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/ADT/PostOrderIterator.h"
22 #include "llvm/Support/CFG.h"
23 #include "llvm/Support/Debug.h"
27 INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
28 "Branch Probability Analysis", false, true)
29 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
30 INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
31 "Branch Probability Analysis", false, true)
33 char BranchProbabilityInfo::ID = 0;
35 // Weights are for internal use only. They are used by heuristics to help to
36 // estimate edges' probability. Example:
38 // Using "Loop Branch Heuristics" we predict weights of edges for the
53 // Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
54 // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
55 static const uint32_t LBH_TAKEN_WEIGHT = 124;
56 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
58 /// \brief Unreachable-terminating branch taken weight.
60 /// This is the weight for a branch being taken to a block that terminates
61 /// (eventually) in unreachable. These are predicted as unlikely as possible.
62 static const uint32_t UR_TAKEN_WEIGHT = 1;
64 /// \brief Unreachable-terminating branch not-taken weight.
66 /// This is the weight for a branch not being taken toward a block that
67 /// terminates (eventually) in unreachable. Such a branch is essentially never
69 static const uint32_t UR_NONTAKEN_WEIGHT = 1023;
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 static const uint32_t FPH_TAKEN_WEIGHT = 20;
78 static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
80 // Standard weight value. Used when none of the heuristics set weight for
82 static const uint32_t NORMAL_WEIGHT = 16;
84 // Minimum weight of an edge. Please note, that weight is NEVER 0.
85 static const uint32_t MIN_WEIGHT = 1;
87 static uint32_t getMaxWeightFor(BasicBlock *BB) {
88 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
92 /// \brief Calculate edge weights for successors lead to unreachable.
94 /// Predict that a successor which leads necessarily to an
95 /// unreachable-terminated block as extremely unlikely.
96 bool BranchProbabilityInfo::calcUnreachableHeuristics(BasicBlock *BB) {
97 TerminatorInst *TI = BB->getTerminator();
98 if (TI->getNumSuccessors() == 0) {
99 if (isa<UnreachableInst>(TI))
100 PostDominatedByUnreachable.insert(BB);
104 SmallPtrSet<BasicBlock *, 4> UnreachableEdges;
105 SmallPtrSet<BasicBlock *, 4> ReachableEdges;
107 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
108 if (PostDominatedByUnreachable.count(*I))
109 UnreachableEdges.insert(*I);
111 ReachableEdges.insert(*I);
114 // If all successors are in the set of blocks post-dominated by unreachable,
115 // this block is too.
116 if (UnreachableEdges.size() == TI->getNumSuccessors())
117 PostDominatedByUnreachable.insert(BB);
119 // Skip probabilities if this block has a single successor or if all were
121 if (TI->getNumSuccessors() == 1 || UnreachableEdges.empty())
124 uint32_t UnreachableWeight =
125 std::max(UR_TAKEN_WEIGHT / UnreachableEdges.size(), MIN_WEIGHT);
126 for (SmallPtrSet<BasicBlock *, 4>::iterator I = UnreachableEdges.begin(),
127 E = UnreachableEdges.end();
129 setEdgeWeight(BB, *I, UnreachableWeight);
131 if (ReachableEdges.empty())
133 uint32_t ReachableWeight =
134 std::max(UR_NONTAKEN_WEIGHT / ReachableEdges.size(), NORMAL_WEIGHT);
135 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReachableEdges.begin(),
136 E = ReachableEdges.end();
138 setEdgeWeight(BB, *I, ReachableWeight);
143 // Propagate existing explicit probabilities from either profile data or
144 // 'expect' intrinsic processing.
145 bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
146 TerminatorInst *TI = BB->getTerminator();
147 if (TI->getNumSuccessors() == 1)
149 if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
152 MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
156 // Ensure there are weights for all of the successors. Note that the first
157 // operand to the metadata node is a name, not a weight.
158 if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
161 // Build up the final weights that will be used in a temporary buffer, but
162 // don't add them until all weihts are present. Each weight value is clamped
163 // to [1, getMaxWeightFor(BB)].
164 uint32_t WeightLimit = getMaxWeightFor(BB);
165 SmallVector<uint32_t, 2> Weights;
166 Weights.reserve(TI->getNumSuccessors());
167 for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
168 ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
172 std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
174 assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
175 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
176 setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
181 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
182 // between two pointer or pointer and NULL will fail.
183 bool BranchProbabilityInfo::calcPointerHeuristics(BasicBlock *BB) {
184 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
185 if (!BI || !BI->isConditional())
188 Value *Cond = BI->getCondition();
189 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
190 if (!CI || !CI->isEquality())
193 Value *LHS = CI->getOperand(0);
195 if (!LHS->getType()->isPointerTy())
198 assert(CI->getOperand(1)->getType()->isPointerTy());
200 BasicBlock *Taken = BI->getSuccessor(0);
201 BasicBlock *NonTaken = BI->getSuccessor(1);
203 // p != 0 -> isProb = true
204 // p == 0 -> isProb = false
205 // p != q -> isProb = true
206 // p == q -> isProb = false;
207 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
209 std::swap(Taken, NonTaken);
211 setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
212 setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
216 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
217 // as taken, exiting edges as not-taken.
218 bool BranchProbabilityInfo::calcLoopBranchHeuristics(BasicBlock *BB) {
219 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
221 Loop *L = LI->getLoopFor(BB);
225 SmallPtrSet<BasicBlock *, 8> BackEdges;
226 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
227 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
229 bool isHeader = BB == L->getHeader();
231 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
232 BasicBlock *Succ = *I;
233 Loop *SuccL = LI->getLoopFor(Succ);
235 ExitingEdges.insert(Succ);
236 else if (Succ == L->getHeader())
237 BackEdges.insert(Succ);
239 InEdges.insert(Succ);
242 if (uint32_t numBackEdges = BackEdges.size()) {
243 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
244 if (backWeight < NORMAL_WEIGHT)
245 backWeight = NORMAL_WEIGHT;
247 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
248 EE = BackEdges.end(); EI != EE; ++EI) {
249 BasicBlock *Back = *EI;
250 setEdgeWeight(BB, Back, backWeight);
254 if (uint32_t numInEdges = InEdges.size()) {
255 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
256 if (inWeight < NORMAL_WEIGHT)
257 inWeight = NORMAL_WEIGHT;
259 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
260 EE = InEdges.end(); EI != EE; ++EI) {
261 BasicBlock *Back = *EI;
262 setEdgeWeight(BB, Back, inWeight);
266 uint32_t numExitingEdges = ExitingEdges.size();
267 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
268 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
269 if (exitWeight < MIN_WEIGHT)
270 exitWeight = MIN_WEIGHT;
272 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
273 EE = ExitingEdges.end(); EI != EE; ++EI) {
274 BasicBlock *Exiting = *EI;
275 setEdgeWeight(BB, Exiting, exitWeight);
282 bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
283 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
284 if (!BI || !BI->isConditional())
287 Value *Cond = BI->getCondition();
288 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
292 Value *RHS = CI->getOperand(1);
293 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
299 switch (CI->getPredicate()) {
300 case CmpInst::ICMP_EQ:
301 // X == 0 -> Unlikely
304 case CmpInst::ICMP_NE:
308 case CmpInst::ICMP_SLT:
312 case CmpInst::ICMP_SGT:
319 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
320 // InstCombine canonicalizes X <= 0 into X < 1.
321 // X <= 0 -> Unlikely
323 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
324 // InstCombine canonicalizes X >= 0 into X > -1.
331 BasicBlock *Taken = BI->getSuccessor(0);
332 BasicBlock *NonTaken = BI->getSuccessor(1);
335 std::swap(Taken, NonTaken);
337 setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
338 setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
343 bool BranchProbabilityInfo::calcFloatingPointHeuristics(BasicBlock *BB) {
344 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
345 if (!BI || !BI->isConditional())
348 Value *Cond = BI->getCondition();
349 FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
354 if (FCmp->isEquality()) {
355 // f1 == f2 -> Unlikely
356 // f1 != f2 -> Likely
357 isProb = !FCmp->isTrueWhenEqual();
358 } else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
361 } else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
368 BasicBlock *Taken = BI->getSuccessor(0);
369 BasicBlock *NonTaken = BI->getSuccessor(1);
372 std::swap(Taken, NonTaken);
374 setEdgeWeight(BB, Taken, FPH_TAKEN_WEIGHT);
375 setEdgeWeight(BB, NonTaken, FPH_NONTAKEN_WEIGHT);
380 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
381 AU.addRequired<LoopInfo>();
382 AU.setPreservesAll();
385 bool BranchProbabilityInfo::runOnFunction(Function &F) {
386 LastF = &F; // Store the last function we ran on for printing.
387 LI = &getAnalysis<LoopInfo>();
388 assert(PostDominatedByUnreachable.empty());
390 // Walk the basic blocks in post-order so that we can build up state about
391 // the successors of a block iteratively.
392 for (po_iterator<BasicBlock *> I = po_begin(&F.getEntryBlock()),
393 E = po_end(&F.getEntryBlock());
395 DEBUG(dbgs() << "Computing probabilities for " << I->getName() << "\n");
396 if (calcUnreachableHeuristics(*I))
398 if (calcMetadataWeights(*I))
400 if (calcLoopBranchHeuristics(*I))
402 if (calcPointerHeuristics(*I))
404 if (calcZeroHeuristics(*I))
406 calcFloatingPointHeuristics(*I);
409 PostDominatedByUnreachable.clear();
413 void BranchProbabilityInfo::print(raw_ostream &OS, const Module *) const {
414 OS << "---- Branch Probabilities ----\n";
415 // We print the probabilities from the last function the analysis ran over,
416 // or the function it is currently running over.
417 assert(LastF && "Cannot print prior to running over a function");
418 for (Function::const_iterator BI = LastF->begin(), BE = LastF->end();
420 for (succ_const_iterator SI = succ_begin(BI), SE = succ_end(BI);
422 printEdgeProbability(OS << " ", BI, *SI);
427 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
430 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
431 const BasicBlock *Succ = *I;
432 uint32_t Weight = getEdgeWeight(BB, Succ);
433 uint32_t PrevSum = Sum;
436 assert(Sum > PrevSum); (void) PrevSum;
442 bool BranchProbabilityInfo::
443 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
444 // Hot probability is at least 4/5 = 80%
445 // FIXME: Compare against a static "hot" BranchProbability.
446 return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
449 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
451 uint32_t MaxWeight = 0;
452 BasicBlock *MaxSucc = 0;
454 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
455 BasicBlock *Succ = *I;
456 uint32_t Weight = getEdgeWeight(BB, Succ);
457 uint32_t PrevSum = Sum;
460 assert(Sum > PrevSum); (void) PrevSum;
462 if (Weight > MaxWeight) {
468 // Hot probability is at least 4/5 = 80%
469 if (BranchProbability(MaxWeight, Sum) > BranchProbability(4, 5))
475 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
476 uint32_t BranchProbabilityInfo::
477 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
479 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
481 if (I != Weights.end())
484 return DEFAULT_WEIGHT;
487 void BranchProbabilityInfo::
488 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
489 Weights[std::make_pair(Src, Dst)] = Weight;
490 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
491 << Dst->getNameStr() << " weight to " << Weight
492 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
496 BranchProbability BranchProbabilityInfo::
497 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
499 uint32_t N = getEdgeWeight(Src, Dst);
500 uint32_t D = getSumForBlock(Src);
502 return BranchProbability(N, D);
506 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
507 const BasicBlock *Src,
508 const BasicBlock *Dst) const {
510 const BranchProbability Prob = getEdgeProbability(Src, Dst);
511 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
512 << " probability is " << Prob
513 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");