1 //===- MachineBranchProbabilityInfo.cpp - Machine Branch Probability Info -===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This analysis uses probability info stored in Machine Basic Blocks.
12 //===----------------------------------------------------------------------===//
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"
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)
27 char MachineBranchProbabilityInfo::ID = 0;
29 void MachineBranchProbabilityInfo::anchor() { }
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);
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);
45 // If the computed sum fits in 32-bits, we're done.
46 if (Sum <= UINT32_MAX)
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;
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;
59 assert(Sum <= UINT32_MAX);
63 uint32_t MachineBranchProbabilityInfo::
64 getEdgeWeight(const MachineBasicBlock *Src,
65 MachineBasicBlock::const_succ_iterator Dst) const {
66 uint32_t Weight = Src->getSuccWeight(Dst);
68 return DEFAULT_WEIGHT;
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
77 return getEdgeWeight(Src, std::find(Src->succ_begin(), Src->succ_end(), Dst));
80 bool MachineBranchProbabilityInfo::isEdgeHot(MachineBasicBlock *Src,
81 MachineBasicBlock *Dst) const {
82 // Hot probability is at least 4/5 = 80%
83 // FIXME: Compare against a static "hot" BranchProbability.
84 return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
88 MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
89 uint32_t MaxWeight = 0;
90 MachineBasicBlock *MaxSucc = 0;
91 for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
92 E = MBB->succ_end(); I != E; ++I) {
93 uint32_t Weight = getEdgeWeight(MBB, I);
94 if (Weight > MaxWeight) {
100 if (getEdgeProbability(MBB, MaxSucc) >= BranchProbability(4, 5))
107 MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
108 MachineBasicBlock *Dst) const {
110 uint32_t D = getSumForBlock(Src, Scale);
111 uint32_t N = getEdgeWeight(Src, Dst) / Scale;
113 return BranchProbability(N, D);
116 raw_ostream &MachineBranchProbabilityInfo::
117 printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
118 MachineBasicBlock *Dst) const {
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");