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, uint32_t &Scale) const {
  31   // First we compute the sum with 64-bits of precision, ensuring that cannot
  32   // overflow by bounding the number of weights considered. Hopefully no one
  33   // actually needs 2^32 successors.
  34   assert(MBB->succ_size() < UINT32_MAX);
  35   uint64_t Sum = 0;
  36   Scale = 1;
  37   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
  38        E = MBB->succ_end(); I != E; ++I) {
  39     uint32_t Weight = getEdgeWeight(MBB, *I);
  40     Sum += Weight;
  41   }
  42
  43   // If the computed sum fits in 32-bits, we're done.
  44   if (Sum <= UINT32_MAX)
  45     return Sum;
  46
  47   // Otherwise, compute the scale necessary to cause the weights to fit, and
  48   // re-sum with that scale applied.
  49   assert((Sum / UINT32_MAX) < UINT32_MAX);
  50   Scale = (Sum / UINT32_MAX) + 1;
  51   Sum = 0;
  52   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
  53        E = MBB->succ_end(); I != E; ++I) {
  54     uint32_t Weight = getEdgeWeight(MBB, *I);
  55     Sum += Weight / Scale;
  56   }
  57   assert(Sum <= UINT32_MAX);
  58   return Sum;
  59 }
  60
  61 uint32_t
  62 MachineBranchProbabilityInfo::getEdgeWeight(MachineBasicBlock *Src,
  63                                             MachineBasicBlock *Dst) const {
  64   uint32_t Weight = Src->getSuccWeight(Dst);
  65   if (!Weight)
  66     return DEFAULT_WEIGHT;
  67   return Weight;
  68 }
  69
  70 bool MachineBranchProbabilityInfo::isEdgeHot(MachineBasicBlock *Src,
  71                                              MachineBasicBlock *Dst) const {
  72   // Hot probability is at least 4/5 = 80%
  73   // FIXME: Compare against a static "hot" BranchProbability.
  74   return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
  75 }
  76
  77 MachineBasicBlock *
  78 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
  79   uint32_t MaxWeight = 0;
  80   MachineBasicBlock *MaxSucc = 0;
  81   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
  82        E = MBB->succ_end(); I != E; ++I) {
  83     uint32_t Weight = getEdgeWeight(MBB, *I);
  84     if (Weight > MaxWeight) {
  85       MaxWeight = Weight;
  86       MaxSucc = *I;
  87     }
  88   }
  89
  90   if (getEdgeProbability(MBB, MaxSucc) >= BranchProbability(4, 5))
  91     return MaxSucc;
  92
  93   return 0;
  94 }
  95
  96 BranchProbability
  97 MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
  98                                                  MachineBasicBlock *Dst) const {
  99   uint32_t Scale = 1;
 100   uint32_t D = getSumForBlock(Src, Scale);
 101   uint32_t N = getEdgeWeight(Src, Dst) / Scale;
 102
 103   return BranchProbability(N, D);
 104 }
 105
 106 raw_ostream &MachineBranchProbabilityInfo::
 107 printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
 108                      MachineBasicBlock *Dst) const {
 109
 110   const BranchProbability Prob = getEdgeProbability(Src, Dst);
 111   OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
 112      << " probability is "  << Prob
 113      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
 114
 115   return OS;
 116 }