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"
16 #include "llvm/Support/Debug.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, uint32_t> *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 uint32_t LBH_TAKEN_WEIGHT = 128;
66 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
68 // Standard weight value. Used when none of the heuristics set weight for
70 static const uint32_t NORMAL_WEIGHT = 16;
72 // Minimum weight of an edge. Please note, that weight is NEVER 0.
73 static const uint32_t 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 uint32_t Weight = BP->getEdgeWeight(Src, Dst);
105 uint32_t 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 uint32_t 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 uint32_t getMaxWeightFor(BasicBlock *BB) const {
126 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
130 BranchProbabilityAnalysis(DenseMap<Edge, uint32_t> *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);
146 } // end anonymous namespace
148 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
149 // leads directly to Return Instruction will not be taken.
150 void BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
151 if (BB->getTerminator()->getNumSuccessors() == 1)
154 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
155 BasicBlock *Succ = *I;
156 if (isReturningBlock(Succ)) {
157 decEdgeWeight(BB, Succ);
162 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
163 // between two pointer or pointer and NULL will fail.
164 void BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
165 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
166 if (!BI || !BI->isConditional())
169 Value *Cond = BI->getCondition();
170 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
174 Value *LHS = CI->getOperand(0);
176 if (!LHS->getType()->isPointerTy())
179 assert(CI->getOperand(1)->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 uint32_t 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 (uint32_t numBackEdges = BackEdges.size()) {
218 uint32_t 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 uint32_t numExitingEdges = ExitingEdges.size();
230 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
231 uint32_t 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 return BPA.runOnFunction(F);
267 uint32_t BranchProbabilityInfo::getSumForBlock(BasicBlock *BB) const {
270 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
271 BasicBlock *Succ = *I;
272 uint32_t Weight = getEdgeWeight(BB, Succ);
273 uint32_t PrevSum = Sum;
276 assert(Sum > PrevSum); (void) PrevSum;
282 uint32_t BranchProbabilityInfo::getBackSumForBlock(BasicBlock *BB) const {
285 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
286 BasicBlock *Pred = *I;
287 uint32_t Weight = getEdgeWeight(Pred, BB);
288 uint32_t PrevSum = Sum;
291 assert(Sum > PrevSum); (void) PrevSum;
297 bool BranchProbabilityInfo::isEdgeHot(BasicBlock *Src, BasicBlock *Dst) const {
298 // Hot probability is at least 4/5 = 80%
299 uint32_t Weight = getEdgeWeight(Src, Dst);
300 uint32_t Sum = getSumForBlock(Src);
302 // FIXME: Implement BranchProbability::compare then change this code to
303 // compare this BranchProbability against a static "hot" BranchProbability.
304 return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
307 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
309 uint32_t MaxWeight = 0;
310 BasicBlock *MaxSucc = 0;
312 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
313 BasicBlock *Succ = *I;
314 uint32_t Weight = getEdgeWeight(BB, Succ);
315 uint32_t PrevSum = Sum;
318 assert(Sum > PrevSum); (void) PrevSum;
320 if (Weight > MaxWeight) {
326 // FIXME: Use BranchProbability::compare.
327 if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
333 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
335 BranchProbabilityInfo::getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const {
337 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
339 if (I != Weights.end())
342 return DEFAULT_WEIGHT;
345 void BranchProbabilityInfo::setEdgeWeight(BasicBlock *Src, BasicBlock *Dst,
347 Weights[std::make_pair(Src, Dst)] = Weight;
348 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
349 << Dst->getNameStr() << " weight to " << Weight
350 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
354 BranchProbability BranchProbabilityInfo::
355 getEdgeProbability(BasicBlock *Src, BasicBlock *Dst) const {
357 uint32_t N = getEdgeWeight(Src, Dst);
358 uint32_t D = getSumForBlock(Src);
360 return BranchProbability(N, D);
363 BranchProbability BranchProbabilityInfo::
364 getBackEdgeProbability(BasicBlock *Src, BasicBlock *Dst) const {
366 uint32_t N = getEdgeWeight(Src, Dst);
367 uint32_t D = getBackSumForBlock(Dst);
369 return BranchProbability(N, D);
373 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
374 BasicBlock *Dst) const {
376 const BranchProbability Prob = getEdgeProbability(Src, Dst);
377 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
378 << " probability is " << Prob
379 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");