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 static const uint32_t FPH_TAKEN_WEIGHT = 20;
80 static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
82 // Standard weight value. Used when none of the heuristics set weight for
84 static const uint32_t NORMAL_WEIGHT = 16;
86 // Minimum weight of an edge. Please note, that weight is NEVER 0.
87 static const uint32_t MIN_WEIGHT = 1;
89 // Return TRUE if BB leads directly to a Return Instruction.
90 static bool isReturningBlock(BasicBlock *BB) {
91 SmallPtrSet<BasicBlock *, 8> Visited;
94 TerminatorInst *TI = BB->getTerminator();
95 if (isa<ReturnInst>(TI))
98 if (TI->getNumSuccessors() > 1)
101 // It is unreachable block which we can consider as a return instruction.
102 if (TI->getNumSuccessors() == 0)
106 BB = TI->getSuccessor(0);
108 // Stop if cycle is detected.
109 if (Visited.count(BB))
116 uint32_t getMaxWeightFor(BasicBlock *BB) const {
117 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
121 BranchProbabilityAnalysis(BranchProbabilityInfo *BP, LoopInfo *LI)
126 bool calcMetadataWeights(BasicBlock *BB);
129 bool calcReturnHeuristics(BasicBlock *BB);
131 // Pointer Heuristics
132 bool calcPointerHeuristics(BasicBlock *BB);
134 // Loop Branch Heuristics
135 bool calcLoopBranchHeuristics(BasicBlock *BB);
138 bool calcZeroHeuristics(BasicBlock *BB);
140 // Floating Point Heuristics
141 bool calcFloatingPointHeuristics(BasicBlock *BB);
143 bool runOnFunction(Function &F);
145 } // end anonymous namespace
147 // Propagate existing explicit probabilities from either profile data or
148 // 'expect' intrinsic processing.
149 bool BranchProbabilityAnalysis::calcMetadataWeights(BasicBlock *BB) {
150 TerminatorInst *TI = BB->getTerminator();
151 if (TI->getNumSuccessors() == 1)
153 if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
156 MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
160 // Ensure there are weights for all of the successors. Note that the first
161 // operand to the metadata node is a name, not a weight.
162 if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
165 // Build up the final weights that will be used in a temporary buffer, but
166 // don't add them until all weihts are present. Each weight value is clamped
167 // to [1, getMaxWeightFor(BB)].
168 uint32_t WeightLimit = getMaxWeightFor(BB);
169 SmallVector<uint32_t, 2> Weights;
170 Weights.reserve(TI->getNumSuccessors());
171 for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
172 ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
176 std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
178 assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
179 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
180 BP->setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
185 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
186 // leads directly to Return Instruction will not be taken.
187 bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
188 if (BB->getTerminator()->getNumSuccessors() == 1)
191 SmallPtrSet<BasicBlock *, 4> ReturningEdges;
192 SmallPtrSet<BasicBlock *, 4> StayEdges;
194 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
195 BasicBlock *Succ = *I;
196 if (isReturningBlock(Succ))
197 ReturningEdges.insert(Succ);
199 StayEdges.insert(Succ);
202 if (uint32_t numStayEdges = StayEdges.size()) {
203 uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
204 if (stayWeight < NORMAL_WEIGHT)
205 stayWeight = NORMAL_WEIGHT;
207 for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
208 E = StayEdges.end(); I != E; ++I)
209 BP->setEdgeWeight(BB, *I, stayWeight);
212 if (uint32_t numRetEdges = ReturningEdges.size()) {
213 uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
214 if (retWeight < MIN_WEIGHT)
215 retWeight = MIN_WEIGHT;
216 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
217 E = ReturningEdges.end(); I != E; ++I) {
218 BP->setEdgeWeight(BB, *I, retWeight);
222 return ReturningEdges.size() > 0;
225 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
226 // between two pointer or pointer and NULL will fail.
227 bool BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
228 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
229 if (!BI || !BI->isConditional())
232 Value *Cond = BI->getCondition();
233 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
234 if (!CI || !CI->isEquality())
237 Value *LHS = CI->getOperand(0);
239 if (!LHS->getType()->isPointerTy())
242 assert(CI->getOperand(1)->getType()->isPointerTy());
244 BasicBlock *Taken = BI->getSuccessor(0);
245 BasicBlock *NonTaken = BI->getSuccessor(1);
247 // p != 0 -> isProb = true
248 // p == 0 -> isProb = false
249 // p != q -> isProb = true
250 // p == q -> isProb = false;
251 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
253 std::swap(Taken, NonTaken);
255 BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
256 BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
260 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
261 // as taken, exiting edges as not-taken.
262 bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
263 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
265 Loop *L = LI->getLoopFor(BB);
269 SmallPtrSet<BasicBlock *, 8> BackEdges;
270 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
271 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
273 bool isHeader = BB == L->getHeader();
275 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
276 BasicBlock *Succ = *I;
277 Loop *SuccL = LI->getLoopFor(Succ);
279 ExitingEdges.insert(Succ);
280 else if (Succ == L->getHeader())
281 BackEdges.insert(Succ);
283 InEdges.insert(Succ);
286 if (uint32_t numBackEdges = BackEdges.size()) {
287 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
288 if (backWeight < NORMAL_WEIGHT)
289 backWeight = NORMAL_WEIGHT;
291 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
292 EE = BackEdges.end(); EI != EE; ++EI) {
293 BasicBlock *Back = *EI;
294 BP->setEdgeWeight(BB, Back, backWeight);
298 if (uint32_t numInEdges = InEdges.size()) {
299 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
300 if (inWeight < NORMAL_WEIGHT)
301 inWeight = NORMAL_WEIGHT;
303 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
304 EE = InEdges.end(); EI != EE; ++EI) {
305 BasicBlock *Back = *EI;
306 BP->setEdgeWeight(BB, Back, inWeight);
310 uint32_t numExitingEdges = ExitingEdges.size();
311 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
312 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
313 if (exitWeight < MIN_WEIGHT)
314 exitWeight = MIN_WEIGHT;
316 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
317 EE = ExitingEdges.end(); EI != EE; ++EI) {
318 BasicBlock *Exiting = *EI;
319 BP->setEdgeWeight(BB, Exiting, exitWeight);
326 bool BranchProbabilityAnalysis::calcZeroHeuristics(BasicBlock *BB) {
327 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
328 if (!BI || !BI->isConditional())
331 Value *Cond = BI->getCondition();
332 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
336 Value *RHS = CI->getOperand(1);
337 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
343 switch (CI->getPredicate()) {
344 case CmpInst::ICMP_EQ:
345 // X == 0 -> Unlikely
348 case CmpInst::ICMP_NE:
352 case CmpInst::ICMP_SLT:
356 case CmpInst::ICMP_SGT:
363 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
364 // InstCombine canonicalizes X <= 0 into X < 1.
365 // X <= 0 -> Unlikely
367 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
368 // InstCombine canonicalizes X >= 0 into X > -1.
375 BasicBlock *Taken = BI->getSuccessor(0);
376 BasicBlock *NonTaken = BI->getSuccessor(1);
379 std::swap(Taken, NonTaken);
381 BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
382 BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
387 bool BranchProbabilityAnalysis::calcFloatingPointHeuristics(BasicBlock *BB) {
388 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
389 if (!BI || !BI->isConditional())
392 Value *Cond = BI->getCondition();
393 FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
394 if (!FCmp || !FCmp->isEquality())
397 BasicBlock *Taken = BI->getSuccessor(0);
398 BasicBlock *NonTaken = BI->getSuccessor(1);
400 // f1 == f2 -> Unlikely
401 // f1 != f2 -> Likely
402 if (FCmp->isTrueWhenEqual())
403 std::swap(Taken, NonTaken);
405 BP->setEdgeWeight(BB, Taken, FPH_TAKEN_WEIGHT);
406 BP->setEdgeWeight(BB, NonTaken, FPH_NONTAKEN_WEIGHT);
411 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
413 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
414 BasicBlock *BB = I++;
416 if (calcMetadataWeights(BB))
419 if (calcLoopBranchHeuristics(BB))
422 if (calcReturnHeuristics(BB))
425 if (calcPointerHeuristics(BB))
428 if (calcZeroHeuristics(BB))
431 calcFloatingPointHeuristics(BB);
437 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
438 AU.addRequired<LoopInfo>();
439 AU.setPreservesAll();
442 bool BranchProbabilityInfo::runOnFunction(Function &F) {
443 LoopInfo &LI = getAnalysis<LoopInfo>();
444 BranchProbabilityAnalysis BPA(this, &LI);
445 return BPA.runOnFunction(F);
448 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
451 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
452 const BasicBlock *Succ = *I;
453 uint32_t Weight = getEdgeWeight(BB, Succ);
454 uint32_t PrevSum = Sum;
457 assert(Sum > PrevSum); (void) PrevSum;
463 bool BranchProbabilityInfo::
464 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
465 // Hot probability is at least 4/5 = 80%
466 uint32_t Weight = getEdgeWeight(Src, Dst);
467 uint32_t Sum = getSumForBlock(Src);
469 // FIXME: Implement BranchProbability::compare then change this code to
470 // compare this BranchProbability against a static "hot" BranchProbability.
471 return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
474 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
476 uint32_t MaxWeight = 0;
477 BasicBlock *MaxSucc = 0;
479 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
480 BasicBlock *Succ = *I;
481 uint32_t Weight = getEdgeWeight(BB, Succ);
482 uint32_t PrevSum = Sum;
485 assert(Sum > PrevSum); (void) PrevSum;
487 if (Weight > MaxWeight) {
493 // FIXME: Use BranchProbability::compare.
494 if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
500 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
501 uint32_t BranchProbabilityInfo::
502 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
504 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
506 if (I != Weights.end())
509 return DEFAULT_WEIGHT;
512 void BranchProbabilityInfo::
513 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
514 Weights[std::make_pair(Src, Dst)] = Weight;
515 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
516 << Dst->getNameStr() << " weight to " << Weight
517 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
521 BranchProbability BranchProbabilityInfo::
522 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
524 uint32_t N = getEdgeWeight(Src, Dst);
525 uint32_t D = getSumForBlock(Src);
527 return BranchProbability(N, D);
531 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
532 BasicBlock *Dst) const {
534 const BranchProbability Prob = getEdgeProbability(Src, Dst);
535 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
536 << " probability is " << Prob
537 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");