MachineBranchProbabilityInfo.cpp

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//===- MachineBranchProbabilityInfo.cpp - Machine Branch Probability Info -===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This analysis uses probability info stored in Machine Basic Blocks.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

INITIALIZE_PASS_BEGIN(MachineBranchProbabilityInfo, "machine-branch-prob",
                      "Machine Branch Probability Analysis", false, true)
INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
                    "Machine Branch Probability Analysis", false, true)

char MachineBranchProbabilityInfo::ID = 0;

void MachineBranchProbabilityInfo::anchor() { }

uint32_t MachineBranchProbabilityInfo::
getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const {
  // First we compute the sum with 64-bits of precision, ensuring that cannot
  // overflow by bounding the number of weights considered. Hopefully no one
  // actually needs 2^32 successors.
  assert(MBB->succ_size() < UINT32_MAX);
  uint64_t Sum = 0;
  Scale = 1;
  for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
       E = MBB->succ_end(); I != E; ++I) {
    uint32_t Weight = getEdgeWeight(MBB, I);
    Sum += Weight;
  }

  // If the computed sum fits in 32-bits, we're done.
  if (Sum <= UINT32_MAX)
    return Sum;

  // Otherwise, compute the scale necessary to cause the weights to fit, and
  // re-sum with that scale applied.
  assert((Sum / UINT32_MAX) < UINT32_MAX);
  Scale = (Sum / UINT32_MAX) + 1;
  Sum = 0;
  for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
       E = MBB->succ_end(); I != E; ++I) {
    uint32_t Weight = getEdgeWeight(MBB, I);
    Sum += Weight / Scale;
  }
  assert(Sum <= UINT32_MAX);
  return Sum;
}

uint32_t MachineBranchProbabilityInfo::
getEdgeWeight(const MachineBasicBlock *Src,
              MachineBasicBlock::const_succ_iterator Dst) const {
  uint32_t Weight = Src->getSuccWeight(Dst);
  if (!Weight)
    return DEFAULT_WEIGHT;
  return Weight;
}

uint32_t MachineBranchProbabilityInfo::
getEdgeWeight(const MachineBasicBlock *Src,
              const MachineBasicBlock *Dst) const {
  // This is a linear search. Try to use the const_succ_iterator version when
  // possible.
  return getEdgeWeight(Src, std::find(Src->succ_begin(), Src->succ_end(), Dst));
}

bool
MachineBranchProbabilityInfo::isEdgeHot(const MachineBasicBlock *Src,
                                        const MachineBasicBlock *Dst) const {
  // Hot probability is at least 4/5 = 80%
  // FIXME: Compare against a static "hot" BranchProbability.
  return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
}

MachineBasicBlock *
MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
  uint32_t MaxWeight = 0;
  MachineBasicBlock *MaxSucc = nullptr;
  for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
       E = MBB->succ_end(); I != E; ++I) {
    uint32_t Weight = getEdgeWeight(MBB, I);
    if (Weight > MaxWeight) {
      MaxWeight = Weight;
      MaxSucc = *I;
    }
  }

  if (getEdgeProbability(MBB, MaxSucc) >= BranchProbability(4, 5))
    return MaxSucc;

  return nullptr;
}

BranchProbability MachineBranchProbabilityInfo::getEdgeProbability(
    const MachineBasicBlock *Src, const MachineBasicBlock *Dst) const {
  uint32_t Scale = 1;
  uint32_t D = getSumForBlock(Src, Scale);
  uint32_t N = getEdgeWeight(Src, Dst) / Scale;

  return BranchProbability(N, D);
}

raw_ostream &MachineBranchProbabilityInfo::printEdgeProbability(
    raw_ostream &OS, const MachineBasicBlock *Src,
    const MachineBasicBlock *Dst) const {

  const BranchProbability Prob = getEdgeProbability(Src, Dst);
  OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
     << " probability is " << Prob
     << (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");

  return OS;
}