lib/CodeGen/MachineLoopInfo.cpp

//===- MachineLoopInfo.cpp - Natural Loop Calculator ----------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines the MachineLoopInfo class that is used to identify natural
// loops and determine the loop depth of various nodes of the CFG.  Note that
// the loops identified may actually be several natural loops that share the
// same header node... not just a single natural loop.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/Analysis/LoopInfoImpl.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

// Explicitly instantiate methods in LoopInfoImpl.h for MI-level Loops.
template class llvm::LoopBase<MachineBasicBlock, MachineLoop>;
template class llvm::LoopInfoBase<MachineBasicBlock, MachineLoop>;

char MachineLoopInfo::ID = 0;
INITIALIZE_PASS_BEGIN(MachineLoopInfo, "machine-loops",
                "Machine Natural Loop Construction", true, true)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_END(MachineLoopInfo, "machine-loops",
                "Machine Natural Loop Construction", true, true)

char &llvm::MachineLoopInfoID = MachineLoopInfo::ID;

bool MachineLoopInfo::runOnMachineFunction(MachineFunction &) {
  releaseMemory();
  LI.analyze(getAnalysis<MachineDominatorTree>().getBase());
  return false;
}

void MachineLoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesAll();
  AU.addRequired<MachineDominatorTree>();
  MachineFunctionPass::getAnalysisUsage(AU);
}

MachineBasicBlock *MachineLoop::getTopBlock() {
  MachineBasicBlock *TopMBB = getHeader();
  MachineFunction::iterator Begin = TopMBB->getParent()->begin();
  if (TopMBB->getIterator() != Begin) {
    MachineBasicBlock *PriorMBB = &*std::prev(TopMBB->getIterator());
    while (contains(PriorMBB)) {
      TopMBB = PriorMBB;
      if (TopMBB->getIterator() == Begin)
        break;
      PriorMBB = &*std::prev(TopMBB->getIterator());
    }
  }
  return TopMBB;
}

MachineBasicBlock *MachineLoop::getBottomBlock() {
  MachineBasicBlock *BotMBB = getHeader();
  MachineFunction::iterator End = BotMBB->getParent()->end();
  if (BotMBB->getIterator() != std::prev(End)) {
    MachineBasicBlock *NextMBB = &*std::next(BotMBB->getIterator());
    while (contains(NextMBB)) {
      BotMBB = NextMBB;
      if (BotMBB == &*std::next(BotMBB->getIterator()))
        break;
      NextMBB = &*std::next(BotMBB->getIterator());
    }
  }
  return BotMBB;
}

MachineBasicBlock *MachineLoop::findLoopControlBlock() {
  if (MachineBasicBlock *Latch = getLoopLatch()) {
    if (isLoopExiting(Latch))
      return Latch;
    else
      return getExitingBlock();
  }
  return nullptr;
}

DebugLoc MachineLoop::getStartLoc() const {
  // Try the pre-header first.
  if (MachineBasicBlock *PHeadMBB = getLoopPreheader())
    if (const BasicBlock *PHeadBB = PHeadMBB->getBasicBlock())
      if (DebugLoc DL = PHeadBB->getTerminator()->getDebugLoc())
        return DL;

  // If we have no pre-header or there are no instructions with debug
  // info in it, try the header.
  if (MachineBasicBlock *HeadMBB = getHeader())
    if (const BasicBlock *HeadBB = HeadMBB->getBasicBlock())
      return HeadBB->getTerminator()->getDebugLoc();

  return DebugLoc();
}

MachineBasicBlock *
MachineLoopInfo::findLoopPreheader(MachineLoop *L,
                                   bool SpeculativePreheader) const {
  if (MachineBasicBlock *PB = L->getLoopPreheader())
    return PB;

  if (!SpeculativePreheader)
    return nullptr;

  MachineBasicBlock *HB = L->getHeader(), *LB = L->getLoopLatch();
  if (HB->pred_size() != 2 || HB->hasAddressTaken())
    return nullptr;
  // Find the predecessor of the header that is not the latch block.
  MachineBasicBlock *Preheader = nullptr;
  for (MachineBasicBlock *P : HB->predecessors()) {
    if (P == LB)
      continue;
    // Sanity.
    if (Preheader)
      return nullptr;
    Preheader = P;
  }

  // Check if the preheader candidate is a successor of any other loop
  // headers. We want to avoid having two loop setups in the same block.
  for (MachineBasicBlock *S : Preheader->successors()) {
    if (S == HB)
      continue;
    MachineLoop *T = getLoopFor(S);
    if (T && T->getHeader() == S)
      return nullptr;
  }
  return Preheader;
}

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void MachineLoop::dump() const {
  print(dbgs());
}
#endif