BranchProbabilityInfo.cpp   [plain text]

//===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Loops should be simplified before this analysis.
//
//===----------------------------------------------------------------------===//

#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Support/Debug.h"

using namespace llvm;

INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
                      "Branch Probability Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
                    "Branch Probability Analysis", false, true)

char BranchProbabilityInfo::ID = 0;

namespace {
// Please note that BranchProbabilityAnalysis is not a FunctionPass.
// It is created by BranchProbabilityInfo (which is a FunctionPass), which
// provides a clear interface. Thanks to that, all heuristics and other
// private methods are hidden in the .cpp file.
class BranchProbabilityAnalysis {

  typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;

  DenseMap<Edge, uint32_t> *Weights;

  BranchProbabilityInfo *BP;

  LoopInfo *LI;


  // Weights are for internal use only. They are used by heuristics to help to
  // estimate edges' probability. Example:
  //
  // Using "Loop Branch Heuristics" we predict weights of edges for the
  // block BB2.
  //         ...
  //          |
  //          V
  //         BB1<-+
  //          |   |
  //          |   | (Weight = 124)
  //          V   |
  //         BB2--+
  //          |
  //          | (Weight = 4)
  //          V
  //         BB3
  //
  // Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
  // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125

  static const uint32_t LBH_TAKEN_WEIGHT = 124;
  static const uint32_t LBH_NONTAKEN_WEIGHT = 4;

  static const uint32_t RH_TAKEN_WEIGHT = 24;
  static const uint32_t RH_NONTAKEN_WEIGHT = 8;

  static const uint32_t PH_TAKEN_WEIGHT = 20;
  static const uint32_t PH_NONTAKEN_WEIGHT = 12;

  static const uint32_t ZH_TAKEN_WEIGHT = 20;
  static const uint32_t ZH_NONTAKEN_WEIGHT = 12;

  // Standard weight value. Used when none of the heuristics set weight for
  // the edge.
  static const uint32_t NORMAL_WEIGHT = 16;

  // Minimum weight of an edge. Please note, that weight is NEVER 0.
  static const uint32_t MIN_WEIGHT = 1;

  // Return TRUE if BB leads directly to a Return Instruction.
  static bool isReturningBlock(BasicBlock *BB) {
    SmallPtrSet<BasicBlock *, 8> Visited;

    while (true) {
      TerminatorInst *TI = BB->getTerminator();
      if (isa<ReturnInst>(TI))
        return true;

      if (TI->getNumSuccessors() > 1)
        break;

      // It is unreachable block which we can consider as a return instruction.
      if (TI->getNumSuccessors() == 0)
        return true;

      Visited.insert(BB);
      BB = TI->getSuccessor(0);

      // Stop if cycle is detected.
      if (Visited.count(BB))
        return false;
    }

    return false;
  }

  uint32_t getMaxWeightFor(BasicBlock *BB) const {
    return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
  }

public:
  BranchProbabilityAnalysis(DenseMap<Edge, uint32_t> *W,
                            BranchProbabilityInfo *BP, LoopInfo *LI)
    : Weights(W), BP(BP), LI(LI) {
  }

  // Return Heuristics
  bool calcReturnHeuristics(BasicBlock *BB);

  // Pointer Heuristics
  bool calcPointerHeuristics(BasicBlock *BB);

  // Loop Branch Heuristics
  bool calcLoopBranchHeuristics(BasicBlock *BB);

  // Zero Heurestics
  bool calcZeroHeuristics(BasicBlock *BB);

  bool runOnFunction(Function &F);
};
} // end anonymous namespace

// Calculate Edge Weights using "Return Heuristics". Predict a successor which
// leads directly to Return Instruction will not be taken.
bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
  if (BB->getTerminator()->getNumSuccessors() == 1)
    return false;

  SmallPtrSet<BasicBlock *, 4> ReturningEdges;
  SmallPtrSet<BasicBlock *, 4> StayEdges;

  for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
    BasicBlock *Succ = *I;
    if (isReturningBlock(Succ))
      ReturningEdges.insert(Succ);
    else
      StayEdges.insert(Succ);
  }

  if (uint32_t numStayEdges = StayEdges.size()) {
    uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
    if (stayWeight < NORMAL_WEIGHT)
      stayWeight = NORMAL_WEIGHT;

    for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
         E = StayEdges.end(); I != E; ++I)
      BP->setEdgeWeight(BB, *I, stayWeight);
  }

  if (uint32_t numRetEdges = ReturningEdges.size()) {
    uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
    if (retWeight < MIN_WEIGHT)
      retWeight = MIN_WEIGHT;
    for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
         E = ReturningEdges.end(); I != E; ++I) {
      BP->setEdgeWeight(BB, *I, retWeight);
    }
  }

  return ReturningEdges.size() > 0;
}

// Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
// between two pointer or pointer and NULL will fail.
bool BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
  BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
  if (!BI || !BI->isConditional())
    return false;

  Value *Cond = BI->getCondition();
  ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
  if (!CI || !CI->isEquality())
    return false;

  Value *LHS = CI->getOperand(0);

  if (!LHS->getType()->isPointerTy())
    return false;

  assert(CI->getOperand(1)->getType()->isPointerTy());

  BasicBlock *Taken = BI->getSuccessor(0);
  BasicBlock *NonTaken = BI->getSuccessor(1);

  // p != 0   ->   isProb = true
  // p == 0   ->   isProb = false
  // p != q   ->   isProb = true
  // p == q   ->   isProb = false;
  bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
  if (!isProb)
    std::swap(Taken, NonTaken);

  BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
  BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
  return true;
}

// Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
// as taken, exiting edges as not-taken.
bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
  uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();

  Loop *L = LI->getLoopFor(BB);
  if (!L)
    return false;

  SmallPtrSet<BasicBlock *, 8> BackEdges;
  SmallPtrSet<BasicBlock *, 8> ExitingEdges;
  SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.

  bool isHeader = BB == L->getHeader();

  for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
    BasicBlock *Succ = *I;
    Loop *SuccL = LI->getLoopFor(Succ);
    if (SuccL != L)
      ExitingEdges.insert(Succ);
    else if (Succ == L->getHeader())
      BackEdges.insert(Succ);
    else if (isHeader)
      InEdges.insert(Succ);
  }

  if (uint32_t numBackEdges = BackEdges.size()) {
    uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
    if (backWeight < NORMAL_WEIGHT)
      backWeight = NORMAL_WEIGHT;

    for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
         EE = BackEdges.end(); EI != EE; ++EI) {
      BasicBlock *Back = *EI;
      BP->setEdgeWeight(BB, Back, backWeight);
    }
  }

  if (uint32_t numInEdges = InEdges.size()) {
    uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
    if (inWeight < NORMAL_WEIGHT)
      inWeight = NORMAL_WEIGHT;

    for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
         EE = InEdges.end(); EI != EE; ++EI) {
      BasicBlock *Back = *EI;
      BP->setEdgeWeight(BB, Back, inWeight);
    }
  }

  uint32_t numExitingEdges = ExitingEdges.size();
  if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
    uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
    if (exitWeight < MIN_WEIGHT)
      exitWeight = MIN_WEIGHT;

    for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
         EE = ExitingEdges.end(); EI != EE; ++EI) {
      BasicBlock *Exiting = *EI;
      BP->setEdgeWeight(BB, Exiting, exitWeight);
    }
  }

  return true;
}

bool BranchProbabilityAnalysis::calcZeroHeuristics(BasicBlock *BB) {
  BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
  if (!BI || !BI->isConditional())
    return false;

  Value *Cond = BI->getCondition();
  ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
  if (!CI)
    return false;

  Value *RHS = CI->getOperand(1);
  ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
  if (!CV || !CV->isZero())
    return false;

  bool isProb;
  switch (CI->getPredicate()) {
  case CmpInst::ICMP_EQ:
    // Equal to zero is not expected to be taken.
    isProb = false;
    break;

  case CmpInst::ICMP_NE:
    // Not equal to zero is expected.
    isProb = true;
    break;

  case CmpInst::ICMP_SLT:
    // Less or equal to zero is not expected.
    // X < 0   ->  Unlikely
    isProb = false;
    break;

  case CmpInst::ICMP_UGT:
  case CmpInst::ICMP_SGT:
    // Greater or equal to zero is expected.
    // X > 0   ->  Likely
    isProb = true;
    break;

  default:
    return false;
  };

  BasicBlock *Taken = BI->getSuccessor(0);
  BasicBlock *NonTaken = BI->getSuccessor(1);

  if (!isProb)
    std::swap(Taken, NonTaken);

  BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
  BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);

  return true;
}


bool BranchProbabilityAnalysis::runOnFunction(Function &F) {

  for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
    BasicBlock *BB = I++;

    if (calcLoopBranchHeuristics(BB))
      continue;

    if (calcReturnHeuristics(BB))
      continue;

    if (calcPointerHeuristics(BB))
      continue;

    calcZeroHeuristics(BB);
  }

  return false;
}

void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
    AU.addRequired<LoopInfo>();
    AU.setPreservesAll();
}

bool BranchProbabilityInfo::runOnFunction(Function &F) {
  LoopInfo &LI = getAnalysis<LoopInfo>();
  BranchProbabilityAnalysis BPA(&Weights, this, &LI);
  return BPA.runOnFunction(F);
}

uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
  uint32_t Sum = 0;

  for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
    const BasicBlock *Succ = *I;
    uint32_t Weight = getEdgeWeight(BB, Succ);
    uint32_t PrevSum = Sum;

    Sum += Weight;
    assert(Sum > PrevSum); (void) PrevSum;
  }

  return Sum;
}

bool BranchProbabilityInfo::
isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
  // Hot probability is at least 4/5 = 80%
  uint32_t Weight = getEdgeWeight(Src, Dst);
  uint32_t Sum = getSumForBlock(Src);

  // FIXME: Implement BranchProbability::compare then change this code to
  // compare this BranchProbability against a static "hot" BranchProbability.
  return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
}

BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
  uint32_t Sum = 0;
  uint32_t MaxWeight = 0;
  BasicBlock *MaxSucc = 0;

  for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
    BasicBlock *Succ = *I;
    uint32_t Weight = getEdgeWeight(BB, Succ);
    uint32_t PrevSum = Sum;

    Sum += Weight;
    assert(Sum > PrevSum); (void) PrevSum;

    if (Weight > MaxWeight) {
      MaxWeight = Weight;
      MaxSucc = Succ;
    }
  }

  // FIXME: Use BranchProbability::compare.
  if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
    return MaxSucc;

  return 0;
}

// Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
uint32_t BranchProbabilityInfo::
getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
  Edge E(Src, Dst);
  DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);

  if (I != Weights.end())
    return I->second;

  return DEFAULT_WEIGHT;
}

void BranchProbabilityInfo::
setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
  Weights[std::make_pair(Src, Dst)] = Weight;
  DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
               << Dst->getNameStr() << " weight to " << Weight
               << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
}


BranchProbability BranchProbabilityInfo::
getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {

  uint32_t N = getEdgeWeight(Src, Dst);
  uint32_t D = getSumForBlock(Src);

  return BranchProbability(N, D);
}

raw_ostream &
BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
                                            BasicBlock *Dst) const {

  const BranchProbability Prob = getEdgeProbability(Src, Dst);
  OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
     << " probability is " << Prob
     << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");

  return OS;
}