llvm / llvm / 3f4c496a9449877189c27c537c0c7699610d6ddb / . / lib / Transforms / Scalar / SampleProfile.cpp

//===- 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; | |

} |