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/Constants.h"
15 #include "llvm/Instructions.h"
16 #include "llvm/Analysis/BranchProbabilityInfo.h"
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/Support/Debug.h"
22 INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
23 "Branch Probability Analysis", false, true)
24 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
25 INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
26 "Branch Probability Analysis", false, true)
28 char BranchProbabilityInfo::ID = 0;
31 // Please note that BranchProbabilityAnalysis is not a FunctionPass.
32 // It is created by BranchProbabilityInfo (which is a FunctionPass), which
33 // provides a clear interface. Thanks to that, all heuristics and other
34 // private methods are hidden in the .cpp file.
35 class BranchProbabilityAnalysis {
37 typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;
39 DenseMap<Edge, uint32_t> *Weights;
41 BranchProbabilityInfo *BP;
46 // Weights are for internal use only. They are used by heuristics to help to
47 // estimate edges' probability. Example:
49 // Using "Loop Branch Heuristics" we predict weights of edges for the
64 // Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
65 // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
67 static const uint32_t LBH_TAKEN_WEIGHT = 124;
68 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
70 static const uint32_t RH_TAKEN_WEIGHT = 24;
71 static const uint32_t RH_NONTAKEN_WEIGHT = 8;
73 static const uint32_t PH_TAKEN_WEIGHT = 20;
74 static const uint32_t PH_NONTAKEN_WEIGHT = 12;
76 static const uint32_t ZH_TAKEN_WEIGHT = 20;
77 static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
79 // Standard weight value. Used when none of the heuristics set weight for
81 static const uint32_t NORMAL_WEIGHT = 16;
83 // Minimum weight of an edge. Please note, that weight is NEVER 0.
84 static const uint32_t MIN_WEIGHT = 1;
86 // Return TRUE if BB leads directly to a Return Instruction.
87 static bool isReturningBlock(BasicBlock *BB) {
88 SmallPtrSet<BasicBlock *, 8> Visited;
91 TerminatorInst *TI = BB->getTerminator();
92 if (isa<ReturnInst>(TI))
95 if (TI->getNumSuccessors() > 1)
98 // It is unreachable block which we can consider as a return instruction.
99 if (TI->getNumSuccessors() == 0)
103 BB = TI->getSuccessor(0);
105 // Stop if cycle is detected.
106 if (Visited.count(BB))
113 uint32_t getMaxWeightFor(BasicBlock *BB) const {
114 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
118 BranchProbabilityAnalysis(DenseMap<Edge, uint32_t> *W,
119 BranchProbabilityInfo *BP, LoopInfo *LI)
120 : Weights(W), BP(BP), LI(LI) {
124 bool calcReturnHeuristics(BasicBlock *BB);
126 // Pointer Heuristics
127 bool calcPointerHeuristics(BasicBlock *BB);
129 // Loop Branch Heuristics
130 bool calcLoopBranchHeuristics(BasicBlock *BB);
133 bool calcZeroHeuristics(BasicBlock *BB);
135 bool runOnFunction(Function &F);
137 } // end anonymous namespace
139 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
140 // leads directly to Return Instruction will not be taken.
141 bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
142 if (BB->getTerminator()->getNumSuccessors() == 1)
145 SmallPtrSet<BasicBlock *, 4> ReturningEdges;
146 SmallPtrSet<BasicBlock *, 4> StayEdges;
148 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
149 BasicBlock *Succ = *I;
150 if (isReturningBlock(Succ))
151 ReturningEdges.insert(Succ);
153 StayEdges.insert(Succ);
156 if (uint32_t numStayEdges = StayEdges.size()) {
157 uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
158 if (stayWeight < NORMAL_WEIGHT)
159 stayWeight = NORMAL_WEIGHT;
161 for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
162 E = StayEdges.end(); I != E; ++I)
163 BP->setEdgeWeight(BB, *I, stayWeight);
166 if (uint32_t numRetEdges = ReturningEdges.size()) {
167 uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
168 if (retWeight < MIN_WEIGHT)
169 retWeight = MIN_WEIGHT;
170 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
171 E = ReturningEdges.end(); I != E; ++I) {
172 BP->setEdgeWeight(BB, *I, retWeight);
176 return ReturningEdges.size() > 0;
179 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
180 // between two pointer or pointer and NULL will fail.
181 bool BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
182 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
183 if (!BI || !BI->isConditional())
186 Value *Cond = BI->getCondition();
187 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
188 if (!CI || !CI->isEquality())
191 Value *LHS = CI->getOperand(0);
193 if (!LHS->getType()->isPointerTy())
196 assert(CI->getOperand(1)->getType()->isPointerTy());
198 BasicBlock *Taken = BI->getSuccessor(0);
199 BasicBlock *NonTaken = BI->getSuccessor(1);
201 // p != 0 -> isProb = true
202 // p == 0 -> isProb = false
203 // p != q -> isProb = true
204 // p == q -> isProb = false;
205 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
207 std::swap(Taken, NonTaken);
209 BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
210 BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
214 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
215 // as taken, exiting edges as not-taken.
216 bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
217 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
219 Loop *L = LI->getLoopFor(BB);
223 SmallPtrSet<BasicBlock *, 8> BackEdges;
224 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
225 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
227 bool isHeader = BB == L->getHeader();
229 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
230 BasicBlock *Succ = *I;
231 Loop *SuccL = LI->getLoopFor(Succ);
233 ExitingEdges.insert(Succ);
234 else if (Succ == L->getHeader())
235 BackEdges.insert(Succ);
237 InEdges.insert(Succ);
240 if (uint32_t numBackEdges = BackEdges.size()) {
241 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
242 if (backWeight < NORMAL_WEIGHT)
243 backWeight = NORMAL_WEIGHT;
245 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
246 EE = BackEdges.end(); EI != EE; ++EI) {
247 BasicBlock *Back = *EI;
248 BP->setEdgeWeight(BB, Back, backWeight);
252 if (uint32_t numInEdges = InEdges.size()) {
253 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
254 if (inWeight < NORMAL_WEIGHT)
255 inWeight = NORMAL_WEIGHT;
257 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
258 EE = InEdges.end(); EI != EE; ++EI) {
259 BasicBlock *Back = *EI;
260 BP->setEdgeWeight(BB, Back, inWeight);
264 uint32_t numExitingEdges = ExitingEdges.size();
265 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
266 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
267 if (exitWeight < MIN_WEIGHT)
268 exitWeight = MIN_WEIGHT;
270 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
271 EE = ExitingEdges.end(); EI != EE; ++EI) {
272 BasicBlock *Exiting = *EI;
273 BP->setEdgeWeight(BB, Exiting, exitWeight);
280 bool BranchProbabilityAnalysis::calcZeroHeuristics(BasicBlock *BB) {
281 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
282 if (!BI || !BI->isConditional())
285 Value *Cond = BI->getCondition();
286 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
290 Value *RHS = CI->getOperand(1);
291 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
292 if (!CV || !CV->isZero())
296 switch (CI->getPredicate()) {
297 case CmpInst::ICMP_EQ:
298 // Equal to zero is not expected to be taken.
302 case CmpInst::ICMP_NE:
303 // Not equal to zero is expected.
307 case CmpInst::ICMP_SLT:
308 // Less or equal to zero is not expected.
313 case CmpInst::ICMP_UGT:
314 case CmpInst::ICMP_SGT:
315 // Greater or equal to zero is expected.
324 BasicBlock *Taken = BI->getSuccessor(0);
325 BasicBlock *NonTaken = BI->getSuccessor(1);
328 std::swap(Taken, NonTaken);
330 BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
331 BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
337 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
339 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
340 BasicBlock *BB = I++;
342 if (calcLoopBranchHeuristics(BB))
345 if (calcReturnHeuristics(BB))
348 if (calcPointerHeuristics(BB))
351 calcZeroHeuristics(BB);
357 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
358 AU.addRequired<LoopInfo>();
359 AU.setPreservesAll();
362 bool BranchProbabilityInfo::runOnFunction(Function &F) {
363 LoopInfo &LI = getAnalysis<LoopInfo>();
364 BranchProbabilityAnalysis BPA(&Weights, this, &LI);
365 return BPA.runOnFunction(F);
368 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
371 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
372 const BasicBlock *Succ = *I;
373 uint32_t Weight = getEdgeWeight(BB, Succ);
374 uint32_t PrevSum = Sum;
377 assert(Sum > PrevSum); (void) PrevSum;
383 bool BranchProbabilityInfo::
384 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
385 // Hot probability is at least 4/5 = 80%
386 uint32_t Weight = getEdgeWeight(Src, Dst);
387 uint32_t Sum = getSumForBlock(Src);
389 // FIXME: Implement BranchProbability::compare then change this code to
390 // compare this BranchProbability against a static "hot" BranchProbability.
391 return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
394 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
396 uint32_t MaxWeight = 0;
397 BasicBlock *MaxSucc = 0;
399 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
400 BasicBlock *Succ = *I;
401 uint32_t Weight = getEdgeWeight(BB, Succ);
402 uint32_t PrevSum = Sum;
405 assert(Sum > PrevSum); (void) PrevSum;
407 if (Weight > MaxWeight) {
413 // FIXME: Use BranchProbability::compare.
414 if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
420 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
421 uint32_t BranchProbabilityInfo::
422 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
424 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
426 if (I != Weights.end())
429 return DEFAULT_WEIGHT;
432 void BranchProbabilityInfo::
433 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
434 Weights[std::make_pair(Src, Dst)] = Weight;
435 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
436 << Dst->getNameStr() << " weight to " << Weight
437 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
441 BranchProbability BranchProbabilityInfo::
442 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
444 uint32_t N = getEdgeWeight(Src, Dst);
445 uint32_t D = getSumForBlock(Src);
447 return BranchProbability(N, D);
451 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
452 BasicBlock *Dst) const {
454 const BranchProbability Prob = getEdgeProbability(Src, Dst);
455 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
456 << " probability is " << Prob
457 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");