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/Function.h"
16 #include "llvm/Instructions.h"
17 #include "llvm/LLVMContext.h"
18 #include "llvm/Metadata.h"
19 #include "llvm/Analysis/BranchProbabilityInfo.h"
20 #include "llvm/Analysis/LoopInfo.h"
21 #include "llvm/Support/CFG.h"
22 #include "llvm/Support/Debug.h"
26 INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
27 "Branch Probability Analysis", false, true)
28 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
29 INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
30 "Branch Probability Analysis", false, true)
32 char BranchProbabilityInfo::ID = 0;
34 // Weights are for internal use only. They are used by heuristics to help to
35 // estimate edges' probability. Example:
37 // Using "Loop Branch Heuristics" we predict weights of edges for the
52 // Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
53 // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
54 static const uint32_t LBH_TAKEN_WEIGHT = 124;
55 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
57 static const uint32_t RH_TAKEN_WEIGHT = 24;
58 static const uint32_t RH_NONTAKEN_WEIGHT = 8;
60 static const uint32_t PH_TAKEN_WEIGHT = 20;
61 static const uint32_t PH_NONTAKEN_WEIGHT = 12;
63 static const uint32_t ZH_TAKEN_WEIGHT = 20;
64 static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
66 static const uint32_t FPH_TAKEN_WEIGHT = 20;
67 static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
69 // Standard weight value. Used when none of the heuristics set weight for
71 static const uint32_t NORMAL_WEIGHT = 16;
73 // Minimum weight of an edge. Please note, that weight is NEVER 0.
74 static const uint32_t MIN_WEIGHT = 1;
76 // Return TRUE if BB leads directly to a Return Instruction.
77 static bool isReturningBlock(BasicBlock *BB) {
78 SmallPtrSet<BasicBlock *, 8> Visited;
81 TerminatorInst *TI = BB->getTerminator();
82 if (isa<ReturnInst>(TI))
85 if (TI->getNumSuccessors() > 1)
88 // It is unreachable block which we can consider as a return instruction.
89 if (TI->getNumSuccessors() == 0)
93 BB = TI->getSuccessor(0);
95 // Stop if cycle is detected.
96 if (Visited.count(BB))
103 static uint32_t getMaxWeightFor(BasicBlock *BB) {
104 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
108 // Propagate existing explicit probabilities from either profile data or
109 // 'expect' intrinsic processing.
110 bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
111 TerminatorInst *TI = BB->getTerminator();
112 if (TI->getNumSuccessors() == 1)
114 if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
117 MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
121 // Ensure there are weights for all of the successors. Note that the first
122 // operand to the metadata node is a name, not a weight.
123 if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
126 // Build up the final weights that will be used in a temporary buffer, but
127 // don't add them until all weihts are present. Each weight value is clamped
128 // to [1, getMaxWeightFor(BB)].
129 uint32_t WeightLimit = getMaxWeightFor(BB);
130 SmallVector<uint32_t, 2> Weights;
131 Weights.reserve(TI->getNumSuccessors());
132 for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
133 ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
137 std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
139 assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
140 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
141 setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
146 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
147 // leads directly to Return Instruction will not be taken.
148 bool BranchProbabilityInfo::calcReturnHeuristics(BasicBlock *BB){
149 if (BB->getTerminator()->getNumSuccessors() == 1)
152 SmallPtrSet<BasicBlock *, 4> ReturningEdges;
153 SmallPtrSet<BasicBlock *, 4> StayEdges;
155 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
156 BasicBlock *Succ = *I;
157 if (isReturningBlock(Succ))
158 ReturningEdges.insert(Succ);
160 StayEdges.insert(Succ);
163 if (uint32_t numStayEdges = StayEdges.size()) {
164 uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
165 if (stayWeight < NORMAL_WEIGHT)
166 stayWeight = NORMAL_WEIGHT;
168 for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
169 E = StayEdges.end(); I != E; ++I)
170 setEdgeWeight(BB, *I, stayWeight);
173 if (uint32_t numRetEdges = ReturningEdges.size()) {
174 uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
175 if (retWeight < MIN_WEIGHT)
176 retWeight = MIN_WEIGHT;
177 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
178 E = ReturningEdges.end(); I != E; ++I) {
179 setEdgeWeight(BB, *I, retWeight);
183 return ReturningEdges.size() > 0;
186 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
187 // between two pointer or pointer and NULL will fail.
188 bool BranchProbabilityInfo::calcPointerHeuristics(BasicBlock *BB) {
189 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
190 if (!BI || !BI->isConditional())
193 Value *Cond = BI->getCondition();
194 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
195 if (!CI || !CI->isEquality())
198 Value *LHS = CI->getOperand(0);
200 if (!LHS->getType()->isPointerTy())
203 assert(CI->getOperand(1)->getType()->isPointerTy());
205 BasicBlock *Taken = BI->getSuccessor(0);
206 BasicBlock *NonTaken = BI->getSuccessor(1);
208 // p != 0 -> isProb = true
209 // p == 0 -> isProb = false
210 // p != q -> isProb = true
211 // p == q -> isProb = false;
212 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
214 std::swap(Taken, NonTaken);
216 setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
217 setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
221 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
222 // as taken, exiting edges as not-taken.
223 bool BranchProbabilityInfo::calcLoopBranchHeuristics(BasicBlock *BB) {
224 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
226 Loop *L = LI->getLoopFor(BB);
230 SmallPtrSet<BasicBlock *, 8> BackEdges;
231 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
232 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
234 bool isHeader = BB == L->getHeader();
236 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
237 BasicBlock *Succ = *I;
238 Loop *SuccL = LI->getLoopFor(Succ);
240 ExitingEdges.insert(Succ);
241 else if (Succ == L->getHeader())
242 BackEdges.insert(Succ);
244 InEdges.insert(Succ);
247 if (uint32_t numBackEdges = BackEdges.size()) {
248 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
249 if (backWeight < NORMAL_WEIGHT)
250 backWeight = NORMAL_WEIGHT;
252 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
253 EE = BackEdges.end(); EI != EE; ++EI) {
254 BasicBlock *Back = *EI;
255 setEdgeWeight(BB, Back, backWeight);
259 if (uint32_t numInEdges = InEdges.size()) {
260 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
261 if (inWeight < NORMAL_WEIGHT)
262 inWeight = NORMAL_WEIGHT;
264 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
265 EE = InEdges.end(); EI != EE; ++EI) {
266 BasicBlock *Back = *EI;
267 setEdgeWeight(BB, Back, inWeight);
271 uint32_t numExitingEdges = ExitingEdges.size();
272 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
273 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
274 if (exitWeight < MIN_WEIGHT)
275 exitWeight = MIN_WEIGHT;
277 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
278 EE = ExitingEdges.end(); EI != EE; ++EI) {
279 BasicBlock *Exiting = *EI;
280 setEdgeWeight(BB, Exiting, exitWeight);
287 bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
288 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
289 if (!BI || !BI->isConditional())
292 Value *Cond = BI->getCondition();
293 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
297 Value *RHS = CI->getOperand(1);
298 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
304 switch (CI->getPredicate()) {
305 case CmpInst::ICMP_EQ:
306 // X == 0 -> Unlikely
309 case CmpInst::ICMP_NE:
313 case CmpInst::ICMP_SLT:
317 case CmpInst::ICMP_SGT:
324 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
325 // InstCombine canonicalizes X <= 0 into X < 1.
326 // X <= 0 -> Unlikely
328 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
329 // InstCombine canonicalizes X >= 0 into X > -1.
336 BasicBlock *Taken = BI->getSuccessor(0);
337 BasicBlock *NonTaken = BI->getSuccessor(1);
340 std::swap(Taken, NonTaken);
342 setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
343 setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
348 bool BranchProbabilityInfo::calcFloatingPointHeuristics(BasicBlock *BB) {
349 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
350 if (!BI || !BI->isConditional())
353 Value *Cond = BI->getCondition();
354 FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
359 if (FCmp->isEquality()) {
360 // f1 == f2 -> Unlikely
361 // f1 != f2 -> Likely
362 isProb = !FCmp->isTrueWhenEqual();
363 } else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
366 } else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
373 BasicBlock *Taken = BI->getSuccessor(0);
374 BasicBlock *NonTaken = BI->getSuccessor(1);
377 std::swap(Taken, NonTaken);
379 setEdgeWeight(BB, Taken, FPH_TAKEN_WEIGHT);
380 setEdgeWeight(BB, NonTaken, FPH_NONTAKEN_WEIGHT);
385 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
386 AU.addRequired<LoopInfo>();
387 AU.setPreservesAll();
390 bool BranchProbabilityInfo::runOnFunction(Function &F) {
391 LastF = &F; // Store the last function we ran on for printing.
392 LI = &getAnalysis<LoopInfo>();
394 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
395 if (calcMetadataWeights(I))
397 if (calcLoopBranchHeuristics(I))
399 if (calcReturnHeuristics(I))
401 if (calcPointerHeuristics(I))
403 if (calcZeroHeuristics(I))
405 calcFloatingPointHeuristics(I);
410 void BranchProbabilityInfo::print(raw_ostream &OS, const Module *) const {
411 OS << "---- Branch Probabilities ----\n";
412 // We print the probabilities from the last function the analysis ran over,
413 // or the function it is currently running over.
414 assert(LastF && "Cannot print prior to running over a function");
415 for (Function::const_iterator BI = LastF->begin(), BE = LastF->end();
417 for (succ_const_iterator SI = succ_begin(BI), SE = succ_end(BI);
419 printEdgeProbability(OS << " ", BI, *SI);
424 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
427 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
428 const BasicBlock *Succ = *I;
429 uint32_t Weight = getEdgeWeight(BB, Succ);
430 uint32_t PrevSum = Sum;
433 assert(Sum > PrevSum); (void) PrevSum;
439 bool BranchProbabilityInfo::
440 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
441 // Hot probability is at least 4/5 = 80%
442 // FIXME: Compare against a static "hot" BranchProbability.
443 return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
446 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
448 uint32_t MaxWeight = 0;
449 BasicBlock *MaxSucc = 0;
451 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
452 BasicBlock *Succ = *I;
453 uint32_t Weight = getEdgeWeight(BB, Succ);
454 uint32_t PrevSum = Sum;
457 assert(Sum > PrevSum); (void) PrevSum;
459 if (Weight > MaxWeight) {
465 // Hot probability is at least 4/5 = 80%
466 if (BranchProbability(MaxWeight, Sum) > BranchProbability(4, 5))
472 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
473 uint32_t BranchProbabilityInfo::
474 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
476 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
478 if (I != Weights.end())
481 return DEFAULT_WEIGHT;
484 void BranchProbabilityInfo::
485 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
486 Weights[std::make_pair(Src, Dst)] = Weight;
487 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
488 << Dst->getNameStr() << " weight to " << Weight
489 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
493 BranchProbability BranchProbabilityInfo::
494 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
496 uint32_t N = getEdgeWeight(Src, Dst);
497 uint32_t D = getSumForBlock(Src);
499 return BranchProbability(N, D);
503 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
504 const BasicBlock *Src,
505 const BasicBlock *Dst) const {
507 const BranchProbability Prob = getEdgeProbability(Src, Dst);
508 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
509 << " probability is " << Prob
510 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");