UnreachableBlockElim.cpp revision 90c579de5a383cee278acc3f7e7b9d0a656e6a35 
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
48    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
49      AU.addPreserved<ProfileInfo>();
50    }
51  };
52}
53char UnreachableBlockElim::ID = 0;
54INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim",
55                "Remove unreachable blocks from the CFG", false, false);
56
57FunctionPass *llvm::createUnreachableBlockEliminationPass() {
58  return new UnreachableBlockElim();
59}
60
61bool UnreachableBlockElim::runOnFunction(Function &F) {
62  SmallPtrSet<BasicBlock*, 8> Reachable;
63
64  // Mark all reachable blocks.
65  for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
66       df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
67    /* Mark all reachable blocks */;
68
69  // Loop over all dead blocks, remembering them and deleting all instructions
70  // in them.
71  std::vector<BasicBlock*> DeadBlocks;
72  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
73    if (!Reachable.count(I)) {
74      BasicBlock *BB = I;
75      DeadBlocks.push_back(BB);
76      while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
77        PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
78        BB->getInstList().pop_front();
79      }
80      for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
81        (*SI)->removePredecessor(BB);
82      BB->dropAllReferences();
83    }
84
85  // Actually remove the blocks now.
86  ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
87  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
88    if (PI) PI->removeBlock(DeadBlocks[i]);
89    DeadBlocks[i]->eraseFromParent();
90  }
91
92  return DeadBlocks.size();
93}
94
95
96namespace {
97  class UnreachableMachineBlockElim : public MachineFunctionPass {
98    virtual bool runOnMachineFunction(MachineFunction &F);
99    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
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
108static RegisterPass<UnreachableMachineBlockElim>
109Y("unreachable-mbb-elimination",
110  "Remove unreachable machine basic blocks");
111
112char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
113
114void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
115  AU.addPreserved<MachineLoopInfo>();
116  AU.addPreserved<MachineDominatorTree>();
117  MachineFunctionPass::getAnalysisUsage(AU);
118}
119
120bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
121  SmallPtrSet<MachineBasicBlock*, 8> Reachable;
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        }
184
185      if (phi->getNumOperands() == 3) {
186        unsigned Input = phi->getOperand(1).getReg();
187        unsigned Output = phi->getOperand(0).getReg();
188
189        MachineInstr* temp = phi;
190        ++phi;
191        temp->eraseFromParent();
192
193        if (Input != Output)
194          F.getRegInfo().replaceRegWith(Output, Input);
195
196        continue;
197      }
198
199      ++phi;
200    }
201  }
202
203  F.RenumberBlocks();
204
205  return DeadBlocks.size();
206}
207