lib/Analysis/BranchProbabilityInfo.cpp

   1 //===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -*- C++ -*-===//
   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 // Loops should be simplified before this analysis.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Instructions.h"
  15 #include "llvm/Analysis/BranchProbabilityInfo.h"
  16 #include <climits>
  17
  18 using namespace llvm;
  19
  20 INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
  21                       "Branch Probability Analysis", false, true)
  22 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  23 INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
  24                     "Branch Probability Analysis", false, true)
  25
  26 char BranchProbabilityInfo::ID = 0;
  27
  28
  29 // Please note that BranchProbabilityAnalysis is not a FunctionPass.
  30 // It is created by BranchProbabilityInfo (which is a FunctionPass), which
  31 // provides a clear interface. Thanks to that, all heuristics and other
  32 // private methods are hidden in the .cpp file.
  33 class BranchProbabilityAnalysis {
  34
  35   typedef std::pair<BasicBlock *, BasicBlock *> Edge;
  36
  37   DenseMap<Edge, unsigned> *Weights;
  38
  39   BranchProbabilityInfo *BP;
  40
  41   LoopInfo *LI;
  42
  43
  44   // Weights are for internal use only. They are used by heuristics to help to
  45   // estimate edges' probability. Example:
  46   //
  47   // Using "Loop Branch Heuristics" we predict weights of edges for the
  48   // block BB2.
  49   //         ...
  50   //          |
  51   //          V
  52   //         BB1<-+
  53   //          |   |
  54   //          |   | (Weight = 128)
  55   //          V   |
  56   //         BB2--+
  57   //          |
  58   //          | (Weight = 4)
  59   //          V
  60   //         BB3
  61   //
  62   // Probability of the edge BB2->BB1 = 128 / (128 + 4) = 0.9696..
  63   // Probability of the edge BB2->BB3 = 4 / (128 + 4) = 0.0303..
  64
  65   static const unsigned int LBH_TAKEN_WEIGHT = 128;
  66   static const unsigned int LBH_NONTAKEN_WEIGHT = 4;
  67
  68   // Standard weight value. Used when none of the heuristics set weight for
  69   // the edge.
  70   static const unsigned int NORMAL_WEIGHT = 16;
  71
  72   // Minimum weight of an edge. Please note, that weight is NEVER 0.
  73   static const unsigned int MIN_WEIGHT = 1;
  74
  75   // Return TRUE if BB leads directly to a Return Instruction.
  76   static bool isReturningBlock(BasicBlock *BB) {
  77     SmallPtrSet<BasicBlock *, 8> Visited;
  78
  79     while (true) {
  80       TerminatorInst *TI = BB->getTerminator();
  81       if (isa<ReturnInst>(TI))
  82         return true;
  83
  84       if (TI->getNumSuccessors() > 1)
  85         break;
  86
  87       // It is unreachable block which we can consider as a return instruction.
  88       if (TI->getNumSuccessors() == 0)
  89         return true;
  90
  91       Visited.insert(BB);
  92       BB = TI->getSuccessor(0);
  93
  94       // Stop if cycle is detected.
  95       if (Visited.count(BB))
  96         return false;
  97     }
  98
  99     return false;
 100   }
 101
 102   // Multiply Edge Weight by two.
 103   void incEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
 104     unsigned Weight = BP->getEdgeWeight(Src, Dst);
 105     unsigned MaxWeight = getMaxWeightFor(Src);
 106
 107     if (Weight * 2 > MaxWeight)
 108       BP->setEdgeWeight(Src, Dst, MaxWeight);
 109     else
 110       BP->setEdgeWeight(Src, Dst, Weight * 2);
 111   }
 112
 113   // Divide Edge Weight by two.
 114   void decEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
 115     unsigned Weight = BP->getEdgeWeight(Src, Dst);
 116
 117     assert(Weight > 0);
 118     if (Weight / 2 < MIN_WEIGHT)
 119       BP->setEdgeWeight(Src, Dst, MIN_WEIGHT);
 120     else
 121       BP->setEdgeWeight(Src, Dst, Weight / 2);
 122   }
 123
 124
 125   unsigned getMaxWeightFor(BasicBlock *BB) const {
 126     return UINT_MAX / BB->getTerminator()->getNumSuccessors();
 127   }
 128
 129 public:
 130   BranchProbabilityAnalysis(DenseMap<Edge, unsigned> *W,
 131                             BranchProbabilityInfo *BP, LoopInfo *LI)
 132     : Weights(W), BP(BP), LI(LI) {
 133   }
 134
 135   // Return Heuristics
 136   void calcReturnHeuristics(BasicBlock *BB);
 137
 138   // Pointer Heuristics
 139   void calcPointerHeuristics(BasicBlock *BB);
 140
 141   // Loop Branch Heuristics
 142   void calcLoopBranchHeuristics(BasicBlock *BB);
 143
 144   bool runOnFunction(Function &F);
 145 };
 146
 147 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
 148 // leads directly to Return Instruction will not be taken.
 149 void BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
 150   if (BB->getTerminator()->getNumSuccessors() == 1)
 151     return;
 152
 153   for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
 154     BasicBlock *Succ = *I;
 155     if (isReturningBlock(Succ)) {
 156       decEdgeWeight(BB, Succ);
 157     }
 158   }
 159 }
 160
 161 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
 162 // between two pointer or pointer and NULL will fail.
 163 void BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
 164   BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
 165   if (!BI || !BI->isConditional())
 166     return;
 167
 168   Value *Cond = BI->getCondition();
 169   ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
 170   if (!CI)
 171     return;
 172
 173   Value *LHS = CI->getOperand(0);
 174   Value *RHS = CI->getOperand(1);
 175
 176   if (!LHS->getType()->isPointerTy())
 177     return;
 178
 179   assert(RHS->getType()->isPointerTy());
 180
 181   BasicBlock *Taken = BI->getSuccessor(0);
 182   BasicBlock *NonTaken = BI->getSuccessor(1);
 183
 184   // p != 0   ->   isProb = true
 185   // p == 0   ->   isProb = false
 186   // p != q   ->   isProb = true
 187   // p == q   ->   isProb = false;
 188   bool isProb = !CI->isEquality();
 189   if (!isProb)
 190     std::swap(Taken, NonTaken);
 191
 192   incEdgeWeight(BB, Taken);
 193   decEdgeWeight(BB, NonTaken);
 194 }
 195
 196 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
 197 // as taken, exiting edges as not-taken.
 198 void BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
 199   unsigned numSuccs = BB->getTerminator()->getNumSuccessors();
 200
 201   Loop *L = LI->getLoopFor(BB);
 202   if (!L)
 203     return;
 204
 205   SmallVector<BasicBlock *, 8> BackEdges;
 206   SmallVector<BasicBlock *, 8> ExitingEdges;
 207
 208   for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
 209     BasicBlock *Succ = *I;
 210     Loop *SuccL = LI->getLoopFor(Succ);
 211     if (SuccL != L)
 212       ExitingEdges.push_back(Succ);
 213     else if (Succ == L->getHeader())
 214       BackEdges.push_back(Succ);
 215   }
 216
 217   if (unsigned numBackEdges = BackEdges.size()) {
 218     unsigned backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
 219     if (backWeight < NORMAL_WEIGHT)
 220       backWeight = NORMAL_WEIGHT;
 221
 222     for (SmallVector<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
 223          EE = BackEdges.end(); EI != EE; ++EI) {
 224       BasicBlock *Back = *EI;
 225       BP->setEdgeWeight(BB, Back, backWeight);
 226     }
 227   }
 228
 229   unsigned numExitingEdges = ExitingEdges.size();
 230   if (unsigned numNonExitingEdges = numSuccs - numExitingEdges) {
 231     unsigned exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
 232     if (exitWeight < MIN_WEIGHT)
 233       exitWeight = MIN_WEIGHT;
 234
 235     for (SmallVector<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
 236          EE = ExitingEdges.end(); EI != EE; ++EI) {
 237       BasicBlock *Exiting = *EI;
 238       BP->setEdgeWeight(BB, Exiting, exitWeight);
 239     }
 240   }
 241 }
 242
 243 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
 244
 245   for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
 246     BasicBlock *BB = I++;
 247
 248     // Only LBH uses setEdgeWeight method.
 249     calcLoopBranchHeuristics(BB);
 250
 251     // PH and RH use only incEdgeWeight and decEwdgeWeight methods to
 252     // not efface LBH results.
 253     calcPointerHeuristics(BB);
 254     calcReturnHeuristics(BB);
 255   }
 256
 257   return false;
 258 }
 259
 260
 261 bool BranchProbabilityInfo::runOnFunction(Function &F) {
 262   LoopInfo &LI = getAnalysis<LoopInfo>();
 263   BranchProbabilityAnalysis BPA(&Weights, this, &LI);
 264   bool ret = BPA.runOnFunction(F);
 265   return ret;
 266 }
 267
 268 // TODO: This currently hardcodes 80% as a fraction 4/5. We will soon add a
 269 // BranchProbability class to encapsulate the fractional probability and
 270 // define a few static instances of the class for use as predefined thresholds.
 271 bool BranchProbabilityInfo::isEdgeHot(BasicBlock *Src, BasicBlock *Dst) const {
 272   unsigned Sum = 0;
 273   for (succ_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I) {
 274     BasicBlock *Succ = *I;
 275     unsigned Weight = getEdgeWeight(Src, Succ);
 276     unsigned PrevSum = Sum;
 277
 278     Sum += Weight;
 279     assert(Sum > PrevSum); (void) PrevSum;
 280   }
 281
 282   return getEdgeWeight(Src, Dst) * 5 > Sum * 4;
 283 }
 284
 285 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
 286   unsigned Sum = 0;
 287   unsigned MaxWeight = 0;
 288   BasicBlock *MaxSucc = 0;
 289
 290   for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
 291     BasicBlock *Succ = *I;
 292     unsigned Weight = getEdgeWeight(BB, Succ);
 293     unsigned PrevSum = Sum;
 294
 295     Sum += Weight;
 296     assert(Sum > PrevSum); (void) PrevSum;
 297
 298     if (Weight > MaxWeight) {
 299       MaxWeight = Weight;
 300       MaxSucc = Succ;
 301     }
 302   }
 303
 304   if (MaxWeight * 5 > Sum * 4)
 305     return MaxSucc;
 306
 307   return 0;
 308 }
 309
 310 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
 311 unsigned
 312 BranchProbabilityInfo::getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const {
 313   Edge E(Src, Dst);
 314   DenseMap<Edge, unsigned>::const_iterator I = Weights.find(E);
 315
 316   if (I != Weights.end())
 317     return I->second;
 318
 319   return DEFAULT_WEIGHT;
 320 }
 321
 322 void BranchProbabilityInfo::setEdgeWeight(BasicBlock *Src, BasicBlock *Dst,
 323                                      unsigned Weight) {
 324   Weights[std::make_pair(Src, Dst)] = Weight;
 325   DEBUG(dbgs() << "setEdgeWeight: " << Src->getNameStr() << " -> "
 326         << Dst->getNameStr() << " to " << Weight
 327         << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
 328 }
 329
 330 raw_ostream &
 331 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
 332                                         BasicBlock *Dst) const {
 333
 334   unsigned Sum = 0;
 335   for (succ_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I) {
 336     BasicBlock *Succ = *I;
 337     unsigned Weight = getEdgeWeight(Src, Succ);
 338     unsigned PrevSum = Sum;
 339
 340     Sum += Weight;
 341     assert(Sum > PrevSum); (void) PrevSum;
 342   }
 343
 344   double Prob = (double)getEdgeWeight(Src, Dst) / Sum;
 345   OS << "probability (" << Src->getNameStr() << " --> " << Dst->getNameStr()
 346      << ") = " << Prob << "\n";
 347
 348   return OS;
 349 }