UnreachableBlockElim.cpp

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//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
//
// 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 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/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
 
namespace {
class UnreachableBlockElimLegacyPass : public FunctionPass {
  bool runOnFunction(Function &F) override {
    return llvm::EliminateUnreachableBlocks(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 = llvm::EliminateUnreachableBlocks(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;
 
  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;
 
  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 (MachineBasicBlock &BB : F) {
    // 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 (MachineBasicBlock *BB : DeadBlocks) {
    // Remove any call site information for calls in the block.
    for (auto &I : BB->instrs())
      if (I.shouldUpdateCallSiteInfo())
        BB->getParent()->eraseCallSiteInfo(&I);
 
    BB->eraseFromParent();
  }
 
  // Cleanup PHI nodes.
  for (MachineBasicBlock &BB : F) {
    // 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);
        Register InputReg = Input.getReg();
        Register 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);
}