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      1 //===- MachineBranchProbabilityInfo.cpp - Machine Branch Probability Info -===//
      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 // This analysis uses probability info stored in Machine Basic Blocks.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
     15 #include "llvm/CodeGen/MachineBasicBlock.h"
     16 #include "llvm/IR/Instructions.h"
     17 #include "llvm/Support/Debug.h"
     18 #include "llvm/Support/raw_ostream.h"
     19 
     20 using namespace llvm;
     21 
     22 INITIALIZE_PASS_BEGIN(MachineBranchProbabilityInfo, "machine-branch-prob",
     23                       "Machine Branch Probability Analysis", false, true)
     24 INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
     25                     "Machine Branch Probability Analysis", false, true)
     26 
     27 char MachineBranchProbabilityInfo::ID = 0;
     28 
     29 void MachineBranchProbabilityInfo::anchor() { }
     30 
     31 uint32_t MachineBranchProbabilityInfo::
     32 getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const {
     33   // First we compute the sum with 64-bits of precision, ensuring that cannot
     34   // overflow by bounding the number of weights considered. Hopefully no one
     35   // actually needs 2^32 successors.
     36   assert(MBB->succ_size() < UINT32_MAX);
     37   uint64_t Sum = 0;
     38   Scale = 1;
     39   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
     40        E = MBB->succ_end(); I != E; ++I) {
     41     uint32_t Weight = getEdgeWeight(MBB, I);
     42     Sum += Weight;
     43   }
     44 
     45   // If the computed sum fits in 32-bits, we're done.
     46   if (Sum <= UINT32_MAX)
     47     return Sum;
     48 
     49   // Otherwise, compute the scale necessary to cause the weights to fit, and
     50   // re-sum with that scale applied.
     51   assert((Sum / UINT32_MAX) < UINT32_MAX);
     52   Scale = (Sum / UINT32_MAX) + 1;
     53   Sum = 0;
     54   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
     55        E = MBB->succ_end(); I != E; ++I) {
     56     uint32_t Weight = getEdgeWeight(MBB, I);
     57     Sum += Weight / Scale;
     58   }
     59   assert(Sum <= UINT32_MAX);
     60   return Sum;
     61 }
     62 
     63 uint32_t MachineBranchProbabilityInfo::
     64 getEdgeWeight(const MachineBasicBlock *Src,
     65               MachineBasicBlock::const_succ_iterator Dst) const {
     66   uint32_t Weight = Src->getSuccWeight(Dst);
     67   if (!Weight)
     68     return DEFAULT_WEIGHT;
     69   return Weight;
     70 }
     71 
     72 uint32_t MachineBranchProbabilityInfo::
     73 getEdgeWeight(const MachineBasicBlock *Src,
     74               const MachineBasicBlock *Dst) const {
     75   // This is a linear search. Try to use the const_succ_iterator version when
     76   // possible.
     77   return getEdgeWeight(Src, std::find(Src->succ_begin(), Src->succ_end(), Dst));
     78 }
     79 
     80 bool
     81 MachineBranchProbabilityInfo::isEdgeHot(const MachineBasicBlock *Src,
     82                                         const MachineBasicBlock *Dst) const {
     83   // Hot probability is at least 4/5 = 80%
     84   // FIXME: Compare against a static "hot" BranchProbability.
     85   return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
     86 }
     87 
     88 MachineBasicBlock *
     89 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
     90   uint32_t MaxWeight = 0;
     91   MachineBasicBlock *MaxSucc = nullptr;
     92   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
     93        E = MBB->succ_end(); I != E; ++I) {
     94     uint32_t Weight = getEdgeWeight(MBB, I);
     95     if (Weight > MaxWeight) {
     96       MaxWeight = Weight;
     97       MaxSucc = *I;
     98     }
     99   }
    100 
    101   if (getEdgeProbability(MBB, MaxSucc) >= BranchProbability(4, 5))
    102     return MaxSucc;
    103 
    104   return nullptr;
    105 }
    106 
    107 BranchProbability MachineBranchProbabilityInfo::getEdgeProbability(
    108     const MachineBasicBlock *Src, const MachineBasicBlock *Dst) const {
    109   uint32_t Scale = 1;
    110   uint32_t D = getSumForBlock(Src, Scale);
    111   uint32_t N = getEdgeWeight(Src, Dst) / Scale;
    112 
    113   return BranchProbability(N, D);
    114 }
    115 
    116 raw_ostream &MachineBranchProbabilityInfo::printEdgeProbability(
    117     raw_ostream &OS, const MachineBasicBlock *Src,
    118     const MachineBasicBlock *Dst) const {
    119 
    120   const BranchProbability Prob = getEdgeProbability(Src, Dst);
    121   OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
    122      << " probability is " << Prob
    123      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
    124 
    125   return OS;
    126 }
    127