UnreachableBlockElim.cpp revision 081c34b725980f995be9080eaec24cd3dfaaf065 
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/Constant.h"
25#include "llvm/Instructions.h"
26#include "llvm/Function.h"
27#include "llvm/Pass.h"
28#include "llvm/Type.h"
29#include "llvm/Analysis/ProfileInfo.h"
30#include "llvm/CodeGen/MachineDominators.h"
31#include "llvm/CodeGen/MachineFunctionPass.h"
32#include "llvm/CodeGen/MachineModuleInfo.h"
33#include "llvm/CodeGen/MachineLoopInfo.h"
34#include "llvm/CodeGen/MachineRegisterInfo.h"
35#include "llvm/Support/CFG.h"
36#include "llvm/Target/TargetInstrInfo.h"
37#include "llvm/ADT/DepthFirstIterator.h"
38#include "llvm/ADT/SmallPtrSet.h"
39using namespace llvm;
40
41namespace {
42  class UnreachableBlockElim : public FunctionPass {
43    virtual bool runOnFunction(Function &F);
44  public:
45    static char ID; // Pass identification, replacement for typeid
46    UnreachableBlockElim() : FunctionPass(ID) {
47      initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry());
48    }
49
50    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
51      AU.addPreserved<ProfileInfo>();
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  ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
89  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
90    if (PI) PI->removeBlock(DeadBlocks[i]);
91    DeadBlocks[i]->eraseFromParent();
92  }
93
94  return DeadBlocks.size();
95}
96
97
98namespace {
99  class UnreachableMachineBlockElim : public MachineFunctionPass {
100    virtual bool runOnMachineFunction(MachineFunction &F);
101    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
102    MachineModuleInfo *MMI;
103  public:
104    static char ID; // Pass identification, replacement for typeid
105    UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
106  };
107}
108char UnreachableMachineBlockElim::ID = 0;
109
110INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
111  "Remove unreachable machine basic blocks", false, false)
112
113char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
114
115void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
116  AU.addPreserved<MachineLoopInfo>();
117  AU.addPreserved<MachineDominatorTree>();
118  MachineFunctionPass::getAnalysisUsage(AU);
119}
120
121bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
122  SmallPtrSet<MachineBasicBlock*, 8> Reachable;
123  bool ModifiedPHI = false;
124
125  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
126  MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
127  MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
128
129  // Mark all reachable blocks.
130  for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
131       I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
132       I != E; ++I)
133    /* Mark all reachable blocks */;
134
135  // Loop over all dead blocks, remembering them and deleting all instructions
136  // in them.
137  std::vector<MachineBasicBlock*> DeadBlocks;
138  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
139    MachineBasicBlock *BB = I;
140
141    // Test for deadness.
142    if (!Reachable.count(BB)) {
143      DeadBlocks.push_back(BB);
144
145      // Update dominator and loop info.
146      if (MLI) MLI->removeBlock(BB);
147      if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
148
149      while (BB->succ_begin() != BB->succ_end()) {
150        MachineBasicBlock* succ = *BB->succ_begin();
151
152        MachineBasicBlock::iterator start = succ->begin();
153        while (start != succ->end() && start->isPHI()) {
154          for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
155            if (start->getOperand(i).isMBB() &&
156                start->getOperand(i).getMBB() == BB) {
157              start->RemoveOperand(i);
158              start->RemoveOperand(i-1);
159            }
160
161          start++;
162        }
163
164        BB->removeSuccessor(BB->succ_begin());
165      }
166    }
167  }
168
169  // Actually remove the blocks now.
170  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
171    DeadBlocks[i]->eraseFromParent();
172
173  // Cleanup PHI nodes.
174  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
175    MachineBasicBlock *BB = I;
176    // Prune unneeded PHI entries.
177    SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
178                                             BB->pred_end());
179    MachineBasicBlock::iterator phi = BB->begin();
180    while (phi != BB->end() && phi->isPHI()) {
181      for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
182        if (!preds.count(phi->getOperand(i).getMBB())) {
183          phi->RemoveOperand(i);
184          phi->RemoveOperand(i-1);
185          ModifiedPHI = true;
186        }
187
188      if (phi->getNumOperands() == 3) {
189        unsigned Input = phi->getOperand(1).getReg();
190        unsigned Output = phi->getOperand(0).getReg();
191
192        MachineInstr* temp = phi;
193        ++phi;
194        temp->eraseFromParent();
195        ModifiedPHI = true;
196
197        if (Input != Output)
198          F.getRegInfo().replaceRegWith(Output, Input);
199
200        continue;
201      }
202
203      ++phi;
204    }
205  }
206
207  F.RenumberBlocks();
208
209  return (DeadBlocks.size() || ModifiedPHI);
210}
211