lib/CodeGen/MachineBranchProbabilityInfo.cpp

   1 //===- MachineBranchProbabilityInfo.cpp - Machine Branch Probability Info -===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This analysis uses probability info stored in Machine Basic Blocks.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
  14 #include "llvm/CodeGen/MachineBasicBlock.h"
  15 #include "llvm/IR/Instructions.h"
  16 #include "llvm/Support/Debug.h"
  17 #include "llvm/Support/raw_ostream.h"
  18
  19 using namespace llvm;
  20
  21 INITIALIZE_PASS_BEGIN(MachineBranchProbabilityInfo, "machine-branch-prob",
  22                       "Machine Branch Probability Analysis", false, true)
  23 INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
  24                     "Machine Branch Probability Analysis", false, true)
  25
  26 cl::opt<unsigned>
  27     StaticLikelyProb("static-likely-prob",
  28                      cl::desc("branch probability threshold in percentage"
  29                               "to be considered very likely"),
  30                      cl::init(80), cl::Hidden);
  31
  32 cl::opt<unsigned> ProfileLikelyProb(
  33     "profile-likely-prob",
  34     cl::desc("branch probability threshold in percentage to be considered"
  35              " very likely when profile is available"),
  36     cl::init(51), cl::Hidden);
  37
  38 char MachineBranchProbabilityInfo::ID = 0;
  39
  40 void MachineBranchProbabilityInfo::anchor() {}
  41
  42 BranchProbability MachineBranchProbabilityInfo::getEdgeProbability(
  43     const MachineBasicBlock *Src,
  44     MachineBasicBlock::const_succ_iterator Dst) const {
  45   return Src->getSuccProbability(Dst);
  46 }
  47
  48 BranchProbability MachineBranchProbabilityInfo::getEdgeProbability(
  49     const MachineBasicBlock *Src, const MachineBasicBlock *Dst) const {
  50   // This is a linear search. Try to use the const_succ_iterator version when
  51   // possible.
  52   return getEdgeProbability(Src, find(Src->successors(), Dst));
  53 }
  54
  55 bool MachineBranchProbabilityInfo::isEdgeHot(
  56     const MachineBasicBlock *Src, const MachineBasicBlock *Dst) const {
  57   BranchProbability HotProb(StaticLikelyProb, 100);
  58   return getEdgeProbability(Src, Dst) > HotProb;
  59 }
  60
  61 MachineBasicBlock *
  62 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
  63   auto MaxProb = BranchProbability::getZero();
  64   MachineBasicBlock *MaxSucc = nullptr;
  65   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
  66        E = MBB->succ_end(); I != E; ++I) {
  67     auto Prob = getEdgeProbability(MBB, I);
  68     if (Prob > MaxProb) {
  69       MaxProb = Prob;
  70       MaxSucc = *I;
  71     }
  72   }
  73
  74   BranchProbability HotProb(StaticLikelyProb, 100);
  75   if (getEdgeProbability(MBB, MaxSucc) >= HotProb)
  76     return MaxSucc;
  77
  78   return nullptr;
  79 }
  80
  81 raw_ostream &MachineBranchProbabilityInfo::printEdgeProbability(
  82     raw_ostream &OS, const MachineBasicBlock *Src,
  83     const MachineBasicBlock *Dst) const {
  84
  85   const BranchProbability Prob = getEdgeProbability(Src, Dst);
  86   OS << "edge " << printMBBReference(*Src) << " -> " << printMBBReference(*Dst)
  87      << " probability is " << Prob
  88      << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
  89
  90   return OS;
  91 }