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/LLVMContext.h"
17 #include "llvm/Metadata.h"
18 #include "llvm/Analysis/BranchProbabilityInfo.h"
19 #include "llvm/Analysis/LoopInfo.h"
20 #include "llvm/Support/Debug.h"
24 INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
25 "Branch Probability Analysis", false, true)
26 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
27 INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
28 "Branch Probability Analysis", false, true)
30 char BranchProbabilityInfo::ID = 0;
33 // Please note that BranchProbabilityAnalysis is not a FunctionPass.
34 // It is created by BranchProbabilityInfo (which is a FunctionPass), which
35 // provides a clear interface. Thanks to that, all heuristics and other
36 // private methods are hidden in the .cpp file.
37 class BranchProbabilityAnalysis {
39 typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;
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(BranchProbabilityInfo *BP, LoopInfo *LI)
123 bool calcMetadataWeights(BasicBlock *BB);
126 bool calcReturnHeuristics(BasicBlock *BB);
128 // Pointer Heuristics
129 bool calcPointerHeuristics(BasicBlock *BB);
131 // Loop Branch Heuristics
132 bool calcLoopBranchHeuristics(BasicBlock *BB);
135 bool calcZeroHeuristics(BasicBlock *BB);
137 bool runOnFunction(Function &F);
139 } // end anonymous namespace
141 // Propagate existing explicit probabilities from either profile data or
142 // 'expect' intrinsic processing.
143 // FIXME: This doesn't correctly extract probabilities for switches.
144 bool BranchProbabilityAnalysis::calcMetadataWeights(BasicBlock *BB) {
145 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
146 if (!BI || !BI->isConditional())
149 MDNode *WeightsNode = BI->getMetadata(LLVMContext::MD_prof);
150 if (!WeightsNode || WeightsNode->getNumOperands() < 3)
153 // Pull the weights out of the metadata. Note that the zero operand is the
155 ConstantInt *Weights[] = {
156 dyn_cast<ConstantInt>(WeightsNode->getOperand(1)),
157 dyn_cast<ConstantInt>(WeightsNode->getOperand(2))
159 if (!Weights[0] || !Weights[1])
162 uint32_t WeightLimit = getMaxWeightFor(BB);
163 BP->setEdgeWeight(BB, BI->getSuccessor(0),
164 Weights[0]->getLimitedValue(WeightLimit));
165 BP->setEdgeWeight(BB, BI->getSuccessor(1),
166 Weights[1]->getLimitedValue(WeightLimit));
171 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
172 // leads directly to Return Instruction will not be taken.
173 bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
174 if (BB->getTerminator()->getNumSuccessors() == 1)
177 SmallPtrSet<BasicBlock *, 4> ReturningEdges;
178 SmallPtrSet<BasicBlock *, 4> StayEdges;
180 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
181 BasicBlock *Succ = *I;
182 if (isReturningBlock(Succ))
183 ReturningEdges.insert(Succ);
185 StayEdges.insert(Succ);
188 if (uint32_t numStayEdges = StayEdges.size()) {
189 uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
190 if (stayWeight < NORMAL_WEIGHT)
191 stayWeight = NORMAL_WEIGHT;
193 for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
194 E = StayEdges.end(); I != E; ++I)
195 BP->setEdgeWeight(BB, *I, stayWeight);
198 if (uint32_t numRetEdges = ReturningEdges.size()) {
199 uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
200 if (retWeight < MIN_WEIGHT)
201 retWeight = MIN_WEIGHT;
202 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
203 E = ReturningEdges.end(); I != E; ++I) {
204 BP->setEdgeWeight(BB, *I, retWeight);
208 return ReturningEdges.size() > 0;
211 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
212 // between two pointer or pointer and NULL will fail.
213 bool BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
214 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
215 if (!BI || !BI->isConditional())
218 Value *Cond = BI->getCondition();
219 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
220 if (!CI || !CI->isEquality())
223 Value *LHS = CI->getOperand(0);
225 if (!LHS->getType()->isPointerTy())
228 assert(CI->getOperand(1)->getType()->isPointerTy());
230 BasicBlock *Taken = BI->getSuccessor(0);
231 BasicBlock *NonTaken = BI->getSuccessor(1);
233 // p != 0 -> isProb = true
234 // p == 0 -> isProb = false
235 // p != q -> isProb = true
236 // p == q -> isProb = false;
237 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
239 std::swap(Taken, NonTaken);
241 BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
242 BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
246 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
247 // as taken, exiting edges as not-taken.
248 bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
249 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
251 Loop *L = LI->getLoopFor(BB);
255 SmallPtrSet<BasicBlock *, 8> BackEdges;
256 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
257 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
259 bool isHeader = BB == L->getHeader();
261 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
262 BasicBlock *Succ = *I;
263 Loop *SuccL = LI->getLoopFor(Succ);
265 ExitingEdges.insert(Succ);
266 else if (Succ == L->getHeader())
267 BackEdges.insert(Succ);
269 InEdges.insert(Succ);
272 if (uint32_t numBackEdges = BackEdges.size()) {
273 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
274 if (backWeight < NORMAL_WEIGHT)
275 backWeight = NORMAL_WEIGHT;
277 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
278 EE = BackEdges.end(); EI != EE; ++EI) {
279 BasicBlock *Back = *EI;
280 BP->setEdgeWeight(BB, Back, backWeight);
284 if (uint32_t numInEdges = InEdges.size()) {
285 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
286 if (inWeight < NORMAL_WEIGHT)
287 inWeight = NORMAL_WEIGHT;
289 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
290 EE = InEdges.end(); EI != EE; ++EI) {
291 BasicBlock *Back = *EI;
292 BP->setEdgeWeight(BB, Back, inWeight);
296 uint32_t numExitingEdges = ExitingEdges.size();
297 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
298 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
299 if (exitWeight < MIN_WEIGHT)
300 exitWeight = MIN_WEIGHT;
302 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
303 EE = ExitingEdges.end(); EI != EE; ++EI) {
304 BasicBlock *Exiting = *EI;
305 BP->setEdgeWeight(BB, Exiting, exitWeight);
312 bool BranchProbabilityAnalysis::calcZeroHeuristics(BasicBlock *BB) {
313 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
314 if (!BI || !BI->isConditional())
317 Value *Cond = BI->getCondition();
318 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
322 Value *RHS = CI->getOperand(1);
323 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
329 switch (CI->getPredicate()) {
330 case CmpInst::ICMP_EQ:
331 // X == 0 -> Unlikely
334 case CmpInst::ICMP_NE:
338 case CmpInst::ICMP_SLT:
342 case CmpInst::ICMP_SGT:
349 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
350 // InstCombine canonicalizes X <= 0 into X < 1.
351 // X <= 0 -> Unlikely
353 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
354 // InstCombine canonicalizes X >= 0 into X > -1.
361 BasicBlock *Taken = BI->getSuccessor(0);
362 BasicBlock *NonTaken = BI->getSuccessor(1);
365 std::swap(Taken, NonTaken);
367 BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
368 BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
374 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
376 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
377 BasicBlock *BB = I++;
379 if (calcMetadataWeights(BB))
382 if (calcLoopBranchHeuristics(BB))
385 if (calcReturnHeuristics(BB))
388 if (calcPointerHeuristics(BB))
391 calcZeroHeuristics(BB);
397 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
398 AU.addRequired<LoopInfo>();
399 AU.setPreservesAll();
402 bool BranchProbabilityInfo::runOnFunction(Function &F) {
403 LoopInfo &LI = getAnalysis<LoopInfo>();
404 BranchProbabilityAnalysis BPA(this, &LI);
405 return BPA.runOnFunction(F);
408 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
411 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
412 const BasicBlock *Succ = *I;
413 uint32_t Weight = getEdgeWeight(BB, Succ);
414 uint32_t PrevSum = Sum;
417 assert(Sum > PrevSum); (void) PrevSum;
423 bool BranchProbabilityInfo::
424 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
425 // Hot probability is at least 4/5 = 80%
426 uint32_t Weight = getEdgeWeight(Src, Dst);
427 uint32_t Sum = getSumForBlock(Src);
429 // FIXME: Implement BranchProbability::compare then change this code to
430 // compare this BranchProbability against a static "hot" BranchProbability.
431 return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
434 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
436 uint32_t MaxWeight = 0;
437 BasicBlock *MaxSucc = 0;
439 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
440 BasicBlock *Succ = *I;
441 uint32_t Weight = getEdgeWeight(BB, Succ);
442 uint32_t PrevSum = Sum;
445 assert(Sum > PrevSum); (void) PrevSum;
447 if (Weight > MaxWeight) {
453 // FIXME: Use BranchProbability::compare.
454 if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
460 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
461 uint32_t BranchProbabilityInfo::
462 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
464 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
466 if (I != Weights.end())
469 return DEFAULT_WEIGHT;
472 void BranchProbabilityInfo::
473 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
474 Weights[std::make_pair(Src, Dst)] = Weight;
475 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
476 << Dst->getNameStr() << " weight to " << Weight
477 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
481 BranchProbability BranchProbabilityInfo::
482 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
484 uint32_t N = getEdgeWeight(Src, Dst);
485 uint32_t D = getSumForBlock(Src);
487 return BranchProbability(N, D);
491 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
492 BasicBlock *Dst) const {
494 const BranchProbability Prob = getEdgeProbability(Src, Dst);
495 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
496 << " probability is " << Prob
497 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");