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 bool BranchProbabilityAnalysis::calcMetadataWeights(BasicBlock *BB) {
144 TerminatorInst *TI = BB->getTerminator();
145 if (TI->getNumSuccessors() == 1)
147 if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
150 MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
154 // Ensure there are weights for all of the successors. Note that the first
155 // operand to the metadata node is a name, not a weight.
156 if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
159 // Build up the final weights that will be used in a temporary buffer, but
160 // don't add them until all weihts are present. Each weight value is clamped
161 // to [1, getMaxWeightFor(BB)].
162 uint32_t WeightLimit = getMaxWeightFor(BB);
163 SmallVector<uint32_t, 2> Weights;
164 Weights.reserve(TI->getNumSuccessors());
165 for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
166 ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
170 std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
172 assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
173 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
174 BP->setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
179 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
180 // leads directly to Return Instruction will not be taken.
181 bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
182 if (BB->getTerminator()->getNumSuccessors() == 1)
185 SmallPtrSet<BasicBlock *, 4> ReturningEdges;
186 SmallPtrSet<BasicBlock *, 4> StayEdges;
188 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
189 BasicBlock *Succ = *I;
190 if (isReturningBlock(Succ))
191 ReturningEdges.insert(Succ);
193 StayEdges.insert(Succ);
196 if (uint32_t numStayEdges = StayEdges.size()) {
197 uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
198 if (stayWeight < NORMAL_WEIGHT)
199 stayWeight = NORMAL_WEIGHT;
201 for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
202 E = StayEdges.end(); I != E; ++I)
203 BP->setEdgeWeight(BB, *I, stayWeight);
206 if (uint32_t numRetEdges = ReturningEdges.size()) {
207 uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
208 if (retWeight < MIN_WEIGHT)
209 retWeight = MIN_WEIGHT;
210 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
211 E = ReturningEdges.end(); I != E; ++I) {
212 BP->setEdgeWeight(BB, *I, retWeight);
216 return ReturningEdges.size() > 0;
219 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
220 // between two pointer or pointer and NULL will fail.
221 bool BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
222 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
223 if (!BI || !BI->isConditional())
226 Value *Cond = BI->getCondition();
227 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
228 if (!CI || !CI->isEquality())
231 Value *LHS = CI->getOperand(0);
233 if (!LHS->getType()->isPointerTy())
236 assert(CI->getOperand(1)->getType()->isPointerTy());
238 BasicBlock *Taken = BI->getSuccessor(0);
239 BasicBlock *NonTaken = BI->getSuccessor(1);
241 // p != 0 -> isProb = true
242 // p == 0 -> isProb = false
243 // p != q -> isProb = true
244 // p == q -> isProb = false;
245 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
247 std::swap(Taken, NonTaken);
249 BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
250 BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
254 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
255 // as taken, exiting edges as not-taken.
256 bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
257 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
259 Loop *L = LI->getLoopFor(BB);
263 SmallPtrSet<BasicBlock *, 8> BackEdges;
264 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
265 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
267 bool isHeader = BB == L->getHeader();
269 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
270 BasicBlock *Succ = *I;
271 Loop *SuccL = LI->getLoopFor(Succ);
273 ExitingEdges.insert(Succ);
274 else if (Succ == L->getHeader())
275 BackEdges.insert(Succ);
277 InEdges.insert(Succ);
280 if (uint32_t numBackEdges = BackEdges.size()) {
281 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
282 if (backWeight < NORMAL_WEIGHT)
283 backWeight = NORMAL_WEIGHT;
285 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
286 EE = BackEdges.end(); EI != EE; ++EI) {
287 BasicBlock *Back = *EI;
288 BP->setEdgeWeight(BB, Back, backWeight);
292 if (uint32_t numInEdges = InEdges.size()) {
293 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
294 if (inWeight < NORMAL_WEIGHT)
295 inWeight = NORMAL_WEIGHT;
297 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
298 EE = InEdges.end(); EI != EE; ++EI) {
299 BasicBlock *Back = *EI;
300 BP->setEdgeWeight(BB, Back, inWeight);
304 uint32_t numExitingEdges = ExitingEdges.size();
305 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
306 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
307 if (exitWeight < MIN_WEIGHT)
308 exitWeight = MIN_WEIGHT;
310 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
311 EE = ExitingEdges.end(); EI != EE; ++EI) {
312 BasicBlock *Exiting = *EI;
313 BP->setEdgeWeight(BB, Exiting, exitWeight);
320 bool BranchProbabilityAnalysis::calcZeroHeuristics(BasicBlock *BB) {
321 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
322 if (!BI || !BI->isConditional())
325 Value *Cond = BI->getCondition();
326 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
330 Value *RHS = CI->getOperand(1);
331 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
337 switch (CI->getPredicate()) {
338 case CmpInst::ICMP_EQ:
339 // X == 0 -> Unlikely
342 case CmpInst::ICMP_NE:
346 case CmpInst::ICMP_SLT:
350 case CmpInst::ICMP_SGT:
357 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
358 // InstCombine canonicalizes X <= 0 into X < 1.
359 // X <= 0 -> Unlikely
361 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
362 // InstCombine canonicalizes X >= 0 into X > -1.
369 BasicBlock *Taken = BI->getSuccessor(0);
370 BasicBlock *NonTaken = BI->getSuccessor(1);
373 std::swap(Taken, NonTaken);
375 BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
376 BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
382 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
384 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
385 BasicBlock *BB = I++;
387 if (calcMetadataWeights(BB))
390 if (calcLoopBranchHeuristics(BB))
393 if (calcReturnHeuristics(BB))
396 if (calcPointerHeuristics(BB))
399 calcZeroHeuristics(BB);
405 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
406 AU.addRequired<LoopInfo>();
407 AU.setPreservesAll();
410 bool BranchProbabilityInfo::runOnFunction(Function &F) {
411 LoopInfo &LI = getAnalysis<LoopInfo>();
412 BranchProbabilityAnalysis BPA(this, &LI);
413 return BPA.runOnFunction(F);
416 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
419 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
420 const BasicBlock *Succ = *I;
421 uint32_t Weight = getEdgeWeight(BB, Succ);
422 uint32_t PrevSum = Sum;
425 assert(Sum > PrevSum); (void) PrevSum;
431 bool BranchProbabilityInfo::
432 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
433 // Hot probability is at least 4/5 = 80%
434 uint32_t Weight = getEdgeWeight(Src, Dst);
435 uint32_t Sum = getSumForBlock(Src);
437 // FIXME: Implement BranchProbability::compare then change this code to
438 // compare this BranchProbability against a static "hot" BranchProbability.
439 return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
442 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
444 uint32_t MaxWeight = 0;
445 BasicBlock *MaxSucc = 0;
447 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
448 BasicBlock *Succ = *I;
449 uint32_t Weight = getEdgeWeight(BB, Succ);
450 uint32_t PrevSum = Sum;
453 assert(Sum > PrevSum); (void) PrevSum;
455 if (Weight > MaxWeight) {
461 // FIXME: Use BranchProbability::compare.
462 if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
468 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
469 uint32_t BranchProbabilityInfo::
470 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
472 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
474 if (I != Weights.end())
477 return DEFAULT_WEIGHT;
480 void BranchProbabilityInfo::
481 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
482 Weights[std::make_pair(Src, Dst)] = Weight;
483 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
484 << Dst->getNameStr() << " weight to " << Weight
485 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
489 BranchProbability BranchProbabilityInfo::
490 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
492 uint32_t N = getEdgeWeight(Src, Dst);
493 uint32_t D = getSumForBlock(Src);
495 return BranchProbability(N, D);
499 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
500 BasicBlock *Dst) const {
502 const BranchProbability Prob = getEdgeProbability(Src, Dst);
503 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
504 << " probability is " << Prob
505 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");