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1 //===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -----------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // Loops should be simplified before this analysis.
11 //
12 //===----------------------------------------------------------------------===//
13 
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"
24 #include "llvm/Support/raw_ostream.h"
25 
26 using namespace llvm;
27 
28 #define DEBUG_TYPE "branch-prob"
29 
30 INITIALIZE_PASS_BEGIN(BranchProbabilityInfoWrapperPass, "branch-prob",
31                       "Branch Probability Analysis", false, true)
32 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
33 INITIALIZE_PASS_END(BranchProbabilityInfoWrapperPass, "branch-prob",
34                     "Branch Probability Analysis", false, true)
35 
36 char BranchProbabilityInfoWrapperPass::ID = 0;
37 
38 // Weights are for internal use only. They are used by heuristics to help to
39 // estimate edges' probability. Example:
40 //
41 // Using "Loop Branch Heuristics" we predict weights of edges for the
42 // block BB2.
43 //         ...
44 //          |
45 //          V
46 //         BB1<-+
47 //          |   |
48 //          |   | (Weight = 124)
49 //          V   |
50 //         BB2--+
51 //          |
52 //          | (Weight = 4)
53 //          V
54 //         BB3
55 //
56 // Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
57 // Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
58 static const uint32_t LBH_TAKEN_WEIGHT = 124;
59 static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
60 
61 /// \brief Unreachable-terminating branch taken weight.
62 ///
63 /// This is the weight for a branch being taken to a block that terminates
64 /// (eventually) in unreachable. These are predicted as unlikely as possible.
65 static const uint32_t UR_TAKEN_WEIGHT = 1;
66 
67 /// \brief Unreachable-terminating branch not-taken weight.
68 ///
69 /// This is the weight for a branch not being taken toward a block that
70 /// terminates (eventually) in unreachable. Such a branch is essentially never
71 /// taken. Set the weight to an absurdly high value so that nested loops don't
72 /// easily subsume it.
73 static const uint32_t UR_NONTAKEN_WEIGHT = 1024*1024 - 1;
74 
75 /// \brief Weight for a branch taken going into a cold block.
76 ///
77 /// This is the weight for a branch taken toward a block marked
78 /// cold.  A block is marked cold if it's postdominated by a
79 /// block containing a call to a cold function.  Cold functions
80 /// are those marked with attribute 'cold'.
81 static const uint32_t CC_TAKEN_WEIGHT = 4;
82 
83 /// \brief Weight for a branch not-taken into a cold block.
84 ///
85 /// This is the weight for a branch not taken toward a block marked
86 /// cold.
87 static const uint32_t CC_NONTAKEN_WEIGHT = 64;
88 
89 static const uint32_t PH_TAKEN_WEIGHT = 20;
90 static const uint32_t PH_NONTAKEN_WEIGHT = 12;
91 
92 static const uint32_t ZH_TAKEN_WEIGHT = 20;
93 static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
94 
95 static const uint32_t FPH_TAKEN_WEIGHT = 20;
96 static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
97 
98 /// \brief Invoke-terminating normal branch taken weight
99 ///
100 /// This is the weight for branching to the normal destination of an invoke
101 /// instruction. We expect this to happen most of the time. Set the weight to an
102 /// absurdly high value so that nested loops subsume it.
103 static const uint32_t IH_TAKEN_WEIGHT = 1024 * 1024 - 1;
104 
105 /// \brief Invoke-terminating normal branch not-taken weight.
106 ///
107 /// This is the weight for branching to the unwind destination of an invoke
108 /// instruction. This is essentially never taken.
109 static const uint32_t IH_NONTAKEN_WEIGHT = 1;
110 
111 /// \brief Calculate edge weights for successors lead to unreachable.
112 ///
113 /// Predict that a successor which leads necessarily to an
114 /// unreachable-terminated block as extremely unlikely.
calcUnreachableHeuristics(const BasicBlock * BB)115 bool BranchProbabilityInfo::calcUnreachableHeuristics(const BasicBlock *BB) {
116   const TerminatorInst *TI = BB->getTerminator();
117   if (TI->getNumSuccessors() == 0) {
118     if (isa<UnreachableInst>(TI) ||
119         // If this block is terminated by a call to
120         // @llvm.experimental.deoptimize then treat it like an unreachable since
121         // the @llvm.experimental.deoptimize call is expected to practically
122         // never execute.
123         BB->getTerminatingDeoptimizeCall())
124       PostDominatedByUnreachable.insert(BB);
125     return false;
126   }
127 
128   SmallVector<unsigned, 4> UnreachableEdges;
129   SmallVector<unsigned, 4> ReachableEdges;
130 
131   for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
132     if (PostDominatedByUnreachable.count(*I))
133       UnreachableEdges.push_back(I.getSuccessorIndex());
134     else
135       ReachableEdges.push_back(I.getSuccessorIndex());
136   }
137 
138   // If all successors are in the set of blocks post-dominated by unreachable,
139   // this block is too.
140   if (UnreachableEdges.size() == TI->getNumSuccessors())
141     PostDominatedByUnreachable.insert(BB);
142 
143   // Skip probabilities if this block has a single successor or if all were
144   // reachable.
145   if (TI->getNumSuccessors() == 1 || UnreachableEdges.empty())
146     return false;
147 
148   // If the terminator is an InvokeInst, check only the normal destination block
149   // as the unwind edge of InvokeInst is also very unlikely taken.
150   if (auto *II = dyn_cast<InvokeInst>(TI))
151     if (PostDominatedByUnreachable.count(II->getNormalDest())) {
152       PostDominatedByUnreachable.insert(BB);
153       // Return false here so that edge weights for InvokeInst could be decided
154       // in calcInvokeHeuristics().
155       return false;
156     }
157 
158   if (ReachableEdges.empty()) {
159     BranchProbability Prob(1, UnreachableEdges.size());
160     for (unsigned SuccIdx : UnreachableEdges)
161       setEdgeProbability(BB, SuccIdx, Prob);
162     return true;
163   }
164 
165   BranchProbability UnreachableProb(UR_TAKEN_WEIGHT,
166                                     (UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) *
167                                         UnreachableEdges.size());
168   BranchProbability ReachableProb(UR_NONTAKEN_WEIGHT,
169                                   (UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) *
170                                       ReachableEdges.size());
171 
172   for (unsigned SuccIdx : UnreachableEdges)
173     setEdgeProbability(BB, SuccIdx, UnreachableProb);
174   for (unsigned SuccIdx : ReachableEdges)
175     setEdgeProbability(BB, SuccIdx, ReachableProb);
176 
177   return true;
178 }
179 
180 // Propagate existing explicit probabilities from either profile data or
181 // 'expect' intrinsic processing.
calcMetadataWeights(const BasicBlock * BB)182 bool BranchProbabilityInfo::calcMetadataWeights(const BasicBlock *BB) {
183   const TerminatorInst *TI = BB->getTerminator();
184   if (TI->getNumSuccessors() == 1)
185     return false;
186   if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
187     return false;
188 
189   MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
190   if (!WeightsNode)
191     return false;
192 
193   // Check that the number of successors is manageable.
194   assert(TI->getNumSuccessors() < UINT32_MAX && "Too many successors");
195 
196   // Ensure there are weights for all of the successors. Note that the first
197   // operand to the metadata node is a name, not a weight.
198   if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
199     return false;
200 
201   // Build up the final weights that will be used in a temporary buffer.
202   // Compute the sum of all weights to later decide whether they need to
203   // be scaled to fit in 32 bits.
204   uint64_t WeightSum = 0;
205   SmallVector<uint32_t, 2> Weights;
206   Weights.reserve(TI->getNumSuccessors());
207   for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
208     ConstantInt *Weight =
209         mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(i));
210     if (!Weight)
211       return false;
212     assert(Weight->getValue().getActiveBits() <= 32 &&
213            "Too many bits for uint32_t");
214     Weights.push_back(Weight->getZExtValue());
215     WeightSum += Weights.back();
216   }
217   assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
218 
219   // If the sum of weights does not fit in 32 bits, scale every weight down
220   // accordingly.
221   uint64_t ScalingFactor =
222       (WeightSum > UINT32_MAX) ? WeightSum / UINT32_MAX + 1 : 1;
223 
224   WeightSum = 0;
225   for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
226     Weights[i] /= ScalingFactor;
227     WeightSum += Weights[i];
228   }
229 
230   if (WeightSum == 0) {
231     for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
232       setEdgeProbability(BB, i, {1, e});
233   } else {
234     for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
235       setEdgeProbability(BB, i, {Weights[i], static_cast<uint32_t>(WeightSum)});
236   }
237 
238   assert(WeightSum <= UINT32_MAX &&
239          "Expected weights to scale down to 32 bits");
240 
241   return true;
242 }
243 
244 /// \brief Calculate edge weights for edges leading to cold blocks.
245 ///
246 /// A cold block is one post-dominated by  a block with a call to a
247 /// cold function.  Those edges are unlikely to be taken, so we give
248 /// them relatively low weight.
249 ///
250 /// Return true if we could compute the weights for cold edges.
251 /// Return false, otherwise.
calcColdCallHeuristics(const BasicBlock * BB)252 bool BranchProbabilityInfo::calcColdCallHeuristics(const BasicBlock *BB) {
253   const TerminatorInst *TI = BB->getTerminator();
254   if (TI->getNumSuccessors() == 0)
255     return false;
256 
257   // Determine which successors are post-dominated by a cold block.
258   SmallVector<unsigned, 4> ColdEdges;
259   SmallVector<unsigned, 4> NormalEdges;
260   for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
261     if (PostDominatedByColdCall.count(*I))
262       ColdEdges.push_back(I.getSuccessorIndex());
263     else
264       NormalEdges.push_back(I.getSuccessorIndex());
265 
266   // If all successors are in the set of blocks post-dominated by cold calls,
267   // this block is in the set post-dominated by cold calls.
268   if (ColdEdges.size() == TI->getNumSuccessors())
269     PostDominatedByColdCall.insert(BB);
270   else {
271     // Otherwise, if the block itself contains a cold function, add it to the
272     // set of blocks postdominated by a cold call.
273     assert(!PostDominatedByColdCall.count(BB));
274     for (BasicBlock::const_iterator I = BB->begin(), E = BB->end(); I != E; ++I)
275       if (const CallInst *CI = dyn_cast<CallInst>(I))
276         if (CI->hasFnAttr(Attribute::Cold)) {
277           PostDominatedByColdCall.insert(BB);
278           break;
279         }
280   }
281 
282   // Skip probabilities if this block has a single successor.
283   if (TI->getNumSuccessors() == 1 || ColdEdges.empty())
284     return false;
285 
286   if (NormalEdges.empty()) {
287     BranchProbability Prob(1, ColdEdges.size());
288     for (unsigned SuccIdx : ColdEdges)
289       setEdgeProbability(BB, SuccIdx, Prob);
290     return true;
291   }
292 
293   BranchProbability ColdProb(CC_TAKEN_WEIGHT,
294                              (CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) *
295                                  ColdEdges.size());
296   BranchProbability NormalProb(CC_NONTAKEN_WEIGHT,
297                                (CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) *
298                                    NormalEdges.size());
299 
300   for (unsigned SuccIdx : ColdEdges)
301     setEdgeProbability(BB, SuccIdx, ColdProb);
302   for (unsigned SuccIdx : NormalEdges)
303     setEdgeProbability(BB, SuccIdx, NormalProb);
304 
305   return true;
306 }
307 
308 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
309 // between two pointer or pointer and NULL will fail.
calcPointerHeuristics(const BasicBlock * BB)310 bool BranchProbabilityInfo::calcPointerHeuristics(const BasicBlock *BB) {
311   const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
312   if (!BI || !BI->isConditional())
313     return false;
314 
315   Value *Cond = BI->getCondition();
316   ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
317   if (!CI || !CI->isEquality())
318     return false;
319 
320   Value *LHS = CI->getOperand(0);
321 
322   if (!LHS->getType()->isPointerTy())
323     return false;
324 
325   assert(CI->getOperand(1)->getType()->isPointerTy());
326 
327   // p != 0   ->   isProb = true
328   // p == 0   ->   isProb = false
329   // p != q   ->   isProb = true
330   // p == q   ->   isProb = false;
331   unsigned TakenIdx = 0, NonTakenIdx = 1;
332   bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
333   if (!isProb)
334     std::swap(TakenIdx, NonTakenIdx);
335 
336   BranchProbability TakenProb(PH_TAKEN_WEIGHT,
337                               PH_TAKEN_WEIGHT + PH_NONTAKEN_WEIGHT);
338   setEdgeProbability(BB, TakenIdx, TakenProb);
339   setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
340   return true;
341 }
342 
343 // Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
344 // as taken, exiting edges as not-taken.
calcLoopBranchHeuristics(const BasicBlock * BB,const LoopInfo & LI)345 bool BranchProbabilityInfo::calcLoopBranchHeuristics(const BasicBlock *BB,
346                                                      const LoopInfo &LI) {
347   Loop *L = LI.getLoopFor(BB);
348   if (!L)
349     return false;
350 
351   SmallVector<unsigned, 8> BackEdges;
352   SmallVector<unsigned, 8> ExitingEdges;
353   SmallVector<unsigned, 8> InEdges; // Edges from header to the loop.
354 
355   for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
356     if (!L->contains(*I))
357       ExitingEdges.push_back(I.getSuccessorIndex());
358     else if (L->getHeader() == *I)
359       BackEdges.push_back(I.getSuccessorIndex());
360     else
361       InEdges.push_back(I.getSuccessorIndex());
362   }
363 
364   if (BackEdges.empty() && ExitingEdges.empty())
365     return false;
366 
367   // Collect the sum of probabilities of back-edges/in-edges/exiting-edges, and
368   // normalize them so that they sum up to one.
369   BranchProbability Probs[] = {BranchProbability::getZero(),
370                                BranchProbability::getZero(),
371                                BranchProbability::getZero()};
372   unsigned Denom = (BackEdges.empty() ? 0 : LBH_TAKEN_WEIGHT) +
373                    (InEdges.empty() ? 0 : LBH_TAKEN_WEIGHT) +
374                    (ExitingEdges.empty() ? 0 : LBH_NONTAKEN_WEIGHT);
375   if (!BackEdges.empty())
376     Probs[0] = BranchProbability(LBH_TAKEN_WEIGHT, Denom);
377   if (!InEdges.empty())
378     Probs[1] = BranchProbability(LBH_TAKEN_WEIGHT, Denom);
379   if (!ExitingEdges.empty())
380     Probs[2] = BranchProbability(LBH_NONTAKEN_WEIGHT, Denom);
381 
382   if (uint32_t numBackEdges = BackEdges.size()) {
383     auto Prob = Probs[0] / numBackEdges;
384     for (unsigned SuccIdx : BackEdges)
385       setEdgeProbability(BB, SuccIdx, Prob);
386   }
387 
388   if (uint32_t numInEdges = InEdges.size()) {
389     auto Prob = Probs[1] / numInEdges;
390     for (unsigned SuccIdx : InEdges)
391       setEdgeProbability(BB, SuccIdx, Prob);
392   }
393 
394   if (uint32_t numExitingEdges = ExitingEdges.size()) {
395     auto Prob = Probs[2] / numExitingEdges;
396     for (unsigned SuccIdx : ExitingEdges)
397       setEdgeProbability(BB, SuccIdx, Prob);
398   }
399 
400   return true;
401 }
402 
calcZeroHeuristics(const BasicBlock * BB)403 bool BranchProbabilityInfo::calcZeroHeuristics(const BasicBlock *BB) {
404   const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
405   if (!BI || !BI->isConditional())
406     return false;
407 
408   Value *Cond = BI->getCondition();
409   ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
410   if (!CI)
411     return false;
412 
413   Value *RHS = CI->getOperand(1);
414   ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
415   if (!CV)
416     return false;
417 
418   // If the LHS is the result of AND'ing a value with a single bit bitmask,
419   // we don't have information about probabilities.
420   if (Instruction *LHS = dyn_cast<Instruction>(CI->getOperand(0)))
421     if (LHS->getOpcode() == Instruction::And)
422       if (ConstantInt *AndRHS = dyn_cast<ConstantInt>(LHS->getOperand(1)))
423         if (AndRHS->getUniqueInteger().isPowerOf2())
424           return false;
425 
426   bool isProb;
427   if (CV->isZero()) {
428     switch (CI->getPredicate()) {
429     case CmpInst::ICMP_EQ:
430       // X == 0   ->  Unlikely
431       isProb = false;
432       break;
433     case CmpInst::ICMP_NE:
434       // X != 0   ->  Likely
435       isProb = true;
436       break;
437     case CmpInst::ICMP_SLT:
438       // X < 0   ->  Unlikely
439       isProb = false;
440       break;
441     case CmpInst::ICMP_SGT:
442       // X > 0   ->  Likely
443       isProb = true;
444       break;
445     default:
446       return false;
447     }
448   } else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
449     // InstCombine canonicalizes X <= 0 into X < 1.
450     // X <= 0   ->  Unlikely
451     isProb = false;
452   } else if (CV->isAllOnesValue()) {
453     switch (CI->getPredicate()) {
454     case CmpInst::ICMP_EQ:
455       // X == -1  ->  Unlikely
456       isProb = false;
457       break;
458     case CmpInst::ICMP_NE:
459       // X != -1  ->  Likely
460       isProb = true;
461       break;
462     case CmpInst::ICMP_SGT:
463       // InstCombine canonicalizes X >= 0 into X > -1.
464       // X >= 0   ->  Likely
465       isProb = true;
466       break;
467     default:
468       return false;
469     }
470   } else {
471     return false;
472   }
473 
474   unsigned TakenIdx = 0, NonTakenIdx = 1;
475 
476   if (!isProb)
477     std::swap(TakenIdx, NonTakenIdx);
478 
479   BranchProbability TakenProb(ZH_TAKEN_WEIGHT,
480                               ZH_TAKEN_WEIGHT + ZH_NONTAKEN_WEIGHT);
481   setEdgeProbability(BB, TakenIdx, TakenProb);
482   setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
483   return true;
484 }
485 
calcFloatingPointHeuristics(const BasicBlock * BB)486 bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
487   const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
488   if (!BI || !BI->isConditional())
489     return false;
490 
491   Value *Cond = BI->getCondition();
492   FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
493   if (!FCmp)
494     return false;
495 
496   bool isProb;
497   if (FCmp->isEquality()) {
498     // f1 == f2 -> Unlikely
499     // f1 != f2 -> Likely
500     isProb = !FCmp->isTrueWhenEqual();
501   } else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
502     // !isnan -> Likely
503     isProb = true;
504   } else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
505     // isnan -> Unlikely
506     isProb = false;
507   } else {
508     return false;
509   }
510 
511   unsigned TakenIdx = 0, NonTakenIdx = 1;
512 
513   if (!isProb)
514     std::swap(TakenIdx, NonTakenIdx);
515 
516   BranchProbability TakenProb(FPH_TAKEN_WEIGHT,
517                               FPH_TAKEN_WEIGHT + FPH_NONTAKEN_WEIGHT);
518   setEdgeProbability(BB, TakenIdx, TakenProb);
519   setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
520   return true;
521 }
522 
calcInvokeHeuristics(const BasicBlock * BB)523 bool BranchProbabilityInfo::calcInvokeHeuristics(const BasicBlock *BB) {
524   const InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator());
525   if (!II)
526     return false;
527 
528   BranchProbability TakenProb(IH_TAKEN_WEIGHT,
529                               IH_TAKEN_WEIGHT + IH_NONTAKEN_WEIGHT);
530   setEdgeProbability(BB, 0 /*Index for Normal*/, TakenProb);
531   setEdgeProbability(BB, 1 /*Index for Unwind*/, TakenProb.getCompl());
532   return true;
533 }
534 
releaseMemory()535 void BranchProbabilityInfo::releaseMemory() {
536   Probs.clear();
537 }
538 
print(raw_ostream & OS) const539 void BranchProbabilityInfo::print(raw_ostream &OS) const {
540   OS << "---- Branch Probabilities ----\n";
541   // We print the probabilities from the last function the analysis ran over,
542   // or the function it is currently running over.
543   assert(LastF && "Cannot print prior to running over a function");
544   for (const auto &BI : *LastF) {
545     for (succ_const_iterator SI = succ_begin(&BI), SE = succ_end(&BI); SI != SE;
546          ++SI) {
547       printEdgeProbability(OS << "  ", &BI, *SI);
548     }
549   }
550 }
551 
552 bool BranchProbabilityInfo::
isEdgeHot(const BasicBlock * Src,const BasicBlock * Dst) const553 isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
554   // Hot probability is at least 4/5 = 80%
555   // FIXME: Compare against a static "hot" BranchProbability.
556   return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
557 }
558 
559 const BasicBlock *
getHotSucc(const BasicBlock * BB) const560 BranchProbabilityInfo::getHotSucc(const BasicBlock *BB) const {
561   auto MaxProb = BranchProbability::getZero();
562   const BasicBlock *MaxSucc = nullptr;
563 
564   for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
565     const BasicBlock *Succ = *I;
566     auto Prob = getEdgeProbability(BB, Succ);
567     if (Prob > MaxProb) {
568       MaxProb = Prob;
569       MaxSucc = Succ;
570     }
571   }
572 
573   // Hot probability is at least 4/5 = 80%
574   if (MaxProb > BranchProbability(4, 5))
575     return MaxSucc;
576 
577   return nullptr;
578 }
579 
580 /// Get the raw edge probability for the edge. If can't find it, return a
581 /// default probability 1/N where N is the number of successors. Here an edge is
582 /// specified using PredBlock and an
583 /// index to the successors.
584 BranchProbability
getEdgeProbability(const BasicBlock * Src,unsigned IndexInSuccessors) const585 BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
586                                           unsigned IndexInSuccessors) const {
587   auto I = Probs.find(std::make_pair(Src, IndexInSuccessors));
588 
589   if (I != Probs.end())
590     return I->second;
591 
592   return {1,
593           static_cast<uint32_t>(std::distance(succ_begin(Src), succ_end(Src)))};
594 }
595 
596 BranchProbability
getEdgeProbability(const BasicBlock * Src,succ_const_iterator Dst) const597 BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
598                                           succ_const_iterator Dst) const {
599   return getEdgeProbability(Src, Dst.getSuccessorIndex());
600 }
601 
602 /// Get the raw edge probability calculated for the block pair. This returns the
603 /// sum of all raw edge probabilities from Src to Dst.
604 BranchProbability
getEdgeProbability(const BasicBlock * Src,const BasicBlock * Dst) const605 BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
606                                           const BasicBlock *Dst) const {
607   auto Prob = BranchProbability::getZero();
608   bool FoundProb = false;
609   for (succ_const_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
610     if (*I == Dst) {
611       auto MapI = Probs.find(std::make_pair(Src, I.getSuccessorIndex()));
612       if (MapI != Probs.end()) {
613         FoundProb = true;
614         Prob += MapI->second;
615       }
616     }
617   uint32_t succ_num = std::distance(succ_begin(Src), succ_end(Src));
618   return FoundProb ? Prob : BranchProbability(1, succ_num);
619 }
620 
621 /// Set the edge probability for a given edge specified by PredBlock and an
622 /// index to the successors.
setEdgeProbability(const BasicBlock * Src,unsigned IndexInSuccessors,BranchProbability Prob)623 void BranchProbabilityInfo::setEdgeProbability(const BasicBlock *Src,
624                                                unsigned IndexInSuccessors,
625                                                BranchProbability Prob) {
626   Probs[std::make_pair(Src, IndexInSuccessors)] = Prob;
627   DEBUG(dbgs() << "set edge " << Src->getName() << " -> " << IndexInSuccessors
628                << " successor probability to " << Prob << "\n");
629 }
630 
631 raw_ostream &
printEdgeProbability(raw_ostream & OS,const BasicBlock * Src,const BasicBlock * Dst) const632 BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
633                                             const BasicBlock *Src,
634                                             const BasicBlock *Dst) const {
635 
636   const BranchProbability Prob = getEdgeProbability(Src, Dst);
637   OS << "edge " << Src->getName() << " -> " << Dst->getName()
638      << " probability is " << Prob
639      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
640 
641   return OS;
642 }
643 
calculate(const Function & F,const LoopInfo & LI)644 void BranchProbabilityInfo::calculate(const Function &F, const LoopInfo &LI) {
645   DEBUG(dbgs() << "---- Branch Probability Info : " << F.getName()
646                << " ----\n\n");
647   LastF = &F; // Store the last function we ran on for printing.
648   assert(PostDominatedByUnreachable.empty());
649   assert(PostDominatedByColdCall.empty());
650 
651   // Walk the basic blocks in post-order so that we can build up state about
652   // the successors of a block iteratively.
653   for (auto BB : post_order(&F.getEntryBlock())) {
654     DEBUG(dbgs() << "Computing probabilities for " << BB->getName() << "\n");
655     if (calcUnreachableHeuristics(BB))
656       continue;
657     if (calcMetadataWeights(BB))
658       continue;
659     if (calcColdCallHeuristics(BB))
660       continue;
661     if (calcLoopBranchHeuristics(BB, LI))
662       continue;
663     if (calcPointerHeuristics(BB))
664       continue;
665     if (calcZeroHeuristics(BB))
666       continue;
667     if (calcFloatingPointHeuristics(BB))
668       continue;
669     calcInvokeHeuristics(BB);
670   }
671 
672   PostDominatedByUnreachable.clear();
673   PostDominatedByColdCall.clear();
674 }
675 
getAnalysisUsage(AnalysisUsage & AU) const676 void BranchProbabilityInfoWrapperPass::getAnalysisUsage(
677     AnalysisUsage &AU) const {
678   AU.addRequired<LoopInfoWrapperPass>();
679   AU.setPreservesAll();
680 }
681 
runOnFunction(Function & F)682 bool BranchProbabilityInfoWrapperPass::runOnFunction(Function &F) {
683   const LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
684   BPI.calculate(F, LI);
685   return false;
686 }
687 
releaseMemory()688 void BranchProbabilityInfoWrapperPass::releaseMemory() { BPI.releaseMemory(); }
689 
print(raw_ostream & OS,const Module *) const690 void BranchProbabilityInfoWrapperPass::print(raw_ostream &OS,
691                                              const Module *) const {
692   BPI.print(OS);
693 }
694 
695 char BranchProbabilityAnalysis::PassID;
696 BranchProbabilityInfo
run(Function & F,AnalysisManager<Function> & AM)697 BranchProbabilityAnalysis::run(Function &F, AnalysisManager<Function> &AM) {
698   BranchProbabilityInfo BPI;
699   BPI.calculate(F, AM.getResult<LoopAnalysis>(F));
700   return BPI;
701 }
702 
703 PreservedAnalyses
run(Function & F,AnalysisManager<Function> & AM)704 BranchProbabilityPrinterPass::run(Function &F, AnalysisManager<Function> &AM) {
705   OS << "Printing analysis results of BPI for function "
706      << "'" << F.getName() << "':"
707      << "\n";
708   AM.getResult<BranchProbabilityAnalysis>(F).print(OS);
709   return PreservedAnalyses::all();
710 }
711