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 "llvm/Analysis/LoopInfo.h"
  17 #include "llvm/Support/Debug.h"
  18
  19 using namespace llvm;
  20
  21 INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
  22                       "Branch Probability Analysis", false, true)
  23 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  24 INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
  25                     "Branch Probability Analysis", false, true)
  26
  27 char BranchProbabilityInfo::ID = 0;
  28
  29 namespace {
  30 // Please note that BranchProbabilityAnalysis is not a FunctionPass.
  31 // It is created by BranchProbabilityInfo (which is a FunctionPass), which
  32 // provides a clear interface. Thanks to that, all heuristics and other
  33 // private methods are hidden in the .cpp file.
  34 class BranchProbabilityAnalysis {
  35
  36   typedef std::pair<BasicBlock *, BasicBlock *> Edge;
  37
  38   DenseMap<Edge, uint32_t> *Weights;
  39
  40   BranchProbabilityInfo *BP;
  41
  42   LoopInfo *LI;
  43
  44
  45   // Weights are for internal use only. They are used by heuristics to help to
  46   // estimate edges' probability. Example:
  47   //
  48   // Using "Loop Branch Heuristics" we predict weights of edges for the
  49   // block BB2.
  50   //         ...
  51   //          |
  52   //          V
  53   //         BB1<-+
  54   //          |   |
  55   //          |   | (Weight = 128)
  56   //          V   |
  57   //         BB2--+
  58   //          |
  59   //          | (Weight = 4)
  60   //          V
  61   //         BB3
  62   //
  63   // Probability of the edge BB2->BB1 = 128 / (128 + 4) = 0.9696..
  64   // Probability of the edge BB2->BB3 = 4 / (128 + 4) = 0.0303..
  65
  66   static const uint32_t LBH_TAKEN_WEIGHT = 128;
  67   static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
  68
  69   // Standard weight value. Used when none of the heuristics set weight for
  70   // the edge.
  71   static const uint32_t NORMAL_WEIGHT = 16;
  72
  73   // Minimum weight of an edge. Please note, that weight is NEVER 0.
  74   static const uint32_t MIN_WEIGHT = 1;
  75
  76   // Return TRUE if BB leads directly to a Return Instruction.
  77   static bool isReturningBlock(BasicBlock *BB) {
  78     SmallPtrSet<BasicBlock *, 8> Visited;
  79
  80     while (true) {
  81       TerminatorInst *TI = BB->getTerminator();
  82       if (isa<ReturnInst>(TI))
  83         return true;
  84
  85       if (TI->getNumSuccessors() > 1)
  86         break;
  87
  88       // It is unreachable block which we can consider as a return instruction.
  89       if (TI->getNumSuccessors() == 0)
  90         return true;
  91
  92       Visited.insert(BB);
  93       BB = TI->getSuccessor(0);
  94
  95       // Stop if cycle is detected.
  96       if (Visited.count(BB))
  97         return false;
  98     }
  99
 100     return false;
 101   }
 102
 103   // Multiply Edge Weight by two.
 104   void incEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
 105     uint32_t Weight = BP->getEdgeWeight(Src, Dst);
 106     uint32_t MaxWeight = getMaxWeightFor(Src);
 107
 108     if (Weight * 2 > MaxWeight)
 109       BP->setEdgeWeight(Src, Dst, MaxWeight);
 110     else
 111       BP->setEdgeWeight(Src, Dst, Weight * 2);
 112   }
 113
 114   // Divide Edge Weight by two.
 115   void decEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
 116     uint32_t Weight = BP->getEdgeWeight(Src, Dst);
 117
 118     assert(Weight > 0);
 119     if (Weight / 2 < MIN_WEIGHT)
 120       BP->setEdgeWeight(Src, Dst, MIN_WEIGHT);
 121     else
 122       BP->setEdgeWeight(Src, Dst, Weight / 2);
 123   }
 124
 125
 126   uint32_t getMaxWeightFor(BasicBlock *BB) const {
 127     return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
 128   }
 129
 130 public:
 131   BranchProbabilityAnalysis(DenseMap<Edge, uint32_t> *W,
 132                             BranchProbabilityInfo *BP, LoopInfo *LI)
 133     : Weights(W), BP(BP), LI(LI) {
 134   }
 135
 136   // Return Heuristics
 137   void calcReturnHeuristics(BasicBlock *BB);
 138
 139   // Pointer Heuristics
 140   void calcPointerHeuristics(BasicBlock *BB);
 141
 142   // Loop Branch Heuristics
 143   void calcLoopBranchHeuristics(BasicBlock *BB);
 144
 145   bool runOnFunction(Function &F);
 146 };
 147 } // end anonymous namespace
 148
 149 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
 150 // leads directly to Return Instruction will not be taken.
 151 void BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
 152   if (BB->getTerminator()->getNumSuccessors() == 1)
 153     return;
 154
 155   for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
 156     BasicBlock *Succ = *I;
 157     if (isReturningBlock(Succ)) {
 158       decEdgeWeight(BB, Succ);
 159     }
 160   }
 161 }
 162
 163 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
 164 // between two pointer or pointer and NULL will fail.
 165 void BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
 166   BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
 167   if (!BI || !BI->isConditional())
 168     return;
 169
 170   Value *Cond = BI->getCondition();
 171   ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
 172   if (!CI || !CI->isEquality())
 173     return;
 174
 175   Value *LHS = CI->getOperand(0);
 176
 177   if (!LHS->getType()->isPointerTy())
 178     return;
 179
 180   assert(CI->getOperand(1)->getType()->isPointerTy());
 181
 182   BasicBlock *Taken = BI->getSuccessor(0);
 183   BasicBlock *NonTaken = BI->getSuccessor(1);
 184
 185   // p != 0   ->   isProb = true
 186   // p == 0   ->   isProb = false
 187   // p != q   ->   isProb = true
 188   // p == q   ->   isProb = false;
 189   bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
 190   if (!isProb)
 191     std::swap(Taken, NonTaken);
 192
 193   incEdgeWeight(BB, Taken);
 194   decEdgeWeight(BB, NonTaken);
 195 }
 196
 197 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
 198 // as taken, exiting edges as not-taken.
 199 void BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
 200   uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
 201
 202   Loop *L = LI->getLoopFor(BB);
 203   if (!L)
 204     return;
 205
 206   SmallVector<BasicBlock *, 8> BackEdges;
 207   SmallVector<BasicBlock *, 8> ExitingEdges;
 208
 209   for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
 210     BasicBlock *Succ = *I;
 211     Loop *SuccL = LI->getLoopFor(Succ);
 212     if (SuccL != L)
 213       ExitingEdges.push_back(Succ);
 214     else if (Succ == L->getHeader())
 215       BackEdges.push_back(Succ);
 216   }
 217
 218   if (uint32_t numBackEdges = BackEdges.size()) {
 219     uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
 220     if (backWeight < NORMAL_WEIGHT)
 221       backWeight = NORMAL_WEIGHT;
 222
 223     for (SmallVector<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
 224          EE = BackEdges.end(); EI != EE; ++EI) {
 225       BasicBlock *Back = *EI;
 226       BP->setEdgeWeight(BB, Back, backWeight);
 227     }
 228   }
 229
 230   uint32_t numExitingEdges = ExitingEdges.size();
 231   if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
 232     uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
 233     if (exitWeight < MIN_WEIGHT)
 234       exitWeight = MIN_WEIGHT;
 235
 236     for (SmallVector<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
 237          EE = ExitingEdges.end(); EI != EE; ++EI) {
 238       BasicBlock *Exiting = *EI;
 239       BP->setEdgeWeight(BB, Exiting, exitWeight);
 240     }
 241   }
 242 }
 243
 244 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
 245
 246   for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
 247     BasicBlock *BB = I++;
 248
 249     // Only LBH uses setEdgeWeight method.
 250     calcLoopBranchHeuristics(BB);
 251
 252     // PH and RH use only incEdgeWeight and decEwdgeWeight methods to
 253     // not efface LBH results.
 254     calcPointerHeuristics(BB);
 255     calcReturnHeuristics(BB);
 256   }
 257
 258   return false;
 259 }
 260
 261 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
 262     AU.addRequired<LoopInfo>();
 263     AU.setPreservesAll();
 264 }
 265
 266 bool BranchProbabilityInfo::runOnFunction(Function &F) {
 267   LoopInfo &LI = getAnalysis<LoopInfo>();
 268   BranchProbabilityAnalysis BPA(&Weights, this, &LI);
 269   return BPA.runOnFunction(F);
 270 }
 271
 272 uint32_t BranchProbabilityInfo::getSumForBlock(BasicBlock *BB) const {
 273   uint32_t Sum = 0;
 274
 275   for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
 276     BasicBlock *Succ = *I;
 277     uint32_t Weight = getEdgeWeight(BB, Succ);
 278     uint32_t PrevSum = Sum;
 279
 280     Sum += Weight;
 281     assert(Sum > PrevSum); (void) PrevSum;
 282   }
 283
 284   return Sum;
 285 }
 286
 287 bool BranchProbabilityInfo::isEdgeHot(BasicBlock *Src, BasicBlock *Dst) const {
 288   // Hot probability is at least 4/5 = 80%
 289   uint32_t Weight = getEdgeWeight(Src, Dst);
 290   uint32_t Sum = getSumForBlock(Src);
 291
 292   // FIXME: Implement BranchProbability::compare then change this code to
 293   // compare this BranchProbability against a static "hot" BranchProbability.
 294   return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
 295 }
 296
 297 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
 298   uint32_t Sum = 0;
 299   uint32_t MaxWeight = 0;
 300   BasicBlock *MaxSucc = 0;
 301
 302   for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
 303     BasicBlock *Succ = *I;
 304     uint32_t Weight = getEdgeWeight(BB, Succ);
 305     uint32_t PrevSum = Sum;
 306
 307     Sum += Weight;
 308     assert(Sum > PrevSum); (void) PrevSum;
 309
 310     if (Weight > MaxWeight) {
 311       MaxWeight = Weight;
 312       MaxSucc = Succ;
 313     }
 314   }
 315
 316   // FIXME: Use BranchProbability::compare.
 317   if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
 318     return MaxSucc;
 319
 320   return 0;
 321 }
 322
 323 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
 324 uint32_t
 325 BranchProbabilityInfo::getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const {
 326   Edge E(Src, Dst);
 327   DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
 328
 329   if (I != Weights.end())
 330     return I->second;
 331
 332   return DEFAULT_WEIGHT;
 333 }
 334
 335 void BranchProbabilityInfo::setEdgeWeight(BasicBlock *Src, BasicBlock *Dst,
 336                                      uint32_t Weight) {
 337   Weights[std::make_pair(Src, Dst)] = Weight;
 338   DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
 339                << Dst->getNameStr() << " weight to " << Weight
 340                << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
 341 }
 342
 343
 344 BranchProbability BranchProbabilityInfo::
 345 getEdgeProbability(BasicBlock *Src, BasicBlock *Dst) const {
 346
 347   uint32_t N = getEdgeWeight(Src, Dst);
 348   uint32_t D = getSumForBlock(Src);
 349
 350   return BranchProbability(N, D);
 351 }
 352
 353 raw_ostream &
 354 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
 355                                             BasicBlock *Dst) const {
 356
 357   const BranchProbability Prob = getEdgeProbability(Src, Dst);
 358   OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
 359      << " probability is " << Prob
 360      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
 361
 362   return OS;
 363 }