1//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This pass is an extremely simple version of the SimplifyCFG pass.  Its sole
10// job is to delete LLVM basic blocks that are not reachable from the entry
11// node.  To do this, it performs a simple depth first traversal of the CFG,
12// then deletes any unvisited nodes.
13//
14// Note that this pass is really a hack.  In particular, the instruction
15// selectors for various targets should just not generate code for unreachable
16// blocks.  Until LLVM has a more systematic way of defining instruction
17// selectors, however, we cannot really expect them to handle additional
18// complexity.
19//
20//===----------------------------------------------------------------------===//
21
22#include "llvm/CodeGen/UnreachableBlockElim.h"
23#include "llvm/ADT/DepthFirstIterator.h"
24#include "llvm/ADT/SmallPtrSet.h"
25#include "llvm/CodeGen/MachineDominators.h"
26#include "llvm/CodeGen/MachineFunctionPass.h"
27#include "llvm/CodeGen/MachineInstrBuilder.h"
28#include "llvm/CodeGen/MachineLoopInfo.h"
29#include "llvm/CodeGen/MachineModuleInfo.h"
30#include "llvm/CodeGen/MachineRegisterInfo.h"
31#include "llvm/CodeGen/Passes.h"
32#include "llvm/CodeGen/TargetInstrInfo.h"
33#include "llvm/IR/CFG.h"
34#include "llvm/IR/Constant.h"
35#include "llvm/IR/Dominators.h"
36#include "llvm/IR/Function.h"
37#include "llvm/IR/Instructions.h"
38#include "llvm/IR/Type.h"
39#include "llvm/InitializePasses.h"
40#include "llvm/Pass.h"
41#include "llvm/Transforms/Utils/BasicBlockUtils.h"
42using namespace llvm;
43
44namespace {
45class UnreachableBlockElimLegacyPass : public FunctionPass {
46  bool runOnFunction(Function &F) override {
47    return llvm::EliminateUnreachableBlocks(F);
48  }
49
50public:
51  static char ID; // Pass identification, replacement for typeid
52  UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
53    initializeUnreachableBlockElimLegacyPassPass(
54        *PassRegistry::getPassRegistry());
55  }
56
57  void getAnalysisUsage(AnalysisUsage &AU) const override {
58    AU.addPreserved<DominatorTreeWrapperPass>();
59  }
60};
61}
62char UnreachableBlockElimLegacyPass::ID = 0;
63INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
64                "Remove unreachable blocks from the CFG", false, false)
65
66FunctionPass *llvm::createUnreachableBlockEliminationPass() {
67  return new UnreachableBlockElimLegacyPass();
68}
69
70PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
71                                                FunctionAnalysisManager &AM) {
72  bool Changed = llvm::EliminateUnreachableBlocks(F);
73  if (!Changed)
74    return PreservedAnalyses::all();
75  PreservedAnalyses PA;
76  PA.preserve<DominatorTreeAnalysis>();
77  return PA;
78}
79
80namespace {
81  class UnreachableMachineBlockElim : public MachineFunctionPass {
82    bool runOnMachineFunction(MachineFunction &F) override;
83    void getAnalysisUsage(AnalysisUsage &AU) const override;
84
85  public:
86    static char ID; // Pass identification, replacement for typeid
87    UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
88  };
89}
90char UnreachableMachineBlockElim::ID = 0;
91
92INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
93  "Remove unreachable machine basic blocks", false, false)
94
95char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
96
97void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
98  AU.addPreserved<MachineLoopInfo>();
99  AU.addPreserved<MachineDominatorTree>();
100  MachineFunctionPass::getAnalysisUsage(AU);
101}
102
103bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
104  df_iterator_default_set<MachineBasicBlock*> Reachable;
105  bool ModifiedPHI = false;
106
107  MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
108  MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
109
110  // Mark all reachable blocks.
111  for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
112    (void)BB/* Mark all reachable blocks */;
113
114  // Loop over all dead blocks, remembering them and deleting all instructions
115  // in them.
116  std::vector<MachineBasicBlock*> DeadBlocks;
117  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
118    MachineBasicBlock *BB = &*I;
119
120    // Test for deadness.
121    if (!Reachable.count(BB)) {
122      DeadBlocks.push_back(BB);
123
124      // Update dominator and loop info.
125      if (MLI) MLI->removeBlock(BB);
126      if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
127
128      while (BB->succ_begin() != BB->succ_end()) {
129        MachineBasicBlock* succ = *BB->succ_begin();
130
131        MachineBasicBlock::iterator start = succ->begin();
132        while (start != succ->end() && start->isPHI()) {
133          for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
134            if (start->getOperand(i).isMBB() &&
135                start->getOperand(i).getMBB() == BB) {
136              start->RemoveOperand(i);
137              start->RemoveOperand(i-1);
138            }
139
140          start++;
141        }
142
143        BB->removeSuccessor(BB->succ_begin());
144      }
145    }
146  }
147
148  // Actually remove the blocks now.
149  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
150    // Remove any call site information for calls in the block.
151    for (auto &I : DeadBlocks[i]->instrs())
152      if (I.shouldUpdateCallSiteInfo())
153        DeadBlocks[i]->getParent()->eraseCallSiteInfo(&I);
154
155    DeadBlocks[i]->eraseFromParent();
156  }
157
158  // Cleanup PHI nodes.
159  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
160    MachineBasicBlock *BB = &*I;
161    // Prune unneeded PHI entries.
162    SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
163                                             BB->pred_end());
164    MachineBasicBlock::iterator phi = BB->begin();
165    while (phi != BB->end() && phi->isPHI()) {
166      for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
167        if (!preds.count(phi->getOperand(i).getMBB())) {
168          phi->RemoveOperand(i);
169          phi->RemoveOperand(i-1);
170          ModifiedPHI = true;
171        }
172
173      if (phi->getNumOperands() == 3) {
174        const MachineOperand &Input = phi->getOperand(1);
175        const MachineOperand &Output = phi->getOperand(0);
176        Register InputReg = Input.getReg();
177        Register OutputReg = Output.getReg();
178        assert(Output.getSubReg() == 0 && "Cannot have output subregister");
179        ModifiedPHI = true;
180
181        if (InputReg != OutputReg) {
182          MachineRegisterInfo &MRI = F.getRegInfo();
183          unsigned InputSub = Input.getSubReg();
184          if (InputSub == 0 &&
185              MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
186              !Input.isUndef()) {
187            MRI.replaceRegWith(OutputReg, InputReg);
188          } else {
189            // The input register to the PHI has a subregister or it can't be
190            // constrained to the proper register class or it is undef:
191            // insert a COPY instead of simply replacing the output
192            // with the input.
193            const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
194            BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
195                    TII->get(TargetOpcode::COPY), OutputReg)
196                .addReg(InputReg, getRegState(Input), InputSub);
197          }
198          phi++->eraseFromParent();
199        }
200        continue;
201      }
202
203      ++phi;
204    }
205  }
206
207  F.RenumberBlocks();
208
209  return (!DeadBlocks.empty() || ModifiedPHI);
210}
211