1 //===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // Loops should be simplified before this analysis.
12 //===----------------------------------------------------------------------===//
14 #include "llvm/Instructions.h"
15 #include "llvm/Analysis/BranchProbabilityInfo.h"
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)
26 char BranchProbabilityInfo::ID = 0;
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 {
35 typedef std::pair<BasicBlock *, BasicBlock *> Edge;
37 DenseMap<Edge, unsigned> *Weights;
39 BranchProbabilityInfo *BP;
44 // Weights are for internal use only. They are used by heuristics to help to
45 // estimate edges' probability. Example:
47 // Using "Loop Branch Heuristics" we predict weights of edges for the
62 // Probability of the edge BB2->BB1 = 128 / (128 + 4) = 0.9696..
63 // Probability of the edge BB2->BB3 = 4 / (128 + 4) = 0.0303..
65 static const unsigned int LBH_TAKEN_WEIGHT = 128;
66 static const unsigned int LBH_NONTAKEN_WEIGHT = 4;
68 // Standard weight value. Used when none of the heuristics set weight for
70 static const unsigned int NORMAL_WEIGHT = 16;
72 // Minimum weight of an edge. Please note, that weight is NEVER 0.
73 static const unsigned int MIN_WEIGHT = 1;
75 // Return TRUE if BB leads directly to a Return Instruction.
76 static bool isReturningBlock(BasicBlock *BB) {
77 SmallPtrSet<BasicBlock *, 8> Visited;
80 TerminatorInst *TI = BB->getTerminator();
81 if (isa<ReturnInst>(TI))
84 if (TI->getNumSuccessors() > 1)
87 // It is unreachable block which we can consider as a return instruction.
88 if (TI->getNumSuccessors() == 0)
92 BB = TI->getSuccessor(0);
94 // Stop if cycle is detected.
95 if (Visited.count(BB))
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);
107 if (Weight * 2 > MaxWeight)
108 BP->setEdgeWeight(Src, Dst, MaxWeight);
110 BP->setEdgeWeight(Src, Dst, Weight * 2);
113 // Divide Edge Weight by two.
114 void decEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
115 unsigned Weight = BP->getEdgeWeight(Src, Dst);
118 if (Weight / 2 < MIN_WEIGHT)
119 BP->setEdgeWeight(Src, Dst, MIN_WEIGHT);
121 BP->setEdgeWeight(Src, Dst, Weight / 2);
125 unsigned getMaxWeightFor(BasicBlock *BB) const {
126 return UINT_MAX / BB->getTerminator()->getNumSuccessors();
130 BranchProbabilityAnalysis(DenseMap<Edge, unsigned> *W,
131 BranchProbabilityInfo *BP, LoopInfo *LI)
132 : Weights(W), BP(BP), LI(LI) {
136 void calcReturnHeuristics(BasicBlock *BB);
138 // Pointer Heuristics
139 void calcPointerHeuristics(BasicBlock *BB);
141 // Loop Branch Heuristics
142 void calcLoopBranchHeuristics(BasicBlock *BB);
144 bool runOnFunction(Function &F);
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)
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);
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())
168 Value *Cond = BI->getCondition();
169 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
173 Value *LHS = CI->getOperand(0);
174 Value *RHS = CI->getOperand(1);
176 if (!LHS->getType()->isPointerTy())
179 assert(RHS->getType()->isPointerTy());
181 BasicBlock *Taken = BI->getSuccessor(0);
182 BasicBlock *NonTaken = BI->getSuccessor(1);
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();
190 std::swap(Taken, NonTaken);
192 incEdgeWeight(BB, Taken);
193 decEdgeWeight(BB, NonTaken);
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();
201 Loop *L = LI->getLoopFor(BB);
205 SmallVector<BasicBlock *, 8> BackEdges;
206 SmallVector<BasicBlock *, 8> ExitingEdges;
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);
212 ExitingEdges.push_back(Succ);
213 else if (Succ == L->getHeader())
214 BackEdges.push_back(Succ);
217 if (unsigned numBackEdges = BackEdges.size()) {
218 unsigned backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
219 if (backWeight < NORMAL_WEIGHT)
220 backWeight = NORMAL_WEIGHT;
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);
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;
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);
243 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
245 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
246 BasicBlock *BB = I++;
248 // Only LBH uses setEdgeWeight method.
249 calcLoopBranchHeuristics(BB);
251 // PH and RH use only incEdgeWeight and decEwdgeWeight methods to
252 // not efface LBH results.
253 calcPointerHeuristics(BB);
254 calcReturnHeuristics(BB);
261 bool BranchProbabilityInfo::runOnFunction(Function &F) {
262 LoopInfo &LI = getAnalysis<LoopInfo>();
263 BranchProbabilityAnalysis BPA(&Weights, this, &LI);
264 bool ret = BPA.runOnFunction(F);
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 {
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;
279 assert(Sum > PrevSum); (void) PrevSum;
282 return getEdgeWeight(Src, Dst) * 5 > Sum * 4;
285 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
287 unsigned MaxWeight = 0;
288 BasicBlock *MaxSucc = 0;
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;
296 assert(Sum > PrevSum); (void) PrevSum;
298 if (Weight > MaxWeight) {
304 if (MaxWeight * 5 > Sum * 4)
310 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
312 BranchProbabilityInfo::getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const {
314 DenseMap<Edge, unsigned>::const_iterator I = Weights.find(E);
316 if (I != Weights.end())
319 return DEFAULT_WEIGHT;
322 void BranchProbabilityInfo::setEdgeWeight(BasicBlock *Src, BasicBlock *Dst,
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"));
331 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
332 BasicBlock *Dst) const {
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;
341 assert(Sum > PrevSum); (void) PrevSum;
344 double Prob = (double)getEdgeWeight(Src, Dst) / Sum;
345 OS << "probability (" << Src->getNameStr() << " --> " << Dst->getNameStr()
346 << ") = " << Prob << "\n";