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 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 = 124 / (124 + 4) = 0.96875
63 // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
65 static const uint32_t LBH_TAKEN_WEIGHT = 124;
66 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
68 static const uint32_t RH_TAKEN_WEIGHT = 24;
69 static const uint32_t RH_NONTAKEN_WEIGHT = 8;
71 static const uint32_t PH_TAKEN_WEIGHT = 20;
72 static const uint32_t PH_NONTAKEN_WEIGHT = 12;
74 static const uint32_t ZH_TAKEN_WEIGHT = 20;
75 static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
77 // Standard weight value. Used when none of the heuristics set weight for
79 static const uint32_t NORMAL_WEIGHT = 16;
81 // Minimum weight of an edge. Please note, that weight is NEVER 0.
82 static const uint32_t MIN_WEIGHT = 1;
84 // Return TRUE if BB leads directly to a Return Instruction.
85 static bool isReturningBlock(BasicBlock *BB) {
86 SmallPtrSet<BasicBlock *, 8> Visited;
89 TerminatorInst *TI = BB->getTerminator();
90 if (isa<ReturnInst>(TI))
93 if (TI->getNumSuccessors() > 1)
96 // It is unreachable block which we can consider as a return instruction.
97 if (TI->getNumSuccessors() == 0)
101 BB = TI->getSuccessor(0);
103 // Stop if cycle is detected.
104 if (Visited.count(BB))
111 uint32_t getMaxWeightFor(BasicBlock *BB) const {
112 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
116 BranchProbabilityAnalysis(BranchProbabilityInfo *BP, LoopInfo *LI)
121 bool calcReturnHeuristics(BasicBlock *BB);
123 // Pointer Heuristics
124 bool calcPointerHeuristics(BasicBlock *BB);
126 // Loop Branch Heuristics
127 bool calcLoopBranchHeuristics(BasicBlock *BB);
130 bool calcZeroHeuristics(BasicBlock *BB);
132 bool runOnFunction(Function &F);
134 } // end anonymous namespace
136 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
137 // leads directly to Return Instruction will not be taken.
138 bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
139 if (BB->getTerminator()->getNumSuccessors() == 1)
142 SmallPtrSet<BasicBlock *, 4> ReturningEdges;
143 SmallPtrSet<BasicBlock *, 4> StayEdges;
145 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
146 BasicBlock *Succ = *I;
147 if (isReturningBlock(Succ))
148 ReturningEdges.insert(Succ);
150 StayEdges.insert(Succ);
153 if (uint32_t numStayEdges = StayEdges.size()) {
154 uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
155 if (stayWeight < NORMAL_WEIGHT)
156 stayWeight = NORMAL_WEIGHT;
158 for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
159 E = StayEdges.end(); I != E; ++I)
160 BP->setEdgeWeight(BB, *I, stayWeight);
163 if (uint32_t numRetEdges = ReturningEdges.size()) {
164 uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
165 if (retWeight < MIN_WEIGHT)
166 retWeight = MIN_WEIGHT;
167 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
168 E = ReturningEdges.end(); I != E; ++I) {
169 BP->setEdgeWeight(BB, *I, retWeight);
173 return ReturningEdges.size() > 0;
176 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
177 // between two pointer or pointer and NULL will fail.
178 bool BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
179 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
180 if (!BI || !BI->isConditional())
183 Value *Cond = BI->getCondition();
184 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
185 if (!CI || !CI->isEquality())
188 Value *LHS = CI->getOperand(0);
190 if (!LHS->getType()->isPointerTy())
193 assert(CI->getOperand(1)->getType()->isPointerTy());
195 BasicBlock *Taken = BI->getSuccessor(0);
196 BasicBlock *NonTaken = BI->getSuccessor(1);
198 // p != 0 -> isProb = true
199 // p == 0 -> isProb = false
200 // p != q -> isProb = true
201 // p == q -> isProb = false;
202 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
204 std::swap(Taken, NonTaken);
206 BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
207 BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
211 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
212 // as taken, exiting edges as not-taken.
213 bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
214 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
216 Loop *L = LI->getLoopFor(BB);
220 SmallPtrSet<BasicBlock *, 8> BackEdges;
221 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
222 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
224 bool isHeader = BB == L->getHeader();
226 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
227 BasicBlock *Succ = *I;
228 Loop *SuccL = LI->getLoopFor(Succ);
230 ExitingEdges.insert(Succ);
231 else if (Succ == L->getHeader())
232 BackEdges.insert(Succ);
234 InEdges.insert(Succ);
237 if (uint32_t numBackEdges = BackEdges.size()) {
238 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
239 if (backWeight < NORMAL_WEIGHT)
240 backWeight = NORMAL_WEIGHT;
242 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
243 EE = BackEdges.end(); EI != EE; ++EI) {
244 BasicBlock *Back = *EI;
245 BP->setEdgeWeight(BB, Back, backWeight);
249 if (uint32_t numInEdges = InEdges.size()) {
250 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
251 if (inWeight < NORMAL_WEIGHT)
252 inWeight = NORMAL_WEIGHT;
254 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
255 EE = InEdges.end(); EI != EE; ++EI) {
256 BasicBlock *Back = *EI;
257 BP->setEdgeWeight(BB, Back, inWeight);
261 uint32_t numExitingEdges = ExitingEdges.size();
262 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
263 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
264 if (exitWeight < MIN_WEIGHT)
265 exitWeight = MIN_WEIGHT;
267 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
268 EE = ExitingEdges.end(); EI != EE; ++EI) {
269 BasicBlock *Exiting = *EI;
270 BP->setEdgeWeight(BB, Exiting, exitWeight);
277 bool BranchProbabilityAnalysis::calcZeroHeuristics(BasicBlock *BB) {
278 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
279 if (!BI || !BI->isConditional())
282 Value *Cond = BI->getCondition();
283 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
287 Value *RHS = CI->getOperand(1);
288 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
294 switch (CI->getPredicate()) {
295 case CmpInst::ICMP_EQ:
296 // X == 0 -> Unlikely
299 case CmpInst::ICMP_NE:
303 case CmpInst::ICMP_SLT:
307 case CmpInst::ICMP_SGT:
314 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
315 // InstCombine canonicalizes X <= 0 into X < 1.
316 // X <= 0 -> Unlikely
318 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
319 // InstCombine canonicalizes X >= 0 into X > -1.
326 BasicBlock *Taken = BI->getSuccessor(0);
327 BasicBlock *NonTaken = BI->getSuccessor(1);
330 std::swap(Taken, NonTaken);
332 BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
333 BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
339 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
341 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
342 BasicBlock *BB = I++;
344 if (calcLoopBranchHeuristics(BB))
347 if (calcReturnHeuristics(BB))
350 if (calcPointerHeuristics(BB))
353 calcZeroHeuristics(BB);
359 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
360 AU.addRequired<LoopInfo>();
361 AU.setPreservesAll();
364 bool BranchProbabilityInfo::runOnFunction(Function &F) {
365 LoopInfo &LI = getAnalysis<LoopInfo>();
366 BranchProbabilityAnalysis BPA(this, &LI);
367 return BPA.runOnFunction(F);
370 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
373 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
374 const BasicBlock *Succ = *I;
375 uint32_t Weight = getEdgeWeight(BB, Succ);
376 uint32_t PrevSum = Sum;
379 assert(Sum > PrevSum); (void) PrevSum;
385 bool BranchProbabilityInfo::
386 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
387 // Hot probability is at least 4/5 = 80%
388 uint32_t Weight = getEdgeWeight(Src, Dst);
389 uint32_t Sum = getSumForBlock(Src);
391 // FIXME: Implement BranchProbability::compare then change this code to
392 // compare this BranchProbability against a static "hot" BranchProbability.
393 return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
396 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
398 uint32_t MaxWeight = 0;
399 BasicBlock *MaxSucc = 0;
401 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
402 BasicBlock *Succ = *I;
403 uint32_t Weight = getEdgeWeight(BB, Succ);
404 uint32_t PrevSum = Sum;
407 assert(Sum > PrevSum); (void) PrevSum;
409 if (Weight > MaxWeight) {
415 // FIXME: Use BranchProbability::compare.
416 if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
422 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
423 uint32_t BranchProbabilityInfo::
424 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
426 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
428 if (I != Weights.end())
431 return DEFAULT_WEIGHT;
434 void BranchProbabilityInfo::
435 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
436 Weights[std::make_pair(Src, Dst)] = Weight;
437 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
438 << Dst->getNameStr() << " weight to " << Weight
439 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
443 BranchProbability BranchProbabilityInfo::
444 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
446 uint32_t N = getEdgeWeight(Src, Dst);
447 uint32_t D = getSumForBlock(Src);
449 return BranchProbability(N, D);
453 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
454 BasicBlock *Dst) const {
456 const BranchProbability Prob = getEdgeProbability(Src, Dst);
457 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
458 << " probability is " << Prob
459 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");