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/Instructions.h"
  15 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
  16 #include "llvm/CodeGen/MachineBasicBlock.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 uint32_t MachineBranchProbabilityInfo::
  30 getSumForBlock(MachineBasicBlock *MBB) const {
  31   uint32_t Sum = 0;
  32
  33   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
  34        E = MBB->succ_end(); I != E; ++I) {
  35     MachineBasicBlock *Succ = *I;
  36     uint32_t Weight = getEdgeWeight(MBB, Succ);
  37     uint32_t PrevSum = Sum;
  38
  39     Sum += Weight;
  40     assert(Sum > PrevSum); (void) PrevSum;
  41   }
  42
  43   return Sum;
  44 }
  45
  46 uint32_t
  47 MachineBranchProbabilityInfo::getEdgeWeight(MachineBasicBlock *Src,
  48                                             MachineBasicBlock *Dst) const {
  49   uint32_t Weight = Src->getSuccWeight(Dst);
  50   if (!Weight)
  51     return DEFAULT_WEIGHT;
  52   return Weight;
  53 }
  54
  55 bool MachineBranchProbabilityInfo::isEdgeHot(MachineBasicBlock *Src,
  56                                              MachineBasicBlock *Dst) const {
  57   // Hot probability is at least 4/5 = 80%
  58   uint32_t Weight = getEdgeWeight(Src, Dst);
  59   uint32_t Sum = getSumForBlock(Src);
  60
  61   // FIXME: Implement BranchProbability::compare then change this code to
  62   // compare this BranchProbability against a static "hot" BranchProbability.
  63   return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
  64 }
  65
  66 MachineBasicBlock *
  67 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
  68   uint32_t Sum = 0;
  69   uint32_t MaxWeight = 0;
  70   MachineBasicBlock *MaxSucc = 0;
  71
  72   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
  73        E = MBB->succ_end(); I != E; ++I) {
  74     MachineBasicBlock *Succ = *I;
  75     uint32_t Weight = getEdgeWeight(MBB, Succ);
  76     uint32_t PrevSum = Sum;
  77
  78     Sum += Weight;
  79     assert(Sum > PrevSum); (void) PrevSum;
  80
  81     if (Weight > MaxWeight) {
  82       MaxWeight = Weight;
  83       MaxSucc = Succ;
  84     }
  85   }
  86
  87   // FIXME: Use BranchProbability::compare.
  88   if ((uint64_t)MaxWeight * 5 >= (uint64_t)Sum * 4)
  89     return MaxSucc;
  90
  91   return 0;
  92 }
  93
  94 BranchProbability
  95 MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
  96                                                  MachineBasicBlock *Dst) const {
  97   uint32_t N = getEdgeWeight(Src, Dst);
  98   uint32_t D = getSumForBlock(Src);
  99
 100   return BranchProbability(N, D);
 101 }
 102
 103 raw_ostream &MachineBranchProbabilityInfo::
 104 printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
 105                      MachineBasicBlock *Dst) const {
 106
 107   const BranchProbability Prob = getEdgeProbability(Src, Dst);
 108   OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
 109      << " probability is "  << Prob
 110      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
 111
 112   return OS;
 113 }