UnreachableBlockElim.cpp revision b12ab97bbdb86e978bbaba62f3dbb402b53010da 
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/CodeGen/MachineFunctionPass.h"
30#include "llvm/CodeGen/MachineRegisterInfo.h"
31#include "llvm/Support/CFG.h"
32#include "llvm/Support/Compiler.h"
33#include "llvm/Target/TargetInstrInfo.h"
34#include "llvm/ADT/DepthFirstIterator.h"
35using namespace llvm;
36
37namespace {
38  class VISIBILITY_HIDDEN UnreachableBlockElim : public FunctionPass {
39    virtual bool runOnFunction(Function &F);
40  public:
41    static char ID; // Pass identification, replacement for typeid
42    UnreachableBlockElim() : FunctionPass((intptr_t)&ID) {}
43  };
44}
45char UnreachableBlockElim::ID = 0;
46static RegisterPass<UnreachableBlockElim>
47X("unreachableblockelim", "Remove unreachable blocks from the CFG");
48
49FunctionPass *llvm::createUnreachableBlockEliminationPass() {
50  return new UnreachableBlockElim();
51}
52
53bool UnreachableBlockElim::runOnFunction(Function &F) {
54  std::set<BasicBlock*> Reachable;
55
56  // Mark all reachable blocks.
57  for (df_ext_iterator<Function*> I = df_ext_begin(&F, Reachable),
58         E = df_ext_end(&F, Reachable); I != E; ++I)
59    /* Mark all reachable blocks */;
60
61  // Loop over all dead blocks, remembering them and deleting all instructions
62  // in them.
63  std::vector<BasicBlock*> DeadBlocks;
64  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
65    if (!Reachable.count(I)) {
66      BasicBlock *BB = I;
67      DeadBlocks.push_back(BB);
68      while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
69        PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
70        BB->getInstList().pop_front();
71      }
72      for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
73        (*SI)->removePredecessor(BB);
74      BB->dropAllReferences();
75    }
76
77  // Actually remove the blocks now.
78  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
79    DeadBlocks[i]->eraseFromParent();
80
81  return DeadBlocks.size();
82}
83
84
85namespace {
86  class VISIBILITY_HIDDEN UnreachableMachineBlockElim :
87        public MachineFunctionPass {
88    virtual bool runOnMachineFunction(MachineFunction &F);
89
90  public:
91    static char ID; // Pass identification, replacement for typeid
92    UnreachableMachineBlockElim() : MachineFunctionPass((intptr_t)&ID) {}
93  };
94}
95char UnreachableMachineBlockElim::ID = 0;
96
97static RegisterPass<UnreachableMachineBlockElim>
98Y("unreachable-mbb-elimination",
99  "Remove unreachable machine basic blocks");
100
101const PassInfo *const llvm::UnreachableMachineBlockElimID = &Y;
102
103bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
104  std::set<MachineBasicBlock*> Reachable;
105
106  // Mark all reachable blocks.
107  for (df_ext_iterator<MachineFunction*> I = df_ext_begin(&F, Reachable),
108         E = df_ext_end(&F, Reachable); I != E; ++I)
109    /* Mark all reachable blocks */;
110
111  // Loop over all dead blocks, remembering them and deleting all instructions
112  // in them.
113  std::vector<MachineBasicBlock*> DeadBlocks;
114  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
115    MachineBasicBlock *BB = I;
116
117    // Test for deadness.
118    if (!Reachable.count(BB)) {
119      DeadBlocks.push_back(BB);
120
121      while (BB->succ_begin() != BB->succ_end()) {
122        MachineBasicBlock* succ = *BB->succ_begin();
123
124        MachineBasicBlock::iterator start = succ->begin();
125        while (start != succ->end() &&
126               start->getOpcode() == TargetInstrInfo::PHI) {
127          for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
128            if (start->getOperand(i).isMBB() &&
129                start->getOperand(i).getMBB() == BB) {
130              start->RemoveOperand(i);
131              start->RemoveOperand(i-1);
132            }
133
134          start++;
135        }
136
137        BB->removeSuccessor(BB->succ_begin());
138      }
139    }
140  }
141
142  // Actually remove the blocks now.
143  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
144    DeadBlocks[i]->eraseFromParent();
145
146  // Cleanup PHI nodes.
147  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
148    MachineBasicBlock *BB = I;
149    // Prune unneeded PHI entries.
150    SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
151                                             BB->pred_end());
152    MachineBasicBlock::iterator phi = BB->begin();
153    while (phi != BB->end() &&
154           phi->getOpcode() == TargetInstrInfo::PHI) {
155      for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
156        if (!preds.count(phi->getOperand(i).getMBB())) {
157          phi->RemoveOperand(i);
158          phi->RemoveOperand(i-1);
159        }
160
161      if (phi->getNumOperands() == 3) {
162        unsigned Input = phi->getOperand(1).getReg();
163        unsigned Output = phi->getOperand(0).getReg();
164
165        MachineInstr* temp = phi;
166        ++phi;
167        temp->eraseFromParent();
168
169        if (Input != Output)
170          F.getRegInfo().replaceRegWith(Output, Input);
171
172        continue;
173      }
174
175      ++phi;
176    }
177  }
178
179  F.RenumberBlocks();
180
181  return DeadBlocks.size();
182}
183
184