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 Loop *L = LI->getLoopFor(BB);
223 SmallPtrSet<BasicBlock *, 8> BackEdges;
224 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
225 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
227 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
228 if (!L->contains(*I))
229 ExitingEdges.insert(*I);
230 else if (L->getHeader() == *I)
231 BackEdges.insert(*I);
236 if (uint32_t numBackEdges = BackEdges.size()) {
237 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
238 if (backWeight < NORMAL_WEIGHT)
239 backWeight = NORMAL_WEIGHT;
241 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
242 EE = BackEdges.end(); EI != EE; ++EI) {
243 BasicBlock *Back = *EI;
244 setEdgeWeight(BB, Back, backWeight);
248 if (uint32_t numInEdges = InEdges.size()) {
249 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
250 if (inWeight < NORMAL_WEIGHT)
251 inWeight = NORMAL_WEIGHT;
253 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
254 EE = InEdges.end(); EI != EE; ++EI) {
255 BasicBlock *Back = *EI;
256 setEdgeWeight(BB, Back, inWeight);
260 if (uint32_t numExitingEdges = ExitingEdges.size()) {
261 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numExitingEdges;
262 if (exitWeight < MIN_WEIGHT)
263 exitWeight = MIN_WEIGHT;
265 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
266 EE = ExitingEdges.end(); EI != EE; ++EI) {
267 BasicBlock *Exiting = *EI;
268 setEdgeWeight(BB, Exiting, exitWeight);
275 bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
276 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
277 if (!BI || !BI->isConditional())
280 Value *Cond = BI->getCondition();
281 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
285 Value *RHS = CI->getOperand(1);
286 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
292 switch (CI->getPredicate()) {
293 case CmpInst::ICMP_EQ:
294 // X == 0 -> Unlikely
297 case CmpInst::ICMP_NE:
301 case CmpInst::ICMP_SLT:
305 case CmpInst::ICMP_SGT:
312 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
313 // InstCombine canonicalizes X <= 0 into X < 1.
314 // X <= 0 -> Unlikely
316 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
317 // InstCombine canonicalizes X >= 0 into X > -1.
324 BasicBlock *Taken = BI->getSuccessor(0);
325 BasicBlock *NonTaken = BI->getSuccessor(1);
328 std::swap(Taken, NonTaken);
330 setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
331 setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
336 bool BranchProbabilityInfo::calcFloatingPointHeuristics(BasicBlock *BB) {
337 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
338 if (!BI || !BI->isConditional())
341 Value *Cond = BI->getCondition();
342 FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
347 if (FCmp->isEquality()) {
348 // f1 == f2 -> Unlikely
349 // f1 != f2 -> Likely
350 isProb = !FCmp->isTrueWhenEqual();
351 } else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
354 } else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
361 BasicBlock *Taken = BI->getSuccessor(0);
362 BasicBlock *NonTaken = BI->getSuccessor(1);
365 std::swap(Taken, NonTaken);
367 setEdgeWeight(BB, Taken, FPH_TAKEN_WEIGHT);
368 setEdgeWeight(BB, NonTaken, FPH_NONTAKEN_WEIGHT);
373 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
374 AU.addRequired<LoopInfo>();
375 AU.setPreservesAll();
378 bool BranchProbabilityInfo::runOnFunction(Function &F) {
379 LastF = &F; // Store the last function we ran on for printing.
380 LI = &getAnalysis<LoopInfo>();
381 assert(PostDominatedByUnreachable.empty());
383 // Walk the basic blocks in post-order so that we can build up state about
384 // the successors of a block iteratively.
385 for (po_iterator<BasicBlock *> I = po_begin(&F.getEntryBlock()),
386 E = po_end(&F.getEntryBlock());
388 DEBUG(dbgs() << "Computing probabilities for " << I->getName() << "\n");
389 if (calcUnreachableHeuristics(*I))
391 if (calcMetadataWeights(*I))
393 if (calcLoopBranchHeuristics(*I))
395 if (calcPointerHeuristics(*I))
397 if (calcZeroHeuristics(*I))
399 calcFloatingPointHeuristics(*I);
402 PostDominatedByUnreachable.clear();
406 void BranchProbabilityInfo::print(raw_ostream &OS, const Module *) const {
407 OS << "---- Branch Probabilities ----\n";
408 // We print the probabilities from the last function the analysis ran over,
409 // or the function it is currently running over.
410 assert(LastF && "Cannot print prior to running over a function");
411 for (Function::const_iterator BI = LastF->begin(), BE = LastF->end();
413 for (succ_const_iterator SI = succ_begin(BI), SE = succ_end(BI);
415 printEdgeProbability(OS << " ", BI, *SI);
420 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
423 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
424 const BasicBlock *Succ = *I;
425 uint32_t Weight = getEdgeWeight(BB, Succ);
426 uint32_t PrevSum = Sum;
429 assert(Sum > PrevSum); (void) PrevSum;
435 bool BranchProbabilityInfo::
436 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
437 // Hot probability is at least 4/5 = 80%
438 // FIXME: Compare against a static "hot" BranchProbability.
439 return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
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 // Hot probability is at least 4/5 = 80%
462 if (BranchProbability(MaxWeight, Sum) > BranchProbability(4, 5))
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->getName() << " -> "
484 << Dst->getName() << " 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,
500 const BasicBlock *Src,
501 const BasicBlock *Dst) const {
503 const BranchProbability Prob = getEdgeProbability(Src, Dst);
504 OS << "edge " << Src->getName() << " -> " << Dst->getName()
505 << " probability is " << Prob
506 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");