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;
35 // Please note that BranchProbabilityAnalysis is not a FunctionPass.
36 // It is created by BranchProbabilityInfo (which is a FunctionPass), which
37 // provides a clear interface. Thanks to that, all heuristics and other
38 // private methods are hidden in the .cpp file.
39 class BranchProbabilityAnalysis {
41 typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;
43 BranchProbabilityInfo *BP;
48 // Weights are for internal use only. They are used by heuristics to help to
49 // estimate edges' probability. Example:
51 // Using "Loop Branch Heuristics" we predict weights of edges for the
66 // Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
67 // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
69 static const uint32_t LBH_TAKEN_WEIGHT = 124;
70 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
72 static const uint32_t RH_TAKEN_WEIGHT = 24;
73 static const uint32_t RH_NONTAKEN_WEIGHT = 8;
75 static const uint32_t PH_TAKEN_WEIGHT = 20;
76 static const uint32_t PH_NONTAKEN_WEIGHT = 12;
78 static const uint32_t ZH_TAKEN_WEIGHT = 20;
79 static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
81 static const uint32_t FPH_TAKEN_WEIGHT = 20;
82 static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
84 // Standard weight value. Used when none of the heuristics set weight for
86 static const uint32_t NORMAL_WEIGHT = 16;
88 // Minimum weight of an edge. Please note, that weight is NEVER 0.
89 static const uint32_t MIN_WEIGHT = 1;
91 // Return TRUE if BB leads directly to a Return Instruction.
92 static bool isReturningBlock(BasicBlock *BB) {
93 SmallPtrSet<BasicBlock *, 8> Visited;
96 TerminatorInst *TI = BB->getTerminator();
97 if (isa<ReturnInst>(TI))
100 if (TI->getNumSuccessors() > 1)
103 // It is unreachable block which we can consider as a return instruction.
104 if (TI->getNumSuccessors() == 0)
108 BB = TI->getSuccessor(0);
110 // Stop if cycle is detected.
111 if (Visited.count(BB))
118 uint32_t getMaxWeightFor(BasicBlock *BB) const {
119 return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
123 BranchProbabilityAnalysis(BranchProbabilityInfo *BP, LoopInfo *LI)
128 bool calcMetadataWeights(BasicBlock *BB);
131 bool calcReturnHeuristics(BasicBlock *BB);
133 // Pointer Heuristics
134 bool calcPointerHeuristics(BasicBlock *BB);
136 // Loop Branch Heuristics
137 bool calcLoopBranchHeuristics(BasicBlock *BB);
140 bool calcZeroHeuristics(BasicBlock *BB);
142 // Floating Point Heuristics
143 bool calcFloatingPointHeuristics(BasicBlock *BB);
145 bool runOnFunction(Function &F);
147 } // end anonymous namespace
149 // Propagate existing explicit probabilities from either profile data or
150 // 'expect' intrinsic processing.
151 bool BranchProbabilityAnalysis::calcMetadataWeights(BasicBlock *BB) {
152 TerminatorInst *TI = BB->getTerminator();
153 if (TI->getNumSuccessors() == 1)
155 if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
158 MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
162 // Ensure there are weights for all of the successors. Note that the first
163 // operand to the metadata node is a name, not a weight.
164 if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
167 // Build up the final weights that will be used in a temporary buffer, but
168 // don't add them until all weihts are present. Each weight value is clamped
169 // to [1, getMaxWeightFor(BB)].
170 uint32_t WeightLimit = getMaxWeightFor(BB);
171 SmallVector<uint32_t, 2> Weights;
172 Weights.reserve(TI->getNumSuccessors());
173 for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
174 ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
178 std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
180 assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
181 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
182 BP->setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
187 // Calculate Edge Weights using "Return Heuristics". Predict a successor which
188 // leads directly to Return Instruction will not be taken.
189 bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
190 if (BB->getTerminator()->getNumSuccessors() == 1)
193 SmallPtrSet<BasicBlock *, 4> ReturningEdges;
194 SmallPtrSet<BasicBlock *, 4> StayEdges;
196 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
197 BasicBlock *Succ = *I;
198 if (isReturningBlock(Succ))
199 ReturningEdges.insert(Succ);
201 StayEdges.insert(Succ);
204 if (uint32_t numStayEdges = StayEdges.size()) {
205 uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
206 if (stayWeight < NORMAL_WEIGHT)
207 stayWeight = NORMAL_WEIGHT;
209 for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
210 E = StayEdges.end(); I != E; ++I)
211 BP->setEdgeWeight(BB, *I, stayWeight);
214 if (uint32_t numRetEdges = ReturningEdges.size()) {
215 uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
216 if (retWeight < MIN_WEIGHT)
217 retWeight = MIN_WEIGHT;
218 for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
219 E = ReturningEdges.end(); I != E; ++I) {
220 BP->setEdgeWeight(BB, *I, retWeight);
224 return ReturningEdges.size() > 0;
227 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
228 // between two pointer or pointer and NULL will fail.
229 bool BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
230 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
231 if (!BI || !BI->isConditional())
234 Value *Cond = BI->getCondition();
235 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
236 if (!CI || !CI->isEquality())
239 Value *LHS = CI->getOperand(0);
241 if (!LHS->getType()->isPointerTy())
244 assert(CI->getOperand(1)->getType()->isPointerTy());
246 BasicBlock *Taken = BI->getSuccessor(0);
247 BasicBlock *NonTaken = BI->getSuccessor(1);
249 // p != 0 -> isProb = true
250 // p == 0 -> isProb = false
251 // p != q -> isProb = true
252 // p == q -> isProb = false;
253 bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
255 std::swap(Taken, NonTaken);
257 BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
258 BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
262 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
263 // as taken, exiting edges as not-taken.
264 bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
265 uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
267 Loop *L = LI->getLoopFor(BB);
271 SmallPtrSet<BasicBlock *, 8> BackEdges;
272 SmallPtrSet<BasicBlock *, 8> ExitingEdges;
273 SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
275 bool isHeader = BB == L->getHeader();
277 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
278 BasicBlock *Succ = *I;
279 Loop *SuccL = LI->getLoopFor(Succ);
281 ExitingEdges.insert(Succ);
282 else if (Succ == L->getHeader())
283 BackEdges.insert(Succ);
285 InEdges.insert(Succ);
288 if (uint32_t numBackEdges = BackEdges.size()) {
289 uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
290 if (backWeight < NORMAL_WEIGHT)
291 backWeight = NORMAL_WEIGHT;
293 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
294 EE = BackEdges.end(); EI != EE; ++EI) {
295 BasicBlock *Back = *EI;
296 BP->setEdgeWeight(BB, Back, backWeight);
300 if (uint32_t numInEdges = InEdges.size()) {
301 uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
302 if (inWeight < NORMAL_WEIGHT)
303 inWeight = NORMAL_WEIGHT;
305 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
306 EE = InEdges.end(); EI != EE; ++EI) {
307 BasicBlock *Back = *EI;
308 BP->setEdgeWeight(BB, Back, inWeight);
312 uint32_t numExitingEdges = ExitingEdges.size();
313 if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
314 uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
315 if (exitWeight < MIN_WEIGHT)
316 exitWeight = MIN_WEIGHT;
318 for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
319 EE = ExitingEdges.end(); EI != EE; ++EI) {
320 BasicBlock *Exiting = *EI;
321 BP->setEdgeWeight(BB, Exiting, exitWeight);
328 bool BranchProbabilityAnalysis::calcZeroHeuristics(BasicBlock *BB) {
329 BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
330 if (!BI || !BI->isConditional())
333 Value *Cond = BI->getCondition();
334 ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
338 Value *RHS = CI->getOperand(1);
339 ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
345 switch (CI->getPredicate()) {
346 case CmpInst::ICMP_EQ:
347 // X == 0 -> Unlikely
350 case CmpInst::ICMP_NE:
354 case CmpInst::ICMP_SLT:
358 case CmpInst::ICMP_SGT:
365 } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
366 // InstCombine canonicalizes X <= 0 into X < 1.
367 // X <= 0 -> Unlikely
369 } else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
370 // InstCombine canonicalizes X >= 0 into X > -1.
377 BasicBlock *Taken = BI->getSuccessor(0);
378 BasicBlock *NonTaken = BI->getSuccessor(1);
381 std::swap(Taken, NonTaken);
383 BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
384 BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
389 bool BranchProbabilityAnalysis::calcFloatingPointHeuristics(BasicBlock *BB) {
390 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
391 if (!BI || !BI->isConditional())
394 Value *Cond = BI->getCondition();
395 FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
400 if (FCmp->isEquality()) {
401 // f1 == f2 -> Unlikely
402 // f1 != f2 -> Likely
403 isProb = !FCmp->isTrueWhenEqual();
404 } else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
407 } else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
414 BasicBlock *Taken = BI->getSuccessor(0);
415 BasicBlock *NonTaken = BI->getSuccessor(1);
418 std::swap(Taken, NonTaken);
420 BP->setEdgeWeight(BB, Taken, FPH_TAKEN_WEIGHT);
421 BP->setEdgeWeight(BB, NonTaken, FPH_NONTAKEN_WEIGHT);
426 bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
427 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
428 if (calcMetadataWeights(I))
430 if (calcLoopBranchHeuristics(I))
432 if (calcReturnHeuristics(I))
434 if (calcPointerHeuristics(I))
436 if (calcZeroHeuristics(I))
438 calcFloatingPointHeuristics(I);
443 void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
444 AU.addRequired<LoopInfo>();
445 AU.setPreservesAll();
448 bool BranchProbabilityInfo::runOnFunction(Function &F) {
449 LastF = &F; // Store the last function we ran on for printing.
450 LoopInfo &LI = getAnalysis<LoopInfo>();
451 BranchProbabilityAnalysis BPA(this, &LI);
452 return BPA.runOnFunction(F);
455 void BranchProbabilityInfo::print(raw_ostream &OS, const Module *) const {
456 OS << "---- Branch Probabilities ----\n";
457 // We print the probabilities from the last function the analysis ran over,
458 // or the function it is currently running over.
459 assert(LastF && "Cannot print prior to running over a function");
460 for (Function::const_iterator BI = LastF->begin(), BE = LastF->end();
462 for (succ_const_iterator SI = succ_begin(BI), SE = succ_end(BI);
464 printEdgeProbability(OS << " ", BI, *SI);
469 uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
472 for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
473 const BasicBlock *Succ = *I;
474 uint32_t Weight = getEdgeWeight(BB, Succ);
475 uint32_t PrevSum = Sum;
478 assert(Sum > PrevSum); (void) PrevSum;
484 bool BranchProbabilityInfo::
485 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
486 // Hot probability is at least 4/5 = 80%
487 // FIXME: Compare against a static "hot" BranchProbability.
488 return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
491 BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
493 uint32_t MaxWeight = 0;
494 BasicBlock *MaxSucc = 0;
496 for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
497 BasicBlock *Succ = *I;
498 uint32_t Weight = getEdgeWeight(BB, Succ);
499 uint32_t PrevSum = Sum;
502 assert(Sum > PrevSum); (void) PrevSum;
504 if (Weight > MaxWeight) {
510 // Hot probability is at least 4/5 = 80%
511 if (BranchProbability(MaxWeight, Sum) > BranchProbability(4, 5))
517 // Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
518 uint32_t BranchProbabilityInfo::
519 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
521 DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
523 if (I != Weights.end())
526 return DEFAULT_WEIGHT;
529 void BranchProbabilityInfo::
530 setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
531 Weights[std::make_pair(Src, Dst)] = Weight;
532 DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
533 << Dst->getNameStr() << " weight to " << Weight
534 << (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
538 BranchProbability BranchProbabilityInfo::
539 getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
541 uint32_t N = getEdgeWeight(Src, Dst);
542 uint32_t D = getSumForBlock(Src);
544 return BranchProbability(N, D);
548 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
549 const BasicBlock *Src,
550 const BasicBlock *Dst) const {
552 const BranchProbability Prob = getEdgeProbability(Src, Dst);
553 OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
554 << " probability is " << Prob
555 << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");