lib/CodeGen/MachineBranchProbabilityInfo.cpp

   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 }