// |
| /// Since this propagation is not guaranteed to finalize for every CFG, we |
| /// only allow it to proceed for a limited number of iterations (controlled |
| /// by -sample-profile-max-propagate-iterations). |
| /// |
| /// FIXME: Try to replace this propagation heuristic with a scheme |
| /// that is guaranteed to finalize. A work-list approach similar to |
| /// the standard value propagation algorithm used by SSA-CCP might |
| /// work here. |
| /// |
| /// Once all the branch weights are computed, we emit the MD_prof |
| /// metadata on BB using the computed values for each of its branches. |
| /// |
| /// \param F The function to query. |
| /// |
| /// \returns true if \p F was modified. Returns false, otherwise. |
| bool SampleProfileLoader::emitAnnotations(Function &F) { |
| bool Changed = false; |
| |
| // Initialize invariants used during computation and propagation. |
| HeaderLineno = getFunctionLoc(F); |
| if (HeaderLineno == 0) |
| return false; |
| |
| DEBUG(dbgs() << "Line number for the first instruction in " << F.getName() |
| << ": " << HeaderLineno << "\n"); |
| |
| // Compute basic block weights. |
| Changed |= computeBlockWeights(F); |
| |
| if (Changed) { |
| // Find equivalence classes. |
| findEquivalenceClasses(F); |
| |
| // Propagate weights to all edges. |
| propagateWeights(F); |
| } |
| |
| return Changed; |
| } |
| |
| char SampleProfileLoader::ID = 0; |
| INITIALIZE_PASS_BEGIN(SampleProfileLoader, "sample-profile", |
| "Sample Profile loader", false, false) |
| INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(PostDominatorTree) |
| INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(AddDiscriminators) |
| INITIALIZE_PASS_END(SampleProfileLoader, "sample-profile", |
| "Sample Profile loader", false, false) |
| |
| bool SampleProfileLoader::doInitialization(Module &M) { |
| auto ReaderOrErr = SampleProfileReader::create(Filename, M.getContext()); |
| if (std::error_code EC = ReaderOrErr.getError()) { |
| std::string Msg = "Could not open profile: " + EC.message(); |
| M.getContext().diagnose(DiagnosticInfoSampleProfile(Filename.data(), Msg)); |
| return false; |
| } |
| Reader = std::move(ReaderOrErr.get()); |
| ProfileIsValid = (Reader->read() == sampleprof_error::success); |
| return true; |
| } |
| |
| FunctionPass *llvm::createSampleProfileLoaderPass() { |
| return new SampleProfileLoader(SampleProfileFile); |
| } |
| |
| FunctionPass *llvm::createSampleProfileLoaderPass(StringRef Name) { |
| return new SampleProfileLoader(Name); |
| } |
| |
| bool SampleProfileLoader::runOnFunction(Function &F) { |
| if (!ProfileIsValid) |
| return false; |
| |
| DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); |
| PDT = &getAnalysis<PostDominatorTree>(); |
| LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); |
| Ctx = &F.getParent()->getContext(); |
| Samples = Reader->getSamplesFor(F); |
| if (!Samples->empty()) |
| return emitAnnotations(F); |
| return false; |
| } |