1 //===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -----------===//
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/Analysis/BranchProbabilityInfo.h"
15 #include "llvm/ADT/PostOrderIterator.h"
16 #include "llvm/Analysis/LoopInfo.h"
17 #include "llvm/IR/CFG.h"
18 #include "llvm/IR/Constants.h"
19 #include "llvm/IR/Function.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/LLVMContext.h"
22 #include "llvm/IR/Metadata.h"
23 #include "llvm/Support/Debug.h"
27 #define DEBUG_TYPE "branch-prob"
29 INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
30 "Branch Probability Analysis", false, true)
31 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
32 INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
33 "Branch Probability Analysis", false, true)
35 char BranchProbabilityInfo::ID = 0;
37 // Weights are for internal use only. They are used by heuristics to help to
38 // estimate edges' probability. Example:
40 // Using "Loop Branch Heuristics" we predict weights of edges for the
55 // Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
56 // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
57 static const uint32_t LBH_TAKEN_WEIGHT = 124;
58 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
60 /// \brief Unreachable-terminating branch taken weight.
62 /// This is the weight for a branch being taken to a block that terminates
63 /// (eventually) in unreachable. These are predicted as unlikely as possible.
64 static const uint32_t UR_TAKEN_WEIGHT = 1;
66 /// \brief Unreachable-terminating branch not-taken weight.
68 /// This is the weight for a branch not being taken toward a block that
69 /// terminates (eventually) in unreachable. Such a branch is essentially never
70 /// taken. Set the weight to an absurdly high value so that nested loops don't
71 /// easily subsume it.
72 static const uint32_t UR_NONTAKEN_WEIGHT = 1024*1024 - 1;
74 /// \brief Weight for a branch taken going into a cold block.
76 /// This is the weight for a branch taken toward a block marked
77 /// cold. A block is marked cold if it's postdominated by a
78 /// block containing a call to a cold function. Cold functions
79 /// are those marked with attribute 'cold'.
80 static const uint32_t CC_TAKEN_WEIGHT = 4;
82 /// \brief Weight for a branch not-taken into a cold block.
84 /// This is the weight for a branch not taken toward a block marked
86 static const uint32_t CC_NONTAKEN_WEIGHT = 64;
88 static const uint32_t PH_TAKEN_WEIGHT = 20;
89 static const uint32_t PH_NONTAKEN_WEIGHT = 12;
91 static const uint32_t ZH_TAKEN_WEIGHT = 20;
92 static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
94 static const uint32_t FPH_TAKEN_WEIGHT = 20;
95 static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
97 /// \brief Invoke-terminating normal branch taken weight
99 /// This is the weight for branching to the normal destination of an invoke
100 /// instruction. We expect this to happen most of the time. Set the weight to an
101 /// absurdly high value so that nested loops subsume it.
102 static const uint32_t IH_TAKEN_WEIGHT = 1024 * 1024 - 1;
104 /// \brief Invoke-terminating normal branch not-taken weight.
106 /// This is the weight for branching to the unwind destination of an invoke
107 /// instruction. This is essentially never taken.
108 static const uint32_t IH_NONTAKEN_WEIGHT = 1;
110 // Standard weight value. Used when none of the heuristics set weight for
112 static const uint32_t NORMAL_WEIGHT = 16;
114 // Minimum weight of an edge. Please note, that weight is NEVER 0.
115 static const uint32_t MIN_WEIGHT = 1;
117 static uint32_t getMaxWeightFor(BasicBlock *BB) {
118 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
122 /// \brief Calculate edge weights for successors lead to unreachable.
124 /// Predict that a successor which leads necessarily to an
125 /// unreachable-terminated block as extremely unlikely.
126 bool BranchProbabilityInfo::calcUnreachableHeuristics(BasicBlock *BB) {
127 TerminatorInst *TI = BB->getTerminator();
128 if (TI->getNumSuccessors() == 0) {
129 if (isa<UnreachableInst>(TI))
130 PostDominatedByUnreachable.insert(BB);
134 SmallVector<unsigned, 4> UnreachableEdges;
135 SmallVector<unsigned, 4> ReachableEdges;
137 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
138 if (PostDominatedByUnreachable.count(*I))
139 UnreachableEdges.push_back(I.getSuccessorIndex());
141 ReachableEdges.push_back(I.getSuccessorIndex());
144 // If all successors are in the set of blocks post-dominated by unreachable,
145 // this block is too.
146 if (UnreachableEdges.size() == TI->getNumSuccessors())
147 PostDominatedByUnreachable.insert(BB);
149 // Skip probabilities if this block has a single successor or if all were
151 if (TI->getNumSuccessors() == 1 || UnreachableEdges.empty())
154 uint32_t UnreachableWeight =
155 std::max(UR_TAKEN_WEIGHT / (unsigned)UnreachableEdges.size(), MIN_WEIGHT);
156 for (SmallVectorImpl<unsigned>::iterator I = UnreachableEdges.begin(),
157 E = UnreachableEdges.end();
159 setEdgeWeight(BB, *I, UnreachableWeight);
161 if (ReachableEdges.empty())
163 uint32_t ReachableWeight =
164 std::max(UR_NONTAKEN_WEIGHT / (unsigned)ReachableEdges.size(),
166 for (SmallVectorImpl<unsigned>::iterator I = ReachableEdges.begin(),
167 E = ReachableEdges.end();
169 setEdgeWeight(BB, *I, ReachableWeight);
174 // Propagate existing explicit probabilities from either profile data or
175 // 'expect' intrinsic processing.
176 bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
177 TerminatorInst *TI = BB->getTerminator();
178 if (TI->getNumSuccessors() == 1)
180 if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
183 MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
187 // Ensure there are weights for all of the successors. Note that the first
188 // operand to the metadata node is a name, not a weight.
189 if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
192 // Build up the final weights that will be used in a temporary buffer, but
193 // don't add them until all weihts are present. Each weight value is clamped
194 // to [1, getMaxWeightFor(BB)].
195 uint32_t WeightLimit = getMaxWeightFor(BB);
196 SmallVector<uint32_t, 2> Weights;
197 Weights.reserve(TI->getNumSuccessors());
198 for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
199 ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
203 std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
205 assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
206 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
207 setEdgeWeight(BB, i, Weights[i]);
212 /// \brief Calculate edge weights for edges leading to cold blocks.
214 /// A cold block is one post-dominated by a block with a call to a
215 /// cold function. Those edges are unlikely to be taken, so we give
216 /// them relatively low weight.
218 /// Return true if we could compute the weights for cold edges.
219 /// Return false, otherwise.
220 bool BranchProbabilityInfo::calcColdCallHeuristics(BasicBlock *BB) {
221 TerminatorInst *TI = BB->getTerminator();
222 if (TI->getNumSuccessors() == 0)
225 // Determine which successors are post-dominated by a cold block.
226 SmallVector<unsigned, 4> ColdEdges;
227 SmallVector<unsigned, 4> NormalEdges;
228 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
229 if (PostDominatedByColdCall.count(*I))
230 ColdEdges.push_back(I.getSuccessorIndex());
232 NormalEdges.push_back(I.getSuccessorIndex());
234 // If all successors are in the set of blocks post-dominated by cold calls,
235 // this block is in the set post-dominated by cold calls.
236 if (ColdEdges.size() == TI->getNumSuccessors())
237 PostDominatedByColdCall.insert(BB);
239 // Otherwise, if the block itself contains a cold function, add it to the
240 // set of blocks postdominated by a cold call.
241 assert(!PostDominatedByColdCall.count(BB));
242 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
243 if (CallInst *CI = dyn_cast<CallInst>(I))
244 if (CI->hasFnAttr(Attribute::Cold)) {
245 PostDominatedByColdCall.insert(BB);
250 // Skip probabilities if this block has a single successor.
251 if (TI->getNumSuccessors() == 1 || ColdEdges.empty())
254 uint32_t ColdWeight =
255 std::max(CC_TAKEN_WEIGHT / (unsigned) ColdEdges.size(), MIN_WEIGHT);
256 for (SmallVectorImpl<unsigned>::iterator I = ColdEdges.begin(),
259 setEdgeWeight(BB, *I, ColdWeight);
261 if (NormalEdges.empty())
263 uint32_t NormalWeight = std::max(
264 CC_NONTAKEN_WEIGHT / (unsigned) NormalEdges.size(), NORMAL_WEIGHT);
265 for (SmallVectorImpl<unsigned>::iterator I = NormalEdges.begin(),
266 E = NormalEdges.end();
268 setEdgeWeight(BB, *I, NormalWeight);
273 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
274 // between two pointer or pointer and NULL will fail.
275 bool BranchProbabilityInfo::calcPointerHeuristics(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);
282 if (!CI || !CI->isEquality())
285 Value *LHS = CI->getOperand(0);
287 if (!LHS->getType()->isPointerTy())
290 assert(CI->getOperand(1)->getType()->isPointerTy());
292 // p != 0 -> isProb = true
293 // p == 0 -> isProb = false
294 // p != q -> isProb = true
295 // p == q -> isProb = false;
296 unsigned TakenIdx = 0, NonTakenIdx = 1;
297 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
299 std::swap(TakenIdx, NonTakenIdx);
301 setEdgeWeight(BB, TakenIdx, PH_TAKEN_WEIGHT);
302 setEdgeWeight(BB, NonTakenIdx, PH_NONTAKEN_WEIGHT);
306 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
307 // as taken, exiting edges as not-taken.
308 bool BranchProbabilityInfo::calcLoopBranchHeuristics(BasicBlock *BB) {
309 Loop *L = LI->getLoopFor(BB);
313 SmallVector<unsigned, 8> BackEdges;
314 SmallVector<unsigned, 8> ExitingEdges;
315 SmallVector<unsigned, 8> InEdges; // Edges from header to the loop.
317 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
318 if (!L->contains(*I))
319 ExitingEdges.push_back(I.getSuccessorIndex());
320 else if (L->getHeader() == *I)
321 BackEdges.push_back(I.getSuccessorIndex());
323 InEdges.push_back(I.getSuccessorIndex());
326 if (BackEdges.empty() && ExitingEdges.empty())
329 if (uint32_t numBackEdges = BackEdges.size()) {
330 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
331 if (backWeight < NORMAL_WEIGHT)
332 backWeight = NORMAL_WEIGHT;
334 for (SmallVectorImpl<unsigned>::iterator EI = BackEdges.begin(),
335 EE = BackEdges.end(); EI != EE; ++EI) {
336 setEdgeWeight(BB, *EI, backWeight);
340 if (uint32_t numInEdges = InEdges.size()) {
341 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
342 if (inWeight < NORMAL_WEIGHT)
343 inWeight = NORMAL_WEIGHT;
345 for (SmallVectorImpl<unsigned>::iterator EI = InEdges.begin(),
346 EE = InEdges.end(); EI != EE; ++EI) {
347 setEdgeWeight(BB, *EI, inWeight);
351 if (uint32_t numExitingEdges = ExitingEdges.size()) {
352 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numExitingEdges;
353 if (exitWeight < MIN_WEIGHT)
354 exitWeight = MIN_WEIGHT;
356 for (SmallVectorImpl<unsigned>::iterator EI = ExitingEdges.begin(),
357 EE = ExitingEdges.end(); EI != EE; ++EI) {
358 setEdgeWeight(BB, *EI, exitWeight);
365 bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
366 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
367 if (!BI || !BI->isConditional())
370 Value *Cond = BI->getCondition();
371 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
375 Value *RHS = CI->getOperand(1);
376 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
382 switch (CI->getPredicate()) {
383 case CmpInst::ICMP_EQ:
384 // X == 0 -> Unlikely
387 case CmpInst::ICMP_NE:
391 case CmpInst::ICMP_SLT:
395 case CmpInst::ICMP_SGT:
402 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
403 // InstCombine canonicalizes X <= 0 into X < 1.
404 // X <= 0 -> Unlikely
406 } else if (CV->isAllOnesValue()) {
407 switch (CI->getPredicate()) {
408 case CmpInst::ICMP_EQ:
409 // X == -1 -> Unlikely
412 case CmpInst::ICMP_NE:
416 case CmpInst::ICMP_SGT:
417 // InstCombine canonicalizes X >= 0 into X > -1.
428 unsigned TakenIdx = 0, NonTakenIdx = 1;
431 std::swap(TakenIdx, NonTakenIdx);
433 setEdgeWeight(BB, TakenIdx, ZH_TAKEN_WEIGHT);
434 setEdgeWeight(BB, NonTakenIdx, ZH_NONTAKEN_WEIGHT);
439 bool BranchProbabilityInfo::calcFloatingPointHeuristics(BasicBlock *BB) {
440 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
441 if (!BI || !BI->isConditional())
444 Value *Cond = BI->getCondition();
445 FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
450 if (FCmp->isEquality()) {
451 // f1 == f2 -> Unlikely
452 // f1 != f2 -> Likely
453 isProb = !FCmp->isTrueWhenEqual();
454 } else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
457 } else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
464 unsigned TakenIdx = 0, NonTakenIdx = 1;
467 std::swap(TakenIdx, NonTakenIdx);
469 setEdgeWeight(BB, TakenIdx, FPH_TAKEN_WEIGHT);
470 setEdgeWeight(BB, NonTakenIdx, FPH_NONTAKEN_WEIGHT);
475 bool BranchProbabilityInfo::calcInvokeHeuristics(BasicBlock *BB) {
476 InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator());
480 setEdgeWeight(BB, 0/*Index for Normal*/, IH_TAKEN_WEIGHT);
481 setEdgeWeight(BB, 1/*Index for Unwind*/, IH_NONTAKEN_WEIGHT);
485 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
486 AU.addRequired<LoopInfo>();
487 AU.setPreservesAll();
490 bool BranchProbabilityInfo::runOnFunction(Function &F) {
491 DEBUG(dbgs() << "---- Branch Probability Info : " << F.getName()
493 LastF = &F; // Store the last function we ran on for printing.
494 LI = &getAnalysis<LoopInfo>();
495 assert(PostDominatedByUnreachable.empty());
496 assert(PostDominatedByColdCall.empty());
498 // Walk the basic blocks in post-order so that we can build up state about
499 // the successors of a block iteratively.
500 for (po_iterator<BasicBlock *> I = po_begin(&F.getEntryBlock()),
501 E = po_end(&F.getEntryBlock());
503 DEBUG(dbgs() << "Computing probabilities for " << I->getName() << "\n");
504 if (calcUnreachableHeuristics(*I))
506 if (calcMetadataWeights(*I))
508 if (calcColdCallHeuristics(*I))
510 if (calcLoopBranchHeuristics(*I))
512 if (calcPointerHeuristics(*I))
514 if (calcZeroHeuristics(*I))
516 if (calcFloatingPointHeuristics(*I))
518 calcInvokeHeuristics(*I);
521 PostDominatedByUnreachable.clear();
522 PostDominatedByColdCall.clear();
526 void BranchProbabilityInfo::print(raw_ostream &OS, const Module *) const {
527 OS << "---- Branch Probabilities ----\n";
528 // We print the probabilities from the last function the analysis ran over,
529 // or the function it is currently running over.
530 assert(LastF && "Cannot print prior to running over a function");
531 for (Function::const_iterator BI = LastF->begin(), BE = LastF->end();
533 for (succ_const_iterator SI = succ_begin(BI), SE = succ_end(BI);
535 printEdgeProbability(OS << " ", BI, *SI);
540 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
543 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
544 uint32_t Weight = getEdgeWeight(BB, I.getSuccessorIndex());
545 uint32_t PrevSum = Sum;
548 assert(Sum > PrevSum); (void) PrevSum;
554 bool BranchProbabilityInfo::
555 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
556 // Hot probability is at least 4/5 = 80%
557 // FIXME: Compare against a static "hot" BranchProbability.
558 return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
561 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
563 uint32_t MaxWeight = 0;
564 BasicBlock *MaxSucc = nullptr;
566 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
567 BasicBlock *Succ = *I;
568 uint32_t Weight = getEdgeWeight(BB, Succ);
569 uint32_t PrevSum = Sum;
572 assert(Sum > PrevSum); (void) PrevSum;
574 if (Weight > MaxWeight) {
580 // Hot probability is at least 4/5 = 80%
581 if (BranchProbability(MaxWeight, Sum) > BranchProbability(4, 5))
587 /// Get the raw edge weight for the edge. If can't find it, return
588 /// DEFAULT_WEIGHT value. Here an edge is specified using PredBlock and an index
589 /// to the successors.
590 uint32_t BranchProbabilityInfo::
591 getEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors) const {
592 DenseMap<Edge, uint32_t>::const_iterator I =
593 Weights.find(std::make_pair(Src, IndexInSuccessors));
595 if (I != Weights.end())
598 return DEFAULT_WEIGHT;
601 uint32_t BranchProbabilityInfo::getEdgeWeight(const BasicBlock *Src,
602 succ_const_iterator Dst) const {
603 return getEdgeWeight(Src, Dst.getSuccessorIndex());
606 /// Get the raw edge weight calculated for the block pair. This returns the sum
607 /// of all raw edge weights from Src to Dst.
608 uint32_t BranchProbabilityInfo::
609 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
611 DenseMap<Edge, uint32_t>::const_iterator MapI;
612 for (succ_const_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
614 MapI = Weights.find(std::make_pair(Src, I.getSuccessorIndex()));
615 if (MapI != Weights.end())
616 Weight += MapI->second;
618 return (Weight == 0) ? DEFAULT_WEIGHT : Weight;
621 /// Set the edge weight for a given edge specified by PredBlock and an index
622 /// to the successors.
623 void BranchProbabilityInfo::
624 setEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors,
626 Weights[std::make_pair(Src, IndexInSuccessors)] = Weight;
627 DEBUG(dbgs() << "set edge " << Src->getName() << " -> "
628 << IndexInSuccessors << " successor weight to "
632 /// Get an edge's probability, relative to other out-edges from Src.
633 BranchProbability BranchProbabilityInfo::
634 getEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors) const {
635 uint32_t N = getEdgeWeight(Src, IndexInSuccessors);
636 uint32_t D = getSumForBlock(Src);
638 return BranchProbability(N, D);
641 /// Get the probability of going from Src to Dst. It returns the sum of all
642 /// probabilities for edges from Src to Dst.
643 BranchProbability BranchProbabilityInfo::
644 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
646 uint32_t N = getEdgeWeight(Src, Dst);
647 uint32_t D = getSumForBlock(Src);
649 return BranchProbability(N, D);
653 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
654 const BasicBlock *Src,
655 const BasicBlock *Dst) const {
657 const BranchProbability Prob = getEdgeProbability(Src, Dst);
658 OS << "edge " << Src->getName() << " -> " << Dst->getName()
659 << " probability is " << Prob
660 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");