UnreachableBlockElim.cpp revision 492d06efde44a4e38a6ed321ada4af5a75494df6 
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/Support/Compiler.h"
37#include "llvm/Target/TargetInstrInfo.h"
38#include "llvm/ADT/DepthFirstIterator.h"
39#include "llvm/ADT/SmallPtrSet.h"
40using namespace llvm;
41
42namespace {
43  class UnreachableBlockElim : public FunctionPass {
44    virtual bool runOnFunction(Function &F);
45  public:
46    static char ID; // Pass identification, replacement for typeid
47    UnreachableBlockElim() : FunctionPass(&ID) {}
48
49    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
50      AU.addPreserved<ProfileInfo>();
51    }
52  };
53}
54char UnreachableBlockElim::ID = 0;
55static RegisterPass<UnreachableBlockElim>
56X("unreachableblockelim", "Remove unreachable blocks from the CFG");
57
58FunctionPass *llvm::createUnreachableBlockEliminationPass() {
59  return new UnreachableBlockElim();
60}
61
62bool UnreachableBlockElim::runOnFunction(Function &F) {
63  SmallPtrSet<BasicBlock*, 8> Reachable;
64
65  // Mark all reachable blocks.
66  for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
67       df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
68    /* Mark all reachable blocks */;
69
70  // Loop over all dead blocks, remembering them and deleting all instructions
71  // in them.
72  std::vector<BasicBlock*> DeadBlocks;
73  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
74    if (!Reachable.count(I)) {
75      BasicBlock *BB = I;
76      DeadBlocks.push_back(BB);
77      while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
78        PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
79        BB->getInstList().pop_front();
80      }
81      for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
82        (*SI)->removePredecessor(BB);
83      BB->dropAllReferences();
84    }
85
86  // Actually remove the blocks now.
87  ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
88  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
89    if (PI) PI->removeBlock(DeadBlocks[i]);
90    DeadBlocks[i]->eraseFromParent();
91  }
92
93  return DeadBlocks.size();
94}
95
96
97namespace {
98  class UnreachableMachineBlockElim : public MachineFunctionPass {
99    virtual bool runOnMachineFunction(MachineFunction &F);
100    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
101    MachineModuleInfo *MMI;
102  public:
103    static char ID; // Pass identification, replacement for typeid
104    UnreachableMachineBlockElim() : MachineFunctionPass(&ID) {}
105  };
106}
107char UnreachableMachineBlockElim::ID = 0;
108
109static RegisterPass<UnreachableMachineBlockElim>
110Y("unreachable-mbb-elimination",
111  "Remove unreachable machine basic blocks");
112
113const PassInfo *const llvm::UnreachableMachineBlockElimID = &Y;
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
124  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
125  MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
126  MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
127
128  // Mark all reachable blocks.
129  for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
130       I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
131       I != E; ++I)
132    /* Mark all reachable blocks */;
133
134  // Loop over all dead blocks, remembering them and deleting all instructions
135  // in them.
136  std::vector<MachineBasicBlock*> DeadBlocks;
137  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
138    MachineBasicBlock *BB = I;
139
140    // Test for deadness.
141    if (!Reachable.count(BB)) {
142      DeadBlocks.push_back(BB);
143
144      // Update dominator and loop info.
145      if (MLI) MLI->removeBlock(BB);
146      if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
147
148      while (BB->succ_begin() != BB->succ_end()) {
149        MachineBasicBlock* succ = *BB->succ_begin();
150
151        MachineBasicBlock::iterator start = succ->begin();
152        while (start != succ->end() &&
153               start->getOpcode() == TargetInstrInfo::PHI) {
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    MachineBasicBlock *MBB = DeadBlocks[i];
172    // If there are any labels in the basic block, unregister them from
173    // MachineModuleInfo.
174    if (MMI && !MBB->empty()) {
175      for (MachineBasicBlock::iterator I = MBB->begin(),
176             E = MBB->end(); I != E; ++I) {
177        if (I->isLabel())
178          // The label ID # is always operand #0, an immediate.
179          MMI->InvalidateLabel(I->getOperand(0).getImm());
180      }
181    }
182    MBB->eraseFromParent();
183  }
184
185  // Cleanup PHI nodes.
186  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
187    MachineBasicBlock *BB = I;
188    // Prune unneeded PHI entries.
189    SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
190                                             BB->pred_end());
191    MachineBasicBlock::iterator phi = BB->begin();
192    while (phi != BB->end() &&
193           phi->getOpcode() == TargetInstrInfo::PHI) {
194      for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
195        if (!preds.count(phi->getOperand(i).getMBB())) {
196          phi->RemoveOperand(i);
197          phi->RemoveOperand(i-1);
198        }
199
200      if (phi->getNumOperands() == 3) {
201        unsigned Input = phi->getOperand(1).getReg();
202        unsigned Output = phi->getOperand(0).getReg();
203
204        MachineInstr* temp = phi;
205        ++phi;
206        temp->eraseFromParent();
207
208        if (Input != Output)
209          F.getRegInfo().replaceRegWith(Output, Input);
210
211        continue;
212      }
213
214      ++phi;
215    }
216  }
217
218  F.RenumberBlocks();
219
220  return DeadBlocks.size();
221}
222