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 DenseMap<Edge, uint32_t> *Weights;
43 BranchProbabilityInfo *BP;
48 // Weights are for internal use only. They are used by heuristics to help to
49 // estimate edges' probability. Example:
51 // Using "Loop Branch Heuristics" we predict weights of edges for the
66 // Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
67 // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
69 static const uint32_t LBH_TAKEN_WEIGHT = 124;
70 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
72 static const uint32_t RH_TAKEN_WEIGHT = 24;
73 static const uint32_t RH_NONTAKEN_WEIGHT = 8;
75 static const uint32_t PH_TAKEN_WEIGHT = 20;
76 static const uint32_t PH_NONTAKEN_WEIGHT = 12;
78 static const uint32_t ZH_TAKEN_WEIGHT = 20;
79 static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
81 // Standard weight value. Used when none of the heuristics set weight for
83 static const uint32_t NORMAL_WEIGHT = 16;
85 // Minimum weight of an edge. Please note, that weight is NEVER 0.
86 static const uint32_t MIN_WEIGHT = 1;
88 // Return TRUE if BB leads directly to a Return Instruction.
89 static bool isReturningBlock(BasicBlock *BB) {
90 SmallPtrSet<BasicBlock *, 8> Visited;
93 TerminatorInst *TI = BB->getTerminator();
94 if (isa<ReturnInst>(TI))
97 if (TI->getNumSuccessors() > 1)
100 // It is unreachable block which we can consider as a return instruction.
101 if (TI->getNumSuccessors() == 0)
105 BB = TI->getSuccessor(0);
107 // Stop if cycle is detected.
108 if (Visited.count(BB))
115 uint32_t getMaxWeightFor(BasicBlock *BB) const {
116 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
120 BranchProbabilityAnalysis(DenseMap<Edge, uint32_t> *W,
121 BranchProbabilityInfo *BP, LoopInfo *LI)
122 : Weights(W), BP(BP), LI(LI) {
126 bool calcMetadataWeights(BasicBlock *BB);
129 bool calcReturnHeuristics(BasicBlock *BB);
131 // Pointer Heuristics
132 bool calcPointerHeuristics(BasicBlock *BB);
134 // Loop Branch Heuristics
135 bool calcLoopBranchHeuristics(BasicBlock *BB);
138 bool calcZeroHeuristics(BasicBlock *BB);
140 bool runOnFunction(Function &F);
142 } // end anonymous namespace
144 // Propagate existing explicit probabilities from either profile data or
145 // 'expect' intrinsic processing.
146 bool BranchProbabilityAnalysis::calcMetadataWeights(BasicBlock *BB) {
147 TerminatorInst *TI = BB->getTerminator();
148 if (TI->getNumSuccessors() == 1)
150 if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
153 MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
157 // Ensure there are weights for all of the successors. Note that the first
158 // operand to the metadata node is a name, not a weight.
159 if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
162 // Build up the final weights that will be used in a temporary buffer, but
163 // don't add them until all weihts are present. Each weight value is clamped
164 // to [1, getMaxWeightFor(BB)].
165 uint32_t WeightLimit = getMaxWeightFor(BB);
166 SmallVector<uint32_t, 2> Weights;
167 Weights.reserve(TI->getNumSuccessors());
168 for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
169 ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
173 std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
175 assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
176 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
177 BP->setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
182 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
183 // leads directly to Return Instruction will not be taken.
184 bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
185 if (BB->getTerminator()->getNumSuccessors() == 1)
188 SmallPtrSet<BasicBlock *, 4> ReturningEdges;
189 SmallPtrSet<BasicBlock *, 4> StayEdges;
191 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
192 BasicBlock *Succ = *I;
193 if (isReturningBlock(Succ))
194 ReturningEdges.insert(Succ);
196 StayEdges.insert(Succ);
199 if (uint32_t numStayEdges = StayEdges.size()) {
200 uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
201 if (stayWeight < NORMAL_WEIGHT)
202 stayWeight = NORMAL_WEIGHT;
204 for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
205 E = StayEdges.end(); I != E; ++I)
206 BP->setEdgeWeight(BB, *I, stayWeight);
209 if (uint32_t numRetEdges = ReturningEdges.size()) {
210 uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
211 if (retWeight < MIN_WEIGHT)
212 retWeight = MIN_WEIGHT;
213 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
214 E = ReturningEdges.end(); I != E; ++I) {
215 BP->setEdgeWeight(BB, *I, retWeight);
219 return ReturningEdges.size() > 0;
222 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
223 // between two pointer or pointer and NULL will fail.
224 bool BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
225 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
226 if (!BI || !BI->isConditional())
229 Value *Cond = BI->getCondition();
230 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
231 if (!CI || !CI->isEquality())
234 Value *LHS = CI->getOperand(0);
236 if (!LHS->getType()->isPointerTy())
239 assert(CI->getOperand(1)->getType()->isPointerTy());
241 BasicBlock *Taken = BI->getSuccessor(0);
242 BasicBlock *NonTaken = BI->getSuccessor(1);
244 // p != 0 -> isProb = true
245 // p == 0 -> isProb = false
246 // p != q -> isProb = true
247 // p == q -> isProb = false;
248 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
250 std::swap(Taken, NonTaken);
252 BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
253 BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
257 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
258 // as taken, exiting edges as not-taken.
259 bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
260 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
262 Loop *L = LI->getLoopFor(BB);
266 SmallPtrSet<BasicBlock *, 8> BackEdges;
267 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
268 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
270 bool isHeader = BB == L->getHeader();
272 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
273 BasicBlock *Succ = *I;
274 Loop *SuccL = LI->getLoopFor(Succ);
276 ExitingEdges.insert(Succ);
277 else if (Succ == L->getHeader())
278 BackEdges.insert(Succ);
280 InEdges.insert(Succ);
283 if (uint32_t numBackEdges = BackEdges.size()) {
284 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
285 if (backWeight < NORMAL_WEIGHT)
286 backWeight = NORMAL_WEIGHT;
288 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
289 EE = BackEdges.end(); EI != EE; ++EI) {
290 BasicBlock *Back = *EI;
291 BP->setEdgeWeight(BB, Back, backWeight);
295 if (uint32_t numInEdges = InEdges.size()) {
296 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
297 if (inWeight < NORMAL_WEIGHT)
298 inWeight = NORMAL_WEIGHT;
300 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
301 EE = InEdges.end(); EI != EE; ++EI) {
302 BasicBlock *Back = *EI;
303 BP->setEdgeWeight(BB, Back, inWeight);
307 uint32_t numExitingEdges = ExitingEdges.size();
308 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
309 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
310 if (exitWeight < MIN_WEIGHT)
311 exitWeight = MIN_WEIGHT;
313 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
314 EE = ExitingEdges.end(); EI != EE; ++EI) {
315 BasicBlock *Exiting = *EI;
316 BP->setEdgeWeight(BB, Exiting, exitWeight);
323 bool BranchProbabilityAnalysis::calcZeroHeuristics(BasicBlock *BB) {
324 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
325 if (!BI || !BI->isConditional())
328 Value *Cond = BI->getCondition();
329 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
333 Value *RHS = CI->getOperand(1);
334 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
340 switch (CI->getPredicate()) {
341 case CmpInst::ICMP_EQ:
342 // X == 0 -> Unlikely
345 case CmpInst::ICMP_NE:
349 case CmpInst::ICMP_SLT:
353 case CmpInst::ICMP_SGT:
360 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
361 // InstCombine canonicalizes X <= 0 into X < 1.
362 // X <= 0 -> Unlikely
364 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
365 // InstCombine canonicalizes X >= 0 into X > -1.
372 BasicBlock *Taken = BI->getSuccessor(0);
373 BasicBlock *NonTaken = BI->getSuccessor(1);
376 std::swap(Taken, NonTaken);
378 BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
379 BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
385 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
387 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
388 BasicBlock *BB = I++;
390 if (calcMetadataWeights(BB))
393 if (calcLoopBranchHeuristics(BB))
396 if (calcReturnHeuristics(BB))
399 if (calcPointerHeuristics(BB))
402 calcZeroHeuristics(BB);
408 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
409 AU.addRequired<LoopInfo>();
410 AU.setPreservesAll();
413 bool BranchProbabilityInfo::runOnFunction(Function &F) {
414 LoopInfo &LI = getAnalysis<LoopInfo>();
415 BranchProbabilityAnalysis BPA(&Weights, this, &LI);
416 return BPA.runOnFunction(F);
419 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
422 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
423 const BasicBlock *Succ = *I;
424 uint32_t Weight = getEdgeWeight(BB, Succ);
425 uint32_t PrevSum = Sum;
428 assert(Sum > PrevSum); (void) PrevSum;
434 bool BranchProbabilityInfo::
435 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
436 // Hot probability is at least 4/5 = 80%
437 uint32_t Weight = getEdgeWeight(Src, Dst);
438 uint32_t Sum = getSumForBlock(Src);
440 // FIXME: Implement BranchProbability::compare then change this code to
441 // compare this BranchProbability against a static "hot" BranchProbability.
442 return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
445 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
447 uint32_t MaxWeight = 0;
448 BasicBlock *MaxSucc = 0;
450 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
451 BasicBlock *Succ = *I;
452 uint32_t Weight = getEdgeWeight(BB, Succ);
453 uint32_t PrevSum = Sum;
456 assert(Sum > PrevSum); (void) PrevSum;
458 if (Weight > MaxWeight) {
464 // FIXME: Use BranchProbability::compare.
465 if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
471 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
472 uint32_t BranchProbabilityInfo::
473 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
475 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
477 if (I != Weights.end())
480 return DEFAULT_WEIGHT;
483 void BranchProbabilityInfo::
484 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
485 Weights[std::make_pair(Src, Dst)] = Weight;
486 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
487 << Dst->getNameStr() << " weight to " << Weight
488 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
492 BranchProbability BranchProbabilityInfo::
493 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
495 uint32_t N = getEdgeWeight(Src, Dst);
496 uint32_t D = getSumForBlock(Src);
498 return BranchProbability(N, D);
502 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
503 BasicBlock *Dst) const {
505 const BranchProbability Prob = getEdgeProbability(Src, Dst);
506 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
507 << " probability is " << Prob
508 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");