1//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass is an extremely simple version of the SimplifyCFG pass.  Its sole
11// job is to delete LLVM basic blocks that are not reachable from the entry
12// node.  To do this, it performs a simple depth first traversal of the CFG,
13// then deletes any unvisited nodes.
14//
15// Note that this pass is really a hack.  In particular, the instruction
16// selectors for various targets should just not generate code for unreachable
17// blocks.  Until LLVM has a more systematic way of defining instruction
18// selectors, however, we cannot really expect them to handle additional
19// complexity.
20//
21//===----------------------------------------------------------------------===//
22
23#include "llvm/CodeGen/Passes.h"
24#include "llvm/ADT/DepthFirstIterator.h"
25#include "llvm/ADT/SmallPtrSet.h"
26#include "llvm/CodeGen/MachineDominators.h"
27#include "llvm/CodeGen/MachineFunctionPass.h"
28#include "llvm/CodeGen/MachineLoopInfo.h"
29#include "llvm/CodeGen/MachineModuleInfo.h"
30#include "llvm/CodeGen/MachineRegisterInfo.h"
31#include "llvm/IR/CFG.h"
32#include "llvm/IR/Constant.h"
33#include "llvm/IR/Dominators.h"
34#include "llvm/IR/Function.h"
35#include "llvm/IR/Instructions.h"
36#include "llvm/IR/Type.h"
37#include "llvm/Pass.h"
38#include "llvm/Target/TargetInstrInfo.h"
39using namespace llvm;
40
41namespace {
42  class UnreachableBlockElim : public FunctionPass {
43    bool runOnFunction(Function &F) override;
44  public:
45    static char ID; // Pass identification, replacement for typeid
46    UnreachableBlockElim() : FunctionPass(ID) {
47      initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry());
48    }
49
50    void getAnalysisUsage(AnalysisUsage &AU) const override {
51      AU.addPreserved<DominatorTreeWrapperPass>();
52    }
53  };
54}
55char UnreachableBlockElim::ID = 0;
56INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim",
57                "Remove unreachable blocks from the CFG", false, false)
58
59FunctionPass *llvm::createUnreachableBlockEliminationPass() {
60  return new UnreachableBlockElim();
61}
62
63bool UnreachableBlockElim::runOnFunction(Function &F) {
64  SmallPtrSet<BasicBlock*, 8> Reachable;
65
66  // Mark all reachable blocks.
67  for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
68       df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
69    /* Mark all reachable blocks */;
70
71  // Loop over all dead blocks, remembering them and deleting all instructions
72  // in them.
73  std::vector<BasicBlock*> DeadBlocks;
74  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
75    if (!Reachable.count(I)) {
76      BasicBlock *BB = I;
77      DeadBlocks.push_back(BB);
78      while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
79        PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
80        BB->getInstList().pop_front();
81      }
82      for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
83        (*SI)->removePredecessor(BB);
84      BB->dropAllReferences();
85    }
86
87  // Actually remove the blocks now.
88  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
89    DeadBlocks[i]->eraseFromParent();
90  }
91
92  return DeadBlocks.size();
93}
94
95
96namespace {
97  class UnreachableMachineBlockElim : public MachineFunctionPass {
98    bool runOnMachineFunction(MachineFunction &F) override;
99    void getAnalysisUsage(AnalysisUsage &AU) const override;
100    MachineModuleInfo *MMI;
101  public:
102    static char ID; // Pass identification, replacement for typeid
103    UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
104  };
105}
106char UnreachableMachineBlockElim::ID = 0;
107
108INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
109  "Remove unreachable machine basic blocks", false, false)
110
111char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
112
113void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
114  AU.addPreserved<MachineLoopInfo>();
115  AU.addPreserved<MachineDominatorTree>();
116  MachineFunctionPass::getAnalysisUsage(AU);
117}
118
119bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
120  SmallPtrSet<MachineBasicBlock*, 8> Reachable;
121  bool ModifiedPHI = false;
122
123  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
124  MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
125  MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
126
127  // Mark all reachable blocks.
128  for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
129       I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
130       I != E; ++I)
131    /* Mark all reachable blocks */;
132
133  // Loop over all dead blocks, remembering them and deleting all instructions
134  // in them.
135  std::vector<MachineBasicBlock*> DeadBlocks;
136  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
137    MachineBasicBlock *BB = I;
138
139    // Test for deadness.
140    if (!Reachable.count(BB)) {
141      DeadBlocks.push_back(BB);
142
143      // Update dominator and loop info.
144      if (MLI) MLI->removeBlock(BB);
145      if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
146
147      while (BB->succ_begin() != BB->succ_end()) {
148        MachineBasicBlock* succ = *BB->succ_begin();
149
150        MachineBasicBlock::iterator start = succ->begin();
151        while (start != succ->end() && start->isPHI()) {
152          for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
153            if (start->getOperand(i).isMBB() &&
154                start->getOperand(i).getMBB() == BB) {
155              start->RemoveOperand(i);
156              start->RemoveOperand(i-1);
157            }
158
159          start++;
160        }
161
162        BB->removeSuccessor(BB->succ_begin());
163      }
164    }
165  }
166
167  // Actually remove the blocks now.
168  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
169    DeadBlocks[i]->eraseFromParent();
170
171  // Cleanup PHI nodes.
172  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
173    MachineBasicBlock *BB = I;
174    // Prune unneeded PHI entries.
175    SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
176                                             BB->pred_end());
177    MachineBasicBlock::iterator phi = BB->begin();
178    while (phi != BB->end() && phi->isPHI()) {
179      for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
180        if (!preds.count(phi->getOperand(i).getMBB())) {
181          phi->RemoveOperand(i);
182          phi->RemoveOperand(i-1);
183          ModifiedPHI = true;
184        }
185
186      if (phi->getNumOperands() == 3) {
187        unsigned Input = phi->getOperand(1).getReg();
188        unsigned Output = phi->getOperand(0).getReg();
189
190        MachineInstr* temp = phi;
191        ++phi;
192        temp->eraseFromParent();
193        ModifiedPHI = true;
194
195        if (Input != Output) {
196          MachineRegisterInfo &MRI = F.getRegInfo();
197          MRI.constrainRegClass(Input, MRI.getRegClass(Output));
198          MRI.replaceRegWith(Output, Input);
199        }
200
201        continue;
202      }
203
204      ++phi;
205    }
206  }
207
208  F.RenumberBlocks();
209
210  return (DeadBlocks.size() || ModifiedPHI);
211}
212