UnreachableBlockElim.cpp

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//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass is an extremely simple version of the SimplifyCFG pass.  Its sole
// job is to delete LLVM basic blocks that are not reachable from the entry
// node.  To do this, it performs a simple depth first traversal of the CFG,
// then deletes any unvisited nodes.
//
// Note that this pass is really a hack.  In particular, the instruction
// selectors for various targets should just not generate code for unreachable
// blocks.  Until LLVM has a more systematic way of defining instruction
// selectors, however, we cannot really expect them to handle additional
// complexity.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/UnreachableBlockElim.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Type.h"
#include "llvm/Pass.h"
using namespace llvm;

static bool eliminateUnreachableBlock(Function &F) {
  df_iterator_default_set<BasicBlock*> Reachable;

  // Mark all reachable blocks.
  for (BasicBlock *BB : depth_first_ext(&F, Reachable))
    (void)BB/* Mark all reachable blocks */;

  // Loop over all dead blocks, remembering them and deleting all instructions
  // in them.
  std::vector<BasicBlock*> DeadBlocks;
  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
    if (!Reachable.count(&*I)) {
      BasicBlock *BB = &*I;
      DeadBlocks.push_back(BB);
      while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
        PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
        BB->getInstList().pop_front();
      }
      for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
        (*SI)->removePredecessor(BB);
      BB->dropAllReferences();
    }

  // Actually remove the blocks now.
  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
    DeadBlocks[i]->eraseFromParent();
  }

  return !DeadBlocks.empty();
}

namespace {
class UnreachableBlockElimLegacyPass : public FunctionPass {
  bool runOnFunction(Function &F) override {
    return eliminateUnreachableBlock(F);
  }

public:
  static char ID; // Pass identification, replacement for typeid
  UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
    initializeUnreachableBlockElimLegacyPassPass(
        *PassRegistry::getPassRegistry());
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addPreserved<DominatorTreeWrapperPass>();
  }
};
}
char UnreachableBlockElimLegacyPass::ID = 0;
INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
                "Remove unreachable blocks from the CFG", false, false)

FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  return new UnreachableBlockElimLegacyPass();
}

PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
                                                FunctionAnalysisManager &AM) {
  bool Changed = eliminateUnreachableBlock(F);
  if (!Changed)
    return PreservedAnalyses::all();
  PreservedAnalyses PA;
  PA.preserve<DominatorTreeAnalysis>();
  return PA;
}

namespace {
  class UnreachableMachineBlockElim : public MachineFunctionPass {
    bool runOnMachineFunction(MachineFunction &F) override;
    void getAnalysisUsage(AnalysisUsage &AU) const override;
    MachineModuleInfo *MMI;
  public:
    static char ID; // Pass identification, replacement for typeid
    UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
  };
}
char UnreachableMachineBlockElim::ID = 0;

INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
  "Remove unreachable machine basic blocks", false, false)

char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;

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

bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
  df_iterator_default_set<MachineBasicBlock*> Reachable;
  bool ModifiedPHI = false;

  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
  MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
  MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();

  // Mark all reachable blocks.
  for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
    (void)BB/* Mark all reachable blocks */;

  // Loop over all dead blocks, remembering them and deleting all instructions
  // in them.
  std::vector<MachineBasicBlock*> DeadBlocks;
  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    MachineBasicBlock *BB = &*I;

    // Test for deadness.
    if (!Reachable.count(BB)) {
      DeadBlocks.push_back(BB);

      // Update dominator and loop info.
      if (MLI) MLI->removeBlock(BB);
      if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);

      while (BB->succ_begin() != BB->succ_end()) {
        MachineBasicBlock* succ = *BB->succ_begin();

        MachineBasicBlock::iterator start = succ->begin();
        while (start != succ->end() && start->isPHI()) {
          for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
            if (start->getOperand(i).isMBB() &&
                start->getOperand(i).getMBB() == BB) {
              start->RemoveOperand(i);
              start->RemoveOperand(i-1);
            }

          start++;
        }

        BB->removeSuccessor(BB->succ_begin());
      }
    }
  }

  // Actually remove the blocks now.
  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
    DeadBlocks[i]->eraseFromParent();

  // Cleanup PHI nodes.
  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    MachineBasicBlock *BB = &*I;
    // Prune unneeded PHI entries.
    SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
                                             BB->pred_end());
    MachineBasicBlock::iterator phi = BB->begin();
    while (phi != BB->end() && phi->isPHI()) {
      for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
        if (!preds.count(phi->getOperand(i).getMBB())) {
          phi->RemoveOperand(i);
          phi->RemoveOperand(i-1);
          ModifiedPHI = true;
        }

      if (phi->getNumOperands() == 3) {
        const MachineOperand &Input = phi->getOperand(1);
        const MachineOperand &Output = phi->getOperand(0);
        unsigned InputReg = Input.getReg();
        unsigned OutputReg = Output.getReg();
        assert(Output.getSubReg() == 0 && "Cannot have output subregister");
        ModifiedPHI = true;

        if (InputReg != OutputReg) {
          MachineRegisterInfo &MRI = F.getRegInfo();
          unsigned InputSub = Input.getSubReg();
          if (InputSub == 0 &&
              MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
              !Input.isUndef()) {
            MRI.replaceRegWith(OutputReg, InputReg);
          } else {
            // The input register to the PHI has a subregister or it can't be
            // constrained to the proper register class or it is undef:
            // insert a COPY instead of simply replacing the output
            // with the input.
            const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
            BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
                    TII->get(TargetOpcode::COPY), OutputReg)
                .addReg(InputReg, getRegState(Input), InputSub);
          }
          phi++->eraseFromParent();
        }
        continue;
      }

      ++phi;
    }
  }

  F.RenumberBlocks();

  return (!DeadBlocks.empty() || ModifiedPHI);
}