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   // FIXME: Compare against a static "hot" BranchProbability.
  59   return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
  60 }
  61
  62 MachineBasicBlock *
  63 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
  64   uint32_t Sum = 0;
  65   uint32_t MaxWeight = 0;
  66   MachineBasicBlock *MaxSucc = 0;
  67
  68   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
  69        E = MBB->succ_end(); I != E; ++I) {
  70     MachineBasicBlock *Succ = *I;
  71     uint32_t Weight = getEdgeWeight(MBB, Succ);
  72     uint32_t PrevSum = Sum;
  73
  74     Sum += Weight;
  75     assert(Sum > PrevSum); (void) PrevSum;
  76
  77     if (Weight > MaxWeight) {
  78       MaxWeight = Weight;
  79       MaxSucc = Succ;
  80     }
  81   }
  82
  83   if (BranchProbability(MaxWeight, Sum) >= BranchProbability(4, 5))
  84     return MaxSucc;
  85
  86   return 0;
  87 }
  88
  89 BranchProbability
  90 MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
  91                                                  MachineBasicBlock *Dst) const {
  92   uint32_t N = getEdgeWeight(Src, Dst);
  93   uint32_t D = getSumForBlock(Src);
  94
  95   return BranchProbability(N, D);
  96 }
  97
  98 raw_ostream &MachineBranchProbabilityInfo::
  99 printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
 100                      MachineBasicBlock *Dst) const {
 101
 102   const BranchProbability Prob = getEdgeProbability(Src, Dst);
 103   OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
 104      << " probability is "  << Prob
 105      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
 106
 107   return OS;
 108 }