UnreachableBlockElim.cpp revision d04a8d4b33ff316ca4cf961e06c9e312eff8e64f 
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/Analysis/Dominators.h"
27#include "llvm/Analysis/ProfileInfo.h"
28#include "llvm/CodeGen/MachineDominators.h"
29#include "llvm/CodeGen/MachineFunctionPass.h"
30#include "llvm/CodeGen/MachineLoopInfo.h"
31#include "llvm/CodeGen/MachineModuleInfo.h"
32#include "llvm/CodeGen/MachineRegisterInfo.h"
33#include "llvm/Constant.h"
34#include "llvm/Function.h"
35#include "llvm/Instructions.h"
36#include "llvm/Pass.h"
37#include "llvm/Support/CFG.h"
38#include "llvm/Target/TargetInstrInfo.h"
39#include "llvm/Type.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      initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry());
49    }
50
51    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
52      AU.addPreserved<DominatorTree>();
53      AU.addPreserved<ProfileInfo>();
54    }
55  };
56}
57char UnreachableBlockElim::ID = 0;
58INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim",
59                "Remove unreachable blocks from the CFG", false, false)
60
61FunctionPass *llvm::createUnreachableBlockEliminationPass() {
62  return new UnreachableBlockElim();
63}
64
65bool UnreachableBlockElim::runOnFunction(Function &F) {
66  SmallPtrSet<BasicBlock*, 8> Reachable;
67
68  // Mark all reachable blocks.
69  for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
70       df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
71    /* Mark all reachable blocks */;
72
73  // Loop over all dead blocks, remembering them and deleting all instructions
74  // in them.
75  std::vector<BasicBlock*> DeadBlocks;
76  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
77    if (!Reachable.count(I)) {
78      BasicBlock *BB = I;
79      DeadBlocks.push_back(BB);
80      while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
81        PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
82        BB->getInstList().pop_front();
83      }
84      for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
85        (*SI)->removePredecessor(BB);
86      BB->dropAllReferences();
87    }
88
89  // Actually remove the blocks now.
90  ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
91  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
92    if (PI) PI->removeBlock(DeadBlocks[i]);
93    DeadBlocks[i]->eraseFromParent();
94  }
95
96  return DeadBlocks.size();
97}
98
99
100namespace {
101  class UnreachableMachineBlockElim : public MachineFunctionPass {
102    virtual bool runOnMachineFunction(MachineFunction &F);
103    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
104    MachineModuleInfo *MMI;
105  public:
106    static char ID; // Pass identification, replacement for typeid
107    UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
108  };
109}
110char UnreachableMachineBlockElim::ID = 0;
111
112INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
113  "Remove unreachable machine basic blocks", false, false)
114
115char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
116
117void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
118  AU.addPreserved<MachineLoopInfo>();
119  AU.addPreserved<MachineDominatorTree>();
120  MachineFunctionPass::getAnalysisUsage(AU);
121}
122
123bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
124  SmallPtrSet<MachineBasicBlock*, 8> Reachable;
125  bool ModifiedPHI = false;
126
127  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
128  MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
129  MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
130
131  // Mark all reachable blocks.
132  for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
133       I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
134       I != E; ++I)
135    /* Mark all reachable blocks */;
136
137  // Loop over all dead blocks, remembering them and deleting all instructions
138  // in them.
139  std::vector<MachineBasicBlock*> DeadBlocks;
140  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
141    MachineBasicBlock *BB = I;
142
143    // Test for deadness.
144    if (!Reachable.count(BB)) {
145      DeadBlocks.push_back(BB);
146
147      // Update dominator and loop info.
148      if (MLI) MLI->removeBlock(BB);
149      if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
150
151      while (BB->succ_begin() != BB->succ_end()) {
152        MachineBasicBlock* succ = *BB->succ_begin();
153
154        MachineBasicBlock::iterator start = succ->begin();
155        while (start != succ->end() && start->isPHI()) {
156          for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
157            if (start->getOperand(i).isMBB() &&
158                start->getOperand(i).getMBB() == BB) {
159              start->RemoveOperand(i);
160              start->RemoveOperand(i-1);
161            }
162
163          start++;
164        }
165
166        BB->removeSuccessor(BB->succ_begin());
167      }
168    }
169  }
170
171  // Actually remove the blocks now.
172  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
173    DeadBlocks[i]->eraseFromParent();
174
175  // Cleanup PHI nodes.
176  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
177    MachineBasicBlock *BB = I;
178    // Prune unneeded PHI entries.
179    SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
180                                             BB->pred_end());
181    MachineBasicBlock::iterator phi = BB->begin();
182    while (phi != BB->end() && phi->isPHI()) {
183      for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
184        if (!preds.count(phi->getOperand(i).getMBB())) {
185          phi->RemoveOperand(i);
186          phi->RemoveOperand(i-1);
187          ModifiedPHI = true;
188        }
189
190      if (phi->getNumOperands() == 3) {
191        unsigned Input = phi->getOperand(1).getReg();
192        unsigned Output = phi->getOperand(0).getReg();
193
194        MachineInstr* temp = phi;
195        ++phi;
196        temp->eraseFromParent();
197        ModifiedPHI = true;
198
199        if (Input != Output) {
200          MachineRegisterInfo &MRI = F.getRegInfo();
201          MRI.constrainRegClass(Input, MRI.getRegClass(Output));
202          MRI.replaceRegWith(Output, Input);
203        }
204
205        continue;
206      }
207
208      ++phi;
209    }
210  }
211
212  F.RenumberBlocks();
213
214  return (DeadBlocks.size() || ModifiedPHI);
215}
216